**2. Experimental**

**1. Introduction**

194 Modern Technologies for Creating the Thin-film Systems and Coatings

sputtering techniques (MST) [16, 17].

porous former agent [22].

tion and distribution of defects.

ments [23–25].

It is known the diversity of materials in use today as bone substitutes. Among them the hydroxyapatite (HA) has deserved special attention because of its excellent biocompatibil‐ ity, almost the same of that of natural bone. Most of commercial HA used in clinical and research applications are in solid and granulated forms with pore sizes between 100 and 150 μm. It has been demonstrated that such a range of pore dimension is appropriate to cause tissue growth in direct applications as bone substitutes [1–5]. HA ceramics and thin films can be synthesized by many methods [3, 4]. The conventional chemical precipitation method is the more extended method [6]. Following chemical precipitation, combination methods and the hydrothermal process are the next most well‐known methods of preparing HA [6–10]. Nevertheless, the scientific community is devoting great efforts looking for new alternative methods in order to obtain hydroxyapatite ceramics with improved microstructural and cor‐ rosion properties. In this way, laser‐assisted bioprinting and pulsed laser deposition tech‐ niques are very promising methods to obtain this kind of hydroxyapatite ceramics and thin films [11–14]. Also, alternating current electric field modified synthesis [15] and magnetron

In this context, several methods of synthesis of HA with appropriately controlled textural characteristics as well as its use as a filler in formulations for systems in orthopedic sur‐ gery have been reported, however, the uniformity of the pore size is a problem unsolved up to now [18–20]. The sol‐gel synthesis of HA thin films and ceramics has attracted much attention because it offers a molecular‐level mixing of the calcium and phosphorus pre‐ cursors, which is capable of improving chemical homogeneity of the resulting HA to a significant extent, in comparison with conventional methods [18–20]. Fortunately, Vila et al*.* have obtained significant progress in recent years using the sol‐gel process [22]. In the context of the present work, they have obtained a bimodal porous process for nanocrystal‐ line hydroxyapatite (HA) coatings with pore sizes in the range of meso/macrometer scale deposited onto Ti6Al4V substrates by the sol‐gel method using nonionic surfactants as the

When phosphates are treated at several temperatures important changes occur in their properties, in particular, in their chemical contents and physicochemical characteristics, which permit an assessment of admixtures in the phosphates and the effects of substitution of the fundamental elements with others. But, on the other hand, the thermal treatment of the material poses a serious problem, due to difficulties to effectively control the gradients of pressure and temperature originated in different parts of the sample in most experi‐

The method for the thermal analysis developed by Rouquerol [26, 27], known as "control rate thermal analysis" (CRTA), has been tested in formulations for the control of textures in solids [28–30]. This method is very useful in cases of complex thermolysis usually lapses through superimposed, parallel or serial reactions. Thermal treatment at a controlled speed can allow the formation of homogeneous porosity and a homogeneous surface in its chemical composi‐

### **2.1. Preparation of hydroxyapatite**

Hydroxyapatite used in this study was obtained by hydrolysis and condensation of suit‐ able precursors following a water‐based sol‐gel process in accordance with the preparation method of Dean Mo Liu et al*.* [33–35]. Triethylphosphite (TEP), C6 H15O<sup>3</sup> P (Aldrich, 98%) and calcium nitrate tetrahydrate, Ca(NO3 )2 ·4H<sup>2</sup> O (Aldrich) were used as precursors of phospho‐ rus and calcium, respectively.

The preparation process includes the following stages: the first stage is the hydrolysis of the precursor of phosphorus. TEP is mixed with ultrapure distilled water under vigorous agita‐ tion. Given the immiscibility between TEP and water, the mixture initially becomes opaque. However, after 24 hours of agitation the emulsion is transformed into a clear dissolution indi‐ cating that the TEP is hydrolyzed completely. In the second stage, the saline precursor of cal‐ cium is added to the medium in a stoichiometric quantity (Ca/P molar ratio = 1.64) using a 4 M aqueous solution of nitrate of calcium. In this step, the agitation is continued for 30 min and then the mixture is left to stand for 24 hours at room temperature. The gelation is guaranteed by the evaporation of the solvent at 80°C, until a viscous liquid is obtained whose volume is about 40% of the initial solution.

### **2.2. Preparation of HA sol‐gel coatings on Ti6Al4V substrates**

Ti6Al4V disks of 2 cm of diameter and 0.4 cm of thickness were polished using different sili‐ con carbide grit up to 1200 grade. The substrates were ultrasonically degreased with acetone for 10 min and washed with distilled water. Finally, the substrates were dried at 200°C for one hour in an air oven to form a titanium oxide layer. The formation of TiO<sup>2</sup> layer might decrease the stress concentration and thermal expansion coefficient mismatch between the coatings and the titanium substrate.

These substrates were dip coated in the HA sol solution, with a dipping and withdraw speed of 12 cm/min. The sol‐coated substrates were then immediately transferred into an air oven and held at 80°C for 30 min to stabilize the deposited layer. To increase the coating thick‐

ness, the above process was repeated three times and finally it was thermally treated under conventional and controlled rate thermal treatments (CRTA). Cross section SEM micrographs revealed that the estimated thickness of all the HA crystalline sol‐gel derived coatings was about 1–2 μm. Nevertheless, these thicknesses were nonuniform due to roughness of the Ti6Al4V substrate.

### **2.3. Crystallization of hydroxyapatite under conventional and controlled rate thermal treatment**

The conventional thermal treatment was carried out in a furnace, burning the precipitate at various temperatures (600, 800°C) for 2 h, at a heating speed of 2°C/min. For the controlled rate thermal analysis (CRTA), samples of 1 g were placed in a quartz sample holder, which was introduced in a programmable tubular oven with Eurotherm control of ±1°C error temperature and connected to a vacuum group, where a Pirani for measuring pressure and a diaphragm with an aperture of 0.1 mm are already present. A home‐made software allows the regulation of the temperature and the measurement of generated pressure. This last parameter is the one that regulates the transformation rate. The basis of this thermal treatment is to control the tem‐ perature and the pressure system, keeping the decomposition rate constant. **Figure 1** shows a photograph of the CRTA equipment and a simplified schematic diagram of the device.

In parallel studies a set of HA sol‐gel coatings previously deposited on Ti6Al4V substrates was densified at the optimal pressure and temperature, which is determined by CRTA. The adequate pressure was accomplished with the use of a vacuum pump connected to the muffle furnace.

### **2.4. Characterization of the powders of hydroxyapatite**

The relation Ca/P was calculated starting from the percentage of Ca, determined by absorp‐ tion spectroscopy in Philips Pye Unicam SP9 at a λ = 422.7 nm and the percentage of P obtained by emission spectrometry in a Perkin Elmer Capture 40 at a λ = 213.6 nm. On the other hand, powders were characterized by infrared spectroscopy (IR) in a PHILIPS FTIR PU 9800, using the method of pills of KBr and X‐ray diffraction (XRD), in a Philips Pye Unicam PW1710, by the method of powders. The thermogravimetric analysis (TGA) was carried out

**Figure 1.** Equipment for controlled rate thermal treatment (CRTA).

with 30 mg of the sample in a SHIMATZU at a speed of 10°C/min, up to 1200°C. The mor‐ phology of powders was examined by scanning electron microscopy (SEM) in a SEM Tescan Vega TS 5130SB. The specific surface (BET) and porosity of the material were determined in Coulter equipment; model Omnisorp TM 100, starting from isotherms of adsorption for N<sup>2</sup> at a temperature of 77 K.

### **2.5. Cytotoxicity/osteoblasts adhesion**

ness, the above process was repeated three times and finally it was thermally treated under conventional and controlled rate thermal treatments (CRTA). Cross section SEM micrographs revealed that the estimated thickness of all the HA crystalline sol‐gel derived coatings was about 1–2 μm. Nevertheless, these thicknesses were nonuniform due to roughness of the

**2.3. Crystallization of hydroxyapatite under conventional and controlled rate thermal** 

The conventional thermal treatment was carried out in a furnace, burning the precipitate at various temperatures (600, 800°C) for 2 h, at a heating speed of 2°C/min. For the controlled rate thermal analysis (CRTA), samples of 1 g were placed in a quartz sample holder, which was introduced in a programmable tubular oven with Eurotherm control of ±1°C error temperature and connected to a vacuum group, where a Pirani for measuring pressure and a diaphragm with an aperture of 0.1 mm are already present. A home‐made software allows the regulation of the temperature and the measurement of generated pressure. This last parameter is the one that regulates the transformation rate. The basis of this thermal treatment is to control the tem‐ perature and the pressure system, keeping the decomposition rate constant. **Figure 1** shows a photograph of the CRTA equipment and a simplified schematic diagram of the device.

In parallel studies a set of HA sol‐gel coatings previously deposited on Ti6Al4V substrates was densified at the optimal pressure and temperature, which is determined by CRTA. The adequate pressure was accomplished with the use of a vacuum pump connected to the muffle

The relation Ca/P was calculated starting from the percentage of Ca, determined by absorp‐ tion spectroscopy in Philips Pye Unicam SP9 at a λ = 422.7 nm and the percentage of P obtained by emission spectrometry in a Perkin Elmer Capture 40 at a λ = 213.6 nm. On the other hand, powders were characterized by infrared spectroscopy (IR) in a PHILIPS FTIR PU 9800, using the method of pills of KBr and X‐ray diffraction (XRD), in a Philips Pye Unicam PW1710, by the method of powders. The thermogravimetric analysis (TGA) was carried out

**2.4. Characterization of the powders of hydroxyapatite**

196 Modern Technologies for Creating the Thin-film Systems and Coatings

**Figure 1.** Equipment for controlled rate thermal treatment (CRTA).

Ti6Al4V substrate.

**treatment**

furnace.

The cytocompatibility of coated samples was analyzed by indirect contact as described in ISO 10993‐5 (ISO Standards 1999). Briefly, HA coated Ti6Al4V alloy were placed in culture plates and incubated in 15 ml of culture medium (DMEM, Dulbecco's Modified Eagle Medium, Gibco) without fetal bovine serum (FBS) for 24 hours at 37°C. The supernatant of this mixture is called pure extract (100%) which then subjected to dilutions of 0, 10, and 50%.

Fibroblast cells (BALB/c, 3T3, ATCC clone A31) were purchased from American Type Culture Collection (ATCC; MD, USA) and seeded in 24‐well plates and cultured in DMEM. 1% antibi‐ otic was added, supplemented with 10% FBS and kept in incubator at 37°C in 5% CO<sup>2</sup> atmo‐ sphere. The culture medium was then replaced by extracts of the material to which 10% FBS was added and after 24 hours, the cells were counted using a Neubauer camera. The number of cells cultured in DMEM containing 10% FBS alone was considered the negative control (corresponding to 0% of the extract dilution).

The experiment was carried out six times, means and standard deviations were subjected to a variance analysis, considering significant differences if *p* < 0.05. Additionally, were seeded, 30.000 cells of pre osteoblasts of femur of Balb/c 3T3 (FOST) in plates with Ti6Al4V coated samples, these were maintained in culture supplement DMEM with 10% FBS at 37°C and atmosphere of 5% CO2 for 24 hours. At the end of this period, the plates were moved and the cells were fixed in 2.5% of glutaraldehyde solution, then subjected to treatment with 0.1 M of cacodylate buffer solution at pH 7.3 for 24 hours.

After the cells were washed twice with buffer solution 0.1 M cacodylate, dehydrated with increasing concentrations of alcohol (50–100%), they were immersed in ethanol‐hexamethyld‐ isilazane absolute solution (50:50 v/v) and then in hexamethyldisilazane (100%) and dried for 24 hours. Finally, metallization of samples with palladium‐gold allowed them to be observed using a scanning electron microscope (SEM).

### **2.6. Corrosion behavior**

The corrosion behavior of the HA film/Ti6Al4V system was evaluated by applying electro‐ chemical impedance spectroscopy (EIS) [21, 31, 32]. These electrochemical measurements were performed using an AutoLab potentiostat/galvanostat PGSTAT30 equipped with a FRA2 frequency response analyzer module (EcoChemie, The Netherlands). A standard three‐elec‐ trode cell was used for this purpose. The working electrode was the investigated sample with an area of 3.14 cm<sup>2</sup> . The reference and the counter‐electrode were a saturated calomel elec‐ trode (SCE) and a large size graphite sheet, respectively. The electrochemical cell was filled with Kokubo's solution. (SBF; pH = 7.4) [36, 37]. The EIS measurements were made at the open circuit potential (OCP). Logarithmic frequency scans were carried out by applying sinusoi‐ dal wave perturbations of ±10 mV in amplitude, in the range of 105 –10‐3Hz. Five impedance sampling points were registered per frequency decade. The impedance data were analyzed by using the ZView software, version 3.5a (Scribner Associates Inc, Southern Pines, NC, USA).

### **3. Results and discussion**

#### **3.1. Characterization of the powders of hydroxyapatite**

The Ca/P rate was determined by chemical analyses (absorption spectroscopy for the Ca and emission spectrometry for the P) of the powder preparations and was 1.64. This rate is appro‐ priate to keep the apatite structure after the thermal treatment.

It has been found reports are scarce in the literature of application of controlled rate thermal treatment (CRTA) technique, for treatments of HA. Consequently, a thermogravimetric anal‐ ysis (TGA) of as‐prepared HA green powders (without a previous thermal treatment) was carried out (**Figure 2**) for determining the characteristic temperatures of HA decomposition. The TGA showed different stages in the thermolysis of HA, the first one is associated with the dehydration. The second one and last one are associated with dehydration‐crystallization. These processes have been studied by other authors [38–40]. However, practically no effort has been dedicated to study the influence of the experimental conditions used for the thermal decomposition on the morphology of the final products.

The results obtained for TGA, were verified by CRTA (**Figure 3**) to obtain characteristic tem‐ peratures and partial pressure for each step of crystallization of the HA. **Table 1** shows the

**Figure 2.** TGA of as‐prepared HA green powders.

**Figure 3.** CRTA of as‐prepared HA green powders.

circuit potential (OCP). Logarithmic frequency scans were carried out by applying sinusoi‐

sampling points were registered per frequency decade. The impedance data were analyzed by using the ZView software, version 3.5a (Scribner Associates Inc, Southern Pines, NC, USA).

The Ca/P rate was determined by chemical analyses (absorption spectroscopy for the Ca and emission spectrometry for the P) of the powder preparations and was 1.64. This rate is appro‐

It has been found reports are scarce in the literature of application of controlled rate thermal treatment (CRTA) technique, for treatments of HA. Consequently, a thermogravimetric anal‐ ysis (TGA) of as‐prepared HA green powders (without a previous thermal treatment) was carried out (**Figure 2**) for determining the characteristic temperatures of HA decomposition. The TGA showed different stages in the thermolysis of HA, the first one is associated with the dehydration. The second one and last one are associated with dehydration‐crystallization. These processes have been studied by other authors [38–40]. However, practically no effort has been dedicated to study the influence of the experimental conditions used for the thermal

The results obtained for TGA, were verified by CRTA (**Figure 3**) to obtain characteristic tem‐ peratures and partial pressure for each step of crystallization of the HA. **Table 1** shows the

–10‐3Hz. Five impedance

dal wave perturbations of ±10 mV in amplitude, in the range of 105

**3.1. Characterization of the powders of hydroxyapatite**

198 Modern Technologies for Creating the Thin-film Systems and Coatings

decomposition on the morphology of the final products.

**Figure 2.** TGA of as‐prepared HA green powders.

priate to keep the apatite structure after the thermal treatment.

**3. Results and discussion**


**Table 1.** Experimental conditions for the CRTA.

experimental parameters used in the different CRTA, which seeks to determine the optimal parameters of crystallization of HA at the lower temperature; where *r*<sup>c</sup> is the controlled rate, *T*cp is the temperature of control of the pressure, *p*<sup>c</sup> is the pressure of control and *T*m is the maximum temperature used to achieve such pressure control.

Analyses by IR and XRD demonstrated that both types of the sample, conventional treated and treated with CRTA, were pure crystalline phases of HA.

In **Figure 4** are shown the IR spectra typical of HA without thermal treatment (HA‐green), conventional thermal treatment (HA‐CTT) and after CRTA (HA‐1 to HA‐4), where the charac‐ teristic bands observed are reported for this material type, corresponding to the fundamental vibrations 3571.46 and 631.73 cm‐1 of the OH‐ and *ν*<sup>3</sup> 1092.75 and 1045.49 cm‐1, *ν*<sup>1</sup> 963.51 cm‐1, *ν*<sup>4</sup> 602.80 and 568.08 cm‐1 of the PO4 3‐ [41].

The bands are very similar in all the IR spectra (**Figure 4**). The wide band from OH‐ vibra‐ tion only could be observed in the green‐HA and in HA‐CTT and was due to hydrate water

**Figure 4.** FTIR spectrum characteristic of an as‐prepared HA‐green sample and HA samples after the thermal treatments.

In **Figure 4** are shown the IR spectra typical of HA without thermal treatment (HA‐green), conventional thermal treatment (HA‐CTT) and after CRTA (HA‐1 to HA‐4), where the charac‐ teristic bands observed are reported for this material type, corresponding to the fundamental

and *ν*<sup>3</sup>

tion only could be observed in the green‐HA and in HA‐CTT and was due to hydrate water

**Figure 4.** FTIR spectrum characteristic of an as‐prepared HA‐green sample and HA samples after the thermal treatments.

The bands are very similar in all the IR spectra (**Figure 4**). The wide band from OH‐

3‐ [41].

1092.75 and 1045.49 cm‐1, *ν*<sup>1</sup>

963.51 cm‐1, *ν*<sup>4</sup>

vibra‐

vibrations 3571.46 and 631.73 cm‐1 of the OH‐

200 Modern Technologies for Creating the Thin-film Systems and Coatings

602.80 and 568.08 cm‐1 of the PO4

**Figure 5.** X‐ray diffraction pattern of as‐prepared HA green powders and HA samples obtained by applying different thermal treatments.

(3442.22 cm‐1). All bands become narrower and more symmetrical for HA‐3 and HA‐4, indi‐ cating an increase in the crystallinity of the material according to results obtained by other authors [42, 43].

**Figure 5** shows X‐ray diffraction, where typical lines for this material appear. In all cases, the signs appeared in X‐ray diffraction corresponds to those reported in Chart No. 9‐432 of ASTM [44, 45].

By applying conventional thermal treatments, HA was typically calcined above 900°C in order to obtain a stoichiometric, apatitic structure. However, it is interesting to note that applying CRTA the degree of crystallization of HA in this study at temperatures as low as 300°C it can be observed in **Figure 6a** and **6b**. The HA here obtained was composed of white tiny crystals, where the particles are fused together and, consequently, they are forming a cluster. This phenomenon might be attributed to a high surface area to volume ratio of ultrafine crys‐ tals related to thermal treatments. It should be also pointed out that the shape of HA grains is quite different from the biological apatite, which mostly exhibits a needle‐like structure. However, we expect that it may be possible to obtain a grain shape similar to biological apa‐ tite by optimizing CRTA conditions.

Results for surface measures by BET method are described in **Table 2**. In the table, the range of specific surface areas achieved after CRTA can be observed. Samples HA‐1 and HA‐2 showed remarkable dependence on pressure for the surface area, increasing almost to double when

**Figure 6.** SEM micrographs under different magnifications of HA‐2 powders prepared using CRTA (Tm = 300°C).


**Table 2.** Determinations of the surface area obtained by the BET method for HA‐CTT and for different CRTA experimental conditions.

this parameter diminishes, at equal temperature of control and final. It is standing out that both samples, in spite of having been treated at 100°C as the temperature of control for pres‐ sure, and at 300°C as the final temperature, crystallized in a pure phase of hydroxyapatite.

In samples HA‐3 and HA‐4, also the same behavior is also observed, that is to say, an increase of the surface area when diminishing the control pressure, in this case it is four times less. Although for both samples, the surface area notably diminishes when increasing the tempera‐ ture of control and the final temperature of the process.

In summary, CRTA technology offers a better resolution and a more detailed interpretation of the decomposition processes of hydroxyapatite via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a control‐ ling parameter on the process of decomposition.

### **3.2. Cytotoxicity and adhesion of osteoblasts**

this parameter diminishes, at equal temperature of control and final. It is standing out that both samples, in spite of having been treated at 100°C as the temperature of control for pres‐ sure, and at 300°C as the final temperature, crystallized in a pure phase of hydroxyapatite.

**Table 2.** Determinations of the surface area obtained by the BET method for HA‐CTT and for different CRTA experimental

**Figure 6.** SEM micrographs under different magnifications of HA‐2 powders prepared using CRTA (Tm = 300°C).

**/g)**

**Sample S (BET) (m2**

HA‐CTT 14.0 HA‐1 34.9 HA‐2 66.7 HA‐3 17.2 HA‐4 26.0

202 Modern Technologies for Creating the Thin-film Systems and Coatings

conditions.

In samples HA‐3 and HA‐4, also the same behavior is also observed, that is to say, an increase of the surface area when diminishing the control pressure, in this case it is four times less. The cytotoxicity assay allows toxicological risk assessment of a material by using cell cultures. Taking into account that migrating substances from biomaterials interact at the cellular level with cell membranes, the cellular organelles (mitochondria and Liposomes), the synthesis of proteins and DNA, cell division and the sequence of DNA, this essay covers from the cell

**Figure 7.** Relative number of cells as a function of exposure to different concentrations of extract for a Ti‐6Al‐4V alloy coated by sol‐gel with HA‐2 film.

**Figure 8.** SEM images of cells morphology cultured on Ti‐6Al‐4V sheets coated by sol‐gel with HA‐2 film.

viability and death to more sophisticated forms that have to do with the cell functionality and genotoxicity.

**Figure 7** shows, the percent (%) of living cells in each one of the tested extracts. The concentra‐ tion of 0 corresponds to the negative control, or cells without being subjected to any concen‐ tration of the extracts.

It can be observed that there is no significant difference (*p* > 0.05) in the number of living cells exposed to different concentrations of the extracts of cultures with alloys coated by HA sol‐ gel, with respect to the control group. This indicates that HA coatings on Ti6Al4V does not affect the viability of the cells evaluated, which demonstrating its cytocompatibility.

SEM (micrographs) of preosteoblast cells femur seeded in Ti6Al4V alloy coated by sol‐gel are shown in **Figure 8**. It is observed that cells were emitting cytoplasmic extensions (filopodia and pseudopodia) that indicate adhesion to the substrate. In addition, mitotic phases were observed (large cells) which suggested that the cells were divided. Apparently, all layers of HA indicate a good biocompatibility because the living cells of osteoblasts hold and spread (propagated) well over all the coating. The observed biocompatibility of HA could be due to roughness and surface porosity that provides sites for attachment and growth of cells.

#### **3.3. Corrosion behavior**

The corrosion protection behavior of the HA films deposited on Ti6Al4V samples was evalu‐ ated by applying electrochemical impedance spectroscopy (EIS). **Figure 9** shows the Bode impedance spectra for the tested samples the in Kokubo's solution at variable immersion time (1 hour and 1, 3, 7, 15 and 30 days).

**Figure 9.** Bode impedance spectra representing the evolution of the impedance modulus (|Z|) in a double‐logarithmic scale and the phase angle in a semi‐logarithmic scale versus frequency for Ti6Al4V/HA coating systems at different immersion time in Kokubo's solution. Coatings: HA‐1 and HA‐2.

viability and death to more sophisticated forms that have to do with the cell functionality and

**Figure 7** shows, the percent (%) of living cells in each one of the tested extracts. The concentra‐ tion of 0 corresponds to the negative control, or cells without being subjected to any concen‐

It can be observed that there is no significant difference (*p* > 0.05) in the number of living cells exposed to different concentrations of the extracts of cultures with alloys coated by HA sol‐ gel, with respect to the control group. This indicates that HA coatings on Ti6Al4V does not

SEM (micrographs) of preosteoblast cells femur seeded in Ti6Al4V alloy coated by sol‐gel are shown in **Figure 8**. It is observed that cells were emitting cytoplasmic extensions (filopodia and pseudopodia) that indicate adhesion to the substrate. In addition, mitotic phases were observed (large cells) which suggested that the cells were divided. Apparently, all layers of HA indicate a good biocompatibility because the living cells of osteoblasts hold and spread (propagated) well over all the coating. The observed biocompatibility of HA could be due to

affect the viability of the cells evaluated, which demonstrating its cytocompatibility.

roughness and surface porosity that provides sites for attachment and growth of cells.

The corrosion protection behavior of the HA films deposited on Ti6Al4V samples was evalu‐ ated by applying electrochemical impedance spectroscopy (EIS). **Figure 9** shows the Bode impedance spectra for the tested samples the in Kokubo's solution at variable immersion time

**Figure 9.** Bode impedance spectra representing the evolution of the impedance modulus (|Z|) in a double‐logarithmic scale and the phase angle in a semi‐logarithmic scale versus frequency for Ti6Al4V/HA coating systems at different

genotoxicity.

tration of the extracts.

204 Modern Technologies for Creating the Thin-film Systems and Coatings

**3.3. Corrosion behavior**

(1 hour and 1, 3, 7, 15 and 30 days).

immersion time in Kokubo's solution. Coatings: HA‐1 and HA‐2.

**Figure 10.** Electrical equivalent circuits used for studying the corrosion behaviour of the Ti6Al4V/hydroxyapatite coating system in contact with Kokubo's solution. One time‐constant circuit (A) and two time‐constant circuit (B).

In these Bode plots, the modulus of the impedance |Z| and the phase angle are represented versus the frequency, the first one in double‐logarithmic scale and the second in semi‐loga‐ rithmic scale.

These impedance spectra can be ascribed to the typical behavior of porous thin films deposited on metal substrates with high corrosion resistance [21, 31, 32, 46, 47]. In the first approach, the single electrical equivalent circuit EEC1 shown in **Figure 10a** can be used to describe the electrochemical behavior of these systems. *R*<sup>s</sup> is associated with the resistance of the electrolyte sited between the working electrode and the reference electrode. The problem is to assign a cor‐ rect physical meaning to the elements *C*1 and *R*2. Respectively, *C*1 could be associated with the coating capacitance or to the double layer capacitance at the base of the pores filled with elec‐ trolyte into the thin film. *R*2 could be ascribed the ionic resistance of the coating pores impreg‐ nated with electrolyte or to charge transfer of the metal/electrolyte interface at the base of the pores. In some cases, the electrical equivalent circuit EEC2 shown in **Figure 10b** could be more convenient to describe the impedance plots of these metal/coating systems [48, 49]. Following the notation of the ZView software [49], *R*<sup>s</sup> is the solution resistance of the bulk electrolyte. *C*coat is the capacitance of the coating. In this case *C*coat is implemented as a constant phase element (CPE). *R*coat is the resistance of the coating and *C*dl represents the double layer capacitance of the electrolyte/metal surface interface. This capacitance is also implemented as a CPE.

As a representative example, **Figure 11** shows fit results obtained by using these two electri‐ cal equivalent circuits and complex nonlinear least‐squares (CNLS) analysis methods. Three types of impedance plots are given in **Figure 11**, i.e.; Nyquist plot (for real and imaginary values of IZI), Bode plot (for IZI versus applied frequency) and the other Bode plot (for the phase angle versus frequency). This example corresponds to the Ti6Al4V/hydroxyapatite sys‐ tem based on the HA‐1 coating after 1 hour in contact with Kokubo's solution. The fit plots generated by the EEC1 and EEC2 electrical equivalent circuits proposed are good. Physical meaning of the values of the electrical elements of the corresponding equivalent circuit and relative errors in % are also good. Finally, the chi‐squared values (χ<sup>2</sup> ) are also very acceptable (9.9 ×10‐4 for EEC1 and 3.9 ×10‐4 for EEC2). It is known that low values of χ<sup>2</sup> are related to a better quality of the fitting results [31, 32].

However due to the uncertainty associated with the difficult interpretation of the results gen‐ erated by these adjustments and simulations, has been more useful to follow the variations of the impedance modulus |Z| at the lowest frequency as a function of exposure time of coat‐ ings to the Kokubo's solution. This parameter has allowed to reevaluate systematically the results obtained with the impedance measurement.

**Figure 11.** Nyquist plots (A), Bode Impedance spectra (B) and fit results obtained by applying the EEC1 and EEC2 electrical equivalent circuits to a Ti6Al4V/hydroxyapatite system based on the HA‐1 coating after 1 hour in contact with Kokubo's solution.

**Table 3** shows the variations of the impedance modulus |Z| at a frequency of 10 mHz with the immersion time for Ti6Al4V/hydroxyapatite systems based on the coatings HA‐1 and HA‐2, respectively. It can observe from the evolution of the values of the parameter |Z|10mHz that both systems show a satisfactory stability when they are tested in a saline solution. Particularly attractive was the protective behavior of the HA‐1 coating whose |Z|10mHz values remained almost constant during the 30 days of the immersion test. However, for the system based on the HA‐2 coating, although very slowly the values of this parameter decrease, down


**Table 3.** Variations of the impedance modulus |Z| for 10 MHz frequency with the immersion time for coatings based on the samples HA‐1 and HA‐2, respectively.

from 7.14 × 105 ohm/cm<sup>2</sup> at the start to 2.88 × 105 ohm/cm<sup>2</sup> at the end of the immersion test (30 days). This behavior can be ascribed to a slow loss of the protection properties of the coating HA‐2 due to the ingress of electrolyte in the coating pores. These results are indi‐ cating that the increase of the control pressure of the CRTA associated with the decrease in specific surface (BET) produces an enhancment of the corrosion protection behavior of the hydroxyapatite coatings. This means that a high specific surface is good for enhancing the adhesion of the preosteoblast cells but provokes a decrease in the corrosion protection of the HA coatings. It is necessary to reach a compromise to balance both properties.

### **4. Conclusion**

the impedance modulus |Z| at the lowest frequency as a function of exposure time of coat‐ ings to the Kokubo's solution. This parameter has allowed to reevaluate systematically the

**Table 3** shows the variations of the impedance modulus |Z| at a frequency of 10 mHz with the immersion time for Ti6Al4V/hydroxyapatite systems based on the coatings HA‐1 and HA‐2, respectively. It can observe from the evolution of the values of the parameter |Z|10mHz that both systems show a satisfactory stability when they are tested in a saline solution. Particularly attractive was the protective behavior of the HA‐1 coating whose |Z|10mHz values remained almost constant during the 30 days of the immersion test. However, for the system based on the HA‐2 coating, although very slowly the values of this parameter decrease, down

**Figure 11.** Nyquist plots (A), Bode Impedance spectra (B) and fit results obtained by applying the EEC1 and EEC2 electrical equivalent circuits to a Ti6Al4V/hydroxyapatite system based on the HA‐1 coating after 1 hour in contact with

**)**

**Table 3.** Variations of the impedance modulus |Z| for 10 MHz frequency with the immersion time for coatings based on

1 hour 6.60 × 105 7.14 × 105 1 day 6.23 105 5.74 × 105 3 days 7.31 × 105 4.10 × 105 7 days 7.00 × 105 3.26 × 105 15 days 6.07 × 105 2.97 × 105 30 days 6.54 × 105 2.88 × 105

**Coating HA‐1 Coating HA‐2**

results obtained with the impedance measurement.

206 Modern Technologies for Creating the Thin-film Systems and Coatings

**Immersion time |Z|10mHz (ohm/cm2**

Kokubo's solution.

the samples HA‐1 and HA‐2, respectively.

In the present work, it was demonstrated the effectiveness and usefulness of the CRTA tech‐ nique, for the preparation of crystallization of powders of synthetic hydroxyapatite and thin films with different specific surface areas, making this technique attractive for medical pur‐ poses. The purity of the phase of the samples obtained by CRTA was proved by IR spectros‐ copy and XRD. Several temperatures of control for pressure and watchword pressures were tested, observing the dependence of the specific surface area to these parameters, making possible to obtain surface areas from 14 up to 66 m<sup>2</sup> /g. It was possible to crystallize pure hydroxyapatite at temperature of 100°C of control of the pressure and 300°C as maximum temperature. Moreover, the results of this study have also indicated that it was also pos‐ sible to cover commercial Ti6Al4V alloy with these sol‐gel‐derived hydroxyapatite thin films. Cytotoxicity tests and corrosion studies showed an improvement for coated surfaces com‐ pared to the base Ti6Al4V alloy. Biocompatibility expressed in terms of adhesion of living cells and their spread on coating was also adequate. According to the ISO 10993‐5 standard, the system was considered nontoxic. The cytocompatibility test shows that the sol‐gel coat‐ ing did not provoke the cell death significantly higher than the control (*p* > 0.05). In addition, the electrochemical impedance spectra confirm that these sol‐gel coatings show promising corrosion protection properties. It can conclude that the sol‐gel process in conjunction with the CRTA method can be a viable alternative for the production of crystallized synthetic hydroxyapatite thin films and ceramics with controlled specific surface and homogeneous distribution of pores, appropriate to be used in bone implantation and other formulations for orthopedic surgery.

### **Acknowledgements**

This work has been supported by the National Program for Materials, Spanish Ministry of Science and Innovation (Project MAT2012‐38541‐C02‐02) and by the Spanish Agency for International Development Cooperation (AECI) under the expert fund project entitled "Development of calcium phosphate ceramics for medical use". SEM images of cells were carried out in COPPE UFRJ.

### **Author details**


### **References**


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**Provisional chapter**

### **Radio Frequency Magnetron Sputter Deposition as a Tool for Surface Modification of Medical Implants Tool for Surface Modification of Medical Implants**

**Radio Frequency Magnetron Sputter Deposition as a** 

Roman Surmenev, Alina Vladescu, Maria Surmeneva, Anna Ivanova, Mariana Braic, Irina Grubova and Cosmin Mihai Cotrut Surmeneva, Anna Ivanova, Mariana Braic, Irina Grubova and Cosmin Mihai Cotrut

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

Roman Surmenev, Alina Vladescu, Maria

http://dx.doi.org/10.5772/66396

#### **Abstract**

[48] B. Chico, J. C. Galván, D. de la Fuente, M. Morcillo, "Electrochemical impedance spec‐ troscopy study of the effect of curing time on the early barrier properties of silane systems applied on steel substrates", *Progress in Organic Coatings*, Vol. 60, Iss. 1, pp.

[49] ZView 3.5a Software, Scribner Association Inc., D. Johnson. Available from: http://www.

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scribner.com/ [accessed: 2016‐09‐19].

212 Modern Technologies for Creating the Thin-film Systems and Coatings

The resent advances in radio frequency (RF)‐magnetron sputtering of hydroxyapatite films are reviewed and challenges posed. The principles underlying RF‐magnetron sput‐ tering used to prepare calcium phosphate‐based, mainly hydroxyapatite coatings, are discussed in this chapter. The fundamental characteristic of the RF‐magnetron sputtering is an energy input into the growing film. In order to tailor the film properties, one has to adjust the energy input into the substrate depending on the desired film properties. The effect of different deposition control parameters, such as deposition time, substrate tem‐ perature, and substrate biasing on the hydroxyapatite (HA) film properties is discussed.

**Keywords:** Hydroxyapatite, magnetron sputtering, corrosion resistance, cell viability

### **1. Introduction**

It is well known that the long‐term success of the dental and orthopedic implants is deter‐ mined by a good osseointegration, which can be guaranteed by a good connection between the bone cell and implant. This connection is dependent on the phenomena which can take place immediately after insertion of the implant in human body. The first process after implantation is the interface between implant and the proteins, by formatting a thin layer which will act as a mediator of a good proliferation of the cells. Thus, protein adsorption determines the nature of the interface between the bone and implant, which will stimulate a fast cell growth, leading to a rapid osseointegration of the implant. In the past few years, it was demonstrated that the osseointegration of the metallic implants could be increased by coating the implant surface

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

with bioactive coatings, which proved to accelerate the bone bonding rate. It was certified by World Biomaterial Congress in 2008 and 2012 and 2016 that this topic is one of the major top‐ ics in biomaterials.

Various different techniques are currently available for deposition of calcium phosphate (CaP), in particular hydroxyapatite (HA) coating, to metallic materials, including plasma spraying, pulsed laser deposition, biomimetic crystallization methods, electrophoretic depo‐ sition, sol‐gel deposition, magnetron sputtering, etc. [1].

Among the listed methods, plasma spraying is the only approach which is commercially approved for HA coatings deposition on metal implants by the food and drug administration (FDA) [1]. The method is based on the formation of a condensed layer of individual particles deposited on a metal substrate. The particles originating from a powder material are carried by a gas stream and passed through electrical plasma produced by a low voltage, high cur‐ rent electrical discharge. During this process, the heated particles crystallize and agglomerate during film formation. The coating features are determined by the chemical and mechanical properties of the used powder material, by the distance between a source and a substrate, the current of the electric arc, the deposition rate, and the work gas composition. Plasma spray‐ ing allows to produce coatings up to 300 μm in thickness. This technique has some significant limitations: poor uniformity in coating thickness and adherence to substrate, low crystallin‐ ity, poor mechanical properties on tensile strength, wear resistance, hardness, toughness, and fatigue [2]. Furthermore, plasma spraying does not allow to produce an uniform HA coating on substrates with complex geometry. Meanwhile, the most important disadvantages of this method are considered to be the presence of impurity phases. A higher temperature (6,000– 10,000°C) is used during plasma spraying, the crystal structure of the HA powder can be easily destabilized and decomposition into mixture of HA, CaO, tricalcium phosphate, and tetra‐ calcium phosphate, and a considerable amount of amorphous phases is occurs [3]. Structural inhomogeneity can lead to differences in coating resorption [3] and a reduction in coating‐ substrate interfacial strength [4, 5]. The alternative coating approaches have been extensively developed and tested to overcome the weaknesses of plasma spraying, namely sol‐gel deposi‐ tion and RF‐magnetron sputtering. An overview of these three techniques is given in **Table 1.**

Sol‐gel deposition is a widespread method to produce CaP coatings [6, 7]. This method is based on the preparation of a suspension (sol) in the dispersion phase with its subsequent transition into a gel and the treatment of a metal surface with the resulting colloid. Thermal treatment at the coating material's crystallization temperature is required as the final step. The method makes it possible to produce a dense CaP coating with the thickness of 0.5–30 μm. Sol‐gel depo‐ sition is a relatively inexpensive technique compared to others. The method has the potential to coat implant with complex shape by using simple setup [8]. Furthermore, it has the benefits of phase and structural uniformity [9, 10]. However, too low processing temperature leads to an amorphous or a nanocrystalline coating structure and requires, therefore, additional annealing of the coating to increase the degree of crystallinity. The major advantages of sol‐gel method are good mechanical properties, corrosion resistance, and adhesion strength due to their nano‐ crystalline structure [11, 12]. However, the sol‐gel deposition has disadvantages such as high permeability, low wear‐resistance, and difficult porosity control, which hinders its commercial


with bioactive coatings, which proved to accelerate the bone bonding rate. It was certified by World Biomaterial Congress in 2008 and 2012 and 2016 that this topic is one of the major top‐

Various different techniques are currently available for deposition of calcium phosphate (CaP), in particular hydroxyapatite (HA) coating, to metallic materials, including plasma spraying, pulsed laser deposition, biomimetic crystallization methods, electrophoretic depo‐

Among the listed methods, plasma spraying is the only approach which is commercially approved for HA coatings deposition on metal implants by the food and drug administration (FDA) [1]. The method is based on the formation of a condensed layer of individual particles deposited on a metal substrate. The particles originating from a powder material are carried by a gas stream and passed through electrical plasma produced by a low voltage, high cur‐ rent electrical discharge. During this process, the heated particles crystallize and agglomerate during film formation. The coating features are determined by the chemical and mechanical properties of the used powder material, by the distance between a source and a substrate, the current of the electric arc, the deposition rate, and the work gas composition. Plasma spray‐ ing allows to produce coatings up to 300 μm in thickness. This technique has some significant limitations: poor uniformity in coating thickness and adherence to substrate, low crystallin‐ ity, poor mechanical properties on tensile strength, wear resistance, hardness, toughness, and fatigue [2]. Furthermore, plasma spraying does not allow to produce an uniform HA coating on substrates with complex geometry. Meanwhile, the most important disadvantages of this method are considered to be the presence of impurity phases. A higher temperature (6,000– 10,000°C) is used during plasma spraying, the crystal structure of the HA powder can be easily destabilized and decomposition into mixture of HA, CaO, tricalcium phosphate, and tetra‐ calcium phosphate, and a considerable amount of amorphous phases is occurs [3]. Structural inhomogeneity can lead to differences in coating resorption [3] and a reduction in coating‐ substrate interfacial strength [4, 5]. The alternative coating approaches have been extensively developed and tested to overcome the weaknesses of plasma spraying, namely sol‐gel deposi‐ tion and RF‐magnetron sputtering. An overview of these three techniques is given in **Table 1.** Sol‐gel deposition is a widespread method to produce CaP coatings [6, 7]. This method is based on the preparation of a suspension (sol) in the dispersion phase with its subsequent transition into a gel and the treatment of a metal surface with the resulting colloid. Thermal treatment at the coating material's crystallization temperature is required as the final step. The method makes it possible to produce a dense CaP coating with the thickness of 0.5–30 μm. Sol‐gel depo‐ sition is a relatively inexpensive technique compared to others. The method has the potential to coat implant with complex shape by using simple setup [8]. Furthermore, it has the benefits of phase and structural uniformity [9, 10]. However, too low processing temperature leads to an amorphous or a nanocrystalline coating structure and requires, therefore, additional annealing of the coating to increase the degree of crystallinity. The major advantages of sol‐gel method are good mechanical properties, corrosion resistance, and adhesion strength due to their nano‐ crystalline structure [11, 12]. However, the sol‐gel deposition has disadvantages such as high permeability, low wear‐resistance, and difficult porosity control, which hinders its commercial

sition, sol‐gel deposition, magnetron sputtering, etc. [1].

214 Modern Technologies for Creating the Thin-film Systems and Coatings

ics in biomaterials.

**Table 1.** The advantages and disadvantages of the most applied methods for HA coating deposition.

application [13]. Meanwhile, annealing can lead to a deterioration of the coating's adhesion. Furthermore, adsorbed organics within the sol‐gel process can also cause coating failure.

Despite all the advantages of the above‐described methods, their most essential limitation is the difficulty to control the phase and chemical composition of a CaP coating. In its turn, RF‐magnetron sputtering allows to control the properties of CaP films within a rather wide range and to form a dense, uniform coating to devices with complex configurations with high adhesion and with uniformity in thickness and composition [17–24].

This high flexibility makes RF‐magnetron sputtering, however, a rather complex method, especially for the deposition of multicomponent materials such as calcium phosphates. There are many process parameters that can have a direct effect on the coating's characteristics. For example, the coating composition can be influenced by the target composition and sputter‐ ing parameters such as gas pressure, substrate bias, and the deposition temperature. During RF‐magnetron sputtering, the dense plasma interacts strongly with the substrate [25], which causes an intense ion bombardment of the growing coating. The energetic particle bombard‐ ment may determine the growth film. During the RF discharge, the positive ions are acceler‐ ated and bombard the substrate with high energies, which are dependent on the discharge excitation frequency. It is plausible that the plasma density is higher in front of the substrate opposite to the plasma torus above the erosion racetrack. Under target erosion zone, the bom‐ bardment of the substrate surface by high energetic oxygen species (O‐ ) occurs, which was confirmed by a number of authors [26–30]. These ions are generated at the target surface, accelerated in the cathode dark space and move with a high energy perpendicular from the target toward the substrate surface [31–33]. Cai et al. [34] described the effect of a local change in the growth rate of ZnO coating in the region of the target erosion zone which is connected with the sputtering of the coating with negatively charged ions. So, the Ar<sup>+</sup> and O− bombard‐ ment is the major part of the energetic particle bombardment occurring in RF discharges. It plays an important role during the film deposition in RF sputtering, because the temperature of the substrate may increase with increasing discharge power. The properties of the RF‐mag‐ netron sputter‐deposited films are highly influenced by the bombardment of the growing film with species from the sputtering target and from the plasma. The latter is determined by the deposition parameters such as the working gas pressure and composition, target‐substrate distance, and substrate bias voltage. Different energetic and thermal circumstances may result in a different final quality and structure of the applied coating. Control of these parameters is essential to modify the HA coating structural properties, its composition, and mechanical characteristics.

This chapter reports on the influence of discharge RF‐power, substrate temperature, and spa‐ tial sample arrangement regarding the target erosion zone on the properties of the CaP films, its mechanical properties, and behavior *in vitro*.

### **2. Literature overview of RF‐magnetron sputtering of CaP coating and its comparison with chemical method**

### **2.1. Principles of RF‐magnetron sputtering**

A magnetron sputtering system is a technological equipment which allows depositing thin films by sputtering of a target material in a magnetron discharge plasma. This type of sys‐ tem is based on the formation of electric and magnetic fields perpendicular to each other in the near‐cathode region. By supplying a voltage between the cathode and the anode, a glow discharge is ignited. When the voltage is applied, the free electrons are repelled from the cathode or target and collide with the atoms of the working gas, creating ions, and new electrons. The positive ions are accelerated toward the target. The collision of the positive, energetic ions with the target leads to its sputtering. Particles removed from the target surface are transported to the substrate and the chamber walls. Not only atoms but also emission of electrons occurs due to the interaction of the ion flux with the target surface. The amount of emitted electrons to each approaching ion is known as the secondary electron emission yield and depends on the properties of the target material, the energy, and the type of bombard‐

ing particles. Secondary electrons are necessary for the ionization of the working gas and the maintenance of the discharge.

RF‐magnetron sputtering, the dense plasma interacts strongly with the substrate [25], which causes an intense ion bombardment of the growing coating. The energetic particle bombard‐ ment may determine the growth film. During the RF discharge, the positive ions are acceler‐ ated and bombard the substrate with high energies, which are dependent on the discharge excitation frequency. It is plausible that the plasma density is higher in front of the substrate opposite to the plasma torus above the erosion racetrack. Under target erosion zone, the bom‐

confirmed by a number of authors [26–30]. These ions are generated at the target surface, accelerated in the cathode dark space and move with a high energy perpendicular from the target toward the substrate surface [31–33]. Cai et al. [34] described the effect of a local change in the growth rate of ZnO coating in the region of the target erosion zone which is connected

ment is the major part of the energetic particle bombardment occurring in RF discharges. It plays an important role during the film deposition in RF sputtering, because the temperature of the substrate may increase with increasing discharge power. The properties of the RF‐mag‐ netron sputter‐deposited films are highly influenced by the bombardment of the growing film with species from the sputtering target and from the plasma. The latter is determined by the deposition parameters such as the working gas pressure and composition, target‐substrate distance, and substrate bias voltage. Different energetic and thermal circumstances may result in a different final quality and structure of the applied coating. Control of these parameters is essential to modify the HA coating structural properties, its composition, and mechanical

This chapter reports on the influence of discharge RF‐power, substrate temperature, and spa‐ tial sample arrangement regarding the target erosion zone on the properties of the CaP films,

**2. Literature overview of RF‐magnetron sputtering of CaP coating and its** 

A magnetron sputtering system is a technological equipment which allows depositing thin films by sputtering of a target material in a magnetron discharge plasma. This type of sys‐ tem is based on the formation of electric and magnetic fields perpendicular to each other in the near‐cathode region. By supplying a voltage between the cathode and the anode, a glow discharge is ignited. When the voltage is applied, the free electrons are repelled from the cathode or target and collide with the atoms of the working gas, creating ions, and new electrons. The positive ions are accelerated toward the target. The collision of the positive, energetic ions with the target leads to its sputtering. Particles removed from the target surface are transported to the substrate and the chamber walls. Not only atoms but also emission of electrons occurs due to the interaction of the ion flux with the target surface. The amount of emitted electrons to each approaching ion is known as the secondary electron emission yield and depends on the properties of the target material, the energy, and the type of bombard‐

) occurs, which was

bombard‐

and O−

bardment of the substrate surface by high energetic oxygen species (O‐

216 Modern Technologies for Creating the Thin-film Systems and Coatings

with the sputtering of the coating with negatively charged ions. So, the Ar<sup>+</sup>

characteristics.

its mechanical properties, and behavior *in vitro*.

**comparison with chemical method**

**2.1. Principles of RF‐magnetron sputtering**

The magnetic field holds electrons in immediate proximity to the target in a so‐called electron "trap" that is created by the intersecting electric and magnetic fields. The electrons oscillate in this trap until several ionizing collisions with atoms of the working gas occur. The plasma is localized above the target surface, due to the presence of the magnetic field. Hence, the target surface is sputtered in areas located between the magnets of the magnetic system. As a result, an erosion zone (racetrack) is created in the form of a closed‐loop path with a shape determined by the magnetic system.

The RF plasma is conducted by electron ionization, which exhibited an oscillating movement at the RF‐magnetron frequency of 13.56 MHz. At this frequency, the ions could not pursue these oscillations due to their mechanical inertia. This excitation is much more effective than the ionization by nonoscillating secondary electrons, leading to decrease of the voltage of the RF discharge. During the positive half‐cycle, the target acts not as a cathode but as an anode. Therefore, the plasma density in front of the substrate is significantly higher for RF. **Figure 1** shows the potential distributions, in which the positive ions (Ar<sup>+</sup> , O<sup>+</sup> , and Ca<sup>+</sup> ) are accelerated in the cathode fall Vp‐Vdc and the target sputtering will take place. At the same time, the elec‐ trons and negative ions (O‐ ) were moved from the target to the substrate, which along to the reflected neutral argon atoms will arrive at the substrate and perform the growth of the coat‐ ing. In **Figure 2**, the effects of energetic particles on a solid surface during ion‐assisted growth during the RF discharge are shown. The secondary and back‐scattered electrons, as well as the reflected ions and neutrals, cause a higher plasma density in front of the substrate for RF excitation and hence a higher ion saturation current to the growing film.

It can be seen that the electrons are kept out of the substrate and only those, which have a sufficiently high energy, will be able to pass through the potential barrier and arrive to the substrate, even if they have a low current. Both neutral species and high‐energy negative ions (i.e., O‐ ) are capable of striking the substrate.

**Figure 1.** Potential distribution in a magnetron sputtering discharge, excited by RF.

**Figure 2.** The effects of energetic particles on a solid surface during ion assisted growth [35].

The properties of the RF‐magnetron sputter‐deposited films are highly influenced by the bom‐ bardment of the growing film with species from the sputtering target and from the plasma. The latter is determined by the deposition parameters such as the working gas pressure and composition, target‐substrate distance, and substrate bias voltage. Control of these parame‐ ters is essential to modify the HA coating structural properties, its composition and mechani‐ cal characteristics. The thermal and energetic conditions at the substrate surface influenced by the different plasma species determine the elementary processes (adsorption, diffusion, and chemical reactions) as well as the microstructure and stoichiometry of the film growth. The energy available per incoming particle and ion‐to‐atom ratio is, therefore, essential in plasma processing of solid surfaces in the case of thin film growth. Functional properties of the thin films are largely determined by the intrinsic coating features, which defined not only by the material properties but, to a large extend, also by the thin film growth mechanism. Passing through several stages, adsorption, nucleation growth, and increase film thickness, a defined coating structure is formed. The extended structure zone model identifies the evolution of a polycrystalline thin film and its relation with the deposition conditions.

#### **2.2. Morphology of RF‐magnetron sputtering of CaP coating**

The surface morphology of the HA coatings appears to play a significant role in implant‐tis‐ sue interaction and osseointegration [1]. RF‐magnetron sputtering allows to deposit dense, uniform coating, without apparent defects (cracks, gas bubbles, and others) keeping the ini‐ tial substrate topography [1, 36]. The latter is beneficial in case of porous scaffolds and other substrates with the complex structure. Meanwhile, it is well known that the coating surface morphology is connected to their growth mechanisms. In this way, it varies according to the deposition process conditions. Most often CaP coatings produced by RF‐magnetron sputter‐ ing at room temperature possess a low crystalline or amorphous structure. It occurs due to the energy flux arriving the substrate at the applied process conditions that is not high enough to ensure crystalline coating formation on the unheated substrate holder. To induce the crystal‐

linity of the coatings and transform the amorphous calcium phosphate into HA, the thermal treatment at *T* > 500°C (*in situ* and *ex situ*) is applied [37–42].

The surface morphology of the coatings to be shown strongly depends on the substrate tem‐ perature. Bramowicz et al. [43] performed the deposition on silicon substrates with the tem‐ perature varied in the range of 400–800°C (**Figure 3**). The sample deposited at 400°C was observed to exhibit some circular cavities. At the increase of the deposition temperature, the cavities started to overlap, leading to the formation of uniform grains with comparable size. At 500°C, a threshold in the growth mode was observed, as the predominant morphology (cavities in otherwise flat surface) turned into a series of convex grains with well‐developed grain boundaries. The authors concluded that the deposited HA coatings exhibit bifractal behavior, their surface topography can be thought of as two interpenetrating spatial struc‐ tures with different characteristic length scales (cavities and clusters of cavities), which inde‐ pendently evolve with the deposition temperature.

The change of the routine of the coating preparation by adjusting the process parameters, such as substrate‐target distance, working gas pressure, bias potential on substrate holder, ensur‐ ing higher energy flux arriving at the substrate allow to obtain crystalline coating at room temperature [44–47]. López et al. [44] published a study on the control of the thermodynamic properties of the plasma to form a coating with higher crystallinity. The authors modified the sputtering geometry by positioning two magnetrons face‐to‐face with a substrate holder kept in a floating electric potential positioned at a right angle to the magnetrons (off‐axis). It was shown that at an RF‐power density of 24 W/cm2 after 180 min of sputtering, the transforma‐ tion of amorphous phase in the coating to the crystalline one occurred. Surmeneva et al. [48] in their work deposited higher crystalline coatings by sputtering a Si‐containing HA target using a setup with an RF‐magnetron source (5.28 MHz) at an RF‐power density of 0.5 W/cm2 and a target‐substrate distance of 40 mm. The experiments were performed with a grounded substrate holder (bias 0 V) and a bias voltage of ‐50 or ‐100 V. The temperature of the substrate

The properties of the RF‐magnetron sputter‐deposited films are highly influenced by the bom‐ bardment of the growing film with species from the sputtering target and from the plasma. The latter is determined by the deposition parameters such as the working gas pressure and composition, target‐substrate distance, and substrate bias voltage. Control of these parame‐ ters is essential to modify the HA coating structural properties, its composition and mechani‐ cal characteristics. The thermal and energetic conditions at the substrate surface influenced by the different plasma species determine the elementary processes (adsorption, diffusion, and chemical reactions) as well as the microstructure and stoichiometry of the film growth. The energy available per incoming particle and ion‐to‐atom ratio is, therefore, essential in plasma processing of solid surfaces in the case of thin film growth. Functional properties of the thin films are largely determined by the intrinsic coating features, which defined not only by the material properties but, to a large extend, also by the thin film growth mechanism. Passing through several stages, adsorption, nucleation growth, and increase film thickness, a defined coating structure is formed. The extended structure zone model identifies the evolution of a

The surface morphology of the HA coatings appears to play a significant role in implant‐tis‐ sue interaction and osseointegration [1]. RF‐magnetron sputtering allows to deposit dense, uniform coating, without apparent defects (cracks, gas bubbles, and others) keeping the ini‐ tial substrate topography [1, 36]. The latter is beneficial in case of porous scaffolds and other substrates with the complex structure. Meanwhile, it is well known that the coating surface morphology is connected to their growth mechanisms. In this way, it varies according to the deposition process conditions. Most often CaP coatings produced by RF‐magnetron sputter‐ ing at room temperature possess a low crystalline or amorphous structure. It occurs due to the energy flux arriving the substrate at the applied process conditions that is not high enough to ensure crystalline coating formation on the unheated substrate holder. To induce the crystal‐

polycrystalline thin film and its relation with the deposition conditions.

**Figure 2.** The effects of energetic particles on a solid surface during ion assisted growth [35].

218 Modern Technologies for Creating the Thin-film Systems and Coatings

**2.2. Morphology of RF‐magnetron sputtering of CaP coating**

**Figure 3.** 10 × 10 μm, 2 planar AFM images of residual surfaces of the HA substrate on Si substrate deposited at: (A) 400°C, (B) 500°C, (C) 600°C, (D) 700°C, and (E) 800°C. Insets: 3D projections of marked areas to show changes in surface morphology [43].

during deposition reached 200°C due to heating by plasma. The effect of ion bombardment on the morphology and microstructure modification of Si‐HA coatings is shown in **Figure 4**. The typical surface morphology of the HA coatings deposited on a flat silicon grounded substrate consisted of mound‐shape grains (**Figure 4a**). By applying a substrate bias a distinct decrease in the morphological features dimension was observed. The coating cross‐section structure was studied by SEM. The samples were prepared by the chemical etching of one half of the coating in 1 M aqueous HCl. The etched regions of the coating are shown in **Figure 4** (right side). The SEM study showed that the coatings deposited on the grounded substrate con‐ sisted of dense columnar grains grown perpendicular to the substrate surface. The columnar structure is the typical characteristic of films deposited by means of magnetron sputtering. The physical reason for the phenomenon of this structure is explained by Krug [49] and Bales and Zangwill [50] as a shadowing effect which can occur if adatoms impinge on the substrate under an angle which deviates from the substrate normal. With negative bias, the columnar structure was completely replaced by a very fine equiaxed grain structure which is reflected in the surface morphology. Therefore, it is considered that due to applying the negative bias, the particles arriving on the growing film have higher enough energy to disrupt column growth and force renucleation. Moreover, an increased ion bombardment may induce coating resput‐ tering effect resulting in flat surface morphology. Thereby, the HA coatings can be deposited by RF‐magnetron sputtering in such a way to control the coating morphology.

#### **2.3. Composition of RF‐magnetron sputtering of CaP coating**

The thermodynamic stability, reactivity, solubility, and mechanical properties of CaPs were reported to strongly depend on the Ca/P ratio [4]. The calcium phosphate with low Ca/P ratio proves to have high dissolution rate. When the Ca/P ratio is equal to 1.67, the stoichiometric compound is obtained, which is referred as hydroxyapatite (HA). In biomedical applications,

**Figure 4.** SEM images of the Si substrate coated with HA coating at the grounded substrate holder (a) 0 V, (b) −50 and (c) −100 V. Left: top view; right: side view after etching. The R<sup>a</sup> was measured before etching [48].

this compound is the most desired to obtain because the Ca/P ratio is close to that of natu‐ ral bone. Thus, the Ca/P ratio is one of the main characteristics of a biocompatible film and it depends on the applied deposition control parameters such as RF‐power, substrate bias, working gas pressure, and configuration of the samples in the vacuum chamber [51]. It is shown that the ratio of elements in the deposited coating may differ substantially from their ratio in the target [24, 28, 52]. It was reported that when sputtering from multicomponent ceramic targets, such as superconducting oxides, HA, and other CaP materials, the alteration in coating composition may occur due to the preferential sputtering, which can initially cause the stoichiometry of the film to deviate from that of the target. However, at steady state, the composition of the sputtered flux must be the same as the target composition unless extensive diffusion occurs in the target [53]. It was also reported that at least 1000 Å (or more of the mul‐ ticomponent target) need to be removed before the coating would reflect the stoichiometry of the bulk target [54]. Thus, the composition of the coating may be quite different from that of the target material, depending on the type of sputtering system, and parameters used for deposition.

during deposition reached 200°C due to heating by plasma. The effect of ion bombardment on the morphology and microstructure modification of Si‐HA coatings is shown in **Figure 4**. The typical surface morphology of the HA coatings deposited on a flat silicon grounded substrate consisted of mound‐shape grains (**Figure 4a**). By applying a substrate bias a distinct decrease in the morphological features dimension was observed. The coating cross‐section structure was studied by SEM. The samples were prepared by the chemical etching of one half of the coating in 1 M aqueous HCl. The etched regions of the coating are shown in **Figure 4** (right side). The SEM study showed that the coatings deposited on the grounded substrate con‐ sisted of dense columnar grains grown perpendicular to the substrate surface. The columnar structure is the typical characteristic of films deposited by means of magnetron sputtering. The physical reason for the phenomenon of this structure is explained by Krug [49] and Bales and Zangwill [50] as a shadowing effect which can occur if adatoms impinge on the substrate under an angle which deviates from the substrate normal. With negative bias, the columnar structure was completely replaced by a very fine equiaxed grain structure which is reflected in the surface morphology. Therefore, it is considered that due to applying the negative bias, the particles arriving on the growing film have higher enough energy to disrupt column growth and force renucleation. Moreover, an increased ion bombardment may induce coating resput‐ tering effect resulting in flat surface morphology. Thereby, the HA coatings can be deposited

by RF‐magnetron sputtering in such a way to control the coating morphology.

The thermodynamic stability, reactivity, solubility, and mechanical properties of CaPs were reported to strongly depend on the Ca/P ratio [4]. The calcium phosphate with low Ca/P ratio proves to have high dissolution rate. When the Ca/P ratio is equal to 1.67, the stoichiometric compound is obtained, which is referred as hydroxyapatite (HA). In biomedical applications,

**Figure 4.** SEM images of the Si substrate coated with HA coating at the grounded substrate holder (a) 0 V, (b) −50 and (c)

was measured before etching [48].

**2.3. Composition of RF‐magnetron sputtering of CaP coating**

220 Modern Technologies for Creating the Thin-film Systems and Coatings

−100 V. Left: top view; right: side view after etching. The R<sup>a</sup>

**Figure 5** shows the typical spectra for the fitted high resolution XPS obtained for O1s, Ca2p, and P2p regions of CaP films deposited by RF‐magnetron sputtering onto titanium substrates. For the studied CaP coating, the O1s envelope (**Figure 5a**) was fitted with energy O1s = 531.9 eV of the calcium in the structure of HA. The energy of Ca2p3/2 = 347.1 eV (**Figure 5b**) was established for all Ca‐O bindings. Finally, the P2p was fitted with two binding energies for the P2p3/2 peak (**Figure 5c**): (i) the P2p3/2 = 132.9 eV was established to the phosphorous bonded to the oxygen in the (PO<sup>4</sup> )3‐—groups in the hydroxyapatite structure and (ii) the P2p3/2 = 133.8 eV was attributed to the P‐O bindings in the calcium phosphate phase. For nanocrystalline HA coatings deposited via the RF‐magnetron sputtering the ratio of Ca/P was reported in the range between 1.6 and 2.9 [47, 55]. The optimum Ca/P ratio was reported to be in the range of 1.67–1.76 [4].

**Figure 5.** XPS: (a) O 1s, (b) Ca 2p, and (c) P 2p spectra of the CaP coating deposited via RF magnetron sputtering onto a titanium substrate [56].

At the low deposition temperature an amorphous coating structure was obtained; the increase of the deposition temperature leads to the Ca/P ratio change in the range of 1.41–1.69 [43]. It was demonstrated that the Ca/P ratio achieved the value of 1.63–1.69 for samples prepared at temperatures between 600 and 800°C. It was found that the Ca/P ratio of the coating is dif‐ ferent than that of the target, due to specific target sputtering mechanisms. Moreover, it was reported that P ions are pumped away before reaching the substrate [17]. In the present case, at relatively low deposition temperature, the deposition conditions yielded Ca‐deficient films, whereas temperature increase resulted in stoichiometric HA films. Possible phenomena caus‐ ing these results include differences in sticking and removal rates of atoms on the growth sur‐ face and gas scattering phenomena [53]. Film growth at relatively high temperatures implies that the sticking probability of the incoming species can be less than unity, which in the case of compound growth can result in modified film composition.

An increase of the negative bias applied to the substrate led to the increase of the coating crystallinity and of Ca/P ratio from 1.53 to 3.88 [52, 55, 56]. Feddes et al. [57] explained this phenomenon by assuming that phosphorus was resputtered from the growing film surface by ion bombardment with the energy determined by the potential drop in the cathode dark sheath. The authors of the study reported that calcium was carried by positively charged radicals (e.g., CaO<sup>+</sup> ) and ions (e.g., Ca<sup>+</sup> and Ca2+) generated in the plasma [57]. Moreover, it is explained that higher negative substrate biasing resulted in higher fluxes of CaO<sup>+</sup> cations onto the surface and it became more difficult for (PO<sup>4</sup> )3− anions to reach the surface, which explained the higher Ca/P ratios at higher negative biases [48].

The composition of the CaP coatings may be controlled by the RF‐magnetron sputtering and may be changed by deposition temperature. The increase of the deposition temperature leads

**Figure 6.** Elemental composition determined by XPS of the CaP coating deposited via RF magnetron sputtering at different deposition temperature onto a silicon substrate [43].

to the Ca/P ratio change in the range 1.41–1.69 [43]. The deposition temperatures influence the Ca/P ratio, which achieves 1.63–1.69, that is very close to the stoichiometric HA (Ca/P = 1.67), for samples prepared at temperatures between 600 and 800°C (**Figure 6**).

### **2.4. Water addition into an working gas atmosphere effect of RF‐magnetron sputtering of CaP coating**

A similar trend was observed for the coatings deposited in H2 O‐containing atmosphere by Ivanova et al. [28] (see **Figure 7**). The Ca/P ratio varied within the range of 1.53–1.70, and first increased with the distance from the center of the substrate holder. The highest Ca/P ratio was obtained for the samples exposed above the racetrack. Feddes et al. [57] reported that the P ions can be resputtered by negatively charged oxygen ions, leading to the variation of the Ca/P. Also Takayangi et al. [58] found that the high‐energy negative ions appear in the erosion area of the oxidized cathode due to a large amount of electrons which are trapped by mag‐ netic field within this zone. So, the negative oxygen ions formed as well as Ar ion bombard‐ ment of the growing HA film caused its stoichiometric deviation from the target composition. So, the RF‐magnetron sputtering is well‐suited method to prepare coatings with different Ca/P molar ratios by variation of the substrate temperature, substrate bias, and position of the sample with regard to the target erosion zone [48].

**Figure 7.** The relation between the sample positions on the substrate holder: (a) and a Ca/P ratio, (b) as function of the time in the race track per period [28].

#### **2.5. Microstructure of RF‐magnetron sputtering of CaP coating**

**Figure 6.** Elemental composition determined by XPS of the CaP coating deposited via RF magnetron sputtering at

At the low deposition temperature an amorphous coating structure was obtained; the increase of the deposition temperature leads to the Ca/P ratio change in the range of 1.41–1.69 [43]. It was demonstrated that the Ca/P ratio achieved the value of 1.63–1.69 for samples prepared at temperatures between 600 and 800°C. It was found that the Ca/P ratio of the coating is dif‐ ferent than that of the target, due to specific target sputtering mechanisms. Moreover, it was reported that P ions are pumped away before reaching the substrate [17]. In the present case, at relatively low deposition temperature, the deposition conditions yielded Ca‐deficient films, whereas temperature increase resulted in stoichiometric HA films. Possible phenomena caus‐ ing these results include differences in sticking and removal rates of atoms on the growth sur‐ face and gas scattering phenomena [53]. Film growth at relatively high temperatures implies that the sticking probability of the incoming species can be less than unity, which in the case

An increase of the negative bias applied to the substrate led to the increase of the coating crystallinity and of Ca/P ratio from 1.53 to 3.88 [52, 55, 56]. Feddes et al. [57] explained this phenomenon by assuming that phosphorus was resputtered from the growing film surface by ion bombardment with the energy determined by the potential drop in the cathode dark sheath. The authors of the study reported that calcium was carried by positively charged

The composition of the CaP coatings may be controlled by the RF‐magnetron sputtering and may be changed by deposition temperature. The increase of the deposition temperature leads

is explained that higher negative substrate biasing resulted in higher fluxes of CaO<sup>+</sup>

and Ca2+) generated in the plasma [57]. Moreover, it

)3− anions to reach the surface, which

cations

of compound growth can result in modified film composition.

222 Modern Technologies for Creating the Thin-film Systems and Coatings

) and ions (e.g., Ca<sup>+</sup>

explained the higher Ca/P ratios at higher negative biases [48].

onto the surface and it became more difficult for (PO<sup>4</sup>

radicals (e.g., CaO<sup>+</sup>

different deposition temperature onto a silicon substrate [43].

At the low energy flux into the substrate, the amorphous HA coating is growing [59]. Thus, the deposition temperature of the HA coating plays an important role to the formation of the crystalline structure, which influence many other properties of the coatings. **Figure 8** shows the evolution of the crystallinity on the deposition temperature. At low temperature, the CaP coating shows only two peaks: (002) and (202). As the deposition temperature increased, more peaks are seen, the (200), (222), (213), and (004) planes) indicating the formation of the crystal‐ line structure. The grain sizes, calculated by the Scherrer formula, increase with the deposi‐ tion temperature, resulting in crystallites aggregation due to the higher adatoms mobility.

**Figure 8.** XRD spectra of the CaP coatings deposited at different deposition temperature [59].

The phase composition and structure of the CaP coating depend on the process conditions as it was mentioned above. RF‐magnetron sputtering allows to deposit the CaP coatings of either amorphous or crystalline structure of a certain phase composition that along with the Ca/P ratio influences the coating behavior *in vitro* and *in vivo* [60]. The high dissolution rates of the amorphous lead to long‐term stability reduction of the implanted devises. With the aim to maintain the HA coating integrity the researchers apply the postdeposition or *in situ* anneal‐ ing of the films. Several authors reported that the transition from amorphous to crystalline coatings can be controlled by the heat‐treatment temperatures and heating environment (air

**Figure 9.** XRD spectra show variations of the intensity distributions, revealing that the studied samples have a different texture [48].

and water vapor) [61–63]. Meanwhile, crystalline coating can be obtained by turning the energy of the bombarding ions or ion to atom ratio arriving the substrate through manipula‐ tion with the substrate bias, working gas composition, pressure, and target‐substrate distance.

The influence of the substrate bias voltage on the structure of the Si‐containing HA coating was studied by Surmeneva et al. [48]. The X‐ray diffractograms of the deposited coatings consisted of the reflexes corresponding well to the expected Bragg peaks for hydroxyapatite (ICDD PDF No. 9‐432) (**Figure 9**). With a grounded substrate, the strongest peak of the Si‐HA coating was the reflection from the (002) plan resolved at 25.9°. Thus, crystallites of Si‐HA preferentially grew in the (002) crystallographic orientation perpendicular to the substrate surface. With an increase in the substrate bias voltage to −100 V, the intensity of the (002) peak relative to the other peaks was observed to decrease. The XRD pattern of the coating at nega‐ tive bias showed broad overlapping peaks around 32°, which indicates the decrease of the crystallite size or/and the presence of the microstress in the film. The average crystallite size as determined by the Scherrer formula was 70 nm for the coating obtained on a grounded substrate (0 V) and 45 nm for the coatings deposited at negative bias (−50 and −100 V). Thus, the enhancement of the energy of the bombarding ions reduces the texture of the film and the crystallite dimension.

The crystalline HA coatings were obtained by RF‐magnetron sputter deposition in water containing atmosphere [28]. It was shown that the HA coatings exhibited considerable change on preferential orientation while the samples approach the target erosion zone. According to XRD analysis with shifting the sample radially from the center of the sub‐ strate holder the texture coefficient of (002) peak decreases while the (300) peak grows up. **Figure 10** shows detailed highlights from the two ultimate cases of the deposited HA coatings preferentially oriented in the (002) and (300) directions. The structural features of

The phase composition and structure of the CaP coating depend on the process conditions as it was mentioned above. RF‐magnetron sputtering allows to deposit the CaP coatings of either amorphous or crystalline structure of a certain phase composition that along with the Ca/P ratio influences the coating behavior *in vitro* and *in vivo* [60]. The high dissolution rates of the amorphous lead to long‐term stability reduction of the implanted devises. With the aim to maintain the HA coating integrity the researchers apply the postdeposition or *in situ* anneal‐ ing of the films. Several authors reported that the transition from amorphous to crystalline coatings can be controlled by the heat‐treatment temperatures and heating environment (air

**Figure 8.** XRD spectra of the CaP coatings deposited at different deposition temperature [59].

224 Modern Technologies for Creating the Thin-film Systems and Coatings

**Figure 9.** XRD spectra show variations of the intensity distributions, revealing that the studied samples have a different

texture [48].

**Figure 10.** Highlights from X‐ray diffractograms of HA films deposited under racetrack (Sample A) and in the centre of the substrate holder (Sample B).

the films were quantitatively studied. The lattice parameters (*a*, *b*, and *c*) of the measured samples were revealed to be higher than that of the bulk HA. The (002) textured coating (Sample B) is characterized by *a* = 9.410 Å and *c* = 6.934 Å; the (300) textured film (Sample A) is with a = 9.490 Å and *c* = 6.925 Å. This behavior is commonly observed in physical vapor deposited thin films and is attributed to the compressive stress arising in the film within growth and the stoichiometric imbalance of the film composition. In this way, the deposi‐ tion conditions which were realized under the racetrack lead to the transformation of the HA film orientation. It is considered that the texture change is resulted by high energy ion bombardment of the growing film deposited under the target erosion zone. This not only affects the deposition rate but also influences the structure and functional properties of the film.

The development of microstructure in the trend of the coating growth was also studied with the help of TEM cross‐section images. **Figure 11** shows the cross‐sectional bright and dark fields of a 250‐nm thick CaP layer prepared by FIB. Note that the coatings had a gradient structure with a nanocrystalline layer at the interface. This result is in good agreement with the results published in reference [48]. The first columnar structure nucleated perpendicu‐ lar to the interface, within the range of 30–50 nm from the interface between the coating and substrate. The CaP film is well defined, dense, and homogenous. In the dark‐field images, the columns have a lateral size of about 40 nm. In **Figure 11a**, the clear structure of HA, with reflections from (100), (002), (211), and (200) planes, was seen. Based on TEM, the average crystal size of the top CaP layer was 30 ± 20 nm. The crystals showed a perfect crystalline structure, being in concordance with the value obtained from the XRD spectra (40 nm).

Also in **Figure 11**, the polycrystalline structure of the HA coating can be observed. Both ED patterns and d‐spacing values (1.90, 2.12, 2.25, 2.82, 3.19, 3.43, 4.10, and 8.20 Å) confirmed that the deposited coatings possess the structure of HA and the absence of other crystalline phases. The physical reason for the phenomenon of this structure is explained by the authors of the study [64] as a shadowing effect which can occur if adatoms impinge on the substrate under an angle which deviates from the substrate normal. The microstructure evolution of the thin film can be described with the structure, zone model (SZM), which characterizes the

**Figure 11.** Cross‐sectional bright field: (a) and dark field, (b) TEM images of a 250 nm thick CaP layer were prepared by FIB. The electron diffraction pattern (insert left image) reveals the presence of a polycrystalline phase [52].

microstructure and texture as a function of the deposition parameters. A good overview of an SZM is given by Mahieu et al. [65–67] developed the extended structure zone model (ESZM) that explains the transformation of the texture and microstructure of thin films as a function of adatom mobility.

the films were quantitatively studied. The lattice parameters (*a*, *b*, and *c*) of the measured samples were revealed to be higher than that of the bulk HA. The (002) textured coating (Sample B) is characterized by *a* = 9.410 Å and *c* = 6.934 Å; the (300) textured film (Sample A) is with a = 9.490 Å and *c* = 6.925 Å. This behavior is commonly observed in physical vapor deposited thin films and is attributed to the compressive stress arising in the film within growth and the stoichiometric imbalance of the film composition. In this way, the deposi‐ tion conditions which were realized under the racetrack lead to the transformation of the HA film orientation. It is considered that the texture change is resulted by high energy ion bombardment of the growing film deposited under the target erosion zone. This not only affects the deposition rate but also influences the structure and functional properties of the

226 Modern Technologies for Creating the Thin-film Systems and Coatings

The development of microstructure in the trend of the coating growth was also studied with the help of TEM cross‐section images. **Figure 11** shows the cross‐sectional bright and dark fields of a 250‐nm thick CaP layer prepared by FIB. Note that the coatings had a gradient structure with a nanocrystalline layer at the interface. This result is in good agreement with the results published in reference [48]. The first columnar structure nucleated perpendicu‐ lar to the interface, within the range of 30–50 nm from the interface between the coating and substrate. The CaP film is well defined, dense, and homogenous. In the dark‐field images, the columns have a lateral size of about 40 nm. In **Figure 11a**, the clear structure of HA, with reflections from (100), (002), (211), and (200) planes, was seen. Based on TEM, the average crystal size of the top CaP layer was 30 ± 20 nm. The crystals showed a perfect crystalline structure, being in concordance with the value obtained from the XRD spectra (40 nm).

Also in **Figure 11**, the polycrystalline structure of the HA coating can be observed. Both ED patterns and d‐spacing values (1.90, 2.12, 2.25, 2.82, 3.19, 3.43, 4.10, and 8.20 Å) confirmed that the deposited coatings possess the structure of HA and the absence of other crystalline phases. The physical reason for the phenomenon of this structure is explained by the authors of the study [64] as a shadowing effect which can occur if adatoms impinge on the substrate under an angle which deviates from the substrate normal. The microstructure evolution of the thin film can be described with the structure, zone model (SZM), which characterizes the

**Figure 11.** Cross‐sectional bright field: (a) and dark field, (b) TEM images of a 250 nm thick CaP layer were prepared by

FIB. The electron diffraction pattern (insert left image) reveals the presence of a polycrystalline phase [52].

film.

The texture change occurred in thin films during their growth is a fundamental issue. It can be determined with the help of several factors, such as precursor adatoms sticking probability, adatom diffusion on the surface, and interaction of high‐energy particles with the surface of the growing film. In the HA structure, the (002) plane has the lowest surface energy [68]. For this reason, to minimize the surface energy, the HA coatings grow on (002) orientation. In the literature, it was reported that the preferred (002) orientation can be thermodynamically changed by increasing oxygen ions bombardment in the deposition process [66]. For example, Van Steenberge et al. [69] demonstrated that the preferential orientation of the CeO2 films pre‐ pared using the reactive magnetron sputtering method can be controlled by increasing oxy‐ gen flow. The influence of different crystalline planes during the collision anisotropy can also be treated as an explanation of the obtained results. The (002) plane of hexagonal structure is the most closely packed and it can be easily damaged by severe bombardment of ions acceler‐ ated in the cathode racetrack giving rise to loosely packed (100) plane [70, 71]. This finding is important, because crystallographic texture of polycrystalline thin film is one of the essen‐ tial microstructural features, which is responsible for its properties. In hexagonal HA, a, b, and c planes exhibit anisotropy in mechanical properties, resolvability, biocompatibility, and absorption ability [72–75]. Naturally occurring apatite crystals frequently exhibit preferred orientations resulting from highly specific biological processes and these preferred orienta‐ tions are believed to affect the biological and biomechanical performance of hard tissue [73, 76, 77]. Moreover, recent investigations suggest that HA with textured in a tailored manner surfaces may enable a new level of control over cellular behavior due to of protein adsorption anisotropy on the different faces of hexagonal HA crystals. Molecular modeling and *in vitro* analysis have shown that acidic bone proteins and other proteins exhibited high affinity to the (100) plane of HA [55]. Moreover, adsorption‐desorption of the protein on nanosurface plays an important role in cell adhesion and mineralization of biomaterials. Thus, by controlling the preferential orientation of sputtered HA coatings, the behavior of the coatings in human body can be tailored, assuring their successful for biomedical applications.

### **2.6. Electrochemical** *in vitro* **tests of RF‐magnetron sputtering of CaP coating**

After implantation in human body, any metallic biomaterials are affected by the action of the body fluids [60]. The metallic biomaterials are degraded by corrosion processes, which disturb the normal body system, leading in the end at the rejection of the implant. For this reason, before the preparation of new biomaterial, it is important to know the effect of the corrosive solutions on its characteristics. In the case of the coatings, the corrosion resistance can be controlled by the adjustment of deposition parameters. The corrosion behavior of the biomaterials at the contact with simulated body solutions (saliva, SBF, PBS, and 0.9% NaCl, etc.) can be evaluated by various techniques; the most used being the potentiodynamic polarization method. The corrosion behavior of hydroxyapatite is influ‐ enced by the many factors such as composition, crystallinity, compactness, and porosity,

**Figure 12.** Evolution of corrosion potential, corrosion current density, and polarization resistance on the deposition temperature of the sputtered hydroxyapatite coatings in Fusayama artificial saliva solution (pH = 5) at 37°C [59].

which are depended on the deposition parameters. For example, Ducheyne et al. found that the stoichiometric hydroxyapatite coatings (Ca/P = 1.67) exhibited low dissolution rate than other types of calcium phosphates [78].

In a previous paper, we demonstrated that the deposition temperature is one of the factors which can affect the corrosion resistance of sputtered hydroxyapatite coatings as following [59]. The increase of the deposition temperature from 400 to 600°C leads to the decrease of the corrosion current density and increase of the polarization resistance (**Figure 12**), indicating an improvement of the corrosion resistance. For further increase of deposition temperature from 600 to 800°C, the corrosion current density and polarization resistance were not affected (**Figure 12**). Comparing the values of corrosion potentials, all the coatings presented more electropositive values than the uncoated Ti alloy (**Figure 12**), demonstrating that the coatings are a good solution for improving corrosion resistance of the Ti6Al4V alloy. The increase of the deposition temperature tends toward more electropositive values for hydroxyapatite, indicating also an enhancement of the corrosion resistance. In the literature, it is commonly admitted that a material is resistant to the corrosion when exhibited more electropositive values for corrosion potential, low values for corrosion current density and high ones for polarization resistance [79, 80]. If we take into account these criteria, it can be observed that the hydroxyapatite prepared between the 600 and 800°C has the best resistance in Fusayama artificial saliva solution (pH = 5) at 37°C, being proper for the dental applications. This results was accounted to the differences in the composition of the samples, the EDS measurements showing that HA‐400 is nonstoichiometric (Ca/P = 1.80) while the HA‐600, HA‐700, and HA‐800 exhibited Ca/P ratio closed to 1.67 [59].

#### **2.7. Mechanical properties of RF‐magnetron sputtering of CaP coating**

To assure the success on long term of the metallic implants coated with HA, the coated surface should exhibit a high hardness, low friction performance and superior bonding strength to the metallic surfaces in order to support potential fatigue stress at the time of surgical procedure or after implantation. Altering the metallic surface texture, namely, the implant roughness, via different pretreatment techniques or/and their combination is the most common used and relatively inexpensive way that can help in tackling above men‐ tioned challenges as the substrate properties play an important role in obtaining the effec‐ tive implant‐tissue interaction and osseointegration.

which are depended on the deposition parameters. For example, Ducheyne et al. found that the stoichiometric hydroxyapatite coatings (Ca/P = 1.67) exhibited low dissolution rate

**Figure 12.** Evolution of corrosion potential, corrosion current density, and polarization resistance on the deposition temperature of the sputtered hydroxyapatite coatings in Fusayama artificial saliva solution (pH = 5) at 37°C [59].

In a previous paper, we demonstrated that the deposition temperature is one of the factors which can affect the corrosion resistance of sputtered hydroxyapatite coatings as following [59]. The increase of the deposition temperature from 400 to 600°C leads to the decrease of the corrosion current density and increase of the polarization resistance (**Figure 12**), indicating an improvement of the corrosion resistance. For further increase of deposition temperature from 600 to 800°C, the corrosion current density and polarization resistance were not affected (**Figure 12**). Comparing the values of corrosion potentials, all the coatings presented more electropositive values than the uncoated Ti alloy (**Figure 12**), demonstrating that the coatings are a good solution for improving corrosion resistance of the Ti6Al4V alloy. The increase of the deposition temperature tends toward more electropositive values for hydroxyapatite, indicating also an enhancement of the corrosion resistance. In the literature, it is commonly admitted that a material is resistant to the corrosion when exhibited more electropositive values for corrosion potential, low values for corrosion current density and high ones for polarization resistance [79, 80]. If we take into account these criteria, it can be observed that the hydroxyapatite prepared between the 600 and 800°C has the best resistance in Fusayama artificial saliva solution (pH = 5) at 37°C, being proper for the dental applications. This results was accounted to the differences in the composition of the samples, the EDS measurements showing that HA‐400 is nonstoichiometric (Ca/P = 1.80) while the HA‐600, HA‐700, and

than other types of calcium phosphates [78].

228 Modern Technologies for Creating the Thin-film Systems and Coatings

HA‐800 exhibited Ca/P ratio closed to 1.67 [59].

tive implant‐tissue interaction and osseointegration.

**2.7. Mechanical properties of RF‐magnetron sputtering of CaP coating**

To assure the success on long term of the metallic implants coated with HA, the coated surface should exhibit a high hardness, low friction performance and superior bonding strength to the metallic surfaces in order to support potential fatigue stress at the time of surgical procedure or after implantation. Altering the metallic surface texture, namely, the implant roughness, via different pretreatment techniques or/and their combination is the most common used and relatively inexpensive way that can help in tackling above men‐ tioned challenges as the substrate properties play an important role in obtaining the effec‐

**Figure 13.** The quantitative comparison of different coating techniques. Reprinted from Mohseni et al. [81].

Substrate topography can be varied via different surface treatment procedures: exposition to abrasive paper (grinding and GR); sand, glass, or ceramic microspheres accelerated toward the surface (sandblasting and SB); exposition to acid chemicals (wet etching and AE); exposi‐ tion to electron beams (EB). It is known that the mechanical properties of biomaterials are strongly governed by the film fabrication method and substrate characteristics. According to Mohseni et al. [81], the sputtering technique provides the highest adhesion of coating to the substrate compared to other methods which can be attributed to the sputter cleaning and ion bombardment processes (**Figure 13**). However, a simple and direct comparison of the effect of different pretreatment methods for enhancing the adhesion strength of magnetron sput‐ tered HA coating is difficult because the deposition parameters are different and the authors use different techniques and experimental equipment to determine the mechanical properties. Nevertheless, the main tasks in the pretreatments of a metal surface prior to coating deposition may be defined as follows: to remove all foreign matter, to render surface suitable (suitable roughness) for the coating, to impart uniformity throughout all treated work piece surfaces.

Nowadays, the most common pretreatment methods of metallic substrate surface used prior to RF‐magnetron coating deposition procedure are GR, SB, AE, and EB treatments [24, 56, 79, 81–84]. A number of studies reported that the combination of these pretreatment techniques


could be more effective for the improvement of hardness, Young's modulus and adhesion of the HA coatings to substrate [56, 85]. **Table 2** summarizes the use of different pretreatment methods prior the deposition of HA‐based RF‐magnetron coating on metallic substrate with comparison on their mechanical properties.

For instance, the increased surface roughness and enhanced mechanical properties of implants by SB and AE, so‐called SLA process [86], were demonstrated by Grubova et al. [85]. Nanohardness H and Young's modulus E of the coatings prepared on Ti after SB with Al2 O3 microspheres of 50 μm diameter followed the etching with a mixture of 1 ml HF + 2 ml HNO3 + 2.5 ml H2 O at the penetration depth of hc = 71.11 ± 2.87 nm were 15.2 ± 0.7 and 147 ± 16 GPa, respectively. The values of Н/Е and Н3 /Е2 for the HA coating (0.101 and 0.164 GPa, respectively) were significantly higher than that of the uncoated substrate (0.038 and 0.005 GPa). Scratch test results revealed that the deposited HA coatings exhibited improved wear resistance and lower friction coefficient. Eventually, the coating was delaminated from the substrate along the scratch path when the load increased up to 3.14 N.

Based on the data obtained in study [24] for pure HA coatings with a thickness of 0.09–2.7 μm prepared by RF‐magnetron sputtering deposition on mechanically polished (GR) NiTi and Ti substrates at a substrate temperature of 500°C in argon atmosphere, we can assume that substrate surface microstructure affected the mechanical properties of HA films, if the film is thinner than about 1 μm. Their hardness and Young's modulus were of about 10 and 110 GPa, respectively. The bond strength of the HA coating to the metallic substrates is affected by its thickness. For example, upon increasing the thickness more than 1.6 μm, the bond strength decreased. The coating with a thickness of less than 1.6 μm was not damaged during the scratch test experiment even at a maximal load of 2 N. No difference was observed between NiTi and Ti substrates [24].

Control of the formation of the surface nanopatterns on Ti via pretreatments can allow varying the grain size of the HA coating. For instance, Grubova et al. [87] investigated the influence of the grain size on the mechanical properties of the nanostructured RF‐magnetron sputter‐deposited Ag‐HA coatings with a concentration of silver in the range of 0.13–0.36 wt% prepared on the Ti substrates treated through SB with Al2 O3 particles (250–320 μm) for 10 s at 0.45 and 0.61 MPа and AE using a 1:2:2.5 mixture of HF (40%), HNO3 (66%), and distilled water. Larger nanostructure sizes were found on the surface of Ti prepared at a lower SB pressure. From the nanoindentation results, it is possible to conclude that smaller grains of the Ag‐HA coatings resulted in signifi‐ cantly higher values of nanohardness and Young's modulus.

The treatment of Ti surfaces by EB has also been used prior the deposition of magnetron HA coatings [56]. EB irradiation of Ti samples has been found to reduce the roughness and to improve the nanohardness of the material [88], allowing for the deposition of smoother HA coatings [56]. For example, Surmeneva et al. [56] studied the nanoindentation hardness and the Young's modulus of the HA coating deposited onto Ti modified by the pulse EB treat‐ ment with an electron energy density of 15 J cm‐2 were determined to be 7.0 ± 0.3 and 124 ± 3 GPa, respectively, which were significantly higher than those of the HA coating on AE Ti in a mixture of HF (48% concentration) and HNO3 (65% concentration) acids; H2 O was set to 1:4:5 in volume. **Figure 14** shows the load‐deformation curves of the tested in [56] surfaces.

**Sample** SB + AE + HA SB + AE + Ag

Pure Ti (Grade 4)

450 ± 60

1.2 ± 0.1

‐ HA

SB + AE + Ag

Pure Ti (Grade 4)

450 ± 60

1.2 ± 0.1

‐ HA

HA EB + HA AE + HA AE + Ag ‐ HA

AE + HA EB + HA GR + HA

Pure Ti (Grade 4)

NiTi and commercially

90 270 450 720 1080 1600 2700

> **Table 2.**

based coatings.

<0.10

pure Ti (Grade 4) i

690 ± 125

21 ± 7

81 ± 4 55 ± 15 165 ± 10 202 ± 10 150 ± 10 152 ± 20 130 ± 30 162 ± 10 The most frequently applied pretreatment techniques prior the RF‐magnetron sputtering and their influence on the mechanical characteristics of deposited HA‐

9 ± 2

120 ± 20

0.080

50.6 × 10‐3

9 ± 1

111 ± 1

0.080

63.2 × 10‐3

13 ± 1

140 ± 10

0.090

112.1 × 10‐3

12 ± 2

130 ± 20

0.090

97.2 × 10‐3

7 ± 2

100 ± 20

0.070

34.5 × 10‐3

5 ± 1

100 ± 10

0.050

12.5 × 10‐3

11 ± 4

100 ± 20

0.110

133.1 × 10‐3

[24]

7.0 ± 0.3

124 ± 3

0.056

22.3 × 10‐3

Pure Ti (Grade 4)

690 ± 125

50 ± 28

150 ± 10

3.7 ± 0.2

85 ± 10

0.043

7 × 10‐3

[56]

Pure Ti (Grade 4)

500

Pure Ti (Grade 4)

500

AZ31 magnesium alloy

700 ± 60

AZ31 magnesium alloy

700 ± 60

100

50 100

50 100

50 100

50

6.0 ± 0.5

96 ± 8

0.062

23.4 × 10‐3

7.2 ± 0.2

122 ± 4

0.060

25.1 × 10‐3

3.6 ± 0.1

82 ± 10

0.047

6.9 × 10‐3

3.8 ± 0.3

90 ± 8

0.047

6.6 × 10‐3

[83]

8.5 ± 1.4

86 ± 5

0.099

82.9 × 10‐3

4.6 ± 0.8

63 ± 5

0.072

23.9 × 10‐3

4.9 ± 2.5

78 ± 16

0.063

19.3 × 10‐3

3.1 ± 2.0

79 ± 11

0.039

4.7 × 10‐3

[121]

230 Modern Technologies for Creating the Thin-film Systems and Coatings

Pure Ti (Grade 4)

500–800

0.8 ± 0.1

71.1 ± 2.9

15.2 ± 0.7

2.8 ± 0.5 5.3 ± 1.2

136 ± 28

0.039

7.8 × 10‐3

94 ± 24

0.030

2.4 × 10‐3

[87]

147 ± 16

0.101

164 × 10‐3

[85]

**Substrate**

*Thickness*

*R***a (μm)**

*S***a, (nm)**

*h***c (nm)**

*Н* **(GPа)**

*Е* **(GPа)**

*Н/Е*

*Н***3***/Е***2 (GPа)**

**Ref.**

**(nm)**

**Figure 14.** Representative load‐displacement curves for uncoated and CaP—coated Ti prepared by AE and pulse EB treatment at a maximum load of 2.5 mN [56].

The CaP coating deposited onto the nanocrystalline EB treated Ti surface is more resistant to plastic deformation than the same coating on the AE Ti substrate. Moreover, Surmeneva et al. [89] also have evaluated the application of negative electrical bias to the Ti substrates preheated to *Т* = 200°C during the Si‐HA coating deposition; the substrate surface was chemi‐ cally etched and then treated with a low energy EB prior to deposition. It was found that for the case of the grounded substrate, the adhesion coefficient is the highest (HSC = 1). With increasing negative bias, the adhesion coefficient HSC lowers to 0.98, indicating a decrease in adhesion. Decreasing adhesion may be associated with an increasing level of microstrains because of a finer grained structure, an increasing volume fraction of defects, and incoherent interfaces, as evidenced by XRD and IR studies of the coating structures. Surmeneva et al.

**Figure 15.** Hardness and elastic modulus of the sputtered CaP coatings prepared at different deposition temperatures; the measurements were performed on the coating deposited on the Si wafers in order to avoid the influence of other factors (e.g., roughness or cast defects occur during the deposition of the coatings on metallic substrates) [59].

found that the HA coating prepared on EB‐treated magnesium AZ31 alloy exhibited higher hardness and the Young's modulus values compared to those of HA coated on untreated AZ 31 alloy. Furthermore, HA coating prepared on treated AZ31 alloy substrate showed the best resistance to plastic deformation than HA coated prepared on treated AZ31 alloy substrate. Although we cannot do the right comparison of the pretreatment methods usually used prior RF‐magnetron coating deposition due to the differences in techniques and experimental equipment for the determination of the mechanical properties, we assume that pretreatments such as AE, SB, GR, and EB treatments and their combinations enhance the bonding strength and hardness of the coating. However, the pretreatments are not the only one way to guaran‐ tee a stable (high hardness and low friction) HA coating on metallic substrate. Using an inter‐ facial layer (such as TiO2 , TiN, SiC, etc.) as the initial coating layer on the substrate followed by HA coating layer also can enhance the bonding strength and mechanical properties as well as post‐treatments [84, 90–93].

In biomedical applications, the mechanical properties of the CaP coatings are important parameters. For success of dental or orthopedic implants, it is important to use a material with high hardness and elastic modulus close to the bone. In the case of CaP, both param‐ eters are influenced by deposition temperature (**Figure 15**). Due to the plastic deformation, we presented the results of the hardness and elastic modulus measured at low load (1 mN). The elastic modulus and hardness values decreased with increasing deposition temperature (**Figure 15**). For both parameters, high values were obtained for the coatings with amorphous structure (sample deposited at 400°C). Note that the crystallinity plays an important role also in the case of mechanical properties. Despite that the high hardness is desired, the elastic modulus should be low, for the biomedical applications. Moreover, the CaP coatings exhib‐ ited a high dissolution rate in contact with human body fluids and it is not desired. Thus, the CaP coatings prepared at high deposition temperature (700 or 800°C) is more proper for coat‐ ing the surface of dental or orthopedic implants.

### **2.8. Behavior** *in vitro* **of RF‐magnetron sputtering of CaP coating**

The CaP coating deposited onto the nanocrystalline EB treated Ti surface is more resistant to plastic deformation than the same coating on the AE Ti substrate. Moreover, Surmeneva et al. [89] also have evaluated the application of negative electrical bias to the Ti substrates preheated to *Т* = 200°C during the Si‐HA coating deposition; the substrate surface was chemi‐ cally etched and then treated with a low energy EB prior to deposition. It was found that for the case of the grounded substrate, the adhesion coefficient is the highest (HSC = 1). With increasing negative bias, the adhesion coefficient HSC lowers to 0.98, indicating a decrease in adhesion. Decreasing adhesion may be associated with an increasing level of microstrains because of a finer grained structure, an increasing volume fraction of defects, and incoherent interfaces, as evidenced by XRD and IR studies of the coating structures. Surmeneva et al.

**Figure 14.** Representative load‐displacement curves for uncoated and CaP—coated Ti prepared by AE and pulse EB

treatment at a maximum load of 2.5 mN [56].

232 Modern Technologies for Creating the Thin-film Systems and Coatings

**Figure 15.** Hardness and elastic modulus of the sputtered CaP coatings prepared at different deposition temperatures; the measurements were performed on the coating deposited on the Si wafers in order to avoid the influence of other factors (e.g., roughness or cast defects occur during the deposition of the coatings on metallic substrates) [59].

Cellular responses to an implanted biomaterial are highly complicated biological and chemi‐ cal processes related to several surface properties [82]. In the literature, it was reported that the Ca and P content affected cell response such as attachment, spreading, and differentiation [61, 94–96]. Moreover, it was demonstrated that the biological properties of Ti or Mg alloys, ceramics, and polymers could be significantly enhanced by substrate coating with CaP thin film [61, 94–96].

*In vitro* cell viability tests studied after 5 days of culture with human osteosarcoma cell line (MG‐63) showed that all of the cells have a good adhesion, spreading, and growth on the sur‐ face of all of the coatings, whatever deposition temperature was (**Figure 16**). Comparing all of the coatings, one may observe that there are no differences between the cell growths after increasing the deposition temperature. On all of the coated surfaces, the cells showed a dense cytoskeletal F‐actin (stained green) and proliferated well, being situated close to each other. For the CaP coatings prepared at 700 and 800°C, more cell nuclei numbers (blue color) were found, indicating that these two coatings have better promoted the cell proliferation. The SEM micrograph of cell growth on the coating prepared at 800°C is presented in **Figure 17**. There

**Figure 16.** Fluorescence micrograph of the osteosarcoma cell growth after 5 days of culture on the uncoated Ti6Al4V substrate and coated at different deposition temperatures. Blue fluorescence represents the nuclei due to Hoechst 33342; green fluorescence is F‐actin fibres due to FITC conjugated phalloidin [59].

are observed many cells well attached and spreading over the whole coated surface, with spindle‐shaped, indicating good biocompatibility. In our previous paper, we demonstrated that this behavior was due to the increase of surface roughness which was increased due to the deposition temperature [59]. Immediately after the implantation, the first contact of the implant surface is with proteins, which are known as a promotor of attachments, spread‐ ing and proliferation of osteoblasts, leading to a successful implantation [97]. The proteins adhere better to the porous or rough surface due to larger contact areas which assure a bigger surface‐cell interface [98, 99].

**Figure 17.** SEM image of the osteosarcoma cell growth after 5 days of culture on the coating prepared at 800°C.

Recent studies have shown that the cell‐substrate interactions depend on the material type and are associated with the surface topography [100, 101], chemical and elemental composi‐ tion [101, 102], dissolution behavior [101, 103], and surface macro and microstructure [101,

Radio Frequency Magnetron Sputter Deposition as a Tool for Surface Modification of Medical Implants http://dx.doi.org/10.5772/66396 235

**Figure 18.** Morphology of MG‐63 cells growth for 1 and 7 days on glass and Ti substrates coated with HA prepared by different methods [48].

are observed many cells well attached and spreading over the whole coated surface, with spindle‐shaped, indicating good biocompatibility. In our previous paper, we demonstrated that this behavior was due to the increase of surface roughness which was increased due to the deposition temperature [59]. Immediately after the implantation, the first contact of the implant surface is with proteins, which are known as a promotor of attachments, spread‐ ing and proliferation of osteoblasts, leading to a successful implantation [97]. The proteins adhere better to the porous or rough surface due to larger contact areas which assure a bigger

**Figure 16.** Fluorescence micrograph of the osteosarcoma cell growth after 5 days of culture on the uncoated Ti6Al4V substrate and coated at different deposition temperatures. Blue fluorescence represents the nuclei due to Hoechst 33342;

green fluorescence is F‐actin fibres due to FITC conjugated phalloidin [59].

234 Modern Technologies for Creating the Thin-film Systems and Coatings

Recent studies have shown that the cell‐substrate interactions depend on the material type and are associated with the surface topography [100, 101], chemical and elemental composi‐ tion [101, 102], dissolution behavior [101, 103], and surface macro and microstructure [101,

**Figure 17.** SEM image of the osteosarcoma cell growth after 5 days of culture on the coating prepared at 800°C.

surface‐cell interface [98, 99].

104]. These physicochemical characteristics can be manipulated during RF‐magnetron sput‐ tering to promote the clinical integration of implants with the surrounding cells and tissue [100, 105]. The response of a human osteoblast‐like cell line (MG‐63) to titanium coated with silicon‐containing calcium phosphate was investigated by Surmeneva et al [48]. *In vitro* cell culture studies showed that cells maintained their natural spindle‐like morphology, are well spread out and adherent to the substrate (**Figure 18**). After 7 days, the cells covered the whole surface coated with HA. All the tested coatings revealed a low toxicity and a very good adhe‐ sion of cells on the surface.

By conducting the biological *in vitro* assay of the HA coatings with DPS cells the surface miner‐ alization of the tested samples was revealed. The mineralized nodules typical for amorphous calcium phosphate (ACP) deposited from solutions were found in the extra‐cellular region during the 3‐day culture (**Figure 19a**) on the poor crystalline HA coating deposited at 200 W and 0.1 Pa on the titanium substrates. The calcification degree in the matrix of fibrous col‐

**Figure 19.** SEM image of the mineralized matrix synthesized by DPS cells cultured on the 440‐nm thick CaP coating. The signs of mineralization are marked by arrows in (a). (b) Reveals the signs of mineralization in the case of uncoated titanium [52].

lagen was significantly lower in the case of the uncoated titanium (**Figure 19b**). It is reported that nodule formation in multilayers of cells is an important factor for *in vitro* mineralization [61, 62]. These results are consistence with those published in Ref. [63]. In the literature, it has been shown that the mineralization process is initiated by controlling the Ca and P concentra‐ tions in the medium [66, 68]. Nodule formation of mineralized is correlated with most of the biological events more or less with a spontaneous precipitation of CaP minerals, depending on the behavior of cellular osteogenic [69, 70]. In other words, poor crystalline HA coatings accelerated the attachment, proliferation, and formation of mineralized nodules by the cells grow on the surface than those grown on uncoated titanium surface.

The geometric and physical characteristics of the CaP film are important for regulation of the cells attachment and migration, such as crystallinity, composition, and thickness. Many scientific papers reported on the effect of crystalline and amorphous CaP coatings prepared by different techniques based on the adhesion, proliferation, and growth of bone‐related cells [106–109]. Anyhow, most of the published papers acknowledged various effects of the CaP coating crystallinity on cell behavior. For example, a low dissolution rate of the coating was found for the coatings with low crystallinity or amorphous ones. On the other hand, the amor‐ phous CaP coatings induce faster bone formation, even though its faster release of calcium ions [110]. Based on some *in vitro* analysis, it was found that the surface chemistry and topography of amorphous HA coatings stimulate the cell attachment, but result in a cytotoxic effect that inhibits proliferation of the cells attached to the coating surfaces [106, 110]. Simultaneously, highly crystalline HA coatings revealed nodule formation [106] and higher rates of osteo‐ genic cell proliferation than the less crystalline or amorphous HA coatings deposited on Ti substrates [106, 110]. It is also reported in reference [107] that the amorphous sputtered CaP coatings constrain the growth and differentiation of rat bone marrow cells and osteoblast‐like cells. On the other hand, crystalline HA coating prove to have better adhesion and accelerates proliferation of bone marrow mesenchymal stem cells compared with an amorphous ACP coating with the same topography and roughness [108].

It is difficult to highlight the most significant properties of the CaP coating that lead to a better cellular response. Furthermore, it is impossible to compare the results of *in vitro* experiments that have been conducted using different methodologies. For instance, the cell types influ‐ enced the Ca2+ and PO<sup>4</sup> 3‐ and by varying the Ca/P ratio, the proliferation and differentiation could be controlled [111]. For this reason, the standardization to establish the effects of CaP on the biological assay is required. Up to date, there are some methodologies, but the published studies on the biological assay of the magnetron sputter‐deposited coating demonstrate that this method can potentially be used for the development of future biomaterials.

## **3. Solutions for improving the properties of RF‐magnetron sputtering of CaP coating**

As it was underlined in the introduction, hydroxyapatite is one of the most extensively used materials in medicine for hard tissues repairment or for enhancing the osseointegration of the metallic implants or their components. It is commonly used due to its chemical simi‐ larities to the component of bones and teeth. However, the high dissolution rate in contact with human fluids, the low mechanical strength and the relatively low bone bonding rate restrict its use as biomaterial. Therefore, the actual challenge in the biomedical field is to find a solution to improve the mentioned properties, but the difficulties are redoubtable, as these surface, characteristics deteriorate in time. A novel approach to obtain these surfaces is to modify HA properties by doping with small amounts of beneficial elements for human bones, as discuss below.

### **3.1. Si or SiC addition**

lagen was significantly lower in the case of the uncoated titanium (**Figure 19b**). It is reported that nodule formation in multilayers of cells is an important factor for *in vitro* mineralization [61, 62]. These results are consistence with those published in Ref. [63]. In the literature, it has been shown that the mineralization process is initiated by controlling the Ca and P concentra‐ tions in the medium [66, 68]. Nodule formation of mineralized is correlated with most of the biological events more or less with a spontaneous precipitation of CaP minerals, depending on the behavior of cellular osteogenic [69, 70]. In other words, poor crystalline HA coatings accelerated the attachment, proliferation, and formation of mineralized nodules by the cells

The geometric and physical characteristics of the CaP film are important for regulation of the cells attachment and migration, such as crystallinity, composition, and thickness. Many scientific papers reported on the effect of crystalline and amorphous CaP coatings prepared by different techniques based on the adhesion, proliferation, and growth of bone‐related cells [106–109]. Anyhow, most of the published papers acknowledged various effects of the CaP coating crystallinity on cell behavior. For example, a low dissolution rate of the coating was found for the coatings with low crystallinity or amorphous ones. On the other hand, the amor‐ phous CaP coatings induce faster bone formation, even though its faster release of calcium ions [110]. Based on some *in vitro* analysis, it was found that the surface chemistry and topography of amorphous HA coatings stimulate the cell attachment, but result in a cytotoxic effect that inhibits proliferation of the cells attached to the coating surfaces [106, 110]. Simultaneously, highly crystalline HA coatings revealed nodule formation [106] and higher rates of osteo‐ genic cell proliferation than the less crystalline or amorphous HA coatings deposited on Ti substrates [106, 110]. It is also reported in reference [107] that the amorphous sputtered CaP coatings constrain the growth and differentiation of rat bone marrow cells and osteoblast‐like cells. On the other hand, crystalline HA coating prove to have better adhesion and accelerates proliferation of bone marrow mesenchymal stem cells compared with an amorphous ACP

It is difficult to highlight the most significant properties of the CaP coating that lead to a better cellular response. Furthermore, it is impossible to compare the results of *in vitro* experiments that have been conducted using different methodologies. For instance, the cell types influ‐

could be controlled [111]. For this reason, the standardization to establish the effects of CaP on the biological assay is required. Up to date, there are some methodologies, but the published studies on the biological assay of the magnetron sputter‐deposited coating demonstrate that

**3. Solutions for improving the properties of RF‐magnetron sputtering of** 

As it was underlined in the introduction, hydroxyapatite is one of the most extensively used materials in medicine for hard tissues repairment or for enhancing the osseointegration of

this method can potentially be used for the development of future biomaterials.

3‐ and by varying the Ca/P ratio, the proliferation and differentiation

grow on the surface than those grown on uncoated titanium surface.

236 Modern Technologies for Creating the Thin-film Systems and Coatings

coating with the same topography and roughness [108].

enced the Ca2+ and PO<sup>4</sup>

**CaP coating**

In the present, the addition of Si into the HA matrix has successful clinic practice as bone graft material in spinal fusion (www.orthoaospl.com). However, this solution is not sufficient for the hard tissue, because they exhibited low hydrophobicity involves a decrease of the proteins adsorption and low mechanical and tribological properties. Thian et al. showed that sputtered Si‐HA coatings could be a good candidate for hard tissue replacement, owing to their ability to form a carbonate‐containing apatite layer rapidly [112]. Azem et al. showed that by SiC addition to the pure HA matrix, their corrosion resistance in artificial saliva and mechanical properties were enhanced, without affect the bioactive abilities of HA [91, 113]. Also, Vladescu et al. reported that the SiC consisting of hydroxyapatite coatings exhibited high adhesion strength to the Ti6Al4V substrates, better corrosion resistance to the SBF attack, better cells proliferation and viability compared with the undoped HA coatings. However, the elastic modulus is still higher than that of the bone [91].

### **3.2. Mg addition**

Some researchers have also proposed the incorporation of Mg in the HA lattice, in order to accelerate the osseointegration process of dental or orthopedic implants. The Mg was selected as dopant because Mg is found in the natural dentin (1.23 wt%) and bone (0.73 wt%). However, a major drawback of the Mg‐HA coatings is their high dissolution rate in physiological solu‐ tions, leading to a high pH value in the surrounding environment, which is detrimental to cell survival [114]. This effect appeared because Mg inhibits the apatite crystallization, leading to a destabilization of the apatite structure, by formation of the *β*‐tricalcium phosphate, known as a phase with high dissolution rate in human body environment [113]. The 1.8 wt% Mg content decreased the compressive strength and hardness of the HA, but these are still higher than the values for bone [115].

### **3.3. Ti addition**

A recent study reported that some of the properties of sputtered HA, such as mechanical properties, *in vitro* and *in vivo* bioactivity, could be enhanced by the incorporation of Ti in the HA lattice. Ribeiro et al. showed that the hydroxyapatite doped with Ti enhanced the protein adsorption, especially the activity of the enzyme GCR, indicating that this coatings may be used as a delivery matrix for biologically active molecules [116].

### **3.4. Ta addition**

Ligot et al. identified that Ta could also be a promising doping element, proving that for Ta content less 4.5 at%, Ta substituted the Ca in the HA cell, and the (Ca + Ta)/P ratio is close to 1.67 (ratio of stoichiometric HA) and exhibited an elastic modulus of 120 GPa, which is com‐ parable to the value of some conventional materials used for load‐bearing implants (stainless steel, Co‐Cr alloys or Ti alloys) for which the elastic modulus ranges between 110 and 232 GPa [117]. Unfortunately, up to date, no information about the effect of Ta addition on the osseointegration ability, bioactivity, or dissolution rate of sputtered HA coatings are avail‐ able. Future work need to be done in this area.

### **3.5. Ag addition**

In the present, in the biomedical applications, especially for dental and orthopedic implants, there is necessary to use surfaces with bioactive and antibacterial surfaces, which helps also the implant osseointegration. The bioactive surface could be achieved by using HA coatings, but the antibacterial or antifungal properties of HA coatings are under devel‐ opment. For example, Park et al. doped HA coatings with different Ag content and found that some of the Ca2+ ions in the hydroxyapatite were replaced with Ag<sup>+</sup> ions [118]. The drawback of this solution is that the hardness and modulus of the Ag doped HA coatings decreased with an increasing Ag content [118], which is not desired for load bearing implants. Ciuca et al. found that the addition of 2.1 at.% Ag is enough for assuring of antifungal activ‐ ity of the HA coating, without affecting the formation of the stoichiometric HA phase [119]. Also, it was demonstrated that the addition of a small Ag content (0.7 at.%) into HA structure could improve the resistance to the *Staphylococcus aureus*, *Streptococcus pyogenes*, *Salmonella typhimurium* attack of hydroxyapatite without any change of the other characteristics such as microchemical, microstructural, or anticorrosive ones of the sputtered hydroxyapatite [120].

### **4. Conclusions**

The fabrication of a controlled crystallinity, chemistry, and stoichiometry of HA nanocoat‐ ings is shown to be possible based on the knowledge gained up to now, which will have a high efficiency in orthopedic implant applications. The HA coatings can be deposited on the surfaces of diverse metallic permanent and biodegradable implants. The complex geometries of implants can be coated by RF‐magnetron sputter, even if this technique is a line‐of‐sight technique. An overall control of the coating properties keeping the adhesion strength to the substrate high makes RF‐magnetron sputtering technique prospective for future commercial applications in the case of biocompatible coating fabrication.

### **Acknowledgements**

This research was supported by the Federal Target Program #14.587.21.0013 (unique application number 2015‐14‐588‐0002‐5599), the Russian President grants МК‐7907.2016.8, МК‐6459.2016.8 and the State order NAUKA #11.1359.2014/K. The work was also supported by the grants of the Romanian National Authority for Scientific Research and Innovation—UEFISCDI, Project numbers PN‐II‐PT‐PCCA‐2014‐212, 44/2016 and 43/2016— INTELBIOCOMP, within PNCDI III and by the Core Program, under the support of ANCSI, project no. PN16.40.01.02.

### **Author details**

**3.4. Ta addition**

**3.5. Ag addition**

**4. Conclusions**

**Acknowledgements**

able. Future work need to be done in this area.

238 Modern Technologies for Creating the Thin-film Systems and Coatings

Ligot et al. identified that Ta could also be a promising doping element, proving that for Ta content less 4.5 at%, Ta substituted the Ca in the HA cell, and the (Ca + Ta)/P ratio is close to 1.67 (ratio of stoichiometric HA) and exhibited an elastic modulus of 120 GPa, which is com‐ parable to the value of some conventional materials used for load‐bearing implants (stainless steel, Co‐Cr alloys or Ti alloys) for which the elastic modulus ranges between 110 and 232 GPa [117]. Unfortunately, up to date, no information about the effect of Ta addition on the osseointegration ability, bioactivity, or dissolution rate of sputtered HA coatings are avail‐

In the present, in the biomedical applications, especially for dental and orthopedic implants, there is necessary to use surfaces with bioactive and antibacterial surfaces, which helps also the implant osseointegration. The bioactive surface could be achieved by using HA coatings, but the antibacterial or antifungal properties of HA coatings are under devel‐ opment. For example, Park et al. doped HA coatings with different Ag content and

The drawback of this solution is that the hardness and modulus of the Ag doped HA coatings decreased with an increasing Ag content [118], which is not desired for load bearing implants. Ciuca et al. found that the addition of 2.1 at.% Ag is enough for assuring of antifungal activ‐ ity of the HA coating, without affecting the formation of the stoichiometric HA phase [119]. Also, it was demonstrated that the addition of a small Ag content (0.7 at.%) into HA structure could improve the resistance to the *Staphylococcus aureus*, *Streptococcus pyogenes*, *Salmonella typhimurium* attack of hydroxyapatite without any change of the other characteristics such as microchemical, microstructural, or anticorrosive ones of the sputtered hydroxyapatite [120].

The fabrication of a controlled crystallinity, chemistry, and stoichiometry of HA nanocoat‐ ings is shown to be possible based on the knowledge gained up to now, which will have a high efficiency in orthopedic implant applications. The HA coatings can be deposited on the surfaces of diverse metallic permanent and biodegradable implants. The complex geometries of implants can be coated by RF‐magnetron sputter, even if this technique is a line‐of‐sight technique. An overall control of the coating properties keeping the adhesion strength to the substrate high makes RF‐magnetron sputtering technique prospective for

This research was supported by the Federal Target Program #14.587.21.0013 (unique application number 2015‐14‐588‐0002‐5599), the Russian President grants МК‐7907.2016.8, МК‐6459.2016.8 and the State order NAUKA #11.1359.2014/K. The work was also supported by the grants of

future commercial applications in the case of biocompatible coating fabrication.

found that some of the Ca2+ ions in the hydroxyapatite were replaced with Ag<sup>+</sup>

Roman Surmenev1 , Alina Vladescu1,2\*, Maria Surmeneva1 , Anna Ivanova1 , Mariana Braic2 , Irina Grubova1 and Cosmin Mihai Cotrut1,3

\*Address all correspondence to: alinava@inoe.ro


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**Provisional chapter**

### **Thin Films for Immobilization of Complexes with Optical Properties Thin Films for Immobilization of Complexes with Optical Properties**

Joana Zaharieva and Maria Milanova Joana Zaharieva and Maria Milanova

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66512

#### **Abstract**

Thin film deposition techniques, such as dip coating, spin coating, and spray pyrolysis, are applied for the production of SiO<sup>2</sup> -, poly-(methylmethacrylate) (PMMA)-, and SiO<sup>2</sup> -/polyester-based "hybrid" matrices. The factors influencing the film properties are briefly discussed. The morphology of the films presented is studied by different microscopy techniques such as atomic force microscopy, electron (scanning and transmission) microscopy, and fluorescence microscopy. The composites based on SiO<sup>2</sup> -, PMMA-, and SiO<sup>2</sup> /polyster "hybrid" matrices possess the optical properties of the immobilized complexes of Ru(II) and Eu(III) with different organic ligands. The preparation of the PMMA matrix by the monomer methylmethacrylate polymerization (instead of using of PMMA solution) caused partial destruction of the less stable complexes and thereby a decrease in the fluorescence intensity.

**Keywords:** dip coating, spin coating, spray pyrolysis, SiO<sup>2</sup> -based films, poly- (methylmethacrylate)-based films, SiO<sup>2</sup> /polyester "hybrid" matrix, morphology, immobilization, composites, optical properties

### **1. Introduction**

The methods applied for thin film production by deposition have been divided into two groups, physical and chemical, based on the nature of the deposition process [1]. Among them, the chemical methods include gas-phase deposition and solution techniques. The solgel process can produce glassy materials by reactions of precursors at room temperature. This process is suitable for thin film fabrication because the sol can easily be deposited on different substrates. It is very easily combined with deposition methods such as dip coating, spin coating, or spray pyrolysis. The convenience of such a combination will be shown in the chapter below along with the characterization of the films by different microscopy techniques such as

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

atomic force microscopy, AFM, and electron microscopy, transmission, TEM and scanning, SEM. The morphology of the sol-gel produced films coated on different substrates as well as their roughness, surface formations, and thickness are crucial parameters for the application of the films. One of the applications of such films is as a support matrix for the immobilization of complexes. The immobilization of complexes with optical properties in films and matrices is among the approaches applied to obtain new materials with interesting optical properties. Besides the fact that the new materials can possess the properties of both the complex and the matrix, the immobilization itself can improve the stability of the complex immobilized and can protect its properties, for instance, the quenching of fluorescence by the environment molecules. Three different types of composites with immobilized complexes were produced: SiO<sup>2</sup> and poly-(methylmethacrylate) (PMMA)-based composites as well as composites based on a SiO<sup>2</sup> /polyester "hybrid" matrix. Complexes with different organic ligands of Ru(II) and Eu(III) were used because of their fluorescence in the visible region of the spectrum. The optical properties of the composites as well as the deposition techniques influencing them are presented.

### **2. Dip-coating technique**

#### **2.1. Advantages of dip-coating technique**

Dip-coating is a low-cost, waste-free process that is easy to scale up and offers a good control on thickness of the films made by it, so it is popular in research and in industrial production as well. It has been demonstrated that the method is good enough to fill porosity, to make nanocomposites, as well as to perform nanocasting [2]. In spite of the ways of deposition proposed, involving a capillary induced convective coating [2], usually dip coating is combined with a sol-gel process and a substrate is immersed and withdrawn from a sol of the precursor at a certain rate, followed by evaporation of the solvent. Different substrates such as glass, polycarbonate and polymethylmethacrylate have been tested for production of transparent films of SiO<sup>2</sup> , ZnO, indium tin oxide [3] as well as substrates such as Si, Si<sup>3</sup> N4 and SiO<sup>2</sup> for layers of γ-Fe<sup>2</sup> O3 nanoparticles [4].

The factors determining the structure of the films produced via dip coating such as structure of the precursors, relative rates of condensation and evaporation, capillary pressure, and substrate withdrawal speed are presented in [5–7]. An essential factor for the film quality is the film thickness, which is determined by the hydrolysis and condensation behavior characteristics of the precursors and depends on their concentration in the starting solution [8–12], the pH of the sol [13–15], the aging time [8, 12–14], the withdrawal speed [16, 17], the number of immersions, and the ratio of water and precursor [8, 13, 14, 16]. By careful control of hydrolysis and condensation reactions of selected precursors, various surface roughnesses and morphologies can be been achieved [18].

Information about the surface morphology including surface area and roughness is provided by an atomic force microscopy, AFM [18–20]. The united power of the SEM, TEM, and AFM methods contributes to the examination of the surface morphology of thin films prepared by the sol-gel technique using different types of precursors and different parameters of the dipcoating deposition procedure [19].

### **2.2. Films, matrices and composites produced by dip coating**

atomic force microscopy, AFM, and electron microscopy, transmission, TEM and scanning, SEM. The morphology of the sol-gel produced films coated on different substrates as well as their roughness, surface formations, and thickness are crucial parameters for the application of the films. One of the applications of such films is as a support matrix for the immobilization of complexes. The immobilization of complexes with optical properties in films and matrices is among the approaches applied to obtain new materials with interesting optical properties. Besides the fact that the new materials can possess the properties of both the complex and the matrix, the immobilization itself can improve the stability of the complex immobilized and can protect its properties, for instance, the quenching of fluorescence by the environment molecules. Three different types of composites with immobilized complexes were produced: SiO<sup>2</sup>

and poly-(methylmethacrylate) (PMMA)-based composites as well as composites based on a

Dip-coating is a low-cost, waste-free process that is easy to scale up and offers a good control on thickness of the films made by it, so it is popular in research and in industrial production as well. It has been demonstrated that the method is good enough to fill porosity, to make nanocomposites, as well as to perform nanocasting [2]. In spite of the ways of deposition proposed, involving a capillary induced convective coating [2], usually dip coating is combined with a sol-gel process and a substrate is immersed and withdrawn from a sol of the precursor at a certain rate, followed by evaporation of the solvent. Different substrates such as glass, polycarbonate and polymethylmethacrylate have been tested for production of transparent

, ZnO, indium tin oxide [3] as well as substrates such as Si, Si<sup>3</sup>

The factors determining the structure of the films produced via dip coating such as structure of the precursors, relative rates of condensation and evaporation, capillary pressure, and substrate withdrawal speed are presented in [5–7]. An essential factor for the film quality is the film thickness, which is determined by the hydrolysis and condensation behavior characteristics of the precursors and depends on their concentration in the starting solution [8–12], the pH of the sol [13–15], the aging time [8, 12–14], the withdrawal speed [16, 17], the number of immersions, and the ratio of water and precursor [8, 13, 14, 16]. By careful control of hydrolysis and condensation reactions of selected precursors, various surface roughnesses and morphologies can be been achieved [18]. Information about the surface morphology including surface area and roughness is provided by an atomic force microscopy, AFM [18–20]. The united power of the SEM, TEM, and AFM methods contributes to the examination of the surface morphology of thin films prepared by the sol-gel technique using different types of precursors and different parameters of the dip-

/polyester "hybrid" matrix. Complexes with different organic ligands of Ru(II) and Eu(III) were used because of their fluorescence in the visible region of the spectrum. The optical properties of the composites as well as the deposition techniques influencing them are presented.

SiO<sup>2</sup>

films of SiO<sup>2</sup>

ers of γ-Fe<sup>2</sup>

O3

coating deposition procedure [19].

**2. Dip-coating technique**

**2.1. Advantages of dip-coating technique**

252 Modern Technologies for Creating the Thin-film Systems and Coatings

nanoparticles [4].


N4

and SiO<sup>2</sup>

for lay-

#### *2.2.1. Sol-gel produced SiO2 -based films and composites by dip coating*

The process conditions as well as the solution properties are factors influencing the thickness and uniformity of thin films. The SiO<sup>2</sup> -based films were made using Si-containing precursors such as the alkoxysilanes TEOS (tetraethoxysilane), OtEOS (octyltriethoxysilane), and MtEOS (methyl triethoxysilane). The latter two are also called organic modified silanes or ormosils because of the C-Si bond in the structure of these hybrid materials. On the surface, the silanol groups are replaced by alkyl groups that have a poor affinity for water so by that they are keeping the sol-gel surface hydrophobic [13]. Films deposited at a withdrawal speed of 0.4 mm/s with variations of the number of the immersions (from 1 to 7) on microscope glass using gels produced from TEOS, OtEOS, or mixtures of TEOS/OtEOS (mole ratio 1:1) as well as TEOS/MtEOS (mole ratio 1:3) were obtained, showing the influence of the alkoxysilane used [19]. It can be seen that the films obtained from pure TEOS sol have smooth, glassy surface (**Figure 1**). By using organic modified silanes (OtEOS, MtEOS), structuring of the surface is observed **(Figures 1–7),** and the appearance of formations with raindrop or ellipsoid shape with a widely varying size is observed; both the size and the concentration of the shapes depend on the film deposition conditions.

**Figure 1.** SEM images of films, obtained from different sols, at a withdrawing speed of 0.4 mm/s and different number of immersions, P [19].

SEM images of layers, prepared from TEOS, TEOS+MtEOS, MtEOS, and OtEOS containing sols with different numbers of immersions and different speeds (**Figure 2**), show the influence of these factors on the film morphology.


**Figure 2.** SEM images (10,000×) of the matrices, produced by dip coating from TEOS/MtEOS sol at different withdrawal speed and number of immersions, P [19].

The ellipsoid or rhombohedral grains observed (approximately 200 nm in size, **Figure 3**) showed a decreasing size and an increasing number per unit surface area with increasing number of immersions (**Figure 2**).

SEM images of layers, prepared from TEOS, TEOS+MtEOS, MtEOS, and OtEOS containing sols with different numbers of immersions and different speeds (**Figure 2**), show the influence

The ellipsoid or rhombohedral grains observed (approximately 200 nm in size, **Figure 3**) showed a decreasing size and an increasing number per unit surface area with increasing

**Figure 2.** SEM images (10,000×) of the matrices, produced by dip coating from TEOS/MtEOS sol at different withdrawal

of these factors on the film morphology.

254 Modern Technologies for Creating the Thin-film Systems and Coatings

number of immersions (**Figure 2**).

speed and number of immersions, P [19].

**Figure 3.** SEM image (30,000×) of the films obtained after (a) 3 and (b) 5 immersions with withdrawing speed (a) 0.3 and (b) 0.4 mm/s, produced from sol TEOS/MtEOS (ellipsoid or rhombohedral grains are approximately 200 nm in size) [19].

The dot distribution can be ascribed to and connected with the randomly formed macroscopic "pores" or hollows observed in the AFM images (**Figure 4**), with a higher concentration and better appearance in samples prepared with larger numbers of immersions.

**Figure 4.** AFM images of films produced by dip coating (withdrawing speed 0.4 mm/s) from TEOS/OtEOS sol (1 immersion), bearing area 0.8 µm<sup>2</sup> . The section analysis is performed along the diagonal of the bearing area [19].

Sol-gel produced SiO<sup>2</sup> -based composites were made by orthosilanes with immobilization of the complex Rudpp, (Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline) dichloride, [Ru(dpp)<sup>3</sup> ] Cl<sup>2</sup> ). The films produced (10–15 mm) from TEOS/OtEOS and Rudpp sols at withdrawal speed of 0.4 mm/s and up to 5 immersions have a good uniformity and thickness of about 500 nm. Smoothness of the films, combined with the good linkage between the layers, is ensured by these conditions [19]. The presence of Rudpp in the films produced from TEOS leads to the appearance (due to microcrystallization of the Rudpp) of dots (**Figure 5b**–**d**) on the otherwise smooth surface of the Rudpp-free dip-produced film (**Figure 5a**).

**Figure 5.** SEM images of layers without and with Ru(dpp), produced from TEOS-containing sols (a) TEOS (1), (b) TEOS-Rudpp (1), (c) TEOS-Rudpp (5), and (d) TEOS-Rudpp (7) (in brackets the number of the immersions) [19].

The synthetic conditions were improved by applying an ultrasound treatment to the sol after its magnetic stirring. It was found that this improves the surface of the films. Uniform, smooth films were obtained, and no microcrystallization of the complex was observed in the SEM or TEM images. Apparently, the ultrasound treatment of the sol is a powerful tool to avoid microcrystallization of the complex, leading to production of high-quality films. Films from non-sonicated sol show some heterogeneity due to microcrystallization of the Rudpp [21] (**Figure 6**), whereas films produced from sonicated sol are entirely homogeneous [19].

**Figure 6.** TEM (a, b) and SEM (c, d) images of TEOS/OtEOS/Rudpp dip-coated films without (a, c) and with (b, d) gel sonication [21].

The presence of OtEOS causes roughness of the films with sharply expressed hills and valleys as observed with AFM (**Figure 7b**). The chain-like surface structure in the OtEOS Rudpp film may be due to the presence of the long CH<sup>3</sup> -(CH<sup>2</sup> )7 radicals that are likely difficult to fit into the SiO<sup>2</sup> network.

appearance (due to microcrystallization of the Rudpp) of dots (**Figure 5b**–**d**) on the otherwise

The synthetic conditions were improved by applying an ultrasound treatment to the sol after its magnetic stirring. It was found that this improves the surface of the films. Uniform, smooth films were obtained, and no microcrystallization of the complex was observed in the SEM or TEM images. Apparently, the ultrasound treatment of the sol is a powerful tool to avoid microcrystallization of the complex, leading to production of high-quality films. Films from non-sonicated sol show some heterogeneity due to microcrystallization of the Rudpp [21] (**Figure 6**), whereas films produced from sonicated sol

**Figure 6.** TEM (a, b) and SEM (c, d) images of TEOS/OtEOS/Rudpp dip-coated films without (a, c) and with (b, d) gel

**Figure 5.** SEM images of layers without and with Ru(dpp), produced from TEOS-containing sols (a) TEOS (1), (b) TEOS-

Rudpp (1), (c) TEOS-Rudpp (5), and (d) TEOS-Rudpp (7) (in brackets the number of the immersions) [19].

smooth surface of the Rudpp-free dip-produced film (**Figure 5a**).

256 Modern Technologies for Creating the Thin-film Systems and Coatings

are entirely homogeneous [19].

sonication [21].

**Figure 7.** AFM images of films produced by dip coating (withdrawing speed 0.4 mm/s) from sols TEOS/OtEOS/Rudpp (a), and OtEOS-Rudpp (b); 1 immersion [19].

#### *2.2.2. Sol-gel produced SiO2 -based matrix made from TEOS with immobilized Еu(DBM)<sup>3</sup> and Eu(DBM)<sup>3</sup> . dpp complexes*

A sol-gel produced SiO<sup>2</sup> -based matrix was used for immobilization of the europium (III) complex with dibenzoylmethane (DBM), Еu(DBM)<sup>3</sup> , and the mixed-ligand complex Eu(DBM)<sup>3</sup> . dpp (dpp is 4,7-diphenyl-1,10-phenanthroline). The synthetic procedure for obtaining the composites is presented in [22]. To a magnetically stirred ethanol-dimethylformamide (DMF) solution (volume ratio 1:4) of the complex (2.5 g/dm<sup>3</sup> ), TEOS was added dropwise until an ethanol:TEOS mole ratio of 4 was obtained. After that water (with pH 8, adjusted by aqueous ammonia solution) was added in the same way, by this reaching mole ratio ethanol:TEOS:water = 4:1:4. Experiments with ratios 8:1:4 were also performed. After 2 h stirring, the sol obtained was aged at 50–70°C for different times. In some experiments, the fresh sol was sonicated for 30 min in an ice-water ultrasound bath. From the prepared gels, films with a typical thickness of ~300 nm were produced from one immersion with a withdrawing speed of 0.2 mm/s. Membranes (1–2 mm in thickness) were prepared by casting of a gel in a Teflon® mould. The influence of the temperature (ambient to 70°C) and time (3 h to 4 weeks) of ageing of the sol before film/membrane preparation and of their drying on the photoluminescence properties was investigated. When adding distilled water to the sol instead of water with pH = 8, a sol with pH of ~5 was obtained. Films produced from such sols showed no photoluminescence; it was concluded that the complex suffered destruction at this pH, by this the importance of pH control is shown [22]. Despite the positive effect of the sonication on the uniform distribution of the complex in the immobilization matrix [19], it caused quenching of the photoluminescence. This was probably due to some destruction of the complexes, and sonication is thus not recommended when these complexes are used. Emission spectra of the pure powdered complexes Eu(DBM)<sup>3</sup> and Eu(DBM)<sup>3</sup> . dpp along with the SiO<sup>2</sup> -based composites containing these complexes are shown in **Figure 8**. The characteristic emission band for Eu3+ is preserved in the spectra after immobilization.

**Figure 8.** Emission spectra of Eu(DBM)<sup>3</sup> (1) and Eu(DBM)<sup>3</sup> . dpp (3) and of the composites containing Eu(DBM)<sup>3</sup> (2) and Eu(DBM)<sup>3</sup> . dpp (4) [22].

When larger amounts of DMF solvent were used than described above, it was found that after 6 weeks storage of the composites, they became opalescent and their optical properties changed. A possible explanation could be the disturbance of the Eu3+ coordination shell caused by the strong donor ability of the DMF. The presence of contaminant ions (using NaOH instead of NH<sup>4</sup> OH for adjustment of the pH for alkoxide hydrolysis) strongly decreases the lifetime of the excitation states to 31 µs for Eu(DBM)<sup>3</sup> and to 19 µs for Eu(DBM)<sup>3</sup> . dpp containing composites, compared with 82 and 318 µs for the pure complexes, respectively.

#### *2.2.3. SiO2 /polyester hybrid (inorganic/organic) matrix with immobilized Rudpp*

Inorganic–organic hybrids combine the functional variety of organic compounds with the benefits of thermally stable and strong inorganic substrates and they can be promising precursors for immobilization matrices [23]. In the review quoted [23] different ways of making such "hybrid" materials are mentioned and among them is the simultaneous condensation of silica and organosilica precursors ("cocondensation"). A similar possibility is given by the application of the polymerized complex method [24], where a formation of polymeric resin is a result of condensation and polyesterification between a hydroxopolycarboxilic acid (preferably citric acid, CA) and a polyvalent alcohol (most often ethylene glycol, EG). If simultaneous hydrolysis of tetraethoxysilane (TEOS), esterification of the hydrolyzed product with CA, and esterification between CA and EG can happen, then it can be expected a formation more complicated structure in comparison with the relatively regular one obtained from pure TEOS. The formed polymer net may ensure suitable places for entrapping a complex and its uniform distribution (which may be very important, for example for gas sensor ). Mole ratios of TEOS:CA:EG = 4:1:1, 4:2:2, 2:1:1, and 1:1:1 were used, and an ultrasound treatment time was 40 min was applied. Substrates such as microscope glass were used. Before deposition, the slides were cleaned by 15 min treatment in an ultrasound bath consecutively with distilled water, methanol, acetone, and finally with distilled water. The factors influencing the properties of the composites synthesized based on SiO<sup>2</sup> /polyester "hybrid" are (i) the mole ratio TEOS:CA:EG, (ii) the sol production method, (iii) the sol sonication, and (iv) the film thickness, determined itself by the sol composition, aging time, and withdrawing speed.

Microcrystallization of Rudpp complex was found to occur in a SiO<sup>2</sup> matrix produced from acid-catalyzed non-sonicated sol [21]; no indications for such a separation were found in the SiO<sup>2</sup> /polyester "hybrid" films. Immobilization in the SiO<sup>2</sup> /polyester films does not significantly change the fluorescent properties of the Rudpp complex, as shown in the fluorescence microscopy images (**Figure 9**).

**Figure 9.** Images from fluorescence microscopy (×160) of films produced on microscope glass by dip coating at mole ratios TEOS:CA:EG = 4:1:1 (a), 4:2:2 (b), 2:1:1 (c), and 1:1:1 (d), respectively [25].

When larger amounts of DMF solvent were used than described above, it was found that after 6 weeks storage of the composites, they became opalescent and their optical properties changed. A possible explanation could be the disturbance of the Eu3+ coordination shell caused by the strong donor ability of the DMF. The presence of contaminant ions (using NaOH

.

composites, compared with 82 and 318 µs for the pure complexes, respectively.

(1) and Eu(DBM)<sup>3</sup>

*/polyester hybrid (inorganic/organic) matrix with immobilized Rudpp*

Inorganic–organic hybrids combine the functional variety of organic compounds with the benefits of thermally stable and strong inorganic substrates and they can be promising precursors for immobilization matrices [23]. In the review quoted [23] different ways of making such "hybrid" materials are mentioned and among them is the simultaneous condensation of silica and organosilica precursors ("cocondensation"). A similar possibility is given by the application of the polymerized complex method [24], where a formation of polymeric resin is a result of condensation and polyesterification between a hydroxopolycarboxilic acid (preferably citric acid, CA) and a polyvalent alcohol (most often ethylene glycol, EG). If simultaneous hydrolysis of tetraethoxysilane (TEOS), esterification of the hydrolyzed product with CA, and esterification between CA and EG can happen, then it can be expected a formation more complicated structure in comparison with the relatively regular one obtained from pure TEOS. The formed polymer net may ensure suitable places for entrapping a complex and its uniform distribution (which may be very important, for example for gas sensor ). Mole ratios

OH for adjustment of the pH for alkoxide hydrolysis) strongly decreases the

and to 19 µs for Eu(DBM)<sup>3</sup>

dpp (3) and of the composites containing Eu(DBM)<sup>3</sup>

.

dpp containing

(2) and

instead of NH<sup>4</sup>

**Figure 8.** Emission spectra of Eu(DBM)<sup>3</sup>

dpp (4) [22].

*2.2.3. SiO2*

Eu(DBM)<sup>3</sup> .

lifetime of the excitation states to 31 µs for Eu(DBM)<sup>3</sup>

258 Modern Technologies for Creating the Thin-film Systems and Coatings

The fluorescence intensity from different regions of a "hybrid" film were followed in time. It can be seen that after a 150 s stabilization period, the fluorescence emission intensity from different regions stays constant within about 1.8% (**Figure 10**) [25].

**Figure 10.** Fluorescence intensity from different regions of a "hybrid" film as a function of time [25].

The matrix composition (ratio TEOS:CA:EG) does not influence the excitation (**Figure 11a**) and emission (**Figure 11b**) spectra of the films and the position of the maxima (310 and 619 nm, respectively).

**Figure 11.** Excitation (a) and emission (b) spectra of Rudpp in hybrid matrix (TEOS:CA:EG = 4:1:1) [25].

The results obtained gives the opportunity to conclude that by formation of SiO<sup>2</sup> /polyester "hybrid" matrix the films can be produced with a good stability, density and homogeneity, as well as with a good adhesion to different substrates such as glass, silica, and ceramics. An additional advantage of that matrix is the uniform distribution of the optically active complex immobilized, especially the fact that it preserves its optical properties. The film morphology strongly depends on the sol composition and deposition mode. Films prepared by dip coating at a withdrawing speed of 0.4 mm s−1 from sonicated sol with molar ratio TEOS:CA:EG = 4:1:1 produced by acid catalysed hydrolysis were found promising as an active component of optical sensors [25].

#### *2.2.4. Composites based on poly-(methylmethacrylate), PMMA, with immobilized complexes*

#### *2.2.4.1. Immobilization of Rudpp in PMMA-based composites*

A PMMA matrix was prepared by a catalytically induced polymerization of the monomer methylmethacrylate (MMA), following a procedure described in [26]. It was found that poly(methylmethacrylate) films produced from monomer, containing 1.5% Rudpp, are dense with a smooth surface and uniform distribution of the complex (**Figure 12a, b**) and fluorescent properties (**Figure 12c**). The fluorescence emission intensity from films produced by polymerization of the monomer MMA was significantly (2.5-fold) weaker in comparison with those prepared from PMMA solution, probably due to partial destruction of the complex.

Using a chloroform solution of PMMA, membranes and thin films of good quality can be obtained. Membranes (0.4–1.5 mm in thickness) were prepared by casting of the PMMA solution or partially polymerized MMA into a Teflon® mould. Both types of samples were dried at 50°C for 24 h. The use of PMMA as an immobilization matrix for Rudpp is a cheap and easy to be applied composite preparation method, ensuring the production of dense and smooth specimens with uniformly distributed optically active complex with the possibility to prepare membranes up to 1 mm thick. No significant interaction of the complex with the matrices takes place, and the emission spectra of the complexes are practically unchanged as a result of immobilization in both types of studied matrices (**Figure 13a, b**). The reported results show the preparation of PMMA by the polymerization of the monomer MMA in the presence of benzoyl peroxide as a polymerization initiator is not a suitable method for the production of Rudpp-PMMA composites due to a partial destruction of the complex.

The matrix composition (ratio TEOS:CA:EG) does not influence the excitation (**Figure 11a**) and emission (**Figure 11b**) spectra of the films and the position of the maxima (310 and 619

The results obtained gives the opportunity to conclude that by formation of SiO<sup>2</sup>

**Figure 11.** Excitation (a) and emission (b) spectra of Rudpp in hybrid matrix (TEOS:CA:EG = 4:1:1) [25].

*2.2.4. Composites based on poly-(methylmethacrylate), PMMA, with immobilized complexes*

A PMMA matrix was prepared by a catalytically induced polymerization of the monomer methylmethacrylate (MMA), following a procedure described in [26]. It was found that poly(methylmethacrylate) films produced from monomer, containing 1.5% Rudpp, are dense with a smooth surface and uniform distribution of the complex (**Figure 12a, b**) and fluorescent properties (**Figure 12c**). The fluorescence emission intensity from films produced by polymerization of the monomer MMA was significantly (2.5-fold) weaker in comparison with those prepared from PMMA solution, probably due to partial destruction of the complex.

Using a chloroform solution of PMMA, membranes and thin films of good quality can be obtained. Membranes (0.4–1.5 mm in thickness) were prepared by casting of the PMMA solution or partially polymerized MMA into a Teflon® mould. Both types of samples were dried at 50°C for 24 h. The use of PMMA as an immobilization matrix for Rudpp is a cheap and easy to be applied composite preparation method, ensuring the production of dense and smooth

*2.2.4.1. Immobilization of Rudpp in PMMA-based composites*

"hybrid" matrix the films can be produced with a good stability, density and homogeneity, as well as with a good adhesion to different substrates such as glass, silica, and ceramics. An additional advantage of that matrix is the uniform distribution of the optically active complex immobilized, especially the fact that it preserves its optical properties. The film morphology strongly depends on the sol composition and deposition mode. Films prepared by dip coating at a withdrawing speed of 0.4 mm s−1 from sonicated sol with molar ratio TEOS:CA:EG = 4:1:1 produced by acid catalysed hydrolysis were found promising as an active component of

/polyester

nm, respectively).

260 Modern Technologies for Creating the Thin-film Systems and Coatings

optical sensors [25].

**Figure 12.** AFM (at different scanning area) (a, b) and fluorescence (x160) (c) images of dip-coated films from Rudppcontaining composite prepared from catalyst-induced polymerization of MMA [26].

**Figure 13.** Emission spectra of a PMMA/Rudpp-containing membrane (excitation at 450 nm) prepared from catalystinduced polymerization of MMA (a) and from PMMA solution (b) [26].

#### *2.2.4.2. Immobilization of Eu(III) beta-diketonates in PMMA*

The optically active complexes (Eu(DBM)<sup>3</sup> , Eu(DBM)<sup>3</sup> . phen, Eu(DBM)<sup>3</sup> . dpp, Eu(TTA)<sup>3</sup> . phen, and Eu(TTA)<sup>3</sup> . dpp) are well-known fluorescent dyes with high fluorescent emission intensity (phen is phenanthroline, and TTA is thenoyltrifluoroacetone). The complexes of the type Eu(DBM)<sup>3</sup> . Q (Q is phen or dpp) are of special interest due to their higher stability (compared to Eu(DBM)<sup>3</sup> ) and increased luminescence intensity [27].

A PMMA matrix was prepared by a catalytically induced polymerization of the monomer MMA [28]. The deposited films were aged at 50°C for 4–6 h. The experiments showed that thicker membranes can be prepared by applying 24 h of aging time at the temperature mentioned. The content of the complex in the final product after evaporation of the solvent was determined to be 1.5%. Using a solution of PMMA, films were deposited and membranes were prepared with a complex concentration in the final matrix of 1.0%. Films were deposited by dip coating at a withdrawal speed of 0.4 mm/s, using the device described in [29]. The films were uniform with a typical thickness of 200 nm. By varying the withdrawal speed, films with thickness of 0.1–1 µm were prepared. Membranes 0.1–1 mm in thickness can be easily obtained by placing the initial complex-binder mixture in a mould or by pouring along glass slides. SEM (**Figure 14**) or AFM (**Figure 15**) showed that no microcrystallization or aggregation (observed for example in [30], reaching 10 nm in size) of the complexes in the matrix took place and that cracks in films and membranes were absent. Qualitative tests with adhesive tape and brass edge showed that films thicker than 400–500 nm have poorer adhesion to the microscope glass.

**Figure 14.** SEM images of dip -coated films of Eu(DBM)<sup>3</sup> (a) and Eu(TTA)<sup>3</sup> . phen (b, c,) in MMA (b) and PMMA (a, c) [28].

**Figure 15.** AFM images of dip-coated thin films of immobilized Eu complexes: Eu(DBM)<sup>3</sup> (a), Eu(DBM)<sup>3</sup> . phen (b), Eu(DBM)<sup>3</sup> . dpp (c), Eu(TTA)<sup>3</sup> . dpp (d) in PMMA, produced from the monomer, and Eu(DBM)<sup>3</sup> (e) in PMMA produced from polymer solution [28].

Embedding of the complexes causes some changes in the excitation spectra but does not influence the emission spectra significantly. The uniform distribution of the complex in the matrix and lack of aggregates, obtained by the applied deposition methods, also have a positive effect on the preservation of the lifetime of the excited states of the embedded complexes (**Figure 16**).

**Figure 16.** Lifetime of the excited state of composite Eu(TTA)<sup>3</sup> . phen in MMA [28].

A PMMA matrix was prepared by a catalytically induced polymerization of the monomer MMA [28]. The deposited films were aged at 50°C for 4–6 h. The experiments showed that thicker membranes can be prepared by applying 24 h of aging time at the temperature mentioned. The content of the complex in the final product after evaporation of the solvent was determined to be 1.5%. Using a solution of PMMA, films were deposited and membranes were prepared with a complex concentration in the final matrix of 1.0%. Films were deposited by dip coating at a withdrawal speed of 0.4 mm/s, using the device described in [29]. The films were uniform with a typical thickness of 200 nm. By varying the withdrawal speed, films with thickness of 0.1–1 µm were prepared. Membranes 0.1–1 mm in thickness can be easily obtained by placing the initial complex-binder mixture in a mould or by pouring along glass slides. SEM (**Figure 14**) or AFM (**Figure 15**) showed that no microcrystallization or aggregation (observed for example in [30], reaching 10 nm in size) of the complexes in the matrix took place and that cracks in films and membranes were absent. Qualitative tests with adhesive tape and brass edge showed that films thicker than 400–500 nm have poorer adhesion to the microscope glass.

(a) and Eu(TTA)<sup>3</sup>

**Figure 15.** AFM images of dip-coated thin films of immobilized Eu complexes: Eu(DBM)<sup>3</sup>

dpp (d) in PMMA, produced from the monomer, and Eu(DBM)<sup>3</sup>

.

phen (b, c,) in MMA (b) and PMMA (a, c) [28].

(a), Eu(DBM)<sup>3</sup>

. phen (b),

(e) in PMMA produced

**Figure 14.** SEM images of dip -coated films of Eu(DBM)<sup>3</sup>

262 Modern Technologies for Creating the Thin-film Systems and Coatings

Eu(DBM)<sup>3</sup> .

dpp (c), Eu(TTA)<sup>3</sup>

from polymer solution [28].

.

The preparation of the matrix by the monomer polymerization leads to partial destruction of the less stable complexes and thereby a decrease in the fluorescence intensity. It was shown that the films produced by dip coating from europium-diketonates–PMMA chloroform solution can be used both as components of luminescent devices and for temperature sensing [28].

### **3. Spin-coating technique for film production**

The basic principles of the spin-coating technique and the parameters controlling the process are presented, for example in Ref. [31], including the spin speed, spin time, acceleration, etc. The process generally involves four stages, namely a dispense stage (or deposition), substrate acceleration (spin-up) stage, a stage of substrate spinning at a constant rate, and evaporation [31, 32]. Evaporation may accompany the other stages [32, 33]. In order to understand the mechanism of thin-film formation by spin coating, the relationship between the thickness and the solvent evaporation rate of spin-coated thin films has been studied [34]. Spin coating combined with the sol-gel process offers a simple, low cost, and highly controlled way of film deposition [35]. In recent years, the method has been used for coating in microelectronics [36] and studied for deposition of transparent conducting oxides [37], for ferroelectric thin films [38], for indium oxide thin films [39], for doped and undoped hematite films [40], for ZnO thin films [35, 41–44], for titanium oxide films [45], for yttria-stabilized zirconia thin films [46], for mesoporous silica thin films on a silicon substrate [47], and so on.

#### **3.1. SiO2 films and composites produced by spin coating**

The spin-coating technique was applied for sol-gel produced SiO<sup>2</sup> -based films. The deposition was done by means of a KW-4A (USA) spin coater at 2000–6000 rpm, spinning time 20–60 s, and 1–3 spinning procedures (0.5 cm<sup>3</sup> sol per procedure) [19]. Different orthosilanes were used for SiO<sup>2</sup> precursors, such as TEOS, OtEOS, or their mixture. The morphology of the films prepared from TEOS and TEOS/OtEOS is shown in the SEM images (**Figure 17**).

**Figure 17.** SEM images of films produced by spin coating at a spinning speed of 3000 min-1 from sols: TEOS (a); TEOS/ OtEOS (b) (spinning time 30 s); TEOS/OtEOS (60 s), at an image angle of 90° (c) or 45°(d) [19].

The alkoxysilane used has an influence on the morphology of the spin-coated films. Films prepared using TEOS are similar to the dip-coated films (part 2.2.1.), that is, the films are dense and uniform (**Figure 17a**). The use of an ormosil-type precursor (TEOS/OtEOS mixture) leads to the production of "structured" films (**Figure 17b**) whose SEM images are different from those of dip-deposited films. The ''structuring'' improves with increasing of the spinning time from 30 to 60 s (**Figure 17b**–**d**), that is, with decreasing film thickness, which is opposite to the effect of the same parameter for dip coating.

A well expressed chain-like surface structure is seen in the AFM image (**Figure 18a**, without Rudpp) where the above mentioned hills and valleys are very regular. The section analysis (**Figure 18b**, with Rudpp) reveals hollows with diameters of 25–60 nm and depths of 1–1.6 nm and differences between the lowest and highest points in the scanned region of 2.8 nm. There is an influence of the spinning time: chains are observed at a spinning time of 60 s that are not noticed at 30 s spinning time (**Figure 17c, d**).

The thickness of the spin-produced films can be controlled by the gel spinning rate and the time. The thickness decreases with increasing spinning rate (2000–4000 min−1) and time (20–40 s). The effect is significant (a factor two) when the spinning time was increased from

**3.1. SiO2**

used for SiO<sup>2</sup>

 **films and composites produced by spin coating**

The spin-coating technique was applied for sol-gel produced SiO<sup>2</sup>

and 1–3 spinning procedures (0.5 cm<sup>3</sup>

264 Modern Technologies for Creating the Thin-film Systems and Coatings

effect of the same parameter for dip coating.

noticed at 30 s spinning time (**Figure 17c, d**).

was done by means of a KW-4A (USA) spin coater at 2000–6000 rpm, spinning time 20–60 s,

The alkoxysilane used has an influence on the morphology of the spin-coated films. Films prepared using TEOS are similar to the dip-coated films (part 2.2.1.), that is, the films are dense and uniform (**Figure 17a**). The use of an ormosil-type precursor (TEOS/OtEOS mixture) leads to the production of "structured" films (**Figure 17b**) whose SEM images are different from those of dip-deposited films. The ''structuring'' improves with increasing of the spinning time from 30 to 60 s (**Figure 17b**–**d**), that is, with decreasing film thickness, which is opposite to the

**Figure 17.** SEM images of films produced by spin coating at a spinning speed of 3000 min-1 from sols: TEOS (a); TEOS/

OtEOS (b) (spinning time 30 s); TEOS/OtEOS (60 s), at an image angle of 90° (c) or 45°(d) [19].

A well expressed chain-like surface structure is seen in the AFM image (**Figure 18a**, without Rudpp) where the above mentioned hills and valleys are very regular. The section analysis (**Figure 18b**, with Rudpp) reveals hollows with diameters of 25–60 nm and depths of 1–1.6 nm and differences between the lowest and highest points in the scanned region of 2.8 nm. There is an influence of the spinning time: chains are observed at a spinning time of 60 s that are not

The thickness of the spin-produced films can be controlled by the gel spinning rate and the time. The thickness decreases with increasing spinning rate (2000–4000 min−1) and time (20–40 s). The effect is significant (a factor two) when the spinning time was increased from

prepared from TEOS and TEOS/OtEOS is shown in the SEM images (**Figure 17**).

precursors, such as TEOS, OtEOS, or their mixture. The morphology of the films


sol per procedure) [19]. Different orthosilanes were

**Figure 18.** AFM images of films produced by spin coating (60 s, 3000 min−1) from TEOS/OtEOS (a) and TEOS/OtEOS-Rudpp sol (b). The section analysis is performed along the diagonal of the bearing area [19].

20 to 30 s. No significant differences were found between films produced from TEOS/OtEOS and pure TEOS at spinning times of 30 and 40 s [21]. The thickness of the produced films (1 × 1 cm), measured by a Talystep profilomer, depends on the deposition conditions and is typically around 300 nm for dip-coated films. The thickness of the spin-deposited films is much larger: from 7 µm (at 0.5 cm<sup>3</sup> gel, 3000 rpm, 30 s) to 19 µm (1.5 cm<sup>3</sup> gel, 2000 rpm, 20 s) [19]. Their morphology depends mainly on the nature of the precursor and somewhat on the deposition method [19].

The ruthenium complex Rudpp was immobilized in a SiO<sup>2</sup> matrix by following a commonly used sol preparation procedure (with 1.25–2.5 g Rudpp/dm<sup>3</sup> sol). It was found that microcrystallization of the complex occurs with formation of randomly distributed crystals of 100–400 nm in size. An ultrasound treatment of the sol by means of an ultrasound disintegrator leads to homogeneous distribution of the complex without observable crystallization (**Figure 18b**).

#### **3.2. Rudpp complex immobilized in a SiO2 /polyester "hybrid" composite**

A SiO<sup>2</sup> /polyester "hybrid" with immobilized Rudpp complex was prepared, using a polished fused silica substrate spinning at 2000 min−1 and a precursor solution with mole ratio TEOS:CA:EG = 1:2:2. The sol aging time was 2 h, the spraying/spinning time was 30 s, and ultrasound treatment time was 40 min [21]. It was found that the spin-produced specimens obtained from sols enriched in CA and EG were not uniform.

### **4. Spray pyrolysis method for film production**

Spray pyrolysis is a simple and inexpensive technique, which does not require high-quality substrates or chemicals for the production of various materials. It is easy for preparing films of any composition, such as thin films with large surface area, ceramic coatings, dense films, porous films at relatively low temperatures, and multilayered films [1].

It has been used for decades in the glass industry and in solar cell production [1], for powder production [48] and for electrodes and counter electrodes for dye-sensitized solar cells [49]. The method is used for deposition of thin ferrite films [50], thin films of the perovskite LaFeO<sup>3</sup> [51], thin films of TiO<sup>2</sup> (pure or modified) [52–55], films of poly-(methyl)methacrylate [28], and thin films of cerium-doped yttrium-iron garnet [56], each of them with potential applications in water purification, oxygen sensing, thermosensors, for deposition of thin yttria-stabilized zirconia films [1, 57, 58], for crystalline and non-crystalline iron oxide (α-Fe<sup>2</sup> O3 ) thin films onto glass substrates at different temperatures [59, 60], highly structured ZnO layers [61], transparent conducting zinc oxide thin films [62], lead(II) oxide thin films [63], nanoporous aluminum oxide [64], europium doped lanthanum oxide films [65], and UV excited green emitting Eu(II) activated BaAl<sup>2</sup> O4 and SrAl<sup>2</sup> O4 [66] and etc. Typical spray pyrolysis equipment consists of an atomizer, precursor solution, substrate heater, and temperature controller. The atomizers usually used in spray pyrolysis technique are mentioned and explained in [1], namely air blast (the liquid is exposed to a stream of air), ultrasonic (ultrasonic frequencies produce the short wavelengths necessary for fine atomization), and electrostatic atomizer (the liquid is exposed to a high electric field).

### **4.1. Factors influencing the properties of the films produced by spray pyrolysis**

The processes involved in the spray pyrolysis technique as well as the effects of spray pyrolysis parameters on film quality such as the influence of the substrate surface temperature on the film roughness, cracking, and crystallinity are discussed in Ref. [1]. The substrate surface temperature is a parameter that determines the film morphology and properties so that by increasing the temperature, the film morphology can be changed from a cracked to a porous microstructure [1]. The influence of different parameters on the thickness, morphology, crystal structure, and adhesion of the films is discussed in [5, 67], and optimal values are given (in brackets) for the nature of the carrier gas (oxygen or nitrogen) and its flow rate (0.5–1.2 dm<sup>3</sup> / min), the substrate temperature during the spraying (350°C or 400°C), the spraying angle (varied in the interval 20°–90°), the distance between the substrate and the nozzle (15–25 cm), the duration of spraying (10–20 s), the interval between the consecutive sprayings (1–5 min), the number of sprayings (1–20 cycles), the postdeposition annealing temperature (350–480°C), and the type of substrate. A comprehensive model for spray pyrolysis using solutions is presented in [67]. Different solutions or suspensions have been used such as ethylene glycol solution of mixed metal citrate complexes [67, 68] and aqueous or methanol suspensions containing TiO<sup>2</sup> and EG or PEG [54]. In some of the experiments, sonication for 20 min by means of ultrasonic disintegrator, UD, 20 (Technopan, Poland) was applied before spraying [54]. The device used [69] is suitable for film deposition. The suspension was passed through a pneumatic nebulizer with a 1 mm nozzle diameter using pressurized O<sup>2</sup> [5, 54] or N<sup>2</sup> [50] as a carrier gas. A nebulizer with nozzle of 0.7 mm in diameter was also used [50]. Microscope slides and optical grade glass of various shapes and sizes were used [51]. The substrate was situated at 20 cm from the nozzle at an angle of 45° and heated at temperatures, depending on the nature of the substrate and of the spraying material and kept within the limits of ±5°C. The suspension was sprayed for 30 s periods, separated by intervals of 5 min. The deposited films were heated at 300–500°C for 1 h [54] or 480–750°C for 0.5–3 h [50] in static air. The prepared films had a very good adhesion to the substrate as demonstrated by the standard tests with scotch tape and brass edge. The film thickness was controlled by the number of spraying cycles [50]. Typically, 10 cycles can be applied to produce 0.5 mg/cm<sup>2</sup> layers of TiO<sup>2</sup> that were approximately 1.5 µm thick [54]. With О<sup>2</sup> as a carrier gas, more uniform films were formed compared with those prepared using N<sup>2</sup> at the same conditions. This is probably due to a more even and complete burning of the organic components in the initial solution, when citric complexes were applied as precursors (**Figure 19**) [51].

The ruthenium complex Rudpp was immobilized in a SiO<sup>2</sup>

266 Modern Technologies for Creating the Thin-film Systems and Coatings

used sol preparation procedure (with 1.25–2.5 g Rudpp/dm<sup>3</sup>

obtained from sols enriched in CA and EG were not uniform.

**4. Spray pyrolysis method for film production**

[1, 57, 58], for crystalline and non-crystalline iron oxide (α-Fe<sup>2</sup>

porous films at relatively low temperatures, and multilayered films [1].

**3.2. Rudpp complex immobilized in a SiO2**

A SiO<sup>2</sup>

thin films of TiO<sup>2</sup>

SrAl<sup>2</sup> O4

tallization of the complex occurs with formation of randomly distributed crystals of 100–400 nm in size. An ultrasound treatment of the sol by means of an ultrasound disintegrator leads to homogeneous distribution of the complex without observable crystallization (**Figure 18b**).

/polyester "hybrid" with immobilized Rudpp complex was prepared, using a polished fused silica substrate spinning at 2000 min−1 and a precursor solution with mole ratio TEOS:CA:EG = 1:2:2. The sol aging time was 2 h, the spraying/spinning time was 30 s, and ultrasound treatment time was 40 min [21]. It was found that the spin-produced specimens

Spray pyrolysis is a simple and inexpensive technique, which does not require high-quality substrates or chemicals for the production of various materials. It is easy for preparing films of any composition, such as thin films with large surface area, ceramic coatings, dense films,

It has been used for decades in the glass industry and in solar cell production [1], for powder production [48] and for electrodes and counter electrodes for dye-sensitized solar cells [49]. The method is used for deposition of thin ferrite films [50], thin films of the perovskite LaFeO<sup>3</sup>

films of cerium-doped yttrium-iron garnet [56], each of them with potential applications in water purification, oxygen sensing, thermosensors, for deposition of thin yttria-stabilized zirconia films

at different temperatures [59, 60], highly structured ZnO layers [61], transparent conducting zinc oxide thin films [62], lead(II) oxide thin films [63], nanoporous aluminum oxide [64], europium

 [66] and etc. Typical spray pyrolysis equipment consists of an atomizer, precursor solution, substrate heater, and temperature controller. The atomizers usually used in spray pyrolysis technique are mentioned and explained in [1], namely air blast (the liquid is exposed to a stream of air), ultrasonic (ultrasonic frequencies produce the short wavelengths necessary for fine atom-

doped lanthanum oxide films [65], and UV excited green emitting Eu(II) activated BaAl<sup>2</sup>

ization), and electrostatic atomizer (the liquid is exposed to a high electric field).

**4.1. Factors influencing the properties of the films produced by spray pyrolysis**

The processes involved in the spray pyrolysis technique as well as the effects of spray pyrolysis parameters on film quality such as the influence of the substrate surface temperature on the film roughness, cracking, and crystallinity are discussed in Ref. [1]. The substrate surface temperature is a parameter that determines the film morphology and properties so that by increasing the temperature, the film morphology can be changed from a cracked to a porous microstructure [1]. The influence of different parameters on the thickness, morphology, crystal

(pure or modified) [52–55], films of poly-(methyl)methacrylate [28], and thin

O3

**/polyester "hybrid" composite**

matrix by following a commonly

sol). It was found that microcrys-

[51],

O4 and

) thin films onto glass substrates

**Figure 19.** AFM images of films deposited on microscope glass slides at a substrate temperature of 350°C; as carrier gas are used O<sup>2</sup> or N<sup>2</sup> , at a flow rate of 1 dm<sup>3</sup> /min; annealing of the films in air for 3 h at 350°C (O<sup>2</sup> -produced films) or 380°C (N<sup>2</sup> -produced films) and at 480°C (rms: root mean square roughness) [51].

The method is suitable for the production of films up to ~700 nm thickness. The film thickness above 700 nm can cause cracking during the thermal treatment, following the deposition [51]. Thin films of La<sup>2</sup> Ti2 O7 were deposited by spray pyrolysis using as starting material the ethylene glycol solutions of La–Ti citric . The films produced on silica glass and Si substrates after 2 h postdeposition annealing at 750°C were with good stoichimetry, homogeneous and with highly crystallinity. It was found that the size of the crystallites was between 25 and 45 nm and it depended on the nature of the substrate and slightly on the conditions for deposition and postdeposition. Using the number of the spraying cycles, the thickness of the films can be controlled (up to 1.2 mm). The size of the grains may also be controlled by the diluents used and by the conditions for annealing [68]. Highly crystalline uniform Y<sup>2</sup> O3 films (0.2 1 µm) were obtained using EG solutions of yttrium citric complexes, demonstrating that thin films can be deposited using spray pyrolysis of nonaqueous solutions of citric complexes as a starting material and using O<sup>2</sup> as a carrier gas. The substrate was heated at 350°C during the deposition, and a postdeposition annealing at 850°C for 2 h was applied [70].

The optimal coating conditions were similar to the ones used in [53]. The suspension was passed through a pneumatic nebulizer with a 1 mm nozzle diameter using pressurized O<sup>2</sup> as a carrier gas. The substrate was situated 20 cm from the nozzle at an angle of 45° and heated at temperatures that depended on the nature of the substrate and of the spraying material. The suspension was sprayed for 30 s periods, separated by intervals of 5 min. The deposited films were heated at 300–500°C for 1 h in static air. The prepared films had a very good adhesion to the substrate as demonstrated by the standard tests with scotch tape and brass edge. The film thickness was controlled by the number of spraying cycles. Typically, 10 cycles can be applied to produce 0.5 mg/cm<sup>2</sup> layers that were approximately 1.5 µm thick [54].

### **4.2. Spray pyrolysis for immobilization of complexes and synthesis of composites with optical properties**

### *4.2.1. PMMA-based composites*

The composite Eu(DBM)<sup>3</sup> /PMMA was obtained by two methods. The first one includes the dissolving of the complex into the MMA monomer solution followed by polymerization, and the second one includes the dissolving of the polymer and the complex in a solvent, followed by the evaporation of the latter. After the solution was prepared, the spray pyrolysis device, described in details in Ref. [69], was applied. The spraying conditions for Eu(DBM)<sup>3</sup> -containing solution of MMA are as follows: a nebulizer with a nozzle diameter of 0.8 mm, under an angle of 45° and at a distance of 20 cm from the substrate surface was used. As a carrier gas, air was applied with a flow rate of 0.7–0.9 dm<sup>3</sup> /min. The substrates used were heated at temperature varied between 50 and 70°C. The spraying time was 15 s, and the interval between different layer depositions was about 30 min. [28]. An example of the morphology of the films can be seen in **Figure 20**.

A tendency for decreasing film roughness when the monomer MMA polymerization was applied can be seen. This is explained by the relatively fast polymerization inside the very small droplets as well as by the sharp temperature decrease in the solution containing oligomerized monomer during deposition. This limits the possibility of formation of long chains and disturbs the structure obtained when PMMA polymer solution is used.

**Figure 20.** AFM-image of Eu(DBM)<sup>3</sup> /PMMA film produced by spray coating [28].

The method is suitable for the production of films up to ~700 nm thickness. The film thickness above 700 nm can cause cracking during the thermal treatment, following the deposi-

the ethylene glycol solutions of La–Ti citric . The films produced on silica glass and Si substrates after 2 h postdeposition annealing at 750°C were with good stoichimetry, homogeneous and with highly crystallinity. It was found that the size of the crystallites was between 25 and 45 nm and it depended on the nature of the substrate and slightly on the conditions for deposition and postdeposition. Using the number of the spraying cycles, the thickness of the films can be controlled (up to 1.2 mm). The size of the grains may also be controlled by the diluents used and by the conditions for annealing [68]. Highly crystalline

demonstrating that thin films can be deposited using spray pyrolysis of nonaqueous solu-

was heated at 350°C during the deposition, and a postdeposition annealing at 850°C for 2

The optimal coating conditions were similar to the ones used in [53]. The suspension was passed through a pneumatic nebulizer with a 1 mm nozzle diameter using pressurized O<sup>2</sup>

a carrier gas. The substrate was situated 20 cm from the nozzle at an angle of 45° and heated at temperatures that depended on the nature of the substrate and of the spraying material. The suspension was sprayed for 30 s periods, separated by intervals of 5 min. The deposited films were heated at 300–500°C for 1 h in static air. The prepared films had a very good adhesion to the substrate as demonstrated by the standard tests with scotch tape and brass edge. The film thickness was controlled by the number of spraying cycles. Typically, 10 cycles can be applied

layers that were approximately 1.5 µm thick [54].

/PMMA was obtained by two methods. The first one includes the

/min. The substrates used were heated at temperature

**4.2. Spray pyrolysis for immobilization of complexes and synthesis of composites with** 

dissolving of the complex into the MMA monomer solution followed by polymerization, and the second one includes the dissolving of the polymer and the complex in a solvent, followed by the evaporation of the latter. After the solution was prepared, the spray pyrolysis device,

solution of MMA are as follows: a nebulizer with a nozzle diameter of 0.8 mm, under an angle of 45° and at a distance of 20 cm from the substrate surface was used. As a carrier gas, air was

varied between 50 and 70°C. The spraying time was 15 s, and the interval between different layer depositions was about 30 min. [28]. An example of the morphology of the films can be

A tendency for decreasing film roughness when the monomer MMA polymerization was applied can be seen. This is explained by the relatively fast polymerization inside the very

described in details in Ref. [69], was applied. The spraying conditions for Eu(DBM)<sup>3</sup>

films (0.2 1 µm) were obtained using EG solutions of yttrium citric complexes,

O7 were deposited by spray pyrolysis using as starting material

as a carrier gas. The substrate

as


tion [51]. Thin films of La<sup>2</sup>

O3

h was applied [70].

to produce 0.5 mg/cm<sup>2</sup>

*4.2.1. PMMA-based composites*

applied with a flow rate of 0.7–0.9 dm<sup>3</sup>

The composite Eu(DBM)<sup>3</sup>

**optical properties**

seen in **Figure 20**.

uniform Y<sup>2</sup>

Ti2

268 Modern Technologies for Creating the Thin-film Systems and Coatings

tions of citric complexes as a starting material and using O<sup>2</sup>

The optical properties of the Eu(DBM)<sup>3</sup> complex and its derivatives as well as of Eu(TTA)<sup>3</sup> . phen after their immobilization both in films and in membranes produced from MMA or PMMA solutions were followed. For both types of matrices, the excitation spectra of the complexes showed some changes (with general pattern preserved), whereas the emission spectra were not disturbed (**Figure 21**) [28].

**Figure 21.** Emission spectra of (a) complex in solid state, (b) immobilized in a matrix from MMA solution, and (c) immobilized in a matrix from PMMA solution [28].

The lifetime is 75, 214, and 265 µs for the pure Eu(DBM)<sup>3</sup> and the complex immobilized in MMA and in PMMA, respectively. The preparation of the matrix by monomer polymerization leads to partial destruction of the less stable complexes and thereby a decrease in the fluorescence intensity.

#### *4.2.2. Spray pyrolysis produced composites based on SiO<sup>2</sup> /polyester "hybrid" matrix*

The synthetic procedure for composited preparation consists of citric acid (a measured amount) dissolved in ethanol under stirring, and of EG added in small portions to the solution obtained. Stirring for 15 min was applied in order to obtain a homogeneous final solution. To the latter one, TEOS was added dropwise in an amount that the desired mole ratio of TEOS:CA:EG:EtOH to be reached. By adding HCl (0.1 M), the pH value was adjusted equal to 2. To obtain a complex concentration of 2.5 g dm−3 sol, an ethanol solution of Rudpp (0.014 g mL−1) was added. The variation of the complex concentration was experimented, but it did not show an effect on the main functional parameters of the films produced.The films produced by spray pyrolysis were uniform, without cracks and with a satisfactory adhesion, and contained pores that were about 100 nm in diameter (**Figure 22**).

**Figure 22.** AFM images of based on SiO<sup>2</sup> polyester hybrid film made by spray pyrolysis [25].

The pores are probably due to the faster evaporation of the excess of EG in the course of spraying on the heated surface. It is concluded that the chain structure is common for sprayproduced films based on SiO<sup>2</sup> /polyester "hybrid" [25].

### **Author details**

Joana Zaharieva\* and Maria Milanova

\*Address all correspondence to: jzaharieva@abv.bg

University of Sofia "St. Kliment Ohridski," Faculty of Chemistry and Pharmacy, Department of Inorganic Chemistry, Sofia, Bulgaria

### **References**

[1] Perednis D, Gauckler LJ. Thin film deposition using spray pyrolysis. J Electroceram. 2005;**14**:103–111.

[2] Grosso D. How to exploit the full potential of the dip-coating process to better control film formation. J Mater Chem. 2011;**21**:17033-17038. doi: 10.1039/C1JM12837J

The lifetime is 75, 214, and 265 µs for the pure Eu(DBM)<sup>3</sup>

*4.2.2. Spray pyrolysis produced composites based on SiO<sup>2</sup>*

270 Modern Technologies for Creating the Thin-film Systems and Coatings

pores that were about 100 nm in diameter (**Figure 22**).

fluorescence intensity.

produced films based on SiO<sup>2</sup>

**Figure 22.** AFM images of based on SiO<sup>2</sup>

Joana Zaharieva\* and Maria Milanova

of Inorganic Chemistry, Sofia, Bulgaria

\*Address all correspondence to: jzaharieva@abv.bg

**Author details**

**References**

2005;**14**:103–111.

MMA and in PMMA, respectively. The preparation of the matrix by monomer polymerization leads to partial destruction of the less stable complexes and thereby a decrease in the

The synthetic procedure for composited preparation consists of citric acid (a measured amount) dissolved in ethanol under stirring, and of EG added in small portions to the solution obtained. Stirring for 15 min was applied in order to obtain a homogeneous final solution. To the latter one, TEOS was added dropwise in an amount that the desired mole ratio of TEOS:CA:EG:EtOH to be reached. By adding HCl (0.1 M), the pH value was adjusted equal to 2. To obtain a complex concentration of 2.5 g dm−3 sol, an ethanol solution of Rudpp (0.014 g mL−1) was added. The variation of the complex concentration was experimented, but it did not show an effect on the main functional parameters of the films produced.The films produced by spray pyrolysis were uniform, without cracks and with a satisfactory adhesion, and contained

The pores are probably due to the faster evaporation of the excess of EG in the course of spraying on the heated surface. It is concluded that the chain structure is common for spray-

polyester hybrid film made by spray pyrolysis [25].

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