**2. Research method**

optical and electrical properties. Because of their large chemical and structural stability, as well as their optical and electrical properties, metallic phthalocyanines (MPcs) have been introduced into polymeric matrices as nanoparticles. A polymeric matrix composite (PMC) is a compound material consisting of a polymeric primary phase, or matrix, which is embedded in a secondary phase based mainly on matrix-reinforcing fibres and particles. The polymeric matrix enhances material stability, as it limits the introduction of environmental oxygen or water, which could reduce the potential usefulness of the MPcs. Nanostructuring also permits two other goals: to achieve optical homogeneity of the polymeric composite medium and to take advantage of specific properties of MPcs in their crystalline form. MPcs are usually ordered in crystalline arrangements, as their aromatic rings stack neatly. Due to the strength of π bonds, MPcs can be accommodated in a large number of different structures, which depend on the substituents they have. The type of structure determines the physical properties of a specific MPc, as well as its applications. The main modes of MPc molecular organization that may be observed are: (i) crystals, which can be in the alpha or beta allotropic forms (the beta polymorph being thermodynamically more stable). The two types are distinguished by the angle formed between the symmetry axis and the stacking direction. Alpha and beta crystals form angles of 26.5 and 45.8°, respectively. (ii) Liquid crystals, where Pcs are substituted by flexible lipophilic chains, which allow the formation by substituents of a quasi-liquid medium surrounding the in-plane aromatic nuclei, which overlap in columns distributed over two-dimensional positions with hexagonal or tetragonal symmetries. (iii) Thin films are solid structures whose thicknesses can be neglected for many physical purposes. In applications involving interaction with electromagnetic waves, thin-film thickness must be of the same order as the wavelength of the interacting disturbance. Thin films represent the Pcs arrangement most commonly considered for electronic applications. (iv) Skewer-structured polymers are obtained by polymerizing MPcs through bridge ligands; due to the variety of ligands that may be used and their properties, the distance between molecules can be controlled rather well and, thanks to the rigidity of the unidirectional connection in this type of structures, very

30 Phthalocyanines and Some Current Applications

good electronic and optical properties can be obtained from the material.

The purpose of this work is to report the generation of MPc crystals, their dispersion into a polymeric matrix and the evaluation of their optical and electrical properties in thin-film form. In this study, a polystyrene polymeric matrix was used. The materials thus obtained were characterized by different methods, including infrared (IR) and ultraviolet-visible (UV-Vis) spectroscopy, as well as scanning electron microscopy (SEM). First nanoparticles were synthesized in a molecular solution obtained from a supersaturated MPc solution. Second a solid composite was prepared by introducing pre-grown colloidal MPc particles into a polymeric matrix in a spin coating process. Spin coating leads to the production of uniform, flat, high-quality films or coatings. This process involves the application of a certain amount of nanoparticles suspended in a polymer and previously solved in an organic solvent. A small amount of the fluid is put on a substrate attached to a plate that is made to rotate at high speed, so that the resulting centripetal force spreads the suspension until the desired film thickness is achieved for the composite material. This process has four stages: deposition, centrifugation, de-centrifugation and evaporation. The evaporation of the fourth stage represents the main thinning mechanism for the film. After the film is deposited, it is annealed for 10 min at 90°C to accelerate matrix polymerization.

#### **2.1. Crystallization process**

To carry out the crystallization of MPcs embedded in a polymeric matrix, the gel crystallization method was used, where a very viscous medium that favours slow crystallization is used to mix the constituent phases, mainly by diffusion. In this method, crystal growth in the gel takes place by diffusion-controlled mass transport. This procedure minimizes the sedimentation and convection effects of traditional crystallization by evaporation methods. One must take into account that the crystallization mechanism consists of three steps, i.e. solution supersaturation, formation of crystalline nuclei and crystal growth. The gel is a means to transport molecules or ions (precipitant agents, shock absorbers), with no or almost no chemical reactivity to molecules and ions that diffuse through their three-dimensional polymeric network. Gels can be classified, according to their preparation method, as chemical or physical. Chemical gels are those obtained by poly-addition processes, like those achieved from neutralization of sodium metasilicate, or by poly-condensation processes, such as those obtained from the hydrolysis reaction of tetramethoxysilane. The physical gels, including agar and agarose, are defined as those where the gelation process is carried out by the variation of some physical parameter, like temperature.

For the current study, tetramethoxysilane gel at 10% volume, with 50% of ethanol for crystallization in FePc capillary tubes, was used. Before introducing the solved gel into the capillary, this tube must be carefully washed with detergent, followed by double-distilled water and then acetone, and finally dried with warm air. The introduction of gel into the capillary is carried out by the application of air pressure with a syringe, taking care to avoid the formation of bubbles in the gel. The gel must occupy the central 4-cm section of the capillary. After the dispersion has gelled (a process which takes about 4 weeks), MPc is added through the ends of the capillary, travelling a distance of 3 cm of length. These MPcs, previously dissolved, must be added in the same way as the gel, by means of air pressure with the help of a syringe, while taking care not to form bubbles. The capillary is then sealed at the two ends and kept at a constant temperature of 22°C, until the product is formed. The conformation of the system used for gel crystallization can be shown in **Figure 1**, where the diagram of the tube used

**Figure 1.** Capillary system used for crystallization.

for the crystallization is divided into three parts, as shown in the figure: one in the middle, where the gel was initially placed and the two ends where the dissolved MPc was placed before the MPc molecules migrated to the gel zone, where they nucleated and grew. This gel-based technique provides continuous control over the crystal or particle growth process, since it becomes possible to increase the growth rate by adding a larger amount of reagents through the ends of the capillary. Moreover, it also reduces the risk of damage to the crystal or the particle that could occur because of physical instabilities in the experimental arrangement, as it avoids the direct manipulation of the grown crystals.

#### **2.2. Thin-film deposition and characterization**

Most of the advanced devices manufactured today depend, at some point of their fabrication, on the synthesis and growth of films or thin layers. For this work, thin-film deposition of FePc particles nucleated and grown in gels was carried out in air by spin coating. The material was deposited onto a Corning 7059 glass, quartz, (100) single-crystalline silicon (c-Si) 200 Ω-cm wafers and ITO-coated glass slides. The quartz and Corning glass substrates were ultrasonically degreased in warm methanol and dried under a nitrogen atmosphere. The silicon substrates were chemically etched with a *p-etch* solution and dried under a nitrogen atmosphere. The composition of the solution was selected to have an FePc: polystyrene ratio of 1:3 in chloroform. The solution was spin coated on the substrates in a two-step process: 2500 rpm for 30 s, followed by annealing at 393 K for 10 min. These processes, spin coating and annealing, were repeated to obtain a suitable thickness. The thicknesses of the films obtained in the present study are shown in **Table 1**. We also report the determination of optical parameters related to the main transitions in the UV-Vis region, as well as the fundamental energy gap calculations for these films. Devices consisting of polystyrene matrix film were placed onto Corning Preparation and Structural Characterization of Metallophthalocyanine Particles Embedded in a Polymer Matrix http://dx.doi.org/10.5772/67576 33


**Table 1.** Characteristic parameters of the FePc/polystyrene films.

glass substrates with a contact conductor of indium tin oxide (ITO) by spin coating. After the deposition, in order to diffuse MPc particles into the polystyrene matrix, the films were heat treated at 393 K for 10 min. The electric conductivity at 298 K of the device was evaluated with a four-point probe; for these measurements, the substrates were ITO-coated glasses with silver strips acting as electrodes. The strips were deposited by the painting process, the current due to hole-injection from positively-biased ITO was measured and the current due to holeinjection from silver was measured by reversing the polarity of the bias voltage [3].

#### **2.3. Instruments**

for the crystallization is divided into three parts, as shown in the figure: one in the middle, where the gel was initially placed and the two ends where the dissolved MPc was placed before the MPc molecules migrated to the gel zone, where they nucleated and grew. This gel-based technique provides continuous control over the crystal or particle growth process, since it becomes possible to increase the growth rate by adding a larger amount of reagents through the ends of the capillary. Moreover, it also reduces the risk of damage to the crystal or the particle that could occur because of physical instabilities in the experimental arrange-

Most of the advanced devices manufactured today depend, at some point of their fabrication, on the synthesis and growth of films or thin layers. For this work, thin-film deposition of FePc particles nucleated and grown in gels was carried out in air by spin coating. The material was deposited onto a Corning 7059 glass, quartz, (100) single-crystalline silicon (c-Si) 200 Ω-cm wafers and ITO-coated glass slides. The quartz and Corning glass substrates were ultrasonically degreased in warm methanol and dried under a nitrogen atmosphere. The silicon substrates were chemically etched with a *p-etch* solution and dried under a nitrogen atmosphere. The composition of the solution was selected to have an FePc: polystyrene ratio of 1:3 in chloroform. The solution was spin coated on the substrates in a two-step process: 2500 rpm for 30 s, followed by annealing at 393 K for 10 min. These processes, spin coating and annealing, were repeated to obtain a suitable thickness. The thicknesses of the films obtained in the present study are shown in **Table 1**. We also report the determination of optical parameters related to the main transitions in the UV-Vis region, as well as the fundamental energy gap calculations for these films. Devices consisting of polystyrene matrix film were placed onto Corning

ment, as it avoids the direct manipulation of the grown crystals.

**2.2. Thin-film deposition and characterization**

**Figure 1.** Capillary system used for crystallization.

32 Phthalocyanines and Some Current Applications

For the preparation of the thin films, a *Best Tools Smart Coater* 200, operating at 400 W, 110 V and 50/60 Hz, was used. FT-IR measurements were obtained with a Nicolet iS5-FT spectrophotometer using KBr pellets for the powders and silicon wafers as substrates for the thin films. Film thickness values were determined by profilometry in a quartz substrate with a *Bruker* profilometer, model DEKTAK XT, with STYLUS, LIS 3, 2 μm RADIUS-Type B. For SEM, a ZEISS EVO LS 10 scanning electron microscope was coupled to a microanalysis system and operated at a voltage of 20 kV and a focal distance of 25 mm, using thin films on a glass substrate. The size and distribution of dispersed particles were observed using a *JEOL* JEM2010 transmission electron microscope (TEM), LaB6 cathode at 200 kV, 105 μA. UV-Vis spectroscopy was carried out in a *Unicam* spectrophotometer, model *UV300*, with a quartz substrate. Electric characterization was performed with a programmable voltage source, an auto-ranging pico-ammeter *Keithley* 4200-SCS-PK1 and a sensing station with a *Next Robotix* lighting controller circuit.
