**3. Lithography using light sensitive precursors of nanofillers**

In situ synthesis of NPs into their matrices is a very powerful lithographic technique because the properties of the nanocomposites can be controlled and tailored locally in a unique way. In this section we will present the laser-induced formation of NPs directly into the polymer matrices following two main strategies. The first one is based on the use of chloroauric acid salt, which is a gold (Au) precursor, introduced in transparent polymer films. Herein, chitosan (CTO) is used as a matrix, which is a very promising polymer due to its biocompatibility. In CTO polymer, Au NPs can be generated by UV irradiation (Miyama & Yonezawa 2004). Using photomasks and UV laser beam we obtain the creation of gold NPs in precise areas of the polymeric film, turning the insulating polymer into electron conducting material. This process allows us to localize and design accurately surface patterns and moreover to tune the metallic particle size in the range of nanoscale by varying the laser irradiation time and energy.

Fig. 9. AFM topography of the nanocomposite SU-8 mixed with iron oxide NPs, vertically

Right Part (°) Left Part (°) Drop Images

Table 1. Contact angle values of the left and right part of the water drop when the magnet is away from the substrate (top raw) and when the magnet in coming closer (down raw). At

In situ synthesis of NPs into their matrices is a very powerful lithographic technique because the properties of the nanocomposites can be controlled and tailored locally in a unique way. In this section we will present the laser-induced formation of NPs directly into the polymer matrices following two main strategies. The first one is based on the use of chloroauric acid salt, which is a gold (Au) precursor, introduced in transparent polymer films. Herein, chitosan (CTO) is used as a matrix, which is a very promising polymer due to its biocompatibility. In CTO polymer, Au NPs can be generated by UV irradiation (Miyama & Yonezawa 2004). Using photomasks and UV laser beam we obtain the creation of gold NPs in precise areas of the polymeric film, turning the insulating polymer into electron conducting material. This process allows us to localize and design accurately surface patterns and moreover to tune the metallic particle size in the range of nanoscale by varying

aligned under the application of an external magnetic field.

127 130

122 135

the right column are demonstrated the two examined frames.

the laser irradiation time and energy.

**3. Lithography using light sensitive precursors of nanofillers** 

Another strategy that we follow for the in situ creation of nanofillers into the polymer matrix is the UV laser irradiation of polymer films containing cadmium thiolate precursors. This method results in the spatially selective formation of cadmium sulphide (CdS) crystalline NPs in the host matrix, through a macroscopically non-destructive procedure for the matrix. Using a pulse by pulse approach, we accomplished the formation of NPs with gradually increasing dimensions, and consequently the progressive change of the emission characteristics of the formed nanocomposites. The optimized combination of irradiation wavelength with polymer matrix gives patterned nanocomposite materials incorporating nanocrystals of high quality, ready to be used in various optical applications.

#### **3.1 Au precursor-based nanocomposite patterns**

The use of polymeric nanocomposite materials is expanding to a huge range of applications since they combine the flexibility, easy processability and low cost of the polymers with the unique properties of the nanofillers. On the other hand the intrinsic insulating characteristic of the polymers limit the possibilities of using polymeric-based systems in devices where electronic conductivity is desired, like sensors, miniaturized electronic chips, etc. In order to overcome this limitation, the use of metallic NPs that exhibit very high electronic conductivity as nanofillers is proved to be a successive strategy (Gelves 2006, 2011, Huang et al 2009).

Our strategy is based on the use of the Au precursor, chloroauric acid salt (HAuCl4), introduced in a transparent polymer film by immersion, and the generation of Au particles in specific areas by means of laser irradiation. The lithographically produced nanocomposite areas have tailored properties, dependent on the density and size of the produced Au NPs. The possibility to produce Au-polymeric nanocomposite materials with enhanced electrical properties, in combination with the spatial control of the specific property by introducing in situ the nanofillers in the desired areas increases enormously the potentiality of such systems in a wide variety of applications.

#### **3.1.1 Spatially controlled in situ formation of Au NPs in chitosan**

Chitosan (CTO) is a natural biodegradable and biocompatible polysaccharide polymer derived from chitin, a linear chain of acetylglucosamine groups, extracted from crustaceans shells and the cell walls of many fungi. CTO is fiberlike and is obtained by the deacetylation process of the natural chitin, a process that gives rise to amine groups which can be used for further functionalization (Yi et al 2005, Luther et al 2005, Su et al 2005, Zhou et al 2006, Zangmeister et al 2006). CTO is becoming widely used due to its potential polysaccharide resource and properties as non-toxicity, excellent processability, adsorption properties, hydrogel behavior, electrospinning, etc (Guibal 2005, Nirmala et al 2011). Its chemical structure is illustrated in figure 10.

CTO has a hydrogel nature resulting in the tendency to absorb ambient moisture or liquids. In this work, the process was optimized and used to introduce gold precursor in CTO polymer thin films. By controlling the immersion time of the polymer film in gold precursor solution, the absorption of gold precursors is highly controlled. The CTO used in this work was purchased from Sigma Aldrich with a degree of deacetylation about 80%. Various concentrations of CTO polymer solutions (0.5%, 1% and 2% wt.) are prepared in acetic acid. CTO polymeric films on glass substrates are obtained by drop-casting or spin-coating. The use of high CTO polymer concentration allows the formation of CTO hydrogel films able to absorb gold precursor crystals behaving as a "gold precursor reservoir". The gold precursor

Laser-Based Lithography for Polymeric Nanocomposite Structures 303

demonstrated. The size of the smallest Au NPs that appear onto the surface of the sample is around 20 nm. Under these experimental conditions, the distribution of Au NPs becomes almost homogeneous after 90 sec and they form a sort of continuous film on the surface of the CTO film. It can also be noted that the form of the NPs seems to change from undefined

Fig. 12. Evolution of the formation of the Au NPs onto the CTO surface upon increasing irradiation time: (a) non-irradiated area, (b) area irradiated for 30 sec, (c) area irradiated for 75 sec. The density of the NPs increases and their size decreases with the irradiation time. In the literature (Miyama & Yonezawa 2004) is mentioned that the growth of Au NPs occurs by aggregation of the photolytically formed Au atoms and clusters (equation 1). Nevertheless, the aggregation mechanism is not confirmed by the AFM images presented in figure 12, where is demonstrated that increasing irradiation time causes the decrease and not the increase of the size of the formed particles. The mechanism occurring in our case seems to be closer to the one reported on the formation of metal NPs by laser ablation in water solutions, where the reduced size of the NPs with increasing irradiation time is explained by their fragmentation due to self-absorption of laser pulses (Mafune´ et al 2000, Shukla & Seal 1999, Videla et al 2010, Kadossov & Burghaus 2010, Kabashin & Meunier 2003). Indeed, the Scanning Electron Microscopy (SEM) images obtained on the same sample irradiated at two different areas for very few seconds (≈5) and 30 sec, respectively, using fluence of 1 J cm-2, demonstrate that the first particles that appear onto the CTO surface are quite big (in the micrometer scale) and then they fragment into smaller ones (Figure 13). A closer look into the NPs clearly demonstrates that some of them are already fragmented into smaller pieces. Due to the irradiation conditions used in our experiments the aggregation of Au atoms and clusters into small particles seems to occur already in the very first seconds of the procedure and then the fragmentation mechanism becomes predominant. It is also interesting to mention that the fragmentation mechanism as described in literature is linked with an increase of local temperature due to the absorption of the laser photons by the Au NPs, which may increase the mobility of the latter towards

a b c

Finally, in figure 14 is demonstrated the intensity of the electronic current that is conducted through a CTO/Au NPs sample as a function of the laser irradiation time, for a laser fluence of ~5 J·cm-2. It is clear that the density increase of the formed Au NPs and the interconnection between them, previously demonstrated in figures 12 and 13, has an effect on the electrons mobility into the samples. Indeed, the current passing through the samples increases almost linearly with the laser irradiation time. The enhancement of the electrical

geometrical structures to spherical shapes after prolonged irradiation.

the surface of the CTO polymeric films.

Fig. 10. Chemical structure of chitosan.

Fig. 11. Lithographic production of Au NPs at adjacent areas (dark spots) of a CTO film incorporating Au precursor, irradiated with increasing times.

used is a chloroauric acid salt (Mw (HAuCl4)=339.5g/mol) and is dissolved in distilled water by providing two solutions (0.01M and 0.02M). Irradiation of the salt embedded in CTO with UV light induces photoreduction of metallic ions (AuCl4-) into metal atoms (Duff et al 1993), clusters, aggregated metal clusters and eventually gold NPs, as described in equation 1. The Au NPs were obtained by irradiating the samples with the third harmonic of a pulsed Nd:YAG laser (λ=355 nm, pulse duration =4-6 ns, repetition rate=10 Hz, Quanta-Ray GCR 190, Spectra Physics) with fluence in the range of 0.5 to 5.0 J·cm-2 through photomasks. The results of the lithographic production of Au NPs at adjacent areas of a representative sample irradiated with increasing irradiation times is shown in Figure 11.

$$\begin{array}{l}\text{RNH}\_3\text{+AuCl}\_4 \rightarrow \text{Au}^0 + \text{RNH}\_3\text{+Cl}\cdot + \text{3Cl} \\\text{Au}^0 \rightarrow \text{Au}\_\text{n}\text{ (cluster)} \rightarrow \text{Au}\_\text{p}\text{ (particles)}\end{array} \tag{1}$$

For the fluence range used in this work the irradiation time needed for the creation of the NPs is very short, in the range of few seconds, and after 2-3 minutes of irradiation the Au/CTO films appear destroyed. The generation of Au NPs at the surface of the CTO-Au polymeric film in function of the irradiation time is illustrated in the AFM images of Figure 12, for a laser fluence of 1 J cm-2. In particular, Fig. 12a illustrates a non-irradiated area with a very smooth surface. Fig. 12b demonstrates an area irradiated for 30 sec. At this stage, the photolysis reaction results in the formation of few Au NPs with low density and sub-micron size. In Fig. 12c is demonstrated the surface of an area after its irradiation for 75 sec. In this figure the Au NPs appear much denser and a clear reduction of their size is also

Fig. 11. Lithographic production of Au NPs at adjacent areas (dark spots) of a CTO film

RNH3+AuCl4 - → Au0 + RNH3+Cl- + 3Cl

For the fluence range used in this work the irradiation time needed for the creation of the NPs is very short, in the range of few seconds, and after 2-3 minutes of irradiation the Au/CTO films appear destroyed. The generation of Au NPs at the surface of the CTO-Au polymeric film in function of the irradiation time is illustrated in the AFM images of Figure 12, for a laser fluence of 1 J cm-2. In particular, Fig. 12a illustrates a non-irradiated area with a very smooth surface. Fig. 12b demonstrates an area irradiated for 30 sec. At this stage, the photolysis reaction results in the formation of few Au NPs with low density and sub-micron size. In Fig. 12c is demonstrated the surface of an area after its irradiation for 75 sec. In this figure the Au NPs appear much denser and a clear reduction of their size is also

Au0 <sup>→</sup> Aun (cluster) → Aup (particles) (1)

used is a chloroauric acid salt (Mw (HAuCl4)=339.5g/mol) and is dissolved in distilled water by providing two solutions (0.01M and 0.02M). Irradiation of the salt embedded in CTO with UV light induces photoreduction of metallic ions (AuCl4-) into metal atoms (Duff et al 1993), clusters, aggregated metal clusters and eventually gold NPs, as described in equation 1. The Au NPs were obtained by irradiating the samples with the third harmonic of a pulsed Nd:YAG laser (λ=355 nm, pulse duration =4-6 ns, repetition rate=10 Hz, Quanta-Ray GCR 190, Spectra Physics) with fluence in the range of 0.5 to 5.0 J·cm-2 through photomasks. The results of the lithographic production of Au NPs at adjacent areas of a representative sample irradiated with increasing irradiation times is shown in Figure 11.

incorporating Au precursor, irradiated with increasing times.

Fig. 10. Chemical structure of chitosan.

demonstrated. The size of the smallest Au NPs that appear onto the surface of the sample is around 20 nm. Under these experimental conditions, the distribution of Au NPs becomes almost homogeneous after 90 sec and they form a sort of continuous film on the surface of the CTO film. It can also be noted that the form of the NPs seems to change from undefined geometrical structures to spherical shapes after prolonged irradiation.

Fig. 12. Evolution of the formation of the Au NPs onto the CTO surface upon increasing irradiation time: (a) non-irradiated area, (b) area irradiated for 30 sec, (c) area irradiated for 75 sec. The density of the NPs increases and their size decreases with the irradiation time.

In the literature (Miyama & Yonezawa 2004) is mentioned that the growth of Au NPs occurs by aggregation of the photolytically formed Au atoms and clusters (equation 1). Nevertheless, the aggregation mechanism is not confirmed by the AFM images presented in figure 12, where is demonstrated that increasing irradiation time causes the decrease and not the increase of the size of the formed particles. The mechanism occurring in our case seems to be closer to the one reported on the formation of metal NPs by laser ablation in water solutions, where the reduced size of the NPs with increasing irradiation time is explained by their fragmentation due to self-absorption of laser pulses (Mafune´ et al 2000, Shukla & Seal 1999, Videla et al 2010, Kadossov & Burghaus 2010, Kabashin & Meunier 2003). Indeed, the Scanning Electron Microscopy (SEM) images obtained on the same sample irradiated at two different areas for very few seconds (≈5) and 30 sec, respectively, using fluence of 1 J cm-2, demonstrate that the first particles that appear onto the CTO surface are quite big (in the micrometer scale) and then they fragment into smaller ones (Figure 13). A closer look into the NPs clearly demonstrates that some of them are already fragmented into smaller pieces. Due to the irradiation conditions used in our experiments the aggregation of Au atoms and clusters into small particles seems to occur already in the very first seconds of the procedure and then the fragmentation mechanism becomes predominant. It is also interesting to mention that the fragmentation mechanism as described in literature is linked with an increase of local temperature due to the absorption of the laser photons by the Au NPs, which may increase the mobility of the latter towards the surface of the CTO polymeric films.

Finally, in figure 14 is demonstrated the intensity of the electronic current that is conducted through a CTO/Au NPs sample as a function of the laser irradiation time, for a laser fluence of ~5 J·cm-2. It is clear that the density increase of the formed Au NPs and the interconnection between them, previously demonstrated in figures 12 and 13, has an effect on the electrons mobility into the samples. Indeed, the current passing through the samples increases almost linearly with the laser irradiation time. The enhancement of the electrical

Laser-Based Lithography for Polymeric Nanocomposite Structures 305

For different applications, the lithographically patterned formation of well-dispersed NCs into polymers is highly requested, since it provides spatially selective tailoring of specific properties of the nanocomposites. Indeed, on one hand a good dispersion of the NCs optimize their quantum size effect, meaning the control of their emission properties. On the other hand, the localization of the NCs in specific sites of the polymer provides the possibility of the direct incorporation of the nanocomposites in various advanced technological devices, such as sensors, biological chips, photoemission devices, etc. In this respect our lithography approach involves the localized in-situ formation of NCs inside polymer matrices by UV laser irradiation of polymer-precursor films. We focus our study on the *in situ* localized formation of NCs of one very promising II–VI semiconductor, the CdS, by the use of pulsed UV laser irradiation of a polymer film incorporating the photosensitive

Cadmium bis-dodecanthiolate Cd(SC12H25)2, (C12), is a photosensitive metal precursor that was mixed with the polymer poly-methylmethacrylate (PMMA) or TOPAS®, a thermoplastic cyclo-olefin copolymer consisting of ethylene and norbonene units, transparent in the visible, in order to produce the CdS NCs after laser irradiation. In particular, 20 wt.% of the metal thiolate precursors was mixed with 80 wt. % of polymer, and then diluted in toluene. The solutions, after being sonicated for 30 min, were cast in Petri capsules. The polymer-precursor films formed after the evaporation of toluene had

For the *in situ* formation of the NCs in the polymer matrix, the films were irradiated through photomasks of different shapes with pulses of Nd:YAG laser (Quanta-Ray PRO-290-30, Spectra Physics) operating at the fourth harmonic, (wavelength 266 nm, pulse duration 8 ns, and repetition rate 2 Hz). The wavelength 266 nm was chosen since the metal precursor exhibits at it enhanced absorption, as demonstrate in figure 15. Both TOPAS and PMMA polymers have intriguing physical properties for a number of applications, taking advantage at the same time of the in situ photoinduced CdS nanocomposite micropatterns

Fig. 15. Normalized absorption spectra of the precursor C12 mixed with the polymers (a),

**3.2.1 Spatially controlled in situ formation of CdS NCs in polymers** 

with accurate control of the NCs size upon UV irradiation.

a b

and of the pure TOPAS and PMMA (b).

metal sulphide precursor.

thickness ∼200 μm.

conductivity in specific areas of nanocomposites by laser induced lithography, can be used for a wide range of applications such as implantable gas sensors, liquid sensors for robotics and nanocircuits.

Fig. 13. SEM images showing the fragmentation of the Au NPs with increasing irradiation time: (a) area irradiated for 5 sec, (b) area irradiated for 30 sec. The density of the NPs increases and their size decreases with the irradiation time.

Fig. 14. Evolution of the current intensity in function of the irradiation time for laser fluence ~5 J·cm-2.

#### **3.2 CdS precursor-based nanocomposite patterns**

Semiconductor nanocrystals (NCs) embedded into polymeric matrices can be exploited in several technological applications, taking advantage of the unique photophysical characteristics of the former due to the quantum confinement effect. In particular, the sizedependent optical properties of the NCs, such as, high emission quantum yields, narrow emission bands, and tunable emission/absorption spectra, have been the topic of many recent research works (Xia et al 2008, Medintz et al, 2005, Bruchez et al 1998, Michalet et al 2005). On the top, the careful selection of the polymer matrices can lead to highly processable nanocomposite materials with increased stability.

conductivity in specific areas of nanocomposites by laser induced lithography, can be used for a wide range of applications such as implantable gas sensors, liquid sensors for robotics

Fig. 13. SEM images showing the fragmentation of the Au NPs with increasing irradiation time: (a) area irradiated for 5 sec, (b) area irradiated for 30 sec. The density of the NPs

Fig. 14. Evolution of the current intensity in function of the irradiation time for laser fluence

Semiconductor nanocrystals (NCs) embedded into polymeric matrices can be exploited in several technological applications, taking advantage of the unique photophysical characteristics of the former due to the quantum confinement effect. In particular, the sizedependent optical properties of the NCs, such as, high emission quantum yields, narrow emission bands, and tunable emission/absorption spectra, have been the topic of many recent research works (Xia et al 2008, Medintz et al, 2005, Bruchez et al 1998, Michalet et al 2005). On the top, the careful selection of the polymer matrices can lead to highly

increases and their size decreases with the irradiation time.

a b

**3.2 CdS precursor-based nanocomposite patterns** 

processable nanocomposite materials with increased stability.

and nanocircuits.

~5 J·cm-2.

For different applications, the lithographically patterned formation of well-dispersed NCs into polymers is highly requested, since it provides spatially selective tailoring of specific properties of the nanocomposites. Indeed, on one hand a good dispersion of the NCs optimize their quantum size effect, meaning the control of their emission properties. On the other hand, the localization of the NCs in specific sites of the polymer provides the possibility of the direct incorporation of the nanocomposites in various advanced technological devices, such as sensors, biological chips, photoemission devices, etc. In this respect our lithography approach involves the localized in-situ formation of NCs inside polymer matrices by UV laser irradiation of polymer-precursor films. We focus our study on the *in situ* localized formation of NCs of one very promising II–VI semiconductor, the CdS, by the use of pulsed UV laser irradiation of a polymer film incorporating the photosensitive metal sulphide precursor.

#### **3.2.1 Spatially controlled in situ formation of CdS NCs in polymers**

Cadmium bis-dodecanthiolate Cd(SC12H25)2, (C12), is a photosensitive metal precursor that was mixed with the polymer poly-methylmethacrylate (PMMA) or TOPAS®, a thermoplastic cyclo-olefin copolymer consisting of ethylene and norbonene units, transparent in the visible, in order to produce the CdS NCs after laser irradiation. In particular, 20 wt.% of the metal thiolate precursors was mixed with 80 wt. % of polymer, and then diluted in toluene. The solutions, after being sonicated for 30 min, were cast in Petri capsules. The polymer-precursor films formed after the evaporation of toluene had thickness ∼200 μm.

For the *in situ* formation of the NCs in the polymer matrix, the films were irradiated through photomasks of different shapes with pulses of Nd:YAG laser (Quanta-Ray PRO-290-30, Spectra Physics) operating at the fourth harmonic, (wavelength 266 nm, pulse duration 8 ns, and repetition rate 2 Hz). The wavelength 266 nm was chosen since the metal precursor exhibits at it enhanced absorption, as demonstrate in figure 15. Both TOPAS and PMMA polymers have intriguing physical properties for a number of applications, taking advantage at the same time of the in situ photoinduced CdS nanocomposite micropatterns with accurate control of the NCs size upon UV irradiation.

Fig. 15. Normalized absorption spectra of the precursor C12 mixed with the polymers (a), and of the pure TOPAS and PMMA (b).

Laser-Based Lithography for Polymeric Nanocomposite Structures 307

pulses. This, in combination with the spatial control of the formation of these NCs thanks to the presented lithographic technique opens the way for the incorporation of such systems in

> 450 500 550 600 650 700 Wavelength /nm

Fig. 17. The change of the fluorescence spectra of C12-TOPAS film with an increasing

The lithographic formation and size tuning of CdS NCs with increasing UV irradiation time of metal precursors occurs also in PMMA, in the same way as in TOPAS. However, intense deteriorations (broadening) are observed in the emission spectra of the NCs formed in PMMA compared to TOPAS upon increasing number of pulses. Figure 18 shows the fluorescence image of a C12-PMMA film irradiated with 80 laser pulses of fluence F=20 mJ·cm-2, at λ=266 nm, and the corresponding emission taken from various areas of the film using a confocal microscope. The emission spectra show the existence of few CdS NCs emitting at the bulk region (NCs diameter > 7 nm) with an emission peak close to 500 nm and FWHM ~32 nm (Figure 18b), while many areas exhibit a very broad emission with the peak around 550 nm (Figure 18b, blue line), representative of the trap states emission formed on the surface of the NCs. Indeed, the quality of the semiconductor NCs is generally studied by their emission characteristics verifying that the broader the emission spectra the higher the number of the trap states on their surfaces (Athanassiou et al 2007, Fragouli et al 2009, 2010, Wang et al 2004, Antoun et al 2007, Wu et al 2000, Khanna et al 2007). After irradiation with the same number of laser pulses but increased laser fluence (F=50 mJ·cm-2, 80 pulses) the characteristic emission of the NCs in PMMA matrix is no more evident in the irradiated area, while the trap state emission is dominant (Figure 18c). Therefore, when PMMA matrix is used the laser irradiation at 266 nm results in the formation of NCs with trap states on their surface, an evidence that becomes clearer as the incident fluence increases. The comparison between the emission spectra of the CdS NCs formed in the TOPAS and in the PMMA matrix shows that they are very narrow and characteristic of CdS

6 pulses

0 pulses

10 pulses 14 pulses 20 pulses

40 pulses

80 pulses

complex multicomponent devides.

number of incident laser pulses.

Normalized Emission /a.u

.

Upon laser irradiation through photomasks the CdS NCs are localized exclusively in the irradiated area, making possible the lithographic patterning of the samples. A characteristic micro pattern of 3 lines is illustrated in figure 16. For the specific pattern the lithographic technique that we used was not involving photomasks but it was done by focusing the laser beam on the C12-TOPAS sample, which was fixed onto a motorized stage moving at constant velocity. The spot size of the beam was ∼0.2 × 0.1 mm2, and the speed of the motor about 0.2 mm·s−1. The lines are fairly clear and their width (240 μm) is constant along the irradiation path. The total number of pulses in each spot area is about 100 and is enough to form CdS NCs, as demonstrated by their emission spectra shown in figure 16. The bright areas correspond to CdS NCs in the bulk region, since the spectrum has the characteristic emission peak at 506 nm, while the dark areas, not being irradiated, have no fluorescence emission in the studied spectral region.

Fig. 16. Fluorescence image of a patterned sample of cadmium thiolate precursor-TOPAS polymer excited with a 405 nm diode laser after irradiation at 266 nm. The narrow fluorescence spectra of the formed CdS NCs are also shown.

The formed CdS NCs following an increasing number of laser pulses are characterized by spatially resolved photoluminescence measurements using a confocal microscope. Indeed, in figure 18 are presented the fluorescence spectra from selected areas of the films irradiated with various laser pulses at a laser fluence of 25 mJ·cm−2. After 6 pulses the emission peak of the formed NCs is close to λ = 440 nm. After successive laser pulses the emission is shifted towards higher wavelengths with the peak reaching 506 nm above 40 laser pulses, which coincides with the emission of the bulk CdS material. It is clearly demonstrated that the luminescence of the samples changes dramatically after the first irradiation pulses, while an increase of the incident pulses causes a red shift to the emission, which is attributed to the increasing dimensions of the CdS NCs. Therefore, the UV irradiation of metal precursor-TOPAS polymer films results in the formation of CdS NCs, with dimensions extending from the quantum size effect range to the bulk, depending on the number of the incident laser

Upon laser irradiation through photomasks the CdS NCs are localized exclusively in the irradiated area, making possible the lithographic patterning of the samples. A characteristic micro pattern of 3 lines is illustrated in figure 16. For the specific pattern the lithographic technique that we used was not involving photomasks but it was done by focusing the laser beam on the C12-TOPAS sample, which was fixed onto a motorized stage moving at constant velocity. The spot size of the beam was ∼0.2 × 0.1 mm2, and the speed of the motor about 0.2 mm·s−1. The lines are fairly clear and their width (240 μm) is constant along the irradiation path. The total number of pulses in each spot area is about 100 and is enough to form CdS NCs, as demonstrated by their emission spectra shown in figure 16. The bright areas correspond to CdS NCs in the bulk region, since the spectrum has the characteristic emission peak at 506 nm, while the dark areas, not being irradiated, have no fluorescence

Fig. 16. Fluorescence image of a patterned sample of cadmium thiolate precursor-TOPAS polymer excited with a 405 nm diode laser after irradiation at 266 nm. The narrow

The formed CdS NCs following an increasing number of laser pulses are characterized by spatially resolved photoluminescence measurements using a confocal microscope. Indeed, in figure 18 are presented the fluorescence spectra from selected areas of the films irradiated with various laser pulses at a laser fluence of 25 mJ·cm−2. After 6 pulses the emission peak of the formed NCs is close to λ = 440 nm. After successive laser pulses the emission is shifted towards higher wavelengths with the peak reaching 506 nm above 40 laser pulses, which coincides with the emission of the bulk CdS material. It is clearly demonstrated that the luminescence of the samples changes dramatically after the first irradiation pulses, while an increase of the incident pulses causes a red shift to the emission, which is attributed to the increasing dimensions of the CdS NCs. Therefore, the UV irradiation of metal precursor-TOPAS polymer films results in the formation of CdS NCs, with dimensions extending from the quantum size effect range to the bulk, depending on the number of the incident laser

fluorescence spectra of the formed CdS NCs are also shown.

emission in the studied spectral region.

pulses. This, in combination with the spatial control of the formation of these NCs thanks to the presented lithographic technique opens the way for the incorporation of such systems in complex multicomponent devides.

Fig. 17. The change of the fluorescence spectra of C12-TOPAS film with an increasing number of incident laser pulses.

The lithographic formation and size tuning of CdS NCs with increasing UV irradiation time of metal precursors occurs also in PMMA, in the same way as in TOPAS. However, intense deteriorations (broadening) are observed in the emission spectra of the NCs formed in PMMA compared to TOPAS upon increasing number of pulses. Figure 18 shows the fluorescence image of a C12-PMMA film irradiated with 80 laser pulses of fluence F=20 mJ·cm-2, at λ=266 nm, and the corresponding emission taken from various areas of the film using a confocal microscope. The emission spectra show the existence of few CdS NCs emitting at the bulk region (NCs diameter > 7 nm) with an emission peak close to 500 nm and FWHM ~32 nm (Figure 18b), while many areas exhibit a very broad emission with the peak around 550 nm (Figure 18b, blue line), representative of the trap states emission formed on the surface of the NCs. Indeed, the quality of the semiconductor NCs is generally studied by their emission characteristics verifying that the broader the emission spectra the higher the number of the trap states on their surfaces (Athanassiou et al 2007, Fragouli et al 2009, 2010, Wang et al 2004, Antoun et al 2007, Wu et al 2000, Khanna et al 2007). After irradiation with the same number of laser pulses but increased laser fluence (F=50 mJ·cm-2, 80 pulses) the characteristic emission of the NCs in PMMA matrix is no more evident in the irradiated area, while the trap state emission is dominant (Figure 18c). Therefore, when PMMA matrix is used the laser irradiation at 266 nm results in the formation of NCs with trap states on their surface, an evidence that becomes clearer as the incident fluence increases. The comparison between the emission spectra of the CdS NCs formed in the TOPAS and in the PMMA matrix shows that they are very narrow and characteristic of CdS

Laser-Based Lithography for Polymeric Nanocomposite Structures 309

the development of nanodevices used in biotechnology, optoelectronics, microfluidics and

The authors would like to thank Dr Rafaella Buonsanti, Dr Gianvito Caputo, Dr P. D Cozzoli, Dr Anna Maria Laera, Professor Leander Tapfer who provided us with diverse colloidal nanofillers or nanocomposite samples and for the fruitful discussions and

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**5. Acknowledgment** 

**6. References** 

in the first case while they are broad and red shifted in the second case. This finding demonstrates that the trap states on the NCs surfaces formed with laser lithography, and thus, the optical quality of the NCs strongly depends on the surrounding matrix.

Fig. 18. (a) Confocal microscopy fluorescence image of the C12-PMMA film after irradiation at 266 nm with 80 incident pulses and fluence F=20 mJ·cm-2. (b) Emission spectra of selected areas of the image; each curve corresponds to the marked area of (a) with the same colour and number. (c) Emission spectra of selected areas of the C12-PMMA film after irradiation at 266 nm with 80 incident pulses and higher fluence F=50 mJ·cm-2.
