Laser-Based Lithography for Polymeric Nanocomposite Structures

material, for the creation of molecular sandwiches, for cells deposition, for microfluidic devices, sensors, photoemitting parts in devices, etc. Herein, we present two different UV light-based lithography techniques for formation of nanocomposite patterns that consist of polymers incorporating inorganic nanofillers. The latter are either colloidal NPs produced by chemical synthesis, or NPs that are formed directly into the polymer matrices starting from an appropriate precursor, using laser light irradiation. Nanotechnology has experienced a rapid growth in the past decade, largely owing to the rapid advances in nanofabrication techniques employed to fabricate nano-devices. Nanofabrication can be divided into two categories: "bottom up" approach using chemical synthesis or self assembly, and "top down" approach using nanolithography, thin film deposition and etching techniques. Both topics are covered, though with a focus on the second category. This book contains twenty nine chapters and aims to provide the fundamentals and recent advances of nanofabrication techniques, as well as its device applications. Most chapters focus on in-depth studies of a particular research field, and are thus targeted for researchers, though some chapters focus on the basics of lithographic techniques accessible for upper year undergraduate students. Divided into five parts, this book covers electron beam, focused ion beam, nanoimprint, deep and extreme UV, X-ray, scanning probe, interference, two-photon, and nanosphere lithography.

material, for the creation of molecular sandwiches, for cells deposition, for microfluidic devices, sensors, photoemitting parts in devices, etc. Herein, we present two different UV light-based lithography techniques for formation of nanocomposite patterns that consist of polymers incorporating inorganic nanofillers. The latter are either colloidal NPs produced by chemical synthesis, or NPs that are formed directly into the polymer matrices starting from an appropriate precursor, using laser light irradiation. In particular the photolithography presented in this work follows two approaches: 1. UV laser photopolymerization: the nanocomposite solutions preexist in the form of colloidal NPs mixed with monomers and photocuring agent and laser light beams induce the patterned structures. Three systems are demonstrated: PMMA with TiO 2 NPs, PMMA with Fe 2 O 3 NPs , SU-8 with Fe 2 O 3 NPs. In the last two systems we can induce further nanopatterning into the patterned polymers structures using an external magnetic field to align the nanofillers into nanowires (NWs). 2. UV laser light-induced formation of NPs into polymers: the starting system is a polymeric film incorporating light-sensitive precursors of NPs and laser light irradiation induces the formation of NPs in specific locations in the polymer matrix. Three systems are demonstrated: Chitosan with Au NPs, TOPAS with CdS NPs, PMMA with CdS NPs. The patterns of nanocomposites produced with the above mentioned lithography methods are categorized according to their properties as follows: patterns with tunable surface properties, magnetic patterns, conducting patterns, patterns with tunable emission. Such patternable composite materials deal with most of the present technologies, including bioengineering and medical instrumentation, packaging, electrical enclosures, sensors, actuators and energy.

Lithography using laser photopolymerization
Lithographic patterning using UV light in combination with monomers and appropriate photoinitiators is a very attractive technology due to low energy consumption, room temperature operation, rapid curing, spatial control, possibility of curing structures on heatsensitive substrates etc. (Andrzejewska 2001). The use of the specific technique for lithography of nanocomposites, permits to realize materials with homogeneous dispersion of the inorganic phase, but also to simultaneously create patterned surfaces in an easy and single step. (Sun et al 2008) The photopolymerization technique involves UV/visible light absorption from the photoinitiators forming radical species (initiation reaction), which are responsible for the initiation of the photopolymerization process due to their addition to the monomer molecules (propagation reaction). The chain growth termination occurs upon annihilation of the radical centres due to radical-radical recombination (termination reaction). Among different light sources, lasers are most widely used when lithography is involved due to their unique characteristics that lead to small, highly controlled, sharp and precise patterns. In the presented work a pulsed laser was used permitting high energy concentrations in very short times, and allowing polymerization to develop to a high extent in the dark periods between consecutive pulses (Van Herk 2000).

TiO 2 -based patterned nanocomposites
In the development of polymeric nanocomposites, semiconductor oxides are extensively studied and applied as nanofillers due to their unique electromagnetic, mechanical, chemical and numerous other properties. In particular, among them titanium dioxide (TiO 2 ) is possibly the most widely used, thanks to two exceptional properties: the photocatalytic activity and the reversible wettability, both activated upon UV irradiation (Fujishima 2000, Nakajima 2000, 2001, Wang 1997, Sakai, 1998, 2001. In this section we focus on the reversible wettability properties of TiO 2 upon excitation with UV laser light. Specifically, nanorods (NRs) of TiO 2 are used as the basic building blocks of photolithographically patterned nanocomposite materials with functional and responsive surfaces, which show UV-sensitive wettability. It has been demonstrated that upon UV irradiation of TiO 2 , oxygen vacancies are created on its surface resulting into the conversion of Ti 4+ into Ti 3+ sites. These sites are favourable for dissociative adsorption of atmospheric water molecules, leading to the formation of a highly hydroxylated (hence, hydrophilic) surface. This procedure is reversible, since upon long term storage (few months) under ambient dark conditions, or for an accelerated process, upon thermal treatment, vacuum storage or visible irradiation (Wang et al., 1999, Nakajima et al., 2001, Sakai et al., 1998, the adsorbed hydroxyl groups can be removed and eventually replaced by ambient oxygen, allowing the initial hydrophobicity to be recovered . Taking advantage of the above described mechanism we present herein how the formed TiO 2 /polymer nanocomposite patterns can serve as paths for the directional movements of water drops onto them.

Photopolymerized patterns of PMMA/TiO 2
Solutions of MMA monomer, TiO 2 NRs, and photoinitiator were prepared in toluene to obtain concentrations of 94 wt.%, 5 wt.% and 1 wt.%, respectively. The photoinitiator was the IRGACURE®1700. It is formed by two distinct molecules, a phosphine oxide derivative and a hydroxyl alkyl phenyl ketone, which acts as co-initiator. Upon UV irradiation, electrons and hydrogen atoms are transferred from the co-initiator molecules to the photoinitiator, generating radicals that initiate the polymerization of the methyl methacrylate (MMA) monomers. All the solutions were stirred and left under dark for several minutes to equilibrate. Glass and silicon substrates were washed with isopropanol and subsequently with acetone and dried with nitrogen. About 200 µL of each solution were initially spin coated on both glass and silicon substrates at 1000 rpm for 20 s. Next, about 40 µL of the same solution were drop casted onto each sample. To obtain photopolymerization, the third harmonic of a pulsed Nd:YAG laser (Quanta-Ray GCR-190, Spectra Physics, energy density = 10.5 mJ·cm -2 , = 355 nm, pulse duration = 6 ns, repetition rate = 10 Hz) was used. In particular, the previously casted samples were irradiated through aluminum masks characterized by different patterns of mm-dimensions. After the photopolymerization, each sample was washed 3 times with methanol to remove unreacted monomer and photoinitiator, and then dried in ambient dark conditions. In Figure 1 are demonstrated characteristic patterns created upon UV laser irradiation of the drop casted solution through a photomask of two parallel lines in the mm range, demonstrating the potentiality of the specific lithography technique. A closer look of the surfaces of the photolithographically produced nanocomposite patterns shown in Figure 1 was performed using lateral force Atomic Force Microscopy (AFM). The obtained AFM images demonstrate that NRs are apparent onto the surface of the photopolymerized MMA-TiO 2 nanocomposite films. In general, they appear quite aggregated, however it is occasionally possible to distinguish also single NRs laying parallel to the substrate (Figure 2a). In contrast, the photopolymerized coatings without TiO 2 NRs appear flat and homogeneous under the AFM (Figure 2b).  In order to enhance the hydrophilicity of the TiO 2 NRs exposed on the surface of the photopolymerized nanocomposites, the prepared patterns were irradiated for diverse time intervals with a pulsed Nd:YAG laser at 355 nm (energy density = 7 mJ·cm -2 , repetition rate = 10 Hz, pulse duration ~ 4-6 ns). Increasing irradiation time up to 90 min under the specific experimental conditions, increases the hydrophilicity of the nanocomposites' surfaces (Villafiorita Monteleone et al 2010). The complete recovery of the initial wettability of the films was achieved by placing them in vacuum at a pressure of 3·10 -3 mbar for 48 hours. We take advantage of the controlled wettability changes to realize liquid flow paths, irradiating adjacent surface areas of the photolithographically produced lines of nanocomposites with increasing time, and creating wettability gradients along their surfaces, essential for liquid droplet motion. An example is demonstrated in Figure 3, where is shown a X×Y = 6x1 mm 2 laser photopolymerized nanocomposite film, where in order to achieve the drop movement along the X-axis, a first area of XxY = 1x1 mm 2 was left without irradiation, the next XxY = 1x1 mm 2 area was irradiated for 20 min and two adjacent areas of XxY = 21 mm 2 each, were irradiated for 40 and 90 min, respectively. The spontaneous water drop movement occurs as shown at the right hand side of Figure 3 ( Figure 3c) because the following criteria essential for the movement are fulfilled: (1) The contact angle hysteresis (Δθ h ) of the rear area, irradiated for less time must be smaller than the difference between the advancing contact angles of the two adjacent areas (nominated Δθ in Figure 3). (2) The side of each irradiated area along the direction of the movement should be smaller than the diameter of the drop on   www.intechopen.com the specific area, so that the front edge of the drop is always in contact with a more hydrophilic area than the back edge (Villafiorita Monteleone et al., 2010). This kind of samples are very versatile and can be used as described above or incorporated in more complicated systems and devices, such as microfluidics, labs on chip etc.

γ-Fe 2 O 3 -based patterned nanocomposites
Magnetic particles are useful in different biomedical applications, such as magnetically induced hyperthermia of malignant tumors, contrast enhancement agents for magnetic resonance imaging (MRI), tissue repair (Gupta et al 2005, Pankhurst et al 2003, and delivery of drugs (Voltairas et al 2002) or nucleic acids (Mair et al 2009, Buerli et al 2007, as they can be manipulated via external magnetic fields to improve transport in biological systems. They are also used in the fabrication of photonic crystals (Ding et al 2009), as nanowire contacts in electronic devices (Bangar et al, 2009), and as device components in microfluidics (Kavcic et al 2009;Fahrni et al 2009). Additionally, there has been growing interest in the use of magnetic particles in the fabrication of nanomotors and nanomachines (Ghosh & Fischer, 2009). Among all magnetic NPs, iron oxides are possibly the most frequently used, due to their high magnetic moment, chemical stability, low toxicity, biocompatibility, easy and economical synthetic procedures. Iron oxides exist in many forms in nature, with magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ) and hematite (α-Fe 2 O 3 ) being the most common (Cornell & Schwertmann 2003). Magnetite is ferrimagnetic at room temperature (Cornell & Schwertmann 2003), and usually particles smaller than ≈6 nm are superparamagnetic at room temperature, although their magnetic properties depend strongly on the methods used in their synthesis (Kado 2008, Zhao et al 2007. At room temperature, maghemite is ferrimagnetic while particles smaller than ≈10 nm are superparamagnetic (Neuberger et al 2005). Aggregation of ultrafine maghemite particles sometimes leads to their magnetic coupling and ordering of their magnetic moment, which is termed superferromagnetism (Cornell & Schwertmann 2003). Hematite is paramagnetic at temperatures above 956 K. At room temperature, it is weakly ferromagnetic and undergoes a phase transition at 260 K to an antiferromagnetic state. The magnetic behavior of hematite depends also on crystallinity, particle size and on the extent of cation substitution (Cornell & Schwertmann 2003). The magnetic properties of iron oxides have been exploited in a broad range of applications including magnetic seals and inks, magnetic recording media, catalysts, ferrofluids, contrast agents for magnetic resonance imaging, therapeutic agents for cancer treatment, and numerous others (Azhar Uddin et al 2008, Cui et al 2006, dos Santos Coelho et al 2008. These applications demand nanomaterials of specific sizes, shapes, surface characteristics, and magnetic properties, so the specific type of iron oxide is chosen every time depending on the field of use. On the top, iron oxide in polymer matrices, forming functional nanocomposites, pave the way to novel plastic devices for gas and vapor sensing, actuation, molecular separation, electromagnetic wave absorption, nonlinear optical systems, and photovoltaic solar cells (Kaushik et al 2009, Merkel et al 2002, Huo et al 2009, Long 2005. Furthermore, the magnetic properties of iron oxide can be used in future generations of electronic, magnetic, and photonic devices for information storage or magnetic imaging (Weller et al 2000. For enhanced performances and occasionally directional, nanocomposite systems require high density of oriented anisotropic magnetic nanostructures, like NWs, in polymer matrices. Indeed, the realization of arrays of NWs is a new interesting solution to obtain novel collective properties, different from those of isolated NPs (Tang and Kotov, 2005).
One-dimensional magnetic NWs can be produced by the assembly of isotropic magnetic NPs, under external magnetic field (MF). This is an attractive technique for the fabrication of NWs, due to its simplicity and at the same time, high effectiveness. In this perspective, several studies have recently demonstrated the possibility of producing oriented magnetic nanocomposites through the dispersion of magnetic NPs in polymer or prepolymer solutions, and subsequent evaporation or polymerization under a weak magnetic field ( Here, we present photolithographically realized patterned nanocomposites of PMMA or SU-8 polymers which incorporate magnetic NWs, formed starting from spherical iron oxide (γ-Fe 2 O 3 ) colloidal NPs. Indeed, applying a homogeneous magnetic field produced by two magnets to the nanocomposites solutions, NWs are formed, which are aligned along the magnetic field lines. We demonstrate that the photolithography process does not affect the NPs alignment, and, more importantly, that it allows the creation of polymeric patterns with magnetic properties in specific areas.

Photopolymerized patterns of PMMA/Fe 2 O 3
Solutions of methyl methacrylate (MMA) monomer mixed with colloidal γ-Fe 2 O 3 NPs in the presence of a photoinitiator IRGACURE®1700 were prepared in chloroform at concentrations of 89.5 wt.%, 10 wt.% and 0.5 wt.%, respectively. The Fe 2 O 3 NPs were previously prepared in chloroform, by modifying a wet-chemical synthetic approach previously reported (Sun et al 2004), in order to obtain hydrophobic-capped Fe 2 O 3 spherical particles with a mean diameter of 10 nm. Oleic acid, oleylamine, and hexadecane-1,2-diol were used as capping molecules of the produced NPs. All solutions were stirred and left in the dark for few minutes to equilibrate. 200 L of each solution were spin-coated at 1000 rpm for 20 s, and subsequently ~40 L of the same solution were drop-casted onto glass substrates. For the alignment of the nanoparticles, the system was subjected to a homogeneous magnetic field (~160 mT), produced by two permanent magnets, applied parallel to the substrate during the deposition, evaporation and photopolymerization processes. The patterned PMMA/Fe 2 O 3 nanocomposites were obtained by irradiating all samples with the third harmonic of a pulsed Nd:YAG laser (Quanta-Ray GCR 190, Spectra Physics) with an energy density of 10.5 mJ·cm -2 ( =355 nm, pulse duration =4-6 ns, repetition rate=10 Hz) for 90 minutes, using aluminum photomasks with different geometries. After the photopolymerization, each sample was washed three times with methanol to remove unreacted monomer and photoinitiator and then dried under ambient dark conditions for 2 days to achieve complete solvent evaporation. In Figure 4 are demonstrated characteristic patterns created upon irradiation of the spin coated and subsequently drop casted MMA/Fe 2 O 3 NPs solution through a photomask of two parallel lines in the mm range.
During the photopolymerization, a homogeneous magnetic field produced by two permanent magnets was applied to the samples, at saturated chloroform atmosphere as shown in Figure 5. All samples prepared with or without the application of the magnetic field were studied under an optical microscope. Figure 6 presents samples with 10 wt. % concentration of NPs. The analysis reveals that in the samples created without the application of the magnetic field (Figures 6a and 6b), the NPs form aggregates randomly distributed in the film. The application of a magnetic field produced by two magnets leads to NPs aligned along the magnetic field lines, forming parallel NWs (Figure 6c and 6d). By these two figures is demonstrated that the formation of magnetic NWs when the samples are placed under the magnetic field occurs both before and after the photopolymerization procedure, with the latter leaving unaffected the geometry of the wires. In particular, Figure  6d shows that the area of the sample that was covered by the mask (bottom part of the picture) is not polymerized and thus, after washing with methanol, the composite is removed and no aligned structures are left onto the substrate.  A detailed topographic study of the photolithographically produced patterns with or without the application of the magnetic field was conducted using AFM (Figures 7a-7d). Figures 7a and 7b show the topography of samples produced in the absence of a magnetic field. The nanofillers of these samples do not show any particular alignment, as expected, but rather organize themselves in order to form aggregates of different sizes distributed all over the surface, in agreement with the results of the optical microscopy examination. On the contrary, the AFM images of the samples realized under the magnetic field produced by two magnets show the presence of parallel structures aligned along the direction of the external magnetic field (Figures 7c and 7d). In particular, parallel lines with thickness of ~1 µm and length of tens of microns clearly appear both in the 2D and in the 3D topography images. The images also demonstrate the presence of bigger aggregated structures with width around 5 µm, which can be attributed to aligned NWs that in some areas of the sample get very closely packed, due to the high NPs concentration. It is very likely that the NWs formation inside the photopatterned polymer are facilitated by the initial organization of the NPs into bigger clusters, as demonstrated already in Figures 7a and 7b. These clusters exhibit higher magnetic moments than the single Fe 2 O 3 NPs, resulting in an increased response to the external magnetic field that leads eventually to chain formation (Lalatonne et al 2004).

Photopolymerized patterns of SU-8/Fe 2 O 3
SU-8 is a commercial biocompatible epoxy-based negative photoresist that is suitable for the microfabrication of high aspect-ratio (>20) structures. When exposed to UV light, its molecular chains crosslink causing the polymerization of the material. Here the SU-8 3050 c d a thick coating. The solution was then spin-coated on a silicon wafer and subsequently left under a magnetic field (400 mT) with a vertical direction with respect to the substrate, resulting in the formation of wire like magnetic structures. These magnetic wires induce magnetic anisotropy so for specific magnetic field orientations the magnetic response is higher . After, the sample was irradiated with a UV lamp mask aligner for 70 sec with energy of 25 mW. Masks of square shaped pillar patterns were used, with 42 µm side and different interpillar distances varying from 14 to 77 µm. The postbake was for 1 min at 65° C and for 4 min at 95° C. Finally, the samples were developed with SU-8 developer (15min) and rinsed with isopropanol with subsequent drying under nitrogen flow. The fabricated nanocomposite pillars formed in this way, had a square side of 42 µm and 40 µm high as shown in the image recorded with Scanning Electron Microscopy (SEM) (Figure 8). The AFM image of figure 9 demonstrates a part of the surface of a nanocomposite pillar in which the NPs are aligned vertically to the substrate. The nanoroughness shown is due to the protruding edges of the formed NWs, demonstrating that the diameters of the NWs are around 100 nm. The magnetic alignment of the iron oxide NPs inside the SU-8 matrix creates a magnetic anisotropy that enhances the possibility for the pillar structures to respond to an external magnetic field. The responsivity of the magnetic pillars to an external magnetic field was checked using water drops placed onto the pillars. In particular, the pillars were inclined in order to observe a difference between the right and the left part of the drops and a static magnet of maximum field strength 500 mT, was moved towards the sample with a direction parallel to it. The test was performed onto magnetic pillars with interpillar distance 14 m. The water contact angle on these pillars is about 130°, while the values of the right and the left contact angle after the tilting of the sample are shown in table 1. After few minutes it was observed a change in the shape of the drop when the magnet was moving towards the drop, while it was recovering when the magnet was far from the substrate. This reversible difference in the shape of the drop indicates the tendency of the drop to move, which is connected with the reversible movement of the magnetic pillars responding to the external magnetic field.
www.intechopen.com  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 the right column are demonstrated the two examined frames.

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.
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.

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, Huang et al 2009. Our strategy is based on the use of the Au precursor, chloroauric acid salt (HAuCl 4 ), 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.

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 used is a chloroauric acid salt (Mw (HAuCl 4 )=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 (AuCl 4 -) 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 ( =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. RNH 3 +AuCl 4 -→ Au 0 + RNH 3 +Cl -+ 3Cl Au 0 → Au n (cluster) → Au p (particles) (1) Fig. 11. Lithographic production of Au NPs at adjacent areas (dark spots) of a CTO film incorporating Au precursor, irradiated with increasing times.
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 www.intechopen.com 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. 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.

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.

a b
www.intechopen.com 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.

Spatially controlled in situ formation of CdS NCs in polymers
Cadmium bis-dodecanthiolate Cd(SC 12 H 25 ) 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 m. 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. 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 mm 2 , and the speed of the motor about 0.2 mm·s −1 . The lines are fairly clear and their width (240 m) 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 = 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 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. 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 =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, 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 www.intechopen.com 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.

Conclusion
In this work are demonstrated two different lithography techniques based on the use of pulsed UV laser light for the creation of well defined polymeric nanocomposite structures.
The presented techniques open up new ways for nanocomposite film patterning with well dispersed and occasionally size controlled NCs. The first technique is the UV laser photopolymerization lithography, where UV laser pulses are used for the polymerization and patterning of nanocomposite solutions of preformed colloidal NPs mixed with monomers and photoinitiators. In the second lithography technique UV laser pulses induce the in situ formation of NPs in specific locations in the polymer matrices, starting from polymeric film incorporating light-sensitive precursors of NPs. Using both techniques we present patterned structures of various nanocomposite systems, such as PMMA with TiO2 NPs, PMMA and SU-8 with Fe 2 O 3 NPs aligned into NWs under external magnetic field, CTO with Au NPs, TOPAS and PMMA with size tuned CdS NPs. The formed nanocomposite structures can exhibit controlled and enhanced surface, optical, electronically conducting and magnetic properties, in an easy and fast way, promising for