**2. 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).

#### **2.1 TiO2-based patterned nanocomposites**

In the development of polymeric nanocomposites, semiconductor oxides are extensively studied and applied as nanofillers due to their unique electromagnetic, mechanical,

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

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 TiO2 NPs, PMMA with Fe2O3 NPs , SU-8 with Fe2O3 NPs. In the last two systems we can induce further nanopatterning into the patterned polymers structures using an external

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

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,

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

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

In the development of polymeric nanocomposites, semiconductor oxides are extensively studied and applied as nanofillers due to their unique electromagnetic, mechanical,

In particular the photolithography presented in this work follows two approaches:

from an appropriate precursor, using laser light irradiation.

**2. Lithography using laser photopolymerization** 

periods between consecutive pulses (Van Herk 2000).

**2.1 TiO2-based patterned nanocomposites** 

with CdS NPs.

actuators and energy.

single step. (Sun et al 2008)

magnetic field to align the nanofillers into nanowires (NWs).

chemical and numerous other properties. In particular, among them titanium dioxide (TiO2) 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, 1998, 1999, Sakai, 1998, 2001, 2003).

In this section we focus on the reversible wettability properties of TiO2 upon excitation with UV laser light. Specifically, nanorods (NRs) of TiO2 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 TiO2, oxygen vacancies are created on its surface resulting into the conversion of Ti4+ into Ti3+ 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 (Caputo et al., 2008). Taking advantage of the above described mechanism we present herein how the formed TiO2/polymer nanocomposite patterns can serve as paths for the directional movements of water drops onto them.

#### **2.1.1 Photopolymerized patterns of PMMA/TiO2**

Solutions of MMA monomer, TiO2 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, λ = 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-TiO2 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 TiO2 NRs appear flat and homogeneous under the AFM (Figure 2b).

Laser-Based Lithography for Polymeric Nanocomposite Structures 293

In order to enhance the hydrophilicity of the TiO2 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** mm2 laser photopolymerized nanocomposite film, where in order to achieve the drop movement along the X-axis, a first area of **XxY** = **1x1** mm2 was left without irradiation, the next **XxY** = **1x1** mm2 area was irradiated for 20 min and two adjacent areas of **XxY** = **21** mm2 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 (Δθh) 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

**1 mm**

Fig. 3. (a) Photolithographically created line of dimensions 6×1 mm2, of PMMA/TiO2 NRs nanocomposite. (b) Graphical representation of the dimensions of the adjacent irradiated areas together with the irradiation times on each area. The pictures of the water drops obtained on each area with their water contact angle and hysteresis values and the

difference between the advancing contact angles of the two adjacent areas are also shown. (c) Side view photographs of a water droplet that is moving on the PMMA/TiO2 surface with UV-induced gradient hydrophilicity. The hydrophilicity increase is in the direction of

θ **= 27°** Δθ**<sup>h</sup> ~ 15°**

**Δθ = 14°**

(c)

**6 mm 1 mm 1 mm 2 mm 2 mm**

> **40 min**

**90 min**

**Δθ = 13°**

θ **= 41°** Δθ**<sup>h</sup> ~ 12°**

**0 min**

θ **= 52°** Δθ**<sup>h</sup> ~9°**

the drop movement, from left to right.

θ **= 89°** Δθ**<sup>h</sup> ~ 6°**

(a)

(b)

**20 min**

**Δθ = 34°**

Fig. 1. Photolithographically created lines of mm-dimensions of PMMA/TiO2 NRs nanocomposites.

Fig. 2. AFM images of (a) photolithographically produced nanocomposite patterns demonstrating NRs onto their surface, and of (b) photopolymerized coatings of PMMA without TiO2 NRs.

Fig. 1. Photolithographically created lines of mm-dimensions of PMMA/TiO2 NRs

Fig. 2. AFM images of (a) photolithographically produced nanocomposite patterns demonstrating NRs onto their surface, and of (b) photopolymerized coatings of PMMA

(b)

nanocomposites.

(a)

without TiO2 NRs.

In order to enhance the hydrophilicity of the TiO2 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** mm2 laser photopolymerized nanocomposite film, where in order to achieve the drop movement along the X-axis, a first area of **XxY** = **1x1** mm2 was left without irradiation, the next **XxY** = **1x1** mm2 area was irradiated for 20 min and two adjacent areas of **XxY** = **21** mm2 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 (Δθh) 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

Fig. 3. (a) Photolithographically created line of dimensions 6×1 mm2, of PMMA/TiO2 NRs nanocomposite. (b) Graphical representation of the dimensions of the adjacent irradiated areas together with the irradiation times on each area. The pictures of the water drops obtained on each area with their water contact angle and hysteresis values and the difference between the advancing contact angles of the two adjacent areas are also shown. (c) Side view photographs of a water droplet that is moving on the PMMA/TiO2 surface with UV-induced gradient hydrophilicity. The hydrophilicity increase is in the direction of the drop movement, from left to right.

Laser-Based Lithography for Polymeric Nanocomposite Structures 295

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 (Park et al 2007, Jestin et al 2008, Fragouli & Buonsanti et al 2010, Fragouli & Torre et al 2010). Here, we present photolithographically realized patterned nanocomposites of PMMA or SU-8 polymers which incorporate magnetic NWs, formed starting from spherical iron oxide (γ-Fe2O3) 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

Solutions of methyl methacrylate (MMA) monomer mixed with colloidal γ-Fe2O3 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 Fe2O3 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 Fe2O3 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 μL of each solution were spin-coated at 1000 rpm for 20 s, and subsequently ~40 μL 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

The patterned PMMA/Fe2O3 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 (λ=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/Fe2O3 NPs solution through a photomask of two parallel lines in the mm

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

magnetic properties in specific areas.

processes.

range.

**2.2.1 Photopolymerized patterns of PMMA/Fe2O3**

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.
