**Laser-Based Lithography for Polymeric Nanocomposite Structures**

Athanassia Athanassiou, Despina Fragouli, Francesca Villafiorita Monteleone, Athanasios Milionis, Fabrizio Spano, Ilker Bayer and Roberto Cingolani *Center for Biomolecular Nanotechnologies @UNILE, Istituto Italiano di Tecnologia (IIT), Via Barsanti, 73010 Arnesano, Lecce Italy* 

### **1. Introduction**

288 Recent Advances in Nanofabrication Techniques and Applications

Zoubir, A.; Richardson, M.; Rivero, C.; Schulte, A.; Lopez, C.; Richardson, K; Ho,

UK

9592

*2009,* pp.44 – 46, ISBN 978-0-9551179-6-1, 30th Aug. – 04th Sept. 2009, London,

N & Vallee, R. (2004). Direct femtosecond laser writing of waveguides in As2S3 thin films, *Optics Letters*, Vol. 29, No. 7, (April 1, 2004), pp. 748-750, ISSN 0146-

> In recent years, the synthesis of polymeric nanocomposites has gained much interest in the scientific community thanks to their unique capability to combine the properties of the host polymer matrices, such as toughness, elasticity, processability, solubility, thermal stability, etc, with those of inorganic nanoparticles (NPs), such as hardness, chemical resistance, optical and electronic properties. Among a variety of nanofillers, semiconductor and metallic NPs are extensively studied and used, because of their unique properties especially in the nanoscale. In this work we deal with polymeric nanocomposites incorporating various nanofillers, each one of them having extremely attractive properties for technological applications. In particular we focus on titanium dioxide (TiO2) NPs due to their unique reversible wettability, iron oxide (Fe2O3) NPs due to their superparamagnetic nature, gold (Au) NPs due to their enhanced electronic conductivity, and cadmium sulphide (CdS) NPs due to their tuned photoemission in the quantum size regime.

> Nanocomposite materials are usually prepared by simple blending of the nanosized inorganic components into polymer solutions. Nevertheless, this method quite often leads to formation of aggregates or macroscopic phase separation, since the control of the dispersion and distribution of the nanofillers within the organic matrices is impossible. To obtain a higher compatibility between the filler and the host polymeric material, and achieve coatings with high content of inorganic particles, the use of polymerization techniques is widely adopted (Fouassier 1995, Decker et al 2005, Lee et al 2006, Wang & Ni 2008). From this point of view, several methods can be considered, depending on the type of monomers and nanomaterials, such as bulk polymerization, photoinitiated polymerization, emulsion polymerization, in situ thermal polymerization, or copolymerization in solution. Among them, the use of ultraviolet (UV) light in combination with proper photoinitiators to produce polymeric nanocomposite films is one of the most rapid and effective method, the main advantage being the creation of well defined patterned structures.

> Patterning of nanocomposites using UV polymerization, the so-called UV photolithography is ideal for the direct incorporation of nanocomposites into specific parts of systems and devices. Photolithographically prepared nanocomposite structures can be used for the selective deposition of molecules, which have specific affinity to the photopolymerized

Laser-Based Lithography for Polymeric Nanocomposite Structures 291

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,

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

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

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,

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

Nakajima 2000, 2001, Wang 1997, 1998, 1999, Sakai, 1998, 2001, 2003).

**2.1.1 Photopolymerized patterns of PMMA/TiO2** 

µL of the same solution were drop casted onto each sample.

demonstrating the potentiality of the specific lithography technique.

appear flat and homogeneous under the AFM (Figure 2b).

onto them.

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:


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.
