Article pubs.acs.org/Langmuir
Hydrophilic/Hydrophobic Film Patterning by Photodegradation of Self-Assembled Alkylsilane Multilayers and Its Applications Lingli Ni,† Céline Dietlin,† Abraham Chemtob,*,† Céline Croutxé-Barghorn,† and Jocelyne Brendlé‡ †
Laboratory of Photochemistry and Macromolecular Engineering, ENSCMu, University of Haute-Alsace, 3 bis rue Alfred Werner, 68093 Mulhouse Cedex, France ‡ Institute of Material Science of Mulhouse, CNRS, UMR 7361, University of Haute-Alsace, 3 bis rue Alfred Werner, 68093 Mulhouse Cedex, France S Supporting Information *
ABSTRACT: While the photopatterning of self-assembled monolayers (SAMs) has been extensively investigated, much less attention has been given to highly ordered multilayer systems. By being both thicker (0.5−2 μm) and more stable (cross-linked) than SAMs, patterned hybrid multilayers lend themselves more easily to the development of technologyrelevant materials and characterization. This paper describes a facile two-step UV approach to patterning an alkylsilane multilayer by combining photoinduced self-assembly for multilayer synthesis and photodegradation through a mask for creating patterns within the film. In this second stage, a spatially resolved removal of the alkyl tail via a photooxidation mechanism took place, yielding regular and uniform silica microdomains. The result was a regular array of features (alkylsiloxane/ silica) differing in chemical composition (hybrid/inorganic), ordering (crystal-like/disordered), and wettability (hydrophobic/ hydrophilic). Such a photopatterned film was of utility for a range of applications in which water droplets, inorganic crystals, or aqueous polymer dispersions were selectively deposited in the hydrophilic silica microwells.
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INTRODUCTION
A promising alternative is the photopatterning of covalently bonded self-assembled multilayers that are both thicker and more robust than SAMs.34 Here, we describe a simple and novel approach of making highly ordered and cross-linked nanolaminated alkylsiloxane on a native silicon oxide surface, followed by exposure to UV light through a mask to create hydrophilic/hydrophobic micropatterns. Figure 1 outlines the procedure. Beginning with a solventless solution of alkylsilane (C12H25Si(OCH3)3) and a iodonium salt photoacid generator (PAG, Ar-I+-Ar SbF6−), we rely on UV light to induce the hydrolysis of the alkoxysilane derivatives using a conventional medium-pressure mercury arc lamp at low irradiance.35,36 Under these conditions, self-assembly of the amphiphilic silsesquioxane species (C12H25Si(OH)3−x(OCH3)x) proceeds without external surfactant organizing the alkyl tails and siloxane heads into the desired multilayer form. In contrast to sequential deposition such as layer-by-layer films,37,38 the ordering process is spontaneous without the need for many repeated deposition steps to build up a micrometer coating thickness.39 Slow polycondensation subsequently fixes the ordered silsesquioxane structure, providing a covalently bonded alkylsiloxane multilayer with high thermal and mechanical stability. In a second step, the robust self-assembled multilayer may be photochemically manipulated in the same way as
Nanopatterning and micropatterning of self-assembled monolayers (SAMs) has become an essential tool for the surface engineering of different types of polar substrates (silicon, glass, metal oxide, etc.).1−3 The fabrication of multifunctional surface patterns is of high interest for applications such as sensors, electronic devices, drug screening, lab on a chip devices, and so on. Among the increasing pool of patterning techniques available, photolithography holds a special place as it is widely implemented in the silicon semiconductor industry.4 Practically, a well-ordered monolayer is first deposited by the immersion of a substrate into a dilute organic solution of precursor molecules (thiol, organosilane, or phosphonic acid). Then, a photomask is positioned on top of the SAMfunctionalized surface, and subsequent UV irradiation causes either a spatially resolved degradation5−15 or chemical modification16−30 of the monolayer. The numerous benefits of this technique, including ease of use, versatility, high resolution, accurate placement features, and wide exposure field, should not overshadow two major limitations. First, monolayers cannot exceed a few nanometers whereas many nanotechnological devices require microscale thickness.31−33 Second, SAMs generally have moderate stability leading to a significant impairment of performance with time, as exemplified by the highly popularized SAMs of alkylthiolate on gold having modest thermal and oxidative resistance. © 2014 American Chemical Society
Received: June 19, 2014 Published: August 1, 2014 10118
dx.doi.org/10.1021/la5023938 | Langmuir 2014, 30, 10118−10126
Langmuir
Article
Figure 1. Two-step synthesis of a photopatterned n-alkylsilane multilayer film using a medium-pressure Hg−Xe arc lamp with adjustable light irradiance. (a) Formation of the lamellar structure via the template-free photoinduced self-assembly process of n-dodecyl trimethoxysilane. (b) Photocalcination under a mask to remove the alkyl chains. The result is a pattern of well-defined amorphous silica channels within the ordered multilayer hybrid film. formation of the multilayer film from the liquid PAG/precursor film (i) and the subsequent degradation patterning (ii). Both processes relied on the same lamp but required different irradiation conditions: irradiance, reflector used, and irradiation time. As shown in Figure 2, a
SAMs.4 In our case, microscale patterns can be fabricated using the same UV lamp at higher irradiance in conjunction with a mask, leading to a removal of the hydrophobic alkyl tail without the need for solvent etching. Very little work has been done on the photolithographic degradation of multilayer films whereas this approach has been extensively studied with SAMs upon deep or vacuum-UV irradiation (100−280 nm), each of which is a more dangerous and expensive light source.5,8 Another difference with respect to photocalcined SAMs is that the exposed regions are not entirely removed, and instead of yielding a clean oxide substrate, well-defined but disordered hydrophilic silicate (SiO2) domains are formed since the lamellar structure collapses. Hence, the result is a pattern of different chemical composition (hybrid/inorganic), ordering (crystal-like/disordered), and wettability (hydrophobic/hydrophilic). This offers promise for many applications in material science. Such a patterned micrometric film has been utilized to fabricate a substrate for the study of condensation figures, the local crystallization of salts, and the patterned deposition of latex particles yielding a polymer film after water evaporation.
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Figure 2. Compact light source combines a Hg−Xe lamp, a cold reflector, and a quartz light guide. The output irradiance as well as the irradiation time can be adjusted.
medium-pressure Hg−Xe arc lamp (L8252, 200 W, Hamamatsu) was connected to a flexible light guide (LC6, Hamamatsu) generating a focused light beam on the film sample placed 3 cm away. Such a spot light source covered a broad continuous spectrum spanning from short-wavelength UV to infrared (185−2000 nm) through a bulb glass made of high-purity fused quartz. Light irradiance was easily adjustable, and the output emission could be shifted toward short and long wavelengths through two types of elliptical cold reflectors, which are also useful in preventing adverse effects from heat (IR radiation). The emission spectra obtained with the short- and longwavelength UV reflectors are depicted in Figure S1 of the Supporting Information (SI), where we can see that the short-UV reflector increases the proportion of light in the 190−210 nm region (needed for the degradation of the organic moieties). As only the light below 300 nm is useful for chemical reactions (PAG photolysis or alkyl chain degradation), the total irradiance below 300 nm (Iλ
