Photopatterning of Multilayer n-Alkylsilane Films - American Chemical

Letter pubs.acs.org/Langmuir

Photopatterning of Multilayer n-Alkylsilane Films Lingli Ni,† Abraham Chemtob,*,† Céline Croutxé-Barghorn,† Jocelyne Brendlé,‡ Loïc Vidal,‡ and Séverinne Rigolet‡ †

Laboratory of Photochemistry and Macromolecular Engineering and ‡Institut de Science des Matériaux de Mulhouse, ENSCMu, University of Haute-Alsace, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France S Supporting Information *

ABSTRACT: Surface photopatterning of organosilane self-assembled monolayers (SAM) has received increasing attention since its introduction 20 years ago. Herein we report for the first time a cost-efficient soft photopatterning technique affording amplified 3D multilayer structures. The essential chemistry relies on a spatially controlled photoacid-catalyzed hydrolysis and polycondensation of nalkyltrimethoxysilane precursors (n-C12H25Si(OCH3)3,). Amphiphilic siloxane species are photogenerated locally and are able to self-assemble spontaneously into a long-range-ordered lamellar mesostructure.

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INTRODUCTION Since the pioneering work of Dulcey et al.,1 the combination of organosilane self-assembled monolayers (SAM) with photolithography techniques has bred a new generation of thin hybrid photoresists exploited for applications in nanotechnology and biology.2,3 Depending on the UV irradiation wavelength, the exposed region of monolayer can be either entirely removed, resulting in clean oxide surfaces (100−280 nm), or photochemically modified (280−400 nm). Photodegraded patterns generated by energetic photons (vacuum UV) have been applied as etch masks,4 hydrophilic/hydrophobic contrast surfaces,5 and for selective metal deposition.6 Alternatively, SAMs bearing photosensitive groups can be subjected to spatially controlled photochemical reactions to create two chemically different film patterns7 endowed with specific binding8 or wetting properties.9 Although micropatterned and nanopatterned SAMs have proven their utility in the fabrication of novel electronic, photonic, and biological devices,10 their practical use is plagued by the inherent thickness of the monolayer, which is only a few nanometers thick. Hence, a great improvement in surface photopatterning would be to extend and amplify the 2D monolayer assemblies into organized 3D structures with thicknesses of several micrometers. Enhanced performance and easier development of practical devices in photonics, electronics, and optics are the expected benefits of this achievement.2,11,12 In this regard, there are few examples of highly ordered multilayers in the literature. The first methodology implies sequential layer-by-layer deposition and chemical modification to create anchoring terminal groups between each layer.11,13−16 A second simplified approach relies on the ability of some surfactants such as organotrihydroxysilanes nCnH2n+1Si(OH)3 arising from the hydrolysis of the initial n© 2012 American Chemical Society

organotrifunctionalsilanes to self-assemble spontaneously in 3D nanostructures.17 Multiple stacks of bilayers with a cross-linked siloxane interlayer can be generated by sol−gel polymerization, in which each layer is composed of highly ordered chains with a structure comparable to that of a single SAM.18,19 Suitable precursors for multilayer film preparation include only longchain n-alkyltrimethoxysilanes18 and (3-glycidoxypropyl)-trimethoxysilane (GPTMS).19 In a single study, patterned nanocrystals were produced with a sol of GPTMS by carrying out its gelation in the confined space of a poly(dimethylsiloxane) micromold,20 but the procedure requires the preparation of a sol under strongly basic conditions as well as very long aging times (10 days). We report herein a simplified methodology for the photopatterning of multilayer films based on n-dodecyltrimethoxysilane (n-C12H25Si(OCH3)3, C12TMS) photoinduced selfassembly, with thicknesses of a few micrometers. The essential chemistry of our approach relies on the fast photoacid-catalyzed hydrolysis and polycondensation of n-alkylsilane reported recently.21,22 Here, we demonstrate that a similar spatially controlled photoprocess can be implemented to form patterned n-dodecylsilsesquioxane lamellar nanostructures.

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EXPERIMENTAL SECTION

Sample Preparation. All of the chemicals were used as received without further purification. In a typical procedure, photoacid generator UV1241 (2% wt, Deuteron) and benzophenone (2% wt, Aldrich) were dissolved in the n-dodecyl trimethoxysilane precursor (C12TMS, 95%, ABCR) to form a photolatent solution in the absence Received: March 19, 2012 Revised: April 19, 2012 Published: April 19, 2012 7129

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Figure 1. (a) Schematic of the preparation process for photopatterned long-range-organized hybrid films based on C12TMS. (b) Optical microscope image of the patterned film. (c) Cross-section determined by an optical profilometry measurement. (d) Simplified representation of the stacked bilayer crystalline phase of the patterned area. of UV light. Then the resulting formulation was deposited onto a glass substrate (Brot) previously treated with a 20 wt % NaOH solution using an Elcometer 4340 automatic film applicator equipped with a wire-wound bar to produce a 4-μm-thick liquid film. Note that silicon wafers were used as substrates for real-time Fourier transform infrared (FT-FTIR) experiments and characterization by small-angle X-ray scattering (SAXS), optical microscopy, and profilometry. Photopatterning was performed using a copper transmission electron microscopy grid (400 mesh square type, 45 × 45 μm2, Oxford Instruments) as a shadow mask, which was pressed in soft contact with the deposited film during UV exposure. Irradiation was performed at room temperature under a mercury−xenon lamp (Hamamatsu L8251, 200 W) coupled to a flexible light guide. The room humidity was checked to be between 30 and 35% with a hygrometer throughout the irradiation. After 960 s of continuous insolation (20 mW/cm2), the TEM grid was smoothly lifted off. The film was subsequently rinsed with acetone to dissolve the unexposed part of the film. Characterization. Time-resolved infrared spectra were recorded by RT-FTIR using a Bruker Vertex 70 spectrophotometer equipped with an MCT detector with a temporal resolution of 0.12 s. The resolution of the infrared spectra was 2 cm−1. This technique is of interest in assessing the sol−gel kinetics and the conformational ordering of the alkyl chains throughout the UV irradiation. A decrease in the intensity of the symmetric methyl CH stretching vibrational band at 2840 cm−1 was exploited to monitor the methoxysilyl hydrolysis of the precursors during UV irradiation. Furthermore, the evolution of the methylene absorbing regions (in the 2800−3000 cm−1 region) was investigated to provide evidence of the gradual photoinduced conformational ordering. SAXS patterns were obtained on a PANalytical X’pert Pro diffractometer with fixed slits using Cu Kα radiation (λ = 1.5418 Å) and θ−2θ mounting. Before analysis, films on silicon wafers were directly deposed on a stainless steel sample holder. Data were collected between 0.5 and 10° 2θ degrees (SAXS) with a scanning step of 0.01° s−1. The morphologies of the samples have been

characterized by scanning electron microscopy (SEM, FEI Quanta 400 microscope working at 30 kV). Because the samples are nonconductive, they have been metalized with gold (15 nm thickness) before analysis. The transmission electron microscopy (TEM) observations were performed with a Phillips CM200 microscope operating at 200 kV. The powders were deposited on the surfaces of copper observation grids directly.

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RESULTS AND DISCUSSION Our procedure does not require the preparation of a sol and begins with the neat precursor including only a lipophilic photoacid generator (PAG, (C12H25)2Φ2I+ SbF6−) in place of mineral or organic acids that are traditionally used in sol−gel chemistry. After deposition on silicon wafers, UV exposure with a conventional medium-pressure mercury lamp with a reflector at 365 nm through a screen mask (TEM grid) creates patterned regions where Brönsted superacids H+SbF6− are liberated locally and can catalyze the sol−gel polymerization. The difference in siloxane condensation result in differential solubility, allowing selective etching of the nonexposed areas (a noncondensed or weakly condensed precursor) with acetone washing. Figure 1a shows a schematic representation of our approach. There is a strong correlation to conventional chemical amplification resists that are similarly driven by the photogeneration of protonic acids fostering a cascade of acidcatalyzed organic reactions.23 The difference here is that inorganic cross-linking is induced by the same catalytic species to create a negative hybrid organic−inorganic resist. An optical micrograph of a UV-irradiated and etched film, as depicted in Figure 1b, illustrates the device-level replication wherein the micrometer-scale pattern (45 μm × 45 μm) 7130

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condensation of 86% was estimated by solid-state 1H MAS NMR (Figure S1) from the ratio of residual silanol groups in the solid network per silicon atom. Additional support for the lamellar structure is furnished by the SEM and TEM images of the scrapped films, as shown in Figure 2b,c, respectively. In addition to its chemical stability, the 3D structure was also characterized by a high optical transparency (Figure S2) and a thermal stability in air up to 215 °C (Figure S3) instead of 150 °C with conventional SAMs based on octadecyltrichlorosilane (OTS).25 As expected, the onset of an ordered mesostructure is consistently accompanied by a progressive conformational ordering of the alkyl chain, as evidenced by in situ timeresolved Fourier transform infrared (FTIR) spectroscopy. Under UV irradiation, the FTIR spectra taken on a time scale of 960 s (Figure 3) in the 2800−3000 cm−1 region show

transferred from the photomask is clearly noticeable. Remarkably, the development phase under UV occurs within minutes, and the etching can be performed immediately after exposure, without the need for thermal consolidation. Profilometry provided a more rigorous examination of the film morphology. Figure 1c is a thickness plot along a section line showing a well-defined relief of ∼3 μm in the illuminated regions. Future studies are required to address whether smaller line features can be obtained. In this regard, the optimization of the PAG concentration is likely to be a key parameter in obtaining better control of superacid diffusion.24 Central evidence establishing the construction of a 3D multilayer assembly with a stacked alkyl bilayer separated by a siloxane network interlayer (Figure 1d) is provided by smallangle X-ray scattering (SAXS) as well as scanning and transmission electron microscopy (SEM and TEM) observations in Figures 2. The SAXS patterns of the UV-irradiated film

Figure 3. Temporal evolution of the antisymmetric (d−) and symmetric (d+) methylene (CH2) IR stretching modes of the nC12H25Si(OCH3)3 film throughout the UV irradiation (Hg−Xe lamp, 20 mW/cm2, 960 s). The distinct and nonresolved band at 2840 cm−1 is assigned to the symmetric CH3 stretching mode of the methoxysilyl (SiO−CH3) groups.

Figure 2. (a) Comparison of the SAXS patterns of the UV-cured C12TMS film (Hg−Xe lamp, 20 mW/cm2, 960 s) before and after acetone etching. (b) SEM image of the as-irradiated film showing the multilayer structure. (c) TEM image of the as-irradiated film also supporting the formation of an ordered lamellar nanostructure.

the characteristic signature due to methylene and methyl CH stretching vibrations.26 Specifically, the strong band maxima at 2854 and 2926 cm−1 assigned to symmetric (d+) and antisymmetric (d−) CH2 stretching modes are known to be sensitive to the alkyl chain conformation.27 From their initial position, a clear shift toward lower frequency is observed during UV irradiation, which is suggestive of an increasing proportion in trans conformers. Their final positions at 2852 and 2923 cm−1 are indicative of all-trans extended dodecyl chains in the as-irradiated film.28 Note finally that the achievement of longrange organization is not straightforward with short-side-chain organosilanes29 such as C12TMS. The light intensity should be optimized to favor slow-enough condensation kinetics. Above 20 mW/cm2, a progressive loss of organization is clearly observed (Figure S4). In contrast, there is almost no effect of film thickness on nanostructuration (Figure S5). Mechanistically, this scenario is consistent with a kinetically driven selfassembly30 and can be understood in terms of a sufficient “lifetime” of the surfactant-like hydrolyzed species (n-C12H25Si-

before and after etching (Figure 2a) similarly show the presence of three discernible peaks at 2.7, 5.2, and 7.7° (2θ) with associated d spacings of 32.8, 16.4, 10.9 Å, respectively. This is typical of lamellar nanostructures wherein the three observed peaks can be straightforwardly indexed to the (100), (200), and (300) Bragg planes. We estimate the d spacing to reflect closely the predicted length of the head-to-head bilayer structure CH3(CH2)11SiOx-OxSi(CH2)11CH3 with alkyl chains in a fully extended trans conformation. The 3D crystalline films have virtually the same structure as a 2D SAM of C12TMS. Although etching does not affect the long-range organization, it causes a slight broadening of the (001) peak at 2.7° (2θ) with a full width at half-maximum (fwhm) from 0.13 to 0.18°, which is indicative of increasing disorder. However, there is a significant structural robustness of the multilayer film, which presumably stems from the cross-linked siloxane interlayer. A degree of 7131

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Monolayers onto Hydroxyl-Terminated Silica. Langmuir 2009, 25, 11592−11597. (9) Zhao, B.; Moore, J. S.; Beebe, D. J. Surface-Directed Liquid Flow Inside Microchannels. Science 2001, 291, 1023−1026. (10) Woodson, M.; Liu, J. Functional Nanostructures from Surface Chemistry Patterning. Phys. Chem. Chem. Phys. 2007, 9, 207−225. (11) Tillman, N.; Ulman, A.; Penner, T. L. Formation of Multilayers by Self-Assembly. Langmuir 1989, 5, 101−111. (12) Fabiano, S.; Pignataro, B. Engineering 3D Ordered Molecular Thin Films by Nanoscale Control. Phys. Chem. Chem. Phys. 2010, 12, 14848−14860. (13) Collins, R. J.; Bae, I. T.; Scherson, D. A.; Sukenik, C. N. Photocontrolled Formation of Hydroxyl-Bearing Monolayers and Multilayers. Langmuir 1996, 12, 5509−5511. (14) Kato, S.; Pac, C. Fabrication of Multilayer Assemblies Based on a Boronate-Terminated Self-Assembled Monolayer. Langmuir 1998, 14, 2372−2377. (15) Maoz, R.; Sagiv, J. Targeted Self-Replication of Silane Multilayers. Adv. Mater. 1998, 10, 580−584. (16) Yam, C. M.; Kakkar, A. K. Molecular Self-Assembly of Dihydroxy-Terminated Molecules via Acid-Base Hydrolytic Chemistry on Silica Surfaces: Step-by-Step Multilayered Thin Film Construction. Langmuir 1999, 15, 3807−3815. (17) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. n-Alkylsiloxanes: From Single Monolayers to Layered Crystals. The Formation of Crystalline Polymers from the Hydrolysis of n-Octadecyltrichlorosilane. J. Am. Chem. Soc. 1997, 119, 3135−3143. (18) Shimojima, A.; Sugahara, Y.; Kuroda, K. Synthesis of Oriented Inorganic-Organic Nanocomposite Films from AlkyltrialkoxysilaneTetraalkoxysilane Mixtures. J. Am. Chem. Soc. 1998, 120, 4528−4529. (19) Menaa, B.; Takahashi, M.; Innocenzi, P.; Yoko, T. Crystallization in Hybrid Organic−Inorganic Materials Induced by Self-Organization in Basic Conditions. Chem. Mater. 2007, 19, 1946− 1953. (20) Takahashi, M.; Figus, C.; Kichob, T.; Enzo, S.; Casula, M.; Valentini, M.; Innocenzi, P. Self-Organized Nanocrystalline Organosilicates in Organic-Inorganic Hybrid Films. Adv. Mater. 2009, 21, 1732−1736. (21) Chemtob, A.; Ni, L.; Croutxé-Barghorn, C.; Demarest, A.; Brendlé, J.; Vidal, L.; Rigolet, S. Self-Organized Poly(n-octadecylsilsesquioxane) Films via Sol−Gel Photopolymerization. Langmuir 2011, 27, 12621−12629. (22) Ni, L.; Chemtob, A.; Croutxé-Barghorn, C.; Brendlé, J.; Vidal, L.; Rigolet, S. Photoinduced Synthesis and Ordering of Lamellar nAlkylsiloxane Films. J. Mater. Chem. 2012, 22, 643−652. (23) Schnabel, W. Polymers and Light: Fundamentals and Technical Applications; Wiley-VCH: Weinheim, Germany, 2007. (24) Doshi, D. A.; Huesing, N. K.; Lu, M.; Fan, H.; Lu, Y.; SimmonsPotter, K.; Potter, B. G.; Hurd, A. J.; Brinker, C. J. Optically Defined Multifunctional Patterning of Photosensitive Thin-Film Silica Mesophases. Science 2000, 290, 107−111. (25) Wang, R.; Baran, G.; Wunder, S. L. Packing and Thermal Stability of Polyoctadecylsiloxane Compared with Octadecylsilane Monolayers. Langmuir 2000, 16, 6298−6305. (26) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. Carbon-Hydrogen Stretching Modes and The Structure of n-Alkyl Chains. 1. Long, Disordered Chains. J. Phys. Chem. 1982, 86, 5145−5150. (27) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Infrared Spectroscopy of Three-Dimensional Self-Assembled Monolayers: nAlkanethiolate Monolayers on Gold Cluster Compounds. Langmuir 1996, 12, 3604−3612. (28) Bantignies, J. L.; Vellutini, L.; Maurin, D.; Hermet, P.; Dieudonné, P.; Wong Chi Man, M.; Bartlett, J. R.; Bied, C.; Sauvajol, J. L.; Moreau, J. l. J. E. Insights into the Self-Directed Structuring of Hybrid Organic−Inorganic Silicas through Infrared Studies. J. Phys. Chem. B 2006, 110, 15797−15802.

(OH)3) before their conversion to a cross-linked siloxane network.

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CONCLUSIONS Patterned self-assembled multilayer films based on ndodecylsilsesquioxane were easily fabricated on silicon wafers using a photoacid-catalyzed sol−gel process. Hierarchical ordering is achieved by combining for the first time organosilane self-assembled nanostructures (on the order of nanometers) and surface photopaterning (on the order of micrometers). The construction of high-quality, close-packed, highly ordered multilayered films by this method relies on both van der Waals-type interactions between alkyl chains and the precise control of the processing conditions because the correlation of chain organization finely depends on hydrolysis−condensation concurrent kinetics. It is believed that this efficient and simple route toward nanostructured hybrid resists can stimulate the developement of novel applications in photonics, microlectronics, and biological sensors, which have already been initiated by 2D phottopaterned SAMs.

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ASSOCIATED CONTENT

S Supporting Information *

1

H MAS solid-state NMR spectrum, UV−vis spectra, thermogravimetric plot of the UV-irradiated and etched films, and SAXS patterns of C12TMS-based films produced at several excitations and various film thicknesses. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +33 3 8933 5030. Fax: +33 3 8933 5017. Notes

The authors declare no competing financial interest.

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REFERENCES

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(29) Besson, E.; Mehdi, A.; Reye, C.; Gaveau, P.; Corriu, R. J. P. SelfAssembly of Layered Organosilicas Based on Weak Intermolecular Interactions. Dalton Trans. 2010, 39, 7534−7539. (30) Lerouge, F.; Cerveau, G.; Corriu, R. J. P. Supramolecular SelfOrganization in Non-Crystalline Hybrid Organic-Inorganic Nanomaterials Induced by Van Der Waals Interactions. New J. Chem. 2006, 30, 1364−1376.

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