AFM evidence of perpendicular orientation of cylindrical craters on hybrid silica thin film templated by triblock copolymer

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1 Available online at Surface Science xxx (2008) xxx xxx AFM evidence of perpendicular orientation of cylindrical craters on hybrid silica thin film templated by triblock copolymer Maggy Dutreilh-Colas a, *, Minhao Yan b, Philippe Labrot c, Nicolas Delorme b, Alain Gibaud b, Jean-Francßois Bardeau b, * a Centre de Recherche sur les Matériaux à Hautes Températures, UPR 4212, Orléans Cedex 02, France b Laboratoire de Physique de l Etat Condensé, UMR CNRS 6087, Université du Maine, Le Mans Cedex 09, France c Centre de Biophysique Moléculaire, UPR 4301, Orléans Cedex 02, France Received 21 September 2007; accepted for publication 25 November 2007 Abstract A simple method for elaborating mesoporous thin films with patterns normal to the substrate has been studied. The structured thin films were prepared by deposition of thin silica film templated by the so-called P123 (EO) 20 -(PO) 69 -(EO) 20 (poly-[ethyleneoxide-co-propyleneoxide-co-ethyleneoxide]) triblock copolymer on monolayers of n-octadecyltrichlorosilane (OTS) assembled onto a silicon wafer. Surface morphologies of films deposited onto silicon wafers modified by this hydrophobic monolayer or cleaned hydrophilic bare silicon wafer were characterized by using atomic force microscopy (AFM). We evidenced that, in the case of hydrophobic modified substrate, organic inorganic hybrid thin films made of P123 species embedded in a silica matrix exhibit regularly-shaped craters in the direction normal to the surface. Our investigation has shown that the diameter of the craters agrees fairly well with the length of the fully extended organic chains of the triblock copolymer. The specific interfacial interactions between both the hydrophilic part of the shell of the surfactant, the native silicon oxide of the silicon wafer and the silanized surface are discussed. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Mesostructured hybrid silica; Self-assembled monolayer; AFM 1. Introduction Self-assembling of surfactant molecules into well-organized mesostructural phases is now an undisputed route for elaborating inorganic nanopatterned surfaces and functionalized new devices at the nanometer scale [1 3]. In order to elaborate structured mesoporous materials, amphiphilic molecular entities such as CTAB (cetyltrimethylammonium bromide) or triblock copolymers (mostly Pluronic P123 or F127) is commonly used given their ability to self-assemble and form hybrid materials (so-called MCM41, SBA-16 or SBA-15 for example). Currently, * Corresponding authors. Present address: Université de Limoges, Faculté des Sciences, 123 Avenue Albert Thomas, LIMOGES cedex, France (M. Dutreilh-Colas). addresses: maggy.colas@unilim.fr (M. Dutreilh-Colas), jeanfrancois.bardeau@univ-lemans.fr (J.-F. Bardeau). highly ordered hybrid mesostructured thin films with 1D, 2D and 3D organization have been made by using a large class of oxides as SiO 2, ZrO 2, CeO 2, TiO 2,Nb 2 O 5 or even heterometalic oxoclusters [4 7]. Control of key parameters such as the composition, relative humidity and temperature is essential to choose the design and construction of ordered mesoporous thin films. If the preparation of surfactant templated mesostructured silica thin films has received much attention, it is because their potential applications as sensors, optoelectronic devices, catalysts are obvious. Nevertheless, one of the remaining challenges was to understand how to control the orientation of the porosity of silica thin films and more especially the porosity normal to the substrate. Since this issue has never been addressed, the description of a simple and efficient route has to be explored to open up both fundamental and new applicative perspectives. In fact, news systems with functionalized porosity easily accessible in concert with their /$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi: /j.susc

2 2 M. Dutreilh-Colas et al. / Surface Science xxx (2008) xxx xxx large surface area will lead to higher sensitivity chemical or biochemical sensors for example. Our previous works were mainly focused on silica thin films using TEOS (tetraethylorthosilicate) as precursor in acidic media and triblock copolymer P123 as the structuring agent. Because of the preferential interactions between PEO (poly-ethyleneoxide) moieties of the copolymer and the silicon oxide native layer of the silicon wafer, the 2D hexagonal phase (p6m symmetry) with cylinders parallel to the surface is usually obtained [8,9]. Similarly behavior with hydrophilic interaction has been reported on polymer thin films elaborated with diblock copolymers where the structure was found strongly dependent on the surface energies of the blocks and on geometrical constraints introduced by the confinement [10]. In this case, authors had been developed two approaches to control the interfacial energies and wetting behavior of polymers in contact with solid surfaces. The first route used an anchored random copolymer [11,12] (the surface energy is neutral with respect to each component of the diblock polymer) and the second one overcame interfacial interactions with electric fields [13 17]. So, in this letter, we present a new and simple method for controlling the orientation of the mesophase of hybrid silica thin films normal to the surface of silicon substrate modified by self-assembled octadecyltrichlorosilane (OTS) monolayer. OTS was selected as a model of highly packed organic monolayer and because of its relative high hydrophobicity. We observed patterned surfaces with craters normal to the surface in the case of mesostructured silica thin films templated by P123 triblock copolymer. These results are discussed in terms of interfacial interactions between the organic species and the modified substrate to which they are attached after controlling different parameters such as the composition of the solutions, the relative humidity and the temperature. 2. Experimental section 2.1. Wafer preparation copolymer P123 purchased from Aldrich and used as received (average molecular weight 5750 g/mol) was stirred for 1h in ethanol and then added to the first solution. So the final molar ratio was 1 TEOS:73 EtOH:8.3 H 2 O:0.01 HCl:0.01 P123. This final solution aged for 5 h was then diluted 1000 times before being deposited onto the OTS functionalized wafers. Deposition was achieved by either dip-coating or spin-coating sol gel mixtures onto the various substrates. The dip-coating was performed at constant withdrawal rate of 5.6 cm/min in a sealed box at controlled relative humidity (RH) fixed at 60% and fixed temperature (298 K). The spin-coated films were coated at 7000 rpm in identical environmental conditions (RH fixed at 60% and temperature fixed at 298 K). The dip and spin-coated films were then annealed under vacuum at 150 C during 24 h Characterization X-ray reflectivity measurements were carried out on a Philips reflectometer at the Laboratorie de Physique de PEO PPO PEO hydrophilic hydrophilic hydrophobic SiO 2 Native SiO 2 layer Silicon Wafer Fig. 1. 2D hexagonal self-assembled triblock copolymer as silica template on a bare silicon wafer. Classical silicon wafers {1 0 0} provided by Siltronix were used as substrates. The wafers were cleaned with ethanol before being irradiated by a UV ozone cleaner (Jelight Company Inc.). Octadecyltrichloro silane (95%) was purchased from Aldrich and was used without additional purification. The OTS silanization was performed by immersing the silicon wafers into a 0.2% solution of OTS in bicyclohexyl (cyclohexylcyclohexane) for 6 min before being ultrasonically rinsed in toluene. R/R F (arb. units) 1E-6 1E-7 1E-8 1E-9 ΔQ = 2π / h OTS 2.2. Initial sol and thin film preparation The initial sols were prepared in two steps. First a prehydrolyzed TEOS sol was prepared by mixing TEOS, EtOH, HCl and H 2 O and stirring for 1 h. The triblock Q z (Å -1 ) Fig. 2. Fresnel normalized X-ray reflectivity results from an OTS-SAM onto silicon (square) together with the calculated curve (full line).

3 M. Dutreilh-Colas et al. / Surface Science xxx (2008) xxx xxx 3 Native SiO 2 1-cleaning process 2-OTS anchored Compact OTS monolayer 3- Dip or spin coating P123/TEOS/EtOH, H 2 O,HCl solution P123 cylinders standing up Fig. 4. AFM topography images of a silica thin film templated by P123 triblock copolymer (a) dip-coated on commercial silicon wafer, (b) dipcoated or (c) spin-coated on silicon substrates functionalized with a submonolayer of n-octadecyltrichlorosilane (OTS). Fig. 3. Schematic representation of our procedure to generate an SiO 2 matrix with surfactant cylinders standing up on silicon substrates functionalized with a submonolayer of n-octadecyltrichlorosilane (OTS). l Etat Condensé (LPEC, Université du Maine, France) with Cu-Ka-radiation (k = 1.54 Å) at 40 kev and 30 ma. The experimental data are shown after the theoretical Fresnel normalization for favoring the observation of the well defined Kiessig fringes [18]. The contact angles measurements were performed at LPEC laboratory on a home-made goniometer built with optical accessories purchase from Nachet. Droplets (2 ll) of ultrapure water (Millipore) with a resistivity higher than 18 MX/cm were used to measure the surface wettability. AFM images were obtained in air using a Digital Instruments Dimension 3100 scanning force microscope, with a Nanoscope IIIa controller, in tapping mode at the Centre de Biophysique Moléculaire (CNRS, Orléans, France). Silicon tips with a spring constant ranging from 21 to 78 N/m as specified by the manufacturer (nanoworld) were used. 3. Results and discussion s, used as received, are commonly hydrophilic as demonstrated by their small water contact angle (20 ). This hydrophilicity is due to silanol groups (Si OH) present on the surface of the native silicon oxide layer covering the silicon wafer. The triblock copolymer P123, we used for templating silica thin films, is made of a PPO core and two PEO parts with hydrophobic/hydrophilic ratio 70:30 in molecular weight. During the self-assembly process, the PPO part of the triblock copolymer is protected from the aqueous medium by the more hydrophilic PEO parts. Cylindrical micelles formed can be described by a core shell model in which the core is made of a PPO hydrophobic surrounded by a PEO hydrophilic shell. Since

4 4 M. Dutreilh-Colas et al. / Surface Science xxx (2008) xxx xxx Fig. 5. (a) AFM topography image of a silica thin film templated by P123 dip-coated on silicon substrates functionalized with a submonolayer of n- octadecyltrichlorosilane (OTS), (b) averaged diameter of the observed cylinder and (c) representation of the triblock copolymer P123 and the total chain length. there is a strong affinity for the PEO shell to the silicon oxide of the silicon wafer, the cylinders naturally fall down to the surface and built some 2D hexagonal array parallel to the substrate as shown on the schematic diagram in Fig. 1. To overcome the specific interfacial interaction between the PEO blocks and the native silicon oxide of the wafer, we functionalized the surface of the wafer by hydrophobic OTS molecules. The thickness of the monolayer was found to be 23 Å after an accurate fitting analysis of the reflectivity curves (Fig. 2) done by the matrix technique [18]. Such a value is consistent with a highly dense OTS monolayer with alkyl chains in all-trans conformation [19,20]. In addition, the high contact angle measurement (110 ) observed for the functionalized surface confirm the novel hydrophobic character of the interface. In a second step, P123-templated silica thin films were either spin-coated or dip-coated on the functionalized surface from an alcoholic P123/TEOS sol in the ratio The TEOS used as a silica precursor was preliminary hydrolyzed under acidic condition in order to favor the formation of a silica matrix surrounding the P123 triblock copolymer cylinders (Fig. 3). The topography image of a silica thin film templated by P123 dip-coated or spin-coated (not shown here) onto a bare silicon wafer shows a typical smooth surface (Fig. 4a) since the domains are preferentially oriented with their tubular micelles aligned with the surface. On the contrary, the hydrophobic functionalization of the silicon wafer surface induces obvious morphological modification of the hybrid film with the appearance of opened craters covering the surface (Figs. 4b and c). The averaged diameter of the cylindrical craters is estimated to 40 nm for both spin and dip-coated films as shown on Fig. 5. The surface roughness (evaluated by the root mean square: RMS) gives a clear indication of the evolution of the films topography. In the case of spin-coated films on OTS, the RMS was found to be 0.2 nm indicating that at this scale the surface topography was comparable to the initial OTS monolayer. On the contrary, the RMS roughness analysis for the dipcoated film revealed higher value (RMS = 0.9 nm) likely due to the presence of small drops on the surface. Therefore the spin-coating technique yielded thin films with homogenous repetitions of the cylinders on the surface after controlling the hydrophobic state of the surface substrate. The diameters of the cylindrical craters agree surprisingly well with the length of the P123 surfactant molecule estimated by a geometric modelization (using the freeware ACD-Chemsketch) in the trans conformation (Fig. 5). To the best of our knowledge it is the first time that such structuration has been experimentally observed with hybrid organic/inorganic mesophased materials deposited on a substrate. Further investigations are necessary if we want to increase the density of the open craters on the surface

5 M. Dutreilh-Colas et al. / Surface Science xxx (2008) xxx xxx 5 and to describe precisely the surfactant self-assembly process. However, as regard to our results, one can already propose a model where Pluronics self organize from aqueous solution onto a hydrophobic substrate. Moreover, as it is known that block copolymers consisting of PEO and PPO show amphiphilic properties which are highly dependent of the temperature conditions [21] and the lengths of the EO-blocks and PO-blocks [22], we could further suggest investigating the effect of a series of Pluronics with constant PO-block length for better understanding the hydrophobic interaction of the chains with the substrate. Finally, our preliminary investigations show that Relative Humidity and temperature annealing time are also key parameters which can tailor the diameter of the open craters since we easily observe a decrease of the pore diameter to a nm. 4. Conclusion We evidenced here a simple method to form mesoporous silica thin films with porosity normal to the substrate in a one step synthesis. In this work, we provided evidence for the feasibility of this approach using hydrophobic selfassembled monolayer grafted to the surface of a silicon wafer. Since the presence of this intermediate layer plays a key role in the appearance of such craters normal to the surface of the substrate, we will intend to increase the density of perpendicular holes and get a local ordering by optimizing the conditions of preparation of mesoporous thin films and eventually the hydrophobicity properties of the surface. Nevertheless, we believe this approach can be extended easily to the preparation of other kind of mesoporous materials and offer a wide applicative perspectives. Acknowledgment This research was supported by the French ACI Nanosciences under project Autofymehypodir. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] G. Stix, Sci. Am. 285 (2001) 32. [3] P. Ball, Nature 413 (2001) 667. [4] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [5] G.J.A.A. Soler-Illia, C. Sanchez, New J. Chem. 24 (2000) 493. [6] E.L. Crepaldi, G.J.A.A. Soler-Illia, D. Grosso, P.A. Albouy, C. Sanchez, Chem.Comm. (2001) [7] E.L. Crepaldi, G.J.A.A. Soler-Illia, A. Bouchara, D. Grosso, D. Durand, C. Sanchez, Angew. Chem. Int. Ed. Engl. 42 (2003) 347. [8] J.-F. Bardeau, A. Gourbil, M. Dutreilh-Colas, S. Dourdain, A. Mehdi, A. Gibaud, Thin Solid Films 495 (2006) 191. [9] S. Dourdain, J.-F. Bardeau, M. Colas, A. Medhi, B. Smarsly, B.M. Ocko, A. Gibaud, A. Appl. Phys. Lett. 86 (2005) [10] M.J. Fasolka, A.M. Mayes, Annu. Rev. Mater. Res. 31 (2001) 323. [11] P. Mansky, Y. Liu, E. Huang, T.P. Russell, C.J. Hawker, Science 275 (1997) [12] E. Huang, L. Rockford, T.P. Russell, C.J. Hawker, Nature 395 (1998) 757. [13] T. Thurn-Albrecht, J. DeRouchey, T.P. Russell, Macromolecules 33 (2000) [14] T.L. Morkved, M. Lu, A. Urbas, E.E. Ehricks, H.M. Jaeger, P. Mansky, T.P. Russell, Science 273 (1996) 931. [15] P. Mansky, J. DeRouchey, T.P. Russell, J. Mays, M. Pitsikalis, T. Morkved, H.M. Jaeger, Macromolecules 31 (1998) [16] T. Xu, H.-C. Kim, J. DeRouchey, C. Seney, C. Leveque, P. Martin, C.M. Stafford, T.P. Russell, Polymer 42 (2001) [17] T. Xu, J.T. Goldbach, M.J. Misner, S. Kim, A. Gibaud, O. Gang, B. Ocko, K.W. Guarini, C.T. Black, C.J. Hawker, T.P. Russell, Macromolecules 37 (2004) [18] G. Vignaud, A. Gibaud, Program Reflex: a matlab routine for the simulation of specular X-ray reflectivity data with the matrix technique. [19] A. Baptiste, A. Gibaud, J.-F. Bardeau, K. Wen, R. Maoz, J. Sagiv, B.M. Ocko, Langmuir 18 (2002) [20] N. Delorme, J.-F. Bardeau, A. Bulou, F. Poncin-Epaillard, Thin Solid Films 496 (2006) 612. [21] P. Alexandridis, T.A. Hatton, Colloids Surf. A: Physicochem. Eng. Aspects 96 (1995) 1. [22] K. Flodström, V. Alfredsson, Micropor. Mesopor. Mater. 59 (2003) 167.