NANO LETTERS
Fabrication of Nanostructured Polymer Surfaces Using Colloidal Lithography and Spin-Coating
2002 Vol. 2, No. 12 1419-1425
Fre´de´ric A. Denis,† Per Hanarp,‡ Duncan S. Sutherland,‡ and Yves F. Dufreˆne*,† Unite´ de Chimie des Interfaces, UniVersite´ Catholique de LouVain, Croix du Sud 2/18, B-1348 LouVain-la-NeuVe, Belgium and Department of Applied Physics, Chalmers UniVersity of Technology, S-412 96 Go¨ teborg, Sweden Received August 14, 2002; Revised Manuscript Received October 12, 2002
ABSTRACT We present an original method for fabricating nanostructured polymer surfaces exhibiting homogeneous chemistry using colloidal lithography combined with polymer spin-coating. The first step of the strategy consists of creating gold surfaces with hemisphere-like protrusions, by adhesion of negatively charged polystyrene colloidal particles onto smooth positively charged surfaces, followed by coating with a thin layer of gold. In the second step, gold substrata are spin-coated with ultrathin polystyrene films. Using X-ray photoelectron spectroscopy and atomic force microscopy, we show that the nanotopography of the obtained polymer surfaces can be modulated by changing three parameters, i.e., the particle diameter, the particle concentration, and the polystyrene film thickness, while keeping the surface chemistry unmodified.
Introduction. Designing polymer materials with controlled surface properties has long been an important issue in basic and applied research.1 More recently, the fabrication of polymer surfaces exhibiting variations of properties on the nanoscale has emerged has an important challenge of nanoscience and nanotechnology.2-4 Strategies that have been explored for fabricating patterned nanostructures (e 100 nm) take advantage of at least one of the following principles: 5-7 (i) interaction of matter (“lithography”) with photons (Xray,8 ultraviolet),9 energetic particles (electrons,10 ions,11 neutral metastable atoms),12 and scanning probes;13,14 (ii) replication against masters via physical contact;15-17 (iii) selfassembly18 of molecules19,20 and nanoscale objects;21 (iv) controlled deposition;22 and (v) size reduction.23 Most of these methods involve sophisticated instrumentation or treatments. Only few of them, i.e., the embossing (or imprinting)15 and the replica molding (or cast molding)16 methods, are suited to create topographical variations at polymer surfaces while keeping the chemistry unmodified. This requirement is important for many applications. For instance, in biomaterial science both surface chemistry and topography are known to affect protein adsorption24-26 and cell adhesion,24,27,28 but the relative importance of these two factors remains poorly understood because most often topographical modifications are accompanied by chemical * Corresponding author: Phone: (32) 10 47 36 00; Fax: (32) 10 47 20 05; E-mail:
[email protected]. † Universite ´ Catholique de Louvain. ‡ Chalmers University of Technology. 10.1021/nl025750g CCC: $22.00 Published on Web 11/08/2002
© 2002 American Chemical Society
variations. Hence, controlling independently the topography and chemistry of polymer surfaces is an important issue. Spin-coating of homopolymer solutions onto flat substrates is commonly used in material engineering and material science to create smooth polymer surfaces. A description of the basic principles of spin-coating is given in Bornside et al.29 Several factors are known to affect the polymer film thickness, among which are the polymer concentration and molar mass, the solution viscosity, the solvent volatility, and the spinning speed.30,31 Because of their great potential applications, “ultrathin” polymer films, with thicknesses in the 1 nm to 200 nm range,31,32 are attracting more and more attention.33 Compared to thicker films, ultrathin films show a different structure, the polymer chains being preferentially extended parallel to the substratum, which in turn confers different thermophysical properties.33-35 The volatility of the solvent can be critical to obtain smooth ultrathin films.36 When polymer solutions are spin-coated onto rough substrata, the polymer films tend to smooth the surface topography, a process referred to as planarization.37 The planarization of substrata that are rough on the micrometer scale has been extensively studied, both from the experimental30,38-40 and theoretical30,37,41,42 points of view. A number of parameters influence the planarization process, among which are the polymer concentration, the polymer molar mass, the solution viscosity, the solvent volatility, the spinning speed, and the geometry and distribution of the substratum surface features. So far, the use of the planariza-
tion process to modulate surface topography on the nanoscale has received little attention. In this paper, we present a novel approach that makes use of colloidal particle assembly, i.e., colloidal lithography, and spin-coating of ultrathin polymer films to create polymer surfaces with modulable nanotopography and homogeneous chemistry. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) are used to characterize the polymer surfaces. The results reveal that by varying the size and concentration of the colloidal particules, on one hand, and the polymer film thickness, on the other hand, it is possible to modulate the surface relief at the nanoscale. Materials and Methods. Metallic surfaces with controlled nanotopography were created using the adhesion of colloidal particles onto solid surfaces as described previously.25,43,44 To this end, silicon wafers (∼5 cm diameter) coated with a 30 nm thick Ti layer using thermal evaporation were coated with a triple layer precursor film (∼1 nm thick)45 to make them positively charged and immersed in an aqueous suspension (pH ) 6) of polystyrene beads of either 23 ( 3 nm diameter (small particles) or 38 ( 5 nm (large particles), at a concentration of either 0.01% (low concentration) or 0.1% (high concentration) by weight (Sulfate Latex, Interfacial Dynamics Corporation (IDC), Portland OR; 60 s adsorption time,
