Exploration of the deposition of submicrometer particles by spin-coating
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The deposition of Cu, Zn, Pt, and Co precursor particles from solution onto a flat silicon wafer using a spin coater was studied. Homogeneously distributed monodisperse particles can be obtained. The dependence of particle size and number density on solution concentration and rotation frequency was investigated. Different solvents and support modifications were studied. The particles were analyzed using dark-field microscopy, scanning electron microscopy, and atomic force microscopy.
I. INTRODUCTION In several areas of science and technology there is a strong interest in particles in the submicrometer size range. The physical and chemical properties of colloidal particles differ considerably from those of bulk material due to the much larger surface to bulk ratio.1'2 The most common way to prepare small particles is by precipitation from solution, as used in the preparation of supported heterogeneous catalysts. When the solvent evaporates, the metal precursor is left behind on the support. Subsequent oxidation/reduction will convert the deposited precursor to its active form.3 Impregnation, however, yields deposits that are usually not homogeneous in particle size and distribution. The problem of inhomogeneity is even larger in the case of "impregnation" of a flat support. In porous supports, the solvent remains inside the pores during drying due to capillary forces. This yields many very small separated droplets, resulting in separately deposited particles inside the pores. During coating of a flat support, however, just a single puddle of liquid exists. This puddle changes in size and shape during drying, which results in an inhomogeneous deposition and a broad particle size distribution. Flat supports can be coated with monodisperse nmsized particles using physical vapor deposition (PVD).4 Control of particle size and density is obtained by variation of substrate temperature and evaporation flux. In this report we will describe a way to coat flat supports homogeneously by deposition from a liquid. Deposition of the liquid phase has some advantage with respect to PVD. Since deposition from the liquid phase resembles the usual preparation of supported catalysts, a more direct link between a flat model and a porous industrial catalyst can be made. Above this, hardly any equipment is needed in the case of deposition from
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Present address: Gorlaeus Laboratories, P.O. Box 9502, 2300 RA Leiden, The Netherlands. J. Mater. Res., Vol. 10, No. 2, Feb 1995 http://journals.cambridge.org
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the liquid phase. In the case of deposition from the liquid, precursors are deposited. These precursors have to be transformed to the final oxide/metal by calcination/reduction. There is the danger that this transformation will introduce contaminants and/or morphology changes, which would be a disadvantage. For deposition from a liquid, control can be achieved by spin coating. Spin coating is commonly used in the microelectronics industry to obtain thin films of polymers or photoresists on substrates.5 To this e
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