Monte Carlo Study of Etching at Silica-Water Interface
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INTRODUCTION Reaction kinetics in confined domains often exhibit anomalous behavior.1-4 In this paper we present a new model resulting in non-classical kinetics in an elementary A+B->0 reaction at a surface/interface with B as adsorbent. Chemistry at a surface or interface is important not only to some traditional applications, such as chromatography and electrophoresis, but also to surface adhesion and to microelectronic structures in silicon wafers. 5 12 Often the system encountered is not a purely two-dimensional system and the material at the surface is not necessarily confined to the surface: it is deposited or adsorbed from the bulk and it can also desorb back into the bulk. We consider the following example: A basic alcohol solution is known to remove the
surface Si-O-Si bonding e.g. the self-assembled monolayer1 3 and, therefore, increase the number of surface silanol functional groups, i.e., the surface negative charges. The etching process of selfassembled-monolayers has been studied experimentally using a SHG (Second Harmonic Generation) technique. 14-15 An anomalous algebraic power law of t112 was observed for the etching, i.e., for the total self-assembled-monolayer mass removal. To explain this unusual power law we proposed a surface adsorption-diffusion-reaction model. We performed preliminary Monte Carlo simulations which agreed well with the experimental kinetics at the surface. In this paper more extensive computer simulations were performed to determine the origin of this anomalous power law. MODEL OF SIMULATION Often, in the study of multi-body chemical reactions kinetics, exact theoretical solutions are difficult, perhaps impossible, to attain. Computer simulation of more complex dynamics becomes a necessary tool for investigating the phenomena of reaction kinetics. Our model is a surface adsorption-diffusion-reaction model. In this model, the reactant does not react with the surface species directly (probably due to steric hindrance), rather, the reactant will have to adsorb onto the surface first, and then diffuse to the reaction site. For simplicity, we adopt the Langmuir adsorption model which assumes that the interaction of adsorbent molecules are the same among 263 Mat. Res Soc. Symp. Proc. Vol. 543 01999 Materials Research Society
all the adsorbent neighbors. The free energy of the adsorption is related to the species solvation energy in the bulk and at the surface. In solution adsorption, when a molecule lands on the surface from the bulk, it has to break up the solvent shell and become solvated at the surface. The energy difference of the above process makes up the free energy in the Langmuir adsorption. Suppose that there are two different domains of sites at the surface, labeled SI and S2, respectively, then the adsorption on these two domains may be different, because the surface solvation could be very different. In our simulations, we imposed a solvent shell for each B landing onto the surface. When B is on the surface, the case of a B landing with no A in the neighborhood (so
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