Atomic-layer titration of surface reactions

Abstract

THE composition of surface layers on solid substrates is critical to many technological problems, such as semiconductor processing and heterogeneous catalysis. Deposited adatoms commonly react with the surface of the substrate to form a new, often two-dimensional structure. The stoichiometry of the surface phase can be difficult to determine by techniques of surface analysis, however; one can monitor the quantity of material deposited (using a quartz microbalance, for example), but the amount of substrate consumed is harder to assess. Here we present a technique for monitoring surface reactions in real time, which allows us to determine the amount of substrate material incorporated into the surface phase. We use the fact that surface reactions commonly involve surface roughening owing to extraction of substrate atoms. By supplying such atoms in advance in the form of sub-monolayer islands, we can titrate the subsequent adatoms against these pre-deposited islands of 'substrate" until a perfectly smooth surface (observed microscopically) is obtained. This should allow accurate characterization of surface reactions with stoichiometric ratios as great as 10:1.