AFM induced formation of SiO2 structures in the electrochemical nanocell

Nanostructures of SiO2 are formed in the so-called nanocell by AFM tip-induced oxidation of Si. The highly resistive electrolyte is formed by a thin water film in wet gas atmosphere. AFM measurements prove that the water film thickness increases with both p(H2O) and electric field strength. Water consumption by Si oxidation and water electrolysis is compensated by field enhanced water condensation from the gas phase. Due to the absence of a reference electrode, current dependent potential drops at the tip and in the water film cannot be compensated. At constant cell voltage DeltaU, the potential drop within the formed oxide Delta phi (ox) depends not only on DeltaU, but on the polarisation time t and the lateral coordinate x as well. Measurements of current transients in the nanocell and a macroscopic cell show that the oxide growth kinetics are similar, but the quantitative result differs due to the 3D structure of the oxide and the variation of Delta phi (ox)(t,x). Investigation of electron transfer reactions in the macro- and nanocell show that anodic oxygen evolution is possible at high electric fields which is explained by hole tunnelling. Direct tunnelling of electrons from the tip to the Si substrate can only take place in the initial stages of oxide formation through few oxide monolayers. Therefore, faradaic processes can be evaluated in the later stage of experiment. For electrochemical nanosystem technology, the oxidation in a one-step process allows the formation of positive Si-structures, while a multistep process followed by etching of the oxide allows the defined formation of pits. (C) 2001 Elsevier Science Ltd. All rights reserved.