Mechanistic investigations of nanometer-scale lithography at liquid-covered graphite surfaces

Recently, atomic and molecular scale features have been produced on surfaces using scanning tunneling microscopy (STM) methods.‘-’ At room temperature, the smallest permanent lithographic process reported to date involves the formation of -4O-A-diam, =:3-A-deep pits on a highly ordered pyrolytic graphite (HOPG) surface.’ These pits were formed by the application of 3-8 V pulses of 10-100 ps duration, while within tunneling distances. Although a substantial fraction of the STM tips successfully generated hundreds of uniform pits when subjected to constant amplitude and constant duration bias pulses,’ the pulse threshold at which the smallest features were observed varied substantially from tip to tip. Additionally, the lithography process was observed to require a humid atmosphere and to exhibit daily fluctuations in pulse bias versus pit size. In order to elucidate the chemistry of this process, we have performed STM lithography studies of HOPG surfaces in contact with water and other organic liquids. For HOPG in H,O(Z), we have observed a welldefined pulse threshold of (4.0 *0.2)V, which reproducibly yielded 7-A-diam features that apparently protruded from the surface by 2 A. Larger voltage pulses yielded pits (with slightly larger dimensions) that were similar to those produced in air or in humid Nz(g) atmospheres. This work demonstrates that subnanometer scale lithography can be accomplished on graphite at room temperature, and that STM studies can provide mechanistic information regarding the chemistry of the liquid-coated and gas phase lithographic processes. A STM specifically designed for imaging surfaces immersed in liquids was employed for this work.’ Bias pulses with a duration of 20 ps and an amplitude