We have investigated the conductance of gold point contacts using a scanning tunneling microscope ͑STM͒ inside a transmission electron microscope ͑TEM͒. Measuring the conductance of these point contacts as a function of radius, we could directly compare it with theories both in the ballistic regime ͑Sharvin͒ as well as in the diffusive regime ͑Maxwell͒. The width of the contacts were between a single atom and 20 nm. Using an interpolation formula ͑Wexler͒ between the two limits, we obtain a mean free path of 4 nm, which is about ten times shorter than the room-temperature bulk value. The low value indicates an enhanced scattering, which is not due to high temperature in the point contact, instead a large number of scattering centers is created during the point contact formation process.
The resolution of an scanning tunneling microscope (STM) when used for topographic measurement is mainly limited by the shape of the tip. We here report on the shape and structure of tips observed with high resolution electron microscopy before and after the use in a STM. Tungsten tips made by electropolishing and Pt–Ir tips made by electropolishing, cutting, and ion milling are studied. During electropolishing, the tungsten tips easily get covered with a rather thick, unwanted contamination and/or oxide layer. We have studied such tips as well as clean tips with transmission electron microscopy before and after they were used in a STM working in ambient air. We find that heavily oxide-covered tips often suffer severe bending on a platinum sample, probably because the oxide layer is deformed (broken) before tunneling can occur, or flattening in the STM. On the contrary, oxide free tips normally do not bend, and tunneling condition with a stable tunnel current is easily established.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.