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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.

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