Tip contamination effects in ambient pressure scanning tunneling microscopy imaging of graphite

We have used electrical noise measurement and transmission electron microscopy to study the condition of metal tips used to obtain contrast current mode scanning tunneling microscope (STM) images of pyrolytic graphite in air. Electrochemically and mechanically sharpened tips of W, W0.97Re0.03, Pt, and Pt0.8Ir0.2, which gave giant corrugations in graphite images, were found to have various forms of carbonaceous contamination on the tip. Contamination in the form of liquid hydrocarbon, graphite clumps, and carbon fibers were observed. Tunneling current noise spectra from these tips exhibited a characteristic 1/f behavior from 10 Hz to 10 kHz. Clean atomic-resolution tips formed by sputtering thin layers of metal (Ni, Au, Pt) on prefabricated tips were found to exhibit lower noise in the tunneling current and ≤1-Å amplitude corrugations. Our interpretation of the present experiment, while similar to earlier explanations that associate the giant corrugations with contamination, differs in that we attribute the large corrugations to deformation of relatively soft carbonaceous material attached to the tip, rather than to elastic deformation of the graphite itself. In addition, we have found that several of the structural artifacts observed in atomic-resolution STM images of graphite can be explained by a moiré effect caused by a double tip.