Spatial variations of current fluctuations in a scanning tunneling microscope

Spatial maps of the power spectral density of the high-frequency fluctuations of the tunnelling current have been obtained with a UHV-STM for two different systems: O2/Cu(1 0 0) and large molecules (blue Landers) on Cu(1 0 0). These “noise” images are strongly correlated with simultaneously acquired “topographic” images down to molecular resolution. The different explanations which are discussed indicate that this method could bring useful information on localized dynamical surface phenomena, not accessible in the usual imaging STM modes, which are intrinsically slow.