Anion or cation molecular layers: Two distinct surfaces in scanning tunneling microscopy images from the organic salt (ET)2KHg(SCN)4

Scanning tunneling micrographs of cleaved (ET${)}_2$KHg(SCN${)}_4$ crystals [where ET is bi(ethylenedithio)tetrathiafulvalene] indicate two distinct types of surfaces. Scans of type I surfaces show a 2D lattice and packing arrangement characteristic of the ET molecular layer; these features are relatively insensitive to reversal of the tip polarity. By contrast, scans of type II surfaces show maxima consistent with either the positions of the K-Hg atoms or with the SCN groups, depending upon the polarity of the tip. This is the first demonstration that a cleaved crystal of this material can present at least two possible surfaces: a complete metallic cation layer, or a complete insulating anion layer.