Scanning tunneling microscopy of molecular adsorbates at the liquid-solid interface: Functional group variations in image contrast

Recent studies have demonstrated that functionalized portions of long-chain hydrocarbons (CH3(CH2)(n)X, where X = SH, S, SS, NH2, Br, I) physisorbed on graphite at the liquid-solid interface exhibit enhanced contrast (increased tunneling current) when imaged by scanning tunneling microscopy(STM). The tunneling mechanism for such molecules can be investigated by examining the interplay between topographic and electronic structure/coupling factors. Utilizing a scanning tunneling spectroscopy methodology, changes in the STM images of two-dimensional thin films of amide-, bromide-, and sulfide-derivatized molecules have been observed as a function of applied bias voltage. For octadecanamide, the relative topographic height (increased tunneling probability) of the functional group compared to that of the hydrocarbon backbone peaks at \V\ = 1.4 V; this differs greatly from the voltage dependence obtained for docosane bromide, where the measured relative topographic height of the bromide end group remains constant for all voltages sampled. These trends are discussed in terms of the geometry of the molecular adsorbate and its electronic coupling to the substrate.