Scanning Tunneling Microscopy of Molecular Adsorbates at the Liquid−Solid Interface:  Functional Group Variations in Image Contrast

Abstract

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.

Ultra-high vacuum scanning tunneling microscopy and theoretical studies of 1-halohexane monolayers on graphite

“…Although a direct comparison of different alkyl derivatives is generally complicated by the presence of both electronic and geometric factors, the image contrast observed in the present work is in agreement with previous observations of bright bromine and iodine substituents (17,41,61) and dark fluorine substituents observed at liquid-solid interfaces (17,62).…”

“…STM images recorded under ambient conditions showed that long 1-bromoalkanes formed self-assembled monolayers at the liquid͞solid interface, where adjacent lamellae are in a head-to-head configuration (as they are in the UHV STM images of the present work) but with a rectangular structure, ϭ 90°( unlike the UHV data where Х 60°) (17,41,61). Energy minimizations based on this initial configuration were found to lead inevitably to the angled structure shown in Fig.…”

“…The present study has shown that a completely ordered and regular angled head-to-head structure results for the 1-chlorohexane, 1-bromohexane, and 1-iodohexane monolayers on graphite under UHV conditions, and a more disordered but tightly packed structure is observed for 1-fluorohexane. In contrast, nearly rectangular structures have been observed in ambient STM images of 1-bromoalkane monolayers at the liquid͞solid interface (i.e., in the presence of solvent) (17,41,61). Further studies are needed to elucidate the ability of solvent to shift the monolayer structure by tilting the balance among these interactions.…”

Electrical Conformational Bistability of Dimesogen Molecules with a Molecular Chord Structure

“…[9][10][11] These studies are complementary to our observations, which show little observable dependence on bias polarity of the mesogen unit in both specimens examined, while the contrast variation is solely associated with the methylene portion. It is therefore reasonable to conclude that the contrast of the mesogen unit does not contribute to the observed contrast reversal of the methylene units.…”

“…[9][10][11] The variation in vertical contrast of the functional groups of various alkane derivatives relative to the methylene units was affected by both topographic and electronic factors, with ionization potential (IP) accounting for the observed contrast. A lower IP leads to higher contrast in the STM images.…”