Source of Image Contrast in STM Images of Functionalized Alkanes on Graphite:  A Systematic Functional Group Approach

Abstract

A series of functionalized alkanes and/or alkyl alcohols have been prepared and imaged by scanning tunneling microscopy (STM) methods on graphite surfaces. The stability of these ordered overlayers has facilitated reproducible collection of STM images at room temperature with submolecular resolution, in most cases allowing identification of individual hydrogen atoms in the alkane chains, but in all cases allowing identification of molecular length features and other aspects of the image that can be unequivocally related to the presence of functional groups in the various molecules of concern. Functional groups imaged in this study include halides (X ) F, Cl, Br, I), amines, alcohols, nitriles, alkenes, alkynes, ethers, thioethers, and disulfides. Except for -Cl and -OH, all of the other functional groups could be distinguished from each other and from -Cl or -OH through an analysis of their STM metrics and image contrast behavior. The dominance of molecular topography in producing the STM images of alkanes and alkanols was established experimentally and also was consistent with quantum chemistry calculations. Unlike the contrast of the methylene regions of the alkyl chains, the STM contrast produced by the various functional groups was not dominated by topographic effects, indicating that variations in local electronic coupling were important in producing the observed STM images of these regions of the molecules. For molecules in which electronic effects overwhelmed topographic effects in determining the image contrast, a simple model is presented to explain the variation in the electronic coupling component that produces the contrast between the various functional groups observed in the STM images. Additionally, the bias dependence of these STM images has been investigated and the contrast vs bias behavior is related to factors involving electron transfer and hole transfer that have been identified as potentially being important in dominating the electronic coupling in molecular electron transfer processes.

Self-Assembly of Small Polycyclic Aromatic Hydrocarbons on Graphite: A Combined Scanning Tunneling Microscopy and Theoretical Approach

“…12 The versatility of STM is limited only by its inability to distinguish different chemical groups or atoms; however related techniques such as scanning tunneling spectroscopy and inelastic electron tunneling spectroscopy can in principle provide unique electronic or vibrational signatures for surface atoms and adsorbate functional groups. 12,[22][23][24][25][26][27][28][29][30] Numerous STM studies have focused on understanding the spontaneous self-assembly of molecules on surfaces, including the physisorbed monolayers formed at the liquid-graphite interface 1,[3][4][5][8][9][10]18,21,28,[31][32][33][34][35][36][37][38][39][40][41] and the vacuum-graphite interface. [23][24][25][42][43][44][45][46][47][48][49][50][51][52] Information about the factors that promote twodimensional self-assembly, such as van der Waals forces, electrostatic forces, and hydrogen-bonded...…”

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

“…However, many subtle modifications of the self-assembled structures have been observed, depending on alkane functionalization and chain length (3, 6, 10-12, 15, 17-19, 28, 35, 36, 41, 46). Specific headgroup interactions (e.g., hydrogen bonding and electrostatic forces) (11,12,17,19,28,41) and geometric subtleties (e.g., odd vs. even chain length) (31, 32, 36) also can have a profound influence on the self-assembly. The resulting set of interactions creates competing constraints, which are evident in the calculated and measured adsorption energies (47).…”

Photoemission electron microscopy of localized surface plasmons in silver nanostructures at telecommunication wavelengths

“…To further characterize the surfaces of the Ag nanoparticles, we perform scanning tunneling microscopy (STM) measurements on a similar sample as used for the PEEM measurements. First, the Au film is imaged ( [34][35][36]. On the nanoparticles though, the STM investigations indicate a clean, metallic surface.…”