Scanning Tunneling Microscopy and Orbital-Mediated Tunneling Spectroscopy of N,N'-Dioctyl-1,8:4,5-naphthaIenediimide Adsorbed on Highly Ordered Pyrolytic Graphite from Various Solvents and in Different Environments

摘要

Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (OMTS) are reported for N,N'-dioctyl-1,8:4,5-naphthalenediimide (diimide) adsorbed on highly ordered pyrolytic graphite (HOPG). The diimide forms well ordered monolayers either at the interface between HOPG and several phenylalkanes, or at the HOPG—air or HOPG—vacuum interface when adsorbed from toluene. Planar adsorption of the diimide ring on HOPG is observed. Hydrogen bonding, O and N interaction with HOPG, and π—π interactions appear to be the primary drivers for determining the monolayer structure which is stable and independent of the adsorption method. This is an unusual example since most alkane-substituted systems studied to date rely on alkane chain interactions (with HOPG and interdigitation) to drive the adsorbate structure on graphite. The observed unit cell has a = 2.0 ± 0.2 nm, b = 1.95 ± 0.2 nm, α = 67 ±2°. The STM imaging is highly bias dependent and appears to be controlled (in the ±2 V bias region) by an unoccupied orbital. Orbital-mediated tunneling spectra reveal a single strong electron affinity band near 3.5 eV below the vacuum level.