Scanning tunneling microscopy and orbital-mediated tunneling spectroscopy of N,N′-dioctyl-1,8:4,5-naphthalenediimide adsorbed on highly ordered pyrolytic graphite from various solvents and in different environments

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, 0 and N interaction with HOPG, and,pi-pi 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, alpha = 67 +/- 2 degrees. 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.