Scanning tunneling microscopy and orbital-mediated tunneling spectroscopy study of 1,5-di(octyloxy)anthracene 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 1,5-di(octyloxy)anthracene (15DA) adsorbed on highly ordered pyrolytic graphite (HOPG). 15DA forms well-ordered monolayers either at the interface between HOPG and 1-phenyloctane or at the HOPG-air or HOPG-vacuum interface when spin doped from octane or dichloromethane. Octyl chain interdigitation and planar adsorption of the anthracene ring combine to produce structures which are stable independent of the adsorption method. The observed unit cell has a = 1.76 +/- 0.06 nm, b = 1.07 +/- 0.06 nm, and alpha = 77 +/- 3 degrees. Molecular orbital calculations are combined with bias-dependent imaging and OMTS results to show that the highest-occupied molecular orbital (HOMO) dominates the tunneling process over the voltage range from -2.5 to +1.0 V. The HOMO of 15DA on HOPG is found to occur at -7.0 eV below the vacuum level, in good agreement with previous UPS studies and with our calculations for the free molecule. We also report the first synthesis of 15DA.