Resonance enhanced scanning tunneling (REST) spectroscopy of molecular aggregates on graphite

Abstract

The effect of photoirradiation of molecular thin layers of 4,4′-azoxyanisole and Malachit Green on highly oriented pyrolytic graphite (HOPG) was investigated by scanning tunneling microscopy (STM) in the wavelength range 300–800 nm. We observed an 10–100 fold increase of the tunneling current at constant position of the STM tip above the surface or an up to 20 nm backtrack of the tip in the constant current mode when the light was resonant with an aggregate absorption. Both changes were time-dependent and mainly caused by photoexcitation of electrons to the conduction band of the molecular aggregates (photoconductivity). At higher light intensity, thermal expansion of the adsorbed molecular layer and the underlying graphite layers in the irradiated area probably also contributes to the increased signal. The increase of the tunneling current due to light absorption by graphite and the resulting thermal expansion at positions in unoccupied HOPG regions without presence of aggregates turned out to be much smaller. We propose the observed local enhancement of the tunneling current upon resonant optical excitation as a method for obtaining UV/VIS or IR optical spectra of aggregates and possibly single molecules in microscopically defined surroundings.