Comparison of ultraviolet photoelectron spectroscopy and scanning tunneling spectroscopy measurements on highly ordered ultrathin films of hexa-<i>peri</i>-hexabenzocoronene on Au(111)

Abstract

Hexa-peri-hexabenzocoronene $({\mathrm{C}}_{42}{\mathrm{H}}_{18},$ HBC) films adsorbed on the Au(111) surface were investigated by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). The results show that both methods give comparable results for the electronic structure of the occupied states, if the STS is performed at appropriate parameters. Additionally, STS gives an immediate insight into the unoccupied states. The highest occupied state of multilayer films is found 1.3--1.4 eV below the Fermi levels and the lowest unoccupied state 1.8 eV above the Fermi level. The resulting transport gap is compared to optical absorption measurements. Work-function changes of -0.8 eV indicate an interface dipole, i.e., vacuum level alignment does not occur at the HBC-Au interface. Compared to UPS, the voltage range of the STS measurements is limited to prevent sample damage.