Comparison of ultraviolet photoelectron spectroscopy and scanning tunneling spectroscopy measurements on highly ordered ultrathin films of hexa-peri-hexabenzocoronene on Au(111) - art. no. 205409

Hexa-peri-hexabenzocoronene (C42H18, HBC) films adsorbed on the Au(111) sur face 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 occupie d states, if the STS is performed at appropriate parameters. Additionally, STS gives an immediate insight into the unoccupied states. The highest occu pied state of multilayer films is found 1.3-1.4 eV below the Fermi levels a nd 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 alignm ent does not occur at the HBC-Au interface. Compared to UPS, the voltage ra nge of the STS measurements is limited to prevent sample damage.