Abstract:
In order to improve the performance of molecular electronics and catalysts, it is important to understand the electronic structures of adsorbed molecules or clusters on surfaces. The work presented in this thesis focuses on the characterization of the electronic properties and their relationship with geometric structures using two-photon photoemission, scanning tunneling microscopy and other surface analysis techniques. Mainly, two types of systems have been studied. One is adsorbed organic molecules (thiophene and aromatic diisocyanides) on a Au(111) surface. On the surface, phenyl diisocyanide molecules lie flat and form one-dimensional molecular chain structures after thermal treatment at 300 K. The molecular chain extends the length of the Au terraces and is composed of alternating diisocyanide molecule and Au adatom in a unit cell. Accompanying the formation of chain structures, photoemission experiments show an unusually large drop in the work function and the appearance of an unoccupied state at 3.3 eV above the Fermi level. Density functional theory calculations support the favorable energetics for diisocyanide-Auad bonding and reveal a dipole moment at the interface that results from the molecular chain structure. The dipole moment explains the observed large work function change. Given their considerable length, tunabilities and novel electronic properties, the self-assembled molecular diisocyanide structures may find application as molecular conductors in nanoelectronics. The other systems investigated are size-selected molybdenum sulfide clusters (MoxSy; x/y = 2/6, 4/6, 6/8, 7/10) deposited on a ultrathin alumina film formed on a NiAl(110) surface. The clusters are produced in a gas-phase magnetron sputtering source and mass-selected prior to being deposited on surface (soft landing). With clusters on the alumina film, the work function determined by 2PPE increases, which is attributed to electron transfer from the alumina substrate to the cluster. The change in work function versus coverage is found to be cluster dependent, but does not simply correlate with cluster size. Interfacial dipole moments derived from the work function measurements are largest for the highly symmetric Mo4S6 and Mo6S8 clusters which have no permanent dipole. These results suggest strong cluster-support interactions that result in significant interfacial charge transfer.
