Atomic scale understanding of nanostructures in a double barrier tunneling junction: Scanning tunneling microscopy of alkali doped buckminsterfullerenes on partially oxidized nickel aluminide(110)

The electronic and vibrational properties of single 4,7,12,15-tetrakis[2,2]paracyclophane molecules, single C60 and C70 molecules, C60 monolayer, and alkali-doped C60 adsorbed on the partially oxidized NiAl(110) surface have been studied by scanning tunneling microscopy and spectroscopy. All experiments were carried out with a homebuilt variable temperature STM housed in an ultrahigh vacuum chamber. The dI/dV and d2I/dV2 signals were acquired with a lock-in amplifier.; Spatially resolved inelastic tunneling microscopy has revealed the spatial distribution of vibrational intensity in different functional groups of the DMAS-PCP molecule at various vibrational modes. The ab initio calculations indicate that the electron-phonon coupling matrix element is a good value to estimate the relative intensity of the vibrational signal in STM-IETS measurements. Due to the unique double barrier tunneling junction configuration consisting of the vacuum gap, the molecules or the nanostructures, and the thin oxide film, interesting phenomena such as vibronic progressions and the Coulomb charging effect have been observed in the conductance measurements. The observation of these phenomena highlights the interplay of the tunneling electron with the vibrational motions of the molecule during the resonant tunneling process and the influence of the electrostatic energy to the tunneling event when the size of the structure is reduced to the nano scale. A distance dependent interaction between alkali adatoms mediated by C60 monolayer was observed. This interaction is characterized by the energy splitting of a single unoccupied resonance of the alkali atoms into the bonding and antibonding states. Due to the indirect interaction through C60 molecules, the energy splitting decays much slower than that in the gas phase.; These results offer valuable knowledge on the electron-phonon coupling within a molecule enhanced by surface phonons, molecular conductance, the role of impurities, and the charging effect in the nano scale electron transport measurements. These studies are of great importance for both the fundamental physics understanding and the future industrial applications.