T. M. Wallis

Single molecule vibrational and electronic analyses of the formation of inorganic complexes: CO bonding to Au and Ag atoms on NiAl(110)

The bonding of CO molecules to single Au and Ag atoms on a NiAl surface was studied with a scanning tunneling microscope. A peak at 35 meV in the vibrational spectra of individual AuCO molecules is assigned to the hindered rotational mode. Vibrational spectra of individual AgCO molecules displayed peaks at 26 meV and 266 meV. These peaks are assigned to the hindered rotational mode and the C–O stretch mode, respectively. Scanning tunneling spectroscopy measurements show an unoccupied electronic resonance about 2.0 eV above the Fermi energy on single Au and Ag atoms. This resonance is quenched when monocarbonyls are formed due to electron donation from the CO ligand into the unoccupied electronic resonance.

Vibrational spectroscopy and imaging of single molecules: Bonding of CO to single palladium atoms on NiAl(110)

A low temperature scanning tunneling microscope (STM) was used to probe the bonding of CO on NiAl(110) and the formation of PdCO and Pd(CO)2 at the single molecule level. While tilting of the two ligands is expected for Pd(CO)2, the observed bending of the CO molecule in PdCO is surprising. The combination of real space topographic imaging and spatially resolved vibrational spectroscopy in a STM provides a unique method in revealing internal bonding configuration and vibrational properties of individual metal carbonyls.

Electronic properties of artificial Au chains with individual Pd impurities

Artificial Au atomic chains with individual Pd impurities were assembled from single metal atoms with a scanning tunneling microscope on a NiAl(110) surface. Scanning tunneling spectroscopy (STS) revealed an electronic resonance 2.15 eV above the Fermi energy localized within 4 A of single Pd atom impurities and two electronic resonances 2.25 eV and 2.95 eV above the Fermi energy localized within 8 A of Pd dimer impurities. The emergence of these localized resonances was studied by STS at each stage of the atom-by-atom assembly. Additionally, conductance images of the chains revealed delocalized electronic density oscillations in the pure Au segments of the chains.

From Single Atoms to One-Dimensional Solids: Artificial Gold Chains on NiAl(110)

The tip of a scanning tunneling microscope (STM) was used to assemble well-defined atomic chains from single Au atoms on a NiAl(110) surface. The electronic properties of the chains are determined by a one-dimensional free-electron like band, gradually developing from a discrete resonance state in Au monomers. The adsorption of a single CO molecule on the chain strongly perturbs the electronic structure by suppressing the interaction between neighboring chain atoms. Vibrational properties of the Au/CO adsorption system were obtained from inelastic electron tunneling spectroscopy with the STM, yielding the energy of the hindered rotational mode for AunCO. The experiments demonstrate the strength of the STM to explore properties of matter in the transition range between single atoms and bulk solids.