We have simultaneously imaged the chemically bound head groups and exposed tail groups in bicomponent alkanethiolate self-assembled monolayers on Au{111} with molecular resolution. This has enabled us to resolve the controversy of scanning tunneling microscopy image interpretation and to measure the molecular polar tilt and azimuthal angles. Our local measurements demonstrate that ordered domains with different superstructures also have varied buried sulfur head group structures.

We present an atomic-scale study of substituent effects in the Ullmann coupling reaction on Cu{111} using low-temperature scanning tunneling microscopy and spectroscopy. We have observed fluorophenyl intermediates and phenyl intermediates as well as biphenyl products on Cu{111} after exposure to 4-fluoro-1-bromobenzene (p-FC(6)H(4)Br) and bromobenzene (C(6)H(5)Br), respectively. When p-FC(6)H(4)Br dissociatively chemisorbs at 298 K on Cu{111}, the relatively weakly bound Br dissociates, and fluorophenyl intermediates are formed. These intermediates couple to form 4,4'-difluorobiphenyl and desorb at temperatures below 370 K. However, by cooling the substrate to low temperature (4 K), we have observed unreacted fluorophenyl intermediates distributed randomly on terraces and at step edges of the Cu{111} surface. Alternatively, at similar coverages of C(6)H(5)Br, we have observed biphenyl distributed on terraces and step edges. In each case, Br adatoms were randomly distributed on the surface. Chemical identification of fluorophenyl and phenyl intermediates and biphenyl products was achieved by vibrational spectroscopy via inelastic tunneling spectroscopy. The strongest vibrational mode in the phenyl species disappears when the tilted intermediates couple to form biphenyl products. We infer that the surface normal component of the dipole moment is important in determining the transition strength in inelastic electron tunneling spectroscopy.

We report diffusion in the tunneling spectra of isolated, ligand-stabilized undecagold (Au11) clusters immobilized by attachment to alpha,omega-alkanedithiolate tethers inserted into alkanethiolate self-assembled monolayers. We use scanning tunneling microscopy and spectroscopy at cryogenic (UHV, 4 K) conditions to measure these clusters' conductance with complete control of their chemical and physical environment; additionally, thermal broadening of their electronic states as well as their mobility is minimized. At low temperature, the Au11 clusters demonstrate Coulomb blockade behavior, with zero-conductance gaps resulting from quantum size effects. Surprisingly, chemically identical and even single particles produced different families of tunneling spectra, comparable to previous results for heterogeneous distributions of particles. We hypothesize that, while these particles are chemically attached to the surface of the SAM for measurement, these assemblies may still be sufficiently dynamic to affect their transport properties significantly.

Long-range electronic interactions between Br adatom islands, which are formed at approximately 600 K, on Cu(111) are mediated by substrate surface-state electrons at that elevated temperature. Using scanning tunneling microscopy at 4 K, we have quantified nearest neighbor island separations and found favored spacings to be half-multiples of the Fermi wavelength of Cu(111). The strong interaction potential and decay length of the interisland interactions are discussed in terms of the interaction of Br with the substrate surface state.

Small aluminum clusters (<30 atoms) have been the subject of extensive study, demonstrating markedly different properties as even a single atom is added or removed. Successfully depositing and characterizing mass-selected clusters are the next steps in producing precise surfaces or cluster-assembled materials that demonstrate properties that differ from those of bulk materials. This proves to be difficult due to the reactivity of these all-metal clusters and their susceptibility to agglomerate in bulk form. In the present study, we have successfully deposited Al17 − onto self-assembled monolayers using reactivity previously characterized in the gas phase.