We have studied the adsorption of benzenethiol molecules on the Au͑111͒ surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61°from the surface normal. As for the self-assembled monolayer ͑SAM͒ structures, the (2)ϫ))R30°herringbone structure is stabilized against the ()ϫ))R30°structure by large steric relaxation. In the most stable (2)ϫ))R30°S AM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21°, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.

No abstract

Knowing how the contact geometry influences the conductance of a molecular wire junction requires both a precise determination of the molecule/metallic-electrode interface structure and an evaluation of the conductance for different contact geometries with a fair accuracy. With a greatly improved method to solve the Lippmann-Schwinger equation, we are able to include at least one atomic layer of each electrode into the extended molecule. The artificial effect of the jellium model used for the electrodes is therefore significantly reduced. Our first-principles calculations on the transport properties of a single benzene dithiolate molecule sandwiched between Au(111) surfaces show that the transmission of the bridge site contact, which is the most stable adsorption configuration in equilibrium, displays different features from those of other configurations, and that the inclusion of the surface layers of Au electrodes into the extended molecule shifts and broadens the transmission peaks due to a stronger and more realistic S-Au bonding. We discuss the geometry dependence of the transport properties by analyzing the density of states of the molecular orbitals.

The transport properties of a biphenyl dithiol (BPD) molecule sandwiched between two gold electrodes are studied using the nonequilibrium Green's function method based on the density functional theory. In particular, their dependence on the dihedral angle (phi=90 degrees -180 degrees ) between two phenyl rings is investigated. While the dihedral-angle dependence of the density of states projected on the BPD molecular orbitals is small, the transport properties change dramatically with phi. The transmission at the Fermi energy exhibits a minimum at phi=90.0 degrees and greatly increases with phi. The ratio of the maximum obtained at phi=180 degrees to the minimum exceeds 100. As an application of this characteristic transport behavior, a BPD molecule functionalized with NH(2) and NO(2) groups is considered. It is found that this molecule works as a nanorectifier.

The adsorption and diffusion of Si atoms on the monohydride terminated Si͑001͒-͑2 3 1͒ surface is investigated using first-principles total-energy calculations. We find that the Si adatom spontaneously segregates one H atom from a surface Si dimer during adsorption, and further captures the remaining H atom of the same Si dimer during surface migration, leading to the most stable adsorption geometry. The migration of the Si adatom is assisted by the mobility of H atoms, being reduced compared with that on the bare Si surface. It is suggested that the reduction of Si diffusion has disruptive effects on Si homoepitaxy. [S0031-9007(97)04691-7]