The dielectric response and electrical properties of junctions based on self-assembled monolayers (SAMs) of the form S(CH2)11X can be controlled by changing the polarizability of X (here X = H, F, Cl, Br, or I). A 1000-fold increase in the tunneling rate and a four-fold increase of the dielectric constant (ε r ) with increasing polarizability of X are found.

It has been demonstrated in experiments that charge transport through self-assembled monolayers (SAMs) of alkanethiolates shows intriguing odd−even effects when the number of methylene groups changes. Most previously reported theoretical investigations were based on semiempirical methods or largely simplified models and the quantum origin of the observed odd−even effects is still unclear. In the current study, we performed ab initio calculations for electronic and transport properties of SAM of alkanethiolates on Ag [111] surface. Extensive density functional theory (DFT) based energy minimizations of the system geometries were conducted to pinpoint the most accurate geometries amenable to experimental observations. The recently proposed dual mean field (DMF) approach that includes bias-induced nonequilibrium effects in density functionals is used to determine current−voltage characteristics. Odd−even effects are observed in both electric currents and binding energies between the SAM and the probing electrode. The significant difference between the tunneling barriers across the "top" contact of odd and even molecular junctions is revealed to be the origin of the odd−even effects in electron transport. Our calculations suggest that the odd−even effects in charge transport in the system under study occur for alkanethiolate molecules with a certain length (10 < n < 19, where n is the number of methylene groups).

Effects of the tensile strain on absorption and diffusion of hydrogen atoms on graphene have been studied by first-principles calculations. Our calculations suggested that there exists a barrier of 0.22 eV for H atom to diffuse from free space to graphene. The barrier originates from the transition of the hybridization of the H-binded carbon atom in graphene from sp2 to sp3, and is robust against the tensile strain. It was also found that, first, the in-plane diffusion of H atoms on graphene is unlikely to happen at low temperature due to the high barrier without or with strain, and second, the tensile strain along the armchair direction greatly decreases the out-plane diffusion barrier of H atoms, making it possible at low temperature. In particular, when the armchair strain is moderate (<10%), we found that the out-plane diffusion of H atoms likely to happen by diffusing through C-C bonds, and for relatively large armchair strain around 15%, the out-plane diffusion will happen though the center of the benzene ring

Electronic properties of two-dimensional 2D graphene superlattice made with partial hydrogenation were thoroughly studied via density functional tight binding approach which incorporates the tight-binding method into the density functional formalism. The 2D pattern of hydrogen atoms on graphene was found to have great effects on electronic structures of graphene superlattice. In particular, the edges of the 2D pattern, armchair or zigzag, are essential for the energy band gap opening, and the energy band gap sensitively depends on the shape, size, and the 2D periodicity of the pattern. Based on these findings, we suggested that the 2D graphene superlattice could be used in fabricating graphene quantum dots or heterojunctions without the need for cutting or etching.

Electronic structures, magnetic properties, and spin-dependent electron transport characteristics of C-doped ZnO nanowires have been investigated via first-principles method based on density functional theory and nonequilibrium techniques of Green's functions. Our calculations show that the doping of carbon atoms in a ZnO nanowire could induce strong magnetic moments in the wire, and the electronic structures as well as the magnetic properties of the system sensitively depend on partial hydrogenation. Based on these findings, we proposed a quasi-1d tunneling magnetic junction made of a partially hydrogenated C-doped ZnO nanowire, which shows a high tunneling magnetoresistance ratio, and could be the building block of a new class of spintronic devices.