Modeling optical properties of silicon clusters by first principles: From a few atoms to large nanocrystals.

Time dependent density functional tight binding (TDDFTB) method is implemented with sparse matrix techniques and improved parallelization algorithms. The method is employed to calculate the optical properties of various Si nanocrystals (NCs). The calculated light absorption spectra of small Si NCs from TDDFTB were found to be comparable with many body perturbation methods utilizing planewave basis sets. For large Si NCs (more than a thousand atoms) that are beyond the reach of conventional approaches, the TDDFTB method is able to produce reasonable results that are consistent with prior experiments. We also employed the method to study the effects of surface chemistry on the optical properties of large Si NCs. We learned that the optical properties of Si NCs can be manipulated with small molecule passivations such as methyl, hydroxyl, amino, and fluorine. In general, the shifts and profiles in the absorption spectra can be tuned with suitably chosen passivants.

Nanowelding of carbon nanotube-metal contacts: an effective way to control the Schottky barrier and performance of carbon nanotube based field effect transistors.

Schottky barriers formed at carbon nanotube (CNT)-metal contacts have been well known to be crucial for the performance of CNT based field effect transistors (FETs). Through first principles calculations we show that a nanowelding process can drastically reduce the Schottky barriers at CNT-metal interfaces, resulting in significantly improved conductivity of CNT-based FETs. The proposed nanowelding can be realized by either laser local heating or a heating process via a controllable pulse current. Results presented in this paper may have great implications in future design and applications of CNT-based electronics.

Sensing with superconducting point contacts.

Superconducting point contacts have been used for measuring magnetic polarizations, identifying magnetic impurities, electronic structures, and even the vibrational modes of small molecules. Due to intrinsically small energy scale in the subgap structures of the supercurrent determined by the size of the superconducting energy gap, superconductors provide ultrahigh sensitivities for high resolution spectroscopies. The so-called Andreev reflection process between normal metal and superconductor carries complex and rich information which can be utilized as powerful sensor when fully exploited. In this review, we would discuss recent experimental and theoretical developments in the supercurrent transport through superconducting point contacts and their relevance to sensing applications, and we would highlight their current issues and potentials. A true utilization of the method based on Andreev reflection analysis opens up possibilities for a new class of ultrasensitive sensors.

C-doped ZnO nanowires: electronic structures, magnetic properties, and a possible spintronic device.

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.

Density Functional Theory for Steady-State Nonequilibrium Molecular Junctions.

We present a density functional theory (DFT) for steady-state nonequilibrium quantum systems such as molecular junctions under a finite bias. Based on the steady-state nonequilibrium statistics that maps nonequilibrium to an effective equilibrium, we show that ground-state DFT (GS-DFT) is not applicable in this case and two densities, the total electron density and the density of current-carrying electrons, are needed to uniquely determine the properties of the corresponding nonequilibrium system. A self-consistent mean-field approach based on two densities is then derived. The theory is implemented into SIESTA computational package and applied to study nonequilibrium electronic/transport properties of a realistic carbon-nanotube (CNT)/Benzene junction. Results obtained from our steady-state DFT (SS-DFT) are compared with those of conventional GS-DFT based transport calculations. We show that SS-DFT yields energetically more stable nonequilibrium steady state, predicts significantly lower electric current, and is able to produce correct electronic structures in local equilibrium under a limiting case.

Tuning the Tunneling Rate and Dielectric Response of SAM-Based Junctions via a Single Polarizable Atom.

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.

Massless Dirac fermions in graphene under an external periodic magnetic field.

By solving the two-component spinor equation for massless Dirac fermions, we show that graphene under a periodic external magnetic field exhibits a unique energy spectrum. At low energies, Dirac fermions are localized inside the magnetic region with discrete Landau energy levels, while at higher energies, Dirac fermions are mainly found in non-magnetic regions with continuous energy bands originating from wavefunctions analogous to particle-in-box states of electrons. These findings offer a new methodology for the control and tuning of massless Dirac fermions in graphene.