Guo-Hui Ding

Fabrication of controllable free-standing ultrathin porous alumina membranes

We present detailed information on the fabrication of free-standing ultrathin porous alumina membranes (PAMs) with controllable thickness of 100–1000 nm. The mechanism of the ultrathin PAM formation has been revealed by a combination study of current–time characteristics and microstructure images. At the beginning of the anodization, V-shape nanopores can be observed due to the alumina formation in both the sidewalls and the barrier layers. As a result of the applied electric field effect, part of the alumina in the sidewalls is dissolved, the nanopores gradually become regularly U-shape and finally grow steadily. Ultrathin PAMs with controllable thickness and morphology have been realized by changing the anodization time and the current density. Furthermore, an improved method has been demonstrated to obtain free-standing ultrathin PAMs by removing unoxided aluminium through a nontoxic mixture solution of saturated CuSO4 and HCl.

Synthesis of ordered large-scale ZnO nanopore arrays

An effective approach is demonstrated for growing ordered large-scale ZnO nanopore arrays through radio-frequency magnetron sputtering deposition on porous alumina membranes (PAMs). The realization of highly ordered hexagonal ZnO nanopore arrays benefits from the unique properties of ZnO (hexagonal structure, polar surfaces, and preferable growth directions) and PAMs (controllable hexagonal nanopores and localized negative charges). Further evidence has been shown through the effects of nanorod size and thermal treatment of PAMs on the yielded morphology of ZnO nanopore arrays. This approach opens the possibility of creating regular semiconducting nanopore arrays for the application of filters, sensors, and templates.

Integration of single-crystalline nanocolumns into highly ordered nanopore arrays

The arrangement of nanostructures into desired well-ordered architectures is crucial for the realization of functional nanodevices and has been the focus of current nanotechnology. Existing physical and chemical approaches have the ability to assemble nanostructures, but it is still a challenge to arrange basic nanostructures into a highly ordered designed pattern. Here, we report a novel method to integrate tin-doped indium oxide single-crystalline nanocolumns into highly ordered two-dimensional nanopore patterns through radio-frequency magnetron sputtering by the aid of porous alumina membranes (PAMs). We have further demonstrated that the morphology of the assembled nanopore arrays is controllable by adjusting either the PAM configurations or sputtering conditions. Our present method provides the possibility of a general approach for nanounit integration, and these assembled regular nanopore arrays pave the way for the application of novel filters and sensors.

Spin-polarized electron transport through graphene nanoribbon with zigzag edges

We investigate the electron transport through a graphene nanoribbon (GNR) junction with zigzag edges. It is shown that the edge state and the magnetization properties of a GNR greatly influence its electron transport properties. By applying sufficient back-gate voltages to the lead parts of a zigzag GNR, the magnetization of the edges is quenched in the leadings parts, but is preserved in the center part of the junction. Without applying external transverse electric field, we show that, by preparing the junction in a ferromagnetic state, a strongly spin-polarized electron current can be obtained even though the incident electron current has no spin polarization.

Indium oxide 'rods in dots' nanostructures

The authors have demonstrated a special indium oxide (In2O3) “rods in dots” nanostructure with high nanorod sheet density of over 1012cm−2. The approach has been realized through depositing controllable individual In2O3 nanorods in both number and shape within a single porous alumina membrane (PAM) nanochannel under radio frequency magnetron sputtering. The authors further discussed in detail effects of the PAM configurations (pore diameter and thickness) and sputtering conditions (substrate temperature and argon pressure) on the formation of the In2O3 nanostructure.The authors have demonstrated a special indium oxide (In2O3) “rods in dots” nanostructure with high nanorod sheet density of over 1012cm−2. The approach has been realized through depositing controllable individual In2O3 nanorods in both number and shape within a single porous alumina membrane (PAM) nanochannel under radio frequency magnetron sputtering. The authors further discussed in detail effects of the PAM configurations (pore diameter and thickness) and sputtering conditions (substrate temperature and argon pressure) on the formation of the In2O3 nanostructure.

Negative differential conductance and hysteretic current switching of benzene molecular junction in a transverse electric field

We study charge transport through single benzene molecular junction (BMJ) directly sandwiched between two platinum electrodes by using a tight-binding model and the non-equilibrium Greens function approach. Pronounced negative differential conductance is observed at finite bias voltage, resulting from charge redistribution in BMJ and a Coulomb blockade effect at the interface of molecule-electrode contacts. In the presence of a transverse electric field, hysteretic switching behavior and large spin-polarization of current are obtained, indicating the potential application of BMJ for acting as a nanoscale current modulator or spintronic molecular device.

The enhanced optical conductivity for zigzag-edge graphene nanoribbons with applied gate voltage

We study the optical absorption properties of zigzag-edge graphene nanoribbons (ZGNRs) taking into account the Coulomb interaction effect in the Hartree-Fock approximation. The optical selection rules for the incident light polarized along the longitudinal and transverse directions are investigated. We demonstrate that the excitations from the edge states are essential for the optical properties of ZGNRs in the neutral case. With the chemical potential shifting away from the Dirac point, the optical conductivity is drastically enhanced in the low frequency region for the transverse polarized incident light.

Spin helix of magnetic impurities in two-dimensional helical metal

The surface state of a topological insulator dubbed as helical metal is a unique metallic system, which exhibits one single Dirac cone and spin-momentum locking. We show that the behaviors of magnetic impurities embedded in this kind of surface states manifest the uniqueness of helical metals among other conventional Dirac materials such as graphene. We find there is a significant Dzyaloshinskii-Moriya (DM) term among the effective interactions between impurity spins mediated by the conduction electrons. For a chain of impurity spins, we show that such a DM interaction gives rise to a single-handed spin helix state, the handedness of which is locked with the sign of the Fermi velocity of the emergent Dirac fermions. We also point out that the polarization of impurity spins can be controlled via electric voltage for dilute magnetic-impurity concentration.

The optical conductivity of bilayer zigzag-edge graphene nanoribbons with external transverse electric fields

The optical absorption properties of bilayer zigzag-edge graphene nanoribbons (BL-ZGNRs) with external transverse electric fields are investigated by taking into account the Coulomb interaction effect in the Hartree-Fock approximation. We study the phase transitions of BL-ZGNRs induced by external electric fields and also the optical selection rules for the incident light polarized along the longitudinal and transverse directions. We find that the excitations from the edge states are crucial for the optical properties of BL-ZGNRs in the antiferromagnetic phase. We show that the low energy part of the optical absorption can be modulated by the external transverse electric field, and there is a broad band low frequency absorption enhancement for the transverse-polarized incident light in the charge-polarized state of BL-ZGNRs.

Transient currents of a single molecular junction with a vibrational mode.

By using a propagation scheme for current matrices and an auxiliary mode expansion method, we investigate the transient dynamics of a single molecular junction coupled with a vibrational mode. Our approach is based on the spinless Anderson-Holstein model and the dressed tunnelling approximation for the electronic self-energy in the polaronic regime. The time-dependent currents after a sudden switching on the tunnelling to leads, an abrupt upward step bias pulse and a step potential on the quantum dot are calculated. We show that the strong electron-phonon interaction greatly influences the nonlinear response properties of the system, and gives rise to interesting characteristics on the time traces of transient currents.