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Wenhui Duan

Publications and source records attributed to Wenhui Duan.

At least 199 records · Page 11Linked to original sources
arXiv2010

Beryllium-dihydrogen complexes on nanostructures

Using the pseudopotential density functional method, we find that a Be atom on a nanostructure with H2 molecules forms a Be-dihydrogen complex through the hybridization of the Be s or p orbits with the H2 sigma orbits and the binding energy of the H2 molecules is in the range of ~0.3 - 0.8 eV/H2. We also study Be-dihydrogen complexes on various nanostructures and demonstrate the feasibility of the application of the complexes to a hydrogen storage medium that operates near room temperature and ambient pressure.

cond-mat.mtrl-sci↗
arXiv2009

Electronic and Magnetic Properties of Partially-Open Carbon Nanotubes

On the basis of the spin-polarized density functional theory calculations, we demonstrate that partially-open carbon nanotubes (CNTs) observed in recent experiments have rich electronic and magnetic properties which depend on the degree of the opening. A partially-open armchair CNT is converted from a metal to a semiconductor, and then to a spin-polarized semiconductor by increasing the length of the opening on the wall. Spin-polarized states become increasingly more stable than nonmagnetic states as the length of the opening is further increased. In addition, external electric fields or chemical modifications are usable to control the electronic and magnetic properties of the system. We show that half-metallicity may be achieved and the spin current may be controlled by external electric fields or by asymmetric functionalization of the edges of the opening. Our findings suggest that partially-open CNTs may offer unique opportunities for the future development of nanoscale electronics and spintronics.

cond-mat.mtrl-sci↗
arXiv2009

Intrinsic anisotropy of thermal conductance in graphene nanoribbons

Thermal conductance of graphene nanoribbons (GNRs) with the width varying from 0.5 to 35 nm is systematically investigated using nonequilibrium Green's function method. Anisotropic thermal conductance is observed with the room temperature thermal conductance of zigzag GNRs up to ~ 30% larger than that of armchair GNRs. At room temperature, the anisotropy is found to disappear until the width is larger than 100 nm. This intrinsic anisotropy originate from different boundary condition at ribbon edges, and can be used to tune thermal conductance, which have important implications for the applications of GNRs in nanoelectronics and thermoelectricity.

cond-mat.mes-hall↗
arXiv2008

Nonequilibrium Green's function method for phonon-phonon interaction and ballistic-diffusive thermal transport

Phonon-phonon interaction is systematically studied by nonequilibrium Green's function (NEGF) formulism in momentum space at finite temperatures. Within the quasi-particle approximation, phonon frequency shift and lifetime are obtained from the retarded self-energy. The lowest order NEGF provides the same phonon lifetime as Fermi's golden rule. Thermal conductance is predicted by the Landauer formula with a phenomenological transmission function. The main advantage of our method is that it covers both ballistic and diffusive limits and thermal conductance of different system sizes can be easily obtained once the mode-dependent phonon mean free path is calculated by NEGF. As an illustration, the method is applied to two one-dimensional atom chain models (the FPU-βmodel and the ϕ^4 model) with an additional harmonic on-site potential. The obtained thermal conductance is compared with that from a quasi-classical molecular dynamics method. The harmonic on-site potential is shown to remove the divergence of thermal conductivity in the FPU-βmodel.

cond-mat.mes-hall↗
arXiv2008

Prediction of Half Metallicity along the Edge of Boron Nitride Zigzag Nanoribbons

First-principles calculations reveal half metallicity in zigzag boron nitride (BN) nanoribbons (ZBNNRs). When the B edge, but not the N edge, of the ZBNNR is passivated, despite being a pure $sp$-electron system, the ribbon shows a giant spin splitting. The electrons at the Fermi level are 100% spin polarized with a half-metal gap of 0.38 eV and its conductivity is dominated by metallic single-spin states. The two states across at the Dirac point have different molecular origins, which signals a switch of carrier velocity. The ZBNNR should be a good potential candidate for widegap spintronics.

cond-mat.mtrl-sci↗
arXiv2008

Adsorption of gas molecules on graphene nanoribbons and its implication for nano-scale molecule sensor

We have studied the adsorption of gas molecules (CO, NO, NO2, O2, N2, CO2, and NH3) on graphene nanoribbons (GNRs) using first principles methods. The adsorption geometries, adsorption energies, charge transfer, and electronic band structures are obtained. We find that the electronic and transport properties of the GNR with armchair-shaped edges are sensitive to the adsorption of NH3 and the system exhibits n type semiconducting behavior after NH3 adsorption. Other gas molecules have little effect on modifying the conductance of GNRs. Quantum transport calculations further indicate that NH3 molecules can be detected out of these gas molecules by GNR based sensor.

cond-mat.mtrl-sci↗
arXiv2008

Suppression of spin-polarization in graphene nanoribbon by edge defect and impurity

We investigate the effect of edge defects (vacancies) and impurities (substitutional dopants) on the robustness of spin-polarization in graphene nanoribbons (GNRs) with zigzag edges, using density-functional-theory calculations. We found that the stability of the spin state and its magnetic moments decrease continuously with increasing concentration of defects or impurities. The system generally becomes non-magnetic at the concentration of one edge defect (impurity) per 10 angstrom. The spin suppression is shown to be caused by reduction and removal of edge states at the Fermi energy. Our analysis implies an important criterion on the GNR samples for spintronics applications.

cond-mat.mtrl-sci↗
arXiv2007

Making a field effect transistor on a single graphene nanoribbon by selective doping

Using first-principle electronic structure calculations, we show a metal- semiconductor transition of a metallic graphene nanoribbon with zigzag edges induced by substitutional doping of Nitrogen or Boron atoms at the edges. A field effect transistor consisting of a metal-semiconductor-metal junction can then be constructed by selective doping of the ribbon edges. The current-voltage characteristics of such a prototype device is determined by the first-principle quantum transport calculations.

cond-mat.mtrl-sci↗
arXiv2005

Modulating transmission properties of nanoscale transistors by dipole effects near contacts

We theoretically demonstrate that a dipole layer on the electrode can modulate the transmission properties of nanoscale devices by influencing the contact properties, through first principles simulations on carbon nanotube based field effect transistors. The dipole layer is realized by potassium adsorption on Au electrodes, which shifts the electrostatic potential at the near contact region significantly. The dipoles parallel to the direction of the bias voltage may act as a supplement to the effect of gate voltages, while the perpendicular dipoles may modify the interface barrier of the contacts.

cond-mat.mtrl-sci↗
arXiv2005

Selective adsorption of first-row atoms on boron nitride nanotubes: the effect of localized states

First-principles calculations reveal that the adsorption of representative first-row atoms with different electronegativity, such as lithium (Li), carbon (C) and fluorine (F), on zigzag single-walled boron nitride nanotubes (BNNTs) exhibits surprising selectivity. The adsorption energy and adsorption site are dependent upon the chemical activity of adsorbate with respect to the B and N atoms in the host tube. In detail, the F atom prefers to be adsorbed on the top of the B atom, the C atom is energetically favorable to stay on the bridge site which is perpendicular to the tube-axis, and the Li atom hardly adheres to the tube (an endothermal reaction). The adsorption behavior of these three types of elements on BNNTs is elucidated based on the frontier molecular orbital theory. In addition, the mechanism of modification of electronic structures of BNNTs by adsorption is probed, and a feasible approach is proposed to tailor the electronic properties of BNNTs.

cond-mat.mtrl-sci↗
arXiv2004

Unusual vortex structure in ultrathin Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ films

Using a first-principles-based approach, we determine the ferroelectric pattern in PbZr$_{0.5}$Ti$_{0.5}$O$_3$ ultrathin film. It is found that vortex stripes are formed in the system and they are responsible for the 180$^\circ$ domains observed. When a local external field is exerted, the vortex stripe transforms into the vortex loop structure, which leads to the formation of a smaller domain with the polarization antiparallel to the field in the center of the field region. This may provide a convenient way to manipulate nanodomains in thin films.

cond-mat.mtrl-sci↗
arXiv2004

Ferroelectricity in Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$ thin films: critical thickness and 180$^\circ$ stripe domains

The ferroelectric properties of disorder Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$ thin films are investigated with Monte Carlo simulations on the basis of a first-principles-derived Hamitonian. It is found that there exists a critical thickness of about three unit cells ($\sim$12 Å) below which the ferroelectricity disappears under the condition that the in-plane polarizations are suppressed by sufficient clamping effect. Above the critical thickness, periodic $180^{\circ}$ stripe domains with out-of-plane polarizations are formed in the systems in order to minimize the energy of the depolarizaing field. The stripe period increases with increasing film thickness. The microscopic mechanism responsible for these phenomena is discussed.

cond-mat.mtrl-sci↗
arXiv2004

Structural Trends Interpretation of the Metal-to-Semiconductor Transition in Deformed Carbon Nanotubes

Two mechanisms that drive metal-to-semiconductor transitions in single-walled carbon nanotubes are theoretically analyzed through a simple tight-binding model. By considering simple structural trends, the results demonstrate that metal-to-semiconductor transitions can be induced more readily in metallic zigzag nanotubes than in armchair nanotubes. Furthermore, it is shown that both mechanisms have the effect of making the two originally equivalent sublattices physically distinguishable.

cond-mat.mtrl-sci↗
arXiv2003

Effects of Finite Deformed Length in Carbon Nanotubes

The effect of finite deformed length is demonstrated by squashing an armchair (10,10) single-walled carbon nanotube with two finite tips. Only when the deformed length is long enough, an effectual metal-semiconductor-metal heterojunction can be formed in the metallic tube. The effect of finite deformed length is explained by the quantum tunnelling effect. Furthermore, some conceptual designs of nanoscale devices are proposed from the metal-semiconductor-metal heterojunction.

cond-mat.mtrl-sci↗
arXiv2003

Huge enhancement of electronmechanical responses in compositionally modulated PZT

Monte Carlo simulations based on a first-principles-derived Hamiltonian are conducted to study the properties of PZT alloys compositionally modulated along the [100] pseudocubic direction near the morphotropic phase boundary (MPB). It is shown that compositional modulation causes the polarization to continuously rotate away from the modulation direction, resulting in the unusual triclinic and C-type monoclinic ground states and huge enhancement of electromechanical responses (the peak of piezoelectric coefficient is as high as 30000 pC/N). The orientation dependence of dipole-dipole interaction in modulated structure is revealed as the microscopic mechanism to be responsible for these anomalies.

cond-mat.mtrl-sci↗
arXiv2003

Electronic-structure theory of crystalline insulators under homogeneous electric field

Based on the conventional energy band theory, an approach is presented to describe the electronic structure of crystalline insulators in the presence of a finite homogeneous electric field. The expression of polarization is derived which extends the ``Berry-phase'' theory of polarization to nonzero fields. The characteristics of the solutions are studied in details and the associations with the existing perturbation theories and computational schemes are discussed.

cond-mat.mtrl-sci↗
arXiv2003

Metal-to-semiconductor transition in squashed armchair carbon nanotubes

We investigate electronic transport properties of the squashed armchair carbon nanotubes, using tight-binding molecular dynamics and Green's function method. We demonstrate a metal-to-semiconductor transistion while squashing the nanotubes and a general mechanism for such transistion. It is the distinction of the two sublattices in the nanotube that opens an energy gap near the Fermi energy. We show that the transition has to be achieved by a combined effect of breaking of mirror symmetry and bond formation between the flattened faces in the squashed nanotubes.

cond-mat.mtrl-sci↗