Zhenhai Wang

Tunable electronic structures of graphene/boron nitride heterobilayers

Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of graphene/boron nitride heterobilayers (C/BN HBLs) can be modulated effectively by tuning the interlayer spacing and stacking arrangement. The HBLs have smaller EEM than that of graphene bilayers (GBLs), and thus higher carrier mobility. For specific stacking patterns, the nearly linear band dispersion relation of graphene monolayer can be preserved in the HBLs accompanied by a small band-gap opening. The tunable band gap and high carrier mobility of these C/BN HBLs are promising for building high-performance nanodevices.

Isoelectronic Doping of Graphdiyne with Boron and Nitrogen: Stable Configurations and Band Gap Modification

Graphdiyne, consisting of sp- and sp(2)-hybridized carbon atoms, is a new member of carbon allotropes which has a natural band gap ~1.0 eV. Here, we report our first-principles calculations on the stable configurations and electronic structures of graphdiyne doped with boron-nitrogen (BN) units. We show that BN unit prefers to replace the sp-hybridized carbon atoms in the chain at a low doping rate, forming linear BN atomic chains between carbon hexagons. At a high doping rate, BN units replace first the carbon atoms in the hexagons and then those in the chains. A comparison study indicates that these substitution reactions may be easier to occur than those on graphene which composes purely of sp(2)-hybridized carbon atoms. With the increase of BN component, the band gap increases first gradually and then abruptly, corresponding to the transition between the two substitution motifs. The direct-band gap feature is intact in these BN-doped graphdiyne regardless the doping rate. A simple tight-binding model is proposed to interpret the origin of the band gap opening behaviors. Such wide-range band gap modification in graphdiyne may find applications in nanoscaled electronic devices and solar cells.

Characterization of mixed Nd:LuxGd1−xVO4 laser crystals

A series of laser crystals Nd:LuxGd1−xVO4 (x=0.14,0.32,0.50,0.61,0.70,0.80) was grown by the Czochralski method. The thermal properties, including the average linear thermal expansion coefficients, thermal diffusion coefficients, specific heats, and thermal conductivities, of the mixed crystals were obtained. The material constants Ms for the thermal stress resistance figure were calculated and showed that the thermal fracture limits of the mixed crystals should be comparable with that of Nd:YVO4. The polarization absorption spectra from 240to1000nm were measured at room temperature and the absorption cross sections at 809nm were calculated. Using the Judd-Ofelt theory, the theoretical radiative lifetimes were calculated and compared with the experimental results. Continuous wave laser performances were achieved with the mixed crystals at the wavelength of 1.06μm when they were pumped by a laser diode. Thermal, optical, and laser properties have shown variation as a function of x and proved that the mixed...

Hybrid density functional study of band alignment in ZnO–GaN and ZnO–(Ga1−xZnx)(N1−xOx)–GaN heterostructures

The band alignment in ZnO-GaN and related heterostructures is crucial for uses in solar harvesting technology. Here, we report our density functional calculations of the band alignment and optical properties of ZnO-GaN and ZnO-(Ga(1-x)Zn(x))(N(1-x)O(x))-GaN heterostructures using a Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. We found that the conventional GGA functionals underestimate not only the band gap but also the band offset of these heterostructures. Using the hybrid functional calculations, we show that the (Ga(1-x)Zn(x))(N(1-x)O(x)) solid solution has a direct band gap of about 2.608 eV, in good agreement with the experimental data. More importantly, this solid solution forms type-II band alignment with the host materials. A GaN-(Ga(1-x)Zn(x))(N(1-x)O(x))-ZnO core-shell solar cell model is presented to improve the visible light absorption ability and carrier collection efficiency.

First-principles study of ZnS nanostructures: nanotubes, nanowires and nanosheets.

We performed first-principles calculations to study the energetics, geometric and electronic properties of zinc sulfide (ZnS) nanostructures. ZnS nanowires (ZnSNWs), nanotubes (ZnSNTs) and nanosheets (ZnSNSs) were considered. Both ZnSNWs and ZnSNTs modeled using hexagonal prisms with the atomic arrangement displaying the characters of wurtzite crystal are more stable than the single-walled ZnS nanotubes presented in previous literature. The energy evolution of ZnSNWs and ZnSNTs as a function of tube diameter and wall thickness was calculated and explained using a simple model. The comparison between the energetics and electronic structures of these ZnS nanostructures was also addressed.

Electronic properties of BN/C nanotube heterostructures

We perform first-principles calculations to investigate the geometric and electronic properties of (10,0) and (5,5) BN/C nanotube heterostructures. We show that both of them have smooth interfaces which are free from bond mismatch and vacancy defect. Interface states appear in the band gaps, due to the discontinuity of π-π bonding of carbon nanotube segments, and exhibit asymmetric distribution in the two segments. The charge redistribution in the region near the interfaces gives rise to a build-in electric field and modulates the static electric potential profiles in the heterostructures. The band scheme diagrams of these heterostructures are also presented.

Can cation vacancy defects induce room temperature ferromagnetism in GaN

The unique properties of gallium nitride (GaN) crystal, such as a wide band-gap and high thermal conductivity, make it ideal material for electronic and optoelectronic devices. Achieving room temperature (RT) ferromagnetism in GaN becomes crucial. In previous works, gallium vacancy (VGa) was expected to be promising for reaching this goal. However, using an accurate hybrid exchange-correlation functional, we show that the largest value of J0 is only 3.3 meV at the VGa density of 1.28 × 1021 cm−3, corresponding to a Curie temperature of 150 K. This suggests that VGa cannot induce RT ferromagnetism at the density lower than that value.

Manifold electronic structure transition of BNC biribbons

Using first principles calculations, we investigate the electronic and magnetic properties of BNC biribbons, a laterally-heterostructured nanoribbon constructed by joining a graphene nanoribbon (GNR) and a BN nanoribbon (BNNR) with zigzag edges. We find that the spin-polarization and electronic structures of the biribbons can be well-tuned by changing the width of the GNR, undergoing manifold transitions from semiconducting to half-metal and ferromagnetic metal. The critical points of GNR width to induce the transitions depend on the interface type (B/C or C/N) rather than the BNNR width. The ground states of metallic BNC biribbons are spin-polarized, forming non-zero magnetic moments. The tunable electronic structures and ferromagnetic ground states make the BNC biribbons promising candidate nanomaterials for building nanoscaled spintronic devices.

Spin-polarization of VGaON center in GaN and its application in spin qubit

VGaON center in cubic gallium nitride is a defect complex composing of a substitutional oxygen atom at nitrogen site (ON) and an adjacent gallium vacancy (VGa). Based on first-principles calculations, we predicted that this VGaON center has much in common with the famous nitrogen-vacancy center in diamond, but the excitation energy is very low. The electron spin-polarization of the centers can be tuned by changing the charge states. The neutral ONVGa center has the v↓ and exy↓ states being well isolated from the bulk bands with appropriate spacing which are suitable for achieving spin qubit operation with low excitation energy.

Theoretical insights into the built-in electric field and band offsets of BN/C heterostructured zigzag nanotubes

We perform first-principles calculations to investigate the band offsets of (9,0) and (10,0) BN/C heterostructured nanotubes with different interfaces. We show that the built-in electric field induced by charge redistribution modulates the band offsets of these nanotubes in different ways. Remarkably enhanced field-emission properties of the heterostructures are also predicted.