Xuejuan Zhang

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.

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.

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.

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.

Size-dependent structural and electronic properties of ZnS nanofilms: An ab initio study

The stable configurations and electronic properties of (0001)/(0001¯)-surfaces-derivated ZnS nanofilms (NFs) were investigated using first-principles calculations. The size-dependent stable configurations of these NFs are characterized by a graphiticlike structure-NF (G-NF), a film terminated by (0001)/(0001¯) surfaces, and a new phase composed of quadrilateral-octagon network-NF (QO-NF), respectively. Their formation energies with respect to bulk crystal are proportional to the inverse of film thickness, except the G-NFs. The QO-NFs are more stable than the polar (0001)/(0001¯) NFs when the film thickness is smaller than 66 A. The (0001)/(0001¯)-terminated NFs are metallic, while the others are direct-band-gap semiconductors. Surface reconstruction and possible stabilization mechanisms of the polar (0001)/(0001¯) surfaces were also discussed.

First-principles study of Co-doped single-walled silicon nanotubes

We performed spin-polarized density functional calculations to study the stable configurations, energetics and electronic structures of Co-doped single-walled silicon nanotubes (CoSi(2)NTs) with the stoichiometry of CoSi(2). We found that the incorporation of Co atoms into the wall of SiNTs not only effectively stabilizes the tubes but also tunes their electronic properties. The formation energies of the CoSi(2)NTs are much lower than those of pristine SiNTs, indicating the plausibility of these tubes. The electronic structures of the CoSi(2)NTs display the characters of metals. This provides a promising synthetic route to stable SiNTs which may find potential applications in building nanoscale devices.