Zhengxiang Gao

Tunable Bandgap in Silicene and Germanene

By using ab initio calculations, we predict that a vertical electric field is able to open a band gap in semimetallic single-layer buckled silicene and germanene. The sizes of the band gap in both silicene and germanene increase linearly with the electric field strength. Ab initio quantum transport simulation of a dual-gated silicene field effect transistor confirms that the vertical electric field opens a transport gap, and a significant switching effect by an applied gate voltage is also observed. Therefore, biased single-layer silicene and germanene can work effectively at room temperature as field effect transistors.

Mixed low-dimensional nanomaterial: 2D ultranarrow MoS2 inorganic nanoribbons encapsulated in quasi-1D carbon nanotubes.

Quasi-one-dimensional nanotubes and two-dimensional nanoribbons are two fundamental forms of nanostructures, and integrating them into a novel mixed low-dimensional nanomaterial is fascinating and challenging. We have synthesized a stable mixed low-dimensional nanomaterial consisting of MoS(2) inorganic nanoribbons encapsulated in carbon nanotubes (which we call nanoburritos). This route can be extended to the synthesis of nanoburritos composed of other ultranarrow transition-metal chalcogenide nanoribbons and carbon nanotubes. The widths of previously synthesized MoS(2) ribbons are greater than 50 nm, while the encapsulated MoS(2) nanoribbons have uniform widths down to 1-4 nm and layer numbers down to 1-3, depending on the nanotube diameter. The edges of the MoS(2) nanoribbons have been identified as zigzag-shaped using both high-resolution transmission electron microscopy and density functional theory calculations.

Tunable and sizable band gap in silicene by surface adsorption

Opening a sizable band gap without degrading its high carrier mobility is as vital for silicene as for graphene to its application as a high-performance field effect transistor (FET). Our density functional theory calculations predict that a band gap is opened in silicene by single-side adsorption of alkali atom as a result of sublattice or bond symmetry breaking. The band gap size is controllable by changing the adsorption coverage, with an impressive maximum band gap up to 0.50 eV. The ab initio quantum transport simulation of a bottom-gated FET based on a sodium-covered silicene reveals a transport gap, which is consistent with the band gap, and the resulting on/off current ratio is up to 108. Therefore, a way is paved for silicene as the channel of a high-performance FET.

Giant magnetoresistance in silicene nanoribbons

By performing first-principle quantum transport calculations, we predict a giant magnetoresistance in zigzag silicene nanoribbons (ZSiNRs) connecting two semi-infinite silicene electrodes through switch of the edge spin direction of ZSiNRs. Spin-filter efficiency of both the antiferromagnetic and ferromagnetic ZSiNRs is sign-changeable with the bias voltage. Therefore, potential application of silicene in spintronics devices is suggested.

Novel One-Dimensional Organometallic Half Metals: Vanadium-Cyclopentadienyl, Vanadium-Cyclopentadienyl-Benzene, and Vanadium-Anthracene Wires

By using the density functional theory, we find that organometallic multidecker sandwich clusters V(2 n+1)Cp(2 n+2), Vn(FeCp2)(n+1) (Cp=cyclopentadienyl), and V(2n)Ant(n+1) (Ant=anthracene) may have linear structures, and their total magnetic moments generally increase with the cluster size. The one-dimensional (VCp)infinity, (VBzVCp)infinity (Bz=benzene), and (V2Ant)infinity wires are predicted to be ferromagnetic half-metals, while the one-dimensional (VCpFeCp)infinity wire is a ferromagnetic semiconductor. The spin transportation calculations show that the finite V2(n+1)Cp2(n+2) and Vn(FeCp2)(n+1) sandwich clusters coupled to gold electrodes are nearly perfect spin-filters.

Functionalized Graphene for High-Performance Two-Dimensional Spintronics Devices

Using first-principles calculations, we explore the possibility of functionalized graphene as a high-performance two-dimensional spintronics device. Graphene functionalized with O on one side and H on the other side in the chair conformation is found to be a ferromagnetic metal with a spin-filter efficiency up to 54% at finite bias. The ground state of graphene semifunctionalized with F in the chair conformation is an antiferromagnetic semiconductor, and we construct a spin-valve device from it by introducing a magnetic field to stabilize its metallic ferromagnetic state. The resulting room-temperature magnetoresistance is up to 2200%, which is 1 order of magnitude larger than the available experimental values.

Sub-10 nm Gate Length Graphene Transistors: Operating at Terahertz Frequencies with Current Saturation

Radio-frequency application of graphene transistors is attracting much recent attention due to the high carrier mobility of graphene. The measured intrinsic cut-off frequency (fT) of graphene transistor generally increases with the reduced gate length (Lgate) till Lgate = 40 nm, and the maximum measured fT has reached 300 GHz. Using ab initio quantum transport simulation, we reveal for the first time that fT of a graphene transistor still increases with the reduced Lgate when Lgate scales down to a few nm and reaches astonishing a few tens of THz. We observe a clear drain current saturation when a band gap is opened in graphene, with the maximum intrinsic voltage gain increased by a factor of 20. Our simulation strongly suggests it is possible to design a graphene transistor with an extraordinary high fT and drain current saturation by continuously shortening Lgate and opening a band gap.

Interfacial Properties of Bilayer and Trilayer Graphene on Metal Substrates

One popular approach to prepare graphene is to grow them on transition metal substrates via chemical vapor deposition. By using the density functional theory with dispersion correction, we systematically investigate for the first time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on metal substrates. Three categories of interfacial structures are revealed. The adsorption of B(T)LG on Al, Ag, Cu, Au, and Pt substrates is a weak physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti, Ni, and Co substrates is a strong chemisorption, and a stacking-insensitive band gap is opened for the two uncontacted layers of TLG. The adsorption of B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for the uncontacted layers. This fundamental study also helps for B(T)LG device study due to inevitable graphene/metal contact.

HALF-METALLIC SILICENE AND GERMANENE NANORIBBONS: TOWARDS HIGH-PERFORMANCE SPINTRONICS DEVICE

By using first-principles calculations, we predict that an in-plane homogenous electrical field can induce half-metallicity in hydrogen-terminated zigzag silicene and germanene nanoribbons (ZSiNRs and ZGeNRs). A dual-gated finite ZSiNR device reveals a nearly perfect spin-filter efficiency (SFE) of up to 99% while a quadruple-gated finite ZSiNR device serves as an effective spin field effect transistor (FET) with an on/off current ratio of over 100 from ab initio quantum transport simulation. This discovery opens up novel prospect of silicene and germanene in spintronics.

Ultra-narrow WS2 nanoribbons encapsulated in carbon nanotubes

Layered tungsten disulfide nanostructures are of both fundamental and technological interest. The widths of currently synthesized WS2 ribbons are in the microscale. By using single-walled carbon nanotubes and double-walled carbon nanotubes as templates, we fabricate WS2 nanoribbons with smooth zigzag edges and uniform widths down to 1–3 nm and layer numbers down to 1–3, dependent on the nanotube diameter. Although bulk WS2 is a nonmagnetic semiconductor, the ultra-narrow free-standing zigzag-edged WS2 nanoribbons turn out to be magnetic or nonmagnetic metals depending on the edge passivation way according to our first-principles calculations, whereas the ultra-narrow armchair-edged WS2 nanoribbons remain nonmagnetic semiconductors with a narrow gap.