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.

We report the electronic structure and optical properties of the recently synthesized stable two-dimensional carbon allotrope-graphdiyne based on first-principles calculations and experimental optical spectrum. Due to the enhanced Coulomb interaction in reduced dimensionality, the band gap of graphdiyne increases to 1.10 eV within the GW many-body theory from a 0.44 eV within the density functional theory. The optical absorption is dominated by excitonic effects with remarkable electron-hole binding energy of over 0.55 eV within the GW-Bethe Salpeter equation calculation. Experimental optical absorption of graphdiyne films is performed and comparison with the theoretical calculations is analyzed in detail.

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.

Opening a tunable and sizable band gap in single-layer graphene (SLG) without degrading its structural integrity and carrier mobility is a significant challenge. Using density functional theory calculations, we show that the band gap of SLG can be opened to 0.16 eV (without an electric field) and 0.34 eV (with a strong electric field) when properly sandwiched between two hexagonal boron nitride single layers. The zero-field band gaps are increased by more than 50% when the many-body effects are included. The ab initio quantum transport simulation of a dual-gated field effect transistor (FET) made of such a sandwich structure reveals an electric-field-enhanced transport gap, and the on/off current ratio is increased by a factor of 8.0 compared with that of a pure SLG FET. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance SLG FETs. NPG Asia Materials (2012) 4, e6; doi:10.1038/am.2012.10; published online 17 February 2012Keywords: density functional theory; electric field; graphene; h-BN sheet; quasiparticle correction; transport properties INTRODUCTION Despite its extremely high carrier mobility (1. 5Â10 4 cm 2 V À1 s À1 for a SiO 2 -supported sample 1 and 2Â10 5 cm 2 V À1 s À1 for a suspended sample 2,3 ), pristine graphene cannot be used for effective roomtemperature field effect transistors (FET) because of its zero band gap. Opening and tailoring a band gap in graphene is probably one of the most important and urgent research topics in the graphene research currently. A large number of methods have been developed to open a band gap in graphene, and these methods can be classified into the following two types, depending on whether they preserve the integrity of the honeycomb structure: in a type I method, the honeycomb structure is destroyed, and in a type II method, the honeycomb structure of graphene is preserved. Typical type I methods include cutting graphene into nanoribbons, 4 making graphene nanomeshes, 5 and chemical functionalization. 6,7 The main disadvantage of the type I methods is that the carrier mobility and on-state current are greatly reduced because the destruction of the honeycomb structure introduces scattering centers, enhances the carrier effective mass and produces a non-tunable band gap.Unlike the type I method, high carrier mobility can be maintained in the type II method because the honeycomb structure is maintained. Typical type II methods include graphene-substrate interaction 8,9 and the application of strain. 10 The graphene band gap induced by a substrate is not tunable. The most effective type II method is the application of an external electric field to the graphene. Both

A red shift of the exciton of ZnO nanowires is efficiently produced by bending strain, as demonstrated by a low-temperature (81 K) cathodoluminescence (CL) study of ZnO nanowires bent into L- or S-shapes. The figure shows a nanowire (Fig. a) with the positions of CL measurements marked. The corresponding CL spectra-revealing a peak shift and broadening in the region of the bend-are shown in Figure b.

We perform first-principles calculations of mechanical and electronic properties of silicene under strains. The in-plane stiffness of silicene is much smaller than that of graphene. The yielding strain of silicene under uniform expansion in the ideal conditions is about 20%. The homogeneous strain can introduce a semimetal-metal transition. The semimetal state of silicene, in which the Dirac cone locates at the Fermi level, can only persist up to tensile strain of 7% with nearly invariant Fermi velocity. For larger strains, silicene changes into a conventional metal. The work function is found to change significantly under biaxial strain. Our calculations show that strain tuning is important for applications of silicene in nanoelectronics

We calculate the electronic structures of the fully bare and half-bare zigzag-edged boron nitride nanoribbons by using density functional theory. We find that the ground states of both the fully bare boron nitride nanoribbons and the boron nitride nanoribbons with a bare N edge and a H-terminated B edge are half-metallic. The alignment of the spin at the bare B edge is antiferromagnetic, while that at the bare N edge is ferromagnetic in the ground states of both the fully bare and half-bare zigzag-edged boron nitride nanoribbons. The H-terminated B or N edge of the half-bare zigzag-edged boron nitride nanoribbon exhibits no magnetism.

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.