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.

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.

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 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.

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We have investigated the effect of uniform plane strain on the electronic properties of monolayer 1T-TiS 2 using first-principles calculations. In the absence of strain, we find monolayer TiS 2 is a semimetal, with a small overlap of the valence band maximum and the conduction band minimum. The band overlap increases under compression; however under tensile, monolayer 1T-TiS 2 experiences a transition from a semimetal to a semiconductor as a band gap emerges. Moreover, the electronic properties change from an indirect to a direct band gap upon application of greater tensile strain. Thus one can modulate the properties of monolayer TiS 2 by applying the appropriate strain, thereby providing a route towards control in optoelectronic devices. † Electronic supplementary information (ESI) available: The complete data set of strain-induced band gaps for mononlayer 1T-TiS 2 as well as the band structure and the density of states for bulk TiS 2 are provided. See