Published: November 03, 2011 r2011 American Chemical Society 113 dx.doi.org/10.1021/nl203065e | Nano Lett. 2012, 12, 113–118 LETTER pubs.acs.org/NanoLett Tunable Bandgap in Silicene and Germanene Zeyuan Ni, Qihang Liu, Kechao Tang, Jiaxin Zheng, Jing Zhou, Rui Qin, Zhengxiang Gao, Dapeng Yu, and Jing Lu* State Key Laboratory of Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, P. R. China b S Supporting Information G raphene has attracted great interest recently for its excep- tional electronic properties. 14 For instance, its charge carriers are massless Dirac fermions, leading to a mobility up to 15 000 cm 2 V 1 s 1 for graphene on SiO 2 substrate and 200 000 cm 2 V 1 s 1 for suspended sample. 14 However, the promising application of graphene in present electronic devices, like field effect transistor (FET), still relies on the opening and controlling of the band gap. Although a band gap is opened in bilayer and multilayer graphene under an external vertical electric field due to the inversion symmetry breaking, 59 monolayer graphene remains zero-gap semimetallic because its two sub- lattices remain equivalent under an external vertical electric field. Consequently, biased monolayer graphene cannot be operated effectively as a FET at room temperature. Other group IV ele- ments, such as silicon and germanium, also have stable honey- comb monolayers (namely silicene and germanene). 10,11 Synthe- sis of pristine, 12 Mg-doped 13 and hydrogenated 14 silicene, and pristine silicene nanoribbon 15 have been reported. Unlike planar graphene monolayers, the most stable silicene and germanene monolayers prefer a low-buckled (LB) structure 1012 (silicene and germanene referred to in the following are those with LB structure). The electronic structures of the silicene and germa- nene are quite similar to that of graphene. 10,11 Namely, silicene and germanene are zero-gap semimetallic, and their charge carriers are also massless fermions because their π and π* bands are linear at the Fermi level (E f ). When a vertical electric field is applied, the atoms in a buckled structure are no longer equiva- lent, and a band gap opening may become possible in silicene and germanene. If so, one can fabricate an FET operating at room temperature out of pure silicene and germanene. In this Letter, we investigate the effects of a vertical electric field on silicene and germanene by means of the density functional theory (DFT) and the nonequilibrium Green’ s function (NEGF) method. A band gap is unambiguously opened, and its size and the effec- tive carrier mass increase linearly with the electric field strength. The effects of the vertical electric field on the transport properties of silicene are subsequently examined by fabricating a prototype of dual-gated silicene FET. A transport gap induced by perpen- dicular electric field is found, accompanied by significant switch- ing effects by gate voltage. Geometry optimization and electronic structure are calculated by using an all-electron double numerical atomic basis set plus polarization (DNP), as implemented in the Dmol 3 package. 16 A 32  32  1 MonkhorstPack 17 k-points grid is used in the first Brillouin zone sampling. A vacuum space of 20 Å is placed to avoid interaction between the monolayer and its periodic images. Both the atomic positions and lattice constant are re- laxed. Transportation properties are calculated by the DFT coupled with NGEF formalism implemented in the ATK 11.2 package. 1820 Both single-ζ (SZ) and double-ζ plus polarization (DZP) basis sets are employed. The k-points of the electrodes and central region, which are generated by the MonkhorstPack scheme as well, are set to 1  300  300 and 1  300  1, respectively. The temperature is set to 300 K. The current is calculated by using the LandauerB€ uttiker formula: 21 I ðV g , V bias Þ¼ 2e h Z þ∞ ∞ fT Vg ðE, V bias Þ½f L ðE  μ L Þ f R ðE  μ R ÞgdE ð1Þ where T V g (E, V bias ) is the transmission probability at a given gate voltage V g and bias voltage V bias , f L/R the FermiDirac Received: September 4, 2011 Revised: October 31, 2011 ABSTRACT: 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. KEYWORDS: Silicene, germanene, band gap, quantum transport, electric field, first-principles calculation