Gate-Controlled Surface Conduction in Na-Doped Bi 2 Te 3 Topological Insulator Nanoplates Yong Wang,* ,†,‡,§ Faxian Xiu, ∥ Lina Cheng, § Liang He, ‡ Murong Lang, ‡ Jianshi Tang, ‡ Xufeng Kou, ‡ Xinxin Yu, ‡ Xiaowei Jiang, ‡ Zhigang Chen, § Jin Zou,* ,§,⊥ and Kang L. Wang* ,‡ † State Key Laboratory for Silicon Materials and Center for Electron Microscopy, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China ‡ Department of Electrical Engineering, University of California, Los Angeles, California 90095, United States § Division of Materials, The University of Queensland, Brisbane QLD 4072, Australia ∥ Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, United States ⊥ Centre for Microcopy and Microanalysis, The University of Queensland, Brisbane QLD 4072, Australia * S Supporting Information ABSTRACT: Exploring exciting and exotic physics, scientists are pursuing practical device applications for topological insulators. The Dirac-like surface states in topological insulators are protected by the time-reversal symmetry, which naturally forbids backscattering events during the carrier transport process, and therefore offers promising applications in dissipationless spintronic devices. Although considerable efforts have been devoted to controlling their surface conduction, limited work has been focused on tuning surface states and bulk carriers in Bi 2 Te 3 nanostructures by external field. Here we report gate-tunable surface conduction in Na-doped Bi 2 Te 3 topological insulator nanoplates. Significantly, by applying external gate voltages, such topological insulators can be tuned from p-type to n-type. Our results render a promise in finding novel topological insulators with enhanced surface states. KEYWORDS: Topological insulator, bismuth telluride, sodium doping, surface states, field-effect transistor I n the past few years, topological insulators with both metallic surface states and insulating bulk states have attracted enormous attention due to their exotic physical properties and potential applications in low dissipation devices and quantum computing, 1−16 particularly after the experimental confirmation of a three-dimensional topological insulator. 17 These novel materials and related hybrid structures are expected to serve as a platform for creating and investigating some mysterious particles such as Axion and Majorana fermions, which have not yet been observed in experiments. 8 Protected by the time- reversal symmetry, the metallic surface states in topological insulators show a single Dirac cone at the Γ point in the energy dispersion spectrum, which excludes the backscattering induced by nonmagnetic impurities. 18 Extensive studies involving angle- resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy, and transport measurements have confirmed the existence of such surface states in Bi 2 Se 3 , 11,19−21 Bi 2 Te 3 , 14,16,22,23 Sb 2 Te 3 , 19 and HgTe quantum well structures. 2 Currently, topological insulator materials are generally produced by mechanical exfoliation, 16,24 molecular beam epitaxy 25,26 and chemical solutions synthesis. 14,27 However, no matter what growth method is employed, the resulting topological insulator materials share a common problem that the bulk contribution usually overwhelms the surface states because of the fact that the Fermi level usually lies in either valence band or conduction band due to the presence of intrinsic or extrinsic defects. 8,28,29 In order to resolve this problem, various approaches have been attempted to reduce the bulk conduction contribution and enhance that of the surface states, including elemental compensating dop- ing 22,28,30,31 and electric gating. 14,32,33 Indeed, considerable progress has been made in terms of shifting the Fermi level (or the chemical potential) into the bulk band gap. However, most of these studies are focused on the photoelectron emission method by ARPES. 22,34 Using electric gating to tune the Fermi level has been realized in Bi 2 Se 3 32,33,35 and in Bi 2 Te 3 . 14 For example, Steinberg et al. 33 swept the Fermi level across the Dirac point by applying double gating in a Bi 2 Se 3 nanodevice. Likewise, by applying a back gate voltage to deplete the bulk carriers, we have previously achieved an enhanced surface conductance in a p-type Bi 2 Te 3 nanoribbon at a positive bias of 80 V. 14 The surface states, however, could not be distinguished under zero and negative gate voltages due to the Fermi level falling into the valence band. The ideal scenario to maximize the surface contribution is to place the Fermi level right at the intrinsic Fermi level where the bulk contribution reaches the minimum. In this work, by using a proper elemental doping, we revealed that the Fermi level of Bi 2 Te 3 can be tuned by Na doping in Received: August 22, 2011 Revised: February 5, 2012 Published: February 7, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 1170 dx.doi.org/10.1021/nl202920p | Nano Lett. 2012, 12, 1170−1175