Dense Electron System from Gate-Controlled Surface Metal− Insulator Transition Kai Liu, †,‡,○ Deyi Fu, †,§,○ Jinbo Cao, †,‡ Joonki Suh, † Kevin X. Wang, † Chun Cheng, † D. Frank Ogletree, ∥ Hua Guo, †,⊥ Shamashis Sengupta, # Asif Khan, ∇ Chun Wing Yeung, ∇ Sayeef Salahuddin, ∇ Mandar M. Deshmukh, # and Junqiao Wu* ,†,‡ † Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States ‡ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § School of Electronic Science and Engineering, Nanjing University, Nanjing, Jiangsu 210093, China ∥ The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States # Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India ∇ Department of Electrical Engineering and Computer Science, University of California, Berkeley, California 94720, United States * S Supporting Information ABSTRACT: Two-dimensional electron systems offer enormous oppor- tunities for science discoveries and technological innovations. Here we report a dense electron system on the surface of single-crystal vanadium dioxide nanobeam via electrolyte gating. The overall conductance of the nanobeam increases by nearly 100 times at a gate voltage of 3 V. A series of experiments were carried out which rule out electrochemical reaction, impurity doping, and oxygen vacancy diffusion as the dominant mechanism for the conductance modulation. A surface insulator-to-metal transition is electrostatically triggered, thereby collapsing the bandgap and unleashing an extremely high density of free electrons from the original valence band within a depth self-limited by the energetics of the system. The dense surface electron system can be reversibly tuned by the gating electric field, which provides direct evidence of the electron correlation driving mechanism of the phase transition in VO 2 . It also offers a new material platform for implementing Mott transistor and novel sensors and investigating low-dimensional correlated electron behavior. KEYWORDS: Vanadium dioxide, 2D electron system, electrostatic gating, metal−insulator transition E lectric-field tuning of surface carrier density is the fundamental mechanism of field-effect transistors and also attracts great attention as a method to control and explore new properties of electronic materials. Typically a high surface carrier density is realized via electrostatic gating, charge transfer, or electronic reconstruction in semiconductors and oxides. 1 Using electric-field tuning, conductive, 1,2 superconducting, 3−5 or magnetic 6 surface layers have been created on materials that originally do not possess these properties. However, the sheet carrier density achieved is typically below ∼10 14 cm −2 limited by factors such as breakdown field of the gate dielectric. 1,3 Higher densities are much desired in the exploration of correlated electron behavior in low dimensions. 7 In this regard, the possibility to induce and tune, by purely electrostatic means, a two-dimensional metallic layer on an insulating substrate is of great potential for reaching an ultrahigh surface carrier density. This could be realized by electric-field tuning of some phase transition materials, for example, vanadium dioxide (VO 2 ), which possesses a metal−insulator phase transition (MIT). Vanadium dioxide exhibits a thermally driven MIT slightly above room temperature, while the role of electron correlation in the transition has been debated for decades. 8−11 Experiments show that the transition from the insulator phase to the metal phase is triggered when a threshold conductivity 12 or threshold free electron density 13 is reached by thermal excitation or compressive stress. However, the electron density threshold for the MIT is too high to be reached in charge accumulation by conventional solid gating. 14,15 As a result, experimental evidence of electrically activated MIT in VO 2 has been limited to those caused by current Joule heating 16 or Poole−Frenkel emission, 17 rather than electrostatic charge injection. Circum- venting this limit, the electric double-layer transistor (EDLT) configuration uses an electrolyte or ionic liquid to produce an ultrahigh electric field on the order of 10 7 V/cm near the surface of the conduction channel, 18 driving electrostatic Received: September 10, 2012 Revised: November 8, 2012 Published: November 19, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 6272 dx.doi.org/10.1021/nl303379t | Nano Lett. 2012, 12, 6272−6277