1 Metal-Insulator Transition in Variably Doped (Bi 1- x Sb x ) 2 Se 3 Nanosheets Chee Huei Lee, †,‡ Rui He, §,* ZhenHua Wang, †,¶ Richard L.J. Qiu, † Ajay Kumar, ‡ Conor Delaney, § Ben Beck, § T. E. Kidd, § C. C. Chancey, § R. Mohan Sankaran, ‡ and Xuan P. A. Gao, †, * † Department of Physics, Case Western Reserve University, Cleveland, OH 44106, U.S.A. ‡ Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, U.S.A. § Department of Physics, University of Northern Iowa, Cedar Falls, IA 50614, U.S.A. ¶ Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China *Email: (R.H.) rui.he@uni.edu ; (X.P.A.G.) xuan.gao@case.edu Abstract Topological insulators are novel quantum materials with metallic surface transport, but insulating bulk behavior. Often, topological insulators are dominated by bulk contributions due to defect induced bulk carriers, making it difficult to isolate the more interesting surface transport characteristics. Here, we report the synthesis and characterization of nanosheets of topological insulator Bi 2 Se 3 with variable Sb-doping level to control the electron carrier density and surface transport behavior. (Bi 1-x Sb x ) 2 Se 3 thin films of thickness less than 10 nm are prepared by epitaxial growth on mica substrates in a vapor transport setup. The introduction of Sb in Bi 2 Se 3 effectively suppresses the room temperature electron density from ~4×10 13 /cm 2 in pure Bi 2 Se 3 (x = 0) to ~2×10 12 /cm 2 in (Bi 1-x Sb x ) 2 Se 3 at x ~0.15, while maintaining the metallic transport behavior. At x ≳ ~0.20, a metal-insulator transition (MIT) is observed indicating that the system