arXiv:0907.3089v1 [cond-mat.mes-hall] 17 Jul 2009 APS/123-QED Systematic control of surface Dirac fermion density on topological insulator Bi 2 Te 3 Y. Xia, 1 D. Qian, 1, 2 D. Hsieh, 1 R. Shankar, 1 H. Lin, 3 A. Bansil, 3 A.V. Fedorov, 4 D. Grauer, 5 Y. S. Hor, 5 R. J. Cava, 5 and M. Z. Hasan 1, 6, 7 1 Joseph Henry Laboratories of Physics, Department of Physics, Princeton University, Princeton, NJ 08544, USA 2 Department of Physics, Shanghai Jiao Tong University, Shanghai 200030, China 3 Department of Physics, Northeastern University, Boston, MA 4 Advanced Light Source, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94305, USA 5 Department of Chemistry, Princeton University, Princeton, NJ 08544, USA 6 Princeton Center for Complex Materials, Princeton University, Princeton, NJ 08544, USA 7 Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ 08544, USA (Dated: July 17, 2009) 1