Phase transition in bulk single crystals and thin films of VO 2 by nano-infrared spectroscopy and imaging Mengkun Liu 1,2 * , Aaron J. Sternbach 1 , Martin Wagner 1 , Tetiana V. Slusar 3 , Tai Kong 4 , Sergey L. Bud’ko 4 , Salinporn Kittiwatanakul 5 , M. M. Qazilbash 6 , Alexander McLeod 1 , Zhe Fei 1 , Elsa Abreu 7, 8 , Jingdi Zhang 1,7 , Michael Goldflam 1 , Siyuan Dai 1 , Guang-Xin Ni 1 , Jiwei Lu 5 , Hans A. Bechtel 9 , Michael C. Martin 9 , Markus B. Raschke 10 , Richard D. Averitt 1 , Stuart A. Wolf 5, 11 , Hyun-Tak Kim 3, 12 , Paul C. Canfield 4 , D. N. Basov 1* 1 Department of Physics, The University of California at San Diego, La Jolla, California 92093, USA. 2 Department of Physics, Stony Brook University, Stony Brook, New York 11794, USA 3 Metal-Insulator Transition Creative Research Center, ETRI, Daejeon 305-350, South Korea 4 Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames Iowa, 50010, USA. 5 Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA. 6 Department of Physics, College of William & Mary, Williamsburg, Virginia 23187, USA 7 Department of Physics, Boston University, Boston, Massachusetts 02215, USA. 8 Institute of Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland 9 Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 10 Department of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, Colorado 80309, USA 11 Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA. 12 School of Advanced Device Technology, Korean University of Science and Technology, Daejeon 305-333, South Korea * mengkun.liu@stonybrook.edu * dbasov@physics.ucsd.edu