1. Department of Physics, University of California at Berkeley, Berkeley, California 94720
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
3. Center of Integrated Nanomechanical Systems (COINS), Berkeley, California 94720
4. The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720
5. School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China
6. Politecnico di Milano, 20133 Milano, Italy