1. Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA.
2. Defense Advanced Research Projects Agency Center for Integrated Micro/Nano-Electromechanical Transducers (iMINT), Columbia University, New York, NY 10027, USA.
3. Center for Nanostructured Materials, Columbia University, New York, NY 10027, USA.
4. Center for Electronic Transport in Molecular Nanostructures, Columbia University, New York, NY 10027, USA.