1. Department of Mechanical and Aerospace Engineering, WVNano Initiative, West Virginia University, Morgantown, West Virginia 26506-6106, Department of Physics, West Virginia University, Morgantown, West Virginia 26506, State Key Laboratory of Silicon Materials, Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, P.R. China, and National Energy Technology Laboratory, U.S. Department of Energy, Morgantown, West Virginia 26507