1. Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
2. Electron Microscope Facility, TIA, AIST, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan
3. Global Zero Emission Research Center, AIST, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan