[1
] Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California 94035,
USA
[2
] Department of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701,
Korea
[3
] National Nanofab Center, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea