a
Department of Electrical and Computer Engineering, University of California, 9500 Gilman Drive, La Jolla, San Diego, USA
E-mail: dwang@ece.ucsd.edu
b
International Center for Materials Nanoarchitectonics (MANA), National Institute for Material Science, Namiki 1-1, Tsukuba, Japan
c
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 109 Remin Street, Changchun, China
d
Materials Science and Engineering, University of California, 9500 Gilman Drive, La Jolla, San Diego, USA
e
California Institute of Telecommunication and Information Technology, University of California, Gilman Dr, La Jolla, San Diego 9500, USA