1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
2. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 P.R. China
3. International Center for Materials Nanoarchitectonics; National Institute for Materials Science; 1-1 Namiki Tsukuba 305-0044 Japan
4. Key Laboratory for the Physics and Chemistry of Nanodevices; Department of Electronics; Peking University; Beijing 100871 P.R. China