1. Department of Materials Science and Engineering, and Inter-University Semiconductor Research Center; College of Engineering; Seoul National University; Seoul 151-744 Republic of Korea
2. Department of Electrical and Computer Engineering; University of Massachusetts; Amherst MA 01003 USA
3. Center for Nanotechnology; NASA Ames Research Center; Moffett Field CA 94035 USA
4. Hewlett Packard Laboratories; Hewlett Packard Enterprise; Palo Alto CA 94304 USA