1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
2 Low Energy Electronic Systems (LEES), Singapore-MIT Alliance for Research and Technology, Singapore 138602, Singapore
3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4 Materials Research Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
5 Current address: Max Planck Institute of Microstructure Physics, Halle (Saale) 06120, Germany
6 Current address: Southern University of Science and Technology, Shenzhen 518055, China
7 e-mail: jmichel@mit.edu