1. Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore
138669, Singapore
2. IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120,
United States
3. Materials Science
and Engineering
Division, Materials Measurement Laboratory, National Institute of Standards and Technology (NIST), 100 Bureau
Drive, Gaithersburg, Maryland 20899-8541, United States