1. Graphene Research Centre and Department of Chemistry; National University of Singapore; 6 Science Drive 2 Singapore 117546 Singapore
2. The Blackett Laboratory; Department of Physics; Imperial College London; London SW7 2AZ UK
3. Chair in Hybrid Nanosystems; Ludwig-Maximilians-Universität München; München 80799 Germany
4. Department of Materials Science & Engineering; National University of Singapore; Singapore 117575 Singapore