1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
2. Graphene Research Centre, National University of Singapore, 6 Science Drive 2, 117546, Singapore
3. Departamento de Física, ICEx, Universidade Federal de Minas Gerais, Avenida Antônio Carlos 6627,
Pampulha, 30123-970, Belo Horizonte, Minas Gerais, Brazil
4. Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore