1. Division of Physics
and Applied Physics, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore 637371
2. Institut
de Ciència de
Materials de Barcelona, ICMAB-CSIC, Campus
de la UAB, 08193 Bellaterra, Catalonia, Spain
3. Laboratorio de Microscopías
Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA) - ARAID
and Departamento de Fisica de la Materia Condensada, Universidad de Zaragoza, 50018 Zaragoza, Spain
4. Institució Catalana de
Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Catalonia, Spain
5. Division of Microelectronics,
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798