1. School
of Electrical and Electronic Engineering, Nanyang Technological University, 639798, Singapore
2. CNRS, Laboratoire de Physique des Interfaces et Couches Minces (LPICM), Ecole Polytechnique, 91128 Palaiseau, France
3. Institut d’Electronique Fondamentale, UMR 8622 CNRS, University Paris-Sud, 91405 Orsay, France