1. State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, China
2. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371
3. NEST Istituto Nanoscienze-CNR and Dipartimento di Fisica dell'Università di Pisa, Largo B. Pontecorvo 3, Pisa 56127, Italy
4. NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798