We wish to acknowledge the funding support for this project from Nanyang Technological University under
the Undergraduate Research Experience on Campus (URECA) program, Singapore A*STAR ‘Advanced Optics in Engineering’ Program under Grant No. 1223600006, funding support from Nanyang Technological University (Startup grant: Lei Wei) and a NRF-CRP grant (Non-volatile logic and memory integrata circuit devices, NRF-CRP9-2011-01). Also, we sincerely appreciate the assistance of Dr Chinkhanlun Guite from
School of Physical and Mathematical Science (SPMS) of Nanyang Technological University for the experiment setup.