1. Electrical Engineering Division; Department of Engineering; University of Cambridge; 9 JJ Thomson Avenue; Cambridge CB3 0FA; UK
2. Thin Film Centre; Scottish Universities Physics Alliance (SUPA); University of the West of Scotland; Paisley PA1 2BE; UK
3. School of Engineering and Physical Sciences; Scottish Universities Physics Alliance (SUPA); Heriot-Watt University; Edinburgh EH14 4AS; UK
4. Centre for Material Research and Innovation; University of Bolton; Deane Road; Bolton BL3 5AB; UK
5. School of Physical Electronics; University of Electronic Science and Technology of China; Chengdu 610054; P. R. China