[1] Univ Calif Riverside, Dept Phys & Astron, Riverside, CA 92521 USA
[2] Univ Calif Los Angeles, Dept Elect Engn, Device Res Lab, Los Angeles, CA 90095 USA
[3] Univ Queensland, Sch Engn, Brisbane, Qld 4072, Australia
[4] Univ Queensland, Ctr Microscopy & Microanal, Brisbane, Qld 4072, Australia