1. NSF Nano-scale Science and Engineering Center (NSEC); University of California Berkeley; Berkeley CA 94720 USA
2. Materials Sciences Division; Lawrence Berkeley National Laboratory; Berkeley CA 94720 USA
3. Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory; Berkeley California 94720 USA
4. Department of Physics; King Abdulaziz University; Jeddah 21589 Saudi Arabia