1. Department of Physics, P. O. Box 11155-9161, Sharif University of Technology, Tehran, Iran, Institute for Nanoscience and Nanotechnology, P. O. Box 14588-89694, Sharif University of Technology, Tehran, Iran, Nano-Electronics and Thin Film Laboratory, Department of Electrical and Computer Engineering, North Kargar Avenue, P. O. Box 14395/515, University of Tehran, Tehran, Iran, and Electronic Research Center, Tehran, Iran