1. a Department of Physics , Beni-Suef Faculty of Science, Cairo University , Beni-Suef , Egypt
2. c Frederick Seitz Materials Research Laboratory , University of Illinois at Urbana-Champaign , 104 S. Goodwin Ave., Urbana , IL , 61801 , USA E-mail:
3. b Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory , Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign , IL 61801 , USA