[1] IBM Corp, Thomas J Watson Res Ctr, Yorktown Hts, NY 10598 USA
[2] Yale Univ, Dept Elect Engn, New Haven, CT 06511 USA
[3] Univ Minnesota, Dept Elect & Comp Engn, Minneapolis, MN 55455 USA
[4] Boston Univ, Boston, MA 02215 USA
[5] Natl Univ Singapore, Graphene Res Ctr, Singapore 117542, Singapore
[6] Natl Univ Singapore, Dept Phys, Singapore 117542, Singapore
[7] Zhejiang Normal Univ, Jinhua 321004, Peoples R China