[1] Yale Univ, Dept Elect Engn, New Haven, CT 06520 USA
[2] Yale Univ, Dept Appl Phys, New Haven, CT 06520 USA
[3] Yale Univ, Dept Phys, New Haven, CT 06520 USA
[4] Rice Univ, Dept Chem, Houston, TX 77005 USA
[5] Rice Univ, Ctr Nanoscale Sci & Technol, Houston, TX 77005 USA