[1] Univ Calif San Diego, Dept Nanoengn, La Jolla, CA 92093 USA
[2] Univ Calif San Diego, Dept Elect & Comp Engn, La Jolla, CA 92093 USA
[3] Univ Penn, Dept Mat Sci & Engn, 3231 Walnut St, Philadelphia, PA 19104 USA
[4] Univ Penn, Dept Phys & Astron, Philadelphia, PA 19104 USA