[1] Univ Wisconsin Madison, Dept Mat Sci & Engn, Madison, WI 53706 USA
[2] Univ Wisconsin Madison, Dept Phys, Madison, WI 53706 USA
[3] Univ Wisconsin Madison, Dept Elect & Comp Engn, Madison, WI 53706 USA
[4] Univ Hong Kong, Fac Sci, Hong Kong, Peoples R China
[5] Univ Hong Kong, Fac Engn, Hong Kong, Peoples R China