Based on a concept published in Nano Letters in 2007 by Zhang and collaborators Lin-Wang Wang (Lawrence Berkeley National Laboratory) and Angelo Mascarenhas (National Renewable Energy Laboratory), the array was fabricated by Zhang's current collaborators Zhiming Wu, Jinjian Zheng, Xiangan Lin, Xiaohang Chen, Binwang Huang, Huiqiong Wang, Kai Huang, Shuping Li and Junyong Kang at the Fujian Key Laboratory of Semiconductor Materials and Applications in the Department of Physics at Xiamen University, China.
Past attempts to use high band gap materials did not actually use the semiconductors to absorb light but instead involved coating them with organic molecules (dyes) that accomplished the photo absorption and simply transmitted electrons to the semiconductor material. In contrast, the team's heterojunction nanowires absorb the light directly and efficiently conduct a current through nano-sized "coaxial" wires, which separate charges by putting the excited electrons in the wires' zinc oxide cores and the "holes" in the zinc selenide shells.
"By making a special heterojunction architecture at the nanoscale, we are also making coaxial nanowires which are good for conductivity," said Zhang. "Even if you have good light absorption and you are creating electron-hole pairs, you need to be able to take them out to the circuit to get current, so we need to have good conductivity. These coaxial nanowires are similar to the coaxial cable in electrical engineering. So basically we have two conducting channels the electron going one way in the core and the hole going the other way in the shell."
The nanowires were created by first growing an array of six-sided zinc oxide crystal "wires" from a thin film of the same material using vapor deposition. The technique created a forest of smooth-sided needle-like zinc oxide crystals with uniform diameters (
|Contact: James Hathaway|
University of North Carolina at Charlotte