Method Positions and Deposits ZnO Nanowires

Daniel S. Burgess

Scientists at the University of Wisconsin-Milwaukee have employed electric fields to align single crystalline ZnO nanowires with support electrodes and electrostatic forces to bind them in place. In a demonstration of the potential utility of their approach, they also discovered that such deposited ZnO nanowires may be used as tips for atomic force microscopes (AFMs) to improve resolution.

The deposition of ZnO nanorods on microscale support electrodes has potential applications in the fabrication of tips for atomic force microscopes. Such tips have been found to yield higher resolutions than standard silicon tips, for reasons yet to be determined.

The technique, said Nikolai Kouklin of the department of electrical engineering, also may be used to create nanowire-based devices such as miniaturized photodetectors and light sources, lasers, and biological and chemical sensors and may have applications in nanolithography.

Kouklin, who developed the technique with Somaditya Sen, explained that attaching nanoscale structures to such microscale supports typically involves time-consuming pick-and-place strategies using scanning electron microscopes equipped with manually directed nanomanipulators or relatively exotic direct-growth techniques that are difficult to control. The new method instead relies on the tendency of high-aspect-ratio nanowires suspended in solution to align with an external electric field and on electrostatic attraction to attach them to the desired microscale support.

In their experiments, the scientists placed droplets of chloroform in which they had suspended ZnO nanowires grown by chemical vapor deposition between a pair of tungsten microelectrodes. They applied a current across the gap to align the nanowires along the field lines parallel to the main axes of the electrodes. As the chloroform evaporated, surface tension would draw the solution up one of the electrodes to form a droplet at its tip, where a nanowire would be deposited.

They discovered that using a 3-μm-long, 20-nm-diameter wire deposited on a silicon AFM tip to image an ion-etched GaN sample with 60-nm-across, 400-nm-deep pores offered higher resolution than using a conventional silicon tip. The reason for the improved performance remains to be determined, but Kouklin suggested that the piezoelectric response of ZnO nanowires or electrostatic interactions involving plasmonic effects at the nanopores may be responsible.

In fact, although the hardness, chemical stability and optical properties of ZnO nanowires make them of interest as high-performance probe tips for AFM and near-field scanning optical microscopy, Kouklin noted that the discovery was serendipitous.

“Our general interest in using this technique lies in developing a future platform for improved biochemical sensing and detection using nanowires as mechanical resonators,” he said.

Applied Physics Letters, Sept. 18, 2006, 123114.