Nanowires Conduct Photocurrent

Photocopiers, infrared detectors, and optical receivers in fiber-optic telecommunication systems all depend on photoconductors–materials that conduct more electric current when exposed to light. Made on a nanometer scale, photoconductors could lead to a variety of tiny optoelectronic devices potentially useful in future generations of nanoelectronics, chemical sensors, and eventually provide clues to the fabrication of tiny solar cells.

Now Japanese researchers, led by Takuzo Aida, a professor in the department of chemistry and biotechnology at the University of Tokyo, have made a photoconductor from two different organic molecules that self-assemble into long, hollow nanotubes. The nanowires conduct almost no current in the dark, but when hit with light, they conduct 10,000 times more current. This could lead to cheap nanodevices that self-assemble out of a chemical solution.

To make a photoconductor, it’s important to have a junction between two segregated layers: one that donates charge and another that accepts it. Previously made photoconductors do not have separate donor and acceptor layers at the nanometer level, says Aida. The new photoconductor, which the researchers describe in this week’s Science, “is the first one that provides a nanoscale donor-acceptor heterojunction and exhibits a photoconductive property,” Aida says.

The researchers create a solution of two organic molecules, trinitrofluorenone (TNF) and hexabenzocoronene (HBC), in a solvent. When they expose this solution to methanol vapors at 25 °C, the organic molecules self-assemble into 16-nanometer-wide hollow tubes. The 3-nanometer-thick walls of the nanotubes are made of TNF layers, which act as the electron-accepting layer, laminating the HBC electron-donating layer.

When the researchers place the photoconductors between electrodes and apply a voltage, almost no current flows. But when illuminated with ultraviolet or visible light, the nanowires conduct electricity. “The electrical current under illumination is four orders of magnitude greater than that in the dark,” Aida says. “Such a large on-off ratio is very important for optoelectronic applications.”

Right now, the nanowires’ conductivity changes in response to light; they do not absorb light to generate electric current as solar cells do. But the layered structure of the nanotubes lays down a blueprint for converting light into electricity, because the interface between the donor and acceptor layers can be thought of as a p-n junction, the basic unit of a solar cell, says Frank Wurthner, a chemistry professor at the University of Wurzburg, in Germany.

Walter Smith, a physicist involved in nanoscale photoconductor research at Haverford College, calls the new work exciting because it’s the first example of a self-assembling system with a well-defined separation between donor and acceptor layers. “People are able to make very small solar cells, but being able to self-assemble them hopefully lowers the manufacture cost,” he says. Self-assembly also gives “extraordinary atomic-level precision in the relative placement of the components linking together to form a structure.”

An important advantage of the Japanese researchers’ method is that the molecules spontaneously assemble when they are exposed to methanol vapors. It’s important to have an external cue that triggers self-assembly, Smith says, because “eventually when we’re trying to build more-complex systems we can use different cues to initiate the self-assembly of different parts of the system.” The Japanese researchers’ work, he adds, “is a big step towards understanding the basic science of what drives self-assembly.”