To build components for flexible electronics, some engineers have turned to thin films made from semiconductor nanocrystals. Unfortunately, many of these nanomaterials lose their unique electronic properties when exposed to air or solvent, making them incompatible with large-scale fabrication methods. Now researchers report a simple treatment that fixes the problem (ACS Nano 2013, DOI: 10.1021/nn403752d).
The treatment could move nanocrystal-based electronics out of the lab and into commercial applications, the researchers say, leading to low-cost, flexible electronics such as solar cells and displays.
Cherie R. Kagan, a materials scientist at the University of Pennsylvania, and her team developed the repair while working with cadmium selenide (CdSe) nanocrystals, which are one of the most intensely studied classes of nanocrystals. Other researchers have explored using the materials to increase the performance of transistors, photovoltaic devices, and optoelectronic systems such as light-emitting diodes. Additionally, CdSe nanocrystals have already found some commercial applications with displays: The company QD Vision has partnered with Sony to sell a device that produces a wider color gamut than is possible with traditional displays.
The problem with CdSe and other nanocrystals, however, is that their high surface-to-volume ratio enhances their reactivity, making them particularly sensitive to water, oxygen, and many solvents. Because exposure to air and solvents leads to surface defects that impede performance, engineers must fabricate devices with the materials under inert atmospheres and in dry conditions. These special conditions aren’t amenable to large-scale fabrication.
Kagan’s group found that they could fix the defects with indium metal. In previous work, the researchers had doped nanocrystal thin films in transistors with indium to improve the materials’ electronic properties. With this new work, the team tried to repair air and solvent defects by evaporating indium metal onto similar nanocrystal thin films and then heating the films. They thought the indium would bond with the films’ surfaces, replacing oxygen and other molecules responsible for the defects. “If you think of [a defect] as a hole you have to plug,” Kagan explains, “the indium kind of comes in and fills that spot.”
To demonstrate the approach, the team tested the performance of thin-film transistors made from CdSe nanocrystals, with and without the indium treatment. The indium-infused films performed better in a number of electrical performance tests. For example, electron mobility was about 50 times greater in the treated films than in the untreated ones. Based on data from voltammetry and ultraviolet-visible spectroscopy, the researchers concluded that the indium treatment repairs the thin films by forcing oxygen and water molecules to desorb from the films’ surfaces.
Kagan says future work will include using the indium fix with more complex circuitry and exploring the possibility of using nanocrystals other than CdSe.
Prior to this work, says Dmitri V. Talapin, a chemist at the University of Chicago, it was unclear whether nanocrystal-based devices could move beyond a lab curiosity and into real-world applications. “This work convincingly shows that these things can operate in air,” he says. “It’s a big deal.”