Issue Date: September 9, 2013 | Web Date: September 5, 2013
Nanocrystal-Based Electronics Repaired With Indium
To build components for improved light-emitting diodes, solar cells, and other electronics, engineers want to use flexible thin films made from semiconductor nanocrystals. Many of these materials unfortunately pick up performance-dwindling defects when exposed to air or solvent, making them incompatible with large-scale fabrication methods. A new treatment infusing the films with indium fixes the problem and could help move nanocrystal-based electronics out of the lab and into broad commercial applications (ACS Nano 2013, DOI: 10.1021/nn403752d).
A team led by Cherie R. Kagan of the University of Pennsylvania developed the repair technique while working with cadmium selenide nanocrystals, one of the most intensely studied types of nanocrystals. The materials are already used in a few commercial applications. The company QD Vision, for example, has partnered with Sony to sell a CdSe-based display that produces a greater color gamut than is possible with traditional displays.
Nanocrystals are favored materials in electronics because their high surface-area-to-volume ratio enhances their reactivity. But that reactivity typically is short-lived—exposure to air and solvents leads to surface defects that ultimately impede performance. Engineers must fabricate devices with the materials under an inert atmosphere and in dry conditions, which often aren’t compatible with large-scale fabrication techniques.
Kagan’s group thought it could fix the defects with indium, a metal commonly used to dope electronic materials to improve their performance. They envisioned that the indium atoms would bind 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 says, “the indium comes in and fills that spot.”
The treatment step is simple: The team evaporates indium metal in the presence of the nanocrystal thin films and then heats the films. Thin-film transistors made using CdSe and infused with indium outperformed untreated transistors in a number of electrical performance tests. For example, electron mobility in the treated films was about 50 times greater than in the untreated ones. On the basis of 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 using nanocrystals other than CdSe.
Prior to this work, says Dmitri V. Talapin, a nanomaterials 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.
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