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Posted: Jul 25, 2016

Coming to a monitor near you: Notorious 'perfection'

(Nanowerk News) Defects, such as missing atoms, are believed to severely limit the photoluminescence efficiency in semiconductors, such as MoS2. Repairing or passivating these defects dramatically improves the photoluminescence efficiency.
A monolayer of a MoS2 was treated by dipping it in a superacid. The superacid filled in missing MoS2 atoms (repairing these defects) and removed contamination that limited the emission efficiency. The treated films exhibited a nearly 100% photoluminescence efficiency (Nano Letters, "Recombination kinetics and effects of superacid treatment in sulfur- and selenium-based transition metal dichalcogenides").
artistic rendering shows a laser beam generating excitons (bound electron and hole) in an atomically thin molybdenum disulfide (MoS<span class='subscript' style=2) semiconductor film" border="0" align="middle">
The artistic rendering shows a laser beam generating excitons (bound electron and hole) in an atomically thin molybdenum disulfide (MoS2) semiconductor film. Relaxation of these excitons normally results in weak light emission (photoluminescence). Scientists found that treating the surface with a superacid repairs most of the defects in the MoS2 film and leads to dramatic enhancement of the photoluminescence efficiency, increasing the percentage of emitted light to nearly 100% of the energy absorbed. (Image: Der-Hsien Lien of University of California-Berkeley, Lawrence Berkeley National Laboratory, King Abdullah University of Science and Technology, and National Taiwan University)
These “perfect,” efficient, thin semiconductor films could lead to flexible LED displays. Because MoS2 is only three atoms thick, it also has low absorption (~10%), displays could be designed such that they become transparent when powered off. It is also promising for the development of advanced computer chips based on low-energy electronic switches such as tunnel transistors, which require device architectures that are much more sensitive to defects than the devices used in current computer chips.
The photoluminescence efficiency is characterized by the ratio of light emitted to the amount of energy deposited into the system. For advanced opto-electronics, one would like the efficiency in a material to be as high as possible (close to 100%). It is believed that the defects limit the luminescence in two-dimensional (2D) semiconductors.
Now researchers led by Lawrence Berkeley National Laboratory and University of California-Berkeley have demonstrated—for the first time—the ability to increase the quantum yield from less than 1% to over 95% for an atomically thin MoS2 semiconductor film.
This discovery offers promising new materials for optoelectronic applications. Enhancement of extremely poor efficiencies to nearly 100% was achieved by repairing defects in the material with a superacid treatment. To repair the defects in the monolayer film, the 2D semiconductor was dipped into a superacid—an acid with an acidity greater than 100% purse sulfuric acid. The superacid loaned protons, filling in missing atoms (defects) in the monolayer MoS2.
Also the superacid cleaned the surface of the semiconductor. This surface treatment resulted in better material quality and higher efficiencies.
These perfect monolayer semiconductors could lead to the development of highly efficient light-emitting diodes, lasers, and solar cells based on 2D materials.
Source: U.S. Department of Energy
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