Coming to a Monitor near you: Defect-free, Molecule-thick Film

Schematic of a laser beam energizing a monolayer semiconductor made up of molybdenum disulfide (MoS2). The red glowing dots are particles excited by the laser. Credit: Der-Hsien Lien Read more at: http://phys.org/news/2015-11-defect-free-molecule-thick.html#jCp

An emerging class of atomically thin materials, monolayer semiconductors has generated a great deal of buzz in the world of materials science. Monolayers hold promise in the development of transparent LED displays, ultra-high efficiency solar cells, photo detectors and nanoscale transistors. Their downside? The films are notoriously riddled with defects, killing their performance.

But a UCLA, Berkeley, and Lawrence Berkeley National Lab team, has found a simple way to fix these defects via an organic superacid. The chemical treatment led to a dramatic 100X increase in the material’s photoluminescence quantum yield, a ratio describing the amount of light generated by the material versus the amount of energy put in. The greater the emission of light, the higher the quantum yield and the better the material quality.

A MoS2 monolayer semiconductor shaped into a Cal logo. The image on the left shows the material before it was treated with superacid. On the right is the monolayer after treatment. The researchers were able to achieve two orders of magnitude improvement in emitted light with the superacid treatment. Credit: Image by Matin Amani Read more at: http://phys.org/news/2015-11-defect-free-molecule-thick.html#jCp

They enhanced quantum yield for molybdenum disulfide, or MoS2, from <1% up to 100% by dipping the material into a superacid called bistriflimide, or TFSI. Their findings, opens the door to the practical application of monolayer materials, such as MoS2, in optoelectronic devices and high-performance transistors. MoS2 is a mere 7/10’s of a nanometer thick.

“Traditionally, the thinner the material, the more sensitive it is to defects,” said Prof Ali Javey. “This study presents the first demonstration of an optoelectronically perfect monolayer, which previously had been unheard of in a material this thin.”
The researchers looked to superacids because they are solutions that “give” protons to other substances ie , protonation. It fills in for the missing atoms at the site of defects + removing unwanted contaminants stuck on the surface, the researchers said.

Monolayer semiconductors have low absorption of light and their ability to withstand twists, bends and other extreme forms of mechanical deformation, which can enable their use in transparent or flexible devices. MoS2 is characterized by molecular layers held together by van der Waals forces. Its thinness also allows highly electrically tunable. For applications such as LED displays, this feature may allow devices to be made where a single pixel could emit a wide range of colors rather than just one by varying the amount of voltage applied.

The lead authors added that the efficiency of an LED is directly related to the photoluminescence quantum yield so, in principle, one could develop high-performance LED displays that are transparent when powered off and flexible using the “perfect” optoelectronic monolayers produced in this study.
This treatment also has revolutionary potential for transistors. As devices in computer chips get smaller and thinner, defects play a bigger role in limiting their performance. http://news.berkeley.edu/2015/11/26/defect-free-monolayer-semiconductor/