Transparent, brighter LEDs may soon become a reality

University of California - Berkeley researchers have discovered a simple way of making monolayer semiconductors - which are less than a nanometre thick - more efficient and defect free.

The finding opens the door to the practical application of monolayer materials, such as MoS2, in devices like LEDs and high-performance transistors.

"One could develop high-performance LED displays that are transparent when powered off and flexible using the 'perfect' optoelectronic monolayers produced in this study," the researchers said.

Monolayer semiconductors have generated a great deal of buzz as they hold promise in the development of transparent LED displays, ultra-high efficiency solar cells, photo detectors and nanoscale transistors.

But the films are notoriously riddled with defects, killing their performance.

The UC-Berkeley team found a simple way to fix these defects through the use of an organic superacid.

The chemical treatment led to a dramatic 100-fold 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.

The researchers enhanced the quantum yield for molybdenum disulfide, or MoS2, from less than one percent up to 100 percent by dipping the material into a superacid called bistriflimide, or TFSI.

"Traditionally, the thinner the material, the more sensitive it is to defects," said principal investigator professor Ali Javey.

"This study presents the first demonstration of an opto-electronically perfect monolayer, which previously had been unheard of in a material this thin," Javey added.

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.

"The defect-free monolayers developed here could solve this problem in addition to allowing for new types of low-energy switches," Javey said.

The study was published in the journal Science.