Schematic of a laser beam energizing a monolayer semiconductor made of molybdenum disulfide (MoS2). The red glowing dots are particles excited by the laser. (Image courtesy of Der-Hsien Lien/UC Berkeley.)
Monolayer semiconductors are an emerging class of atomically thin materials. Scientists and engineers are pursuing these semiconductors for their low light absorption and their durability under mechanical deformation.
These characteristics make monolayer semiconductors a promising technology for the development of flexible and transparent LED displays, ultra-high efficiency solar cells, photo detectors and nanoscale transistors.
But for all their potential, these films have one impediment: they are typically riddled with defects on the nanoscale.
Now a simple way to fix these defects using an organic super acid is under development by engineers at the University of California-Berkeley. Using a chemical treatment with a super acid, the material’s photoluminescence quantum yield experiences a 100-fold increase.
A material’s photoluminescence quantum yield is the ratio of the light generated by a material versus the amount of energy put into the material. The greater the light emission, the higher the quantum yield will be and the higher the quality.
In its experiments, the team enhanced the quantum yield for a monolayer made of molybdenum disulfide (MoS2) from one percent up to 100 percent by dipping the material into a super acid called bistriflimide (TFSI).
The UC Berkeley team used super acids because these solutions have a propensity to protonate or “give” protons to other substances. The main effect is that the “given” hydrogen atoms will fill in for the missing atoms at the site of the defect. This process will also remove contaminants stuck to the surface of the material.
"Traditionally, the thinner the material, the more sensitive it is to defects," said Ali Javey, UC Berkeley professor of electrical engineering and computer sciences. "This study presents the first demonstration of an optoelectronically perfect monolayer, which previously had been unheard of in a material this thin."
MoS2 is characterized by molecular layers that are held together with van der Waals forces.
A benefit of a material this thin is that it is electrically tunable. This means that for applications such as LED displays, a single pixel could be made to emit a wide range of colors by varying the amount of voltage applied.
The researchers pointed out that the LED efficiency is directly linked to the photoluminescence quantum yield. They envision, for example, the development of high-performance LED displays that could be transparent when powered off or displays and solar cells that are extremely flexible due to the use of optoelectric monolayers.
There are also potentially revolutionary applications for the development of transistors. As computer chips become smaller and thinner, defects in semiconductors begin to play a more significant role in limiting performance. A defect-free semiconductor will enable more reliable and efficient chips, leading to even greater innovations in electronic devices.
The team’s paper describing its research is published in Science and can be read here. For more information, visit the University of California – Berkeley and the Javey Research Group websites.