US team makes defect-free MoS2 film
Schematic of a laser beam energising a monolayer semiconductor made up of MoS2. The red glowing dots are particles excited by the laser.
Monolayer semiconductors such as MoS2 have generated a great deal of interest for developing transparent LED displays, ultra-high efficiency solar cells, photo detectors and nanoscale transistors. But the films suffer from defects, which adversely affects their performance.
Now a research team, led by engineers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory, has found a simple way to fix defects in MoS2. The chemical treatment, which involves dipping the material into an organic superacid called bistriflimide (or TFSI), led to a 100-fold increase in the material's photoluminescence quantum yield.
Quantum yield is 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 MoS2, from less than 1 percent up to 100 percent.
According to the team, these findings, published in the November 27 issue of Science, open the door to the practical application of monolayer materials, such as MoS2, in optoelectronic devices and high-performance transistors.
"Traditionally, the thinner the material, the more sensitive it is to defects," said principal investigator Ali Javey, UC Berkeley professor of electrical engineering and computer sciences and a faculty scientist at Berkeley Lab. "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, by definition, they are solutions with a propensity to 'give' protons, often in the form of hydrogen atoms, to other substances. This chemical reaction, called protonation, has the effect of filling in for the missing atoms at the site of defects as well as removing unwanted contaminants stuck on the surface, the researchers said.
Co-lead authors of the paper are UC Berkeley PhD student Matin Amani, visiting PhD student Der-Hsien Lien and postdoctoral fellow Daisuke Kiriya.
They noted that scientists have been pursuing monolayer semiconductors because of their 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, specifically, is characterised by molecular layers held together by van der Waals forces, a type of atomic bonding between each layer that is atomically sharp. An added benefit of having a material that is so thin is that it is 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 colours 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. "The defect-free monolayers developed here could solve this problem in addition to allowing for new types of low-energy switches," said Javey.
Other co-authors include researchers from National Taiwan University, the University of Texas and the US Army Research Laboratory in Maryland.
The work was funded by the US Department of Energy, the National Science Foundation Center for Energy Efficient Electronics and Science at UC Berkeley, Samsung and the Center for Low Energy System Technology.
