Monolayers have the potential to be used in transparent LED displays, nanoscale transistors, photodetectors, and ultra-high efficiency solar cells. Scientists have suggested that their low absorption of light and their ability to withstand bends, twists, and other extreme kinds of mechanical deformation, may eventually enable their use in transparent or flexible devices. However, so far the films are usually riddled with defects, killing their performance.
A research team led by engineers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory, has devised a simple way to fix these defects using an organic superacid. According to the researchers, the chemical treatment led to a 100-fold increase in the material’s photoluminescence quantum yield. Quantum yield is a ratio of the amount of light generated by a material versus the energy input.
The greater the light output, the higher the quantum yield and the better the material quality. The researchers reportedly improved the quantum yield for molybdenum disulfide (MoS2) from below 1 percent up to 100 percent. They achieved this improvement by dipping the material into a superacid called bistriflimide, or TFSI.
The findings will be published in the Nov. 27 issue of Science. Co-lead authors of the paper are UC Berkeley Ph.D. student Matin Amani, visiting Ph.D. student Der-Hsien Lien and postdoctoral fellow Daisuke Kiriya.
The researchers chose to examine superacids because, by definition, their solutions have a propensity to “give” protons, frequently as hydrogen atoms, to other substances in a chemical reaction, called protonation. According to the researchers, protonation effectively fills in for the missing atoms at defect sites and removes unwanted contaminants from the surface.
The technique could lead to practical uses of monolayer materials, such as MoS2, in optoelectronic devices and high-performance transistors. MoS2 is a just 0.7 nanometers thick. For comparison, a human DNA strand measures 2.5 nanometers in diameter.
Ali Javey, UC Berkeley professor of electrical engineering and computer sciences and a faculty scientist at Berkeley Lab stated, “This study presents the first demonstration of an optoelectronically perfect monolayer, which previously had been unheard of in a material this thin.”
A material that is so thin is highly electrically tunable making it potentially suitable for applications such as LED displays in which a a varying amount of voltage could be applied to a single pixel to emit a wide range of colors rather than just one.
The lead authors also noted that an LED’s efficiency is directly related to the photoluminescence quantum yield. So, in theory, “perfect” monolayers such as those used in the study could be used to develop high-performance LED displays, which are flexible and are transparent when powered off.
This treatment also holds tremendous potential for smaller, lower power transistors. “The defect-free monolayers developed here could solve this problem in addition to allowing for new types of low-energy switches,” said Javey.
“Near-unity photoluminescence quantum yield in MoS2” Science, www.sciencemag.org/lookup/doi/10.1126/science.aad2114