An emerging class of atomically thin materials known as 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 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 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 1 percent up to 100 percent by dipping the material into a superacid called bistriflimide, or TFSI.
Their findings, to be published in the Nov. 27 issue of Science, opens the door to the practical application of monolayer materials, such as MoS2, in optoelectronic devices and high-performance transistors. MoS2 is a mere seven-tenths of a nanometer thick. For comparison, a strand of human DNA is 2.5 nanometers in diameter.
"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 Ph.D. student Matin Amani, visiting Ph.D. 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 characterized 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 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.
"The defect-free monolayers developed here could solve this problem in addition to allowing for new types of low-energy switches," said Javey.
Xiang Zhang and Eli Yablonovitch, faculty members with joint appointments at UC Berkeley's College of Engineering and Berkeley Lab, co-authored this paper. Other co-authors include researchers from National Taiwan University, the University of Texas and the U.S. Army Research Laboratory in Maryland.
The work was funded by the U.S. 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.
Sarah Yang | EurekAlert!
A new tool for discovering nanoporous materials
23.05.2017 | Ecole Polytechnique Fédérale de Lausanne
Did you know that packaging is becoming intelligent through flash systems?
23.05.2017 | Heraeus Noblelight GmbH
An international team of physicists has monitored the scattering behaviour of electrons in a non-conducting material in real-time. Their insights could be beneficial for radiotherapy.
We can refer to electrons in non-conducting materials as ‘sluggish’. Typically, they remain fixed in a location, deep inside an atomic composite. It is hence...
Two-dimensional magnetic structures are regarded as a promising material for new types of data storage, since the magnetic properties of individual molecular building blocks can be investigated and modified. For the first time, researchers have now produced a wafer-thin ferrimagnet, in which molecules with different magnetic centers arrange themselves on a gold surface to form a checkerboard pattern. Scientists at the Swiss Nanoscience Institute at the University of Basel and the Paul Scherrer Institute published their findings in the journal Nature Communications.
Ferrimagnets are composed of two centers which are magnetized at different strengths and point in opposing directions. Two-dimensional, quasi-flat ferrimagnets...
An Australian-Chinese research team has created the world's thinnest hologram, paving the way towards the integration of 3D holography into everyday...
In the race to produce a quantum computer, a number of projects are seeking a way to create quantum bits -- or qubits -- that are stable, meaning they are not much affected by changes in their environment. This normally needs highly nonlinear non-dissipative elements capable of functioning at very low temperatures.
In pursuit of this goal, researchers at EPFL's Laboratory of Photonics and Quantum Measurements LPQM (STI/SB), have investigated a nonlinear graphene-based...
Dental plaque and the viscous brown slime in drainpipes are two familiar examples of bacterial biofilms. Removing such bacterial depositions from surfaces is...
23.05.2017 | Event News
22.05.2017 | Event News
17.05.2017 | Event News
23.05.2017 | Physics and Astronomy
23.05.2017 | Life Sciences
23.05.2017 | Medical Engineering