Schematic of a
laser beam energizing a monolayer semiconductor made of
molybdenum
disulfide, or MoS2. The red glowing dots are particles excited by
the laser. (Image
by Der-Hsien Lien)
(November 26, 2015) 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 UC 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.
Shown is a MoS2
monolayer semiconductor shaped into a Cal logo. The image on the left
shows the material
before it was treated with superacid. On the right is the monolayer after
treatment. The
researchers were able to achieve two orders of magnitude improvement in
emitted light with
the superacid treatment. (Image by Matin Amani)
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.