This light micrograph shows two-dimensional crystals of the thin-film semiconductor molybdenum disulfide. Image: Hisato Yamaguchi (Los Alamos National Laboratory, USA).Based on a study of the optical properties of novel ultrathin semiconductors, researchers at Ludwig-Maximilians-Universitaet (LMU) in Munich, Germany, have developed a method for the rapid and efficient characterization of these materials.
Chemical compounds based on elements that belong to the so-called transition metals can be processed to yield atomically-thin, two-dimensional crystals known as transition metal dichalcogenides (TMDs), which are semiconductors with surprising optical properties. Using their novel optical characterization method, a team of LMU physicists led by Alexander Högele, in cooperation with colleagues in the US, has now explored the properties of thin-film semiconductors made from TMDs. The researchers report their findings in a paper in Nature Nanotechnology.
These semiconductors exhibit a remarkably strong interaction with light and therefore have great potential for applications in the field of opto-electronics. In particular, the electrons in these materials can be excited with polarized light.
"Circularly polarized light generates charge carriers that exhibit either left- or right-handed circular motion," explains Högele. "The associated angular momentum is quantized and described by the so-called valley index, which can be detected as valley polarization." Just like quantum mechanical spin, the valley index can be used to encode information, making it useful for for applications such as quantum computing.
However, recent studies of the valley index in TMD semiconductors have produced controversial results, with different groups worldwide reporting inconsistent values for the degree of valley polarization. With the aid of their newly-developed polarimetric method and using monolayers of the semiconducting TMD molybdenum disulfide as a model system, the LMU researchers have now clarified the reasons for these discrepancies.
"Response to polarized light turns out to be very sensitive to the quality of the crystals, and can thus vary significantly within the same crystal," says Högele. "The interplay between crystal quality and valley polarization will allow us to measure rapidly and efficiently those properties of the sample that are relevant for applications based on the valley quantum degree of freedom."
The new characterization method can be applied to other monolayer semiconductors and systems made up of several different materials. In the future, this will enable the functionalities of devices based on atomically-thin semiconductors – such as novel types of LEDs – to be characterized swiftly and economically.
This story is adapted from material from Ludwig-Maximilians-Universitaet, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.