news digest ♦ LEDs Looking at 2D molybdenum
disulphide in a new way Researchers have observed the 1D edge states vital to nanoelectronic and photonic applications
The drive to develop ultra-small and ultrafast electronic devices using a single atomic layer of semiconductors, such as transition metal dichalcogenides, has received a significant boost.
Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have recorded the first observations of a strong nonlinear optical resonance along the edges of a single layer of molybdenum disulphide.
The existence of these edge states is key to the use of molybdenum disulphide in nanoelectronics, as well as a catalyst for the hydrogen evolution reaction in fuel cells, desulphurisation and other chemical reactions.
“We observed strong nonlinear optical resonances at the edges of a two-dimensional crystal of molybdenum disulphide” says Xiang Zhang, a faculty scientist with Berkeley Lab’s Materials Sciences Division who led this study. “These one-dimensional edge states are the result of electronic structure changes and may enable novel nanoelectronics and photonic devices. These edges have also long been suspected to be the active sites for the electrocatalytic hydrogen evolution reaction in energy applications. We also discovered extraordinary second harmonic light generation properties that may be used for the in situ monitoring of electronic changes and chemical reactions that occur at the one-dimensional atomic edges.”
Zhang, who also holds the Ernest S. Kuh Endowed Chair Professor at the University of California (UC) Berkeley and directs the National Science Foundation’s Nano-scale Science and Engineering Centre, is the corresponding author of a paper in Science describing this research.
Emerging two-dimensional semiconductors are prized in the electronics industry for their superior energy efficiency and capacity to carry much higher current densities than silicon. Only a single molecule thick, they are well-suited for integrated optoelectronic devices.
Until recently, graphene has been the unchallenged superstar of 2D materials, but today there is considerable attention focused on 2D semiconducting crystals that consist of a single layer of transition metal atoms, such as molybdenum, tungsten or niobium, sandwiched between two layers of chalcogen atoms, such as sulphur or selenium.
Featuring the same flat hexagonal “honeycombed” structure as graphene and many of the same electrical advantages, these transition metal dichalcogenides, unlike graphene, have direct energy bandgaps. This facilitates their application in transistors and other electronic devices, particularly LEDs.
A new SHG imaging technique allows rapid and all-optical determination of the crystal orientations of 2D semiconductor membranes at a large scale, providing the knowledge needed to use these materials in nanoelectronic devices
“Our nonlinear optical imaging technique is a non-invasive, fast, easy metrologic approach to the study of 2D atomic materials,” says Xiaobo Yin, the lead author of the Science paper and a former member of Zhang’s research group who is now on the faculty at the University of Colorado, Boulder.
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www.compoundsemiconductor.net June 2014
(From left) Yu Ye, Xiaobo Yin, Xiang Zhang and Ziliang Ye used second-harmonic generation imaging to discover strong nonlinear optical resonance along the edges of single layers of molybdenum disulphide. (Photo by Roy Kaltschmidt)
Full realisation of the vast potential of transition metal dichalcogenides will only come with a better understanding of the domain orientations of their crystal structures that give rise to their exceptional properties.
Until now, however, experimental imaging of these three-atom- thick structures and their edges have been limited to scanning tunnelling microscopy and transmission electron microscopy, technologies that are often difficult to use.
Nonlinear optics at the crystal edges and boundaries enabled Zhang and his collaborators to develop a new imaging technique based on second-harmonic generation (SHG) light emissions that can easily capture the crystal structures and grain orientations with an optical microscope.
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