The development of novel nanoelectronics and photonic devices has been given a boost by research into the edge states of molybdenum disulphide.
Scientists with the US Department of Energy’s (DOE) Lawrence Berkeley National Laboratory 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 the material in nanoelectronics, as well as a catalyst for the hydrogen evolution reaction in fuel cells, desulphurisation and other chemical reactions.
The research was described in a paper in Science. ‘We observed strong nonlinear optical resonances at the edges of a two-dimensional crystal of molybdenum disulphide,’ said 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.’
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 light-emitting diodes.
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.
‘Our nonlinear optical imaging technique is a non-invasive, fast, easy metrologic approach to the study of 2D atomic materials,’ said Xiaobo Yin, the lead author of the Science paper. ‘We don't need to prepare the sample on any special substrate or vacuum environment, and the measurement won't perturb the sample during the imaging process. This advantage allows for in-situ measurements under many practical conditions. Furthermore, our imaging technique is an ultrafast measurement that can provide critical dynamic information, and its instrumentation is far less complicated and less expensive compared with scanning tunnelling microscopy and transmission electron microscopy.’
For the SHG imaging of molybdenum disulphide, Zhang and his collaborators illuminated sample membranes that are only three atoms thick with ultrafast pulses of infrared light. The nonlinear optical properties of the samples yielded a strong SHG response in the form of visible light that is both tunable and coherent. The resulting SHG-generated images enabled the researchers to detect ‘structural discontinuities’ or edges along the 2D crystals only a few atoms wide where the translational symmetry of the crystal was broken.
‘By analysing the polarised components of the SHG signals, we were able to map the crystal orientation of the molybdenum disulphide atomic membrane,’ said Ziliang Ye, the co-lead author of the paper. ‘This allowed us to capture a complete map of the crystal grain structures, colour-coded according to crystal orientation. We now have a real-time, non-invasive tool that allows us explore the structural, optical, and electronic properties of 2D atomic layers of transition metal dichalcogenides over a large area.’