Valleytronics, based on the valley degree of freedom, is a promising candidate for next-generation information devices beyond traditional field-effect transistors. In recent years, the emergence of two-dimensional materials such as transition metal dichalcogenides (TMDs) has provided a new platform for the research of valleytronics. In these materials, valley-polarized information can be easily written and read by optical approaches. Specifically, interlayer excitons (IXs) in TMDs-based heterostructures carry valley-polarized information and thus would find promising applications in valleytronics taking advantage of their long lifetime.
Previous studies have demonstrated that IXs exhibit a large valley-polarization degree that can be tuned in a wide range by external electric field, magnetic field and twist-angle engineering. Although considerable progress has been made in valleytronics, nonvolatile device that is indispensable for valleytronics has not been achieved up to date.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Dehui Li from School of Optical and Electronic Information, Huazhong University of Science and Technology, China, and co-workers have developed a nonvolatile approach for the modulation of IXs. They also demonstrated a nonvolatile valley-addressable memory, which would prompt relevant investigations on valleytronics. These scientists summarize the new findings of their work:
“We find the emission characteristics of IXs in WS2/WSe2 heterostructures exhibit a large excitonic/valley-polarized hysteresis upon cyclic-voltage sweeping, which is ascribed to the chemical-doping of O2/H2O redox couple trapped between WSe2 and substrate. Taking advantage of the large hysteresis, a nonvolatile valley-addressable memory is successfully demonstrated. The valley-polarized information can be non-volatilely switched by electrical gating with retention time exceeding 60 minutes.”
“The method of creating the hysteresis effect is very simple, which only needs to fully clean the SiO2/Si substrate with oxygen plasma. The nonvolatile control of IXs provides a new method for the research of valleytronics, and is expected to promote the development of valleytronic devices, especially for nonvolatile valleytronic memories.” they added.
“In this study, although the electrons injected by electrochemical method are not valley-polarized, the remaining electrons generated after a process of circularly-polarized excitation and IXs recombination should theoretically have nonvolatile valley-polarization properties, which would play an important role in the application of future valleytronic devices. In addition, through structure optimization, it is possible for us to realize a nonvolatile device that can switch between positive and negative valley-polarized degrees, which would be a vital breakthrough for valleytronics and is worthy of further studies.” the scientists forecast.