image: a, Schematic of the device which is composed by a WS2/WSe2 heterostructure on a oxygen-plasma cleaned SiO2/Si substrate. b-c, Photon energy and absolute circular polarization degree of the IXs as a function of Vg, showing a large hysteresis. d, PL spectra of the IXs at 0 V with different scanning sequences (0V-1 ~ -60 V ~ 0V-2 ~ 60 V ~ 0V-3). The 0V-2 spectrum blueshifts for about 20 meV in comparison to 0V-1 and 0V-3. e-f, Schematic of the origin of the hysteresis effect. Chemical-doped electrons lift the Fermi level up and shift IXs from spin-triplet state (IXS) to spin-singlet state (IXT). When those electrons are released, the IXs return from IXT to IXS. The orange dashed line stands for the upper spin-splitting conduction band (CB) of WS2. Red and blue arrows represent the recombination paths of the IXs. view more
Credit: by Tong Ye, Yongzhuo Li, Junze Li, Hongzhi Shen, Junwen Ren, Cun-Zheng Ning, Dehui Li
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.
Journal
Light Science & Applications