Van der Waals HSs stacked by TMDs monolayers enable the generation of long-lived IXs with a large binding energy of about 150 meV1, and a long diffusion distance over five micrometres2, further extending the already appealing properties of the constituent TMDs monolayers. Since IXs are composed of electrons and holes that are resided in neighboring layers, their physical properties strongly depend on the layer configurations and external fields or dopings3-5. Through electrical field or doping, we can modulate the emission intensity and wavelength of the IX1,6, and even switch its polarization7. Recently, IX in the HSs stacked by other layered materials such as 2D perovskites and InSe with TMDs monolayer has been demonstrated and can be utilized in mid-infrared photodetections and valleytronics8,9. In particular, dark IX with a long lifetime exhibits unique merit in valleytronic devices. Nevertheless, the manipulation of dark exciton is hard and study on nonvolatile valleytronic memory remains elusive. In this work, we have successfully achieved nonvolatile valleytronic memory based on IX in TMDs HSs, which would greatly prompt relevant investigations on valleytronics.
The HS is formed by a monolayer WS2 (top) and a monolayer WSe2 (bottom), both of which are contacted with an electrode (Fig. 1a). By applying voltage between the electrode and the heavily-doped Si substrate, we can control the doping level of the device when performing optical measurements. Fig. 1b shows the optical microscopy image of the device. The WS2 and WSe2 sheets are mechanically exfoliated from their respective bulk crystals and then transferred on a SiO2/p++-Si substrate through dry-transfer technique10. The edges of the two sheets are intentionally aligned to improve interlayer coupling3.
IX in the WS2/WSe2 HS. Fig. 1c shows the PL spectra of the HS, from which we can observe a severe PL quenching and redshift of the intralayer excitonic peaks, together with the appearance of a low-energy peak at 1.4 eV. The quenching and redshift of the intralayer excitonic peaks can be attributed to the interlayer charge transfer11,12 and modified dielectric environment13,14, respectively. We ascribe the peak at 1.4 eV to the emission of IX according to previous reports15,16. The excitation-power dependent PL spectra further verify its interlayer nature (Fig. 1d). The IX emission peak shows a blueshift with the increase of excitation power, which is due to many-body effect arising from the repulsive interaction between the dipole-aligned IX4,17,18.
Excitonic hysteresis of the IX. To explore gate-dependent features of the IX emission, we measured the PL spectra of the device under cyclic Vg, which scans first from 0 V to 60 V, then down to 0 V, then to −60 V, and finally back to 0 V (Fig. 2a). The IX emission peak shows a redshift and the emission intensity is enhanced with the decrease of Vg, and vice versa. The redshift of the IX emission peak with Vg can be ascribed to the Stark effect7,19, which is further verified by the opposite shift trend of the IX emission peak in the devices with stacking order inversed (Fig. S1). Interestingly, the IX emission peak exhibits a strong hysteresis upon cyclic-voltage sweeping. As marked by black arrows in Fig. 2a, the peak position of the IX at middle 0 V (0V-2) cannot return to the same value of initial 0 V (0V-1), until a upward scanning and finally back to 0 V (0V-3). The gate-dependent photon energy and PL intensity can be seen more clearly in Fig. 2b and 2c. For a simple discussion, we only compare the states at 0V-2 and 0V-3. The photon energy of 0V-2 is blueshifted by about 20 meV with respect to that of 0V-3. Meanwhile, the PL intensity of 0V-2 is weaker than 0V-3 with a contrast ratio (I3/I2) of about 1.7. It is worth noting that the light intensity increases non-monotonously as Vg decreases from 0 V to −60 V, indicating the occurrence of chemical doping20,21, which will be discussed in the following.
The IX emission peak of 0V-2 exhibits an asymmetric lineshape, which can be decomposed to two Gaussian peaks (Fig. 2d). The energy difference of the two peaks is about 20 meV that is consistent with the splitting energy of the conduction band of WS222,23, strongly suggesting the occurrence of dark excitons. This peculiar phenomenon can be understood from the chemical-doping20,21 induced band-filling effect5,24. Due to the type-II band alignment, electrically-doped electrons and holes reside only in the WS2 and WSe2 layer respectively, as illustrated in Fig. 2e and 2f. When the device is chemically n-doped, the Fermi level will be lift up and the IX will shift to a dark state (IXD), which has an inefficient PL yield because of inversed spin in dark excitons. Contrarily, when the chemically-doped electrons are released, the IX will return to a bright state (IXB). Therefore, the IX emission peaks in 0V-1 and 0V-3 spectra are attributed to bright-exciton emission, and that in 0V-2 spectra is resulted from dark exciton. The difference of light intensities between 0V-3 and 0V-1 might be due to different levels of chemical doping at the initial and final sweeping stages.
Mechanism of the excitonic hysteresis. Electrical hysteresis is very common in two-dimensional material devices, such as graphene and TMDs based field-effect transistors25,26. Generally, electrical hysteresis is attributed to the chemical-doping effect by doping species (O2 and H2O) that are bound at the device/substrate interface, and/or on the surface of the device27-30. In our case, we propose that the excitonic hysteresis mentioned above is originated from the same scenario. Since our measurements were performed in high vacuum (~10-7 Torr), the influence of the molecules on the device surface can be safely neglected. Therefore, the excitonic hysteresis is more likely due to the O2/H2O molecules that are trapped at the interface between the HS and substrate. To clarify this, we examine the gate-dependent PL spectra of the individual WSe2 region (Fig. 3a), because WSe2 is in the bottom of the HS and directly contacts the SiO2/Si substrate. Additionally, we conducted a control experiment with WSe2 monolayer on a hydrophobic substrate (Fig. 3b).
Interestingly, the emission features of the intralayer excitons in WSe2 are closely correlated to that of IX. As Vg decreases from 0 V to –60 V, the emission of positive trions ( X+) is gradually enhanced, while the peak of neutral excitons (X0) is suppressed, indicating an efficient hole doping (detailed data is provided in Fig. S2). Peculiarly, as Vg increases from −60 V back to 0 V, the evolution track is asymmetric to that from 0 V to –60 V. The trion emission peak is firstly weakened, then enhanced and redshifted with the increase of Vg. The asymmetric evolution strongly indicates the occurrence of negative trions X- and suggests that the WSe2 is chemically n-doped30,31 at 0V-2. When Vg increases from 0 V to 60 V, the peak is redshifted further, but with emission intensity weakened because of Coulomb screening from the free electrons32. When voltage scans backward from 60 V to 0 V, the peak shows a blueshift and emission intensity becomes weaker while the peak is gradually enhanced, indicating that the chemically-doped charges have been released. All the above features are well consistent with the previously mentioned chemical-doping induced band-filling mechanism.
To validate such speculation, we then focus on the PL spectra of a control device with monolayer WSe2 on a hydrophilic substrate (Fig. 3b). The evolution track of X+and X0emission are roughly symmetric along the black dashed line at about –50 V. The slight deviation of the symmetry line at –50 V (rather than –60 V) might be due to trace O2/H2O molecules that are absorbed on WSe2 before the transfer procedure. Besides, the track of X- is symmetrical about the dashed line at 60 V. Such symmetric evolution suggests that the excitonic hysteresis is largely suppressed, thus proving the important role of H2O in our observations. The broad PL peak centered at about 1.65 eV might be due to local-state exciton33, which is out of the scope of this study.
The surface of SiO2 is usually covered with a layer of silanol groups (≡ Si - OH), especially after it is treated by piranha solution or plasma cleaner20,30. With these silanol groups, SiO2/Si substrates are easily bound by ambient O2 and H2O molecules34. As shown in Fig. 3c, the electrochemical potential of the redox couple (O2/H2O) is about –5.3 eV20,35, which is slightly higher than the valence band of WSe2 (about –5.46 eV)36,37. Electrons spontaneously transfer from O2/H2O to WSe2, making monolayer WSe2 initially n-doped (detailed information is provided in Fig. S2), and resulting in the deviated symmetry at −50 V in Fig. 3b. The applied negative gate voltages would force electrons further transferring from O2/H2O to WSe2. Those chemically-doped electrons balance out the gate modulation, resulting in the non-monotonic behavior of IX in 0 ~ −60 V range (Fig. 2c) and the excitonic hysteresis. The chemical-doping effect also explains why emission maintains its intensity from 0 V to –60 V for WSe2 on the hydrophilic substrate (Fig. 3a) but greatly suppressed on the hydrophobic substrate (Fig. 3b). When applying positive gate voltages, the chemical-doped charges are driven back from WSe2 to the O2/H2O redox couple. As a consequence, the intralayer excitons of WSe2 return to the initial state whereas the IX returns to the bright state. This control experiment further verifies the chemical-doping mechanism and well explains the origin of the excitonic hysteresis of IX shown in Fig. 2. To confirm the scenario, we made a WS2/WSe2 HS on a hydrophobic substrate, which shows no hysteresis (Fig. S3). In addition, we also fabricated a WS2/WSe2/BN HS on a hydrophilic substrate with WS2/WSe2 HS partially separated from the substrate by a thin layer BN. For this device, the excitonic hysteresis is observed in the region where WS2/WSe2 HS directly contacts with substrate, but absent in the BN-insulated region, which further supports that the excitonic hysteresis originates from the interface trap states (Fig. S4).
Hysteresis of circular polarization degree of the IX. To study the chemical-doping effect on the chirality features of IX, we measured the helicity-resolved PL spectra of the device (Fig. 4a). Interestingly, the IX peak exhibits a negative circular polarization in contrast to that of intralayer excitons in WSe2 and WS2, which can be ascribed to the interlayer quantum interference imposed by the atomic registry between the constituent layers38. To qualify the valley polarization, the degree of circular polarization has been introduced, which can be defined as PC = (I+ - I-)/(I+ + I-) , where I+ (I-) denotes the intensity of co-polarized (cross-polarized) PL component with the excitation. The IX peak exhibitsPC = -12.3%, while the intralayer excitonic peak of WS2 and WSe2 shows PC = 15% and 7.1% , respectively.
The valley polarization of IX can also be electrically controlled by Vg. As shown in Fig. 4b, the circular polarization rate shows strong voltage dependence (the full data set is provided in Fig. S5). The absolute polarization degree is greatly suppressed at −60 V (p-doping), but enhanced at 60 V (n-doping). Interestingly, the helicity of the IX emission also exhibits a strong hysteresis. The polarization degree of 0V-2 is much larger than that of 0V-1 and 0V-3. This observation further supports the presence of dark exciton7,39, because it has a long lifetime that suppresses valley depolarization40,41, as illustrated in Fig.4c and 4d. Due to long-range electron-hole exchange40, electrons in the K valley are scattered from the conduction band to the valence band, whereas electrons in the K’ valley are scattered from the valence band to the conduction band. This process can also be regarded as virtual recombination of an exciton in the K valley and generation of another exciton in the K’ valley, or vice versa. The long-range electron-hole exchange typically takes a few picoseconds41, and the lifetime of the bright and dark IX is about several nanoseconds and microseconds39, respectively. Therefore, the valley depolarization is strong for bright IX (Fig. 4c) but largely suppressed for long-lived dark IX (Fig. 4d). As a result, the IX can be switched between the bright and dark states by cyclic Vg, leading to the presence of the hysteresis of circular polarization degree. We define the helicity contrast as , where and is the circular polarization degree of 0V-2 and 0V-3 state, respectively. The helicity contrast is about 1.8, indicating potential application of valleytronic information processing and storage.
IX based valleytronic memory.
Due to the strong interaction with light, exciton-based information can be detected by photon energy, PL intensity and chirality contrast through optical approaches. To demonstrate the valley-encoding ability of the device, we measured the time-dependent PL spectra under circular excitation ( ), as shown in Fig. 5a. As gate voltage cyclically changes among −60 V, 0 V, 60 V and 0 V, the photon energy of the IX emission periodically shifts among 1.38 eV, 1.42 eV, 1.45 eV, and 1.40 eV, which are analogous to the performance of conventional electronic devices under “write”, “read” and “erase” operations. In addition, the emission intensity also periodically changes in response to those memory operations. Specifically, the PL intensity of the bright state (1.40 eV) and the dark state (1.42 eV) can be utilized for information storage, because the intensity levels can persist for a long time with no power consumption. Intriguingly, as the detection helicity switches between and , the PL intensity of the 0 and 1 states exhibit helicity-resolved features. There are four intensity levels emerged, which can be defined as “00”, “01”, “10”and “11”, indicating valley-encoding abilities of the device. Based on this feature, we can selectively encode/address the valley-polarized information by helicity excitation/ detection.
To evaluate the retention time of the encoded information, we then prolong the reading-operation time, as shown in Fig. 5b. Surprisingly, the 1 and 0 excitonic states can persist for at least 60 minutes, holding great promise for nonvolatile valleytronic memories. As a matter of fact, the retention time should be much longer than 60 minutes, as can be seen in a logarithmic-timescale plot (Fig. S6a). We also note that the 0 (1) state varies dynamically before reaching a steady state. This is probably due to the charging/discharging process of the device, as confirmed by the features of time-dependent leakage current (Fig. S6b). More important, the information encoding ability of the device can persist up to 250 K, which shows great promising for high temperature valleytronic devices (supplementary materials Fig. S7).