Two-dimensional multibit optoelectronic memory with broadband spectrum distinction

Optoelectronic memory plays a vital role in modern semiconductor industry. The fast emerging requirements for device miniaturization and structural flexibility have diverted research interest to two-dimensional thin layered materials. Here, we report a multibit nonvolatile optoelectronic memory based on a heterostructure of monolayer tungsten diselenide and few-layer hexagonal boron nitride. The tungsten diselenide/boron nitride memory exhibits a memory switching ratio approximately 1.1 × 106, which ensures over 128 (7 bit) distinct storage states. The memory demonstrates robustness with retention time over 4.5 × 104 s. Moreover, the ability of broadband spectrum distinction enables its application in filter-free color image sensor. This concept is further validated through the realization of integrated tungsten diselenide/boron nitride pixel matrix which captured a specific image recording the three primary colors (red, green, and blue). The heterostructure architecture is also applicable to other two-dimensional materials, which is confirmed by the realization of black phosphorus/boron nitride optoelectronic memory.

respectively. The graph was spread separately with an interval of 6.2x10 3 s. In order to further investigate the non-volatility of our memory, we analyzed its charge retention time.
The memory was kept in the absence of external perturbation (no voltage and no light) after programming, and the storage currents were readout in a fixed interval (3.1×10 3 s).
Supplementary Fig.4 shows the storage currents as a function of waiting time up to 4.34x10 4 s at three different programming gates (Vpro = 0 V, -40 V, -80 V). To simplify the graph, we plotted the data with an interval of 6.2x10 3 s. The storage currents almost keep unchanged in the time range 4.34x10 4 s for all the programming gates, which verifies the memory is retained in the absence of external perturbation.

Supplementary Note 1 | Raman and PL spectra of the exfoliated WSe2 and BN Samples
Supplementary Fig. 1a shows the Raman spectrum of the exfoliated WSe2, with two characteristic peaks located at 250 cm -1 and 261 cm -1 respectively. There is no distinct peak at 316 cm -1 , indicating the monolayer nature of the WSe2 flake 3 . This result is further confirmed by the PL spectrum ( Supplementary Fig. 1b) Fig.1c). Supplementary Fig.1d illustrates the Raman spectrum of the BN flake, with one characteristic peak at 1365 cm -1 .

Supplementary Note 2 | Saturation of the storage current
In our experiment, we observed that the storage current gradually saturates with increasing the pulse number, which is shown in Supplementary Fig.6a. The storage current almost maintains when the storage level is more than 130 ( Supplementary Fig.6b). In our WSe2/BN memory, the photon-excited electrons in BN conduction band can transfer into WSe2 driven by the electric field, leaving the positive charges localized in middle of the BN bandgap. It is worth noting that these localized positive charges in BN can effectively screen the negative gate and hence weaken the electric field exerting on WSe2 during the programming process.
The elimination of the effective electric field in BN symbolizes the termination of the programming process and results in the saturation of the storage current.

Supplementary Note 3 | Noise calculations for reliable storage states
Supplementary Fig.7a shows an example about the calculation of the storage currents and standard deviations (STDs) for states 58 and 59. When the gate voltage is switched to 50 V after programming, the electron-domination current increases sharply followed by stabilization. Both the average storage currents and their STDs are calculated in the area where the current becomes stable (encircled by a rectangle in Supplementary Fig.7a). The

Supplementary Note 4 | Gate controlled BP/BN memory
The initial transfer curve presents a current minimum of 15 V. When increasing the Vpro to -60 V, the current minimum progressively moves to -48 V. This suggests a significant electron-doping effect in BP modulated by backgate, a characteristic similar to that of the WSe2/BN memory. The dynamic behavior shows that the storage current increases stepwise with the increase of Vpro, consistent with the transfer characteristics evolution. 7 distinguishable storage states are observed in Supplementary Fig.15b through applying 7 different Vpro. The switching ratio rises in a nearly linear trend with respect to Vpro, from 49 to 415 ( Supplementary Fig.15c).
It should be noted that the erasing time of the BP/BN memory (150 s) is longer comparing to that of the WSe2/BN memory. We propose that the long erasing time in BP/BN memory is mainly due to the electron trapping states in BP which was studied before 5,6 . During The photo responsivity displays nearly linear dependence on the programming gate, progressively increasing from 2×10 6 AW -1 (Vpro = -10 V) to 1.2×10 7 AW -1 (Vpro = -80 V).
This extraordinary photo responsivity is comparable to or even larger than the results of recently reported ultrasensitive BP photodetectors 7,8 , indicating BP/BN heterostructure as a promising candidate for photodetector application.