Optoelectronic memory devices, whose states can be controlled using electrical optical signals, are receiving much attention for their potential applications in image sensing and parallel data transmission and processes. Here, we report MoS 2 -based devices with top floating gates of Au, graphene, and MoS 2 . Unlike conventional floating gate memory devices, our devices have the photoresponsive floating gate at the top, acting as a charge trapping layer. Stable and reliable switching with an on/off ratio of ∼10 6 and a retention time of >10 4 s is achieved by illumination with 405 nm light pulses as well as application of gate voltage pulses. However, upon illumination with 532 or 635 nm light pulses, multilevel optical memory effects are observed, which are dependent on the wavelength and the optical exposure dosage. In addition, compared to the device employing a graphene floating gate, the device with an MoS 2 floating gate is more sensitive to light, suggesting that the multilevel optical memory properties originate from photoexcited carriers in the top floating gate and can be modulated by adjusting the top floating gate materials. The structure of the top floating gate may open up a new way to novel optoelectronic memory devices.

We fabricated MoS-based flash memory devices by stacking MoS and hexagonal boron nitride (hBN) layers on an hBN/Au substrate and demonstrated that these devices can emulate various biological synaptic functions, including potentiation and depression processes, spike-rate-dependent plasticity, and spike-timing dependent plasticity. In particular, compared to a flash memory device prepared on an hBN substrate, the device fabricated on the hBN/Au exhibited considerably more symmetric and linear bidirectional gradual conductance change curves, which may be attributed to the device structure incorporating double floating gate. For the device on the hBN/Au, electron transfers may occur between the floating gate MoS and Au, as well as between the floating gate MoS and the channel MoS, allowing for more control over electron tunneling and injection. To test our hypothesis, we also fabricated a MoS-based flash memory device on an hBN/Pd substrate and found behavior similar to the device fabricated on hBN/Au. Our results demonstrate that flexible synaptic electronics may be implemented using MoS-based flash memory devices with double floating gates.

We developed Schottky junction photovoltaic cells based on multilayer MoWSe with x = 0, 0.5, and 1. To generate built-in potentials, Pd and Al were used as the source and drain electrodes in a lateral structure, and Pd and graphene were used as the bottom and top electrodes in a vertical structure. These devices exhibited gate-tunable diode-like current rectification and photovoltaic responses. MoWSe Schottky diodes with Pd and Al electrodes exhibited higher photovoltaic efficiency than MoSe and WSe devices with Pd and Al electrodes, likely because of the greater adjusted band alignment in MoWSe devices. Furthermore, we showed that MoWSe-based vertical Schottky diodes yield a power conversion efficiency of ∼16% under 532 nm light and ∼13% under a standard air mass 1.5 spectrum, demonstrating their remarkable potential for photovoltaic applications.

We investigated the relationship between defects in graphene and NO2 gas sensitivity of graphene-based gas sensors. Defects were introduced by hydrogen plasma or ultraviolet (UV)/ozone treatment. As the defect concentration increased, the sensitivity was enhanced, and sub-ppb level detection limit was achieved. UV irradiation was used for recovery at room temperature. However, defects generated by ozone treatment, like graphene oxide, were reduced back to graphene by UV irradiation, so the ozone-treated graphene sensor was not stable over time. In contrast, the response of the hydrogenated graphene sensor was very repeatable because defects generated by hydrogenation was stable enough not to be dehydrogenated by UV irradiation. These results demonstrate that the hydrogenated graphene sensor is a highly sensitive and stable NO2 sensor at room temperature.

The temperature dependence of electrical transport properties was investigated for multilayered MoS2 field effect transistor devices with thicknesses of 3-22 nm. Some devices showed typical n-type semiconducting behavior, while others exhibited metal-insulator crossover (MIC) from metallic to insulating conduction at finite temperatures. The latter effect occurred near zero gate voltage or at high positive gate voltages. Analysis of Raman spectroscopy revealed the key difference that devices with MIC have a metallic 1T phase as well as a semiconducting 2H phase, whereas devices without the MIC did not have a metallic 1T phase. These results suggest that the metallic 1T phase may contribute to inducing the MIC.

Near-infrared photodetectors are developed using graphene/MoS2 and WSe2/MoS2 vertical heterojunctions. These heterojunctions exhibit diode-rectifying behavior in the dark and enhanced photocurrent upon near-infrared irradiation. The photocurrent increases with increasing near-infrared power, leading to the photoresponsibility of 0.14 and 0.3 A W−1 for the graphene/MoS2 and WSe2/MoS2 heterojunctions, respectively, which are much higher than the photoresponsibility reported for a multilayer MoS2 phototransistor.