Despite numerous studies on two-dimensional van der Waals heterostructures, a full understanding of the charge transport and photoinduced current mechanisms in these structures, in particular, associated with charge depletion/inversion layers at the interface remains elusive. Here, we investigate transport properties of a prototype multilayer MoS/WSe heterojunction via a tunable charge inversion/depletion layer. A charge inversion layer was constructed at the surface of WSe due to its relatively low doping concentration compared to that of MoS, which can be tuned by the back-gate bias. The depletion region was limited within a few nanometers in the MoS side, while charges are fully depleted on the whole WSe side, which are determined by Raman spectroscopy and transport measurements. Charge transport through the heterojunction was influenced by the presence of the inversion layer and involves two regimes of tunneling and recombination. Furthermore, photocurrent measurements clearly revealed recombination and space-charge-limited behaviors, similar to those of the heterostructures built from organic semiconductors. This contributes to research of various other types of heterostructures and can be further applied for electronic and optoelectronic devices.

RT), precluding the use of these materials to practical implementations. [2][3][4][5]14] The ferromagnetic state in magnetic metaldoped oxides and nitrides is available at RT but is localized to aggregated metal oxide/nitride nanoparticles without a longrange magnetic order. [4] The ferromagnetic state in van der Waals 2D materials has been observed recently in the monolayer limit. [15][16][17][18][19][20][21] Intrinsic CrI 3 and CrGeTe 3 semiconductors reveal ferromagnetism but the T c is still low below 60 K. [20,21] In contrast, monolayer VSe 2 and MnSe 2 are ferromagnetic metals with T c above RT but incapable of controlling its carrier density. [22,23] Moreover, the long-range ferromagnetic order in doped diluted chalcogenide semiconductors has not been demonstrated at RT. [24][25][26][27][28] The key research target is to realize the long-range order ferromagnetism, T c over RT, and semiconductor with gate tunability. Here, we unambiguously observe tunable magnetic domains by a gate bias above RT in diluted V-doped WSe 2 , while maintaining the semiconducting characteristic of WSe 2 with a high on/off current ratio of five orders of magnitude. Figure 1a illustrates the schematic for the synthesis of V-doped monolayer WSe 2 via chemical vapor deposition (CVD). A metal precursor solution prepared by mixing V and W liquid sources at a given atomic ratio was spin-coated on SiO 2 substrate and the substrate was introduced into the CVD chamber with selenium. The metal precursors get decomposed into metal oxides at growth temperature, resulting in monolayer V x W 1−x Se 2 , followed by selenization. The atomic ratio of V to W sources in precursor solution can be precisely controlled from 0.1% to 40%, while the hexagonal flakes are retained in a monolayer form (the optical image in Figure 1a; Figure S1, Supporting Information). Meanwhile, the dendritic and multilayer flakes are partially generated at higher V-concentration. The V atoms are incorporated into monolayer WSe 2 with V/W contents similar to nominal values, as confirmed by X-ray photoelectron spectroscopy ( Figure S2, Supporting Information). With low V-doping concentration, the hexagonal V-doped WSe 2 flake is a single crystal confirmed by previous TEM study. [29] To study the doping effect of vanadium to the electronic properties of WSe 2 , field effect transistors (FETs) of V-doped mono layer WSe 2 were fabricated (Figure 1b). The CVD-grown pristine WSe 2 manifests a p-type semiconductor with a threshold voltage at −50 V. The threshold voltage is shifted to −10 V for Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room temperature with longrange ferromagnetic order is still debatable to date. Here, the room-temperature ferromagnetic domains with long-range order in semiconducting V-doped WSe 2 monolayer synthesized by chemical vapor deposition are reported. Ferromagnetic order is manifested using magnetic force microscopy up to 360 K, while retaining high on/off current rati...

cells, [11] and memory. [12,13] Recently, vdWs heterostructure-based nonvolatile optical memory have been investigated for broad potential applications in imaging sensors, [14] logic gates, [15] optoelectronic demodulators, [16] and synaptic devices for neuromorphic systems. [17,18] These 2D vdWs materials and their hybrids are considered to be an ideal platform for nonvolatile optical memory owing to their strong light-matter interactions [19][20][21] and significant photogenerated charge trapping derived from their very large surface-to-volume ratio. [22][23][24] In addition, the mechanical strength and atomic thickness of 2D vdWs materials allow for device miniaturization in flexible and wearable optoelectronics. [25][26][27] The demonstration of 2D vdWs materials-based optical memory in the FETs of few-layer copper indium selenide (CuIn 7 Se 11 ) has been reported [14] along with hybrids of graphene/MoS 2[5] on a silicon substrate. These devices exhibit low optical switching on/off ratios (<1 [5] and ≈10 [14] ), high off-currents (≈400 µA [5] and 20 pA [14] ), and short retention times (50 s [14] ), preventing their use in high quality image sensors and multilevel storage devices.Using oxygen plasma treatments to create more charge trap sites at SiO 2 surface, monolayer MoS 2 -FET-based optical memory on silicon substrate has exhibited a long retention time of ≈10 4 s. [28] However, the data storage capacity of eight levels of the material remains limited for practical applications due to moderate switching on/off ratio of ≈4700. Importantly, the memory function of these devices relies on charge trapping of photoexcited carriers in defects and impurities on either the surface or material/SiO 2 interface. [5,28] This results in short retention times and sensitivity to environmental factors. A charge trapping layer acting as a floating gate, instead of charge trapping at the materials/SiO 2 interface, can be introduced via gold nanoparticle/crosslinked poly(4-vinylphenol)/MoS 2 heterojunction-FETs, which significantly increase both the switching on/off ratio (≈10 6 ) and retention time (>10 4 s). [29] Similarly, by storing charge in the hexagonal boron nitride (h-BN) dielectric layer, the WSe 2 /h-BN-FET-based optical memory on silicon also exhibited a high on/off ratio of 1.1 × 10 6 , realizing a data storage capability of up to 128 distinct states. [30] Despite the long retention time and high on/off ratios of the recently developed vdWs heterostructure-based optical memory 2D van der Waals (vdWs) heterostructures exhibit intriguing optoelectronic properties in photodetectors, solar cells, and light-emitting diodes. In addition, these materials have the potential to be further extended to optical memories with promising broadband applications for image sensing, logic gates, and synaptic devices for neuromorphic computing. In particular, high programming voltage, high off-power consumption, and circuital complexity in integration are primary concerns in the development of three-terminal optical memory devices....

Because of strong Coulomb interaction in two-dimensional van der Waals-layered materials, the trap charges at the interface strongly influence the scattering of the majority carriers and thus often degrade their electrical properties. However, the photogenerated minority carriers can be trapped at the interface, modulate the electron-hole recombination, and eventually influence the optical properties. In this study, we report the role of the hole trap sites on the inconsistency in the electrical and optical phenomena between two systems with different interfacial trap densities, which are monolayer MoS-based field-effect transistors (FETs) on hexagonal boron nitride (h-BN) and SiO substrates. Electronic transport measurements indicate that the use of h-BN as a gate insulator can induce a higher n-doping concentration of the monolayer MoS by suppressing the free-electron transfer from the intrinsically n-doped MoS to the SiO gate insulator. Nevertheless, optical measurements show that the electron concentration in MoS/SiO is heavier than that in MoS/h-BN, manifested by the relative red shift of the A Raman peak. The inconsistency in the evaluation of the electron concentration in MoS by electrical and optical measurements is explained by the trapping of the photogenerated holes in the spatially modulated valence band edge of the monolayer MoS caused by the local strain from the SiO/Si substrate. This photoinduced electron doping in MoS/SiO is further confirmed by the development of the trion component in the power-dependent photoluminescence spectra and negative shift of the threshold voltage of the FET after illumination.