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...

Vertically stacked two-dimensional van der Waals (vdW) heterostructures, used to obtain homogeneity and band steepness at interfaces, exhibit promising performance for band-to-band tunneling (BTBT) devices. Esaki tunnel diodes based on vdW heterostructures, however, yield poor current density and peak-to-valley ratio, inferior to those of three-dimensional materials. Here, we report the negative differential resistance (NDR) behavior in a WSe2/SnSe2 heterostructure system at room temperature and demonstrate that heterointerface control is one of the keys to achieving high device performance by constructing WSe2/SnSe2 heterostructures in inert gas environments. While devices fabricated in ambient conditions show poor device performance due to the observed oxidation layer at the interface, devices fabricated in inert gas exhibit extremely high peak current density up to 1460 mA/mm2, 3–4 orders of magnitude higher than reported vdW heterostructure-based tunnel diodes, with a peak-to-valley ratio of more than 4 at room temperature. Besides, Pd/WSe2 contact in our device possesses a much higher Schottky barrier than previously reported Cr/WSe2 contact in the WSe2/SnSe2 device, which suppresses the thermionic emission current to less than the BTBT current level, enabling the observation of NDR at room temperature. Diode behavior can be further modulated by controlling the electrostatic doping and the tunneling barrier as well.

van der Waals heterostructures (vdWHs) of metallic (m-) and semiconducting (s-) transition-metal dichalcogenides (TMDs) exhibit an ideal metal/semiconductor (M/S) contact in a field-effect transistor. However, in the current two-step chemical vapor deposition process, the synthesis of m-TMD on pregrown s-TMD contaminates the van der Waals (vdW) interface and hinders the doping of s-TMD. Here, NbSe 2 /Nb-doped-WSe 2 metal-doped-semiconductor (M/d-S) vdWHs are created via a one-step synthesis approach using a niobium molar ratio-controlled solution-phase precursor. The one-step growth approach synthesizes Nb-doped WSe 2 with a controllable doping concentration and metal/doped-semiconductor vdWHs. The hole carrier concentration can be precisely controlled by controlling the Nb/(W + Nb) molar ratio in the precursor solution from ∼3 × 10 11 /cm 2 at Nb-0% to ∼1.38 × 10 12 /cm 2 at Nb-60%; correspondingly, the contact resistance R C value decreases from 10 888.78 at Nb-0% to 70.60 kΩ.μm at Nb-60%. The Schottky barrier height measurement in the Arrhenius plots of ln(I sat /T 2 ) versus q/K B T demonstrated an ohmic contact in the NbSe 2 /W x Nb 1−x Se 2 vdWHs. Combining p-doping in WSe 2 and M/d-S vdWHs, the mobility (27.24 cm 2 V −1 s −1 ) and on/off ratio (2.2 × 10 7 ) are increased 1238 and 4400 times, respectively, compared to that using the Cr/pure-WSe 2 contact (0.022 cm 2 V −1 s −1 and 5 × 10 3 , respectively). Together, the R C value using the NbSe 2 contact shows 2.46 kΩ.μm, which is ∼29 times lower than that of using a metal contact. This method is expected to guide the synthesis of various M/d-S vdWHs and applications in future high-performance integrated circuits.

Graphene is one of the most promising materials for photodetectors due to its ability to convert photons into hot carriers within approximately 50 fs and generate long-lived thermalized states with lifetimes longer than 1 ps. In this study, we demonstrate a wide range of vertical photodetectors having a graphene/h-BN/Au heterostructure in which an hexagonal boron nitride (h-BN) insulating layer is inserted between an Au electrode and graphene photoabsorber. The photocarriers effectively tunnel through the small hole barrier (1.93 eV) at the Au/h-BN junction while the dark carriers are highly suppressed by a large electron barrier (2.27 eV) at the graphene/h-BN junction. Thus, an extremely low dark current of ∼10 −13 A is achieved, which is 8 orders of magnitude lower than that of graphene lateral photodetector devices (∼10 −5 A). Also, our device displays an asymmetric photoresponse behavior due to photothermionic emission at the graphene/h-BN and Au/h-BN junctions. The asymmetric behavior generates additional thermal carriers (hot carriers) to enable our device to generate photocurrents that can overcome the Schottky barrier. Furthermore, our device shows the highest value of the I ph /I dark ratio of ∼225 at 7 nm thick h-BN insulating layer, which is 3 orders of magnitude larger than that of the previously reported graphene lateral photodetectors without any active materials. In addition, we achieve a fast response speed of 12 μs of rise time and 5 μs of fall time, which are about 100 times faster than those of other graphene integrated photodetectors.