While two-dimensional (2D) van der Waals (vdW) layered materials are promising channel materials for wearable electronics and energy-efficient field-effect transistors (FETs), large hysteresis and large subthreshold swing induced by either dangling bonds at gate oxide dielectrics and/or trap molecules in bubbles at vdW interface are a serious drawback, hampering implementation of the 2D-material based FETs in real electronics. Here, we report a monolayer MoS2 FET with near-zero hysteresis reaching 0.15% of the sweeping range of the gate bias, a record-value observed so far in 2D FETs. This was realized by squeezing the MoS2 channel between top h-BN layer and bottom h-BN gate dielectrics and further removing the trap molecules in bubbles at the vdW interfaces via post-annealing. By segregating the bubbles out to the edge of the channel, we also obtain excellent switching characteristics with a minimum subthreshold swing of 63 mV/dec, an average subthreshold slope of 69 mV/dec for a current range of four orders of magnitude at room temperature, and a high on/off current ratio of 108 at a small operating voltage (<1 V). Such a near-zero hysteresis and a near-ideal subthreshold limit originate from the reduced trap density of ~5.2  ×  109 cm−2 eV−1, a thousand times smaller than previously reported values.

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.

Two-dimensional (2D) van der Waals (vdW) heterostructures with pre-determined properties are key ingredients for the success of advanced electronics and optoelectronics. The construction of vdW heterostructures is a prerequisite to obtain the desired performance with high quality. A typical dry/wet transfer technique is a promising route to physically stack vdW heterostructures via a gentle-energy fabrication procedure, allowing the fabrication of atomically sharp and thin heterointerfaces. This strategy has gained considerable attention for intriguing physics phenomena such as superconductivity, topological insulator, valleytronics, and interweaving proximity effect. It also offers various possibilities to construct sophisticated electrical, optical, energy harvesting, and memory devices. Here, we review the state-of-the-art transfer techniques and describe their advantages and drawbacks. We also discuss the transfer methodologies of particular purposes, which are extremely desired for further exploration of the vdW heterostructures such as the integration of diverse functional substrates, passivation of air-sensitive materials, twistronics, vdW contacts by 3D metal, and hybrid devices with 1D or 3D materials. We finally provide potential transfer approaches inspired from our experience, thereby considerably optimizing and simplifying the process.

building blocks for numerous applications in diodes, transistors, photodetectors, and solar cells. [5][6][7][8][9] In addition, the charge transfer originating from the thermionic emission or quantum tunneling is affected by the band bending near the heterojunction, which relies on the Fermi level difference between materials.Previously, carrier transport properties through a heterojunction appear to be in principle material-dependent behaviors. Recent reports show the possibility to integrate multiple functions in a certain vdW heterostructure; for example, black phosphorus (BP) of different thicknesses are employed to provide a tunable Fermi level for the desired band bending. [10,11] Nevertheless, BP suffers from sensitivity in air. [12] Besides, the use of mechanically exfoliated BP is not tenable for controlling the exact flake thickness, which directly determines its Fermi level, let alone realize accessible integration. Chemical vapor deposition (CVD) technique is a promising approach in a wafer-scale synthesis for industrial applications. Moreover, introducing the dopant during CVD growth is an effective way to change the carrier density in the material and thus modulate the position of the Fermi level. [13] Here, we report a reliable and repeatable method to synthesize vanadium (V) substituted WSe 2 monolayer by CVD and demonstrate multifunctional p-n diode behaviors in V-doped WSe 2 /SnSe 2 heterostructures. A liquid precursor with tungsten host and vanadium dopant atoms is adopted in our approach with only two zones for the precise control of V-doping concentration in WSe 2 . The coverage of the grown-WSe 2 flakes is increased up to 90% at a high liquid precursor concentration ( Figure S1, Supporting Information). In addition to the p-type V-doped WSe 2 monolayer, multilayer SnSe 2 as an n-type material is introduced to construct the p-n junction. We observe the diverse p-n diode behaviors in V-WSe 2 /SnSe 2 devices with various V-doping concentrations at room temperature, including quantum tunneling p-n diodes for the forward rectification, backward rectification, negative differential resistance (NDR), and ohmic resistance. Figure 1a,b is the schematics for synthesizing V-doped WSe 2 monolayer with the CVD approach. Liquid precursor containing solutions of ammonium metatungstate (AMT: W-precursor) and ammonium metavanadate (AMV: V-precursor) together with a promoter of alkali metal (NaOH) and iodixanol is spin-casted on SiO 2 /Si substrate and then introduced into a two-zone furnace CVD for the selenization 2D van der Waals layered heterostructures allow for a variety of energy band offsets, which help in developing valuable multifunctional devices. However, p-n diodes, which are typical and versatile, are still limited by the material choice due to the fixed band structures. Here, the vanadium dopant concentration is modulated in monolayer WSe 2 via chemical vapor deposition to demonstrate tunable multifunctional quantum tunneling diodes by vertically stacking SnSe 2 layers at room temperature. This...

Quantum tunneling with band-structure engineering has been feasibly developed for many applications in electrical, optoelectrical, and magnetic devices. It relies on layer-by-layer design and fabrication, which is an interdisciplinary research field covering material science and technology. Ever since the discovery of two-dimensional (2D) layered materials, tunneling devices based on 2D van der Waals (vdW) heterostructures have been extensively studied as potential next-generation devices. 2D materials are thin at the atomic scale and extremely flat without surface dangling bonds. Because of these unique characteristics, 2D vdW heterostructures offer superior tunneling performance that reaches the benchmark of traditional Si technology and possess additional ability to scale down device size. Here, we comprehensively review quantum tunneling in 2D vdW heterostructures, in addition to their unique mechanisms and applications. Moreover, we analyze the possibilities and challenges currently faced by 2D tunneling devices and provide a perspective on their exploitation for advanced future applications. The investigation of technology-and performancecontrol of 2D tunneling devices is at their beginning stages; however, these devices should emerge as competitive candidates for realizing low-power supply, fast-speed capability, and high-frequency operating devices.

Monolayer molybdenum disulfide (MoS 2) possesses a desirable direct bandgap with moderate carrier mobility, whereas graphene (Gr) exhibits a zero bandgap and excellent carrier mobility. Numerous approaches have been suggested for concomitantly realizing high on/off current ratio and high carrier mobility in field-effect transistors, but little is known to date about the effect of twodimensional layered materials. Herein, we propose a Gr/MoS 2 heterojunction platform, i.e., junction field-effect transistor (JFET), that enhances the carrier mobility by a factor of ~ 10 (~ 100 cm 2 V −1 s −1) compared to that of monolayer MoS 2 , while retaining a high on/off current ratio of ~ 10 8 at room temperature. The Fermi level of Gr can be tuned by the wide back-gate bias (V BG) to modulate the effective Schottky barrier height (SBH) at the Gr/MoS 2 heterointerface from 528 meV (n-MoS 2 /p-Gr) to 116 meV (n-MoS 2 /n-Gr), consequently enhancing the carrier mobility. The double humps in the transconductance derivative profile clearly reveal the carrier transport mechanism of Gr/MoS 2 , where the barrier height is controlled by electrostatic doping.

Neutron-transmutation doping (NTD) has been demonstrated for the first time in this work for substitutional introduction of tin (Sn) shallow donors into two-dimensional (2D) layered indium selenide (InSe) to manipulate electron transfer and charge carrier dynamics. Multidisciplinary study including density functional theory, transient optical absorption, and FET devices have been carried out to reveal that the field effect electron mobility of the fabricated phototransistor is increased 100-fold due to the smaller electron effective mass and longer electron life time in the Sn-doped InSe. The responsivity of the Sn-doped InSe based phototransistor is accordingly enhanced by about 50 times, being as high as 397 A/W. The results show that NTD is a highly effective and controllable doping method, possessing good compatibility with the semiconductor manufacturing process, even after device fabrication, and can be carried out without introducing any contamination, which is radically different from traditional doping methods.

Since atomically thin two-dimensional (2D) graphene was successfully synthesized in 2004, it has garnered considerable interest due to its advanced properties. However, the weak optical absorption and zero bandgap strictly limit its further development in optoelectronic applications. In this regard, other 2D materials, including black phosphorus (BP), transition metal dichalcogenides (TMDCs), 2D Te nanoflakes, and so forth, possess advantage properties, such as tunable bandgap, high carrier mobility, ultra-broadband optical absorption, and response, enable 2D materials to hold great potential for next-generation optoelectronic devices, in particular, mid-infrared (MIR) band, which has attracted much attention due to its intensive applications, such as target acquisition, remote sensing, optical communication, and night vision. Motivated by this, this article will focus on the recent progress of semiconducting 2D materials in MIR optoelectronic devices that present a suitable category of 2D materials for light emission devices, modulators, and photodetectors in the MIR band. The challenges encountered and prospects are summarized at the end. We believe that milestone investigations of 2D materials beyond graphene-based MIR optoelectronic devices will emerge soon, and their positive contribution to the nano device commercialization is highly expected.