Charge Transport in MoS2/WSe2 van der Waals Heterostructure with Tunable Inversion Layer

Negative Differential Resistance with Ultralow Peak-to-Valley Voltage Difference in Td-WTe2/2H-MoS2 Heterostructure

Negative differential resistance (NDR) devices with a low peak-to-valley voltage difference (ΔV) exhibit a high cut off frequency and low power consumption efficiency, which is significant for fabricating high-performance oscillators. However, achieving an ultralow ΔV is challenging. In this work, we report the first construction of an NDR device utilizing a van der Waals heterostructure composed of semimetallic Td-WTe2 and semiconducting 2H-MoS2. Our findings reveal that the narrow energy region of the decreasing density of states (DOS) above the Fermi level of WTe2 acts as a narrow band gap, facilitating type-III band alignment with MoS2 and enabling band-to-band tunneling-based NDR transport. Notably, the NDR device exhibits an ultralow ΔV of approximately 0.01 V, which is at least an order of magnitude lower than previously reported values. This work not only introduces a new approach for NDR device fabrication but also provides new insights into the pivotal role of Td-WTe2 in NDR transport.

Composition Modulation-Mediated Band Alignment Engineering from Type I to Type III in 2D vdW Heterostructures

Band alignment engineering is crucial for facilitating charge separation and transfer in optoelectronic devices, which ultimately dictates the behavior of Van der Waals heterostructures (vdWH)-based photodetectors and light emitting diode (LEDs). However, the impact of the band offset in vdWHs on important figures of merit in optoelectronic devices has not yet been systematically analyzed. Herein, the regulation of band alignment in WSe2/Bi2Te3- xSex vdWHs (0 ≤ x ≤ 3) is demonstrated through the implementation of chemical vapor deposition (CVD). A combination of experimental and theoretical results proved that the synthesized vdWHs can be gradually tuned from Type I (WSe2/Bi2Te3) to Type III (WSe2/Bi2Se3). As the band alignment changes from Type I to Type III, a remarkable responsivity of 58.12 A W-1 and detectivity of 2.91×1012 Jones (in Type I) decrease in the vdWHs-based photodetector, and the ultrafast photoresponse time is 3.2 µs (in Type III). Additionally, Type III vdWH-based LEDs exhibit the highest luminance and electroluminescence (EL) external quantum efficiencies (EQE) among p-n diodes based on Transition Metal Dichalcogenides (TMDs) at room temperature, which is attributed to band alignment-induced distinct interfacial charge injection. This work serves as a valuable reference for the application and expansion of fundamental band alignment principles in the design and fabrication of future optoelectronic devices.

Adaptative machine vision with microsecond-level accurate perception beyond human retina

Visual adaptive devices have potential to simplify circuits and algorithms in machine vision systems to adapt and perceive images with varying brightness levels, which is however limited by sluggish adaptation process. Here, the avalanche tuning as feedforward inhibition in bionic two-dimensional (2D) transistor is proposed for fast and high-frequency visual adaptation behavior with microsecond-level accurate perception, the adaptation speed is over 104 times faster than that of human retina and reported bionic sensors. As light intensity changes, the bionic transistor spontaneously switches between avalanche and photoconductive effect, varying responsivity in both magnitude and sign (from 7.6 × 104 to -1 × 103 A/W), thereby achieving ultra-fast scotopic and photopic adaptation process of 108 and 268 μs, respectively. By further combining convolutional neural networks with avalanche-tuned bionic transistor, an adaptative machine vision is achieved with remarkable microsecond-level rapid adaptation capabilities and robust image recognition with over 98% precision in both dim and bright conditions.

J-MISFET Hybrid Dual-Gate Switching Device for Multifunctional Optoelectronic Logic Gate Applications

High-performance and low operating voltage are becoming increasingly significant device parameters to meet the needs of future integrated circuit (IC) processors and ensure their energy-efficient use in upcoming mobile devices. In this study, we suggest a hybrid dual-gate switching device consisting of the vertically stacked junction and metal-insulator-semiconductor (MIS) gate structure, named J-MISFET. It shows excellent device performances of low operating voltage (<0.5 V), drain current ON/OFF ratio (∼4.7 × 105), negligible hysteresis window (<0.5 mV), and near-ideal subthreshold slope (SS) (60 mV/dec), making it suitable for low-power switching operation. Furthermore, we investigated the switchable NAND/NOR logic gate operations and the photoresponse characteristics of the J-MISFET under the small supply voltage (0.5 V). To advance the applications further, we successfully demonstrated an integrated optoelectronic security logic system comprising 2-electric inputs (for encrypted data) and 1-photonic input signal (for password key) as a hardware security device for data protection. Thus, we believe that our J-MISFET, with its heterogeneous hybrid gate structures, will illuminate the path toward future device configurations for next-generation low-power electronics and multifunctional security logic systems in a data-driven society.

Electrically tunable interlayer recombination and tunneling behavior in WSe2/MoS2 heterostructure for broadband photodetector

Electrically tunable band structure and light-matter interaction are of great importance in designing novel devices and constructing high-integrated and high-performance photodetector systems in the future. However, tunable mechanisms on the layered semiconductor, especially the heterojunction, are still unclear. Herein, the WSe2/MoS2 phototransistor with dual-gated configuration is fabricated, and its electrical and photoelectrical conversion has been studied to show large tunability. It was found that conduction and rectification characteristics can be tuned by dual gates showing four states, p-i, p-n, i-n, and n-n, as a result of the charging and depletion of WSe2 and MoS2. The rectifying ratio can be modulated across a large range from 102.5 to 10-3.2. Its photoelectronic characteristics were observed to exhibit bipolar and wavelength-dependent behaviors. The interlayer recombination of charge carriers dominates the photoresponse of the device under the illumination of visible light, while it is dominated by interlayer tunneling under the illumination of near-infrared wavelengths. This bipolar photoresponse is associated with different states of band alignment, which can be switched by dual-gating modulation. Finally, by tuning the gate voltage, responsivities reach 27 445 A W-1 and 2827 A W-1 at wavelengths of 400 and 1010 nm at room temperature, respectively, which directly extends the response region from visible light to near-infrared.

Room Temperature Negative Differential Resistance with High Peak Current in MoS2/WSe2 Heterostructures

Two-dimensional transition metal dichalcogenide (2D TMD) semiconductors allow facile integration of p- and n-type materials without a lattice mismatch. Here, we demonstrate gate-tunable n- and p-type junctions based on vertical heterostructures of MoS2 and WSe2 using van der Waals (vdW) contacts. The p-n junction shows negative differential resistance (NDR) due to Fowler-Nordheim (F-N) tunneling through the triangular barrier formed by applying a global back-gate bias (VGS). We also show that the integration of hexagonal boron nitride (h-BN) as an insulating tunnel barrier between MoS2 and WSe2 leads to the formation of sharp band edges and unintentional inelastic tunnelling current. The devices based on vdW contacts, global VGS, and h-BN tunnel barriers exhibit NDR with a peak current (Ipeak) of 315 μA, suggesting that the approach may be useful for applications.

Interlayer Coupling in Anisotropic/Isotropic Van der Waals Heterostructures of ReS2 and WS2

Van der Waals (vdW) heterostructures are composed of atomically thin layers assembled through weak (vdW) force, which have opened a new era for integrating materials with distinct properties and specific applications. However, few studies have focused on whether and how anisotropic materials affect heterostructure system. The study introduces anisotropic and isotropic materials in a heterojunction system to change the in-plane symmetry, offering a new degree of freedom for modulating its properties. The sample is fabricated by manually stacking ReS2 and WS2 flakes prepared by mechanical exfoliation. Raman spectra and photoluminescence measurements confirm the formation of an effective heterojunction, indicating interlayer coupling of the system. The anisotropy and asymmetry of the WS2 -ReS2 heterostructure system can be adjusted by the introduction of isotropic WS2 and anisotropic ReS2 , which can be proved by the change of the polarized Raman pattern. In the transient absorption measurement, the transient absorption spectra of WS2 -ReS2 heterostructure are red-shifted compared to those of WS2 monolayer, and the charge transfer is observed in the heterostructure. These results show the potential of anisotropic 2D materials in anisotropy modulation of heterostructures, which may promote future electronic or photonic application.

Effect of external electric field on the electronic properties of the AlAs/SiC van der Waals heterostructure

Type-II van der Waals (vdW) heterostructures are regarded as the optimum candidates for unipolar electronic device applications due to their capacity for spontaneous electron-hole separation. Here, we studied the electronic properties of the AlAs/SiC vdW heterostructure via density functional theory calculations. Results show that the conduction band minimum (CBM) and valence band maximum (VBM) of this heterostructure are mainly contributed by different materials, illustrating that the AlAs/SiC heterostructure has a type-II band alignment. Interestingly, this heterostructure possesses flat valence bands near the Fermi level. In addition, under the modulation of external electric field ranging between -1 V Å-1∼0.8 V Å-1, the band gap of the heterostructure can be tuned continuously, while the band structure maintains a stable type-II band alignment with flat top valence bands. When the electric field exceeds -1 or 0.8 V Å-1, the heterostructure transitions from semiconductor material to metal, indicating the tunability of electronic properties under external fields. These results indicate that the AlAs/SiC heterostructure shows great potential for application in high-performance optoelectronic devices and a strong correlation may exist in this system.

Dual transmission channels at metal-MoS2/WSe2 hetero-bilayer interfaces

van der Waals heterostructures (vdWHs) open the possibility of creating novel semiconductor materials at the atomic scale that demonstrate totally new physics and enable unique functionalities, and have therefore attracted great interest in the fields of advanced electronic and optoelectronic devices. However, the interactions between metals and vdWHs semiconductors require further investigation as they directly affect or limit the advancement of high-performance electronic devices. Here we study the contact behavior of MoS₂/WSe₂ vdWHs in contact with a series of bulk metals using ab initio electronic structure calculations and quantum transport simulations. Our study shows that dual transmission paths for electrons and holes exist at the metal-MoS₂/WSe₂ hetero-bilayer interfaces. In addition, the metal-induced bandgap state (MIGS) of the original monolayer disappears due to the creation of the heterolayer, which weakens the Fermi level pinning (FLP) effect. We also find that the creation of the heterolayer causes a change in the Schottky barrier height (SBH) of the non-ohmic contact systems, whilst this does not occur so easily in the ohmic contact systems. In addition, our results indicate that when Al, Ag and Au are in contact with a MoS₂/WSe₂ hetero-bilayer semiconductor, a low contact barrier exists throughout the whole transmission process causing the charge to tunnel to the MoS₂ layer, irrespective of whether the MoS₂ is in contact with the metals as the nearest layer or as the next-nearest layer. Our work not only offers new insights into electrical contact issues between metals and hetero-bilayer semiconductors, but also provides guidance for the design of high-performance vdWHs semiconductor devices.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Chemical Dopant-Free Controlled MoTe2/MoSe2 Heterostructure toward a Self-Driven Photodetector and Complementary Logic Circuits

Two-dimensional (2D) van der Waals heterostructures based on transition metal dichalcogenides are expected to be unique building blocks for next-generation nanoscale electronics and optoelectronics. The ability to control the properties of 2D heterostructures is the key for practical applications. Here, we report a simple way to fabricate a high-performance self-driven photodetector based on the MoTe₂/MoSe₂ p-n heterojunction, in which the hole-dominated transport polarity of MoTe₂ is easily achieved via a straightforward thermal annealing treatment in air without any chemical dopants or special gases needed. A high photoresponsivity of 0.72 A W^-1, an external quantum efficiency up to 41.3%, a detectivity of 7 × 10¹¹ Jones, and a response speed of 120 μs are obtained at zero bias voltage. Additionally, this doping method is also utilized to realize a complementary inverter with a voltage gain of 24. By configuring 2D p-MoTe₂ and n-MoSe₂ on demand, logic functions of NAND and NOR gates are also accomplished successfully. These results present a significant potential toward future larger-scale heterogeneously integrated 2D electronics and optoelectronics.

High-Performance and Polarization-Sensitive Imaging Photodetector Based on WS2 /Te Tunneling Heterostructure

Next-generation imaging systems require photodetectors with high sensitivity, polarization sensitivity, miniaturization, and integration. By virtue of their intriguing attributes, emerging 2D materials offer innovative avenues to meet these requirements. However, the current performance of 2D photodetectors is still below the requirements for practical application owing to the severe interfacial recombination, the lack of photoconductive gain, and insufficient photocarrier collection. Here, a tunneling dominant imaging photodetector based on WS₂ /Te heterostructure is reported. This device demonstrates competitive performance, including a remarkable responsivity of 402 A W^-1 , an outstanding detectivity of 9.28 × 10¹³ Jones, a fast rise/decay time of 1.7/3.2 ms, and a high photocurrent anisotropic ratio of 2.5. These outstanding performances can be attributed to the type-I band alignment with carrier transmission barriers and photoinduced tunneling mechanism, allowing reduced interfacial trapping effect, effective photoconductive gains, and anisotropic collection of photocarriers. Significantly, the constructed photodetector is successfully integrated into a polarized light imaging system and an ultra-weak light imaging system to illustrate the imaging capability. These results suggest the promising application prospect of the device in future imaging systems.

Measuring Band Modulation of MoS2 with Ferroelectric Gates

Electronic properties of two-dimensional (2D) materials can be significantly tuned by an external electric field. Ferroelectric gates can provide a strong polarization electric field. Here, we report the measurements of the band structure of few-layer MoS₂ modulated by a ferroelectric P(VDF-TrFE) gate with contact-mode scanning tunneling spectroscopy. When P(VDF-TrFE) is fully polarized, an electric field up to ∼0.62 V/nm through the MoS₂ layers is inferred from the measured band edges, which affects the band structure significantly. First, strong band bending in the vertical direction signifies the Franz-Keldysh effect and a large extension of the optical absorption edge. Photons with energy of half the band gap are still absorbed with 20% of the absorption probability of photons at the band gap. Second, the electric field greatly enlarges the energy separations between the quantum-well subbands. Our study intuitively demonstrates the great potential of ferroelectric gates in band structure manipulation of 2D materials.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

Highly Sensitive MoS2 Photodetectors Enabled with a Dry-Transferred Transparent Carbon Nanotube Electrode

Fabricating electronic and optoelectronic devices by transferring pre-deposited metal electrodes has attracted considerable attention, owing to the improved device performance. However, the pre-deposited metal electrode typically involves complex fabrication procedures. Here, we introduce our facile electrode fabrication process which is free of lithography, lift-off, and reactive ion etching by directly press-transferring a single-walled carbon nanotube (SWCNT) film. We fabricated Schottky diodes for photodetector applications using dry-transferred SWCNT films as the transparent electrode to increase light absorption in photoactive MoS₂ channels. The MoS₂ flake vertically stacked with an SWCNT electrode can exhibit excellent photodetection performance with a responsivity of ∼2.01 × 10³ A/W and a detectivity of ∼3.2 × 10¹² Jones. Additionally, we carried out temperature-dependent current-voltage measurement and Fowler-Nordheim (FN) plot analysis to explore the dominant charge transport mechanism. The enhanced photodetection in the vertical configuration is found to be attributed to the FN tunneling and internal photoemission of charge carriers excited from indium tin oxide across the MoS₂ layer. Our study provides a novel concept of using a photoactive MoS₂ layer as a tunneling layer itself with a dry-transferred transparent SWCNT electrode for high-performance and energy-efficient optoelectronic devices.

High-Gain MoS2/Ta2NiSe5 Heterojunction Photodetectors with Charge Transfer and Suppressing Dark Current

Emerging two-dimensional narrow band gap materials with tunable band gaps and unique electrical and optical properties have shown tremendous potential in broadband photodetection. Nevertheless, large dark currents severely hinder the performance of photodetectors. Here, a MoS₂/Ta₂NiSe₅ van der Waals heterostructure device was successfully fabricated with a high rectification ratio of ∼10⁴ and an ultralow reverse bias current of the pA level. Excitingly, the charge transfer and the generation of the built-in electric field of heterostructures have been proved by theory and experiment, which effectively suppress dark currents. The dark current of the heterostructure reduces by nearly 10⁴ compared with the pure Ta₂NiSe₅ photodetector at V_ds = 1 V. The MoS₂/Ta₂NiSe₅ device exhibits excellent photoelectric performance with the maximum responsivity of 515.6 A W^-1 and 0.7 A W^-1 at the wavelengths of 532 and 1064 nm under forward bias, respectively. In addition, the specific detectivity is up to 3.1 × 10¹³ Jones (532 nm) and 2.4 × 10⁹ Jones (1064 nm). Significantly, the device presents an ultra-high gain of 6 × 10⁷ and an exceptional external quantum efficiency of 1.2 × 10⁵% under 532 nm laser irradiation. The results reveal that the MoS₂/Ta₂NiSe₅ heterostructure provides an essential platform for the development and application of high-performance broadband optoelectronic devices.

Ultrafast Drift Current Terahertz Emission Amplification in the Monolayer WSe2/Si Heterostructure

Two-dimensional transition metal dichalcogenides (TMDs) have great potential application for seamless on-chip integration due to their strong photon-electron-spin-valley coupling. However, the contact-free measurements of the valley-coupled photocurrent in TMDs is still challenging. Here, ultrafast terahertz emission spectroscopy is employed to investigate the photocurrent dynamics in monolayer WSe₂, and an interface-induced drift current amplification is found in the WSe₂/Si heterostructure. The amplification of terahertz emission comes from the photocurrent enlarged by band bending in the WSe₂ and Si junction, and the amplification ratio increase further near the valley resonant transition of WSe₂. In addition, the valley-momentum locked photocurrent in the WSe₂/Si heterostructure reserves the same chirality with monolayer WSe₂ at room temperature. These findings could provide a new method for manipulating valley-momentum locked photocurrent by photon helicity and open new avenues for TMD-based valley-polarized terahertz emission devices.

Van der Waals-Interface-Dominated All-2D Electronics

The interface is the device. As the feature size rapidly shrinks, silicon-based electronic devices are facing multiple challenges of material performance decrease and interface quality degradation. Ultrathin 2D materials are considered as potential candidates in future electronics by their atomically flat surfaces and excellent immunity to short-channel effects. Moreover, due to naturally terminated surfaces and weak van der Waals (vdW) interactions between layers, 2D materials can be freely stacked without the lattice matching limit to form high-quality heterostructure interfaces with arbitrary components and twist angles. Controlled interlayer band alignment and optimized interfacial carrier behavior allow all-2D electronics based on 2D vdW interfaces to exhibit more comprehensive functionality and better performance. Especially, achieving the same computing capacity of multiple conventional devices with small footprint all-2D devices is considered to be the key development direction of future electronics. Herein, the unique properties of all-2D vdW interfaces and their construction methods are systematically reviewed and the main performance contributions of different vdW interfaces in 2D electronics are summarized, respectively. Finally, the recent progress and challenges for all-2D vdW electronics are discussed, and how to improve the compatibility of 2D material devices with silicon-based industrial technology is pointed out as a critical challenge.

Miniaturized spectrometers with a tunable van der Waals junction

Miniaturized computational spectrometers, which can obtain incident spectra using a combination of device spectral responses and reconstruction algorithms, are essential for on-chip and implantable applications. Highly sensitive spectral measurement using a single detector allows the footprints of such spectrometers to be scaled down while achieving spectral resolution approaching that of benchtop systems. We report a high-performance computational spectrometer based on a single van der Waals junction with an electrically tunable transport-mediated spectral response. We achieve high peak wavelength accuracy (∼0.36 nanometers), high spectral resolution (∼3 nanometers), broad operation bandwidth (from ∼405 to 845 nanometers), and proof-of-concept spectral imaging. Our approach provides a route toward ultraminiaturization and offers unprecedented performance in accuracy, resolution, and operation bandwidth for single-detector computational spectrometers.

Nondestructive and local mapping photoresponse of WSe2 by electrostatic force microscopy

We report a local mapping photoresponse of WSe₂ using a second-harmonic (2w) channel based on nondestructive electrostatic force microscopy (EFM). The 2w signals resulting from interaction between WSe₂ and EFM tip are intrinsically related to the electrical conductivity of WSe₂. The photoresponse images and rise/decay time constants of WSe₂ are obtained by local mapping 2w signals under illumination. We observe that the local photoresponse signals of WSe₂ increase with the positive tip gate voltage while the WSe₂ shows a p-type behavior in dark conditions We find that the reduced mobility of the photogenerated charge carriers resulting from the enhanced carrier scattering in the accumulation regime of WSe₂ is responsible for the gate-dependent photoresponse behavior. Our results provide a deep understanding the intrinsic optoelectrical properties of WSe₂ and contribute to the developments in the optoelectronic devices based on van der Waals layered materials.

An Ultralow Power Mixed Dimensional Heterojunction Transistor Based on the Charge Plasma pn Junction

Development of a reliable doping method for 2D materials is a key issue to adopt the materials in the future microelectronic circuits and to replace the silicon, keeping the Moore's law toward the sub-10 nm channel length. Especially hole doping is highly required, because most of the transition metal dichalcogenides (TMDC) among the 2D materials are electron-doped by sulfur vacancies in their atomic structures. Here, hole doping of a TMDC, tungsten disulfide (WS₂ ) using the silicon substrate as the dopant medium is demonstrated. An ultralow-power current sourcing transistor or a gated WS₂ pn diode is fabricated based on a charge plasma pn heterojunction formed between the WS₂ thin-film and heavily doped bulk silicon. An ultralow switchable output current down to 0.01 nA µm^-1 , an off-state current of ≈1 × 10^-14 A µm^-1 , a static power consumption range of  1 fW µm^-1 -1 pW µm^-1 , and an output current ratio of 10³ at 0.1 V supply voltage are achieved. The charge plasma heterojunction allows a stable (less than 3% variation) output current regardless of the gate voltage once it is turned on.

High-Efficiency WSe2 Photovoltaic Devices with Electron-Selective Contacts

A rapid surge in global energy consumption has led to a greater demand for renewable energy to overcome energy resource limitations and environmental problems. Recently, a number of van der Waals materials have been highlighted as efficient absorbers for very thin and highly efficient photovoltaic (PV) devices. Despite the predicted potential, achieving power conversion efficiencies (PCEs) above 5% in PV devices based on van der Waals materials has been challenging. Here, we demonstrate a vertical WSe₂ PV device with a high PCE of 5.44% under one-sun AM1.5G illumination. We reveal the multifunctional nature of a tungsten oxide layer, which promotes a stronger internal electric field by overcoming limitations imposed by the Fermi-level pinning at WSe₂ interfaces and acts as an electron-selective contact in combination with monolayer graphene. Together with the developed bottom contact scheme, this simple yet effective contact engineering method improves the PCE by more than five times.

Mixed-Dimensional 1D/2D van der Waals Heterojunction Diodes and Transistors in the Atomic Limit

Inverting a semiconducting channel is the basis of all field-effect transistors. In silicon-based metal-oxide-semiconductor field-effect transistors (MOSFETs), a gate dielectric mediates this inversion. Access to inversion layers may be granted by interfacing ultrathin low-dimensional semiconductors in heterojunctions to advance device downscaling. Here we demonstrate that monolayer molybdenum disulfide (MoS₂) can directly invert a single-walled semiconducting carbon nanotube (SWCNT) transistor channel without the need for a gate dielectric. We fabricate and study this atomically thin one-dimensional/two-dimensional (1D/2D) van der Waals heterojunction and employ it as the gate of a 1D heterojunction field-effect transistor (1D-HFET) channel. Gate control is based on modulating the conductance through the channel by forming a lateral p-n junction within the CNT itself. In addition, we observe a region of operation exhibiting a negative static resistance after significant gate tunneling current passes through the junction. Technology computer-aided design (TCAD) simulations confirm the role of minority carrier drift-diffusion in enabling this behavior. The resulting van der Waals transistor architecture thus has the dual characteristics of both field-effect and tunneling transistors, and it advances the downscaling of heterostructures beyond the limits of dangling bonds and epitaxial constraints faced by III-V semiconductors.

A GaSe/Si-based vertical 2D/3D heterojunction for high-performance self-driven photodetectors

We report on the fabrication of a vertical 2D/3D heterojunction diode between gallium selenide (GaSe) and silicon (Si), and describe its photoresponse properties. Kelvin probe force microscopy (KPFM) has been employed to investigate the surface potentials of the GaSe/Si heterostructure, leading to the evaluation of the value of the conduction band offset at the heterostructure interface. The current-voltage measurements on the heterojunction device display a diode-like nature. This diode-like nature is attributed to the type-II band alignment that exists at the p-n interface. The key parameters of a photodetector, such as photoresponsivity, detectivity, and external quantum efficiency, have been calculated for the fabricated device and compared with those of other similar devices. The photodetection measurements of the GaSe/Si heterojunction diode show excellent performance of the device, with high photoresponsivity, detectivity, and EQE values of ∼2.8 × 10³ A W^-1, 6.2 × 10¹² Jones, and 6011, respectively, at a biasing of -5 V. Even at zero biasing, a high photoresponsivity of 6 A W^-1 was obtained, making it a self-powered device. Therefore, the GaSe/Si self-driven heterojunction diode has promising potential in the field of efficient optoelectronic devices.

Visible and infrared dual-band imaging via Ge/MoS2 van der Waals heterostructure

Multispectral photodetectors are emerging devices capable of detecting photons in multiple wavelength ranges, such as visible (VIS), near infrared (NIR), etc. Image data acquired with these photodetectors can be used for effective object identification and navigations owing to additional information beyond human vision, including thermal image and night vision. However, these capabilities are hindered by the structural complexity arising from the integration of multiple heterojunctions and selective absorbers. In this paper, we demonstrate a Ge/MoS₂ van der Waals heterojunction photodetector for VIS- and IR-selective detection capability under near-photovoltaic and photoconductive modes. The simplified single-polarity bias operation using single pixel could considerably reduce structural complexity and minimize peripheral circuitry for multispectral selective detection. The proposed multispectral photodetector provides a potential pathway for the integration of VIS/NIR vision for application in self-driving, surveillance, computer vision, and biomedical imaging.

2D Cu9 S5 /PtS2 /WSe2 Double Heterojunction Bipolar Transistor with High Current Gain

Bipolar junction transistor (BJT) as one important circuit element is now widely used in high-speed computation and communication for its capability of high-power signal amplification. 2D materials and their heterostructures are promising in building high-amplification and high-frequency BJTs because they can be naturally thin and highly designable in tailoring components properties. However, currently the low emitter injection efficiency results in only moderate current gain achieved in the pioneer researches, severely restraining its future development. Herein, it is shown that an elaborately designed double heterojunction bipolar transistor (DHBT) can greatly promote the injection efficiency, improving the current gain by order of magnitude. In this DHBT high-doping-density wide-bandgap 2D Cu₉ S₅ is used as emitter and narrow-bandgap PtS₂ as base. This heterostructure efficiently suppresses the reverse electron flux from base and increase the injection efficiency. Consequently, the DHBT achieves an excellent current gain (β ≈ 910). This work systematically explores the electrical behavior of 2D materials based DHBT, and provides deep insight of the architecture design for building high gain DHBT, which may promote the applications of 2Dheterojunctions in the fields of integrated circuits.

Visualizing Band Profiles of Gate-Tunable Junctions in MoS2/WSe2 Heterostructure Transistors

Heterostructure devices based on two-dimensional materials have been under intensive study due to their intriguing electrical and optical properties. One key factor in understanding these devices is their nanometer-scale band profiles, which is challenging to obtain in devices. Here, we use a technique named contact-mode scanning tunneling spectroscopy to directly visualize the band profiles of MoS₂/WSe₂ heterostructure devices at different gate voltages with nanometer resolution. The long-held view of a conventional p-n junction in the MoS₂/WSe₂ heterostructure is reexamined. Due to strong inter- and intralayer charge transfer, the MoS₂ layer in contact with WSe₂ is found to convert from n-type to p-type, and a series of gate-tunable p-n and p-p⁺ junctions are developed in the devices. Highly conductive edges are also discovered which could strongly affect the device properties.

Coherent Elliptically Polarized Terahertz Wave Generation in WSe2 by Linearly Polarized Femtosecond Laser Excitation

Coherent polarization control of terahertz (THz) wave radiation in both the time-domain and the frequency-domain is significant in information technology, material science, and spectroscopic analysis. Elliptically polarized THz radiation is generally limited to chiral materials induced by circularly polarized light excitation. Herein, we demonstrate the coherent elliptically polarized THz radiation from few-layer tungsten diselenide (WSe₂) in both the time-domain and the frequency-domain under linearly polarized femtosecond laser excitation. This coherent elliptical THz radiation is mainly dominated by in-plane anisotropic shift current and out-of-plane drift current, which is verified by the THz radiation dependence on the pump laser polarization angles, incident angles, and sample azimuthal angles systematically. The ellipticity and major axis direction of the elliptical THz wave can be efficiently controlled by either pump light polarization or sample azimuthal angle due to the controllable amplitudes and phases of two coherent orthogonal THz wave components. Our finding provides a method to distinguish drift and shift photocurrents in different directions and offers a unique design concept for elliptical THz generation with two-dimensional (2D) material physics.

Oxygen Vacancy Defects and a Field Effect-Mediated ZnO/WO2.92 Heterojunction for Enhanced Corrosion Resistance

The heterojunction constructed by tungsten oxide and zinc oxide materials can improve the problem of easy deactivation of electrons, which is a new and effective strategy for realizing anticorrosion. Here, the ZnO/WO_2.92 heterojunction modified by oxygen vacancies (OVs) serving as the photoelectric conversion center was not consumed to provide continuous light-induced protection for steel, and the impedance value was increased by 185.35% compared to that of epoxy resin after 72 h of corrosion. The enhanced anticorrosion activity was due to OV modification leading to oxygen adsorption and electron capture, which inhibited the cathodic corrosion reaction and effectively hindered electron transport. Additionally, the localized surface plasmon resonance effect produced by OVs improved light utilization efficiency and increased electron density, which enabled numerous photoelectrons to gather on the surface of the iron substrate to reduce the corrosion rate of metals. Besides, the cascade effect of the ZnO/WO_2.92 heterojunction promoted the transfer of e^-/h⁺ to form an electric field that allowed the directional flow of electrons to inhibit the anode dissolution process. Thus, exploring the corrosion reaction involving OVs and heterojunction structures was of great significance to the development of nonsacrificial and efficient anticorrosion materials.

Graphene/SnS2 van der Waals Photodetector with High Photoresponsivity and High Photodetectivity for Broadband 365-2240 nm Detection

The fabrication of graphene/SnS₂ van der Waals photodetectors and their photoelectrical properties are systematically investigated. It was found that a dry transferred graphene/SnS₂ van der Waals heterostructure had a broadband sensing range from ultraviolet (365 nm) to near-infrared (2.24 μm) and respective improved responsivities and photodetectivities of 7.7 × 10³ A/W and 8.9 × 10¹³ jones at 470 nm and 2 A/W and 1.8 × 10¹⁰ jones at 1064 nm. Moreover, positive and negative photoconductance effects were observed when the photodetectors were illuminated by photon sources with energies greater and smaller than the bandgap of SnS₂, respectively. The photoresponsivity (R) versus incident power density (P) follows the empirical law R ∝ P_in^β, with β > -1 for positive photoconductance effects and β < -1 for negative photoconductance effects. On the basis of the Fowler-Nordheim tunneling model and a Poisson and drift-diffusion simulation, we show quantitatively that the barrier height and barrier width of the heterostructure photodetector could be controlled by a laser and an external electrical field through a photogating effect generated by carriers trapped at the interface, which could be used to tune the separation and transport of photogenerated carriers. Our results may be useful for the design of high performance van der Waals heterojunction photodetectors.

Photovoltaic Field-Effect Photodiodes Based on Double van der Waals Heterojunctions

High performance photodetectors based on van der Waals heterostructures (vdWHs) are crucial to developing micro-nano-optoelectronic devices. However, reports show that it is difficult to balance fast response and high sensitivity. In this work, we design a photovoltaic field-effect photodiode (PVFED) based on the WSe₂/MoS₂/WSe₂ double vdWHs, where the photovoltage that originated from one vdWH modulates the optoelectronic characteristics of another vdWH. The proposed photodiode exhibits an excellent self-powered ability with a high responsivity of 715 mA·W^-1 and fast response time of 45 μs. This work demonstrates an efficient method that optimizes the photoelectric performance of vdWH by introducing the photovoltaic field effect.

One-Step Synthesis of NbSe2/Nb-Doped-WSe2 Metal/Doped-Semiconductor van der Waals Heterostructures for Doping Controlled Ohmic Contact

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, NbSe₂/Nb-doped-WSe₂ 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 WSe₂ 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 × 10¹¹/cm² at Nb-0% to ∼1.38 × 10¹²/cm² 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/T²) versus q/K_BT demonstrated an ohmic contact in the NbSe₂/W_xNb_1-xSe₂ vdWHs. Combining p-doping in WSe₂ and M/d-S vdWHs, the mobility (27.24 cm² V^-1 s^-1) and on/off ratio (2.2 × 10⁷) are increased 1238 and 4400 times, respectively, compared to that using the Cr/pure-WSe₂ contact (0.022 cm² V^-1 s^-1 and 5 × 10³, respectively). Together, the R_C value using the NbSe₂ 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.

All-Electrical High-Sensitivity, Low-Power Dual-Mode Gas Sensing and Recovery with a WSe2/MoS2 pn Heterodiode

Two-dimensional MoS₂ gas sensors have conventionally relied on a change in field-effect-transistor (FET) channel resistance or in the Schottky contact/pn homojunction barrier. We demonstrate an enhancement in sensitivity (6×) and dynamic response along with a reduction in detection limit (8×) and power (10⁴×) in a gate-tunable type-II WSe₂(p)/MoS₂(n) heterodiode gas sensor over an MoS₂ FET on the same flake. Measurements for varying NO₂ concentration, gate bias, and MoS₂ flake thickness, reinforced with first-principles calculations, indicate dual-mode operation due to (i) a series resistance-based exponential change in the high-bias thermionic current (high sensitivity), and (ii) a heterointerface carrier concentration-based linear change in near-zero-bias interlayer recombination current (low power) resulting in sub-100 μW/cm² power consumption. Fast and gate-bias tunable recovery enables an all-electrical, room-temperature dynamic operation. Coupled with the sensing of trinitrotoluene (TNT) molecules down to 80 ppb, this study highlights the potential of the WSe₂/MoS₂ pn heterojunction as a simple, low-overhead, and versatile chemical-sensing platform.

Application of Pulsed Laser Deposition in the Preparation of a Promising MoSx/WSe2/C(В) Photocathode for Photo-Assisted Electrochemical Hydrogen Evolution

We studied the possibility of using pulsed laser deposition (PLD) for the formation of a MoS_x/WSe2 heterostructure on a dielectric substrate. The heterostructure can be employed for effective solar water splitting to produce hydrogen. The sapphire substrate with the conducting C(B) film (rear contact) helped increase the formation temperature of the WSe₂ film to obtain the film consisting of 2H-WSe₂ near-perfect nanocrystals. The WSe₂ film was obtained by off-axis PLD in Ar gas. The laser plume from a WSe₂ target was directed along the substrate surface. The preferential scattering of selenium on Ar molecules contributed to the effective saturation of the WSe₂ film with chalcogen. Nano-structural WSe₂ film were coated by reactive PLD with a nanofilm of catalytically active amorphous MoS_x~4. It was established that the mutual arrangement of energy bands in the WSe₂ and MoS_x~4 films facilitated the separation of electrons and holes at the interface and electrons moved to the catalytically active MoS_x~4. The current density during light-assisted hydrogen evolution was above ~3 mA/cm² (at zero potential), whilst the onset potential reached 400 mV under irradiation with an intensity of 100 mW/cm² in an acidic solution. Factors that may affect the HER performance of MoS_x~4/WSe₂/C(В) structure are discussed.

Broken-Gap PtS2/WSe2 van der Waals Heterojunction with Ultrahigh Reverse Rectification and Fast Photoresponse

Broken-gap van der Waals (vdW) heterojunctions based on 2D materials are promising structures to fabricate high-speed switching and low-power multifunctional devices thanks to its charge transport versus quantum tunneling mechanism. However, the tunneling current is usually generated under both positive and negative bias voltage, resulting in small rectification and photocurrent on/off ratio. In this paper, we report a broken-gap vdW heterojunction PtS₂/WSe₂ with a bilateral accumulation region design and a big band offset by utilizing thick PtS₂ as an effective carrier-selective contact, which exhibits an ultrahigh reverser rectification ratio approaching 10⁸ and on/off ratio over 10⁸ at room temperature. We also find excellent photodetection properties in such a heterodiode with a large photocurrent on/off ratio over 10⁵ due to its ultralow forward current and a comparable photodetectivity of 3.8 × 10¹⁰ Jones. In addition, the response time of such a photodetector reaches 8 μs owing to the photoinduced tunneling mechanism and reduced interface trapping effect. The proposed heterojunction not only demonstrates the high-performance broken-gap heterodiode but also provides in-depth understanding of the tunneling mechanism in the development of future electronic and optoelectronic applications.

Plasmonic enhancement of photocurrent generation in two-dimensional heterostructure of WSe2/MoS2

Enhancing the photoresponse of single-layered semiconductor materials is a challenge for high-performance photodetectors due to atomically thickness and limited quantum efficiency of these devices. Band engineering in heterostructure of transition metal chalcogenides (TMDs) can sort out part of this challenge. Here, we address this issue by utilizing the plasmonics phenomenon to enrich the optoelectronics property of the WSe₂/MoS₂heterojunction and further enhancement of photoresponse. The introduced approach presents a contamination-free, tunable and efficient way to improve light interactions with heterojunction devices. The results showed a 3600-fold enhancement in photoresponsivity and a 46-fold increase in external quantum efficiency (549%) along with a fast photoresponse time (~2μs) and light polarization dependence. This improvement may assign to multiple light scatterings by the Au nanoarrays and creation of strong local electrical fields (hot spots) at the interfaces of the gold nanoarrays and the TMDs heterostructure. The high-energy electrons (hot electrons) originating from hot spots surmount easily to conduction bands of heterojunction which is leading to a remarkable enhancement of photocurrent. The plasmons assisted photoresponse strategy can be easily matched with the semiconductor industry to boost the performance of optoelectronics devices for practical applications.

Directly Probing Interfacial Coupling in a Monolayer MoSe2 and CuPc Heterostructure

It is of great importance to develop useful methods to evaluate interfacial coupling strength noninvasively for exploring and optimizing heterointerface functionality. Recently, organic-inorganic van der Waals (vdW) heterostructures (HSs) composed of organic semiconductors and transition-metal dichalcogenides (TMD) have shown great potential for developing next-generation flexible optical, electrical, and optoelectrical devices. Since vdW coupling dominates the property of such a vdW HS, it is crucial to develop a method to evaluate its interfacial coupling strength noninvasively. In this work, by combining electrical force microscopy (EFM) and Raman and photoluminescence spectroscopic measurements, we were able to directly probe the coupling strength between monolayer MoSe₂ and a copper phthalocyanine (CuPc) thin film. Especially, we also found a new Raman mode in HS due to the Davydov splitting of the CuPc thin film via strong interfacial coupling between the two materials. This new Raman mode was thus utilized as a probe to reveal the modulation of the coupling strength by changing post-treatment conditions. All of these results indicate that the method developed here is capable of evaluating the coupling strength of the MoSe₂/CuPc HS effectively and innovatively, which aids in providing deep insights into such hybrid vdW HSs for future optical and optoelectrical applications.

Strain-Modulated Photoelectric Responses from a Flexible α-In2Se3/3R MoS2 Heterojunction

Semiconducting piezoelectric α-In₂Se₃ and 3R MoS₂ have attracted tremendous attention due to their unique electronic properties. Artificial van der Waals (vdWs) heterostructures constructed with α-In₂Se₃ and 3R MoS₂ flakes have shown promising applications in optoelectronics and photocatalysis. Here, we present the first flexible α-In₂Se₃/3R MoS₂ vdWs p-n heterojunction devices for photodetection from the visible to near infrared region. These heterojunction devices exhibit an ultrahigh photoresponsivity of 2.9 × 10³ A W^-1 and a substantial specific detectivity of 6.2 × 10¹⁰ Jones under a compressive strain of - 0.26%. The photocurrent can be increased by 64% under a tensile strain of + 0.35%, due to the heterojunction energy band modulation by piezoelectric polarization charges at the heterojunction interface. This work demonstrates a feasible approach to enhancement of α-In₂Se₃/3R MoS₂ photoelectric response through an appropriate mechanical stimulus.

Low-Temperature Synthesis of Wafer-Scale MoS2-WS2 Vertical Heterostructures by Single-Step Penetrative Plasma Sulfurization

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their synergetic effects with other 2D materials, such as graphene and hexagonal boron nitride, in TMD-based heterostructures. Therefore, it is important to understand the physical properties of TMD-TMD vertical heterostructures for their applications in next-generation electronic devices. However, the conventional synthesis process of TMD-TMD heterostructures has some critical limitations, such as nonreproducibility and low yield. In this paper, we synthesize wafer-scale MoS₂-WS₂ vertical heterostructures (MWVHs) using plasma-enhanced chemical vapor deposition (PE-CVD) via penetrative single-step sulfurization discovered by time-dependent analysis. This method is available for fabricating uniform large-area vertical heterostructures (4 in.) at a low temperature (300 °C). MWVHs were characterized using various spectroscopic and microscopic techniques, which revealed their uniform nanoscale polycrystallinity and the presence of vertical layers of MoS₂ and WS₂. In addition, wafer-scale MWVHs diodes were fabricated and demonstrated uniform performance by current mapping. Furthermore, mode I fracture tests were performed using large double cantilever beam specimens to confirm the separation of the MWVHs from the SiO₂/Si substrate. Therefore, this study proposes a synthesis mechanism for TMD-TMD heterostructures and provides a fundamental understanding of the interfacial properties of TMD-TMD vertical heterostructures.

Photoelectrical Dynamics Uplift in Perovskite Solar Cells by Atoms Thick 2D TiS2 Layer Passivation of TiO2 Nanograss Electron Transport Layer

A deficiency in the photoelectrical dynamics at the interface due to the surface traps of the TiO₂ electron transport layer (ETL) has been the critical factor for the inferiority of the power conversion efficiency (PCE) in the perovskite solar cells. Despite its excellent energy level alignment with most perovskite materials, its large density of surface defect as a result of sub lattice vacancies has been the critical hurdle for an efficient photovoltaic process in the device. Here, we report that atoms thick 2D TiS₂ layer grown on the surface of a (001) faceted and single-crystalline TiO₂ nanograss (NG) ETL have effectively passivated the defects, boosting the charge extractability, carrier mobility, external quantum efficiency, and the device stability. These properties allow the perovskite solar cells (PSCs) to produce a PCE as high as 18.73% with short-circuit current density (J_sc), open-circuit voltage (V_oc), and fill-factor (FF) values as high as 22.04 mA/cm², 1.13 V, and 0.752, respectively, a 3.3% improvement from the pristine TiO₂-NG-based PSCs. The present approach should find an extensive application for controlling the photoelectrical dynamic deficiency in perovskite solar cells.

Laser Writable Multifunctional van der Waals Heterostructures

Achieving multifunctional van der Waals nanoelectronic devices on one structure is essential for the integration of 2D materials; however, it involves complex architectural designs and manufacturing processes. Herein, a facile, fast, and versatile laser direct write micro/nanoprocessing to fabricate diode, NPN (PNP) bipolar junction transistor (BJT) simultaneously based on a pre-fabricated black phosphorus/molybdenum disulfide heterostructure is demonstrated. The PN junctions exhibit good diode rectification behavior. Due to different carrier concentrations of BP and MoS₂ , the NPN BJT, with a narrower base width, renders better performance than the PNP BJT. Furthermore, the current gain can be modulated efficiently through laser writing tunable base width W_B , which is consistent with the theoretical results. The maximum gain for NPN and PNP is found to be ≈41 (@W_B ≈600 nm) and ≈12 (@W_B ≈600 nm), respectively. In addition, this laser write processing technique also can be utilized to realize multifunctional WSe₂ /MoS₂ heterostructure device. The current work demonstrates a novel, cost-effective, and universal method to fabricate multifunctional nanoelectronic devices. The proposed approach exhibits promise for large-scale integrated circuits based on 2D heterostructures.

Probing the Importance of Charge Balance and Noise Current in WSe2/WS2/MoS2 van der Waals Heterojunction Phototransistors by Selective Electrostatic Doping

Heterojunction structures using 2D materials are promising building blocks for electronic and optoelectronic devices. The limitations of conventional silicon photodetectors and energy devices are able to be overcome by exploiting quantum tunneling and adjusting charge balance in 2D p-n and n-n junctions. Enhanced photoresponsivity in 2D heterojunction devices can be obtained with WSe2 and BP as p-type semiconductors and MoS2 and WS2 as n-type semiconductors. In this study, the relationship between photocurrent and the charge balance of electrons and holes in van der Waals heterojunctions is investigated. To observe this phenomenon, a p-WSe2/n-WS2/n-MoS2 heterojunction device with both p-n and n-n junctions is fabricated. The device can modulate the charge carrier balance between heterojunction layers to generate photocurrent upon illumination by selectively applying electrostatic doping to a specific layer. Using photocurrent mapping, the operating transition zones for the device is demonstrated, allowing to accurately identify the locations where photocurrent generates. Finally, the origins of flicker and shot noise at the different semiconductor interfaces are analyzed to understand their effect on the photoresponsivity and detectivity of unit active area (2.5 µm2, λ = 405 nm) in the p-WSe2/n-WS2/n-MoS2 heterojunction device.

Observing the Layer-Number-Dependent Local Dielectric Response of WSe2 by Electrostatic Force Microscopy

We investigate the layer-number-dependent dielectric response of WSe₂ by measuring the phase shift (Φ) through an electrostatic force microscopy (EFM). The measured Φ results stem mainly from the capacitive coupling between the tip and WSe₂ based on the plane capacitor model, leading to changes in the second derivative of the capacitance (C'') values, which increase in a few layers and saturate to the bulk value under an applied EFM tip bias. The C'' value is related to the dielectric polarization, reflecting the charge carrier concentration and mobility of WSe₂ flakes with different numbers of layers. This implies that the dielectric constant of WSe₂ shows layer-number-dependent behavior which increases with the number of layers, approaching the bulk value. Furthermore, we also construct a spatially resolved C'' map to observe the local dielectric response of WSe₂ flakes. Our work could be significant in that it can improve the performance of novel electronic devices based on the controllable dielectric properties of 2D vdW semiconductor materials.

Ultrasensitive Phototransistor Based on WSe2-MoS2 van der Waals Heterojunction

Band engineering using the van der Waals heterostructure of two-dimensional materials allows for the realization of high-performance optoelectronic devices by providing an ultrathin and uniform PN junction with sharp band edges. In this study, a highly sensitive photodetector based on the van der Waals heterostructure of WSe₂ and MoS₂ was developed. The MoS₂ was utilized as the channel for a phototransistor, whereas the WSe₂-MoS₂ PN junction in the out-of-plane orientation was utilized as a charge transfer layer. The vertical built-in electric field in the PN junction separated the photogenerated carriers, thus leading to a high photoconductive gain of 10⁶. The proposed phototransistor exhibited an excellent performance, namely, a high photoresponsivity of 2700 A/W, specific detectivity of 5 × 10¹¹ Jones, and response time of 17 ms. The proposed scheme in conjunction with the large-area synthesis technology of two-dimensional materials contributes significantly to practical photodetector applications.

Distinctive Field-Effect Transistors and Ternary Inverters Using Cross-Type WSe2/MoS2 Heterojunctions Treated with Polymer Acid

The electrical and optical characteristics of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be improved by surface modification. In this study, distinctive field-effect transistors (FETs) were realized by forming cross-type 2D WSe₂/MoS₂ p-n heterojunctions through surface treatment using poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-PMAA). The FETs were applied to new ternary inverters as multivalued logic circuits (MVLCs). Laser confocal microscope photoluminescence spectroscopy indicated the generation of trions in the WSe₂ and MoS₂ layers, and the intensity decreased after PMMA-co-PMAA treatment. For the cross-type WSe₂/MoS₂ p-n heterojunction FETs subjected to PMMA-co-PMAA treatment, the channel current and the region of anti-ambipolar transistor characteristics increased considerably, and ternary inverter characteristics with three stable logic states, "1", "1/2", and "0", were realized. Interestingly, the intermediate logic state 1/2, which results from the negative differential transconductance characteristics, was realized by the turn-on of all component FETs, as the current of the FETs increased after PMMA-co-PMAA treatment. The electron-rich carboxyl acid moieties in PMMA-co-PMAA can undergo coordination with the metal Mo or W atoms present in the Se or S vacancies, respectively, resulting in the modulation of charge density. These features yielded distinctive FETs and ternary inverters for MVLCs using cross-type WSe₂/MoS₂ heterojunctions.

Van der Waals Heterostructures for High-Performance Device Applications: Challenges and Opportunities

The discovery of two-dimensional (2D) materials with unique electronic, superior optoelectronic, or intrinsic magnetic order has triggered worldwide interest in the fields of material science, condensed matter physics, and device physics. Vertically stacking 2D materials with distinct electronic and optical as well as magnetic properties enables the creation of a large variety of van der Waals heterostructures. The diverse properties of the vertical heterostructures open unprecedented opportunities for various kinds of device applications, e.g., vertical field-effect transistors, ultrasensitive infrared photodetectors, spin-filtering devices, and so on, which are inaccessible in conventional material heterostructures. Here, the current status of vertical heterostructure device applications in vertical transistors, infrared photodetectors, and spintronic memory/transistors is reviewed. The relevant challenges for achieving high-performance devices are presented. An outlook into the future development of vertical heterostructure devices with integrated electronic and optoelectronic as well as spintronic functionalities is also provided.

Emerging Opportunities for Electrostatic Control in Atomically Thin Devices

Electrostatic control of charge carrier concentration underlies the field-effect transistor (FET), which is among the most ubiquitous devices in the modern world. As transistors and related electronic devices have been miniaturized to the nanometer scale, electrostatics have become increasingly important, leading to progressively sophisticated device geometries such as the finFET. With the advent of atomically thin materials in which dielectric screening lengths are greater than device physical dimensions, qualitatively different opportunities emerge for electrostatic control. In this Review, recent demonstrations of unconventional electrostatic modulation in atomically thin materials and devices are discussed. By combining low dielectric screening with the other characteristics of atomically thin materials such as relaxed requirements for lattice matching, quantum confinement of charge carriers, and mechanical flexibility, high degrees of electrostatic spatial inhomogeneity can be achieved, which enables a diverse range of gate-tunable properties that are useful in logic, memory, neuromorphic, and optoelectronic technologies.

Observation of double indirect interlayer exciton in WSe2/WS2 heterostructure

Interlayer excitons (IX) are produced by the spatially separated electron-hole pairs due to the robust Coulomb interactions in van der Waals transition metal dichalcogenide (TMDC) heterostructures (HS_S). IX is characterized by a larger binding energy, and its lifetime is orders of magnitude longer than that of the direct excitons, providing a significant platform for the manufacture of long-lived exciton devices and the exploration of exciton quantum gas. However, the studies are restricted to the single interlayer exciton, and the simultaneous capture and study of double IX remain challenging in the WSe₂/WS₂ HS. Here, we demonstrate the existence of double indirect IX in the WSe₂/WS₂ HS with the emission centers at 1.4585eV (∼25.9meV wide) and 1.4885 eV (∼14.4 meV wide) at cryogenic temperature. Interestingly, the intensities of the double IX emission peaks are almost equal, and the energy difference between them is in a good agreement with the cleavage value of the WS₂ conduction band (CB). Additionally, diverse types of excitons in the individual materials were successfully observed in the PL spectra at 8 K. Such unique double IX features, in combination with excellent exciton identification, open up new opportunities for further investigations for new physical properties of TMDCs and explorations for the technological innovation of exciton devices.

Mixed-Dimensional Van der Waals Heterostructure Photodetector

Van der Waals (vdW) heterostructures, integrated two-dimensional (2D) materials with various functional materials, provide a distinctive platform for next-generation optoelectronics with unique flexibility and high performance. However, exploring the vdW heterostructures combined with strongly correlated electronic materials is hitherto rare. Herein, a novel temperature-sensitive photodetector based on the GaSe/VO₂ mixed-dimensional vdW heterostructure is discovered. Compared with previous devices, our photodetector exhibits excellent enhanced performance, with an external quantum efficiency of up to 109.6% and the highest responsivity (358.1 mA·W^-1) under a 405 nm laser. Interestingly, we show that the heterostructure overcomes the limitation of a single material under the interaction between VO₂ and GaSe, where the photoresponse is highly sensitive to temperature and can be further vanished at the critical value. The metal-insulator transition of VO₂, which controls the peculiar band-structure evolution across the heterointerface, is demonstrated to manipulate the photoresponse variation. This study enables us to elucidate the method of manipulating 2D materials by strongly correlated electronic materials, paving the way for developing high-performance and special optoelectronic applications.

Breaking symmetry in device design for self-driven 2D material based photodetectors

The advent of graphene and other two-dimensional (2D) materials offers great potential for optoelectronic applications. Various device structures and novel mechanisms have been proposed to realize photodetectors with unique detecting properties. In this minireview, we focus on a self-driven photodetector that has great potential for low-power or even powerless operation required in the internet of things and wearable electronics. To address the general principle of self-driven properties, we propose and elaborate the concept of symmetry breaking in 2D material based self-driven photodetectors. We discuss various mechanisms of breaking symmetry for self-driven photodetectors, including asymmetrical contact engineering, field-induced asymmetry, PN homojunctions, and PN heterostructures. Typical device examples based on these mechanisms are reviewed and compared. The performance of current self-driven photodetectors is critically assessed and future directions are discussed towards the target application fields.