Band Alignment in WSe2-Graphene Heterostructures

Nonlinear Nano-Imaging of Interlayer Coupling in 2D Graphene-Semiconductor Heterostructures

The emergent electronic, spin, and other quantum properties of 2D heterostructures of graphene and transition metal dichalcogenides are controlled by the underlying interlayer coupling and associated charge and energy transfer dynamics. However, these processes are sensitive to interlayer distance and crystallographic orientation, which are in turn affected by defects, grain boundaries, or other nanoscale heterogeneities. This obfuscates the distinction between interlayer charge and energy transfer. Here, nanoscale imaging in coherent four-wave mixing (FWM) and incoherent two-photon photoluminescence (2PPL) is combined with a tip distance-dependent coupled rate equation model to resolve the underlying intra- and inter-layer dynamics while avoiding the influence of structural heterogeneities in mono- to multi-layer graphene/WSe2  heterostructures. With selective insertion of hBN spacer layers, it is shown that energy, as opposed to charge transfer, dominates the interlayer-coupled optical response. From the distinct nano-FWM and -2PPL tip-sample distance-dependent modification of interlayer and intralayer relaxation by tip-induced enhancement and quenching, an interlayer energy transfer time ofτ ET ≈ ( 0 . 35 - 0.15 + 0.65 ) $\tau _{\rm ET} \approx (0.35^{+0.65}_{-0.15})$  ps consistent with recent reports is derived. As a local probe technique, this approach highlights the ability to determine intrinsic sample properties even in the presence of large sample heterogeneity.

Evidence for Exciton Crystals in a 2D Semiconductor Heterotrilayer

Two-dimensional (2D) transition metal dichalcogenides (TMDC) and their moiré interfaces have been demonstrated for correlated electron states, including Mott insulators and electron/hole crystals commensurate with moiré superlattices. Here we present spectroscopic evidence for ordered bosons─interlayer exciton crystals in a WSe2/MoSe2/WSe2 trilayer, where the enhanced Coulomb interactions over those in heterobilayers have been predicted to result in exciton ordering. Ordered interlayer excitons in the trilayer are characterized by negligible mobility and by sharper PL peaks persisting to an exciton density of nex ∼ 1012 cm-2, which is an order of magnitude higher than the corresponding limit in the heterobilayer. We present evidence for the predicted quadrupolar exciton crystal and its transitions to dipolar excitons either with increasing nex or by an applied electric field. These ordered interlayer excitons may serve as models for the exploration of quantum phase transitions and quantum coherent phenomena.

Visualizing moiré ferroelectricity via plasmons and nano-photocurrent in graphene/twisted-WSe2 structures

Ferroelectricity, a spontaneous and reversible electric polarization, is found in certain classes of van der Waals (vdW) materials. The discovery of ferroelectricity in twisted vdW layers provides new opportunities to engineer spatially dependent electric and optical properties associated with the configuration of moiré superlattice domains and the network of domain walls. Here, we employ near-field infrared nano-imaging and nano-photocurrent measurements to study ferroelectricity in minimally twisted WSe2. The ferroelectric domains are visualized through the imaging of the plasmonic response in a graphene monolayer adjacent to the moiré WSe2 bilayers. Specifically, we find that the ferroelectric polarization in moiré domains is imprinted on the plasmonic response of the graphene. Complementary nano-photocurrent measurements demonstrate that the optoelectronic properties of graphene are also modulated by the proximal ferroelectric domains. Our approach represents an alternative strategy for studying moiré ferroelectricity at native length scales and opens promising prospects for (opto)electronic devices.

Exciton Superposition across Moiré States in a Semiconducting Moiré Superlattice

Moiré superlattices of semiconducting transition metal dichalcogenides enable unprecedented spatial control of electron wavefunctions, leading to emerging quantum states. The breaking of translational symmetry further introduces a new degree of freedom: high symmetry moiré sites of energy minima behaving as spatially separated quantum dots. We demonstrate the superposition between two moiré sites by constructing a trilayer WSe2/monolayer WS2 moiré heterojunction. The two moiré sites in the first layer WSe2 interfacing WS2 allow the formation of two different interlayer excitons, with the hole residing in either moiré site of the first layer WSe2 and the electron in the third layer WSe2. An electric field can drive the hybridization of either of the interlayer excitons with the intralayer excitons in the third WSe2 layer, realizing the continuous tuning of interlayer exciton hopping between two moiré sites and a superposition of the two interlayer excitons, distinctively different from the natural trilayer WSe2.

GaN/Gr (2D)/Si (3D) Combined High-Performance Hot Electron Transistors

To overcome the problem of minority carrier storage time in bipolar transistors, a hot electron transistor (HET) has been proposed. This device has the advantage of high working speed and some complex logic functions can be completed by using one component. Here, we demonstrate a mixed-dimensional HET composed of GaN/AlN microwires, graphene (Gr), and Si. The electrons between GaN/AlN are injected into graphene by an F-N tunneling mechanism to achieve high speed hot electrons, then cross graphene by ballistic transport, and are collected in a nearly lossless manner through a low-barrier Si. Therefore, the device shows a record DC gain of 16.2, a collection efficiency close to the limit of 99.9% based on the graphene hot electron transistor (GHET), an emitter current density of about 68.7 A/cm², and a high on/off current ratio reaching ∼10⁷. Meanwhile, the current saturation range is wide, beyond those of most GHETs. It has potential applications as a power amplifier.

Ambipolar charge-transfer graphene plasmonic cavities

Plasmon polaritons in van der Waals materials hold promise for various photonics applications^1-4. The deterministic imprinting of spatial patterns of high carrier density in plasmonic cavities and nanoscale circuitry can enable the realization of advanced nonlinear nanophotonic⁵ and strong light-matter interaction platforms⁶. Here we demonstrate an oxidation-activated charge transfer strategy to program ambipolar low-loss graphene plasmonic structures. By covering graphene with transition-metal dichalcogenides and subsequently oxidizing the transition-metal dichalcogenides into transition-metal oxides, we activate charge transfer rooted in the dissimilar work functions between transition-metal oxides and graphene. Nano-infrared imaging reveals ambipolar low-loss plasmon polaritons at the transition-metal-oxide/graphene interfaces. Further, by inserting dielectric van der Waals spacers, we can precisely control the electron and hole densities induced by oxidation-activated charge transfer and achieve plasmons with a near-intrinsic quality factor. Using this strategy, we imprint plasmonic cavities with laterally abrupt doping profiles with nanoscale precision and demonstrate plasmonic whispering-gallery resonators based on suspended graphene encapsulated in transition-metal oxides.

Monolayer Fullerene Networks as Photocatalysts for Overall Water Splitting

Photocatalytic water splitting can produce hydrogen in an environmentally friendly way and provide alternative energy sources to reduce global carbon emissions. Recently, monolayer fullerene networks have been successfully synthesized [Hou et al. Nature2022, 606, 507], offering new material candidates for photocatalysis because of their large surface area with abundant active sites, feasibility to be combined with other 2D materials to form heterojunctions, and the C₆₀ cages for potential hydrogen storage. However, efficient photocatalysts need a combination of a suitable band gap and appropriate positions of the band edges with sufficient driving force for water splitting. In this study, I employ semilocal density functional theory and hybrid functional calculations to investigate the electronic structures of monolayer fullerene networks. I find that only the weakly screened hybrid functional, combined with time-dependent Hartree–Fock calculations to include the exciton binding energy, can reproduce the experimentally obtained optical band gap of monolayer C₆₀. All the phases of monolayer fullerene networks have suitable band gaps with high carrier mobility and appropriate band edges to thermodynamically drive overall water splitting. In addition, the optical properties of monolayer C₆₀ are studied, and different phases of fullerene networks exhibit distinct absorption and recombination behavior, providing unique advantages either as an electron acceptor or as an electron donor in photocatalysis.

Novel Graphene Adjustable-Barrier Transistor with Ultra-High Current Gain

A graphene-based three-terminal barristor device was proposed to overcome the low on/off ratios and insufficient current saturation of conventional graphene field-effect transistors. In this study, we fabricated and analyzed a novel graphene-based transistor, which resembles the structure of the barristor but uses a different operating condition. This new device, termed graphene adjustable-barriers transistor (GABT), utilizes a semiconductor-based gate rather than a metal-insulator gate structure to modulate the device currents. The key feature of the device is the two graphene-semiconductor Schottky barriers with different heights that are controlled simultaneously by the gate voltage. Due to the asymmetry of the barriers, the drain current exceeds the gate current by several orders of magnitude. Thus, the GABT can be considered an amplifier with an alterable current gain. In this work, a silicon-graphene-germanium GABT with an ultra-high current gain (I_D/I_G up to 8 × 10⁶) was fabricated, and the device functionality was demonstrated. Additionally, a capacitance model is applied to predict the theoretical device performance resulting in an on-off ratio above 10⁶, a swing of 87 mV/dec, and a drive current of about 1 × 10⁶ A/cm².

Optical absorption of interlayer excitons in transition-metal dichalcogenide heterostructures

Interlayer excitons, electron-hole pairs bound across two monolayer van der Waals semiconductors, offer promising electrical tunability and localizability. Because such excitons display weak electron-hole overlap, most studies have examined only the lowest-energy excitons through photoluminescence. We directly measured the dielectric response of interlayer excitons, which we accessed using their static electric dipole moment. We thereby determined an intrinsic radiative lifetime of 0.40 nanoseconds for the lowest direct-gap interlayer exciton in a tungsten diselenide/molybdenum diselenide heterostructure. We found that differences in electric field and twist angle induced trends in exciton transition strengths and energies, which could be related to wave function overlap, moiré confinement, and atomic reconstruction. Through comparison with photoluminescence spectra, this study identifies a momentum-indirect emission mechanism. Characterization of the absorption is key for applications relying on light-matter interactions.

Field-Dependent Band Structure Measurements in Two-Dimensional Heterostructures

In electronic and optoelectronic devices made from van der Waals heterostructures, electric fields can induce substantial band structure changes which are crucial to device operation but cannot usually be directly measured. Here, we use spatially resolved angle-resolved photoemission spectroscopy to monitor changes in band alignment of the component layers, corresponding to band structure changes of the composite heterostructure system, that are produced by electrostatic gating. Our devices comprise graphene on a monolayer semiconductor, WSe₂ or MoSe₂, atop a boron nitride dielectric and a graphite gate. Applying a gate voltage creates an electric field that shifts the semiconductor bands relative to those in the graphene by up to 0.2 eV. The results can be understood in simple terms by assuming that the materials do not hybridize.

Strongly correlated excitonic insulator in atomic double layers

Excitonic insulators (EIs) arise from the formation of bound electron-hole pairs (excitons)^1,2 in semiconductors and provide a solid-state platform for quantum many-boson physics^3-8. Strong exciton-exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations^8-11. Although spectroscopic signatures of EIs have been reported^6,12-14, conclusive evidence for strongly correlated EI states has remained elusive. Here we demonstrate a strongly correlated two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. A quasi-equilibrium spatially indirect exciton fluid is created when the bias voltage applied between the two electrically isolated TMD layers is tuned to a range that populates bound electron-hole pairs, but not free electrons or holes^15-17. Capacitance measurements show that the fluid is exciton-compressible but charge-incompressible-direct thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing exotic quantum phases of excitons⁸, as well as multi-terminal exciton circuitry for applications^18-20.

Dual-Mode Learning of Ambipolar Synaptic Phototransistor Based on 2D Perovskite/Organic Heterojunction for Flexible Color Recognizable Visual System

Artificial intelligence vision systems (AIVSs) with information sensing, processing, and storage functions are increasingly gaining attention in the science and technology community. Although synapse phototransistor (SPT) is one of the essential components in AIVSs, solution-processed large-area photonic synapses that can detect and recognize multi-wavelength light are highly desirable. One of the major challenges in this area is the inability of the available materials to distinguish colors from the visible light to the near-infrared (NIR) light for single carrier (hole-only or electron-only) SPTs owing to lack of cognitive elements. Herein, 2D perovskite/organic heterojunction (PEA₂ SnI₄ /Y6) ambipolar SPTs (POASPTs) are developed via solution process. The POASPTs can display dual-mode learning process, which can convert light signals into postsynaptic currents with excitement/inhibition modes (hole-transporting region) or inhibition/excitement (electron-transporting region). The POASPTs exhibit high responsivity to visible light (10⁴ A W^-1 ) and NIR light (200 A W^-1 ), and effectively perform learning and memory simultaneously. The flexible POASPT arrays can successfully recognize the images of different colors of light. This study reveals that the fabricated POASPTs have great potentials in the development of large-area, high-efficiency, and low-cost AIVSs.

Ambipolar Channel p-TMD/n-Ga2 O3 Junction Field Effect Transistors and High Speed Photo-sensing in TMD Channel

Highly crystalline 2D/3D-mixed p-transition metal dichalcogenide (TMD)/n-Ga₂ O₃ heterojunction devices are fabricated by mechanical exfoliation of each p- and n-type material. N-type β-Ga₂ O₃ and p-type TMD separately play as a channel for junction field effect transistors (JFETs) with each type of carriers as well as materials for a heterojunction PN diode. The work thus mainly focuses on such ambipolar channel transistors with two different types of channel in a single device architecture. For more extended applications, the transparency of high energy band gap β-Ga₂ O₃ (E_g  ≈ 4.8 eV) is taken advantage of, firstly to measure the electrical energy gap of p-TMDs receiving visible or near infrared (NIR) photons through the β-Ga₂ O₃ . Next, the p-TMD/n-Ga₂ O₃ JFETs are put to high speed photo-sensing which is achieved from the p-TMD channel under reverse bias voltages on n-Ga₂ O₃ . The photo-switching cutoff frequency appears to be ≈16 and 29 kHz for visible red and NIR illuminations, respectively, on the basis of -3 dB photoelectric power loss. Such a high switching speed of the JFET is attributed to the fast transport of photo-carriers in TMD channels. The 2D/3D-mixed ambipolar channel JFETs and their photo-sensing applications are regarded novel, promising, and practically easy to achieve.

Bilayer Wigner crystals in a transition metal dichalcogenide heterostructure

One of the first theoretically predicted manifestations of strong interactions in many-electron systems was the Wigner crystal^1-3, in which electrons crystallize into a regular lattice. The crystal can melt via either thermal or quantum fluctuations⁴. Quantum melting of the Wigner crystal is predicted to produce exotic intermediate phases^5,6 and quantum magnetism^7,8 because of the intricate interplay of Coulomb interactions and kinetic energy. However, studying two-dimensional Wigner crystals in the quantum regime has often required a strong magnetic field^9-11 or a moiré superlattice potential^12-15, thus limiting access to the full phase diagram of the interacting electron liquid. Here we report the observation of bilayer Wigner crystals without magnetic fields or moiré potentials in an atomically thin transition metal dichalcogenide heterostructure, which consists of two MoSe₂ monolayers separated by hexagonal boron nitride. We observe optical signatures of robust correlated insulating states at symmetric (1:1) and asymmetric (3:1, 4:1 and 7:1) electron doping of the two MoSe₂ layers at cryogenic temperatures. We attribute these features to bilayer Wigner crystals composed of two interlocked commensurate triangular electron lattices, stabilized by inter-layer interaction¹⁶. The Wigner crystal phases are remarkably stable, and undergo quantum and thermal melting transitions at electron densities of up to 6 × 10¹² per square centimetre and at temperatures of up to about 40 kelvin. Our results demonstrate that an atomically thin heterostructure is a highly tunable platform for realizing many-body electronic states and probing their liquid-solid and magnetic quantum phase transitions^4-8,17.

Impact of Al2O3 Passivation on the Photovoltaic Performance of Vertical WSe2 Schottky Junction Solar Cells

Transition metal dichalcogenide (TMD) materials have emerged as promising candidates for thin-film solar cells due to their wide bandgap range across the visible wavelengths, high absorption coefficient, and ease of integration with both arbitrary substrates and conventional semiconductor technologies. However, reported TMD-based solar cells suffer from relatively low external quantum efficiencies (EQE) and low open circuit voltage due to unoptimized design and device fabrication. This paper studies Pt/WSe₂ vertical Schottky junction solar cells with various WSe₂ thicknesses in order to find the optimum absorber thickness. Also, we show that the devices' photovoltaic performance can be improved via Al₂O₃ passivation, which increases the EQE up to 29.5% at 410 nm wavelength incident light. The overall resulting short circuit current improves through antireflection coating, surface doping, and surface trap passivation effects. Thanks to the Al₂O₃ coating, this work demonstrates a device with an open circuit voltage (V_OC) of 380 mV and a short circuit current density (J_SC) of 10.7 mA/cm². Finally, the impact of Schottky barrier height inhomogeneity at the Pt/WSe₂ contact is investigated as a source of open circuit voltage lowering in these devices.

Quantitative Nanoinfrared Spectroscopy of Anisotropic van der Waals Materials

Anisotropic dielectric tensors of uniaxial van der Waals (vdW) materials are difficult to investigate at infrared frequencies. The small dimensions of high-quality exfoliated crystals prevent the use of diffraction-limited spectroscopies. Near-field microscopes coupled to broadband lasers can function as Fourier transform infrared spectrometers with nanometric spatial resolution (nano-FTIR). Although dielectric functions of isotropic materials can be readily extracted from nano-FTIR spectra, the in- and out-of-plane permittivities of anisotropic vdW crystals cannot be easily distinguished. For thin vdW crystals residing on a substrate, nano-FTIR spectroscopy probes a combination of sample and substrate responses. We exploit the information in the screening of substrate resonances by vdW crystals to demonstrate that both the in- and out-of-plane dielectric permittivities are identifiable for realistic spectra. This novel method for the quantitative nanoresolved characterization of optical anisotropy was used to determine the dielectric tensor of a bulk 2H-WSe₂ microcrystal in the mid-infrared.

A Logic-Memory Transistor with the Integration of Visible Information Sensing-Memory-Processing

To meet the demands of future intelligent application scenarios, the time-efficient information acquisition and energy-efficient data processing capabilities of terminal electronic systems are indispensable. However, in current commercial visual systems, the visible information is collected by image sensors, converted into digital format data, and transferred to memory units and processors for subsequent processing tasks. As a result, most of the time and energy are wasted in the data conversion and movement, which leads to large time latency and low energy efficiency. Here, based on 2D semiconductor WSe2, a logic-memory transistor that integrates visible information sensing-memory-processing capabilities is successfully demonstrated. Furthermore, based on 3 × 3 fabricated devices, an artificial visible information sensing-memory-processing system is proposed to perform image distinction tasks, in which the time latency and energy consumption caused by data conversion and movement can be avoided. On the other hand, the logic-memory transistor can also execute digital logic processing (logic) and logic results storage (memory) at the same time, such as AND logic function. Such a logic-memory transistor could provide a compact approach to develop next-generation efficient visual systems.

Highly energy-tunable quantum light from moiré-trapped excitons

Photon antibunching, a hallmark of quantum light, has been observed in the correlations of light from isolated atomic and atomic-like solid-state systems. Two-dimensional semiconductor heterostructures offer a unique method to create a quantum light source: Moiré trapping potentials for excitons are predicted to create arrays of quantum emitters. While signatures of moiré-trapped excitons have been observed, their quantum nature has yet to be confirmed. Here, we report photon antibunching from single moiré-trapped interlayer excitons in a heterobilayer. Via magneto-optical spectroscopy, we demonstrate that the discrete anharmonic spectra arise from bound band-edge electron-hole pairs trapped in moiré potentials. Last, we exploit the large permanent dipole of interlayer excitons to achieve large direct current (DC) Stark tuning up to 40 meV. Our results confirm the quantum nature of moiré-confined excitons and open opportunities to investigate their inhomogeneity and interactions between the emitters or energetically tune single emitters into resonance with cavity modes.

Dynamic band alignment modulation of ultrathin WOx/ZnO stack for high on/off ratio field-effect switching applications

A two-dimensional (2D) WO_x/ZnO stack reveals a unique carrier transport behavior, which can be utilized as a novel device element to achieve a very high on/off ratio (>10⁶) and an off current density lower than 1 nA cm^-2. These unique behaviors are explained by a dynamic band alignment between WO_x and ZnO, which can be actively modulated by a gate bias. The performance of FET utilizing the WO_x/ZnO stack is comparable to those of other 2D heterojunction devices; however, it has a unique benefit in terms of process integration because of very low temperature process capability (T < 110 °C). The high on/off switching with extremely low off current density utilizing the dynamic band alignment modulation at the WO_x/ZnO stack can be a very useful element for future device applications, especially in monolithic 3D integration or flexible electronics.

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.

Far-infrared and terahertz emitting diodes based on graphene/black-P and graphene/MoS2 heterostructures

We propose the far-infrared and terahertz emitting diodes (FIR-EDs and THz-EDs) based on the graphene-layer/black phosphorus (GL/b-P) and graphene-layer/MoS₂ (GL/MoS₂) heterostructures with the lateral hole and vertical electron injection and develop their device models. In these EDs, the GL serves as an active region emitting the FIR and THz photons. Depending on the material of the electron injector, the carriers in the GL can be either cooled or heated dictated by the interplay of the vertical electron injection and optical phonon recombination. The proposed EDs based on GL/b-P heterostructures can be efficient sources of the FIP and THz radiation operating at room temperature.

Performance Degradation in Graphene-ZnO Barristors Due to Graphene Edge Contact

The physical and chemical characteristics of the edge states of graphene have been studied extensively as they affect the electrical properties of graphene significantly. Likewise, the edge states of graphene in contact with semiconductors or transition-metal dichalcogenides (TMDs) are expected to have a strong influence on the electrical properties of the resulting Schottky junction devices. We found that the edge states of graphene form chemical bonds with the ZnO layer, which limits the modulation of the Fermi level at the graphene-semiconductor junction, in a manner similar to Fermi level pinning in silicon devices. Therefore, we propose that graphene-based Schottky contact should be accomplished with minimal edge contact to reduce the limits imposed on the Fermi level modulation; this hypothesis has been experimentally verified, and its microscopic mechanism is further theoretically examined.

Interface-Induced WSe2 In-plane Homojunction for High-Performance Photodetection

2D transition metal dichalcogenides (TMDCs) have been extensively attractive for nano-electronics and nano-optoelectronics due to their unique properties. Especially, WSe₂, having bipolar carrier transport ability and sizable bandgap, is a promising candidate for future photodetectors. Here, we report an in-plane WSe₂ homojunction formed by the interface gate of the substrate. In this architecture, an insulated h-BN flake was used to make only part of WSe₂ flake contact substrate directly. Finally, the structures of WSe₂/substrate and WSe₂/h-BN/substrate construct an in-plane homojunction. Interestingly, the device can operate in both photovoltaic and photoconductive modes at different biases. As a result, a responsivity of 1.07 A W^-1 with a superior detectivity of over 10¹² jones and a fast response time of 106 μs are obtained simultaneously. Compared with previously reported methods adopted by chemical doping or electrostatic gating with extra bias voltages, our design provides a more facile and efficient way for the development of high-performance WSe₂-based photodetectors.

Charge transfers and charged defects in WSe2/graphene-SiC interfaces

We report on Kelvin probe force microscopy (KPFM) and density functional theory (DFT) investigations of charge transfers in vertical heterojunctions between tungsten diselenide (WSe₂) layers and graphene on silicon carbide substrates. The experimental data reveal the existence of an interface dipole, which is shown by DFT to originate from the neutralization of the graphene n-doping by an electron transfer towards the transition metal dichalcogenide (TMD) layer. The relative vacuum level shift probed by KPFM between the TMD and the substrate stays constant when passing from monolayer to bilayer graphene, which confirms that the Schottky-Mott model can be rigorously applied to these interfaces by taking into account the charge transfer from the substrate to the TMD. DFT calculations show that the first TMD layer absorbs almost all the excess charges contained in the graphene, and that the second TMD layer shall not play a significant role in the electrostatics of the system. Negatively charged defect at the TMD edges contribute however to the electrostatic landscape probed by KPFM on both TMD layers.

Ultrafast 27 GHz cutoff frequency in vertical WSe2 Schottky diodes with extremely low contact resistance

Ultra-thin two-dimensional semiconducting crystals in their monolayer and few-layer forms show promising aspects in nanoelectronic applications. However, the ultra-thin nature of two-dimensional crystals inevitably results in high contact resistance from limited channel/contact volume as well as device-to-device variability, which seriously limit reliable applications using two-dimensional semiconductors. Here, we incorporate rather thick two-dimensional layered semiconducting crystals for reliable vertical diodes showing excellent Ohmic and Schottky contacts. Using the vertical transport of WSe₂, we demonstrate devices which are functional at various frequency ranges from megahertz AM demodulation of audio signals, to gigahertz rectification for fifth-generation wireless electronics, to ultraviolet-visible photodetection. The WSe₂ exhibits an excellent Ohmic contact to bottom platinum electrode with record-low contact resistance (~50 Ω) and an exemplary Schottky junction to top transparent conducting oxide electrode. Our semitransparent vertical WSe₂ Schottky diodes could be a key component of future high frequency electronics in the era of fifth-generation wireless communication.

Optoelectronics of Multijunction Heterostructures of Transition Metal Dichalcogenides

Among p-n junction devices with multilayered heterostructures with WSe₂ and MoSe₂, a device with the MoSe₂-WSe₂-MoSe₂ (NPN) structure showed a remarkably high photoresponse, which was 1000 times higher than the MoSe₂-WSe₂ (NP) structure. The ideality factor of the NPN structure was estimated to be ∼1, lower than that of the NP structure. It is claimed that the NPN structure formed a thinner depletion region than that of the NP structure because of the difference of carrier concentrations of MoSe₂ and WSe₂. Hence, the built-in electric field was weaker, and the motion of the photocarriers was facilitated. These behaviors were confirmed experimentally from a photocurrent mapping analysis and Kelvin probe force microscopy. The work function depended on the wavelength of the illuminator, and quasi-Fermi level was estimated. The surface photovoltage on the MoSe₂ region was higher than that on WSe₂ because the lower bandgap of MoSe₂ induces more electron-hole pair generation.

Influence of a substrate on ultrafast interfacial charge transfer and dynamical interlayer excitons in monolayer WSe2/graphene heterostructures

Efficient interfacial light-electric interconversion in van der Waals heterostructures is critical for their optoelectronic applications. Using time-resolved terahertz spectroscopy and transient absorption spectroscopy, the charge transfer and the dynamical interlayer excitons were investigated in the heterostructures comprising monolayer WSe₂ and monolayer graphene with varying stacking order on a sapphire substrate. Herein, a more comprehensive understanding of ultrafast charge transfer and exciton dynamics in two-dimensional heterostructures is shown. Owing to the effective electric field induced by the sapphire substrate, the WSe₂/graphene heterostructure exhibits positive terahertz photoconductivity after photoexcitation, while negative terahertz photoconductivity is observed in the graphene/WSe₂ heterostructure. The transient absorption spectra indicate that the exciton lifetimes also exhibit a considerable difference, where WSe₂/graphene exhibits the longest exciton lifetime, followed by monolayer WSe₂, while graphene/WSe₂ exhibits the shortest lifetime. These observations provide a new idea for using van der Waals heterostructures in electronic and photonic devices.

Approaching the Collection Limit in Hot Electron Transistors with Ambipolar Hot Carrier Transport

Hot electron transistors (HETs) containing two-dimensional (2D) materials promise great potential in high-frequency analog and digital applications. Here, we experimentally demonstrate all-2D van der Waals (vdW) HETs formed by graphene, hBN, and WSe₂, in which the polarity of carriers could be tuned by changing bias conditions. We proposed a theoretical model to distinguish hot hole and hot electron components in the ambipolar vdW HET. Importantly, both hot hole and hot electron modes are achieved with pronounced saturation behavior as well as record-high collection efficiency approaching theoretical limits (99.9%) at room temperature. The vdW HETs show a maximum output current density of 400 A/cm². The observed ambipolar hot carrier transport with high collection efficiency is promising for high-speed nanoelectronics and 2D hot electron spectroscopy.

Infrared Interlayer Exciton Emission in MoS_{2}/WSe_{2} Heterostructures

We report light emission around 1 eV (1240 nm) from heterostructures of MoS_{2} and WSe_{2} transition metal dichalcogenide monolayers. We identify its origin in an interlayer exciton (ILX) by its wide spectral tunability under an out-of-plane electric field. From the static dipole moment of the state, its temperature and twist-angle dependence, and comparison with electronic structure calculations, we assign this ILX to the fundamental interlayer transition between the K valleys in this system. Our findings gain access to the interlayer physics of the intrinsically incommensurate MoS_{2}/WSe_{2} heterostructure, including moiré and valley pseudospin effects, and its integration with silicon photonics and optical fiber communication systems operating at wavelengths longer than 1150 nm.

Electrical control of interlayer exciton dynamics in atomically thin heterostructures

A van der Waals heterostructure built from atomically thin semiconducting transition metal dichalcogenides (TMDs) enables the formation of excitons from electrons and holes in distinct layers, producing interlayer excitons with large binding energy and a long lifetime. By employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate that neutral interlayer excitons can propagate across the entire sample and that their propagation can be controlled by excitation power and gate electrodes. We also use devices with ohmic contacts to facilitate the drift motion of charged interlayer excitons. The electrical generation and control of excitons provide a route for achieving quantum manipulation of bosonic composite particles with complete electrical tunability.

Nonlinear Optical Response in Graphene/WX2 (X = S, Se, and Te) van der Waals Heterostructures

Light-frequency conversion based on two-dimensional (2D) materials is of great importance for modern nano- and integrated photonics. Herein, we report both the intrinsic (from the pure WX₂ (X = S, Se, and Te)) and extrinsic (from the interface of graphene/WX₂) second-order nonlinear coefficient tensor from graphene/WX₂ van der Waals (vdW) heterostructures by first-principles calculations. The prominent peaks in the dispersion relation of the intrinsic second-order nonlinear coefficient in monolayer WX₂ are due to the Van Hove singularity in the high-symmetry point or along the high-symmetry line with high joint density of states. The enhanced nonlinear optical response in the infrared band can be achieved in graphene/WS₂ vdW heterostructures, resulting from the interlayer charge transfer between graphene and WS₂. The value of the intrinsic second-order nonlinear coefficients of graphene/WSe₂ vdW heterostructures is 1.5 times larger than that of pure monolayer WSe₂ at the band gap energy of monolayer WSe₂ because of the enhanced carrier generation after the heterostructure formation. Different from pure monolayer WX₂, azimuthal angle-dependent second harmonic generation from graphene/WX₂ vdW heterostructures exhibits extraordinary rotational symmetry at different photon energies, which can be used to deduce the extrinsic second-order nonlinear coefficient. These results pave the way for the nonlinear optical coefficient design based on 2D heterostructures for nonlinear nanophotonics and integrated devices.

Control of the metal/WS2 contact properties using 2-dimensional buffer layers

Transition metal dichalcogenides (TMDC) have recently attracted much attention as a promising platform for the realization of 2-dimensional (2D) electronic devices. One of the major challenges for their wide-scale application is the control of the potential barrier at the metal/TMDC junction. Using conductive atomic force microscopy (c-AFM) we have investigated modifications of the Schottky barrier height (SBH) across a Pt/WS2 junction by the introduction of thin buffer layers of graphene and MoSe2. While graphene greatly reduces the contact resistance in both bias directions, thin layers of MoSe2 lower the Schottky barrier and leave the rectifying properties of the junction intact. We have studied the dependence of the transport properties on the thickness of the graphene and MoSe2 buffer layers. In both cases, the charge transport characteristics can be tailored by varying the buffer layer thickness. The edge of single layer graphene is observed to form an ohmic contact to the underlying WSe2 substrate. This study demonstrates that the introduction of atomically thin MoSe2 and graphene buffer layers is a feasible and elegant method to control the Schottky barrier when contacting TMDCs. The results are important for the fabrication of devices utilizing 2D materials.

Symmetric Ultrafast Writing and Erasing Speeds in Quasi-Nonvolatile Memory via van der Waals Heterostructures

Due to the large gap in timescale between volatile memory and nonvolatile memory technologies, quasi-nonvolatile memory based on 2D materials has become a viable technology for filling the gap. By exploiting the elaborate energy band structure of 2D materials, a quasi-nonvolatile memory with symmetric ultrafast write-1 and erase-0 speeds and long refresh time is reported. Featuring the 2D semifloating gate architecture, an extrinsic p-n junction is used to charge or discharge the floating gate. Owing to the direct injection or recombination of charges from the floating gate electrode, the erasing speed is greatly enhanced to nanosecond timescale. Combined with the ultrafast write-1 speed, symmetric ultrafast operations on the nanosecond timescale are achieved, which are ≈10⁶ times faster than other memories based on 2D materials. In addition, the refresh time after a write-1 operation is 219 times longer than that of dynamic random access memory. This performance suggests that quasi-nonvolatile memory has great potential to decrease power consumption originating from frequent refresh operations, and usher in the next generation of high-speed and low-power memory technology.

Twist-Angle-Dependent Optoelectronics in a Few-Layer Transition-Metal Dichalcogenide Heterostructure

Lattice matching has been supposed to play an important role in the coupling between two materials in a vertical heterostructure (HS). To investigate this role, we fabricated a heterojunction device with a few layers of p-type WSe₂ and n-type MoSe₂ with different crystal orientation angles. The crystal orientations of WSe₂ and MoSe₂ were estimated using high-resolution X-ray diffraction. Heterojunction devices were fabricated with twist angles of 0, 15, and 30°. The I- V curve of the sample with the twist angle of 0° under the dark condition showed a diodelike behavior. The strong coupling due to lattice matching caused a well-established p-n junction. In cases of 15 and 30° samples, the van der Waals gap was built because of lattice mismatching, which resulted in the formation of a potential barrier. However, when the light-emitting diode light of 365 nm (3.4 eV) was illuminated, it was possible for excited electrons and holes to jump beyond the potential barrier and the current flowed well in both forward and reverse directions. The effects of the twist angle were analyzed by spectral responsivity and external quantum efficiency, where it was found that the untwisted HS exhibited higher sensitivity under IR illumination, whereas the twisting effect was not noticeable under UV illumination. From photoluminescence and Raman spectroscopy studies, it was confirmed that the twisted HS showed a weak coupling because of the lattice mismatch.

Observation of giant spin–orbit interaction in graphene and heavy metal heterostructures

Graphene is a promising material demonstrating some interesting phenomena such as the spin Hall effect, bipolar transistor effect, and non-trivial topological states. However, graphene has an intrinsically small spin–orbit interaction (SOI), making it difficult to apply in spintronic devices. The electronic band structure of graphene makes it possible to develop a systematic method to enhance SOI extrinsically. In this study, we designed a graphene field-effect transistor with a Pb layer intercalated between graphene (Gr) and Au layers and studied the effect on the strength of the SOI. The SOI in our system was significantly increased to 80 meV, which led to a giant non-local signal (∼180 Ω) at room temperature due to the spin Hall effect. Further, we extract key parameters of spin transport from the length and width dependence of non-local measurement. To support these findings, we also measured the temperature and gate-dependent weak localization (WL) effect. We obtained the magnitude of the SOI and spin relaxation time of Gr via quantitative analysis of WL. The SOI magnitudes estimated from the non-local signal and the WL effect are close in value. The enhancement of the SOI of Gr at room temperature is a potential simple manipulation method to explore the use of this material for spin-based applications.

We used Pb as an intercalated layer between the graphene and Au and measured the spin–orbit interaction in local and non-local measurement configurations.

WSe2/Au vertical Schottky junction photodetector with low dark current and fast photoresponse

Atomically thin two-dimensional materials including graphene, transition metal dichalcogenides, black phosphorus and so forth have been considered as promising channel medias for electronic and optoelectronic devices in the past few years. However, the poor photoresponse time and the large dark current are the two major issues which greatly block their applications. Here, we report a vertical Au-WSe₂-ITO (indium tin oxide) Schottky junction photodetector with a broadband photoresponse from 550-950 nm and a stable photovoltaic responsivity of ∼0.1 A W^-1. The fast photoresponse of ∼50 μs and low dark current of ∼1 pA are achieved at the vertical Au-WSe₂-ITO photodetector. These results indicate that metal contact to a 2D material-based vertical Schottky junction can achieve an excellent photoelectric response.

High-Performance Two-Dimensional Schottky Diodes Utilizing Chemical Vapour Deposition-Grown Graphene-MoS2 Heterojunctions

Heterostructures based on two-dimensional (2D) materials have attracted enormous interest as they display unique functionalities and have potential to be applied in next-generation electronics. In this report, we fabricated three types of heterostructures based on chemical vapor deposition-grown graphene and MoS₂. A significant rectification was observed in the Au-MoS₂-Gr heterojunction, with a rectification ratio over 2 × 10⁴. The rectifying behavior is reproducible among nearly all 44 devices and is attributed to an asymmetrical Schottky barrier at Au-MoS₂ and MoS₂-graphene contacts. This rectification can be tuned by external gating and laser illumination, which have different impact on the rectifying ratio. This modulation of the Schottky barrier is evidenced by output characteristics of two symmetrical heterostructures: Au-MoS₂-Au and Gr-MoS₂-Gr field-effect transistors. The effective heights of MoS₂-graphene and MoS₂-Au Schottky barriers and their response to back-gate voltage and laser irradiation were extracted from output characteristics of Au-MoS₂-Au and Gr-MoS₂-Gr field-effect transistors. The tuned Schottky barriers could be explained by the Fermi level change of graphene and MoS₂. These results contributed to our understanding of 2D heterostructures and have potential applications in novel electronics and optoelectronics.

The Auger process in multilayer WSe2 crystals

Multilayer WSe2 with a larger optical density of states and absorbance is regarded as a better candidate than its monolayer counterpart for next generation optoelectronic devices, however insight into carrier dynamics is still lacking. Herein, we experimentally observed an anomalous PL quenching with decreasing temperature for multilayer WSe2. At a low temperature (77 K), the Auger processes govern carrier recombination in multilayer WSe2, which are induced by a phonon bottleneck effect and strong photon absorption, and lead to PL quenching. From transient absorption spectroscopy, two distinct Auger processes are observed: a fast one (1-2 ps) and a slow one (>190 ps), which are caused by two different deep midgap defect-levels in WSe2. Based on the Auger recombination model, these two Auger rates are quantitatively estimated at ∼6.69 (±0.05) × 10-2 and 1.22 (±0.04) × 10-3 cm2 s-1, respectively. Our current observations provide an important supplement for optimizing the optical and electric behaviors in multilayer WSe2 based devices.

Tunable WSe2-CdS mixed-dimensional van der Waals heterojunction with a piezo-phototronic effect for an enhanced flexible photodetector

Due to the absence of bond fracture and atomic reconstruction under strain, vdWs structures hold great promise in flexible electronic/optoelectronic applications. Besides all-2D heterojunctions, the dangling-bond-free surfaces of 2D materials also enable vdWs interaction with other materials of different dimensionalities, forming mixed-dimensional vdWs heterostructures. Such structures allow a much broader selection of materials and may provide a promising approach to compensate for the intrinsic weakness of 2D crystals before realizing their full potential. In this study, we present the fabrication of a WSe2-CdS mixed-dimensional vdWs p-n heterojunction for flexible photodetection. A strain-tunable vdWs interface was demonstrated and the photoresponse was dramatically enhanced with the piezo-phototronic effect. The photocurrent can be increased by ∼110% under a compressive strain of -0.73% and the corresponding photoresponsivity reaches up to 33.4 A W-1. The enhancement originates from realigned local energy-band tilting at the WSe2-CdS interface by strain-induced piezopolarization, which promotes the transport process of photoexcited carriers. Our work provides a new route to a tunable vdWs interface other than with electrostatic gating, which may inspire the development of novel flexible vdWs optoelectronics.

Strongly Enhanced Tunneling at Total Charge Neutrality in Double-Bilayer Graphene-WSe_{2} Heterostructures

We report the experimental observation of strongly enhanced tunneling between graphene bilayers through a WSe_{2} barrier when the graphene bilayers are populated with carriers of opposite polarity and equal density. The enhanced tunneling increases sharply in strength with decreasing temperature, and the tunneling current exhibits a vertical onset as a function of interlayer voltage at a temperature of 1.5 K. The strongly enhanced tunneling at overall neutrality departs markedly from single-particle model calculations that otherwise match the measured tunneling current-voltage characteristics well, and suggests the emergence of a many-body state with condensed interbilayer excitons when electrons and holes of equal densities populate the two layers.

Tunable Γ-K Valley Populations in Hole-Doped Trilayer WSe_{2}

We present a combined experimental and theoretical study of valley populations in the valence bands of trilayer WSe_{2}. Shubnikov-de Haas oscillations show that trilayer holes populate two distinct subbands associated with the K and Γ valleys, with effective masses 0.5m_{e} and 1.2m_{e}, respectively; m_{e} is the bare electron mass. At a fixed total hole density, an applied transverse electric field transfers holes from Γ orbitals to K orbitals. We are able to explain this behavior in terms of the larger layer polarizability of the K orbital subband.

Gate Modulation of the Spin-orbit Interaction in Bilayer Graphene Encapsulated by WS2 films

Graphene has gigantic potential in the development of advanced spintronic devices. The interfacial interactions of graphene with semiconducting transition metal dichalcogenides improve the electronic properties drastically, making it an intriguing candidate for spintronic applications. Here, we fabricated bilayer graphene encapsulated by WS₂ layers to exploit the interface-induced spin-orbit interaction (SOI). We designed a dual gated device, where the SOI is tuned by gate voltages. The strength of induced SOI in the bilayer graphene is dramatically elevated, which leads to a strong weak antilocalization (WAL) effect at low temperature. The quantitative analysis of WAL demonstrates that the spin relaxation time is 10 times smaller than in bilayer graphene on conventional substrates. To support these results, we also examined Shubnikov-de Haas (SdH) oscillations, which give unambiguous evidence of the zero-field spin-splitting in our bilayer graphene. The spin-orbit coupling constants estimated by two different measurements (i.e., the WAL effect and SdH oscillations) show close values as a function of gate voltage, supporting the self-consistency of this study’s experimental results. The gate modulation of the SOI in bilayer graphene encapsulated by WS₂ films establishes a novel way to explore the manipulation of spin-dependent transport through an electric field.

Electrical Tuning of Interlayer Exciton Gases in WSe2 Bilayers

van der Waals heterostructures formed by stacking two-dimensional atomic crystals are a unique platform for exploring new phenomena and functionalities. Interlayer excitons, bound states of spatially separated electron-hole pairs in van der Waals heterostructures, have demonstrated potential for rich valley physics and optoelectronics applications and been proposed to facilitate high-temperature superfluidity. Here, we demonstrate highly tunable interlayer excitons by an out-of-plane electric field in homobilayers of transition metal dichalcogenides. Continuous tuning of the exciton dipole from negative to positive orientation has been achieved, which is not possible in heterobilayers due to the presence of large built-in interfacial electric fields. A large linear field-induced redshift up to ∼100 meV has been observed in the exciton resonance energy. The Stark effect is accompanied by an enhancement of the exciton recombination lifetime by more than two orders of magnitude to >20 ns. The long recombination lifetime has allowed the creation of an interlayer exciton gas with density as large as 1.2 × 10¹¹ cm^-2 by moderate continuous-wave optical pumping. Our results have paved the way for the realization of degenerate exciton gases in atomically thin semiconductors.

Novel magnetically separable Fe3O4–WSe2/NG photocatalysts: synthesis and photocatalytic performance under visible-light irradiation

Visible light responsive Fe₃O₄–WSe₂/NG (nitrogen doped graphene oxide) heterojunction nanocomposites were synthesized by a hydrothermal synthesis route, in which Fe₃O₄ and WSe₂ particles were coated on the surface of NG.

Highly Efficient and Air-Stable Infrared Photodetector Based on 2D Layered Graphene-Black Phosphorus Heterostructure

The presence of a direct band gap and high carrier mobility in few-layer black phosphorus (BP) offers opportunities for using this material for infrared (IR) light detection. However, the poor air stability of BP and its large contact resistance with metals pose significant challenges to the fabrication of highly efficient IR photodetectors with long lifetimes. In this work, we demonstrate a graphene-BP heterostructure photodetector with ultrahigh responsivity and long-term stability at IR wavelengths. In our device architecture, the top layer of graphene functions not only as an encapsulation layer but also as a highly efficient transport layer. Under illumination, photoexcited electron-hole pairs generated in BP are separated and injected into graphene, significantly reducing the Schottky barrier between BP and the metal electrodes and leading to efficient photocurrent extraction. The graphene-BP heterostructure phototransistor exhibits a long-term photoresponse at near-infrared wavelength (1550 nm) with an ultrahigh photoresponsivity (up to 3.3 × 10³ A W^-1), a photoconductive gain (up to 1.13 × 10⁹), and a rise time of about 4 ms. Considering the thickness-dependent band gap in BP, this material represents a powerful photodetection platform that is able to sustain high performance in the IR wavelength regime with potential applications in remote sensing, biological imaging, and environmental monitoring.