Van Der Waals Heterojunctions Research Articles

In situ edge extraction enabled by reconfigurable van der Waals infrared photodetectors

At present, the widely used artificial intelligence image perception technology is composed of discrete detection and processing components, which is mismatching the demand for increasingly sizeable redundant image information processing. The transmission of non-critical information between components limits the efficiency of image perception systems. Inspired by the human vision system, which can extract the key features and reduce the transmission burden at the low-level detection end, we propose a symmetric structured mixed-dimensional n/p/n [n-molybdenum disulfide (MoS2)/p-germanium (Ge)/n-MoS2] interdigital van der Waals heterojunction infrared photodetector that can be tuned by bias voltage in response polarity and magnitude. The MoS2/Ge/MoS2 infrared photodetector has a bias-symmetric optoelectronic response and covers the detection band from visible to short-wave infrared. Through the joint detection of a pair of MoS2/Ge/MoS2 infrared photodetectors, we demonstrate the image in situ edge extraction at the detection end, which provides key features for high-level processing. This work has discovered the potential of retina-inspired infrared photodetectors on a 2D/3D integration platform, providing distinct opportunities for a neuromorphic visual perception hardware.

Strain‐Modulated Ferromagnetism at an Intrinsic van der Waals Heterojunction

AbstractThe van der Waals interaction enables atomically thin layers of exfoliated 2D materials to be interfaced in heterostructures with relaxed epitaxy conditions, however, the ability to exfoliate and freely stack layers without any strain or structural modification is by no means ubiquitous. In this work, the piezoelectricity of the exfoliated van der Waals piezoelectric α‐In2Se3 is utilized to modify the magnetic properties of exfoliated Fe3GeTe2, a van der Waals ferromagnet, resulting in increased domain wall density, reductions in the transition temperature ranging from 5 to 20 K, and an increase in the magnetic coercivity. Structural modifications at the atomic level are corroborated by a comparison to a graphite/α‐In2Se3 heterostructure, for which a decrease in the Tuinstra‐Koenig ratio is found. Magnetostrictive ferromagnetic domains are also observed, which may contribute to the enhanced magnetic coercivity. Density functional theory calculations and atomistic spin dynamic simulations show that the Fe3GeTe2 layer is compressively strained by 0.4%, reducing the exchange stiffness and magnetic anisotropy. The incorporation of α‐In2Se3 may be a general strategy to electrostatically strain interfaces within the paradigm of hexagonal boron nitride‐encapsulated heterostructures, for which the atomic flatness is both an intrinsic property and paramount requirement for 2D van der Waals heterojunctions.

Demystifying the modulation effects of the graphene transport layers on the interlayer charge transfer mechanism and broadband optical properties of MoS2/graphene/WSe2 Van der Waals heterojunctions

2D materials and heterojunctions have extraordinary potential in the field of next-generation integrated photodetectors. Recently, inserting graphene (Gra) into Van der Waals heterojunctions as transport layers has been proven to be an effective method for improving the responsivity and response speed of photodetectors. However, how did the physical mechanisms caused by the insertion (interlayer coupling and electron transfer) regulate its optical properties are unclear. Meanwhile, whether the insertion will modulate the broadband optical properties of the heterojunction should also be carefully verified. In this work, the broadband (1.25–6.50 eV) electronic band structures and exciton properties of MoS2/WSe2 and MoS2/Gra/WSe2 were studied using spectroscopic ellipsometry, meanwhile Raman and PL spectroscopy were used to assist in analysis. We found that graphene insertion not only promote the spontaneous charge transfer from WSe2 to MoS2, but also effectively inject electrons into the MoS2 layer, which benefits the interlayer charge separation and net charge accumulation in MoS2. Moreover, the almost stable CP energies near the band edge represent that graphene insertion does not change the electronic band structures of MoS2/WSe2. Furthermore, due to the increasing effective dielectric screening induced by graphene insertion, the exciton binding energies redshift and the exciton transition energy blueshift consequently.

Electrical characterization of multi-gated WSe2/MoS2 van der Waals heterojunctions

Vertical stacking of different two-dimensional (2D) materials into van der Waals heterostructures exploits the properties of individual materials as well as their interlayer coupling, thereby exhibiting unique electrical and optical properties. Here, we study and investigate a system consisting entirely of different 2D materials for the implementation of electronic devices that are based on quantum mechanical band-to-band tunneling transport such as tunnel diodes and tunnel field-effect transistors. We fabricated and characterized van der Waals heterojunctions based on semiconducting layers of WSe2 and MoS2 by employing different gate configurations to analyze the transport properties of the junction. We found that the device dielectric environment is crucial for achieving tunneling transport across the heterojunction by replacing thick oxide dielectrics with thin layers of hexagonal-boronnitride. With the help of additional top gates implemented in different regions of our heterojunction device, it was seen that the tunneling properties as well as the Schottky barriers at the contact interfaces could be tuned efficiently by using layers of graphene as an intermediate contact material.

Tunable Interfacial Electronic and Photoexcited Carrier Dynamics of an S-Scheme MoSi2N4/SnS2 Heterojunction.

Exploring and designing an efficient S-scheme heterojunction photocatalyst for water splitting are crucial. Herein, we report the interfacial electronics, photoexcited carrier dynamics, and photocatalytic performance for water splitting of the MoSi2N4/SnS2 van der Waals heterojunction under the modulation of an electric field and biaxial strain. Our results show that the MoSi2N4/SnS2 heterojunction has a direct band gap of 0.41 eV and obeys the S-scheme charge transfer mechanism. Further calculations of the photoexcited carrier dynamics demonstrate that the interfacial carrier recombination time is 7.22 ps, which is shorter than the electron (hole) transfer time of 39.5 ps (566 ps). Moreover, under the effect of a positive electric field and tensile strain, the S-scheme MoSi2N4/SnS2 heterojunction exhibits excellent visible-light absorption, satisfactory band-edge potentials, tunable interfacial charge transfer, and spontaneous hydrogen evolution reaction activity. The calculated STH efficiency indicates that a tensile strain of 2% is the most effective means of improving the photocatalytic performance of the S-scheme MoSi2N4/SnS2 heterojunction.

One-step synthesis of 2D/2D gC3N4/MoS2 composites for effective photocatalytic hydrogen evolution

Two-dimensional (2D) heterostructures have proven their efficiency in separating charges. Herein, a gC3N4/MoS2 (CN/MoS2) photocatalyst, in the form of 2D/2D van der Waals heterojunction with a type-II band alignment, was synthesized using a novel one-step method. The resulting in-situ formed heterojunction interface enhances electron transfer from gC3N4 to MoS2. The CN/MoS2 photocatalyst exhibits exceptional activity in generating hydrogen from water under simulated solar light. Its hydrogen production rate was 1285 μmol/h/g, which was approximately 3 times higher compared to gC3N4. This superiority is due to a significant increase in photogenerated carrier density and efficient charge separation facilitated by the type-II heterojunction. This study demonstrates a straightforward one-step approach for synthesizing 2D/2D composite photocatalysts, highlighting the effective synergy between different 2D materials in promoting hydrogen generation.

Co-optimization of g-C3N4 with prolonging exciton lifetime strategy and co-catalyst strategy for enhanced photocatalytic H2 evolution activity

In this work, the photocatalytic activity of g-C3N4 is co-optimized for the first time using a strategy of prolonging exciton lifetime and a co-catalyst strategy. Amorphous CoB nanoparticles are loaded on OCN-K-CN, which is an O, K doped g-C3N4 material with van der Waals heterojunctions inside. The optimized OCN-K-CN-CoB sample, containing 12 wt% CoB and 1.25% K2SO4 in precursors, show photocatalytic H2 evolution rate approximately 27 times higher than that of GCN. The mechanism of co-optimization is detailedly investigated through comparative experiments on light absorption, charge separation performance, and HER activity, with optimization strategy and CoB loading ratio as controlled variables. New roles of the two strategies have been revealed: "flux-limited reduction activity sites" for CoB and ''buffer-enhancer'' for OCN-K-CN. These new roles illustrate a hidden relationship between the optimizing effects of the two strategies and can explain a surprising advantage of this co-optimization: achieving near-optimal hydrogen evolution activity with significantly reduced co-catalyst loading ratio.

Black Arsenic Phosphorus Mid-Wave Infrared Barrier Detector with High Detectivity at Room Temperature.

The barrier structure is designed to enhance the operating temperature of the infrared detector, thereby improving the efficiency of collecting photogenerated carriers and reducing dark current generation, without suppressing the photocurrent. However, the development of barrier detectors using conventional materials is limited due to the strict requirements for lattice and band matching. In this study, a high-performance unipolar barrier detector is designed utilizing a black arsenic phosphorus/molybdenum disulfide/black phosphorus van der Waals heterojunction. The device exhibits a broad response bandwidth ranging from visible light to mid-wave infrared (520nm to 4.6 µm), with a blackbody detectivity of 2.7× 1010 cmHz-1/2 W-1 in the mid-wave infrared range at room temperature. Moreover, the optical absorption anisotropy of black arsenic phosphorus enables polarization resolution detection, achieving a polarization extinction ratio of 35.5 at 4.6 µm. Mid-wave infrared imaging of the device is successfully demonstrated at room temperature, highlighting the significant potential of barrier devices based on van der Waals heterojunctions in mid-wave infrared detection.

Thermal Rectification across an Asymmetric Layer Carbon Nanotube van der Waals Heterostructure.

The exceptional thermal conductivity and strength of carbon nanotubes (CNTs) position them as outstanding materials for thermal conduction. The intriguing properties introduced by van der Waals (vdW) heterojunctions have also captured the interest of researchers. However, further refinement of the research concerning the integration of these two elements is required. In our study, a vdW heterostructure with asymmetric layer nesting of multiwalled CNTs (ALCNTs) is devised, with a specific focus on the model's heat flux and thermal rectification (TR) properties, which are analyzed using nonequilibrium molecular dynamics (NEMD). Notably, the greatest TR ratio is observed in the connection of three-layer and single-layer ALCNTs. Moreover, multilayer variable-length nested models exhibit a sluggish TR ratio. An examination of the interface thermal resistance (ITR) reveals that the maximum ITR in the multilayer nested model resides at the rightmost interface. However, it is essential to highlight that the determinant of the TR ratio and heat flux in the multilayer nested model is not the maximum ITR of the rightmost interface but rather the ITR of the outermost layer on the left. Additionally, the impacts of the defect density, length, temperature difference, and hydrogenation on the model's heat flux and TR are explored, yielding noteworthy conclusions. For instance, defects in the outer CNT have a minimal influence on the heat flux and TR compared with those in the inner CNT. As the length increases, the heat flux initially decreases and then increases. Hydrogenation significantly enhances the model's heat flux but does not favor the TR. Our study contributes to advancing the understanding of CNT vdW heterojunctions and offers valuable insights for their practical applications.

Steep-slope vertical-transport transistors built from sub-5 nm Thin van der Waals heterostructures

Two-dimensional (2D) semiconductor-based vertical-transport field-effect transistors (VTFETs) – in which the current flows perpendicularly to the substrate surface direction – are in the drive to surmount the stringent downscaling constraints faced by the conventional planar FETs. However, low-power device operation with a sub-60 mV/dec subthreshold swing (SS) at room temperature along with an ultra-scaled channel length remains challenging for 2D semiconductor-based VTFETs. Here, we report steep-slope VTFETs that combine a gate-controllable van der Waals heterojunction and a metal-filamentary threshold switch (TS), featuring a vertical transport channel thinner than 5 nm and sub-thermionic turn-on characteristics. The integrated TS-VTFETs were realised with efficient current switching behaviours, exhibiting a current modulation ratio exceeding 1 × 108 and an average sub-60 mV/dec SS over 6 decades of drain current. The proposed TS-VTFETs with excellent area- and energy-efficiency could help to tackle the performance degradation-device downscaling dilemma faced by logic transistor technologies.

Optoelectronic Synapses Based on MXene/Violet Phosphorus van der Waals Heterojunctions for Visual-Olfactory Crossmodal Perception

HighlightsThe photoelectric response of violet phosphorus (VP) is significantly enhanced by the MXene/VP van der Waals heterojunctions and the highest photoresponsivity of VP is demonstrated.The first VP-based optoelectronic synapse with essential synaptic behaviours is demonstrated.Proof-of-concept visual-olfactory crossmodal perception based on MXene/VP optoelectronic synapses is explored to mimic neuromorphic vision with multi-sensory interactions.

High-performance photodetector based on ReS2/WSe2/Te dual van der Waals heterojunctions

High-performance photodetector based on ReS2/WSe2/Te dual van der Waals heterojunctions

Excellent spin diode and spin-dependent Seebeck effects in hetero-junctions based on ferromagnetic Cr3M4 (M = Se, Te)

Ferromagnetic (FM) half-metals have been regarded as the most promising candidates for spin injection into semiconductors due to their completely spin-polarized electronic states around the Fermi level. To explore the potential application of Cr3M4 (M = Se, Te) in spintronic devices, which are experimentally synthesized FM bulks as well as half-metallic monolayers with high Curie temperature, we propose to design a Bi2Se3/Cr3Se4 van der Waals hetero-junction and a 2D Cr3S4/Cr3Te4 hetero-junction motivated by the recent report on controlling synthesis of Cr x M y based heterostructures. First-principles calculations combined with non-equilibrium Green’s function uncover the perfect spin filtering and spin diode effect in Bi2Se3/Cr3Se4 van der Waals hetero-junction. More interestingly, the Cr3S4/Cr3Te4 hetero-junction shows an excellent spin-dependent Seebeck effect, in which spin-dependent currents with opposite spin orientation can be driven by a temperature gradient to flow in the opposite transport direction. These effects can be understood from the calculated spin-dependent band structures and transmission spectrum. Our results put forward a promising designing and application of spintronic devices based on Cr3M4 (M = Se, Te) FM materials.

Highly efficient tunable photodetector with a bipolar response in van der Waals heterojunctions

Highly efficient tunable photodetector with a bipolar response in van der Waals heterojunctions

Enhancing Visible-Light Absorption of 2D Carbon Nitride by Constructing 2D/2D van der Waals Heterojunctions of Carbon Nitride/Nitrogen-Superdoped Graphene.

Carbon nitride sheets (CNs) down to the two-dimensional (2D) limit have been widely used in photoelectric conversion due to their inherent band gap and extremely short charge-carrier diffusion distance. However, the utilization of visible light remains low due to the rapid recombination of photogenerated electron-hole pairs and enlarged band gap. Here, atomically thin 2D/2D van der Waals heterojunctions (vdWHs) of N-superdoped graphene (NG) and CNs (CNs/NG) are fabricated via a facile electrostatic self-assembly method. Our results revealed that the vdWHs can increase the visible-light absorption of CNs by extending the absorption edge from 455 to up to 490 nm. The recombination of photogenerated electron-hole pairs is inhibited because superdoped N in CNs/NG facilitates the transmission of photogenerated carriers in the melon chain. This study opens a new avenue for narrowing the band gap and promoting photoexcited carrier separation in carbon-nitride-based materials.

Electronic properties, interface contact and transport properties of strain-modulated MS2/borophosphene and MSeS/borophosphene (M = Cr, Mo, W) heterostructure: Insights from first-principles

Controlling the interface contact performance to form low-resistance ohmic contact is of great importance in designing low-power two-dimensional (2D) devices. Here, we explore a 2D semi-metal with Dirac cones, borophosphene, for different types of contact with MS2, MSeS/borophosphene (M = Cr, Mo, W) using density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. The results indicate that under no strain, different van der Waals heterojunctions (vdWHs) exhibit an n-type (p-type) Schottky contact type, with the CrS2/borophosphene vdWHs exhibiting a lower barrier height (Φn = 0.08 eV). Next, we find that the in-plane strain is more efficient than the vertical strain. Under in-plane strain modulation, the contact type will change from Schottky to Ohmic contacts as the tensile strain increases. Similarly, the contact type will change from Schottky to semiconductor as the compressive strain increases. Under the influence of vertical strain, the CrS2/borophosphene vdWHs transition into ohmic contact only when the interlayer spacing increases to above 0.4 Å. Finally, we study the ohmic contact formation and transport characteristics based on the device angle. These findings demonstrate that interface contact types can be effectively regulated by strain, which is crucial for the design and manufacturing of novel nano electronic devices composed of borophosphene based vdWHs.

Prediction and simulation of a potential barrier block in Van der Waals heterojunction photodetectors.

The pBp structure can effectively suppress the dark current of a photodetector by blocking the majority of carriers. However, it is a big challenge to carry out large-scale simulation optimization for two-dimensional (2D) pBp heterojunction photodetectors due to a lack of the device models. Here, a numerical simulation model of the 2D pBp heterojunction was established based on the finite element method to solve this problem. Using this model, the spatial distribution of the energy band is clarified for each layer. The concentration of nonuniformly distributed electrons, induced by the incident light and bias voltage, is obtained by solving the diffusion and drift equations. The characteristics of the photocurrent and the dark current could be presented and the quantum efficiency could be calculated by counting the ratio of the number of carriers collected at the terminals and the carriers photogenerated. The material parameters could be modified for the optimization of the simulation and prediction. In using our model, a B P/M o S 2/graphene photodetector was constructed, and the simulation results show that it works effectively under a reverse bias ranging from -0.3 to 0V. The external quantum efficiency is 18%, while the internal efficiency approaches 85%. The doping in the barrier region definitely does not affect the dark current and the photocurrent. These results are similar to experimental results published earlier. In addition, with the BP bandgap width of 0.8eV and incident wavelength of 1.7µm, the dark current density predicted by the model could reach 3.3×10-8 A/c m 2, which is two orders lower than the reported 2D photodetectors at room temperature. This proposed model provides a way to design 2D pBp heterojunction photodetectors.

2D/1D PbI2/Sb2S3 van der Waals heterojunction for highly sensitive and broadband photodetectors

A 2D/1D PbI2/Sb2S3 van der Waals heterojunction was successfully used to construct a photodetector device with high sensitivity and broad spectral response.

Two-dimensional direct Z-scheme AlN/GaS-SiP heterojunctions enhance photocatalytic hydrogen production from water: a DFT study.

Photocatalytic water splitting for hydrogen production offers a feasible solution to the problems of energy shortages and environmental pollution. However, its low photocatalytic efficiency limits the application of this technology in real world scenarios. In this study, a two-dimensional AlN/PSi-GaS-I van der Waals heterojunction is constructed and the properties of water photolysis are studied based on first-principles calculations. The results demonstrate that AlN/PSi-GaS-I exhibits exceptional photocatalytic performance with good stability, a narrow bandgap, appropriate band-edge position, a broader light absorption range and efficient separation of photogenerated electron-hole pairs. Moreover, the Gibbs free energies of different intermediates throughout the entire reaction process are calculated based on type-II and Z-scheme reaction mechanisms. By comparing the free energy barriers of the two pathways, it is observed that the Z-scheme reaction pathway has a lower energy barrier. Consequently, it can be concluded that AlN/PSi-GaS-I belongs to the direct Z-scheme heterojunction. These findings suggest that AlN/PSi-GaS-I exhibits an enhanced redox capacity, efficiently driving the water splitting reaction. More excitingly, the AlN/PSi-GaS-I can undergo spontaneous photocatalytic reactions under acidic conditions when provided with adequate optical driving force. This study not only proves that AlN/PSi-GaS-I is a promising high-efficiency photocatalyst for water splitting, but also describes a method for determining direct Z-scheme heterojunctions, which offers theoretical guidance for the design of efficient and stable photocatalysts.

A bicomponent synergistic MoxW1-xS2/aluminum nitride vdW heterojunction for enhanced photocatalytic hydrogen evolution: a first principles study.

The coupling of two-dimensional van der Waals heterojunctions is an effective way to achieve photocatalytic hydrogen production. This paper designs the MoxW1-xS2/AlN (x = 0, 0.25, 0.5, 0.75, 1) van der Waals heterojunction as a possible photocatalytic material. By using first-principles calculations, the effects of different Mo/W ratios on the band gap and photocatalytic hydrogen production performance of heterojunctions were investigated. The results show that the heterojunction is a direct Z-scheme photocatalyst and can achieve overall water splitting. By calculating the absorption spectrum, it is found that the heterojunction has a wider visible light absorption range when the bimetal is added, and there is still a strong absorption peak at 615 nm. With the increase of the Mo atom ratio, the absorption spectrum is red-shifted. The Gibbs free energy of the two-component Mo0.5W0.5S2/AlN heterojunction is only -0.028 eV. Our work provides a new perspective for the modification of 2D transition metal dichalcogenide photocatalytic heterojunctions.