vdW Heterojunctions Research Articles

Full van der Waals Ambipolar Ferroelectric Configurable Optical Hetero-Synapses for In-Sensor Computing.

The rapid development of visual neuromorphic hardware can be attributed to their ability to capture, store and process optical signals from the environment. The main limitation of existing visual neuromorphic hardware is that the realization of complex functions is premised on the increase of manufacturing cost, hardware volume and energy consumption. In this study, we demonstrated an optical synaptic device based on a three-terminal van der Waals (vdW) heterojunction that can realize the sensing functions of light wavelength and intensity as well as short-term and long-term synaptic plasticity. In the image recognition task, we constructed an optical reservoir neural network (ORNN) and a visible light communication system (VLC) composed of this optical synaptic device. The ORNN has a recognition rate of up to 84.9% for 50 000 color images in 10 categories in the CIFAR-10 color image dataset, and the VLC system can achieve high-speed transmission with an ultra-low power consumption of only 0.4 nW. This work shows that through reasonable design, vdW heterojunction structures have great application potential in low-power multifunctional fusion application tasks such as visual bionics.

Electrically reconfigurable MoO3/InSe van der Waals heterojunctions

Abstract Van der Waals (vdW) heterojunctions formed by stacking different layers of two-dimensional atomic crystals with atomically sharp interface and versatile band alignment have been considered as promising candidates for construction of nanoelectronic and nanophotonic devices. Here, we demonstrate the electrically reconfigurable behavior in MoO3/InSe vdW heterojunction with a type-III broken-gap band alignment. The electrical reconfigurability is enabled by the ambipolar transport property of InSe, which is achieved through the use of Pt as contact electrodes. By electrostatically doping the indium selenide (InSe), the reconfigurable MoO3/InSe heterojunctions can be converted between p-n and n+-n junctions. As a current rectifier, the MoO3/InSe heterojunction shows rectification ratios of ~ 107 and ~104 for forward and backward bias conditions, respectively. As a photodetector, the MoO3/InSe heterojunction shows stable photo-switching behavior with a ~105 on/off ratio and an ultralow dark current (≈10 fA).The response time is measured to be 500 μs and 300 μs for rise and fall processes, respectively. These results highlight the role of MoO3 with high electron affinity in construction of vdW heterostructure with type-III band alignment and provide a new platform for high performance optoelectronic devices.

Large out-of-plane piezoelectric response and ultra-low polarization transition barriers in two-dimensional MoGe2N4/MoSi2N4 heterostructures

With the help of the first principle calculations, two-dimensional (2D) MoGe2N4/MoSi2N4 van der Waals (vdW) heterojunction was confirmed as a type II bandgap arrangement with distinct piezoelectric properties dependent on the stacking orders and interlayer interactions. This structural change is facilitated by slip interactions between the layers and is characterized by a process with a low energy barrier, allowing for efficient and responsive phase transition of structures. Notably, this heterojunction exhibits a particularly large out-of-plane piezoelectric coefficient, d33, within a finite thickness. Concurrently, the piezoelectric coefficient can be enhanced by adjusting the vertical strain to a specific value, and the out-of-plane piezoelectric coefficient d33 can be as high as 73.28 pm/V, which is larger than the values of the available 2D materials or their heterojunctions. In addition, we simulated a piezoelectric actuator on a two-dimensional scale, where the deformation of the upper surface of the piezoelectric brake was linearly modulated by adjusting the driving voltage. Thus, our research paves the way for the conceptualization and creation of cutting-edge nanoelectronic devices, electromechanical systems, and multifunctional atomic-scale appliances.

Electronic structures and optoelectronic properties of self-powered black phosphorus/InSe heterojunction: A time-domain ab initio perspective

The design and research of multifunctional, high-performance optoelectronic devices are greatly facilitated by the vdW heterojunctions of 2D materials. This study thoroughly investigates the electronic structures and optoelectronic properties of a BP/InSe heterojunction using first-principles and molecular dynamics methods. The van der Waals (vdW) heterojunction exhibits a built-in electric field and demonstrates strong self-powered capabilities. The NAMD analysis shows that the BP/InSe heterojunction effectively suppresses the recombination of electron-hole pairs, and has a fast photoresponse at zero bias voltage. In addition, the application of biaxial strain effectively adjusts the electronic properties of BP/InSe heterojunction. With the increase of compressive strain, the self-powered capability and NIR light capture of BP/InSe heterojunction are obviously enhanced. This study offers valuable insights and effective strategies for advancing high-performance, self-powered optoelectronic devices using two-dimensional van der Waals (vdW) heterojunctions.

Electrostatic Gating-Dependent Multiple Band Alignments in Ferroelectric α-In2Se3/α-Te van der Waals Heterostructures.

The two-dimensional ferroelectric van der Waals (vdW) heterojunction has been recognized as one of the most promising combinations for emerging ferroelectric memory materials due to its noncovalent bonding and flexible stacking of various materials. In this work, the first-principles calculations were performed to study the stable geometry and electronic structure of α-In2Se3/α-Te, incorporating the vdW correction via the DFT-D2 method. The reversal of the polarization direction in α-In2Se3 can induce a transition in the heterostructure from metallic to semiconductor, accompanied by a shift from type-III to type-I band alignment. These changes are attributed to variations in interfacial charge transfer. Analysis of the modulation effects of external electric fields reveals that the P↑ α-In2Se3/α-Te configuration maintains metallic, whereas the P↓ α-In2Se3/α-Te configuration exhibits a linear reduction in band gap. Furthermore, both heterostructural configurations will undergo transitions to type-II band alignment transitions at 0.2 V Å-1 and within a range from 0.2 to 0.3 V Å-1 under external electric fields. Our findings offer valuable insights for applications such as ferroelectric memory and static gate devices with multiband alignment.

Susceptible Detection of Organic Molecules Based on C3B/Graphene and C3N/Graphene van der Waals Heterojunction Gas Sensors.

Constructing van der Waals (vdW) heterostructures is a prospective approach that is essential for developing a new generation of functional two-dimensional (2D) materials and designing new conceptual nanodevices. Using density-functional theory combined with a nonequilibrium Green's function approach allows for the theoretical and systematic exploration of the electronic structure, transport properties, and sensitivity of organic small molecules adsorbed on 2D C3B/graphene (Gra) and C3N/Gra vdW heterojunctions. Calculations show the metallic properties of C3B/Gra and C3N/Gra after the formation of heterojunctions. Interestingly, the heterojunctions C3B/Gra (C3N/Gra) for the adsorption of small organic molecules (C2H2, C2H4, CH3OH, CH4, and HCHO) at the C3B (C3N) side are sensitive to the chemisorption of C2H2 and C2H4. Similarly, the Gra/C3B is chemisorbed for both C2H2 and C2H4 when adsorbed on Gra side, while it is only chemisorbed for C2H2 in Gra/C3N. Interestingly, all heterojunctions on different sides are physisorbed for CH3OH, CH4, and HCHO. Furthermore, the calculated I-V curves demonstrate that the devices based on the adsorption of C2H2 and C2H4 at each side of the heterojunction have remarkable anisotropy, in with the current being considerably greater in the zigzag direction than in the armchair direction. More specifically, with C2H2 adsorbed on the Gra side, the sensitivity along the armchair direction is up to 85.0% for Gra/C3B and close to 100% for Gra/C3N. This study reveals that C3B/Gra (C3N/Gra) heterojunctions with high selectivity, high anisotropy, and excellent sensitivity are highly prospective 2D materials for applications, which further contributes new insights into the development of future electronic nanodevices.

Contributions of Edge and Internal Atoms to the Friction of Two-Dimensional Heterojunctions.

The weak interlayer interaction and strong intralayer bonding in Van der Waals (vdW) layered materials give rise to ultralow friction at incommensurate contact interfaces, a phenomenon known as structural superlubricity. This phenomenon is complicated by the interplay between atomic degrees of freedom, twist angle, and normal force. In this Letter, we exploit naturally occurring cracks in vdW crystals and microfabrication techniques to qualitatively separate the contributions of edge, complete moiré tile, and incomplete moiré rim regions. It is observed that friction from the incomplete moiré rim region, scaling linearly with the rim area, plays a dominant role in determining the twist angle dependence of friction. Interestingly, despite lower friction contributions from complete moirés, friction from a complete moiré tile is independent of moiré size; consequently, friction from the moiré tile scales linearly with the total number of moirés. By integrating the contributions from edge, moiré tile, and incomplete moiré rim, a friction law for vdW heterojunctions is proposed. Finally, friction changes from positively to negatively correlating with normal force, contingent on the suppression of moiré ridge and the dissipation from edge atoms.

Vertical van der Waals Integrated p‐W0.09Re0.91S2 /GaN Heterojunction for Ultra‐High Detectivity UV Image Sensing

AbstractThe relentless advancement of van der Waals (vdW) heteroepitaxy technology has charted an expansive horizon for the integration and functionalization of heterogeneous materials. In this research, a 2D/3D vdW heterojunction photodetector based on p‐W0.09Re0.91S2/n‐GaN integrated on a Free‐standing (FS)‐GaN substrate, encompassing horizontal, quasi‐vertical, and vertical structures is have successfully fabricated. By incorporating a suite of performance enhancement strategies, including mixed‐dimensional stacking, p‐type doping, type‐II band alignment, and vertical structure design, the developed vertical structure photodetector has exhibited exceptional performance. Specifically, the detector achieves a high Responsivity of up to 497.60 A W−1, an impressive specific Detectivity of 8.41 × 1013 Jones, and a fast response speed (rise/decay time of 10 ms/20 ms). Additionally, the device is successfully applied in the realm of single‐pixel imaging, substantiating its potential for practical applications. The findings of this work not only signify notable strides in integration and optoelectronic performance within the optoelectronic device domain but also portend novel breakthroughs in imaging technology applications, bestowing renewed vigor and endless potential upon the evolution of this field.

Electronic properties of InSe/CNT heterojunctions with the modulation of electric field and vacancy defects

We investigate van der Waals (vdW) heterojunctions by combining InSe and zigzag carbon nanotubes (CNT(n,0)) by first-principle calculations. When n ranges from 5 to 7, The heterojunctions show n-type Schottky contact. However, for n of 8, 9, and 11, the heterojunctions still retain the characteristic of semiconductors with bandgaps. The metallized InSe/CNT(10,0) heterojunction has the most amount of charge transfer and the highest tunneling probability. Ohmic contact can be formed in InSe/CNT(n,0) (n = 5–7) heterojunctions under the external electric field. The charge transfer is enhanced and Schottky barrier heights are significantly reduced in heterojunctions with Se vacancy defect. In vacancy defect causes the disappearance of Schottky barrier because of metallization of InSe and more charge transfer than Se vacancy defect in InSe/CNT. Our findings provide a direction for the application of InSe/CNT in tunable nanoelectronic devices.

Optical quantification of the weak van der Waals coupling between multilayer antimonene and bilayer MoS2 using ultrafast coherent vibration spectroscopy.

Antimonene, a promising conductor for next-generation 2D-based devices, has its contact resistance significantly influenced by the van der Waals (vdW) interaction within its heterostructure. In this study, we report the quantification of the vdW coupling between multilayer antimonene and bilayer MoS2 by ultrafast coherent vibration spectroscopy. By utilizing a femtosecond laser, we excited coherent acoustic vibrations in the multilayer-antimonene on substrate-supported bilayer MoS2, and the relative displacement at the vdW heterojunction was detected with the aid of bilayer MoS2. The photoexcited strain pulse generated in the multilayer-antimonene was observed as it transported to the bilayer MoS2, explaining the distortion at the beginning of the oscillation. By analyzing the thickness-dependent oscillation frequencies, we determine the effective vdW elastic constant between multilayer-antimonene and MoS2 to be (1.9 ± 0.2) × 1018N/m3. This non-destructive optical technique offers a significant advance in the evaluation of vdW interactions at 2D metal-semiconductor interfaces.

Highly sensitive full solar-blind ultraviolet spectrum detection and imaging based on PdSe2/Ga2O3 vdW heterojunction.

Solar-blind ultraviolet (UV) photodetectors are in great demand for both military and civilian applications. Here, we have successfully demonstrated the synthesis of the Sn-doped Ga2O3 films with controllable bandgaps to construct PdSe2/Ga2O3 van der Waals (vdW) heterojunctions achieving highly sensitive full solar-blind UV spectrum detection. The assembled device demonstrates a full solar-blind UV spectral self-powered response, with a large responsivity of 123.5 mA/W, a high specific detectivity of 1.63 × 1013 Jones, and a rapid response time of 0.15/2.3 ms. Importantly, an outstanding solar-blind UV imaging application based on an integrated PdSe2/Ga2O3 device array has been demonstrated. Our work paves a feasible path toward achieving highly sensitive solar-blind UV detecting and imaging based on wide-bandgap Ga2O3 films.

Stable WSeTe/PtTe2 van der Waals heterojunction: Excellent mechanical, electronic, and optical properties and device applications

Janus WSeTe and PtTe2 monolayers have attracted much attention due to their outstanding electronic properties. To explore their combined advantages and to find new physics, we construct WSeTe/PtTe2 vdW heterojunctions and use the most stable configurations A4 and B1 to explore their various physical properties and device applications. It is found that both A4 and B1 hold exceptional mechanical properties. They are type-II semiconductors, and types of band alignment and band gap size can be tuned flexibly by external electric field and vertical strain. Based on such a property, we design a strain-controlled high on–off-ratio mechanical switching device. They also show an excellent light response and high photoelectric power conversion efficiency (PCE), which can be tuned greatly by the external electric field and vertical strain as well. For example, light adsorption in visible and infrared region can be improved greatly by the compressive strain. Particularly, we find for B1 in the intrinsic state or with a small tensile strain of 0.3 Å, or for A4 with small electric field of −0.3 V/Å, they all hold type-II band alignment and have very high PCE and large visible light absorption, based on these results, we propose high-performance solar cell devices.

Active Control of Temperature‐Sensitive GaN‐Graphene van der Waals Heterojunctions Integrated Metasurfaces: A Platform for Multifunctional Micro–Nanophotonic Devices

AbstractVan der Waals (vdW) heterojunctions composed of GaN/graphene have high transmittance and excellent carrier transport properties. The combination of multidimensional hybrid heterojunctions with metasurfaces can open up many fascinating prospects for novel optical components over a broad range of the electromagnetic spectrum. This work experimentally demonstrates a multifunctional temperature‐sensitive meta‐device based on GaN/graphene vdW heterojunctions integrated with a metasurface. Notably, it is discovered that the conductivity of the vdW heterojunctions increases rapidly when it is excited by a thermal signal, resulting in a significant change in the relative phase retardation as well as amplitude modulation of an incident THz wave. Then a continuous wavelet transform is used instead of the traditional Fourier transform, and the two‐dimensional wavelet coefficient card is built to achieve fast detection over a wider range of temperature. Simultaneously, the variation of temperature dynamically controls the contributions of the multipoles, eventually determining the active switching of exotic anapoles with extreme non‐radiative confinement to highly radiative electric dipoles. This work offers the possibility of designing novel chip‐scale multifunctional thermal tuning devices and promotes the potential application of active micro‐nanophotonic devices in temperature sensors, terahertz modulators, and dynamic near‐field imaging.

Research on the robust electrical contact properties and favorable transport characteristics of two-dimensional WB4/MoSi2N4 van der Waals heterostructures

Ensuring optimal interface contact performance is crucial for achieving low-resistance ohmic contact in the design of two-dimensional (2D) electronic devices. In this study, we systematically investigate the contact properties of van der Waals (vdW) heterojunctions formed by compositing the semi-metal WB4 with Dirac cones and the semiconductor MoSi2N4 through first-principles calculations. A thorough investigation has been carried out on the electronic structure and transport characteristics at the interface. Our research findings indicate that the WB4/MoSi2N4 combination demonstrates robust ohmic contact performance, maintaining ohmic or near-ohmic p-type Schottky contacts over a wide range when exposed to electric field, varying layer spacing, and biaxial strain. This unique characteristic is associated with the Fermi level pinning (FLP), where there are gap states present at the interface. Here, the FLP plays a positive role in maintaining ohmic contact. In addition, different derivatives of MoSi2N4 are combined with WB4 to create heterostructures and study the changes in contact performance. Our research shows that the robust ohmic contact of WB4/MoSi2N4 will have excellent prospects for application in future electronic devices.

First principles study on the type-II g-C6N6/PtSSe heterojunction: A potential photocatalyst for overall water splitting

Constructing suitable van der Waals heterojunction is considered to be an efficient way to develop and design new multifunctional devices. In this work, DFT calculations have been preformed to investigate the stacking structures, electronic, optical as well as catalytic properties of vertical heterojunctions (g-C6N6/PtSSe) constructed from graphene-like C6N6 layer and PtSSe monolayer. It is predicted that Se-side contacted heterojunction (CN/SePtS) is a typical type-II semiconductor with band edges satisfying the conditions of overall water splitting, thus can be a potential photocatalyst. In addition, CN/SePtS also possesses good absorption ranging from ultraviolet to visible light regions. It is also found that strain engineering can further tune the electronic and optical properties of the heterojunction and even improve its visible light absorption. By considering the effect of optical driving force, CN/SePtS heterojunction with 4 % biaxial tensile strain is recommended to be a good candidate for overall water splitting photocatalyst. Our research can provide a guidance for designing new optoelectronic devices and photocatalysts based on g-C6N6/PtSSe vdW heterojunctions.

High Sensitive and Stable UV-Vis Photodetector Based on MoS2/MoO3 vdW Heterojunction.

The development of new high-performance photodetectors (PDs) is currently focused on achieving small size, low power consumption, low cost, and large bandwidth. Two-dimensional (2D) materials and heterostructures offer promising approaches for the future development of optoelectronic devices. However, there has been limited research on 2D wide-bandgap semiconductor heterostructures. In this study, we successfully constructed a MoS2/MoO3 vdW heterojunction PD. This PD exhibited excellent response and significant photovoltaic behavior in the ultraviolet (UV) to visible (Vis) range. Under 365 nm UV light and 1 V bias voltage, the PD demonstrated a high responsivity of 645 mA/W, a high specific detectivity of 8.98 × 1010 Jones, and fast response speeds of 55.9/59.6 ms. At 0 V bias voltage, the responsivity reached as high as 157 mA/W. Furthermore, the PD exhibited remarkable stability in its performance. These outstanding characteristics can be attributed to the strong internal electric field created by the type II heterojunction structure and the chemical stability of the materials. This work opens a route for the application of 2D wide-bandgap semiconductor materials in optoelectronic devices.

Monomolecular Membrane-Assisted Growth of Antimony Halide Perovskite/MoS2 Van der Waals Epitaxial Heterojunctions with Long-Lived Interlayer Exciton.

Epitaxial growth stands as a key method for integrating semiconductors into heterostructures, offering a potent avenue to explore the electronic and optoelectronic characteristics of cutting-edge materials, such as transition metal dichalcogenide (TMD) and perovskites. Nevertheless, the layer-by-layer growth atop TMD materials confronts a substantial energy barrier, impeding the adsorption and nucleation of perovskite atoms on the 2D surface. Here, we epitaxially grown an inorganic lead-free perovskite on TMD and formed van der Waals (vdW) heterojunctions. Our work employs a monomolecular membrane-assisted growth strategy that reduces the contact angle and simultaneously diminishing the energy barrier for Cs3Sb2Br9 surface nucleation. By controlling the nucleation temperature, we achieved a reduction in the thickness of the Cs3Sb2Br9 epitaxial layer from 30 to approximately 4 nm. In the realm of inorganic lead-free perovskite and TMD heterojunctions, we observed long-lived interlayer exciton of 9.9 ns, approximately 36 times longer than the intralayer exciton lifetime, which benefited from the excellent interlayer coupling brought by direct epitaxial growth. Our research introduces a monomolecular membrane-assisted growth strategy that expands the diversity of materials attainable through vdW epitaxial growth, potentially contributing to future applications in optoelectronics involving heterojunctions.

Schottky junction-mediated carrier separation in BiOBr/Ti3C2Tx van der Waals heterojunction photocatalyst for increased removal of Cr(VI) under visible-light

The photocatalytic removal of pollutants can integrate efficient solar energy utilization with wastewater treatment. Herein, we report a simple strategy to construct BiOBr/Ti3C2Tx composite photocatalysts consisting of self-assembled van der Waals (vdw) heterojunction structures containing Schottky junctions. The vdW interaction between BiOBr and Ti3C2Tx and the structural stability of the catalysts were validated by density functional theory (DFT) calculations and an ab initio molecular dynamic simulation (AIMD). Compared to BiOBr, the BiOBr/Ti3C2Tx composites exhibited an enhanced light absorption capability and photocurrent response, along with a lower carrier recombination rate and charge-transfer resistance. The BiOBr/Ti3C2Tx activity for the photoreduction of hexavalent chromium (Cr(VI)) under visible-light irradiation considerably surpassed that of the original BiOBr (53.9 %). Using the BTC-0.8 catalyst (with a Ti3C2Tx: BiOBr mass ratio of 0.8 %) optimal reduction efficiency (94.0 %) for Cr(VI) with a reaction rate approximately 3.5 times that of pure BiOBr. The BTC-0.8 catalyst was viable over a wide pH range and maintained an 83.9 % photoreduction rate after four cycling experiments. The exceptional photocatalytic ability of BiOBr/Ti3C2Tx was achieved because Ti3C2Tx served as an electron injection layer that separated photogenerated carriers. The Schottky junction constructed within the BiOBr/Ti3C2Tx vdW heterojunctions suppressed electron backflow, promoting carrier separation. The excellent photocatalytic performance and strong cyclic stability of BiOBr/Ti3C2Tx demonstrated the considerable application potential of this photocatalyst.

Spontaneous enhanced photocatalytic overall water splitting on AlO/WSi2N4 vdW heterojunction

Completely achieving spontaneous water splitting without sacrificing co-catalysts poses a significant challenge, as factors such as electrical, optical, and catalytic activity can limit the conversion efficiency of solar energy to hydrogen. Herein, we propose a type-II van der Waals heterostructure composed of vertically stacked AlO and WSi2N4, and systematically investigate its mechanism and potential for catalyzing water splitting. Photon-induced photocatalytic water splitting takes advantage of the staggered energy bands of the heterojunction, effectively promoting the separation of photogenerated carriers and thus enhancing catalytic efficiency. In neutral water environment, the band edge position of AlO/WSi2N4 aligns with the redox potential required for water decomposition. Thermodynamic analysis of the oxygen and hydrogen evolution reactions indicates that the external potential provided by photogenerated carriers is expected to trigger spontaneous water decomposition. Moreover, with its high electron mobility (4800.62 cm2/Vs) and a solar-to-hydrogen energy conversion efficiency of up to 19.98 %, the AlO/WSi2N4 heterojunction demonstrates substantial potential as a photoelectric catalyst. This research introduces a promising avenue for efficient photocatalytic production of clean energy.

On-chip integrated GeSe2/Si vdW heterojunction for ultraviolet-enhanced broadband photodetection, imaging, and secure optical communication

On-chip integrated GeSe2/Si vdW heterojunction for ultraviolet-enhanced broadband photodetection, imaging, and secure optical communication