Van Der Waals Heterojunctions Research Articles

Near space detection technology combining van der Waals heterojunction photodetectors with long-range infrared target detection algorithms

To test the target detection accuracy of infrared detectors in practical applications, this study proposes a near space detection (NSD) technology that integrates van der Waals heterojunction photodetectors with long-range infrared target detection algorithms (ITDAs). A van der Waals heterojunction infrared photodetector (MoS2/CdTe) is prepared by combining molybdenum disulfide thin film and cadmium telluride substrate, and depositing gold electrodes on both substrates. A fusion algorithm of spatiotemporal target detection network and local contrast method is proposed for long-distance detection of weak infrared targets, and a near-space infrared detection system is designed. The data showed that the MoS2/CdTe heterojunction photodetector had a detection range of 200–1700 nm and a response of 36.7 mA/W, demonstrating excellent optoelectronic performance. The false alarm rates of the long-distance ITDA were 0.006% and 0.009%, respectively, which are significantly better than the target detection algorithm based on local contrast. In the testing of the near-space infrared detection system, the errors in the shortwave and longwave field of view tests were 2.5% and 5.2%, respectively, and the resolution test errors were 3.6% and 3.7%. The infrared detection system meets the performance standards, has high detection ability and stability, and provides a more effective solution for NSD.

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.

High performance Ge/MoS2 heterojunction photodetector with a short active region

We present a Ge/MoS2 van der Waals heterojunction photodetector with a short active region constructed using a transfer process. The device exhibits broadband, self-powered, superior device performance within the visible to infrared wavelength (500–1700 nm) operated in a photovoltaic mode. Intriguingly, a sharp increased gain of 10 556 (93) with a varied breakdown voltage of −8.02 V (−6.25 V) under the 700 nm (1550 nm) laser illumination was observed, which was interpreted as the synergistic effect of both soft and avalanche breakdown behavior. These results imply disposable high-sensitivity broadband light-detection potentials with a simple Ge/MoS2 heterojunction, exempting it from the complex and strict construction requirement of conventional avalanche photodetectors.

Controllable electrical contact characteristics of graphene/Ga2X3 (X = S, Se) ferroelectric heterojunctions

Reducing the interface barrier between metals and semiconductors is crucial for designing high-performance optoelectronic devices based on van der Waals heterojunctions (HJs). This study proposes four models of HJs composed of graphene (GR) and Ga2X3 (X = S, Se) and systematically investigates their interface electronic properties, along with strain engineering and electric field effects. The results indicated that exploiting the interface dipole-induced potential step allows modulation of the Schottky barrier height (SBH) and contact type of the HJs by altering the contact interfaces. In the BGR/Ga2S3 HJs (BGR means GR positioned at the bottom of Ga2X3), only a small positive (negative) electric field is required to realize the transition from n-type Schottky to p-type Schottky (Ohmic) contacts. Also, strain engineering provides additional means for flexible and controllable contact types, facilitating the design of reversible logic circuits. It indicates the physical insights and strategic interventions of GR/Ga2X3 HJs tunable SBH and offers theoretical guidance for the design of two-dimensional ferroelectric nanodevices with high-quality electrical contact interfaces.

2D Reconfigurable van der Waals Heterojunction for Logic Gate Circuits and Wide-Spectrum Photodetectors via Sulfur Substitution and Band Matching.

The attractive physical properties of two-dimensional (2D) semiconductors in group IVA-VIA have been fully revealed in recent years. Combining them with 2D ambipolar materials to construct van der Waals heterojunctions (vdWHs) can offer tremendous opportunities for designing multifunctional electronic and optoelectronic devices, such as logic switching circuits, half-wave rectifiers, and broad-spectrum photodetectors. Here, an optimized SnSe0.75S0.25 is grown to design a SnSe0.75S0.25/MoTe2 vdWH for logic operation and wide-spectrum photodetection. Benefiting from the excellent gate modulation under the appropriate sulfur substitution and type-II band alignment, the device exhibits reconfigurable antiambipolar and ambipolar transfer behaviors at positive and negative source-drain voltage (Vds), enabling stable XNOR logic operation. It also features a gate-modulated positive and negative rectifying behavior with rectification ratios of 265:1 and 1:196, confirming its potential as half-wave logic rectifiers. Besides, the device can respond from visible to infrared wavelength up to 1400 nm. Under 635 nm illumination, the maximum responsivity of 1.16 A/W and response time of 657/500 μs are achieved at the Vds of -2 V. Furthermore, due to the strong in-plane anisotropic structure of SnSe0.75S0.25-alloyed nanosheet and narrow bandgap of 2H-MoTe2, it shows a broadband polarization-sensitive function with impressive photocurrent anisotropic ratios of 15.6 (635 nm), 7.0 (808 nm), and 3.7 (1310 nm). The direction along the maximum photocurrent can be reconfigurable depending on the wavelengths. These results indicate that our designed alloyed SnSe0.75S0.25/MoTe2 vdWH has reconfigurable logic operation and broadband photodetection capabilities in 2D multifunctional integrated circuits.

Enhanced water decontamination via photogenerated electron delocalization of π → π* and D-π-A synergistically

Mixed-dimensional van der Waals heterojunctions (MD-vdWhs), known for exceptional electron transfer and charge separation capabilities, remain underexplored in photocatalysis. In this study, we leveraged the synergistic effect of intermolecular π → π* and D-π-A dual channels to fabricate novel MD-vdWhs. Owing to the synergistic effect, it exhibits superior electron transfer and delocalization ability, thereby enhancing its photocatalytic performance. The Optimal photocatalyst can degrade 98.78 % of 20 mg/L tetracycline (TC) within 15 min. Additionally, we introduced a novel proof strategy for investigating the photoelectron transfer path, creatively demonstrating the synergistic dual channels effect, which can be attributed to the carbonyl density and light-excitation degree. Notably, even under low-power light sources, it achieved complete inactivation of Escherichia coli within just 7 mins, far surpassing current cutting-edge research. This theoretical framework holds promise for broader applications within related studies.

Enhanced figure of merit in two-dimensional ZrNiSn nanosheets for thermoelectric applications

A novel two-dimensional (2D) half-Heusler ZrNiSn nanosheet for thermoelectric applications was designed from bulk half-Heusler ZrNiSn through first-principles calculation. Investigation of bulk half-Heusler and 2D nanosheet ZrNiSn was performed with the Quantum Espresso code based on a density functional theory plane wave basis set. Electronic band structure and density of states calculations were used to study the confinement effects. On moving from bulk to 2D a change of structure is observed from face-centered cubic to trigonal due to confinement effects. The semiconducting nature of bulk ZrNiSn is undisturbed while moving to a 2D nanosheet; however, the band gap is widened from 0.46 to 1.3 eV due to the restricted motion of electrons in one direction. Compared with bulk ZrNiSn, 2D nanosheets were found to have a higher Seebeck coefficient a lower thermal conductivity and higher figure of merit, which makes 2D ZrNiSn nanosheets suitable for thermoelectric applications. Atomically thin 2D structures with a flat surface have the potential to form van der Waals heterojunctions, paving the way for device fabrication at the nanoscale level.

Gain-type photodetector with GFET-coupled MoS2/WSe2 heterojunction

Due to its exceptional photovoltaic properties and freedom from crystal lattice matching constraints, transition-metal dichalcogenides (TMDCs) have gained increased attention as a prospective two-dimensional (2D) layered material. Van der Waals heterojunctions (vdWHs) constituted of these materials afford chances for the advancement of high-performance photodetectors and solar cells. However, low light absorption efficiency and electron/hole traps at heterointerfaces make it difficult to improve the response speed and sensitivity of TMDCs vdWH based photodetectors. Here, we designed and realized a GFET-coupled MoS2/WSe2 heterojunction gain photodetector. An interfacial amplification effect is induced when a graphene field-effect transistor (GFET) is coupled to a MoS2/WSe2 heterojunction because of the lengthy carrier lifetime of MoS2 and the ultrahigh mobility of graphene. This effect leads to a high gain for our device, enhancing the photovoltaic response. Compared to the pristine MoS2/WSe2 heterojunction, the device exhibits two to three orders of magnitude improvement in responsivity, up to 50 A/W. Rising and decaying times of 67 μs and 2 μs are also achieved, respectively. By utilizing the strong correlation between the incident light spot position and the photocurrent, the device also enables sensitive detection of the spot position. This research offers an attainable method for developing gain-type, high-speed photodetectors.

Triazine-based covalent organic framework/ g-C3N4 heterojunction toward highly efficient photoactivation of peroxydisulfate for sulfonamides degradation

Rational design of multifunctional catalysts to realize the synergistic combination of photocatalysis and peroxydisulfate (PDS) activation process is a promising approach for the effective removal of recalcitrant pollutants. Herein, metal-free covalent organic framework/g-C3N4 (COF/CN) van der Waals heterojunction was synthesized by the in-situ polymerization on g-C3N4. The van der Waals force and π-π conjugation effect contributes to promoting the interfacial migration and separation of photoinduced carries. The photocatalytic removal efficiency of sulfamethazine (SMT) by COF/CN was increased by 4.9 and 6.6-folds compared to those of COF and g-C3N4, respectively. In the optimal COF/CN-3/PDS/Vis process, SO4•− and 1O2 were identified as the major contributors to SMT degradation. The relationship between the molecular reactivity of sulfonamides (SAs) and reaction kinetics was elucidated by density functional theory (DFT) calculations at the electron level. The degradation rates and pathways depend on the electronic distribution and reactive sites of SAs with five- or six-membered heterocyclic ring substituents. The nonradical attack on the aniline and the breakage of C-N bond were the common traits of SAs degradation, whereas a unique Smiles-type pathway occurred in SAs with six-membered heterocyclic substituents. This work expected to promote the insight for the interaction between molecular structures of pollutants and reactive species in wastewater purification.

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.

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.

High-Spike Barrier Photodiodes Based on 2D Te/WS2 Heterostructures.

Two-dimensional (2D) van der Waals (vdWs) heterojunctions have been actively investigated in low-power-consumption and fast-response photodiodes owing to their atomically smooth interfaces and ultrafast interfacial charge transfer. However, achieving ultralow dark current and ultrafast photoresponse in the reported photovoltaic devices remains a challenge as the large built-in electric field in a heterojunction can not only speed up photocarrier transport but also increase the minority-carrier dark current. Here, we propose a high-spike barrier photodiode that can achieve both an ultralow dark current and an ultrafast response. The device is fabricated by the Te/WS2 heterojunction, while the band alignment can transition from type-II to type-I with a high electron barrier and a large hole built-in electronic field. The high electron barrier can greatly reduce the drift current of minority carriers and the generation current of the thermal carriers, while the large built-in electronic field can still speed up the photocarrier transport. The designed Te/WS2 vdWs photodiode yields an ultralow dark current of 8 × 10-14 A and an ultrafast photoresponse of 10/13 μs. Furthermore, a high-performance visible-light imager with a pixel resolution of 100 × 40 is demonstrated using the Te/WS2 vdWs photodiode. This work provides a comprehensive understanding of designing 2D-material-based photovoltaics with excellent overall performance.

Programmable Interfacial Band Configuration in WS2/Bi2O2Se Heterojunctions.

van der Waals heterojunctions based on transition-metal dichalcogenides (TMDs) offer advanced strategies for manipulating light-emitting and light-harvesting behaviors. A crucial factor determining the light-material interaction is in the band alignment at the heterojunction interface, particularly the distinctions between type-I and type-II alignments. However, altering the band alignment from one type to another without changing the constituent materials is exceptionally difficult. Here, utilizing Bi2O2Se with a thickness-dependent band gap as a bottom layer, we present an innovative strategy for engineering interfacial band configurations in WS2/Bi2O2Se heterojunctions. In particular, we achieve tuning of the band alignment from type-I (Bi2O2Se straddling WS2) to type-II and finally to type-I (WS2 straddling Bi2O2Se) by increasing the thickness of the Bi2O2Se bottom layer from monolayer to multilayer. We verified this band architecture conversion using steady-state and transient spectroscopy as well as density functional theory calculations. Using this material combination, we further design a sophisticated band architecture incorporating both type-I (WS2 straddles Bi2O2Se, fluorescence-quenched) and type-I (Bi2SeO5 straddles WS2, fluorescence-recovered) alignments in one sample through focused laser beam (FLB). By programming the FLB trajectory, we achieve a predesigned localized fluorescence micropattern on WS2 without changing its intrinsic atomic structure. This effective band architecture design strategy represents a significant leap forward in harnessing the potential of TMD heterojunctions for multifunctional photonic applications.

Construction of covalent organic framework and g-C3N4 heterojunction for photocatalytic degradation of tetracycline and photocatalytic production of hydrogen peroxide

Organic semiconductors, notably graphitic carbon nitride (g-C3N4, CN), are increasingly recognized as effective photocatalysts for environmental remediation and energy harvesting. In this study, we introduce a Van der Waals metal-free heterojunction (TTO/CN), composed of g-C3N4 and a vinyl-linked covalent organic framework (COF), fabricated via a simple self-assembly process. This novel heterojunction is adeptly utilized for the photocatalytic degradation of tetracycline (TC) in aqueous environments and for the generation of hydrogen peroxide (H2O2). The synthesized TTO/CN heterojunction demonstrates an expanded specific surface area and broadened absorption spectrum in the visible light range, coupled with enhanced efficiency in photogenerated carrier separation and transfer. These attributes collectively contribute to its remarkable photocatalytic performance. Notably, the TTO/CN-1 variant emerges as a highly efficient photocatalyst, exhibiting degradation rate constants for TC that surpass those of TTO-COF and CN by 2.0 and 6.2 times, respectively. Furthermore, TTO/CN-1 distinguishes itself as a dual-functional photocatalyst, showcasing exceptional activity in H2O2 production (745.3 μM·h−1, or 1863.3 μmol·g−1·h−1). This research heralds a potent strategy for the design and synthesis of van der Waals heterojunction photocatalysts, and the distinctive heterostructures of COF and CN hold significant promise for enhancing the performance and broadening the applicability of CN-based photocatalysts.

Polarized Photodetectors Based on 2D 2H‐MoTe2/1T'‐MoTe2/MoSe2 Van Der Waals Heterojunction

AbstractThe van der Waals heterojunction based on transition metal dichalcogenides has broad research value and application prospects as the basic material for advanced near‐infrared polarized photodetectors. a type II heterojunction based on the 2H‐MoTe2/1T′‐MoTe2/MoSe2 structure is reported for photovoltaic photodetectors. Notably, this device generates a self‐powered photocurrent, eliminating the need for an external bias or gate voltage. This device has the ability to detect polarized light, and its photocurrent anisotropy ratio is 55. The device performance is significantly amplified due to the proficient facilitation of electron‐hole separation through the type II band structure of MoSe2 at the bottom and 2H‐MoTe2 at the top, as well as enhanced exciton splitting by 1T′‐MoTe2 situated in the middle. Consequently, the device demonstrates exceptional proficiency, presenting a noteworthy response rate of 0.76 A W−1, a high detection rate of 3 × 109 Jones, an elevated EQE of 71%, and rapid rising and falling response speeds of 13 ms/10 ms respectively. Moreover, the device's anisotropy ratio of photocurrent highlights its sensitivity to polarized light, making it a promising candidate for applications in polarized photodetection. Overall, this innovative heterostructure opens new avenues for exploring and developing advanced optoelectronic devices based on 2D van der Waals materials.

Multifunction realization in MoS2/WS2/h-BN heterojunction: Integrated self-powered high-performance photodetection, visualization, nonvolatile memory, and synaptic simulation

Nowadays, efficient performance, functional diversification, device miniaturization and systematic integration are the trends for developing new electronic information technology. However, photodetectors with only optoelectronic detection functions cannot satisfy the growing demands of the multifunction required in single devices. This paper presents a MoS2/WS2/h-BN van der Waals heterojunction device realizing multifunctional applications which integrated self-powered high-performance photodetection, visualization, nonvolatile memory, and synaptic simulation. The heterojunction achieves a high responsivity of 9.28 A/W, an excellent specific detectivity of 6.1×1012 Jones, a large external quantum efficiency of 2358 %, a considerable on/off ratio of 7×105 and an ultrafast response time of 0.6 μs at 1 V bias under 488 nm laser irradiation. Besides, the photodetector shows excellent photovoltaic characteristics with a short-circuit current of 0.22 μA and an open-circuit voltage of 0.34 V. Moreover, the device also shows a favorable optical response to 532 and 633 nm lasers. The device also realizes visualization and can be used as an imaging sensor. In addition, the device integrates nonvolatile memory function due to the charge storage effect of h-BN and h-BN/SiO2 interface. Furthermore, biological synaptic functions, such as the memory process, short- and long-term plasticity, and double pulse facilitation, are also successfully simulated. This work realizes the high-performance and multifunctional applications of MoS2/WS2/h-BN device based on a new strategy of utilizing h-BN as a heterojunction substrate.

Van der Waals heterojunctions comprising double/single-walled carbon nanotubes: Insights into heat flux and thermal rectification

Van der Waals (vdWs) heterojunctions constructed from double/single-walled carbon nanotubes (DSCNTs) demonstrate exceptional thermal rectification (TR) ratios, coupled with a straightforward fabrication process, making them highly suitable for thermal rectifier applications. In this study, we employed a Nonequilibrium Molecular Dynamics method to investigate the heat flux and TR across DSCNTs with varying interlayer spacings. Our findings reveal that at smaller interlayer spacings, high temperatures readily overcome vdWs interactions. Conversely, increasing the interlayer spacing initially impedes this effect, though elevated temperatures can surmount this barrier. Significantly, at specific combinations of temperature and interlayer spacing, the left side in DSCNTs establishes interlayer phonons transport channels, facilitating forward heat flux but impeding the backward. This mechanism results in an exceptionally high TR ratio, reaching up to 757 %. Additionally, altering the chiral configurations of the CNTs in the inner and outer layers, while preserving interlayer spacing, further enhances the TR ratio and reduces the temperature threshold required to achieve peak TR ratio. Our findings contribute valuable insights into the phonon transport mechanisms within one-dimensional vdWs heterojunctions.

Low-cost and efficient all group-IV visible/shortwave infrared dual-band photodetector.

Low-cost broadband photodetectors (PDs) based on group-IV materials are highly demanded. Herein, a vertical all group-IV graphene-i-n (Gr-i-n) structure based on sputtering-grown undoped Ge0.92Sn0.08/Ge multiple quantum wells (MQWs) on n-Ge substrate was proposed to realize efficient visible/shortwave infrared (VIS/SWIR) dual-band photoresponse. Harnessing Gr-germanium tin (GeSn)/Ge MQWs van der Waals heterojunctions, an extended surface depletion region was established, facilitating separation and transportation of photogenerated carriers at VIS wavelengths. Consequently, remarkable VIS/SWIR dual-band response ranging from 400 to 2000 nm with a rapid response time of 23 μs was achieved. Compared to the PD without Gr, the external quantum efficiency at 420, 660, and 1520 nm was effectively enhanced by 10.2-, 5.2-, and 1.2-fold, reaching 40, 42, and 50%, respectively. This research paves the way for the advancement of all group-IV VIS/SWIR broadband PDs and presents what we believe to be a novel approach to the design of low-cost broadband PDs.

GeC/SiCx van der Waals heterojunction: Applications for water splitting and solar cell

We construct GeC/SiCx(x = 1,2,7) van der Waals heterojunctions to study the photoelectric properties. The first-principle calculations show that three heterojunctions have the Z-type band alignment by analyzing the charge transfer paths. The designed heterojunctions possess good optical absorption in the visible light region, suggesting potential applications in photocatalysis, solar cells, and other photovoltaic devices. It is found that GeC/SiC and GeC/SiC7 heterojunctions fulfill the requirements as photocatalysts from PH = 0 to PH = 14. Although the GeC/SiC2 heterojunction fulfills the requirements as a photocatalyst from PH = 6 to PH = 10, the GeC/SiC2-based solar cell presents excellent performance, whose power conversion efficiency reaches 24.6 % theoretically. The results demonstrate that GeC/SiCx (x = 1, 2, 7) is a potential candidate for photocatalytic and solar cell materials in the future.