Progress and Insight of Van der Waals Heterostructures Containing Interlayer Transition for Near Infrared Photodetectors

Van der Waals (vdWs) heterostructures enable bandgap engineering of different 2D materials to realize the interlayer transition via type-II band alignment leading to broaden spectrum that is beyond the cut-off wavelength of individual 2D materials. Interlayer transition has a significant effect on the optoelectronic performance of vdWs heterostructure devices, and strong interlayer transition in 2D vdWs heterojunction is always demandable for sufficient charge transfer and rapid speed response. Herein, a state-of-the-art review is presented on recent progress on interlayer transition in vdWs heterostructures for near-infrared (NIR) photodetectors. First, the general synthesis techniques for vdWs heterostructures, band alignments in the vdWs heterostructures are provided. Then, the mechanism of interlayer transition in vdWs heterostructure and recent progress on interlayer transition in vdWs heterostructures for NIR photodetectors are summarized. Afterward, some worthy applications of NIR photodetectors are reviewed in related areas of this topic. At the last, an outlook, challenges, and future research directions of vdWs heterostructures for photodetectors at broaden response spectrum are presented.     

Anomalous Dimensionality-Driven Phase Transition of MoTe2 in Van der Waals Heterostructure

Phase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface-to-volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality-driven phase transition of molybdenum ditelluride (MoTe₂) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single-crystal 2H-MoTe₂ transformed into polycrystalline T_d-MoTe₂ with tilt-angle grain boundaries of 60°-glide-reflection and 120°-twofold rotation. In contrast to conventional nanomaterials, the hBN-encapsulated MoTe₂ exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe₂ has a higher 2H-to-T_d phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe₂ with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless T_d contacts for 2H-MoTe₂ transistors can be fabricated by using the dimensionality-driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi-phase 2D electronics.     

Fully-Depleted PdTe2/WSe2 van der Waals Field Effect Transistor with High Light On/Off Ratio and Broadband Detection

Due to its unique band structure and topological properties, the 2D topological semimetal exhibits potential applications in photoelectric detection, polarization sensitive imaging, and Schottky barrier diodes. However, its inherent large dark current hinders the further improvement of the performance of the semimetal-based photodetectors. In this study, a van der Waals (vdWs) field effect transistor (FET) composed of semimetal PdTe₂ and transition metal dichalcogenides (TMDs) WSe₂ is fabricated, which exhibits high sensitivity photoelectric detection performance in a wide band from visible light (405 nm) to mid-infrared (5 µm). The dark current and the noise level in the device are greatly suppressed by the effective control of the gate. Benefiting from the extremely low dark current (1.2 pA), the device achieves an optical on/off ratio up to 10⁶, a high detectivity of 9.79 × 10¹³ Jones and a rapid response speed (219 and 45 µs). This research demonstrates the latent capacity of the 2D topological semimetal/TMDs vdWs FET for broadband, high-performance, and miniaturized photodetection.     

2D Layered Material‐Based van der Waals Heterostructures for Optoelectronics

Van der Waals heterostructures (vdWHs) based on 2D layered materials with selectable materials properties pave the way to integration at the atomic scale, which may give rise to fresh heterostructures exhibiting absolutely novel physics and versatility. This feature article reviews the state‐of‐the‐art research activities that focus on the 2D vdWHs and their optoelectronic applications. First, the preparation methods such as mechanical transfer and chemical vapor deposition growth are comprehensively outlined. Then, unique energy band alignments generated in 2D vdWHs are introduced. Furthermore, this feature article focuses on the applications in light‐emitting diodes, photodetectors, and optical modulators based on 2D vdWHs with novel constructions and mechanisms. The recently reported novel constructions of the devices are introduced in three primary aspects: light‐emitting diodes (such as single defect light‐emitting diodes, circularly polarized light emission arising from valley polarization), photodetectors (such as photo‐thermionic, tunneling, electrolyte‐gated, and broadband photodetectors), and optical modulators (such as graphene integrated with silicon technology and graphene/hexagonal boron nitride (hBN) heterostructure), which show promising applications in the next‐generation optoelectronics. Finally, the article provides some conclusions and an outlook on the future development in the field.     

2D van der Waals Heterojunction Nanophotonic Devices: From Fabrication to Performance

2D van der Waals heterojunctions (vdWhs) are a novel type of metamaterial that are flexible, adjustable, and easy to assemble. Using weak van der Waals forces (vdWfs), layered 2D materials can stack freely to form vdWhs with atomic level flat interfaces. By using different 2D materials and specific stacking methods, their unique properties can be organically combined, to exhibit more abundant optical properties. In fact, nanophotonic devices based on 2D vdWhs have developed rapidly and made significant progress. Therefore, the main progress of 2D vdWhs nanophotonic devices in recent years, including the preparation methods of 2D vdWhs and the performance improvements of various nanophotonic devices, is reviewed. Lastly, the prospects of 2D vdWhs nanophotonic devices are discussed.     

Photoinduced Contact Evolution and Junction Rearrangement in Two-Dimensional van der Waals Heterostructure

Devices based on 2DMs van der Waals (vdW) heterostructures always compose of multiple contacts. Due to the instability of nanoscale 2DMs and interfaces, these contacts can be affected by the operation-induced photo or thermal effect. They can trigger the evolution of junctions and rearrange the junctions across a device, which are detrimental for applications. Herein, vdW heterostructure of indium selenide (InSe) and black phosphorus (BP) on Au electrodes are investigated to reveal the contact evolution and its relation to device performance. During operation, light irradiation changes the I–V characteristics from symmetry to strong rectification. Photocurrent mapping and Kelvin-probe force microscopy (KPFM) reveal triple junctions in this heterostructure, i.e., Au-InSe junction, InSe homojunction, and InSe-BP heterojunction. The variation of I–V characteristics of vdW heterostructure is ascribed to the evolution of Au-InSe junction from quasi-ohmic junction with a near-zero work function difference (Δφ) to a strong Schottky junction (Δφ = ≈0.27 eV). The stabilized device demonstrates distinguished time-domain response at individual junctions and overall device, indicating the evolution of contacts and the consequent opposite junction directions degrade the overall device performance. This research emphasizes the importance of dealing with heterogeneous contacts and junction directions in designing vdW heterostructure photodetectors.     

A Quaternary van der Waals Ferromagnetic Semiconductor AgVP2Se6

The recent realization of 2D magnetism in van der Waals (vdWs) magnets holds promise for future information technology. However, the vdWs semiconducting ferromagnets, which remain rare, are especially important in developing 2D magnetic devices with new functionalities due to the possibility of simultaneous control of the carrier charge and spin. Metal thiophosphate (MTP), a multifunctional vdWs material system that combines the sought‐after properties of complex oxides, is a promising vdWs magnet system. Here, single crystals of a novel vdWs ferromagnetic semiconductor MTP AgVP₂Se₆ with a room‐temperature resistivity of 1 Ω m are successfully synthesized. Due to the nature of vdWs bonding along the c‐axis, the magnetic properties of the few‐layer AgVP₂Se₆ with different thicknesses are characterized on the exfoliated samples. The AgVP₂Se₆ flakes exhibit significant thickness‐dependent magnetic properties, and a rectangular hysteresis loop with a large coercive field of 750 Oe at 2 K and an undiminished Curie temperature of 19 K are observed in the 6.7 nm AgVP₂Se₆ flake. The discovered vdWs ferromagnet AgVP₂Se₆ with semiconducting behavior will provide alternative platforms for exploring 2D magnetism and potential applications in spintronic devices.     

Piezo‐Phototronic Effect for Enhanced Flexible MoS2/WSe2 van der Waals Photodiodes

The recent discoveries of transition‐metal dichalcogenides (TMDs) as novel 2D electronic materials hold great promise to a rich variety of artificial van der Waals (vdWs) heterojunctions and superlattices. Moreover, most of the monolayer TMDs become intrinsically piezoelectric due to the lack of structural centrosymmetry, which offers them a new degree of freedom to interact with external mechanical stimuli. Here, fabrication of flexible vdWs p–n diode by vertically stacking monolayer n‐MoS₂ and a few‐layer p‐WSe₂ is achieved. Electrical measurement of the junction reveals excellent current rectification behavior with an ideality factor of 1.68 and photovoltaic response is realized. Performance modulation of the photodiode via piezo‐phototronic effect is also demonstrated. The optimized photoresponsivity increases by 86% when introducing a −0.62% compressive strain along MoS₂ armchair direction, which originates from realigned energy‐band profile at MoS₂/WSe₂ interface under strain‐induced piezoelectric polarization charges. This new coupling mode among piezoelectricity, semiconducting, and optical properties in 2D materials provides a new route to strain‐tunable vdWs heterojunctions and may enable the development of novel ultrathin optoelectronics.     

Manipulation of Magnetic Skyrmion in a 2D van der Waals Heterostructure via Both Electric and Magnetic Fields

As a promising candidate for the much-desired low power consumption spintronic devices, 2D magnetic van der Waals material also provides a versatile platform for the design and control of topological spin textures. In this work on WTe₂/CrCl₃ bilayer van der Waals heterostructures, a complete Néel-type skyrmion–bimeron–ferromagnet phase transition is demonstrated, accompanied by the evolution of the topological number. This cyclic transition, mediated by a perpendicular magnetic field, is largely driven by the competition between the out-of-plane magnetocrystalline anisotropy and magnetic dipole–dipole interaction. In the presence of a driving current, the Néel-type skyrmion gains a higher velocity yet larger skyrmion Hall angle, in comparison to the bimeron. By incorporating a ferroelectric CuInP₂S₆ monolayer as a substrate, writing and erasing of skyrmions may be regulated using a ferroelectric polarization. This work sheds light on a novel approach to the design and control of magnetic skyrmions on 2D van der Waals materials.     

Interlayer Transition in a vdW Heterostructure toward Ultrahigh Detectivity Shortwave Infrared Photodetectors

Van der Waals (vdW) heterostructures of 2D atomically thin layered materials (2DLMs) provide a unique platform for constructing optoelectronic devices by staking 2D atomic sheets with unprecedented functionality and performance. A particular advantage of these vdW heterostructures is the energy band engineering of 2DLMs to achieve interlayer excitons through type‐II band alignment, enabling spectral range exceeding the cutoff wavelengths of the individual atomic sheets in the 2DLM. Herein, the high performance of GaTe/InSe vdW heterostructures device is reported. Unexpectedly, this GaTe/InSe vdWs p–n junction exhibits extraordinary detectivity in a new shortwave infrared (SWIR) spectrum, which is forbidden by the respective bandgap limits for the constituent GaTe (bandgap of ≈1.70 eV in both the bulk and monolayer) and InSe (bandgap of ≈1.20–1.80 eV depending on thickness reduction from bulk to monolayer). Specifically, the uncooled SWIR detectivity is up to ≈10¹⁴ Jones at 1064 nm and ≈10¹² Jones at 1550 nm, respectively. This result indicates that the 2DLM vdW heterostructures with type‐II band alignment produce an interlayer exciton transition, and this advantage can offer a viable strategy for devising high‐performance optoelectronics in SWIR or even longer wavelengths beyond the individual limitations of the bandgaps and heteroepitaxy of the constituent atomic layers.     

Mixed‐Dimensional 1D ZnO–2D WSe2 van der Waals Heterojunction Device for Photosensors

2D layered van der Waals (vdW) atomic crystals are an emerging class of new materials that are receiving increasing attention owing to their unique properties. In particular, the dangling‐bond‐free surface of 2D materials enables integration of differently dimensioned materials into mixed‐dimensional vdW heterostructures. Such mixed‐dimensional heterostructures herald new opportunities for conducting fundamental nanoscience studies and developing nanoscale electronic/optoelectronic applications. This study presents a 1D ZnO nanowire (n‐type)–2D WSe₂ nanosheet (p‐type) vdW heterojunction diode for photodetection and imaging process. After amorphous fluoropolymer passivation, the ZnO–WSe₂ diode shows superior performance with a much‐enhanced rectification (ON/OFF) ratio of over 10⁶ and an ideality factor of 3.4–3.6 due to the carbon–fluorine (C? F) dipole effect. This heterojunction device exhibits spectral photoresponses from ultraviolet (400 nm) to near infrared (950 nm). Furthermore, a prototype visible imager is demonstrated using the ZnO–WSe₂ heterojunction diode as an imaging pixel. To the best of our knowledge, this is the first demonstration of an optoelectronic device based on a 1D–2D hybrid vdW heterojunction. This approach using a 1D ZnO–2D WSe₂ heterojunction paves the way for the further development of electronic/optoelectronic applications using mixed‐dimensional vdW heterostructures.     

Polarity‐Controlled Attachment of Cytochrome C for High‐Performance Cytochrome C/Graphene van der Waals Heterojunction Photodetectors

Biomolecule/graphene van der Waals heterojunction provides a generic platform for designing high‐performance, flexible, and scalable optoelectronics. A key challenge is, in controllable attachment, the biomolecules to form a desired interfacial electronic structure for a high‐efficiency optoelectronic process of photoabsorption, exciton dissociation into photocarriers, carrier transfer, and transport. Here, it is shown that a polarity‐controlled attachment of the Cytochrome c (Cyt c) biomolecules can be achieved on the channel of graphene field effect transistors (GFET). High‐efficiency charge transfer across the formed Cyt c/graphene interface is demonstrated when Cyt c attaches with positively charged side to GFET as predicted by molecular dynamics simulation and confirmed experimentally. This Cyt c/GFET van der Waals heterojunction nanohybrid photodetector exhibits a spectral photoresponsivity resembling the absorption spectrum of the Cyt c, confirming the role of Cty c as the photosensitizer in the device. The high visible photoresponsivity up to 7.57 × 10⁴ A W^?1 can be attributed to the high photoconductive gain in exceeding 10⁵ facilitated by the high carrier mobility in graphene. This result therefore demonstrates a viable approach in synthesis of the biomolecule/graphene van der Waals heterojunction optoelectronics using polarity‐controlled biomolecule attachment, which can be expanded for on‐chip printing of high‐performance molecular optoelectronics.     

InSe Schottky Diodes Based on Van Der Waals Contacts

2D semiconductors are excellent candidates for next‐generation electronics and optoelectronics thanks to their electrical properties and strong light‐matter interaction. To fabricate devices with optimal electrical properties, it is crucial to have both high‐quality semiconducting crystals and ideal contacts at metal‐semiconductor interfaces. Thanks to the mechanical exfoliation of van der Waals crystals, atomically thin high‐quality single‐crystals can easily be obtained in a laboratory. However, conventional metal deposition techniques can introduce chemical disorder and metal‐induced mid‐gap states that induce Fermi level pinning and can degrade the metal‐semiconductor interfaces, resulting in poorly performing devices. In this article, the electrical contact characteristics of Au–InSe and graphite–InSe van der Waals contacts, obtained by stacking mechanically exfoliated InSe flakes onto pre‐patterned Au or graphite electrodes without the need for lithography or metal deposition is explored. The high quality of the metal‐semiconductor interfaces obtained by van der Waals contact allows to fabricate high‐quality Schottky diodes based on the Au–InSe Schottky barrier. The experimental observation indicates that the contact barrier at the graphite–InSe interface is negligible due to the similar electron affinity of InSe and graphite, while the Au–InSe interfaces are dominated by a large Schottky barrier.     

Graphene-Enhanced Metal Transfer Printing for Strong van der Waals Contacts between 3D Metals and 2D Semiconductors

2D semiconductors have shown great potentials for ultra-short channel field-effect transistors (FETs) in next-generation electronics. However, because of intractable surface states and interface barriers, it is challenging to realize high-quality contacts with low contact resistances for both p- and n- 2D FETs. Here, a graphene-enhanced van der Waals (vdWs) integration approach is demonstrated, which is a multi-scale (nanometer to centimeter scale) and reliable (≈100% yield) metal transfer strategy applicable to various metals and 2D semiconductors. Scanning transmission electron microscopy imaging shows that 2D/2D/3D semiconductor/graphene/metal interfaces are atomically flat, ultraclean, and defect-free. First principles calculations indicate that the sandwiched graphene monolayer can eliminate gap states induced by 3D metals in 2D semiconductors. Through this approach, Schottky barrier-free contacts are realized on both p- and n-type 2D FETs, achieving p-type MoTe₂, p-type black phosphorus and n-type MoS₂ FETs with on-state current densities of 404, 1520, and 761 µA µm⁻¹, respectively, which are among the highest values reported in literature.     

Ultrasensitive Photoresponsive Devices Based on Graphene/BiI3 van der Waals Epitaxial Heterostructures

In recent years, bismuth iodide (BiI₃), a layered metal halide semiconducting light absorber with a wide bandgap of ≈1.8 eV and strong optical absorption in the visible region, has received greater attention for photovoltaic applications. In this study, ultrasensitive visible‐light photodetectors with graphene/BiI₃ vertical heterostructures are achieved by van der Waals epitaxies. The BiI₃ films deposited on graphene show flatter morphologies and significantly better crystallinities than that of BiI₃ films on SiO₂ substrates, mainly due to weak van der Waals interactions at the graphene/BiI₃ interface. Hybrid photodetectors with highly crystalline graphene/BiI₃ heterostructures demonstrate an ultrahigh responsivity of 6 × 10⁶ A W^?1, shot‐noise‐limited detectivity of 7 × 10¹⁴ Jones, and a relatively short response time of ≈8 ms. Compared to most previously reported graphene‐based hybrid photodetectors, these devices have comparable photosensitivities but a faster response speed and lower operation voltage, which is quite promising for ultralow intensity visible‐light sensors. Moreover, the electronic structure and interfacial chemistry at the graphene/BiI₃ heterojunctions are investigated using photoemission spectroscopy. The results give clear evidence that no chemical interactions occur between graphene and BiI₃, resulting in the van der Waals epitaxial growth, and the measured band bending consistently illustrates that a photoinduced charge transfer occurs at the graphene/BiI₃ interface.     

Electrical Conduction at the Interface between Insulating van der Waals Materials

Emergent properties of 2D materials attract considerable interest in condensed matter physics and materials science due to their distinguished features that are missing in their bulk counterparts. A mainstream in this research field is to broaden the scope of material to expand the horizons of the research area, while developing functional interfaces between different 2D materials is another indispensable research direction. Here, the emergence of electrical conduction at the interface between insulating 2D materials is demonstrated. A new class of van der Waals heterostructures consisting of two sets of insulating transition‐metal dichalcogenides, group‐VI WSe₂ and group‐IV TMSe₂ (TM = Zr, Hf), is developed via molecular‐beam epitaxy, and it is found that those heterostructures are highly conducting although all the constituent materials are highly insulating. The WSe₂/ZrSe₂ interface exhibits more conducting behavior than the WSe₂/HfSe₂ interface, which can be understood by considering the band alignments between constituent materials. Moreover, by increasing Se flux during heterostructure fabrication, the WSe₂/ZrSe₂ interface becomes more conducting, reaching nearly metallic behavior. Further improvement of the crystalline quality as well as exploring different material combinations are expected to lead to metallic conduction, providing a novel functionality emerging at van der Waals heterostructures.     

2D van der Waals Heterojunction of Organic and Inorganic Monolayers for High Responsivity Phototransistors

Van der Waals (vdW) heterostructures composing of organic molecules with inorganic 2D crystals open the door to fabricate various promising hybrid devices. Here, a fully ordered organic self-assembled monolayer (SAM) to construct hybrid organic–inorganic vdW heterojunction phototransistors for highly sensitive light detection is used. The heterojunctions, formed by layering MoS₂ monolayer crystals onto organic [12-(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)dodecyl)]phosphonic acid SAM, are characterized by Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy. Remarkably, this vdW heterojunction transistor exhibits a superior photoresponsivity of 475 A W⁻¹ and enhanced external quantum efficiency of 1.45 × 10⁵%, as well as an extremely low dark photocurrent in the pA range. This work demonstrates that hybridizing SAM with 2D materials can be a promising strategy for fabricating diversified optoelectronic devices with unique properties.     

A MoS2 and Graphene Alternately Stacking van der Waals Heterostructure for Li+/Mg2+ Co-Intercalation

Owing to the low-cost, dendrite-free formation, and high volumetric capacity, rechargeable Li⁺/Mg²⁺ hybrid-ion batteries (LMIBs) have attracted great attention and are regarded as promising energy storage devices. However, due to the strong Coulombic interaction of Mg²⁺ with host materials, the traditional “Daniell Type” LMIBs with only Li⁺ intercalation usually cannot ensure a satisfactory energy density. Herein, graphene monolayers are arranged intercalating into MoS₂ interlamination to construct van der Waals heterostructures (MoS₂/G VH). This operation transforms the construction of ion channels from pristine interlamination of two MoS₂ monolayers to the interlamination of MoS₂ monolayer with graphene monolayer, thereby greatly reducing ion diffusion energy barriers. Compared with pristine MoS₂, the MoS₂/G VH can obviously reduce the migration energy barriers of Li⁺ (from 0.67 to 0.09 eV) and Mg²⁺ (from 1.01 to 0.21 eV). Moreover, it is also demonstrated that MoS₂/G VHs realize Li⁺/Mg²⁺ co-intercalation even at a rate current of 1000 mA g⁻¹. As expected, the MoS₂/G VH exhibits superior electrochemical performance with a reversible capacity of 145.8 mAh g⁻¹ at 1000 mA g⁻¹ after 2200 cycles, suggesting the feasibility of potential applications for high-performance energy storage devices.     

Junction Field-Effect Transistors Based on PdSe2/MoS2 Heterostructures for Photodetectors Showing High Responsivity and Detectivity

2D materials have shown great promise for next-generation high-performance photodetectors. However, the performance of photodetectors based on 2D materials is generally limited by the tradeoff between photoresponsivity and photodetectivity. Here, a novel junction field-effect transistor (JFET) photodetector consisting of a PdSe₂ gate and MoS₂ channel is constructed to realize high responsivity and high detectivity through effective modulation of top junction gate and back gate. The JFET exhibits high carrier mobility of 213 cm² V⁻¹ s⁻¹. What is more, the high responsivity of 6 × 10² A W⁻¹, as well as the high detectivity of 10¹¹ Jones, are achieved simultaneously through the dual-gate modulation. The high performance is attributed to the modulation of the depletion region by the dual-gate, which can effectively suppress the dark current and enhance the photocurrent, thereby realizing high detectivity and responsivity. The JFET photodetector provides a new approach to realize photodetectors with high responsivity and detectivity.