Photovoltaic Field-Effect Photodiodes Based on Double van der Waals Heterojunctions

Dual-Electrically Configurable MoTe2/In2S3 Phototransistor toward Multifunctional Applications

Photodetectors, essential for a wide range of optoelectronic applications in both military and civilian sectors, face challenges in balancing responsivity, detectivity, and response time due to their inherent unidirectional carrier transport mechanism. Multifunctional photodetectors that address these trade-offs are highly sought after for their potential to reduce costs, simplify system design, and surpass Moore's Law limitations. Herein, we present a multimodal phototransistor based on a 2D MoTe2/In2S3 heterostructure. Through dual electrical modulation employing bias voltage and gate voltage, we engineer the energy band to achieve switchable photoresponse mechanisms between photoconductive and photovoltaic modes. In photoconductive mode, the device exhibits a responsivity of 320 A/W and a specific detectivity of 1.2 × 1013 Jones. Meanwhile, in photovoltaic mode, it exhibits a light on/off ratio of 2 × 105 and response speed of 0.68/0.60 ms. These capabilities enable multifunctional applications such as high-resolution imaging across various wavelengths, a conceptual optoelectronic logic gate, and dual-channel optical communication. This work makes an advancement in the development of future multifunctional optoelectronic devices.

AC Characteristics of van der Waals Bipolar Junction Transistors Using an MoS2/WSe2/MoS2 Heterostructure

Two-dimensional layered materials, characterized by their atomically thin thicknesses and surfaces that are free of dangling bonds, hold great promise for fabricating ultrathin, lightweight, and flexible bipolar junction transistors (BJTs). In this paper, a van der Waals (vdW) BJT was fabricated by vertically stacking MoS2, WSe2, and MoS2 flakes in sequence. The AC characteristics of the vdW BJT were studied for the first time, in which a maximum common emitter voltage gain of around 3.5 was observed. By investigating the time domain characteristics of the device under various operating frequencies, the frequency response of the device was summarized, which experimentally proved that the MoS2/WSe2/MoS2 BJT has voltage amplification capability in the 0-200 Hz region. In addition, the phase response of the device was also investigated. A phase inversion was observed in the low-frequency range. As the operating frequency increases, the relative phase between the input and output signals gradually shifts until it is in phase at frequencies exceeding 2.3 kHz. This work demonstrates the signal amplification applications of the vdW BJTs for neuromorphic computing and wearable healthcare devices.

Phase-Controlled Growth of 1T'-MoS2 Nanoribbons on 1H-MoS2 Nanosheets

2D heterostructures are emerging as alternatives to conventional semiconductors, such as silicon, germanium, and gallium nitride, for next-generation electronics and optoelectronics. However, the direct growth of 2D heterostructures, especially for those with metastable phases still remains challenging. To obtain 2D transition metal dichalcogenides (TMDs) with designed phases, it is highly desired to develop phase-controlled synthetic strategies. Here, a facile chemical vapor deposition method is reported to prepare vertical 1H/1T' MoS2 heterophase structures. By simply changing the growth atmosphere, semimetallic 1T'-MoS2 can be in situ grown on the top of semiconducting 1H-MoS2, forming vertical semiconductor/semimetal 1H/1T' heterophase structures with a sharp interface. The integrated device based on the 1H/1T' MoS2 heterophase structure displays a typical rectifying behavior with a current rectifying ratio of ≈103. Moreover, the 1H/1T' MoS2-based photodetector achieves a responsivity of 1.07 A W-1 at 532 nm with an ultralow dark current of less than 10-11 A. The aforementioned results indicate that 1H/1T' MoS2 heterophase structures can be a promising candidate for future rectifiers and photodetectors. Importantly, the approach may pave the way toward tailoring the phases of TMDs, which can help us utilize phase engineering strategies to promote the performance of electronic devices.

High-Performance Self-Powered WSe2/ReS2 Photodetector Enabled via Surface Charge Transfer Doping

Two-dimensional (2D) van der Waals heterostructures based on various 2D transition metal dichalcogenides are widely used in photodetection applications. However, their response time and photoresponsivity are limited, posing a challenge for their applications in high-sensitivity photodetection. Surface charge transfer doping (SCTD) has emerged as a novel doping approach for low-dimensional materials with high specific surface area and attracted considerable attention, as it is simple and effective, does not damage the lattice, and considers various types of dopants. Herein, we prepare p-i-n junction-based photodetectors via the SCTD of WSe2/ReS2 heterojunctions using p-type dopant F4-TCNQ molecules, where doped WSe2 serves as a p-type semiconductor, undoped WSe2 acts as an intrinsic layer, and ReS2 functions as an n-type semiconductor. The surface-charge-transfer-doped WSe2/ReS2 heterojunction leads to a reduction in the Schottky barrier and an increase in the built-in electric field compared with the as-fabricated heterojunction. In the photovoltaic mode and under 785 nm laser illumination, the photodiode exhibits an increase in responsivity from 0.08 to 0.29 A/W, specific detectivity from 1.89 × 1012 to 8.02 × 1012 Jones, and the external quantum efficiency from 12.67 to 46.29%. Additionally, the p-i-n structure expands the depletion region width, resulting in a photovoltaic response time of 7.56/6.48 μs and a -3 dB cutoff frequency of over 85 kHz, an order of magnitude faster than the pristine response time. Herein, we derive an effective and simple scheme for designing high-performance, low-power optoelectronic devices based on 2D van der Waals heterostructures.

Optical Enhancement of Indirect Bandgap 2D Transition Metal Dichalcogenides for Multi-Functional Optoelectronic Sensors

The unique electrical and optical properties of transition metal dichalcogenides (TMDs) make them attractive nanomaterials for optoelectronic applications, especially optical sensors. However, the optical characteristics of these materials are dependent on the number of layers. Monolayer TMDs have a direct bandgap that provides higher photoresponsivity compared to multilayer TMDs with an indirect bandgap. Nevertheless, multilayer TMDs are more appropriate for various photodetection applications due to their high carrier density, broad spectral response from UV to near-infrared, and ease of large-scale synthesis. Therefore, this review focuses on the modification of the optical properties of devices based on indirect bandgap TMDs and their emerging applications. Several successful developments in optical devices are examined, including band structure engineering, device structure optimization, and heterostructures. Furthermore, it introduces cutting-edge techniques and future directions for optoelectronic devices based on multilayer TMDs.

Strong interlayer coupling in p-Te/n-CdSe van der Waals heterojunction for self-powered photodetectors with fast speed and high responsivity

Self-driven photodetectors, which can detect optical signals without external voltage bias, are highly attractive in the field of low-power wearable electronics and internet of things. However, currently reported self-driven photodetectors based on van der Waals heterojunctions (vdWHs) are generally limited by low responsivity due to poor light absorption and insufficient photogain. Here, we report p-Te/n-CdSe vdWHs utilizing non-layered CdSe nanobelts as efficient light absorption layer and high mobility Te as ultrafast hole transporting layer. Benefiting from strong interlayer coupling, the Te/CdSe vdWHs exhibit stable and excellent self-powered characteristics, including ultrahigh responsivity of 0.94 A W^-1, remarkable detectivity of 8.36 × 10¹² Jones at optical power density of 1.18 mW cm^-2 under illumination of 405 nm laser, fast response speed of 24 µs, large light on/off ratio exceeding 10⁵, as well as broadband photoresponse (405-1064 nm), which surpass most of the reported vdWHs photodetectors. In addition, the devices display superior photovoltaic characteristics under 532 nm illumination, such as large V_oc of 0.55 V, and ultrahigh I_sc of 2.73 µA. These results demonstrate the construction of 2D/non-layered semiconductor vdWHs with strong interlayer coupling is a promising strategy for high-performance and low-power consumption devices.

Integration of Self-Passivated Topological Electrodes for Advanced 2D Optoelectronic Devices

With the rapid development of two-dimensional semiconductor technology, the inevitable chemical disorder at a typical metal-semiconductor interface has become an increasingly serious problem that degrades the performance of 2D semiconductor optoelectronic devices. Herein, defect-free van der Waals contacts have been achieved by utilizing topological Bi₂ Se₃ as the electrodes. Such clean and atomically sharp contacts avoid the consumption of photogenerated carriers at the interface, enabling a markedly boosted sensitivity as compared to counterpart devices with directly deposited metal electrodes. Typically, the device with 2D WSe₂ channel realizes a high responsivity of 20.5 A W^-1 , an excellent detectivity of 2.18 × 10¹²  Jones, and a fast rise/decay time of 41.66/38.81 ms. Furthermore, high-resolution visible-light imaging capability of the WSe₂ device is demonstrated, indicating its promising application prospect in future optoelectronic systems. More inspiringly, the topological electrodes are universally applicable to other 2D semiconductor channels, including WS₂ and InSe, suggesting its broad applicability. These results open fascinating opportunities for the development of high-performance electronics and optoelectronics.

High-Performance and Polarization-Sensitive Imaging Photodetector Based on WS2 /Te Tunneling Heterostructure

Next-generation imaging systems require photodetectors with high sensitivity, polarization sensitivity, miniaturization, and integration. By virtue of their intriguing attributes, emerging 2D materials offer innovative avenues to meet these requirements. However, the current performance of 2D photodetectors is still below the requirements for practical application owing to the severe interfacial recombination, the lack of photoconductive gain, and insufficient photocarrier collection. Here, a tunneling dominant imaging photodetector based on WS₂ /Te heterostructure is reported. This device demonstrates competitive performance, including a remarkable responsivity of 402 A W^-1 , an outstanding detectivity of 9.28 × 10¹³ Jones, a fast rise/decay time of 1.7/3.2 ms, and a high photocurrent anisotropic ratio of 2.5. These outstanding performances can be attributed to the type-I band alignment with carrier transmission barriers and photoinduced tunneling mechanism, allowing reduced interfacial trapping effect, effective photoconductive gains, and anisotropic collection of photocarriers. Significantly, the constructed photodetector is successfully integrated into a polarized light imaging system and an ultra-weak light imaging system to illustrate the imaging capability. These results suggest the promising application prospect of the device in future imaging systems.

Integration of photovoltaic and photogating effects in a WSe2/WS2/p-Si dual junction photodetector featuring high-sensitivity and fast-response

Two-dimensional (2D) material-based van der Waals (vdW) heterostructures with exotic semiconducting properties have shown tremendous potential in next-generation photovoltaic photodetectors. Nevertheless, these vdW heterostructure devices inevitably suffer from a compromise between high sensitivity and fast response. Herein, an ingenious photovoltaic photodetector based on a WSe₂/WS₂/p-Si dual-vdW heterojunction is demonstrated. First-principles calculations and energy band profiles consolidate that the photogating effect originating from the bottom vdW heterojunction not only strengthens the photovoltaic effect of the top vdW heterojunction, but also suppresses the recombination of photogenerated carriers. As a consequence, the separation of photogenerated carriers is facilitated and their lifetimes are extended, resulting in higher photoconductive gain. Coupled with these synergistic effects, this WSe₂/WS₂/p-Si device exhibits both high sensitivity (responsivity of 340 mA W^-1, a light on/off ratio greater than 2500, and a detectivity of 3.34 × 10¹¹ Jones) and fast response time (rise/decay time of 657/671 μs) under 405 nm light illumination in self-powered mode. Finally, high-resolution visible-light and near-infrared imaging capabilities are demonstrated by adopting this dual-heterojunction device as a single pixel, indicating its great application prospects in future optoelectronic systems.

Recent progress of two-dimensional heterostructures for thermoelectric applications

The rapid development of synthesis and fabrication techniques has opened up a research upsurge in two-dimensional (2D) material heterostructures, which have received extensive attention due to their superior physical and chemical properties. Currently, thermoelectric energy conversion is an effective means to deal with the energy crisis and increasingly serious environmental pollution. Therefore, an in-depth understanding of thermoelectric transport properties in 2D heterostructures is crucial for the development of micro-nano energy devices. In this review, the recent progress of 2D heterostructures for thermoelectric applications is summarized in detail. Firstly, we systematically introduce diverse theoretical simulations and experimental measurements of the thermoelectric properties of 2D heterostructures. Then, the thermoelectric applications and performance regulation of several common 2D materials, as well as in-plane heterostructures and van der Waals heterostructures, are also discussed. Finally, the challenges of improving the thermoelectric performance of 2D heterostructures materials are summarized, and related prospects are described.

Rational Design of WSe2 /WS2 /WSe2 Dual Junction Phototransistor Incorporating High Responsivity and Detectivity

The excellent semiconducting properties and ultrathin morphological characteristics allow van der Waals (vdW) heterostructures based on 2D materials to be promising channel materials for the next-generation optoelectronic devices, especially in photodetectors. Although various 2D heterostructure-based photodetectors have been developed, the unavoidable trade-off between responsivity and detectivity remains a critical issue for these devices. Here, an ingenious phototransistor based on WSe₂ /WS₂ /WSe₂ dual-vdW heterostructures is constructed, performing both high responsivity and detectivity. In the charge neutrality point (gate voltage of -15 V and bias voltage of 1 V), this device demonstrates a pronounced photosensitivity, accompanying with high detectivity of 1.9 × 10¹⁴  Jones, high responsivity of 35.4 A W^-1 , and fast rise/fall time of 3.2/2.5 ms at 405 nm with power density of 60 µW cm^-2 . Density functional theory calculations, energy band profiles, and optoelectronic characteristics jointly verify that the high performance is ascribed to the distinctive device design, which not only facilitates the separation of photogenerated carriers but also produces a strong photogating effect. As a feasible application, an automotive radar system is demonstrated, proving that the device has considerable potential for application in vehicle intelligent assisted driving.

Dielectric Contrast Tailoring for Polarized Photosensitivity toward Multiplexing Optical Communications and Dynamic Encrypt Technology

A selective-area oxidation strategy is developed to polarize high-symmetry 2D layered materials (2DLMs). The dichroic ratio of the derived O-WS₂/WS₂ photodetector reaches ∼8, which is competitive among state-of-the-art polarization photodetectors. Finite-different time-domain simulations consolidate that the polarization-sensitive photoresponse is associated with anisotropic spacial confinement, which gives rise to distinct dielectric contrasts for linearly polarized light of various directions and thus the polarization-dependent near-field distribution. Furthermore, selective-area oxidation treatment brings about dual effects, comprising the in situ formation of seamless in-plane WS₂ homojunctions by thickness tailoring and the formation of out-of-plane O-WS₂/WS₂ heterojunctions. As a consequence, the recombination of photocarriers is markedly suppressed, resulting in outstanding photosensitivity with the optimized responsivity, external quantum efficiency, and detectivity of 0.161 A/W, 49.4%, and 1.4 × 10¹¹ Jones for an O-WS₂/WS₂ photodetector in a self-powered mode. A scheme of multiplexing optical communications is revealed by harnessing the intensity and polarization state of light as independent transmission channels. Furthermore, dynamic encryption by leveraging the polarization state as a secret key is proposed. In the end, broad universality is reinforced through the induction of linear dichroism within 2D WSe₂ crystals. On the whole, this study provides an additional perspective on polarization optoelectronics based on 2DLMs.

Recent Progress in Fabrication and Physical Properties of 2D TMDC-Based Multilayered Vertical Heterostructures

Two-dimensional (2D) vertical heterojunctions (HSs), which are usually fabricated by vertically stacking two layers of transition metal dichalcogenide (TMDC), have been intensively researched during the past years. However, it is still an enormous challenge to achieve controllable preparation of the TMDC trilayer or multilayered van der Waals (vdWs) HSs, which have important effects on physical properties and device performance. In this review, we will introduce fundamental features and various fabrication methods of diverse TMDC-based multilayered vdWs HSs. This review focuses on four fabrication methods of TMDC-based multilayered vdWs HSs, such as exfoliation, chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD). The latest progress in vdWs HS-related novel physical phenomena are summarized, including interlayer excitons, long photocarrier lifetimes, upconversion photoluminescence, and improved photoelectrochemical catalysis. At last, current challenges and prospects in this research field are provided.

MoS2/WSe2 vdW Heterostructures Decorated with PbS Quantum Dots for the Development of High-Performance Photovoltaic and Broadband Photodiodes

van der Waals heterostructures (vdWHs) overcoming the lattice and processing limitations of conventional heterostructures provide an opportunity to develop high-performance 2D vdWH solar cells and photodiodes. However, it is challenging to improve the sensitivity and response speed of 2D vdWH photovoltaic devices due to the low light absorption efficiency and electron/hole traps in heterointerfaces. Here, we design a PbS/MoS₂/WSe₂ heterostructure photodiode in which a light-sensitive PbS quantum dot (QD) layer combined with a MoS₂/WSe₂ heterostructure significantly enhances the photovoltaic response. The electron current in the heterostructure is increased by the effective collection of photogenerated electrons induced by PbS QDs. The device exhibits a broadband photovoltaic response from 405 to 1064 nm with a maximum responsivity of 0.76 A/W and a specific detectivity of 5.15 × 10¹¹ Jones. In particular, the response speed is not limited by multiple electron traps in the PbS QDs/2D material heterointerface, and a fast rising/decaying time of 43/48 μs and a -3 dB cutoff frequency of over 10 kHz are achieved. The negative differential capacitance and frequency dependence of capacitance demonstrate the presence of interface states in the MoS₂/WSe₂ heterointerface that hamper the improvement of the response speed. The scheme to enhance photovoltaic performance without sacrificing response speed provides opportunities for the development of high-performance 2D vdWH optoelectronic devices.

A Submicrosecond-Response Ultraviolet-Visible-Near-Infrared Broadband Photodetector Based on 2D Tellurosilicate InSiTe3

2D material (2DM) based photodetectors with broadband photoresponse are of great value for a vast number of applications such as multiwavelength photodetection, imaging, and night vision. However, compared with traditional photodetectors based on bulk material, the relatively slow speed performance of 2DM based photodetectors hinders their practical applications. Herein, a submicrosecond-response photodetector based on ternary telluride InSiTe₃ with trigonal symmetry and layered structure was demonstrated in this study. The InSiTe₃ based photodetectors exhibit an ultrafast photoresponse (545-576 ns) and broadband detection capabilities from the ultraviolet (UV) to the near-infrared (NIR) optical communication region (365-1310 nm). Besides, the photodetector presents an outstanding reversible and stable photoresponse in which the response performance remains consistent within 200 000 cycles of switch operation. These significant findings suggest that InSiTe₃ can be a promising candidate for constructing fast response broadband 2DM based optoelectronic devices.

Vertical 1D/2D Heterojunction Architectures for Self-Powered Photodetection Application: GaN Nanorods Grown on Transition Metal Dichalcogenides

Van der Waals (vdW) heterojunctions based on two-dimensional (2D) transition metal dichalcogenide (TMD) materials have attracted the attention of researchers to conduct fundamental investigations on emerging physical phenomena and expanding diverse nano-optoelectronic devices. Herein, the quasi-van der Waals epitaxial (QvdWE) growth of vertically aligned one-dimensional (1D) GaN nanorod arrays (NRAs) on TMDs/Si substrates is reported, and their vdW heterojunctions in the applications of high-performance self-powered photodetection are demonstrated accordingly. Such 1D/2D hybrid systems fully combine the advantages of the strong light absorption of 1D GaN nanoarrays and the excellent electrical properties of 2D TMD materials, boosting the photogenerated current density, which demonstrates a light on/off ratio above 10⁵. The device exhibits a competitive photovoltaic photoresponsivity over 10 A W^-1 under a weak detectable light signal without any external bias, which is attributed to the efficient photogenerated charge separation under the strong built-in potential from the type-II band alignment of GaN NRAs/TMDs. This work presents a QvdWE route to prepare 1D/2D heterostructures for the fabrication of self-powered photodetectors, which shows promising potentials for practical applications of space communications, sensing networks, and environmental monitoring.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.