Contemporary innovations in two-dimensional transition metal dichalcogenide-based P–N junctions for optoelectronics

Abstract

Two-dimensional transition metal dichalcogenides (2D-TMDCs) with various physical characteristics have attracted significant interest from the scientific and industrial worlds in the years following Moore's law. The p–n junction is one of the earliest electrical components to be utilized in electronics and optoelectronics, and modern research on 2D materials has renewed interest in it. In this regard, device preparation and application have evolved substantially in this decade. 2D TMDCs provide unprecedented flexibility in the construction of innovative p–n junction device designs, which is not achievable with traditional bulk semiconductors. It has been investigated using 2D TMDCs for various junctions, including homojunctions, heterojunctions, P–I–N junctions, and broken gap junctions. To achieve high-performance p–n junctions, several issues still need to be resolved, such as developing 2D TMDCs of superior quality, raising the rectification ratio and quantum efficiency, and successfully separating the photogenerated electron–hole pairs, among other things. This review comprehensively details the various 2D-based p–n junction geometries investigated with an emphasis on 2D junctions. We investigated the 2D p–n junctions utilized in current rectifiers and photodetectors. To make a comparison of various devices easier, important optoelectronic and electronic features are presented. We thoroughly assessed the review's prospects and challenges for this emerging field of study. This study will serve as a roadmap for more real-world photodetection technology applications.

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Ehsan Elahi

Ehsan Elahi is a PhD student who is currently working in the Department of Physics (Sejong University). He obtained his M.Sc. in Electronics from Quaid-i-Azam University Islamabad Pakistan and his M.Phil. from Riphah International University Lahore Campus, Punjab, Pakistan. Ehsan's research interests include 2D materials (TMDCs), 2D Ferromagnetic materials, Electronics, Optoelectronics, Spintronics, and Spin-caloritronics.

Muneeb Ahmad

Mr Muneeb Ahmad is currently working as an MS-Ph.D. student in the Nano-device Laboratory, Department of Semiconductor Systems Engineering, Sejong University, South Korea. He obtained his BS in Materials Science and Engineering from the Institute of Space Technology, Islamabad, Pakistan. His main area of work is resistive memory devices, transistors, and solar cells.

Muhammad Rabeel

Mr Muhammad Rabeel is a dedicated Ph.D. student at the Nano-device Laboratory in the Department of Electrical Engineering at Sejong University, South Korea. He obtained his master's degree from the School of Chemical & Materials Engineering (SCME) at the National University of Sciences and Technology (NUST) in Islamabad, Pakistan. His research interests primarily revolve around the fascinating world of 2D materials and their diverse applications. Specifically, he has a strong passion for exploring their potential in sensing applications, as well as their use in the development of flexible photodetectors, logic gates, and photocatalysis.

Sidra Saleem

Sidra Saleem is a Ph.D. student in the Department of Energy Storage and Conversion Engineering at Jeonbuk National University, Jeonju, South Korea. Her research primarily focused on water splitting, specifically addressing the Hydrogen Evolution Reaction (HER), Oxygen Evolution Reaction (OER), and Oxygen Reduction Reaction (ORR) for energy storage devices.

Van Huy Nguyen

Dr Van Huy Nguyen is a research associate at the University of Manchester, UK. He obtained his Ph.D. in Nanotechnology and Advanced Materials Engineering from Sejong University, South Korea, in 2023. His primary research interests are focused on 2D materials, including graphene and TMDCs, based on their heterostructures for energy harvesting and storage devices.

Sikandar Aftab

Sikandar Aftab earned his Ph.D. in Physics from the Department of Physics and Graphene Research Institute, Sejong University, South Korea. He also completed a two-year post-doctoral fellowship at Simon Fraser University in Burnaby, Canada. Dr Aftab is currently employed as an Assistant Professor at Sejong University's Department of Intelligent Mechatronics Engineering in Seoul, South Korea. His research interests include nanodevices based on two-dimensional materials heterostructures, energy storage devices and solar cells.

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1. Introduction

Two-dimensional (2D) layered materials are recognized as potential prospects in the fields of electronics and optoelectronics owing to their striking physical characteristics.^1–6 2D materials have established a significant deal of devotion in the fields of spintronics,^7–12 electronics,^13–17 and optoelectronics.^15,18–24 Scientists were sure that heat fluctuation made it difficult for nanoparticles to maintain stability, which ultimately caused a breakdown. The thermally stable graphene in this scenario has greatly surprised scientists, spurring the rapid growth of the 2D materials research field over the past few decades.^25,26 Because graphene shows a lack of bandgap configuration, it should potentially be competent in reacting to all photons, giving it the benefit of wide band revealing from ultraviolet (UV) to infrared (IR) and to the terahertz range.^27–29 Apart from its extensive band response qualities, pure monolayer graphene has high transparency, with a wide band absorption of 2.3%.³⁰ With such high transmittance, graphene has the potential to be used as a transparent electrode, particularly in devices with large absorption areas, such as solar cells and capacitors.^31,32 Additionally, by changing the electronic Fermi level, it is possible to alter the broadband absorption of graphene, which may then control the photon transition in graphene nanosheets.^33,34 With the contact of incoming light, energy might be transferred among carriers, photons, and phonons in graphene via charge carrier transition (electrons and holes). Macroscopical mechanisms, such as plasmons,^35,36 non-linear optical characteristics,^37–39 and photon absorption,^40,41 have been observed previously.

Group IV (Si, Ge, and Sn) element-based 2D materials have recently been developed. The structure of silicene and germanene is comparable to that of graphene, with an in-plane honeycomb lattice and van der Waals (vdW's) bonds linking neighboring layers. Because of their compatibility with Si technology, they have the potential to be used in integrated circuits. Another type of 2D material, black phosphorus (BP),^42,43 was initially produced over a century ago and has recently sparked considerable attention because of its direct band gap in bulk and single-layer ranging from 0.33 to more than 1 eV. Therefore, BP can be used in mid-infrared photodetectors:⁴⁴ a monolayer of BP has been exfoliated using the scotch tape technique and has a thickness of 0.7–0.85 nm. Regarding the electric characteristics of BP, it has been determined that the electron and hole mobilities are greater than 1000 cm² V⁻¹ s⁻¹ under ambient conditions with an extraordinary on/off ratio from the application viewpoint with its suitable band structure, which makes it appropriate for photodetection.^45,46 In addition, MXenes,^47–49 which include carbonitrides, transition-metal carbides, and nitrides, have been thoroughly researched for use in electrical and optoelectronic appliances.^50,51 Owing to the intrinsic p-type layered semiconductor characteristics, ultralow thermal conductivity, and near-infrared narrow band gap, tin selenide (SnSe) nanosheets have gained considerable attention.^52–56 When combined with additional benefits, such as chemical stability, earth abundance, cheap cost, and nontoxicity, SnSe has a lot of promise for use in upcoming electronic circuit applications. InSe-based devices have undergone a comprehensive investigation of the visible spectrum, but they have not yet been thoroughly investigated for near-infrared (NIR) applications.^57,58 The absence of on-chip sources and the elevated cost of manufacturing place further restrictions on optoelectronic devices for detection and sensing applications in the mid-infrared region.^59–61 Most InSe-based optoelectronic devices are transistor-based, necessitating strong electrical bias gating and bias voltage.^58,62,63 In another class, depending on the layer thickness, 2D transition metal dichalcogenide (TMDC)^{15–17,20,24,64,65} materials have band gaps between 1 and 2 eV. A broad range of applications, including photodetectors,^66,67 are possible owing to the band gap variation between multilayer and monolayer direct gaps^68,69 and field-effect transistor (FET),^{16,65,70–73} as well as optical communication.^74,75 The fundamental components of diodes, transistors, photodetectors, and solar cells, which make up advanced electronics and optoelectronic relevance,^20,23 are junctions that contain homo- and hetero-types.^24,76–78 Significant studies have focused on the p–n interface made of various 2D materials, creating a novel material manifesto for investigating novel physical features and innovative device applications.^79–85 It has been proven that layered MoS₂, a member of the TMDC family, is a prominent material with outstanding electrical and optical characteristics.^2,86,87 Thus, an innovative material, which provides a way for optoelectronic applications, is offered by the construction of p–n junctions using SnSe and MoS₂ nanosheet. It is known that SnSe/MoS₂ vertical p–n junctions are grown using a two-step CVD process, resulting in enhanced optoelectronic capabilities in the devised devices.⁸⁸ A p–n interface consisting of 2D materials^89–94 is a type of junction formed by combining a region of p-type layered material with a region of n-type material. The resulting junction has interesting electrical properties, including the ability to allow current to flow in only one direction. This property makes p–n junctions useful in an extensive range of electronic devices, including diodes, transistors, and solar cells. In a homojunction, when a semiconductor junction has a p-region and an n-region formed of the same 2D material, the term “p–n junction” is used. In other words, the junction is made up of the same kind of 2D materials on both sides but with varying degrees of doping, as shown in Fig. 1(a).^95–97 The movement of charge carriers (electrons and holes) through the junction is governed by the potential barrier that is created at the junction owing to the difference in doping levels. A heterojunction p–n is a form of interface in which the p-region and n-region are constructed of separate 2D materials, as shown in Fig. 1(b).^93,98,99 The p- and n-regions of heterojunction p–n are generally made up of materials with differing bandgaps and lattice architectures. The energy difference between the valence band (VB: where electrons are forcefully bound) and the conduction band (CB: where electrons are free to flow) is defined as the bandgap. The difference in bandgaps between a heterojunction's p-region and n-region provides a potential energy barrier at the interface. There are various advantages to employing a heterojunction p–n over a homojunction p–n diode. For instance, the difference in bandgaps provides improved charge carrier flow management and can lower leakage currents. Second, it allows for the engineering of band alignment, which can aid in the efficient flow of electrons or holes across junctions. A PIN junction¹⁰⁰ is a type of semiconductor junction that consists of three layers: a p-type substance, an intrinsic (I) layer, and an n-type material. The name “PIN” is derived from the arrangement of these layers, as shown in Fig. 1(c). Because the intrinsic layer is usually broader than the P and N layers, it might operate as a barrier for the majority of carriers (electrons in the n area and holes in the p region). This barrier prohibits electron and hole recombination, resulting in a depletion zone inside the intrinsic layer. PIN junctions provide various benefits over traditional p–n junctions. The larger intrinsic layer enhances the area available for charge carrier separation and decreases the likelihood of recombination. This feature makes the PIN junction ideal for high-frequency signal applications, such as photodetectors and high-speed diodes. It improves the reaction speed, sensitivity, and efficiency of the device. A “broken gap p–n junction” refers to a type of p–n junction where the bandgap of a 2D material changes abruptly across the junction interface, as shown in Fig. 1(d).^94,101,102 The p–n junctions with broken gaps are commonly employed in devices such as tunnel diodes and quantum well lasers. The tunneling phenomenon occurs in tunnel diodes owing to the broken gap p–n junction, which allows charge carriers to travel across the junction even when their energy levels do not align. Quantum well lasers, however, use the p–n junction's shattered gap to generate quantum confinement and regulate the light output at certain wavelengths. Overall, the broken gap p–n junction is a specialized arrangement with distinct properties when compared to normal p–n junctions, enabling specialized applications in electrical and optoelectronic devices.

Fig. 1 Various types of junctions. (a) Homojunction p–n. (b) Heterojunction p–n. (c) PIN junction. (d) Broken-gap p–n junction.

This study considers current developments in reconfigurable electrical devices built on 2D materials. We explored 2D material-based p–n heterostructures and devices for photodetection. We explain the essentials of 2D materials, including their configuration, electrical and optoelectronic characteristics, and interactions with hybrid systems. Next, several topologies and associated nanodevices for photodetection, such as p–n junction diode, are described in depth. The device's performances were analyzed and summarized. This review, which is certainly of considerable interest to developers of p–n junction-based photodetectors, highlights the key features of 2D material heterostructure-based photodetectors.

2. Preparation techniques for heterojunctions

Several approaches for integrating 2D materials have been suggested to create a 2D heterostructure using TMDCs, including mechanical transfer and stacking procedures with exfoliated layers,¹⁰³ chemical vapor deposition (CVD),^83,104–106 Molecular Beam Epitaxy (MBE),^107–110 and phase transition.¹¹¹

2.1. Mechanical exfoliation and restacking

One of the earliest methods of fabricating 2D materials is the exfoliation of monolayer graphene. With a thorough investigation of graphene and its related variants as well as other 2D materials with an analogous arrangement, such as the hexagonal boron nitride (h-BN),¹¹² TMDCs,¹¹³ and “BP”,¹¹⁴ mechanically exfoliated and incredibly nano structural devices with remarkable performance were produced. The excellent optical and optoelectronic capabilities of these 2D materials have attracted increased attention in the study of optoelectronic devices.¹¹⁵ Owing to accessibility and versatility, mechanical exfoliation has developed into a simple method for producing 2D materials.¹¹⁶ Mechanical exfoliation is a straightforward technique for creating new layers of 2D materials (from one to many layers) while retaining crystal symmetry and other properties.¹¹⁷ The mechanical exfoliation process of the 2D materials is shown in Fig. 2(a). Adhesive tape is pressed into the bulk material, and the top layer becomes a segment of the tape. Once more, the tape is pushed with layers on the plane of the substrate. The bottom layer turns into part of the substrate when detached off. GaSe/SnS₂ vertical heterojunction device with a clearly enhanced color contrast between the layer interface is shown in the optical microscope (OM) image form in Fig. 2(b). The devices were fabricated using the micromechanical cleavage technique. Fig. 2(c) shows that mechanical exfoliation was employed to create the high-quality multilayer WSe₂ and α-In₂Se₃ flakes utilized in this study.¹¹⁸ Furthermore, PET has a comparatively extraordinary Young's modulus, ensuring that the produced strain during bending may be effectively transmitted to the 2D layers. Flakes of α-In₂Se₃ and WSe₂ were exfoliated onto SiO₂/Si substrates to begin the device's construction. Moreover, a PET substrate was used to produce Ti/Au (10 nm/50 nm) electrodes using conventional photolithography and e-beam evaporation. However, mechanically exfoliated WSe₂–MoS₂ p–n junction was investigated by Shin et al. The optical image of the fabricated device is demonstrated in Fig. 2(d).¹¹⁹ The WSe₂–MoS₂ p–n interface served as a charge transfer layer, while the MoS₂ served as the phototransistor's channel. With a high photoresponsivity of 2700 A W⁻¹, a specific detectivity of 5 × 10¹¹ Jones, and a reaction time of 17 ms, the suggested p–n device demonstrated exceptional performance. The p-GeSe/n-WS₂ diode has remarkable optoelectronic and rectification properties as the optical image of the designed device (mechanically exfoliated), as illustrated in Fig. 2(e).¹²⁰ The diode displayed effective photodetection with encouraging merit numbers. Investigation of the photoresponse is performed using illuminations of λ = 850, 530 and 365 nm. At 850 nm, the diode has a strong response of 845 mA W⁻¹. The diode also has a detectivity of 3.28 × 10⁸ Jones. Remote sensing, target tracking, and computer vision are all possible uses of near-infrared polarization-sensitive photodetectors because of their benefits in efficiently recording light signals while blocking unwanted light. By stacking multilayer p-type GeSe atop n-type MoTe₂ with a mechanical exfoliation approach, a self-powered photodetector that is 2D polarization sensitive is developed, as shown in Fig. 2(f).¹²¹ Broadband spectrum representation from the visible (405 nm) to near-IR (1310 nm) wavelength range is displayed by the device. The responsivity (R) and detectivity (D*) at zero bias and 808 nm light can be as high as 52 mA W⁻¹ and 4.1 × 10¹¹ Jones, respectively. An efficient method for creating self-powered devices with incorporated angle-resolved optoelectronic devices is shown in this study by employing GeSe/MoTe₂ heterojunctions.

Fig. 2 (a) Process of mechanical exfoliation.¹²² (b) Optical microscope image of a fabricated GaSe/SnS₂ p–n heterostructure by exfoliation. This figure is reproduced from ref. 123, copyright, Appl. Surface Sci., 2021, 535, 147480. (c) Optical image of mechanically exfoliated α-In₂Se₃/WSe₂ heterojunction under tensile strain. This figure is reproduced by ref. 118, copyright, Adv. Opt. Mater., 2021. (d) The device is based on WSe₂−MoS₂ vdW's heterostructure. This figure is replicated by ref. 119, copyright, Nano Lett., 2020, 20.8. (e) Optical picture of a Pd/Au and Cr/Au electrode p-GeSe/n-WS₂ diode. This figure is reproduced by the authorization of ref. 120, copy right, Adv. Mater. Interfaces, 2020. (f) Image of a GeSe/MoTe₂ p–n junction device taken using an optical microscope. This figure is replicated by ref. 121, copyright, Adv. Mater. Interfaces, 2022.

2.2. Chemical vapor deposition (CVD)

Several 2D material-based FETs have been recognized as having potential for usage in multifunctional devices for optoelectronics, photonics and nanoelectronics at the nanoscale^69,124 since scientists began studying graphene as a photodiode in 2009.^18,125 TMDC materials, particularly monolayer MoSe₂ and WSe₂, have attracted a lot of interest as next-generation materials. Their semiconducting properties, direct bandgap, substantial absorption coefficient, and band-gap features are adjustable by changing the thickness of the material as well as electrically by the field effect, and they are ideal materials for ultrathin electronic and optoelectronic ICs.^124,126 However, whether used in a device with greater sensitivity or with a broad absorption spectrum, single 2D TMDC materials have some limits.^126,127 Thus, the hybridization of materials with various optical characteristics significantly simplifies optical engineering.¹²⁸ Various methods have been employed for this purpose. The CVD approach, which is both cost effective and time efficient, is commonly employed for the fabrication of 2D heterostructures.^83,105 Vertical structures associated with vdW forces or covalently bonded lateral heterostructures can be created via CVD. The lateral heterojunctions are covalently bonded, providing stronger contact surfaces than the vdW heterojunction.^129,130 This connection not only benefits from electrical qualities but also enhances the optical features, such as the value of D*, R, and boot the response of the device.^80,131 Because of their atomically sharp interfaces, these heterojunctions have high energy transfer effectiveness at the nanoscale.^132,133 Owing to its advantages in large-area heterostructure production, the CVD technique has attracted considerable interest. A wide range of 2D vertical heterostructures has been magnificently produced in recent years using the CVD technique.^134–141 Zhou et al. suggested a widespread CVD approach for 2D material manufacturing and demonstrated regulated development of over “40” types of 2D materials and their related heterostructures as well.⁴ However, in previous studies on 2D heterostructures, the optical characteristics largely depend on the fundamental characteristics of 2D materials, which may be considerably enhanced by constructing perfect p–n diodes utilizing 2D heterostructure doping control. In the intended devices, the CVD development of SnSe/MoS₂ vertical p–n junctions produced improved optoelectronic capabilities. The SnSe nanosheets are of excellent feature, and the junction interface is very efficient according to a thorough analysis of the crystalline phase and morphology of the heterojunction. This study expands the potential for heterojunctions to develop 2D materials with various crystal arrangements and creates a new method for building highly effective electrical and optoelectronic devices.⁸⁸

Li et al. demonstrated vdW's development of GaSe/MoSe₂ vertical heterojunctions, which broadened device applications in the optoelectronic sector.^138,142 The related growth trend of 2D materials was demonstrated by Zhang et al., who attained 100% overlap ReS₂/WS₂ vertical arrangement with a large-scale crystal size.¹⁴² Li et al. effectively achieved a flexible assemblage of diverse TMDCs into heterostructures based on the operative modulation of the diffusion barrier of the functioning clusters.¹⁴¹ However, the majority of the described heterostructures are n–n junctions with very little Fermi level difference, causing inadequate photo response actions because there are few p-type materials, and controlled development is challenging. From this perspective, developing p–n junctions will be crucial for integrated optoelectronics in the future. We reviewed here a high-performance photodiode made of Nb-doped p-type WSe₂ and inherently n-type MoSe₂ produced using doping-controlled CVD. The reduced chemical reactivity of WO₃ and Nb₂O₅ compared to MoO₃ allows for sequential, intrinsic development of the inner MoSe₂ layer at low temperature and Nb-doped growth of the outer WSe₂ layer at elevated temperature during synthesis.^133,135,143 Specific mixed water-soluble sources were employed to illustrate the p–n heterostructure of the synthetic monolayer TMDCs. Doping heterostructures with Nb atoms were also attempted throughout the synthesis. Because Nb has one valence electron less than tungsten (W) when it shares electrons with Se atoms, one of the bonds has a missing electron, resulting in a hole carrier.^144,145 Consequently, the Nb atoms in WSe₂ can shift their type from ambipolar to p-type dominant. Fig. 3(a) depicts a diagram of the one-step lateral heterojunction CVD setup. An in-plane heterostructure flake is observed schematically in Fig. 3(c). Fig. 3(b and d) depict optical microscope images of an Nb-doped WSe₂–MoSe₂ heterojunction formed on a SiO₂/Si substrate. The atomic force microscope (AFM) picture in tapping mode, as represented in Fig. 3(e), however, displays no color contrast between the outside and inner domains. This suggests that the color variation in the optical microscope pictures was generated by an in-plane structure rather than by a bilayer of vertical heterostructures. The height profile suggests that the flake thickness was around 0.9 nm, equating to a single layer of TMDCs, as illustrated in Fig. 3(f). Consequently, lateral 2D p–n diodes of p-type doped WSe₂ and inherently n-type MoSe₂ are created, resulting in a near-unity ideality factor (1.3) and an extremely high forward/reverse current ratio (10⁴). The high built-in potential at the p–n depleted layer (V_OC = 0.52 V) in this perfect 2D p–n photodiode proficiently controls the dark current in the reverse bias region (B ∼ 100 fA at an overall V_DS of 0 V to about −10 V) and boosts the photocurrent. Thus, this device has a high I_light/I_dark ratio (10⁵) and ultra-high detectivity (5.78 10¹⁵ Jones), which are roughly 100 times greater than those of previously published lateral 2D p–n photodiodes.¹⁴⁶ These exceptional results demonstrate that doping-controlled TMDC-based p–n heterojunctions are favorable applicants for future-advanced optoelectronics. This study suggests a viable method for employing a doping-controlled 2D p–n diode as a post-silicon optoelectronic device.¹⁴⁶

Fig. 3 (a) One-step chamber arrangement for direct CVD development of the Nb-doped WSe₂-MoSe₂ p–n structure is depicted schematically. Gas concentration is regulated using a mass flow controller (MFC). (b) Image of an Nb-doped WSe₂–MoSe₂ based on various p–n structures with an optical microscope. The scale bar is 10 μm. (c) An in-plane heterostructure flake is observed schematically. A magnified optical microscopic picture, with a scale bar of 3 μm. (e) Image of AFM scanning and (f) height pattern along the red solid line. This shows that the flake's thickness matches the TMDC monolayer and that the flake grew as an in-plane structure. This figure is replicated by ref. 146, copyright, J. Mater. Chem. C, 2021, 9.10.

2.3. Molecular beam epitaxy (MBE)

In the fields of materials science and semiconductor technology, Molecular Beam Epitaxy (MBE) is an approach used to create thin films of crystalline materials with exact monitoring of their structure and chemistry.^147–151 Owing to the ultra-high vacuum used in molecular beam epitaxy (MBE), there has recently been a lot of scientific interest in the production of large-area and high-quality epilayers of different TMDCs.^152–155 The fact that MBE offers several crucial competencies, including the ability to precisely regulate the composition, thickness, fundamental phases, and other features of TMDCs, makes it a perfect tool for investigating novel physics, looking into technological possibilities, and investigating basic sciences that may lead to intriguing purposes. Twenty years ago, MBE started making epitaxial TMDC films.¹⁵⁶ By successfully growing epitaxial MoSe₂ thin films and epitaxial NbSe₂ thin films for the first time on a cleaved 2H MoS₂ surface, Koma et al.¹⁵⁷ accomplished ground-breaking work. Heterostructures based on 2D materials have recently been studied. Lin et al.¹⁵⁸ presented significant advancements in thin-film synthesis and processing methods that exhibit exceptional manipulation and dependability for substitutional doping of TMDCs thin film monolayers. WTe_2−xSe_x− and V_1−xMo_xSe₂^159,160 structures are among the several 2D alloys that MBE can generate with exquisite controllability. Using MBE, Ohtake et al.¹⁶¹ grew a heterostructure consisting of layered MoSe₂ and MoTe₂ monolayers (MLs) in alternating orientations. Despite the significant lattice mismatch of 6.51%, the alternately stacked MLs were produced in a nearly perfect layer-by-layer manner at least up to six MLs. The current observation has a significant meaning that the lattice mismatch also influences the structural quality of epitaxial films in the 2D materials framework.¹⁶¹ When the ultra-high quality of the substance's ingredients and strict control over layer thickness is necessary, MBE is recognized as a potential synthesis method for large-scale TMDC thin films and nano heterostructures.¹⁶²

3. Techniques for generating 2D homojunctions

To further investigate fundamental features and device applications, high-quality 2D homojunction preparation is necessary. We summarized only a few methods here.¹⁶³ As illustrated in Fig. 4, the described methods for fabricating 2D homojunctions may now be broadly categorized into four groups: vapor-phase deposition,^164,165 lithium intercalation,¹⁶⁶ laser irradiation,¹⁶⁷ and doping engineering.¹⁶⁸ Chemical doping,¹⁶⁹ electrostatic doping,¹⁷⁰ and photodoping^171,172 are the three subcategories of doping engineering strategies based on variations in doping techniques. The ultimate kind of homojunction structure is primarily determined by applying these production techniques. Chemical doping techniques, including surface modulation, intercalation, and replacement, offer a potent and stable way to modify a semiconductor's electrical behavior. Using a local ionic gate, solid-state split gate, or ferroelectric polarization, it is often possible to modify the carrier concentration and type in distinct areas of the same material to produce electrostatic doping. The photoinduced electrons in h-BN are transferred into the channels, causing n-type doping, and this phenomenon has been frequently observed in graphene/h-BN¹⁷³ and WSe₂/h-BN¹⁷⁴ heterostructures. The idea was used to build a p–n homojunction after additional development.^171,172 It is noteworthy that UV light with high photon energy is typically utilized in photodoping. Heterophase homojunctions and layer-engineered homojunctions are often created via vapor-phase deposition. The fabrication of heterophase homojunction's mostly involves lithium intercalation and laser irradiation. Doping engineering methods are used to manufacture p–n homojunction.¹⁶³ 2D homojunction unique structures and features may provide a viable foundation for future scientific study and commercial applications. Furthermore, we anticipate that 2D homojunction's will advance rapidly as a result of the collaborative efforts of several scientific experts.

Fig. 4 Preparation strategies for 2D homojunctions. This figure is reproduced from ref. 163, copyright, Adv. Mater..

4. Homojunctions based on TMDCs

The accessibility of intrinsic p- and n-type 2D materials, such as p-type tungsten diselenide (WSe₂) and n-type tungsten disulfide (WS₂), is one of the main benefits of TMDCs. TMDC materials are highly desirable for a collection of applications involving FETs,¹⁷⁵ photodetectors,¹⁷⁶ photovoltaics,^177,178 and sensors.^179,180 The introduction of 2D TMDCs might address a long-standing difficulty in traditional p–n junctions built on bulk semiconductors, which have significant limits in the pursuit of low-power feasting, reduced size, and applied applications necessitating durability and versatility.¹⁸¹ An effective technique is to produce p–n TMDC junctions for photovoltaic device applications, in which the photoinduced electron–hole pairs may be separated swiftly.¹⁸² Building vertical heterojunctions with separated p and n TMDCs by vdW's bonding is a common method for forming p–n junctions.¹⁸³ 2D WSe₂ has considerable promise in producing a p–n interface for next-generation photovoltaic devices because of its strong light absorption and distinctive band formation, which permits straightforward bipolar manipulation of the carrier type.^184,185 Surface charge transfer is an excellent way to alter the electrical characteristics of TMDCs by efficiently convincing a change in the Fermi level to accomplish the alteration from p- to n-type carriers in low-dimensional TMDCs. Cetyltrimethyl ammonium bromide (CTAB) solutions are extensively utilized in the preparation of innovative hybrid nanomaterials for various applications, including photoluminescence, photocatalysis, battery, and photovoltaic cells as traditional and economical active substances.

Hence, the improved WSe₂ p–n junction is specifically created with fewer trap states in the junction system. The devices showed significant performance with a ratio (I_light/I_dark) of ≈10⁵, a value of R of 30 A W⁻¹ (7989% EQE), and a D* of more than 10¹¹ Jones. The electrical characteristics, including mobility, ON-current, and threshold voltage, were carefully regulated by altering the CTAB's immersing time. These findings point to an effective method for achieving adjustable doping of 2D materials, as well as the prospective uses of WSe₂ in various p–n junction-based electronic devices, such as tunnel diodes, LEDs, photodetectors, and solar cells. The schematic of the device is demonstrated in Fig. 5(a). To attain the p–n junction, half portion of the WSe₂ is capped by spin-coated PMMA to avert electron doping and preserve the pristine hole characteristics; however, the remaining channel is adsorbed with CTAB to attain n-type doping. After that, the p–n junctions made of WSe₂ are immersed in the CTAB solution for 2 hours to ensure that CTAB molecules are efficiently adsorbed. The inset of Fig. 5(b) displays the optical microscope image of the device after doping. The I–V> curves of the pristine and CTAB-doped devices show that the CTAB-doped WSe₂ exhibits an extraordinary rectifying conduct with the ratio (I_forward/I_reverse) of approximately 10³ as opposed to the essentially symmetrical structure of the pristine one, revealing an appropriate p–n diode arrangement. The Shockley diode equation is used to determine the ideality factor η of the p–n junction to assess the diode property:

where I_s represents reverse saturation current, q represents electron charge, V_ds represents drain voltage, η represents the ideality factor, k represents the Boltzmann constant and T represents Kelvin temperature.¹⁸⁶ According to the estimates, the ideality factor of this WSe₂ p–n junction is 1.64, which indicates that the current in this device is primarily carried by diffusion under forward bias rather than by impurities already present.^185,187 This indicates a high-quality p–n interface with fewer trap states in its junction regime. Fig. 5(c) depicts a summary of the time dependence of the diode current with varied power density, where the photocurrent may be regulated from 1.75 to 291.12 nA by modulating the light density from 0.50 to 166.90 mW cm⁻². To assess the optical performance of the WSe₂ photodetector, the highest EQE and detectivities (D*) are derived using the following equations:where e is the charge, c is the speed of light, h is the Planck constant, I_ph is the photocurrent (|I_light − I_dark|), R is the photoresponsivity, and A is the device's effective area.¹⁸⁸ The maximal EQE at 0.5 mW cm⁻² is calculated to reach up to 7337%, which is unexpectedly greater than many earlier studies. This confirms the efficient creation, separation, and collection of photo carriers at p–n junctions once more. Even though it tends to decrease with increasing light power density, it achieves a surprisingly high EQE of more than 3400% at all measured power densities without back-gate voltages. Other photoconductors have also shown a decrease in responsiveness and EQE at greater illumination powers, which is attributed to a decrease in the rate of photogenerated carriers present for extraction under high photon flux.¹⁸⁹ Specific detectivities (D*) are another useful metric for assessing the device's lowest detectable signal. D* has a significant value; roughly 10¹¹ Jones was attained at all evaluated power densities, demonstrating a superior ability to detect weak signals. Furthermore, the investigation considers the WSe₂ p–n junction's photoelectric response properties as a function of bias voltages. As anticipated, when the laser is turned on, the supplied bias voltage causes the photocurrent to surge abruptly because of an increase in carrier drift velocity. The thermionic and tunneling mechanisms combine to increase the efficiency of electron transfers to semiconductors in the process for the enhancement of transport characteristics by chemical doping. Recently, many methods have been used to alter conduction polarity, producing a homogenous p–n junction. InSe-based devices have undergone a comprehensive investigation of the visible spectrum, but they have not yet been thoroughly investigated for near-infrared (NIR) applications.^57,58 The absence of on-chip sources and the high cost of manufacturing further restrict optoelectronic devices for modulation, detection, and sensing things in the mid-infrared region.^59–61 The majority of InSe-based optoelectronic devices are transistor-based, necessitating strong electrical bias gating and bias voltage.^58,62,63 A NIR application-specific self-driven p-InSe/n-InSe-based photodetector was explored.¹⁹⁰ The vertical heterostructure stack is made of p- and n-doped 2D InSe flakes that demonstrate an innovative fast and sensitive InSe heterojunction-based NIR photodetector appropriate for low-power optical detector devices with a responsivity improvement of about 3 times and a response time reduction of about 3.5 times. This capacity paves the way for non-external gain, low noise, and low photon flux photodetectors. In this delicate zero-bias voltage domain, researchers also showed millisecond response speeds. Wearable biosensors, three-dimensional (3D) sensing, and remote gas sensing are just a few of the applications that might benefit from such sensitive photodetection potential in the technically significant NIR wavelength area at low form factors. Electronic architecture may be made smaller using only one nanoflake lateral p–n diode (in-plane) made of a 2D material. In this instance, a unique lateral homojunction p–n diode comprising a single WSe₂ nanoflake is produced by photoinduced doping through the illumination-induced optical excitation of defect states in an h-BN nanoflake. By partly stacking the WSe₂ nanoflake on the h-BN and Si substrates, this lateral diode is made by utilizing a mechanical exfoliation process. By inverting the carrier type in the region of the WSe₂ film on the h-BN substrate, a built-in potential difference is produced, which is detected by Kelvin probe force microscopy ranging from 5.0 to 4.50 eV. It is discovered that there is a ∼492 mV contact potential difference at the junction of p-WSe₂ and n-WSe₂. With an ideality factor of 1.1, the lateral diode exhibits an exceptional rectification ratio of up to ∼3.9 × 10⁴. Upon illumination of the radiant light, a typical self-biased photovoltaic conduct is observed at the p–n junction, where a positive open-circuit voltage (V_oc) is developed at I_ds = 0 V and a negative short-circuit current (I_sc) is observed at V_ds = 0 V. Future devices can use a self-powered photoelectric system by implementing the I_sc and V_oc established by the suggested homojunction diode's built-in potential when it is illuminated.

Fig. 5 (a) Constructed WSe₂ p–n junction schematic view with light illumination; the inset displays the doping technique. (b) Pristine and CTAB-doped p–n junction log-scale I–V> curves, and the device's optical microscope image are presented in the inset. (c) Summarized photocurrent data over time. These figures (a–c) are reproduced with the permission of ref. 193, copyright, Adv. Mater. (d) Schematic representation of a device based on homojunction WSe₂ flake. (e) WSe₂-based homojunction p–n diode optical image. (f) Rectification ratio of the WSe₂-based homojunction device as a function of V_bg. These figures (d–) are reused with the permission of ref. 204, copyright, ACS Appl. Mater. Interfaces, 2020, 12.37. (g and h) Optical image (right side) and 3D graphic (left side) structure of a WSe₂ FET device undergoing treatment with O₂ plasma are displayed. Channels 1–2, 2–3, and 3–4 are indicated as n-WSe₂, pn-WSe₂, and p-WSe₂. (i) Time-dependent analyses of the photocurrent in the p–n device were made with 532 nm laser illumination at fixed V_g = 40 V. These figures (g–i) are replicated by the reference,⁹⁶copyright, Nanoscale 2023, 15.10. 4940–4950.

Future electrical and optoelectronic applications can successfully create planar homojunction devices using the suggested doping approach without the need for a photoresist. The schematic of the purposed device is demonstrated in Fig. 5(d), where a part of the WSe₂ layer was placed on the h-BN flake and a remaining portion of WSe₂ was placed on the Si/SiO₂ substrate. Fig. 5(e) shows a representation of the optical image of the fabricated device that contains the p–n homojunction based on WSe₂. A lateral WSe₂ p–n diode's rectifying performance was assessed by changing the V_bg from −80 to +40 V. The diode current significantly falls in the reverse bias when V_bg increases from −80 to −30 V, leading to an increase in the rectification ratio of up to 3.9 × 10⁴, as illustrated in Fig. 5(f). This device showed better performance compared to previous reports using other doping approaches, such as chemical doping.^191,192

Conventional silicon (Si) microelectronic doping strategies are inappropriate with atomically thick 2D TMDCs, making them difficult to construct high-quality 2D homogenous p–n junctions. WSe₂ is an interesting 2D material with bipolar transport properties and thickness-dependent band gaps of 1.7 eV (direct, monolayer) and roughly 1.2 eV (indirect, multilayer), in addition to predicted electron and hole motilities of around 250 and 270 cm² V⁻¹ s⁻¹, respectively.^193–195 Owing to its distinct physical characteristics, WSe₂ has made it possible to create an extensive range of electronic and optoelectronic devices with high-quality optoelectronic performance. The application of WSe₂ optoelectronic devices has recently been very interesting in novel artificial configurations based on vdW's vertical heterojunctions or lateral p–n heterostructures.^{184,193,196,197} In a previous study, Jo and colleagues showed that a triphenylphosphine n-doping technique significantly increased the photoresponsivity of WSe₂/h-BN-based p–n heterojunction.¹⁹⁸ A WSe₂-ZnO p–n heterojunction photodetector constructed by fusing p-type WSe₂ and n-type ZnO had an ultra-high value of R of about 4.83 × 10³ A W⁻¹ under 405 nm light irradiation according to Guo and colleagues.¹⁹⁹ According to Liu et al., under 633 nm irradiation, a photodetector based on a WSe₂–Bi₂Te₃ p–n heterojunction may provide a quick reaction time of ∼210 μs and a suitable value of R of about ∼20.5 A W⁻¹.¹³⁹ These findings show that the effective creation of heterojunctions can substantially enhance the photoresponse properties of TMDC-based devices. Conversely, these devices still struggle with inevitable residues and are problematic in accurately localizing target materials onto the complex flake alignment procedures of other flakes. Additionally, a vdW gap among junctions may prevent carrier charge transport, which is usually unwanted for device efficiency. As a substitute technique, homogeneous junctions provide neat and self-aligned interfaces, which have inherent advantages over heterogeneous junctions. Numerous doping schemes and electrical tuning techniques, such as work function engineering methods, have been investigated in an effort to conquer the inherent performance limitations of 2D TMDCs and fix the contact issues on WSe₂ functional devices.^200–202 These techniques, however, are quite difficult and require high temperature or vacuum process conditions, seriously restricting their practical use in the manufacture of high-performance WSe₂-based devices. Transition metal oxides (TMOs) have recently been proven to be efficient p-type contacts and dopants in organic circuitry and 2D material-based devices.²⁰³ The manufacturing of a 2D TMDC p–n homojunction with low R_c is still constrained by technical issues despite several encouraging results obtained thus far. Another approach, by applying O₂ plasma treatment, was possible to readily allow the development of hole transport and therefore attain in situ p-type semiconductor properties in WSe₂. This method was shown to be effective and durable. Through the transport of electrons from the WSe₂ to the top layer of oxidized WO_x, the stoichiometric oxidation of WSe₂ into WO_x strongly generates hole doping in the neighboring WSe₂. Additionally, the results showed that the hole mobility in a p-type-doped WSe₂ transistor increased significantly from ∼22 to ∼157 cm² V⁻¹ s⁻¹ owing to the alteration of the SB to an Ohmic contact resulting from plasma conduction. Thus, under 532 nm illumination, the since-designed p–n homojunction displays a better EQE of 228%, a superb value of R of about ∼7.1 × 10⁴ mA W⁻¹, and a photodetectivity of ∼3 × 10³ Jones. The optical visualization (top view) and schematic representation of the constructed WSe₂ device are displayed in Fig. 5(g and h). Then, under various operating conditions, these WSe₂ devices underwent exposure to O₂ plasma. Notably, portions designated with electrodes 1 to 2 are protected by a PMMA layer, leaving areas marked with electrodes 3 to 4 exposed for O₂ plasma therapy. The O₂ molecule was then adsorbed onto the upper surface of the exposing WSe₂, where an oxygen atom replaced the top layer of the Se atom. On top of the WSe₂ layer, the WO_x layer was then created. In the O₂ plasma-treated (plasma-protected) area, p-type (n-type) dominating conduction appears because of O₂ plasma-induced p-doping (creation of a WO_x layer on top of WSe₂).

The device performance results of this approach demonstrate that it is the best way to develop high-performance p–n diode devices by operating 2D TMDCs that self-oxidize. This approach may be adapted to other TMDCs for potential multifunctional electrical and optoelectronic purposes. Here, the WSe₂ p–n junction photocurrent was additionally determined at a constant gate bias of 40 V and was time- and source–drain voltage-dependent (Fig. 5(i)). The photocurrent improvement was observed under varied V_D and V_G situations as V_G swept from +40 to −40 V. The highest photocurrent is specifically attained at a gate bias voltage of 40 V. At both the V_G < 0 V and V_G > 0 V sides, significant alterations in the gate-dependent photocurrent may be perceived. These findings imply that the gated electric field can successfully alter the photocurrent response. Additionally, under continuous light irradiation, it is observed that the photocurrent gradually decreases over time, suggesting potential charge-trapping effects. A 40 V gate voltage results in a greater concentration of carriers and a smaller potential barrier at the contact interface. Consequently, carriers can fill the charge trap states substantially more easily, somewhat decreasing the photocurrent. The greater value of R and EQE values additionally indicate that the plasma-treated WSe₂ p–n device has strong photoresponsivity. This proposed WSe₂ p–n device has a D* value of around 3 × 10³ Jones. These findings demonstrate the value of O₂ plasma treatment in the progress of WSe₂-based optoelectronic devices. These findings support the feasibility of generating WSe₂-based optoelectronic logic purposes and the practical application of the local plasma process for WSe₂ flakes to modify the electrical characteristics of WSe₂. We summarized the data of various homojunction p–n developments, as shown in Table 1.

Table 1 Key features of a homojunction-based photodetector recently described

Materials	Wavelength (λ)	Responsivity	Detectivity (D*) Jones (J)	EQE (%)	Ref.
Thin WSe2/thick WSe2	532 nm	0.7 mW^−1	—	0.1	205
WSe2 p–n	<750 nm	16 mA W^−1	—	3	124, 185 and 206
Graphene p–n	Visible	10 mA W^−1		2.5	207–209
MoS2 p–n diode	500 nm	5.07 A W^−1	∼3 × 10^10	∼1200%	191
PdSe2	(532–1470 nm)	53 mA W^−1	5.17 × 10^11	—	210
WSe2	410 nm	11.2 mA W^−1	4.4 × 10^10	2.6	211
MoTe2	405–1550 nm	1200 mA W^−1	10^12	—	212
WSe2	520–852 nm	2000 mA W^−1	7.2 × 10^10	420	213
WSe2 (chemical doping)	450 nm	30 A W^−1	10^11	7989	193
WSe2 (P–I–N)	450 nm	0.1 A W^−1	2.2 × 10^13	—	214
MoTe2	1260–1510 nm	156 mA W^−1	-—	13.1	215
MoTe2 (P–I–N)	400–1200 nm	1.10 A W^−1	3.0 × 10^12	—	24
BP	520–1450 nm	1.06 A W^−1	1.27 × 10^11	90.8	216
MoS2	640–1040 nm	7 × 10^4 A W^−1	3.5 × 10^14	10	217
MoS2	300–700 nm	5.07 A W^−1	5.0 × 10^10	∼7000	191

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5. Heterostructure p–n junctions based on 2D materials

A potential geometry for obtaining broadband photodetection using a mix of extensive-bandgap and narrow-bandgap operational elements is a heterojunction based on 2D materials. However, because air bubbles and wrinkles are bound to enter the heterojunction, it is challenging to control the interface conditions. High sensitivity, quick reaction times, and a wide response range are all essential for various applications, including optical signal reception in modems,²¹⁸ image recognition in cameras^219,220 and infrared temperature observation in electronic thermometers.^221–223 Owing to their unique optoelectronic characteristics, 2D materials are currently a viable solution for the functional layer to achieve excellent detectivity, quick reaction times, and broad detection ranges. The heterojunction framework, which commonly combines a broader bandgap and a narrow bandgap material via a mechanical exfoliation process, is the standard configuration of 2D-based photodetectors.^224,225 However, this approach cannot produce efficiency at a high rate particularly when constructing heterojunctions calls for various materials.^226–228 Furthermore, the inadequate interface situations of the heterojunction, which are exactly connected to D* and reaction time, are at all times influenced by bubbles and crinkles created through the stamping process.²²⁹ Furthermore, because of the high binding energy between the multiple layers, it is typically hard to exfoliate thin flakes.²³⁰ However, a thick flake results in a reduction in the detection region and sensitivity.

According to a recent study, a synthesis technique that combines exfoliation and CVD can be used to create photodetectors for GaSe/PtSe₂ heterojunctions. At a 10 V bias and in the avalanche mode, the devices exhibit maximum responsiveness and detectivity, which are around 1.7 A W⁻¹ and 3.51 × 10¹² Jones, respectively. Additionally, a quick response time of 20 μs was attained. The design of the GaSe/PtSe₂ photodetector's final device construction is demonstrated in Fig. 6(a). The mechanically exfoliated GaSe sheet was first stamped with PDMS onto a Si/SiO₂ substrate. Then, using conventional photolithography, a specified region was etched on GaSe for the following Pt film. However, a soft oxygen plasma phase was added before the deposition of the Pt film to produce a thin oxidation layer of GaO_x that was interspersed between GaSe and Pt. The GaSe/Pt structure was then placed in a CVD system for additional selenization to create a heterojunction consisting of GaSe/PtSe₂. As depicted in Fig. 6(b), the optical representation of the device shape is displayed, and the testing region includes the entire junction. As described in earlier investigations, the work function of GaSe and PtSe₂ is utilized here.^231–233 The GaSe energy band diagrams of /PtSe₂ heterojunction were created in avalanche and self-driven modes. The schematic without bias condition is shown in Fig. 6(c). A built-in electric field pointing from PtSe₂ to GaSe might be produced owing to the band alignment after contact depending on the Fermi level of GaSe and PtSe₂. The built-in electric field inside the junction region may separate the photo-induced electron–hole pairs, which can then be introduced as photocurrent into the electrical circuit. After the device has reached the avalanche condition under a significant reverse bias, the separated electron–hole pairs accelerate because of the strong external electric field in preparation for another collision (Fig. 6(d)). Excellent responsiveness and quick response times are mostly due to this. The device based on the optical response properties of the GaSe/PtSe₂ heterojunction device is next assessed using a 520 nm laser diode and a supercontinuum light source with a wavelength ranging from 800 to 1450 nm. As depicted in Fig. 6(e), the log-scale findings are displayed in the inset, and the I–V> curve is compared with the bias voltage from −10 V to 10 V in the dark and under 520 nm illumination. Under laser irradiation, an increase is observed at zero bias, demonstrating the device's self-driven functioning. The photocurrent demonstrates a substantial increase as the bias voltage increases. In particular, the photocurrent under reverse bias is substantially larger than that under positive bias because of the heterojunction nature at the GaSe/PtSe₂ interface. The device current switches the ON–OFF rate over 1000 times at a bias of −10 V (10⁻⁹ A in the dark and 10⁻⁶ A in the light), demonstrating a high sensitivity. The heterostructure's time-resolved photocurrent characteristics at a bias of −10 V are demonstrated in Fig. 6(f) by modulated light. It is possible to distinguish between “ON” and “OFF” currents, exhibiting high repeatability and stability. As demonstrated in Fig. 6(g), photocurrent switching tests using various laser wavelengths are carried out to display the broadband photoresponse at the NIR waveband. The supplied reverse-bias voltage may divide and drift the photo-generated carriers in the PtSe₂ layer (bandgap: ∼0.1 eV), which are subsequently collected by the counter electrodes. One could anticipate that a PtSe₂-based device will optically respond to IR light because multilayer PtSe₂ has a thickness-dependent and small band gap. This research indicates that the device has a relatively low response at 1550 nm. The device displayed a quick reaction time of 20 μs and a wide photodetection range of 405–1550 nm. Additionally, an image sensor's potential has been demonstrated with respectable resolution in work. To establish a favorable junction requirement for the next advancement of electrical and photoelectric attributes, this heterojunction manufacturing approach offers an alternate choice. One of the most interesting future directions in this field is researching approaches to multifunctional vdW heterojunction devices. GeAs/ReS₂ heterojunction, which is achieved here by varying the doping amount of GeAs, serves various purposes in forward rectifying diodes. The GeAs/ReS₂ forward rectifying diode demonstrates extremely sensitive photodetection in a broad spectrum of up to 1550 nm, which corresponds to a short-wave infrared (SWIR) zone. Additionally, the heterojunction displays significant polarization-sensitive photodetection performance with a dichroic photocurrent ratio of 1.7 because of two robust anisotropic materials, GeAs and ReS₂. The fabricated device schematic vertically stacked ReS₂/GeAs vdW heterojunction is presented in Fig. 6(h). AFM was performed to determine the heterojunction's thickness; GeAs and ReS₂ had thicknesses of 8.4 and 11.2 nm, respectively, equivalent to 10 and 15 layers, respectively. The AFM image is illustrated in Fig. 6(i). Optoelectronic characteristics were explored in the structure of GeAs/ReS₂-based devices. Under the light of a laser with 532, 638, and 1550 nm wavelengths, the photoresponse was examined. The authors obtained the power-dependent R, EQE and D* of the heterojunction detectors illuminated by a 532 nm laser to thoroughly assess the photoelectric efficiency. The value of responsivity for that device was about 6.86 × 10³ mA W⁻¹. Fig. 6(j) depicts EQE at various powers of incident light, which may reach 1639%. D* is a crucial parameter to reflect the least detectable signal and is equal to 1.2 × 10⁹, showing improved weak signal detection performance. This research suggests a useful technique for a multipurpose design vdW heterojunction device that will render applications in practice much easier.²³⁴ We summarized the comparative analysis of the p–n junction established on various 2D materials, as represented in Table 2.

Fig. 6 (a) Schematic p–n made of GaSe/PtSe₂ in 3D form. (b) Optical image of the p–n-based photodetector. (c) GaSe/PtSe₂ energy band diagram schematic. (d) Energy band diagram schematic when a reverse voltage is applied. (e) I–V> properties in the presence of visible 520 nm laser illumination (red) and complete darkness (black). (f) Test of the switching cycle of a laser using a 520 nm laser and a −10 V bias voltage. (g) Heterojunction photodetector broad response. This figure is replicated from ref. 219, copyright, Mater. Design 2023, 228. (h) Schematic illustration of GeAS/ReSe₂ junction-based device. (i) AFM image of the device based on GeAs/ReS₂ heterostructure, where both flakes are marked by dotted lines. (j) EQE and D* depend on power. Figures (h–j) are reproduced with the permission of ref. 234, copyright, Small, 2023.

Table 2 A comparative summary of the fundamental p–n performance metrics

Materials	Wavelength (λ)	Responsivity	Detectivity (D^*) Jones	EQE (%)	Response time	Ref.
GaSe/PtSe2	520 nm	1.78 A W^−1	3.5 × 10^12	405	20 μs	219
GaSe/WS2	410 nm	149 A W^−1	4.3 × 10^12	450	37 μs	235
WSe2/ReS2	600 nm	3 A W^−1	8.39 × 10^10	600	5 μs	197
BP/MoS2	Near IR (532 nm)	22.3 A W^−1	3.1 × 10^11	∼1000	15 μs/70 μs	236
GaSe/SnS2	633 nm	∼35 A W^−1	8.2 × 10^13	62	—	123
ReS2/Te	458 nm	3 × 10^6 AW^−1	1 × 10^11	3 × 10^4	—	102
WSe2−MoS2	872 nm	2700 A W^−1	5 × 10^11	—	10 ms	119
p-GeSe/n-WS2	850 nm	845 mA W ^−1	3.28 × 10^8	—	10 ms/14 ms	120
WSe2/SnS2	520 nm	108.7 A W^−1	4.7 × 10^10	—	500 μs/600 μs	237
MoTe2/MoS2	550 nm	0.26 A W^−1	—	58.7	0.2s/0.1s	238
In2Se3/GaN	365nm	∼70 mA W^−1	∼10^11	∼30	0.2s/0.42s	239
PtTe2/Si-	980 nm	406 mA W ^−1	3.62 × 10^12	—	2.4/32 μs	240
ReS2/MoS2	532–785 nm	0.197 A W^−1	10^12	35% to 39%	13/15 μs	241
PtSe2/GaAs	200–1200 nm	0.262 A W^−1	2.52 × 10^12	—	5.5/6.5 μs	233
MoS2/CdTe	200–1700 nm	0.036 A W^−1	6.1 × 10^10	—	0.043/0.082 (ms)	242
WSe2/SnSe2	532–1550 nm	588 A W^−1	4.4 × 10^10	1367	13/14 (ms)	243
WSe2/SnS2	550 nm	244 A W^−1	1.29 × 10^13	—	13 (ms)	244
GaTe/InSe	532–1550 nm	1.5 A W^−1	10^14	—	360 (ms)	245
PdSe2/MoS2	532 nm	600 A W^−1	10^11	—	100/37 (ms)	246
BP/WSe2	400–1550 nm	10^3 A W^−1	10^14	∼10^6	0.8/0.8 (ms)	247
GeAs/ReS2	532 nm	6.86 × 10^3 mA W^−1	1.2 × 10^9	1639%	20/30 s	234
	1550 nm	1.02 × 10^2 mA W^−1	2.16 × 10^8		96/98 ms
GaSe/MoS2	520 nm	0.67 A W^−1	2.3 × 10^11	∼160%	1.4 s	248

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6. P–I–N junction based on 2D materials

Recently, many methods have been used to alter conduction polarity, producing a homogenous p–n junction.^191,206,249 A MoTe₂-based device with a thickness on the order of nanometers may be used for short-wave infrared (SWIR) recognition in the range of even more than about 1100 nm, which is a wavelength range that Si-based devices cannot adequately report. This is because MoTe₂ has a smaller bandgap (1 eV) compared to Si. This detection might be crucial in demonstrating the value of 2D-layered TMDCs over Si-based sensors for light.^250–255 The recognition of visible to 1300 nm IR photons by a MoTe₂ PIN junction diode is reviewed herein. Initially, p-type thin MoTe₂ with 2D-like characteristics was selectively H-doped to create a p–n junction. The p–n diode also features split gates to independently regulate the E_F level of the p and n regions. Therefore, by varying the gate voltages, three distinct areas of P⁺, virtually intrinsic (I), and N⁺ conduction may be created. In this case, the intrinsic (I) region develops because the split gate generation of P⁺ and N⁺ regions leads to further hole and electron depletion close to the pre-existing p–n junction, which previously had a depletion area from electron–hole recompense. For instance, the original p–n junction mode's response in the 1300 nm SWIR region is insufficient, but the response in the PIN junction diode regulated by two split gates increases by an order of magnitude. Although there have been few investigations of PIN structures in a thin TMDC semiconductor that resembles a 2D,^{189,254,256,257} it is generally known and comprehensible that PIN diodes have better responsiveness than p–n diodes in general elements of device physics. Fig. 7(a) displays an optical microscope image of our MoTe₂-based PIN diode with two splitting gates (gate voltages: V_GN and V_GP) for the N⁺ and P⁺ regions, respectively, at the top and bottom of the device. Fig. 7(b) shows a 3D representation of a PIN device with a 50 nm thick ALD Al₂O₃ gate insulator made on a glass substrate using dry transfer and standard photolithography. The n-MoTe₂ area is achieved using the top 30 nm thick ALD layer and a Ti contact for the n terminal,^258,259 whereas the thin p-MoTe₂ region contains Pt contact electrodes, and the p-polarity is improved via polymer CYTOP deposition. The p–n and PIN working modes of our MoTe₂ photodetector are shown graphically in the schematic band diagram (Fig. 7(c)). The p–n mode requires two terminals and simple reverse-bias conditions to work, but the PIN mode combines V_GP and V_GN to generate P⁺, N⁺, and I regions. Based on the gate patterning, the I region comprises a small gap (<1 μm). Consequently, compared to a simple p–n junction, this area is exposed to a greater electric field when biasing is reversed. The remarkable value of R of the PIN-mode MoTe₂ for 1300 nm SWIR photons is this study's most intriguing finding. Fig. 7(d) and (e) display photoinduced current plots for the p–n mode for 1300 nm SWIR radiation with no gate voltage (V_GN = V_GP = 0 V) as a dependence on V_a and optical power density. V_OC and I_SC for the p–n mode are almost zero, but at larger reverse voltages, a modest photocurrent is observed. However, in PIN-mode operation, V_OC, and I_SC are clearly attained at 0.25 V and 50 pA, and a much-improved photo I–V> trend is monitored with varied optical power density. The PIN mode (gating) exhibits a photocurrent and R larger than those of the p–n mode (GND) at a V_a of −1.5 V according to the photocurrent vs. power density visualizations displayed in Fig. 7(f). The 1300 nm SWIR area with a far larger response in the PIN mode than anticipated is still a mystery. It is strongly hypothesized that the Franz–Keldysh effect might cause a decrease in the bandgap in MoTe₂. When the bandgap of 7 nm thick MoTe₂ is about 0.95 eV, or the energy at a wavelength of 1300 nm, efficient absorption always requires an E_g less than 0.95 eV. Thus, the SWIR laser photons at 1300 nm may not be successfully gripped by the material. Because of the Franz–Keldysh effect, it is predicted that E_g may drop if a strong E-field were applied to the same thin MoTe₂.²⁶⁰ In fact, even for the p–n mode, a slightly greater responsivity is produced at a reverse voltage of V_a = −1.5 V than at V_a = 0 V. The E_g of MoTe₂ may be lowered below 0.95 eV (hv_FK) owing to the introduction of a considerably higher E-field in the “I” region of the PIN diode, leading to the efficient absorption of 1300 nm SWIR photons there. We thus conclude that the PIN diode with two split gates, which exhibits greater responsiveness over the full detection range, is innovative and significant for the recognition of 1300 nm SWIR light. Herein, we review a multilayer WSe₂ diode based on a lateral PIN homojunction.²¹⁴ The photodiode is manufactured using particular doping; more precisely, the contact areas are treated with self-aligning surface plasma, while the WSe₂ channel is left intrinsic. Electrical testing of such a diode indicates a perfect rectifying behavior with an ideality factor of 1.14 and a current on/off ratio as high as 1.2 × 10⁶. The diode exhibits good photo-detecting capabilities when working in the photovoltaic mode under 450 nm light irradiation, with a V_oc of 340 mV, R of 0.1A W⁻¹, and a D* of 2.2 × 10¹³ Jones. The lateral PIN design also considerably reduces slow photoresponse dynamics, such as photocarrier diffusion in undepleted areas and photocarrier trapping/de-trapping caused by doping process-induced defect states. Fig. 7(g) generally depicts the final constructed device's three-dimensional structure. The devices described in this study used self-aligned surface plasma processing before metal contact deposition to use the WO_x and WSe_2−y interfacial layers as p-type and n-type dopants separately. A 2 mm diameter homogeneous circular light point lit the entire device region. The output curves, as shown in Fig. 7(h), clearly demonstrate an increase in the current when illuminated. The output characteristics show a significant nonzero I_sc at zero bias voltage when compared with the dark situation curve, with 3.3 mW cm⁻² light illumination yielding a peak value of 0.57 μA. The time-dependent photoresponse was elaborated with 450 nm radiant light, and the rise and fall times are represented in Fig. 7(i), as 264 ns and 552 ns, respectively. This study indicates a significant field for fabricating p–i–n interfaces based on TMDCs by attaining selective doping using a simple applied plasma treatment approach.²¹⁴

Fig. 7 (a) MoTe₂ PIN diode optical microscope photograph and cross-sectional schematic. (b) PIN diode schematic in three dimensions. (c) Schematic band diagrams for the PN and PIN operation modes. (d) Photoinduced logarithmic I–V> curves of the device for the p–n mode at different power densities with a wavelength of 1300 nm. (e) Linear scale representation, which denotes the I_sc and V_oc. (f) Photocurrent I_ph is enhanced with radiant laser power. The blue (PIN mode) and orange (p–n state) traces demonstrate that the PIN mode's photoresponse is significantly greater than that of the p–n state. (f) Photocurrent as a function of power intensity. These figures (a–e) are replicated from ref. 261, copyright, Adv. Opt. Mater. (g) Optical image demonstration of a PIN device based on homojunction WSe₂. The scale bar is 10 micro-meters. (h) WSe₂ PIN junction diode's output characteristics under 450 nm irradiation as the power density varies. (i) Extended photoresponse curve at 20 kHz indicating the rise time (τ_r) and fall time (τ_f). The figures (g–i) are reproduced with the permission of ref. 214, copyright, ACS Nano.

7. Broken gap p–n junction for fast photoresponse

The broken-gap vdw's heterostructure consisting of 2D material favorable for manufacturing fast tuning and low-power consumption appliances acknowledges its charge transfer vs. quantum tunneling phenomenon. Recently, broken gap vdW heterostructures (vdWHs) constructed on multiple SnSe₂ and other 2D material combinations have been created using various tuning techniques (chemical doping, gating coupling, and modification of thickness).^262–264 These heterostructure devices have received attention because of the tunneling processes they possess, which may be utilized to create Esaki diodes and hetero-tunnel field effect transistors (hetero-TFETs).^264,265 However, the interface consisting of SnSe₂ has a wide depletion zone that often hinders operational gain because SnSe₂ has poor mobility and low carrier concentration.^262,265 When compared to SnSe₂, BP has higher mobility and higher hole carriers,²⁶⁶ making it an excellent channel material for creating broken gap vdWHs. Consequently, devices produced from 2D BP are becoming increasingly desirable.^267,268 To build multivalued logic and binary inverters, for instance, broken-gap vdWHs made of MoS₂ and BP have been constructed for particularly effective properties with a similar rectification ratio (5 × 10⁵) and high current on/off ratio (10⁷).²⁶⁸ However, BP experiences ecological instability. Additionally, for a vdWH heterodiode, a VB or CB offset among the two stacking materials affects the rectifying operation in addition to the Fermi level.²⁶⁸ However, the aforementioned vdWH frequently has a very tiny CB offset in which the forward and reverse currents simultaneously grow, limiting the rectification ability of the device. Additionally, a photodetector's enhanced drain current results in a significant dark current, which further lowers the ratio of photocurrent on to photocurrent off.^101,269

We considered a broken-gap vdWH that formed a significant CB offset to block forward current by combining a thick n-type PtS₂ with a thin p-type WSe₂.²⁷⁰ Consequently, the heterodiode's rectification ratio may be significantly improved. Through photo-dominated tunneling transport, the forward photocurrent may be substantially enhanced under low illumination. Additionally, its photogenerated tunneling electrons may easily mix with the opposing majority hole carriers in the WSe₂ accumulation zone, thereby reducing the interface trapping effect and considerably enhancing the reaction speed. Fig. 8(a) illustrates a schematic of a PtS₂–WSe₂-based broken gap heterojunction device under illumination. Fig. 8(b) displays the heterodiode device's I_ds–V_ds properties on a logarithmic scale at different gate voltages. The amazing thing is that an evident backward diode was created using ultralow forward currents of less than 0.2 pA and extremely large reverse currents of up to 20 μA. Thus, an extremely high reverse rectification ratio of close to 10⁸ may be estimated using the reverse-to-forward current ratio. This heterodiode had a greater rectification ratio of many orders of magnitude when compared to previous reverse diodes made using traditional bulk materials and other 2D materials, as described in previous studies.^271–274 The surface potential across PtS₂ and WSe₂ was measured using Kelvin probe force microscopy (KPFM) to better comprehend the tunnel operation of the PtS₂/WSe₂ broken gap vdWH heterodiode. According to the KPFM finding, when PtS₂ and WSe₂ were arranged together, the WSe₂ electrons flowed into PtS₂ and attained an equilibrium state, creating more electrons in PtS₂ and holes in WSe₂. Owing to the formation of the bilateral accumulation zone at the interface, broken-gap band alignments with significant band bending may emerge. Fig. 8(e) illustrates how, at a low negative bias, electrons move from a WSe₂ valence band maximum (VBM) to a PtS₂ conduction band minimum (CBM) via band-to-band tunneling (BTBT). The thick PtS₂, which has mobility, can act as a useful carrier-selective contact, which helps to increase the reverse current. As shown in Fig. 8(d), if the negative bias voltage is increased, the energy band of WSe₂ continues to increase and the energy band of PtS₂ decreases, creating a wide tunneling window and a greater reverse current. The decrease in the energy band of WSe₂ and the increase in PtS₂ cause the tunneling window to disappear when a positive bias voltage is applied. Meanwhile, extreme band bending (Fig. 8(f)) restricts hot carrier emission. Thus, with a positive bias voltage, ultrasmall forward currents may be achieved. Additionally, the approximate formula for the tunneling current likelihood in 2D materials is

where ħ and W denote the decreased Planck constant and the barrier width, respectively. Here, m* represents effective and qV_D − E represents the tunneling barrier height of the electron with energy E.²⁷⁵ We may infer from the aforementioned equation that the tunneling probability varies exponentially with 1/W. As was already established, thick PtS₂ exhibits little modulation, whereas comparatively thin WSe₂ may efficiently modify the position of the Fermi level with applied gate voltage (V_bg). The Fermi level variance between PtS₂ and WSe₂ typically reduces by improving the negative back-gate voltage, resulting in a lower barrier width. A small, precisely focused laser beam is also employed to generate spatially resolved photocurrent mapping with zero bias. The heterostructure area and separated PtS₂ and WSe₂ flakes are included in the photocurrent mapping together with the photoresponse at any given place on the whole device. As shown in Fig. 8(c), practically all the photocurrents were derived in the heterojunction area. Furthermore, the photoresponse of the PtS₂/WSe₂ heterodiode was investigated. As illustrated in Fig. 8(g), it can be observed that the drain current may swiftly change to the corresponding permanent value when the laser is in the on/off state, suggesting that the permanence and responsiveness are excellent at various power densities. The related acquired photoresponsivity of 1.7 A W⁻¹ and value of D* of 3.8 × 10¹⁰ Jones were obtained.²⁷⁶ The study emphasizes the heterodiode's broad-bandwidth optical detection capacity, which suggests a wide range of recognition from visible light to near-IR light for future practical appliances.

Fig. 8 (a) Schematic representing the tunneling diode based on broken gap Vdw's heterojunction (PtS₂–WSe₂). (b) I_dsvs. V_ds curves with respect to gate voltages in logarithmic scale. (c) Scanning photocurrent mapping with a laser wavelength of 532 nm at V_ds = 0 and V_g = 0. (d–f) Energy band diagram at different biases of voltage (V_ds). (g) Photo current of the heterostructure-based device. These figures (a–g) are replicated from ref. 270, copyright, ACS Nano, 2021. (h) Schematic of the device based on materials MoTe₂–SnSe₂. (i) Optical representation of the SnSe₂/MoTe₂ electronic device. (j) Photovoltaic effect based on SnSe₂/MoTe₂ heterostructure. I_ds–V_ds curves at 0.2 mW laser power at various V_gs demonstrate variability in the photovoltaic effect. This figure is replicated from ref. 277, copyright, Nano Lett.

A broken gap in the heterostructures of SnSe₂ and MoTe₂ with gate-controlled interface topologies was explored owing to their electrical and optoelectronic features.²⁷⁷ This device can function as an Esaki diode and a reverse diode according to the interband tunneling current, and at room temperature, its peak-to-valley current ratio is close to 5.7. Additionally, the heterojunction displays a detectivity of up to 7.5 × 10¹² Jones when subjected to an 811 nm laser beam. The SnSe₂/MoTe₂ heterostructure device is schematically shown in Fig. 8(h), along with top representations of the atomic structures of 1T-SnSe₂ and 2H-MoTe₂.^278–280 An optical representation of the SnSe₂/MoTe₂ heterojunction is presented in Fig. 8(i). Furthermore, the photovoltaic effect of the SnSe₂/MoTe₂ heterojunctions was observed. Fig. 8(j) displays the I_ds–V_ds curves of the 811 nm laser-irradiated device at various V_gs ranging from −40 to 40 V, illustrating its photovoltaic response. Particularly, the change in gate bias from −40 to 40 V caused the V_oc to shift from −100 to 20 mV. As the built-in potentials in n-type materials are larger than those in p-type materials, the V_OC (photovoltaic effect) is caused by the intrinsic potential at the p–n interface and is often detected under various drain biases. Additionally, it is noteworthy that a photovoltaic effect with a fill factor greater than 41% underscored the tremendous scope of optoelectronic applications. This work not only gives a thorough grasp of broken-band orientation and its potential uses but also exhibits outstanding performance in optoelectronics predicated on the SnSe₂/MoTe₂ heterojunction.

8. Self-powered p–n junction for photodetection

Polarization-sensitive detectors based on p–n have attracted increased attention as an essential application scenario owing to their suitability for light detection in complicated settings.^{10,11,281,282} The polarization-sensitive 2D photodetector extracts polarization information from radiant light and shields the impurity signal to progress the signal-to-noise ratio, which necessitates the application of semiconductor materials possessing an inherent photoresponse to polarized light, such as BP, GeAs, ReS₂, and GeSe.²⁸³ The polarization ratio was increased to more than 22 by fabricating more BP/MoS₂ heterojunction photodiodes.²⁸⁴ Wang et al. revealed that the unique GeSe-based polarized photodetector responded anisotropically to 532, 638, and 808 nm light, with an optimal polarization ratio of 2.16 at 808 nm.²⁸⁵ Furthermore, many polarized photodetectors constructed using 2D-layered materials limit themselves to the visible band, restricting their angle-dependent optoelectronic applications in the IR region.²⁸⁵ Furthermore, the polarization sensitivity for certain 2D materials, such as SnS and ReS₂, is very low owing to their intrinsic smaller mobility and high electron–hole recombination rate.²⁸⁵ A heterostructure composed of multilayer GeSe and 2H-MoTe₂ was developed to provide polarization-sensitive and self-powered photodetection with outstanding implementation in the near-infrared spectral range, as discussed below.¹²¹ Owing to the low bandgap of both elements, the detection spectrum ranges from visible to near-IR light (405–1310 nm) at zero bias. The device has a rapid reaction time of 26 ms at a particular near-IR wavelength of 808 nm and an excellent detectivity of 4.1 × 10¹¹ Jones. Using GeSe's anisotropic optical absorption, the resulting heterojunction device obtained substantial polarization sensitivities of 5.4 and 1.8 at 635 and 1310 nm, respectively. Using the assembly of anisotropic/isotropic 2D semiconductors, the self-powered and polarization-sensitive photoresponse in the near-IR range can allow polarization-related optoelectronic presentations in IC's. Fig. 9(a) depicts the schematic design of the GeSe/MoTe₂ heterojunction device. Fig. 9(b) depicts the current–voltage (I–V>) properties of GeSe/MoTe₂ vdW's heterojunctions in logarithmic and linear scales under dark conditions. This diode exhibits evident current rectification conduct with a rectification ratio of roughly 300. The ideality factor is determined to be η = 1.24, which is near to η = 1, indicating that this device has a good-quality connection and that diffusion current, not recombination current, controls the rectifying performance of GeSe/MoTe₂ devices.²⁸⁶ The time-resolved photoresponses under visible (405 and 635 nm) and near-IR (808, 980, 1064, and 1310 nm) light stimulation were implemented to test the device's self-powered photodetection capabilities, as illustrated in Fig. 9(c) at zero bias. This demonstrates a rapid and strong photocurrent with a wide spectral coverage range of 405 nm–1310 nm. It is also notable that the photoswitching ratio in infrared light can surpass 10⁴ owing to the ultralow dark current of less than 0.1 pA in the self-driven state. To quantify the optoelectronic performance of the GeSe/MoTe₂ heterostructure, the device was evaluated using a particular near-IR light with a wavelength of 808 nm. Fig. 9(d) and (e) show the I–V> curves of the heterostructure device under 808 nm laser irradiation with an improvement in light power density from 1.25 to 437.3 mW cm⁻² on linear and logarithmic scales. This GeSe/MoTe₂ vdW heterojunction exhibits both a sensitive photoresponse and a significant photovoltaic effect. The photoresponse is owing to the high absorption coefficients of GeSe and MoTe₂ at 808 nm light, but the substantial photovoltaic effect is caused by the built-in electric field in the GeSe/MoTe₂ heterojunction's depletion zone. Both the V_oc and I_sc increase when an 808 nm laser with enhanced optical power density is used, as shown in Fig. 9(f), with a maximum I_sc of 4.58 nA and a maximum V_oc of 0.43 V attained under strong light power. The following equation is utilized: I_sc = cP_in^λ, where I_sc denotes a power law connection with a light power density (P_in), c denotes the proportionality constant and λ denotes the fitting exponent. The λ is observed to be ≈0.7, suggesting that the defect level trapping charge behavior affects the photoexcited carriers in the same way as it does in other p–n heterojunctions.²⁸⁷ This work produces vdW heterojunctions with low bandgaps composed of in-plane anisotropic GeSe layers and isotropic MoTe₂ layers, suggesting an efficient design for self-powered, broadband, and polarization-sensitive photodetection applications.

Fig. 9 (a) Schematic design of the GeSe/MoTe₂ structure-based device. (b) The I-V curve of the device is linear and log (coordinates) under dark conditions. (c) Time-resolved photoresponse of the device under multiple wavelengths of light illumination. (d) GeSe/MoTe₂ heterojunction device I-V curves. (e) Logarithmic scale. At room temperature, I_sc and V_oc are proportional to the laser power density. (f) V_oc and I_sc dependency on light power density. Figure a–f is reproduced with the permission of ref. 121, copyright, Adv. Mater. Interfaces, 2022. (g) GeSe–SnS₂ hetero-structured illustrated graphically in three dimensions. (h) GeSe/SnS₂ photodetectors are dependent on the time photocurrent at V_bias = 0 V and various light power densities at 532 nm. (i) Consequence of light power density, the responsivity (R) and I_ph/I_dark ratio. This figure is replicated from ref. 288, copyright, Adv. Mater. Interfaces.

Here, the vertically stacked multilayer GeSe/SnS₂ design adjustable current transport and autonomous optoelectrical characteristics have been revealed.²⁸⁸ In addition, owing to the huge band offset and the effective carrier separation method, the device's photovoltaic characteristics include a very less dark current of 30 fA, maximum responsiveness of 130 mA W⁻¹, and a high I_on/I_off ratio of 105 at 532 nm. GeSe/SnS₂ vdW's heterojunction-based device is shown in 3D in Fig. 9(g). Because of its increased state density, excessive charge storage ability, and the existence of charge-neutral sites (i.e., an equal number of electrons and holes), multilayer graphene (MLG) specifically serves as a bottom gate semimetal.^289,290 Smooth h-BN can aid in trapping residual charges effectively and decreasing surface trap scattering compared to SiO₂ with surface dangling bonds, resulting in more effective control of the transport current at less power use.²⁹¹ The time-resolved photoresponse at various light power densities with V_bias = 0 V is shown in Fig. 9(h). Owing to fewer trap states at the GeSe/SnS₂ junction, it can be shown that the photo-switching mechanism is robust and reproducible.²⁸⁶ Because of an increase in Auger recombination sites, R_{532 nm} typically decreases in Fig. 9(i) (left axis) from 130 to 66.7 mA W⁻¹ when the light power density increases from 4.1 to 101.48 mW cm⁻². The photocurrent at 532 nm in Fig. 9(i) (right axis), which exhibits a sublinear behavior with an exponent of 0.87, is effectively fitted with a power law with the following formula: I_ph ∝ P^α (where P is the radiant light power densities and the exponent defines photoelectric conversion efficiency). This shows the heterojunction's capacity to effectively transform incident photons into electron–hole pairs. The exponent α < 1 reveals that additional Auger recombination sites and trapped states affect photogenerated carriers throughout the transport period. It has been discovered that the GeSe/SnS₂ vdWs device polarity-switchable conduct and self-driven photodetection ability can, hopefully, extend and improve forthcoming multipurpose-integrated devices. The summarized data related to the self-powered junction are described in Table 3.

Table 3 A recently outlined self-powered photodetector's salient characteristics

Materials	Wavelength (λ)	Responsivity (R)	Detectivity (D*) Jones	Response time	Ref.
GeSe/MoS2	532 nm	105 mA W^−1	1.46 × 10^10	—	286
MoTe2/MoS2	473 nm	150 mA W^−1	—	6.8 × 10^−3 s	292
BP/InSe	455 nm	11.7 mA W^−1	—	32 × 10^−3 s	293
Se/WS2	405 nm	196 mA W^−1	1.3 × 10^10	—	294
SnS/InSe	635 nm	215 mA W^−1	1.05 × 10^10	4.2 s	295
GaTe/InSe	405 nm	13.8 mA W^−1	—	20 × 10^−3 s	93
AsP/MoS2	520 nm	300 mA W^−1	—	9 × 10^−6 s	274
Te/WS2	635 nm	471 mA W^−1	1.24 × 10^−10	14.7 × 10^−3 s	296
WSe2/Ta2NiSe5/WSe2	635 nm	436 mA W^−1	0.99 × 10^12	0.46 × 10^−3 s	297
WSe2/Ta2NiSe5/WSe2	808 nm	37.22 A W^−1	8.44 × 10^10	—	297
α-In2Se3/PtSe2	405–1550 nm	0.08 A W^−1	5 × 10^11	127/104 μs	298
PtTe2/MoTe2	635 nm	1600 mA W^−1	8.16 × 10^11	23 μs	299
	1310 nm	1200 mA W^−1	5.90 × 10^11	23 μs
SnS2/MXene Nb2C	365 nm	102.44 μA W^−1	7.48 × 10^12	—	300

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9. The p–n junction for flexible electronics

Owing to their remarkable qualities, 2D materials provide a foundation that enables the development of flexible functional mechanisms with a range of features. A flexible α-In₂Se₃/WSe₂ vdW's heterostructure interaction with the piezo-phototronic effect is explored, and the device's strain-regulated photo behavior is tested.¹¹⁸ Owing to the noncentrosymmetric construction of the III–VI compound α-In₂Se₃, the strain-induced piezo-potential modifies the band slope close to the p–n junction interface, enhancing the piezo-phototronic effect's capability to separate photogenerated electron–hole pairs and their related transport features. A tensile strain of 0.433% may be used to increase the photocurrent of the device by around 18 times. Additionally, the device exhibited outstanding values of R and D*, which may be as high as 4.61 × 10⁵ A W⁻¹ and 4.34 × 10¹⁴ Jones, respectively. These findings suggest that the strain-controlled vdW p–n hybrid junction photodetector may be made possible by the piezo-phototronic effect, which offers an innovative conception of a 2D material-based vdW heterostructure modification toward outstanding performance elastic optoelectronics.¹¹⁸ The schematic of the device based on the structure α-In₂Se₃/WSe₂ is shown in Fig. 10(a). For practical uses, photoresponsivity must be independent of the intensity of the light source. However, as the data were not investigated under comparable conditions, a simple comparison of the photoresponsivity of photodetectors constructed with various architectures is inaccurate. Additionally, the following equation may be used to determine D*: D* = (Af)^0.5/NEP, where A is the photodetector's functional area, f is its electrical bandwidth, and NEP is its noise equivalent power; this information is depicted in Fig. 10(b). At the lowest optical density of 0.069 W cm⁻² and 0.433% tensile strain, the D* decreases under compressive strain and increases with tensile strain, which reaches the highest value of 4.34 × 10¹⁴ Jones. Under continuous on/off illumination, the device's photo-switching properties and time-dependent reliability were evaluated. The recognition of periodic switching photoresponse at zero voltage bias is shown in Fig. 10(c). The augmented I_ph was achieved with a growing tensile strain, while the original I_ph occurred abruptly upon the light's lighting and vanished when the light was switched off. These findings imply that the viability of vdW p–n heterostructures may be extensively extended to various piezoelectric 2D materials, which may offer guidance for producing flexible nano-optoelectronic devices with superior performance.

Fig. 10 (a) Schematic illustration of the p–n diode device under red LED radiant. (b) D*'s dependency on optical power intensity under various strain settings when biased by −2 V. (c) Photoresponse evaluation in the absence of external voltage bias under irradiation of 782 W cm⁻² optical intensity. This figure is replicated from ref. 118, copyright Adv. Opt. Mater., 2021.

10. Challenges, solutions, and future perspectives

The development of innovative p–n interfaces in recent years has used the ultrathin structure of 2D materials. Top-down and bottom-up manufacturing processes have both been competent in producing high-quality p–n junctions with exceptional optoelectronic characteristics. We evaluated the current advances in 2D p–n junctions in this review, including the most often utilized materials and fabrication processes, as well as patterns of 2D p–n junctions from the literature to be appropriate for various distinct junction topologies. Different applications have been made possible by these designs, and we have addressed studies that employ 2D p–n junctions as rectifiers and photodetectors/solar cells, respectively. Finally, a comparison of the significant merit metrics of the various literary techniques is given. The ability to create exceptional new p–n junctions with 2D materials continues to open fascinating scientific avenues for both fundamental research and practical applications. We also conclude that the MoTe₂ PIN diode with two split gates is innovative and exciting for the detection of 1300 nm SWIR radiation, demonstrating improved responsivity throughout the whole recognition range, regardless of the paucity of reports on PIN structures in 2D-like thin TMDC semiconductors. Additionally, we believe that this research may facilitate the integration of Si photodetectors on Si devices.

The incorporation of 2D p–n junctions into electronic appliances still faces several obstacles despite the extensive number of studies included in this review. The manufacturing of 2D p–n devices on a wider scale and the deterioration of 2D materials owing to the environment are the two biggest obstacles. The large-scale manufacture of customized vdW's heterostructures with precise interfaces presents the first major difficulty. VdW's epitaxial techniques provide even more potential for producing 2D heterostructures of superior quality. Combining the development of single 2D materials, such as MoS₂, with numerous doping processes (mainly chemical or electrostatic) provides a second potential method for increasing the construction of 2D p–n junctions. The environmental deterioration of many well-known 2D materials is a second problem. For instance, BP in its ultrathin form has the propensity to absorb moisture when exposed to air, which impairs the substance's electrical characteristics.^301,302 The most widely acknowledged method for the deterioration of BP includes the material's interaction with oxygen, which modifies the material's characteristics.^302,303 Encasing the air-sensitive substance among two flakes of h-BN in an environment free of oxygen and moisture is one technique to stop this deterioration.^304,305 Therefore, scaling up encapsulating techniques is a focus of 2D materials research. The active search for novel 2D materials that do not have degrading issues and might originate from either synthesis (such as TiS₃) or natural sources is one alternative strategy that is currently being researched (e.g. franckeite).^306,307 In addition to traditional optoelectronic uses, 2D p–n junctions still have an extensive range of unanswered questions and potential applications. For instance, 2D p–n junction thermoelectric applications have not yet been fully studied. The conventional Peltier device, which is mostly used in electronics for cooling (and less often for heating), is composed of a p-type semiconductor and an n-type semiconductor thermally and electrically coupled in parallel. Combining atomically thin cooling (or heating) components with other desirable features of 2D materials, such as great transparency or flexibility, might be accomplished via van der Waals heterostructures. Another intriguing path is researching innovative p–n junction shapes (such as the recently observed circular p–n junctions in graphene^308,309) or cutting-edge 2D p–n junction-based devices, such as memory or logic gates.^82,268 More genuine possibilities than those mentioned are likely still concealed in 2D materials, and 2D p–n junctions have great potential for practical uses. Particularly intriguing as building blocks for flexible and transparent electrical components, such as solar cells or light-emitting diodes, are these 2D junctions.^310,311 The ultrathin nature of 2D p–n junctions makes them useful for nanophotonics as well as applications in light sensing and harvesting.^312,313 2D p–n junctions may be utilized as photodetectors, and several material arrangements are available that can be employed to construct devices sensitive to wavelengths spanning from the IR to ultraviolet range.^236,314,315

TMDs have recently attracted significant interest because of their distinct electronic, optical, and catalytic characteristics. TMDCs are layered materials made up of chalcogen atoms and transition metal atoms sandwiched between them. They display various junctions, such as semiconductor–semiconductor, semiconductor–metal, and metal–metal junctions. TMDCs exhibit intriguing characteristics, such as superconductivity and magnetism, in metal–metal junctions. The underlying mechanisms of these phenomena have been thoroughly studied, and researchers have considered how they might be used in spintronics, energy storage, and quantum computing. TMDC-based metal–semiconductor junctions have also received considerable attention. Researchers have created novel functionalities, such as Schottky barriers, rectifying behavior, and enhanced photoresponse, by fusing various transition metals using semiconducting TMDCs. These junctions show promise for use in photovoltaics, optoelectronics, and electronic devices. Different TMDCs have been used to create semiconductor–semiconductor junctions, and these junctions have been studied owing to their potential to produce heterostructures with specialized electronic properties. Researchers have created interlayer exciton formation, bandgap engineering, and effective charge transfer by stacking layers of various TMDCs. These heterostructures offer the potential to create sensors, photodetectors, and transistors of the future.

In several critical areas, future developments in TMDC junctions are anticipated. To increase the number of feasible junctions and optimize their properties, research on new TMDC materials and their combinations will first continue. Research on novel 2D materials other than TMDCs, such as transition metal oxides and nitrides, is also included to create hybrid junctions with improved functionalities. The second goal is to comprehend the underlying physics governing TMDC junctions and to create precise theoretical simulations and models. Consequently, it is easier to predict and explain the characteristics of TMDC junctions and to design novel devices with the desired functionalities. Additionally, a significant research area is the incorporation of TMDC junctions into usable devices. The creation of functional devices with high performance and reproducibility is made possible by the development of methods for large-scale synthesis, precise positioning, and controlled alignment of TMDC layers. Additionally, the use of TMDC junctions in cutting-edge industries, such as wearable electronics, quantum information processing, and neuromorphic computing, will be investigated. TMDCs are intriguing candidates for these applications owing to their special characteristics and compatibility with flexible substrates. Finally, efforts will be focused on finding solutions for the problems that TMDC junctions face, including environmental stability, contact resistance, and scalability. TMDC junction technology will advance owing to methods for passivating TMDC surfaces, enhancing interface quality, and creating scalable fabrication techniques. Finally, recent developments in TMDC junctions have shown enormous potential for various applications. The achievement of TMDC-based devices with superior performance and new functionalities will result from continued research and development in this field.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through a large group Research Project under grant number RGP.2/575/44.

This work is also supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. RS-2022-00165798).

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