A Dual-Gate MoS2 Photodetector Based on Interface Coupling Effect

Engineering Graphene Phototransistors for High Dynamic Range Applications

Phototransistors are light-sensitive devices featuring a high dynamic range, low-light detection, and mechanisms to adapt to different ambient light conditions. These features are of interest for bioinspired applications such as artificial and restored vision. In this work, we report on a graphene-based phototransistor exploiting the photogating effect that features picowatt- to microwatt-level photodetection, a dynamic range covering six orders of magnitude from 7 to 107 lux, and a responsivity of up to 4.7 × 103 A/W. The proposed device offers the highest dynamic range and lowest optical power detected compared to the state of the art in interfacial photogating and further operates air stably. These results have been achieved by a combination of multiple developments. For example, by optimizing the geometry of our devices with respect to the graphene channel aspect ratio and by introducing a semitransparent top-gate electrode, we report a factor 20-30 improvement in responsivity over unoptimized reference devices. Furthermore, we use a built-in dynamic range compression based on a partial logarithmic optical power dependence in combination with control of responsivity. These features enable adaptation to changing lighting conditions and support high dynamic range operation, similar to what is known in human visual perception. The enhanced performance of our devices therefore holds potential for bioinspired applications, such as retinal implants.

Polarizable Nonvolatile Ferroelectric Gating in Monolayer MoS2 Phototransistors

Given the requirements for power and dimension scaling, modulating channel transport properties using high gate bias is unfavorable due to the introduction of severe leakages and large power dissipation. Hence, this work presents an ultrathin phototransistor with chemical-vapor-deposition-grown monolayer MoS2 as the channel and a 10.2 nm thick Al:HfO2 ferroelectric film as the dielectric. The proposed device is meticulously modulated utilizing an Al:HfO2 nanofilm, which passivates traps and suppresses charge Coulomb scattering with Al doping, efficiently improving carrier transport and inhibiting leakage current. Furthermore, a bipolar pulses excitable polarization method is developed to induce a nonvolatile electrostatic field. The MoS2 channel is fully depleted by the switchable and stable floating gate originating from remanent polarization, leading to a high detectivity of 2.05 × 1011 Jones per nanometer of gating layer (Jones nm-1) and photocurrent on/off ratio >104 nm-1, which are superior to the state-of-the-art phototransistors based on two-dimensional (2D) materials and ferroelectrics. The proposed polarizable nonvolatile ferroelectric gating in a monolayer MoS2 phototransistor promises a potential route toward ultrasensitive photodetectors with low power consumption that boast of high levels of integration.

Recent advances in 2D transition metal dichalcogenide-based photodetectors: a review

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as a highly promising platform for the development of photodetectors (PDs) owing to their remarkable electronic and optoelectronic properties. Highly effective PDs can be obtained by making use of the exceptional properties of 2D materials, such as their high transparency, large charge carrier mobility, and tunable electronic structure. The photodetection mechanism in 2D TMD-based PDs is thoroughly discussed in this article, with special attention paid to the key characteristics that set them apart from PDs based on other integrated materials. This review examines how single TMDs, TMD-TMD heterostructures, TMD-graphene (Gr) hybrids, TMD-MXene composites, TMD-perovskite heterostructures, and TMD-quantum dot (QD) configurations show advanced photodetection. Additionally, a thorough analysis of the recent developments in 2D TMD-based PDs, highlighting their exceptional performance capabilities, including ultrafast photo response, ultrabroad detectivity, and ultrahigh photoresponsivity, attained through cutting-edge methods is provided. The article conclusion highlights the potential for ground-breaking discoveries in this fast developing field of research by outlining the challenges faced in the field of PDs today and providing an outlook on the prospects of 2D TMD-based PDs in the future.

2D Dual Gate Field-Effect Transistor Enabled Versatile Functions

Advanced computing technologies such as distributed computing and the Internet of Things require highly integrated and multifunctional electronic devices. Beyond the Si technology, 2D-materials-based dual-gate transistors are expected to meet these demands due to the ultra-thin body and the dangling-bond-free surface. In this work, a molybdenum disulfide (MoS2 ) asymmetric-dual-gate field-effect transistor (ADGFET) with an In2 Se3 top gate and a global bottom gate is designed. The independently controlled double gates enable the device to achieve an on/off ratio of 106 with a low subthreshold swing of 94.3 mV dec-1 while presenting a logic function. The coupling effect between the double gates allows the top gate to work as a charge-trapping layer, realizing nonvolatile memory (105 on/off ratio with retention time over 104 s) and six-level memory states. Additionally, ADGFET displays a tunable photodetection with the responsivity reaching the highest value of 857 A W-1 , benefiting from the interface coupling between the double gates. Meanwhile, the photo-memory property of ADGFET is also verified by using the varying exposure dosages-dependent illumination. The multifunctional applications demonstrate that the ADGFET provides an alternative way to integrate logic, memory, and sensing into one device architecture.

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

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

Integrated Logic Circuits Based on Wafer-Scale 2D-MoS2 FETs Using Buried-Gate Structures

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) materials, such as molybdenum disulfide (MoS2), stand out due to their atomically thin layered structure and exceptional electrical properties. Consequently, they could potentially become one of the main materials for future integrated high-performance logic circuits. However, the local back-gate-based MoS2 transistors on a silicon substrate can lead to the degradation of electrical characteristics. This degradation is caused by the abnormal effect of gate sidewalls, leading to non-uniform field controllability. Therefore, the buried-gate-based MoS2 transistors where the gate electrodes are embedded into the silicon substrate are fabricated. The several device parameters such as field-effect mobility, on/off current ratio, and breakdown voltage of gate dielectric are dramatically enhanced by field-effect mobility (from 0.166 to 1.08 cm2/V·s), on/off current ratio (from 4.90 × 105 to 1.52 × 107), and breakdown voltage (from 15.73 to 27.48 V) compared with a local back-gate-based MoS2 transistor, respectively. Integrated logic circuits, including inverters, NAND, NOR, AND, and OR gates, were successfully fabricated by 2-inch wafer-scale through the integration of a buried-gate MoS2 transistor array.

High photoresponsivity MoS2phototransistor through enhanced hole trapping HfO2gate dielectric

Phototransistor using 2D semiconductor as the channel material has shown promising potential for high sensitivity photo detection. The high photoresponsivity is often attributed to the photogating effect, where photo excited holes are trapped at the gate dielectric interface that provides additional gate electric field to enhance channel charge carrier density. Gate dielectric material and its deposition processing conditions can have great effect on the interface states. Here, we use HfO2gate dielectric with proper thermal annealing to demonstrate a high photoresponsivity MoS2phototransistor. When HfO2is annealed in H2atmosphere, the photoresponsivity is enhanced by an order of magnitude as compared with that of a phototransistor using HfO2without annealing or annealed in Ar atmosphere. The enhancement is attributed to the hole trapping states introduced at HfO2interface through H2annealing process, which greatly enhances photogating effect. The phototransistor exhibits a very large photoresponsivity of 1.1 × 107A W-1and photogain of 3.3 × 107under low light illumination intensity. This study provides a processing technique to fabricate highly sensitive phototransistor for low optical power detection.

Large area roll-to-roll printed semiconducting carbon nanotube thin films for flexible carbon-based electronics

A universal roll-to-roll (R2R) printing approach was developed to construct large area (8 cm × 14 cm) semiconducting single-walled carbon nanotube (sc-SWCNT) thin films on flexible substrates (such as polyethylene terephthalate (PET), paper, and Al foils) at a printing speed of 8 m min^-1 using highly concentrated sc-SWCNT inks and crosslinked poly-4-vinylphenol (c-PVP) as the adhesion layer. Bottom-gated and top-gated flexible printed p-type TFTs based on R2R printed sc-SWCNT thin films exhibited good electrical properties with a carrier mobility of ∼11.9 cm² V^-1 s^-1, I_on/I_off ratios of ∼10⁶, small hysteresis, and a subthreshold swing (SS) of 70-80 mV dec^-1 at low gate operating voltages (±1 V), and excellent mechanical flexibility. Furthermore, the flexible printed complementary metal oxide semiconductor (CMOS) inverters demonstrated rail-to-rail voltage output characteristics under an operating voltage as low as V_DD = -0.2 V, a voltage gain of 10.8 at V_DD = -0.8 V, and power consumption as low as 0.056 nW at V_DD = -0.2 V. To the best of our knowledge, the electrical properties of the printed SWCNT TFTs (such as I_on/I_off ratio, mobility, operating voltage, and mechanical flexibility) and printed CMOS inverters based on the R2R printed sc-SWCNT active layer in this work are excellent compared to those of R2R printed SWCNT TFTs reported in the literature. Consequently, the universal R2R printing method reported in this work could promote the development of fully printed low-cost, large-area, high-output, and flexible carbon-based electronics.

Photogating Effect-Driven Photodetectors and Their Emerging Applications

Rather than generating a photocurrent through photo-excited carriers by the photoelectric effect, the photogating effect enables us to detect sub-bandgap rays. The photogating effect is caused by trapped photo-induced charges that modulate the potential energy of the semiconductor/dielectric interface, where these trapped charges contribute an additional electrical gating-field, resulting in a shift in the threshold voltage. This approach clearly separates the drain current in dark versus bright exposures. In this review, we discuss the photogating effect-driven photodetectors with respect to emerging optoelectrical materials, device structures, and mechanisms. Representative examples that reported the photogating effect-based sub-bandgap photodetection are revisited. Furthermore, emerging applications using these photogating effects are highlighted. The potential and challenging aspects of next-generation photodetector devices are presented with an emphasis on the photogating effect.

Multiwavelength High-Detectivity MoS2 Photodetectors with Schottky Contacts

Photodetection is one of the vital functions for the multifunctional "More than Moore" (MtM) microchips urgently required by Internet of Things (IoT) and artificial intelligence (AI) applications. The further improvement of the performance of photodetectors faces various challenges, including materials, fabrication processes, and device structures. We demonstrate in this work MoS₂ photodetectors with a nanoscale channel length and a back-gate device structure. With the mechanically exfoliated six-monolayer-thick MoS₂, a Schottky contact between source/drain electrodes and MoS₂, a high responsivity of 4.1 × 10³ A W^-1, and a detectivity of 1.34 × 10¹³ cm Hz^1/2 W^-1 at 650 nm were achieved. The devices are also sensitive to multiwavelength lights, including 520 and 405 nm. The electrical and optoelectronic properties of the MoS₂ photodetectors were studied in depth, and the working mechanism of the devices was analyzed. The photoinduced Schottky barrier lowering (PIBL) was found to be important for the high performance of the phototransistor.

Substrate effects on the speed limiting factor of WSe2 photodetectors

We investigate the time-resolved photoelectric response of WSe₂ crystals on glass and flexible polyimide substrates to determine the effect of a changed dielectric environment on the speed of the photodetectors. We show that varying the substrate material can alter the speed-limiting mechanism: while the detectors on polyimide are RC limited, those on glass are limited by slower excitonic diffusion processes. We attribute this to a shortening of the depletion layer at the metal electrode/WSe₂ interface caused by the higher dielectric screening of glass compared to polyimide. The photodetectors on glass show a tunable bandwidth, which can be increased to 2.6 MHz with increasing the electric field.

A 619-pixel machine vision enhancement chip based on two-dimensional semiconductors

The rapid development of machine vision applications demands hardware that can sense and process visual information in a single monolithic unit to avoid redundant data transfer. Here, we design and demonstrate a monolithic vision enhancement chip with light-sensing, memory, digital-to-analog conversion, and processing functions by implementing a 619-pixel with 8582 transistors and physical dimensions of 10 mm by 10 mm based on a wafer-scale two-dimensional (2D) monolayer molybdenum disulfide (MoS₂). The light-sensing function with analog MoS₂ transistor circuits offers low noise and high photosensitivity. Furthermore, we adopt a MoS₂ analog processing circuit to dynamically adjust the photocurrent of individual imaging sensor, which yields a high dynamic light-sensing range greater than 90 decibels. The vision chip allows the applications for contrast enhancement and noise reduction of image processing. This large-scale monolithic chip based on 2D semiconductors shows multiple functions with light sensing, memory, and processing for artificial machine vision applications, exhibiting the potentials of 2D semiconductors for future electronics.

Mixed-Dimensional Anti-ambipolar Phototransistors Based on 1D GaAsSb/2D MoS2 Heterojunctions

The incapability of modulating the photoresponse of assembled heterostructure devices has remained a challenge for the development of optoelectronics with multifunctionality. Here, a gate-tunable and anti-ambipolar phototransistor is reported based on 1D GaAsSb nanowire/2D MoS₂ nanoflake mixed-dimensional van der Waals heterojunctions. The resulting heterojunction shows apparently asymmetric control over the anti-ambipolar transfer characteristics, possessing potential to implement electronic functions in logic circuits. Meanwhile, such an anti-ambipolar device allows the synchronous adjustment of band slope and depletion regions by gating in both components, thereby giving rise to the gate-tunability of the photoresponse. Coupled with the synergistic effect of the materials in different dimensionality, the hybrid heterojunction can be readily modulated by the external gate to achieve a high-performance photodetector exhibiting a large on/off current ratio of 4 × 10⁴, fast response of 50 μs, and high detectivity of 1.64 × 10¹¹ Jones. Due to the formation of type-II band alignment and strong interfacial coupling, a prominent photovoltaic response is explored in the heterojunction as well. Finally, a visible image sensor based on this hybrid device is demonstrated with good imaging capability, suggesting the promising application prospect in future optoelectronic systems.

Enhancing and Broadening the Photoresponse of Monolayer MoS2 Based on Au Nanoslit Array

Two-dimensional molybdenum disulfide (MoS₂), featuring unique optoelectronic properties, has attracted tremendous interest in developing novel photodetection devices. However, the limited light absorption and small carrier transport rate of the monolayer MoS₂ result in low photoresponse, and the large band gap limits its detection range in the visible region. In this study, we propose a nanoslit array-MoS₂ hybrid device architecture with enhanced and broadened photoresponse. The nanoslit array can localize free-space light to achieve strong interactions with MoS₂, and acts as the channel to improve charge transport. As a result, the Au nanoslit array-MoS₂ hybrid detector exhibits a nearly 100-fold increase in photocurrent compared to the pure MoS₂ device. More importantly, the hybrid device can broaden the photoresponse to the optical communication band of 1550 nm which is lower than the band gap of MoS₂, by efficiently utilizing the hot carriers generated by the Au nanoslits. The experimental results are supported by both theoretical analysis and numerical simulation. Since our demonstration leverages the engineering of the hybrid photodetectors with metal nanostructures rather than semiconductor materials, it should be universal and applicable to other devices for broadband, high-efficiency photoelectric conversion.

Wafer-Scale Demonstration of MBC-FET and C-FET Arrays Based on Two-Dimensional Semiconductors

Two-dimentional semiconductors have shown potential applications in multi-bridge channel field-effect transistors (MBC-FETs) and complementary field-effect transistors (C-FETs) due to their atomic thickness, stackability, and excellent electrical properties. However, the exploration of MBC-FET and C-FET based on large-scale 2D semiconductors is still lacking. Here, based on a reliable vertical stacking of wafer-scale 2D semiconductors, large-scale MBC-FETs and C-FETs using n-type MoS₂ and p-type MoTe₂ are successfully fabricated. The drive current of an MBC-FET with two layers of MoS₂ channel (20 µm/10 µm) is up to 60 µA under 1 V bias. Compared with the single-gate MoS₂ FET, the carrier mobility of MBC-FET is 2.3 times higher and the sub-threshold swing is 70% smaller. Furthermore, NAND and NOR logic circuits are also constructed based on two vertically stacked MoS₂ channels. Then, C-FET arrays are fabricated by 3D integrating n-type MoS₂ FET and p-type MoTe₂ FET, which exhibit a voltage gain of 7 V/V when V_DD  = 4 V. In addition, this C-FET device can directly convert light signals to an electrical digital signal within a single device. The demonstration of MBC-FET and C-FET based on large-scale 2D semiconductors will promote the application of 2D semiconductors in next-generation circuits.

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

The evolutionary success in information technology has been sustained by the rapid growth of sensor technology. Recently, advances in sensor technology have promoted the ambitious requirement to build intelligent systems that can be controlled by external stimuli along with independent operation, adaptivity, and low energy expenditure. Among various sensing techniques, field-effect transistors (FETs) with channels made of two-dimensional (2D) materials attract increasing attention for advantages such as label-free detection, fast response, easy operation, and capability of integration. With atomic thickness, 2D materials restrict the carrier flow within the material surface and expose it directly to the external environment, leading to efficient signal acquisition and conversion. This review summarizes the latest advances of 2D-materials-based FET (2D FET) sensors in a comprehensive manner that contains the material, operating principles, fabrication technologies, proof-of-concept applications, and prototypes. First, a brief description of the background and fundamentals is provided. The subsequent contents summarize physical, chemical, and biological 2D FET sensors and their applications. Then, we highlight the challenges of their commercialization and discuss corresponding solution techniques. The following section presents a systematic survey of recent progress in developing commercial prototypes. Lastly, we summarize the long-standing efforts and prospective future development of 2D FET-based sensing systems toward commercialization.

Engineering Top Gate Stack for Wafer-Scale Integrated Circuit Fabrication Based on Two-Dimensional Semiconductors

In recent years, two-dimensional (2D) semiconductors have attracted considerable attention from both academic and industrial communities. Recent research has begun transforming from constructing basic field-effect transistors (FETs) into designing functional circuits. However, device processing remains a bottleneck in circuit-level integration. In this work, a non-destructive doping strategy is proposed to modulate precisely the threshold voltage (V_TH) of MoS₂-FETs in a wafer scale. By inserting an Al interlayer with a varied thickness between the high-k dielectric and the Au top gate (TG), the doping could be controlled. The full oxidation of the Al interlayer generates a surplus of oxygen vacancy (Vo) in the high-k dielectric layer, which further leads to stable electron doping. The proposed strategy is then used to optimize an inverter circuit by matching the electrical properties of the load and driver transistors. Furthermore, the doping strategy is used to fabricate digital logic blocks with desired logic functions, which indicates its potential to fabricate fully integrated multistage logic circuits based on wafer-scale 2D semiconductors.

A floating gate negative capacitance MoS2 phototransistor with high photosensitivity

Monolayer MoS₂ exhibits interesting optoelectronic properties that have been utilized in applications such as photodetectors and light emitting diodes. For image sensing applications, improving the light sensitivity relies on achieving a low dark current that enables the detection weak light signals. Although previous reports on improving the detectivity have been explored with heterostructures and pn junction devices, some of these approaches lack CMOS compatibility processing and sufficient low dark current suppression. Steep slope transistors that overcome the Boltzmann tyranny can further enhance the performance in photodetectors by providing efficient extraction of photogenerated charges. Here, we report a monolayer MoS₂ floating gate negative capacitance phototransistor with the integration of a hafnium-zirconium oxide ferroelectric capacitor. In this study, a SS_min of 30 mV dec^-1, very low dark currents of 10^-13-10^-14 A, and a high detectivity of 7.2 × 10¹⁵ cm Hz^1/2 W^-1 were achieved under weak light illumination due to an enhancement in the photogating effect. In addition, its potential as an optical memory and as an optical synapse with excellent long-term potentiation characteristics in an artificial neural network was also explored. Overall, this device structure offers high photosensitivity to weak light signals for future low-powered optoelectronic applications.

Mechanism, Material, Design, and Implementation Principle of Two-Dimensional Material Photodetectors

Two-dimensional (2D) materials may play an important role in future photodetectors due to their natural atom-thin body thickness, unique quantum confinement, and excellent electronic and photoelectric properties. Semimetallic graphene, semiconductor black phosphorus, and transition metal dichalcogenides possess flexible and adjustable bandgaps, which correspond to a wide interaction spectrum ranging from ultraviolet to terahertz. Nevertheless, their absorbance is relatively low, and it is difficult for a single material to cover a wide spectrum. Therefore, the combination of phototransistors based on 2D hybrid structures with other material platforms, such as quantum dots, organic materials, or plasma nanostructures, exhibit ultra-sensitive and broadband optical detection capabilities that cannot be ascribed to the individual constituents of the assembly. This article provides a comprehensive and systematic review of the recent research progress of 2D material photodetectors. First, the fundamental detection mechanism and key metrics of the 2D material photodetectors are introduced. Then, the latest developments in 2D material photodetectors are reviewed based on the strategies of photocurrent enhancement. Finally, a design and implementation principle for high-performance 2D material photodetectors is provided, together with the current challenges and future outlooks.

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

High performance photodetectors based on van der Waals heterostructures (vdWHs) are crucial to developing micro-nano-optoelectronic devices. However, reports show that it is difficult to balance fast response and high sensitivity. In this work, we design a photovoltaic field-effect photodiode (PVFED) based on the WSe₂/MoS₂/WSe₂ double vdWHs, where the photovoltage that originated from one vdWH modulates the optoelectronic characteristics of another vdWH. The proposed photodiode exhibits an excellent self-powered ability with a high responsivity of 715 mA·W^-1 and fast response time of 45 μs. This work demonstrates an efficient method that optimizes the photoelectric performance of vdWH by introducing the photovoltaic field effect.

Tunable Linearity of High-Performance Vertical Dual-Gate vdW Phototransistors

Layered 2D semiconductors have been widely exploited in photodetectors due to their excellent electronic and optoelectronic properties. To improve their performance, photogating, photoconductive, photovoltaic, photothermoelectric, and other effects have been used in phototransistors and photodiodes made with 2D semiconductors or hybrid structures. However, it is difficult to achieve the desired high responsivity and linear photoresponse simultaneously in a monopolar conduction channel or a p-n junction. Here, dual-channel conduction with ambipolar multilayer WSe₂ is presented by employing the device concept of dual-gate phototransistor, where p-type and n-type channels are produced in the same semiconductor using opposite dual-gating. It is possible to tune the photoconductive gain using a vertical electric field, so that the gain is constant with respect to the light intensity-a linear photoresponse, with a high responsivity of ≈2.5 × 10⁴ A W^-1 . Additionally, the 1/f noise of the device is kept at a low level under the opposite dual-gating due to the reduction of current and carrier fluctuation, resulting in a high detectivity of ≈2 × 10¹³ Jones in the linear photoresponse regime. The linear photoresponse and high performance of the dual-gate WSe₂ phototransistor offer the possibility of achieving high-resolution and quantitative light detection with layered 2D semiconductors.

Fast and Low-Cost Synthesis of MoS2 Nanostructures on Paper Substrates for Near-Infrared Photodetectors

Recent advances in the production and development of two-dimensional transition metal dichalcogenides (2D TMDs) allow applications of these materials, with a structure similar to that of graphene, in a series of devices as promising technologies for optoelectronic applications. In this work, molybdenum disulfide (MoS2) nanostructures were grown directly on paper substrates through a microwave-assisted hydrothermal synthesis. The synthesized samples were subjected to morphological, structural, and optical analysis, using techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman. The variation of synthesis parameters, as temperature and synthesis time, allowed the manipulation of these nanostructures during the growth process, with alteration of the metallic (1T) and semiconductor (2H) phases. By using this synthesis method, two-dimensional MoS2 nanostructures were directly grown on paper substrates. The MoS2 nanostructures were used as the active layer, to produce low-cost near-infrared photodetectors. The set of results indicates that the interdigital MoS2 photodetector with the best characteristics (responsivity of 290 mA/W, detectivity of 1.8 × 109 Jones and external quantum efficiency of 37%) was obtained using photoactive MoS2 nanosheets synthesized at 200 °C for 120 min.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.