Broadband, Ultra-High-Responsive Monolayer MoS2/SnS2 Quantum-Dot-Based Mixed-Dimensional Photodetector

Two-Dimensional Transition Metal Dichalcogenides (2D TMDs) Coupled With Zero-Dimensional Nanomaterials (0D NMs) for Advanced Photodetection

The integration of 2D transition metal dichalcogenides (TMDs) with other materials presents a promising approach to overcome inherent limitations and enable the development of novel functionalities. In particular, 0D nanomaterials (0D NMs) offer notable advantages for photodetection, including broadband light absorption, size-dependent optoelectronic properties, high quantum efficiency, and good compatibility. Herein, the integration of 0D NMs with 2D TMDs to develop high-performance photodetectors is reviewed. The review provides a comprehensive overview of different types of 0D NMs, including plasma nanoparticles (NPs), up-conversion NPs, quantum dots (QDs), nanocrystals (NCs), and small molecules. The discussion starts with an analysis of the mechanism of 0D NMs on 2D TMDs in photodetection, exploring various strategies for improving the performance of hybrid 2D TMDs/0D NMs. Recent advancements in photodetectors combining 2D TMDs with 0D NMs are investigated, particularly emphasizing critical factors such as photosensitivity, photogain, specific detectivity, and photoresponse speed. The review concludes with a summary of the current status, highlighting the existing challenges and prospective developments in the advancement of 0D NMs/2D TMDs-based photodetectors.

Recent Progress in Photodetectors: From Materials to Structures and Applications

Photodetectors are critical components in a wide range of applications, from imaging and sensing to communications and environmental monitoring. Recent advancements in material science have led to the development of emerging photodetecting materials, such as perovskites, polymers, novel two-dimensional materials, and quantum dots, which offer unique optoelectronic properties and high tunability. This review presents a comprehensive overview of the synthesis methodologies for these cutting-edge materials, highlighting their potential to enhance photodetection performance. Additionally, we explore the design and fabrication of photodetectors with novel structures and physics, emphasizing devices that achieve high figure-of-merit parameters, such as enhanced sensitivity, fast response times, and broad spectral detection. Finally, we discuss the demonstration of new applications enabled by these advanced photodetectors, including flexible and wearable devices, next-generation imaging systems, and environmental sensing technologies. Through this review, we aim to provide insights into the current trends and future directions in the field of photodetection, guiding further research and development in this rapidly evolving area.

Gate modulation of barrier height of unipolar vertically stacked monolayer ReS2/MoS2 heterojunction

This study investigates vertically stacked CVD grown ReS2/MoS2 unipolar heterostructure device as Field Effect Transistor (FET) device wherein ReS2 on top acts as drain and MoS2 at bottom acts as source. The electrical measurements of ReS2/MoS2 FET device were carried out and variation in Ids (drain current) Vs Vds (drain voltage) for different Vgs (gate voltage) revealing the n-type device characteristics. Furthermore, the threshold voltage was calculated at the gate bias voltage corresponding to maximum transconductance (gm) value which is ~ 12 V. The mobility of the proposed ReS2/MoS2 heterojunction FET device was calculated as 60.97 cm2 V-1 s-1. The band structure of the fabricated vDW heterostructure was extracted utilizing ultraviolet photoelectron spectroscopy and the UV-visible spectroscopy revealing the formation of 2D electron gas (2DEG) at the ReS2/MoS2 interface which explains the high carrier mobility of the fabricated FET. The field effect behavior is studied by the modulation of the barrier height across heterojunction and detailed explanation is presented in terms of the charge transport across the heterojunction.

Enhancing 2D Photonics and Optoelectronics with Artificial Microstructures

By modulating subwavelength structures and integrating functional materials, 2D artificial microstructures (2D AMs), including heterostructures, superlattices, metasurfaces and microcavities, offer a powerful platform for significant manipulation of light fields and functions. These structures hold great promise in high-performance and highly integrated optoelectronic devices. However, a comprehensive summary of 2D AMs remains elusive for photonics and optoelectronics. This review focuses on the latest breakthroughs in 2D AM devices, categorized into electronic devices, photonic devices, and optoelectronic devices. The control of electronic and optical properties through tuning twisted angles is discussed. Some typical strategies that enhance light-matter interactions are introduced, covering the integration of 2D materials with external photonic structures and intrinsic polaritonic resonances. Additionally, the influences of external stimuli, such as vertical electric fields, enhanced optical fields and plasmonic confinements, on optoelectronic properties is analysed. The integrations of these devices are also thoroughly addressed. Challenges and future perspectives are summarized to stimulate research and development of 2D AMs for future photonics and optoelectronics.

High-performance flexible resistive random-access memory based on SnS2 quantum dots with a charge trapping/de-trapping effect

The application of resistive random-access memory (RRAM) in storage and neuromorphic computing has attracted widespread attention. Benefitting from the quantum effect, transition metal dichalcogenides (TMD) quantum dots (QDs) exhibit distinctive optical and electronic properties, which make them promising candidates for emerging RRAM. Here, we show a high-performance forming-free flexible RRAM based on high-quality tin disulfide (SnS2) QDs prepared by a facile liquid phase method. The RRAM device demonstrates high flexibility with a large on/off ratio of ∼106 and a long retention time of over 3 × 104 s. The excellent switching behavior of the memristor is elucidated by a charge trapping/de-trapping mechanism where the SnS2 QDs act as charge trapping centers. This study is of significance for the understanding and development of TMD QD-based flexible memristors.

Boosting the UV-vis-NIR Photodetection Performance of MoS2 through the Cavity Enhancement Effect and Bulk Heterojunction Strategy

Navigating more effective methods to enhance the photon utilization of photodetectors poses a significant challenge. This study initially investigates the impact of morphological alterations in 2H-MoS2 on photodetector (PD) performance. The results reveal that compared to layered MoS2 (MoS2 NLs), MoS2 nanotubes (MoS2 NTs) impart a cavity enhancement effect through multiple light reflections. This structural feature significantly enhances the photodetection performance of the MoS2-based PDs. We further employ the heterojunction strategy to construct Y-TiOPc NPs:MoS2 NTs, utilizing Y-TiOPc NPs (Y-type titanylphthalocyanine) as the vis-NIR photosensitizer and MoS2 NTs as the photon absorption enhancer. This approach not only addresses the weak absorption of MoS2 NTs in the near-infrared region but also enhances carrier generation, separation, and transport efficiency. Additionally, the band bending phenomenon induced by trapped-electrons at the interface between ITO and the photoactive layer significantly enhances the hole tunneling injection capability from the external circuit. By leveraging the synergistic effects of the aforementioned strategies, the PD based on Y-TiOPc NPs:MoS2 NTs (Y:MT-PD) exhibits superior photodetection performance in the wavelength range of 365-940 nm compared to MoS2 NLs-based PD and MoS2 NTs-based PD. Particularly noteworthy are the peak values of key metrics for Y:MT-PD, such as EQE, R, and D* that are 4947.6%, 20588 mA/W, and 1.94 × 1012 Jones, respectively. The multiperiod time-resolved photocurrent response curves of Y:MT-PD also surpass those of the other two PDs, displaying rapid, stable, and reproducible responses across all wavelengths. This study provides valuable insights for the further development of photoactive materials with a high photon utilization efficiency.

Band Alignment Transition and Enhanced Performance in Vertical SnS2/MoS2 van der Waals Photodetectors

The strong light-matter interaction and naturally passivated surfaces of van der Waals materials make heterojunctions of such materials ideal candidates for high-performance photodetectors. In this study, we fabricated SnS2/MoS2 van der Waals heterojunctions and investigated their photoelectric properties. Using an applied gate voltage, we can effectively alter the band arrangement and achieve a transition in type II and type I junctions. It is found that the SnS2/MoS2 van der Waals heterostructures are type II heterojunctions when the gate voltage is above -25 V. Below this gate voltage, the heterojunctions become type I. Photoelectric measurements under various wavelengths of incident light reveal enhanced sensitivity in the ultraviolet region and a broadband sensing range from 400 to 800 nm. Moreover, due to the transition from type II to type I band alignment, the measured photocurrent saturates at a specific gate voltage, and this value depends crucially on the bias voltage and light wavelength, providing a potential avenue for designing compact spectrometers.

Mixed-Dimensional van der Waals Heterostructure (2D ReS2/0D MoS2 Quantum Dots)-Based Broad Spectral Range with Ultrahigh-Responsive Photodetector

The remarkable properties of two-dimensional (2D) materials have led to significant advancements in photodetection and optoelectronics research. Currently, there are many successful methods that are employed to improve the responsivity of photodetectors, but the limited spectral range of the device remains a limitation. This work demonstrates the development of a mixed-dimensional (2D/0D) hybrid photodetector device fabricated using chemical vapor deposition (CVD)-grown monolayer ReS2 and solution-processed MoS2 quantum dots (QDs). The mixed dimensionality of 2D (ReS2) and zero-dimensional (0D) MoS2 QDs assist in improving the spectral range of the device [ultraviolet (360 nm) to near-infrared (780 nm)]. Further, due to the work function difference between ReS2 and MoS2 QDs, the built-in electric field across the mixed-dimensional interface promotes effective charge separation and migration, resulting in improved responsivities of the device. The calculated responsivities of the fabricated photodetector are 5.4 × 102, 3.3 × 102, and 2.6 × 102 A/W when subjected to visible, UV, and NIR light illumination, which is remarkable when compared to the existing reports on broadband photodetection. The mixed-dimensionality heterostructure coupled with contact engineering paves the way for highly responsive broadband photodetectors for potential applications in security, healthcare, etc.

Engineering surface state density of monolayer CVD grown 2D MoS2 for enhanced photodetector performance

This study demonstrates the effect of nitrogen doping on the surface state densities (Nss) of monolayer MoS2 and its effect on the responsivity and the response time of the photodetector. Our experimental results shows that by doping monolayer MoS2 by nitrogen, the surface state (Nss) increases thereby increasing responsivity. The mathematical model included in the paper supports the relation of photocurrent gain and its dependency on trap level which states that the increasing the trap density increases the photocurrent gain and the same is observed experimentally. The experimental results at room temperature revealed that nitrogen doped MoS2 have a high NSS of 1.63 X 1013 states/m2/eV compared to undoped MoS2 of 4.2 x 1012 states/m2/eV. The increase in Nss in turn is the cause for rise in trap states which eventually increases the value of photo responsivity from 65.12 A/W (undoped MoS2) to 606.3 A/W (nitrogen doped MoS2). The response time calculated for undoped MoS2 was 0.85 sec and for doped MoS2 was 0.35 sec. Finally, to verify the dependence of surface states on the responsivity, the surface states were varied by varying temperature and it was observed that upon increment in temperature, the surface states decreases which causes the responsivity values also to decrease.

Environmental-Friendly Ag2S QD-Multilayer MoSe2 van Der Waals Heterostructure for High-Performance Broadband Photodetection

Broadband photodetectors have drawn intensive attention owing to their wide application prospects in optical communication, imaging, astronomy, and so on. Two-dimensional transition-metal dichalcogenides (TMDs) are considered as highly potential candidates for photodetection applications, benefiting from their excellent photoelectric properties. However, most of the photodetectors based on TMDs suffer from low performance in the near-infrared (NIR) region due to the weak optical absorption efficiency near their absorption band edge, which severely constrains their usage for broadband optoelectronics. Here, by taking advantage of the high absorption coefficient and environment-friendly property of Ag2S quantum dots (QDs), the hybrid of multilayer MoSe2/Ag2S QDs is demonstrated with a high-performance broadband photodetection capability (532-1270 nm). The favorable energy band alignment of MoSe2/Ag2S QDs facilitates effective separation and collection of photogenerated carriers, and the heterostructure device exhibits significant enhancement of performance compared to the bare MoSe2 device. High responsivity, detectivity, and external quantum efficiency of 25.5 A/W, 1.45 × 1011 Jones, and 1070% are obtained at a low working voltage of 1 V under 980 nm illumination. The responsivity of the device can reach up to 1.2 A/W at 1270 nm wavelength, which is competitive to the commercial NIR photodetectors. Meanwhile, broadband imaging capability is demonstrated. Our work may open up a facile and eco-friendly approach to construct high-performance broadband photodetectors for next-generation compact optoelectronic applications.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

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.

Direct Z-Scheme Heterostructure of Vertically Oriented SnS2 Nanosheet on BiVO4 Nanoflower for Self-Powered Photodetectors and Water Splitting

The construction of nanostructured Z-scheme heterostructure is a powerful strategy for realizing high-performance photoelectrochemical (PEC) devices such as self-powered photodetectors and water splitting. Considering the band structure and internal electric field direction, BiVO4 is a promising candidate to construct SnS2 -based heterostructure. Herein, the direct Z-scheme heterostructure of vertically oriented SnS2 nanosheet on BiVO4 nanoflower is rationally fabricated for efficient self-powered PEC photodetectors. The Z-scheme heterostructure is identified by ultraviolet photoelectron spectroscopy, photoluminescence spectroscopy, PEC measurement, and water splitting. The SnS2 /BiVO4 heterostructure shows a superior photodetection performance such as excellent photoresponsivity (10.43 mA W-1 ), fast response time (6 ms), and long-term stability. Additionally, by virtue of efficient Z-scheme charge transfer and unique light-trapping nanostructure, the SnS2 /BiVO4 heterostructure also displays a remarkable photocatalytic hydrogen production rate of 54.3 µmol cm-2 h-1 in Na2 SO3 electrolyte. Furthermore, the synergistic effect between photo-activation and bias voltage further improves the PEC hydrogen production rate of 360 µmol cm-2 h-1 at 0.8 V, which is an order of magnitude above the BiVO4 . The results provide useful inspiration for designing direct Z-scheme heterostructures with special nanostructured morphology to signally promote the performance of PEC devices.

Transition Metal Ions Influence the Performance of Photodetector of Two-Dimensional CdS Nanoplatelets

Transition metal-doped two-dimensional (2D) semiconductor nanoplatelets (NPLs) with atomically precise thickness have attracted much research interest due to their inherent photo-physical properties. In this work, we have synthesized 2D Cu-doped CdS NPLs, investigated the charge transfer dynamics using ultrafast transient absorption spectroscopy, and fabricated an efficient photodetector device. A large Stoke's shifted emission at ~685 nm with an average lifetime of about ~1.45 μs is observed in Cu-doped CdS NPLs. Slower bleach recovery kinetics leads to large charge carrier separation in Cu-doped NPLs which is beneficial for photodetector applications. Cu-doped NPLs-based photodetectors exhibit high photocurrent, fast response (~120 ms), ~600 times higher photoresponsivity, and ~300 times higher detectivity (~4.1×1013 Jones) than undoped CdS NPLs. These excellent properties of Cu-doped CdS NPLs make this material an efficient alternative for next-generation optoelectronic devices.

High-Responsivity and Broadband MoS2 Photodetector Using Interfacial Engineering

Combining MoS2 with mature silicon technology is an effective method for preparing high-performance photodetectors. However, the previously studied MoS2/silicon-based heterojunction photodetectors cannot simultaneously demonstrate high responsivity, a fast response time, and broad spectral detection. We constructed a broad spectral n-type MoS2/p-type silicon-based heterojunction photodetector. The SiO2 dielectric layer on the silicon substrate was pretreated with soft plasma to change its thickness and surface state. The pretreated SiO2 dielectric layer and the silicon substrate constitute a multilayer heterostructure with a high carrier concentration and responsiveness. Taking silicon-based and n-type MoS2 heterojunction photodetectors as examples, its responsivity can reach 4.05 × 104 A W1- at 637 nm wavelength with a power density of 2 μW mm-2, and the detectable spectral range is measured from 447 to 1600 nm. This pretreated substrate was proven applicable to other n-type TMDCs, such as MoTe2, ReS2, etc., with certain versatility.

Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

Ultrahigh-Sensitivity and Fast-Speed Solar-Blind Ultraviolet Photodetector Based on a Broken-Gap van der Waals Heterodiode

Broad-bandgap semiconductor-based solar-blind ultraviolet (SBUV) photodetectors have attracted considerable research interest because of their broad applications in missile plume tracking, flame detectors, environmental monitoring, and optical communications due to their solar-blind nature and high sensitivity with low background radiation. Owing to its high light absorption coefficient, abundance, and wide tunable bandgap of 2-2.6 eV, tin disulfide (SnS₂) has emerged as one of the most promising compounds for application in UV-visible optoelectronic devices. However, SnS₂ UV detectors have some undesirable properties such as slow response speed, high current noise level, and low specific detectivity. This study reports a metal mirror-enhanced Ta_0.01W_0.99Se₂/SnS₂ (TWS) van der Waals heterodiode-based SBUV photodetector with an ultrahigh photoresponsivity (R) of ∼1.85 × 10⁴ AW^-1 and a fast speed with rising time (τ_r) of 3.3 μs and decay time (τ_d) of 3.4 μs. Notably, the TWS heterodiode device exhibits a significantly low noise equivalent power of ∼1.02 × 10^-18 W Hz^-1/2 and a high specific detectivity of ∼3.65 × 10¹⁴ cm Hz^1/2 W^-1. This study provides an alternative method for designing fast-speed SBUV photodetectors with enormous potential in applications.

Impact of coherent core/shell architecture on fast response in InP-based quantum dot photodiodes

Solution-processed, cadmium-free quantum dot (QD) photodiodes are compatible with printable optoelectronics and are regarded as a potential candidate for wavelength-selective optical sensing. However, a slow response time resulting from low carrier mobility and a poor dissociation of charge carriers in the optically active layer has hampered the development of the QD photodiodes with nontoxic device constituents. Herein, we report the first InP-based photodiode with a multilayer device architecture, working in photovoltaic mode in photodiode circuits. The photodiode showed the fastest response speed with rising and falling times of τ _r = 4 ms and τ _f = 9 ms at a voltage bias of 0 V at room temperature in ambient air among the Cd-free photodiodes. The single-digit millisecond photo responses were realized by efficient transportation of the photogenerated carriers in the optically active layer resulting from coherent InP/ZnS core/shell QD structure, fast separation of electron and hole pairs at the interface between QD and Al-doped ZnO layers, and optimized conditions for uniform deposition of each thin film. The results suggested the versatility of coherent core/shell QDs as a photosensitive layer, whose structures allow various semiconductor combinations without lattice mismatch considerations, towards fast response, high on/off ratios, and spectrally tunable optical sensing.

Geometry-asymmetric photodetectors from metal-semiconductor-metal van der Waals heterostructures

The functional diversities of two-dimensional (2D) material devices with simple architectures are ultimately limited by immature doping techniques. An alternative strategy is to use geometry-asymmetric metal-semiconductor-metal (GA-MSM) structures, which enable the basic functions of semiconductor junctions such as rectification and photovoltaics. Here, the mixed-dimensional van der Waals heterostructures (MDvdWHs) based on the separation and self-assembly of p-type SnS layered nanosheets (NSs) and n-type SnS₂ nanoparticles (NPs) are obtained using an aqueous phase exfoliation (APE) method. Due to the surface charge transfer doping, the carrier transport mechanism of devices based on MDvdWHs turns from thermionic field emission (TFE) to thermionic emission (TE), with the rectification factor (I_forward/I_reverse) changing from 0.7 to 3. To further illustrate the experimental results, the generic current transport models of GA-MSM devices have been established based on the TE and TFE mechanisms in which the TE and TFE mechanisms lead to opposite rectification phenomena in good agreement with the experimental results. The GA-MSM devices show a photovoltaic effect with a high responsivity of 35 A W^-1 and detectivity of 3.4 × 10¹¹ cm Hz^1/2 W^-1. This study not only provides a novel strategy to design photovoltaic devices with MDvdWHs, but more importantly, we have established fundamental models for the rectification behavior of GA-MSM devices.

Vertically oriented SnS2 on MoS2 nanosheets for high-photoresponsivity and fast-response self-powered photoelectrochemical photodetectors

Van der Waals heterostructures have great potential for the emerging self-powered photoelectrochemical photodetectors due to their outstanding photoelectric conversion capability and efficient interfacial carrier transportation. By considering the band alignment, structural design, and growth optimization, the heterostructures of vertically oriented SnS₂ with different densities on MoS₂ nanosheets are designed and fabricated using a two-step epitaxial growth method. Compared with SnS₂, MoS₂, and low density-vertical SnS₂/MoS₂ heterostructure, the high density-vertical SnS₂/MoS₂ heterostructure exhibits largely enhanced self-powered photodetection performances, such as a giant photocurrent density (∼932.8 μA cm^-2), an excellent photoresponsivity (4.66 mA W^-1), and an ultrafast response/recovery time (3.6/6.4 ms) in the ultraviolet-visible range. This impressive enhancement of high density-vertical SnS₂/MoS₂ photodetectors is mainly ascribed to the essentially improved charge transfer and carrier transport of type-II band alignment heterostructures and the efficient light absorption from the unique light-trapping structure. In addition, the photoelectrocatalytic water splitting performance of the high density-vertical SnS₂/MoS₂ heterostructure also benefits from the type-II band alignment and the light-trapping structure. This work provides valuable inspiration for the design of two-dimensional optoelectronic and photoelectrochemical devices with improved performance by the morphology and heterostructure design.