Wafer-Scale Patterning Synthesis of Two-Dimensional WSe2 Layers by Direct Selenization for Highly Sensitive van der Waals Heterojunction Broadband Photodetectors

Simulating and Implementing Broadband van der Waals Artificial Visual Synapses Based on Photoconductivity and Pyroconductivity Mechanisms

With advancements in artificial neural networks and information processing technology, a variety of neuromorphic synaptic devices have been proposed to emulate human sensory systems, with vision being a crucial information source. Moreover, as practical applications become increasingly complex, the need for multifunctional visual synapses to expand the application range becomes urgent. This study introduces a MoS2/WSe2 van der Waals (vdW) heterojunction and utilizes it to replicate artificial visual synapses by harnessing the cooperative effect of photoconductivity and pyroconductivity mechanisms. By adjusting the optical power, pulse width, and pulse number of the optical stimulus, the heterojunction effectively simulates synaptic properties. Under the combined action of an external electric field and the built-in electric field (Ebi), the heterojunction exhibits broadband synaptic properties in the visible to near-infrared spectrum (405-1550 nm) while consuming low power of 0.3-1.1 pJ per spike. The heterojunction can detect ultraweak optical signals at 660 nm with an optical power intensity of 14 μW/cm2, displaying a high specific detectivity (D*) of 3.98 × 1011 Jones. Furthermore, at 405, 808, 1064, and 1550 nm, the D* of the heterojunction is 4.16 × 1011, 3.61 × 109, 4.96 × 107, and 1.64 × 107 Jones, respectively. Visual synaptic devices based on the MoS2/WSe2 vdW heterojunction hold significant promise for the future development of integrated sensing and memory processing devices.

Toward Mass Production of Transition Metal Dichalcogenide Solar Cells: Scalable Growth of Photovoltaic-Grade Multilayer WSe2 by Tungsten Selenization

Semiconducting transition metal dichalcogenides (TMDs) are promising for high-specific-power photovoltaics due to their desirable band gaps, high absorption coefficients, and ideally dangling-bond-free surfaces. Despite their potential, the majority of TMD solar cells to date are fabricated in a nonscalable fashion, with exfoliated materials, due to the lack of high-quality, large-area, multilayer TMDs. Here, we present the scalable, thickness-tunable synthesis of multilayer WSe2 films by selenizing prepatterned tungsten with either solid-source selenium at 900 °C or H2Se precursors at 650 °C. Both methods yield photovoltaic-grade, wafer-scale WSe2 films with a layered van der Waals structure and superior characteristics, including charge carrier lifetimes up to 144 ns, over 14× higher than those of any other large-area TMD films previously demonstrated. Simulations show that such carrier lifetimes correspond to ∼22% power conversion efficiency and ∼64 W g-1 specific power in a packaged solar cell, or ∼3 W g-1 in a fully packaged solar module. The results of this study could facilitate the mass production of high-efficiency multilayer WSe2 solar cells at low cost.

Composition Modulation-Mediated Band Alignment Engineering from Type I to Type III in 2D vdW Heterostructures

Band alignment engineering is crucial for facilitating charge separation and transfer in optoelectronic devices, which ultimately dictates the behavior of Van der Waals heterostructures (vdWH)-based photodetectors and light emitting diode (LEDs). However, the impact of the band offset in vdWHs on important figures of merit in optoelectronic devices has not yet been systematically analyzed. Herein, the regulation of band alignment in WSe2/Bi2Te3- xSex vdWHs (0 ≤ x ≤ 3) is demonstrated through the implementation of chemical vapor deposition (CVD). A combination of experimental and theoretical results proved that the synthesized vdWHs can be gradually tuned from Type I (WSe2/Bi2Te3) to Type III (WSe2/Bi2Se3). As the band alignment changes from Type I to Type III, a remarkable responsivity of 58.12 A W-1 and detectivity of 2.91×1012 Jones (in Type I) decrease in the vdWHs-based photodetector, and the ultrafast photoresponse time is 3.2 µs (in Type III). Additionally, Type III vdWH-based LEDs exhibit the highest luminance and electroluminescence (EL) external quantum efficiencies (EQE) among p-n diodes based on Transition Metal Dichalcogenides (TMDs) at room temperature, which is attributed to band alignment-induced distinct interfacial charge injection. This work serves as a valuable reference for the application and expansion of fundamental band alignment principles in the design and fabrication of future optoelectronic devices.

Enhanced photoelectric performance of Bi2O2Se/CuInP2S6 heterojunction via ferroelectric polarization in two-dimensional CuInP2S6

Two-dimensional (2D) materials have drawn tremendous interest as promising materials for photoelectric devices due to their extraordinary properties. As an outstanding 2D photoelectric material, Bi2O2Se (BOS) has exhibited good performance and great potential in photoelectric applications. In this report, we have constructed a photoelectric heterojunction based on BOS and CuInP2S6 (CIPS) nanosheets to achieve enhanced photoelectric performance. With modulation of the ferroelectric-polarization-induced built-in electric field in CIPS, the photogenerated carriers in BOS are effectively separated to form a stable current that is independent of the applied voltage, so that the photoelectric performance of the heterojunction is significantly improved. The photoresponsivity (R), external quantum efficiency (EQE), and normalized detectivity (D*) are calculated and analyzed to evaluate the photodetection performance of the heterojunction. Results demonstrate excellent photoelectric performance of BOS/CIPS heterojunction under irradiation of light from ultraviolet (365 nm), visible (405/550/650 nm) to near-infrared (980 nm). R, EQE, and D* are up to 338.94 A W-1, 7.65 × 104%, and 3.99 × 1010 Jones, respectively, under the condition of 550 nm and 0.24 W m-2. Meanwhile, the measured rise and fall times of the heterojunction reach 2.74 and 4.82 ms, respectively, indicating its fast photoelectric response. This work provides an effective approach to enhance the photoelectric response and stability of BOS via the ferroelectric-polarization-induced built-in electric field of CIPS.

Progress in Advanced Infrared Optoelectronic Sensors

Infrared optoelectronic sensors have attracted considerable research interest over the past few decades due to their wide-ranging applications in military, healthcare, environmental monitoring, industrial inspection, and human-computer interaction systems. A comprehensive understanding of infrared optoelectronic sensors is of great importance for achieving their future optimization. This paper comprehensively reviews the recent advancements in infrared optoelectronic sensors. Firstly, their working mechanisms are elucidated. Then, the key metrics for evaluating an infrared optoelectronic sensor are introduced. Subsequently, an overview of promising materials and nanostructures for high-performance infrared optoelectronic sensors, along with the performances of state-of-the-art devices, is presented. Finally, the challenges facing infrared optoelectronic sensors are posed, and some perspectives for the optimization of infrared optoelectronic sensors are discussed, thereby paving the way for the development of future infrared optoelectronic sensors.

Two-Step Conversion of Metal and Metal Oxide Precursor Films to 2D Transition Metal Dichalcogenides and Heterostructures

The widely studied class of two-dimensional (2D) materials known as transition metal dichalcogenides (TMDs) are now well-poised to be employed in real-world applications ranging from electronic logic and memory devices to gas and biological sensors. Several scalable thin film synthesis techniques have demonstrated nanoscale control of TMD material thickness, morphology, structure, and chemistry and correlated these properties with high-performing, application-specific device metrics. In this review, the particularly versatile two-step conversion (2SC) method of TMD film synthesis is highlighted. The 2SC technique relies on deposition of a solid metal or metal oxide precursor material, followed by a reaction with a chalcogen vapor at an elevated temperature, converting the precursor film to a crystalline TMD. Herein, the variables at each step of the 2SC process including the impact of the precursor film material and deposition technique, the influence of gas composition and temperature during conversion, as well as other factors controlling high-quality 2D TMD synthesis are considered. The specific advantages of the 2SC approach including deposition on diverse substrates, low-temperature processing, orientation control, and heterostructure synthesis, among others, are featured. Finally, emergent opportunities that take advantage of the 2SC approach are discussed to include next-generation electronics, sensing, and optoelectronic devices, as well as catalysis for energy-related applications.

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.

Ferroelectricity-Driven Self-Powered Weak Temperature and Broadband Light Detection in MoS2/CuInP2S6/WSe2 van der Waals Heterojunction Nanoarchitectonics

Two-dimensional ferroelectric materials enrich the modulation degrees of freedom in self-powered van der Waals temperature/light detectors by incorporating pyroelectric and bulk photovoltaic effects. However, in addition to the low polarization, the practical applications of these materials are limited due to the significant challenge posed by their ultrathin nature, which affects their polarization stability. In this report, we introduce a design for a dual heterostructure-stabilized van der Waals heterojunction that addresses this challenge by improving the performance and extending the operational lifetime of self-powered van der Waals temperature/light detectors. The design is demonstrated using the MoS2/CuInP2S6 (CIPS)/WSe2 van der Waals heterojunction, which exhibits sensitivity to small temperature changes induced by weak light across the ultraviolet to mid-infrared spectrum. It can generate a noticeable pyroelectric current without the need for an external voltage, and its pyroelectric coefficient exceeds 130 and 978 μC/m2 K for 45 and 70 nm CIPS, respectively. The heterojunction offers high detection accuracy, with a temperature variation sensitivity as small as 0.1 K and an optical power intensity detection range from low to 1 μW/cm2. Additionally, the heterojunction exhibits exceptional detectivity (D*) for different light wavelengths. Remarkably, the self-powered detection performance remains stable for months without obvious degradation in the natural environment. These results offer a promising solution for high-performance, self-sustaining temperature/light detection applications and pave the way for the development of future ferroelectricity-driven photodetection technologies.