Polarization- and Gate-Tunable Optoelectronic Reverse in 2D Semimetal/Semiconductor Photovoltaic Heterostructure

Modulation anisotropy of nanomaterials toward monolithic integrated polarization-sensitive photodetectors

By virtue of the unique ability of providing additional information beyond light intensity and spectra, polarization-sensitive photodetectors could precisely identify targets in several concealed, camouflaged, and non-cooperative backgrounds, making them highly suitable for potential applications in remote sensing, astronomical detection, medical diagnosis, etc. Therefore, to provide a comprehensive design guideline for a wide range of interdisciplinary researchers, this review provides a general overview of state-of-the-art linear, circular, and full-Stokes polarization-sensitive photodetectors. In particular, from the perspectives of technological progress and the development of nanoscience, the detailed discussion focuses on strategies to simplify high-performance polarization-sensitive photodetectors, reducing their size and achieving a smaller volume. In addition, to lay a solid foundation for modulating the properties of future nanostructure-based polarization-sensitive photodetectors, insights into light-matter interactions in low-symmetry materials and asymmetric structures are provided here. Meanwhile, the corresponding opportunities and challenges in this research field are identified.

Ultrahigh Exchange Bias Field/Coercive Field Ratio in In Situ Formed Two-Dimensional Magnetic Te-Cr2O3/Cr5Te6 Heterostructures

The exchange bias (EB) effect is a fundamental magnetic phenomenon, in which the exchange bias field/coercive field ratio (|HEB/HC|) can improve the stability of spintronic devices. Two-dimensional (2D) magnetic heterostructures have the potential to construct low-power and high-density spintronic devices, while their typically air unstable and |HEB/HC| lesser, limiting the possibility of applications. Here, 2D Cr5Te6 nanosheets have been systematically synthesized with an in situ formed ≈2 nm-thick Te doped Cr2O3 layer (Te-Cr2O3) on the upper surface by chemical vapor deposition (CVD) method. The strong and air stable EB effect, achieving a |HEB/HC| of up to 80% under an ultralow cooling field of 0.01 T, which is greater than that of the reported 2D magnetic heterostructures. Meanwhile, the uniformity of thickness and chemical composition of the Te-Cr2O3 layer can be controlled by the growth conditions which are highly correlated with the EB effect of 2D Te-Cr2O3/Cr5Te6 heterostructures. First-principles calculations show that the Te-Cr2O3 can provide uncompensated spins in the Cr2O3, thus forming strong spin pinning effect. The systematical investigation of the EB effect in 2D Te-Cr2O3/Cr5Te6 heterostructures with high |HEB/HC| will open up exciting opportunities in low-power and high-stability 2D spintronic devices.

2D Perovskite Heterojunction-Based Self-Powered Polarized Photodetectors with Controllable Polarization Ratio Enabled by Ferro-Pyro-Phototronic Effect

Metal halide perovskites (MHPs) are commonly used in polarization-sensitive photodetectors (PDs) for applications such as polarization imaging, remote sensing, and optical communication. Although various methods exist to adjust the polarization-sensitive photocurrent, a universal and effective approach for continuous control of MHPs' optoelectronic and polarized properties is lacking. A universal strategy to electrically modulate the polarization ratio (PR) of self-powered polarized PDs using the ferro-pyro-phototronic effect (FPPE) in 2D perovskites is presented. By varying the amplitude and direction of ferroelectric polarization voltage, the built-in electric field in the heterojunction can be modulated, allowing for controllable PR regulation and adjustable polarization characteristics. Moreover, the polarized pyroelectric photoresponses are realized, significantly enhancing the responsivity, response speed of the polarized PDs. Both the pyroelectric currents and photocurrents exhibit obvious polarization characteristics. This method's versatility is demonstrated by creating three additional quasi-2D MHP ferroelectric-based polarized-sensitive PDs. A proof-of-concept for encrypted optical communication is achieved using the UV-sensitive PDs as light-sensing units. These findings highlight FPPE's potential to enhance ferroelectric device polarization control, enabling high-performance and self-powered polarization photodetection.

Switchable Photovoltaic Effect Induced by Light Intensity

Photovoltaic devices capable of reversible photovoltaic polarity through external signal modulation may enable multifunctional optoelectronic systems. However, such devices are limited to those induced by gate voltage, electrical poling, or optical wavelength by using complicated device architectures. Here, we show that the photovoltaic polarity is also switchable with the intensity of incident light. The modulation in light intensity induces photovoltaic polarity switching in geometrically asymmetric MoS2 Schottky photodiodes, explained by the asymmetric lowering of the Schottky barrier heights due to the trapping of photogenerated holes at the MoS2/Cr interface states. An applied gate voltage can further modulate the carrier concentration in the MoS2 channel, providing a method to tune the threshold light intensity of polarity switching. Finally, a bidirectional optoelectronic logic gate with "AND" and "OR" functions was demonstrated within a single device.

Interface Engineering for Efficient Photocarrier Generation and Transfer in Strongly Coupled Metallic/Semiconducting 1T'/2H MoS2 Heterobilayers

Developing alternative two-dimensional (2D) metallic/semiconducting (M/S) van der Waals heterostructures (vdWHs) along with an understanding of interfacial photocarrier behavior is crucial for designing high-performance optoelectronic devices. Here, we comprehensively explored the photophysical model of photocarrier generation and interfacial transfer in as-grown 2D 1T'/2H MoS2 vdWHs using various spectroscopic characterizations. We demonstrated the transitions of activated photocarrier transfer trajectories by tuning the pump photon energies across the 2H MoS2 bandgap. The importance of confined bilayer transfer systems and strong interlayer coupling at vdW interfaces for transfer efficiency was elucidated. Additionally, the fluorophlogopite substrate was found to be an external method for regulating photocarrier generation in individual 2H layers through the p-doping effect at the substrate-2H layer interfaces, and this influence was alleviated after introducing the 2H-1T' vdW interface. Particularly, 1T' MoS2 as a broadband hot carrier absorber enabled the ultrafast (∼133 fs) injection and extraction of energetic hot carriers into the 2H layer via a photothermionic emission mechanism, achieving a high efficiency of ∼41% under 900 nm photoexcitation at room temperature. Our work offers fundamental insights into the complex interfacial carrier photophysics in 2D M/S vdWHs, providing a way of constructing advanced multifunctional devices by using these emerging materials as active components and interface engineering.

An Ultrasensitive Bi2O2Se/In2S3 Photodetector with Low Detection Limit and Fast Response toward High-Precision Unmanned Driving

The machine vision utilized in unmanned driving systems must possess the ability to accurately perceive scenes under low-light illumination conditions. To achieve this, photodetectors with low detection limits and a fast response are essential. Current systems rely on avalanche diodes or lidars, which come with the drawbacks of increased energy consumption and complexity. Here, we present an ultrasensitive photodetector based on a two-dimensional (2D) Bi2O2Se/In2S3 heterostructure, incorporating a homotype unilateral depletion band design. This innovative architecture effectively modulates the transport of both free and photoexcited carriers, suppressing the dark current and facilitating the rapid and efficient separation of photocarriers. Owing to these features, this device exhibits a responsivity of 144 A/W, a specific detectivity of 1.2 × 1014 Jones, and a light on/off ratio of 1.1 × 105. These metrics rank among the top values reported for state-of-the-art 2D devices. Moreover, this device also demonstrates a fast response time of 170/296 μs and a low noise equivalent power of 0.57 fW/Hz1/2, attributes that endow it with ultraweak light imaging capabilities. Furthermore, we have successfully integrated this device into an unmanned driving system, providing a perspective on the design and fabrication of future optoelectronic devices.

Two-dimensional layered material photodetectors: what could be the upcoming downstream applications beyond prototype devices?

With distinctive advantages spanning excellent flexibility, rich physical properties, strong electrostatic tunability, dangling-bond-free surface, and ease of integration, 2D layered materials (2DLMs) have demonstrated tremendous potential for photodetection. However, to date, most of the research enthusiasm has been merely focused on developing novel prototype devices. In the past few years, researchers have also been devoted to developing various downstream applications based on 2DLM photodetectors to contribute to promoting them from fundamental research to practical commercialization, and extensive accomplishments have been realized. In spite of the remarkable advancements, these fascinating research findings are relatively scattered. To date, there is still a lack of a systematic and profound summarization regarding this fast-evolving domain. This is not beneficial to researchers, especially researchers just entering this research field, who want to have a quick, timely, and comprehensive inspection of this fascinating domain. To address this issue, in this review, the emerging downstream applications of 2DLM photodetectors in extensive fields, including imaging, health monitoring, target tracking, optoelectronic logic operation, ultraviolet monitoring, optical communications, automatic driving, and acoustic signal detection, have been systematically summarized, with the focus on the underlying working mechanisms. At the end, the ongoing challenges of this rapidly progressing domain are identified, and the potential schemes to address them are envisioned, which aim at navigating the future exploration as well as fully exerting the pivotal roles of 2DLMs towards the practical optoelectronic industry.

Realization of High-Performance Self-Powered Polarized Photodetection with Large Temperature Window in a 2D Polar Perovskite

Polarization photodetection taking advantage of the anisotropy of 2D materials shines brilliantly in optoelectronic fields owing to differentiating optical information. However, the previously reported polarization detections are mostly dependent on external power sources, which is not conducive to device integration and energy conservation. Herein, a 2D polar perovskite (CBA)2CsPb2Br7 (CCPB, CBA = 4-chlorobenzyllamine) has been successfully synthesized, which shows anticipated bulk photovoltaic effect (BPVE) with an open-circuited photovoltage up to ≈0.2 V. Devices based on CCPB monomorph fulfill a fascinating self-powered polarized photodetection with a large polarization ratio of 2.7 at room temperature. Moreover, CCPB features a high phase-transition temperature (≈475 K) which prompts such self-powered polarized photodetection in a large temperature window of device operation, since BPVE generated by spontaneous polarization can only exist in the polar structure prior to the phase transition. Further computational investigation reveals the introduction of CBA+ with a large dipole moment contributes to quite large polarization (17.5 µC cm-2) and further super high phase transition temperature of CCPB. This study will promote the application of 2D perovskite materials for self-powered polarized photodetection in high-temperature conditions.