Large Lateral Photovoltage Observed in MoS2 Thickness-Modulated ITO/MoS2/p-Si Heterojunctions

Polarization-Resolved Position-Sensitive Self-Powered Binary Photodetection in Multilayer Janus CrSBr

Recent progress in polarization-resolved photodetection based on low-symmetry 2D materials has formed the basis of cutting-edge optoelectronic devices, including quantum optical communication, 3D image processing, and sensing applications. Here, we report an optical polarization-resolving photodetector (PD) fabricated from multilayer semiconducting CrSBr single crystals with high structural anisotropy. We have demonstrated self-powered photodetection due to the formation of Schottky junctions at the Au-CrSBr interfaces, which also caused the photocurrent to display a position-sensitive and binary nature. The self-biased CrSBr PD showed a photoresponsivity of ∼0.26 mA/W with a detectivity of 3.4 × 108 Jones at 514 nm excitation of fluency (0.42 mW/cm2) under ambient conditions. The optical polarization-induced photoresponse exhibits a large dichroic ratio of 3.4, while the polarization is set along the a- and the b-axes of single-crystalline CrSBr. The PD also showed excellent stability, retaining >95% of the initial photoresponsivity in ambient conditions for more than five months without encapsulation. Thus, we demonstrate CrSBr as a fascinating material for ultralow-powered optical polarization-resolving optoelectronic devices for cutting-edge technology.

An ultrafast and self-powered MoSxSe2-x/Si photodetector with high light-trapping structures and a SiOx interface layer

MoSxSe2-x nanofilms, as a typical metal dichalcogenide, have attracted great interest, due to their adjustable bandgap and distinctive electronic and optical properties. However, the inherent bandgap of MoSxSe2-x and the strong interface recombination impede the actualization of a high-sensitivity photodetector (PD). Few-layer MoSxSe2-x nanofilms were prepared with vertically orientation at 450 °C, which would be a less restrictive choice of substrates. Herein, a self-powered MoSxSe2-x/SiOx/Si photodetector was fabricated which exhibits unprecedented performance with excellent reproducibility and stability from 405 nm to 980 nm, a high responsivity (0.450 A W-1), normalized detectivity (4.968 × 1012 Jones) and ultrafast photoresponse (τr = 1.20 μs, τf = 4.92 μs) at zero bias under 980 nm incident laser illumination with a density of 200 μW cm-2. Significantly, the self-powered PD is capable of detecting ultraweak IR signals below 200 μW cm-2 with high on-off ratios. More importantly, an oxidized atomic layer is generated through the wet oxidation in the Piranha solution. The PD can work well at high frequencies even at 100 kHz, which shows its potential application in high-frequency photoelectric devices and health monitors. Summing up, this work not only suggests that an ultrathin SiOx interface layer can reduce carrier recombination via simple interface engineering, but also proposes a novel strategy for the preparation of high-performance and low-cost optoelectronic devices.

Preparation of a novel Bi9O7.5S6/SnS composite film with improved photoelectric properties

Atomically thin two-dimensional (2D) bismuth oxychalcogenides have been considered as promising candidates for high-speed and low-power photoelectronic devices due to their high charge carrier mobility and excellent environmental stability. However, the photoelectric performance of their bulk materials still falls short of expectations. Herein, a novel Bi9O7.5S6/SnS composite film with a type-II heterojunction was successfully prepared by combining hydrothermal and knife-coating techniques. The crystal structure, morphology, and optical properties were systematically investigated. Under 1 V bias voltage, the photocurrent of the Bi9O7.5S6/SnS composite film can be obtained as 107 μA cm-2, which is about 29.9 times and 93.9 times higher than that of bare Bi9O7.5S6 and SnS, respectively. The type-II heterojunction has played a significant role in improving the photoelectric performance of the Bi9O7.5S6/SnS composite film by facilitating the separation and transfer of photo-generated carriers. This work sheds light on the design and development of new bismuth-based composite materials for advanced photoelectric and photocatalytic applications.

Large lateral photovoltaic effect with ultrafast optical relaxation time in SnS2/n-Si junctions

A large lateral photovoltaic effect (LPE) with a fast optical response time is necessary to develop high-performance position-sensitive detectors. In this paper, we report an LPE with a high self-powered position sensitivity and ultrafast optical relaxation time in S n S 2/n-S i junctions prepared using pulsed laser deposition. A large built-in electric field was generated at the S n S 2/S i interface, which resulted in a large LPE with a positional sensitivity of up to 116 mV/mm. Furthermore, the measurement circuit with multiple parallel resistors had a strong influence on the ultrafast optical response time of the LPE and the fastest optical relaxation time observed was ∼0.44µs. Our results suggest that the S n S 2/S i junction would be a promising candidate for a wide range of optoelectronic device applications.

A Time-Division Position-Sensitive Detector Image System for High-Speed Multitarget Trajectory Tracking

High-speed trajectory tracking with real-time processing capability is particularly important in the fields of pilotless automobiles, guidance systems, robotics, and filmmaking. The conventional optical approach to high-speed trajectory tracking involves charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) image sensors, which suffer from trade-offs between resolution and framerates, complexity of the system, and enormous data-analysis processes. Here, a high-speed trajectory tracking system is designed by using a time-division position-sensitive detector (TD-PSD) based on a graphene-silicon Schottky heterojunction. Benefiting from the high-speed optoelectronic response and sub-micrometer positional accuracy of the TD-PSD, multitarget real-time trajectory tracking is realized, with a maximum image output framerate of up to 62 000 frames per second. Moreover, multichannel trajectory tracking and image-distortion correction functionalities are realized by TD-PSD systems through frequency-related image preprocessing, which significantly improves the capacity of real-time information processing and image quality in complicated light environments.

Multifunctional high-performance position sensitive detector based on a Sb2Se3-nanorod/CdS core-shell heterojunction

Sb₂Se₃ exhibits fascinating optical and electrical properties owing to its unique one-dimensional crystal structure. In this study, a Sb₂Se₃-nanorod/CdS core-shell heterostructure was successfully constructed, and the lateral photovoltaic effect (LPE), as well as the lateral photocurrent and photoresistance effects, were first studied. The measurements indicate that this heterojunction exhibits excellent lateral photoelectric performance in a broad range of 405-1064 nm with the best position sensitivities (PSs) of 525.9 mV/mm, 79.1 µA/mm, and 25.6 kΩ/mm for the lateral photovoltage, photocurrent, and photoresistance, respectively, while the nonlinearity is maintained below 7%, demonstrating its great potential in a novel high-performance multifunctional position sensitive detector (PSD). Moreover, this PSD could work well at different frequencies with good stability and repeatability, and the rise and fall times were deduced to be 48 and 180 µs, respectively. Besides, large linear working distances are achieved in this heterojunction PSD, and the PS can still reach 75.5 mV/mm even at an ultra-large working distance of 9 mm. These outstanding performances can be attributed to the high-quality Sb₂Se₃ nanorod arrays and the fast charge-carrier separation and transport properties of this core-shell heterojunction. This study provides important ideas for developing high-performance, broadband, large working distances, and ultrafast multifunctional PSDs based on the new core-shell heterostructure.

Fabrication and enhanced photoelectric properties of a novel Bi9O7.5S6/CdS composite film

A novel Bi9O7.5S6/CdS composite film with a type-II heterojunction was presented with a superior photoelectric response and photostability under visible-light irradiation.

High-performance broadband position-sensitive detector based on lateral photovoltaic effect of PbSe heterostructure

PbSe has attracted considerable attention due to its promising applications in optoelectronics and energy harvesting. In this work, we explore the lateral photovoltaic effect (LPE) of PbSe films with a simple PbSe/Si heterostructure under nonuniform light illumination and zero-bias conditions. The LPE response is strongly dependent on the thickness of the PbSe film, but always shows a linear dependence on the laser spot position in an ultra-large working size of 5 mm and exhibits a wide photoresponse ranging from visible to near-infrared. The maximum position sensitivity can reach up to 190 mV/mm for the 15-nm-thick PbSe device at 1064 nm and nonlinearity is less than 4%, demonstrating its new potential application in novel position sensitive detectors (PSDs). Besides, the device also shows an ultrafast response speed, with the rise and fall time of ∼40 µs and ∼105 µs, respectively, and excellent reproducibility. These results bring great inspirations for developing high-performance broadband and self-powered PSDs based on the PbSe/Si heterostructure.

A Photovoltaic Self-Powered Gas Sensor Based on All-Dry Transferred MoS2 /GaSe Heterojunction for ppb-Level NO2 Sensing at Room Temperature

Traditional gas sensors are facing the challenge of low power consumption for future application in smart phones and wireless sensor platforms. To solve this problem, self-powered gas sensors are rapidly developed in recent years. However, all reported self-powered gas sensors are suffering from high limit of detection (LOD) toward NO2 gas. In this work, a photovoltaic self-powered NO2 gas sensor based on n-MoS2 /p-GaSe heterojunction is successfully prepared by mechanical exfoliation and all-dry transfer method. Under 405 nm visible light illumination, the fabricated photovoltaic self-powered gas sensors show a significant response toward ppb-level NO2 with short response and recovery time and high selectivity at room temperature (25 °C). It is worth mentioning that the LOD toward NO2 of this device is 20 ppb, which is the lowest of the reported self-powered room-temperature gas sensors so far. The discussed devices can be used as building blocks to fabricate more functional Internet of things devices.

Visualization of band offsets at few-layer MoS2/Ge heterojunction

The visualization of band alignment for designing heterostructures between transition metal dichalcogenides and germanium plays a vital role in a deeper understanding of carrier dynamics at the heterointerface. Here, to study the band alignment across the MoS₂/Ge heterojunction, we have deposited a wafer-scale highly crystalline few atomic layers MoS₂film via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The Raman and XRD spectra of as-fabricated MoS₂/Ge heterojunction expose the presence of highly crystalline few atomic layer MoS₂on top of Ge substrate. Interestingly, we found a type-II band alignment at the MoS₂/Ge heterointerface having valence band, and conduction band offset values of 0.88 and 0.21 eV, which can provide very efficient recombination through spatially confining charge carriers. The calculation of band offset parameters offers a promising way for device engineering across the MoS₂/Ge heterojunction interface. Moreover, to demonstrate the practicability of the fabricated heterostructure, we explored the suitability of our device for broadband photodetection applications.

High-performance position-sensitive detector based on the lateral photoelectrical effect of two-dimensional materials

Two-dimensional (2D) materials such as graphene and transition-metal chalcogenides have been extensively studied because of their superior electronic and optical properties. Recently, 2D materials have shown great practical application in position-sensitive detectors (PSDs), originating from the lateral photoelectrical effect of the materials or junctions. The high position sensitivity and ultrafast photoresponse of PSDs based on 2D materials, especially compatibility with Si technology, may enable diverse optoelectronic applications. In this review, recent studies of PSDs based on 2D materials are summarized, providing a promising route for high-performance PSDs.

3C-SiC/Si Heterostructure: An Excellent Platform for Position-Sensitive Detectors Based on Photovoltaic Effect

Single-crystalline silicon carbide (3C-SiC) on the Si substrate has drawn significant attention in recent years due to its low wafer cost and excellent mechanical, chemical, and optoelectronic properties. However, the applications of the structure have primarily been focused on piezoresistive and pressure sensors, bio-microelectromechanical system, and photonics. Herein, we report another promising application of the heterostructure as a laser spot position-sensitive detector (PSD) based on the lateral photovoltaic effect (LPE) under nonuniform optical illuminations at zero-bias conditions. The LPE shows a linear dependence on spot positions, and the sensitivity is found to be as high as 33 mV/mm under an illumination of 2.8 W/cm² (635 nm). The structure also exhibits a linear dependence of the LPE over a large distance (7 mm) between two electrodes, which is crucial for PSDs as the region with a linear dependence of LPE is only usable for PSDs. The LPE at different spot positions and under different illumination conditions have been investigated and explained based on the energy-band analysis. The temperature dependence of the LPE and position sensitivity is also investigated. Furthermore, the two-dimensional mapping of the lateral photovoltages reveals the potential for utilizing the 3C-SiC/Si heterostructure to detect the laser spot position precisely on a plane.

High sensitivity Si-based photodetection with nanoscale protective layer based on interface states

A large lateral photovoltaic effect has been observed at the interface of SiO₂/p-Si/SiO₂ structure. Different from the traditional Schottky junction or PN junction device, this photovoltage is mainly dominated by the interface states existing at SiO₂/p-Si interface, where the covered nanoscale SiO₂ layer brings this device a stable and high photoelectric performance. These interface states can be explained as built-in field caused by the band bending at interface, which regulates the generation and diffusion of photo induced carriers. In this study, we discuss clearly the factors that impact greatly on the photovoltage output and sensitivity, including the oxide thickness, resistivity and tunneling effect. We believe this simple but efficient device will be beneficial for the exploring in photoelectric detection.

A Transparent Photonic Artificial Visual Cortex

Mimicking brain-like functionality with an electronic device is an essential step toward the design of future technologies including artificial visual and memory applications. Here, a proof-of-concept all-oxide-based (NiO/TiO₂ ) highly transparent (54%) heterostructure is proposed and demonstrated, which mimics the primitive functions of the visual cortex. Specifically, orientation selectivity and spatiotemporal processing similar to that of the visual cortex are demonstrated using direct optical stimuli under the self-biased condition due to photovoltaic effect, illustrating an energy-efficient approach for neuromorphic computing. The photocurrent of the device can be modulated from zero to 80 µA by simply rotating the slit by 90°. The device shows fast rise and fall times of 3 and 6 ms, respectively. Based on Kelvin probe force measurements, the observed results are attributed to a lateral photovoltaic effect. This highly transparent, self-biased, photonic triggered device paves the way for the advancement of energy-efficient neuromorphic computation.

High-performance position-sensitive detector based on the lateral photovoltaic effect in MoSe2/p-Si junctions

Optoelectronic position-sensitive detectors (PSDs) based on the lateral photovoltaic effect (LPE) have been a focus of research due to their ability to detect very small displacements. In this paper, we investigate the LPE properties of MoSe₂/p-Si junctions prepared using pulsed laser deposition. The LPE shows a good linear dependence with the position of the laser spot. A large positional sensitivity and a fast optical relaxation time of 563  mV mm^-1 and 2 μs, respectively, were observed in the MoSe₂ (10 nm)/p-Si junction. The influence of the laser power and the wavelength on the LPE suggests that the observed response originates from the photoelectric effect. The large positional sensitivity and fast relaxation time of the LPE make the MoSe₂/p-Si junction a promising candidate for PSDs.

Growth Mechanisms and Electronic Properties of Vertically Aligned MoS2

Thin films of layered semiconductors emerge as highly promising materials for energy harvesting and storage, optoelectronics and catalysis. Their natural propensity to grow as oriented crystals and films is one of their distinct properties under recent focal interest. Specifically, the reaction of transition metal films with chalcogen vapor can result in films of vertically aligned (VA) layers, while metal-oxides react with chalcogens in vapor phase to produce horizontally aligned crystals and films. The growth mechanisms of vertically oriented films are not yet fully understood, as well as their dependence on the initial metal film thickness and growth conditions. Moreover, the resulting electronic properties and the role of defects and disorder had not yet been studied, despite their critical influence on catalytic and device performance. In this work, we study the details of oriented growth of MoS₂ with complementary theoretical and experimental approaches. We present a general theoretical model of diffusion-reaction growth that can be applied to a large variety of layered materials synthesized by solid-vapor reaction. Moreover, we inspect the relation of electronic properties to the structure of vertically aligned MoS₂ and shed light on the density and character of defects in this material. Our measurements on Si-MoS₂ p-n hetero-junction devices point to the existence of polarizable defects that impact applications of vertical transition-metal dichalcogenide materials.

Ultrahigh, Ultrafast, and Self-Powered Visible-Near-Infrared Optical Position-Sensitive Detector Based on a CVD-Prepared Vertically Standing Few-Layer MoS2/Si Heterojunction

MoS2, as a typical transition metal dichalcogenide, has attracted great interest because of its distinctive electronic, optical, and catalytic properties. However, its advantages of strong light absorption and fast intralayer mobility cannot be well developed in the usual reported monolayer/few-layer structures, which make the performances of MoS2-based devices undesirable. Here, large-area, high-quality, and vertically oriented few-layer MoS2 (V-MoS2) nanosheets are prepared by chemical vapor deposition and successfully transferred onto an Si substrate to form the V-MoS2/Si heterojunction. Because of the strong light absorption and the fast carrier transport speed of the V-MoS2 nanosheets, as well as the strong built-in electric field at the interface of V-MoS2 and Si, lateral photovoltaic effect (LPE) measurements suggest that the V-MoS2/Si heterojunction is a self-powered, high-performance position sensitive detector (PSD). The PSD demonstrates ultrahigh position sensitivity over a wide spectrum, ranging from 350 to 1100 nm, with position sensitivity up to 401.1 mV mm-1, and shows an ultrafast response speed of 16 ns with excellent stability and reproducibility. Moreover, considering the special carrier transport process in LPE, for the first time, the intralayer and the interlayer transport times in V-MoS2 are obtained experimentally as 5 and 11 ns, respectively.

Highly Enhanced H2 Sensing Performance of Few-Layer MoS2/SiO2/Si Heterojunctions by Surface Decoration of Pd Nanoparticles

A novel few-layer MoS₂/SiO₂/Si heterojunction is fabricated via DC magnetron sputtering technique, and Pd nanoparticles are further synthesized on the device surface. The results demonstrate that the fabricated sensor exhibits highly enhanced responses to H₂ at room temperature due to the decoration of Pd nanoparticles. For example, the Pd-decorated MoS₂/SiO₂/Si heterojunction shows an excellent response of 9.2 × 10³% to H₂, which is much higher than the values for the Pd/SiO₂/Si and MoS₂/SiO₂/Si heterojunctions. In addition, the H₂ sensing properties of the fabricated heterojunction are dependent largely on the thickness of the Pd-nanoparticle layer and there is an optimized Pd thickness for the device to achieve the best sensing characteristics. Based on the microstructure characterization and electrical measurements, the sensing mechanisms of the Pd-decorated MoS₂/SiO₂/Si heterojunction are proposed. These results indicate that the Pd decoration of few-layer MoS₂/SiO₂/Si heterojunctions presents an effective strategy for the scalable fabrication of high-performance H₂ sensors.

Large Lateral Photovoltaic Effect in MoS2/GaAs Heterojunction

Molybdenum disulfide (MoS₂) nanoscaled films are deposited on GaAs substrates via magnetron sputtering technique, and MoS₂/GaAs heterojunctions are fabricated. The lateral photovoltaic effect (LPE) of the fabricated MoS₂/GaAs heterojunctions is investigated. The results show that a large LPE can be obtained in the MoS₂/n-GaAs heterojunction. The LPE exhibits a linear dependence on the position of the laser illumination and the considerably high sensitivity of 416.4 mV mm^-1. This sensitivity is much larger than the values in other reported MoS₂-based devices. Comparatively, the LPE in the MoS₂/p-GaAs heterojunction is much weaker. The mechanisms to the LPE are unveiled by constructing the energy-band alignment of the MoS₂/GaAs heterojunctions. The excellent LPE characteristics make MoS₂ films combined with GaAs semiconductors promising candidates for the application of high-performance position-sensitive detectors.

Sb2S3 thickness-dependent lateral photovoltaic effect and time response observed in glass/FTO/CdS/Sb2S3/Au structure

As an interesting one dimensional ribbon material, Sb₂S₃ has recently attracted much attention in recent years due to its exciting optical properties. However, Sb₂S₃-based photovoltaic or photoelectronic devices are still in research, and there are many things unknown to us and need to be well studied. In this work, the glass/FTO/CdS/Sb₂S₃/Au structures were successfully prepared with different Sb₂S₃ thicknesses, and the lateral photovoltaic effect (LPE) was firstly observed in this structure, suggesting its great potential in position sensitivity detectors (PSD). It is demonstrated that the crystallinity of Sb₂S₃ film increases, and Sb₂S₃ film tends to be vertical ribbon orientation with increasing thickness. Owing to the strong light absorption of the thicker Sb₂S₃ film and its one dimensional ribbon like crystal structure, the LPE in the glass/FTO/CdS/Sb₂S₃/Au structure improves with increasing Sb₂S₃ thickness from 350 nm to 800 nm, and the glass/FTO/CdS/Sb₂S₃(800 nm) structure exhibits an unprecedented performance with position sensitivity as large as 2230.4 mV/mm. Moreover, the time response of photovoltage was also firstly measured in this structure, it is observed that both the rise time and the fall time decrease with increasing thickness from 350 nm to 800 nm, and then increase quickly for 1100 nm film, further verifing that the Sb₂S₃ thickness-dependent LPE is strongly dependent on the carriers' longitudinal transport time. The very large LPE and the relatively fast response speed observed in the glass/FTO/CdS/Sb₂S₃(800 nm)/Au structure unveils its great potential applications in the optoelectronic detectors and also bring an insight that the suitable thickness is very crucial in Sb₂S₃-based devices.