Growth of Wafer-Scale Standing Layers of WS2 for Self-Biased High-Speed UV-Visible-NIR Optoelectronic Devices

Ultrathin Bi2O2Se/Si Heterojunction Photodetector with Tunneling Oxide Passivation for Enhanced Optoelectronic Performance

Two-dimensional (2D) materials have garnered significant attention for next-generation optoelectronic devices due to their exceptional physical properties. This study introduces a high-performance ultrathin Bi2O2Se/Si heterojunction photodetector with tunneling oxide passivation, fabricated using a transfer-free pulsed laser deposition method. The Bi2O2Se layer exhibits strong air stability and compatibility for practical applications. By incorporating a thin SiO2 tunneling layer, the heterostructure achieves a low dark current (∼22.3 nA/cm2), a high on/off ratio (∼8 × 106), and a responsivity of 23.0 A/W. Compared to traditional CdS/Si devices, this photodetector demonstrates superior performance, including faster response time and higher stability. These findings underscore the potential of Bi2O2Se/Si heterostructures for advanced photonic and optoelectronic applications.

High performance in the DC sputtering-fabricated Au/WS2 optoelectronic device

A key component of the transition metal dichalcogenides (TMDCs) family, tungsten disulfide (WS2) offers exceptional optoelectronic characteristics and significant promise for high-sensitivity photodetection. This study utilized chemical vapor deposition (CVD) to grow WS2 nanoflakes on a SiO2/Si substrate. The WS2/SiO2/Si substrate was then coated with a thin layer of gold (Au) to create Au/WS2 nanosheets. The resultant samples' crystalline structure and optical properties were investigated using Raman, UV-visible, and X-ray diffraction spectroscopies. Finally, we characterized the photodetectors' current and voltage under various power and wavelength settings. Following the deposition of the Au layer, the photodetector's responsivity, detectivity, and external quantum efficiency (EQE) all improved. Under a 635 nm laser with a power density of 10 mW/cm2, the Au/WS2 photodetector exhibited a high EQE of 138.91, a specific detectivity of 11 × 10¹⁰ Jones, and a responsivity of 71.09 mA/W. The current-time (It) measurement evaluates that Au/WS2/SiO2/Si photodetector responds better and at a faster response speed. Consequently, this hybrid material shows great potential for future optical device applications.

Advanced T-In2Se3/M-WS2/B-WSe2 Photodetectors Enabled by Cascaded Band Tailoring and Charge Reservoir Engineering

Taking advantage of their unparalleled electrostatic and optoelectronic properties, 2D layered materials (2DLMs) have emerged as alluring building blocks for crafting advanced photodetectors. Nevertheless, preceding research has predominantly concentrated on rudimentary designs incorporating single-channel or single-junction setups, failing to exert the full potency of 2DLMs. Therefore, there is still an imperative requirement to develop innovative device architectures grounded in novel physical mechanisms. Herein, a T-In2Se3/M-WS2/B-WSe2 heterojunction photodetector boasting pronounced gate-tunability is devised, achieving remarkable light on/off ratio of 5.8 × 104 and detectivity of 1.1 × 1013 Jones at Vgs = -25 V, alongside competitive responsivity and gain of 633 A W-1 and 1943 at Vgs = 30 V. Energy band analysis has determined that the former is associated with the synergy of the cascaded band alignment and the high degree of depletion effect, while the latter is ascribed to the intermediate electron reservoir enabling high-efficiency spacial separation of photoexcited electron-hole pairs. Leveraging this device as the pivotal sensing component, proof-of-concept applications spanning broadband optoelectronic imaging and automatic driving are demonstrated. This study presents a novel paradigm for constructing 2DLM-based photodetectors with outstanding comprehensive performance, thereby establishing a fascinating platform capable of catering to the diverse demands of next-generation optoelectronic industry.

Advances in borophene based photodetectors for a sustainable tomorrow: a comprehensive review

Borophene, with its unique properties such as excellent conductivity, high thermal stability, and tunable electronic band structure, holds immense promise for advancing photodetector technology. These qualities make it an attractive material for enhancing the efficiency and performance of photodetectors across various wavelengths. Research so far has highlighted borophene's potential in improving sensitivity, response time, and overall functionality in optoelectronic devices. However, to fully realize the potential of borophene-based photodetectors, several challenges must be addressed. A major hurdle is the reproducibility and scalability of borophene synthesis, which is essential for its widespread use in practical applications. Furthermore, understanding the underlying physics of borophene and optimizing the device architecture are critical for achieving consistent performance under different operating conditions. These challenges must be overcome to enable the effective integration of borophene into commercial photodetector devices. A thorough evaluation of borophene-based photodetectors is necessary to guide future research and development in this field. This review will provide a detailed account of the current synthesis methods, discuss the experimental results, and identify the challenges that need to be addressed. Additionally, the review will explore potential strategies to overcome these obstacles, paving the way for significant advancements in solar cells, light-based sensors, and environmental monitoring systems. By addressing these issues, the development of borophene-based photodetectors could lead to substantial improvements in optoelectronic technology, benefiting various applications and industries.

Vertical Barrier Heterostructures for Reliable and High-Performance Self-Powered Infrared Detection

With their fascinating properties, emerging two-dimensional (2D) materials offer innovative ways to prepare high-performance infrared (IR) detectors. However, the current performance of 2D IR photodetectors is still below the requirements for practical application owing to the severe interfacial recombination, sharply raised contact resistance, and deteriorated metal conductivity at nanoscale. Here, we introduce a vertical barrier heterojunction with a structure of PtSe2/GaAs that combines the excellent optoelectronic properties of transition metal sulfides with topological semi-metals, which allows for an adjustable bandgap and high carrier mobility. The heterojunction was fabricated using the wet transfer method. The heterostructures show significant rectification behaviors and photovoltaic effects, which allow it to operate as a self-driven photodetector at zero bias. The photoresponse parameters at 850 nm with zero bias voltage are 67.2 mA W-1, 6.7 × 1012 Jones, 9.8%, 3.8 × 105, 164 μs, and 198 μs for the responsivity, specific detectivity, external quantum efficiency, Ilight/Idark ratio, rise time, and fall time, respectively. Moreover, the heterojunction is highly sensitive to a wide spectral band from ultraviolet to near-infrared (360-1550 nm). At the same time, this heterostructure demonstrates significant potential for applications in IR polarized light detection and room-temperature high-resolution IR imaging. The excellent properties of the heterojunction make it well-suited for high-performance, self-powered IR detection.

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.

Interfacial Effect on the Transient Dielectric Function and Charge Transfer in a Monolayer WS2/Si Heterojunction

Monolayer tungsten disulfide (WS2) is a highly promising material for silicon photonics. Thus, the WS2/Si interface plays a very important role due to the interfacial complex effects and abundant states. Among them, the effect of charge transfer on exciton dynamics and the optoelectronic property is determined by the dielectric function, which is very crucial for the performance of optoelectronic devices. However, research on the exciton dynamics or the transient dielectric function of WS2 in such WS2/Si junctions is still rare. In this work, both the transient dielectric function and charge transfer of WS2/Si heterojunctions are analyzed based on the transient reflectance spectra measured by the pump-probe spectrometer. The dynamic processes of the A exciton, affected by charge transfer within the WS2/Si heterojunction, are interpreted. Moreover, the transient dielectric function of WS2 is quantitatively analyzed. The dielectric function of WS2 exhibits a notable 19% change, persisting for more than 180 ps within the WS2/Si heterojunction. These findings can pave the way for the advancement of silicon photonic devices based on WS2.

Broadband high-performance vertical WS1.08Se0.92/Si heterojunction photodetector with MXene electrode

Vertical semiconductor van der Waals heterojunctions are essential for fabricating high-performance photodetectors. However, the range of the spectral response and defect states of semiconductor materials are two critical factors affecting the performance of photodetectors. In this work, the spectral response range of WS2was changed through WS2band gap regulation, and a self-powered vertical WS1.08Se0.92/Si heterojunction photodetector with MXene electrode was prepared by synthesizing WS1.08Se0.92film on Si substrate and vertically stacking Ti3C2TxMXene on the film. Due to the electron collection of MXene and the wonderful junction quality of WS1.08Se0.92/Si, the photodetector can detect near-infrared light in the range of 980-1310 nm, which exceed the detection limit of WS1.08Se0.92. And the device had high sensitivity in the broadband. The responsivity was 4.58 A W-1, the specific detectivity was 4.58 × 1011Jones, the on/off ratio was 4.95 × 103, and the fast response time was 9.81/9.03μs. These properties are superior to previously reported WS2-based photodetectors. Vertical structure, Energy band tuning, and MXene electrode provide a new idea for preparing broadband high-performance and self-powered photodetector.

Uncooled Mid-Infrared Sensing Enabled by Chip-Integrated Low-Temperature-Grown 2D PdTe2 Dirac Semimetal

Next-generation mid-infrared (MIR) imaging chips demand free-cooling capability and high-level integration. The rising two-dimensional (2D) semimetals with excellent infrared (IR) photoresponses are compliant with these requirements. However, challenges remain in scalable growth and substrate-dependence for on-chip integration. Here, we demonstrate the inch-level 2D palladium ditelluride (PdTe2) Dirac semimetal using a low-temperature self-stitched epitaxy (SSE) approach. The low formation energy between two precursors facilitates low-temperature multiple-point nucleation (∼300 °C), growing up, and merging, resulting in self-stitching of PdTe2 domains into a continuous film, which is highly compatible with back-end-of-line (BEOL) technology. The uncooled on-chip PdTe2/Si Schottky junction-based photodetector exhibits an ultrabroadband photoresponse of up to 10.6 μm with a large specific detectivity. Furthermore, the highly integrated device array demonstrates high-resolution room-temperature imaging capability, and the device can serve as an optical data receiver for IR optical communication. This study paves the way toward low-temperature growth of 2D semimetals for uncooled MIR sensing.

A fully printed ultrafast Si/WS2 quantum dot photodetector with very high responsivity over the UV to near-infrared region

Two-dimensional (2D) semiconducting material-based photodetectors (PDs) with high responsivity and fast photo-response are of great interest for various applications such as optical communications, biomedical imaging, security surveillance, environmental monitoring, etc. Additive manufacturing such as 2D printing is a potentially less cumbersome and cost-effective alternative to conventional microdevice fabrication processes used in the production of PDs. Here, we have fabricated a Si/WS2 quantum dot-based heterostructure PD with a very short electrode gap of 40 μm by a simple printing process. The printed p-Si/n-WS2 PD shows an excellent photo-to-dark current ratio of 5121 under 405 nm illumination (23.8 mW cm-2). The printed photodetector exhibits a peak responsivity of 126 A W-1 and a peak detectivity of 9.24 × 1012 Jones over a very broad wavelength range (300-1100 nm), which is much superior to commercial Si PDs. A high external quantum efficiency of 3.9 × 104% and an ultrafast photoresponse (7.8 μs rise time and 9.5 μs fall time) make the device an attractive candidate as an efficient photodetector. The origin of high-performance photodetection is traced to a nearly defect-free interface at the heterojunction, leading to highly efficient charge separation and high photocurrent. Finally, the 2D-printed device exhibits good photodetection even in self-powered conditions, which is very attractive.

High-Performance Graphene Nanowalls/Si Self-Powered Photodetectors with HfO2 as an Interfacial Layer

Graphene/silicon (Si) heterojunction photodetectors are widely studied in detecting of optical signals from near-infrared to visible light. However, the performance of graphene/Si photodetectors is limited by defects created in the growth process and surface recombination at the interface. Herein, a remote plasma-enhanced chemical vapor deposition is introduced to directly grow graphene nanowalls (GNWs) at a low power of 300 W, which can effectively improve the growth rate and reduce defects. Moreover, hafnium oxide (HfO₂) with thicknesses ranging from 1 to 5 nm grown by atomic layer deposition has been employed as an interfacial layer for the GNWs/Si heterojunction photodetector. It is shown that the high-k dielectric layer of HfO₂ acts as an electron-blocking and hole transport layer, which minimizes the recombination and reduces the dark current. At an optimized thickness of 3 nm HfO₂, a low dark current of 3.85 × 10^-10, with a responsivity of 0.19 AW^-1, a specific detectivity of 1.38 × 10¹² as well as an external quantum efficiency of 47.1% at zero bias, can be obtained for the fabricated GNWs/HfO₂/Si photodetector. This work demonstrates a universal strategy to fabricate high-performance graphene/Si photodetectors.

Integration of photovoltaic and photogating effects in a WSe2/WS2/p-Si dual junction photodetector featuring high-sensitivity and fast-response

Two-dimensional (2D) material-based van der Waals (vdW) heterostructures with exotic semiconducting properties have shown tremendous potential in next-generation photovoltaic photodetectors. Nevertheless, these vdW heterostructure devices inevitably suffer from a compromise between high sensitivity and fast response. Herein, an ingenious photovoltaic photodetector based on a WSe₂/WS₂/p-Si dual-vdW heterojunction is demonstrated. First-principles calculations and energy band profiles consolidate that the photogating effect originating from the bottom vdW heterojunction not only strengthens the photovoltaic effect of the top vdW heterojunction, but also suppresses the recombination of photogenerated carriers. As a consequence, the separation of photogenerated carriers is facilitated and their lifetimes are extended, resulting in higher photoconductive gain. Coupled with these synergistic effects, this WSe₂/WS₂/p-Si device exhibits both high sensitivity (responsivity of 340 mA W^-1, a light on/off ratio greater than 2500, and a detectivity of 3.34 × 10¹¹ Jones) and fast response time (rise/decay time of 657/671 μs) under 405 nm light illumination in self-powered mode. Finally, high-resolution visible-light and near-infrared imaging capabilities are demonstrated by adopting this dual-heterojunction device as a single pixel, indicating its great application prospects in future optoelectronic systems.

Uniformity of HfO2 Thin Films Prepared on Trench Structures via Plasma-Enhanced Atomic Layer Deposition

In this study, we assessed the physical and chemical properties of HfO₂ thin films deposited by plasma-enhanced atomic layer deposition (PEALD). We confirmed the self-limiting nature of the surface reactions involved in the HfO₂ thin film's growth by tracing the changes in the growth rate and refractive index with respect to the different dose times of the Hf precursor and O₂ plasma. The PEALD conditions were optimized with consideration of the lowest surface roughness of the films, which was measured by atomic force microscopy (AFM). High-resolution X-ray photoelectron spectroscopy (XPS) was utilized to characterize the chemical compositions, and the local chemical environments of the HfO₂ thin films were characterized based on their surface roughness and chemical compositions. The surface roughness and chemical bonding states were significantly influenced by the flow rate and plasma power of the O₂ plasma. We also examined the uniformity of the films on an 8″ Si wafer and analyzed the step coverage on a trench structure of 1:13 aspect ratio. In addition, the crystallinity and crystalline phases of the thin films prepared under different annealing conditions and underlying layers were analyzed.

Vertically oriented ReS2(1-x)Se2x nanosheet-formed porous arrays on SiO2/Si substrates for ultraviolet-visible photoelectric detection

Rhenium (Re)-based transition metal dichalcogenides (TMDs) have excellent in-plane anisotropic optical and electrical properties. However, their distorted octahedral (1T') structure and weak interlayer coupling easily lead to anisotropic and out-of-plane growth, which makes it particularly difficult to prepare large-area Re-based TMD continuous porous films on SiO₂/Si substrates. In this work, ReS₂ films are synthesized on SiO₂/Si substrates by using tellurium (Te) powder-assisted chemical vapor deposition, and then the films are selenized to synthesize a series of continuous large-area ReS_2(1-x)Se_2x (x = 0, 0.34, 0.56, 0.84, and 0.91) nanosheet-formed porous films. Furthermore, prototype ReS_2(1-x)Se_2x photodetectors with different Se compositions are fabricated. The surface morphology, high quality crystallization and compositions are confirmed by various characterization techniques. The ReS_2(1-x)Se_2x photodetectors based on these films show excellent ultraviolet-visible (UV-vis) spectral responses and self-powered characteristics. The response time is faster, and the photocurrent increases with the Se composition. Due to the Schottky barrier generated by the Ag-ReS_2(1-x)Se_2x interface, the device without bias voltage has a superior responsivity (121.9 mA W^-1), high detectivity (5.27 × 10¹² Jones), good on/off ratio (1.2 × 10³) and fast response time (rising/decay times, 30/60 ms) under 365 nm light irradiation. This simple and controllable method opens up a new way to produce high-quality vertically oriented ReS_2(1-x)Se_2x porous arrays on SiO₂/Si substrates for next-generation application in UV-vis photodetectors.

Non-cytotoxic Dy3+ activated La10W22O81 nanophosphors for UV based cool white LEDs and anticancer applications

White-light-emitting La₁₀W₂₂O₈₁ (LWO): xDy³⁺ (0.5 ≤ x ≤ 10 mol%) nanocrystalline phosphors were developed by a facile hydrothermal assisted solid-state reaction. X-ray diffraction (XRD) pattern indicated that the prepared samples adopted orthorhombic crystal structures. The agglomeration of uniform nanorods was identified from the FE-SEM analysis of the optimized LWO: 1.5 mol% Dy³⁺ nanocrystalline phosphors. Additionally, transmission electron microscope, scanning transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy were employed to explore the surface morphology, size, interplanar distance, and chemical composition with valence states of the LWO: 1.5 mol% Dy³⁺ phosphors, respectively. By exciting with 387 nm, the LWO: Dy³⁺ emission spectra showed two intense peaks at 476 nm (⁴F_9/2→⁶H_15/2) and 571 nm (⁴F_9/2→⁶H_13/2) and a shoulder peak at 659 nm (⁴F_9/2→⁶H_11/2). Optimum emission intensity was achieved for 1.5 mol% Dy³⁺ in the LWO host lattice. The luminescence quenching beyond 1.5 mol% Dy³⁺ is attributed to the dipole-dipole interactions when the Dy³⁺ (donor) and Dy³⁺ (acceptor) ions are at a critical distance of 58.53 Å. Photometric studies were conducted to evaluate the performance and practical applicability of the phosphors. The CIE chromaticity diagram suggests that the LWO: 1.5 mol% Dy³⁺ nanophosphor conspicuously exhibits cool white light. Therefore, this material could be a promising and potential white light-emitting nanocrystalline phosphor material for white light emitting diodes (LEDs) under near-UV excitation. In addition, the toxicity of the optimized nanophosphor in normal WI-38 lung fibroblast cells and MCF-7 breast cancer cells was examined. Surprisingly, LWO: 1.5 mol% Dy³⁺ nanophosphor was found to be non-cytotoxic to normal cells, but extremely toxic to cancer cells. Therefore, the nanophosphor materials can be considered potential candidates for biomedical applications, particularly for cancer treatment.

Synthesis of WS2 by Chemical Vapor Deposition: Role of the Alumina Crucible

The role of the alumina crucible for the tungsten disulfide (WS2) growth on silicon dioxide substrates (SiO2/Si) under atmospheric pressure chemical vapor deposition (APCVD) was investigated. Both synthesis and properties of the APCVD-WS2 depend on the number of growth cycles when using the same alumina crucible. It was discovered that there is an ideal condition for the material’s synthesis, which is characterized by an increase in the photoluminescence (PL) yield and larger WS2 triangles. It usually happens for the first three growth cycles. For the fourth cycle and beyond, the PL decreases gradually. Simultaneously, atomic force microscopy images revealed no important changes in the topography of the WS2 flakes. As a function of the number of synthesis cycles, the progressive decrease in PL yield could be associated with materials with a higher density of defects, as identified by the LA(M)/A1g(M)−LA(M) ratio from Raman data using the green line.

A defect-induced broadband photodetector based on WS2/pyramid Si 2D/3D mixed-dimensional heterojunction with a light confinement effect

Broadband photodetection is of vital importance for both civil and technological applications. The widespread use of commercial photodiodes based on traditional semiconductors (e.g. GaN, Si, and InGaAs) is limited to the relatively narrow response range. In this work, we have demonstrated a self-driven and broadband photodetector based on WS₂/pyramid Si 2D/3D mixed-dimensional van der Waals (vdW) heterojunction, which is assembled by directly transferring 2D WS₂ film on 3D pyramid Si. Thanks to the enhanced light absorption with the pyramid Si structure, the defect-induced narrowed bandgap of the WS₂ film, and high-quality vdW heterojunction, impressive device performances in terms of a large responsivity of 290 mA W^-1, a high specific detectivity of up to 2.6 × 10¹⁴ Jones and an ultrabroad response spectrum ranging from 265 nm to 3.0 μm are achieved at zero bias. Importantly, the photodetector can function as an infrared imaging cell with a high spatial resolution. The totality of these excellent features confirms that the demonstrated WS₂/pyramid Si 2D/3D mixed-dimensional vdW heterojunction device may hold great promise for applications in high-performance broadband infrared photodetection and imaging.

van der Waals Epitaxial Growth of Borophene on a Mica Substrate toward a High-Performance Photodetector

The emergence of borophene has triggered soaring interest in the investigation of its superior structural anisotropy, a novel photoelectronic property for diverse potential applications. However, the structural instability and need of a metal substrate for depositing borophene restrict its large-scale applications toward high-performance electronic and optoelectric devices. van der Waals epitaxy is regarded as an efficient technique for growing superb two-dimensional materials onto extensive functional substrates, but the preparation of stable and controllable borophene on nonmetallic substrates is still not reported. Here, we demonstrate that borophene films can be synthesized onto a mica substrate by van der Waals epitaxy, where hydrogen and NaBH₄ are respectively used as the carrier gas and the boron source. The lattice structure of the as-synthesized borophene coincides with the predicted α'-boron sheet. The borophene-based photodetector shows an excellent photoresponsivity of 1.04 A W^-1 and a specific detectivity of 1.27 × 10¹¹ Jones at a reversed bias of 4 V under illumination of a 625 nm light-emitting diode, which are remarkably superior to those of reported boron nanosheets. This work facilitates further studies of borophene toward its attractive properties and applications in novel optoelectronic devices and integrated circuits.

Unfolding photophysical properties of poly(3-hexylthiophene)-MoS2organic-inorganic hybrid materials: an application to self-powered photodetectors

Self-powered photodetectors have grown as inevitable members of the optoelectronic device family. However, it is still challenging to achieve self-powered photodetection with good responsivity in the visible spectrum region. Herein, we report solution-processable poly(3-hexylthiophene) (P3HT)-molybdenum disulfide (MoS₂) organic-inorganic hybrid material, which can be used as the active layer in self-powered photodetectors. The morphological and structural properties of the synthesized P3HT-MoS₂hybrid material has been discussed using atomic force microscopy and transmission electron microscopy, respectively. The hybrid material loaded with 1 wt% MoS₂has shown an enhancement in the self-assembly of polymer in the form of fibrillar formation and excellent structural features in terms ofπ-conjugation. The self-powered photodetectors have been fabricated in indium tin oxide (ITO) coated glass/P3HT-MoS₂/Al configuration. The merit of P3HT-MoS₂hybrid photodetectors is measured under the illumination of 470, 530, and 627 nm light in ambient conditions. P3HT-MoS₂photodetectors show significantly higher responsivity and detectivity. The photo responsivity and detectivity in P3HT-MoS₂devices are found to be 271.2 mA W^-1and 4.4 × 10¹⁰jones at zero bias, respectively, for 470 nm light with the optical power density of 74.1μW cm^-2. Furthermore, the photocurrent switching behaviour at periodic illuminations of 1 Hz has also been examined for P3HT-MoS₂self-powered photodetectors.

Ultrabroadband and High-Detectivity Photodetector Based on WS2/Ge Heterojunction through Defect Engineering and Interface Passivation

Broadband photodetectors are of great importance for numerous optoelectronic applications. Two-dimensional (2D) tungsten disulfide (WS₂), an important family member of transition-metal dichalcogenides (TMDs), has shown great potential for high-sensitivity photodetection due to its extraordinary properties. However, the inherent large bandgap of WS₂ and the strong interface recombination impede the actualization of high-sensitivity broadband photodetectors. Here, we demonstrate the fabrication of an ultrabroadband WS₂/Ge heterojunction photodetector through defect engineering and interface passivation. Thanks to the narrowed bandgap of WS₂ induced by the vacancy defects, the effective surface modification with an ultrathin AlO_x layer, and the well-designed vertical n-n heterojunction structure, the WS₂/AlO_x/Ge photodetector exhibits an excellent device performance in terms of a high responsivity of 634.5 mA/W, a large specific detectivity up to 4.3 × 10¹¹ Jones, and an ultrafast response speed. Significantly, the device possesses an ultrawide spectral response spanning from deep ultraviolet (200 nm) to mid-wave infrared (MWIR) of 4.6 μm, along with a superior MWIR imaging capability at room temperature. The detection range has surpassed the WS₂-based photodetectors in previous reports and is among the broadest for TMD-based photodetectors. Our work provides a strategy for the fabrication of high-performance ultrabroadband photodetectors based on 2D TMD materials.

A reasonably designed 2D WS2 and CdS microwire heterojunction for high performance photoresponse

Heterojunctions based on low-dimensional materials can combine the superiorities of each component and realize novel properties. Herein, a mixed-dimensional heterojunction comprising multilayer WS₂, CdS microwire, and few-layer WS₂ has been demonstrated. The working mechanism and its application in a photodetector are investigated. The multilayer WS₂ and CdS microwire are utilized to provide efficient light absorption, while the few-layer WS₂ is utilized to passivate interfacial impurity scattering. In addition, based on the reasonable band alignment of the components, three built-in electric fields are formed, which efficiently separate the photo-generated carriers and enhance the photocurrent. In particular, the photo-generated electrons are trapped in CdS, while the photo-generated holes circulate in the external circuit, leading to a high photoconductivity gain. Motivated by these, we constructed a device that exhibits a photoresponsivity of ∼4.7 A W^-1, a response/recovery time of 13.7/15.8 ms, and a detectivity of 3.4 × 10¹² Jones, which are much better than the counterparts. All of these clearly demonstrate the importance of advanced device designs for realizing high performance optoelectronic devices.

A mechanism for the variation in the photoelectric performance of a photodetector based on CVD-grown 2D MoS2

Two-dimensional transition-metal dichalcogenides are considered as promising candidates for next-generation flexible nanoelectronics owing to their compelling properties. The photoelectric performance of a photodetector based on CVD-grown 2D MoS₂ was studied. It is found that annealing treatment can make the photoresponsivity and specific detectivity of the CVD-grown 2D MoS₂ based photodetector increase from 0.1722 A W^-1 and 10^14.65 Jones to 0.2907 A W^-1 and 10^14.84 Jones, respectively, while vulcanization can make the rise response time and fall response time decrease from 0.9013 s and 2.173 s to 0.07779 s and 0.08616 s, respectively. A method to determine the O-doping concentration in the CVD-grown 2D MoS₂ has been obtained. The criterion for the CVD-grown 2D MoS₂ to transition from an oxygen-doped state to a pure state has been developed. A mechanism explaining the variation in the photoelectric performance of the CVD-grown 2D MoS₂ has been proposed. The CVD-grown 2D MoS₂ and the annealed CVD-grown 2D MoS₂ are oxygen-doped MoS₂ while the vulcanized CVD-grown 2D MoS₂ is pure MoS₂. The variation in the photoelectric performance of CVD-grown 2D MoS₂ results from differences in the O-doping concentration and the bandgap.

Vertically-Oriented WS2 Nanosheets with a Few Layers and Its Raman Enhancements

Vertically-oriented two-dimensional (2D) tungsten disulfide (WS2) nanosheets were successfully grown on a Si substrate at a temperature range between and 550 °C via the direct chemical reaction between WCl6 and S in the gas phase. The growth process was carefully optimized by adjusting temperature, the locations of reactants and substrate, and carrier gas flow. Additionally, vertically-oriented 2D WS2 nanosheets with a few layers were tested as a surface-enhanced Raman scattering substrate for detecting rhodamine 6G (R6G) molecules where enhancement occurs from chemical enhancement by charge transfer transition from semiconductor). Raman spectra of R6G molecules adsorbed on vertically-oriented 2D WS2 nanosheets exhibited strong Raman enhancement effects up to 9.2 times greater than that on the exfoliated WS2 monolayer flake sample. From our results, we suggest that the WS2 nanosheets can be an effective surface-enhanced Raman scattering substrate for detecting target molecules.

Room-Temperature Gas Sensors Under Photoactivation: From Metal Oxides to 2D Materials

Room-temperature gas sensors have aroused great attention in current gas sensor technology because of deemed demand of cheap, low power consumption and portable sensors for rapidly growing Internet of things applications. As an important approach, light illumination has been exploited for room-temperature operation with improving gas sensor's attributes including sensitivity, speed and selectivity. This review provides an overview of the utilization of photoactivated nanomaterials in gas sensing field. First, recent advances in gas sensing of some exciting different nanostructures and hybrids of metal oxide semiconductors under light illumination are highlighted. Later, excellent gas sensing performance of emerging two-dimensional materials-based sensors under light illumination is discussed in details with proposed gas sensing mechanism. Originated impressive features from the interaction of photons with sensing materials are elucidated in the context of modulating sensing characteristics. Finally, the review concludes with key and constructive insights into current and future perspectives in the light-activated nanomaterials for optoelectronic gas sensor applications.

Wafer-size growth of 2D layered SnSe films for UV-Visible-NIR photodetector arrays with high responsitivity

Due to its excellent electrical and optical properties, tin selenide (SnSe), a typical candidate of two-dimensional (2D) semiconductors, has attracted great attention in the field of novel optoelectronics. However, the large-area growth of high-quality SnSe films still remains a great challenge, which limits their practical applications. Here, wafer-size SnSe ultrathin films with high uniformity and crystallization were deposited via a scalable magnetron sputtering method. The results showed that the SnSe photodetector was highly sensitive to a broad range of wavelengths in the UV-visible-NIR range, especially showing an extremely high responsivity of 277.3 A W-1 with the corresponding external quantum efficiency of 8.5 × 104% and detectivity of 7.6 × 1011 Jones. These figures of merits are among the best performances for the sputter-fabricated 2D photodetector devices. The photodetecting mechanisms based on a photogating effect induced by the trapping effect of localized defects are discussed in detail. The results indicate that the few-layered SnSe films obtained from sputtering growth have great potential in the design of high-performance photodetector arrays.

Robust Sample Preparation of Large-Area In- and Out-of-Plane Cross Sections of Layered Materials with Ultramicrotomy

Layered materials (LMs) such as graphene or MoS2 have attracted a great deal of interest recently. These materials offer unique functionalities due to their structural anisotropy characterized by weak van der Waals bonds along the out-of-plane axis and covalent bonds in the in-plane direction. A central requirement to access the structural information on complex nanostructures built upon LMs is to control the relative orientation of each sample prior to their inspection, e.g., with transmission electron microscopy (TEM). However, developing sample preparation methods that result in large inspection areas and ensure full control over the sample orientation while avoiding damage during the transfer to the TEM grid is challenging. Here, we demonstrate the feasibility of deploying ultramicrotomy for the preparation of LM samples in TEM analyses. We show how ultramicrotomy leads to the reproducible large-scale production of both in-plane and out-of-plane cross sections, with bulk vertically oriented MoS2 and WS2 nanosheets as a proof of concept. The robustness of the prepared samples is subsequently verified by their characterization by means of both high-resolution TEM and Raman spectroscopy measurements. Our approach is fully general and should find applications for a wide range of materials as well as of techniques beyond TEM, thus paving the way to the systematic large-area mass-production of cross-sectional specimens for structural and compositional studies.

Defect-induced room temperature ferromagnetism in Cu-doped In2S3 QDs

The practical application of existing diluted magnetic semiconductors (DMSs) depends crucially on improving their room temperature ferromagnetism. Doping, as an effective method, can be used to modulate the physical properties of semiconducting materials. Herein, we report on the observation of significant RTFM in a III-VI semiconductor compound doped with nonmagnetic impurities, Cu-doped In2S3 quantum dots (QDs) grown by a gas-liquid phase chemical deposition method. The effect of Cu doping on the electronic structure and optical and magnetic properties of In2S3 is studied systematically. The UV-vis and photoluminescence (PL) spectra reveal that Cu-doped In2S3 can moderately benefit the optical properties of pristine In2S3. Magnetic measurements show that the pristine In2S3 and Cu-doped In2S3 QDs exhibit obvious RTFM, which is ascribed to the role of intrinsic defects in accordance with the bound-magnetic-polaron (BMP) theory. Furthermore, first-principles calculations based on the spin density functional theory indicate that In vacancies and their complexes with Cu dopants play a crucial role in inducing ferromagnetism. These results suggest that the Cu-doped In2S3 QDs are promising candidates for spintronics and magneto-optical applications.

TiO2@TiO2-xHx core-shell nanoparticle film/Si heterojunction for ultrahigh detectivity and sensitivity broadband photodetector

A simple hydrogenation treatment is used to synthesize unique oxygen-deficient TiO₂ with a core/shell structure (TiO₂@TiO_2-xH_x), superior to the high H₂-pressure process (under 20 bar for five days). It is demonstrated that oxygen-deficient TiO₂ nanoparticle film/Si heterojunction possesses improved photoresponse performance compared to the untreated TiO₂ nanoparticle film/Si heterojunction. Particularly, under 900 nm of 0.5 μW cm^-2, the oxygen-deficient TiO₂ nanoparticle film (TiO₂@TiO_2-xH_x core-shell nanoparticle film)/Si heterojunction shows high responsivity (R) of 336 A W^-1, prominent sensitivity (S) of 1.3 × 10⁷ cm² W^-1, accompanied with a fast rise and decay time of 6 and 5 ms, respectively. Significantly, the detectivity (D*) of the photodetector is up to 1.17 × 10¹⁴ cm Hz^1/2 W^-1, which is better than that reported in metal oxide nanomaterials/Si heterojunction photodetectors, and is 4-5 orders of magnitude higher than some 2D nanosheets/Si heterojunctions of 10⁹-10¹⁰ cm Hz^1/2 W^-1, indicating the excellent ability to detect weak signals. The oxygen vacancies generated in amorphous shell TiO_2-xH_x make the Fermi level of TiO_2-x shift near the conduction band minimum and can lead to reduced dark current. The high absorption and reduced dark current of the heterojunction ensure excellent photoresponse properties of oxygen-deficient TiO₂ nanoparticle film/Si heterojunction. The H-reduced oxygen-deficient amorphous shell may be an excellent candidate to enhance the photoresponse performance of metal oxide/Si heterojunction.

Synthesis of Wafer-Scale Monolayer WS2 Crystals toward the Application in Integrated Electronic Devices

Two-dimensional transition-metal dichalcogenides (TMDCs) possess unique electronic and optical properties, which open up a new opportunity for atomically thin optoelectronic devices. Synthesizing large-scale monolayer TMDCs on the SiO₂/Si substrate is crucial for practical applications, however, it remains a big challenge. In this work, a method which combines chemical vapor deposition (CVD) and thermal evaporation was employed to grow monolayer tungsten disulfide (WS₂) crystals. Through controlling the density and the distribution of W precursors, a wafer-scale continuous uniform WS₂ film was achieved, with the structural and spectral characterizations confirming a monolayer configuration and a high crystalline quality. Wafer-scale field-effect transistor arrays based on the monolayer WS₂ were fabricated. The devices show superior electrical performances, and the maximal mobility is almost 1 order of magnitude higher than those of CVD-grown large-scale TMDC devices reported so far.

Chemical exfoliation efficacy of semiconducting WS2 and its use in an additively manufactured heterostructure graphene–WS2–graphene photodiode

In the present work, various chemical exfoliation routes for semiconducting two-dimensional (2D) layered material WS₂ are explored, which include magnetic stirring (MS), shear mixing (SM), and horn-tip (HT) sonication. Current–voltage measurements, Raman spectroscopy, and photoluminescence (PL) spectroscopy were used to characterize the drop-casted WS₂ nanosheets produced by these three techniques and our analysis revealed that HT sonication produced the most optimal dispersions. Heterostructure photodetector devices were then fabricated using inkjet printing of the HT sonicated dispersions of WS₂ and graphene. The photodetector device performance was measured using a stream of ON/OFF light pulses generated using a red laser with wavelength λ ∼ 660 nm, and an arbitrary waveform generator. From this analysis, the photoresponsivity and detectivity of the graphene–WS₂–graphene heterostructure devices were calculated to be ∼0.86 A W⁻¹ and ∼10¹³ Jones, respectively. Capacitance–voltage (C–V) and C–frequency (f) measurements were also conducted, where the V was swept from –6 V to +6 V, while the change in C was measured from f ∼ 20 kHz up to 3 MHz to gain insights into the nature of the graphene–WS₂ interface. From the C–V measurements, the C plateaued at ∼324.3 pF from ∼−4 V to +4 V for the lowest f of 20 kHz and it reduced to ∼200 pF from −6 V to ∼−4 V, and similarly from ∼4 V to 6 V, C was ∼190 pF. The decrease in C for V > +4 V and V < −4 V was attributed to the reduction of the interfacial barrier at the electrodes which is suggestive of a Schottky-based photodiode at the graphene–WS₂ interface. A sharp decrease in C from ∼315.75 pF at 25.76 kHz to ∼23.79 pF at 480 kHz (at 0 V bias) from the C–f measurements suggests a strong effect of interface trap density on C built-up at the graphene–WS₂ interface and the ensuing Schottky barrier height. Our work confirms the excellent potential of solution-cast, trilayer graphene–WS₂–graphene heterostructures as a promising photodetector platform using additively manufactured inkjet printing.

Among magnetic stirring (MS), shear mixing (SM), and horn-tip (HT) sonication for the chemical exfoliation of semiconducting WS₂, HT sonication resulted in stable dispersions which were used to demonstrate ink-jet printed graphene–WS₂–graphene photodiodes.