Fast Photoresponse Research Articles

Controllable Synthesis, Polar Behavior and Photoelectric Properties of BiOCl Microplates

Bismuth oxychloride (BiOCl) square microplates were prepared via a facile hydrothermal method. The X-ray diffraction patterns of the samples reveal a tetragonal BiOCl phase, and the scanning electron microscopy images show plate-like structures with large lateral size of 3~6 μm and thickness in the range of 100~300 nm. The effects of surfactant, reaction temperature and duration on the morphology of BiOCl powders are systematically investigated. The polar behavior of a BiOCl single-crystalline microplate is examined by using piezoresponse force microscopy evidenced over 80 pm displacement under 40 V bias voltage. In addition, the photoelectric performance of the BiOCl microplates is evaluated by using electrochemical workstation with three-electrode system, and large photocurrent densities (over 0.5 μA/cm2) and fast photoresponse (0.7~1.1 s) are detected by applying both 365 nm monochromatic light and sunlight illumination. The surface potential changes of BiOCl microplate under different light condition, characterized by in-situ Kelvin probe force microscopy, further verify the separation ability of the photo-induced charge carriers. These findings would be beneficial for further design photocatalytic and piezocatalytic materials.

Pinhole-free 2D Ruddlesden–Popper perovskite layer with close packed large crystalline grains, suitable for optoelectronic applications

Here, we achieved pinhole-free 2D Ruddlesden–Popper Perovskite (RPP) BA2PbI4 layers with close packed crystalline grains with dimension of about 30 × 30 µm2, which have been demonstrated to be favorable for optoelectronic applications, such as fast response RPP-based metal/semiconductor/metal photodetectors. We explored affecting parameters in hot casting of BA2PbI4 layers, and proved that oxygen plasma treatment prior to hot casting plays a significant role to achieve high quality close packed polycrystalline RPP layers at lower hot cast temperatures. Moreover, we demonstrate that crystal growth of 2D BA2PbI4 can be dominantly controlled by the rate of solvent evaporation through substrate temperature or rotational speed, while molarity of the prepared RPP/DMF precursor is the dominant factor that determines the RPP layer thickness, and can affect the spectral response of the realized photodetector. Benefiting from the high light absorption and inherent chemical stability of 2D RPP layers, we achieved high responsivity and stability, and fast response photodetection from perovskite active layer. We achieved a fast photoresponse with rise and fall times of 189 µs and 300 µs, and the maximum responsivity of 119 mA/W and detectivity of 2.15 × 108 Jones in response to illumination wavelength of 450 nm. The presented polycrystalline RPP-based photodetector benefits from a simple and low-cost fabrication process, suitable for large area production on glass substrate, a good stability and responsivity, and a promising fast photoresponse, even around that of exfoliated single crystal RPP-based counterparts. However, it is well known that exfoliation methods suffer from poor repeatability and scalability, which make them incompatible with mass production and large area applications.

Giant Superlinear Power Dependence of Photocurrent Based on Layered Ta2 NiS5 Photodetector.

Photodetector based on two-dimensional (2D) materials is an ongoing quest in optoelectronics. 2D photodetectors are generally efficient at low illuminating power but suffer severe recombination processes at high power, which results in the sublinear power-dependent photoresponse and lower optoelectronic efficiency. The desirable superlinear photocurrent is mostly achieved by sophisticated 2D heterostructures or device arrays, while 2D materials rarely show intrinsic superlinear photoresponse. This workreports the giant superlinear power dependence of photocurrent based on multilayer Ta2 NiS5 . While the fabricated photodetector exhibits good sensitivity (3.1 mS W-1 per □) and fast photoresponse (31 µs), the bias-, polarization-, and spatial-resolved measurements point to an intrinsic photoconductive mechanism. By increasing the incident power density from 1.5 to 200 µW µm-2 , the photocurrent power dependence varies from sublinear to superlinear. At higher illuminating conditions, prominent superlinearity is observed with a giant power exponent of γ = 1.5. The unusual photoresponse can be explained by a two-recombination-center model where density of states of the recombination centers (RC) effectively closes all recombination channels. The photodetector is integrated into camera for taking photos with enhanced contrast due to superlinearity. This work provides an effective route to enable higher optoelectronic efficiency at extreme conditions.

Fast-Response Micro-Phototransistor Based on MoS2/Organic Molecule Heterojunction.

Over the past years, molybdenum disulfide (MoS2) has been the most extensively studied two-dimensional (2D) semiconductormaterial. With unique electrical and optical properties, 2DMoS2 is considered to be a promising candidate for future nanoscale electronic and optoelectronic devices. However, charge trapping leads to a persistent photoconductance (PPC), hindering its use for optoelectronic applications. To overcome these drawbacks and improve the optoelectronic performance, organic semiconductors (OSCs) are selected to passivate surface defects, tune the optical characteristics, and modify the doping polarity of 2D MoS2. Here, we demonstrate a fast photoresponse in multilayer (ML) MoS2 by addressing a heterojunction interface with vanadylphthalocyanine (VOPc) molecules. The MoS2/VOPc van der Waals interaction that has been established encourages the PPC effect in MoS2 by rapidly segregating photo-generated holes, which move away from the traps of MoS2 toward the VOPc molecules. The MoS2/VOPc phototransistor exhibits a fast photo response of less than 15 ms for decay and rise, which is enhanced by 3ordersof magnitude in comparison to that of a pristine MoS2-based phototransistor (seconds to tens of seconds). This work offers a means to realize high-performance transition metal dichalcogenide (TMD)-based photodetection with a fast response speed.

Ternary GePdS3: 1D van der Waals Nanowires for Integration of High-Performance Flexible Photodetectors

One-dimensional (1D) van der Waals (vdW) materials are anticipated to leverage for high-performance, giant polarized, and hybrid-dimension photodetection owing to their dangling-bond free surface, intrinsic crystal structure, and weak vdW interaction. However, only a few related explorations have been conducted, especially in the field of flexible and integrated applications. Here, high-quality 1D vdW GePdS3 nanowires were synthesized and proven to be an n-type semiconductor. The Raman vibration and band gap (1.37-1.68 eV, varying from bulk to single chain) of GePdS3 were systemically studied by experimental and theoretical methods. The photodetector based on a single GePdS3 nanowire possesses fast photoresponse at a broadband spectrum of 254-1550 nm. The highest responsivity and detectivity reach up to ∼219 A/W and ∼2.7 × 1010 Jones (under 254 nm light illumination), respectively. Furthermore, an image sensor with 6 × 6 pixels based on GePdS3 nanowires is integrated on a flexible polyethylene terephthalate (PET) substrate and exhibits sensitive and homogeneous detection at 808 nm light. These results indicate that the ternary noble metal chalcogenides show great potential in flexible and broadband optoelectronics applications.

Fast and broadband photoresponse in CdO thin film

Thermal annealing is recognized as an effective tool to manage the structural and electronic properties of oxide thin films. Combined with the atmospheric environment control, additional issues concerning surface effects act to change the performance of oxides for photodetectors purposes. The present investigation systematically studies morphological and optical properties and electrical transport of CdO thin films using an as-grown reference and annealed samples at 500 °C under O2 and N2 atmospheres. As-grown CdO presents fast and high photoresponses in a wide range of the visible spectrum, with an amplitude that reaches nearly 3000%. Also, a transition from negative to positive photoresistance was observed for temperatures below 230 K. From the analysis of the photoluminescence spectra, we noted two activation energies for as-grown CdO, whereas annealed samples presented only one. Therefore, the additional defect level, reduced after annealing, is responsible for the huge amplitude as well as the negative to positive transition in the photoresistance of as-grown CdO film.

Vertical asymmetric metal-semiconductor-metal photodiode based on β-Ga2O3 thin films fabricated via solution process for arc discharge detection

Arc discharge detection is a preeminent strategy for diagnosing aging of high-power electric equipment. Therefore, the timely detection of arc discharge can prevent immense disasters, such as massive fire accidents. Ultraviolet C (UVC) photodetectors (PD) are effective devices for optical arc discharge detection because of their solar blindness, which enables noise-free and highly selective arc discharge detection. In this study, we developed a solution-processed β-Ga2O3-based vertical asymmetric metal-semiconductor-metal (MSM) UVC PD as a high-performance and inexpensive UVC detector. The high bandgap of photoactive β-Ga2O3 (4.7–5.1 eV) enables selective UVC detection. The Schottky barrier heights at two different metal-semiconductor junctions (PEDOT:PSS/β-Ga2O3 and ITO/β-Ga2O3) were systematically modulated by varying the β-Ga2O3 film annealing temperature and thickness, which enabled fast photoresponse and self-powered operation, as demonstrated by various optical and electrical analyses. The optimized device exhibited a fast photoresponse (rise/fall time of 67/53 ms) and high responsivity (41 mA W−1) and detectivity (1.5×1011 Jones) in self-powered mode. The instantaneous arc discharge was successfully detected using the β-Ga2O3 based vertical MSM PD, demonstrating its feasibility for arc discharge detection. Thus, this study provides new insights into inexpensive self-powered PDs that can be implemented for the facile and prompt monitoring of aging electric equipment and prevention of potential fire accidents.

Gate-Tunable van der Waals Photodiodes with an Ultrahigh Peak-to-Valley Current Ratio.

Photodetectors and imagers based on 2D layered materials are currently subject to a rapidly expanding application space, with an increasing demand for cost-effective and lightweight devices. However, the underlying carrier transport across the 2D homo- or heterojunction channel driven by the external electric field, like a gate or drain bias, is still unclear. Here, a visible-near infrared photodetector based on van der Waals stacked molybdenum telluride (MoTe2 ) and black phosphorus (BP) is reported. The type-I and type-II band alignment can be tuned by the gate and drain voltage combined showing a dynamic modulation of the conduction polarity and negative differential transconductance. The heterojunction devices show a good photoresponse to light illumination ranging from 520-2000nm. The built-in potential at the MoTe2 /BP interface can efficiently separate photoexcited electron-hole pairs with a high responsivity of 290mAW-1 , an external quantum efficiency of 70%, and a fast photoresponse of 78µs under zero bias.

Cu2O quantum dots modified α-Ga2O3 nanorod arrays as a heterojunction for improved sensitivity of self-powered photoelectrochemical detectors

Long distance charge transfer will lead to the recombination of carriers, which seriously limits the practical application of photoelectrochemical (PEC) detector. In this work, a self-powered solar-blind photodetector based on a mixed dimensional α-Ga2O3 nanorod arrays and Cu2O quantum dots has been prepared by a simple hydrothermal method and a two-part impregnation method. Cu2O quantum dots can form type Ⅱ heterojunction with α-Ga2O3 and effectively reduce the photoinduced electron and hole recombination due to their short carrier transport distances. Just because of this, α-Ga2O3 photodetectors modified by Cu2O quantum dots present special self-power optical response at 254 nm, with optical flow densities of 13.88 μA/cm2, large responsivity of 4.57 mA/W, high detection sensitivity of 2.107 × 109 Jones and fast photoresponse time of 0.815 s/0.978 s. This kind of thought provides a simple and feasible method for the preparation of α-Ga2O3 and other semiconductor oxides based on PEC photodetectors.

Ultrafast response self-powered UV photodetectors based on GaS/GaN heterojunctions

Self-powered ultraviolet (UV) photodetectors (PDs) based on GaN have been of great importance in the application of UV communication, thanks to its wide direct bandgap and strong resistance to irradiation. However, current self-powered GaN-based heterojunction UV photodetectors could not meet the requirement of fast photoresponse. Herein, type-II pn heterojunction GaS/GaN-based self-powered PDs have been proposed with a naturally p-type doping GaS thin film grown on n-type GaN via chemical vapor deposition. The electronic and optical properties of GaS/GaN heterojunction were investigated via experiments and the density functional theory. Afterward, as-prepared GaS/GaN-based PDs reveal an excellent self-powered photosensitivity/detectivity of 6.26 mA W−1/8.29 × 109 Jones at 0 V at 365 nm, ultrafast response speed with a rise/fall time of 48/80 μs as well as an amazing rejection ratio (R365 nm/R500 nm) of 3.42 × 104, and a fine rectification ratio of 105.9. This work provides a feasible method to synthesize high-performance GaS/GaN heterojunctions and demonstrates their enormous potential in ultrafast response self-powered UV photodetection.

A solar-blind photodetector with ultrahigh rectification ratio and photoresponsivity based on the MoTe2/Ta:β-Ga2O3 pn junction

High-performance solar-blind photodetectors have attracted increasing attention due to their important implications in military, medical, and industrial applications. Among a large number of wide-bandgap semiconductors, the quasi-two-dimensional β-Ga2O3 material is considered as a novel candidate for the solar-blind region (200–280 nm) owing to its suitable bandgap and superior optoelectronic properties. Herein, we report an outstanding solar-blind photodetector based on the MoTe2/Ta:β-Ga2O3 pn junction, which exhibits excellent optoelectrical properties with a superhigh rectification ratio ∼1.0 × 107, an ultrahigh responsivity (R ∼358.9 A/W), a large detectivity (D* ∼3.1 × 1012 Jones), an excellent external quantum efficiency (EQE ∼1.76 × 105%), a fast photoresponse (21.1/84.5 ms), and a high rejection ratio (∼1.1 × 103) under forward bias (1.5 V) and deep-ultraviolet (UV) illumination. These superior photoresponses result from the rapid separation of photoexcited electron-hole pairs driven by the synergistic effect of the forward bias and built-in electric field of the pn junction. Moreover, the MoTe2/Ta:β-Ga2O3 pn junction photodetector also demonstrates competitive optoelectronic characteristics in self-powered mode with an ultralow dark current of 8.5 fA and a fairly high R of 9.2 mA/W. As a result, this work provides an alternative solution for designing and fabricating high-performance solar-blind photodetectors with potential applications in UV imaging, environmental monitoring, and insecure communication.

N-ion-implanted ZnO microtubes for highly-efficient UV detection

Here we report a N-ion-implanted ZnO microtube fabricated by optical vapor supersaturated precipitation (OVSP) and ion implantation techniques. The N ions with concentration of ∼2.4 × 1023 cm−3 were mainly located below 100 nm of the surface in the acceptor-rich ZnO (A-ZnO) microtubes. Due to the presence of N in ZnO lattice as NO defects, an ∼15-fold enhancement of near-band edge (NBE)/deep-level emission (DLE) intensity ratio was achieved. The yellow luminescence and the anomalous negative thermal quenching (NTQ) effect further confirmed the NO defect level of 1.3 eV above the valence band maximum in A-ZnO:N microtubes. The implantation of N ions can expand the depletion layer between A-ZnO:N detector and atmospheric species, by which fast photoresponse down to 0.1 s was realized by a metal-semiconductor-metal (MSM) UV detector. Compared with the A-ZnO UV detector, the photoresponsivity of the MSM UV detector was enhanced by ∼10 folds. This work opens up new opportunities to design effective UV detectors for a variety of practical applications.

Manipulation of ferroelectric polarization and light absorption in non-noble metal/Aurivillius ferroelectric hybrid to enhance self-driven photodetection by coupling ferro-pyro-phototronic and plasmonic effect

Aurivillius ferroelectrics materials with layer perovskite structure, spontaneous polarization and ferro-pyro-phototronic effect afford a promising avenue for self-powered photodetectors. Nevertheless, poor photo-response caused by weak remanent polarization and low absorption coefficient hinders their further application. Herein, we demonstrate an excellent self-powered photodetector in nanosheets-assembled Bi microspheres array/Nd-doped Bi4Ti3O12 [Bi3.8Nd0.2Ti3O12] nanofibers heterostructured, which can rationally couple between ferroelectric polarization and ferro-pyro-phototronic effect in Aurivillius ferroelectric Bi3.8Nd0.2Ti3O12 and plasmonic effect in non-noble metal Bi. Firstly, an A-site doping with lanthanoid cation Nd3+ and dimension lowering strategy is proposed to improve the ferroelectric properties by reducing the oxygen vacancy concentration and coercive force field of Bi4Ti3O12 nanofibers through electrospinning method. Furthermore, the Bi nanosheets-assembled microspheres array allowing multiple reflection and scattering of incident light is introduced to enhance the local electromagnetic field and light absorption of Bi3.8Nd0.2Ti3O12. Ultimately, the Bi@Bi3.8Nd0.2Ti3O12 with negatively polarized obtains a superior zero-bias photocurrent of -500 nA under 390 nm laser illumination, which is 195.31 and 22.16 times higher than unpolarized Bi4Ti3O12 and Bi3.8Nd0.2Ti3O12, respectively. Simultaneously, an outstanding detectivity (8.11 ×1011 Jones) and fast photoresponse time (57/114 µs) are correspondingly achieved. Our research provides a reliable way to simultaneously realize large ferroelectric polarization and high light absorption for jointly manipulation of electrons and photons in self-powered optoelectronic devices.

Skin-Like Near-Infrared II Photodetector with High Performance for Optical Communication, Imaging, and Proximity Sensing

Skin-like second-near-infrared (NIR-II) photodetectors are key requirements for a flexible photodetection system that can be seamlessly integrated with three-dimensional geometries to accurately distinguish and acquire real-time various information. However, such devices exhibiting simultaneously high photosensitivity, fast photoresponse, and excellent flexibility still remain elusive. Here, we design and fabricate such skin-like NIR-II photodetectors and arrays with a simple bilayer configuration by combining a highly conducting MXene (Ti3C2Tx) and photosensitive lead sulfide quantum dots (PbS QDs), where highly promoted photocarrier separation at the interface of bilayers and efficient transport across the spatially separated layers could be achieved. Moreover, the bilayer architecture together with the strong interface bonding between the MXene and PbS QDs ensure a high-quality interface, leading to simultaneously improved optoelectronic performance and mechanical stability. Thus, the resulting device exhibits a high responsivity (1000 mA W–1), a fast photoresponse (30 ms), and excellent mechanical stability (>95% performance retention after 500 cycle bending), which renders this skin-like NIR-II photodetector highly suitable for fabricating various practical and high-performance flexible wireless photodetection systems for optical communication, NIR-II imaging, and proximity sensing in different environments. We anticipate that this simple yet high-performance skin-like MXene/PbS QDs-based photodetector would hold great potential in the applications of next-generation flexible optoelectronics.

Ultrafast Charge Transfer 2D MoS2 /Organic Heterojunction for Sensitive Photodetector.

The 2D MoS2 with superior optoelectronic properties such as high charge mobility and broadband photoresponse has attracted broad research interests in photodetectors (PD). However, due to the atomic thin layer of 2D MoS2 , its pure photodetectors usually suffer from inevitable drawbacks such as large dark current, and intrinsically slow response time. Herein, a new organic material BTP-4F with high mobility is successfully stacked with 2D MoS2 film to form an integrated 2D MoS2 /organic P-N heterojunction, facilitating efficient charge transfer as well as significantly suppressed dark current. As a result, the as-obtained 2D MoS2 /organic (PD) has exhibited excellent response and fast response time of 332/274 µs. The analysis validated photogenerated electron transition from this monolayer MoS2 to subsequent BTP-4F film, whereas the transited electron is originated from the A- exciton of 2D MoS2 by temperature-dependent photoluminescent analysis. The ultrafast charge transfer time of ≈0.24ps measured by time-resolved transient absorption spectrum is beneficial for efficient electron-hole pair separation, greatly contributing to the obtained fast photoresponse time of 332/274 µs. This work can open a promising window to acquire low-cost and high-speed (PD).

Photochromic performance of hydrogel based on deep eutectic solvent induced water soluble Cu-doped WO3 hybrids with antibacterial property

A novel fast photoresponse and self-fading photochromic hydrogel system was fabricated based on Cu-doped WO 3 , which prepared by employing DESs as co-solvent. The photochromic hydrogel displayed remarkable antibacterial ability and non-cytotoxicity, great potential in the applications of stimuli-responsive materials such as smart windows, repeatable erasing paper and photochromic sensors. • Deep eutectic solvents (DESs) as a green co-solvent was employed to prepare Cu-doped WO 3 photochromic materials with excellent water solubility. • The fabricated P(AM-HEA)-Cu-WO 3 -X photochromic hydrogels exhibit rapid photoresponse and enhanced self-fading rate. • The deep-blue colored P(AM-HEA)-Cu-WO 3 -25% hydrogel could come back to its original colorless state within 40 min. • The mechanism of photochromic and self-fading behaviors for P(AM-HEA)-Cu-WO 3 -X photochromic hydrogels was detailed analyzed. • The remarkable antibacterial performance and cell viability possessed in hydrogels also confirmed. A novel photochromic and self-bleaching hydrogel was prepared based on the Cu-doped tungsten oxide (WO 3 ) hybrid encapsulated in the co-polymer matrix polymerized using acrylamide (AM) and hydroxyethyl acrylate (HEA) as monomers. The Cu-WO 3 -25% hybrid was generated by introducing Cu species into the lattice of WO 3 with hexagonal-like prismatic structure and acted as light absorber. Given that WO 3 and Cu-doped WO 3 hybrids possess superior water solubility by using water and deep eutectic solvents (DESs, ChCl:EG) containing sufficient hydrogen bond as co-solvent, the high transparency of 90% of the hydrogels was achieved. As compared with P(AM-HEA)-WO 3 -based hydrogel, the P(AM-HEA)-Cu-WO 3 -X hydrogel exhibited the enhanced coloration degree under UV irradiation. The P(AM-HEA)-Cu-WO 3 -25% photochromic hydrogel changed from colorless to blue immediately once illuminated by UV light, and turns into dark-blue with the irradiation time going on. The self-bleaching process indicates it only takes 40 min for the P(AM-HEA)-Cu-WO 3 -25% hydrogel to return to the original colorless transparent state. Furthermore, the hydrogel systems exhibit efficient antibacterial activities against bacteria of both E. coli and S. aureus , accompanying with outstanding biocompatibility. As such, the fabricated hydrogel demonstrates rapid photoresponse, self-bleaching process, superior antibacterial performance, and cell viability, showing a great potential application as stimuli-responsive materials.

Lateral WSe2 Homojunction through Metal Contact Doping: Excellent Self‐powered Photovoltaic Photodetector

AbstractHere an IR‐heating chemical vapor deposition (CVD) approach enabling fast 2D‐growth of WSe2 thin films is reported, and the great potential of metal contact doping in building CVD‐grown WSe2‐based lateral homojunction is demonstrated by contacting with TiN/Ni metals in favor of holes/electrons injection. Shortening nanosheet channel to ≈2 µm leads to pronounced enhancement in the performance of diode. The fabricated WSe2‐based diode exhibits high rectification ratios without the need of gate modulation and can work efficiently as photovoltaic cell, with maximum open circuit voltage reaching up to 620 mV and a high power conversion efficiency over 15%, empowering it as superb self‐powered photodetector for visible to near‐infrared lights, with photoresponsivity over 0.5 A W−1 and a fast photoresponse speed of 10 µs under 520 nm illumination. It is of practical significance to achieve well‐performed photovoltaic devices with CVD‐grown WSe2 using fab‐friendly metals and simple processing, which will help pave the way toward future mass production of optoelectronic chips.

Hybrid Reduced Graphene Oxide/GaN Nanocolumns on Flexible Niobium Foils for Efficient Photoelectrochemical Water Splitting

We present the photoelectrochemical (PEC) water-splitting properties of pristine and reduced graphene oxide (rGO)-coated GaN nanocolumns (NCs) on flexible niobium (Nb) metal foils. The structural, optical, and electronic structure analyses of rGO-coated GaN-NCs on Nb foils revealed the formation of a rGO/GaN-NCs hybrid structure. Further, the valence-band studies of pure GaN-NCs shows valence-band maxima at ∼3.0 eV below the Fermi level, which decreased to ∼2.8 eV for rGO/GaN-NCs. The PEC measurement performed on pristine GaN-NCs under standard (1 Sun) conditions shows an effective photocatalytic nature with a photocurrent density of ∼60 μA/cm2 at 0.8 V (vs Ag/AgCl) in a 1 M oxalic acid electrolyte, which increases to ∼110 μA/cm2 for rGO/GaN-NCs. The efficient PEC characteristics of rGO/GaN-NCs are attributed to the effective charge separation/transport of photogenerated carriers. Furthermore, transient photocurrent measurement reveals that hybrid and pristine GaN-NCs on flexible metal foil have a fast and stable photoresponse in an aqueous solution. The development of a hybrid nitride nanostructure-based PEC device on flexible metal foils paves the way toward developing scalable PEC devices for hydrogen production applications.

Direct Z-Scheme SnSe2/SnSe Heterostructure Passivated by Al2O3 for Highly Stable and Sensitive Photoelectrochemical Photodetectors.

To mimic the natural photosynthesis system, a Z-scheme heterostructure is proposed as a viable and effective strategy for efficient solar energy utilization such as photocatalysis and photoelectrochemical (PEC) water splitting due to the high carrier separation efficiency, fast charge transport, strong redox, and wide light absorption. However, it remains a huge challenge to form a direct Z-scheme heterostructure due to the internal electric-field restriction and vital band-alignment at the interface. Herein, the van der Waals heterostructure based on the allotrope SnSe2 and SnSe is designed and synthesized by a two-step vapor phase deposition method to overcome the limitation in the formation of the Z-scheme heterostructure for the first time. The Z-scheme heterostructure of SnSe2/SnSe is confirmed by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, PEC measurement, density functional theory calculations, and water splitting. Strikingly, the PEC photodetectors based on the Z-scheme heterostructure show a synergistic effect of superior stability from SnSe and fast photoresponse from SnSe2. As such, the SnSe2/SnSe Z-scheme heterostructure shows a good photodetection performance in the ultraviolet to visible wavelength range. Furthermore, the photodetector shows a faster response/recovery time of 13/14 ms, a higher photosensitivity of 529.13 μA/W, and a higher detectivity of 4.94 × 109 Jones at 475 nm compared with those of single components. Furthermore, the photodetection stability of the SnSe2/SnSe is also greatly improved by a-thin-Al2O3-layer passivation. The results imply the promising rational design of a direct Z-scheme heterostructure with efficient charge transfer for high performance of optoelectronic devices.

Gate-tunable self-driven photodetector based on asymmetric monolayer WSe2 channel

Two-dimensional (2D) self-driven photodetectors have attracted extensive attention due to the fast photoresponse, low energy consumption and high flexibility. Recently, the metal–semiconductor-metal (MSM) structures with asymmetrical 2D semiconductor channel are exploited for the self-driven photodetection. However, the mechanism is still in dispute and the channel has not yet been forward to monolayer. Here, a self-driven MSM photodetector is fabricated with the directly-grown monolayer WSe2 which is asymmetrically patterned by the controllable scratching lithography. The self-driven behavior mainly originates from the asymmetry of both conduction channel and the Schottky junctions. With the photovoltaic effect driven by asymmetry, the device exhibits linear photoresponse with responsivity of 1.4 mA/W, detectivity of 1010 Jones, dark current level of 0.1 pA and response speed of tens of milliseconds. Moreover, performances show highly gate-tunable characteristics due to the tunable Schottky barriers. This work provides a new route towards self-driven photodetectors based on 2D materials.