High photosensitivity few-layered MoSe2 back-gated field-effect phototransistors

First-principles study of the effect of doping on the optoelectronic properties of defective monolayers of MoSe2

CONTEXT

In this paper, the structural stability, electronic structure, and optical properties of monolayer MoSe2 doped with C, O, Si, S, and Te atoms, respectively, under defective conditions are investigated based on first principles. It is found that the system is more structurally stable when defecting a single Se atom as compared to defecting a single Mo or two Se atoms. The electronic structure analysis of the system reveals that intrinsic MoSe2 is a direct bandgap semiconductor. The bandgap value of the system decreases with a single Se atom defect and introduces two new impurity energy levels in the conduction band. The defective systems doped with C and Si atoms all exhibit P-type doping. The total density of states of intrinsic MoSe2 is mainly contributed by the Mo-d and Se-p orbitals, and new density of state peaks appears near the conduction band after the defects of Se atoms. The total density of states of the defective system doped by each atom is mainly contributed by Mo-d, Se-p, and the result of the p orbital contribution of each dopant atom. By analyzing the dielectric function of each system, it is found that the intrinsic MoSe2 has the lowest static permittivity and the C-doped defect system has the highest static permittivity, which reaches 21.42. The C- and Si-doped defect systems are the first to start absorbing the light, and the intrinsic MoSe2 absorbs the light later, with its absorption edge starting at 1.25 eV. In the visible range, the reflection peaks of the systems move toward the high-energy region and the blue-shift phenomenon occurs. It is hoped that applying modification means to modulate the physical properties of the two-dimensional materials will provide some theoretical basis for broadening the application of monolayer MoSe2 in the field of optoelectronic devices.

METHODS

This study utilizes the first principle computational software package MS8.0 (Materials studio8.0) under density functional theory (DFT). The exchange-correlation potential (GGA-PBE) is described by the Perdew-Burke-Ernzerhof correlation function in CASTEP, and the potential function adopts the ultrasoft pseudopotential in the inverse space formulation. The plane wave truncation energy Ecut is set to 400 eV, the K-point is taken as 5 × 5 × 1, and the force convergence criterion is 0.05 eV/Å. The convergence accuracy of the total energy of the system is less than 1.0 × 10-5 eV/atom, the tolerance shift is less than 0.002 Å, and the stress deviation is less than 0.1 GPa. The vacuum layer is taken as 15 Å, which is intended to minimize the interlayer force. The vacuum layer was set to 15 Å to avoid the effect of layer-to-layer interaction forces in the crystal cell.

2D Materials for Efficient Photodetection: Overview, Mechanisms, Performance and UV-IR Range Applications

Two-dimensional (2D) materials have been widely used in photodetectors owing to their diverse advantages in device fabrication and manipulation, such as integration flexibility, availability of optical operation through an ultrabroad wavelength band, fulfilling of photonic demands at low cost, and applicability in photodetection with high-performance. Recently, transition metal dichalcogenides (TMDCs), black phosphorus (BP), III-V materials, heterostructure materials, and graphene have emerged at the forefront as intriguing basics for optoelectronic applications in the field of photodetection. The versatility of photonic systems composed of these materials enables their wide range of applications, including facilitation of chemical reactions, speeding-up of responses, and ultrasensitive light detection in the ultraviolet (UV), visible, mid-infrared (MIR), and far-infrared (FIR) ranges. This review provides an overview, evaluation, recent advancements as well as a description of the innovations of the past few years for state-of-the-art photodetectors based on two-dimensional materials in the wavelength range from UV to IR, and on the combinations of different two-dimensional crystals with other nanomaterials that are appealing for a variety of photonic applications. The device setup, materials synthesis, operating methods, and performance metrics for currently utilized photodetectors, along with device performance enhancement factors, are summarized.

Low Power Consumption Gate-Tunable WSe2/SnSe2 Van Der Waals Tunnel Field-Effect Transistor

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have attracted attention as promising next-generation electronic devices and sensors. In this study, we fabricated a novel nanoelectronic device based on a black-phosphorus-gated WSe2/SnSe2 van der Waals (vdW) tunnel field-effect transistor (TFET), where hexagonal boron nitride (h-BN) was used as the gate insulator. We performed morphological, electrical, and optoelectronic characterizations. The p-WSe2/n-SnSe2 heterostructure-based TFET exhibited p-type behavior with a good dependence on the gate voltage. The TFET device showed a trend toward negative differential resistance (NDR) originating from band-to-band tunneling, which can be tuned by applying a gate voltage. The optoelectronic performance of the TFET device was low, with a maximum photoresponsivity of 11 mA W−1, owing to the large device length. The results obtained herein promote the integration of black phosphorus into low-energy-consumption 2D vdW TFETs.

Photovoltaic Characteristics of GaSe/MoSe2 Heterojunction Devices

The two-dimensional materials have the thickness of an atomic layer level and are expected as alternative materials for future electronics and optoelectronics due to their specific properties. Especially recently, transition metal monochalcogenides and dichalcogenides have attracted attention. Since these materials have a band gap unlike graphene and exhibit a semiconductor property even in a single layer, application to a new flexible optoelectronics is expected. In this study, the photovoltaic characteristics of a GaSe/MoSe2 heterojunction device using two-dimensional semiconductors, p-type GaSe and n-type MoSe2, were investigated. The heterojunction device was prepared by transferring GaSe and MoSe2 onto the substrate which the titanium electrodes were fabricated through a mechanical peeling method. The current-voltage characteristics of the GaSe/MoSe2 heterojunction device were measured in a dark condition and under light irradiation using a solar simulator. The irradiation light intensity was changed from 0.5 to 1.5 sun. It was found that when the illuminance was increased in this illuminance range, both the short-circuit current and the open-circuit voltage increased. The open-circuit voltage and the energy conversion efficiency were 0.41 V and 0.46% under 1.5 sun condition, respectively.

Origin of phonon-limited mobility in two-dimensional metal dichalcogenides

Metal dichalcogenides are novel two-dimensional (2D) semiconductors after the discovery of graphene. In this article, phonon-limited mobility for six kinds of 2D semiconductors with the composition of MX₂is reviewed, in which M (Cr, Mo and W) is the transition metal, and X (S and Se) is the chalcogen element. The review is divided into three parts. In the first part, we briefly introduce the calculation method of mobility, including the empirical model and Boltzmann transport theory (BTE). The application scope, merits and limitations of these methods are summarized. In the second part, we explore empirical models to calculate the mobility of MX₂, including longitudinal acoustic phonon, optical phonon (OP) and polar optical phonon (POP) models. The contribution of multi-valley to mobility is reviewed in the calculation. The differences between static and high-frequency dielectric constants (Δϵ) are only 0.13 and 0.03 for MoS₂and WS₂. Such a low value indicates that the polarization hardly changes in the external field. So, their mobility is not determined by POP, but by deformation potential models. Different from GaAs, POP scattering plays a decisive role in its mobility. Our investigations also reveal that the scattering from POP cannot be ignored in CrSe₂, MoSe₂and WSe₂. In the third parts, we investigate the mobility of MX₂using electron-phonon coupling matrix element, which is based on BTE from the framework of a many-body quantum-field theory. Valence band splitting of MoS₂and WS₂is induced by spin-orbit coupling effect, which leads to the increase of hole mobility. In particular, we review in detail the theoretical and experimental results of MoS₂mobility in recent ten years, and its mobility is also compared with other materials to deepen the understanding.

Ultrasensitive Photodetection in MoS2 Avalanche Phototransistors

Recently, there have been numerous studies on utilizing surface treatments or photosensitizing layers to improve photodetectors based on 2D materials. Meanwhile, avalanche breakdown phenomenon has provided an ultimate high-gain route toward photodetection in the form of single-photon detectors. Here, the authors report ultrasensitive avalanche phototransistors based on monolayer MoS2 synthesized by chemical vapor deposition. A lower critical field for the electrical breakdown under illumination shows strong evidence for avalanche breakdown initiated by photogenerated carriers in MoS2 channel. By utilizing the photo-initiated carrier multiplication, their avalanche photodetectors exhibit the maximum responsivity of ≈3.4 × 107 A W-1 and the detectivity of ≈4.3 × 1016 Jones under a low dark current, which are a few orders of magnitudes higher than the highest values reported previously, despite the absence of any additional chemical treatments or photosensitizing layers. The realization of both the ultrahigh photoresponsivity and detectivity is attributed to the interplay between the carrier multiplication by avalanche breakdown and carrier injection across a Schottky barrier between the channel and metal electrodes. This work presents a simple and powerful method to enhance the performance of photodetectors based on carrier multiplication phenomena in 2D materials and provides the underlying physics of atomically thin avalanche photodetectors.

The Electrochemical Stability of Starch Carbon as an Important Property in the Construction of a Lithium-Ion Cell

This paper shows use of starch-based carbon (CSC) and graphene as the anode electrode for lithium-ion cell. To describe electrochemical stability of the half-cell system and kinetic parameters of charging process in different temperatures, electrochemical impedance spectroscopy (EIS) measurement was adopted. It has been shown that smaller resistances are observed for CSC. Additionally, Bode plots show high electrochemical stability at higher temperatures. The activation energy for the SEI (solid–electrolyte interface) layer, charge transfer, and electrolyte were in the ranges of 24.06–25.33, 68.18–118.55, and 13.84–15.22 kJ mol⁻¹, respectively. Moreover, the activation energy of most processes is smaller for CSC, which means that this electrode could serve as an eco-friendly biodegradable lithium-ion cell element.

Blackbody-sensitive room-temperature infrared photodetectors based on low-dimensional tellurium grown by chemical vapor deposition

Blackbody-sensitive room-temperature infrared detection is a notable development direction for future low-dimensional infrared photodetectors. However, because of the limitations of responsivity and spectral response range for low-dimensional narrow bandgap semiconductors, few low-dimensional infrared photodetectors exhibit blackbody sensitivity. Here, highly crystalline tellurium (Te) nanowires and two-dimensional nanosheets were synthesized by using chemical vapor deposition. The low-dimensional Te shows high hole mobility and broadband detection. The blackbody-sensitive infrared detection of Te devices was demonstrated. A high responsivity of 6650 A W-1 (at 1550-nm laser) and the blackbody responsivity of 5.19 A W-1 were achieved. High-resolution imaging based on Te photodetectors was successfully obtained. All the results suggest that the chemical vapor deposition-grown low-dimensional Te is one of the competitive candidates for sensitive focal-plane-array infrared photodetectors at room temperature.

Ultrafast Photocurrent Response and High Detectivity in Two-Dimensional MoSe2-based Heterojunctions

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have garnered great attention on account of their novel properties and potential to advance modern technology. Recent studies have demonstrated that TMDCs can be utilized to create high-performing heterostructures with combined functionality of the individual layers and new phenomena at these interfaces. Here, we report an ultrafast photoresponse within MoSe₂-based heterostructures in which heavily p-doped WSe₂ and MoS₂ flakes share an undoped MoSe₂ channel, allowing us to directly compare the optoelectronic properties of MoSe₂-based heterojunctions with different 2D materials. Strong photocurrent signals have been observed in both MoSe₂-WSe₂ and MoSe₂-MoS₂ heterojunctions with a photoresponse time constant of ∼16 μs, surmounting previous MoSe₂-based devices by three orders of magnitude. Further studies have shown that the fast response is independent of the integrated 2D materials (WSe₂ or MoS₂) but is likely attributed to the high carrier mobility of 260 cm² V^-1 s^-1 in the undoped MoSe₂ channel as well as the greatly reduced Schottky barriers and near absence of interface states at MoSe₂-WSe₂/MoS₂ heterojunctions, which lead to reduced carrier transit time and thus short photocurrent response time. Lastly, a high detectivity on the order of ∼10¹⁴ Jones has been achieved in MoSe₂-based heterojunctions, which supersedes current industry standards. These fundamental studies not only shed light on photocurrent generation mechanisms in MoSe₂-based heterojunctions but also open up new avenues for engineering future high-performance 2D optoelectronic devices.

Dynamic Control of High-Range Photoresponsivity in a Graphene Nanoribbon Photodetector

Graphene has been demonstrated to be a promising material for optoelectronics and photodetection devices because of its ultra-broadband optical absorption and high carrier mobility. However, its integration with optoelectronic systems has been limited by the zero-bandgap and the lack of a gain mechanism. Herein, we demonstrate a novel photodetector based on the graphene nanoribbons (GRNs) with a sizable bandgap. Utilizing trapping charge at the interface between SiO₂ and light-doped silicon, an ultrahigh gain of 22,400 has been obtained. Our devices show an enhanced photoresponsivity (~ 800 AW^-1) while the response speed is still fast (up to 10 μs). This photoresponsivity is about two orders of magnitude higher compared to that of a previous graphene-based photodetector. The photodetector exhibits a wide-range tunability via source-drain bias and back gate voltage. Our work addresses key challenges for the photodetectors and potentially provides the desired pathway toward practical application of graphene photodetectors that can be externally manipulated by an electric field with fast response speed and high sensitivity.

A hydrothermally synthesized MoS2(1-x)Se2x alloy with deep-shallow level conversion for enhanced performance of photodetectors

Photoelectric detectors based on binary transition metal chalcogenides have attracted widespread attention in recent years. However, due to the high-temperature synthesis of binary TMD, high-density deep-level defect states may be generated, leading to poor responsiveness or a long response time. Besides, the addition of an alloy will change the DLDSs from deep to shallow energy levels caused by S vacancies. In this paper, MoS_2(1-x)Se_2x nanostructures were synthesized by a hydrothermal method, and a novel type of photodetector was fabricated by using the synthesized material as a light sensitive material. The MoSSe-based photodetector not only has a high photocurrent, but also exhibits a wide spectral response in the range of 405 nm to 808 nm. At the same time, it can achieve a responsivity of 1.753 mA W^-1 under 660 nm laser irradiation of 1.75 mW mm^-2. Therefore, this work can be considered as a method of constructing a new type of photodetector with a simple process and low cost.

Single-photon emitters in hexagonal boron nitride: a review of progress

This report summarizes progress made in understanding properties such as zero-phonon-line energies, emission and absorption polarizations, electron-phonon couplings, strain tuning and hyperfine coupling of single photon emitters in hexagonal boron nitride. The primary aims of this research are to discover the chemical nature of the emitting centres and to facilitate deployment in device applications. Critical analyses of the experimental literature and data interpretation, as well as theoretical approaches used to predict properties, are made. In particular, computational and theoretical limitations and challenges are discussed, with a range of suggestions made to overcome these limitations, striving to achieve realistic predictions concerning the nature of emitting centers. A symbiotic relationship is required in which calculations focus on properties that can easily be measured, whilst experiments deliver results in a form facilitating mass-produced calculations.

High Photosensitive Indium-Gallium-Zinc Oxide Thin-Film Phototransistor with a Selenium Capping Layer for Visible-Light Detection

Visible light can be detected using an indium-gallium-zinc oxide (IGZO)-based phototransistor, with a selenium capping layer (SCL) that functions as a visible light absorption layer. Selenium (Se) exhibits photoconductive properties as its conductivity increases with illumination. We report an IGZO phototransistor with an SCL (SCL/IGZO phototransistor) that demonstrated optimal photoresponse characteristics when the SCL was 150 nm thick. The SCL/IGZO phototransistor exhibited a photoresponsivity of 1.39 × 10³ A/W, photosensitivity of 4.39 × 10⁹, detectivity of 3.44 × 10¹³ Jones, and external quantum efficiency of 3.52 × 10³% when illuminated by green light (532 nm). Ultraviolet-visible spectroscopy and ultraviolet photoelectron spectroscopy analysis showed that Se has a narrow energy band gap, in which visible light is absorbed and forms a p-n junction with IGZO so that photogenerated electron-hole pairs are easily separated, which makes recombination more challenging. We show that electrons generated in the SCL flow through the IGZO layer, which enables the phototransistor to detect visible light. Furthermore, the SCL/IGZO phototransistor exhibited excellent durability and reversibility owing to the constant light and dark current and the time-dependent photoresponse characteristics over 8000 s when a red light (635 nm) source was turned on and off at a frequency of 0.1 Hz.

A First-Principles Study of Nonlinear Elastic Behavior and Anisotropic Electronic Properties of Two-Dimensional HfS2

We utilize first principles calculations to investigate the mechanical properties and strain-dependent electronic band structure of the hexagonal phase of two dimensional (2D) HfS₂. We apply three different deformation modes within -10% to 30% range of two uniaxial (D1, D2) and one biaxial (D3) strains along x, y, and x-y directions, respectively. The harmonic regions are identified in each deformation mode. The ultimate stress for D1, D2, and D3 deformations is obtained as 0.037, 0.038 and 0.044 (eV/Ang3), respectively. Additionally, the ultimate strain for D1, D2, and D3 deformation is obtained as 17.2, 17.51, and 21.17 (eV/Ang3), respectively. In the next step, we determine the second-, third-, and fourth-order elastic constants and the electronic properties of both unstrained and strained HfS₂ monolayers are investigated. Our findings reveal that the unstrained HfS₂ monolayer is a semiconductor with an indirect bandgap of 1.12 eV. We then tune the bandgap of HfS₂ with strain engineering. Our findings reveal how to tune and control the electronic properties of HfS₂ monolayer with strain engineering, and make it a potential candidate for a wide range of applications including photovoltaics, electronics and optoelectronics.

Sub-millimeter size high mobility single crystal MoSe2 monolayers synthesized by NaCl-assisted chemical vapor deposition

Monolayer MoSe₂ is a transition metal dichalcogenide with a narrow bandgap, high optical absorbance and large spin-splitting energy, giving it great promise for applications in the field of optoelectronics. Producing monolayer MoSe₂ films in a reliable and scalable manner is still a challenging task as conventional chemical vapor deposition (CVD) or exfoliation based techniques are limited due to the small domains/nanosheet sizes obtained. Here, based on NaCl assisted CVD, we demonstrate the simple and stable synthesis of sub-millimeter size single-crystal MoSe₂ monolayers with mobilities ranging from 38 to 8 cm² V⁻¹ s⁻¹. The average mobility is 12 cm² V⁻¹ s⁻¹. We further determine that the optical responsivity of monolayer MoSe₂ is 42 mA W⁻¹, with an external quantum efficiency of 8.22%.

Sub-millimeter single crystal MoSe₂ monolayers with a mobility of 38 cm² V⁻¹ s⁻¹ and responsivity of 42 mA W⁻¹ were synthesized by NaCl-assisted chemical vapor deposition.

Seeded growth of high-quality transition metal dichalcogenide single crystals via chemical vapor transport

Seeded chemical vapor transport growth gives high-quality and millimeter-sized transition metal dichalcogenide single crystals in a short period.

Recent advances in synthesis and biosensors of two-dimensional MoS2

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous research interests due to their exciting optical properties, large surface area, intercalatable morphologies and excellent electrochemically catalytic activity. Acting as the most typical member in TMDCs family, layer-dependent molybdenum disulfide (MoS₂) with particular direct bandgap of 1.8 eV in monolayer has been widely applied in various biosensors with high sensitivity and selectivity. In this review, the preparation methods of MoS₂, together with MoS₂-based biosensors for detecting cells and biomolecules (such as glucose, DNA and antigens) would be summarized. In addition, the current challenges and future perspectives are outlined for the applications of biosensors based on 2D MoS₂.

Near-Infrared Photoelectric Properties of Multilayer Bi2O2Se Nanofilms

The near-infrared (NIR) photoelectric properties of multilayer Bi₂O₂Se nanofilms were systematically studied in this paper. Multilayer Bi₂O₂Se nanofilms demonstrate a sensitive photo response to NIR, including a high photoresponsivity (~ 101 A/W), a quick response time (~ 30 ms), a high external quantum efficiency (~ 20,300%), and a high detection rate (1.9 × 10¹⁰ Jones). These results show that the device based on multilayer Bi₂O₂Se nanofilms might have great potentials for future applications in ultrafast, highly sensitive NIR optoelectronic devices.

Self-Powered Broad-band Photodetectors Based on Vertically Stacked WSe2/Bi2Te3 p-n Heterojunctions

Semiconducting p-n heterojunctions, serving as the basic unit of modern electronic devices, such as photodetectors, solar-energy conversion devices, and light-emitting diodes (LEDs), have been extensively investigated in recent years. In this work, high performance self-powered broad-band photodetectors were fabricated based on vertically stacked p-n heterojunctions though combining p-type WSe₂ with n-type Bi₂Te₃ via van der Waals (vdW) epitaxial growth. Devices based on the p-n heterojunction show obvious current rectification behaviors in the dark and superior photovoltaic characteristics under light irradiation. A maximum short circuit current of 18 nA and open circuit voltage of 0.25 V can be achieved with the illumination light of 633 nm (power density: 26.4 mW/cm²), which are among the highest values compared with the ever reported 2D vdW heterojunctions synthesized by chemical vapor deposition (CVD) method. Benefiting from the broad-band absorption of the heterostructures, the detection range can be expanded from the visible to near-infrared (375-1550 nm). Moreover, ascribing to the efficient carriers separation process at the junction interfaces, the devices can be further employed as self-powered photodetectors, where a fast response time (∼210 μs) and high responsivity (20.5 A/W at 633 nm and 27 mA/W at 1550 nm) are obtained under zero bias voltage. The WSe₂/Bi₂Te₃ p-n heterojunction-based self-powered photodetectors with high photoresponsivity, fast photoresponse time, and broad spectral response will find potential applications in high speed and self-sufficient broad-band devices.

Spatial Mapping of Thermal Boundary Conductance at Metal-Molybdenum Diselenide Interfaces

Improving the thermal transport across interfaces is a necessary consideration for micro- and nanoelectronic devices and necessitates accurate measurement of the thermal boundary conductance (TBC) and understanding of transport mechanisms. Two-dimensional transition-metal dichalcogenides (TMDs) have been studied extensively for their electrical properties, including the metal-TMD electrical contact resistance, but the thermal properties of these interfaces are significantly less explored irrespective of their high importance in their electronic devices. We isolate individual islands of MoSe₂ grown by chemical vapor deposition using photolithography and correlate the 2D variation of TBC with optical microscope images of the MoSe₂ islands. We measure the 2D spatial variation of the TBC at metal-MoSe₂-SiO₂ interfaces using a modified time-domain thermoreflectance (TDTR) technique, which requires much less time than full TDTR scans. The thermoreflectance signal at a single probe delay time is compared with a correlation curve, which enables us to estimate the change in the signal with respect to the TBC at the metal-MoSe₂-SiO₂ interface as opposed to recording the decay of the thermoreflectance signal over delay times of several nanoseconds. The results show a higher TBC across the Ti-MoSe₂-SiO₂ interface compared to Al-MoSe₂-SiO₂. An image-clustering method is developed to differentiate the TBC for different numbers of MoSe₂ layers, which reveals that the TBC in single-layer regions is higher than that in the bilayer. We perform traditional TDTR measurements over a range of delay times and verify that TBC is higher at the Ti-MoSe₂-SiO₂ interface compared to Al-MoSe₂-SiO₂, highlighting the importance of the choice of metal for heat dissipation at electrical contacts in TMD devices.

Multilayer ReS2 Photodetectors with Gate Tunability for High Responsivity and High-Speed Applications

Rhenium disulfide (ReS₂) is an attractive candidate for photodetection applications owing to its thickness-independent direct band gap. Despite various photodetection studies using two-dimensional semiconductors, the trade-off between responsivity and response time under varying measurement conditions has not been studied in detail. This report presents a comprehensive study of the architectural, laser power and gate bias dependence of responsivity and speed in supported and suspended ReS₂ phototransistors. Photocurrent scans show uniform photogeneration across the entire channel because of enhanced optical absorption and a direct band gap in multilayer ReS₂. A high responsivity of 4 A W^-1 (at 50 ms response time) and a low response time of 20 μs (at 4 mA W^-1 responsivity) make this one of the fastest reported transition-metal dichalcogenide photodetectors. Occupancy of intrinsic (bulk ReS₂) and extrinsic (ReS₂/SiO₂ interface) traps is modulated using gate bias to demonstrate tunability of the response time (responsivity) over 4 orders (15×) of magnitude, highlighting the versatility of these photodetectors. Differences in the trap distributions of suspended and supported channel architectures, and their occupancy under different gate biases enable switching the dominant operating mechanism between either photogating or photoconduction. Further, a new metric that captures intrinsic photodetector performance by including the trade-off between its responsivity and speed, besides normalizing for the applied bias and geometry, is proposed and benchmarked for this work.

Flexible Vertical p-n Diode Photodetectors with Thin N-type MoSe2 Films Solution-Processed on Water Surfaces

Two-dimensional (2D) nanosheets of transition metal dichalcogenides (TMDs) are of significant interest for potential photoelectronic applications. However, the fabrication of solution-processed arrays of mechanically flexible thin TMD films-based vertical type p-n junction photodetectors over a large area is a great challenge. Our method is based on controlled solvent evaporation of MoSe₂ suspension spread on water surface. Single or few-layered MoSe₂ nanosheets modified with the dispersant amine-terminated poly(styrene) (PS-NH₂) were homogeneously deposited and stacked on water upon solvent evaporation, giving rise to uniform MoSe₂/PS-NH₂ composite films that can be readily transferred onto other substrates. A p-n junction vertical diode of Al/p-type Si/p-type poly(9,9-di- n-octylfluorenyl-2,7-diyl)/n-type MoSe₂ composite/Au stacked from bottom to top exhibited characteristic rectifying current behavior upon voltage sweep with a rectification ratio of 10³. Subsequent illumination of near-infrared light on the device resulted in a substantially enhanced dark current of approximately 10³ times greater than that of the nonexposed device. The photodetection performance, that is, switching time, responsivity, and detectivity, were 100.0 ms, 2.5 AW^-1, and 2.34 × 10¹⁴, respectively. Furthermore, the performance of mechanically flexible photodetectors devices was comparable with that of the devices fabricated on the hard Si substrate even after 1000 bending cycles at a bending diameter of 7.2 mm.

Accumulation-Mode Two-Dimensional Field-Effect Transistor: Operation Mechanism and Thickness Scaling Rule

Understanding the operation mode of a two-dimensional (2D) material-based field-effect transistor (FET) is one of the most essential issues in the study of electronics and physics. The existing Schottky barrier FET model for devices with global back gate and metallic contacts overemphasizes the metal/2D contact effect, and the widely observed residual conductance cannot be explained by this model. Here, an accumulation-mode (ACCU) FET model, which directly reveals 2D channel transport properties, is developed based on a partial top-gate MoS₂ FET with metallic contacts and a channel thickness of 0.65-118 nm. The operation mechanism of an ACCU-FET is validated and clarified by carefully performed capacitance measurements. A depletion capacitance-quantum capacitance transition is observed. After the analysis of the MoS₂ ACCU-FET, we have confirmed that most 2D FETs show an accumulation-mode behavior. The universal thickness scaling rule of 2D-FETs is then proposed, which provides guidance for future research on 2D materials.

Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS₂, MoSe₂, WS₂ and WSe₂

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS₂, MoSe₂, WS₂, and WSe₂, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS₂, MoSe₂, WS₂, and WSe₂.

Ultrafast and highly sensitive infrared photodetectors based on two-dimensional oxyselenide crystals

Infrared light detection and sensing is deeply embedded in modern technology and human society and its development has always been benefitting from the discovery of various photoelectric materials. The rise of two-dimensional materials, thanks to their distinct electronic structures, extreme dimensional confinement and strong light–matter interactions, provides a material platform for next-generation infrared photodetection. Ideal infrared detectors should have fast respond, high sensitivity and air-stability, which are rare to meet at the same time in one two-dimensional material. Herein we demonstrate an infrared photodetector based on two-dimensional Bi₂O₂Se crystal, whose main characteristics are outstanding in the whole two-dimensional family: high sensitivity of 65 AW⁻¹ at 1200 nm and ultrafast photoresponse of ~1 ps at room temperature, implying an intrinsic material-limited bandwidth up to 500 GHz. Such great performance is attributed to the suitable electronic bandgap and high carrier mobility of two-dimensional oxyselenide.

Two-dimensional (2D) bismuth oxyselenide crystals with suitable electronic band-gap and ultrahigh carrier mobility enable near-infrared photodetection. Here, the authors report an infrared photodetector based on 2D-bismuth oxyselenide with high responsivity, ultrafast photoresponse of ~ 1 ps at room temperature and a detectable frequency limit of up to 500 GHz.

Metal-agglomeration-suppressed growth of MoS2 and MoSe2 films with small sulfur and selenium molecules for high mobility field effect transistor applications

This work reports a breakthrough technique for achieving high quality and uniform molybdenum dichalcogenide (MoX2 where X = S, Se) films on large-area wafers via metal-agglomeration-suppressed growth (MASG) with small chalcogen (X-) molecules at growth temperatures (TG) of 600 °C or lower. In order to grow MoS2 films suitable for field effect transistors (FETs), S-molecules should be pre-deposited on Mo films at 60 °C prior to heating the substrate up to TG. The pre-deposited S-molecules successfully suppressed the agglomeration of Mo during sulfurization and prevented the formation of protruding islands in the resultant sulfide films. The small X-molecules supplied from a thermal cracker reacted with Mo-precursor film to form MoX2. The film quality strongly depends on the temperatures of cracking and reservoir zones, as well as TG. The MoS2 film grown at 570 °C showed a thickness variation of less than 3.3% on a 6 inch-wafer. The mobility and on/off current ratio of 6.1 nm-MoS2 FET at TG = 570 °C were 59.8 cm2 V-1 s-1 and 105, respectively. The most significant advantages of the MASG method proposed in this work are its expandability to various metal dichalcogenides on larger substrates as well as a lower TG enabled by using reactive small molecules supplied from a cracker, for which temperature is independently controlled.

High-Responsivity Multilayer MoSe2 Phototransistors with Fast Response Time

There is a great interest in phototransistors based on transition metal dichalcogenides because of their interesting optoelectronic properties. However, most emphasis has been put on MoS₂ and little attention has been given to MoSe₂, which has higher optical absorbance. Here, we present a compelling case for multilayer MoSe₂ phototransistors fabricated in a bottom-gate thin-film transistor configuration on SiO₂/Si substrates. Under 650-nm-laser, our MoSe₂ phototransistor exhibited the best performance among MoSe₂ phototransistors in literature, including the highest responsivity (1.4 × 10⁵ AW^-1), the highest specific detectivity (5.5 × 10¹³ jones), and the fastest response time (1.7 ms). We also present a qualitative model to describe the device operation based on the combination of photoconductive and photogating effects. These results demonstrate the feasibility of achieving high performance in multilayer MoSe₂ phototransistors, suggesting the possibility of further enhancement in the performance of MoSe₂ phototransistors with proper device engineering.

Large-area synthesis and photoelectric properties of few-layer MoSe2 on molybdenum foils

Compared with MoS₂ and WS₂, selenide analogs have narrower band gaps and higher electron mobilities, which make them more applicable to real electrical devices. In addition, few-layer metal selenides have higher electrical conductivity, carrier mobility and light absorption than the corresponding monolayers. However, the large-scale and high-quality growth of few-layer metal selenides remains a significant challenge. Here, we develop a facile method to grow large-area and highly crystalline few-layer MoSe₂ by directly selenizing the Mo foil surface at 550 °C within 60 min under ambient pressure. The atomic layers were controllably grown with thicknesses between 3.4 and 6 nm, which just met the thickness range required for high-performance electrical devices. Furthermore, we fabricated a vertical p-n junction photodetector composed of few-layer MoSe₂ and p-type silicon, achieving photoresponsivity higher by two orders of magnitude than that of the reported monolayer counterpart. This technique provides a feasible approach towards preparing other 2D transition metal dichalcogendes for device applications.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Interstitial Mo-Assisted Photovoltaic Effect in Multilayer MoSe2 Phototransistors

Thin-film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe₂ ) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W^-1 ), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe₂ TFTs is sought. A unique structural characteristic of the APCVD-grown MoSe₂ is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe₂ phototransistors is identified, suggesting a novel route to high-performance, multifunctional 2D material devices for future wearable sensor applications.

Black Phosphorus: Optical Characterization, Properties and Applications

The rise of black phosphorus (BP) as a new family member of two-dimensional (2D) materials brings new concepts and applications to the field, because of the infrared band gap and anisotropic properties of such materials. Among many excellent properties of BP, the optical property attracts special attention in recent years. Optical methods have been widely and successfully used in characterizing BP, not only to obtain the structural information (such as thickness and crystalline orientation), but also to probe the fundamental properties of BP in terms of the behavior of electrons, phonons, excitons etc. In this Review, a comprehensive understanding about the optical characterization of BP such as Raman, absorption, and photoluminescence is presented. Also, the unique optical properties and applications explored in recent years are reviewed.

Recent Progress on Localized Field Enhanced Two-dimensional Material Photodetectors from Ultraviolet-Visible to Infrared

Two-dimensional (2D) materials have drawn tremendous attention in recent years. Being atomically thin, stacked with van der Waals force and free of surface chemical dangling bonds, 2D materials exhibit several distinct physical properties. To date, 2D materials include graphene, transition metal dichalcogenides (TMDS), black phosphorus, black P_(1-x) As_x , boron nitride (BN) and so forth. Owing to their various bandgaps, 2D materials have been utilized for photonics and optoelectronics. Photodetectors based on 2D materials with different structures and detection mechanisms have been established and present excellent performance. In this Review, localized field enhanced 2D material photodetectors (2DPDs) are introduced with sensitivity over the spectrum from ultraviolet, visible to infrared in the sight of the influence of device structure on photodetector performance instead of directly illustrating the detection mechanisms. Six types of localized fields are summarized. They are: ferroelectric field, photogating electric field, floating gate induced electrostatic field, interlayer built-in field, localized optical field, and photo-induced temperature gradient field, respectively. These localized fields are proved to effectively promote the detection ability of 2DPDs by suppressing background noise, enhancing optical absorption, improving electron-hole separation efficiency, amplifying photoelectric gain and/or extending the detection range.

Centimeter-Scale Deposition of Mo0.5W0.5Se2 Alloy Film for High-Performance Photodetectors on Versatile Substrates

Because of their great potential for academic investigation and practical application in next-generation optoelectronic devices, ternary layered semiconductors have attracted considerable attention in recent years. Similar to the applications of traditional layered materials, practical applications of ternary layered semiconductor alloys require the synthesis of large-area samples. Here, we report the preparation of centimeter-scale and high-quality Mo_0.5W_0.5Se₂ alloy films on both a rigid SiO₂/Si substrate and a flexible polyimide (PI) substrate. Then, photodetectors based on these alloy films are fabricated, which are capable of conducting broad-band photodetection from ultraviolet to near-infrared region (370-808 nm) with high performance. The photodetector on SiO₂/Si substrates demonstrates a high responsivity (R) of 77.1 A/W, an outstanding detectivity (D*) of 1.1 × 10¹² Jones, and a fast response time of 8.3 ms. These figures-of-merit are much superior to those of the counterparts of binary material-based devices. Moreover, the photodetector on PI substrates also achieves high performance (R = 63.5 A/W, D* = 3.56 × 10¹² Jones). And no apparent degradation in the device properties is observed even after 100 bending cycles. These results make Mo_0.5W_0.5Se₂ alloy a highly qualified candidate for next-generation optoelectronic applications.

Two-dimensional transition metal dichalcogenide-based counter electrodes for dye-sensitized solar cells

Dye-sensitized solar cell using counter electrode based on transition metal dichalcogenides.

Ultrafast, Broadband Photodetector Based on MoSe2/Silicon Heterojunction with Vertically Standing Layered Structure Using Graphene as Transparent Electrode

A MoSe2/Si heterojunction photodetector is constructed by depositing MoSe2 film with vertically standing layered structure on Si substrate. Graphene transparent electrode is utilized to further enhance the separation and transport of photogenerated carriers. The device shows excellent performance in terms of wide response spectrum of UV-visible-NIR, high detectivity of 7.13 × 1010 Jones, and ultrafast response speed of ≈270 ns, unveiling the great potential for the heterojunction for high-performance optoelectronic devices.

Ultrathin 2D Photodetectors Utilizing Chemical Vapor Deposition Grown WS2 With Graphene Electrodes

In this report, graphene (Gr) is used as a 2D electrode and monolayer WS2 as the active semiconductor in ultrathin photodetector devices. All of the 2D materials are grown by chemical vapor deposition (CVD) and thus pose as a viable route to scalability. The monolayer thickness of both electrode and semiconductor gives these photodetectors ∼2 nm thickness. We show that graphene is different to conventional metal (Au) electrodes due to the finite density of states from the Dirac cones of the valence and conduction bands, which enables the photoresponsivity to be modulated by electrostatic gating and light input control. We demonstrate lateral Gr-WS2-Gr photodetectors with photoresponsivities reaching 3.5 A/W under illumination power densities of 2.5 × 10(7) mW/cm(2). The performance of monolayer WS2 is compared to bilayer WS2 in photodetectors and we show that increased photoresponsivity is achieved in the thicker bilayer WS2 crystals due to increased optical absorption. This approach of incorporating graphene electrodes in lateral TMD based devices provides insights on the contact engineering in 2D optoelectronics, which is crucial for the development of high performing ultrathin photodetector arrays for versatile applications.

Thickness dependence on the optoelectronic properties of multilayered GaSe based photodetector

Two-dimensional (2D) layered materials exhibit unique optoelectronic properties at atomic thicknesses. In this paper, we fabricated metal-semiconductor-metal based photodetectors using layered gallium selenide (GaSe) with different thicknesses. The electrical and optoelectronic properties of the photodetectors were studied, and these devices showed good electrical characteristics down to GaSe flake thicknesses of 30 nm. A photograting effect was observed in the absence of a gate voltage, thereby implying a relatively high photoresponsivity. Higher values of the photoresponsivity occurred for thicker layers of GaSe with a maximum value 0.57 AW(-1) and external quantum efficiency of of 132.8%, and decreased with decreasing GaSe flake thickness. The detectivity was 4.05 × 10(10) cm Hz(1/2) W(-1) at 532 nm laser wavelength, underscoring that GaSe is a promising p-type 2D material for photodetection applications in the visible spectrum.

Black Phosphorus Nanosheets: Synthesis, Characterization and Applications

Black phosphorus (BP) is an emerging two-dimensional (2D) material with a natural bandgap, which has unique anisotropy and extraordinary physical properties. Due to its puckered structure, BP exhibits strong in-plane anisotropy unlike other layered materials. The bandgap tunability of BP enables a wide range of ultrafast electronics and high frequency optoelectronic applications ranging from telecommunications to thermal imaging covering the nearly entire electromagnetic spectrum, whereas no other 2D material has this functionality. Here, recent advances in the synthesis, fabrication, anisotropic physical properties, and BP-based devices including field effect transistors (FETs) and photodetectors, are discussed. Recent passivation approaches to address the degradation of BP, which is one of the main challenges to bring this material into real world applications, are also introduced. Finally, a comment is made on the recent developments in other emerging applications, future outlook and challenges ahead in BP research.

Observation of Switchable Photoresponse of a Monolayer WSe2-MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging

In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane p-n junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe2-MoS2 lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe2 and MoS2, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are observed to be selectively switched on and off, allowing for the realization of a hyper-resolution array of the switchable photodiode pixels. This experimental approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolutions. Comparative 2D Fourier analysis of physical height and current images shows high spatial frequency variations in substrate/MoS2 (or WSe2) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures.

Toward High-Performance Top-Gate Ultrathin HfS2 Field-Effect Transistors by Interface Engineering

Top-gate HfS2 field-effect transistors (FETs) with 5 nm HfO2 as dielectrics are successfully demonstrated, with on/off ratio of 10(5) and subthreshold swing of 95 mV dec(-1) . Moreover, due to the self-functionalization of HfS2 , uniform and ultrathin HfO2 film free of pinhole-like defects could be deposited on HfS2 , which is dramatically different from other transition metal dichalcogenide FETs.

Quasi Two-Dimensional Dye-Sensitized In2O3 Phototransistors for Ultrahigh Responsivity and Photosensitivity Photodetector Applications

We report the development of dye-sensitized thin-film phototransistors consisting of an ultrathin layer (<10 nm) of indium oxide (In2O3) the surface of which is functionalized with a self-assembled monolayer of the light absorbing organic dye D102. The resulting transistors exhibit a preferential color photoresponse centered in the wavelength region of ∼500 nm with a maximum photosensitivity of ∼10(6) and a responsivity value of up to 2 × 10(3) A/W. The high photoresponse is attributed to internal signal gain and more precisely to charge carriers generated upon photoexcitation of the D102 dye which lead to the generation of free electrons in the semiconducting layer and to the high photoresponse measured. Due to the small amount of absorption of visible photons, the hybrid In2O3/D102 bilayer channel appears transparent with an average optical transmission of >92% in the wavelength range 400-700 nm. Importantly, the phototransistors are processed from solution-phase at temperatures below 200 °C hence making the technology compatible with inexpensive and temperature sensitive flexible substrate materials such as plastic.