2D Transition Metal Dichalcogenides Research Articles

Noise‐Reduced WSe2 Phototransistors for Enhanced Photodetection Performance via Suppression of Metal‐Induced Gap States

AbstractPhototransistors are critical components in optoelectronics, and 2D transition metal dichalcogenides (TMDC), such as tungsten diselenide (WSe2), show promise for phototransistor applications due to their strong light‐matter interaction, unique excitonic properties, and high surface‐to‐volume ratio. In 2D TMDC‐based phototransistors, 1/f noise, caused by complex defect states, acts as a dominant low‐frequency noise (LFN) and is crucial for obtaining accurate photodetection characteristics. However, many studies still overlook LFN and focus on enhancing photocurrent or response time. In this study, the importance of LFN analysis is highlighted in WSe2 phototransistors and demonstrate reduced noises and enhanced photodetection performance through the suppression of metal‐induced gap states (MIGS) that act as noise sources by utilizing semimetal bismuth (Bi) contact. The WSe2 phototransistors demonstrated ≈1000 times lower noise, 100 times higher responsivity, and 10 times higher specific detectivity than devices with conventional metal contacts. The results of this study suggest that reducing LFN in photodetection devices, such as by suppressing MIGS, can be an efficient way to enhance device performance.

Effect of Fabricating Process on the Performance of Two-Dimensional p-Type WSe2 Field Effect Transistors.

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as WSe2, are promising candidates for next-generation integrated circuits. However, the dependence of intrinsic properties of TMD devices on various processing steps remains largely unexplored. Here, using pristine p-type WSe2 devices as references, we comprehensively studied the influence of each step in traditional nanofabrication methods on device performance. Our findings reveal that electron beam exposure significantly alters the electrical conductivity of WSe2 due to the doping and diffusion effects of electrons. During ultraviolet lithography, the bilayer WSe2 device immersed in a 4‰ NaOH developer also showed substantial quality degradation (40%-84%). In this case, we combined laser patterning with the transfer electrode method to fabricate a high-performance device with a current density of 278.5 μA/μm and an on/off ratio of 3.9 × 107. This work reveals the influence of the nanofabrication process on TMD devices and guides for improving device performance.

Enhancing Dynamic Range in Low-Noise 2D-Integrated Organic Photodiodes by Mitigating Langevin Recombination.

Organic photodiodes (OPDs) are a significant focus for the next-generation of light-detection technologies. However, organic semiconductors in OPDs still face key challenges, such as low carrier mobilities and limited efficiency in generating photon-induced signals, which affect the detectable resolution and dynamic range. In this study, the characterization of the interaction between organic polymeric bulk heterojunctions and two-dimensional (2D) transition metal dichalcogenides (MoS2) reveals an enhancement in photocurrent due to improved photogeneration dynamics (e.g., reduction of bimolecular recombination and enhancing charge carrier transfer). Consequently, the optimized 2D MoS2-additive OPD achieved an exceptionally high linear dynamic range (LDR) exceeding 174 dB and an outstanding specific detectivity (D*) of 3.21 × 1012 Jones, while reaching femto-scale noise levels. This presents the potential of state-of-the-art OPDs for various light signal applications.

Defect-assisted reversible phase transition in mono- and few-layer ReS2

2D transition metal dichalcogenide (TMD) materials have attracted interest due to their remarkable excitonic, optical, electrical, and mechanical properties, which are dependent on their crystal structure. Consequently, controlling the crystal structure of these materials is essential for fine-tuning their performance, e.g., linear and nonlinear optical, as well as charge transport properties. While various phase-switching TMD materials are available, their transitions are often irreversible. Here, we investigate the mechanism of a light-induced reversible phase transition in mono- and bilayer rhenium disulfide (ReS2). Our observations, based on transmission electron microscopy, nonlinear spectroscopy, and density functional theory, reveal a transition from the ground T″ (double-distorted T) to the metastable H′ (distorted H) phase under femtosecond laser irradiation or influence of highly-energetic electrons. We show that the formation of sulfur vacancies facilitates this phenomenon. Our findings pave the way toward manipulating the crystal structure of ReS2 and possibly its heterostructures.

High-Specific Power Flexible Photovoltaics from Large-Area MoS2 for Space Applications.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 are excellent candidates for photovoltaic (PV) applications. Here, we present the modeling, fabrication, and characterization of large-area CVD-grown MoS2-based flexible PV on an off-the-shelf, 3 μm-thick flexible colorless polyimide with polyimide encapsulation designed for space structures. The devices are characterized under 1 sun AM0 illumination and show a V OC of 0.180 V and a specific power of 0.001 kW/kg for a subnanometer-thick, single MoS2 monolayer absorber. Model projections indicate that the polyimide encapsulant introduces negligible absorption loss, and up to 12.97 kW/kg specific power is attainable for a 100 nm-thick MoS2 absorber layer. The devices maintain their performance after repetitive bending down to 5 mm bend radius. An increase in performance is measured after radiation exposure to 1 MeV e- fluence, which is partially attributed to defect healing. Techno-economic analysis shows that even with a lower efficiency, the specific power of a 2D PV array designed for a 6U CubeSat is 2 orders of magnitude higher, and the cost to deploy in space is 2 orders of magnitude less than that of a Si panel used in space. This indicates that the 2D TMDC-based PV has great potential for space applications.

Two-dimensional Czochralski growth of single-crystal MoS2.

Batch production of single-crystal two-dimensional (2D) transition metal dichalcogenides is one prerequisite for the fabrication of next-generation integrated circuits. Contemporary strategies for the wafer-scale high-quality crystallinity of 2D materials centre on merging unidirectionally aligned, differently sized domains. However, an imperfectly merged area with a translational lattice brings about a high defect density and low device uniformity, which restricts the application of the 2D materials. Here we establish a liquid-to-solid crystallization in 2D space that can rapidly grow a centimetre-scale single-crystal MoS2 domain with no grain boundaries. The large MoS2 single crystal obtained shows superb uniformity and high quality with an ultra-low defect density. A statistical analysis of field effect transistors fabricated from the MoS2 reveals a high device yield and minimal variation in mobility, positioning this FET as an advanced standard monolayer MoS2 device. This 2D Czochralski method has implications for fabricating high-quality and scalable 2D semiconductor materials and devices.

Unified transmission electron microscopy with the glovebox integrated system for investigating air-sensitive two-dimensional quantum materials.

Transmission electron microscopy (TEM) is an indispensable tool for elucidating the intrinsic atomic structures of materials and provides deep insights into defect dynamics, phase transitions, and nanoscale structural details. While numerous intriguing physical properties have been revealed in recently discovered two-dimensional (2D) quantum materials, many exhibit significant sensitivity to water and oxygen under ambient conditions. This inherent instability complicates sample preparation for TEM analysis and hinders accurate property measurements. This review highlights recent technical advancements to preserve the intrinsic structures of water- and oxygen-sensitive 2D materials for atomic-scale characterizations. A critical development discussed in this review is implementing an inert gas-protected glovebox integrated system (GIS) designed specifically for TEM experiments. In addition, this review emphasizes air-sensitive materials such as 2D transition metal dichalcogenides, transition metal dihalides and trihalides, and low-dimensional magnetic materials, demonstrating breakthroughs in overcoming their environmental sensitivity. Furthermore, the progress in TEM characterization enabled by the GIS is analyzed to provide a comprehensive overview of state-of-the-art methodologies in this rapidly advancing field.

Controllable spin rectification behavior of vertical and lateral VSe2/WSe2 heterojunction Schottky diodes.

Heterojunctions (HJs) based on two-dimensional (2D) transition metal dichalcogenides are considered promising candidates for next-generation electronic and optoelectronic devices. Here, vertical (V-type) and lateral (L-type) HJ diodes based on metallic 1T-VSe2 and semiconducting 2H-WSe2 with out-of-plane and in-plane contacts are designed. First-principles quantum transport simulations reveal that both V- and L-type VSe2/WSe2 HJ diodes form p-type Schottky contacts. Under zero gate voltage, V-type VSe2/WSe2 HJ Schottky diodes exhibit superior spin rectification behavior compared to L-type, with rectification ratios approaching 109 and 106, respectively. At 300 K, the ideality factor of the V-type diode is lower than that of the L-type and reaches the ideal state at 478 and 510 K, respectively. Notably, positive gate voltage can reverse the rectification direction in both diodes and weaken the rectifying effect in V-type devices. Conversely, negative gate voltage significantly increases the current in both diodes and enhances the rectification ratio of the L-type device to 109. These findings provide insights into the spin-dependent rectification behavior of V- and L-type VSe2/WSe2 HJs in Schottky diodes, offering theoretical guidance for exploring magnetic nanoscale devices based on 2D materials.

Understanding the trends of phase transition in d^n (n= 0 to 6) transition metal disulfides under carrier injection: A perspective from d-orbital filling

Two-dimensional (2D) transition metal dichalcogenides have different structural phases including the H-phase, T-phase, and various distorted phases. Structural phase transitions between phases are of significant importance for realizing unique electronic...

Layer-number-dependent photoswitchability in 2D MoS2-diarylethene hybrids.

Molybdenum disulfide (MoS2) is a notable two-dimensional (2D) transition metal dichalcogenide (TMD) with properties ideal for nanoelectronic and optoelectronic applications. With growing interest in the material, it is critical to understand its layer-number-dependent properties and develop strategies for controlling them. Here, we demonstrate a photo-modulation of MoS2 flakes and elucidate layer-number-dependent charge transfer behaviors. We fabricated hybrid structures by functionalizing MoS2 flakes with a uniform layer of photochromic diarylethene (DAE) molecules that can switch between closed- and open-form isomers under UV and visible light, respectively. We discovered that the closed-form DAE quenches the photoluminescence (PL) of monolayer MoS2 when excited at 633 nm and that the PL fully recovers after DAE isomerization into the open-form. Similarly, the electric conductivity of monolayer MoS2 is drastically enhanced when interacting with the closed-form isomers. In contrast, photoinduced isomerization did not modulate the properties of the hybrids made of MoS2 bilayers and trilayers. Density functional theory (DFT) calculations revealed that a hole transfer from monolayer MoS2 to the closed-form isomer took place due to energy level alignments, but such interactions were prohibited with open-form DAE. Computational results also indicated negligible charge transfer at the hybrid interfaces with bilayer and trilayer MoS2. These findings highlight the critical role of layer-number-dependent interactions in MoS2-DAE hybrids, offering valuable insights for the development of advanced photoswitchable devices.

Manipulation of trions to enhance the excitonic emission in monolayer p-MoS2 and its hetero-bilayer by reverse charge injection.

Monolayer 2D transition metal dichalcogenides (TMDs) are known for their direct bandgaps and pronounced excitonic effects, which facilitate efficient light absorption and high photoluminescence (PL). In this study, we report a significant enhancement in PL emission from monolayers of p-type molybdenum disulfide (p-MoS2), fabricated on conductive substrates-such as indium tin oxide (ITO) and gold (Au). We attribute this behaviour to the reverse injection of charge carriers from substrates to p-MoS2 and the subsequent localization of electrons and holes in the substrate and p-MoS2, respectively. Such injection of charge carriers was suppressed when few-layer graphene (FLG) was used as a barrier layer. Further investigation of the PL emission characteristics from a vertically stacked hetero-bilayer (the p-n interface) of p-MoS2 and n-MoSe2 revealed a single resonant high-emission PL peak at 1.64 eV with the PL emission from this heterostructure significantly higher than that from free-standing monolayers. This finding contrasts sharply with the PL quenching often seen in hetero-bilayers with an n-n interface. These findings offer valuable insights into the fundamental optical and electronic properties of 2D TMDs and their heterostructures, which are essential for optimizing these materials for optoelectronic applications.

Unveiling the Advancements in Electrochemical Performance of 2D Transition Metal Dichalcogenides as an Electrode Material in Asymmetric Supercapacitors

Unveiling the Advancements in Electrochemical Performance of 2D Transition Metal Dichalcogenides as an Electrode Material in Asymmetric Supercapacitors

Defect dependent electronic properties of two-dimensional transition metal dichalcogenides (2H, 1T, and 1T' phases).

Transition metal dichalcogenides (TMDs) exhibit a wide range of electronic properties due to their structural diversity. Understanding their defect-dependent properties might enable the design of efficient, bright, and long-lifetime quantum emitters. Here, we use density functional theory (DFT) calculations to investigate the 2H, 1T, and 1T' phases of MoS2, WS2, MoSe2, WSe2 and the effect of defect densities on the electronic band structures, focusing on the influence of chalcogen vacancies. The 2H phase, which is thermodynamically stable, is a direct band gap semiconductor, while the 1T phase, despite its higher formation energy, exhibits metallic behavior. 1T phases with spin-orbit coupling show significant band inversions of 0.61, 0.77, 0.24 and 0.78 eV for MoS2, MoSe2, WS2 and WSe2, respectively. We discovered that for all four MX2 systems, the energy difference between 2H, 1T and 1T phases decreases with increasing concentration of vacancies (from 3.13% to 21.88%). Our findings show that the 2H phase also has minimum energy values depending on vacancies. TMDs containing W were found to have a wider bandgap compared to those containing Mo. The band gap of 2H WS2 decreased from 1.81 eV (1.54 eV with SOC included) under GGA calculations to a range of 1.37 eV to 0.79 eV, while the band gap of 2H MoSe2 reduced from 1.43 eV (1.31 eV with SOC) under GGA to a range of 0.98 eV to 0.06 eV, depending on the concentration. Our findings provide guidelines for experimental screening of 2D TMD defects, paving the way for the development of next-generation spintronic, electronic, and optoelectronic devices.

Strain‐Modulated Dominant Response Band of Self‐Powered Photodetector Based on WSe2 Lateral PN Homojunction

AbstractFlexible devices based on 2D materials have shown promising application capacity in next‐generation optoelectronics. The lack of inversion centrosymmetry renders odd‐layered 2D transition metal dichalcogenides (TMDs) to be piezoelectric, which means the properties modulation of them gets rid of the limit to the gate voltage and they can be directly gated by external strain. Here, a self‐powered photodetector based on WSe2 lateral PN homojunction is constructed, which exhibits excellent current rectification behavior with a rectification ratio of 1.8 × 103. Further, under the modulation of uniaxial tensile strain, a novel phenomenon that the dominant response waveband can be tuned from 550 to 800 nm by 1.04% tensile strain is observed. The maximum photoresponsivity to 800 nm incident laser reach 216.7 mA W−1 with 455% improvement has been demonstrated when a 1.04% tensile strain is applied. This work provides an example of multi‐band response light detection with strain manipulation on a single photodetector device, which shows significant prospect in adaptable artificial vision application.

Two-Dimensional Transition Metal Dichalcogenide: Synthesis, Characterization, and Application in Candlelight OLED.

Low-color-temperature candlelight organic light-emitting diodes (OLEDs) offer a healthier lighting alternative by minimizing blue light exposure, which is known to disrupt circadian rhythms, suppress melatonin, and potentially harm the retina with prolonged use. In this study, we explore the integration of transition metal dichalcogenides (TMDs), specifically molybdenum disulfide (MoS2) and tungsten disulfide (WS2), into the hole injection layers (HILs) of OLEDs to enhance their performance. The TMDs, which are known for their superior carrier mobility, optical properties, and 2D layered structure, were doped at levels of 0%, 5%, 10%, and 15% in PEDOT:PSS-based HILs. Our findings reveal that OLEDs doped with 10% MoS2 exhibit notable enhancements in power efficacy (PE), current efficacy (CE), and external quantum efficiency (EQE) of approximately 39%, 21%, and 40%, respectively. In comparison, OLEDs incorporating 10% of WS2 achieve a PE of 28%, a CE of 20%, and an EQE of 35%. The enhanced performance of the MoS2-doped devices is attributed to their superior hole injection and balanced carrier transport properties, resulting in more efficient operation. These results highlight the potential of incorporating 2D TMDs, especially MoS2, into OLED technology as a promising strategy to enhance energy efficiency. This approach aligns with environmental, social, and governance (ESG) goals by emphasizing reduced environmental impact and promoting ethical practices in technology development. The improved performance metrics of these TMD-doped OLEDs suggest a viable path towards creating more energy-efficient and health-conscious lighting solutions.

Highly Oriented WS2 Monolayers for High-Performance Electronics.

2Dtransition-metal dichalcogenide (TMDC) semiconductors represent the most promising channel materials for post-silicon microelectronics due to their unique structure and electronic properties. However, it remains challenging to synthesize wide-bandgap TMDCs monolayers featuring large areas and high performance simultaneously. Herein, highly oriented WS2 monolayers are reproducibly synthesized through a templated growth strategy on vicinal C/A-plane sapphire wafers. Various spectroscopic characterizations confirm the high crystallographic orientation and uniformity across the entire wafers. Electronic measurements for samples transferred onto SiO2/Si substrates reveal high average field-effect mobilities of 62 and 180 cm2V-1s-1 at room temperature and 8 K, respectively. On hexagonal boron nitride substrates, these mobilities increase to 94 and 473 cm2V-1s-1, respectively. A record high saturation current density of 675µAµm-1 is observed, outperforming the index required for high-density integration circuits in IRDS 2025. This work paves the way for the application of wide-bandgap TMDC monolayers in post-silicon electronics.

Van der Waals growth of monolayer transition metal dichalcogenide superconductors on ultra-flat graphene

Abstract Two-dimensional (2D) superconductors have desirable physical properties and a wide range of potential applications. However, many 2D superconducting materials have poor structural integrity and performance at thicknesses down to monolayer. Therefore, they are still hardly exploited in large-scale practical applications. In this study, we demonstrate a growth strategy based on the two-step vapor deposition process and van der Waals (vdW) ultra-flat graphene as a buffer layer, and thereafter realize the wafer-scale monolayer transition metal dichalcogenide (TMD) films, especially for 2D TMD superconductors. By comparing different non-vdW and vdW growth substrates, we demonstrate that ultra-flat graphene films provide an atomically smooth surface and effectively isolates oxygen-dangling bonds, and finally promote the superconducting behaviours of monolayer NbSe2 and TaS2 films. This growth strategy could also be extended to other wafer-scale TMD films and be of both fundamental and technological significance in the development of wafer-scale monolayer TMD-based devices.

Two-Dimensional Transition Metal Dichalcogenides (2D TMDs) Coupled With Zero-Dimensional Nanomaterials (0D NMs) for Advanced Photodetection.

The integration of 2D transition metal dichalcogenides (TMDs) with other materials presents a promising approach to overcome inherent limitations and enable the development of novel functionalities. In particular, 0D nanomaterials (0D NMs) offer notable advantages for photodetection, including broadband light absorption, size-dependent optoelectronic properties, high quantum efficiency, and good compatibility. Herein, the integration of 0D NMs with 2D TMDs to develop high-performance photodetectors is reviewed. The review provides a comprehensive overview of different types of 0D NMs, including plasma nanoparticles (NPs), up-conversion NPs, quantum dots (QDs), nanocrystals (NCs), and small molecules. The discussion starts with an analysis of the mechanism of 0D NMs on 2D TMDs in photodetection, exploring various strategies for improving the performance of hybrid 2D TMDs/0D NMs. Recent advancements in photodetectors combining 2D TMDs with 0D NMs are investigated, particularly emphasizing critical factors such as photosensitivity, photogain, specific detectivity, and photoresponse speed. The review concludes with a summary of the current status, highlighting the existing challenges and prospective developments in the advancement of 0D NMs/2D TMDs-based photodetectors.

Robust Ferroelectricity in Nonstoichiometric 2D AgCr1-xS2 via Chemical Vapor Deposition.

Ferroelectricity in two-dimensional (2D) materials at room temperature has attracted significant interest due to their substantial potential for applications in non-volatile memory, nanoelectronics, and optoelectronics. The intrinsic tendency of 2D materials toward nonstoichiometry results in atomic configurations that differ from those of their stoichiometric counterparts, thereby giving rise to potential ferroelectric polarization properties. However, reports on the emergence of room temperature ferroelectric effects in nonstoichiometric 2D materials remain limited. This study reports the observation of room temperature ferroelectricity in nonstoichiometric AgCr1-xS2 ternary 2D transition metal dichalcogenides synthesized via chemical vapor deposition. The noncentrosymmetric crystal structure and switchable ferroelectric polarization are confirmed through second harmonic generation (SHG) and piezoresponse force microscopy (PFM) measurements. It is determined that the primary cause of ferroelectric polarization is the interlayer movement of ordered asymmetric Ag atoms under the influence of numerous chromium (Cr) vacancies along with interlayer atom displacement. Furthermore, two types of electrical devices based on in-plane (IP) and out-of-plane (OOP) polarization are demonstrated. This work offers a new perspective for fabricating ternary ultrathin 2D transition metal dichalcogenides ferroelectric materials and presents a potential pathway for creating exceptional multifunctional materials.

Efficient Gate-Tunable Hot-Carrier Photocurrent from Perovskite Multiple Quantum Wells.

Hot-carrier relaxation above the bandgap results in significant energy losses, making the extraction of hot carriers a critical challenge for efficient hot-carrier photocurrent generation in devices. In this study, we observe long-lived hot carriers in the metal-halide perovskite multiple quantum wells, (BA)2(MA)n-1PbnI3n+1 (n = 3), and demonstrate effective hot-hole photocurrent generation using 2DMoS₂ as an extraction layer. A high external quantum efficiency of short-circuit hot-carrier photocurrent of up to 35.4% is achieved. Further enhancement in photocurrent efficiency and open-circuit photovoltage is achieved when a gate electric field is applied, resulting in an external quantum efficiency of up to 61.9%. Evidence of hot-hole extraction is validated through operando transient reflection measurements on the working devices, with studies that depend on wavelength, carrier density, and gate voltage. DFT calculations on the heterostructure devices under different bias voltages further elucidate the mechanism of hot-hole extraction enhancement. These findings underscore the potential of perovskite multiple quantum wells as long-lived hot-carrier generators and highlight the role of 2D transition metal dichalcogenide semiconductors as efficient hot-carrier extraction electrodes for low-power optoelectronics.