2D Transition Metal Dichalcogenides Research Articles

Boltzmann Switching MoS2 Metal-Semiconductor Field-Effect Transistors Enabled by Monolithic-Oxide-Gapped Metal Gates at the Schottky-Mott Limit.

A gate stack that facilitates a high-quality interface and tight electrostatic control is crucial for realizing high-performance and low-power field-effect transistors (FETs). However, when constructing conventional metal-oxide-semiconductor structures with two-dimensional (2D) transition metal dichalcogenide channels, achieving these requirements becomes challenging due to inherent difficulties in obtaining high-quality gate dielectrics through native oxidation or film deposition. Here, a gate-dielectric-less device architecture of van der Waals Schottky gated metal-semiconductor FETs (vdW-SG MESFETs) using a molybdenum disulfide (MoS2) channel and surface-oxidized metal gates such as nickel and copper is reported. Benefiting from the strong SG coupling, these MESFETs operate at remarkably low gate voltages, <0.5V. Notably, they also exhibit Boltzmann-limited switching behavior featured by a subthreshold swing of ≈60mVdec-1 and negligible hysteresis. These ideal FET characteristics are attributed to the formation of a Fermi-level (EF) pinning-free gate stack at the Schottky-Mott limit. Furthermore, authors experimentally and theoretically confirm that EF depinning can be achieved by suppressing both metal-induced and disorder-induced gap states at the interface between the monolithic-oxide-gapped metal gate and the MoS2 channel. This work paves a new route for designing high-performance and energy-efficient 2D electronics.

Optical Signal Investigation of Monolayer MoS2 Grown Via Glass-Assisted CVD On Patterned Surfaces

Enhancing photoluminescence (PL) in single-layer transition metal dichalcogenides has garnered significant interest, particularly for advancing high-performance 2D electronics and optoelectronics. The combination of surface engineering and contemporary growth methods has provided a platform for investigating optical signals. In this study, we present variations in PL and Raman signals of single-layer MoS2 flakes grown conformally using the glass-assisted CVD method on square-patterned surfaces with varying well depths. PL spectroscopy revealed a systematic and pronounced enhancement in intensities as the valley thickness decreased from 285 nm to 225 nm. Conversely, for the hill regions of the samples, the PL intensity initially increased with decreasing valley thickness and then decreased, despite the hill regions having a constant thickness of 300 nm. On the other hand, PL maps did not exhibit a systematic dependence of intensities on the hill-valley thickness distinction, contrary to expected results based on literature data for similar materials on flat surfaces. The origin of the intensity oscillations was attributed to possible mechanisms, including thickness-dependent interference and strain-related exciton funneling effects. Additionally, Raman measurements revealed irregular variations in intensity in hill regions, dependent on the thicknesses of the underlying SiO2 layers. Furthermore, we observed that the sizes of the flakes increased as the well depths of the underlying patterned surface decreased. This phenomenon might be attributed to alterations in the carrier gas flow pattern and varying temperature gradients between the hills and valleys. These results hold substantial potential to open new avenues for the integration of 2D transition metal dichalcogenides into on-chip electronic and optoelectronic devices.

Synthesis and Characterization of 2D Ternary Compound TMD Materials Ta3VSe8

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are garnering considerable scientific interest, prompting discussion regarding their prospective applications in the fields of nanoelectronics and spintronics while also fueling groundbreaking discoveries in phenomena such as the fractional quantum anomalous Hall effect (FQAHE) and exciton dynamics. The abundance of binary compound TMDs, such as MX2 (M = Mo, W; X = S, Se, Te), has unlocked myriad avenues of exploration. However, the exploration of ternary compound TMDs remains relatively limited, with notable examples being Ta2NiS5 and Ta2NiSe5. In this study, we report the synthesis of a new 2D ternary compound TMD materials, Ta3VSe8, employing the chemical vapor transport (CVT) method. The as-grown bulk crystal is shiny and can be easily exfoliated. The crystal quality and structure are verified by X-ray diffraction (XRD), while the surface morphology, stoichiometric ratio, and uniformity are determined by scanning electron microscopy (SEM). Although the phonon property is found stable at different temperatures, magneto-resistivity evolves. These findings provide a possible approach for the realization and exploration of ternary compound TMDs.

Lowering the Schottky Barrier Height by Quasi-van der Waals Contacts for High-Performance p-Type MoTe2 Field-Effect Transistors.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) offer advantages over traditional silicon in future electronics but are hampered by the prominent high contact resistance of metal-TMD interfaces, especially for p-type TMDs. Here, we present high-performance p-type MoTe2 field-effect transistors via a nondestructive van der Waals (vdW) transfer process, establishing low contact resistance between the 2D MoTe2 semiconductor and the PtTe2 semimetal. The integration of PtTe2 as contacts in MoTe2 field-effect transistors leads to significantly improved electrical characteristics compared to conventional metal contacts, evidenced by a mobility increase to 80 cm2 V-1 s-1, an on-state current rise to 5.0 μA/μm, and a reduction in Schottky barrier height (SBH) to 48 meV. Such a low SBH in quasi-van der Waals contacts can be assigned to the low electrical resistivity of PtTe2 and the high efficiency of carrier injection at the 2D semimetal/2D semiconductor interfaces. Imaging via transmission electron microscopy reveals that the 2D semimetal/two-dimensional semiconductor interfaces are atomically flat and exceptionally clean. This interface engineering strategy could enable low-resistance contacts based on vdW architectures in a facile manner, providing opportunities for 2D materials for next-generation optoelectronics and electronics.

An ultra energy-efficient hardware platform for neuromorphic computing enabled by 2D-TMD tunnel-FETs

Brain-like energy-efficient computing has remained elusive for neuromorphic (NM) circuits and hardware platform implementations despite decades of research. In this work we reveal the opportunity to significantly improve the energy efficiency of digital neuromorphic hardware by introducing NM circuits employing two-dimensional (2D) transition metal dichalcogenide (TMD) layered channel material-based tunnel-field-effect transistors (TFETs). Our novel leaky-integrate-fire (LIF) based digital NM circuit along with its Hebbian learning circuitry operates at a wide range of supply voltages, frequencies, and activity factors, enabling two orders of magnitude higher energy-efficient computing that is difficult to achieve with conventional material and/or device platforms, specifically the silicon-based 7 nm low-standby-power FinFET technology. Our innovative 2D-TFET based NM circuit paves the way toward brain-like energy-efficient computing that can unleash major transformations in future AI and data analytics platforms.

Solution-Processable and Printable Two-Dimensional Transition Metal Dichalcogenide Inks.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with layered crystal structures have been attracting enormous research interest for their atomic thickness, mechanical flexibility, and excellent electronic/optoelectronic properties for applications in diverse technological areas. Solution-processable 2D TMD inks are promising for large-scale production of functional thin films at an affordable cost, using high-throughput solution-based processing techniques such as printing and roll-to-roll fabrications. This paper provides a comprehensive review of the chemical synthesis of solution-processable and printable 2D TMD ink materials and the subsequent assembly into thin films for diverse applications. We start with the chemical principles and protocols of various synthesis methods for 2D TMD nanosheet crystals in the solution phase. The solution-based techniques for depositing ink materials into solid-state thin films are discussed. Then, we review the applications of these solution-processable thin films in diverse technological areas including electronics, optoelectronics, and others. To conclude, a summary of the key scientific/technical challenges and future research opportunities of solution-processable TMD inks is provided.

Low-Temperature Synthesis of WSe2 by the Selenization Process under Ultrahigh Vacuum for BEOL Compatible Reconfigurable Neurons.

Low-temperature large-area growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) is critical for their integration with silicon chips. Especially, if the growth temperatures can be lowered below the back-end-of-line (BEOL) processing temperatures, the Si transistors can interface with 2D devices (in the back end) to enable high-density heterogeneous circuits. Such configurations are particularly useful for neuromorphic computing applications where a dense network of neurons interacts to compute the output. In this work, we present low-temperature synthesis (400 °C) of 2D tungsten diselenide (WSe2) via the selenization of the W film under ultrahigh vacuum (UHV) conditions. This simple yet effective process yields large-area, homogeneous films of 2D TMDs, as confirmed by several characterization techniques, including reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and different spectroscopy methods. Memristors fabricated using the grown WSe2 film are leveraged to realize a novel compact neuron circuit that can be reconfigured to enable homeostasis.

Band Gap Engineering of MoxW1-xS2 Alloy Monolayers with Wafer-Scale Uniformity.

Modulating the band gap of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is critical for their application in a wider spectral range. Alloying has been demonstrated as an effective method for regulating the band gap of 2D TMDC semiconductors. The fabrication of large-area 2D TMDC alloy films with centimeter-scale uniformity is fundamental to the application of integrated devices. Herein, we report a liquid-phase precursor one-step chemical vapor deposition (CVD) method for fabricating a MoxW1-xS2 alloy monolayer with a large size and an adjustable band gap. Good crystalline quality and high uniformity on a wafer scale enable the continuous adjustment of its band gap in the range of 1.8-2.0 eV. Density functional theory calculations provided a deep understanding of the Raman-active vibration modes of the MoxW1-xS2 alloy monolayer and the change in the conductivity of the alloy with photon energy. The synthesis of large-area MoxW1-xS2 alloy monolayers is a critical step toward the application of 2D layered semiconductors in practical optoelectronic devices.

Magnetic proximity effect on the spin-valley coupling in two-dimensional Cr2Ge2Te6/2H-TMD van der Waals heterostructures

Two-dimensional (2D) transition metal dichalcogenides with 2H-phases, as a unique platform of valleytronics, display valley polarization and the well-known anomalous valley Hall effect when stacking with 2D magnetic substrates. In this study, we employ first-principles calculations to investigate the magnetic states, band structures, and magnetic proximity-dependent valley properties of 2D van der Waals heterostructures Cr2Ge2Te6/2H-MX2 (M = Mo, W, and X = S, Se, Te). Our findings reveal that the heterostructures possess stacking-dependent spontaneous valley polarization as well as pristine perpendicular magnetic anisotropy. Additionally, the Berry curvature and circular polarization demonstrate the presence of spin–momentum coupling characteristics, while the calculated non-zero Hall voltage indicates that the anomalous valley Hall effect can be achieved in valley-polarized systems. Furthermore, due to the strain effect and the electronic polarization at the interface, Cr2Ge2Te6/2H-MX2 heterostructures undergo the transition from semiconductors to semimetals upon substitution of early chalcogen elements. These calculations provide valuable insights for practical applications of valleytronics in 2D van der Waals heterostructure systems.

Near-field coupling of interlayer excitons in MoSe2/WSe2 heterobilayers to surface plasmon polaritons.

Two-dimensional (2D) transition metal dichalcogenides have emerged as promising quantum functional blocks benefitting from their unique combination of spin, valley, and layer degrees of freedom, particularly for the tremendous flexibility of moiré superlattices formed by van der Waals stacking. These degrees of freedom coupled with the enhanced Coulomb interaction in 2D structures allow excitons to serve as on-chip information carriers. However, excitons are spatially circumscribed due to their low mobility and limited lifetime. One way to overcome these limitations is through the coupling of excitons with surface plasmon polaritons (SPPs), which facilitates an interaction between remote quantum states. Here, we showcase the successful coupling of SPPs with interlayer excitons in molybdenum diselenide/tungsten diselenide heterobilayers. Our results indicate that the valley polarization can be efficiently transferred to SPPs, enabling preservation of polarization information even after propagating tens of micrometers.

High-Performance Contact-Doped WSe2 Transistors Using TaSe2 Electrodes.

Two-dimensional (2D) transitional metal dichalcogenides (TMDs) have garnered significant attention due to their potential for next-generation electronics, which require device scaling. However, the performance of TMD-based field-effect transistors (FETs) is greatly limited by the contact resistance. This study develops an effective strategy to optimize the contact resistance of WSe2 FETs by combining contact doping and 2D metallic electrode materials. The contact regions were doped using a laser, and the metallic TaSe2 flakes were stacked on doped WSe2 as electrodes. Doping the contact areas decreases the depletion width, while introducing the TaSe2 contact results in a lower Schottky barrier. This method significantly improves the electrical performance of the WSe2 FETs. The doped WSe2/TaSe2 contact exhibits an ultralow Schottky barrier height of 65 meV and a contact resistance of 11 kΩ·μm, which is a 50-fold reduction compared to the conventional Cr/Au contact. Our method offers a way on fabricating high-performance 2D FETs.

Transient Nanoscopy of Exciton Dynamics in 2D Transition Metal Dichalcogenides.

The electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides are dominated by strong excitonic resonances. Exciton dynamics plays a critical role in the functionality and performance of many miniaturized 2D optoelectronic devices; however, the measurement of nanoscale excitonic responses remains challenging. Here, a near-field transient nanoscopy is reported to probe exciton dynamics beyond the diffraction limit. Exciton recombination and exciton-exciton annihilation processes in monolayer and bilayer MoS2 are studied as the proof-of-concept demonstration. Moreover, with the capability to access local sites, intriguing exciton dynamics near the monolayer-bilayer interface and at the MoS2 nano-wrinkles are resolved. Such nanoscale resolution highlights the potential of this transient nanoscopy for fundamental investigation of exciton physics and further optimization of functional devices. This article is protected by copyright. All rights reserved.

Nb Doping and Alloying of 2D WS2 by Atomic Layer Deposition for 2D Transition Metal Dichalcogenide Transistors and HER Electrocatalysts.

We utilize plasma-enhanced atomic layer deposition to synthesize two-dimensional Nb-doped WS2 and NbxW1-xSy alloys to expand the range of properties and improve the performance of 2D transition metal dichalcogenides for electronics and catalysis. Using a supercycle deposition process, films are prepared with compositions spanning the range from WS2 to NbS3. While the W-rich films form crystalline disulfides, the Nb-rich films form amorphous trisulfides. Through tuning the composition of the films, the electrical resistivity is reduced by 4 orders of magnitude compared to pure ALD-grown WS2. To produce Nb-doped WS2 films, we developed a separate ABC-type supercycle process in which a W precursor pulse precedes the Nb precursor pulse, thereby reducing the minimum Nb content of the film by a factor of 3 while maintaining a uniform distribution of the Nb dopant. Initial results are presented on the electrical and electrocatalytic performances of the films. Promisingly, the NbxW1-xSy films of 10 nm thickness and composition x ≈ 0.08 are p-type semiconductors and have a low contact resistivity of (8 ± 1) × 102 Ω cm to Pd/Au contacts, demonstrating their potential use in contact engineering of 2D TMD transistors.

Logic-in-memory application of ferroelectric-based WS2-channel field-effect transistors for improved area and energy efficiency

In this study, we applied ferroelectrics to the gate stack of Field Effect Transistors (FETs) with a 2D transition-metal dichalcogenide (TMDC) channel, actively researching for sub-2nm technology node implementation. Subsequently, we analyzed the circuit characteristics of Logic-in-Memory (LiM) operation and utilized LiM features after applying ferroelectrics to achieve a single-device configuration. Based on well-calibrated simulations, we performed compact modeling in a circuit simulator to depict the temperature-dependent electrical characteristics of ferroelectric FETs with a double gate structure and 2D channel (DG 2D-FeFET) in sub-2nm dimensions. Through this, we have confirmed that the 2D FeFET-based LiM technology, designed for the 2 nm technology node, exhibits superior characteristics in terms of delay, power/energy consumption, and circuit area under all temperature conditions, compared to the conventional CMOS technology based on 2D FETs. This verification serves as proof of the future technological potential of 2D-FeFET in extremely scaled-down technology nodes.

A comparative study on 2D materials with native high-[formula omitted] oxides for sub-10 nm transistors

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with native high-κ oxides have presented a new avenue towards the development of next-generation ultra-scaled field-effect transistors (FETs). These materials have been experimentally shown to form a natively compatible oxide layer with a high dielectric constant, which can help scale down both the transistor size and the supply voltage. We present a material and device performance study into the use of several of these materials – namely HfS2, HfSe2, ZrS2, ZrSe2 – as channels in sub-10 nm FETs. All four materials exhibit isotropic transport at 10 nm channel length with ON currents over 1000 μA/μm but show anisotropic transport and degraded ON currents at 5 nm channel length. In general, the sulfide family excels in terms of subthreshold characteristics at sub-10 nm channel lengths. HfS2, in particular, surpasses all the other materials in terms of ON currents and subthreshold swing (SS), allowing it to also achieve excellent intrinsic performance. We have identified HfS2 as a superior material within this TMD family for sub-10 nm FETs.

Trifluoromethylation of 2D Transition Metal Dichalcogenides: A Mild Functionalization and Tunable p‐Type Doping Method

Chemical modification is a powerful strategy for tuning the electronic properties of 2D semiconductors. Here we report the electrophilic trifluoromethylation of 2D WSe2 and MoS2 under mild conditions using the reagent trifluoromethyl thianthrenium triflate (TTT). Chemical characterization and density functional theory calculations reveal that the trifluoromethyl groups bind covalently to surface chalcogen atoms as well as oxygen substitution sites. Trifluoromethylation induces p‐type doping in the underlying 2D material, enabling the modulation of charge transport and optical emission properties in WSe2. This work introduces a versatile and efficient method for tailoring the optical and electronic properties of 2D transition metal dichalcogenides.

Trifluoromethylation of 2D Transition Metal Dichalcogenides: A Mild Functionalization and Tunable p-Type Doping Method.

Chemical modification is a powerful strategy for tuning the electronic properties of 2D semiconductors. Here we report the electrophilic trifluoromethylation of 2D WSe2 and MoS2 under mild conditions using the reagent trifluoromethyl thianthrenium triflate (TTT). Chemical characterization and density functional theory calculations reveal that the trifluoromethyl groups bind covalently to surface chalcogen atoms as well as oxygen substitution sites. Trifluoromethylation induces p-type doping in the underlying 2D material, enabling the modulation of charge transport and optical emission properties in WSe2. This work introduces a versatile and efficient method for tailoring the optical and electronic properties of 2D transition metal dichalcogenides.

NaCl-assisted chemical vapor deposition growth of MoSe2 and MoSe2/h-BN heterostructure

As a typical two-dimensional (2D) transition metal dichalcogenides (TMDCs), MoSe2 has attracted lots of interest due to its electronic and optical properties in recent years. The controllable synthesis of 2D MoSe2 and its heterostructure is of considerable importance for achieving diverse applications. Herein, we realized the controlled growth of MoSe2 by atmospheric pressure chemical vapor deposition (APCVD) method through using NaCl as the promoter. Raman and photoluminescence (PL) spectroscopy have been used to investigate the optical properties of MoSe2 domains with different layers. The dependence of structures on synthetic parameter is systematically investigated by tuning the growth conditions. On this basis, the monolayer MoSe2 was constructed on CVD-grown hexagonal boron nitride (h-BN) by an all-CVD approach. Detailed characterizations indicate that MoSe2 deposited on h-BN exhibits improved optical properties as compared to those on SiO2/Si. Our results could enrich the understanding for growth of TMDCs, advocating further investigation of their intrinsic properties and a variety of device applications.

Exciton Dynamics of TiOPc/WSe2 Heterostructure.

The van der Waals (vdW) heterostructures composed of two-dimensional (2D) transition metal dichalcogenides (TMDs) and organic semiconductors demonstrate numerous compelling optoelectronic properties. However, the influence of the vdW epitaxial effect and temperature on the optoelectronic properties and interface exciton dynamics of heterostructures remains unclear. This study systematically investigates the fluorescence properties of TiOPc/WSe2 heterostructure. Comprehensive spectral characterization elucidates that the emission behavior of the TiOPc/WSe2 heterostructure arises from charge/energy transfer at the heterostructure interfaces and the structural ordering of the organic layer on the 2D monolayer WSe2 induced by vdW epitaxy. The interface exciton dynamic features probed by ultrafast transient spectroscopy reveal that the face-to-face molecular stacking configuration of TiOPc exhibits ultrafast exciton dynamics. In particular, we observe picosecond-scale absorption of organic molecular dimer cations, providing direct evidence of interface charge transfer at room temperature. Moreover, energy transfer from the TiOPc to WSe2 may exist based on the tunability in the fluorescence emission of the TiOPc/WSe2 heterostructure as the temperature changes. This study unveils the critical role of vdW epitaxy and temperature in the exciton dynamics of organic/2D TMDs hybrid systems and provides guidance for studying interlayer charge and energy transfer in organic/inorganic heterostructures.

Metallic layered VSe2 saturable absorber based single- and dual-wavelength ultrafast fiber laser

Vanadium selenide (VSe2) is a special two-dimensional (2D) transition metal dichalcogenide (TMD) material with metallic properties similar to graphene, which has been proved to have saturable absorption properties and used in mode-locked fiber lasers. As an excellent saturable absorber (SA) material, VSe2 has advantage of ultrafast carrier recovery time and broadband optical response range. However, the potential application of VSe2 in the field of ultrafast photonics has not been fully developed so far. In this work, the as-prepared VSe2 nanosheets were deposited on tapered fiber by optical deposition to obtain VSe2 SA. By inserting VSe2 SA into an Er-doped fiber laser, the mode-locked pulses with a central wavelength of 1574.4 nm can be obtained. The pulse width of the laser is measured to be ∼680 fs which is narrowest in the reported VSe2 SA-based lasers. In addition, the unique dual-wavelength mode-locked pulse is also obtained by further adjusting intracavity dispersion and polarization. This kind of dual-wavelength pulse is valuable in high order harmonic generation enhancement, terahertz wave generation, dense wavelength division multiplexing, dual-frequency comb and other applications. Our results show that metallic VSe2 is of great potential value in ultrafast fiber laser.