2D WSe2 Research Articles

Mixed-dimensional heterojunction by 3D CdS nanowire arrays bridged with 2D WSe2 for ultrafast photoelectric gas sensor

Mixed-dimensional heterojunction by 3D CdS nanowire arrays bridged with 2D WSe2 for ultrafast photoelectric gas sensor

High-Performance Gate-Voltage-Tunable Photodiodes Based on Nb2Pd3Se8/WSe2 Mixed-Dimensional Heterojunctions.

The mixed-dimensional (MD) van der Waals (vdWs) heterojunction for photodetectors has garnered significant attention owing to its exceptional compatibility and superior quality. Low-dimensional material heterojunctions exhibit unique photoelectric properties attributed to their nanoscale thickness and vdWs contact surfaces. In this work, a novel MD vdWs heterojunction composed of one-dimensional (1D) Nb2Pd3Se8 nanowires and two-dimensional (2D) WSe2 nanosheets is proposed. The heterojunction's energy band engineering is accomplished by manipulating the Fermi level of the bipolar 2D material via gate voltage, resulting in a rectification characteristic that can be adjusted with gate voltage. Under 685 nm laser irradiation, it demonstrates exceptional self-powered photodetection performance, attaining a photoresponsivity of 1.45 A W-1, an ultrahigh detectivity of 6.8 × 1012 Jones, and an ultrafast response time of 37/64 μs at zero bias. In addition, a broadband photodetector from 255 to 1064 nm is realized. These results demonstrate the great potential of Nb2Pd3Se8/WSe2 MD heterostructures for advanced electronic and optoelectronic devices.

Parallel Logic Operations in Electrically Tunable Two-Dimensional Homojunctions.

Two-dimensional materials show great potential for future electronics beyond silicon materials. Here, we report an exotic multiple-port device based on multiple electrically tunable planar p-n homojunctions formed in a two-dimensional (2D) ambipolar semiconductor, tungsten diselenide (WSe2). In this device, we prepare multiple gates consisting of a global gate and several local gates, by which electrostatically induced holes and electrons are simultaneously accumulated in a WSe2 channel, and furthermore, at the boundaries, p-n junctions are formed as directly visualized by Kelvin probe force microscopy. Therefore, in addition to the gate voltages in our device, the drain/source bias can also be used to switch the 2D WSe2 channel on/off due to the rectification effect of the formed p-n junctions. More importantly, when the voltage on the global gate electrode is altered, all p-n junctions are affected, which makes it possible to perform parallel logic operations.

Two-Dimensional ZrS2 and HfS2 for Making Sub-10 nm High-Performance P-Type Transistors.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have been recognized as reliable candidates for future sub-10 nm physical gate length field-effect transistors (FETs). However, the device performance of 2D P-type devices is far inferior to that of N-type devices, which seriously hinders the development of complementary metal-oxide-semiconductor (CMOS) integrated circuits. Herein, we presented that two new 2D TMDC channel materials, ZrS2 and HfS2, can realize high-performance P-type MOSFETs through first-principles quantum transport simulations. Different from the 2D MoS2 and WSe2, the continuous in-plane p-orbitals at the valence band edge of 2D ZrS2 and HfS2 lead to a small hole effective mass of 0.24 m0. As a result, 2D ZrS2 and HfS2 P-type MOSFETs with 10 nm gate length possess an on-state current (Ion) as high as 2000 μA/μm. Moreover, even when the gate length shrinks to 5 nm, the Ion can also reach ∼1500 μA/μm with the energy delay product ranging from 3 × 10-30 to 1 × 10-29 Js/μm, which are better than many other 2D P-type MOSFETs like MoS2 and WSe2. Our work demonstrates that 2D ZrS2 and HfS2 are competitive channel materials for constructing future sub-10 nm P-type high-performance FETs.

Low-temperature photoluminescence and Raman study of monolayer WSe2 for photocarrier dynamics and thermal conductivity

Low-temperature PL analysis reveals an intriguing temperature-dependent emission pattern in WSe2: excitonic dominance above the 150 K Debye temperature, a balance between excitonic and trionic emissions at 150 K, and trionic dominance below this threshold. At lower temperatures, both excitons and trions display linearly polarized emissions, with polarization increasing from 0% at 300 K to 23% (excitons) and 7% (trions) at 150 K, and 12% for trions at 90 K. Moreover, the synthesized monolayer of WSe2 exhibits high thermal conductivity (246 W m−1 K−1 for A1g and 185 W m−1 K−1 for E2g1 modes). This property is attributed to Se vacancies and defects at triangle edges, which redirect phonons, reducing scattering and enabling efficient heat transport along boundaries. The unveiling of these novel insights within the synthesized 2D WSe2 material holds significant promise for its potential applications in nano-optoelectronics. Its demonstrated efficiency in dissipating heat, coupled with improved thermal stability, suggests the possibility of employing it in future devices. This could facilitate compact designs and the miniaturization of advanced technological tools, showcasing the material's potential for practical implementation.

Ultra-High-k Ferroelectric BaTiO3 Perovskite in the Gate Stack for Two-Dimensional WSe2 p-Type High-Performance Transistors.

The experimental demonstration of a p-type 2D WSe2 transistor with a ferroelectric perovskite BaTiO3 gate oxide is presented. The 30 nm thick BaTiO3 gate stack shows a robust ferroelectric hysteresis with a remanent polarization of 20 μC/cm2 and further enables a capacitance equivalent thickness of 0.5 nm in the hybrid WSe2/BaTiO3 stack due to its high dielectric constant of 323. We demonstrate one of the best ON currents for perovskite gate 2D transistors in the literature. This is enabled by high-quality epitaxial growth of BaTiO3 and a single 2D layer transfer based fabrication method that is shown to be amenable to silicon platforms. This demonstration is an important milestone toward the integration of crystalline complex oxides with 2D channel materials for scaled CMOS and low-voltage ferroelectric logic applications.

Direct Measurement of the Thermal Expansion Coefficient of Epitaxial WSe2 by Four-Dimensional Scanning Transmission Electron Microscopy.

Current reports of thermal expansion coefficients (TEC) of two-dimensional (2D) materials show large discrepancies that span orders of magnitude. Determining the TEC of any 2D material remains difficult due to approaches involving indirect measurement of samples that are atomically thin and optically transparent. We demonstrate a methodology to address this discrepancy and directly measure TEC of nominally monolayer epitaxial WSe2 using four-dimensional scanning transmission electron microscopy (4D-STEM). Experimentally, WSe2 from metal-organic chemical vapor deposition (MOCVD) was heated through a temperature range of 18-564 °C using a barrel-style heating sample holder to observe temperature-induced structural changes without additional alterations or destruction of the sample. By combining 4D-STEM measurements with quantitative structural analysis, the thermal expansion coefficient of nominally monolayer polycrystalline epitaxial 2D WSe2 was determined to be (3.5 ± 0.9) × 10-6 K-1 and (5.7 ± 2) × 10-5 K-1 for the in- and out-of-plane TEC, respectively, and (3.6 ± 0.2) × 10-5 K-1 for the unit cell volume TEC, in good agreement with historically determined values for bulk crystals.

Photoluminescence of Monolayer WSe2 Enhanced by the Exciton Funnel Effect and the Interfacial Carrier Tunneling Effect When Integrated with 3D Si Wrinkled Structures

Quantum optics and photonic quantum-information technologies require emitters that have good stability and brightness, coupled with fabrication scalability and on-chip integrability. Most quantized emitters are presently based on 1D and 3D sources. Recently, monolayer transition metal dichalcogenides (TMDCs) hosting spatially localized excitons with narrow linewidths have garnered great interest. Advantages such as large binding energies and long room-temperature lifetimes of intralayer excitons suggest that TMDCs are promising candidates for use in optical devices. Here, we propose an emitter based on a 2D WSe2 semiconductor monolayer integrated with a periodic 3D Si-based wrinkled pattern. Carriers confined within the wrinkled pattern can be electrically and optically pumped, and funneled, to boost emission from the 2D WSe2 layer. This in turn acts as a monochromated quantum light source for the Si or any Si-based quantum optic and photonic information technologies. The brightness of the emission is enhanced by a factor greater than 40 compared with monolayer WSe2 on conventional flat SiGe. Moreover, these monolayer 2D/3D semiconductor composite heterostructures are fully scalable and promisingly efficient chip-integrated emitters.

DFT-Assisted Investigation of the Electric Field and Charge Density Distribution of Pristine and Defective 2D WSe2 by Differential Phase Contrast Imaging.

Most properties of solid materials are defined by their internal electric field and charge density distributions which so far are difficult to measure with high spatial resolution. Especially for 2D materials, the atomic electric fields influence the optoelectronic properties. In this study, the atomic-scale electric field and charge density distribution of WSe2 bi- and trilayers are revealed using an emerging microscopy technique, differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). For pristine material, a higher positive charge density located at the selenium atomic columns compared to the tungsten atomic columns is obtained and tentatively explained by a coherent scattering effect. Furthermore, the change in the electric field distribution induced by a missing selenium atomic column is investigated. A characteristic electric field distribution in the vicinity of the defect with locally reduced magnitudes compared to the pristine lattice is observed. This effect is accompanied by a considerable inward relaxation of the surrounding lattice, which according to first principles DFT calculation is fully compatible with a missing column of Se atoms. This shows that DPC imaging, as an electric field sensitive technique, provides additional and remarkable information to the otherwise only structural analysis obtained with conventional STEM imaging.

Hydrothermal Synthesis and Characterization of WSe2 Nanosheets: A Promising Approach for Wearable Photodetector Applications.

The two-dimensional (2D) WSe2 nanostructure was successfully synthesized via the hydrothermal method and subjected to comprehensive characterization using various spectroscopic techniques. X-ray diffraction (XRD) analysis confirmed the formation of nanosheets with a hexagonal crystal structure having a space symmetry of P63/mmc. Scanning electron microscopy (SEM) images showed irregular and nonuniform morphology. The size of the 2D nanosheets was determined using transmission electron microscopy (TEM) providing insights intotheir physical characteristics. The optical spectrum analysis yielded a discernible band gap value of 2.1 eV, as determined by the Tauc equation. Photoluminescence (PL) spectra display an emission at a wavelength of 610 nm, showing a broad emission associated with self-trapped excitons. Under excitation at λexc = 360 nm, PL emission spectra displayed a distinct peak at 610 nm, demonstrating the ability of the nanostructure to emit vivid red light. Photometric analysis underscored the potential of this nanostructure as a prominent red-light source for diverse display applications. The optimized photodetection performance of a device showcases a photoresponsivity of approximately 1.25 × 10-3 AW-1 and a detectivity of around 5.19 × 108 Jones at a wavelength of 390 nm. Additionally, the quantum efficiency is reported to be approximately 6.99 × 10-3 at a wavelength of 635 nm. These findings highlight the capability of the device for efficient photoconversion at specified wavelengths, indicating potential applications in sensing, imaging, and optical communication. The combination of structural, morphological, and optical characterizations highlights the suitability of 2D WSe2 nanostructure for practical optoelectronic applications, particularly in display technologies.

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

AbstractChemical 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.

Ultrathick MA2N4(M'N) Intercalated Monolayers with Sublayer‐Protected Fermi Surface Conduction States: Interconnect and Metal Contact Applications

AbstractRecent discovery of ultrathick MoSi2N4(MoN)n monolayers open up an exciting platform to engineer two‐dimensional (2D) material properties via intercalation architecture. In this study, a series of ultrathick MA2N4(M'N) monolayers (M, M' = Mo, W; A = Si, Ge) is computationally investigated under both homolayer and heterolayer intercalation architectures, in which the same and different species of transition metal nitride inner core sublayer are intercalated by outer passivating nitride sublayers, respectively. The MA2N4(M'N) are stable metallic monolayers with excellent mechanical strength. Intriguingly, the metallic states around Fermi level are localized within the inner core sublayer. Carrier conduction mediated by electronic states around the Fermi level is thus spatially insulated from the external environment by the native outer nitride sublayers, suggesting the potential of MA2N4(M'N) in back‐end‐of‐line metal interconnect applications. N and Si (or Ge) vacancy defects at the outer sublayers create ‘punch through’ states around the Fermi level that bridges the carrier conduction in the inner core sublayer and the outer environment, forming an electrical contact akin to the ‘via' structures of metal interconnects. It is further shown that MoSi2N4(MoN) can serve as a quasi‐Ohmic contact to 2D WSe2. These findings reveal the potential of ultrathick MA2N4(MN) monolayers in interconnect and metal contact applications.

Metatungstate Chemical Vapor Deposition of WSe2: Substrate Effects, Shapes, and Morphologies

Owing to their exceptional properties, which are usually determined by the growth conditions, 2D transition metal dichalcogenides (TMDCs) offer numerous research directions for applications in the fields of spintronics, valleytronics, and optoelectronics. Here, we focus on the chemical vapor deposition (CVD) synthesis of WSe2 (tungsten diselenide) nanoclusters/nanoflakes by using a liquid precursor for tungsten (ammonium metatungstate) on Si/SiO2, fused silica, and sapphire substrates. Various WSe2 clusters with different sizes, thicknesses, and geometries were analyzed by means of optical and atomic force microscopy (AFM) and Raman spectroscopy. The observed structures were mostly WSe2 multilayers; however, monolayer formations were also found. They showed significant morphological differences, as well as wide nucleation density and size variations, possibly related to precursor/substrate surface interactions under the same CVD synthesis conditions. The largest WSe2 domains with a lateral size of up to hundreds of micrometers were observed on sapphire, probably caused by a higher growth rate of singular nucleation sites. WSe2 domains with irregular and triangular shapes were simultaneously identified on fused silica, whereas multilayered pyramidal WSe2 structures dominated in the case of Si/SiO2 substrates. The application of polarized Raman spectroscopy to precisely determine and differentiate the characteristic vibrational modes (A1g, E2g, and 2LA(M)) enabled the unambiguous identification of 2D and/or multilayered WSe2 formations with a high crystallinity level. The presented comparative analysis of samples prepared in relatively simple synthesis conditions (moderate working temperatures and ambient pressure) provides a base for further progress of the facile metatungstate CVD method and relevant opportunities for the exploration of 2D TMDC materials.

Plasmonically enhancing 2D WSe2 light absorption for photodetection

Plasmonically enhancing 2D WSe2 light absorption for photodetection

Intrinsic and Strain-Dependent Properties of Suspended WSe2 Crystallites toward Next-Generation Nanoelectronics and Quantum-Enabled Sensors.

Two-dimensional (2D) layered materials exhibit great potential for high-performance electronics, where knowledge of their thermal and phononic properties is critical toward understanding heat dissipation mechanisms, considered to be a major bottleneck for current generation nanoelectronic, optoelectronic, and quantum-scale devices. In this work, noncontact Raman spectroscopy was used to analyze thermal properties of suspended 2D WSe2 membranes to access the intrinsic properties. Here, the influence of electron-phonon interactions within the parent crystalline WSe2 membranes was deciphered through a comparative analysis of extrinsic substrate-supported WSe2, where heat dissipation mechanisms are intimately tied to the underlying substrate. Moreover, the excitonic states in WSe2 were analyzed by using temperature-dependent photoluminescence spectroscopy, where an enhancement in intensity of the localized excitons in suspended WSe2 was evident. Finally, phononic and electronic properties in suspended WSe2 were examined through nanoscale local strain engineering, where a uniaxial force was induced on the membrane using a Au-coated cantilever within an atomic force microscope. Through the fundamental analysis provided here with temperature and strain-dependent phononic and optoelectronic properties in suspended WSe2 nanosheets, the findings will inform the design of next-generation energy-efficient, high-performance devices based on WSe2 and other 2D materials, including for quantum applications.

First-principles calculations on monolayer WX2 (X = S, Se) as an effective drug delivery carrier for anti-tuberculosis drugs.

Tuberculosis (TB) remains a major global health concern, necessitating the exploration of novel drug delivery systems to combat the challenges posed by conventional approaches. We investigated the potential of monolayer transition metal dichalcogenides (TMDs) as an innovative platform for efficient and targeted delivery of antituberculosis drugs. Specifically, the electronic and optical properties of prominent TB drugs, isoniazid (INH) and pyrazinamide (PZA), adsorbed on tungsten diselenide (WSe2) and tungsten disulfide (WS2) monolayers were studied using first-principles calculations based on density functional theory (DFT). The investigation revealed that the band gaps of WSe2 and WS2 monolayers remain unaltered upon adsorption of PZA or INH, with negative adsorption energy indicating stable physisorption. We explored different vertical and horizontal configurations, and the horizontal ones were more stable. When INH and PZA drugs were horizontally adsorbed together on WSe2, the most stable configuration was found with an adsorption energy of -2.35 eV. Moreover, the adsorbed drugs could be readily released by light within the visible or near-infrared (NIR) wavelength range. This opened up possibilities for their potential application in photothermal therapy, harnessing the unique properties of these 2D materials. The comprehensive analysis of the band structures and density of states provides valuable insights into how the drug molecules contributed to the conduction and valence bands. The optical responses of anti-TB drugs adsorbed in 2D WSe2 and WS2 were similar to those of pristine 2D WSe2 and WS2. We demonstrated the temperature-dependent release mechanism of our 2D WSe2 and WS2 drug complexes, confirming the feasibility of releasing the discussed anti-tuberculosis drugs by generating heat through photothermal therapy. These findings hold significant promise for developing innovative drug delivery systems that have enhanced efficacy for targeted and low-toxic TB treatment.

Bidirectional rectifier with gate voltage control based on Bi2O2Se/WSe2 heterojunction

Two-dimensional (2D) WSe2 has received increasing attention due to its unique optical properties and bipolar behavior. Several WSe2-based heterojunctions exhibit bidirectional rectification characteristics, but most devices have a lower rectification ratio. In this work, the Bi2O2Se/WSe2 heterojunction prepared by us has a type Ⅱ band alignment, which can vastly suppress the channel current through the interface barrier so that the Bi2O2Se/WSe2 heterojunction device has a large rectification ratio of about 105. Meanwhile, under different gate voltage modulation, the current on/off ratio of the device changes by nearly five orders of magnitude, and the maximum current on/off ratio is expected to be achieved 106. The photocurrent measurement reveals the behavior of recombination and space charge confinement, further verifying the bidirectional rectification behavior of heterojunctions, and it also exhibits excellent performance in light response. In the future, Bi2O2Se/WSe2 heterojunction field-effect transistors have great potential to reduce the volume of integrated circuits as a bidirectional controlled switching device.

Single-crystalline WSe2 nanoflakes as efficient electrocatalysts

Microwave-assisted synthesis of 2D WSe2 nanoflakes from a single molecular source for efficient electrocatalysis.

Signaling redox probe/DNA aptamer complexes on a new POP/2D WSe2 composite-based immunosensor towards the simultaneous detection of three-protein overexpression as an alternative severe SARS-COV-2 infection diagnosis

The coronavirus disease 2019 (COVID-19) has become a global pandemic in which patients with symptoms are infected with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Clinical investigation of human immune response cytokines or/and overexpressed proteins at trace cut-off levels can be valuable to monitor body function failures and, eventually, improve the prognosis for patients. Nevertheless, the currently available technologies are still limited. Herein, we provide the first sandwich-like immunosensor that has the potential to detect multiple protein biomarkers in a single measurement. The sandwich-like structure was created by coating a three-screen printed carbon electrode (3SPCE) array with a porous organic polymer (POP)/2D WSe2 nanocomposite. This system enables the simultaneous immunoassay of C-reactive protein (CRP), native cardiac troponin I (cTnI), and procalcitonin (PCT), which together provide a panel of symptom-diagnostic biomarkers for the presence and severity of COVID-19 infection. The biomarker detection principle relies on three antibodies as bioreceptor probes bound on the modified 3SPCE and three aptamers to generate three redox probe complexes as signal amplifiers. After incorporating three redox species (Cd2+, methylene blue (MB), and Ag+) into the aptamer structure, well-differentiated oxidation peaks can be achieved, indicating the concentrations of these three proteins. Under the optimized conditions of the stepwise fabrications, the linear dynamic range of three-protein detection by our proposed multiplex biosensor ranges from 100 to 50,000 pg mL–1. The limits of detection (LODs) are as low as 62, 81, and 44 pg mL–1 for the detection of CRP, cTnI, and PCT, respectively. As proof of the concept, this multiplexed assay demonstrates outstanding analytical performance with excellent reproducibility, high sensitivity and selectivity, and acceptable recoveries of the detection in human serum. Therefore, the multiplexed sensor could be an alternative tool in the field of clinical analysis for the diagnosis of COVID-19 severity or other emerging infectious diseases.