Transition Metal Dichalogenides Research Articles

Effects of Exchange Correlation Functional (Vwdf3) on the Structural, Elastic, and Electronic Properties of Transition Metal Dichalogenides

In this research, the effects of Van der Waals forces on the structural, elastic, electronic, and optical properties of bulk transition metals dichalcogenides (TMDs) were studied using a novel exchange-correlation functional, vdW-DF3. This new functional tries to correct the hidden Van der Waals problems which are not reported by the previous exchange functionals. Molybdenum dichalcogenide, MoX 2 (X = S, Se, Te) was chosen as a representative transition metal dichalcogenide to compare the performance of the newly designed functional with the other two popular exchange-correlation functional; PBE and rVV10. From the results so far obtained, the analysis of the structural properties generally revealed better performance by vdW-DF3 via the provision of information on lattice parameters very closer to the experimental value. For example, the lattice constant obtained by vdW-DF3 was 3.161 Å which is very close to 3.163 Å and 3.160 Å experimental and theoretical values respectively. Calculations of the electronic properties revealed good performance by vdW-DF3 functional. Furthermore, new electronic features were revealed for MoX2 (X = S, Se, Te). In terms of optical properties, PBE functional demonstrates lower absorption than vdW-DF3, as such it can be reported that vdW-DF3 improves photon absorption by TMDs. However, our results also revealed that vdW-DF3 performed well for MoS2 than for MoSe2 and MoTe2 because of the lower density observed for the S atom in MoS2.

Optical Mode Tuning of Monolayer Tungsten Diselenide (WSe2) by Integrating with One-Dimensional Photonic Crystal through Exciton-Photon Coupling.

Two-dimensional materials, such as transition metal dichalogenides (TMDs), are emerging materials for optoelectronic applications due to their exceptional light–matter interaction characteristics. At room temperature, the coupling of excitons in monolayer TMDs with light opens up promising possibilities for realistic electronics. Controlling light–matter interactions could open up new possibilities for a variety of applications, and it could become a primary focus for mainstream nanophotonics. In this paper, we show how coupling can be achieved between excitons in the tungsten diselenide (WSe2) monolayer with band-edge resonance of one-dimensional (1-D) photonic crystal at room temperature. We achieved a Rabi splitting of 25.0 meV for the coupled system, indicating that the excitons in WSe2 and photons in 1-D photonic crystal were coupled successfully. In addition to this, controlling circularly polarized (CP) states of light is also important for the development of various applications in displays, quantum communications, polarization-tunable photon source, etc. TMDs are excellent chiroptical materials for CP photon emitters because of their intrinsic circular polarized light emissions. In this paper, we also demonstrate that integration between the TMDs and photonic crystal could help to manipulate the circular dichroism and hence the CP light emissions by enhancing the light–mater interaction. The degree of polarization of WSe2 was significantly enhanced through the coupling between excitons in WSe2 and the PhC resonant cavity mode. This coupled system could be used as a platform for manipulating polarized light states, which might be useful in optical information technology, chip-scale biosensing and various opto-valleytronic devices based on 2-D materials.

Charged Bosons Made of Fermions in Bilayer Structures with Strong Metallic Screening.

Two-dimensional monolayer structures of transition metal dichalogenides (TMDs) have been shown to allow many higher-order excitonic bound states, including trions (charged excitons), biexcitons (excitonic molecules), and charged biexcitons. We report here experimental evidence and the theoretical basis for a new bound excitonic complex, consisting two free carriers bound to an exciton in a bilayer structure. Our experimental measurements on structures made using two different materials show a new spectral line at the predicted energy with two different TMD materials (MoSe2 and WSe2) with both n- and p-doping if and only if all the required theoretical conditions for this complex are fulfilled, in particular, only in the presence of a parallel metal layer that significantly screens the repulsive interaction between the like-charge carriers. Because these four-carrier bound states are charged bosons, they could eventually be the basis for a new path to superconductivity without Cooper pairing.

Electronic and transport properties of TMDC planar superlattices: effective Hamiltonian approach

We model and study the electronic and transport properties of a planar 2D superlattice (PSL) structure containing laterally arranged alternating ribbons of Transition Metal DiChalogenides (TMDC). Within governed effective Hamiltonian we derived adopted transfer matrix formalism to obtain dispersion relation and electronic band structure with wave functions, transmission probability and Fano spectrum. Surprisingly, spin orbit coupling has considerable opposite effects on valence bands shift in TMDC-PSL that is blue and red for k and k’ valleys, respectively. The amount of contribution of each ribbon determines the transmission spectrum and the transport feature. We observed that outside the band gap, the Fano factor changes from 1 to smaller values gradually, that indicates the ballistic transport. To give real aspect to our model, the effect of structural disorder and defect are addressed in details. Interestingly, we found that main gap is not dependent on structural disorder and electron incident angle. Our study presents an efficient way to control the key parameters in conductivity and band structure of TMDC-PSL in the view of optoelectronics applications.

Layer-dependent optoelectronic properties of black phosphorus

The layer-dependent optoelectronic properties of monolayer, bilayer and trilayer black phosphorus (BP) are studied by using the first-principles calculations based on density functional theory (DFT). The valence band splits and the density of states (DOS) in the conduction band obviously shift to the Fermi surface with the increased layer number. The atomic p orbital of BP plays an decisive role in determining the electronic and optical properties, which are drastically different from those of graphene and transition metal dichalogenides (TMDs). The increase of the layer number leads to the metal characteristics. The extinction coefficient and photoconductivity show strong optical responses to the ultraviolet (UV) light, which further increase with the number of layers. BP layers can reflect UV rays effectively because of their metallic properties in the UV energy range. Our study shows that the interlayer interaction can intensely change the electronic and optical properties of BP.

Device Architecture for Visible and Near-Infrared Photodetectors Based on Two-Dimensional SnSe2 and MoS2: A Review

While band gap and absorption coefficients are intrinsic properties of a material and determine its spectral range, response time is mainly controlled by the architecture of the device and electron/hole mobility. Further, 2D-layered materials such as transition metal dichalogenides (TMDCs) possess inherent and intriguing properties such as a layer-dependent band gap and are envisaged as alternative materials to replace conventional silicon (Si) and indium gallium arsenide (InGaAs) infrared photodetectors. The most researched 2D material is graphene with a response time between 50 and 100 ps and a responsivity of <10 mA/W across all wavelengths. Conventional Si photodiodes have a response time of about 50 ps with maximum responsivity of about 500 mA/W at 880 nm. Although the responsivity of TMDCs can reach beyond 104 A/W, response times fall short by 3–6 orders of magnitude compared to graphene, commercial Si, and InGaAs photodiodes. Slow response times limit their application in devices requiring high frequency. Here, we highlight some of the recent developments made with visible and near-infrared photodetectors based on two dimensional SnSe2 and MoS2 materials and their performance with the main emphasis on the role played by the mobility of the constituency semiconductors to response/recovery times associated with the hetero-structures.

Electrodeposition of MoS2 from Dichloromethane

The electrodeposition of MoS2 from dichloromethane (CH2Cl2) using tetrabutylammonium tetrathiomolybdate ([NnBu4]2[MoS4]) as a single source precursor is presented. The electrodeposition of MoS2 from CH2Cl2 requires addition of a proton donor to the electrolyte and trimethylammonium chloride (Me3NHCl) was used for this purpose. Electrochemical Quartz Crystal Microbalance (EQCM) experiments have been employed for a detailed study of the electrochemical mechanism and to study the role of the proton donor. EQCM reveals cathodic electrodeposition of MoS2 and anodic deposition of MoS3 as well as an additional corrosion process where the deposited MoS3 strips back into solution. The electrodeposited MoS2 films are amorphous in nature. All the films were found to be homogeneous in composition across the electrode area and to be reproducible between experiments. Annealing of the as-deposited films under a sulfur atmosphere results in crystalline MoS2 as confirmed by energy dispersive X-ray spectroscopy (EDX), Raman spectroscopy and X-ray diffraction. The deposited films were smooth and planar, as observed with scanning electron microscopy (SEM), indicating a layer-by-layer growth typical of transition metal dichalogenides.

PVP intercalated metallic WSe2 as NIR photothermal agents for efficient tumor ablation

Transition metal dichalogenides (TMDCs) with unique layered structures hold promising potential as transducers for photothermal therapy. However, the low photothermal conversion efficiency and poor stability in some cases limit their practical applications. Herein, we demonstrate the fabrication of ultrathin homogeneous hybridized TMDC nanosheets and their use for highly efficient photothermal tumor ablation. In particular, the nanosheets were composed of metallic WSe2 intercalated with polyvinylpyrrolidone (PVP), which was facilely prepared through a solvothermal process from the mixture of selenourea crystals, WCl6 powder along with PVP polymeric nanogel. Our characterizations revealed that the obtained nanosheets exhibited excellent photothermal conversion efficiency, therapeutic demonstration with improved biocompatibility and physiological stability attributing to the combined merits of metallic phase of WSe2 and hydrophilic PVP insertion. Both the histological analysis of vital organs and in vitro/in vivo tests confirmed the nanosheets as actively effective and biologically safe in this phototherapeutic technique. Findings from this non-invasive experiment clearly emphasize the explorable therapeutic efficacy of the layered-based hybrid agents in future cancer treatment planning procedures.

Thermally Induced 2D Alloy-Heterostructure Transformation in Quaternary Alloys.

Composition and phase specific 2D transition metal dichalogenides (2D TMDs) with a controlled electronic and chemical structure are essential for future electronics. While alloying allows bandgap tunability, heterostructure formation creates atomically sharp electronic junctions. Herein, the formation of lateral heterostructures from quaternary 2D TMD alloys, by thermal annealing, is demonstrated. Phase separation is observed through photoluminescence and Raman spectroscopy, and the sharp interface of the lateral heterostructure is examined via scanning transmission electron microscopy. The composition-dependent transformation is caused by existence of miscibility gap in the quaternary alloys. The phase diagram displaying the miscibility gap is obtained from the reciprocal solution model based on density functional theory and verified experimentally. The experiments show direct evidence of composition-driven heterostructure formation in 2D atomic layer systems.

1305 nm Few‐Layer MoTe2‐on‐Silicon Laser‐Like Emission

AbstractThe missing piece in the jigsaw of silicon photonics is a light source that can be easily incorporated into the standard silicon fabrication process. Here, silicon laser‐like emission is reported that employs few‐layer semiconducting transition metal dichalogenides of molybdenum ditelluride (MoTe2) as a gain material in a silicon photonic crystal L3 nanocavity. An optically pumped MoTe2‐on‐silicon laser‐like emission at 1305 nm, i.e. in the center of the “O‐band” of optical communications, is demonstrated at room temperature and with a threshold power density of 1.5 kW/cm2. The surprising insight is that, contrary to common understanding, a monolayer MoTe2 is not required to achieve higher efficiency laser‐like operation. Instead, few‐layer MoTe2 offers a higher overlap between the two dimensional (2D) gain material and the optical mode for sufficient gain. The ability to use few‐layer material opens new opportunities for deploying manufacturing methods such as chemical vapor deposition and thereby brings 2D‐on‐silicon devices a step closer to becoming a scalable technology.

Low-Temperature Synthesis of Near-Monodisperse Globular MoS2 Nanoparticles with Sulphur Powders

Nanoparticles (NPs) with high uniformity have been extensively investigated for their excellent chemical stability. Near-monodisperse globular MoS2 NPs were prepared with sulphur powders (SPs) as a sulphur source by a one-pot polyol-mediated process without surfactants, transfer agents and toxic agents at 170–190[Formula: see text]C. The as-processed SPs greatly affected the formation of the MoS2 NPs after low-activity sulphur (S8)n was reassembled from common SPs (S[Formula: see text]. The average size of MoS2 NPs can be reduced remarkably from 100–200[Formula: see text]nm to 50[Formula: see text]nm by introducing low amounts of MnCl2. A preliminary four-step growth mechanism based on the aggregation-coalescence model was also proposed. This green and simple method may be an alternative to the common hot-injection and heating-up methods for the preparation of monodisperse NPs, particularly transition metal dichalogenides.

Carrier transfer across a 2D-3D semiconductor heterointerface: The role of momentum mismatch

Two-dimensional (2D) transition metal dichalogenides exhibit a unique band structure: In contrast to many direct-gap classical semiconductors, their band-gap minimum is not at the center of the Brillouin zone, but at finite values of the $k$ vector. We report on clear indications that this momentum mismatch fundamentally influences the carrier transfer between a 2D ${\mathrm{WS}}_{2}$ crystal and a three-dimensional (3D) GaN layer: Populating different local band extrema of the ${\mathrm{WS}}_{2}$ in $k$ space by selective laser excitation leads to a pronounced difference in the ${\mathrm{WS}}_{2}$ photoluminescence signal. These findings may be of high importance for future 2D-3D semiconductor devices.

Surface science studies of elementary processes in photoelectrochemistry: adsorption of electrolyte components on layered transition metal dichalogenides

The application of surface science techniques to investigate elementary processes in photoelectrochemistry is shown based on a number of experiments performed during the last years. For these experiments, layered chalcogenide WSe2(0 0 0 1) van der Waals surfaces are used as semiconductor substrates as they provide ideal surface properties for fundamental studies. After a short introduction to the experimental techniques, we will present results of adsorption and coadsorption experiments performed to investigate different aspects of semiconductor/electrolyte contacts. For H2O and Br2 adsorbed as single species and coadsorbed with each other, the electronic states involved in contact formation can be identified and different mechanisms of interface interaction have been deduced. Na and H2O coadsorption experiments give information about solute solvent interaction as, e.g. solvation energies. Finally, a complex electrolyte phase containing all relevant species has been prepared by coadsorbing Br2, Na and H2O.

Phenomenology of antiferromagnetic metal-insulator transition in 3d transition metal dichalcogenides

A phenomenological theory is set up to describe the metal-insulator transition at T = 0 between phases which both exhibit cooperative antiferromagnetic ordering. A prediction which emerges relates the antiferromagnetic moment to Fermiology in the metal. Possible experiments on 3d transition metal dichalogenides are thereby suggested.