Properties Of Transition Metal Dichalcogenides Research Articles

Tunable Ferromagnetism and Thermally Induced Spin Flip in Vanadium-Doped Tungsten Diselenide Monolayers at Room Temperature.

The outstanding optoelectronic and valleytronic properties of transition metal dichalcogenides (TMDs) have triggered intense research efforts by the scientific community. An alternative to induce long-range ferromagnetism (FM) in TMDs is by introducing magnetic dopants to form a dilute magnetic semiconductor. Enhancing ferromagnetism in these semiconductors not only represents a key step toward modern TMD-based spintronics, but also enables exploration of new and exciting dimensionality-driven magnetic phenomena. To this end, tunable ferromagnetism at room temperature and a thermally induced spin flip (TISF) in monolayers of V-doped WSe2 are shown. As vanadium concentration increases, the saturation magnetization increases, which is optimal at ≈4 at% vanadium; the highest doping level ever achieved for V-doped WSe2 monolayers. The TISF occurs at ≈175 K and becomes more pronounced upon increasing the temperature toward room temperature. The TISF can be manipulated by changing the vanadium concentration. The TISF is attributed to the magnetic-field- and temperature-dependent flipping of the nearest W-site magnetic moments that are antiferromagnetically coupled to the V magnetic moments in the ground state. This is fully supported by a recent spin-polarized density functional theory study. The findings pave the way for the development of novel spintronic and valleytronic nanodevices and stimulate further research.

Band structure engineering through van der Waals heterostructing superlattices of two‐dimensional transition metal dichalcogenides

AbstractThe indirect‐to‐direct band‐gap transition in transition metal dichalcogenides (TMDCs) from bulk to monolayer, accompanying with other unique properties of two‐dimensional materials, has endowed them great potential in optoelectronic devices. The easy transferability and feasible epitaxial growth pave a promising way to further tune the optical properties by constructing van der Waals heterostructures. Here, we performed a systematic high‐throughput first‐principles study of electronic structure and optical properties of the layer‐by‐layer stacking TMDCs heterostructing superlattices, with the configuration space of [(MX2)n(M′X′2)10−n] (M/M′ = Cr, Mo, W; X/X′ = S, Se, Te; n = 0‐10). Our calculations involving long‐range dispersive interaction show that the indirect‐to‐direct band‐gap transition or even semiconductor‐to‐metal transition can be realized by changing component compositions of superlattices. Further analysis indicates that the indirect‐to‐direct band‐gap transition can be ascribed to the in‐plane strain induced by lattice mismatch. The semiconductor‐to‐metal transition may be attributed to the band offset among different components that is modified by the in‐plane strain. The superlattices with direct band‐gap show quite weak band‐gap optical transition because of the spacial separation of the electronic states involved. In general, the layers stacking‐order of superlattices results in a small up to 0.2 eV band gap fluctuation because of the built‐in potential. Our results provide useful guidance for engineering band structure and optical properties in TMDCs heterostructing superlattices.image

Correlating structural, electronic, and magnetic properties of epitaxial VSe2 thin films

The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono- and bilayer $VSe_2$ films grown using molecular beam epitaxy. $VSe_2$ has recently attracted much attention due to reports of emergent ferromagnetism in the 2D limit. To understand this important compound, high quality 1T and distorted 1T films were grown at temperatures of 200 $^\text{o}$C and 450 $^\text{o}$C respectively and studied using 4K Scanning Tunneling Microscopy/Spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T-film shows a CDW pattern with 4a $\times$ 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wavevector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T $VSe_2$ film is weakly ferromagnetic, with ~ $3.5 {\mu}_B$ per unit similar to previous reports.

Strain-dependent luminescence and piezoelectricity in monolayer transition metal dichalcogenides

The modification of optical and electronic properties of transition metal dichalcogenides via mechanical deformation has been widely studied. Their ability to withstand large deformations before rupture has enabled large tunability of the bandgap, and further, the spatially varying strain has been shown to control the spatial distribution of the bandgap and lead to effects such as carrier funneling. Monolayer transition metal dichalcogenides exhibit a significant piezoelectric effect that could couple to a spatially inhomogeneous strain distribution to influence electronic and optical behavior. We investigate both experimentally and theoretically an example case of photoluminescence in structures with a strain distribution similar to that employed in single-photon emitters but generated here via nanoindentation. Using a mechanical model for strain induced by nanoindentation, we show that piezoelectricity can result in charge densities reaching 1012 e/cm2 and can generate electrostatic potential variations on the order of ±0.1 V across the suspended monolayer. We analyze the implications of these results for luminescence and exciton transport in monolayer transition metal dichalcogenides with spatially varying strain.

Microscopic Modeling of Pump–Probe Spectroscopy and Population Inversion in Transition Metal Dichalcogenides

Optical properties of transition metal dichalcogenide (TMD) monolayers are dominated by excitonic effects. These are significantly altered at high carrier densities, leading to energy renormalization, absorption bleaching, and even optical gain. Such effects are experimentally accessible in ultra‐fast pump–probe measurements. Herein, the semiconductor Bloch equations are combined with the generalized Wannier equation to investigate the effect that excited carriers have on the excitonic properties of TMD monolayers. In particular, the dynamics of carrier occupation, energy renormalization, and absorption bleaching as well as population inversion and optical gain are investigated.

Visible range photoresponse of vertically oriented on-chip MoS2 and WS2 thin films

The excellent electrical properties of transition metal dichalcogenide (TMD) 2D materials promise a competitive alternative to traditional semiconductor materials for applications in optoelectronics, chemical sensing, as well as in energy harvesting and conversion. As the typical synthesis methods of TMDs produce nanoparticles, such as single or multi-layered nanoflakes, subsequent strenuous integration steps are necessary to obtain devices. Direct synthesis of the material on substrates would simplify the process and provide the means for large-scale integration and production of practical devices. In our approach, we synthesize MoS2 and WS2 thin films with a simple sulfurization of the respective metal films deposited by sputtering on Si/SiO2 chips, and study their optoelectrical properties at wavelengths of 661 nm, 552 nm, and 401 nm using pulsed lasers. Both TMD thin films are found to show photoresponsivities of up to ∼5 × 10−6 A W−1 with corresponding quantum efficiencies of ∼10−5, which are unexpectedly moderate, and can be attributed to their columnar microstructure, in which the basal planes of the hexagonal lattices are perpendicular to the substrate, thus, limiting the electron transport in the films parallel to the plane of the substrate.

Transition Metal Dichalcogenides for the Application of Pollution Reduction: A Review.

The material characteristics and properties of transition metal dichalcogenide (TMDCs) have gained research interest in various fields, such as electronics, catalytic, and energy storage. In particular, many researchers have been focusing on the applications of TMDCs in dealing with environmental pollution. TMDCs provide a unique opportunity to develop higher-value applications related to environmental matters. This work highlights the applications of TMDCs contributing to pollution reduction in (i) gas sensing technology, (ii) gas adsorption and removal, (iii) wastewater treatment, (iv) fuel cleaning, and (v) carbon dioxide valorization and conversion. Overall, the applications of TMDCs have successfully demonstrated the advantages of contributing to environmental conversation due to their special properties. The challenges and bottlenecks of implementing TMDCs in the actual industry are also highlighted. More efforts need to be devoted to overcoming the hurdles to maximize the potential of TMDCs implementation in the industry.

(Invited) Utilizing the Properties of Transition Metal Dichalcogenides and Black Phosphorus for Novel Device and Circuit Applications

Many novel low-dimensional materials have been explored over the course of the past two decades, and it has widely been acknowledged that one-dimensional (1D) materials like nanotubes and nanowires and two-dimensional (2D) materials like transition metal dichalcogenides (TMDs) and black phosphorus (BP) offer unique opportunities and challenges when it comes to electronic applications. One aspect of 1D and 2D materials that makes them unique is the opportunity to build three-terminal transistor devices from them that show excellent response to the gate terminal due to the ultra-thin body of these materials. This tight gate control allows in particular achieving so-called ambipolar device characteristics that enable electron or hole currents to flow through the channel, depending on the chosen gate voltage polarity. However, in general, having both current polarities being present in the same device is not desirable for complementary metal-oxide semiconductor (CMOS) type circuits. In my presentation I will discuss how a properly designed device structure enables suppression of one type of carrier without losing the ability of generating n-type or p-type field-effect transistors (FETs) at will from the same channel material in the same gate geometry. Moreover, I will discuss how this concept translates into an opportunity to camouflage the circuit performance through the formation of devices that are only revealing their n-type or p-type nature through application of the correct gate and drain voltage polarity. Moreover, I will discuss in the context of the device performance the importance of gated contacts and will elucidate how in the past wrong mobility values were extracted by ignoring a particular feature of contacts to ultra-thin body FETs.

(Invited) Ab Initio Exciton and Phonon Dynamics in Transition Metal Dichalcogenides

Interest in the properties of transition metal dichalcogenides (TMDs) has increased due to the discovery of the coupling between spin and valley degrees of freedom, which can be seen experimentally using a circularly polarised laser. After excitation the newly formed carrier populations must move towards the other valley until balance is reached. However, this relaxation process is not entirely understood in the literature, where the relative importance of the electron-electron (e-e) or electron-phonon (e-p) interactions is still a subject of debate. Previous works on WSe2 [A. Molina- Sánchez, et al - Nano letters, 2017] have shown that the e-p interaction is a good candidate to describe the relaxation process. Using a fully ab-initio framework based on the Baym-Kadanoff equations [P. M. M. C. de Melo and A. Marini, Phys. Rev. B 93, 155102 (2016)] we study the influence of the e-p interaction on MoSe2 after its excitation by a laser field. We show how phonons allow carrier relaxation and how the Kerr signal and total magnetisation are affected at different temperatures, with the latter exhibiting a non-monotonic behaviour as the temperature increases.

Van der Waals Epitaxial Formation of Atomic Layered α-MoO3 on MoS2 by Oxidation.

The electronic, catalytic, and optical properties of transition metal dichalcogenides (TMDs) are significantly affected by oxidation, and using oxidation to tune the properties of TMDs has been actively explored. In particular, because transition metal oxides (TMOs) are promising hole injection layers, a TMD-TMO heterostructure can be potentially applied as a p-type semiconductor. However, the oxidation of TMDs has not been clearly elucidated because of the structural instability and the extremely small quantity of oxides formed. Here, we reveal the phases and morphologies of oxides formed on two-dimensional molybdenum disulfide (MoS2) using transmission electron microscopy analysis. We find that MoS2 starts to oxidize around 400 °C to form orthorhombic-phase molybdenum trioxide (α-MoO3) nanosheets. The α-MoO3 nanosheets so formed are stacked layer-by-layer on the underlying MoS2 via van der Waals interaction and the nanosheets are aligned epitaxially with six possible orientations. Furthermore, the band gap of MoS2 is increased from 1.27 to 3.0 eV through oxidation. Our study can be extended to most TMDs to form TMO-TMD heterostructures, which are potentially interesting as p-type transistors, gas sensors, or photocatalysts.

Morphological and Spectroscopic Characterizations of Monolayer and Few-Layer MoS2 and WSe2 Nanosheets under Oxygen Plasma Treatment with Different Excitation Power: Implications for Modulating Electronic Properties

Modulating the electronic properties of transition metal dichalcogenides (TMDCs) by oxygen plasma treatment has attracted great attention. However, the effect of excitation power on oxidation and e...

Transition-Metal Substitution-Induced Lattice Strain and Electrical Polarity Reversal in Monolayer WS2.

The physical and chemical properties of transition metal dichalcogenides can be effectively tuned by doping or alloying, which is essential for their practical applications. However, the microstructure evolutions and their effects on the physical properties induced by alloying from hetero-atoms with different outermost electronic structures are still unclear. Here, we synthesized Nb-substituted WS2 with various Nb concentrations showing unusual changes of optical behaviors and continuous electrical polarity reversal. The fully softened Raman mode, rapidly quenched photoluminescence, and severe electron scattering can be attributed to the combined effects of charge doping and lattice strain caused by atomic Nb doping. Three types of substitution modes of Nb atoms in the WS2 lattice were observed directly from atomic-resolution scanning transmission electron microscopy. Density functional theory calculations further confirm the role of lattice strain in the evolutions of optical and electrical characteristics. With increasing Nb concentration, n-type, ambipolar, and p-type field-effect transistors can be achieved, indicating the capacity of this doping method to engineer the properties of two-dimensional materials for future electronic applications.

A review on graphene based transition metal oxide composites and its application towards supercapacitor electrodes

Two-dimensional (2D) graphene and its outstanding properties such as, enormous conductivities, mechanical strength, optical and electrical properties brought it one of the most studied materials for the production of energy using renewable resources. The composites of graphene with the same morphological materials such as layered transition metal oxides will be the most aspiring combination to boost their conducting, optoelectronic and mechanical properties. Furthermore, the transition metal chalcogenides materials attracted significant attention due to their atomically thin layers and enormous optical, conducting and electrical properties. The layered materials offer mostly atomically thin 2D plane to the charge carrier with superior conducting properties. The thickness at atomic level, band gap, spin orbit coupling electronic and optoelectronics properties of transition metal dichalcogenides makes them one of the promising entities in energy harvesting technologies. The review suggests some strategies to improve the transition metals-composites stability conducting properties to be tailored in various applications.

Chalcogen Incorporation Process during High-Vacuum Conversion of Bulk Mo Oxides to Mo Dichalcogenides

The optical, electronic, catalytic, and mechanical properties of transition metal dichalcogenide (TMD) bulk crystals and monolayers have been examined extensively. TMD materials can be prepared thr...

A non-rigid shift of band dispersions induced by Cu intercalation in 2H-TaSe2

The intercalation of metal is a promising method for the modulating electronic properties in transition metal dichalcogenides (TMDs). However, there still lacks enough knowledge about how the intercalated atoms directly impact the two-dimensional structural layers and modulate the band structures therein. Taking advantage of X-ray absorption fine structure and angle-resolved photoemission spectroscopy, we studied how Cu intercalation influences the host TaSe2 layers in Cu0.03TaSe2 crystals. The intercalated Cu atoms form bonds with Se of the host layers, and there is charge transfer from Cu to Se. By examining the changes of band dispersions, we show that the variation of electronic structures is beyond a simple rigid band model with merely charge doping effect. This work reveals that the unusual change of band dispersions is associated with the formation of bonds between the intercalated metal elements and anion ions in the host layers, and provides a reference for the comprehensive understanding of the electronic structures in intercalated materials.

Pressure-induced electronic and isostructural phase transitions in PdPS: Raman, x-ray, and first-principles study

Application of pressure is known to be an effective tool for tuning structural and electronic properties of transition metal dichalcogenides. In this work we present evolution of PdPS with pressure using Raman spectroscopy and synchrotron x-ray diffraction up to 26 GPa, complemented with first-principles theoretical analysis of PdPS under pressure up to 36 GPa. Raman spectra reveal changes in the pressure derivatives of Raman frequencies at P�2, 11, and 21 GPa, suggesting three isostructural electronic phase transitions in PdPS. The pressure-dependent x-ray diffraction shows a sudden rise in the bulk modulus from 90±3 to 123±7 GPa occurring at P�11 GPa. Using first-principles density functional theory calculations, we demonstrate that the low-pressure phase transitions are associated with changes in direct band gap at I� point while the high-pressure (at P=21 GPa) transition is associated with semiconductor to semimetal transition. From analysis of PdPS at higher pressures, we predict a structural phase transition in PdPS at P�32 GPa from orthorhombic to monoclinic structure.

Second harmonic generation of two-dimensional layered materials: characterization, signal modulation and enhancement

Two-dimensionl (2D) layered transition metal dichalcogenides (TMDCs) have received great attention in integrated on-chip photonic and photoelectric applications due to their unique physical properties including indirect-to-direct optical bandgap transition, broad bandgap from visible band to near-infrared band, as well as their excellent optoelectric properties derived from the 2D confinement. Recently, with the in-depth study of their fundament nonlinear optical properties, these 2D layered TMDCs have displayed significant potential applications in nonlinear optical devices. In this review, we focus on recent research progress of second harmonic generation (SHG) studies of TMDCs. Firstly, we briefly introduce the basic theory of nonlinear optics (mainly about SHG). Secondly, the several intrinsic SHG relative properties in TMDCs including layer dependence, polarization dependence, exciton resonance effect, valley selection rule are discussed. Thirdly, the latest SHG modulation and enhancement studies are presented, where the electric field, strain, plasmonic structure and micro-cavity enhancement are covered. Finally, we will summarize and give a perspective of possible research direction in the future. We believe that a more in-depth understanding of the SHG process in 2D layered TMDCs as well as the material structure and modulation effects paves the way for further developing the ultra-thin, multifunctional 2D nonlinear optical devices.

Elemental Substitution of Two-Dimensional Transition Metal Dichalcogenides (MoSe2 and MoTe2): Implications for Enhanced Gas Sensing.

The quest for a suitable material with the potential of capturing toxic nitrogen-containing gases (NH3, NO, and NO2) has motivated us to explore the structural, electronic, and gas-sensing properties of transition metal dichalcogenides (TMDs); MoSe2 and MoTe2. Spin-polarized density functional theory (DFT) calculations demonstrate weak binding of nitrogen-containing gases (NCGs) with the pristine TMDs, which limits the use of the latter as efficient sensing materials. However, suitable elemental substitutions improve the binding mechanism enormously. Our dispersion-corrected DFT calculations revealed that Se (Te) substitution with Ge (Sb) in MoSe2 (MoTe2) not only enhances the binding energies but also causes a significant variation in the electronic properties and work functions. A charge-transfer mechanism based on Bader analysis indicates that transfer of charges from MoSe2-Ge (MoTe2-Sb) to the NCGs is responsible for the improvement in the binding characteristics. Based on our findings, it is evident that 2.08% of elemental substitutional makes both MoSe2 and MoTe2 promising materials for NH3, NO, and NO2 gas sensing.

Gate‐Tunable Electrical Transport in Thin 2M‐WS2 Flakes

AbstractElectrolyte gating has been employed as an effective way to modulate the electronic properties of transition metal dichalcogenides (TMDs) by carrier doping over a wide range. Here, the carrier density of a new metastable phase of TMD material 2M‐WS2 is controlled by electrolyte gating to achieve reversible transitions between the superconducting state, metallic state, and insulating state. Pristine 2M‐WS2 has a superconducting transition temperature (Tc) of 8.9 K with a hole‐type carrier density of 9.05 × 1021 cm−3 at 100 K. A gate voltage (Vg) is applied between the sample and a side gate, which are both immersed in a droplet of gel‐like Li‐ion electrolyte (LiClO4 dissolved in polyethylene oxide matrix). When Vg = −3.5 V, the sample is in its pristine superconducting phase. With increasing Vg, the lithium ions gradually intercalate into the layered sample and tune it from a superconductor to an insulator. By precise tuning of Vg in this device, the entire phase diagram of 2M‐WS2 over a large range of carrier density is obtained.

First-principles study of mechanical and electronic properties of bent monolayer transition metal dichalcogenides

The mechanical and electronic properties of transition metal dichalcogenide (TMD) monolayers corresponding to transition groups IV, VI, and X are explored under mechanical bending from first principles calculations using the strongly constrained and appropriately normed (SCAN) meta-GGA (MGGA). SCAN provides an accurate description of the phase stability of the TMD monolayers. Our calculated lattice parameters and other structural parameters agree well with experiment. We find that bending stiffness (or flexural rigidity) increases as the transition metal group goes from IV to X to VI, with the exception of PdTe$_2$. Variation in mechanical properties (local strain, physical thickness) and electronic properties (local charge density, band structure) with bending curvature is discussed. The local strain profile of these TMD monolayers under mechanical bending is highly non-uniform. The mechanical bending tunes not only the thickness of the TMD monolayers but also the local charge distribution as well as the band structures, adding more functionalization options to these materials.