Transition Metal Ditellurides Research Articles

Microscopic growth mechanism and edge states of monolayer 1T′-MoTe2

Transition metal ditellurides (TMTDs) have versatile physical properties, including non-trivial topology, Weyl semimetal states and unique spin texture. Controlled growth of high-quality and large-scale monolayer TMTDs with preferred crystal phases is crucial for their applications. Here, we demonstrate the epitaxial growth of 1T′-MoTe2 on Au (111) and graphitized silicon carbide (Gr/SiC) by molecular beam epitaxy (MBE). We investigate the morphology of the grown 1T′-MoTe2 at the atomic level by scanning tunnelling microscopy (STM) and reveal the corresponding microscopic growth mechanism. It is found that the unique ordered Te structures preferentially deposited on Au (111) regulate the growth of monolayer single crystal 1T′-MoTe2, while the Mo clusters were preferentially deposited on the Gr/SiC substrate, which impedes the ordered growth of monolayer MoTe2. We confirm that the size of single crystal 1T′-MoTe2 grown on Au (111) is nearly two orders of magnitude larger than that on Gr/SiC. By scanning tunnelling spectroscopy (STS), we observe that the STS spectrum of the monolayer 1T′-MoTe2 nano-island at the edge is different from that at the interior, which exhibits enhanced conductivity.

Trends in CO2 Reduction on Transition Metal Dichalcogenide Edges

Electrocatalytic CO2 reduction is a promising solution to close the anthropogenic carbon cycle. However, linear scaling relations among reaction intermediates of the CO2 reduction reaction (CO2RR) and the competing hydrogen evolution reaction (HER) limits the Faradaic efficiencies and selectivities of traditional transition metal catalysts. The two-dimensional transition metal dichalcogenides (TMDCs) is a promising class of catalysts for the reaction. Selected transition metal dichalcogenides have been shown experimentally to effectively reduce CO2 and in some cases produce “beyond CO” products. The majority of theoretical investigations have focused on MoS2 and WS2, materials that have been widely studied for various applications, e.g desulfurization, HER, Li metal batteries and optical devices. In this computational study, we go beyond Mo and W and study the edge configurations, adsorption properties and aqueous stability of a range of TMDC materials in the 2H phase. We show that the most stable edge configuration is highly dependent on the specific composition of the material, while adsorption properties of H, CO, and COOH vary significantly with edge configuration. We furthermore find that, while a linear scaling among H and CO adsorbates is observed within each group of the sulfides, selenides, and tellurides, respectively, the scaling is shifted toward weaker H bonding and stronger CO bonding going from sulfides to selenides to tellurides. A similar trend is observed for CO/COOH for some edge configurations. The results indicate that the transition metal ditellurides are a promising class of materials with the potential of reducing CO2 further than CO while limiting HER.

Growth of bilayer MoTe2 single crystals with strong non-linear Hall effect

The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T′ MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer Td phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer Td MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers.

Light-Tunable Charge Density Wave Orders in MoTe2 and WTe2 Single Layers

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with light-tunable distortion, periodicity, electronic structure and bandgap. Moreover, optical pumping of the semimetallic 1T' phases generates a transient charge ordered metallic phase composed of 2D diamond clusters. For each transient phase we identify the critical fluence at which it is observed and the specific optical and Raman fingerprints to directly compare with future ultrafast pump-probe experiments. Our work demonstrates that it is possible to stabilize charge density waves even in insulating 2D transition metal dichalcogenides by ultrafast irradiation.

First-principles study of the crystal structure, electronic structure, and transport properties of NiTe2 under pressure

Recent experiments showed the distinct observations on the transition metal ditelluride NiTe$_2$ under pressure: one reported a superconducting phase transition at 12 GPa, whereas another observed a sign reversal of Hall resistivity at 16 GPa without the appearance of superconductivity. To clarify the controversial experimental phenomena, we have carried out first-principles electronic structure calculations on the compressed NiTe$_2$ with structure searching and optimization. Our calculations show that the pressure can transform NiTe$_2$ from a layered P-3m1 phase to a cubic Pa-3 phase at $\sim$10 GPa. Meanwhile, both the P-3m1 and Pa-3 phases possess nontrivial topological properties. The calculated superconducting $T_c$'s for these two phases based on the electron-phonon coupling theory both approach 0 K. Further magnetic transport calculations reveal that the sign of Hall resistance for the Pa-3 phase is sensitive to the pressure and the charge doping, in contrast to the case of the P-3m1 phase. Our theoretical predictions on the compressed NiTe$_2$ wait for careful experimental examinations.

Hyperbolicity in two-dimensional transition metal ditellurides induced by electronic bands nesting

Naturally occurring hyperbolic plasmonic media is rare, and was only recently observed in the 1T$'$ phase of WTe$_2$. We elucidate on the physical origin of this strong infrared hyperbolic response, and attribute it to band-nested anisotropic interband transitions. Such phenomenon does not occur in general anisotropic materials, at least not in their pristine state. However, band-nested anisotropic interband transitions can in principle be induced via proper electronic band nesting. We illustrate this principle and demonstrate a topological elliptic-to-hyperbolic transition in MoTe$_2$ via strain engineering, which is otherwise non-hyperbolic.

Ferromagnetism in PtTe2 monolayer introduced by doping 3d transition metal atoms and group VA and VIIB atoms

Most of the two-dimensional layered materials discovered so far are intrinsically nonmagnetic. Introducing ferromagnetism into these materials will expand their application greatly in the field of spintronics. Here we study the introduction of ferromagnetism into the PtTe2 monolayer via doping the 3d transition metal atoms and the group VA and VIIB atoms by means of the first-principles calculations. Results show that the magnetic order can be introduced in the V-, Mn-, Fe-, Cu- and Zn-doped PtTe2 monolayers, among which the V-, Cu- and Zn-doped monolayers have ferromagnetic ground state, and the Mn- and Fe-doped monolayers have antiferromagnetic ordering in ground state. Among the group VA atom doped systems, N-, P- and As-doped monolayers exhibit ferromagnetic, and Sb doped monolayer is a nonmagnetic metal. All the VIIB atom doped PtTe2 monolayers are nonmagnetic, while the doping makes the PtTe2 monolayer transit from an indirect bandgap semiconductor to metal. The findings obtained in this work may initiate new experimental exploration and expand the application of the layered transition-metal ditellurides in spintronics.

A Symmetry-Breaking Phase in Two-Dimensional FeTe2 with Ferromagnetism above Room Temperature.

Recently, ferromagnetism observed in monolayer two-dimensional (2D) materials has attracted attention due to the promise of its application in next-generation spintronics. Here, we predict a symmetry-breaking phase in 2D FeTe2 that differs from conventional transition metal ditellurides shows superior stability and room-temperature ferromagnetism. Through density functional theory calculations, we find the exchange interactions in FeTe2 consist of short-range superexchange and long-range oscillatory exchanges mediated by itinerant electrons. For six nearest neighbors, the exchange constants are calculated to be 50.95, 33.41, 2.70, 11.02, 14.46, and -4.12 meV. Furthermore, the strong relativistic effects on Te2+ induce giant out-of-plane exchange anisotropy and open up a significantly large spin wave gap (ΔSW) of 1.22 meV. All of this leads to robust ferromagnetism with the Tc surpassing 423 K, which is predicted by the renormalization group Monte Carlo method, sufficiently higher than room temperature. Our findings shed light on the promising future of FeTe2 in 2D magnetic research and spintronic applications.

Low-temperature synthesis and electrocatalytic application of large-area PtTe2 thin films

The synthesis of transition metal dichalcogenides (TMDs) has been a primary focus for 2D nanomaterial research over the last 10 years, however, only a small fraction of this research has been concentrated on transition metal ditellurides. In particular, nanoscale platinum ditelluride (PtTe2) has rarely been investigated, despite its potential applications in catalysis, photonics and spintronics. Of the reports published, the majority examine mechanically-exfoliated flakes from chemical vapor transport (CVT) grown crystals. This method produces high quality-crystals, ideal for fundamental studies. However, it is very resource intensive and difficult to scale up meaning there are significant obstacles to implementation in large-scale applications.In this report, the synthesis of thin films of PtTe2 through the reaction of solid-phase precursor films is described. This offers a production method for large-area, thickness-controlled PtTe2, potentially suitable for a number of applications. These polycrystalline PtTe2 films were grown at temperatures as low as 450 °C, significantly below the typical temperatures used in the CVT synthesis methods. Adjusting the growth parameters allowed the surface coverage and morphology of the films to be controlled. Analysis with scanning electron- and scanning tunneling microscopy indicated grain sizes of above 1 µm could be achieved, comparing favorably with typical values of ∼50 nm for polycrystalline films. To investigate their potential applicability, these films were examined as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The films showed promising catalytic behavior, however, the PtTe2 was found to undergo chemical transformation to a substoichiometric chalcogenide compound under ORR conditions. This study shows while PtTe2 is stable and highly useful for in HER, this property does not apply to ORR, which undergoes a fundamentally different mechanism. This study broadens our knowledge on the electrocatalysis of TMDs.

Sub-picosecond photo-induced displacive phase transition in two-dimensional MoTe2

Photo-induced phase transitions (PIPTs) provide an ultrafast, energy-efficient way for precisely manipulating the topological properties of transition-metal ditellurides and can be used to stabilize a topological phase in an otherwise semiconducting material. Using first-principles calculations, we demonstrate that the PIPT in monolayer MoTe2 from the semiconducting 2H phase to the topological 1T′ phase can be triggered purely by electronic excitations that soften multiple lattice vibrational modes. These softenings, driven by a Peierls-like mechanism within the conduction bands, lead to structural symmetry breaking within sub-picosecond timescales, which is shorter than the timescale of a thermally driven phase transition. The transition is predicted to be triggered by photons with energies over 1.96 eV, with an associated excited carrier density of 3.4 × 1014 cm−2, which enables a controllable phase transformation by varying the laser wavelength. Our results provide insight into the underlying physics of the phase transition in 2D transition-metal ditellurides and show an ultrafast phase-transition mechanism for manipulation of the topological properties of 2D systems.

Electronic structure and magnetism of MTe2 (M = Ti, V, Cr, Mn, Fe, Co and Ni) monolayers

We study the electronic structure and magnetism of monolayer 3d transition-metal ditellurides MTe2 (M = Ti, V, Cr, Mn, Fe, Co and Ni) in trigonal prismatic H- and/or octahedral T-phase by means of the first-principles calculations. The results show that H-VTe2, T-VTe2, H-FeTe2 and T-MnTe2 monolayers exhibit intrinsic ferromagnetism, and the others have no ferromagnetism. The exchange splitting of V, Mn and Fe 3d orbitals is responsible for ferromagnetism. The exchange constant and the Curie temperature are estimated by using 2D Ising model and mean field theory. Among the four ferromagnetic monolayers, the H-VTe2 monolayer has the largest exchange constant and the corresponding Curie temperature is near room temperature. Calculations also show that the T-VTe2, H-VTe2 and H-FeTe2 monolayers have in-plane easy magnetization direction, while the easy direction of the T-MnTe2 monolayer is perpendicular to the layer. Moreover, we analyze the relative exchange strength of the four ferromagnetic monolayers based on the competition between the through-bond ferromagnetic interaction and the through-space antiferromagnetic interaction according to the Goodenough–Kanamori rules. The magnetocrystalline anisotropy is explained qualitatively based on the second-order perturbation theory from the spin-orbit coupling between 3d orbitals of M atoms.

Wafer-scale production of patterned transition metal ditelluride layers for two-dimensional metal–semiconductor contacts at the Schottky–Mott limit

A key challenge in the development of two-dimensional (2D) devices is the fabrication of metal–semiconductor junctions with minimal contact resistance and depinned energy levels. An ideal solution for practical applications is to make contacts between 2D van der Waals semiconductors and 2D van der Waals metals. Here we report the wafer-scale production of patterned layers of metallic transition metal ditellurides on different substrates. Our tungsten ditelluride and molybdenum ditelluride layers, which are grown using a tellurization process applied to a precursor transition metal layer, have an electronic performance comparable to that of mechanically exfoliated flakes and can be combined with the 2D semiconductor molybdenum disulfide. The resulting metal–semiconductor junctions are free from significant disorder effects and Fermi-level pinning, and are used to create monolayer molybdenum disulfide field-effect transistors. The Schottky barrier heights of the devices also largely follow the trend of the Schottky–Mott limit. Two-dimensional metallic WTe2 and MoTe2 layers can be combined with a semiconducting MoS2 monolayer to create metal–semiconductor junctions that are free from substantial disorder effects and Fermi-level pinning.

Transition metal ditellurides make for better 2D contacts

Metal–semiconductor junctions formed between a transition metal ditelluride and monolayer molybdenum disulfide exhibit nearly ideal Schottky–Mott conditions.

Pressure-driven Lifshitz transition in type-II Dirac semimetal NiTe2

Band engineering in layered transition metal dichalcogenides (TMDs) leads to a variety of emergent phenomena and has obtained considerable attention recently. Transition metal ditelluride ${\mathrm{NiTe}}_{2}$ has been discovered experimentally to be a type-II Dirac semimetal at ambient pressure, and was predicted to display superconductivity in the monolayer limit. Here we systematically investigate the structural and electronic properties of type-II Dirac semimetal ${\mathrm{NiTe}}_{2}$ under high pressure. Room-temperature synchrotron x-ray diffraction and Raman scattering measurements reveal the stability of the pristine hexagonal phase up to 52.2 GPa, whereas both the pressure coefficient and linewidth of Raman mode ${E}_{g}$ exhibit anomalies at a critical pressure ${P}_{\mathrm{c}}\ensuremath{\sim}16\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Meantime, Hall resistivity measurement indicates that the hole-dominated behavior maintains up to 15.6 GPa and transforms into electron-dominated behavior at higher pressures. Our findings consistently demonstrate a pressure-induced Lifshitz transition in type-II Dirac semimetal ${\mathrm{NiTe}}_{2}$.

Type-II Dirac semimetal candidates ATe2 ( A= Pt, Pd): A de Haas-van Alphen study

We report on a magnetotransport and quantum oscillations study on high quality single crystals of the transition metal di-tellurides ${\mathrm{PtTe}}_{2}$ and ${\mathrm{PdTe}}_{2}$. The de Haas-van Alphen (dHvA) oscillations in the magnetization measurements on ${\mathrm{PtTe}}_{2}$ reveal a complicated, anisotropic band structure characterized by low effective masses and high mobilities for the carriers. Extracted transport parameters for ${\mathrm{PtTe}}_{2}$ reveal a strong anisotropy which might be related to the tilted nature of the Dirac cone. Using a Landau level fan diagram analysis we find at least one Fermi surface orbit with a Berry phase of $\ensuremath{\pi}$ consistent with Dirac electrons for both ${\mathrm{PtTe}}_{2}$ and ${\mathrm{PdTe}}_{2}$. The light effective mass and high mobility are also consistent with Dirac electrons in ${\mathrm{PtTe}}_{2}$. Our results therefore suggest that similar to ${\mathrm{PdTe}}_{2}$, ${\mathrm{PtTe}}_{2}$ might also be a three-dimensional Dirac semimetal.

Single-Crystalline Nanobelts Composed of Transition Metal Ditellurides.

Following the celebrated discovery of graphene, considerable attention has been directed toward the rich spectrum of properties offered by van der Waals crystals. However, studies have been largely limited to their 2D properties due to lack of 1D structures. Here, the growth of high-yield, single-crystalline 1D nanobelts composed of transition metal ditellurides at low temperatures (T ≤ 500 °C) and in short reaction times (t ≤ 10 min) via the use of tellurium-rich eutectic metal alloys is reported. The synthesized semimetallic 1D products are highly pure, stoichiometric, structurally uniform, and free of defects, resulting in high electrical performances. Furthermore, complete compositional tuning of the ternary ditelluride nanobelts is achieved with suppressed phase separation, applicable to the creation of unprecedented low-dimensional materials/devices. This approach may inspire new growth/fabrication strategies of 1D layered nanostructures, which may offer unique properties that are not available in other materials.

Synthesis of Atomically Thin Transition Metal Ditelluride Films by Rapid Chemical Transformation in Solution Phase

The controlled synthesis of large-area, atomically thin molybdenum and tungsten ditelluride (MoTe2 and WTe2) crystals is crucial for their emerging applications based on the attractive electronic properties. However, the solution phase synthesis of high-quality and large-area MoTe2 or WTe2 ultrathin films have not been achieved yet. In this study, we synthesized for the first time, large-area atomically thin MoTe2 and WTe2 films in solution phase, through rapid crystal formation directly on a conducting substrate. For the synthesis, we developed a new Te precursor. The crystal growth involves an in situ chemical transformation from Te nanoparticles into MoTe2 or WTe2 thin films. The synthesis enables precise control of the number of atomic layers over a large area, from a monolayer to multilayers. Micropatterned MoTe2 thin films are also readily synthesized in situ using the same process. The photodetector made of 3-layer semiconducting MoTe2 thin films exhibits high photoresponsivity (Rλ) over a broad sp...

Cytotoxicity of Group 5 Transition Metal Ditellurides (MTe2 ; M=V, Nb, Ta).

Much research effort has been put in to study layered compounds with transition metal dichalcogenides (TMDs) being one of the most studied compounds. Due to their extraordinary properties such as excellent electrochemical properties, tuneable band gaps, and low shear resistance due to weak van der Waals interactions between layers, TMDs have been found to have wide applications such as electrocatalysts for hydrogen evolution reactions, supercapacitors, biosensors, field-effect transistors (FETs), photovoltaics, and lubricant additives. In very recent years, Group 5 transition metal ditellurides have received an immense amount of research attention. However to date, little has been known of the potential toxicities posed by these materials. As such, we conducted the cytotoxicity study by incubating various concentrations of the Group 5 transition metal ditellurides (MTe2 ; M=V, Nb, Ta) with human lung carcinoma epithelial A549 cells for 24 hours and the remaining cell viabilities after treatment was measured. Our findings indicate that VTe2 is highly toxic whereas NbTe2 and TaTe2 are deemed to exhibit mild toxicities. This study constitutes an exemplary first step towards the understanding of the Group 5 transition metal ditellurides' toxicity effects in preparation for their possible future commercialisation.

Unconventionally Layered CoTe2 and NiTe2 as Electrocatalysts for Hydrogen Evolution.

Two members of the transition metal ditelluride family, CoTe2 and NiTe2 , exist in multiple structures encompassing marcasite-, pyrite- and CdI2 related structures. The allotrope modification is influenced by weak changes in stoichiometry and synthesis. It is crucial to emphasize that the CdI2 structure type is manifested by NiTe2 while the CoTe2 adopts a related structure for a non-stoichiometric composition with ratio below 1:1.8. The obtained structure is based on LiTiS2 which is derived from CdI2 structure, however contains a polymeric cobalt network. Despite the atypical nature of their layered structure, layered phases of CoTe1.8 and NiTe2 are rarely cast into the spotlight. Here, layered CoTe1.8 and NiTe2 are investigated for their electrochemical and electrocatalytic properties. In electrocatalytic aspects, layered CoTe1.8 and NiTe2 demonstrate low overpotentials and small Tafel slopes that are quintessential features of hydrogen evolution electrocatalysts. These findings impart fundamental insights to the transition metal ditelluride family and affirm the prospective use of layered CoTe1.8 and NiTe2 in electrochemical applications.

What drives superconductivity in Pt-doped IrTe2?

The emergency of superconductivity at the edge of charge density wave (CDW) state by pressure or chemical doping in 1T transition metal dichalcogenides (TMDs) has attracted much attention in past decade [1, 2]. The interplay of CDW and superconductivity has triggered the hot debate: why does superconductivity arise after suppressing the CDW and what’s the driving force of the charge modulation? For the 5d transition metal ditelluride IrTe2, a layered compound with trigonal symmetry, previous studies have shown an ordered superstructure below *280 K, and a superconducting state when doped or intercalated with Pd or Pt [3, 4]. In IrTe2, the Fermi surface nesting has been excluded as the impetus of superstructure; instead, splitting of Te orbitals by the crystal field [5] or destabilization of polymeric Te–Te networks was proposed [6], evident from the visualized periodic dimerization of Te atoms by scanning tunneling microscopy (STM) [7]. Meanwhile, a charge disproportionated CDW by covalent bonding Ir dimers was also presented [8]. However, though the origin of the superstructure formation becomes more and more clear, the underlying mechanism of the appearance of superconductivity by doping Pd or Pt is still obscure. Recently, Ruan et al. [9] have studied the structural phase transition and electronic structure evolution in pure and Pt-doped IrTe2 single crystals using low-temperature STM combined with first-principles calculations. The authors presented a clear evidence for a commensurate 1/6 charge modulation as the ground state of high-quality IrTe2 single crystals, consistent with previous reports [7, 10]. By substituting Pt for Ir, the system exhibits longrange ordered 1/5 phase and then goes to a new threefold quasiperiodic phase. In addition to a full picture of phase evolution, the most important finding is that the Pt dopants actually act as chemical strain, generating local impurity states rather than injecting itinerant charge carriers [9]. The weakened intralayer Ir dimerization effectively leads to the recovery of polymeric Te bonds across layers, lifting the density of states at Fermi level and making the system more three-dimensional [9]. Therefore, it turns out that the higher dimensionality tips the balance in favor of superconductivity in this system. This work presents a coherent story and also stimulates us to think more about the role of strain in modulating new order.