Raman signatures of inversion symmetry breaking and structural phase transition in type-II Weyl semimetal MoTe2

Harnessing Physical Entropy Noise in Structurally Metastable 1T' Molybdenum Ditelluride for True Random Number Generation

True random numbers are crucial for various research and engineering problems. Their generation depends upon a robust physical entropy noise. Here, we present true random number generation from the conductance noise probed in structurally metastable 1T' molybdenum ditelluride (MoTe2). The noise, fitting a Poisson process, is proved to be a robust physical entropy noise at low and even cryogenic temperatures. Noise characteristic analyses suggest the noise may originate from the polarization variations of the underlying ferroelectric dipoles in 1T' MoTe2. We demonstrate the noise allows for true random number generation, and this facilitates their use as the seed for generating high-throughput secure random numbers exceeding 1 Mbit/s, appealing for practical applications in, for instance, cryptography where data security is now critical. As an example, we show biometric information safeguarding in neural networks by using the random numbers as the mask, proving a promising data security measure in big data and artificial intelligence.

Quantum-Confined Lifshitz Transition on Weyl Semimetal Td-MoTe2

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal Td-MoTe2 by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.

Lithiation Induced Phases in 1T'-MoTe2 Nanoflakes

Multiple polytypes of MoTe2 with distinct structures and intriguing electronic properties can be accessed by various physical and chemical approaches. Here, we report electrochemical lithium (Li) intercalation into 1T'-MoTe2 nanoflakes, leading to the discovery of two previously unreported lithiated phases. Distinguished by their structural differences from the pristine 1T' phase, these distinct phases were characterized using in situ polarization Raman spectroscopy and in situ single-crystal X-ray diffraction. The lithiated phases exhibit increasing resistivity with decreasing temperature, and their carrier densities are two to 4 orders of magnitude smaller than the metallic 1T' phase, as probed through in situ Hall measurements. The discovery of these gapped phases in initially metallic 1T'-MoTe2 underscores electrochemical intercalation as a potent tool for tuning the phase stability and electron density in two-dimensional (2D) materials.

Halide Anions Tuning of Lead-Free Perovskite-Type Ferroelectric Semiconductors with Inverse High-Temperature Symmetry Breaking

There is a noticeable gap in the literature regarding research on halogen-substitution-regulated ferroelectric semiconductors featuring multiple phase transitions. Here, a new category of 1D perovskite ferroelectrics (DFP)2SbX5 (DFP+ = 3,3-difluoropyrrolidium, X- = I-, Br-, abbreviated as I-1 and Br-2) with twophase transitions (PTs) is reported. The first low-temperature PT is a mmmFmm2 ferroelectric PT, while the high-temperature PT is a counterintuitive inverse temperature symmetry-breaking PT. By the substitution of iodine with bromine, the Curie temperature (Tc) significantly increases from 348 K of I-1 to 374 K of Br-2. Their ferroelectricity and pyroelectricity are improved (Ps value from 1.3 to 4.0 µC cm-2, pe value from 0.2 to 0.48 µC cm-2 K-1 for I-1 and Br-2), while their optical bandgaps increased from 2.1 to 2.7 eV. A critical slowing down phenomenon is observed in the dielectric measurement of I-1 while Br-2 exhibits the ferroelastic domain. Structural and computational analyses elucidate that the order-disorder movement of cations and the distortion of the chain perovskite [SbX5]2- anions skeleton lead to PT. The semiconductor properties are determined by [SbX5]2- anions. The findings contribute to the development of ferroelectric semiconductors and materials with multiple PTs and provide materials for potential applications in the optoelectronic field.

Spatially Controlled Phase Transition in MoTe2 Driven by Focused Ion Beam Irradiations

Phase transitions play an important role in tuning the physical properties of two-dimensional (2D) materials as well as developing their high-performance device applications. Here, we reported the observation of a phase transition in few-layered MoTe2 flakes by the irradiation of gallium (Ga+) ions using a focused ion beam (FIB) system. The semiconducting 2H phase of MoTe2 can be controllably converted to the metallic 1T'-like phase via Te defect engineering during irradiations. By taking advantage of the nanometer-sized Ga+ ion probe proved by FIB, in-plane 1T'-2H homojunctions of MoTe2 at submicrometer scale can be fabricated. Furthermore, we demonstrate the improvement of device performance (on-state current over 2 orders of magnitude higher) in MoTe2 transistors using the patterned 1T'-like phase regions as contact electrodes. Our study provides a new strategy to drive the phase transitions in MoTe2, tune their properties, and develop high-performance devices, which also extends the applications of FIB technology in 2D materials and their devices.

Directionally-Resolved Phononic Properties of Monolayer 2D Molybdenum Ditelluride (MoTe2) under Uniaxial Elastic Strain

Understanding the phonon characteristics of two-dimensional (2D) molybdenum ditelluride (MoTe2) under strain is critical to manipulating its multiphysical properties. Although there have been numerous computational efforts to elucidate the strain-coupled phonon properties of monolayer MoTe2, empirical validation is still lacking. In this work, monolayer 1H-MoTe2 under uniaxial strain is studied via in situ micro-Raman spectroscopy. Directionally dependent monotonic softening of the doubly degenerate in-plane E2g1 phonon mode is observed with increasing uniaxial strain, where the E2g1 peak red-shifts -1.66 ± 0.04 cm-1/% along the armchair direction and -0.80 ± 0.07 cm-1/% along the zigzag direction. The corresponding Grüneisen parameters are calculated to be 1.09 and 0.52 along the armchair and zigzag directions, respectively. This work provides the first empirical quantification and validation of the orientation-dependent strain-coupled phonon response in monolayer 1H-MoTe2 and serves as a benchmark for other prototypical 2D transition-metal tellurides.

Interface-induced enhanced room temperature ferromagnetism in hybrid transition metal dichalcogenides

Magnetic semiconductors with both electron charge and spin features exhibit tremendous potential in spintronics. Although defective transition-metal dichalcogenides are promising with induced room temperature (RT) magnetic moments, impacts of the defect type and underlying mechanisms remain unclear. Herein, two strategies involving elemental substitution and epitaxial growth have been explored to synthesize alloyed and hybrid MoSe2-xSx with lattice distortion and artificial interfaces respectively. Both experimental measurements and first-principle calculations demonstrate induced magnetism in the resultant MoSe2-xSx with the magnetization intensity closely associated to the atomic structure. The alloyed MoSe2-xSx exhibits satisfactory structural stability and atomic magnetic moments due to the Mo 4d orbital splitting induced by lattice distortion. Nevertheless, both enhanced RT ferromagnetism and thermomagnetic stability can be achieved for the hybrid MoSe2-xSx resulted from stronger localized spin polarization at the MoSe2/MoS2 interfaces. As such the work not only sheds light on the mechanisms underlying defect-induced ferromagnetism in transition-metal dichalcogenides, but also proposes an interface engineering strategy to induce significant ferromagnetism for multi-fields including spintronics, multiferroics and valleytronics.

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2

In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional (2D) material, MoTe2, caused by the phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semimetallic 1T'-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 to 63 meV during phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semimetallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 μA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30%, respectively. Our findings are distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials and a cutting-edge approach to optimize the efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

Emergent layer stacking arrangements in c-axis confined MoTe2

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe2 possesses two stacking arrangements, the ferroelectric Weyl semimetal Td phase and the higher-order topological insulator 1T' phase. However, in thin flakes of MoTe2, it is unclear if the layer stacking follows the Td, 1T', or an alternative stacking sequence. Here, we use atomic-resolution scanning transmission electron microscopy to directly visualize the MoTe2 layer stacking. In thin flakes, we observe highly disordered stacking, with nanoscale 1T' and Td domains, as well as alternative stacking arrangements not found in the bulk. We attribute these findings to intrinsic confinement effects on the MoTe2 stacking-dependent free energy. Our results are important for the understanding of exotic physics displayed in MoTe2 flakes. More broadly, this work suggests c-axis confinement as a method to influence layer stacking in other 2D materials.

A Centimeter-Scale Type-II Weyl Semimetal for Flexible and Fast Ultra-Broadband Photodetection from Ultraviolet to Sub-Millimeter Wave Regime

Flexible photodetectors with ultra-broadband sensitivities, fast response, and high responsivity are crucial for wearable applications. Recently, van der Waals (vdW) Weyl semimetals have gained much attention due to their unique electronic band structure, making them an ideal material platform for developing broadband photodetectors from ultraviolet (UV) to the terahertz (THz) regime. However, large-area synthesis of vdW semimetals on a flexible substrate is still a challenge, limiting their application in flexible devices. In this study, centimeter-scale type-II vdW Weyl semimetal, Td -MoTe2 films, are grown on a flexible mica substrate by molecular beam epitaxy. A self-powered and flexible photodetector without an antenna demonstrated an outstanding ability to detect electromagnetic radiation from UV to sub-millimeter (SMM) wave at room temperature, with a fast response time of ≈20 µs, a responsivity of 0.53 mA W-1 (at 2.52 THz), and a noise-equivalent power (NEP) of 2.65 nW Hz-0.5 (at 2.52 THz). The flexible photodetectors are also used to image shielded items with high resolution at 2.52 THz. These results can pave the way for developing flexible and wearable optoelectronic devices using direct-grown large-area vdW semimetals.

Enhanced Superconductivity and Upper Critical Field in Ta-Doped Weyl Semimetal Td -MoTe2

2D transition metal dichalcogenides are promising platforms for next-generation electronics and spintronics. The layered Weyl semimetal (W,Mo)Te₂ series features structural phase transition, nonsaturated magnetoresistance, superconductivity, and exotic topological physics. However, the superconducting critical temperature of the bulk (W,Mo)Te₂ remains ultralow without applying a high pressure. Here, the significantly enhanced superconductivity is observed with a transition temperature as large as about 7.5 K in bulk Mo_{1- x} Ta_x Te₂ single crystals upon Ta doping (0 ≤ x ≤ 0.22), which is attributed to an enrichment of density of states at the Fermi level. In addition, an enhanced perpendicular upper critical field of 14.5 T exceeding the Pauli limit is also observed in T_d -phase Mo_{1- x} Ta_x Te₂ (x = 0.08), indicating the possible emergence of unconventional mixed singlet-triplet superconductivity owing to the inversion symmetry breaking. This work provides a new pathway for exploring the exotic superconductivity and topological physics in transition metal dichalcogenides.

Gate and Temperature Driven Phase Transitions in Few-Layer MoTe2

MoTe₂ has a stable hexagonal semiconducting phase (2H) as well as two semimetallic phases with monoclinic (1T') and orthorhombic (T_d) structures. A structural change can thus be accompanied by a significant change in electronic transport properties. The two semimetallic phases are connected by a temperature driven transition and could exhibit topological properties. Here we make extensive Raman measurements as a function of layer thickness, temperature, and electrostatic doping on few layer 2H-MoTe₂ and also on 1T'-MoTe₂ and T_d-WTe₂. Recent work in MoTe₂ has raised the possibility of a 2H-1T' transition through technology compatible pathways. It has been claimed that such a transition, of promise for device applications, is activated by electrostatic gating. We investigate this claim and find that few-layer tellurides are characterized by high mobility of Te ions, even in ambient conditions and especially through the variation of external parameters like electric field or temperature. These can generate Te clusters, vacancies at crystalline sites, and facilitate structural transitions. We however find that the purported 2H-1T' transition in MoTe₂ cannot be obtained by a pure electrostatic field.

Origin of the Anomalous Electrical Transport Behavior in Fe-Intercalated Weyl Semimetal Td -MoTe2

Weyl semimetal T_d -MoTe₂ has recently attracted much attention due to its intriguing electronic properties and potential applications in spintronics. Here, Fe-intercalated T_d -Fe_x MoTe₂ single crystals (0 < x < 0.15 ) are grown successfully. The electrical and thermoelectric transport results consistently demonstrate that the phase transition temperature T_S is gradually suppressed with increasing x. Theoretical calculation suggests that the increased energy of the T_d phase, enhanced transition barrier, and more occupied bands in 1T' phase is responsible for the suppression in T_S . In addition, a ρ^α -lnT behavior induced by Kondo effect is observed with x ≥ 0.08, due to the coupling between conduction carriers and the local magnetic moments of intercalated Fe atoms. For T_d -Fe_0.15 MoTe₂ , a spin-glass transition occurs at ≈10 K. The calculated band structure of T_d -Fe_0.25 MoTe₂ shows that two flat bands exist near the Fermi level, which are mainly contributed by the d_yz and d x 2 - y 2 ${{\rm{d}}_{{x^2} - {y^2}}}$ orbitals of the Fe atoms. Finally, the electronic phase diagram of T_d -Fe_x MoTe₂ is established for the first time. This work provides a new route to control the structural instability and explore exotic electronic states for transition-metal dichalcogenides.

Laser-Induced Phase Transition and Patterning of hBN-Encapsulated MoTe2

Transition metal dichalcogenides exhibit phase transitions through atomic migration when triggered by various stimuli, such as strain, doping, and annealing. However, since atomically thin 2D materials are easily damaged and evaporated from these strategies, studies on the crystal structure and composition of transformed 2D phases are limited. Here, the phase and composition change behavior of laser-irradiated molybdenum ditelluride (MoTe₂ ) in various stacked geometry are investigated, and the stable laser-induced phase patterning in hexagonal boron nitride (hBN)-encapsulated MoTe₂ is demonstrated. When air-exposed or single-side passivated 2H-MoTe₂ are irradiated by a laser, MoTe₂ is transformed into Te or Mo₃ Te₄ due to the highly accumulated heat and atomic evaporation. Conversely, hBN-encapsulated 2H-MoTe₂ transformed into a 1T' phase without evaporation or structural degradation, enabling stable phase transitions in desired regions. The laser-induced phase transition shows layer number dependence; thinner MoTe₂ has a higher phase transition temperature. From the stable phase patterning method, the low contact resistivity of 1.13 kΩ µm in 2H-MoTe₂ field-effect transistors with 1T' contacts from the seamless heterophase junction geometry is achieved. This study paves an effective way to fabricate monolithic 2D electronic devices with laterally stitched phases and provides insights into phase and compositional changes in 2D materials.

Probing electron-phonon and phonon-phonon coupling in type-II Dirac semi-metal NiTe2via temperature-dependent Raman spectroscopy

We report the temperature-dependent structural characterization of type-II Dirac semimetal NiTe₂in the form of a bulk single crystal and a nanoflake (200 nm thick). Detailed x-ray diffraction study along with Rietveld refinement analysis reveals superior crystallinity and linear thermal expansion coefficient (α_T) of 5.56 × 10^-6and 22.5 × 10^-6K^-1along a or b and c lattice directions, respectively. Temperature evolution of Raman spectra shows non-linear variations in the phonon frequency and full-width half maxima of the out-of-plane A1gand in-plane E_gmodes. Raman mode E2g1, corresponding to an in-plane vibration, disappears on decreasing the thickness from bulk to nanoflake. Quantitative analysis with anharmonic model yields dominating electron-phonon interaction over phonon-phonon interaction mediated by three- and four-phonon processes.

In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe2

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe₂) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H-T_d interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe₂ with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the T_d-to-2H phase transition initiates at phase boundaries at low temperatures (200-225 °C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-T_d-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D heterophase devices with atomically sharp and coherent interfaces.

Suppressing non-radiative recombination in metal halide perovskite solar cells by synergistic effect of ferroelasticity

The low fraction of non-radiative recombination established the foundation of metal halide perovskite solar cells. However, the origin of low non-radiative recombination in metal halide perovskite materials is still not well-understood. Herein, we find that the non-radiative recombination in twinning-tetragonal phase methylammonium lead halide (MAPbI_xCl_3-x) is apparently suppressed by applying an electric field, which leads to a remarkable increase of the open-circuit voltage from 1.12 V to 1.26 V. Possible effects of ionic migration and light soaking on the open-circuit voltage enhancement are excluded experimentally by control experiments. Microscopic and macroscopic characterizations reveal an excellent correlation between the ferroelastic lattice deformation and the suppression of non-radiative recombination. The calculation result suggests the existence of lattice polarization in self-stabilizable deformed domain walls, indicating the charge separation that facilitated by lattice polarization is accountable for the suppressed non-radiative recombination. This work provides an understanding of the excellent performance of metal halide perovskite solar cells.

Anomalous Hall effect in topological Weyl and nodal-line semimetal Heusler compound Co2VAl

Magnetic topological semimetals (TSMs) with broken time-reversal symmetry are very rare and have drawn significant attention in condensed matter physics due to their numerous intriguing topological properties. Among these various magnetic TSMs, Co₂-based full Heusler compounds are of current interest, since a few of these materials exhibit Weyl and nodal fermions in their topological band structure. In this work, we report a comprehensive study of anomalous Hall effect (AHE) in the ferromagnetic full Heusler compound Co₂VAl. Recent studies indicate that the intrinsic AHE is closely related to the Berry curvature of the occupied electronic Bloch states. The present study of Co₂VAl attempts to understand and explore the possibility of topology-induced AHE. The anomalous Hall resistivityρxyAis observed to scale quadratically with the longitudinal resistivityρ_xx. Our experimental results also reveal that the anomalous Hall conductivity (AHC) is ∼85 cm^-1at 2 K with an intrinsic contribution of ∼75.6 S cm^-1, and is nearly insensitive to temperature. The first principle calculations note that the Berry curvature originated from a gapped nodal line and symmetry-protected Weyl nodes near the Fermi level (EF) is the main source of AHE in this compound. Thus, this investigation on Co₂VAl discloses that it is a ferromagnetic Weyl and nodal-line TSM. The theoretically calculated AHC is in well agreement with the experimentally obtained AHC.

Atomic and Electronic Manipulation of Robust Ferroelectric Polymorphs

Polymorphism allows the symmetry of the lattice and spatial charge distributions of atomically thin materials to be designed. While various polymorphs for superconducting, magnetic, and topological states have been extensively studied, polymorphic control is a challenge for robust ferroelectricity in atomically thin geometries. Here, the atomic and electric manipulation of ferroelectric polymorphs in Mo_{1- x} W_x Te₂ is reported. Atomic manipulation for polymorphic control via chemical pressure (substituting tungsten for molybdenum atoms) and charge density modulation can realize tunable polar lattice structures and robust ferroelectricity up to T = 400 K with a constant coercive field in an atomically thin material. Owing to the effective inversion symmetry breaking, the ferroelectric switching withstands a charge carrier density of up to 1.1 × 10¹³ cm^-2 , developing an original diagram for ferroelectric switching in atomically thin materials.

Traversing Double-Well Potential Energy Surfaces: Photoinduced Concurrent Intralayer and Interlayer Structural Transitions in XTe2 (X = Mo, W)

The microscopic arrangement of atoms and molecules is the determining factor in how materials behave and perform; i.e., the structure determines the property, a traditional paradigm in materials science. Photoexcitation-driven manipulation of the crystal structure and associated electronic properties in quantum materials provides opportunities for the exploration of exotic physics and practical applications; however, a generalized mechanism for such symmetry engineering is absent. Here, by ultrafast electron diffraction, structure factor calculation, and TDDFT-MD simulations, we report the photoinduced concurrent intralayer and interlayer structural transitions in the Td and 1T' phases of XTe₂ (X = Mo, W). We discuss the modification of multiple quantum electronic states associated with the intralayer and interlayer structural transitions, such as the topological band inversion and the higher-order topological state. The twin structures and the stacking faults in XTe₂ are also identified by ultrafast structural responses. The comprehensive study of the ultrafast structural response in XTe₂ suggests the traversal of all double-well potential energy surfaces (DWPES) by laser excitation, which is expected to be an intrinsic mechanism in the field of photoexcitation-driven global/local symmetry engineering and also a critical ingredient inducing the exotic properties in the non-equilibrium state in a large number of material systems.

Large Two-Magnon Raman Hysteresis Observed in a Magnetically Uncompensated Hematite Coating across the Morin Transition

A temperature-dependent Raman experiment between 80 and 600 K was performed in a nanoparticulated coating of single-phase hematite grown on a silica substrate. In that range, a thermal Raman shift hysteresis was identified in the vibrational modes that accompanies the Morin transition, observing large effects in the two-magnon Raman frequency position and in its relative intensity. Interestingly, no decrease in coercivity occurs when the hematite crosses the Morin transition below 230 K. The spin-flop processes produced in the coating leads to a strong decompensation of the surface spins, generating a ferromagnetic component over the whole temperature range studied. Such unusual effects might be promoted by a certain degree of structural disorder and the stresses produced by the nanoparticulation growth approach of the hematite coating. As a result, a high stability of the two-magnon excitation is obtained over a wide temperature range and considerable advances are made for the development of spintronic devices based on semiconductor antiferromagnetic materials.

First principles assessment of the phase stability and transition mechanisms of designated crystal structures of pristine and Janus transition metal dichalcogenides

Two-dimensional Transition Metal Dichalcogenides (TMDs) possessing extraordinary physical properties at reduced dimensionality have attracted interest due to their promise in electronic and optical device applications. However, TMD monolayers can show a broad range of different properties depending on their crystal phase; for example, H phases are usually semiconductors, while the T phases are metallic. Thus, controlling phase transitions has become critical for device applications. In this study, the energetically low-lying crystal structures of pristine and Janus TMDs are investigated by using ab initio Nudged Elastic Band and molecular dynamics simulations to provide a general explanation for their phase stability and transition properties. Across all materials investigated, the T phase is found to be the least stable and the H phase is the most stable except for WTe₂, while the T' and T'' phases change places according to the TMD material. The transition energy barriers are found to be large enough to hint that even the higher energy phases are unlikely to undergo a phase transition to a more stable phase if they can be achieved except for the least stable T phase, which has zero barrier towards the T' phase. Indeed, in molecular dynamics simulations the thermodynamically least stable T phase transformed into the T' phase spontaneously while in general no other phase transition was observed up to 2100 K for the other three phases. Thus, the examined T', T'' and H phases were shown to be mostly stable and do not readily transform into another phase. Furthermore, so-called mixed phase calculations considered in our study explain the experimentally observed lateral hybrid structures and point out that the coexistence of different phases is strongly stable against phase transitions. Indeed, stable complex structures such as metal-semiconductor-metal architectures, which have immense potential to be used in future device applications, are also possible based on our investigation.

Ultrafast and stable phase transition realized in MoTe2-based memristive devices

Phase engineering of two-dimensional transition metal dichalcogenides has received increasing attention in recent years due to its atomically thin nature and polymorphism. Here, we first realize an electric-field-induced controllable phase transition between semiconducting 2H and metallic 1T' phases in MoTe₂ memristive devices. The device performs stable bipolar resistive switching with a cycling endurance of over 10⁵, an excellent retention characteristic of over 10⁵ s at an elevated temperature of 85 °C and an ultrafast switching of ∼5 ns for SET and ∼10 ns for RESET. More importantly, the device works in different atmospheres including air, vacuum and oxygen, and even works with no degradation after being placed in air for one year, indicating excellent surrounding and time stability. In situ Raman analysis reveals that the stable resistive switching originates from a controllable phase transition between 2H and 1T' phases. Density functional theory calculations reveal that the Te vacancy facilitates the phase transition in MoTe₂ through decreasing the barrier between 2H and 1T' phases, and serving as nucleation sites due to the elimination of repulsive forces. This electric-field-induced controllable phase transition in MoTe₂ devices offers new opportunities for developing reliable and ultrafast phase transition devices based on atomically thin membranes.

Persistence of Monoclinic Crystal Structure in 3D Second-Order Topological Insulator Candidate 1T'-MoTe2 Thin Flake Without Structural Phase Transition

A van der Waals material, MoTe2 with a monoclinic 1T' crystal structure is a candidate for 3D second-order topological insulators (SOTIs) hosting gapless hinge states and insulating surface states. However, due to the temperature-induced structural phase transition, the monoclinic 1T' structure of MoTe2 is transformed into the orthorhombic Td structure as the temperature is lowered, which hinders the experimental verification and electronic applications of the predicted SOTI state at low temperatures. Here, systematic Raman spectroscopy studies of the exfoliated MoTe2 thin flakes with variable thicknesses at different temperatures, are presented. As a spectroscopic signature of the orthorhombic Td structure of MoTe2 , the out-of-plane vibration mode D at ≈ 125 cm-1 is always visible below a certain temperature in the multilayer flakes thicker than ≈ 27.7 nm, but vanishes in the temperature range from 80 to 320 K when the flake thickness becomes lower than ≈ 19.5 nm. The absence of the out-of-plane vibration mode D in the Raman spectra here demonstrates not only the disappearance of the monoclinic-to-orthorhombic phase transition but also the persistence of the monoclinic 1T' structure in the MoTe2 thin flakes thinner than ≈ 19.5 nm at low temperatures down to 80 K, which may be caused by the high enough density of the holes introduced during the gold-enhanced exfoliation process and exposure to air. The MoTe2 thin flakes with the low-temperature monoclinic 1T' structure provide a material platform for realizing SOTI states in van der Waals materials at low temperatures, which paves the way for developing a new generation of electronic devices based on SOTIs.

Thickness-dependent in-plane anisotropy of GaTe phonons

Gallium Telluride (GaTe), a layered material with monoclinic crystal structure, has recently attracted a lot of attention due to its unique physical properties and potential applications for angle-resolved photonics and electronics, where optical anisotropies are important. Despite a few reports on the in-plane anisotropies of GaTe, a comprehensive understanding of them remained unsatisfactory to date. In this work, we investigated thickness-dependent in-plane anisotropies of the 13 Raman-active modes and one Raman-inactive mode of GaTe by using angle-resolved polarized Raman spectroscopy, under both parallel and perpendicular polarization configurations in the spectral range from 20 to 300 cm^-1. Raman modes of GaTe revealed distinctly different thickness-dependent anisotropies in parallel polarization configuration while nearly unchanged for the perpendicular configuration. Especially, three A_g modes at 40.2 ([Formula: see text]), 152.5 ([Formula: see text]), and 283.8 ([Formula: see text]) cm^-1 exhibited an evident variation in anisotropic behavior as decreasing thickness down to 9 nm. The observed anisotropies were thoroughly explained by adopting the calculated interference effect and the semiclassical complex Raman tensor analysis.

MoTe2: Semiconductor or Semimetal?

Transition metal tellurides (TMTs) have attracted intense interest due to their intriguing physical properties arising from their diverse phase topologies. To date, a wide range of physical properties have been discovered for TMTs, including that they can act as topological insulators, semiconductors, Weyl semimetals, and superconductors. Among the TMT families, MoTe₂ is a representative material because of its Janus nature and rich phases. In this Perspective, we first introduce phase structures in monolayer and bulk MoTe₂ and then summarize MoTe₂ synthesis strategies. We highlight recent advances of Janus MoTe₂ in terms of material structures and emerging quantum states. We also provide insight into the opportunities and challenges faced by MoTe₂-associated device design and applications.

Atmospheric-pressure CVD growth of two-dimensional 2H- and 1 T'-MoTe2films with high-performance SERS activity

Two-dimensional (2D) molybdenum ditelluride (MoTe₂) is a member of the transition-metal dichalcogenides family, which is an especially promising platform for surface-enhanced Raman scattering (SERS) applications, due to its excellent electronic properties. However, the synthesis of large-area highly crystalline 2D MoTe₂with controllable polymorphism is a huge challenge due to the small free energy difference (∼40 meV per unit cell) between semiconducting 2H-MoTe₂and semi-metallic 1 T'-MoTe₂. Herein, we report an optimized route for the synthesis of 2H- and 1 T'-MoTe₂films by atmospheric-pressure chemical vapor deposition. The SERS study of the as-grown MoTe₂films was carried out using methylene blue (MB) as a probe molecule. The Raman enhancement factor on 1 T'-MoTe₂was found to be three times higher than that on 2H-MoTe₂and the 1 T'-MoTe₂film is an efficient Raman-enhancing substrate that can be used to detect MB at nanomolar concentrations. Our study also imparts knowledge on the significance of a suitable combination of laser excitation wavelength and molecule-material platform for achieving ultrasensitive SERS-based chemical detection.

Field induced hysteretic structural phase switching and possible CDW in Re-doped MoTe2

Novel electronic systems displaying exotic physical properties can be derived from complex topological materials through chemical doping. MoTe₂, the candidate type-II Weyl semimetal shows dramatically enhanced superconductivity up to 4.1 K upon Re doping in Mo sites. Based on bulk transport and local scanning tunnelling microscopy here we show that Re doping also leads to the emergence of a possible charge density wave phase in Re_0.2Mo_0.8Te₂. In addition, the tunnellingI-Vcharacteristics display non-linearity and hysteresis which is commensurate with a hysteresis observed in the change in tip-height (z) as a function of applied voltageV. The observations indicate an electric field induced hysteretic switching consistent with piezoelectricity and possible ferroelectricity.

Manipulating Weyl quasiparticles by orbital-selective photoexcitation in WTe2

Optical control of structural and electronic properties of Weyl semimetals allows development of switchable and dissipationless topological devices at the ultrafast scale. An unexpected orbital-selective photoexcitation in type-II Weyl material WTe2 is reported under linearly polarized light (LPL), inducing striking transitions among several topologically-distinct phases mediated by effective electron-phonon couplings. The symmetry features of atomic orbitals comprising the Weyl bands result in asymmetric electronic transitions near the Weyl points, and in turn a switchable interlayer shear motion with respect to linear light polarization, when a near-infrared laser pulse is applied. Consequently, not only annihilation of Weyl quasiparticle pairs, but also increasing separation of Weyl points can be achieved, complementing existing experimental observations. In this work, we provide a new perspective on manipulating the Weyl node singularity and coherent control of electron and lattice quantum dynamics simultaneously.

Synthesis of Large-Scale Monolayer 1T'-MoTe2 and Its Stabilization via Scalable hBN Encapsulation

Out of the different structural phases of molybdenum ditelluride (MoTe2), the distorted octahedral 1T' possesses great interest for fundamental physics and is a promising candidate for the implementation of innovative devices such as topological transistors. Indeed, 1T'-MoTe2 is a semimetal with superconductivity, which has been predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. Large instability of monolayer 1T'-MoTe2 in environmental conditions, however, has made its investigation extremely challenging so far. In this work, we demonstrate homogeneous growth of large single-crystal (up to 500 μm) monolayer 1T'-MoTe2 via chemical vapor deposition (CVD) and its stabilization in air with a scalable encapsulation approach. The encapsulant is obtained by electrochemically delaminating CVD hexagonal boron nitride (hBN) from copper foil, and it is applied on the freshly grown 1T'-MoTe2 via a top-down dry lamination step. The structural and electrical properties of encapsulated 1T'-MoTe2 have been monitored over several months to assess the degree of degradation of the material. We find that when encapsulated with hBN, the lifetime of monolayer 1T'-MoTe2 successfully increases from a few minutes to more than a month. Furthermore, the encapsulated monolayer can be subjected to transfer, device processing, and heating and cooling cycles without degradation of its properties. The potential of this scalable heterostack is confirmed by the observation of signatures of low-temperature phase transition in monolayer 1T'-MoTe2 by both Raman spectroscopy and electrical measurements. The growth and encapsulation methods reported in this work can be employed for further fundamental studies of this enticing material as well as facilitate the technological development of monolayer 1T'-MoTe2.

Unidirectional Alignment of AgCN Microwires on Distorted Transition Metal Dichalcogenide Crystals

Van der Waals epitaxy on the surface of two-dimensional (2D) layered crystals has gained significant research interest for the assembly of well-ordered nanostructures and fabrication of vertical heterostructures based on 2D crystals. Although van der Waals epitaxial assembly on the hexagonal phase of transition metal dichalcogenides (TMDCs) has been relatively well characterized, a comparable study on the distorted octahedral phase (1T' or T_d) of TMDCs is largely lacking. Here, we investigate the assembly behavior of one-dimensional (1D) AgCN microwires on various distorted TMDC crystals, namely 1T'-MoTe₂, T_d-WTe₂, and 1T'-ReS₂. The unidirectional alignment of AgCN chains is observed on these crystals, reflecting the symmetry of underlying distorted TMDCs. Polarized Raman spectroscopy and transmission electron microscopy directly confirm that AgCN chains display the remarkable alignment behavior along the distorted chain directions of underlying TMDCs. The observed unidirectional assembly behavior can be attributed to the favorable adsorption configurations of 1D chains along the substrate distortion, which is supported by our theoretical calculations and observation of similar assembly behavior from different cyanide chains. The aligned AgCN microwires can be harnessed as facile markers to identify polymorphs and crystal orientations of TMDCs.

Structural Phase Transition and Interlayer Coupling in Few-Layer 1T' and Td MoTe2

We performed polarized Raman spectroscopy on mechanically exfoliated few-layer MoTe₂ samples and observed both 1T' and T_d phases at room temperature. Few-layer 1T' and T_d MoTe₂ exhibited a significant difference especially in interlayer vibration modes, from which the interlayer coupling strengths were extracted using the linear chain model: strong in-plane anisotropy was observed in both phases. Furthermore, temperature-dependent Raman measurements revealed a peculiar phase transition behavior in few-layer 1T' MoTe₂. In contrast to bulk 1T' MoTe₂ crystals, where the phase transition to the T_d phase occurs at ∼250 K, the temperature-driven phase transition to the T_d phase is increasingly suppressed as the thickness is reduced, and the transition and the critical temperature varied dramatically from sample to sample even for the same thickness. Raman spectra of intermediate phases that correspond to neither 1T' nor T_d phase with different interlayer vibration modes were observed, which suggests that several metastable phases exist with similar total energies.

Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3

The discovery of 2-dimensional (2D) materials, such as CrI₃, that retain magnetic ordering at monolayer thickness has resulted in a surge of both pure and applied research in 2D magnetism. Here, we report a magneto-Raman spectroscopy study on multilayered CrI₃, focusing on two additional features in the spectra that appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI₃.

Engineering Weyl Phases and Nonlinear Hall Effects in T_{d}-MoTe_{2}

MoTe_{2} has recently attracted much attention due to the observation of pressure-induced superconductivity, exotic topological phase transitions, and nonlinear quantum effects. However, there has been debate on the intriguing structural phase transitions among various observed phases of MoTe_{2} and their connection to the underlying topological electronic properties. In this work, by means of density-functional theory calculations, we investigate the structural phase transition between the polar T_{d} and nonpolar 1T^{'} phases of MoTe_{2} in reference to a hypothetical high-symmetry T_{0} phase that exhibits higher-order topological features. In the T_{d} phase we obtain a total of 12 Weyl points, which can be created/annihilated, dynamically manipulated, and switched by tuning a polar phonon mode. We also report the existence of a tunable nonlinear Hall effect in T_{d}-MoTe_{2} and propose the use of this effect as a probe for the detection of polarity orientation in polar (semi)metals. By studying the role of dimensionality, we identify a configuration in which a nonlinear surface response current emerges. The potential technological applications of the tunable Weyl phase and the nonlinear Hall effect are discussed.

Thickness dependent transition from the 1T' to Weyl semimetal phase in ultrathin MoTe2: electrical transport, noise and Raman studies

Bulk 1T'-MoTe₂ shows a structural phase transition from the 1T' to Weyl semimetallic (WSM) T_d phase at ∼240 K. This phase transition and transport properties in the two phases have not been investigated on ultra-thin crystals. Here we report electrical transport, 1/f noise and Raman studies on ultra-thin 1T'-MoTe₂ (∼5 to 16 nm thick) field-effect transistor (FETs) devices as a function of temperature. The electrical resistivities for a thickness of 16 nm and 11 nm show maxima at temperatures of 208 K and 178 K, respectively, making a transition from the semiconducting to semi-metallic phase, hitherto not observed in bulk samples. Raman frequencies and linewidths for an 11 nm thick crystal show a change around 178 K, attributed to the additional contribution to the phonon self-energy due to the enhanced electron-phonon interaction in the WSM phase. Furthermore, the resistivity at low temperature shows an upturn below 20 K along with the maximum in the power spectral density of the low frequency 1/f noise. The latter rules out the metal-insulator transition (MIT) being responsible for the upturn of resistivity below 20 K. The low temperature resistivity follows ρ∝ 1/T, changing to ρ∝T with increasing temperature supports electron-electron interaction physics at electron-hole symmetric Weyl nodes below 20 K. These observations will pave the way to unravel the properties of the WSM state in layered ultra-thin van der Waals materials.

Polarization-resolved broadband time-resolved optical spectroscopy for complex materials: application to the case of MoTe2 polytypes

Time-resolved optical spectroscopy (TR-OS) has emerged as a fundamental spectroscopic tool for probing complex materials, to both investigate ground-state-related properties and trigger phase transitions among different states with peculiar electronic and lattice structures. We describe a versatile approach to perform polarization-resolved TR-OS measurements, by combining broadband detection with the capability to simultaneously probe two orthogonal polarization states. This method allows us to probe, with femtoseconds resolution, the frequency-resolved reflectivity or transmittivity variations along two mutually orthogonal directions, matching the principal axis of the crystal structure of the material under scrutiny. We report on the results obtained by acquiring the polarization-dependent transient reflectivity of two polytypes of the MoTe₂ compound, with 2H and 1T' crystal structures. We reveal marked anisotropies in the time-resolved reflectivity signal of 1T'-MoTe₂, which are connected to the crystal structure of the compound. Polarization- and time- resolved spectroscopic measurements can thus provide information about the nature and dynamics of both the electronic and crystal lattice subsystems, advancing the comprehension of their inter-dependence, in particular in the case of photoinduced phase transitions; in addition, they provide a broadband measurement of transient polarization rotations.

A Review of Chinese Raman Spectroscopy Research Over the Past Twenty Years

This paper introduces the major Chinese research groups in the fields of biomedicine, food safety, environmental testing, material research, archaeological and cultural relics, gem identification, forensic science, and other research areas of Raman spectroscopy and combined methods spanning the two decades from 1997 to 2017. Briefly summarized are the research directions and contents of the major Chinese Raman spectroscopy research groups, giving researchers engaged in Raman spectroscopy research a more comprehensive understanding of the state of Chinese Raman spectroscopy research and future development trends to further develop Raman spectroscopy and its applications.

Quasi-Two-Dimensional Magnon Identification in Antiferromagnetic FePS3via Magneto-Raman Spectroscopy

Recently it was discovered that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer, opening many avenues in fundamental physics and potential applications of these fascinating materials. One such material is FePS3, a large spin (S=2) Mott insulator where the Fe atoms form a honeycomb lattice. In the bulk, FePS3 has been shown to be a quasi-two-dimensional-Ising antiferromagnet, with additional features in the Raman spectra emerging below the Néel temperature (T N ) of approximately 120 K. Using magneto-Raman spectroscopy as an optical probe of magnetic structure, we show that one of these Raman-active modes in the magnetically ordered state is actually a magnon with a frequency of ≈3.7 THz (122 cm-1). Contrary to previous work, which interpreted this feature as a phonon, our Raman data shows the expected frequency shifting and splitting of the magnon as a function of temperature and magnetic field, respectively, where we determine the g-factor to be ≈2. In addition, the symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of FePS3.

Oxygen-Sensitive Layered MoTe2 Channels for Environmental Detection

The oxygen (O₂)-dependent resistance change of multilayered molybdenum ditelluride (MoTe₂) channels was characterized. A variation of the channel resistance could reproducibly determine relative O₂ content (denoted as the O₂ index). We found that Joule heating in a layered MoTe₂ field-effect transistor caused the O₂ index to decrease drastically from 100 to 12.1% in back gate modulation. Furthermore, Joule heating caused effective O₂ desorption from the MoTe₂ surface and repeatable O₂ detection by multilayered MoTe₂ channels was realized. This work not only explored the influence of O₂ on the electrical properties of multilayered MoTe₂ channels but also revealed that MoTe₂ channels are promising for sensing O₂ in an environmental condition.

Strong and Tunable Electrical Anisotropy in Type-II Weyl Semimetal Candidate WP2 with Broken Inversion Symmetry

A transition metal diphosphide, WP₂ , is a candidate for type-II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP₂ , spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP₂ electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle-resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first-principle studies of WP₂ are reported. The energies of the observed Raman-active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first-principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP₂ . Moreover, the measured ratio (R_c /R_a ) between the crystalline c-axis and a-axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in-plane electrical resistivity anisotropy among the reported type-II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first-principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the R_c /R_a of WP₂ at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so-far-strongest and magnetic-field-tunable electrical resistivity anisotropy found in WP₂ can serve as a degree of freedom for tuning the electrical properties of type-II WSMs, which paves the way for the development of novel electronic applications based on type-II WSMs.

Spatially dispersive circular photogalvanic effect in a Weyl semimetal

Weyl semimetals (WSMs) are gapless topological states of matter with broken inversion and/or time reversal symmetry. WSMs can support a circulating photocurrent when illuminated by circularly polarized light at normal incidence. Here, we report a spatially dispersive circular photogalvanic effect (s-CPGE) in a WSM that occurs with a spatially varying beam profile. Our analysis shows that the s-CPGE is controlled by a symmetry selection rule combined with asymmetric carrier excitation and relaxation dynamics. By evaluating the s-CPGE for a minimal model of a WSM, a frequency-dependent scaling behaviour of the photocurrent is obtained. Wavelength-dependent measurements from the visible to mid-infrared range show evidence of Berry curvature singularities and band inversion in the s-CPGE response. We present the s-CPGE as a promising spectroscopic probe for topological band properties, with the potential for controlling photoresponse by patterning optical fields on topological materials to store, manipulate and transmit information.

In-Plane Anisotropic Properties of 1T'-MoS2 Layers

Crystal phases play a key role in determining the physicochemical properties of a material. To date, many phases of transition metal dichalcogenides have been discovered, such as octahedral (1T), distorted octahedral (1T'), and trigonal prismatic (2H) phases. Among these, the 1T' phase offers unique properties and advantages in various applications. Moreover, the 1T' phase consists of unique zigzag chains of the transition metals, giving rise to interesting in-plane anisotropic properties. Herein, the in-plane optical and electrical anisotropies of metastable 1T'-MoS₂ layers are investigated by the angle-resolved Raman spectroscopy and electrical measurements, respectively. The deconvolution of J₁ and J₂ peaks in the angle-resolved Raman spectra is a key characteristic of high-quality 1T'-MoS₂ crystal. Moreover, it is found that its electrocatalytic performance may be affected by the crystal orientation of anisotropic material due to the anisotropic charge transport.

Vibrational properties of germanane and fluorinated germanene in the chair, boat, and zigzag-line configurations

The electronic and vibrational properties of germanane and fluorinated germanene are studied within density functional theory (DFT) and density functional perturbation theory frameworks. Different structural configurations of germanane and fluorinated germanene are investigated. The energy difference between the different configurations are consistently smaller than the energy of thermal fluctuations for all the analyzed DFT functionals LDA, GGA, and hybrid functionals, which implies that, in principle, it is possible to find these different configurations in different regions of the sample as minority phases or local defects. We calculate the Raman and infrared spectra for these configurations by using ab initio calculations and compare it with available experimental spectra for germanane. Our results show the presence of minority phases compatible with the configurations analyzed in this work. As these low energy configurations are metastable the present work shows that the synthesis of these energy competing phases is feasible by selectively changing the synthesis conditions, which is an opportunity to expand in this way the availability of new two-dimensional compounds.

Elastic Properties and Fracture Behaviors of Biaxially Deformed, Polymorphic MoTe2

Biaxial deformation of suspended membranes widely exists and is used in nanoindentation to probe elastic properties of structurally isotropic two-dimensional (2D) materials. However, the elastic properties and, in particular, the fracture behaviors of anisotropic 2D materials remain largely unclarified in the case of biaxial deformation. MoTe₂ is a polymorphic 2D material with both isotropic (2H) and anisotropic (1T' and T_d) phases and, therefore, an ideal system of single-stoichiometric materials with which to study these critical issues. Here, we report the elastic properties and fracture behaviors of biaxially deformed, polymorphic MoTe₂ by combining temperature-variant nanoindentation and first-principles calculations. It is found that due to similar atomic bonding, the effective moduli of the three phases deviate by less than 15%. However, the breaking strengths of distorted 1T' and T_d phases are only half the value of 2H phase due to their uneven distribution of bonding strengths. Fractures of both isotropic 2H and anisotropic 1T' phases obey the theorem of minimum energy, forming triangular and linear fracture patterns, respectively, along the orientations parallel to Mo-Mo zigzag chains. Our findings not only provide a reference database for the elastic behaviors of versatile MoTe₂ phases but also illuminate a general strategy for the mechanical investigation of any isotropic and anisotropic 2D materials.

An ultrafast symmetry switch in a Weyl semimetal

Topological quantum materials exhibit fascinating properties^1-3, with important applications for dissipationless electronics and fault-tolerant quantum computers^4,5. Manipulating the topological invariants in these materials would allow the development of topological switching applications analogous to switching of transistors⁶. Lattice strain provides the most natural means of tuning these topological invariants because it directly modifies the electron-ion interactions and potentially alters the underlying crystalline symmetry on which the topological properties depend^7-9. However, conventional means of applying strain through heteroepitaxial lattice mismatch¹⁰ and dislocations¹¹ are not extendable to controllable time-varying protocols, which are required in transistors. Integration into a functional device requires the ability to go beyond the robust, topologically protected properties of materials and to manipulate the topology at high speeds. Here we use crystallographic measurements by relativistic electron diffraction to demonstrate that terahertz light pulses can be used to induce terahertz-frequency interlayer shear strain with large strain amplitude in the Weyl semimetal WTe₂, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements indicate that this transition is associated with a symmetry change to a centrosymmetric, topologically trivial phase. We further show that such shear strain provides an ultrafast, energy-efficient way of inducing robust, well separated Weyl points or of annihilating all Weyl points of opposite chirality. This work demonstrates possibilities for ultrafast manipulation of the topological properties of solids and for the development of a topological switch operating at terahertz frequencies.

Orientation-dependent optical characterization of atomically thin transition metal ditellurides

Molybdenum ditellurides (MoTe2) have recently attracted attention owing to their excellent structurally tunable nature between 1T'(metallic)- and 2H(semiconducting)-phases; thus, the controllable fabrication and critical identification of MoTe2 are highly desired. Here, we semi-controllably synthesized 1T'- and 2H-MoTe2 crystals using the atmospheric pressure chemical vapor deposition (APCVD) technique and studied their grain-orientation dependency using polarization-sensitive optical microscopy, Raman scattering, and second-harmonic generation (SHG) microspectroscopy. The polycrystalline 1T'-MoTe2 phase with quasi-1D "Mo-Mo" zigzag chains showed anisotropic optical absorption, leading to a clear visualization of the lattice domains. On the other hand, 2H-MoTe2 lattice grains did not exhibit any discernible difference under polarized light illumination. The combined aforementioned microscopy techniques could be used as an easy-to-access and non-destructive tool for a quick and solid identification of intended lattice orientation development in industry-scale MoTe2 crystal manufacturing.

Recent Progress of Janus 2D Transition Metal Chalcogenides: From Theory to Experiments

Since the discovery of graphene, 2D materials with various properties have gained increasing attention in fields such as novel electronic, optic, spintronic, and valleytronic devices. As an important derivative of 2D materials, Janus 2D materials, such as Janus transition metal chalcogenides (TMDs), have become a research hot spot in recent years. Janus 2D materials with mirror asymmetry display novel properties, such as the Rashba effect and normal piezoelectric polarization, providing great promise for their application in sensors, actuators, and other electromechanical devices. Here, the current theoretical and experimental progresses made in the development of Janus 2D TMDs, including their structure and stability, electronic properties, fabrication, and the results of their characterization are reported. Finally, the future prospects for the further development of Janus 2D materials are considered.

Atomic Insights into Phase Evolution in Ternary Transition-Metal Dichalcogenides Nanostructures

Phase engineering through chemical modification can significantly alter the properties of transition-metal dichalcogenides, and allow the design of many novel electronic, photonic, and optoelectronics devices. The atomic-scale mechanism underlying such phase engineering is still intensively investigated but elusive. Here, advanced electron microscopy, combined with density functional theory calculations, is used to understand the phase evolution (hexagonal 2H→monoclinic T'→orthorhombic T_d ) in chemical vapor deposition grown Mo_1-x W _x Te₂ nanostructures. Atomic-resolution imaging and electron diffraction indicate that Mo_1-x W _x Te₂ nanostructures have two phases: the pure monoclinic phase in low W-concentrated (0 < x ≤ 10 at.%) samples, and the dual phase of the monoclinic and orthorhombic in high W-concentrated (10 < x < 90 at.%) samples. Such phase coexistence exists with coherent interfaces, mediated by a newly uncovered orthorhombic phase T_d '. T_d ', preserves the centrosymmetry of T' and provides the possible phase transition path for T'→T_d with low energy state. This work enriches the atomic-scale understanding of phase evolution and coexistence in multinary compounds, and paves the way for device applications of new transition-metal dichalcogenides phases and heterostructures.

Anisotropic Broadband Photoresponse of Layered Type-II Weyl Semimetal MoTe2

Photodetectors based on Weyl semimetal promise extreme performance in terms of highly sensitive, broadband and self-powered operation owing to its extraordinary material properties. Layered Type-II Weyl semimetal that break Lorentz invariance can be further integrated with other two-dimensional materials to form van der Waals heterostructures and realize multiple functionalities inheriting the advantages of other two-dimensional materials. Herein, we report the realization of a broadband self-powered photodetector based on Type-II Weyl semimetal T_d -MoTe₂ . The prototype metal-MoTe₂ -metal photodetector exhibits a responsivity of 0.40 mA W^-1 and specific directivity of 1.07 × 10⁸ Jones with 43 μs response time at 532 nm. Broadband responses from 532 nm to 10.6 μm are experimentally tested with a potential detection range extendable to far-infrared and terahertz. Furthermore, we identify the response of the detector is polarization angle sensitive due to the anisotropic response of MoTe₂ . The anisotropy is found to be wavelength dependent, and the degree of anisotropy increases as the excitation wavelength gets closer to the Weyl nodes. In addition, with power and temperature dependent photoresponse measurements, the photocurrent generation mechanisms are investigated. Our results suggest this emerging class of materials can be harnessed for broadband angle sensitive, self-powered photodetection with decent responsivities.