Anomalous in-plane anisotropic Raman response of monoclinic semimetal 1 T´-MoTe 2

Strain-Induced 2H to 1T' Phase Transition in Suspended MoTe2 Using Electric Double Layer Gating

MoTe2 can be converted from the semiconducting (2H) phase to the semimetallic (1T') phase by several stimuli including heat, electrochemical doping, and strain. This type of phase transition, if reversible and gate-controlled, could be useful for low-power memory and logic. In this work, a gate-controlled and fully reversible 2H to 1T' phase transition is demonstrated via strain in few-layer suspended MoTe2 field effect transistors. Strain is applied by the electric double layer gating of a suspended channel using a single ion conducting solid polymer electrolyte. The phase transition is confirmed by simultaneous electrical transport and Raman spectroscopy. The out-of-plane vibration peak (A1g)─a signature of the 1T' phase─is observed when VSG ≥ 2.5 V. Further, a redshift in the in-plane vibration mode (E2g) is detected, which is a characteristic of a strain-induced phonon shift. Based on the magnitude of the shift, strain is estimated to be 0.2-0.3% by density functional theory. Electrically, the temperature coefficient of resistance transitions from negative to positive at VSG ≥ 2 V, confirming the transition from semiconducting to metallic. The approach to gate-controlled, reversible straining presented here can be extended to strain other two-dimensional materials, explore fundamental material properties, and introduce electronic device functionalities.

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.

Thermodynamics and Kinetics in Anisotropic Growth of One-Dimensional Midentropy Nanoribbons

One-dimensional (1D) materials demonstrate anisotropic in-plane physical properties that enable a wide range of functionalities in electronics, photonics, valleytronics, optoelectronics, and catalysis. Here, we undertake an in-depth study of the growth mechanism for equimolar midentropy alloy of (NbTaTi)_0.33S₃ nanoribbons as a model system for 1D transition metal trichalcogenide structures. To understand the thermodynamic and kinetic effects in the growth process, the energetically preferred phases at different synthesis temperatures and times are investigated, and the phase evolution is inspected at a sequence of growth steps. It is uncovered that the dynamics of the growth process occurs at four different stages via preferential incorporation of chemical species at high-surface-energy facets. Also, a sequence of temperature and time dependent nonuniform to uniform phase evolutions has emerged in the composition and structure of (NbTaTi)_0.33S₃ which is described based on an anisotropic vapor-solid (V-S) mechanism. Furthermore, direct evidence for the 3D structure of the charge density wave (CDW) phase (width less than 100 nm) is provided by three-dimensional electron diffraction (3DED) in individual nanoribbons at cryogenic temperature, and detailed comparisons are made between the phases obtained before and after CDW transformation. This study provides important fundamental information for the design and synthesis of future 1D alloy structures.

Density Functional Theory Approximations and Experimental Investigations on Co1-xMoxTe2 Alloy Electrocatalysts Tuning the Overall Water Splitting Reactions

Understanding the relationship between electronic structure, surface characteristic, and reaction process of a catalyst helps to architect proficient electrodes for sustainable energy development. The highly active and stable catalysts made of earth-abundant materials provide a great endeavor toward green hydrogen production. Herein, we assembled the Co_1-xMo_xTe (x = 0-1) nanoarray structures into a bifunctional electrocatalyst to achieve high-performance hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) kinetics under alkaline conditions. The designed Co_0.75Mo_0.25Te and Co_0.50Mo_0.50 electrocatalysts require minimum overpotential and Tafel slope for high-efficacy HER and OER, respectively. Furthermore, we constructed a Co_0.50Mo_0.50Te₂∥Co_0.50Mo_0.50Te₂ device for overall water splitting with an overpotential of 1.39 V to achieve a current density of 10 mA cm^-2, which is superior to noble electrocatalyst performance, with stable reaction throughout the 50 h continuous process. Density functional theory approximations and Gibbs free energy calculations validate the enhanced water splitting reaction catalyzed by the Co_0.50Mo_0.50Te₂ nanoarrays. The partial replacement of Co atoms with Mo atoms in the Co_0.50Mo_0.50Te₂ structure substantially enhances the water electrolysis kinetics through the synergistic effects between the combined metal atoms and the bonded chalcogen.

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.

Recent advances in 2D TMD circular photo-galvanic effects

Two-dimensional (2D) layered semiconductors are appealing materials for high-specific-power photovoltaic systems due to their unique optoelectronic properties. The 2D materials can be naturally thin, and their properties can be altered in a variety of ways. Therefore, these materials may be used to develop high-performance opto-spintronic and photovoltaic devices. The most recent and promising strategies were used to induce circular photo-galvanic effects (CPGEs) in 2D TMD materials with broken inversion symmetry. The majority of quantum devices were manufactured by mechanical exfoliation to investigate the electrical behavior of ultrathin 2D materials. The investigation of CPGEs in 2D materials could enable the exploration of spin-polarized optoelectronics to produce more energy-efficient computing systems. The current research on nanomaterial-based materials paves the way for developing materials to store, manipulate, and transmit information with better performance. Finally, this study concludes by summarizing the current challenges and prospects.

Raman Optical Activity of 1T-TaS2

Measurements of optical activity can be readily performed in transparent matter by means of a rotation of transmitted light polarization. In the case of opaque bulk materials, such measurements cannot be performed, making it difficult to assess possible chiral properties. In this work, we present full angular polarization dependencies of the Raman modes of bulk 1T-TaS₂, which has recently been suggested to have chiral properties after pulsed laser excitation. We found that a mechanical rotation of the sample does not alter polarization-resolved Raman spectra, which can only be explained by introducing an antisymmetric Raman tensor, frequently used to describe Raman optical activity (ROA). Raman spectra obtained under circularly polarized excitation demonstrate that 1T-TaS₂ indeed shows ROA, providing strong evidence that 1T-TaS₂ is chiral under the used conditions of laser excitation. Our results suggest that ROA may be used as a universal tool to study chiral properties of quantum materials.

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.

Controlled Synthesis of MoxW1-xTe2 Atomic Layers with Emergent Quantum States

Recently, new states of matter like superconducting or topological quantum states were found in transition metal dichalcogenides (TMDs) and manifested themselves in a series of exotic physical behaviors. Such phenomena have been demonstrated to exist in a series of transition metal tellurides including MoTe₂, WTe₂, and alloyed Mo_xW_1-xTe_2. However, the behaviors in the alloy system have been rarely addressed due to their difficulty in obtaining atomic layers with controlled composition, albeit the alloy offers a great platform to tune the quantum states. Here, we report a facile CVD method to synthesize the Mo_xW_1-xTe₂ with controllable thickness and chemical composition ratios. The atomic structure of a monolayer Mo_xW_1-xTe₂ alloy was experimentally confirmed by scanning transmission electron microscopy. Importantly, two different transport behaviors including superconducting and Weyl semimetal states were observed in Mo-rich Mo_0.8W_0.2Te₂ and W-rich Mo_0.2W_0.8Te₂ samples, respectively. Our results show that the electrical properties of Mo_xW_1-xTe₂ can be tuned by controlling the chemical composition, demonstrating our controllable CVD growth method is an efficient strategy to manipulate the physical properties of TMDCs. Meanwhile, it provides a perspective on further comprehension and sheds light on the design of devices with topological multicomponent TMDC materials.

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.

A Library of Atomically Thin 2D Materials Featuring the Conductive-Point Resistive Switching Phenomenon

Non-volatile resistive switching (NVRS) is a widely available effect in transitional metal oxides, colloquially known as memristors, and of broad interest for memory technology and neuromorphic computing. Until recently, NVRS was not known in other transitional metal dichalcogenides (TMDs), an important material class owing to their atomic thinness enabling the ultimate dimensional scaling. Here, various monolayer or few-layer 2D materials are presented in the conventional vertical structure that exhibit NVRS, including TMDs (MX₂ , M = transitional metal, e.g., Mo, W, Re, Sn, or Pt; X = chalcogen, e.g., S, Se, or Te), TMD heterostructure (WS₂ /MoS₂ ), and an atomically thin insulator (h-BN). These results indicate the universality of the phenomenon in 2D non-conductive materials, and feature low switching voltage, large ON/OFF ratio, and forming-free characteristic. A dissociation-diffusion-adsorption model is proposed, attributing the enhanced conductance to metal atoms/ions adsorption into intrinsic vacancies, a conductive-point mechanism supported by first-principle calculations and scanning tunneling microscopy characterizations. The results motivate further research in the understanding and applications of defects in 2D materials.

Symmetry induced phonon renormalization in few layers of 2H-MoTe2 transistors: Raman and first-principles studies

Understanding of electron-phonon coupling (EPC) in two-dimensional (2D) materials manifesting as phonon renormalization is essential to their possible applications in nanoelectronics. Here we report in situ Raman measurements of electrochemically top-gated 2, 3 and 7 layered 2H-MoTe₂ channel based field-effect transistors. While the [Formula: see text] and B_2g phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (p) up to ∼2.3 × 10¹³/cm², A_1g shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the [Formula: see text] mode on the number of layers in these 2D crystals confirms that hole doping occurs primarily in the top two layers, in agreement with recent predictions. We present first-principles density functional theory analysis of bilayer MoTe₂ that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with [Formula: see text] or B_2g modes as compared with the A_1g mode originates from the in-plane orbital character and symmetry of the states at valence band maximum. The contrast between the manifestation of EPC in monolayer MoS₂ and those observed here in a few-layered MoTe₂ demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.

Atomic-Precision Repair of a Few-Layer 2H-MoTe2 Thin Film by Phase Transition and Recrystallization Induced by a Heterophase Interface

2D semiconductors have emerged as promising candidates for post-silicon nanoelectronics, owing to their unique properties and atomic thickness. However, in the handling of 2D material, various forms of macroscopic damage, such as cracks, wrinkles, and scratches, etc., are usually introduced, which cause adverse effects on the material properties and device performance. Repairing such macroscopic damage is crucial for improving device performance and reliability, especially for large-scale 2D device arrays. Here, a method is demonstrated repair damage to few-layer 2H-MoTe₂ films with atomic precision, and its mechanism is elucidated. The repaired 2H-MoTe₂ inherits the lattice orientation of the adjacent original 2H-MoTe₂ , thereby forming an atomically perfect lattice at the repaired interface. The time-evolution experiments show that the interface between the 2H- and early formed 1T'-MoTe₂ plays an important role in the subsequent phase transition and recrystallization. Electrical measurements on the original MoTe₂ , repaired MoTe₂ , and cross-interface regions show unobservable differences, indicating that the repaired MoTe₂ has the same electrical quality as the original one and the interface does not introduce extra scattering centers for carrier transport. The findings provide an effective strategy for macroscopic damage repair of few-layer 2H-MoTe₂ , which paves the way for its practical application in advanced electronics and optoelectronics.

Direct Observation of the Linear Dichroism Transition in Two-Dimensional Palladium Diselenide

The linear dichroism (LD) transition within anisotropic photonic materials displays promising prospects for applications in polarization-wavelength-selective detectors, optical switching, and optical communication. In conventional two-dimensional (2D) anisotropic materials, the LD is predominantly uniaxial over a broad spectrum of wavelengths and arises principally from the reduced symmetry of the materials. However, the LD transition behavior in crystalline 2D materials remains elusive. Here, we demonstrate the observation of a unique LD conversion phenomenon at a wavelength of 472 nm in palladium diselenide (PdSe₂) using polarization-resolved absorption spectroscopy. This material exhibits prominent anisotropic responses and a high absorption ratio of α_y/α_x ≈ 1.11 at 364 nm, 1.15 at 532 nm, and 0.84 at 633 nm. We propose that this abnormal LD conversion behavior originates from the forceful localization rules at different parallel energy bands that exist within this material. Furthermore, the robust periodicity of A_g and B_1g modes in polarization-resolved Raman spectroscopy is in good agreement with the theoretical structure symmetry analysis. This indicates the strong intrinsic LD effect in the anisotropic nature of PdSe₂, which offers a macrolevel determination of crystal orientations. Such unique LD conversion features, in combination with strong LD effects, enable the air-stable PdSe₂ to be a potential candidate for technological innovations in multispectral imaging, sensing, and polarization-sensitive and wavelength-controllable photoelectronic applications.

Biaxial strain modulated the electronic structure of hydrogenated 2D tetragonal silicene

Silicene-based materials have attracted great attention due to their easier incorporation into silicon-based devices and components. In addition to the reported hydrogenated 2D tetragonal silicene (γ-SiH), we propose two stable atomic configurations of hydrogenated 2D tetragonal silicene (α-SiH and β-SiH) based on first-principles calculation. The calculated results indicate hydrogenation can effectively open the band gap of 2D tetragonal silicene, α-SiH is a semiconductor with a direct band gap of 2.436 eV whereas β-SiH is indirect band gap of 2.286 eV. We also find that the electronic band structure of α-SiH, β-SiH and γ-SiH can be modulated via biaxial strain. By applying biaxial strain in the range of -10% to 12%, the band gap of α-SiH, β-SiH and γ-SiH can be tuned in a range of 1.732-2.585 eV. Furthermore, direct-indirect or indirect-direct transition can be induced under biaxial strain, showing a high degree of flexibility in electronic band structure. The research not only broadens the diversity of hydrogenated 2D tetragonal silicenes, but also provides more possibilities of their applications in spintronic devices.

In-Plane Anisotropic Raman Response and Electrical Conductivity with Robust Electron-Photon and Electron-Phonon Interactions of Air Stable MoO2 Nanosheets

The monoclinic m-MoO₂ shows strong in-plane anisotropy that is essential to design anisotropic nanodevices. However, there is not yet detailed study on the anisotropy of m-MoO₂ nanostructures. Here, the in-plane anisotropic Raman spectra and electrical conductivity of single-crystal m-MoO₂ nanosheets were investigated systematically for the first time. Angle-resolved polarized Raman spectroscopy (ARPRS) shows distinct dependence on the excitation laser wavelength and sample thickness, demonstrating the robust wavelength-dependent optical absorption, electron-photon/electron-phonon interactions, and anisotropic phonon polarization in m-MoO₂ nanosheets. Such results agree well with the semiclassical Placzek model. Moreover, the angle-dependent electrical conductivity of starburst-like m-MoO₂ nanosheet devices shows a strong anisotropy with a conductivity ratio (σ_max/σ_min) of up to 10.1, which is the largest value in the previously reported 2D materials. This work underscores the importance of understanding the in-plane anisotropic light-matter interactions for the application of m-MoO₂ nanosheets in anisotropic plasmonics and artificial synapse.

Millimeter-Scale Single-Crystalline Semiconducting MoTe2 via Solid-to-Solid Phase Transformation

Among the Mo- and W-based two-dimensional (2D) transition metal dichalcogenides, MoTe₂ is particularly interesting for phase-engineering applications, because it has the smallest free energy difference between the semiconducting 2H phase and metallic 1T' phase. In this work, we reveal that, under the proper circumstance, Mo and Te atoms can rearrange themselves to transform from a polycrystalline 1T' phase into a single-crystalline 2H phase in a large scale. We manifest the mechanisms of the solid-to-solid transformation by conducting density functional theory calculations, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The phase transformation is well described by the time-temperature-transformation diagram. By optimizing the kinetic rates of nucleation and crystal growth, we have synthesized a single-crystalline 2H-MoTe₂ domain with a diameter of 2.34 mm, a centimeter-scale 2H-MoTe₂ thin film with a domain size up to several hundred micrometers, and a seamless 1T'-2H MoTe₂ coplanar homojunction. The 1T'-2H MoTe₂ homojunction provides an elegant solution for ohmic contact of 2D semiconductors. The controlled solid-to-solid phase transformation in 2D limit provides a new route to realize wafer-scale single-crystalline 2D semiconductor and coplanar heterostructure for 2D circuitry.

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.

Tunable Out-of-Plane Piezoelectricity in Thin-Layered MoTe2 by Surface Corrugation-Mediated Flexoelectricity

Piezoelectricity crystallographically exists only in the in-plane direction in two-dimensional transition metal dichalcogenides. Here, we demonstrated flexoelectricity-tunable out-of-plane piezoelectricity in semiconducting 2H-MoTe₂ flakes by creating surface corrugation. In particular, the strong out-of-plane piezoelectricity and its spatial variation depending on local flexoelectricity was observed even though crystallographically there exists only in-plane piezoelectricity. Surface corrugation-mediated flexoelectricity tuning can be applied to other two-dimensional or thin-layered materials and, furthermore, the results could provide useful information on the interweaving nature between mechanical stimulus and electric dipole in low-dimensional materials.