Elastic and electronic tuning of magnetoresistance in MoTe2

Probing the Strain Direction-Dependent Nonmonotonic Optical Bandgap Modulation of Layered Violet Phosphorus

Recent theoretical investigations have well-predicted strain-induced nonmonotonic optical band gap variations in low-dimensional materials. However, few two-dimensional (2D) materials are experimentally confirmed to exhibit nonmonotonic optical band gap variation under varying strain. Here, a strain-induced nonmonotonic optical bandgap variation in violet phosphorus (VP) nanosheets is observed, as evidenced by photoluminescence spectroscopy, which is reported in a few other 2D materials in knowledge. The optical bandgap variations are characterized to show the modulation rates of 41 and -24 meV/% with compression and tensile strains, respectively. Remarkably, first-principle calculations predict and clarify the nonmonotonic modulation accurately, highlighting its relationship with the strain direction-dependent asymmetric distribution of conduction band minimum wavefunctions, demonstrating that this unique nonmonotonic optical bandgap modulation is determined by the distinctive crystal structure of VP. This work provides a deep insight into the design of 2D materials toward optoelectronic and photoelectrochemical applications via strain engineering.

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.

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.

Wrinkles, Ridges, Miura-Ori, and Moiré Patterns in MoSe2 Using Neural Networks

Effects of lateral compression on out-of-plane deformation of two-dimensional MoSe₂ layers are investigated. A MoSe₂ monolayer develops periodic wrinkles under uniaxial compression and Miura-Ori patterns under biaxial compression. When a flat MoSe₂ monolayer is placed on top of a wrinkled MoSe₂ layer, the van der Waals (vdW) interaction transforms wrinkles into ridges and generates mixed 2H and 1T phases and chain-like defects. Under a biaxial strain, the vdW interaction induces regions of Miura-Ori patterns in bilayers. Strained systems analyzed using a convolutional neural network show that the compressed system consists of semiconducting 2H and metallic 1T phases. The energetics, mechanical response, defect structure, and dynamics are analyzed as bilayers undergo wrinkle-ridge transformations under uniaxial compression and moiré transformations under biaxial compression. Our results indicate that in-plane compression can induce self-assembly of out-of-plane metasurfaces with controllable semiconducting and metallic phases and moiré patterns with unique optoelectronic properties.

Extraordinary Nonlinear Optical Interaction from Strained Nanostructures in van der Waals CuInP2S6

Local strain engineering and structural modification of 2D materials furnish benevolent control over their optoelectronic properties and provide an exciting approach to tune light-matter interaction in layered materials. Application of strain at the nanoscale is typically obtained through permanently deformed nanostructures such as nanowrinkles, which yield large band gap modulation, photoluminescence enhancement, and surface potential. Ultrathin transition metal dichalcogenides (TMDs) have been greatly analyzed for such purposes. Herein, we extend strain-induced nanoengineering to an emerging 2D material, CuInP₂S₆ (CIPS), and visualize extraordinary control over nonlinear light-matter interaction. Wrinkle nanostructures exhibit ∼160-fold enhancement in second harmonic generation (SHG) compared to unstrained regions, which is additionally influenced by a change in the dielectric environment. The SHG enhancement was significantly modulated by the percentage of applied strain which was numerically estimated. Furthermore, polarization-dependent SHG revealed quenching and enhancement in the parallel and perpendicular directions, respectively, due to the direction of the compressive vector. Our work provides an important advancement in controlling optoelectronic properties beyond TMDs for imminent applications in flexible electronics.

Magnetoresistance of Ni/WSe2/Ni junctions: robustness against the thickness of WSe2

Magnetoresistive materials are vital for the development of storage devices. Using the first-principles transport simulations with nonequilibrium Green's function calculation, we investigate the magnetoresistive properties of Ni/WSe₂/Ni junctions withm-layers of WSe₂(m= 1, 2, ⋯ ,6). Form≤ 2, the junctions are metallic inspite of the semiconducting nature of few-layer WSe₂. However, the junctions exhibit transport gaps form> 2. Interestingly, magnetoresistance of the junctions stays around 6% when there are more than one layer of WSe₂in the center, which is closely related to the robust spacial variation of interfacial properties and can be attributed to no spin flipping in tunneling regions. Our results suggest that Ni/WSe₂/Ni junctions have a robust magnetoresistance which is insensitive to the thickness of WSe₂.

Chalcogen···Chalcogen Bonding in Molybdenum Disulfide, Molybdenum Diselenide and Molybdenum Ditelluride Dimers as Prototypes for a Basic Understanding of the Local Interfacial Chemical Bonding Environment in 2D Layered Transition Metal Dichalcogenides

An attempt was made, using computational methods, to understand whether the intermolecular interactions in the dimers of molybdenum dichalcogenides MoCh2 (Ch = chalcogen, element of group 16, especially S, Se and Te) and similar mixed-chalcogenide derivatives resemble the room temperature experimentally observed interactions in the interfacial regions of molybdenites and their other mixed-chalcogen derivatives. To this end, MP2(Full)/def2-TVZPPD level electronic structure calculations on nine dimer systems, including (MoCh2)2 and (MoChCh′2)2 (Ch, Ch′ = S, Se and Te), were carried out not only to demonstrate the energetic stability of these systems in the gas phase, but also to reproduce the intermolecular geometrical properties that resemble the interfacial geometries of 2D layered MoCh2 systems reported in the crystalline phase. Among the six DFT functionals (single and double hybrids) benchmarked against MP2(full), it was found that the double hybrid functional B2PLYPD3 has some ability to reproduce the intermolecular geometries and binding energies. The intermolecular geometries and binding energies of all nine dimers are discussed, together with the charge density topological aspects of the chemical bonding interactions that emerge from the application of the quantum theory of atoms in molecules (QTAIM), the isosurface topology of the reduced density gradient noncovalent index, interaction region indicator and independent gradient model (IGM) approaches. While the electrostatic surface potential model fails to explain the origin of the S···S interaction in the (MoS2)2 dimer, we show that the intermolecular bonding interactions in all nine dimers examined are a result of hyperconjugative charge transfer delocalizations between the lone-pair on (Ch/Ch′) and/or the π-orbitals of a Mo–Ch/Ch′ bond of one monomer and the dπ* anti-bonding orbitals of the same Mo–Ch/Ch′ bond in the second monomer during dimer formation, and vice versa. The HOMO–LUMO gaps calculated with the MN12-L functional were 0.9, 1.0, and 1.1 eV for MoTe2, MoSe2 and MoS2, respectively, which match very well with the solid-state theoretical (SCAN-rVV10)/experimental band gaps of 0.75/0.88, 0.90/1.09 and 0.93/1.23 eV of the corresponding systems, respectively. We observed that the gas phase dimers examined are perhaps prototypical for a basic understanding of the interfacial/inter-layer interactions in molybdenum-based dichalcogenides and their derivatives.

Exceptional Elasticity of Microscale Constrained MoS2 Domes

The outstanding mechanical performances of two-dimensional (2D) materials make them appealing for the emerging fields of flextronics and straintronics. However, their manufacturing and integration in 2D crystal-based devices rely on a thorough knowledge of their hardness, elasticity, and interface mechanics. Here, we investigate the elasticity of highly strained monolayer-thick MoS₂ membranes, in the shape of micrometer-sized domes, by atomic force microscopy (AFM)-based nanoindentation experiments. A dome's crushing procedure is performed to induce a local re-adhesion of the dome's membrane to the bulk substrate under the AFM tip's load. It is worth noting that no breakage, damage, or variation in size and shape are recorded in 95% of the crushed domes upon unloading. Furthermore, such a procedure paves the way to address quantitatively the extent of the van der Waals interlayer interaction and adhesion of MoS₂ by studying pull-in instabilities and hysteresis of the loading-unloading cycles. The fundamental role and advantage of using a superimposed dome's constraint are also discussed.

Strain-driven phase transition and spin polarization of Re-doped transition-metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDCs) are promising in spintronics due to their spin-orbit coupling, but their intrinsic non-magnetic properties limit their further development. Here, we focus on the energy landscapes of TMDC (MX2, M = Mo, W and X = S, Se, Te) monolayers by rhenium (Re) substitution doping under axial strains, which controllably drive 1H ↔ 1Td structural transformations. For both 1H and 1Td phases without strain, Re-doped TMDCs have an n-type character and are non-magnetic, but the tensile strain could effectively induce and modulate the magnetism. Specifically, 1H-Re0.5Mo0.5S2 gets a maximum magnetic moment of 0.69 μB at a 6% uniaxial tensile strain along the armchair direction; along the zigzag direction it exhibits a significant magnetic moment (0.49 μB) at a 2.04% uniaxial tensile strain but then exhibits no magnetism in the range of [5.10%, 7.14%]. By contrast, for 1Td-Re0.5Mo0.5S2 a critical uniaxial tensile strain along the zigzag direction reaches up to ∼9.18%, and a smaller uniaxial tensile strain (∼5.10%) along the zigzag direction is needed to induce the magnetism in 1Td-Re0.5M0.5Te2. The results reveal that the magnetism of Re-doped TMDCs could be effectively induced and modulated by the tensile strain, suggesting that strain engineering could have significant applications in doped TMDCs.

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T') phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T') phase. Bottom-up synthesis of materials in the 1T(1T') phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T') phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control.

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.

Polar and phase domain walls with conducting interfacial states in a Weyl semimetal MoTe2

Much of the dramatic growth in research on topological materials has focused on topologically protected surface states. While the domain walls of topological materials such as Weyl semimetals with broken inversion or time-reversal symmetry can provide a hunting ground for exploring topological interfacial states, such investigations have received little attention to date. Here, utilizing in-situ cryogenic transmission electron microscopy combined with first-principles calculations, we discover intriguing domain-wall structures in MoTe2, both between polar variants of the low-temperature(T) Weyl phase, and between this and the high-T higher-order topological phase. We demonstrate how polar domain walls can be manipulated with electron beams and show that phase domain walls tend to form superlattice-like structures along the c axis. Scanning tunneling microscopy indicates a possible signature of a conducting hinge state at phase domain walls. Our results open avenues for investigating topological interfacial states and unveiling multifunctional aspects of domain walls in topological materials.

Anomalous Photothermoelectric Transport Due to Anisotropic Energy Dispersion in WTe2

Band structures are vital in determining the electronic properties of materials. Recently, the two-dimensional (2D) semimetallic transition metal tellurides (WTe₂ and MoTe₂) have sparked broad research interest because of their elliptical or open Fermi surface, making distinct from the conventional 2D materials. In this study, we demonstrate a centrosymmetric photothermoelectric voltage distribution in WTe₂ nanoflakes, which has not been observed in common 2D materials such as graphene and MoS₂. Our theoretical model shows the anomalous photothermoelectric effect arises from an anisotropic energy dispersion and micrometer-scale hot carrier diffusion length of WTe₂. Further, our results are more consistent with the anisotropic tilt direction of energy dispersion being aligned to the b-axis rather than the a-axis of the WTe₂ crystal, which is consistent with the previous first-principle calculations as well as magneto-transport experiments. Our work shows the photothermoelectric current is strongly confined to the anisotropic direction of the energy dispersion in WTe₂, which opens an avenue for interesting electro-optic applications such as electron beam collimation and electron lenses.

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.

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Here, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

Spin-Resolved Electronic Response to the Phase Transition in MoTe_{2}

The semimetal MoTe_{2} is studied by spin- and angle-resolved photoemission spectroscopy across the centrosymmetry-breaking structural transition temperature of the bulk. A three-dimensional spin-texture is observed in the bulk Fermi surface in the low temperature, noncentrosymmetric phase that is consistent with first-principles calculations. The spin texture and two types of surface Fermi arc are not completely suppressed above the bulk transition temperature. The lifetimes of quasiparticles forming the Fermi arcs depend on thermal history and lengthen considerably upon cooling toward the bulk structural transition. The results indicate that a new form of polar instability exists near the surface when the bulk is largely in a centrosymmetric phase.

Origin of extremely large magnetoresistance in the candidate type-II Weyl semimetal MoTe2-x

The recent observation of extremely large magnetoresistance (MR) in the transition-metal dichalcogenide MoTe₂ has attracted considerable interest due to its potential technological applications as well as its relationship with novel electronic states predicted for a candidate type-II Weyl semimetal. In order to understand the origin of the MR, the electronic structure of MoTe_2−x (x = 0.08) is systematically tuned by application of pressure and probed via its Hall and longitudinal conductivities. With increasing pressure, a monoclinic-to-orthorhombic (1 T′ to T_d) structural phase transition temperature (T*) gradually decreases from 210 K at 1 bar to 58 K at 1.1 GPa, and there is no anomaly associated with the phase transition at 1.4 GPa, indicating that a T = 0 K quantum phase transition occurs at a critical pressure (P_c) between 1.1 and 1.4 GPa. The large MR observed at 1 bar is suppressed with increasing pressure and is almost saturated at 100% for P > P_c. The dependence on magnetic field of the Hall and longitudinal conductivities of MoTe_2−x shows that a pair of electron and hole bands are important in the low-pressure T_d phase, while another pair of electron and hole bands are additionally required in the high-pressure 1 T′ phase. The MR peaks at a characteristic hole-to-electron concentration ratio (n_c) and is sharply suppressed when the ratio deviates from n_c within the T_d phase. These results establish the comprehensive temperature-pressure phase diagram of MoTe_2−x and underscore that its MR originates from balanced electron-hole carrier concentrations.