Transition Metal Monochalcogenides Research Articles

Van der Waals Engineering of Charge Density Waves in One-Dimensional Nb6Te6 Nanowires.

One-dimensional (1D) systems have played a crucial role in the development of fundamental physics and practical applications. Recently, transition metal monochalcogenide (TMM) wires based on molybdenum (Mo) and tungsten (W) have emerged as promising platforms for investigating 1D physics in pure van der Waals (vdW) platforms. Here, we report on the bottom-up fabrication of Nb6Te6 wires down to the single-wire limit. The unique properties of Nb6Te6 single wire enable the realization of 1D charge density wave (CDW) phases in an isolated single TMM wire. Moreover, we revealed the appealing regulation of 1D CDW orders by van der Waals interactions at either the 1D-2D interface (i.e., rotation of a single wire along its wire axis) or the 1D-1D interface. Two rotation angles (30° and 0°) give rise to 3 × 1 and zigzag chain CDW morphologies, respectively, which exhibit pronounced differences in atomic displacement by a factor of 2. The interwire vdW coupling overwhelms its counterpart at the 1D-2D interface, thus locking the rotation angle (at 0°) as well as the interwire atomic registries. In contrast, interestingly, the phases of the charge oscillations are independent of the adjacent wires. The ability to tailor 1D charge orders provides a crucial addition to the toll set of vdW integrations beyond two-dimensional materials.

Strain-induced superstructure evolution and Te vacancy accumulation in AgTe monolayer grown on Ag(111) substrate

Two-dimensional (2D) materials have attracted increasing attention for frontier science and industrial applications in the past few decades. As a 2D materials with honeycomb lattice, AgTe monolayer grown on Ag (111) substrate belongs to the transition metal monochalcogenides (TMMs) family possessing various superstructures and exotic electronic structure. In this report, the AgTe monolayer superstructure is mainly studied using scanning tunneling microscope/spectroscopy (STM/S) measurements. With the increase of Te deposition, the planar structure of the AgTe monolayer becomes the one-dimensional (1D) moiré pattern structure, which transformation is related to the change of strain combing with theoretical analysis. When there is an isotropic strain on the surface, the AgTe monolayer is a flat honeycomb structure. In contrast, the lattice structural strain is released under a uniaxial strain as the AgTe monolayer is transformed into a 1D moiré pattern structure. In addition, Te vacancies in the AgTe monolayer are also affected by the synergistic effect of strain and Ag (111) substrate, and the concentration of Te vacancy changes significantly with the strain transformation. Moreover, both experimental statistics and theoretical calculations verify that Te vacancy tend to appear in the bright ridge (BR) region of the AgTe monolayer with a 1D moiré pattern structure, which is also the result of strain modulation. Our findings open up a new thought for strain transformation control of 2D materials driven by elemental concentration.

Dual Higgs modes entangled into a soliton lattice in CuTe

Recently discovered Higgs particle is a key element in the standard model of elementary particles and its analogue in materials, massive Higgs mode, has elucidated intriguing collective phenomena in a wide range of materials with spontaneous symmetry breaking such as antiferromagnets, cold atoms, superconductors, superfluids, and charge density waves (CDW). As a straightforward extension beyond the standard model, multiple Higgs particles have been considered theoretically but not yet for Higgs modes. Here, we report the real-space observations, which suggest two Higgs modes coupled together with a soliton lattice in a solid. Our scanning tunneling microscopy reveals the 1D CDW state of an anisotropic transition metal monochalcogenide crystal CuTe is composed of two distinct but degenerate CDW structures by the layer inversion symmetry broken. More importantly, the amplitudes of each CDW structure oscillate in an out-of-phase fashion to result in a regular array of alternating domains with repeating phase-shift domain walls. This unusual finding is explained by the extra degeneracy in CDWs within the standard Landau theory of the free energy. The multiple and entangled Higgs modes demonstrate how novel collective modes can emerge in systems with distinct symmetries broken simultaneously.

Ionic liquid gating induced self-intercalation of transition metal chalcogenides

Ionic liquids provide versatile pathways for controlling the structures and properties of quantum materials. Previous studies have reported electrostatic gating of nanometer-thick flakes leading to emergent superconductivity, insertion or extraction of protons and oxygen ions in perovskite oxide films enabling the control of different phases and material properties, and intercalation of large-sized organic cations into layered crystals giving access to tailored superconductivity. Here, we report an ionic-liquid gating method to form three-dimensional transition metal monochalcogenides (TMMCs) by driving the metals dissolved from layered transition metal dichalcogenides (TMDCs) into the van der Waals gap. We demonstrate the successful self-intercalation of PdTe2 and NiTe2, turning them into high-quality PdTe and NiTe single crystals, respectively. Moreover, the monochalcogenides exhibit distinctive properties from dichalcogenides. For instance, the self-intercalation of PdTe2 leads to the emergence of superconductivity in PdTe. Our work provides a synthesis pathway for TMMCs by means of ionic liquid gating driven self-intercalation.

Non-centrosymmetric Weyl semimetal state and strain effect in the twisted-brick phase transition metal monochalcogenides.

Weyl semimetals are a class of gapless electronic excitation topological quantum materials upon breaking time-reversal or inversion symmetry. Here, we demonstrate the existence of the Weyl semimetal state in the non-centrosymmetric twisted-brick phase MoTe theoretically. The topological properties and strain effects of MoTe have been systematically studied based on first-principles calculations and the Wannier-based tight-binding method. In the absence of spin-orbit coupling (SOC), MoTe exhibits gapless nodal loop states related to the mirror reflection symmetry. When the SOC is turned on, the two nodal loops split into 22 pairs of Weyl points (WPs) with opposite chirality. When the effect of uniaxial (εz) strain is taken into account, the Weyl semimetal phase of MoTe shows great robustness and striking tunable topological strength. In particular, the total number of WPs changes significantly under strain. MoTe under +4% and +8% uniaxial strains have only four pairs of WPs with a relatively large separation in momentum space. These results show that MoTe under weak strain is a promising partly ideal type I Weyl semimetal candidate, while the isolog structure WTe both opens a direct gap with and without SOC, showing a compensated semimetal state.

Electronic properties of monolayer copper selenide with one-dimensional moiré patterns

Strain engineering is a vital way to manipulate the electronic properties of two-dimensional (2D) materials. As a typical representative of transition metal mono-chalcogenides (TMMs), a honeycomb CuSe monolayer features with one-dimensional (1D) moiré patterns owing to the uniaxial strain along one of three equivalent orientations of Cu(111) substrates. Here, by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/S) experiments and density functional theory (DFT) calculations, we systematically investigate the electronic properties of the strained CuSe monolayer on the Cu(111) substrate. Our results show the semiconducting feature of CuSe monolayer with a band gap of 1.28 eV and the 1D periodical modulation of electronic properties by the 1D moiré patterns. Except for the uniaxially strained CuSe monolayer, we observed domain boundary and line defects in the CuSe monolayer, where the biaxial-strain and strain-free conditions can be investigated respectively. STS measurements for the three different strain regions show that the first peak in conduction band will move downward with the increasing strain. DFT calculations based on the three CuSe atomic models with different strain inside reproduced the peak movement. The present findings not only enrich the fundamental comprehension toward the influence of strain on electronic properties at 2D limit, but also offer the benchmark for the development of 2D semiconductor materials.

Time-reversal symmetry breaking in the superconducting state of ScS

We have studied the electronic properties of ScS, a transition-metal monochalcogenide with a rocksalt crystal structure, using magnetization, specific heat, transport, and muon spin rotation/relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements. All measurements confirm the bulk superconductivity in ScS with a transition temperature of ${T}_{C}=5.1(5)$ K. Specific heat together with transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements indicate a full gap, while our zero-field $\ensuremath{\mu}\mathrm{SR}$ study reveals the presence of spontaneous static or quasistatic magnetic fields emerging when entering the superconducting state. We discuss various possible microscopic origins of the observed time-reversal symmetry breaking. As none of them can be readily reconciled with a conventional pairing mechanism, this introduces ScS as a candidate material for unconventional superconductivity.

Two-dimensional Dirac nodal line state protected against spin-orbit coupling in MoTe monolayer

We theoretically study a kind of protected double-nodal line state in two-dimensional (2D) transition metal monochalcogenide MoTe by using symmetry analysis and first-principles calculation. Among this kind of materials, there are various intriguing electronic properties, such as the long-range Coulomb interaction. The 2D layer four-fold degeneracy nodal line material is susceptible to the perturbation of strong spin-orbit coupling (SOC). We find that the Dirac degeneracy of two low-energy bands around the S point of the Brillouin zone is robust against SOC in P21/m-MoTe. The degeneracy nodal lines are protected by nonsymmorphic screw-rotational crystal symmetry and time-reversal symmetry. Due to the protection of symmetries, the Dirac double node line states under SOC are different from the accidental band touching. Furthermore, several isolog structures of P21/m-MoTe are also discussed. We find that the fourfold degeneracy nodal line state of MoSe also is robust against SOC, similar to MoTe.

Phase transition of a MoS2 monolayer through top layer desulfurization by He+ ion irradiation

Two-dimensional (2D) metal monochalcogenides have recently attracted significant interest following the extensive and intensive research into transition metal dichalcogenides (TMDs). However, the formation of transition metal monochalcogenide remains relatively unstudied. Here, we investigate the structural and electronic changes of the MoS2 monolayer by removing the top sulfur layer using low-energy He+ ion sputtering. As a result, the substoichiometric MoSx surface induces semiconducting to the metallic phase transition. Under ambient conditions, the oxidized MoSx surface restores a semiconducting state with narrowed bandgap, p-type conduction, or possibly a semimetallic state. Our findings provide an effective way to form and improve the functionality of Janus TMD monolayers.

Weyl semimetal states in transition metal monochalcogenide superlattices AX/BX (A, B = Cr, Mo, W, A ≠ B; X = Se, Te)

Based on first-principles calculations and the Wannier-function-based tight-binding method, we investigate the topological electronic properties of designed noncentrosymmetric transition metal monochalcogenide superlattices AX/BX (A, B = Cr, Mo, W, A ≠ B; X = Se, Te). Here, we mainly consider time-reversal symmetry (TRS) MoTe/WTe and MoSe/WSe and TRS broken CrTe/WTe and CrTe/MoTe, which are the Weyl and magnetic Weyl semimetals, respectively. The considered systems are nodal line semimetals with nodal lines on three mirror planes of the Brillouin zone of hexagonal lattice in the absence of spin–orbit coupling (SOC). When considering the SOC, each nodal line opens a gap except for some pairs of Weyl points (WPs) on the kz ≠ 0 planes and the number of WPs is material-dependent. We identify the termination-dependent Fermi arc connection patterns on the (001) and (00) surfaces for four Weyl semimetal superlattices AX/BX. These results provide a theoretical basis to realize the possible applications of these superlattice materials in future topological electronic devices.

Robust nanosecond laser passively Q-switched by tin selenide nanoflowers

We report the generation of the robust nanosecond Nd:YVO4 solid-state laser and Er3+:ZBLAN fiber laser passively Q-switched by the tin selenide (SnSe) nanoflowers saturable absorbers. The SnSe nanoflowers prepared via chemical precipitation method exhibit broadband nonlinear optical absorption performance, and can modulate the separate lasers to deliver stable nanosecond pulse ∼1 µm and ∼2.8 µm successfully. The experimental results show that the low-dimensional transition-metal monochalcogenides hold great potential for the broadband robust saturable absorbers, and may pave an avenue toward developing high-performance broadband optoelectronic devices.

Se-concentration dependent superstructure transformations of CuSe monolayer on Cu(111) substrate

As one of the most distinctive members of the monolayer transition metal monochalcogenides (TMM) family, the CuSe monolayer with a honeycomb structure has drawn much attention in the past few years. Depending on the Se concentration, the CuSe monolayer has two distinct superstructures on a Cu(111) substrate, a one dimensional (1D) moiré pattern, and two dimensional (2D) periodic nanopores. Here, we devise a strategy for simultaneous fabrication of the two superstructures of the CuSe monolayer on a Cu(111) substrate via artificially creating a density gradient of the Se concentration by an off-centered deposition method. At the boundary of the two superstructures, an intermediate state of the CuSe monolayer with a 2D hexagonal moiré pattern connected by six twisted petal-like stripes is observed. High-resolution scanning tunneling microscopy characterizations of three distinct CuSe monolayer superstructures demonstrate that the Se density can effectively modulate the stress in the CuSe monolayer formed by the lattice mismatch, driving the superstructure transformation from 1D moiré pattern through 2D intermediate states to 2D periodic nanopores. In addition, scanning tunneling spectroscopy measurements show that the intermediate state features a semiconducting behavior with a band gap of ∼2.0 eV. Our findings open up a new route for superstructure transformation control of 2D materials.

Observation of Exchange Bias in Antiferromagnetic Cr0.79Se Due to the Coexistence of Itinerant Weak Ferromagnetism at Low Temperatures.

We report on the structural, electrical transport, and magnetic properties of antiferromagnetic transition-metal monochalcogenide Cr0.79Se. Different from the existing off-stoichiometric compositions, Cr0.79Se is found to be synthesized into the same NiAs-type hexagonal crystal structure as that of CrSe. Resistivity data suggest Cr0.79Se to be a Fermi-liquid-type metal at low temperatures, while at intermediate temperatures, the resistivity depends sublinearly on the temperature. Eventually, at elevated temperatures, the rate of change of resistivity rapidly decreases with increasing temperature. Magnetic measurements suggest a transition from the paramagnetic phase to an antiferromagnetic phase at a Néel temperature of 225 K. Further reduction of the sample temperature results in the coexistence of weak ferromagnetism along with the antiferromagnetic phase below 100 K. As a result, below 100 K, we identify a significant exchange bias due to the interaction between the ferro- and antiferromagnetic phases. In addition, from temperature-dependent X-ray diffraction measurements, we observe that the NiAs-type structure is stable up to as high as 600 °C.

Oxidations of two-dimensional semiconductors: Fundamentals and applications

Since the discovery of graphene, two-dimensional (2D) semiconductors have been attracted intensive interest due to their unique properties. They have exhibited potential applications in next generation electronic and optoelectronic devices. However, most of the 2D semiconductor are known to suffer from the ambient oxidation which degrade the materials and therefore hinder us from the intrinsic materials’ properties and the optimized performance of devices. In this review, we summarize the recent progress on both fundamentals and applications of the oxidations of 2D semiconductors. We begin with the oxidation mechanisms in black phosphorus, transition metal dichalcogenides and transition metal monochalcogenides considering the factors such as oxygen, water, and light. Then we show the commonly employed passivation techniques. In the end, the emerging applications utilizing controlled oxidations will be introduced.

Two-Dimensional Gallium Sulfide Nanoflakes for UV-Selective Photoelectrochemical-type Photodetectors.

Two-dimensional (2D) transition-metal monochalcogenides have been recently predicted to be potential photo(electro)catalysts for water splitting and photoelectrochemical (PEC) reactions. Differently from the most established InSe, GaSe, GeSe, and many other monochalcogenides, bulk GaS has a large band gap of ∼2.5 eV, which increases up to more than 3.0 eV with decreasing its thickness due to quantum confinement effects. Therefore, 2D GaS fills the void between 2D small-band-gap semiconductors and insulators, resulting of interest for the realization of van der Waals type-I heterojunctions in photocatalysis, as well as the development of UV light-emitting diodes, quantum wells, and other optoelectronic devices. Based on theoretical calculations of the electronic structure of GaS as a function of layer number reported in the literature, we experimentally demonstrate, for the first time, the PEC properties of liquid-phase exfoliated GaS nanoflakes. Our results indicate that solution-processed 2D GaS-based PEC-type photodetectors outperform the corresponding solid-state photodetectors. In fact, the 2D morphology of the GaS flakes intrinsically minimizes the distance between the photogenerated charges and the surface area at which the redox reactions occur, limiting electron–hole recombination losses. The latter are instead deleterious for standard solid-state configurations. Consequently, PEC-type 2D GaS photodetectors display a relevant UV-selective photoresponse. In particular, they attain responsivities of 1.8 mA W–1 in 1 M H2SO4 [at 0.8 V vs reversible hydrogen electrode (RHE)], 4.6 mA W–1 in 1 M Na2SO4 (at 0.9 V vs RHE), and 6.8 mA W–1 in 1 M KOH (at 1.1. V vs RHE) under 275 nm illumination wavelength with an intensity of 1.3 mW cm–2. Beyond the photodetector application, 2D GaS-based PEC-type devices may find application in tandem solar PEC cells in combination with other visible-sensitive low-band-gap materials, including transition-metal monochalcogenides recently established for PEC solar energy conversion applications.

Ultra-thin 2D transition metal monochalcogenide crystals by planarized reactions

We here present a planarized solid-state chemical reaction that can produce transition metal monochalcogenide (TMMC) 2D crystals with large lateral extent and finely controllable thickness down to individual layers. The enhanced lateral diffusion of a gaseous reactant at the interface between a solid precursor and graphene was found to provide a universal route towards layered TMMCs of different compositions. A unique layer-by-layer growth mechanism yields atomically abrupt crystal interfaces and kinetically controllable thickness down to a single TMMC layer. Our approach stabilizes 2D crystals with commonly unattainable thermodynamic phases, such as β-Cu2S and γ-CuSe, and spectroscopic characterization reveals ultra-large phase transition depression and interesting electronic properties. The presented ability to produce large-scale 2D crystals with high environmental stability was applied to highly sensitive and fast optoelectronic sensors. Our approach extends the morphological, compositional, and thermodynamic complexity of 2D materials.

Anisotropic Interlayer Exciton in GeSe/SnS van der Waals Heterostructure.

Stacking two or more two-dimensional materials to form a heterostructure is becoming the most effective way to generate new functionalities for specific applications. Herein, using GW and Bethe-Salpeter equation simulations, we demonstrate the generation of linearly polarized, anisotropic intra- and interlayer excitonic bound states in the transition metal monochalcogenide (TMC) GeSe/SnS van der Waals heterostructure. The puckered structure of TMC results in the directional anisotropy in band structure and in the excitonic bound state. Upon the application of compressive/tensile biaxial strain dramatic variation (∓3%) in excitonic energies, the indirect-to-direct semiconductor transition and the red/blue shift of the optical absorption spectrum are observed. The variations in excitonic energies and optical band gap have been attributed to the change in effective dielectric constant and band dispersion upon the application of biaxial strain. The generation and control over the interlayer excitonic energies can find applications in optoelectronics and optical quantum computers and as a gain medium in lasers.

Mechanical behaviors of MoS nanowires under tension from molecular dynamics simulations

As a new class of one-dimensional (1D) transition-metal monochalcogenides (TMMs) nanowires (NWs), the recently synthesized MoS NWs exhibit potential applications in two-dimensional integrated circuit. However, their mechanical behaviors remain almost unexplored. In this paper, the mechanical behaviors of MoS NWs under tensile loading are studied by classical molecular dynamics simulations together with first-principles calculations. A novel phase transformation is observed in the MoS NWs when the strain applied to them is larger than a critical value, which results in tension-induced hardening in their tensile modulus. Due to the existence of phase transformation, a complicated mechanical response is observed in MoS NWs during the entire loading process, which successively experiences the linear, plateau, stress hardening and fracture stages. The influence of various factors including the strain rate and temperature on the mechanical properties of MoS NWs such as phase transformation, Young’s modulus, fracture strength and fracture strain are also examined. The mechanical properties of MoS NWs obtained in this paper will be beneficial to their future applications in the semiconducting field. It is also expected that the results of the tension-induced phase transformation and its influence on the material properties observed in the present MoS NWs can be further extended to other 1D TMMs such as MoTe and SnSe NWs, since these 1D TMMs are found to possess the similar structure.

ZnS Nanospheres for Optical Modulator in an Erbium‐Doped Fiber Laser

AbstractZinc sulfide (ZnS), which belongs to transition metal monochalcogenides, is a semiconductor material with wide direct band gap. It can potentially show some special applications (such as luminescence, phosphor, sensors, infrared window materials, photocatalysis) by changing the morphology, size, and crystal structure of semiconductor materials. However, ZnS nanospheres have not been studied as optical modulators until now. Herein, ZnS nanospheres are synthesized by the hydrothermal method and are used to realize optical modulators in an Er‐doped fiber laser. The evanescent field effect is utilized to incorporate the ZnS nanospheres on a tapered fiber. Furthermore, with the increase in pump power, the modulation interval gradually decreases to a minimum of 34.36 ns corresponding to the modulation frequency of 29.1 MHz, which is the highest modulation frequency to our knowledge in a ring cavity all‐fiber laser. These results demonstrate ZnS nanospheres together with the interaction of dispersion and nonlinearity in optical fibers can modulate the proposed lasers. This not only provides a new method for controlling the power and frequency of all optical modulators, but also marks an important step for ZnS materials in optics research and device applications.

A type of novel Weyl semimetal candidate: layered transition metal monochalcogenides Mo2XY (X, Y = S, Se, Te, X ≠ Y).

Based on ab initio calculations and the Wannier-based tight-binding method, we studied the topological electronic properties and strain modulation of transition metal monochalcogenides (TMM) Mo2XY (X, Y = S, Se, Te, X ≠ Y). These materials are nodal line semimetals in the absence of spin-orbit coupling (SOC). The presence of SOC turns them into Weyl semimetals with 24 Weyl nodes located in the kz≠ 0 planes and related by time-reversal, rotation C3z, and mirror symmetries. The maximal separation between two neighboring Weyl points with opposite chirality is of the order of magnitude of 0.10 Å-1, which can be readily accessed by angle-resolved photoemission spectroscopy (ARPES). The Weyl semimetal phase shows great robustness and demonstrates different responses under uniaxial and biaxial strain. Intriguingly, the location of the Weyl point changes significantly, resulting in a striking modulation of topological properties under in-plane biaxial strain. Our finding provides a realistic and promising platform for studying and manipulating the behavior of Weyl fermions in experiments.