Two-dimensional (2D) materials have become a fertile playground for the exploration and manipulation of novel collective electronic states. Recent experiments have unveiled a variety of robust 2D orders in highly-crystalline materials ranging from magnetism to ferroelectricity and from superconductivity to charge density wave (CDW) instability. The latter, in particular, appears in diverse patterns even within the same family of materials with isoelectronic species. Furthermore, how they evolve with dimensionality has so far remained elusive. Here we propose a general framework that provides a unfied picture of CDW ordering in the 2H polytype of four isoelectronic transition metal dichalcogenides 2H-MX$_2$ (M=Nb, Ta and X=S, Se). We first show experimentally that whilst NbSe$_2$ exhibits a strongly enhanced CDW order in the 2D limit, the opposite trend exists for TaSe$_2$ and TaS$_2$, with CDW being entirely absent in NbS$_2$ from its bulk to the monolayer. Such distinct behaviours are then demonstrated to be the result of a subtle, yet profound, competition between three factors: ionic charge transfer, electron-phonon coupling, and the spreading extension of the electronic wave functions. Despite its simplicity, our approach can, in essence, be applied to other quasi-2D materials to account for their CDW response at different thicknesses, thereby shedding new light on this intriguing quantum phenomenon and its underlying mechanisms.
Two-dimensional transition-metal dichalcogenides (TMDs) are gaining increasing attention as alternative to graphene for their very high potential in optoelectronics applications. Here we consider two prototypical metallic 2D TMDs, NbSe$_2$ and TaS$_2$. Using a first-principles approach, we investigate the properties of the localised intraband $d$ plasmon that cannot be modelled on the basis of the homogeneous electron gas. Finally, we discuss the effects of the reduced dimensionality on the plasmon dispersion through the interplay between interband transitions and local-field effects. This res…
We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter $α$, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of $α$ is highlighted by its independence of the physical mechanisms underlying the confinement …
Finding stimuli capable of driving an imbalance of spin-polarised electrons within a solid is the central challenge in the development of spintronic devices. However, without the aid of magnetism, routes towards this goal are highly constrained with only a few suitable pairings of compounds and driving mechanisms found to date. Here, through spin- and angle-resolved photoemission along with density functional theory, we establish how the $p$-derived bulk valence bands of semiconducting 1T-HfSe$_2$ possess a local, ground-state spin texture spatially confined within each Se-sublayer due to stro…
Defect-induced states, defect-induced phase transition and excitonic states in bent transition metal dichalcogenide (TMD) nanoribbons: density functional vs. many body theory
Using first-principles atomistic simulations, we study the response of atomically-thin layers of transition metal dichalcogenides (TMDs) - a new class of two-dimensional inorganic materials with unique electronic properties - to electron irradiation. We calculate displacement threshold energies for atoms in 21 different compounds and estimate the corresponding electron energies required to produce defects. For a representative structure of MoS2, we carry out high-resolution transmission electron microscopy experiments and validate our theoretical predictions via observations of vacancy formati…
We analyze the dielectric-function spectra of low dimensional transition metal dichalcogenides (TMDCs) using a fully analytical model of the complex dielectric function that is applicable in fractional dimensional space. We extract the dimensionalities of the $A$ and $B$ excitons as well as their Lorentzian broadening widths by fitting the model to experimental data in the spectral range of photon energies (1.5 - 3 eV). Our results show the significant contribution of the lowest ground exciton state to the dielectric properties of exemplary monolayer materials (MoS$_2$, MoSe$_2$ and WSe$_2$). …
We have performed angle-resolved photoemission spectroscopy on transition-metal dichalcogenide 1$T$-HfTe$_2$ to elucidate the evolution of electronic states upon potassium (K) deposition. In pristine HfTe$_2$, an in-plane hole pocket and electron pockets are observed at the Brillouin-zone center and corner, respectively, indicating the semimetallic nature of bulk HfTe$_2$, with dispersion perpendicular to the plane. In contrast, the band structure of heavily K-dosed HfTe$_2$ is obviously different from that of bulk, and resembles the band structure calculated for monolayer HfTe$_2$. It was als…
Metallic transition metal dichalcogenides like tantalum diselenide (TaSe$_{2}$) exhibit exciting behaviors at low temperatures, including the emergence of charge density wave (CDW) states. In this work, density functional theory (DFT) is used to investigate how structural, electronic, and Raman spectral properties of the CDW configuration change as a function of thickness. Such findings highlight the influence of dimensionality change (from 2D to 3D) and van der Waals (vdW) interactions on the system properties. The vdW effect is most strongly present in bulk TaSe$_{2}$ in the spectral range 1…
Second-harmonic generation (SHG) is a well-known nonlinear spectroscopy method to probe electronic structure, specifically, in transition metal dichalcogenide (TMDC) monolayers. This work investigates the nonlinear dynamics of a strongly excited TMDC monolayer by solving the time evolution equations for the density matrix. It is shown that the presence of excitons qualitatively changes the nonlinear dynamics leading, in particular, to a huge enhancement of the nonlinear signal as a function of the dielectric environment. It is also shown that the SHG polarization angular diagram and its depend…
Electronic interactions associated with atomic adsorbates on transition metal dichalcogenides such as MoS2 can induce massive electronic reconstruction that results in the formation of a new type of heavily doped transition metal dichalcogenide that exhibits characteristics of a two-dimensional electron gas. The impact of quantum confinement and reduced dimensionality on the carrier dynamics in such two-dimensional systems is at present not known, but is of paramount importance if they are to find application in optoelectronic devices. Here we show by a combination of angle-resolved photoemiss…
Layered transition metal dichalcogenides have rich phase diagram and they feature two dimensionality on numerous physical properties. Co1/3NbS2 is one of the newest members of this family where Co atoms are intercalated into the Van der Waals gaps between NbS2 layers. We study the three-dimensional electronic band structure of Co1/3NbS2 using both surface and bulk sensitive angle-resolved photoemission spectroscopy. We show that the electronic bands do not fit into the rigid-band-shift picture after the Co intercalation. Instead, Co1/3NbS2 displays a different orbital character near the Fermi …
Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit i…
We perform ab initio calculations of the coupling between electrons and small-momentum polar-optical phonons in monolayer transition metal dichalcogenides of the 2H type: MoS2, MoSe2, MoTe2, WS2, and WSe2. The so-called Fröhlich interaction is fundamentally affected by the dimensionality of the system. In a plane-wave framework with periodic boundary conditions, this coupling is affected by the spurious interaction between the 2D material and its periodic images. To overcome this, we perform density functional perturbation theory calculations with a truncated Coulomb interaction in the out-of-…
Vacancies in materials structure -- lowering its atomic density -- take the system closer to the atomic limit, to which all systems are topologically trivial. Here we show a mechanism of mediated interaction between vacancies inducing a topologically non-trivial phase. Within an {\it ab initio} approach we explore topological transition dependence with the vacancy density in transition metal dichalcogenides. As a case of study, we focus on the PtSe$_2$, to which pristine form is a trivial semiconductor with an energy gap of $1.2$\,eV. The vacancies states lead to a large topological gap of $18…
Electronic and topological properties of materials are derived from the interplay between crystalline symmetry and dimensionality. Simultaneously introducing 'forbidden' symmetries via quasiperiodic ordering with low-dimensionality into a material system promises the emergence of new physical phenomena. Here, we isolate a two-dimensional chalcogenide quasicrystal and approximant, and investigate associated electronic and topological properties. Ultra-thin two-dimensional layers of the materials with a composition close to Ta1.6Te, derived from a layered transition metal dichalcogenide, are iso…
Deciphering Mechanoluminescence: How the Nature of Mechanical Stress and Structural Dimensionality Shape Mechanisms and Responses
Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251（雷恩大学，法国国家科学研究中心，雷恩物理研究所）；Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226（雷恩大学，法国国家科学研究中心，雷恩化学科学研究所）；Otto Schott Institute of Materials Research, Friedrich Schiller University Jena（耶拿弗里德里希·席勒大学奥托·肖特材料研究所）
We demonstrate that the Einstein relation for the diffusion of a particle in the random energy landscape with the Gaussian density of states is an exclusive 1D property and does not hold in higher dimensions. We also consider the analytical properties of the particle velocity and diffusivity for the limit of weak driving force and establish connection between these properties and dimensionality and spatial correlation of the random energy landscape.
LaOBiPbS$_3$ is a kind of pnictogen-dichalcogenide layered compounds, which have recently been experimentally investigated as thermoelectric materials owing to their low thermal conductivity and high controllability of constituent elements. However, thermoelectric performance of LaOBiPbS$_3$ is at present not very high and that of its analogous compounds remains to be unknown. In this study, we theoretically investigate thermoelectric properties of 24 possible variations of the constituent elements in LaOBiPbS$_3$ from the viewpoint of the electronic structure. We find that some compounds can …
Dynamic control of conductivity and optical properties via atomic structure changes is of tremendous technological importance in information storage. Energy consumption considerations provide a driving force toward employing thin materials in devices. Monolayer transition metal dichalcogenides are nearly atomically-thin materials that can exist in multiple crystal structures, each with distinct electrical properties. Using density functional approaches, we discover that electrostatic gating device configurations have the potential to drive structural semiconductor-to-semimetal phase transition…
The dynamics of the photoinduced commensurate to incommensurate charge density wave (CDW) phase transition in 4Hb-TaSe2 are investigated by femtosecond electron diffraction. In the perturbative regime the CDW reforms on a 150 ps timescale, which is two orders of magnitude slower than in other transition-metal dichalcogenides. We attribute this to a weak coupling between the CDW carrying T-layers and thus demonstrate the importance of three-dimensionality for the existence of CDWs. With increasing optical excitation the phase transition is achieved showing a second order character in contrast t…
The existence of a charge density wave (CDW) in transition metal dichalcogenide CuS$_2$ has remained undetermined since its first experimental synthesis nearly 50 years ago. Despite conflicting experimental literature regarding its low temperature structure, there exists no theoretical study of the phonon properties and lattice stability of this material. By studying the first-principles electronic structure and phonon properties of CuS$_2$ at various electronic temperatures, we identify temperature-sensitive soft phonon modes which unveil a previously unreported Kohn anomaly at approximately …
Typical Raman spectra of transition metal dichalcogenides (TMDs) display two prominent peaks, E2g and A1g, that are well separated from each other. We find that these modes are degenerate in bulk WSe2 yielding one single Raman peak. As the dimensionality is lowered, the observed peak splits in two as a result of broken degeneracy. In contrast to our experimental findings, our phonon dispersion calculations reveal that these modes remain degenerate independent of the number of layers. Interestingly, for minuscule biaxial strain the degeneracy is preserved but once the crystal symmetry is broken…
Despite inherently poor interlayer conductivity, photodetectors made from few-layer devices of 2D transition metal dichalcogenides (TMDs) such as WSe$_2$ and MoS$_2$ can still yield a desirably fast ($\leq$90 ps) and efficient ($ε$$>$40\%) photoresponse. By combining ultrafast photocurrent (U-PC) and transient absorption (TA) microscopy, the competing electronic escape and recombination rates are unambiguously identified in otherwise complex kinetics. Both the U-PC and TA response of WSe$_2$ yield matching interlayer electronic escape times that accelerate from 1.6 ns to 86 ns with applied $E$…
Transition metal dichalcogenides are promising candidates to show long-range ferromagnetic order in the single-layer limit. Based on ab initio calculations, we report the emergence of a charge density wave (CDW) phase in monolayer 1T-CrTe$_2$. We demonstrate that this phase is the ground state in the single-layer limit at any strain value. We obtain an optical phonon mode of $1.96$ THz that connects CDW phase with the undistorted 1T phase. Localization of the $a_{1g}$ orbital of CrTe$_2$ produces an out-of-plane orientation of the magnetic moments, circumventing the restrictions of the Mermin-…
The semimetallic or semiconducting nature of the transition metal dichalcogenide 1$T$-TiSe$_2$ remains under debate after many decades mainly due to the fluctuating nature of its 2 $\times$ 2 $\times$ 2 charge-density-wave (CDW) phase at room-temperature. In this letter, using angle-resolved photoemission spectroscopy, we unambiguously demonstrate that the 1$T$-TiSe$_2$ normal state is semimetallic with an electron-hole band overlap of $\sim$110 meV by probing the low-energy electronic states of the perturbed CDW phase strongly doped by alkali atoms. Our study not only closes a long-standing d…
Nano-thick metallic transition metal dichalcogenides such as VS$_{2}$ are essential building blocks for constructing next-generation electronic and energy-storage applications, as well as for exploring unique physical issues associated with the dimensionality effect. However, such 2D layered materials have yet to be achieved through either mechanical exfoliation or bottom-up synthesis. Herein, we report a facile chemical vapor deposition route for direct production of crystalline VS$_{2}$ nanosheets with sub-10 nm thicknesses and domain sizes of tens of micrometers. The obtained nanosheets fea…
Two-dimensional transition metal dichalcogenides show great potential as promising thermoelectric materials due to their lower dimensionality, the unique density of states and quantum confinement of carriers. The effect of mechanical strain on the thermoelectric performances of monolayer WS 2 has been investigated using density functional theory associated with semiclassical Boltzmann transport theory. The variation of Seebeck coefficient and band gap with applied strain has followed the same type of trend. For n-type material the relaxation time scaled power factor(S 2 σ/τ) increases by the a…
Layered quasi-two-dimensional transition metal dichalcogenides (TMDCs), which can be readily made in ultrathin films, offer excellent opportunities for studying how dimensionality affects electronic structure and physical properties. Among all TMDCs, NbSe2 is of special interest; bulk NbSe2 hosts a charge-density-wave phase at low temperatures and has the highest known superconducting transition temperature, and these properties can be substantially modified in the ultrathin film limit. Motivated by these effects, we report herein a study of few-layer NbSe2 films, with a well-defined single-do…
Combination of low-dimensionality and electron correlation is vital for exotic quantum phenomena such as the Mott-insulating phase and high-temperature superconductivity. Transition-metal dichalcogenide (TMD) 1T-TaS2 has evoked great interest owing to its unique nonmagnetic Mott-insulator nature coupled with a charge-density-wave (CDW). To functionalize such a complex phase, it is essential to enhance the CDW-Mott transition temperature TCDW-Mott, whereas this was difficult for bulk TMDs with TCDW-Mott < 200 K. Here we report a strong-coupling 2D CDW-Mott phase with a transition temperature on…
Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe2 on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe2. Renormalization group analysis confirms that imperfect nesting in three…
Nanomaterials based on MoS2 are remarkably versatile; MoS2 nanoparticles are proven catalysts for processes such as hydrodesulphurization and the hydrogen evolution reaction, and transition metal dichalcogenides in general have recently emerged as novel 2D components for nanoscale electronics and optoelectronics. The properties of such materials are intimately related to their structure and dimensionality. For example, only the edges exposed by MoS2 nanoparticles (NPs) are catalytically active, and extended MoS2 systems show different character (direct or indirect gap semiconducting, or metall…
Layered transition metal dichalcogenides (TMDs) are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single 2D layer of NbSe2 by means of low temperature scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectro…
Transition metal dichalcogenides (TMDs) are a class of widely studied 2D layered materials which exist in various polymorphs. The 1T' phase of MoTe2 is of prime importance as it has been reported to show quantum spin hall (QSH) behavior with a fairly large band-gap of ~ 60 meV, in contrast to most QSH materials known. It is noteworthy that though the monolayer 1T'-MoTe2 was initially predicted to show the QSH behavior, recent theoretical studies claim that the few-layered counterparts also exhibit higher order topological behavior. Besides, 1T'-MoTe2 also undergoes a hysteretic phase transitio…
