Anionic polymeric bonds in transition metal ditellurides

Charge density waves in two-dimensional transition metal dichalcogenides

Charge density wave (CDW is one of the most ubiquitous electronic orders in quantum materials. While the essential ingredients of CDW order have been extensively studied, a comprehensive microscopic understanding is yet to be reached. Recent research efforts on the CDW phenomena in two-dimensional (2D) materials provide a new pathway toward a deeper understanding of its complexity. This review provides an overview of the CDW orders in 2D with atomically thin transition metal dichalcogenides (TMDCs) as the materials platform. We mainly focus on the electronic structure investigations on the epitaxially grown TMDC samples with angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy as complementary experimental tools. We discuss the possible origins of the 2D CDW, novel quantum states coexisting with them, and exotic types of charge orders that can only be realized in the 2D limit.

Superconductivity in Te-Deficient ZrTe2

We present structural, electrical, and thermoelectric potential measurements on high-quality single crystals of ZrTe_1.8 grown from isothermal chemical vapor transport. These measurements show that the Te-deficient ZrTe_1.8, which forms the same structure as the nonsuperconducting ZrTe₂, is superconducting below 3.2 K. The temperature dependence of the upper critical field (H _c2) deviates from the behavior expected in conventional single-band superconductors, being best described by an electron-phonon two-gap superconducting model with strong intraband coupling. For the ZrTe_1.8 single crystals, the Seebeck potential measurements suggest that the charge carriers are predominantly negative, in agreement with the ab initio calculations. Through first-principles calculations within DFT, we show that the slight reduction of Te occupancy in ZrTe₂ unexpectedly gives origin to density of states peaks at the Fermi level due to the formation of localized Zr-d bands, possibly promoting electronic instabilities at the Fermi level and an increase at the critical temperature according to the standard BCS theory. These findings highlight that the Te deficiency promotes the electronic conditions for the stability of the superconducting ground state, suggesting that defects can fine-tune the electronic structure to support superconductivity.

A Novel 19 $\sqrt {19} $ × 19 $\sqrt {19} $ Superstructure in Epitaxially Grown 1T-TaTe2

The spontaneous formation of electronic orders is a crucial element for understanding complex quantum states and engineering heterostructures in 2D materials. A novel 19 $\sqrt {19} $ × 19 $\sqrt {19} $ charge order in few-layer-thick 1T-TaTe₂ transition metal dichalcogenide films grown by molecular beam epitaxy, which has not been realized, is report. The photoemission and scanning probe measurements demonstrate that monolayer 1T-TaTe₂ exhibits a variety of metastable charge density wave orders, including the 19 $\sqrt {19} $ × 19 $\sqrt {19} $ superstructure, which can be selectively stabilized by controlling the post-growth annealing temperature. Moreover, it is found that only the 19 $\sqrt {19} $ × 19 $\sqrt {19} $ order persists in 1T-TaTe₂ films thicker than a monolayer, up to 8 layers. The findings identify the previously unrealized novel electronic order in a much-studied transition metal dichalcogenide and provide a viable route to control it within the epitaxial growth process.

Direct Observation of Orbital Driven Strong Interlayer Coupling in Puckered Two-Dimensional PdSe2

Interlayer coupling between individual unit layers is known to be critical in manipulating the layer-dependent properties of two-dimensional (2D) materials. While recent studies have revealed that several 2D materials with significant degrees of interlayer interaction (such as black phosphorus) show strongly layer-dependent properties, the origin based on the electronic structure is drawing intensive attention along with 2D materials exploration. Here, the direct observation of a highly dispersive single electronic band along the interlayer direction in puckered 2D PdSe₂ as an experimental hallmark of strong interlayer couplings is reported. Remarkably large band dispersion along the k_z -direction near Fermi level, which is even wider than the in-plane one, is observed by the angle-resolved photoemission spectroscopy measurement. Employing X-ray absorption spectroscopy and density functional theory calculations, it is revealed that the strong interlayer coupling in 2D PdSe₂ originates from the unique directional bonding of Pd d orbitals associated with unexpected Pd 4d⁹ configuration, which consequently plays a decisive role for the strong layer-dependency of the band gap.

Spatially Separated Superconductivity and Enhanced Charge-Density-Wave Ordering in an IrTe2 Nanoflake

The interplay among collective electronic states like superconductivity (SC) and charge density wave (CDW) is of significance in transition metal dichalcogenides. To date, a consensus on the relationship between SC and CDW has not been established in IrTe₂. Here we use the Au-assisted exfoliation method to cleave IrTe₂ down to 10 nm. A striking feature is the concurrence of phase separation in a single piece of nanoflake, i.e., the superconducting (P3̅m1) and CDW (P3̅) phases. In the former area, the dimensional fluctuations suppress the CDW ordering and induce SC at 3.5 K. The CDW area at the phase boundary shows enhanced T_CDW at 605 K (T_CDW = 280 K in the bulk phase), which is accompanied by a unique wrinkle. Detailed analyses suggest that the strain-induced bond breaking of Te-Te dimers favors the CDW. Our works provide compelling evidence of competition between SC and CDW in IrTe₂.

Conversion-Type Nonmetal Elemental Tellurium Anode with High Utilization for Mild/Alkaline Zinc Batteries

Zinc ion batteries (ZIBs), generally established on an excessive metallic Zn anode and aqueous electrolytes, suffer from severe dendrites and gassing issues at Zn side, resulting in poor cycling life. Substituting Zn metal anode with non-Zn ones is a promising strategy for solving these problems, whereas this is still restricted by the limited anode alternatives. Herein, by replacing metal Zn with chalcogen element tellurium (Te), a conversion-type Te-based ZIB is reported that can work in both mild and alkaline electrolytes. As expected, the as-assembled mild Te/MnO₂ and alkaline Te/Ni(OH)₂ cells deliver remarkable capacities up to 106 and 161 mAh g^-1 _{anode+cathode} , respectively, with a high utilization of anode (50.1% for the Te/MnO₂ and 38.9% for the Te/Ni(OH)₂ ), which surpass all ZIBs. Ultralong cycling life (over 75% capacity retention after 5000 cycles) is achieved in the two systems, benefiting from the stable conversion mechanisms (mild: Te to ZnTe₂ to ZnTe; alkaline: ZnTe to Te to TeO₂ ) with thoroughly eliminated dendrites and gassing. Moreover, high gravimetric energy density of ZIBs is also achieved, which are 176.3 Wh kg^-1 _{anode+cathdoe} (Te/Ni(OH)₂ ) and 81 Wh Kg^-1 _{anode+cathode} (Te/MnO₂ ), respectively. This work sheds light on the development of advanced conversion-type anode for high-performance batteries with superior stability.

Transport phenomena of Cu-S-Te chalcogenide nanocomposites: frequency response and AC conductivity

In this work, the development and electrical characterization of several chalcogenide nanocomposites have been reported. X-ray diffraction (XRD) has been used to reveal their microstructures. Mott's variable range hopping model has been used to interpret the DC conductivity data of the nanocomposites at lower temperatures. The DC conductivity data at higher temperatures has been explained well using Greave's model. To explain the AC conductivity data, the Meyer-Neldel (MN) conduction rule has been employed. The AC conductivity spectra at different temperatures have been analyzed using Almond-West formalism. Different conduction models, namely, correlated barrier hopping (CBH) and modified non-overlapping small polaron tunneling (NSPT), have been used to interpret the conduction mechanism of the nanocomposites. Scaling of the AC conductivity spectra reveals that the electrical relaxation process is independent of temperature, but depends on the nanocomposite composition. The conductivity mechanism is explained using a schematic structural model.

Direct Visualization of Trimerized States in 1T^{'}-TaTe_{2}

Transition-metal dichalcogenides containing tellurium anions show remarkable charge-lattice modulated structures and prominent interlayer character. Using cryogenic scanning transmission electron microscopy (STEM), we map the atomic-scale structures of the high temperature (HT) and low temperature (LT) modulated phases in 1T^{'}-TaTe_{2}. At HT, we directly show in-plane metal distortions which form trimerized clusters and staggered, three-layer stacking. In the LT phase at 93 K, we visualize an additional trimerization of Ta sites and subtle distortions of Te sites by extracting structural information from contrast modulations in plan-view STEM data. Coupled with density functional theory calculations and image simulations, this approach opens the door for atomic-scale visualizations of low temperature phase transitions and complex displacements in a variety of layered systems.

Aqueous Zinc-Tellurium Batteries with Ultraflat Discharge Plateau and High Volumetric Capacity

Traditional aqueous zinc-ion batteries (ZIBs) based on ion-intercalation or surface redox behaviors at the cathode side suffer severely from an unsatisfactory specific capacity and unstable output potential. Herein, these issues are applied to a conversion-type zinc-tellurium (Zn-Te) battery. Typically, this battery works based on a two-step solid-to-solid conversion with the successive formation of zinc ditelluride (ZnTe₂ ) and zinc telluride (ZnTe). It delivers an ultrahigh volumetric capacity of 2619 mAh cm^-3 (419 mAh g^-1 ), 74.1% of which is from the first conversion (Te to ZnTe₂ ) with an ultraflat discharge plateau. Though reported first in a challenging aqueous environment, this Zn-Te battery demonstrates an excellent capacity retention of >82.8% after 500 cycles, which results from the elimination of the notorious "shuttle effect" due to the solid-to-solid conversion behaviors. In addition, a quasi-solid-state Zn-Te battery is also fabricated, exhibiting superior flexibility, robustness, and good electrochemical performance. This work develops a novel cathode material based on conversion-type ion-storage mechanism. The system is attractive due to its ultrastable energy output, which is rarely reported for ZIBs.

Layered Noble Metal Dichalcogenides: Tailoring Electrochemical and Catalytic Properties

Owing to the anisotropic nature, layered transition metal dichalcogenides (TMDs) have captured tremendous attention for their promising uses in a plethora of applications. Currently, bulk of the research is centered on Group 6 TMDs. Layered noble metal dichalcogenides, in particular the noble metal tellurides, belong to a subset of Group 10 TMDs, wherein the transition metal is a noble metal of either palladium or platinum. We address here a lack of contemporary knowledge on these compounds by providing a comprehensive study on the electrochemistry of layered noble metal tellurides, PdTe₂ and PtTe₂, and their efficiency as electrocatalysts toward the hydrogen evolution reaction (HER). Observed parallels in the electrochemical peaks of the noble metal tellurides are traced to the tellurium electrochemistry. PdTe₂ and PtTe₂ can be discriminated by their distinct reduction peaks in the first cathodic scans. Considering the influence of the metal component, PtTe₂ outperforms PdTe₂ in aspects of charge transfer and electrocatalysis. The heterogeneous electron transfer (HET) rate of PtTe₂ is an order of magnitude faster than PdTe₂, and a lower HER overpotential of 0.54 V versus reversible hydrogen electrode (RHE) at a current density of -10 mA cm^-2 is evident in PtTe₂. On PdTe₂ and PtTe₂ surfaces, adsorption via the Volmer process has been identified as the limiting step for HER. A general phenomenon for the noble metal tellurides is that faster HET rates are observed upon electrochemical reductive pretreatment, whereas slower HET rates occur when the noble metal tellurides are oxidized during pretreatment. PtTe₂ becomes successfully activated for HER when subject to oxidative treatment, whereas oxidized or reduced PdTe₂ shows a deactivated HER performance. These findings provide fundamental insights that are pivotal to advancing the field of the underemphasized TMDs. Furthermore, electrochemical tuning as a means to tailor specific properties of the TMDs is advantageous for the development of their future applications.

Nanoscale Superconducting Honeycomb Charge Order in IrTe2

Entanglement of charge orderings and other electronic orders such as superconductivity is in the core of challenging physics issues of complex materials including high temperature superconductivity. Here, we report on the observation of a unique nanometer scale honeycomb charge ordering of the cleaved IrTe2 surface, which hosts a superconducting state. IrTe2 was recently established to exhibit an intriguing cascade of stripe charge orders. The stripe phases coexist with a hexagonal phase, which is formed locally and falls into a superconducting state below 3 K. The atomic and electronic structures of the honeycomb and hexagon pattern of this phase are consistent with the charge order nature, but the superconductivity does not survive on neighboring stripe charge order domains. The present work provides an intriguing physics issue and a new direction of functionalization for two-dimensional materials.

Origin of first-order-type electronic and structural transitions in IrTe2

We have explored the origin of unusual first-order-type electronic and structural transitions in IrTe2, based on the first-principles total energy density functional theory analysis. We have clarified that the structural transition occurs through the interplay among the charge density wavelike lattice modulation with q1/5=(1/5,0,1/5), in-plane dimer ordering, and the uniform lattice deformation. The Ir-Ir dimer formation via a molecular-orbital version of the Jahn-Teller distortion in the Ir-Ir zigzag stripe is found to play the most important role in producing the charge disproportionation state. Angle-resolved photoemission spectroscopy reveals the characteristic features of structural transition, which are in good agreement with the density functional theory bands obtained by the band-unfolding technique.

Hysteretic melting transition of a soliton lattice in a commensurate charge modulation

We report on the observation of the hysteretic transition of a commensurate charge modulation in IrTe2 from transport and scanning tunneling microscopy (STM) studies. Below the transition (TC≈275  K on cooling), a q=1/5 charge modulation was observed, which is consistent with previous studies. Additional modulations [qn=(3n+2)(-1)] appear below a second transition at TS≈180  K on cooling. The coexistence of various modulations persists up to TC on warming. The atomic structures of charge modulations and the temperature-dependent STM studies suggest that 1/5 modulation is a periodic soliton lattice that partially melts below TS on cooling. Our results provide compelling evidence that the ground state of IrTe2 is a commensurate 1/6 charge modulation, which originates from the periodic dimerization of Te atoms visualized by atomically resolved STM images.

Anionic depolymerization transition in IrTe2

Selenium substitution drastically increases the transition temperature of iridium ditelluride (IrTe(2)) to a diamagnetic superstructure from 278 to 560 K. Transmission electron microscopy experiments revealed that this enhancement is accompanied by the evolution of nonsinusoidal structure modulations from q = 1/5(101) to q = 1/6(101) types. These comprehensive results are consistent with the concept of the destabilization of polymeric Te-Te bonds at the transition, the temperature of which is increased by chemical and hydrostatic pressure and by the substitution of Te with the more electronegative Se. This temperature-induced depolymerization transition in IrTe(2) is unique in crystalline inorganic solids.

The crystal structure of Cu2.31TeS0.32 - a condensed cluster compound

The crystal structure of Cu2.31TeS0.32, a sulfur-poor member of the solid solution series Cu61.3Te18.3S20.5 - Cu61.9Te35.7S2.36 prepared at 675°C, was determined and refined from single-crystal diffractometer X-ray data (R = 0.044). The quenched statistical cubic structure (a = 14.836(4)Å, space group F4132) consists at room temperature of two interpenetrating tetrahedral frameworks of icosahedral tellurium cages which enclose icosahedral clusters of half-occupied Cu positions centred by additional, fully occupied central copper sites. In one subset, statistically occupied sulfur tetrahedra occur in the tellurium cages, centred on this central Cu site.

The Cu2.31TeS0.32 structure is a stuffed derivative of Cr3Si (structure type A13) and shows affinities to the structures of Cu9.1TeSb3, the Ag8GeTe6 family and some metal clusters, especially [Cu26Se13(PEt2Ph)14] and [Au13Cl2(PMe2Ph)10].

Structural Trends and Chemical Bonding in Te-Doped Silicon Clathrates

The recently discovered tellurium-doped silicon clathrates, Te7+xSi20-x and Te16Si38, both low- and high-temperature forms (cubic and rhombohedral, respectively), exhibit original structures that are all derived from the parent type I clathrate G8Si46 (G = guest atom). The similarities and differences between the structures of these compounds and that of the parent one are analyzed and discussed on the basis of charge distribution and chemical bonding considerations. Because of the particular character of the Te atom, these compounds appear to be at the border between the clathrate and polytelluride families.

Vacancy Ordering and Bonding Competition in the Group 9 Tellurides M(x)()Te(2) (M = Rh, Ir; 0.75 </= x </= 2): A Theoretical Study

The Rh-Te and Ir-Te binary systems for 50-78 atom % Te show remarkable differences in their phase and structural features at temperatures below 1100 degrees C. Extended Hückel calculations are employed to investigate the influence of various orbital interactions on these differences. In general, a strong interrelationship among valence electron count, orbital characteristics at and near the Fermi levels, and relative strengths of M-Te, Te-Te, and M-M orbital interactions control the occurrence and structures of various M(x)()Te(2) compounds (0.75 </= x </= 2). Stronger Ir-Te than Rh-Te orbital interactions lead to the different low-temperature structures of IrTe(2) (CdI(2)-type) and RhTe(2) (pyrite-type), but then short and intermediate-range Te-Te interactions lead to the pyrite-type structure for the defect phases M(1)(-)(u)()Te(2). At temperatures above 600 degrees C, RhTe(2) (pyrite-type) is unstable relative to disproportionation to the "stuffed" CdI(2)-type Rh(1+)(x)()Te(2) and the defect pyrite-type Rh(1)(-)(u)()Te(2). The Rh-rich phases, Rh(1+)(x)()Te(2), show ordered vacancies in alternating layers of octahedral holes and can be formulated as (Rh(3))(x)()(Rh)(1)(-)(2)(x)()Te(2) (x </= (1)/(2)) and (Rh(3))(1)(-)(x)()(Rh)(4)(x)()(-)(2)Te(2) (x >/= (1)/(2)) to emphasize the occurrence of linear Rh(3) units in their structures. The pattern of vacancies in these structures follows the preference of Rh(4)(n)()(+3) oligomers over Rh(4)(n)()(+1) chains. Charge-iterative calculations of Rh atomic orbital energies in Rh(1+)(x)()Te(2) (x = 0.0, 0.5, 1.0) were carried out to analyze the electronic properties of Rh throughout the series. As x increases, Rh-Te orbital interactions become less attractive and the concentration of Rh-Rh repulsive interactions grows. Both effects control the maximum value of x (observed to be 0.84) for this series and influence the pattern of occupied octahedral holes in the close-packed tellurium matrix.

Effect of atmospheric composition and pressure on the laser ablation of (GeTe)(85)Sn(15) chalcogenide thin films

Laser ablation of (GeTe)(85)Sn(15) thin films as a function of atmospheric exposure was monitored in real time by transient reflectivity. The observed optical changes were correlated with microstructural analysis. Among the key findings were that the presence of water in the atmosphere during laser irradiation of a thin-film structure reduced the incident laser power required for ablation by as much as a factor of 2. The magnitude of the effect was dependent on both H(2)O vapor pressure and duration of exposure to the vapor. The reduction of laser power necessary to ablate was partially reversed by exposure of the thin-film structure to vacuum. Significantly, exposure to other (dry) gases such as N(2) did not change the ablation threshold from that observed in vacuum. We determined that dome formation and ablation occurred at lower temperatures in the presence of water. In addition, the power necessary to crystallize the amorphous chalcogenide layer in the structure was independent of atmospheric composition or pressure. Microstructure analysis showed the presence of H(2)O fostered the formation of a nonuniform distribution of the chalcogenide material in the ablated region. The experimental results are consistent with our model that ablation is assisted by high pressures produced by vaporization of absorbed liquid water.