X-ray photoelectron spectra and electronic structure of zirconium trisulfide and triselenide

X-ray photoelectron spectra of ZrS₃ and ZrSe₃ have been recorded. These compounds may be regarded as Zr(Ch₂)Ch (Ch=S,Se) with both (Ch₂) groups and isolated Ch atoms. An unambiguous assignment of the chalcogen core levels was made by comparing the spectra with those of ZrS₂ and ZrSe₂ (where all chalcogen atoms are isolated). The levels of the (Ch₂) groups are found at higher binding energies than those of the isolated Ch atoms, which is consistent with a larger negative charge on the isolated atoms. The structures of the valence bands of ZrS₃ and ZrSe₃ are discussed in terms of a molecular-orbital scheme for the (Ch₂) groups. The exciton peaks observed in the optical absorption spectra of ZrS₃ and ZrSe₃ are assigned as due to excitations of the (Ch₂) groups.     

Electrical properties of zirconium diselenide single crystals grown by iodine transport method

Single crystals of zirconium diselenide (ZrSe₂) were grown by chemical vapour transport method using iodine as the transporting agent. The crystals were found to exhibit metallic behaviour in the temperature range 77–300 K and semiconducting nature in 300–443 K range. The measurements of thermoelectric power and conductivity enabled the determination of both carrier mobility and carrier concentration. The variation of carrier mobility and carrier concentration with temperature indicates the presence of deep trapping centres and their reduction with temperature in these crystals.     

Universal Imaging of Full Strain Tensor in 2D Crystals with Third‐Harmonic Generation

Quantitatively mapping and monitoring the strain distribution in 2D materials is essential for their physical understanding and function engineering. Optical characterization methods are always appealing due to unique noninvasion and high‐throughput advantages. However, all currently available optical spectroscopic techniques have application limitation, e.g., photoluminescence spectroscopy is for direct‐bandgap semiconducting materials, Raman spectroscopy is for ones with Raman‐active and strain‐sensitive phonon modes, and second‐harmonic generation spectroscopy is only for noncentrosymmetric ones. Here, a universal methodology to measure the full strain tensor in any 2D crystalline material by polarization‐dependent third‐harmonic generation is reported. This technique utilizes the third‐order nonlinear optical response being a universal property in 2D crystals and the nonlinear susceptibility has a one‐to‐one correspondence to strain tensor via a photoelastic tensor. The photoelastic tensor of both a noncentrosymmetric D_3h WS₂ monolayer and a centrosymmetric D_3d WS₂ bilayer is successfully determined, and the strain tensor distribution in homogenously strained and randomly strained monolayer WS₂ is further mapped. In addition, an atlas of photoelastic tensors to monitor the strain distribution in 2D materials belonging to all 32 crystallographic point groups is provided. This universal characterization on strain tensor should facilitate new functionality designs and accelerate device applications in 2D‐materials‐based electronic, optoelectronic, and photovoltaic devices.     

Probing the Domain Architecture in 2D α‐Mo2C via Polarized Raman Spectroscopy

MXenes are emerging 2D materials with intriguing properties such as excellent stability and high conductivity. Here, a systematic study on the Raman spectra of 2D α‐Mo₂C (molybdenum carbide), a promising member in MXene family, is conducted. Six experimentally observed Raman modes from ultrathin α‐Mo₂C crystal are first assigned with the assistance of phonon dispersion calculated from density functional theory. Angle‐resolved polarized Raman spectroscopy indicates the anisotropy of α‐Mo₂C in the b–c plane. Raman spectroscopy is further used to study the unique domain structures of 2D α‐Mo₂C crystals grown by chemical vapor deposition. A Raman mapping investigation suggests that most of the α‐Mo₂C flakes contain multiple domains and the c‐axes of neighboring domains tend to form a 60° or 120° angle, due to the weak Mo? C bonds in this interstitial carbide and the low formation energy of the carbon chains along three equivalent directions. This study demonstrates that polarized Raman spectroscopy is a powerful and effective way to characterize the domain structures in α‐Mo₂C, which will facilitate the further exploration of the domain‐structure‐related properties and potential applications of α‐Mo₂C.     

Exfoliated Mesoporous 2D Covalent Organic Frameworks for High-Rate Electrochemical Double-Layer Capacitors

The electrochemical double-layer capacitors (EDLCs) are highly demanded electrical energy storage devices due to their high power density with thousands of cycle life compared with pseudocapacitors and batteries. Herein, a series of capacitor cells composed of exfoliated mesoporous 2D covalent organic frameworks (e-COFs) that are able to perform excellent double-layer charge storage is reported. The selected mesoporous 2D COFs possess eclipsed AA layer-stacking mode with 3.4 nm square-like open channels, favorable BET surface areas (up to 1170 m² g⁻¹), and high thermal and chemical stabilities. The COFs via the facile, scalable, and mild chemical exfoliation method are further exfoliated to produce thin-layer structure with average thickness of about 22 nm. The e-COF-based capacitor cells achieve high areal capacitance (5.46 mF cm⁻² at 1,000 mV s⁻¹), high gravimetric power (55 kW kg⁻¹), and relatively low τ₀ value (121 ms). More importantly, they perform nearly an ideal DL charge storage at high charge–discharge rate (up to 30 000 mV s⁻¹) and maintain almost 100% capacitance stability even after 10 000 cycles. This study thus provides insights into the potential utilization of COF materials for EDLCs.     

Synthesis and properties of Ca<Subscript>2.8</Subscript>Ln<Subscript>0.2</Subscript>Co<Subscript>4</Subscript>O<Subscript>9 + δ</Subscript> (Ln = La,Nd, Sm,Tb-Er) solid solutions

Ca_2.8Ln_0.2Co₄O_{9 + δ} (Ln = La, Nd, Sm, Tb-Er) solid solutions have been prepared via a citrate route and their structural parameters have been determined. Their thermal expansion, thermoelectric power, thermal conductivity, and electrical conductivity have been measured above room temperature. The results demonstrate that all of the materials obtained are p-type semiconductors. Their unit-cell parameters decrease with a decrease in the ionic radius of the Ln³⁺ cation substituted for Ca²⁺, and their thermoelectric power increases with an increase in the number of f electrons of the Ln³⁺ cation. We have determined the electron and phonon contributions to the thermal conductivity of the materials and evaluated the thermoelectric power factor and thermoelectric figure of merit of the oxide ceramics. The highest thermoelectric power factors are offered by the Ca_2.8Tb_0.2Co₄O_{9 + δ} and Ca_2.8Er_0.2Co₄O_{9 + δ} solid solutions: 0.27 and 0.29 mW/(m K²), respectively, at T = 1100 K.     

Charge and Sodium Ordering in β-Na0.33V2O3

Polarized Raman and optical spectra for the quasi one-dimensional metallic vanadate β-Na_0.33V₂O₃ are reported for various temperatures. The spectra are discussed in the light of the sodium and charge ordering transitions occurring in this material, and demonstrate the presence of strong electron–phonon coupling.     

Engineering Multiple Microstructural Defects for Record-Breaking Thermoelectric Properties of Chalcopyrite Cu1-xAgxGaTe2

Defect engineering for vacancies, holes, nano precipitates, dislocations, and strain are efficient means of suppressing lattice thermal conductivity. Multiple microstructural defects are successfully designed in Cu_1-xAg_xGaTe₂ (0 ≤ x ≤ 0.5) solid solutions through high-ratio alloying and vibratory ball milling, to achieve ultra-low thermal conductivity and record-breaking thermoelectric performance. Extremely low total thermal conductivities of 1.28 W m⁻¹ K⁻¹ at 300 K and 0.40 W m⁻¹ K⁻¹ at 873 K for the Cu_0.5Ag_0.5GaTe₂ are observed, which are ≈79% and ≈58% lower than that of the CuGaTe₂ matrix. Multiple phonon scattering mechanisms are collectively responsible for the reduction of thermal conductivity in this work. On one hand, large amounts of nano precipitates and dislocations are formed via vibrating ball milling followed by the low-temperature hot press, which can enhance phonon scattering. On the other hand, the difference in atomic sizes, distorted chemical bonds, elements fluctuation, and strained domains are caused by the high substitution ratio of Ag and also function as a center for the strong phonon scattering. As a result, the Cu_0.7Ag_0.3GaTe₂ exhibits a record high ZT_max of ≈1.73 at 873 K and ZT_ave of ≈0.69 between 300–873 K, which are the highest values of CuGaTe₂-based thermoelectric materials.     

Raman spectra and structural analysis in ZrOxNy thin films

Raman spectroscopy has been used as a local probe to characterize the structural evolution of magnetron-sputtered decorative zirconium oxynitride ZrO_xN_y films which result from an increase of reactive gas flow in the deposition. The lines shapes, the frequency position and widths of the Raman bands show a systematic change as a function of the reactive gas flow (a mixture of both oxygen and nitrogen). The as-deposited zirconium nitride film presents a Raman spectrum with the typical broadened bands, due to the disorder induced by N vacancies. The recorded Raman spectrum of the zirconium oxide film is typical of the monoclinic phase of ZrO₂, which is revealed also by X-ray diffraction. Raman spectra of zirconium oxynitride thin films present changes, which are found to be closely related with the oxygen content in films and the subsequent structural changes.     

Strain effects on thermoelectric properties of two-dimensional materials

Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride (hBN), phosphorene, transition metal dichalcogenides (e.g., MoS₂, WS₂, etc.), metal oxides (e.g., MoO₃) have attracted much attention recently due to their extraordinary structural, mechanical and physical properties. In particular, 2D materials have shown great potential for thermal management and thermoelectric energy generation due to their fascinating electrical and thermal transport properties, which can lead to a significantly large figure-of-merit. Also due to their large stretchability, 2D materials are promising for using strain engineering to tune and modulate their electronic and thermal properties, which can further enhance their figure-of-merit. In this article, we give a review on the recent advances in the study of strain-engineering on the thermoelectric properties of 2D materials. We first review some important aspects in thermoelectric effects, such as Peltier effect, Seebeck effect, the coefficient of performance and figure-of-merit (ZT) and discuss why 2D materials are ideal candidates for thermal management and thermoelectric applications. We then briefly discuss the strain (stress) generation in 2D materials and their structure integrity under strain (stress). Next, we discuss how strain affects the electronic properties of 2D materials, followed by the discussion on the effects of strain on the thermal properties of 2D materials. Subsequently, we discuss the strain effects on two important thermoelectric properties, Seebeck coefficient and figure-of-merit ZT. Finally, we present our conclusions and future perspective.     

Lattice vibrations and Raman scattering in two-dimensional layered materials beyond graphene

We review lattice vibrational modes in atomically thin two-dimensional (2D) layered materials, focusing on 2D materials beyond graphene, such as group VI transition metal dichalcogenides, topological insulator bismuth chalcogenides, and black phosphorus. Although the composition and structure of those materials are remarkably different, they share a common and important feature, i.e., their bulk crystals are stacked via van der Waals interactions between “layers” while each layer is comprised of one or more atomic planes. First, we review the background of some 2D materials (MX₂, M = Mo, W; X = S, Se, Te. Bi₂X₃, X = Se, Te. Black phosphorus), including crystalline structures and stacking order. We then review the studies on vibrational modes of layered materials and nanostructures probed by the powerful yet nondestructive Raman spectroscopy technique. Based on studies conducted before 2010, recent investigations using more advanced techniques have pushed the studies of phonon modes in 2D layered materials to the atomically thin regime, down to monolayers. We will classify the recently reported general features into the following categories: phonon confinement effects and electron–phonon coupling, anomalous shifts in high-frequency intralayer vibrational modes and surface effects, reduced dimensionality and lower symmetry, the linear chain model and the substrate effect, stacking orders and interlayer shear modes, polarization dependence, and the resonance effect. Within the seven categories, both intralayer and interlayer vibrational modes will be discussed. The comparison between different materials will be provided as well.

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Development of novel thermoelectric materials by reduction of lattice thermal conductivity

Abstract

Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a ‘natural superlattice’ in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered TX₂ structure which has a general formula of (MX)_1+x(TX₂)_n (M=Pb, Bi, Sn, Sb or a rare earth element; T=Ti, V, Cr, Nb or Ta; X=S or Se and n=1, 2, 3). We demonstrate that one of the intercalation compounds (SnS)_1.2(TiS₂)₂ has better thermoelectric properties compared with pure TiS₂ in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities.     

Development of novel thermoelectric materials by reduction of lattice thermal conductivity

Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a ‘natural superlattice’ in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered TX₂ structure which has a general formula of (MX)_1+x(TX₂)_n (M=Pb, Bi, Sn, Sb or a rare earth element; T=Ti, V, Cr, Nb or Ta; X=S or Se and n=1, 2, 3). We demonstrate that one of the intercalation compounds (SnS)_1.2(TiS₂)₂ has better thermoelectric properties compared with pure TiS₂ in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities.     

Surface Tension Components Based Selection of Cosolvents for Efficient Liquid Phase Exfoliation of 2D Materials

A proper design of direct liquid phase exfoliation (LPE) for 2D materials as graphene, MoS₂, WS₂, h‐BN, Bi₂Se₃, MoSe₂, SnS₂, and TaS₂ with common cosolvents is carried out based on considering the polar and dispersive components of surface tensions of various cosolvents and 2D materials. It has been found that the exfoliation efficiency is enhanced by matching the ratio of surface tension components of cosolvents to that of the targeted 2D materials, based on which common cosolvents composed of IPA/water, THF/water, and acetone/water can be designed for sufficient LPE process. In this context, the library of low‐toxic and low‐cost solvents with low boiling points for LPE is infinitely enlarged when extending to common cosolvents. Polymer‐based composites reinforced with a series of different 2D materials are compared with each other. It is demonstrated that the incorporation of cosolvents‐exfoliated 2D materials can substantially improve the mechanical and thermal properties of polymer matrices. Typically, with the addition of 0.5 wt% of such 2D material as MoS₂ nanosheets, the tensile strength and Young's modulus increased up to 74.85% and 136.97%, respectively. The different enhancement effect of 2D materials is corresponded to the intrinsic properties and LPE capacity of 2D materials.     

Realizing zT of 2.3 in Ge1−x−ySbxInyTe via Reducing the Phase‐Transition Temperature and Introducing Resonant Energy Doping

GeTe with rhombohedral‐to‐cubic phase transition is a promising lead‐free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon–phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge_1?xSb_xTe with optimized carrier concentration, a record‐high figure‐of‐merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase‐transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high‐density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high‐performance Pb‐free thermoelectric materials.     

High purity synthesis of ZrB2 by a combined ball milling and carbothermal method: Structural and magnetic properties

The present work provides a new insight into the high purity synthesis of zirconium diboride (ZrB₂) powders and a method of controlling impurity during the synthesis process. The single phase ZrB₂ nano-powder was synthesized by a combined ball milling and carbothermal method using zirconium oxide (ZrO₂), boron oxide (B₂O₃) and carbon (C) as starting materials. The reaction pathway, phase purity, and morphology of the ZrB₂ produced are elucidated from X-ray diffraction (XRD) and scanning electron microscopy studies. The details of the impure phases generated during synthesis were obtained from multi-phase Rietveld refinements of XRD data. Experiments revealed that the method of synthesis carried out at 1750?°C involving ZrB₂:B₂O₃:C at a molar ratio of 1:4.5:7.5 could produce highly pure ZrB₂ nano-powders of 67?nm average crystallite size. The magnetometry studies on such pure form of ZrB₂ nano-powders indicated that both paramagnetic and diamagnetic characteristics coexisted in ZrB₂, which could be attributed to its polycrystallinity.     

Infra-red transmitting glass-ceramics of the As-Ge-Se system nucleated by zirconium selenide

The aim of this study was to produce an infra-red transmitting chalcogenide glass-ceramic nucleated by an appropriate nucleant. In order to crystallize chalcogenide glasses in a controlled manner a preliminary series of experiments, in which the metals of group IVB of the periodic table of elements were used as nucleants, showed that zirconium is an effective additive. Studies of glass compositionsxZrSe₂-(100–x) [As_0.1Ge_0.3Se_0.6] indicated that for values ofx 0.25 and 0.50 mol%, glass-ceramics with high thermomechanical properties were produced. By heat treating at 400° C for 15 h, the glass As_0.1Ge_0.3Se_0.6 with 0.25 mol% ZrSe₂ was transformed into a glass-ceramic with a fracture toughness of 0.848 MN m^–3/2, a sag point of 530° C and satisfactory infra-red transmission. X-ray diffraction analysis revealed that the precipitated crystal phase was germanium selenide (GeSe₂). The parasite absorption band with a maximum around 800cm^–1 generally present in the infra-red spectra was eliminated by adding 0.1 wt% metallic zirconium. The infra-red transmision and thermomechanical properties for glass-ceramics of compositions with 0.50 mol% ZrSe₂ were poorer than for those with 0.25 mol% ZrSe₂, for the same heat treatment condtions. The kinetics and activation energies of crystallization were studied by means of electron microscopy. The mechanism of controlled crystallization proceeds by the precipitation of crystalline GeSe₂ on the ZrSe₂ nuclei formed by heat treatment of the initial chalcogenide glass supersaturated in ZrSe₂.     

3D Anisotropic Thermal Conductivity of Exfoliated Rhenium Disulfide

ReS₂ represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single‐crystalline ReS₂ in a distorted 1T phase is determined at room temperature for the in‐plane directions parallel and perpendicular to the Re‐chains, and the through‐plane direction using time‐domain thermoreflectance. ReS₂ is prepared in the form of flakes having thicknesses of 60–450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in‐plane thermal conductivity is higher along the Re‐chains, (70 ± 18) W m^?1 K^?1, as compared to transverse to the chains, (50 ± 13) W m^?1 K^?1. As expected from the weak interlayer bonding, the through‐plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m^?1 K^?1, resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in‐plane directions. The thermal conductivity and interface thermal conductance of ReS₂ are discussed relative to the other 2D materials.     

Synthesis and properties of Na<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>CoO<Subscript>2</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0.55, 0.89) oxide thermoelectrics

Na_xCoO₂ (x = 0.55, 0.89) sodium cobaltites have been prepared by solid-state reactions; their structural parameters have been determined; their microstructure has been studied; and their thermal (thermal expansion, thermal diffusivity, and thermal conductivity), electrical (electrical conductivity and thermoelectric power), and functional (power factor, thermoelectric figure of merit, and self-compatibility factor) properties have been investigated in air at temperatures from 300 to 1100 K. The results demonstrate that, with increasing sodium content, the electrical conductivity and thermoelectric power of the materials increase and their thermal conductivity decreases. As a result, the power factor and thermoelectric figure of merit of the Na_0.89CoO₂ ceramic at a temperature of 1100 K reach 0.829 mW/(m K²) and 1.57, respectively. The electron and phonon (lattice) contributions to the thermal conductivity of the ceramics have been separately assessed, and their linear thermal expansion coefficients have been evaluated.     

Triboelectric Series of 2D Layered Materials

Recently, as applications based on triboelectricity have expanded, understanding the triboelectric charging behavior of various materials has become essential. This study investigates the triboelectric charging behaviors of various 2D layered materials, including MoS₂, MoSe₂, WS₂, WSe₂, graphene, and graphene oxide in a triboelectric series using the concept of a triboelectric nanogenerator, and confirms the position of 2D materials in the triboelectric series. It is also demonstrated that the results are obviously related to the effective work functions. The charging polarity indicates the similar behavior regardless of the synthetic method and film thickness ranging from a few hundred nanometers (for chemically exfoliated and restacked films) to a few nanometers (for chemical vapor deposited films). Further, the triboelectric charging characteristics could be successfully modified via chemical doping. This study provides new insights to utilize 2D materials in triboelectric devices, allowing thin and flexible device fabrication.