Defect Chemistry for Thermoelectric Materials

Suppressing Se Vacancies in Sb2Se3 Photocathode by In Situ Annealing with Moderate Se Vapor Pressure for Efficient Photoelectrochemical Water Splitting

Sb2Se3 emerges as a promising material for solar energy conversion devices. Unfortunately, the common deep-level defect VSe (selenium vacancy) in Sb2Se3 results in a low solar conversion efficiency. The post selenization process has been widely adopted for suppressing VSe. However, the effect of selenization on suppressing VSe is often compromised and even more VSe are induced due to defect-correlation. Herein, high-quality Sb2Se3 films are prepared using an unconventional selenization process, with precisely regulating in situ annealing Se vapor pressure. It is found that moderate Se vapor pressure annealing can efficiently suppress VSe by overcoming defect-correlation, as well as promotes grain growth and forms a better heterojunction band alignment. Consequently, the Sb2Se3 photocathode shows a high-level photocurrent of 19.5 mA cm-2 at 0 VRHE, an onset potential of 0.40 VRHE and a half-cell solar-to-hydrogen conversion efficiency of 1.9%, owing to the inhibited charge recombination, excellent charge transport and interface charge extraction. This work provides a significant insight to suppress deep-level defect VSe by adjusting Se vapor pressure for efficient Sb2Se3 photocathode.

Achieving ZT > 1 in Cu and Ga Co-doped Ag6Ge10P12 with Superior Mechanical Performance and Its Fundamental Physical Properties toward Practical Thermoelectric Device Applications

Recently, phosphorus-based compounds have emerged as potential candidates for thermoelectric materials. One of the key challenges facing this field is to achieve ZT > 1, which is the benchmark for thermoelectric device applications. In this study, it is demonstrated that the thermoelectric performance of environmentally friendly Ag6Ge10P12 is enhanced by co-doping Cu and Ga. The mechanical properties, coefficient of linear thermal expansion, work function, and compatibility factor are comprehensively clarified to provide guidelines for reliable device applications. The peak and average dimensionless figures of merit of Ag5.85Cu0.15Ge9.875Ga0.125P12 reach 1.04 at 723 K and 0.63 at 300-723 K, respectively, which are the highest values for phosphorus-based thermoelectric materials. The Young's modulus, Vickers microhardness, fracture toughness, and compressive strength of Ag5.85Cu0.15Ge9.875Ga0.125P12 are 132 GPa, 589, 1.23 MPa m1/2, and 219 MPa, respectively, which are superior to those of typical state-of-the-art thermoelectric materials. The remarkable thermoelectric and mechanical performance of Ag5.85Cu0.15Ge9.875Ga0.125P12 mean that it is a promising candidate for medium-temperature thermoelectric conversion. Ti, V, Rh, and Pt are suitable for electrodes without exfoliation under thermal expansion and with ohmic contacts to Ag5.85Cu0.15Ge9.875Ga0.125P12 in terms of the coefficient of linear thermal expansion and work function. Considering that the compatibility factor of Ag5.85Cu0.15Ge9.875Ga0.125P12 is approximately 2.8, half-Heusler, skutterudite, and magnesium silicide-stannide compounds are suitable n-type thermoelectric counterpart materials in thermoelectric devices. These insights will lead to the development of phosphorus-based thermoelectric materials toward practical thermoelectric device applications.

Principles and Methods for Improving the Thermoelectric Performance of SiC: A Potential High-Temperature Thermoelectric Material

Thermoelectric materials that can convert thermal energy to electrical energy are stable and long-lasting and do not emit greenhouse gases; these properties render them useful in novel power generation devices that can conserve and utilize lost heat. SiC exhibits good mechanical properties, excellent corrosion resistance, high-temperature stability, non-toxicity, and environmental friendliness. It can withstand elevated temperatures and thermal shock and is well suited for thermoelectric conversions in high-temperature and harsh environments, such as supersonic vehicles and rockets. This paper reviews the potential of SiC as a high-temperature thermoelectric and third-generation wide-bandgap semiconductor material. Recent research on SiC thermoelectric materials is reviewed, and the principles and methods for optimizing the thermoelectric properties of SiC are discussed. Thus, this paper may contribute to increasing the application potential of SiC for thermoelectric energy conversion at high temperatures.

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing.

Two-dimensional Mo1-xB2 with ordered metal vacancies obtained for advanced thermoelectric applications based on first-principles calculations

The study and development of high thermoelectric properties is crucial for the next generation of microelectronic and wearable electronics. Derived from the recent experimental realization of layers of transition metal molybdenum and boride, we report the theoretical realization of advanced thermoelectric properties in two-dimensional (2D) transition metal boride Mo1-xB2 (x = 0, 0.05, 0.10, 0.125, 0.15)-based defect sheets. The introduction of metal vacancies results in stronger d-p exchange interactions and hybridization between the Mo-d and B-p atoms. Meanwhile, the ordered metal vacancies enabled transition metal borides (n-type Mo0.9B2) to widen the d-bandwidth and raise the d-band center, leading to a relatively high carrier mobility of 3262 cm2 V-1 s-1 and conductivity twice that of a bug-free n-type MoB2 layer, which indicates that it presents good electronic and thermal transport properties. Furthermore, investigations of the thermoelectric performance exhibit a maximum ZT of up to 3.29, which is superior to those of currently reported 2D materials. Modulation by defect engineering suggests that 2D transition metal boride sheets with ordered metal vacancies have promising applications in microelectronics, wearable electronics and thermoelectric devices.

Computational approach to enhance thermoelectric performance of Ag2Se by S and Te substitutions

Designing an n-type thermoelectric material with a high thermoelectric figure of merit at near room temperature is extremely challenging. Generally, pristine Ag2Se reveals unusually low thermal conductivity along with a high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudo-ternary phase (Ag2Se0.5Te0.25S0.25) that exhibits significantly high thermoelectric performance (zT ∼ 2.1) even at 400 K. First-principles calculation reveals that the Rashba type of spin-dependent band spitting, which originates due to sulfur and tellurium substitution, helps to improve the thermopower magnitude. We also show that the intrinsic carrier mobility is not only controlled by the carrier effective mass but is substantially limited by longitudinal acoustic and optical phonon modes, which is an extension of the deformation potential theory. Locally off-center sulfur atoms, together with the increase in configurational entropy via substitution of Te and S atoms in Ag2Se, lead to a drastic reduction in the lattice thermal conductivity (klat ∼ 0.34 W m-1 K-1 at 400 K). The Rashba effect coupled with the configurational entropy synergistically results in a high thermoelectric figure of merit in the n-type thermoelectric material working in the near-room-temperature regime.

Extremely Low Lattice Thermal Conductivity and Significantly Enhanced Near-Room-Temperature Thermoelectric Performance in α-Cu2Se through the Incorporation of Porous Carbon

In this work, a series of Cu2Se/x wt % porous carbon (PC) (x = 0, 0.2, 0.4, 0.6, 0.8, 1) composite materials were synthesized by ball milling and spark plasma sintering (SPS). The highly ordered porous carbon was synthesized by a hydrothermal method using mesoporous silica (SBA-15) as the template. X-ray diffraction results show that the incorporation of porous carbon induces a phase transition of Cu2Se from the β phase to the α phase. Meanwhile, the addition of porous carbon reduces the carrier concentration from 2.7 × 1021 to 2.45 × 1020 cm-3 by 1 order of magnitude. The decrease of the carrier concentration leads to the reduction of electrical conductivity and the increase of the Seebeck coefficient, which results in the enhancement of the power factor. On the other hand, the incorporation of porous carbon into Cu2Se increases the porosity of the composites and also introduces more interfaces between the two materials, which is evidenced by positron annihilation lifetime measurements. Both pores and interfaces greatly enhance phonon scattering, leading to extremely low lattice thermal conductivity. In addition, the decrease of electrical conductivity also causes a sufficient reduction in electronic thermal conductivity. Due to the above synergistic effects, the thermoelectric performance of the Cu2Se/PC composite is significantly enhanced with a maximum ZT value of 0.92 at 403 K in the Cu2Se/1 wt % PC composite, which is close to that of the Bi2Te3-based materials. Our work shows that α-Cu2Se has great potential for near-room-temperature thermoelectric materials.

Review on design strategies and applications of flexible cellulose‑carbon nanotube functional composites

Combining the excellent biocompatibility and mechanical flexibility of cellulose with the outstanding electrical, mechanical, optical and stability properties of carbon nanotubes (CNTs), cellulose-CNT composites have been extensively studied and applied to many flexible functional materials. In this review, we present advances in structural design strategies and various applications of cellulose-CNT composites. Firstly, the structural characteristics and corresponding treatments of cellulose and CNTs are analyzed, as are the potential interactions between the two to facilitate the formation of cellulose-CNT composites. Then, the design strategies and processing techniques of cellulose-CNT composites are discussed from the perspectives of cellulose fibers at the macroscopic scale (natural cotton, hemp, and other fibers; recycled cellulose fibers); nanocellulose at the micron scale (nanofibers, nanocrystals, etc.); and macromolecular chains at the molecular scale (cellulose solutions). Further, the applications of cellulose-CNT composites in various fields, such as flexible energy harvesting and storage devices, strain and humidity sensors, electrothermal devices, magnetic shielding, and photothermal conversion, are introduced. This review will help readers understand the design strategies of cellulose-CNT composites and develop potential high-performance applications.

The manipulation of natural mineral chalcopyrite CuFeS2via mechanochemistry: properties and thermoelectric potential

In this study, the properties of the natural mineral chalcopyrite CuFeS2 after mechanical activation in a planetary mill were studied. The intensity of mechanical activation was controlled by changing the revolutions of the mill in the range 100-600 min-1. A series of characterization techniques, such as XRD, SEM, TEM, TA (DTA, TG, and DTG), particle size analysis, and UV-vis spectroscopy was applied and reactivity studies were also performed. Several new features were revealed for the mechanically activated chalcopyrite, e.g. the poly-modal distribution of produced nanoparticles on the micrometer scale, agglomeration effects by prolonged milling, possibility to modify the shape of the particles, X-ray amorphization and a shift from a non-cubic (tetragonal) structure to pseudo-cubic structure. The thermoelectric response was evaluated on the "softly" compacted powder via the spark plasma sintering method (very short holding time, low sintering temperature, and moderate pressure) by measuring the Seebeck coefficient and electrical and thermal conductivity above room temperature. The milling process produced samples with lower resistivity compared to the original non-activated sample. The Seebeck data close to zero confirmed the "compensated" character of natural chalcopyrite, reflecting its close-to stoichiometric composition with low concentration of both n- and p-type charge carriers. Alternatively, an evident correlation between thermal conductivity and energy supply by milling was observed with the possibility of band gap manipulation, which is associated with the energy delivered by the milling procedure.

Defect Engineering in Biomedical Sciences

With the promotion of nanochemistry research, large numbers of nanomaterials have been applied in vivo to produce desirable cytotoxic substances in response to endogenous or exogenous stimuli for achieving disease-specific therapy. However, the performance of nanomaterials is a critical issue that is difficult to improve and optimize under biological conditions. Defect-engineered nanoparticles have become the most researched hot materials in biomedical applications recently due to their excellent physicochemical properties, such as optical properties and redox reaction capabilities. Importantly, the properties of nanomaterials can be easily adjusted by regulating the type and concentration of defects in the nanoparticles without requiring other complex designs. Therefore, this tutorial review focuses on biomedical defect engineering and briefly discusses defect classification, introduction strategies, and characterization techniques. Several representative defective nanomaterials are especially discussed in order to reveal the relationship between defects and properties. A series of disease treatment strategies based on defective engineered nanomaterials are summarized. By summarizing the design and application of defective engineered nanomaterials, a simple but effective methodology is provided for researchers to design and improve the therapeutic effects of nanomaterial-based therapeutic platforms from a materials science perspective.

Correlation of rattlers with thermal transport and thermoelectric performance

The presence of rattlers in the host-guest structure has sparked great interest in the field of thermoelectrics, as it allows for the suppression of thermal transport in materials through vigorous anharmonic vibrations. This work predicts a ternary half-Heusler compound, LiAgTe, with good thermoelectric properties and high-temperature stability, which possesses a host-guest structure. Furthermore, it provides a detailed analysis of the role of rattlers in the transport process. By microscopically exploring rattlers, we have revealed that rattlers (Ag atoms), while suppressing the thermal transport properties of the host framework, provide a significant enhancement of the electronic transport capability through the provision of nearly free weakly bound electrons. Using self-consistent phonon theory combined with compressive sensing lattice dynamics method, we captured the exact lattice thermal conductivity considering quartic anharmonicity and four-phonon scattering, and obtained the electronic transport parameters through the calculation of τe, which includes full anisotropic acoustic deformation potential scattering, polar optical phonon scattering, and ionized impurity scattering. We systematically dissected the role of rattlers in the host-guest structure by combining methods such as electron local function, Bader charge density, and Vibration visualization. The anharmonic vibrations of rattlers enhance the temperature response of scattering, resulting in rapid deterioration of thermal transport at high temperatures. Moreover, the extended d-orbital electrons of the rattlers, together with the p-orbital electrons of the Te atom in the host framework, result in the coexistence of maximum degeneracy and high dispersion bands in the valence band, which greatly enhances the electronic transport properties.

Non-Equilibrium Thermodynamics of Heat Transport in Superlattices, Graded Systems, and Thermal Metamaterials with Defects

In this review, we discuss a nonequilibrium thermodynamic theory for heat transport in superlattices, graded systems, and thermal metamaterials with defects. The aim is to provide researchers in nonequilibrium thermodynamics as well as material scientists with a framework to consider in a systematic way several nonequilibrium questions about current developments, which are fostering new aims in heat transport, and the techniques for achieving them, for instance, defect engineering, dislocation engineering, stress engineering, phonon engineering, and nanoengineering. We also suggest some new applications in the particular case of mobile defects.

Toward High Conversion Efficiency of Thermoelectric Modules through Synergistical Optimization of Layered Materials

Waste-heat electricity generation using high-efficiency solid-state conversion technology can significantly decrease dependence on fossil fuels. Here, a synergistical optimization of layered half-Heusler (hH) materials and module to improve thermoelectric conversion efficiency is reported. This is realized by manufacturing multiple thermoelectric materials with major compositional variations and temperature-gradient-coupled carrier distribution by one-step spark plasma sintering. This strategy provides a solution to overcome the intrinsic concomitants of the conventional segmented architecture that only considers the matching of the figure of merit (zT) with the temperature gradient. The current design is dedicated to temperature-gradient-coupled resistivity and compatibility matching, optimum zT matching, and reducing contact resistance sources. By enhancing the quality factor of the materials by Sb-vapor-pressure-induced annealing, a superior zT of 1.47 at 973 K is achieved for (Nb, Hf)FeSb hH alloys. Along with the low-temperature high-zT hH alloys of (Nb, Ta, Ti, V)FeSb, the single stage layered hH modules are developed with efficiencies of ≈15.2% and ≈13.5% for the single-leg and unicouple thermoelectric modules, respectively, under ΔT of 670 K. Therefore, this work has a transformative impact on the design and development of next-generation thermoelectric generators for any thermoelectric material families.

High-Performance Industrial-Grade p-Type (Bi,Sb)2 Te3 Thermoelectric Enabled by a Stepwise Optimization Strategy

As the sole dominator of the commercial thermoelectric (TE) market, Bi₂ Te₃ -based alloys play an irreplaceable role in Peltier cooling and low-grade waste heat recovery. Herein, to improve the relative low TE efficiency determined by the figure of merit ZT, an effective approach is reported for improving the TE performance of p-type (Bi,Sb)₂ Te₃ by incorporating Ag₈ GeTe₆ and Se. Specifically, the diffused Ag and Ge atoms into the matrix conduce to optimized carrier concentration and enlarge the density-of-states effective mass while the Sb-rich nanoprecipitates generate coherent interfaces with little loss of carrier mobility. The subsequent Se dopants introduce multiple phonon scattering sources and significantly suppress the lattice thermal conductivity while maintaining a decent power factor. Consequently, a high peak ZT of 1.53 at 350 K and a remarkable average ZT of 1.31 (300-500 K) are attained in the Bi_0.4 Sb_1.6 Te_0.95 Se_0.05  + 0.10 wt% Ag₈ GeTe₆ sample. Most noteworthily, the size and mass of the optimal sample are enlarged to Ø40 mm-200 g and the constructed 17-couple TE module exhibits an extraordinary conversion efficiency of 6.3% at ΔT = 245 K. This work demonstrates a facile method to develop high-performance and industrial-grade (Bi,Sb)₂ Te₃ -based alloys, which paves a strong way for further practical applications.

Ag2 Q-Based (Q = S, Se, Te) Silver Chalcogenide Thermoelectric Materials

Thermoelectric technology provides a promising solution to sustainable energy utilization and scalable power supply. Recently, Ag₂ Q-based (Q = S, Se, Te) silver chalcogenides have come forth as potential thermoelectric materials that are endowed with complex crystal structures, high carrier mobility coupled with low lattice thermal conductivity, and even exceptional plasticity. This review presents the latest advances in this material family, from binary compounds to ternary and quaternary alloys, covering the understanding of multi-scale structures and peculiar properties, the optimization of thermoelectric performance, and the rational design of new materials. The "composition-phase structure-thermoelectric/mechanical properties" correlation is emphasized. Flexible and hetero-shaped thermoelectric prototypes based on Ag₂ Q materials are also demonstrated. Several key problems and challenges are put forward concerning further understanding and optimization of Ag₂ Q-based thermoelectric chalcogenides.

Theoretical study of Cr2X3S3(X = Br, I) monolayers for thermoelectric and spin caloritronics properties

High curie temperature 2D materials are important for the progress of the field of spin caloritronics. The spin Seebeck effect and conventional thermoelectric figure of merit (ZT) can give a great insight into how these 2D magnetic materials will perform in spin caloritronics applications. Here in this paper, we have systematically studied 2D Janus monolayers based on CrX₃monolayers. We obtain a ZT of 0.31 and 0.21 for the Cr₂Br₃S₃and Cr₂I₃S₃Janus monolayers. The spin Seebeck coefficient obtained at room temperature is also very high (∼1570μVK^-1in the hole-doped region and ∼1590μVK^-1in the electron-doped region). The thermal conductivity of these monolayers (∼22 Wm^-1K^-1for Cr₂Br₃S₃and ∼16 Wm^-1K^-1for Cr₂I₃S₃) are also very similar to other 2D semiconductor transition metals chalcogenides. These findings suggest a high potential for these monolayers in the spin caloritronics field.

Highly Thermoelectric ZnO@MXene (Ti3C2Tx) Composite Films Grown by Atomic Layer Deposition

Due to its unique high conductivity and flexibility, the two-dimensional MXene material (Ti₃C₂T_x) is expected to possess great potential in the thermoelectric field. However, the low thermoelectric performance from high thermal conductivity and a low Seebeck coefficient has limited its practical application. In this report, we demonstrate the uniform growth of ZnO layers on the laminar Ti₃C₂T_x membrane by atomic layer deposition (ALD). Benefiting from the low-temperature deposition characteristics of the ALD technique, the ZnO@Ti₃C₂T_x composite films maintain the basic apparent morphology of the original films after the deposition. We reveal that the Schottky barrier formed between ZnO and Ti₃C₂T_x exhibits an energy-filtering effect, significantly enhancing the Seebeck coefficient to result in more than a double increase in the power factor. Meanwhile, the strong phonon-interface scattering between ZnO and Ti₃C₂T_x is found to reduce the thermal conductivity of the composite films by a factor of four as compared to pure Ti₃C₂T_x ones, further improving the overall thermoelectric properties of the ZnO@Ti₃C₂T_x composite films. Our investigation provides an ALD-based strategy for growing wide band gap layers on the narrow band gap films to improve the thermoelectric performance of various MXene materials.

Thermodefect voltage in graphene nanoribbon junctions

Thermoelectric junctions are often made of components of different materials characterized by distinct transport properties. Single material junctions, with the same type of charge carriers, have also been considered to investigate various classical and quantum effects on the thermoelectric properties of nanostructured materials. We here introduce the concept of defect-induced thermoelectric voltage, namely,thermodefect voltage, in graphene nanoribbon (GNR) junctions under a temperature gradient. Our thermodefect junction is formed by two GNRs with identical properties except the existence of defects in one of the nanoribbons. At room temperature the thermodefect voltage is highly sensitive to the types of defects, their locations, as well as the width and edge configurations of the GNRs. We computationally demonstrate that the thermodefect voltage can be as high as 1.7 mV K^-1for 555-777 defects in semiconducting armchair GNRs. We further investigate the Seebeck coefficient, electrical conductance, and electronic thermal conductance, and also the power factor of the individual junction components to explain the thermodefect effect. Taken together, our study presents a new pathway to enhance the thermoelectric properties of nanomaterials.

Bulk Superlattice Analogues for Energy Conversion

Energy storage and conversion in a clean, efficient, and safe way is the core appeal of a modern sustainable society, which is built on the development of multifunctional materials. Superlattice structures can integrate the advantage of their sublayers while new phenomena may arise from the interface, which play key roles in modern semiconductor technology; however, additional concerns such as stability and yield challenge their large-scale applications in industrial products. In this Perspective we focus our interest on a distinctive category of easily available multilayered inorganic materials that have well-defined subunit structures and can be regarded as bulk superlattice analogues. We illustrate several specific combining forms of subunits in bulk superlattice analogues, including soft/rigid sublayers, electron/phonon transport sublayers, quasi-two-dimensional layers, and intercalated metal layers. We hope to provide insights into material design and broaden the application scope in the field of energy conversion by integrating the versatility of subunits into these bulk superlattice analogues.

Mixed-Valence CsCu4Se3: Large Phonon Anharmonicity Driven by the Hierarchy of the Rigid [(Cu+)4(Se2-)2](Se-) Double Anti-CaF2 Layer and the Soft Cs+ Sublattice

Crystalline solids that exhibit inherently low lattice thermal conductivity (κ_lat) have attracted a great deal of attention because they offer the only independent control for pursuing a high thermoelectric figure of merit (ZT). Herein, we report the successful preparation of CsCu₄Q₃ (Q = S (compound 1), Se (compound 2)) with the aid of a safe and facile boron-chalcogen method. The single-crystal diffraction data confirm the P4/mmm hierarchical structures built up by the mixed-valence [(Cu⁺)₄(Q^2-)₂](Q^-) double anti-CaF₂ layer and the NaCl-type Cs⁺ sublattice involving multiple bonding interactions. The electron-poor compound CsCu₄Q₃ features Cu-Q antibonding states around E_F that facilitates a high σ value of 3100 S/cm in 2 at 323 K. Significantly, the ultralow κ_lat value of 2, 0.20 W/m/K at 650 K (70% lower than that of Cu₂Se), is mainly driven by the vibrational coupling of the rigid double anti-CaF₂ layer and the soft NaCl-type sublattice. The hierarchical structure increases the bond multiplicity, which eventually leads to a large phonon anharmonicity, as evidenced by the effective scattering of the low-lying optical phonons to the heat-carrying acoustic phonons. Consequently, the acoustic phonon frequency in 2 drops sharply from 118 cm^-1 (of Cu₂Se) to 48 cm^-1. In addition, the elastic properties indicate that the hierarchical structure largely inhibits the transverse phonon modes, leading to a sound velocity (1571 m/s) and a Debye temperature (189 K) lower than those of Cu₂Se (2320 m/s; 292 K).

ZnIn2 S4 -Based Photocatalysts for Energy and Environmental Applications

As a fascinating visible-light-responsive photocatalyst, zinc indium sulfide (ZnIn₂ S₄ ) has attracted extensive interdisciplinary interest and is expected to become a new research hotspot in the near future, due to its nontoxicity, suitable band gap, high physicochemical stability and durability, ease of synthesis, and appealing catalytic activity. This review provides an overview on the recent advances in ZnIn₂ S₄ -based photocatalysts. First, the crystal structures and band structures of ZnIn₂ S₄ are briefly introduced. Then, various modulation strategies of ZnIn₂ S₄ are outlined for better photocatalytic performance, which includes morphology and structure engineering, vacancy engineering, doping engineering, hydrogenation engineering, and the construction of ZnIn₂ S₄ -based composites. Thereafter, the potential applications in the energy and environmental area of ZnIn₂ S₄ -based photocatalysts are summarized. Finally, some personal perspectives about the promises and prospects of this emerging material are provided.

More Se Vacancies in Sb2 Se3 under Se-Rich Conditions: An Abnormal Behavior Induced by Defect-Correlation in Compensated Compound Semiconductors

It was believed that the Se-rich synthesis condition can suppress the formation of deep-level donor defect V_Se (selenium vacancy) in Sb₂ Se₃ and is thus critical for fabricating high-efficiency Sb₂ Se₃ solar cells. However, here it is shown that by first-principles calculations the density of V_Se increases unexpectedly to 10¹⁶ cm^-3 when the Se chemical potential increases, so Se-rich condition promotes rather than suppresses the formation of V_Se . Therefore, high density of V_Se is thermodynamically inevitable, no matter under Se-poor or Se-rich conditions. This abnormal behavior can be explained by a physical concept "defect-correlation", i.e., when donor and acceptor defects compensate each other, all defects become correlated with each other due to the formation energy dependence on Fermi level which is determined by densities of all ionized defects. In quasi-1D Sb₂ Se₃ , there are many defects and the complicated defect-correlation can give rise to abnormal behaviors, e.g., lowering Fermi level and thus decreasing the formation energy of ionized donor V_Se ²⁺ in Se-rich Sb₂ Se₃ . Such behavior exists also in Sb₂ S₃ . It explains the recent experiments that the extremely Se-rich condition causes the efficiency drop of Sb₂ Se₃ solar cells, and demonstrates that the common chemical intuition and defect engineering strategies may be invalid in compensated semiconductors.

Defect engineering in thermoelectric materials: what have we learned?

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Electron Beam Irradiation-Induced Formation of Defect-Rich Zeolites under Ambient Condition within Minutes

Zeolites are a well-known family of microporous aluminosilicate crystals with a wide range of applications. Their industrial synthetic method under hydrothermal condition requires elevated temperature and long crystallization time and is therefore quite energy-consuming. Herein, we utilize high-energy electron beam irradiation generated by an industrial accelerator as a distinct type of energy source to activate the formation reaction of Na-A zeolite. The initial efforts afford an attractive reaction process that can be achieved under ambient conditions and completed within minutes with almost quantitative yield, leading to notable energy saving of one order of magnitude compared to the hydrothermal reaction. More importantly, electron beam irradiation simultaneously exhibits an etching effect during the formation of zeolite generating a series of crystal defects and additional pore windows that can be controlled by irradiation dose. These observations give rise to significantly enhanced surface area and heavy metal removal capabilities in comparison with Na-A zeolite synthesized hydrothermally. Finally, we show that this method can be applied to many other types of zeolites.

One-Dimensional Frenkel Chain Defects in CsBi4Te6

Understanding the detailed process of spontaneous formation of intrinsic defects and their ability to tune the electronic structures in functional materials has become a key prerequisite for their technological applications. Here, by using in situ scanning tunneling microscopy, we report the observation of one-dimensional Frenkel chain defects on the cleaved CsBi₄Te₆ surface due to the migration of Te atoms for the first time. Further scanning tunneling spectroscopy measurements clearly revealed a self-electron doping effect of the Frenkel chain defects, which could directly affect their thermoelectric and superconducting properties. The unique one-dimensional Frenkel tellurium atomic chain defect and its doping effect on the electronic structure observed here not only shed light on tuning the electric properties of a series of tellurides but also possess profound implications for enriching the microscopic details of defect chemistry and materials science.

Bismuth Doping in Nanostructured Tetrahedrite: Scalable Synthesis and Thermoelectric Performance

In this study, we demonstrate the feasibility of Bi-doped tetrahedrite Cu₁₂Sb_4-xBi_xS₁₃ (x = 0.02-0.20) synthesis in an industrial eccentric vibratory mill using Cu, Sb, Bi and S elemental precursors. High-energy milling was followed by spark plasma sintering. In all the samples, the prevailing content of tetrahedrite Cu₁₂Sb₄S₁₃ (71-87%) and famatinite Cu₃SbS₄ (13-21%), together with small amounts of skinnerite Cu₃SbS₃, have been detected. The occurrence of the individual Cu-Sb-S phases and oxidation states of bismuth identified as Bi⁰ and Bi³⁺ are correlated. The most prominent effect of the simultaneous milling and doping on the thermoelectric properties is a decrease in the total thermal conductivity (κ) with increasing Bi content, in relation with the increasing amount of famatinite and skinnerite contents. The lowest value of κ was achieved for x = 0.2 (1.1 W m^-1 K^-1 at 675 K). However, this sample also manifests the lowest electrical conductivity σ, combined with relatively unchanged values for the Seebeck coefficient (S) compared with the un-doped sample. Overall, the lowered electrical performances outweigh the benefits from the decrease in thermal conductivity and the resulting figure-of-merit values illustrate a degradation effect of Bi doping on the thermoelectric properties of tetrahedrite in these synthesis conditions.

Enhancing the Thermoelectric Performance of Mg2Sn Single Crystals via Point Defect Engineering and Sb Doping

Mg₂Sn is a potential thermoelectric (TE) material that exhibits environmental compatibility. In this study, we fabricated Sb-doped Mg₂Sn (Mg₂Sn_1-xSb_x) single-crystal ingots and demonstrated the enhancement of TE performance via point defect engineering and Sb doping. The Mg₂Sn_1-xSb_x single-crystal ingots exhibited considerably enhanced electrical conductivity because of the donor-doping effect in addition to high carrier mobility. Moreover, the Mg₂Sn_1-xSb_x single-crystal ingots contained Mg vacancy (V_Mg) as a point defect. The introduced V_Mg and doped Sb atoms formed nanostructures, both acting as phonon-scattering centers. Consequently, lower lattice thermal conductivity was achieved for the Mg₂Sn_1-xSb_x single-crystal ingots compared with polycrystalline counterparts. Owing to the significant enhancement in the electrical conductivity and the reduction in the lattice thermal conductivity, the maximum power factor of 5.1(4) × 10^-3 W/(K² m) and the maximum dimensionless figure of merit of 0.72(5) were achieved for the Mg₂Sn_0.99Sb_0.01 single-crystal ingot, which are higher than those of single-phase Mg₂Sn_1-xSb polycrystals.

Defect Compensation Weakening Induced Mobility Enhancement in Thermoelectric BiTeI by Iodine Deficiency

Carrier mobility (weighted mobility more specifically) of thermoelectrics fundamentally determines its power factor, representing a new cut-in point to optimize the thermoelectric performance. However, researches on enhancing the carrier mobility to improve power factor has been overlooked. In present work, we highlight a significant mobility enhancement in BiTeI by introducing I deficiency, which improves the power factor and final ZT value. A defect compensation weakening mechanism is adopted that the induced I vacancies reduce the concentration of intrinsic I Te • and Te I ' antisite defects, which weakens the donor-acceptor defect compensation and suppresses the defects-induced carrier scattering. As a result, the carrier mobility is obviously enhanced in I-deficient samples, which ensures an effectively improved power factor and final ZT. A maximum ZT of 0.57 is achieved at 570 K perpendicular to the pressing direction, which is superior to pristine BiTeI and among the highest values reported for bulk BiTeI-based thermoelectric materials. Present work opens up a new avenue for thermoelectric optimization mainly by mobility enhancement.

Ternary Compounds Cu3RTe3 (R = Y, Sm, and Dy): A Family of New Thermoelectric Materials with Trigonal Structures

In this work, we report a series of Cu₃RTe₃ (R = Y, Sm, and Dy) ternary compounds with a trigonal structure (R3̅) as a family of new thermoelectric materials. First-principles calculations show that Cu₃RTe₃ (R = Y, Sm, and Dy) compounds are semiconductors with similar band structures and moderate band gaps (0.69-0.82 eV). The synthesized polycrystalline Cu₃RTe₃ (R = Y, Sm, and Dy) compounds possess moderate carrier concentrations (0.8-2.2 × 10²⁰ cm^-3) and density-of-state effective masses (around 1.1 m_e), yielding decent electrical transport performance. Furthermore, intrinsically low lattice thermal conductivities, below 1 W m^-1 K^-1 at 300-900 K, originating from the heavy average atomic masses and large number of atoms in the unit cell, are observed for Cu₃RTe₃ (R = Y, Sm, and Dy). Finally, Cu₃DyTe₃ demonstrates a peak dimensionless figure of merit of 0.9 at 900 K, which is among the highest reported for the Cu/Ag-based tellurides.

A high-performance and flexible thermoelectric generator based on the solution-processed composites of reduced graphene oxide nanosheets and bismuth telluride nanoplates

The fabrication of a flexible thermoelectric generator (TEG) with both high power output and good flexibility has drawn considerable attention. Solution-processed inorganic nanocrystals have good processibility in interface to retain excellent electrical properties of nanocrystals and can be processed into thin films on a flexible substrate by an easy scale-up printing or coating method. However, a high-performance TEG device based on inorganic solution-processed materials also poses challenges when it comes to flexibility of the whole device. Herein, flexible planar TEG devices are fabricated by printing an ink mixture comprising solution-processed bismuth telluride (Bi2Te3) nanoplates with reduced-graphene oxide (rGO) nanosheets onto flexible polyimide substrates. The interface treatment by hot ethylenediamine and the appropriate amount of rGO contribute to the high electrical properties of the material. Also, when rGO nanosheets of 1% mass ratio are added, the optimum power output of the corresponding rGO/Bi2Te3 TEG device with six elements reaches ∼1.72 μW at a temperature difference of 20 K. Moreover, owing to the contribution from flexible rGO nanosheets, the suitable thickness of each element, and the artful connection of elements with a soft copper wire in the devices, the 1% rGO/Bi2Te3 TEG device was found to be robust, and its electrical resistance merely changes by 2% after bending 1000 cycles on 5 mm in bending. These inorganic-based TEGs with both high performance and good flexibility will promote the development of new generation energy devices in the field of flexible electronics.

Decoupling and Coupling of the Host-Dopant Interaction by Manipulating Dopant Movement in Core/Shell Quantum Dots

Doping through the incorporation of transition metal ions allows for the emergence of new optical, electrical, and magnetic properties in quantum dots (QDs). While dopants can be introduced into QDs through many synthetic methods, the control of dopant location and host-dopant (H-D) coupling through directional dopant movement is still largely unexplored. In this work, we have studied dopant behaviors in Mn:CdS/ZnS core/shell QDs and found that dopant transport behavior is very sensitive to the temperature and microenvironments within the QDs. The migration of Mn toward the alloyed interface of the core/shell QDs, below a temperature boundary (T_b) at ∼200 °C, weakens the H-D interactions. At temperatures higher than the T_b, however, dopant ejection and global alloying of CdS/ZnS QDs can occur, leading to stronger H-D coupling. The behavior of incorporated dopants inside QDs is fundamentally important for understanding doping mechanisms and the host-dopant interaction-dependent properties of doped nanomaterials.

Boosting the Thermoelectric Performance of Calcium Cobaltite Composites through Structural Defect Engineering

Misfit-layered Ca₃Co₄O₉ as a p-type semiconductor is difficult to commercialize because of its relatively poor performance. Here, Ca_2.7-xLa_xAg_0.3Co₄O₉/Ag composites prepared by spark plasma sintering were systematically investigated in terms of La³⁺ dopant levels and nano-sized Ag compacts. Multiscale microstructures of stacking fault, dislocation, and oxygen vacancy-linked defects could be recognized as an effective strategy for tuning the transport of charge carriers and phonon scattering. An increasing concentration of charge carriers was caused by the introduction of nano-sized Ag particles at the grain boundary. The multiscale structural defects served as phonon scattering centers to reduce the thermal conductivity. Finally, the Ca_2.61La_0.09Ag_0.3Co₄O₉/Ag sample exhibited a maximum ZT of 0.35 at 1073 K. The results suggest that the interplay of structural defects provides an impetus for a huge improvement in thermoelectric performance.

Control of the Thermoelectric Properties of Mg2Sn Single Crystals via Point-Defect Engineering

Mg₂Sn is a potential thermoelectric (TE) material that can directly convert waste heat into electricity. In this study, Mg₂Sn single-crystal ingots are prepared by melting under an Ar atmosphere. The prepared ingots contain Mg vacancies (V_Mg) as point defects, which results in the formation of two regions: an Mg₂Sn single-crystal region without V_Mg (denoted as the single-crystal region) and a region containing V_Mg (denoted as the V_Mg region). The V_Mg region is embedded in the matrix of the single-crystal region. The interface between the V_Mg region and the single-crystal region is semi-coherent, which does not prevent electron carrier conduction but does increase phonon scattering. Furthermore, electron carrier concentration depends on the fraction of V_Mg, reflecting the acceptor characteristics of V_Mg. The maximum figure of merit zT_max of 1.4(1) × 10⁻² is realised for the Mg₂Sn single-crystal ingot by introducing V_Mg. These results demonstrate that the TE properties of Mg₂Sn can be optimised via point-defect engineering.

Defects Engineering with Multiple Dimensions in Thermoelectric Materials

Going through decades of development, great progress in both theory and experiment has been achieved in thermoelectric materials. With the growing enhancement in thermoelectric performance, it is also companied with the complexation of defects induced in the materials. 0D point defects, 1D linear defects, 2D planar defects, and 3D bulk defects have all been induced in thermoelectric materials for the optimization of thermoelectric performance. Considering the distinct characteristics of each type of defects, in-depth understanding of their roles in the thermoelectric transport process is of vital importance. In this paper, we classify and summarize the defect-related physical effects on both band structure and transport behavior of carriers and phonons when inducing different types of defects. Recent achievements in experimental characterization and theoretical simulation of defects are also summarized for accurately determining the type of defects serving for the design of thermoelectric materials. Finally, based on the current theoretical and experimental achievements, strategies engaged with multiple dimensional defects are reviewed for thermoelectric performance optimization.

Decoupling Thermoelectric Performance and Stability in Liquid-Like Thermoelectric Materials

Liquid-like materials are one family of promising thermoelectric materials discovered in the past years due to their advantanges of ultrahigh thermoelectric figure of merit (zT), low cost, and environmental friendliness. However, their practial applications are greatly limited by the low service stability from the Cu/Ag metal deposition under large current and/or temperature gradient. Both high zT for high efficiency and large critical voltage for good stability are required for liquid-like materials, but they are usually strongly correlated and hard to be tuned individually. Herein, based on the thermodynamic analysis, it is shown that such a correlation can be decoupled through doping immobile ions into the liquid-like sublattice. Taking Cu2- δ S as an example, doping immobile Fe ions in Cu1.90S scarcely degrades the initial large critical voltage, but significantly enhances the zT to 1.5 at 1000 K by tuning the carrier concentration to the optimal range. Combining the low-cost and environmentally friendly features, these Fe-doped Cu2- δ S-based compounds show great potential in civil applications. This study sheds light on the realization of both good stability and high performance for many other liquid-like thermoelectric materials that have not been considered for real applications before.

Plasma treated Bi2WO6 ultrathin nanosheets with oxygen vacancies for improved photocatalytic CO2 reduction

Ar-plasma treatment quickly and effectively increased the amount of oxygen vacancies on the surface of Bi₂WO₆. In photocatalytic CO₂ reduction, the CO generation rate of Bi₂WO₆ with abundant surface oxygen vacancies increased by 2.4 times.

Synergistically optimizing the thermoelectric properties of polycrystalline Ag8SnSe6 by introducing additional Sn

Introducing additional Sn into polycrystalline Ag₈SnSe₆ could manipulate self-defects and improve the crystallinity, and the peak ZT value is significantly improved.

Gas-Sensing Activity of Amorphous Copper Oxide Porous Nanosheets

In this paper, the gas-sensing properties of copper oxide porous nanosheets in amorphous and highly crystalline states were comparatively investigated on the premise of almost the same specific surface area, morphology and size. Unexpectedly, the results show that amorphous copper oxide porous nanosheets have much better gas sensing properties than highly crystalline copper oxide to a serious of volatile organic compounds, and the lowest detection limit (LOD) of the amorphous copper oxide porous nanosheets to methanal is even up to 10 ppb. By contrast, the LOD of the highly crystalline copper oxide porous nanosheets to methanal is 95 ppb. Experiments prove that the oxygen vacancies contained in the amorphous copper oxide porous nanosheets play a key role in improving gas sensitivity, which greatly improve the chemical activity of the materials, especially for the adsorption of molecules containing oxygen-groups such as methanal and oxygen.

Compositional Tailoring for Realizing High Thermoelectric Performance in Hafnium-Free n-Type ZrNiSn Half-Heusler Alloys

Compositional tailoring enables fine-tuning of thermoelectric (TE) transport parameters by synergistic modulation of electronic and vibrational properties. In the present work, the aspects of compositionally tailored defects have been explored in ZrNiSn-based half-Heusler (HH) TE materials to achieve high TE performance and cost effectiveness in n-type Hf-free HH alloys. In off-stoichiometric Ni-rich ZrNi_1+xSn alloys in a low Ni doping limit (x < 0.1), excess Ni induces defects (Ni/vacancy antisite + interstitials), which tend to cause band structure modification. In addition, the structural similarity of HH and full-Heusler (FH) compounds and formation energetics lead to an intrinsic phase segregation of FH nanoscale precipitates that are coherently dispersed within the ZrNiSn HH matrix as nanoclusters. A consonance was achieved experimentally between these two competing mechanisms for optimal HH composition having both FH precipitates and Ni/vacancy antisite defects in the HH matrix by elevating the sintering temperature up to the solubility limit range of the ZrNiSn system. Defect-mediated optimization of electrical and thermal transport via carrier concentration tuning, energy filtering, and possibly all scale-hierarchical architecture resulted in a maximum ZT ≈ 1.1 at 873 K for the optimized ZrNi_1.03Sn composition. Our findings highlight the realistic prospect of enhancing TE performance via compositional engineering approach for wide applications of TE.

Filiform Metal Silver Nanoinclusions To Enhance Thermoelectric Performance of P-type Ca3Co4O9+δ Oxide

Cd doping and metallic Ag additives in Ca₃Co₄O_9+δ polycrystalline materials are shown to result in improved thermoelectric (TE) transport properties. Carrier concentration and mobility were optimized through the combination of doping and compositional modulation approaches. The formation of filiform Ag nanoinclusions between the interlayers and grain boundaries enhances the anisotropic carrier transport, leading to higher carrier mobility. A spin entropy enhancement due to the change of the net valence of Co induced by Cd substitution on the Ca site was confirmed by X-ray photoelectron spectroscopy. High carrier mobility and enhanced spin entropy results in higher electrical conductivity and Seebeck coefficient, leading to the increase of the power factor. In conjunction, mass fluctuation between Cd and Ca on the same crystal site along with the increase of metallic Ag nanoinclusions effectively lowers thermal conductivity. Consequently, the figure-of-merit, zT, has been improved to 0.31 at 950 K for 10 wt % Ag-modified Ca_2.9Cd_0.1Co₄O_9+δ specimen, which is a significant improvement compared to the pristine material. This dual-mode control of electron and phonon transport by including Ag additives and Cd doping offers an approach for tuning the correlated TE parameters.

Charge Compensation Modulation of the Thermoelectric Properties in AgSbTe2 via Mn Amphoteric Doping

In thermoelectric research, the introduction of a dopant can suppress lattice thermal conductivity (κ₁) through phonon scattering and optimize the power factor (PF) by changing the behavior of carriers, which are the key prerequisites for high thermoelectric performance. However, the electrical thermal conductivity (κ_e) can also increase with the increase of electrical conductivity (σ), which may override the optimization in PF and be detrimental to the improvement of final ZT. In this work, we highlight an amphoteric doping method by using Mn atoms to substitute both Ag and Sb atoms in AgSbTe₂. The Mn_Sb positive doping in p-type AgSbTe₂ can improve the σ through increasing the hole concentration while maintaining a relative high Seebeck coefficient (S), thus substantially improving the PF. On the other hand, the Mn_Ag negative doping can introduce electrons into the matrix, which will recombine with the major hole carriers and lead to a decrease of σ to suppress exorbitant κ_e induced by the Mn_Sb doping. The combination of the both functions by Mn amphoteric doping can further improve the thermoelectric property through charge compensation modulation. By virtue of amphoteric doping, though σ is decreased, PF is further optimized because of increased S, while the total thermal conductivity (κ_total) is further decreased due to suppressed κ_e and additional phonon scattering, which are beneficial for the improvement of the final ZT value. As a result, 5 mol % Mn_Ag-Mn_Sb amphoteric doping AgSbTe₂ sample achieves a maximum ZT value of ∼0.74 at 550 K, which is higher than that of the pristine sample and other Mn monodoped counterparts. The present work suggests charge compensation modulation via amphoteric doping as an effective avenue to simultaneously achieve low thermal conductivity and high power factor for better thermoelectric performance.

Enhanced Thermoelectric Performance of Quaternary Cu2-2 xAg2 xSe1- xS x Liquid-like Chalcogenides

Liquid-like binary Cu_2-δX (X = S, Se, and Te) chalcogenides and their ternary solid solutions have gained notable attention in thermoelectrics due to their interesting and abnormal thermal and electrical transport properties. However, previous studies mainly focus on a single element alloying at either an anion or cation site whereas the investigation on cation/anion co-alloying is very rare so far. Here, a series of quaternary Cu_{2-2 x}Ag_{2 x}Se_{1- x}S _x ( x = 0.01, 0.03, 0.05, 0.1, 0.15) liquid-like copper chalcogenide materials have been fabricated and the effects of Ag/S co-alloying on the thermoelectric properties of Cu₂Se have been systematically studied. It is found that all compounds are mixed phases at room temperature but single cubic phase at high temperatures. The introduction of Ag and S in Cu₂Se brings about a large mass fluctuation rather than strain field fluctuation that effectively suppresses the lattice thermal conductivity. Furthermore, on increasing the Ag and S contents, the high electrical conductivity of pristine Cu₂Se is well tuned to the optimal range derived from the single parabolic band model analysis. Consequently, a peak zT of 1.6 at 900 K is achieved in Cu_1.8Ag_0.2Se_0.9S_0.1, which is about 33% higher than that of binary Cu₂Se.