High thermoelectric and reversible p-n-p conduction type switching integrated in dimetal chalcogenide

Room-Temperature Synthesis and Low Thermal Conductivity in Nanocrystalline Ag3CuS2

Noble-metal-based chalcogenide materials recently gained massive attention in the field of thermoelectrics. In most cases, materials are synthesized using (i) high-temperature solid-state reactions or (ii) soft chemical methods where temperature requirements are lower than those of solid-state reactions (generally below 400 °C). Herein, we present a simple, surfactant-free, room-temperature, and energy-efficient synthesis of Ag3CuS2 nanocrystals. The present synthesis technique is scalable and capable of gram-scale production. A spark plasma sintering (SPS) pressed sample exhibits ultralow thermal conductivity (∼0.31 W/mK at room temperature). We found that Ag3CuS2 exhibits low sound velocity, as well as a non-Debye-like behavior based on a low-temperature heat capacity measurement. A high degree of anharmonicity of bonding, soft vibrations modes, and nanoscale grain boundary scattering in Ag3CuS2 lead to ultralow thermal conductivity, which can be important for thermoelectrics, optoelectronics, and thermal barrier coating applications.

Electron Localization-Triggered Proton Pumping Toward Cu Single Atoms for Electrochemical CO2 Methanation of Unprecedented Selectivity

Slow multi-proton coupled electron transfer kinetics and unexpected desorption of intermediates severely hinder the selectivity of CO2 methanation. In this work, a one-stone-two-bird strategy of pumping protons and improving adsorption configuration/capability enabled by electron localization is developed to be highly efficient for CH4 electrosynthesis over Cu single atoms anchored on bismuth vacancies of BiVO4 (Bi1-xVO4─Cu), with superior kinetic isotope effect and high CH4 Faraday efficiency (92%), far outperforming state-of-the-art electrocatalysts for CO2 methanation. Control experiments and theoretical calculations reveal that the bismuth vacancies (VBi) not only act as active sites for H2O dissociation but also induce electron transfer toward Cu single-atom sites. The VBi-induced electron localization pumps *H from VBi sites to Cu single atoms, significantly promoting the generation and stabilization of the pivotal intermediate (*CHO) for highly selective CH4 electrosynthesis. The metal vacancies as new initiators show enormous potential in the proton transfer-involved hydrogenative conversion processes.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Two-step phase manipulation by tailoring chemical bonds results in high-performance GeSe thermoelectrics

In thermoelectrics, phase engineering serves a crucial function in determining the power factor by affecting the band degeneracy. However, for low-symmetry compounds, the mainstream one-step phase manipulation strategy, depending solely on the valley or orbital degeneracy, is inadequate to attain a high density-of-states effective mass and exceptional zT. Here, we employ a distinctive two-step phase manipulation strategy through stepwise tailoring chemical bonds in GeSe. Initially, we amplify the valley degeneracy via CdTe alloying, which elevates the crystal symmetry from a covalently bonded orthorhombic to a metavalently bonded rhombohedral phase by significantly suppressing the Peierls distortion. Subsequently, we incorporate Pb to trigger the convergence of multivalence bands and further enhance the density-of-states effective mass by moderately restraining the Peierls distortion. Additionally, the atypical metavalent bonding in rhombohedral GeSe enables a high Ge vacancy concentration and a small band effective mass, leading to increased carrier concentration and mobility. This weak chemical bond along with strong lattice anharmonicity also reduces lattice thermal conductivity. Consequently, this unique property ensemble contributes to an outstanding zT of 0.9 at 773 K for Ge0.80Pb0.20Se(CdTe)0.25. This work underscores the pivotal role of the two-step phase manipulation by stepwise tailoring of chemical bonds in improving the thermoelectric performance of p-bonded chalcogenides.

Advances in thermoelectric AgBiSe2: Properties, strategies, and future challenges

Thermoelectric materials are attracting considerable attention to alleviate the global energy crisis by enabling the direct conversion of heat into electricity. As a class of I-V-VI2 semiconductors, AgBiSe2 is expected to be the potential thermoelectric material to replace conventional PbTe-based compounds due to its non-toxic and abundant nature of its constituent elements. This review article summarizes the fundamental properties of AgBiSe2, thermoelectric properties, the effect of different dopants on its transport properties and entropy engineering for cubic phase stabilization with the detailed description of related techniques used to analyze the properties of AgBiSe2. The current thermoelectric figure-of-merit and approaches to further improve performance and operational stability are also discussed.

Effects of Intermixing in Sb2Te3/Ge1+xTe Multilayers on the Thermoelectric Power Factor

Over the past few decades, telluride-based chalcogenide multilayers, such as PbSeTe/PbTe, Bi₂Te₃/Sb₂Te₃, and Bi₂Te₃/Bi₂Se₃, were shown to be promising high-performance thermoelectric films. However, the stability of performance in operating environments, in particular, influenced by intermixing of the sublayers, has been studied rarely. In the present work, the nanostructure, thermal stability, and thermoelectric power factor of Sb₂Te₃/Ge_1+xTe multilayers prepared by pulsed laser deposition are investigated by transmission electron microscopy and Seebeck coefficient/electrical conductivity measurements performed during thermal cycling. Highly textured Sb₂Te₃ films show p-type semiconducting behavior with superior power factor, while Ge_1+xTe films exhibit n-type semiconducting behavior. The elemental mappings indicate that the as-deposited multilayers have well-defined layered structures. Upon heating to 210 °C, these layer structures are unstable against intermixing of sublayers; nanostructural changes occur on initial heating, even though the highest temperature is close to the deposition temperature. Furthermore, the diffusion is more extensive at domain boundaries leading to locally inclined structures there. The Sb₂Te₃ sublayers gradually dissolve into Ge_1+xTe. This dissolution depends markedly on the relative Ge_1+xTe film thickness. Rather, full dissolution occurs rapidly at 210 °C when the Ge_1+xTe sublayer is substantially thicker than that of Sb₂Te₃, whereas the dissolution is very limited when the Ge_1+xTe sublayer is substantially thinner. The resulting variations of the nanostructure influence the Seebeck coefficient and electrical conductivity and thus the power factor in a systematic manner. Our results shed light on a previously unreported correlation of the power factor with the nanostructural evolution of unstable telluride multilayers.

A Switchable One-Compound Diode

A diode requires the combination of p- and n-type semiconductors or at least the defined formation of such areas within a given compound. This is a prerequisite for any IT application, energy conversion technology, and electronic semiconductor devices. Since the discovery of the pnp-switchable compound Ag₁₀ Te₄ Br₃ in 2009, it is in principle possible to fabricate a diode from a single material without adjusting the semiconduction type by a defined doping level. Often a structural phase transition accompanied by a dynamic change of charge carriers or a charge density wave within certain substructures are responsible for this effect. Unfortunately, the high pnp-switching temperature between 364 and 580 K hinders the application of this phenomenon in convenient devices. This effect is far removed from a suitable operation temperature at ambient conditions. Ag₁₈ Cu₃ Te₁₁ Cl₃  is a room temperature pnp-switching material and the first single-material position-independent diode. It shows the highest ever reported Seebeck coefficient drop that takes place within a few Kelvin. Combined with its low thermal conductivity, it offers great application potential within an accessible and applicable temperature window. Ag₁₈ Cu₃ Te₁₁ Cl₃ and pnp-switching materials have the potential for applications and processes where diodes, transistors, or any defined charge separation with junction formation are utilized.

Suppressing Charged Cation Antisites via Se Vapor Annealing Enables p-Type Dopability in AgBiSe2 -SnSe Thermoelectrics

Cation disordering is commonly found in multinary cubic compounds, but its effect on electronic properties has been neglected because of difficulties in determining the ordered structure and defect energetics. An absence of rational understanding of the point defects present has led to poor reproducibility and uncontrolled conduction type. AgBiSe₂ is a representative compound that suffers from poor reproducibility of thermoelectric properties, while the origins of its intrinsic n-type conductivity remain speculative. Here, it is demonstrated that cation disordering is facilitated by Bi_Ag charged antisite defects in cubic AgBiSe₂ which also act as a principal donor defect that greatly controls the electronic properties. Using density functional theory calculations and in situ Raman spectroscopy, how saturation annealing with selenium vapor can stabilize p-type conductivity in cubic AgBiSe₂ alloyed with SnSe at high temperatures is elucidated. With stable and controlled hole concentration, a peak is observed in the weighted mobility and the density-of-states effective mass in AgBiSnSe₃ , implying an increased valley degeneracy in this system. These findings corroborate the importance of considering the defect energetics for exploring the dopability of ternary thermoelectric chalcogenides and engineering electronic bands by controlling self-doping.

Recent advances in biological applications of nanomaterials through defect engineering

In recent years, defect engineering sprung up in the artificial nanomaterials (NMs) has attracted significant attention, since the physical and chemical properties of NMs could be largely optimized based on the rational control of different defect types and densities. Defective NMs equipped with the improved electric and catalytic ability, would be widely utilized as the photoelectric device and catalysts to alleviate the growing demands of industrial production and environmental treatments. In particular, considering that the features of targeting, adsorptive, loading and optical could be adjusted by the introduction of defects, numerous defective NMs are encouraged to be applied in the biological fields including bacterial inactivation, cancer therapy and so on. And this review is devoted to summarize the recent biological applications of NMs with abundant defects. Moreover, the opportunity of these defective NMs released into the surrounding environment continue to increase, the direct and indirect contact with biological molecules and organisms would be inevitable. Due to its high reactivity and adsorption triggered by defects, NMs tend to exhibit overestimate biological behaviors and effects on organisms. Thus, the sections regarding toxicological effects of NMs with abundant defects are also carried out to supplement the safety assessments of NMs and guide further applications in the industrial production and living.

Heterointerface Engineering in Electromagnetic Absorbers: New Insights and Opportunities

Electromagnetic (EM) absorbers play an increasingly essential role in the electronic information age, even toward the coming "intelligent era". The remarkable merits of heterointerface engineering and its peculiar EM characteristics inject a fresh and infinite vitality for designing high-efficiency and stimuli-responsive EM absorbers. However, there still exist huge challenges in understanding and reinforcing these interface effects from the micro and macro perspectives. Herein, EM response mechanisms of interfacial effects are dissected in depth, and with a focus on advanced characterization as well as theoretical techniques. Then, the representative optimization strategies are systematically discussed with emphasis on component selection and structural design. More importantly, the most cutting-edge smart EM functional devices based on heterointerface engineering are reported. Finally, current challenges and concrete suggestions are proposed, and future perspectives on this promising field are also predicted.

From Three-Dimensional Clathrates to Two-Dimensional Zintl Phases AMSb2 (A = Rb, Cs; M = Ga, In) Composed of Pentagonal M-Sb Rings

Three new antimonide Zintl phases, RbGaSb₂, CsGaSb₂, and CsInSb₂, were discovered during exploration of corresponding A-M-Sb (A = Rb, Cs; M = Ga, In) ternary systems while searching for new clathrates. The AGaSb₂ phases crystallize in the tetragonal space group P4₂/nmc (No. 137) in the LiBS₂ structure type, while CsInSb₂ crystallizes in lower symmetry in the orthorhombic space group Cmce (No. 64) in the KGaSb₂ structure type with additional disorder of one of the Cs sites. The crystal structures of all three reported AMSb₂ compounds are composed of two-dimensional [MSb₂]^- tetrahedral layers separated by Rb⁺ or Cs⁺ cations. [MSb₂]^- layers are built from fused M-Sb pentagons and hexagons, which are also the main structural units for A₈M₂₇Sb₁₉ clathrate cages. The semiconductor nature of AMSb₂ was suggested by band structure calculations and confirmed by transport property characterization. CsGaSb₂ is a rare example of an n-type pnictide Zintl phase. All reported compounds exhibit low thermal conductivity typical for complex antimonides of heavy elements.

Boosting the Dispersity of Metallic Ag Nanoparticles and Ozone Decomposition Performance of Ag-Mn Catalysts via Manganese Vacancy-Dependent Metal-Support Interactions

Ozone (O₃) removal has important implications for environmental protection and human health, and Ag-Mn catalysts have shown promising O₃ decomposition. Catalysts with Ag supported on porous cube-like α-Mn₂O₃ (Ag/Mn-C) with high utilization of Ag were prepared by the impregnation method and showed excellent O₃ decomposition activity. Physicochemical characterizations demonstrated that metallic Ag nanoparticles (Ag_n⁰) were mainly anchored on manganese vacancies, forming Ag-O-Mn bonds between Ag_n⁰ and α-Mn₂O₃-C. The abundant manganese vacancies of α-Mn₂O₃-C can lead to Ag_n⁰ with a smaller particle size and more uniform dispersion, thereby resulting in markedly enhanced O₃ decomposition performance compared to Ag_n⁰ with a large particle size and uneven distribution on rod-like α-Mn₂O₃ (Ag/Mn-R). Under a relative humidity of 65% and a space velocity of 1,110,000 h^-1, the conversion of 40 ppm O₃ over the 2%Ag/Mn-C catalyst within 6 h (98%) at 30 °C was more than twice as high as that of the 2%Ag/Mn-R catalyst (42%). The study provides guidance for the design of highly efficient Ag-based catalysts and the understanding of the microstructure of supported catalysts.

Long-range ordering and local structural disordering of BiAgSe2 and BiAgSeTe thermoelectrics

Thermoelectric materials are promising for energy harvesting using waste heat. The thermal management of the thermoelectric materials attract scientific and technological interests. The narrow bandgap semiconductor BiAgSe₂ is a good candidate for thermoelectric materials due to its ultralow thermal conductivity. The mother compound BiAgSe₂ crystallizes in hexagonal symmetry at room temperature, but experiences structural transitions to cubic phase at high temperature. By contrast, the daughter compound BiAgSeTe exhibits long range ordering and crystallizes into cubic phase at room temperature. Nevertheless, the local structural disorderings due to the Bi³⁺ and Ag⁺ anti-site defects, as well as local structural distortions, are ubiquitous in both parent BiAgSe₂ and BiAgSeTe. BiAgSeTe exhibits distinct transport properties owing to the disordering-induced drastic changes in the electronic band structure, as well as the scattering dictated by the point defects. It is suggested that BiAgSe₂ and BiAgSeTe could be good candidates for phonon glass and crystal glass (PGEC)-type thermoelectrics.

Ion Dynamics and Polymorphism in Cu20Te11Cl3

Coinage metal polychalcogenide halides are an intriguing class of materials, and many representatives are solid ion conductors and thermoelectric materials. The materials show high ion mobility, polymorphism, and various attractive interactions in the cation and anion substructures. Especially the latter feature leads to complex electronic structures and the occurrence of charge-density waves (CDWs) and, as a result, the first p-n-p switching materials. During our systematic investigations for new p-n-n switching materials in the Cu-Te-Cl phase diagram, we were able to isolate polymorphic Cu₂₀Te₁₁Cl₃, which we characterized structurally and with regard to its electronic and thermoelectric properties. Cu₂₀Te₁₁Cl₃ is trimorphic, with phase transitions occurring at 288 and 450 K. The crystal structures of two polymorphs, the α phase, stable above 450 K, and the β polymorph (288-450 K), are reported, and the complex structure chemistry featuring twinning upon a phase change is illustrated. We identified a dynamic cation substructure and a static anion substructure for all polymorphs, characterizing Cu₂₀Te₁₁Cl₃ as a solid Cu-ion conductor. Temperature-dependent measurements of the Seebeck coefficient and total conductivity were performed and substantiated a linear response of the Seebeck coefficient, a lack of CDWs, and no p-n-p switching. Reasons for a lack of CDWs in Cu₂₀Te₁₁Cl₃ are discussed and illustrated in the context of existing p-n-p switching materials.

Defect engineering in polymeric carbon nitride photocatalyst: Synthesis, properties and characterizations

Polymer carbon nitride (CN) has unique structure and electronic properties, making it attractive in photocatalysis fields. However, the photocatalytic efficiency of the pristine CN photocatalyst is still unsatisfactory. In this regard, the introduction of vacancy defects can effectively tune photoelectric properties of CN photocatalyst through tailoring the electronic structure and bandgap engineering. In this review, the effect of vacancy defects on CN is reviewed from the aspects of light absorption, charge separation and surface photoreactivity of CN. Meanwhile, the current progress in the design of vacancy defects with the classified carbon vacancies (CVs), nitrogen vacancies (NVs), amino and cyano groups on CN to boost the photocatalytic performance is summarized. Furthermore, various characterization methods have been summarized and highlighted, including microscopic characterization (SEM, TEM, AFM, HAADF-STEM), spectroscopic characterization (XRD, FTIR, XAFS, XANES, EPR, PAS, XPS, raman spectroscopy, solid-state NMR spectroscopy), elemental analysis, and computational characterization. Finally, the future opportunities and challenges of CN photocatalysts designed with vacancies and defects are proposed to highlight the development direction of this research field.

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.

Manganese vacancy-confined single-atom Ag in cryptomelane nanorods for efficient Wacker oxidation of styrene derivatives

Single-atom catalysts provide a pathway to elucidate the nature of catalytically active sites. However, keeping them stabilized during operation proves to be challenging. Herein, we employ cryptomelane-type octahedral molecular sieve nanorods featuring abundant manganese vacancy defects as a support, to periodically anchor single-atom Ag. The doped Ag atoms with tetrahedral coordination are found to locate at cation substitution sites rather than being supported on the catalyst surface, thus effectively tuning the electronic structure of adjacent manganese atoms. The resulting unique Ag–O–MnO_x unit functions as the active site. Its turnover frequency reaches 1038 h⁻¹, one order of magnitude higher than for previously reported catalysts, with 90% selectivity for anti-Markovnikov phenylacetaldehyde. Mechanistic studies reveal that the activation of styrene on the ensemble site of Ag–O–MnO_x is significantly promoted, which can accelerate the oxidation of styrene and, in particular, the rate-determining step of forming the epoxide intermediate. Such an extraordinary electronic promotion can be extended to other single-atom catalysts and paves the way for their practical applications.

Manganese vacancy-confined single-atom Ag in cryptomelane nanorods efficiently catalyses Wacker oxidation of styrene derivatives.

Defect engineering and characterization of active sites for efficient electrocatalysis

Electrocatalysis plays a decisive role in various energy-related applications. Engineering the active sites of electrocatalysts is an important aspect to promote their catalytic performance. In particular, defect engineering provides a feasible and efficient approach to improve the intrinsic activities and increase the number of active sites in electrocatalysts. In this review, recent investigations on defect engineering of a wide range of electrocatalysts such as metal-free carbon materials, transition metal oxides, transition metal dichalcogenides and metal-organic frameworks (MOFs) will be summarized. Different defect creation strategies will be outlined, for example, heteroatom doping and removal, plasma irradiation, hydrogenation, amorphization, phase transition and reduction treatment. In addition, we will overview the commonly used advanced characterization techniques that could confirm the existence and identify the detailed structures, types and concentration of defects in electrocatalysts. The defect characterization tools are beneficial for gaining an in-depth understanding of defects on electrocatalysis and thus could reveal the structure-performance relationship. Finally, the major challenges and future development directions on defect engineering of electrocatalysts will be discussed.

Temperature-driven n-p conduction type switching without structural transition in a Cu-rich chalcogenide, NaCu5S3

We report for the first time the discovery of reversible n-p conduction type switching in a chalcogenide, NaCu5S3, without structural transition. AC impedance and first-principles simulations of the ionic migration confirmed the local melting trends of the hexagonal copper lattice at high temperatures, which could result in superionic conductivity within NaCu5S3.

Ultrahigh Average ZT Realized in p-Type SnSe Crystalline Thermoelectrics through Producing Extrinsic Vacancies

Crystalline SnSe has been revealed as an efficient thermoelectric candidate with outstanding performance. Herein, record-high thermoelectric performance is achieved among SnSe crystals via simply introducing a small amount of SnSe₂ as a kind of extrinsic defect dopant. This excellent performance mainly arises from the largely enhanced power factor by increasing the carrier concentration high as 6.55 × 10¹⁹ cm^-3, which was surprisingly promoted by introducing extrinsic SnSe₂ even though pristine SnSe₂ is an n-type conductor. The optimized carrier concentration promotes a deeper Fermi level and activates more valence bands, leading to an extraordinary room-temperature power factor ∼54 μW cm^-1 K^-2 through enlarging the band effective mass and Seebeck coefficient. As a result, on the basis of simultaneously depressed thermal conductivity induced from both Sn vacancies and SnSe₂ microdomains, maximum ZT values ∼0.9-2.2 and excellent average ZT > 1.7 among the working temperature range are achieved in Na doped SnSe crystals with 2% extrinsic SnSe₂. Our investigation illustrates new approaches on improving thermoelectric performance through introducing defect dopants, which might be well-implemented in other thermoelectric systems.

Cation Disorder and Local Structural Distortions in AgxBi1-xS2 Nanoparticles

By combining X-ray absorption fine structure and X-ray diffraction measurements with density functional and molecular dynamics simulations, we study the structure of a set of Ag_xBi_1−xS₂ nanoparticles, a materials system of considerable current interest for photovoltaics. An apparent contradiction between the evidence provided by X-ray absorption and diffraction measurements is solved by means of the simulations. We find that disorder in the cation sublattice induces strong local distortions, leading to the appearance of short Ag–S bonds, the overall lattice symmetry remaining close to hexagonal.

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.

Controlling the thermoelectric power of silicon–germanium alloys in different crystalline phases by applying high pressure

Si–Ge crystals are promising materials for use in various stress-controlled electronic junctions for next-generation nanoelectronic devices.

Boosting visible-light-driven catalytic hydrogen evolution via surface Ti3+ and bulk oxygen vacancies in urchin-like hollow black TiO2 decorated with RuO2 and Pt dual cocatalysts

A novel hollow urchin-like black RuO₂/TiO₂/Pt nanomaterial with surface Ti³⁺ and bulk single-electron oxygen vacancies (Vo·) was used for enhancing the hydrogen evolution performance under visible light.

Doping-Induced Polymorph and Carrier Polarity Changes in Thermoelectric Ag(Bi,Sb)Se2 Solid Solution

Silver bismuth diselenide (AgBiSe₂) is an n-type thermoelectric material that exhibits a complex structural phase transition from the hexagonal to cubic phase, while silver antimony diselenide (AgSbSe₂) is a p-type thermoelectric material that crystallizes in the cubic phase at all temperatures. Here, we investigate the crystal structure and thermoelectric properties of Ag(Bi,Sb)Se₂ solid solution, employing AgBi_0.9Sb_0.1Se₂ and AgBi_0.7Sb_0.3Se₂ as representative samples. The carrier polarity of AgBi_0.9Sb_0.1Se₂ is converted from the n-type to p-type by Pb doping, accompanied by a polymorphic change to the cubic phase. It is difficult to obtain highly conductive p-type hexagonal AgBiSe₂-based materials, although first-principles calculations predict high-performance thermoelectric properties for these systems. We also demonstrate that cubic AgBi_0.7Sb_0.3Se₂ undergoes a polymorphic change to the hexagonal phase upon Nb doping. The present study show that polymorphic changes inevitably occurred upon Pb/Nb doping to optimize thermoelectric properties of Ag(Bi,Sb)Se₂ solid solution.

Enhancement in thermoelectric properties due to Ag nanoparticles incorporated in Bi2Te3 matrix

The present study aims to see the enhancement in thermoelectric properties of bismuth telluride (Bi₂Te₃) annealed at different temperatures (573 and 773 K) through silver (Ag) nano-inclusions (0, 2, 5, 10, 15 and 20 wt %). Transmission electron microscopy (TEM) images of Ag incorporated in Bi₂Te₃ annealed at 573 K shows tubular, pentagonal, trigonal, circular and hexagonal nanoparticles with sizes of 6-25 nm (for 5 wt % Ag ) and 7-30 nm (for 20 wt % Ag). Ag incorporated in Bi₂Te₃ annealed at 773 K shows mainly hexagonally shaped structures with particle sizes of 2-20 nm and 40-80 nm (for 5 wt % Ag) and 10-60 nm (for 20 wt % Ag). Interestingly, the samples annealed at 573 K show the highest Seebeck coefficient (S, also called thermopower) at room temperature (p-type behavior) for 5% Ag which is increased ca. five-fold in comparison to Ag-free Bi₂Te₃, whereas for samples with the same content (5% Ag) annealed at 773 K the increment in thermopower is only about three-fold with a 6.9-fold enhancement of the power factor (S ²σ). The effect of size and shape of the nanoparticles on thermoelectric properties can be understood on the basis of a carrier-filtering effect that results in an increase in thermopower along with a control over the reduction in electrical conductivity to maintain a high power factor yielding a high figure of merit.

Pressure-Driven Reversible Switching between n- and p-Type Conduction in Chalcopyrite CuFeS2

Temperature-dependent switching between p- and n-type conduction is a newly observed phenomenon in very few Ag-based semiconductors, which may promote fascinating applications in modern electronics. Pressure, as an efficient external stimulus that has driven collective phenomena such as spin-crossover and Mott transition, is also expected to initialize a conduction-type switching in transition metal-based semiconductors. Herein, we report the observation of a pressure-driven dramatic switching between p- and n-type conduction in chalcopyrite CuFeS₂ associated with a structural phase transition. Under compression around 8 GPa, CuFeS₂ undergoes a phase transition with symmetry breakdown from space group I-42 d to space group I-4 accompanying with a remarkable volume shrinkage of the FeS₄ tetrahedra. A high-to-low spin-crossover of Fe²⁺ ( S = 2 to S = 0) is manifested along with this phase transition. Instead of pressure-driven metallization, a surprising semiconductor-to-semiconductor transition is observed associated with the structural and electronic transformations. Significantly, both photocurrent and Hall coefficient measurements confirm that CuFeS₂ undergoes a reversible pressure-driven p- n conduction type switching accompanying with the structural phase transition. The absence of cationic charge transfer between copper and iron during the phase transition is confirmed by both X-ray absorption near-edge spectra (Cu/Fe, K-edge) and total-fluorescence-yield X-ray absorption spectra (Fe, K-edge) results, and the valence distribution maintains Cu²⁺Fe²⁺S₂ in the high-pressure phase. The observation of an abrupt pressure-driven p- n conduction type switching in a transition metal-based semiconductor paves the way to novel pressure-responsive switching devices.

Low thermal conductivity and high figure of merit for rapidly synthesized n-type Pb1-xBixTe alloys

High figures of merit of n-type Pb1-xBixTe alloys have been achieved by rapid synthesis at low temperature. The effects of Bi dopant and microwave hydrothermal technology on microstructure and thermoelectric performance have been studied. The solid solubility limit of Bi in PbTe is between x = 0.02 and 0.03. Homogenous nanopowders of about 70 nm have been synthesized by the microwave hydrothermal method. When followed by hot pressing, sub-microscale grain sizes are also formed for Pb1-xBixTe alloys. With increase in Bi, the carrier concentration is improved within the solubility limit. This leads to low electrical resistivity and higher power factor at high temperature. A higher power factor of 8.5 μW cm-1 K-2 is obtained for x = 0.02 sample at 623 K. In addition, the introduction of Bi effectively prohibits the p-n transition and bipolar thermal conductivity of pristine PbTe. Thus, a low lattice thermal conductivity of 0.68 W m-1 K-1 is achieved at 673 K, combining scattering of alloys, grain boundaries, dislocations and defects. As a result, the highest peak figure of merit, i.e., zT = 0.62 at 673 K is achieved for Pb0.98Bi0.02Te sample, which is comparable with that of Bi-doped PbTe alloys synthesized by the conventional melting method. Thus, the right synthesis conditions of the microwave hydrothermal method can rapidly result in thermoelectric materials with comparable figures of merit.

Tuning of p-n-p-Type Conduction in AgCuS through Cation Vacancy: Thermopower and Positron Annihilation Spectroscopy Investigations

Understanding the complex phenomenon behind the structural transformations is a key requisite to developing important solid-state materials with better efficacy such as transistors, resistive switches, thermoelectrics, etc. AgCuS, a superionic semiconductor, exhibits temperature-dependent p-n-p-type conduction switching and a colossal jump in thermopower during an orthorhombic to hexagonal superionic transition. Tuning of p-n-p-type conduction switching in superionic compounds is fundamentally important to realize the correlation between electronic/phonon dispersion modulation with changes in the crystal structure and bonding, which might contribute to the design of better thermoelectric materials. Herein, we have created extrinsic Ag/Cu nonstoichiometry in AgCuS, which resulted in the vanishing of p-n-p-type conduction switching and improved its thermoelectric properties. We have performed the selective removal of cations and measured their temperature-dependent thermopower and Hall coefficient, which demonstrates only p-type conduction in the Ag_{1- x}CuS and AgCu_{1- x}S samples. The removal of Cu is much more efficient in arresting conduction switching, whereas in the case of Ag vacancy, p-n-p-type conduction switching vanishes at higher vacant concentrations. Positron annihilation spectroscopy measurements have been done to shed further light on the mechanisms behind this structural transition-dependent conduction switching. Cation (Ag⁺/Cu⁺) nonstoichiometry in AgCuS significantly increases the vacancy concentration, hence, the p-type carriers, which is confirmed by positron annihilation spectroscopy and Hall measurement. The Ag_{1- x}CuS and AgCu_{1- x}S samples exhibit ultralow thermal conductivity (∼0.3-0.5 W/m·K) in the 290-623 K temperature range because of the low-energy cationic sublattice vibration that arises as a result of the movement of loosely bound Ag/Cu within the stiff S sublattice.

Soft Phonon Modes Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe

Crystalline solids with intrinsically low lattice thermal conductivity (κ_L ) are crucial to realizing high-performance thermoelectric (TE) materials. Herein, we show an ultralow κ_L of 0.35 Wm^-1  K^-1 in AgCuTe, which has a remarkable TE figure-of-merit, zT of 1.6 at 670 K when alloyed with 10 mol % Se. First-principles DFT calculation reveals several soft phonon modes in its room-temperature hexagonal phase, which are also evident from low-temperature heat-capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow κ_L in the room-temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. Despite the cation disorder at elevated temperatures, the crystalline conduits of the rigid anion sublattice give a high power factor.

Matildite Contact with Media: First-Principles Study of AgBiS2 Surfaces and Nanoparticle Morphology

Motivated by the interest in AgBiS₂ material for solar light harvesting applications, a detailed bulk first-principles quantum mechanical study of its surface properties is presented. Density functional theory based calculations with the Perdew-Burke-Ernzerhof functional have been carried out for different surface orientations and terminations of the matildite polymorph. From the results, two particularly stable facets are predicted to dominate Wulff shaped AgBiS₂ nanoparticles. These are the (001) type nonpolar surface and the (111) polar terminations where facets are exposed containing solely Ag or S atoms. The Wulff equilibrium shape is predicted to be a cube with only two edges capped. This particular shape explains a previously reported surface enrichment of Ag with respect to Bi of ∼1.5. The (001) surfaces display an ionic character similar to bulk AgBiS₂, with a low work function of 4.31 eV, although the inspection of the density of states (DOS) reveals a bandgap increased by 0.3 eV compared to bulk. This surface effect could explain the bulk wavelength overestimation of the absorption coefficient decay, as previously determined. Last but not least, the DOS of the (111) polar termination reveals a metallic character, where Fermi level is located below that on the (001) surfaces. Possible implications of the different electronic structure of these surfaces in the reported photoactivity are discussed.

Effect of Te substitution on crystal structure and transport properties of AgBiSe2thermoelectric material

Reduced lattice thermal conductivity of Te-substituted AgBiSe₂was qualitatively described using the point defect scattering model.

Comparing the role of annealing on the transport properties of polymorphous AgBiSe2 and monophase AgSbSe2

AgBiSe₂ and AgSbSe₂, two typical examples of Te-free I–V–VI₂ chalcogenides, are drawing much attention due to their promising thermoelectric performance. Both compounds were synthesized via melting and consolidated by spark plasma sintering. The role of annealing on the transport properties of polymorphous AgBiSe₂ and monophase AgSbSe₂ was studied. Annealing has a greater impact on AgBiSe₂ than AgSbSe₂, which is ascribed to the temperature dependent phase transition of AgBiSe₂. Unannealed AgBiSe₂ shows p–n switching, but annealed AgBiSe₂ exhibits n-type semiconducting behavior over the whole measurement temperature range. By performing high-temperature Hall measurements, we attribute this intriguing variation to the change in the amount of Ag vacancies and mid-temperature rhombohedral phase after annealing. Both AgBiSe₂ and AgSbSe₂ exhibit low thermal conductivity values, which are ∼0.40–0.50 W m⁻¹ K⁻¹ for AgSbSe₂ and ∼0.45–0.70 W m⁻¹ K⁻¹ for AgBiSe₂, respectively. The maximum ZT value of AgBiSe₂ is enhanced from 0.18 to 0.21 after annealing. Pristine AgSbSe₂ presents a ZT value as high as 0.60 at 623 K, although slight deterioration emerges after annealing.

Annealing treatment has different impact on the transport properties of polymorphous AgBiSe₂ and monophase AgSbSe₂.

Positron Annihilation Spectroscopic Investigation on the Origin of Temperature-Dependent Electrical Response in Methylammonium Lead Iodide Perovskite

Organic-inorganic hybrid perovskite has appeared as one of the leading materials for realizing solution-based high-performing optoelectronic devices. The charge transport properties in this class of material are quite intriguing and still need to be carefully investigated. The temperature-dependent electrical property of methylammonium lead iodide (CH₃NH₃PbI₃) has been investigated by employing positron annihilation spectroscopy (PAS), which unambiguously reveals the gradual formation of open volume defects with the enhancement in temperature. The high-temperature ionic conductivity is due to the generation of both cationic (CH₃NH₃⁺) and anionic (I^-) vacancies, possibly because of the elimination of methylammonium iodide (CH₃NH₃I) as identified from the coincidence Doppler broadening (CDB) of the positron annihilation spectroscopy. Further, the evolution of temperature-dependent defect density and corresponding electrical responses has been correlated with the structural phase transitions of CH₃NH₃PbI₃. This is the first ever report of temperature-dependent PAS measurement on hybrid lead halide perovskites to understand the nature and the origin of its electrical characteristics arising due to the variation in temperature.

Vacancy and anti-site disorder scattering in AgBiSe2 thermoelectrics

AgBiSe₂ has recently been shown to exhibit promising thermoelectric properties due to the low intrinsic thermal conductivity, stemming from a large degree of lattice anharmonicity. While samples synthesized via solid-state routes usually exhibit n-type behavior, p-type transport is seen in samples based on solution synthetic routes possibly due to Ag vacancies. Using a combined approach of synchrotron diffraction, thermoelectric transport measurements and thermal transport modeling, we show the influence of synthetically induced Ag vacancies on the structure of AgBiSe₂ and the thermoelectric transport. We identify the degree of anti-site disorder of Ag and Bi due to the occurring phase transformation and the influence of the vacancy content on metal ordering. Additionally, we show that anti-site disorder and vacancies act as scattering centers for phonons, leading to enhanced point defect scattering in this interesting thermoelectric material.

Large change in thermopower with temperature driven p–n type conduction switching in environment friendly BaxSr2−xTi0.8Fe0.8Nb0.4O6 double perovskites

Temperature driven p–n type conduction switching in combination with colossal change in thermo-power in Ba_xSr_2−xTi_0.8Fe_0.8Nb_0.4O₆ (BSTFN) double perovskites.

Syntheses, structures, and thermoelectric properties of ternary tellurides: RECuTe2 (RE = Tb–Er)

Four new compounds with the general formula RECuTe₂ and the influence of the electronegativity of rare-earth metals on thermoelectric properties are reported for the first time.

Bandgap engineering by cationic disorder: case study on AgBiS2

The influence of cationic disorder on the electronic structure of ternary compounds, here exemplified on AgBiS₂ material, is studied by means of accurate first principles periodic density functional theory based calculations.

Ion exchange-prepared NaSbSe2 nanocrystals: electronic structure and photovoltaic properties of a new solar absorber material

We report the calculated electronic structure, syntheses and photovoltaic properties of a new ternary solar absorber material NaSbSe₂.