High-performance thermoelectric silver selenide thin films cation exchanged from a copper selenide template

Deviceization of high-performance and flexible Ag2Se films for electronic skin and servo rotation angle control

Ag2Se shows significant potential for near-room-temperature thermoelectric applications, but its performance and device design are still evolving. In this work, we design a novel flexible Ag2Se thin-film-based thermoelectric device with optimized electrode materials and structure, achieving a high output power density of over 65 W m-2 and a normalized power density up to 3.68 μW cm-2 K-2 at a temperature difference of 42 K. By fine-tuning vapor selenization time, we strengthen the (013) orientation and carrier mobility of Ag2Se films, reducing excessive Ag interstitials and achieving a power factor of over 29 μW cm-1 K-2 at 393 K. A protective layer boosts flexibility of the thin film, retaining 90% performance after 1000 bends at 60°. Coupled with p-type Sb2Te3 thin films and rational simulations, the device shows rapid human motion response and precise servo motor control, highlighting the potential of high-performance Ag2Se thin films in advanced applications.

Precise synthesis of copper selenide nanowires with tailored Cu vacancies through photo-induced reduction for thermoelectric applications

Nanostructuring in α-Cu2Se while optimizing carrier concentration holds the promise of realizing further high thermoelectric performance at near room temperature. Nevertheless, controlling the amounts of Cu vacancies, which work as acceptors, in nanostructures is considerably more intricate than in bulk materials. Hence, controlling the amounts of Cu vacancies while maintaining the α-phase and nanostructure shape poses a formidable challenge. In this study, we synthesized Cu2+x Se nanowires (NWs) with various amounts of Cu vacancies at room temperature by the photoreduction method and investigated their thermoelectric properties. Cu2+x Se NWs exhibited a comparable thermoelectric power factor to that of the polycrystalline films fabricated at higher temperature. The achievement of the high power factor despite low-temperature fabrication is attributed to the precise synthesis of Cu2+x Se NWs with various amounts of Cu vacancies. We also investigated the reaction process of Cu2.00Se NWs in detail by observing the reaction intermediates. It was found that photoreduction occurred with Cu2+ ions adsorbed on Se NWs, leading to the reaction of Cu2+ ions and Se NWs without Cu deficiency. Namely, this photoreduction under the adsorbed conditions realized the control of Cu vacancies in Cu2+x Se NWs.

Solvent-assisted synthesis of Ag2Se and Ag2S nanoparticles on carbon fabric for enhanced thermoelectric performance

The challenge of developing low-cost, highly flexible, and high-performance thermoelectric (TE) materials persists due to the low thermoelectric efficiency of conducting polymers and the inflexibility of inorganic materials. In this study, we successfully integrated Ag2Se and Ag2S with highly conductive carbon fabric (CF) to produce a flexible thermoelectric material. A facile one-step solvothermal method was employed to synthesize the Ag2Se-CF and Ag2S-CF, which were then subjected to X-ray analysis to confine the phase formation of Ag2Se and Ag2S on the carbon fabric. The analysis revealed that Ag2Se and Ag2S nanoparticles were tightly packed on the surface of carbon fabric, and compositional analysis confirmed the interaction between the material and carbon fabric. The thermoelectric properties of Ag2Se-CF and Ag2S-CF were significantly altered due to carrier concentration and mobility variations, resulting in a low power factor of 6.7 μW/mK2 for Ag2Se-CF and a high-power factor of 24 μW/mK2 at 373 K for Ag2S-CF. The growth of Ag2Se-CF and Ag2S-CF on carbon fabric led to an enhancement in their thermoelectric properties. Further, TE legs were fabricated using the Ag2Se-CF (p-type) and Ag2S-CF (n-type), and the fabricated legs exhibited an output voltage of ∼20 mV to ∼86.65 mV at a temperature gradient (ΔT) of 3-8 K. This work represents a cutting-edge approach to the fabrication of high-performance, wearable thermoelectric devices.

Architected Metal Selenides via Sequential Cation and Anion Exchange on Self-Organizing Nanocomposites

Shape-preserving conversion reactions have the potential to unlock new routes for self-organization of complex three-dimensional (3D) nanomaterials with advanced functionalities. Specifically, developing such conversion routes toward shape-controlled metal selenides is of interest due to their photocatalytic properties and because these metal selenides can undergo further conversion reactions toward a wide range of other functional chemical compositions. Here, we present a strategy toward metal selenides with controllable 3D architectures using a two-step self-organization/conversion approach. First, we steer the coprecipitation of barium carbonate nanocrystals and silica into nanocomposites with controllable 3D shapes. Second, using a sequential exchange of cations and anions, we completely convert the chemical composition of the nanocrystals into cadmium selenide (CdSe) while preserving the initial shape of the nanocomposites. These architected CdSe structures can undergo further conversion reactions toward other metal selenides, which we demonstrate by developing a shape-preserving cation exchange toward silver selenide. Moreover, our conversion strategy can readily be extended to convert calcium carbonate biominerals into metal selenide semiconductors. Hence, the here-presented self-assembly/conversion strategy opens exciting possibilities toward customizable metal selenides with complex user-defined 3D shapes.

Thermoelectric Silver-Based Chalcogenides

Heat is abundantly available from various sources including solar irradiation, geothermal energy, industrial processes, automobile exhausts, and from the human body and other living beings. However, these heat sources are often overlooked despite their abundance, and their potential applications remain underdeveloped. In recent years, important progress has been made in the development of high-performance thermoelectric materials, which have been extensively studied at medium and high temperatures, but less so at near room temperature. Silver-based chalcogenides have gained much attention as near room temperature thermoelectric materials, and they are anticipated to catalyze tremendous growth in energy harvesting for advancing internet of things appliances, self-powered wearable medical systems, and self-powered wearable intelligent devices. This review encompasses the recent advancements of thermoelectric silver-based chalcogenides including binary and multinary compounds, as well as their hybrids and composites. Emphasis is placed on strategic approaches which improve the value of the figure of merit for better thermoelectric performance at near room temperature via engineering material size, shape, composition, bandgap, etc. This review also describes the potential of thermoelectric materials for applications including self-powering wearable devices created by different approaches. Lastly, the underlying challenges and perspectives on the future development of thermoelectric materials are discussed.

Molecular Decomposition Routes of Diaryl Diselenide Precursors in Relation to the Phase Determination of Copper Selenides

¹H nuclear magnetic resonance (NMR), ⁷⁷Se NMR, and powder X-ray diffraction (XRD) were used to monitor the thermal decomposition of diphenyl and dibenzyl diselenide precursors toward the synthesis of copper selenides. Copper was found to promote the decomposition of both precursors. The inorganic nanocrystals and organic byproducts were sensitive to the specific diaryl diselenides and the presence of oleylamine and copper. Molecular mechanistic routes are proposed. Berzelianite (Cu_1.8Se), klockmannite (CuSe), umangite (Cu₃Se₂), and both petřı́čekite (m-CuSe₂) and krutaite (p-CuSe₂) were identified as products. Multistep transformations between phases were discovered through reactions with the organoselenium precursors, and organic decomposition products are proposed.

Facile one pot synthesis of highly photoresponsive coinage metal selenides (Cu1.8Se and Ag2Se) achieved through novel Cu and Ag pyridylselenolates as molecular precursors

Copper selenide (Cu_1.8Se) and silver selenide (Ag₂Se) have garnered unprecedented attention as efficient absorber materials for cost-effective and sustainable solar cells. Phase pure preparation of these exotic materials in a nano-regime is highly desirable. This account outlines a simple and easily scalable pathway to Cu_1.8Se and Ag₂Se nanocrystals using novel complexes [Cu{2-SeC₅H₂(Me-4,6)₂N}]₄ (1), [Ag{2-SeC₅H₂(Me-4,6)₂N}]₆ (2) and [Ag{2-SeC₅H₃(Me-5)N}]₆·2C₆H₅CH₃ (3·2C₆H₅CH₃) as single source molecular precursors (SSPs). Structural studies revealed that the Cu and Ag complexes crystallize into tetrameric and hexameric forms, respectively. This observed structural diversity in the complexes has been rationalized via DFT calculations and attributed to metal-metal bond endorsed energetics. The thermolysis at relatively lower temperature in oleylamine of complex 1 afforded cubic berzelianite Cu_1.8Se and complexes 2 and 3 produced orthorhombic naumannite Ag₂Se nanocrystals. The low temperature synthesis of these nanocrystals seems to be driven by the observed preformed Cu₄Se₄ and Ag₆Se₆ core in the complexes which have close resemblance with the bulk structure of the final materials (Cu_1.8Se and Ag₂Se). The crystal structure, phase purity, morphology, elemental composition and band gap of these nanocrystals were determined from pXRD, electron microscopy (SEM and TEM), EDS and DRS-UV, respectively. The band gap of these nanocrystals lies in the range suitable for solar cell applications. Finally, these nanocrystal-based prototype photo-electrochemical cells exhibit high photoresponsivity and stability under alternating light and dark conditions.

Intraband Transitions of Nanocrystals Transforming from Lead Selenide to Self-doped Silver Selenide Quantum Dots by Cation Exchange

In search of heavy metal-free mid-IR active colloidal materials, self-doped silver selenide colloidal quantum dots (CQDs) can be an alternative offering tunable mid-IR wavelength with a narrow bandwidth. One of the challenges in the study of the intraband transition is developing a method to widen the intraband transition energy range as well as reducing the toxicity of the materials. Here, we present Ag_xSe (x > 2) CQDs exhibiting an intraband transition up to 0.39 eV, produced by the cation exchange (CE) method from PbSe CQDs. The major electronic transition efficiently changes from the SWIR band gap of PbSe CQDs to the mid-IR intraband transition of the Ag_xSe CQDs by the CE. The intraband exciton is verified by examining the absorption and emission of the CE Ag_xSe CQDs as well as their applications on electrochemical mid-IR luminescence and mid-IR intraband photodetectors.

Prediction of stable silver selenide-based energy materials sustained by rubidium selenide alloying

Silver selenide (Ag2Se) is a ductile material with a low lattice thermal conductivity that can be a valuable substitute for both PbSe and Bi2Se3 for Pb toxicity free and Bi scarcity.

Crystal Structure of Colloidally Prepared Metastable Ag2Se Nanocrystals

Structural polymorphism is known for many bulk materials; however, on the nanoscale metastable polymorphs tend to form more readily than in the bulk, and with more structural variety. One such metastable polymorph observed for colloidal Ag₂Se nanocrystals has traditionally been referred to as the "tetragonal" phase. While there are reports on the chemistry and properties of this metastable polymorph, its crystal structure, and therefore electronic structure, has yet to be determined. We report that an anti-PbCl₂-like structure type (space group P2₁/n) more accurately describes the powder X-ray diffraction and X-ray total scattering patterns of colloidal Ag₂Se nanocrystals prepared by several different methods. Density functional theory (DFT) calculations indicate that this anti-PbCl₂-like Ag₂Se polymorph is a dynamically stable, narrow-band-gap semiconductor. The anti-PbCl₂-like structure of Ag₂Se is a low-lying metastable polymorph at 5-25 meV/atom above the ground state, depending on the exchange-correlation functional used.