Tin Selenide Molecular Precursor for the Solution Processing of Thermoelectric Materials and Devices

The Latest Advances in Ink-Based Nanogenerators: From Materials to Applications

Nanogenerators possess the capability to harvest faint energy from the environment. Among them, thermoelectric (TE), triboelectric, piezoelectric (PE), and moisture-enabled nanogenerators represent promising approaches to micro-nano energy collection. These nanogenerators have seen considerable progress in material optimization and structural design. Printing technology has facilitated the large-scale manufacturing of nanogenerators. Although inks can be compatible with most traditional functional materials, this inevitably leads to a decrease in the electrical performance of the materials, necessitating control over the rheological properties of the inks. Furthermore, printing technology offers increased structural design flexibility. This review provides a comprehensive framework for ink-based nanogenerators, encompassing ink material optimization and device structural design, including improvements in ink performance, control of rheological properties, and efficient energy harvesting structures. Additionally, it highlights ink-based nanogenerators that incorporate textile technology and hybrid energy technologies, reviewing their latest advancements in energy collection and self-powered sensing. The discussion also addresses the main challenges faced and future directions for development.

Bottom-Up Synthesis of SnTe-Based Thermoelectric Composites

There is a need for the development of lead-free thermoelectric materials for medium-/high-temperature applications. Here, we report a thiol-free tin telluride (SnTe) precursor that can be thermally decomposed to produce SnTe crystals with sizes ranging from tens to several hundreds of nanometers. We further engineer SnTe-Cu₂SnTe₃ nanocomposites with a homogeneous phase distribution by decomposing the liquid SnTe precursor containing a dispersion of Cu_1.5Te colloidal nanoparticles. The presence of Cu within the SnTe and the segregated semimetallic Cu₂SnTe₃ phase effectively improves the electrical conductivity of SnTe while simultaneously reducing the lattice thermal conductivity without compromising the Seebeck coefficient. Overall, power factors up to 3.63 mW m^-1 K^-2 and thermoelectric figures of merit up to 1.04 are obtained at 823 K, which represent a 167% enhancement compared with pristine SnTe.

Large-Scale Colloidal Synthesis of Chalcogenides for Thermoelectric Applications

A simple and effective preparation of solution-processed chalcogenide thermoelectric materials is described. First, PbTe, PbSe, and SnSe were prepared by gram-scale colloidal synthesis relying on the reaction between metal acetates and diphenyl dichalcogenides in hexadecylamine solvent. The resultant phase-pure chalcogenides consist of highly crystalline and defect-free particles with distinct cubic-, tetrapod-, and rod-like morphologies. The powdered PbTe, PbSe, and SnSe products were subjected to densification by spark plasma sintering (SPS), affording dense pellets of the respective chalcogenides. Scanning electron microscopy shows that the SPS-derived pellets exhibit fine nano-/micro-structures dictated by the original morphology of the key constituting particles, while the powder X-ray diffraction and electron microscopy analyses confirm that the SPS-derived pellets are phase-pure materials, preserving the structure of the colloidal synthesis products. The resultant solution-processed PbTe, PbSe, and SnSe exhibit low thermal conductivity, which might be due to the enhanced phonon scattering developed over fine microstructures. For undoped n-type PbTe and p-type SnSe samples, an expected moderate thermoelectric performance is achieved. In contrast, an outstanding figure-of-merit of 0.73 at 673 K was achieved for undoped n-type PbSe outperforming, the majority of the optimized PbSe-based thermoelectric materials. Overall, our findings facilitate the design of efficient solution-processed chalcogenide thermoelectrics.

Electronic Structure-, Phonon Spectrum-, and Effective Mass- Related Thermoelectric Properties of PdXSn (X = Zr, Hf) Half Heuslers

We hereby discuss the thermoelectric properties of PdXSn(X = Zr, Hf) half Heuslers in relation to lattice thermal conductivity probed under effective mass (hole/electrons) calculations and deformation potential theory. In addition, we report the structural, electronic, mechanical, and lattice dynamics of these materials as well. Both alloys are indirect band gap semiconductors with a gap of 0.91 eV and 0.82 eV for PdZrSn and PdHfSn, respectively. Both half Heusler materials are mechanically and dynamically stable. The effective mass of electrons/holes is (0.13/1.23) for Zr-type and (0.12/1.12) for Hf-kind alloys, which is inversely proportional to the relaxation time and directly decides the electrical/thermal conductivity of these materials. At 300K, the magnitude of lattice thermal conductivity observed for PdZrSn is 15.16 W/mK and 9.53 W/mK for PdHfSn. The highest observed ZT value for PdZrSn and PdHfSn is 0.32 and 0.4, respectively.

High-Performance Thermoelectrics Based on Solution-Grown SnSe Nanostructures

Two-dimensional layered tin selenide (SnSe) has attracted immense interest in thermoelectrics due to its ultralow lattice thermal conductivity and high thermoelectric performance. To date, the majority of thermoelectric studies of SnSe have been based on single crystals. However, because synthesizing SnSe single crystals is an expensive, time-consuming process that requires high temperatures and because SnSe single crystals have relatively weaker mechanical stability, they are not favorable for scaling up synthesis, commercialization, or practical applications. As a result, research on nanocrystalline SnSe that can be produced in large quantities by simple and low-temperature solution-phase synthesis is needed. In this Perspective, we discuss the progress in thermoelectric properties of SnSe with a particular emphasis on nanocrystalline SnSe, which is grown in solution. We first describe the state-of-the-art high-performance single crystal and polycrystals of SnSe and their importance and drawbacks and discuss how nanocrystalline SnSe can solve some of these challenges. We illustrate different solution-phase synthesis procedures to produce various SnSe nanostructures and discuss their thermoelectric properties. We also highlight a unique solution-phase synthesis technique to prepare CdSe-coated SnSe nanocomposites and its unprecedented thermoelectric figure of merit (ZT) of 2.2 at 786 K, as reported in this issue of ACS Nano. In general, solution synthesis showed excellent control over nanoscale grain growth, and nanocrystalline SnSe shows ultralow thermal conductivity due to strong phonon scattering by the nanoscale grain boundaries. Finally, we offer insight into the opportunities and challenges associated with nanocrystalline SnSe synthesized by the solution route and its future in thermoelectric energy conversion.

Bottom-Up Engineering Strategies for High-Performance Thermoelectric Materials

The recent advancements in thermoelectric materials are largely credited to two factors, namely established physical theories and advanced materials engineering methods. The developments in the physical theories have come a long way from the "phonon glass electron crystal" paradigm to the more recent band convergence and nanostructuring, which consequently results in drastic improvement in the thermoelectric figure of merit value. On the other hand, the progresses in materials fabrication methods and processing technologies have enabled the discovery of new physical mechanisms, hence further facilitating the emergence of high-performance thermoelectric materials. In recent years, many comprehensive review articles are focused on various aspects of thermoelectrics ranging from thermoelectric materials, physical mechanisms and materials process techniques in particular with emphasis on solid state reactions. While bottom-up approaches to obtain thermoelectric materials have widely been employed in thermoelectrics, comprehensive reviews on summarizing such methods are still rare. In this review, we will outline a variety of bottom-up strategies for preparing high-performance thermoelectric materials. In addition, state-of-art, challenges and future opportunities in this domain will be commented.

Tin-selenide as a futuristic material: properties and applications

SnSe/SnSe2 is a promising versatile material with applications in various fields like solar cells, photodetectors, memory devices, lithium and sodium-ion batteries, gas sensing, photocatalysis, supercapacitors, topological insulators, resistive switching devices due to its optimal band gap. In this review, all possible applications of SnSe/SnSe2 have been summarized. Some of the basic properties, as well as synthesis techniques have also been outlined. This review will help the researcher to understand the properties and possible applications of tin selenide-based materials. Thus, this will help in advancing the field of tin selenide-based materials for next generation technology.

Biomimetic Approach to Facilitate the High Filler Content in Free-Standing and Flexible Thermoelectric Polymer Composite Films Based on PVDF and Ag2Se Nanowires

A high filler content is often needed in polymer composite-based thermoelectric (TE) films to improve their performance. Nevertheless, this often leads to poor processability and poor mechanical performance. Herein, a biomimetic approach is adopted to facilitate the filler content up to 90.5 wt % in free-standing and flexible n-type PVDF/Ag₂Se TE films, where PVDF dendricolloids are a solution mixed with Ag₂Se nanowires (NWs), followed by filtration. These soft dendric nanoparticles within PVDF dendricolloids have high adhesivity and strong network-building ability, which allows the formation of "grapevine-grape"-like networks with soft dendritic particles and inorganic TE fillers as "grapevine" and "manicure finger grapes", respectively. The maximum power factor of 189.02 μW m^-1 K^-2 is achieved for a PVDF/Ag₂Se mass ratio of 1:9.5 at 300 K. Meanwhile, excellent flexibility with only 15.8% decrease in electrical conductivity after 1000 bending cycles was observed. These properties at such a high filler content are attributed to the long-range grapevine-like network of soft PVDF dendritic particles and entanglement between numerous Ag₂Se NWs. This work carves a path to fabricate high-performance free-standing flexible n-type TE composite films as well as other functional polymer composites requiring high inorganic filler loading.