Tunable quantum gaps to decouple carrier and phonon transport leading to high-performance thermoelectrics

Into the Void: Single Nanopore in Colloidally Synthesized Bi2Te3 Nanoplates with Ultralow Lattice Thermal Conductivity

Bi2Te3 is a well-known thermoelectric material that was first investigated in the 1960s, optimized over decades, and is now one of the highest performing room-temperature thermoelectric materials to-date. Herein, we report on the colloidal synthesis, growth mechanism, and thermoelectric properties of Bi2Te3 nanoplates with a single nanopore in the center. Analysis of the reaction products during the colloidal synthesis reveals that the reaction progresses via a two-step nucleation and epitaxial growth: first of elemental Te nanorods and then the binary Bi2Te3 nanoplate growth. The rates of epitaxial growth can be controlled during the reaction, thus allowing the formation of a single nanopore in the center of the Bi2Te3 nanoplates. The size of the nanopore can be controlled by changing the pH of the reaction solution, where larger pores with diameter of ∼50 nm are formed at higher pH and smaller pores with diameter of ∼16 nm are formed at lower pH. We propose that the formation of the single nanopore is mediated by the Kirkendall effect and thus the reaction conditions allow for the selective control over pore size. Nanoplates have well-defined hexagonal facets as seen in the scanning and transmission electron microscopy images. The single nanopores have a thin amorphous layer at the edge, revealed by transmission electron microscopy. Thermoelectric properties of the pristine and single-nanopore Bi2Te3 nanoplates were measured in the parallel and perpendicular directions. These properties reveal strong anisotropy with a significant reduction to thermal conductivity and increased electrical resistivity in the perpendicular direction due to the higher number of nanoplate and nanopore interfaces. Furthermore, Bi2Te3 nanoplates with a single nanopore exhibit ultralow lattice thermal conductivity values, reaching ∼0.21 Wm-1K-1 in the perpendicular direction. The lattice thermal conductivity was found to be systematically lowered with pore size, allowing for the realization of a thermoelectric figure of merit, zT of 0.75 at 425 K for the largest pore size.

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.

Nanoscale fabrication of heterostructures in thermoelectric SnTe

Enhancing the performance of thermoelectric materials is demanded to develop strategies for introducing multidimensional microstructures into materials to induce full-scale phonon scattering while ensuring electrical transport performance. Herein, a previously unreported rhombohedral h-SnTe (R3̄m) has been achieved in the nanoscale dimension by the electron beam irradiation of β-SnTe (Fm3̄m) materials. The h-SnTe structure contains interlayer van der Waals gaps and exhibits metallic behavior evaluated by density-functional theory calculations, which coherently appears in the narrow-band semiconductor β-SnTe matrix. Our results provide a strategy for modifying the properties of SnTe-based thermoelectric materials and designing nanostructured chalcogenide heterostructures via electron beam irradiation.

Decoupling Carrier-Phonon Scattering Boosts the Thermoelectric Performance of n-Type GeTe-Based Materials

The coupled relationship between carrier and phonon scattering severely limits the thermoelectric performance of n-type GeTe materials. Here, we provide an efficient strategy to enlarge grains and induce vacancy clusters for decoupling carrier-phonon scattering through the annealing optimization of n-type GeTe-based materials. Specifically, boundary migration is used to enlarge grains by optimizing the annealing time, while vacancy clusters are induced through the aggregation of Ge vacancies during annealing. Such enlarged grains can weaken carrier scattering, while vacancy clusters can strengthen phonon scattering, leading to decoupled carrier-phonon scattering. As a result, a ratio between carrier mobility and lattice thermal conductivity of ∼492.8 cm3 V-1 s-1 W-1 K and a peak ZT of ∼0.4 at 473 K are achieved in Ge0.67Pb0.13Bi0.2Te. This work reveals the critical roles of enlarged grains and induced vacancy clusters in decoupling carrier-phonon scattering and demonstrates the viability of fabricating high-performance n-type GeTe materials via annealing optimization.

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.

Swift Assembly of Adaptive Thermocell Arrays for Device-Level Healable and Energy-Autonomous Motion Sensors

The evolution of wearable technology has prompted the need for adaptive, self-healable, and energy-autonomous energy devices. This study innovatively addresses this challenge by introducing an MXene-boosted hydrogel electrolyte, which expedites the assembly process of flexible thermocell (TEC) arrays and thus circumvents the complicated fabrication of typical wearable electronics. Our findings underscore the hydrogel electrolyte's superior thermoelectrochemical performance under substantial deformations and repeated self-healing cycles. The resulting hydrogel-based TEC yields a maximum power output of 1032.1 nW under the ΔT of 20 K when being stretched to 500% for 1000 cycles, corresponding to 80% of its initial state; meanwhile, it sustains 1179.1 nW under the ΔT of 20 K even after 60 cut-healing cycles, approximately 92% of its initial state. The as-assembled TEC array exhibits device-level self-healing capability and high adaptability to human body. It is readily applied for touch-based encrypted communication where distinct voltage signals can be converted into alphabet letters; it is also employed as a self-powered sensor to in-situ monitor a variety of body motions for complex human actions. The swift assembly approach, combined with the versatile functionality of the TEC device, paves the way for future advancements in wearable electronics targeting at fitness monitoring and human-machine interfaces.

MXene and Carbon-Based Electrodes of Thermocells for Continuous Thermal Energy Harvest

Low-grade heat represents a significant form of energy loss; thermocells (TECs) utilizing the thermogalvanic effect can convert thermal energy into electricity without generating vibrations, noise, or waste emissions, making them a promising energy conversion technology for efficiently harvesting low-grade heat. Despite recent advancements, the reliance on high-cost platinum electrodes in TECs has considerably hindered their widespread adoption. Developing cost-effective electrodes that maintain the same thermoelectrochemical performance is crucial for the successful application of TECs. In this review article, the exploration of MXene materials as TEC electrodes is discussed first, emphasizing the immense potential of the MXene family for low-grade heat harvesting applications. Next, recent research on carbon-based electrodes is summarized, and morphological and structural optimizations are comprehensively discussed aiming at enhancing the thermoelectrochemical performance of TECs. In the concluding section, the challenges are outlined and future perspectives are offered, which provide valuable insights into the ongoing development of high-performance TEC electrodes using MXene and carbon-based materials.

Charge transfer engineering to achieve extraordinary power generation in GeTe-based thermoelectric materials

By the fine manipulation of the exceptional long-range germanium-telluride (Ge─Te) bonding through charge transfer engineering, we have achieved exceptional thermoelectric (TE) and mechanical properties in lead-free GeTe. This chemical bonding mechanism along with a semiordered zigzag nanostructure generates a notable increase of the average zT to a record value of ~1.73 in the temperature range of 323 to 773 K with ultrahigh maximum zT ~ 2.7. In addition, we significantly enhanced the Vickers microhardness numbers (H_v) to an extraordinarily high value of 247 H_v and effectively eliminated the thermal expansion fluctuation at the phase transition, which was problematic for application, by the present charge transfer engineering process and concomitant formation of microstructures. We further fabricated a single-leg TE generator and obtained a conversion efficiency of ~13.4% at the temperature difference of 463 K on a commercial instrument, which is located at the pinnacle of TE conversion.

High thermoelectric efficiency realized in SnSe crystals via structural modulation

Crystalline thermoelectrics have been developed to be potential candidates for power generation and electronic cooling, among which SnSe crystals are becoming the most representative. Herein, we realize high-performance SnSe crystals with promising efficiency through a structural modulation strategy. By alloying strontium at Sn sites, we modify the crystal structure and facilitate the multiband synglisis in p-type SnSe, favoring the optimization of interactive parameters μ and m^*. Resultantly, we obtain a significantly enhanced PF ~85 μW cm^-1 K^-2, with an ultrahigh ZT ~1.4 at 300 K and ZT_ave ~2.0 among 300-673 K. Moreover, the excellent properties lead to single-leg device efficiency of ~8.9% under a temperature difference ΔT ~300 K, showing superiority among the current low- to mid-temperature thermoelectrics, with an enhanced cooling ΔT_max of ~50.4 K in the 7-pair thermoelectric device. Our study further advances p-type SnSe crystals for practical waste heat recovery and electronic cooling.

Abnormal behavior of preferred formation of the cationic vacancies from the interior in a γ-GeSe monolayer with the stereo-chemical antibonding lone-pair state

Two-dimensional (2D) materials tend to have the preferable formation of vacancies at the outer surface. Here, contrary to the normal notion, we reveal a type of vacancy that thermodynamically initiates from the interior part of the 2D backbone of germanium selenide (γ-GeSe). Interestingly, the Ge-vacancy (V_Ge) in the interior part of γ-GeSe possesses the lowest formation energy amongst the various types of defects considered. We also find a low diffusion barrier (1.04 eV) of V_Ge, which is half of those of sulfur vacancies in MoS₂. The facile formation of mobile V_Ge is rooted in the antibonding coupling of the lone-pair Ge 4s and Se 4p states near the valence band maximum, which also exists in other gamma-phase MX (M = Sn, Ge; X = S, Te). The V_Ge is accompanied by a shallow acceptor level in the band gap and induces strong infrared light absorption and p-type conductivity. The V_Ge located in the middle cationic Ge sublattice is well protected by the surface Se layers - a feature that is absent in other atomically thin materials. Our work suggests that the unique well-buried inner V_Ge, with the potential of forming structurally protected ultrathin conducting filaments, may render the GeSe layer an ideal platform for quantum emitting, memristive, and neuromorphic applications.

Advances in Versatile GeTe Thermoelectrics from Materials to Devices

Driven by the intensive efforts in the development of high-performance GeTe thermoelectrics for mass-market application in power generation and refrigeration, GeTe-based materials display a high figure of merit of >2.0 and an energy conversion efficiency beyond 10%. However, a comprehensive review on GeTe, from fundamentals to devices, is still needed. In this regard, the latest progress on the state-of-the-art GeTe is timely reviewed. The phase transition, intrinsic high carrier concentration, and multiple band edges of GeTe are fundamentally analyzed from the perspectives of the native atomic orbital, chemical bonding, and lattice defects. Then, the fabrication methods are summarized with a focus on large-scale production. Afterward, the strategies for enhancing electronic transports of GeTe by energy filtering effect, resonance doping, band convergence, and Rashba band splitting, and the methods for strengthening phonon scatterings via nanoprecipitates, planar vacancies, and superlattices, are comprehensively reviewed. Besides, the device assembly and performance are highlighted. In the end, future research directions are concluded and proposed, which enlighten the development of broader thermoelectric materials.