Interface Engineering in Solution-Processed Nanocrystal Thin Films for Improved Thermoelectric Performance

Solution-synthesized nanostructured materials with high thermoelectric performance

Facing the growing scarcity of traditional fossil fuels and the inefficiency of energy utilization, thermoelectric materials have garnered increasing attention due to their ability to convert between electrical and thermal energy. However, the strong coupling between thermoelectric parameters presents a significant challenge for simultaneously reducing thermal conductivity and maintaining electrical performance in bulk materials. The solution-based synthesis of nanostructured materials offers a promising approach for the decoupling regulation of electronic and phonon transport properties by regulating grain size and morphology, second phases, and surface ligands. The strategies for optimizing thermoelectric performance outlined above are founded upon several pivotal elements: the enhancement of grain boundary effects, precise regulation of grain stacking, utilization of heterogeneous interface effects, and generation of metastable phases and novel structural configurations facilitated by ligand management approaches. We have also comprehensively addressed the challenges associated with solution-based synthesis, particularly material oxidation and grain coarsening, along with their corresponding mitigation strategies. In addition, machine learning can effectively accelerate solution synthesis and the exploration of composite materials. This review summarizes and generalizes the research related to these strategies, providing recommendations for future research directions based on observed trends.

Recent Progress on Layered Sn and Pb-Based Mono Chalcogenides: Synthesis, Structure, Optical, and Thermoelectric Properties and Related Applications

The research on two-dimensional materials has gained significant traction due to their potential for thermoelectric, optical, and other properties. The development of two-dimensional (2D) nanostructured-based TE generators and photodetectors has shown promising results. Over the years, researchers have played a crucial role in advancing this field, enhancing the properties of 2D materials through techniques such as doping, alloying, and various growth methods. Among these materials, black phosphorus, transition metal dichalcogenides, graphene, and IVA-VIA compounds stand out for their remarkable electronic, mechanical, and optical properties. This study presents a comprehensive review of the progress in the field, focusing on IVA-VIA compounds and their applications in TE and photodetector technologies. We summarize recent advancements in enhancing these materials' TE and optical properties and provide an overview of various synthesis techniques for their fabrication. Additionally, we highlight their potential applications as photodetectors in the infrared spectrum. This comprehensive review aims to equip researchers with a deep understanding of the TE and optical properties of 2DMs and their potential applications and to inspire further advancements in this field of research.

Recent Progress in Colloidal Quantum Dot Thermoelectrics

Semiconducting colloidal quantum dots (CQDs) represent an emerging class of thermoelectric materials for use in a wide range of future applications. CQDs combine solution processability at low temperatures with the potential for upscalable manufacturing via printing techniques. Moreover, due to their low dimensionality, CQDs exhibit quantum confinement and a high density of grain boundaries, which can be independently exploited to tune the Seebeck coefficient and thermal conductivity, respectively. This unique combination of attractive attributes makes CQDs very promising for application in emerging thermoelectric generator (TEG) technologies operating near room temperature. Herein, recent progress in CQDs for application in emerging thin-film thermoelectrics is reviewed. First, the fundamental concepts of thermoelectricity in nanostructured materials are outlined, followed by an overview of the popular synthetic methods used to produce CQDs with controllable sizes and shapes. Recent strides in CQD-based thermoelectrics are then discussed with emphasis on their application in thin-film TEGs. Finally, the current challenges and future perspectives for further enhancing the performance of CQD-based thermoelectric materials for future applications are discussed.

Deposition of ultra-thin coatings by a nature-inspired Spray-on-Screen technology

Nanometre-thick, ultrathin coatings applied over a large area are of paramount importance for various application fields such as biomedicine, space and automotive, organic electronics, memory devices, or energy storage devices. So far wet chemical deposition as a cost-effective, scalable, and versatile method can only be used for thicker deposits. Here the formation of uniform ultra-thin coatings with thicknesses below 15 nm using a nature-inspired, roll-to-roll compatible Spray-on-Screen (SoS) technology is reported. For this, the finite micro-droplet generation of Ultrasonic Spray Coating (USSC) is combined with the coating formation from a screen printing mesh. Hydrophobic micro-threads of the mesh, resembling the micro-hair on the legs of water striders, produce millidroplets from micro droplets, and when applying an external pressure to the mesh, dynamic wetting is enforced. The proposed technology is applicable for a wide variety of substrates and applications. It is shown by theory and experiment that ultra-thin coatings below 5 nm homogeneous over a large area can be deposited without the use of extended ink formulation or high substrate temperatures during or after deposition. This simple yet effective technique enables the deposition of ultra-thin films on any substrates, and is very promising to fabricate the organic, inorganic electronics devices and batteries cost effectively.

Rachith Kumar and coworkers report a bio-inspired coating technique able to deposit uniform films with thicknesses below 15 nm on various substrates. This method will not require the use of extended ink formulation or high substrate temperature as existing techniques do, potentially reducing the fabrication cost of future electronic devices and batteries.

Excellent thermoelectric properties of the Tl2S3 monolayer for medium-temperature applications

Exploring materials with high thermoelectric (TE) performance can alleviate energy pressure and protect the environment, and thus, TE materials have attracted extensive attention in the new energy field. In this paper, we systematically study the TE properties of Tl₂S₃ using first-principles combined with Boltzmann transport theory (BTE). The calculation results show an excellent power factor (1.12 × 10^-2 W m^-1 K^-2) and ultra-low lattice thermal conductivity (k_l = 0.88 W m^-1 K^-1) at room temperature. Through analysis, we attribute the ultra-low k_l of Tl₂S₃ to the lower phonon group velocity (v_g) and larger phonon anharmonicity. Meanwhile, discussion of chemical bonding showed that the filling of the anti-bonding state leads to the weakening of the Tl-S chemical bond, resulting in low v_g. Furthermore, this research also investigates the scattering processes (the out-of-plane acoustic mode (ZA) + optical mode (O) → O (ZA + O → O), the in-plane transverse acoustic mode (TA) + O → O (TA + O → O), and the in-plane longitudinal acoustic mode (LA) + O → O (LA + O → O)), from which we find that 2D Tl₂S₃ possesses strong acoustic-optical scattering. Based on the analysis of electron transport properties under electron-phonon coupling, 2D Tl₂S₃, as a novel TE material, exhibits a ZT value as high as 2.8 at 400 K. Our calculations suggest that Tl₂S₃ is a potential TE material at medium temperature.

Solution Phase Growth and Ion Exchange in Microassemblies of Lead Chalcogenide Nanoparticles

We demonstrate the synthesis of micron-sized assemblies of lead chalcogenide nanoparticles with controlled morphology, crystallinity, and composition through a facile room-temperature solution phase reaction. The amine-thiol solvent system enables this synthesis with a unique oriented attachment growth mechanism of nanoparticles occurring on the time scale of the reaction itself, forming single-crystalline microcubes of PbS, PbSe, and PbTe materials. Increasing the rate of reaction by changing reaction parameters further allows disturbing the oriented attachment mechanism, which results in polycrystalline microassemblies with uniform spherical morphologies. Along with polycrystallinity, due to the differences in reactivities of each chalcogen in the solution, a different extent of hollow-core nature is observed in these microparticles. Similar to morphologies, the composition of such microparticles can be altered through very simplistic room-temperature solution phase coprecipitation, as well as ion-exchange reactions. While coprecipitation reactions are successful in synthesizing core-shell microstructures of PbSe-PbTe materials, the use of solution phase ion-exchange reaction allows for the exchange of not only Te with Se but also Ag with Pb inside the core of the PbTe microparticles. Despite exchanging one Pb with two Ag cations, the hollow-core nature of particles aids in the retention of the original uniform microparticle morphology.

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.

Modifying the Thermoelectric Transport of Sb2Te3 Thin Films via the Carrier Filtering Effect by Incorporating Size-Selected Gold Nanoparticles

Hot energy carrier filtering as a means to improve the thermoelectric (TE) property in Sb₂Te₃ thin film samples having size-selected Au nanoparticles (NPs) is investigated in the present study. Nonagglomerated Au NPs with a very narrow size distribution grown by an integrated gas-phase synthesis setup are incorporated into the Sb₂Te₃ thin film synthesized by RF magnetron sputtering. TE properties have been investigated as a function of size-selected Au NP concentrations and compared with that of a nanocomposite sample having non-size-selected Au NPs. An increase in the Seebeck coefficient and power factor, along with a slight decrease in electrical conductivity, is observed for samples with a NP size of minimum variance. Further, the Kelvin probe force microscopy and conducting atomic force microscopy techniques were employed to understand the nature of the interface and charge transport across the Sb₂Te₃ matrix and Au NPs. The study provides an opportunity to modulate the TE properties in Sb₂Te₃ thin films by constructing a metal-semiconductor heterostructure through controlling the concentration and randomness to achieve a high TE performance.

Evolution map of the memristor: from pure capacitive state to resistive switching state

Memristors possess great application prospects in terabit nonvolatile storage devices, memory-in-logic algorithmic chips and bio-inspired artificial neural network systems. However, "what is the origin state of the memristor?" has remained an unanswered question for half a century. While many applications rely on the memristor, its origin state is becoming a fundamental issue. Herein, we reveal a new state, the pure capacitance state (PCS), which occurs before the memristor is triggered, and the origin state of the memristor can be verified in the memory cells through controlling the ambience parameters. Discovery of the PCS, a missing earlier stage of the memristor, completes the whole evolution map of the memristor from the very beginning to the final developed state.

Thermal-assisted self-assembly: a self-adaptive strategy towards large-area uniaxial organic single-crystalline microribbon arrays

Uniaxial organic single-crystalline microribbon arrays (OSCMAs) are a class of highly desirable materials for a variety of optoelectronic applications due to their favorable molecular orientations along the long axes of the ribbons. Up to now, great endeavors have been made and several solution-processing techniques have been proposed to grow uniaxial OSCMAs. However, the crystal growth parameters are tuned non-synergistically in these techniques, resulting in challenging growth condition control. Herein, we report a self-adaptive thermal-assisted self-assembly (TASA) strategy to realize the synergistic control of key crystal growth parameters for the facile yet controllable production of centimeter-sized uniaxial OSCMAs from the solution. In the TASA strategy, key crystal growth parameters, such as solvent evaporation, nucleation and crystal growth, are controlled synergistically by the temperature gradient. As a result, the TASA strategy is self-adaptive, and it shows a large temperature and concentration tolerance. Organic phototransistors (OPTs) based on the uniaxial OSCMAs produced by the TASA strategy exhibit an unprecedented photosensitivity of 1.36 × 10⁸, a high responsivity of 845 A W^-1 and a high detectivity of 1.98 × 10¹⁵ Jones.

Induced charge electro-osmotic particle separation

Vortex-based separation is a promising method in particle-particle separation and has only been demonstrated theoretically some years ago. To date, a continuous-flow separation device based on vortices has not been conceived because many known vortices were either unstable or controlling them lacked precision. Electro-convection from induced charge electro-osmosis (ICEO) has advantages, such as adjustable flow profiles, long-range actuation, and long-lived vortices, and offers an alternative means of particle separation. We found though a different ICEO focusing behaviour of particles whereby particles were trapped and concentrated in two vortex cores. Encouraged by these features of ICEO vortices, we proposed a direct method for particle separation in continuous flow. In various experiments, we first characterized the ICEO-induced focusing performances of various kinds of particle samples in a straight channel embedded with an individual central bipolar electrode, presenting a justifiable explanation. Second, the combined dependences of ICEO particle separation on the sample size and mass density were investigated. Third, an application to cell purification was performed in which we obtained a purity surpassing 98%. Finally, we investigated the ICEO characteristics of nanoparticles, exploiting our method in isolating nanoscale objects by separating 500 nm and 5 μm polystyrene beads, gaining clear separation. Certain features of this method, such as having ease of operation, simple structure, and continuous flow, and being prefocusing free and physical property-based, indicate its good potential in tackling environmental monitoring, cell sorting, chemical analysis, isolation of uniform-sized graphene and transesterification of micro-algal lipids to biodiesel.

Hierarchically hydrogen-bonded graphene/polymer interfaces with drastically enhanced interfacial thermal conductance

Interfacial thermal transport is a critical physical process determining the performance of many material systems with small-scale features. Recently, self-assembled monolayers and polymer brushes have been widely used to engineer material interfaces presenting unprecedented properties. Here, we demonstrate that poly(vinyl alcohol) (PVA) monolayers with hierarchically arranged hydrogen bonds drastically enhance interfacial thermal conductance by a factor of 6.22 across the interface between graphene and poly(methyl methacrylate) (PMMA). The enhancement is tunable by varying the number of grafted chains and the density of hydrogen bonds in the unique hierarchical hydrogen bond network. The extraordinary enhancement results from a synergy of hydrogen bonds and other structural and thermal factors including molecular morphology, chain orientation, interfacial vibrational coupling and heat exchange. Two types of hydrogen bonds, i.e. PVA-PMMA hydrogen bonds and PVA-PVA hydrogen bonds, are analyzed and their effects on various structural and thermal properties are systematically investigated. These results are expected to provide new physical insights for interface engineering to achieve tunable thermal management and energy efficiency in a wide variety of systems involving polymers and biomaterials.

A guard to reduce the accidental oxidation of PbTe nanocrystals

In the synthesis of lead telluride nanocrystals (PbTe NCs), oxidized PbTe is commonly regarded as a waste material as this will reduce the performance of pure PbTe NCs. The waste is normally thrown away, leading to potential environment risks and is less economical in terms of atom usage. Conventional anti-oxidation methods such as inert gas flow or sealed systems cannot deal with leaking or accidental contamination. To solve this problem, by simulating accidental oxidation, we utilized a cheap and easily-performed strategy to reduce the oxidation to a very low level. Further analysis indicates that this anti-oxidation effect should be due to interactions between the double bonds from the coating ligands and the extended π bonds from the benzene rings. This strategy increases the synthesis efficiency of the reactants and reduces the environmental pollution risk.

A Solution Processable High-Performance Thermoelectric Copper Selenide Thin Film

A solid-state thermoelectric device is attractive for diverse technological areas such as cooling, power generation and waste heat recovery with unique advantages of quiet operation, zero hazardous emissions, and long lifetime. With the rapid growth of flexible electronics and miniature sensors, the low-cost flexible thermoelectric energy harvester is highly desired as a potential power supply. Herein, a flexible thermoelectric copper selenide (Cu₂ Se) thin film, consisting of earth-abundant elements, is reported. The thin film is fabricated by a low-cost and scalable spin coating process using ink solution with a truly soluble precursor. The Cu₂ Se thin film exhibits a power factor of 0.62 mW/(m K² ) at 684 K on rigid Al₂ O₃ substrate and 0.46 mW/(m K² ) at 664 K on flexible polyimide substrate, which is much higher than the values obtained from other solution processed Cu₂ Se thin films (<0.1 mW/(m K² )) and among the highest values reported in all flexible thermoelectric films to date (≈0.5 mW/(m K² )). Additionally, the fabricated thin film shows great promise to be integrated with the flexible electronic devices, with negligible performance change after 1000 bending cycles. Together, the study demonstrates a low-cost and scalable pathway to high-performance flexible thin film thermoelectric devices from relatively earth-abundant elements.