The conversion of thermal energy to electricity and vice versa by means of solid state thermoelectric devices is extremely appealing. However, its cost-effectiveness is seriously hampered by the ...relatively high production cost and low efficiency of current thermoelectric materials and devices. To overcome present challenges and enable a successful deployment of thermoelectric systems in their wide application range, materials with significantly improved performance need to be developed. Nanostructuration can help in several ways to reach the very particular group of properties required to achieve high thermoelectric performances. Nanodomains inserted within a crystalline matrix can provide large charge carrier concentrations without strongly influencing their mobility, thus allowing to reach very high electrical conductivities. Nanostructured materials contain numerous grain boundaries that efficiently scatter mid- and long-wavelength phonons thus reducing the thermal conductivity. Furthermore, nanocrystalline domains can enhance the Seebeck coefficient by modifying the density of states and/or providing type- and energy-dependent charge carrier scattering. All these advantages can only be reached when engineering a complex type of material, nanocomposites, with exquisite control over structural and chemical parameters at multiple length scales. Since current conventional nanomaterial production technologies lack such level of control, alternative strategies need to be developed and adjusted to the specifics of the field. A particularly suitable approach to produce nanocomposites with unique level of control over their structural and compositional parameters is their bottom-up engineering from solution-processed nanoparticles. In this work, we review the state-of-the-art of this technology applied to the thermoelectric field, including the synthesis of nanoparticles of suitable materials with precisely engineered composition and surface chemistry, their combination and consolidation into nanostructured materials, the strategies to electronically dope such materials and the attempts to fabricate thermoelectric devices using nanoparticle-based nanopowders and inks.
We report a procedure to prepare highly monodisperse copper telluride nanocubes, nanoplates, and nanorods. The procedure is based on the reaction of a copper salt with trioctylphosphine telluride in ...the presence of lithium bis(trimethylsilyl)amide and oleylamine. CuTe nanocrystals display a strong near-infrared optical absorption associated with localized surface plasmon resonances. We exploit this plasmon resonance for the design of surface-enhanced Raman scattering sensors for unconventional optical probes. Furthermore, we also report here our preliminary analysis of the use of CuTe nanocrystals as cytotoxic and photothermal agents.
A procedure for the continuous production of Cu2ZnSnS4 (CZTS) nanoparticles with controlled composition is presented. CZTS nanoparticles were prepared through the reaction of the metals' amino ...complexes with elemental sulfur in a continuous-flow reactor at moderate temperatures (300–330 °C). High-resolution transmission electron microscopy and X-ray diffraction analysis showed the nanocrystals to have a crystallographic structure compatible with that of the kesterite. Chemical characterization of the materials showed the presence of the four elements in each individual nanocrystal. Composition control was achieved by adjusting the solution flow rate through the reactor and the proper choice of the nominal precursor concentration within the flowing solution. Single-particle analysis revealed a composition distribution within each sample, which was optimized at the highest synthesis temperatures used.
The efficient conversion between thermal and electrical energy by means of durable, silent and scalable solid-state thermoelectric devices has been a long standing goal. While nanocrystalline ...materials have already led to substantially higher thermoelectric efficiencies, further improvements are expected to arise from precise chemical engineering of nanoscale building blocks and interfaces. Here we present a simple and versatile bottom-up strategy based on the assembly of colloidal nanocrystals to produce consolidated yet nanostructured thermoelectric materials. In the case study on the PbS-Ag system, Ag nanodomains not only contribute to block phonon propagation, but also provide electrons to the PbS host semiconductor and reduce the PbS intergrain energy barriers for charge transport. Thus, PbS-Ag nanocomposites exhibit reduced thermal conductivities and higher charge carrier concentrations and mobilities than PbS nanomaterial. Such improvements of the material transport properties provide thermoelectric figures of merit up to 1.7 at 850 K.
Large amounts of waste heat generated in our fossil-fuel based economy can be converted into useful electric power by using thermoelectric generators. However, the low-efficiency, scarcity, high-cost ...and poor production scalability of conventional thermoelectric materials are hindering their mass deployment. Nanoengineering has proven to be an excellent approach for enhancing thermoelectric properties of abundant and cheap materials such as silicon. Nevertheless, the implementation of these nanostructures is still a major challenge especially for covering the large areas required for massive waste heat recovery. Here we present a family of nano-enabled materials in the form of large-area paper-like fabrics made of nanotubes as a cost-effective and scalable solution for thermoelectric generation. A case study of a fabric of p-type silicon nanotubes was developed showing a five-fold improvement of the thermoelectric figure of merit. Outstanding power densities above 100 W/m
at 700 °C are therefore demonstrated opening a market for waste heat recovery.
Morphology is a key parameter in the design of novel nanocrystals and nanomaterials with controlled functional properties. Here, we demonstrate the potential of foreign metal ions to tune the ...morphology of colloidal semiconductor nanoparticles. We illustrate the underlying mechanism by preparing copper selenide nanocubes in the presence of Al ions. We further characterize the plasmonic properties of the obtained nanocrystals and demonstrate their potential as a platform to produce cubic nanoparticles with different composition by cation exchange.
A synthetic route for producing Cu2ZnGeSe4 nanocrystals with narrow size distributions and controlled composition is presented. These nanocrystals were used to produce densely packed nanomaterials by ...hot-pressing. From the characterization of the thermoelectric properties of these nanomaterials, Cu2ZnGeSe4 is demonstrated to show excellent thermoelectric properties. A very preliminary adjustment of the nanocrystal composition has already resulted in a figure of merit of up to 0.55 at 450 °C.
The bottom-up assembly of nanocrystals provides access to a three-dimensional composition control at the nanoscale not attainable by any other technology. In particular, colloidal ...nanoheterostructures, with intrinsic multiphase organization, are especially appealing building blocks for the bottom-up production of nanocomposites. In the present work, we use PbTe–PbS as the model material system and thermoelectricity as the paradigmatic application to investigate the potential of the bottom-up assembly of core–shell nanoparticles to produce functional nanocomposites. With this goal in mind, a rapid, high-yield and scalable colloidal synthetic route to prepare grams of PbTe@PbS core–shell nanoparticles with unprecedented narrow size distributions and exceptional composition control is detailed. PbTe@PbS nanoparticles were used as building blocks for the bottom-up production of PbTe–PbS nanocomposites with tuned composition. In such PbTe–PbS nanocomposites, synergistic nanocrystal doping effects result in up to 10-fold higher electrical conductivities than in pure PbTe and PbS nanomaterials. At the same time, the acoustic impedance mismatch between PbTe and PbS phases and a partial phase alloying provide PbTe–PbS nanocomposites with strongly reduced thermal conductivities. As a result, record thermoelectric figures of merit (ZT) of ∼1.1 were obtained from undoped PbTe and PbS phases at 710 K. These high ZT values prove the potential of the proposed processes to produce efficient functional nanomaterials with programmable properties.
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