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Nanostructured thermoelectric materials: current research and future challenge

Thermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high-performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar-thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.

https://espace.library.uq.edu.au/view/UQ:722903/UQ722903_OA.pdf

High Performance Thermoelectric Materials: Progress and Their Applications

SnTe is a potentially attractive thermoelectric because it is the lead-free rock-salt analogue of PbTe. However, SnTe is a poor thermoelectric material because of its high hole concentration arising from inherent Sn vacancies in the lattice and its very high electrical and thermal conductivity. In this study, we demonstrate that SnTe-based materials can be controlled to become excellent thermoelectrics for power generation via the successful application of several key concepts that obviate the well-known disadvantages of SnTe. First, we show that Sn self-compensation can effectively reduce the Sn vacancies and decrease the hole carrier density. For example, a 3 mol % self-compensation of Sn results in a 50% improvement in the figure of merit ZT. In addition, we reveal that Cd, nominally isoelectronic with Sn, favorably impacts the electronic band structure by (a) diminishing the energy separation between the light-hole and heavy-hole valence bands in the material, leading to an enhanced Seebeck coefficien...

High Thermoelectric Performance of p-Type SnTe via a Synergistic Band Engineering and Nanostructuring Approach

We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically struc...

Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence

The chemistry, material processing and fundamental understanding of colloidal semiconductor nanocrystals (quantum dots) are advancing at an astounding rate, bringing the prospects of widespread commercialization of these novel and exciting materials ever closer. Interest in narrow bandgap nanocrystals in particular has intensified in recent years, and the results of research worldwide point to the realistic prospects of applications for these materials in solar cells, infrared optoelectronics (e.g. lasers, optical modulators, photodetectors and photoimaging devices), low cost/large format microelectronics, and in biological imaging and biosensor systems to name only some technologies. Improvements in fundamental understanding and material quality are built on a vast body of experience spread over many different methods of colloidal synthetic growth, each with their own strengths and weaknesses for different materials and sometimes with regard to particular applications. The nanocrystal growth expertise is matched by a rapidly expanding, and highly interdisciplinary, understanding of how best to assemble these materials into films or hybrid composites and thereby into useful devices, and again there are many different strategies that can be adopted. In this review we have attempted to survey and compare the recent work on colloidal synthesis, film and nanocrystal composite material fabrication, concentrating on narrow bandgap chalcogenide materials and some of their topical applications in the solar energy and biological fields. Since these applications are attracting rising interest across a wide range of disciplines, from the biological sciences, device engineering, and materials processing fields as well as the physics and synthetic chemistry communities, we have endeavoured to make the review of these narrow bandgap nanomaterials both comprehensive and accessible to newcomers to the area.

Narrow bandgap colloidal metal chalcogenide quantum dots: synthetic methods, heterostructures, assemblies, electronic and infrared optical properties

The dependencies of the thermoelectric properties of n-PbTe∕p-SnTe∕n-PbTe heterostructures on the SnTe quantum well width (dSnTe=0.5–6.0nm) at fixed PbTe barrier layers thicknesses were studied. It was established that the thickness dependencies of the Seebeck coefficient, electrical conductivity, the Hall coefficient, charge carrier mobility, and the thermoelectric power factor are distinctly nonmonotonic. The observed effect is attributed to the size quantization of the energy spectrum of the hole gas in a SnTe quantum well.

Quantum size effects in n-PbTe∕p-SnTe∕n-PbTe heterostructures

Semimetal–semiconductor transition in thin Bi films

The problem of doping of non-stoichiometric phases

The dependences of the electrical conductivity, the Hall coefficient, and the Seebeck coefficient on the layer thickness d (d = 18−600 nm) of p-type topological insulator Bi2Te3 thin films grown by thermal evaporation in vacuum on glass substrates were obtained at room temperature. In the thickness range of d = 18–100 nm, sustained oscillations with a substantial amplitude were revealed. The observed oscillations are well approximated by a harmonic function with a period Δd = (9.5 ± 0.5) nm. At d > 100 nm, the transport coefficients practically do not change as d is increased. The oscillations of the kinetic properties are attributed to the quantum size effects due to the hole confinement in the Bi2Te3 quantum wells. The results of the theoretical calculations of Δd within the framework of a model of an infinitely deep potential well are in good agreement with the experimental results. It is suggested that the substantial amplitude of the oscillations and their sustained character as a function of d are c...

E. I. Rogacheva

Papers

Quantum size effects in n-PbTe∕p-SnTe∕n-PbTe heterostructures

Semimetal–semiconductor transition in thin Bi films

The problem of doping of non-stoichiometric phases

Thickness dependent quantum oscillations of transport properties in topological insulator Bi2Te3 thin films

Growth and structure of thermally evaporated Bi2Te3 thin films