Semiconductor nanowires represent an important class of nanostructure building block for photovoltaics as well as direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting. In addition, we have also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced due to phonon scattering, pointing to a very promising approach to design better thermoelectrical materials. It is important to note that the engines that generate most of the world's power typically operate at only 30–40 per cent efficiency, releasing roughly 15 terawatts of heat to the environment. If this “wasted heat” could be recycled, the impact globally would be enormous. Our silicon nanowire thermoelectric technology could have a significant impact in alternative energy generation.

Semiconductor nanowires for energy conversion.

Layered MAX phases are exfoliated into 2D single layers and multilayers, so-called MXenes. Using fi rst-principles calculations, the formation and electronic properties of various MXene systems, M 2 C (M = Sc, Ti, V, Cr, Zr, Nb, Ta) and M 2 N (M = Ti, Cr, Zr) with surfaces chemically functionalized by F, OH, and O groups, are examined. Upon appropriate surface functionalization, Sc 2 C, Ti 2 C, Zr 2 C, and Hf 2 C MXenes are expected to become semiconductors. It is also derived theoretically that functionalized Cr 2 C and Cr 2 N MXenes are magnetic. Thermoelectric calculations based on the Boltzmann theory imply that semiconducting MXenes attain very large Seebeck coeffi cients at low temperatures.

Novel Electronic and Magnetic Properties of Two‐Dimensional Transition Metal Carbides and Nitrides

The field of thermoelectrics has progressed enormously and is now growing steadily because of recently demonstrated advances and strong global demand for cost-effective, pollution-free forms of energy conversion. Rapid growth and exciting innovative breakthroughs in the field over the last 10-15 years have occurred in large part due to a new fundamental focus on nanostructured materials. As a result of the greatly increased research activity in this field, a substantial amount of new data--especially related to materials--have been generated. Although this has led to stronger insight and understanding of thermoelectric principles, it has also resulted in misconceptions and misunderstanding about some fundamental issues. This article sets out to summarize and clarify the current understanding in this field; explain the underpinnings of breakthroughs reported in the past decade; and provide a critical review of various concepts and experimental results related to nanostructured thermoelectrics. We believe recent achievements in the field augur great possibilities for thermoelectric power generation and cooling, and discuss future paths forward that build on these exciting nanostructuring concepts.

Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features

Extraordinary electron systems can be generated at well-defined interfaces between complex oxides. In recent years, progress has been achieved in exploring and making use of the fundamental properties of such interfaces, and it has become clear that these electron systems offer the potential for possible future devices. We trace the state of the art of this emerging field of electronics and discuss some of the challenges and pitfalls that may lie ahead.

http://science.sciencemag.org/content/sci/327/5973/1607.full.pdf

Oxide Interfaces—An Opportunity for Electronics

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Advanced Thermoelectric Design: From Materials and Structures to Devices

Enhancement of the Seebeck coefficient (S ) without reducing the electrical conductivity (sigma) is essential to realize practical thermoelectric materials exhibiting a dimensionless figure of merit (ZT=S2 x sigma x T x kappa-1) exceeding 2, where T is the absolute temperature and kappa is the thermal conductivity. Here, we demonstrate that a high-density two-dimensional electron gas (2DEG) confined within a unit cell layer thickness in SrTiO(3) yields unusually large |S|, approximately five times larger than that of SrTiO(3) bulks, while maintaining a high sigma2DEG. In the best case, we observe |S|=850 microV K-1 and sigma2DEG=1.4 x 10(3) S cm-1. In addition, by using the kappa of bulk single-crystal SrTiO(3) at room temperature, we estimate ZT approximately 2.4 for the 2DEG, corresponding to ZT approximately 0.24 for a complete device having the 2DEG as the active region. The present approach using a 2DEG provides a new route to realize practical thermoelectric materials without the use of toxic heavy elements.

Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3

We report two-dimensional Seebeck coefficients (∣S∣2D) of [(SrTiO3)x∕(SrTi0.8Nb0.2O3)y]20 (x=1–60, y=1–20) superlattices, which were grown on the (100) face of insulating LaAlO3 substrates to clarify the origin of the giant ∣S∣2D values of the SrTiO3 superlattices [H. Ohta et al., Nat. Mater. 6, 129 (2007)]. The ∣S∣2D values of the [(SrTiO3)17∕(SrTi0.8Nb0.2O3)y]20 superlattices increased proportionally to y−0.5 and reached 320μVK−1 (y=1), which is approximately five times larger than that of the SrTi0.8Nb0.2O3 bulk (∣S∣3D=61μVK−1). The slope of the log∣S∣2D-logy plots was −0.5, proving that the density of states in the ground state for SrTiO3 increases inversely proportionally to y. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25nm (16 unit cells of SrTiO3).

Enhanced Seebeck coefficient of quantum-confined electrons in SrTiO3∕SrTi0.8Nb0.2O3 superlattices

Herein we report the carrier transport properties of [(SrTiO3)x/(SrTi0.8Nb0.2O3)1]20 (x=0–50) superlattices at high temperatures (T=300–900 K). Significant structural changes were not observed in the superlattices after annealing at 900 K in a vacuum. The Seebeck coefficient of the [(SrTiO3)20/(SrTi0.8Nb0.2O3)1]20 superlattice, which was 300 µVK-1 at room temperature, gradually increased with temperature and reached 450 µVK-1 at 900 K, which is ~3 times larger than that of bulk SrTi0.8Nb0.2O3. These observations provide clear evidence that the superlattice is stable and exhibits a giant Seebeck coefficient even at high temperature.

https://iopscience.iop.org/article/10.1143/APEX.1.015007/pdf

Thermal Stability of Giant Thermoelectric Seebeck Coefficient for SrTiO3/SrTi0.8Nb0.2O3 Superlattices at 900 K

Critical thickness for giant thermoelectric Seebeck coefficient of 2DEG confined in SrTiO3/SrTi0.8Nb0.2O3 superlattices

Disclosed is a thermoelectric material having a novel constitution. Specifically disclosed is a thermoelectric material having a superlattice structure wherein a barrier layer composed of an insulating SrTiO3 and a quantum well layer composed of SrTiO3 which has become semiconductive by being doped with an n-type impurity are arranged. In this thermoelectric material, the thickness of the quantum well layer is not more than 4 times the unit lattice thickness of the SrTiO3 which has become semiconductive by being doped with an n-type impurity.

Yoriko Mune

Papers

Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3

Enhanced Seebeck coefficient of quantum-confined electrons in SrTiO3∕SrTi0.8Nb0.2O3 superlattices

Thermal Stability of Giant Thermoelectric Seebeck Coefficient for SrTiO3/SrTi0.8Nb0.2O3 Superlattices at 900 K

Critical thickness for giant thermoelectric Seebeck coefficient of 2DEG confined in SrTiO3/SrTi0.8Nb0.2O3 superlattices

Thermoelectric material, infrared sensor and image forming device