Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

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Alternative Plasmonic Materials: Beyond Gold and Silver

A dimensionless thermoelectric figure-of-merit (ZT) of 0.95 in p-type nanostructured bulk silicon germanium (SiGe) alloys is achieved, which is about 90% higher than what is currently used in space flight missions, and 50% higher than the reported record in p-type SiGe alloys. These nanostructured bulk materials were made by using a direct current-induced hot press of mechanically alloyed nanopowders that were initially synthesized by ball milling of commercial grade Si and Ge chunks with boron powder. The enhancement of ZT is due to a large reduction of thermal conductivity caused by the increased phonon scattering at the grain boundaries of the nanostructures combined with an increased power factor at high temperatures.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/NanoLetters%208_4670_2008.pdf

Enhanced thermoelectric figure-of-merit in nanostructured p-type silicon germanium bulk alloys.

The field of thermoelectric energy conversion is reviewed from both a theoretical and an experimental standpoint. The basic theory is introduced and the thermodynamic and solid state views are compared. An overview of the development of thermoelectric materials is presented with particular emphasis being placed on the most recent developments in high-temperature semiconductors. A number of possible device applications are discussed and the successful use and suitability of these devices for space power is manifest.

https://iopscience.iop.org/article/10.1088/0034-4885/51/4/001/pdf

Materials for thermoelectric energy conversion

Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

The thermal conductivity of Si–Ge superlattices with superlattice periods 30  2 × 109 W m−2 K−1 at 200 K. Superlattices with relatively longer periods, L>130 A, have smaller thermal conductivities than the short-period samples. This unexpected result is attributed to a strong disruption of the lattice vibrations by extended defects produced during lattice-mismatched growth.

Thermal conductivity of Si–Ge superlattices

The thermal resistivity, Seebeck coefficient, electrical resistivity, and Hall mobility of Ge‐Si alloys have been measured throughout the Ge‐Si alloy system as functions of impurity concentration in the range 2×1018−4×1020cm−3, and of temperature in the range 300°–1300°K. A qualitative interpretation of these properties is given. For power conversion, boron and phosphorus were found to be useful p‐type and n‐type impurities, respectively, because of their high solid solubilities. At 1200°K, the maximum values of the dimensionless figure of merit zT were 0.8 for p‐type Ge0.15‐Si0.85 alloy doped to 2.1×1020cm−3 holes, and 1.0 for n‐type Ge0.15‐Si0.85 alloy doped to 2.7×1020cm−3 electrons. The maximum over‐all efficiency of a stable generator operating between 300°–1200°K, using the best p‐type and n‐type materials was computed to be 10%.

J. P. Dismukes

Papers

Thermal and Electrical Properties of Heavily Doped Ge‐Si Alloys up to 1300°K