M. S. Dresselhaus

Bulk nanostructured thermoelectric materials: current research and future prospects

Thermoelectrics have long been recognized as a potentially transformative energy conversion technology due to their ability to convert heat directly into electricity. Despite this potential, thermoelectric devices are not in common use because of their low efficiency, and today they are only used in niche markets where reliability and simplicity are more important than performance. However, the ability to create nanostructured thermoelectric materials has led to remarkable progress in enhancing thermoelectric properties, making it plausible that thermoelectrics could start being used in new settings in the near future. Of the various types of nanostructured materials, bulk nanostructured materials have shown the most promise for commercial use because, unlike many other nanostructured materials, they can be fabricated in large quantities and in a form that is compatible with existing thermoelectric device configurations. The first generation of these materials is currently being developed for commercialization, but creating the second generation will require a fundamental understanding of carrier transport in these complex materials which is presently lacking. In this review we introduce the principles and present status of bulk nanostructured materials, then describe some of the unanswered questions about carrier transport and how current research is addressing these questions. Finally, we discuss several research directions which could lead to the next generation of bulk nanostructured materials.

Characterizing carbon nanotube samples with resonance Raman scattering

The basic concepts and characteristics of Raman spectra from carbon nanotubes (both isolated and bundled) are presented. The general characteristics of the radial breathing mode, tangential mode (G band), disorder-induced mode (D-band) and other Raman features are presented, with the focus directed toward their use for carbon nanotube characterization. Polarization analysis, surface enhanced Raman spectroscopy and complementary optical techniques are also discussed in terms of their advantages and limitations.

Recent developments in thermoelectric materials

Abstract Efficient solid state energy conversion based on the Peltier effect for cooling and the Seebeck effect for power generation calls for materials with high electrical conductivity σ, high Seebeck coefficient S, and low thermal conductivity k. Identifying materials with a high thermoelectric figure of merit Z(= S2σ/k) has proven to be an extremely challenging task. After 30 years of slow progress, thermoelectric materials research experienced a resurgence, inspired by the developments of new concepts and theories to engineer electron and phonon transport in both nanostructures and bulk materials. This review provides a critical summary of some recent developments of new concepts and new materials. In nanostructures, quantum and classical size effects provide opportunities to tailor the electron and phonon transport through structural engineering. Quantum wells, superlattices, quantum wires, and quantum dots have been employed to change the band structure, energy levels, and density of states of electrons, and have led to improved energy conversion capability of charged carriers compared to those of their bulk counterparts. Interface reflection and the scattering of phonons in these nanostructures have been utilised to reduce the heat conduction loss. Increases in the thermoelectric figure of merit based on size effects for either electrons or phonons have been demonstrated. In bulk materials, new synthetic routes have led to engineered complex crystal structures with the desired phonon-glass electron-crystal behaviour. Recent studies on new materials have shown that dimensionless figure of merit (Z ×temperature) values close to 1·5 could be obtained at elevated temperatures. These results have led to intensified scientific efforts to identify, design, engineer and characterise novel materials with a high potential for achieving ZT much greater than 1 near room temperature.

Vapor-grown carbon fibers (VGCFs): Basic properties and their battery applications

Abstract Submicron vapor grown carbon fibers (VGCFs) obtained by a floating growth method were evaluated in terms of their microstructural development with heat treatment temperature (HTT), physical properties of a single fiber and of the bulk state, and additional effects, such as the filler in the electrode of a lead–acid battery and a Li-ion battery system. Its desirable properties, such as relatively high mechanical strength and electrical conductivity, both in the single fiber state and in the bulk state, including their very special network-like morphology, improved the performance of the electrodes in lead–acid batteries and Li-ion batteries, especially their cycle characteristics.

Model for Raman scattering from incompletely graphitized carbons

Abstract Laser Raman spectroscopy has been applied to the study of various carbons ranging from highly graphitized to amorphous materials. A calculation is presented for such materials based on the assumption of short correlation lengths for normal modes that break the wave-vector selection rules. This calculation leads to expressions for the first-order Raman scattering intensity in terms of the vibrational density of states of a single graphite layer weighted by the electron-phonon couping of the modes to the electromagnetic radiation.

Use of quantum‐well superlattices to obtain a high figure of merit from nonconventional thermoelectric materials

Currently, the materials with the highest thermoelectric figure of merit (ZT) are one‐band materials. The presence of both electrons and holes lowers ZT, so two‐band materials such as semimetals are not useful thermoelectric materials. However, by preparing these materials in the form of two‐dimensional quantum‐well superlattices, it is possible to separate the two bands and transform the material to an effective one‐carrier system. We have investigated theoretically the effect of such an approach and our results indicate that a significant increase in ZT may be achieved. This result allows the possibility of using a new class of materials as thermoelectric refrigeration elements.

Anomalous potential barrier of double-wall carbon nanotube

Abstract The stable structure of a double-wall carbon nanotube (DWNT) is calculated for various chirality pairs, (n,m) – (n ′ ,m ′ ) , of inner and outer constituent layers. The stability of a double-wall nanotube is found not to depend on chirality, but rather on the diameter difference between inner and outer layers. However, the potential barrier for the relative displacement of the inner and outer nanotube layers is found to depend significantly on the chirality difference of the pair. Mechanical motions like a bolt–nut pair or discrete rotations can be expected for special pairs of chiralities in double-wall nanotubes, and these special motions will be important for nano-technology.

Structural characterization of cup-stacked-type nanofibers with an entirely hollow core

Straight long carbon nanofibers with a large hollow core obtained by a floating reactant method show a stacking morphology of truncated conical graphene layers, which in turn exhibit a large portion of open edges on the outer surface and also in the inner channels. Through a judicious choice of oxidation conditions, nanofibers with increased active edge sites are obtained without disrupting the fiber’s morphology. A graphitization process induces a morphological change from a tubular type to a reversing saw-toothed type and the formation of loops along the inner channel of the nanofibers, accompanied by a decrease in interlayer spacing.

Bulk morphology and diameter distribution of single-walled carbon nanotubes synthesized by catalytic decomposition of hydrocarbons

Long and wide ropes/ribbons of single-walled carbon nanotube (SWNT) bundles with rope diameters of 100 mu m and lengths to 3 cm were synthesized by the catalytic decomposition of hydrocarbons. These ropes/ribbons consist of roughly-aligned bundles of aligned SWNTs. The SWNT diameters, as determined from HRTEM images, are 1.69 +/- 0.34 nm, which, in combination with resonant Raman scattering measurements, indicates that our SWNTs have larger diameters than those synthesized by other techniques. The larger SWNTs are promising for gas-storage applications. The easy manipulation of the ropes offer special opportunities for their characterization and applications

Defect characterization in graphene and carbon nanotubes using Raman spectroscopy

This review discusses advances that have been made in the study of defect-induced double-resonance processes in nanographite, graphene and carbon nanotubes, mostly coming from combining Raman spectroscopic experiments with microscopy studies and from the development of new theoretical models. The disorder-induced peak frequencies and intensities are discussed, with particular emphasis given to how the disorder-induced features evolve with increasing amounts of disorder. We address here two systems, ion-bombarded graphene and nanographite, where disorder is represented by point defects and boundaries, respectively. Raman spectroscopy is used to study the ‘atomic structure’ of the defect, making it possible, for example, to distinguish between zigzag and armchair edges, based on selection rules of phonon scattering. Finally, a different concept is discussed, involving the effect that defects have on the lineshape of Raman-allowed peaks, owing to local electron and phonon energy renormalization. Such effects can be observed by near-field optical measurements on the G′ feature for doped single-walled carbon nanotubes.