G. Dresselhaus

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.

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

Phonon modes in carbon nanotubules

Abstract The relationship between the phonon modes in carbon nanotubules and those in a graphene sheet is studied by use of the zone-folding method. The modes identified with pure translations and rotations are identified. General results relating to the symmetry properties of the Raman and infrared activity of the phonon modes are deduced. It is shown that as the diameter of a tubule increases, the number of Raman and infrared active modes is essentially unchanged despite the increase in the number of atoms per unit cell. Explicit phonon dispersion curves are presented for a small diameter armchair carbon tubule.

Electronic structure of double‐layer graphene tubules

The electronic structure of coaxial, graphene double‐layer tubules is predicted for various combinations of metallic and insulating constituent inner and outer monolayers, depending on the diameter and chirality of the tubule. For the examples chosen, some of the energy bands of the inner and outer tubules are coupled to each other by commensurate interlayer interactions. Nevertheless, because of symmetry, the energy bands of metallic monolayer tubules remain metallic even after interlayer interactions are considered. The possible implications of these results on molecular metal‐insulator devices are discussed.

Fracture modes in brittle coatings with large interlayer modulus mismatch

Fracture modes in a model glass–polymer coating–substrate system indented with hard spheres are investigated. The large modulus mismatch between the glass and polymer results in distinctive transverse fracture modes within the brittle coating: exaggerated circumferential (C) ring cracks that initiate at the upper coating surface well outside the contact (as opposed to the near-contact Hertzian cone fractures observed in monolithic brittle materials); median–radial (M) cracks that initiate at the lower surface (i.e., at the substrate interface) on median planes containing the contact axis. Bonding between the coating and substrate is sufficiently strong as to preclude delamination in our system. The transparency of the constituent materials usefully enables in situ identification and quantification of the two transverse fracture modes during contact. The morphologies of the cracks and the corresponding critical indentation loads for initiation are measured over a broad range of coating thicknesses (20 mm to 5.6 mm), on coatings with like surface flaw states, here ensured by a prebonding abrasion treatment. There is a well-defined, broad intermediate range where the indented coating responds more like a flexing plate than a Hertzian contact, and where the M and C cracks initiate in close correspondence with a simple critical stress criterion, i.e., when the maximum tensile stresses exceed the bulk strength of the (abraded) glass. In this intermediate range the M cracks generally form first—only when the flaws on the lower surface are removed (by etching) do the C cracks form first. Finite element modeling is used to evaluate the critical stresses at crack initiation and the surface locations of the crack origins. Departures from the critical stress condition occur at the extremes of very thick coatings (monolith limit) and very thin coatings (thin-film limit), where stress gradients over the flaw dimension are large. Implications of the results concerning practical coating systems are considered.

Raman scattering from nanoscale carbons generated in a cobalt-catalyzed carbon plasma

Abstract Carbonaceous material including nanoscale soot, carbon-coated nanoscale Co particles and nanotubes have been generated from a dc arc discharge between carbon electrodes. Sharp first- and second-order lines are observed in the Raman scattering spectra of the arc-derived carbons only when Co metal is present in the core of the anode. The sharp lines in the Raman spectrum of the Co-catalyzed, arc-derived carbons have not been observed previously in carbonaceous materials and are tentatively assigned to carbon nanotubes on the basis of a zone-folded model for the vibrational spectra of armchair tubules.

Mechanism for displacive excitation of coherent phonons in Sb, Bi, Te, and Ti2O3

Coherent phonons in Sb, Bi, Te, and Ti2O3 can be generated impulsively, and detected in the time domain through reflectivity modulation using 60 fs pulses of laser light at 2 eV. Experimental data for these opaque solids suggest that a direct Raman excitation mechanism is not responsible for coherent phonon generation. Rather, the excitation is attributed to an electronically induced displacement of the ion equilibrium coordinates.

Raman Scattering in Fullerenes

Since the discovery in 1990 of a relatively simple arc method to synthesize gram quantities of carbon cage molecules (fullerenes), considerable research effort has been expended to understand the molecular and solid-state properties of fullerenes and fullerene-derived materials. Raman scattering has played an important role in this effort which has focused on the fullerene C60, and to a lesser extent C70. From Raman spectra and their comparison with model andab initiocalculations, fundamental questions regarding the intramolecular bonds, the structure and properties of crystalline, fullerene-derived solids, including the superconducting K3C60 and Rb3C60 compounds, have been addressed and largely understood. Raman scattering has also been used to probe pressure- and temperature-driven phase transitions and the photopolymerization of fullerenes in the solid state, in addition to the bonding of fullerene molecules to metal substrates. In this review the highlights of this Raman scattering research are discussed. Brief introductory remarks concerning the discovery and structure of fullerenes are also given.

Optical Properties of MS 2 (M = Mo, W) Inorganic Fullerenelike and Nanotube Material Optical Absorption and Resonance Raman Measurements

The optical properties of inorganic fullerene-like and nanotube MS 2 (M = Mo, W) material are studied through absorption and resonance Raman, and compared to those of the corresponding bulk material. The absorption measurements show that the semiconductivity is preserved. Nevertheless, the positions of the excitons are altered in comparison to the bulk. The Raman spectra of the nanoparticles show a close correspondence to that of the bulk. However, the first-order peaks are broadened and, under resonance conditions, new peaks are observed. The new peaks are assigned to disorder-induced zone edge phonons.

Determination of nanotubes properties by Raman spectroscopy.

The basic concepts and characteristics of Raman spectra from single–wall carbon nanotubes (SWNTs, both isolated and bundled) are presented. The physical properties of the SWNTs are introduced, followed by the conceptual framework and characteristics of their Raman spectra. Each Raman feature, namely the radial breathing mode, the tangential G band, combination modes and disorder–induced bands are discussed, addressing their physical origin, as well as their capability for characterizing SWNT properties.