High resolution far infrared absorption measurements were carried out for single walled and double walled carbon nanotubes samples (SWCNT and DWCNT) encased in a polyethylene matrix to investigate the temperature and bundling effects on the low frequency phonons associated with the low frequency circumferential vibrations. At a temperature where kBT is significantly lower than the phonon energy, the broad absorption features as observed at room temperature become well resolved phonon transitions. For a DWCNT sample whose inner tubes have a similar diameter distribution as the SWCNT sample studied, a series of sharp features were observed at room temperature at similar positions as for the SWCNT samples studied. The narrow linewidth is attributed to the fact that the inner tubes are isolated from the polyethylene matrix and the weak inter-tubule interactions. More systematic studies will be required to better understand the effects of inhomogeneous broadening and thermal-excitation on the detailed position and lineshape of the low frequency phonon features in carbon nanotubes.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

The following article is based on the plenary presentation given by Mildred S. Dresselhaus of the Massachusetts Institute of Technology on November 29, 2004, at the Materials Research Society Fall Meeting in Boston. Advanced materials offer new promise for addressing some of the grand societal challenges of our future, including that of global energy. This article will review opportunities that have opened up at the nanoscale, with materials of reduced dimensionality and enhanced surface-to-volume ratio. Some examples of research accomplishments and opportunities at the nanoscale will be described, with special attention given to the potential for advanced materials and nanoscience to have an impact on the grand challenges related to a sustainable energy supply for the 21st century and beyond.

Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena with both bulk samples containing nanoscale constituents and nanoscale materials exhibiting enhanced thermoelectric performance in their own right. Prior theoretical and experimental proof of principle studies on isolated quantum well and quantum wire samples have now evolved into studies on bulk samples containing nanostructured constituents. In this review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications. A review of some of the results obtained to date are presented.

The use of resonance Raman spectroscopy (RRS) to study and characterize single wall carbon nanotubes (SWNTs) is discussed, focusing on preliminary efforts for the development of the RRS to characterize defects in SWNTs. The disorder-induced D-band, disorder-induced peaks just above the first-order allowed graphite G-band, as well as the intermediate frequency modes (IFMs) appearing between the RBM and the D/G spectral region are addressed. RRS on nanographite ribbons and on a step-like defect in highly ordered pyrolytic graphite (HOPG) sheds light into the problem of characterizing specific defects in nano-related carbons.

We present a detailed photoluminescence study of a (6, 5) enriched DNA-wrapped single wall carbon nanotube (DNA-CNT) solid sample and an as-produced DNA-CNT solution. Multiple strong PL peaks were observed at excitation energies that do not correspond to the interband electronic transitions that are strongly enhanced by electronic van Hove singularities. These strong PL peaks are assigned to different mechanisms of excitation and relaxation, including one phonon, two phonon, hot luminescence processes, as well as radiative and non-radiative energy transferring mechanisms between neighboring nanotubes. These processes are assigned to different channels of phonon-assisted electron relaxation. The study shows that the electronic relaxation processes observed in PL can be used as a means to probe different physical interactions between photons, electrons, and phonons that are not separately identified in bulk semiconducting materials.

Double-walled carbon nanotubes (DWNTs), synthesized by the catalytic decomposition of methane, were explored by resonance Raman spectroscopy using different energies for laser excitation. Based on the radial breathing mode frequencies, the indices of the two layers of a DWNT were approximately assigned, depending on the interlayer separation of the two coaxial layers of the DWNT, which ranged from 0.34 to 0.40 nm. From the tentatively assigned results, it was found that the two walls of the DWNT are not strongly selected by chirality and diameter. The results, however, suggest that, for the tubes that are resonant with the available laser excitation energies, most of the outer layers of the observed DWNTs in our samples are semiconducting, while the inner layers of the observed DWNTs are either semiconducting or metallic based on the assembled DWNTs. The characteristics of the G, D, and G′ band of the DWNTs are discussed, and a double peak feature in the D and G′ band, originating from the inner and outer layers of the DWNTs, is reported.

The heat-treatment retention time effect of carbonized polyvinylidenechloride (PVDC) was investigated. Homogeneous PVDC with a crystallite size of 267 Å was used as a precursor material for an electric double-layer capacitor electrode. The P-120m material, which was heat treated for 120 min at 700 °C, shows a larger specific capacitance than any other material in this study. It shows the largest values reported up to now, reaching values as high as 100.2 F/g for a two-electrode system, which is equivalent to 400.8 F/g in a conventional three-compartment electrode system. It is difficult to distinguish the difference in the pore-size distribution by way of gas adsorption as the retention time is varied. However, the difference can be clarified using a novel method based on the analysis of transmission electron microscopy images. As the retention time for heat treatment increases, the pore size grows through the coalescence of small pores. Furthermore, a new concept for the electric double-layer capacitance is suggested on the basis of analysis of the transmission electron microscopy observations.

Low dimensionality provides opportunities to modify the properties of bulk materials dramatically and to control materials properties independently in a manner that is not possible for bulk materials. The special characteristics of low dimensional materials to enhance thermoelectric performance have already been demonstrated in quantum wells, quantum wires and quantum dots. The main focus of this review is a summary of advances made in the modeling of quantum dot superlattice nanowires. Several new research directions for low dimensional thermoelectricity or inspired by this research are briefly mentioned.

An incoherent particle model has been developed to predict the phonon thermal conductivity of nanowires and superlattice nanowires. It is argued that the surface roughness of most real nanowires prevents the formation of idealized confined dispersion relations for typical temperatures and diameters. Instead, the three-dimensional bulk dispersion is used, thus addressing only classical size effects. Four adjustable parameters capture the effects of diameter, superlattice, Umklapp, impurity, and alloy scattering. Predictions are compared with experimental data for nanowires and superlattice nanowires down to 22 nm diameter and 20 K, and are in good agreement above ∼40 nm diameter. The analysis suggests that ideal low thermal conductivity nanowires for thermoelectric applications would have small-diameter, alternating alloy segments that are acoustically dissimilar but electrically similar.

High figure of merit has been reported in superlattice structures in recent years mainly due to a large reduction in thermal conductivity, while maintaining or even enhancing the power factor. These findings suggest the possibility of using nanocomposites to reduce a material's thermal conductivity and thereby increasing the thermoelectric figure of merit. The current work reports on the thermal conductivity of various Si-Ge nanocomposites synthesized in a simple and straightforward process that allows one to exploit nanoscale physics while rapidly producing macroscale devices. Composite synthesis consisted of combining Si nanoparticles approximately 70 nm in diameter with micron-sized Ge particles in various atomic ratios via hotpressing to form macroscale samples. Room-temperature thermal conductivity was measured for each of the nanocomposite samples and compared with the values of SiGe bulk alloys. In this preliminary work, we observed a significant reduction in bulk thermal conductivity in such materials.

Three different types of carbon nanotubes being considered for bio-recognition experiments were studied using resonance Raman spectroscopy. Raman spectroscopy, taken using several laser excitation energies, has been shown to provide an effective characterization tool for these carbon nanotubes. The technique yields structural information that both complements and corroborates structural information obtained using electron microscopy techniques, such as TEM, SEM, and AFM.

The optical absorption from an indirect L-T point valence band transition in bismuth nanowires is simulated based on a surface sensitive term, which is very weak in bulk bismuth, but effectively couples the L and T point bands in bismuth nanowires. The experimental Fourier transform infrared absorption spectra in the energy range 0.08 to 0.5 eV are compared to theoretical simulations for absorption resulting from indirect interband transitions between subbands at the L point valence band to subbands at the T point valence band as the polarization, tellurium doping, and wire diameter are varied. The simulated absorption from this indirect transition reproduces many of the experimentally observed trends and allows us to conclude that the dominant feature in the experimental absorption spectra of Bi nanowires results from an indirect L-T point transition which is enhanced in Bi nanowires.

Copper-doped carbon aerogels, which were prepared by sol-gel polymerization of a mixture of resorcinol, formaldehyde and copper ions, followed by supercritical drying with liquid CO2 and carbonization at different temperatures under a N2 atmosphere, were characterized by magnetic susceptibility and temperature-dependent conductivity measurements. The experimental results show that the magnetic susceptibilities (χ) of all the copper-doped carbon aerogels are larger than that of the blank carbon aerogel. The temperature-dependent magnetic susceptibilities of the copper-doped carbon aerogels are well fit to a Curie function in the low temperature region (below 45 K). According to the Curie constant obtained, the spin concentration (N) of the copper-doped carbon aerogels was calculated and the results show that the spin concentration of doped samples increases at first with an increase in the copper concentration used in doping, and then drops from the maximum in N and χ when the copper concentration is further increased above 0.1 wt%. Magnetic susceptibility results suggest that copper-doped carbon aerogels carbonized at 800 °C are more disordered than those carbonized at 1000 °C. The conductivity of these copper-doped carbon aerogels is apparently higher than that of the blank carbon aerogel, especially at low temperature (T < 45 K). The temperature-dependent resistivities of both the copper-doped and the blank carbon aerogels follow a linear functional form log ρ(T) ∼ (1/T)1/2 with two distinct regions below and above 45 K, indicating that the conduction of both the copper-doped and the blank carbon aerogels obey a tunneling and/or a hopping mechanism.

Activated milled mesophase carbon fibers (AC-mMPCF, MP-series) show a higher specific capacitance in spite of a smaller specific surface area than those of powder-type activated carbons (AC-series). This phenomenon can be interpreted to mean that it is difficult to predict the capacitance of an electric double-layer capacitor (EDLC) knowing just the surface area and the pore size. More information is needed about other inherent characteristics of the samples, for example, the equivalent series resistance (ESR), shape of the pores, etc. We investigate here other characteristics of the samples. Consequently, it was deduced that the MP-series of EDLCs have a slit pore shape, which affects the accessibility of the electrolyte ion onto the electrode surface. Moreover, the MP-series of materials have suitable ESR values, and these material properties themselves should be considered as factors that affects the deterioration of the specific capacitance of EDLCs.

The use of Raman spectroscopy as a characterization tool for individual single wall carbon nanotubes is briefly reviewed. New physical phenomena occurring at the single nanotube level are discussed, with special emphasis given to the use of resonance Raman scattering for the structural determination of (n, m) for individual nanotubes, based on diameter and chirality dependent phenomena associated with the radial breathing mode, the G-band and the G[prime]-band features. Examples are given to show how single nanotube spectroscopy provides insight into the use of Raman spectroscopy for the characterization of nanotube bundles and for the study of new physical phenomena occurring at the single nanotube level.

We analyze the dependence of the second-order G'-band profile in terms of their (n,m) indices by measuring the resonance Raman spectra of several semiconducting and metalic isolated single wal carbon nanotubes. We show that this profile is very sensitive to the electronic structure, thus making it possible to get structural (n,m) information and to probe the splitting of the van Hove singularities in the electronic density of states due to the trigona warping effect.

We present results from resonant Raman spectroscopy on the graphite-like G band by measuring Raman spectra on isolated single wal carbon nanotubes (SWNTs). We discuss the G-band lineshape dependence on nanotube diameter and chirality, as well as polarization studies related to the antenna effect. Symmetry selection rules, dipolar and multipolar antenna behaviors are discussed. Spectra at the single nanotube level are related to spectra observed from SWNT bundles.

The double resonance Raman spectra of the disorder-induced D-band and some other non-zone-center phonon modes are discussed for two-dimensional (2D) graphite and to some extent for single wall carbon nanotubes (SWNTs). The phonon dispersion relations of graphite can be determined using Raman spectroscopy b y measuring the non-zone center Raman phonon frequencies in combination with theoretically determined phonon q-vectors. We report a t of the phonon dispersion relations to experimental Raman spectra which were previously observed but have not y et been assigned to speci c phonon branches. We found that the D-band and the G0-band of 2D graphite consist of, respectively, two and one Raman Lorentzian peaks, while 3D graphite shows two G0-band Lorentzian peaks. The appearance of two G0 peaks in the resonance Raman spectra of SWNTs may come from resonances of one laser line with two di erent van Hov e singularities.

The potential of Bi and Bi1−x Sbx nanowire arrays for thermoelectric applications is discussed. The advantages of bismuth as a low dimensional thermoelectric material are enumerated and the role of modeling is emphasized. The advantages of using the Sb concentration as well as the wire diameter as materials parameters for optimizing the thermoelectric performance of these nanowires are discussed, with particular emphasis given to the development of a high performance p-type nanowire thermoelectric material.