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The massless Dirac spectrum of electrons in single-layer graphene has been thoroughly studied both theoretically and experimentally. Although a subject of considerable theoretical interest, experimental investigations of the richer electronic structure of few-layer graphene (FLG) have been limited. Here we examine FLG graphene crystals with Bernal stacking of layer thicknesses N = 1,2,3, ... 8 prepared using the mechanical exfoliation technique. For each layer thickness N, infrared conductivity measurements over the spectral range of 0.2-1.0 eV have been performed and reveal a distinctive band structure, with different conductivity peaks present below 0.5 eV and a relatively flat spectrum at higher photon energies. The principal transitions exhibit a systematic energy-scaling behavior with N. These observations are explained within a unified zone-folding scheme that generates the electronic states for all FLG materials from that of the bulk 3D graphite crystal through imposition of appropriate boundary conditions. Using the Kubo formula, we find that the complete infrared conductivity spectra for the different FLG crystals can be reproduced reasonably well within the framework a tight-binding model. electronic structures | infrared spectroscopy | zone-folding method doi/ 10.1073/pnas.1004595107

We report a simple but powerful method for engineering multiwalled carbon nanotubes (MWNTs) by using manipulation by an atomic-force microscope. The successive shell-by-shell extraction process of ultralong MWNTs allows the exposure of the innermost single-walled carbon nanotubes (SWNTs), which have diameters as small as [approximately equal to] 0.4 nm. The inner-shell extraction process changes the electrical characteristics of the MWNTs. Whereas the outer hollowed-out nanotubes show either metallic or semiconducting character, the innermost SWNTs of small diameter exhibit predominantly metallic transport properties. atomic-force microscope | double-walled carbon nanotube | extraction | single-walled carbon nanotube | resonance Raman spectroscopy

Research on plasmons of gold nanoparticles has gained broad interest in nanoscience. However, ultrasmall sizes near the metal-to-nonmetal transition regime have not been explored until recently due to major synthetic difficulties. Herein, intriguing electron dynamics in this size regime is observed in atomically precise Au333(SR)79 nanoparticles. Femtosecond transient-absorption spectroscopy reveals an unprecedented relaxation process of 4-5 ps--a fast phonon-phonon relaxation process, together with electron-phonon coupling (~1 ps) and normal phonon-phonon coupling (>100 ps) processes. Three types of -R capped Au333(SR)79 all exhibit two plasmon-bleaching signals independent of the -R group as well as solvent, indicating plasmon splitting and quantum effect in the ultrasmall core of Au333(SR)79. This work is expected to stimulate future work on the transition-size regime of nanometals and discovery of behavior of nascent plasmons. [ABSTRACT FROM AUTHOR]

Atomically precise gold nanocluster based on linear assembly of repeating icosahedrons (clusters of clusters) is a unique type of linear nanostructure, which exhibits strong near-infrared absorption as their free electrons are confined in a one-dimensional quantum box. Little is known about the carrier dynamics in these nanoclusters, which limit their energy-related applications. Here, we reported the observation of exciton localization in triicosahedral Au37 nanoclusters (0.5 nm in diameter and 1.6 nm in length) by measuring femtosecond and nanosecond carrier dynamics. Upon photoexcitation to S1 electronic state, electrons in Au37 undergo ~100-ps localization from the two vertexes of three icosahedrons to one vertex, forming a long-lived S1* state. Such phenomenon is not observed in Au25 (dimer) and Au13 (monomer) consisting of two and one icosahedrons, respectively. We have further observed temperature dependence on the localization process, which proves it is thermally driven. Two excited-state vibration modes with frequencies of 20 and 70 cm-1 observed in the kinetic traces are assigned to the axial and radial breathing modes, respectively. The electron localization is ascribed to the structural distortion of Au37 in the excited state induced by the strong coherent vibrations. The observed electron localization phenomenon provides unique physical insight into one-dimensional gold nanoclusters and other nanostructures, which will advance their applications in solar-energy storage and conversion. [ABSTRACT FROM AUTHOR]

We report a simple but powerful method for engineering multi-walled carbon nanotubes (MWNTs) by using manipulation by an atomic-force microscope. The successive shell-by-shell extraction process of ultralong MWNTs allows the exposure of the innermost single-walled carbon nanotubes (SWNTs), which have diameters as small as ≈0.4 nm. The inner-shell extraction process changes the electrical characteristics of the MWNTs. Whereas the outer hollowed-out nanotubes show either metallic or semiconducting character, the innermost SWNTs of small diameter exhibit predominantly metallic transport properties. [ABSTRACT FROM AUTHOR]

Research on plasmons of gold nanoparticles has gained broad interest in nanoscience. However, ultrasmall sizes near the metal-to-nonmetal transition regime have not been explored until recently due to major synthetic difficulties. Herein, intriguing electron dynamics in this size regime is observed in atomically precise Au 333 (SR) 79 nanoparticles. Femtosecond transient-absorption spectroscopy reveals an unprecedented relaxation process of 4-5 ps-a fast phonon-phonon relaxation process, together with electron-phonon coupling (∼1 ps) and normal phonon-phonon coupling (>100 ps) processes. Three types of -R capped Au 333 (SR) 79 all exhibit two plasmon-bleaching signals independent of the -R group as well as solvent, indicating plasmon splitting and quantum effect in the ultrasmall core of Au 333 (SR) 79 This work is expected to stimulate future work on the transition-size regime of nanometals and discovery of behavior of nascent plasmons., Competing Interests: The authors declare no conflict of interest.

Atomically precise gold nanocluster based on linear assembly of repeating icosahedrons (clusters of clusters) is a unique type of linear nanostructure, which exhibits strong near-infrared absorption as their free electrons are confined in a one-dimensional quantum box. Little is known about the carrier dynamics in these nanoclusters, which limit their energy-related applications. Here, we reported the observation of exciton localization in triicosahedral Au 37 nanoclusters (0.5 nm in diameter and 1.6 nm in length) by measuring femtosecond and nanosecond carrier dynamics. Upon photoexcitation to S 1 electronic state, electrons in Au 37 undergo ∼100-ps localization from the two vertexes of three icosahedrons to one vertex, forming a long-lived S 1 * state. Such phenomenon is not observed in Au 25 (dimer) and Au 13 (monomer) consisting of two and one icosahedrons, respectively. We have further observed temperature dependence on the localization process, which proves it is thermally driven. Two excited-state vibration modes with frequencies of 20 and 70 cm -1 observed in the kinetic traces are assigned to the axial and radial breathing modes, respectively. The electron localization is ascribed to the structural distortion of Au 37 in the excited state induced by the strong coherent vibrations. The observed electron localization phenomenon provides unique physical insight into one-dimensional gold nanoclusters and other nanostructures, which will advance their applications in solar-energy storage and conversion., Competing Interests: The authors declare no conflict of interest.