Fundamental optical processes in armchair carbon nanotubes

Abstract

Single-wall carbon nanotubes provide ideal model 1-D condensed matter systems in which to address fundamental questions in many-body physics, while, at the same time, they are leading candidates for building blocks in nanoscale optoelectronic circuits. Much attention has been recently paid to their optical properties, arising from 1-D excitons and phonons, which have been revealed via photoluminescence, Raman scattering, and ultrafast optical spectroscopy of semiconducting carbon nanotubes. On the contrary, dynamical properties of metallic nanotubes have been poorly explored, although they are expected to provide a novel setting for the study of electron-hole pairs in the presence of degenerate 1-D electrons. In particular, (n,n)-chirality, or armchair, metallic nanotubes are truly gapless with massless carriers, ideally suited for dynamical studies of Tomonaga-Luttinger liquids. Unfortunately, progress towards such studies has been slowed by the inherent problem of nanotube synthesis whereby both semiconducting and metallic nanotubes are produced. Here, we use post-synthesis separation methods based on density gradient ultracentrifugation and DNAbased ion-exchange chromatography to produce aqueous suspensions strongly enriched in armchair nanotubes. Through resonant Raman spectroscopy of the radial breathing mode phonons, we provide macroscopic and unambiguous evidence that density gradient ultracentrifugation can enrich armchair nanotubes. Furthermore, using conventional, optical absorption spectroscopy in the nearinfrared and visible range, we show that interband absorption in armchair nanotubes is strongly excitonic. Lastly, by examining the G-band mode in Raman spectra, we determine that observation of the broad, lower frequency (G − ) feature is a result of resonance with non-armchair "metallic" nanotubes. These findings regarding the fundamental optical absorption and scattering processes in metallic carbon nanotubes lay the foundation for further spectroscopic studies to probe many-body physical phenomena in one dimension.

Asymmetric excitation profiles in the resonance Raman response of armchair carbon nanotubes

“…The RBM frequencies found for the (8,8), (7,7), and (6,6), are 219, 248, and 287 cm −1 , respectively and match those found in previous work 7,19,20,26,27 . The (5,5) RBM frequency is found here to be 338 cm −1 .…”

“…The (8,8) shows a small level of (10,4) contamination (RBM feature at 236 cm −1 ). Absence of ad- (8,8), (7,7), (6,6), and (5,5), respectively. The (7,7) and (6,6) samples were enriched by DNAbased ion-exchange chromatography 20 and the (8,8) and (5,5) samples were enriched by aqueous two-phase extraction 21 .…”

“…Two peaks in the REPs are observed for each of the armchair chiralities and define the incoming (lower energy peak) and outgoing (higher energy peak) resonances. The spacing of the two peaks is that of the 2.55 E x c it a t io n E n e r g y ( e V ) (a) (8,8) (b) (7,7) (c) (6,6) (d) (5,5) FIG. 3.…”

“…This is now possible for specific metallic species as a result of recent advances in separation techniques that yield samples enriched in armchair structures [19][20][21] . Here, we present resonance Raman excitation spectra for the radial breathing mode (RBM) and G-band phonons for the (5,5), (6,6), (7,7), and (8,8) armchair nanotubes. The high-frequency (∼1580 cm −1 ) G-band allowed us to resolve the incoming and outgoing resonance peaks in the excitation profile.…”

“…We performed tunable resonance Raman spectroscopy on samples highly enriched in the (5,5), (6,6), (7,7), and (8,8) armchair structures of metallic single-wall carbon nanotubes. We present Raman excitation profiles (REPs) for both the radial breathing mode and G-band phonons of these species.…”