Broadening of van Hove Singularities Measured by Photoemission Spectroscopy of Single- and Mixed-Chirality Single-Walled Carbon Nanotubes

The
occupied valence electronic states of single-walled carbon nanotubes
(SWCNTs) are responsible for their optoelectronic properties and are
unique for each SWCNT chirality. Photoemission spectroscopy (PES)
is one of the few methods capable of directly measuring the electron
density in the valence states of materials, but there are only a few
reports which have observed the valence states of SWCNTs and no examples
for single-chirality SWCNTs. Here, we prepare single- and mixed-chirality
SWCNT films and characterize their valence states using PES. Chirality-pure
SWCNTs were isolated using both gel permeation chromatography and
single-stranded DNA-facilitated aqueous two-phase extraction from
starting materials consisting of mixed-chirality species. Chirality
separation and purity was confirmed with UV–vis–NIR
absorption spectroscopy. SWCNT films were prepared for the single-chirality
species (10,3), (7,6), (7,3), (6,5), (8,3), (9,1) along with SWCNT
chirality mixture of metallic and semiconducting SWCNTs, and as-synthesized
mixtures possessing a range of SWCNT diameter. PES using synchrotron
radiation was completed for all samples with survey and C 1s core-level
spectra obtained to confirm SWCNT coverage, defect level, and purity.
Valence band PES was obtained to characterize the valence electronic
states and showed significant broadening of the signal, in comparison
to calculated density of states, which could not be accounted for
by instrument resolution. An inverse diameter dependence of the broadening
was observed with greater broadening for smaller-diameter SWCNTs.
The broadening is hypothesized to be related to the photohole lifetime,
which was found to be significantly longer for wide-diameter SWCNTs.
The diameter dependence of the broadening and photohole lifetimes
is discussed in terms of both Tomonaga–Luttinger and Landau
theories of Fermi liquids.