How to recognize clustering of luminescent defects in single-wall carbon nanotubes

Semiconducting single-wall carbon nanotubes (SWCNTs) are a
promising material platform for near-infrared in vivo imaging,
optical sensing, and single-photon emission at telecommunication
wavelengths. The functionalization of SWCNTs with luminescent
defects can lead to significantly enhanced photoluminescence (PL)
properties due to efficient trapping of highly mobile excitons and
red-shifted emission from these trap states. Among the most
studied luminescent defect types are oxygen and aryl defects that
have largely similar optical properties. So far, no direct comparison
between SWCNTs functionalized with oxygen and aryl defects
under identical conditions has been performed. Here, we employ
a combination of spectroscopic techniques to quantify the number
of defects, their distribution along the nanotubes and thus their
exciton trapping efficiencies. The different slopes of Raman D/G+
ratios versus calculated defect densities from PL quantum yield
measurements indicate substantial dissimilarities between oxygen
and aryl defects. Supported by statistical analysis of single-nano-
tube PL spectra at cryogenic temperatures they reveal clustering of
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oxygen defects. The clustering of 2–3 oxygen defects, which act as
a single exciton trap, occurs irrespective of the functionalization
method and thus enables the use of simple equations to determine
the density of oxygen defects and defect clusters in SWCNTs
based on standard Raman spectroscopy. The presented analytical
approach is a versatile and sensitive tool to study defect distribution
and clustering in SWCNTs and can be applied to any new functio-
nalization method.