Optical Property Tuning of Single-Wall Carbon Nanotubes by Endohedral Encapsulation of a Wide Variety of Dielectric Molecules

Specific
and tunable modification to the optical properties of
single-wall carbon nanotubes (SWCNTs) is demonstrated through direct
encapsulation into the nanotube interior of guest molecules with widely
varying static dielectric constants. Filled through simple ingestion
of the guest molecule, each SWCNT population is demonstrated to display
a robust modification to absorbance, fluorescence, and Raman spectra.
Over 30 distinct compounds, covering static dielectric constants from
1.8 to 109, are inserted in large diameter SWCNTs (d = 1.104–1.524 nm) and more than 10 compounds in small diameter
SWCNTs (d = 0.747–1.153 nm), demonstrating
that the general effect of filler dielectric on the nanotube optical
properties is a monotonic energy reduction (red-shifting) of the optical
transitions with increased magnitude of the dielectric constant. Systematic
fitting of the two-dimensional fluorescence–excitation and
Raman spectra additionally enables determination of the critical filling
diameter for each molecule and distinguishing of overall trends from
specific guest–host interactions. Comparisons to predictions
from existing theory are presented, and specific guest molecule/SWCNT
chirality combinations that disobey the general trend and theory are
identified. A general increase of the fluorescence intensity and line
narrowing is observed for low dielectric constants, with long linear
alkane filled SWCNTs exhibiting emission intensities approaching those
of empty SWCNTs. These results demonstrate an exploitable modulation
in the optical properties of SWCNTs and provide a foundation for examining
higher-order effects, such as due to nonbulk-like molecule stacking,
in host–guest interactions in well-controlled nanopore size
materials.