4 research outputs found
Photoluminescence Imaging of Polyfluorene Surface Structures on Semiconducting Carbon Nanotubes: Implications for Thin Film Exciton Transport
Single-walled
carbon nanotubes (SWCNTs) have potential to act as
light-harvesting elements in thin film photovoltaic devices, but performance
is in part limited by the efficiency of exciton diffusion processes
within the films. Factors contributing to exciton transport can include
film morphology encompassing nanotube orientation, connectivity, and
interaction geometry. Such factors are often defined by nanotube surface
structures that are not yet well understood. Here, we present the
results of a combined pumpāprobe and photoluminescence imaging
study of polyfluorene (PFO)-wrapped (6,5) and (7,5) SWCNTs that provide
additional insight into the role played by polymer structures in defining
exciton transport. Pumpāprobe measurements suggest exciton
transport occurs over larger length scales in films composed of PFO-wrapped
(7,5) SWCNTs, compared to those prepared from PFO-bpy-wrapped (6,5)
SWCNTs. To explore the role the difference in polymer structure may
play as a possible origin of differing transport behaviors, we performed
a photoluminescence imaging study of individual polymer-wrapped (6,5)
and (7,5) SWCNTs. The PFO-bpy-wrapped (6,5) SWCNTs showed more uniform
intensity distributions along their lengths, in contrast to the PFO-wrapped
(7,5) SWCNTs, which showed irregular, discontinuous intensity distributions.
These differences likely originate from differences in surface coverage
and suggest the PFO wrapping on (7,5) nanotubes produces a more open
surface structure than is available with the PFO-bpy wrapping of (6,5)
nanotubes. The open structure likely leads to improved <i>intertube</i> coupling that enhances exciton transport within the (7,5) films,
consistent with the results of our pumpāprobe measurements
Polymer-Free Carbon Nanotube Thermoelectrics with Improved Charge Carrier Transport and Power Factor
Semiconducting
single-walled carbon nanotubes (s-SWCNTs) have recently
attracted attention for their promise as active components in a variety
of optical and electronic applications, including thermoelectricity
generation. Here we demonstrate that removing the wrapping polymer
from the highly enriched s-SWCNT network leads to substantial improvements
in charge carrier transport and thermoelectric power factor. These
improvements arise primarily from an increase in charge carrier mobility
within the s-SWCNT networks because of removal of the insulating polymer
and control of the level of nanotube bundling in the network, which
enables higher thin-film conductivity for a given carrier density.
Ultimately, these studies demonstrate that highly enriched s-SWCNT
thin films, in the complete absence of any accompanying semiconducting
polymer, can attain thermoelectric power factors in the range of ā¼400
Ī¼WāÆm<sup>ā1</sup>āÆK<sup>ā2</sup>, which is on par with that of some of the best single-component
organic thermoelectrics demonstrated to date
Isolation of >1 nm Diameter Single-Wall Carbon Nanotube Species Using Aqueous Two-Phase Extraction
In this contribution we demonstrate the effective separation of single-wall carbon nanotube (SWCNT) species with diameters larger than 1 nm through multistage aqueous two-phase extraction (ATPE), including isolation at the near-monochiral species level up to at least the diameter range of SWCNTs synthesized by electric arc synthesis (1.3ā1.6 nm). We also demonstrate that refined species are readily obtained from both the metallic and semiconducting subpopulations of SWCNTs and that this methodology is effective for multiple SWCNT raw materials. Using these data, we report an empirical function for the necessary surfactant concentrations in the ATPE method for separating different SWCNTs into either the lower or upper phase as a function of SWCNT diameter. This empirical correlation enables predictive separation design and identifies a subset of SWCNTs that behave unusually as compared to other species. These results not only dramatically increase the range of SWCNT diameters to which species selective separation can be achieved but also demonstrate that aqueous two-phase separations can be designed across experimentally accessible ranges of surfactant concentrations to controllably separate SWCNT populations of very small (ā¼0.62 nm) to very large diameters (>1.7 nm). Together, the results reported here indicate that total separation of all SWCNT species is likely feasible by the ATPE method, especially given future development of multistage automated extraction techniques
Low-Temperature Single Carbon Nanotube Spectroscopy of sp<sup>3</sup> Quantum Defects
Aiming
to unravel the relationship between chemical configuration
and electronic structure of sp<sup>3</sup> defects of aryl-functionalized
(6,5) single-walled carbon nanotubes (SWCNTs), we perform low-temperature
single nanotube photoluminescence (PL) spectroscopy studies and correlate
our observations with quantum chemistry simulations. We observe sharp
emission peaks from individual defect sites that are spread over an
extremely broad, 1000ā1350 nm, spectral range. Our simulations
allow us to attribute this spectral diversity to the occurrence of
six chemically and energetically distinct defect states resulting
from topological variation in the chemical binding configuration of
the monovalent aryl groups. Both PL emission efficiency and spectral
line width of the defect states are strongly influenced by the local
dielectric environment. Wrapping the SWCNT with a polyfluorene polymer
provides the best isolation from the environment and yields the brightest
emission with near-resolution limited spectral line width of 270 Ī¼eV,
as well as spectrally resolved emission wings associated with localized
acoustic phonons. Pump-dependent studies further revealed that the
defect states are capable of emitting single, sharp, isolated PL peaks
over 3 orders of magnitude increase in pump power, a key characteristic
of two-level systems and an important prerequisite for single-photon
emission with high purity. These findings point to the tremendous
potential of sp<sup>3</sup> defects in development of room temperature
quantum light sources capable of operating at telecommunication wavelengths
as the emission of the defect states can readily be extended to this
range <i>via</i> use of larger diameter SWCNTs