25 research outputs found
Spontaneous Partition of Carbon Nanotubes in Polymer-Modified Aqueous Phases
The
distribution of nanoparticles in different aqueous environments
is a fundamental problem underlying a number of processes, ranging
from biomedical applications of nanoparticles to their effects on
the environment, health, and safety. Here, we study distribution of
carbon nanotubes (CNTs) in two immiscible aqueous phases formed by
the addition of polyethylene glycol (PEG) and dextran. This well-defined
model system exhibits a strikingly robust phenomenon: CNTs spontaneously
partition between the PEG- and the dextran-rich phases according to
nanotubeās diameter and metallicity. Thermodynamic analysis
suggests that this chirality-dependent partition is determined by
nanotubeās intrinsic hydrophobicity and reveals two distinct
regimes in hydrophobicity-chirality relation: a small diameter (<1
nm) regime, where curvature effect makes larger diameter tubes more
hydrophobic than small diameter ones, and a large diameter (>1.2
nm)
regime, where nanotubeās polarizability renders semiconducting
tubes more hydrophobic than metallic ones. These findings reveal a
general rule governing CNT behaviors in aqueous phase and provide
an extremely simple way to achieve spatial separation of CNTs by their
electronic structures
A Low Energy Route to DNA-Wrapped Carbon Nanotubes via Replacement of Bile Salt Surfactants
DNA-wrapped carbon
nanotubes are a class of bionano hybrid molecules
that have enabled carbon nanotube sorting, controlled assembly, and
biosensing and bioimaging applications. The current method of synthesizing
these hybrids via direct sonication of DNA/nanotube mixtures is time-consuming
and not suitable for high-throughput synthesis and combinatorial sequence
screening. Additionally, the direct sonication method does not make
use of nanotubes presorted by extensively developed surfactant-based
methods, is not effective for large diameter (>1 nm) tubes, and
cannot
maintain secondary and tertiary structural and functional domains
present in certain DNA sequences. Here, we report a simple, quick,
and robust process to produce DNA-wrapped carbon nanotube hybrids
with nanotubes of broad diameter range and DNA of arbitrary sequence.
This is accomplished by exchanging strong binding bile salt surfactant
coating with DNA in methanol/water mixed solvent and subsequent precipitation
with isopropyl alcohol. The exchange process can be completed within
10 min and converts over 90% nanotubes into the DNA wrapped form.
Applying the exchange process to nanotubes presorted by surfactant-based
methods, we show that the resulting DNA-wrapped carbon nanotubes can
be further sorted to produce nanotubes with defined handedness, helicity,
and endohedral filling. The exchange method greatly expands the structural
and functional variety of DNA-wrapped carbon nanotubes and opens possibilities
for DNA-directed assembly of structurally sorted nanotubes and high-throughput
screening of properties that are controlled by the wrapping DNA sequences
Differentiating Left- and Right-Handed Carbon Nanotubes by DNA
New structural characteristics emerge
when solid-state crystals
are constructed in lower dimensions. This is exemplified by single-wall
carbon nanotubes, which exhibit a degree of freedom in handedness
and a multitude of helicities that give rise to three distinct types
of electronic structures: metals, quasi-metals, and semiconductors.
Here we report the use of intrinsically chiral single-stranded DNA
to achieve simultaneous handedness and helicity control for all three
types of nanotubes. We apply polymer aqueous two-phase systems to
select special DNA-wrapped carbon nanotubes, each of which we argue
must have an ordered DNA structure that binds to a nanotube of defined
handedness and helicity and resembles a well-folded biomacromolecule
with innate stereoselectivity. We have screened over 300 short single-stranded
DNA sequences with palindrome symmetry, leading to the selection of
more than 20 distinct carbon nanotube structures that have defined
helicity and handedness and cover the entire chiral angle range and
all three electronic types. The mechanism of handedness selection
is illustrated by a DNA sequence that adopts two distinct folds on
a pair of (6,5) nanotube enantiomers, rendering them large differences
in fluorescence intensity and chemical reactivity. This result establishes
a first example of functionally distinguishable left- and right-handed
carbon nanotubes. Taken together, our work demonstrates highly efficient
enantiomer differentiation by DNA and offers a first comprehensive
solution to achieve simultaneous handedness and helicity control for
all three electronic types of carbon nanotubes
Rod Hydrodynamics and Length Distributions of Single-Wall Carbon Nanotubes Using Analytical Ultracentrifugation
Because of their repetitive chemical
structure, extreme rigidity,
and the separability of populations with varying aspect ratio, SWCNTs
are excellent candidates for use as model rodlike colloids. In this
contribution, the sedimentation velocities of length and density sorted
single-wall carbon nanotubes (SWCNTs) are compared to predictions
from rod hydrodynamic theories of increasing complexity over a range
of aspect ratios from <50 to >400. Independently measuring all
contributions to the sedimentation velocity besides the shape factor,
excellent agreement is found between the experimental findings and
theoretical predictions for numerically calculated hydrodynamic radius
values and for multiterm analytical expansion approximations; values
for the hydrodynamic radii in these cases are additionally found to
be consistent with the apparent hydrated particle radius determined
independently by buoyancy measurements. Lastly, we utilize this equivalency
to calculate the apparent distribution of nanotube lengths in each
population from their sedimentation coefficient distribution without
adjustable parameters, achieving excellent agreement with distributions
from atomic force microscopy. The method developed herein provides
an alternative for the ensemble measurement of SWCNT length distributions
and others rodlike particles
Correction to āNature of Record Efficiency Fluid-Processed NanotubeāSilicon Heterojunctionsā
Correction to āNature of Record Efficiency
Fluid-Processed NanotubeāSilicon Heterojunctions
Nature of Record Efficiency Fluid-Processed NanotubeāSilicon Heterojunctions
Although there has been significant
recent progress in improving
performance, the precise classification of nanotubeāsilicon
heterojunctions remains ambiguous. Here, we use type, chirality, and
length-purified single-walled carbon nanotubes to clarify the nature
of these devices. Our junctions are assembled from freestanding nanotube
sheets that show remarkable stability in response to repeated crumpling
and folding during fluid processing, making the films well suited
to flexible platforms. Despite modest ideality factors, the best diodes
meet or exceed state-of-the-art characteristics, but with a surprising
dependence on sample type. The data further suggest that these devices
can be simultaneously categorized as either Schottky or pān
junctions, and we use scaling arguments to model the behavior over
a broad range of sheet resistance and film thickness in a manner that
highlights the critical role of nanotube midgap states. Our results
demonstrate how band gap engineering can optimize these devices while
emphasizing the important role of the junction morphology
Analyzing Surfactant Structures on Length and Chirality Resolved (6,5) Single-Wall Carbon Nanotubes by Analytical Ultracentrifugation
The structure and density of the bound interfacial surfactant layer and associated hydration shell were investigated using analytical ultracentrifugation for length and chirality purified (6,5) single-wall carbon nanotubes (SWCNTs) in three different bile salt surfactant solutions. The differences in the chemical structures of the surfactants significantly affect the size and density of the bound surfactant layers. As probed by exchange of a common parent nanotube population into sodium deoxycholate, sodium cholate, or sodium taurodeoxycholate solutions, the anhydrous density of the nanotubes was least for the sodium taurodeoxycholate surfactant, and the absolute sedimentation velocities greatest for the sodium cholate and sodium taurodeoxycholate surfactants. These results suggest that the thickest interfacial layer is formed by the deoxycholate, and that the taurodeoxycholate packs more densely than either sodium cholate or deoxycholate. These structural differences correlate well to an observed 25% increase in fluorescence intensity relative to the cholate surfactant for deoxycholate and taurodeoxycholate dispersed SWCNTs displaying equivalent absorbance spectra. Separate sedimentation velocity experiments including the density modifying agent iodixanol were used to establish the buoyant density of the (6,5) SWCNT in each of the bile salt surfactants; from the difference in the buoyant and anhydrous densities, the largest hydrated diameter is observed for sodium deoxycholate. Understanding the effects of dispersant choice and the methodology for measurement of the interfacial density and hydrated diameter is critical for rationally advancing separation strategies and applications of nanotubes
Intensity Ratio of Resonant Raman Modes for (<i>n</i>,<i>m</i>) Enriched Semiconducting Carbon Nanotubes
Relative
intensities
of resonant Raman spectral features, specifically
the radial breathing mode (RBM) and G modes, of 11, chirality-enriched,
single-wall carbon nanotube (SWCNT) species were established under
second-order optical transition excitation. The results demonstrate
an under-recognized complexity in the evaluation of Raman spectra
for the assignment of (<i>n</i>,<i>m</i>) population
distributions. Strong chiral angle and mod dependencies affect the
intensity ratio of the RBM to G modes and can result in misleading
interpretations. Furthermore, we report five additional (<i>n</i>,<i>m</i>) values for the chirality-dependent G<sup>+</sup> and G<sup>ā</sup> Raman peak positions and intensity ratios;
thereby extending the available data to cover more of the smaller
diameter regime by including the (5,4) second-order, resonance Raman
spectra. Together, the Raman spectral library is demonstrated to be
sufficient for decoupling G peaks from multiple species <i>via</i> a spectral fitting process, and enables fundamental characterization
even in mixed chiral population samples
van der Waals SWCNT@BN Heterostructures Synthesized from Solution-Processed Chirality-Pure Single-Wall Carbon Nanotubes
Single-wall carbon nanotubes in boron nitride (SWCNT@BN)
are one-dimensional
van der Waals heterostructures that exhibit intriguing physical and
chemical properties. As with their carbon nanotube counterparts, these
heterostructures can form from different combinations of chiralities,
providing rich structures but also posing a significant synthetic
challenge to controlling their structure. Enabled by advances in nanotube
chirality sorting, clean removal of the surfactant used for solution
processing, and a simple method to fabricate free-standing submonolayer
films of chirality pure SWCNTs as templates for the BN growth, we
show it is possible to directly grow BN on chirality enriched SWCNTs
from solution processing to form van der Waals heterostructures. We
further report factors affecting the heterostructure formation, including
an accelerated growth rate in the presence of H2, and significantly
improved crystallization of the grown BN, with the BN thickness controlled
down to one single BN layer, through the presence of a Cu foil in
the reactor. Transmission electron microscopy and electron energy-loss
spectroscopic mapping confirm the synthesis of SWCNT@BN from the solution
purified nanotubes. The photoluminescence peaks of both (7,5)- and
(8,4)-SWCNT@BN heterostructures are found to redshift (by ā¼10
nm) relative to the bare SWCNTs. Raman scattering suggests that the
grown BN shells pose a confinement effect on the SWCNT core
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