4 research outputs found
Interfacial Surfactant Ordering in Thin Films of SDS-Encapsulated Single-Walled Carbon Nanotubes
The molecular self-assembly of surfactants
on the surface of single-walled
carbon nanotubes (SWCNT) is currently a common strategy for the tuning
of nanotube properties and the stabilization of carbon nanotube dispersions.
Here, we report direct measurements of the degree of interfacial ordering
for sodium dodecyl sulfate (SDS) surfactants adsorbed on colloidal,
single-chirality enriched, SWCNTs within a solid film and investigate
the dependence of surface alkyl chain order on the surfactant concentration
in the precursor solution. The degree of order for the SWCNT-bound
SDS molecules, is probed by vibrational sum frequency generation (VSFG)
spectroscopy. We find concrete evidence for the presence of highly
ordered surface structures at sufficiently high SDS concentrations,
attributed here to cylindrical-like micelle assemblies with the SWCNT
at the core. As the SDS concentration decreases, the interfacial order
is found to decrease as well, generating a more disordered or random
adsorption of surfactants on the nanotube surfaces
Aggregated States of Chalcogenorhodamine Dyes on Nanocrystalline Titania Revealed by Doubly Resonant Sum Frequency Spectroscopy
Broadband
doubly resonant sum frequency generation (DR-SFG) spectroscopy
was used to probe selectively the vibrational signatures of a series
of chalcogenorhodamine dyes tethered to TiO<sub>2</sub> either as
monomers or as H-aggregates. This selectivity was achieved by virtue
of the vibronic coupling associated with the unique electronic transitions
for the corresponding states. It was found that clear spectral differences
and polarization dependences were observed contingent on whether the
outgoing SFG wavelength was in resonance with either the monomer or
H-aggregate electronic excited states. Our results also indicate that
the orientation and proximity of the xanthylium core to the semiconducting
surface play an important role in this vibrational–electronic
coupling and may affect the character of the charge-transfer complex.
Our DR-SFG results also provide important clues into the anchoring
motifs for carboxylic and phosphonic acid groups
Tuning the Surface Ordering of Self-Assembled Ionic Surfactants on Semiconducting Single-Walled Carbon Nanotubes: Concentration, Tube Diameter, and Counterions
We report direct
spectroscopic measurements of the macromolecular
organization of ionic surfactants on the surface of semiconducting
single-walled carbon nanotubes (SWCNTs) within solution-processed
thin films. By using vibrational sum frequency generation (VSFG) spectroscopy,
sensitive measurements of interfacial surfactant ordering were obtained
as a function of surfactant concentration for sodium dodecyl sulfate
(SDS)-encapsulated (6,5) and (7,6) SWCNTs with and without excess
electrolytes. Anionic surfactants are known to effectively stabilize
SWCNTs. The current models suggest a strong influence of the dispersion
conditions on the surfactant interfacial macromolecular organization
and self-assembly. Direct experimental probes of such an organization
using nanotubes of specific chirality are needed to validate the existing
models. We found that as the bulk SDS concentration increases near
the surfactant critical micelle concentration, the interfacial ordering
increased, approaching the formation of cylindrical-like micelles
with the nanotube at the core. At the higher surfactant concentrations
measured here, the (6,5) SWCNTs produced more ordered structures relative
to those with the (7,6) SWCNTs. The relatively larger-diameter (7,6)
chiral tubes support enhanced van der Waals (vdW) interactions between
the tube carbon surface and the surfactant methylene chain groups
that likely increase the density of gauche defects. A new effect arises
when the precursor solution is exposed to a small concentration of
divalent Ca<sup>2+</sup> counterions. We postulate that a salt-bridging
configuration on such highly curved surfaces decreases the ordering
of interfacial surfactant molecules, resulting in compact, disordered
structures. However, this phenomenon was not observed with excess
Na<sup>+</sup> ions at the same ionic strength. Instead, a modest
increase in surfactant ordering was observed with the excess monovalent
electrolyte. These results provide new insights for thin film solution
processing of vdW nanomaterials and demonstrate that VSFG is a sensitive
probe of surfactant organization on nanostructures
Accurate Line Shapes from Sub‑1 cm<sup>–1</sup> Resolution Sum Frequency Generation Vibrational Spectroscopy of α‑Pinene at Room Temperature
Despite the importance of terpenes
in biology, the environment,
and catalysis, their vibrational spectra remain unassigned. Here,
we present subwavenumber high-resolution broad-band sum frequency
generation (HR-BB-SFG) spectra of the common terpene (+)-α-pinene
that reveal 10 peaks in the C–H stretching region at room temperature.
The high spectral resolution resulted in spectra with more and better
resolved spectral features than those of the Fourier transform infrared,
femtosecond stimulated Raman spectra in the bulk condensed phase and
those of the conventional BB-SFG and scanning SFG spectroscopy of
the same molecule on a surface. Experiment and simulation show the
spectral line shapes with HR-BB-SFG to be accurate. Homogeneous vibrational
decoherence lifetimes of up to 1.7 ps are assigned to specific oscillators
and compare favorably to lifetimes computed from density functional
tight binding molecular dynamics calculations. Phase-resolved spectra
provided their orientational information. We propose the new spectroscopy
as an attractive alternative to time domain vibrational spectroscopy
or heterodyne detection schemes for studying vibrational energy relaxation
and vibrational coherences in molecules at molecular surfaces or interfaces