8 research outputs found
Clusters and Inverse Emulsions from Nanoparticle Surfactants in Organic Solvents
A method is presented for the synthesis
of self-assembling nanoparticle
surfactants in nonpolar organic solvents. The method relies on the
control of long-range steric repulsion imparted by grafted polystyrene
and short-range attraction from short-chain thiol molecules with an
alcohol or carboxylic functionality. Similar to water-based nanoparticle
surfactants, these oil-dispersed materials are found to cluster in
dispersion and also to stabilize oilâwater interfaces to form
water-in-oil emulsions. The clustering process is characterized with
dynamic light scattering (DLS), small-angle X-ray scattering (SAXS),
UVâvis spectroscopy, and transmission electron microscopy (TEM).
Thermogravimetric analysis (TGA) is used to quantify the surface concentration
of grafted polymer, which is found to be a parameter of critical importance
for the formation of stable clusters. The clustering kinetics and
dispersion stability are both affected by the polymer molecular weight,
surface concentration, and chemical structure of the thiol molecules
that induce particle attraction. Nanometer-sized water-in-oil emulsions
are formed by sonication in the presence of nanoparticle surfactants.
A large broadening of the optical absorption spectrum in the NIR region
is observed because of changes in the collective surface plasmon resonance
of the gold particle shell
Efficient Electrosteric Assembly of Nanoparticle Heterodimers and Linear Heteroassemblies
Bottom-up
approaches to the synthesis of nanostructures are of
particular interest because they offer several advantages over the
traditional top-down approaches. In this work, we present a new method
to self-assemble nanoparticles into controlled heteroaggregates. The
technique relies on carefully balancing attractive electrostatic forces
with repulsive steric hindrance that is provided by surface-grafted
polyethylene glycol (PEG). Two different-sized gold nanoparticles
(GNPs) were used as a model system: 13 nm GNPs were functionalized
with PEG-thiol and mercapto dodecanoic acid, while 7 nm GNPs were
functionalized with PEG-thiol and (11- Mercaptoundecyl)Âtrimethylammonium
bromide. When mixed together, these oppositely charged particles self-assemble
into stable colloidal structures (i.e., nanoclusters) whose structure
depends strongly on the surface concentration of PEG. Smaller structures
are obtained as the PEG surface concentration increases because steric
hindrance dominates and prevents uncontrolled aggregation. In particular,
under the right conditions, we were able to selectively synthesize
heterodimers (which are effectively Janus particles) and linear heteroassemblies.
This method is scalable, and it provides a step forward in bottom-up
synthesis of nanomaterials
Correlating Structure and Photocurrent for Composite Semiconducting Nanoparticles with Contrast Variation Small-Angle Neutron Scattering and Photoconductive Atomic Force Microscopy
Aqueous dispersions of semiconducting nanoparticles have shown promise as a robust and scalable platform for the production of efficient polymer/fullerene active layers in organic photovoltaic applications. Semiconducting nanoparticles are a composite of both an n-type and p-type semiconductor contained within a single nanoparticle. In order to realize efficient organic solar cells from these materials, there is a need to understand how the size and internal distribution of materials within each nanoparticle contributes to photocurrent generation in a nanoparticle-derived device. Therefore, characterizing the internal distribution of conjugated polymer and fullerene within the dispersion is the first step to improving performance. To date, study of polymer/fullerene structure within these nanoparticles has been limited to microscopy techniques of deposited nanoparticles. In this work, we use contrast variation with small-angle neutron scattering to determine the internal distribution of poly(3-hexylÂthiophene) and [6,6]phenyl-C<sub>61</sub>-butyric acid methyl ester inside the composite nanoparticles as a function of formulation while in dispersion. On the basis of these measurements, we connect the formulation of these nanoparticles with their internal structure. Using electrostatic deposited monolayers of these nanoparticles, we characterize intrinsic charge generation using photoconductive atomic force microscopy and correlate this with structures determined from small-angle neutron scattering measurements. These techniques combined show that the best performing composite nanoparticles are those that have a uniform distribution of conjugated polymer and fullerene throughout the nanoparticle volume such that electrons and holes are easily transported out of the particle
Structure Characterization and Properties of MetalâSurfactant Complexes Dispersed in Organic Solvents
This work describes the synthesis
and characterization of metalâsurfactant
complexes. Dioctyl sulfosuccinate and dodecylbenzenesulfonate are
associated with multivalent aluminum, iron, and vanadium ions using
an ion exchange reaction. The metal complexes are dispersible in various
organic solvents. In solvents with low polarity, the complexes form
âinverseâ macromolecular structures with multiple metal
ions. In contrast, in alcohols, the complex size is reduced, showing
a more disperse conformation. The metal and surfactant ions are still
strongly bonded to each other in all the solvents probed. Small-angle
X-ray and neutron scattering (SAXS and SANS) are used to characterize
the structures. Simultaneous fitting of neutron and X-ray scattering
spectra is performed in order to obtain an accurate description of
the system. Scattering results are also validated by performing molecular
dynamics (MD) simulations. The conductive and electrochemical properties
of the complexes in solution are also evaluated. The dispersion of
metalâorganic complexes significantly increases electric conductivity,
and some metal ions in the core of the complexes are shown to be electrochemically
active in apolar solvents
Polypyrrole-Coated Perfluorocarbon Nanoemulsions as a Sono-Photoacoustic Contrast Agent
A new
contrast agent for combined photoacoustic and ultrasound
imaging is presented. It has a liquid perfluorocarbon (PFC) core of
about 250 nm diameter coated by a 30 nm thin polypyrrole (PPy) doped
polymer shell emulsion that represents a broadband absorber covering
the visible and near-infrared ranges (peak optical extinction at 1050
nm). When exposed to a sufficiently high intensity optical or acoustic
pulse, the droplets vaporize to form microbubbles providing a strong
increase in imaging sensitivity and specificity. The threshold for
contrast agent activation can further drastically be reduced by up
to 2 orders of magnitude if simultaneously exposing them with optical
and acoustic pulses. The selection of PFC core liquids with low boiling
points (i.e., perfluorohexane (56 °C), perfluoropentane (29 °C),
and perfluorobutane (â2 °C)) facilitates activation and
reduces the activation threshold of PPy-coated emulsion contrast agents
to levels well within clinical safety limits (as low as 0.2 MPa at
1 mJ/cm<sup>2</sup>). Finally, the potential use of these nanoemulsions
as a contrast agent is demonstrated in a series of phantom imaging
studies
Designing Two-Dimensional Protein Arrays through Fusion of Multimers and Interface Mutations
We have combined fusion of oligomers with cyclic symmetry
and alanine substitutions to eliminate clashes and produce proteins
that self-assemble into 2-D arrays upon addition of calcium ions.
Using TEM, AFM, small-angle X-ray scattering, and fluorescence microscopy,
we show that the designed lattices which are 5 nm high with <i>p</i>3 space group symmetry and 7.25 nm periodicity self-assemble
into structures that can exceed 100 ÎŒm in characteristic length.
The versatile strategy, experimental approach, and hexagonal arrays
described herein should prove valuable for the engineering of functional
nanostructured materials in 2-D
Understanding Interfacial Alignment in Solution Coated Conjugated Polymer Thin Films
Domain
alignment in conjugated polymer thin films can significantly enhance
charge carrier mobility. However, the alignment mechanism during meniscus-guided
solution coating remains unclear. Furthermore, interfacial alignment
has been rarely studied despite its direct relevance and critical
importance to charge transport. In this study, we uncover a significantly
higher degree of alignment at the top interface of solution coated
thin films, using a donorâacceptor conjugated polymer, polyÂ(diketopyrrolopyrrole-<i>co</i>-thiophene-<i>co</i>-thienoÂ[3,2-<i>b</i>]Âthiophene-<i>co</i>-thiophene) (DPP2T-TT), as the model
system. At the molecular level, we observe in-plane ÏâÏ
stacking anisotropy of up to 4.8 near the top interface with the polymer
backbone aligned parallel to the coating direction. The bulk of the
film is only weakly aligned with the backbone oriented transverse
to coating. At the mesoscale, we observe a well-defined fibril-like
morphology at the top interface with the fibril long axis pointing
toward the coating direction. Significantly smaller fibrils with poor
orientational order are found on the bottom interface, weakly aligned
orthogonal to the fibrils on the top interface. The high degree of
alignment at the top interface leads to a charge transport anisotropy
of up to 5.4 compared to an anisotropy close to 1 on the bottom interface.
We attribute the formation of distinct interfacial morphology to the
skin-layer formation associated with high Peclet number, which promotes
crystallization on the top interface while suppressing it in the bulk.
We further infer that the interfacial fibril alignment is driven by
the extensional flow on the top interface arisen from increasing solvent
evaporation rate closer to the meniscus front
Solvatochromism and Conformational Changes in Fully Dissolved Poly(3-alkylthiophene)s
Absorption spectroscopy is commonly
utilized to probe optical properties
that can be related, among other things, to the conformation of single,
isolated conjugated polymer chains in solution. It is frequently suggested
that changes in peak positions of optical spectra result from variations
in the stiffness of polymer chains in solution because this modifies
the conjugation length. In this work we utilize ultravioletâvisible
(UVâvis) spectroscopy, small angle neutron scattering (SANS),
and all atom molecular dynamic (AA-MD) simulations to closely probe
the relationship between the conformation of single-chains of polyÂ(3-alkylthiophene)Âs
(P3ATs) and their optical properties. SANS results show variations
in the radius of gyration and Kuhn length as a function of alkyl chain
length, and structure, as well as the solvent environment. Furthermore,
both SANS and MD simulations show that dissolved P3HT chains are more
rigid in solvents where self-assembly and crystallization are possible.
Shifts in P3AT optical properties were also observed for different
solvent environments. However, these changes were not correlated to
the changes in polymer conformation. Furthermore, changes in optical
properties could not be perfectly described by generalized solventâsolute
interactions. AA-MD simulations provide new insights into specific
polymerâsolvent interactions not accounted for in generalized
solvatochromic theory. This work highlights the need for experiments
and molecular simulations that further inform the specific role of
solvent molecules on local polymer conformation and on optical properties