11 research outputs found
Processing Approaches for the Defect Engineering of Lamellar-Forming Block Copolymers in Thin Films
The in-plane connectivity and continuity
of lamellar-forming polystyrene-<i>block</i>-polyÂ(methyl
methacrylate) copolymer domains in thin
films depend on the density and relative population of defects in
the self-assembled morphology. Here we varied film thickness, degree
of polymerization, thermal annealing time, and annealing temperature
in order to engineer the defect densities and topology of the lamellar
morphology. Assembly in thicker films leads to lower defect densities
and thus reduced connectivity of the lamellar domains, which is considered
in the context of the activation energies and driving forces for defect
annihilation. Systems with smaller degrees of polymerization were
also found to achieve lower defect densities and reduced domain connectivity.
Most importantly, the relative populations of each type of defect
were unaffected by the defect density, and these morphologies had
similar long-range continuities. Controlling processing conditions
such as thermal annealing time and temperature, in comparison, was
ineffective at tuning the defect density of block copolymer lamellae
because quasi-equilibrium morphologies were rapidly achieved and subsequently
remained quasi-static. These results provide a framework for selecting
the composition, degree of polymerization, and processing parameters
for lamellar-forming block copolymers in thin films for applications
that either require low defect densities (e.g., in the directed assembly
of microelectronic architectures) or benefit from high defect densities
(e.g., in network structures for transport)
Classifying the Shape of Colloidal Nanocrystals by Complex Fourier Descriptor Analysis
The optical, electrical, magnetic, and catalytic properties
of
colloidal nanocrystals are intimately tied to their form, in particular
their physical size and shape. Synthetic techniques have been developed
to produce metallic and semiconducting nanomaterials with well-controlled
forms; however, characterization tools for describing shape have remained
limited to small samples and lack the quantitative rigor necessary
for a universal classification scheme. Here complex Fourier descriptors
are shown to be a quantitative and high-throughput approach for classifying
the shape of colloidal nanocrystals. Large, monodisperse, and polydisperse
ensembles of CdSe nanocrystals are characterized with respect to shape
and categorized as circles, triangles, squares, rods, and pentagonal
or hexagonal platelets. These results suggest that classification
of shape by Fourier descriptor analysis may in the near future be
a powerful tool for continuous monitoring of synthesis, purification,
or packaging/integration processes during industrial-scale production
of nanomaterials
Seed-Mediated Growth of Shape-Controlled Wurtzite CdSe Nanocrystals: Platelets, Cubes, and Rods
Prior
investigations into the synthesis of colloidal CdSe nanocrystals
with a wurtzite crystal structure (wz-CdSe) have given rise to well-developed
methods for producing particles with anisotropic shapes such as rods,
tetrapods, and wires; however, the synthesis of other shapes has proved
challenging. Here we present a seed-mediated approach for the growth
of colloidal, shape-controlled wz-CdSe nanoparticles with previously
unobserved morphologies. The synthesis, which makes use of small (2–3
nm) wz-CdSe nanocrystals as nucleation sites for subsequent growth,
can be tuned to selectively yield colloidal wz-CdSe nanocubes and
hexagonal nanoplatelets in addition to nanorod and bullet-shaped particles.
We thoroughly characterize the morphology and crystal structures of
these new shapes, as well as discuss possible growth mechanisms in
the context of control over surface chemistry and the nucleation stage
Role of Dimension and Spatial Arrangement on the Activity of Biocatalytic Cascade Reactions on Scaffolds
Despite
broad interest in engineering enzyme cascades on surfaces
(i.e., for multistep biocatalysis, enzyme-mediated electrocatalysis,
biosensing, and synthetic biology), there is a fundamental gap in
understanding how the local density and spatial arrangement of enzymes
affect overall activity. In this work, the dependence of the overall
activity of a cascade reaction on the geometric arrangement and density
of enzymes immobilized on a two-dimensional scaffold was elucidated
using kinetic Monte Carlo simulations. Simulations were specifically
used to track the molecular trajectories of the reaction species and
to investigate the turnover frequency of individual enzymes on the
surface under diffusion-limited and reaction-limited conditions for
random, linear striped, and hexagonal arrangements of the enzymes.
Interestingly, the simulation results showed that, under diffusion-limited
conditions, the overall cascade activity was only weakly dependent
on spatial arrangement and, specifically, nearest-neighbor distance
for high enzyme surface coverages. This dependence becomes negligible
for reaction-limited conditions, implying that the spatial arrangement
has only a minimal impact on cascade activity for the length scales
studied here, which has important practical implications. These results
suggest that, at short length scales (i.e., sub 10 nm dimensions)
and high enzyme densities, sophisticated approaches for controlling
enzyme spatial arrangement have little benefit over random immobilization.
Moreover, our findings suggest that engineering artificial cascades
with enhanced activity will likely require direct molecular channeling
rather than a reliance on free molecular diffusion
Nanoscale Topography Influences Polymer Surface Diffusion
Using high-throughput single-molecule tracking, we studied the diffusion of poly(ethylene glycol) chains at the interface between water and a hydrophobic surface patterned with an array of hexagonally arranged nanopillars. Polymer molecules displayed anomalous diffusion; in particular, they exhibited intermittent motion (<i>i.e.</i>, immobilization and “hopping”) suggestive of continuous-time random walk (CTRW) behavior associated with desorption-mediated surface diffusion. The statistics of the molecular trajectories changed systematically on surfaces with pillars of increasing height, exhibiting motion that was increasingly subdiffusive and with longer waiting times between diffusive steps. The trajectories were well-described by kinetic Monte Carlo simulations of CTRW motion in the presence of randomly distributed permeable obstacles, where the permeability (the main undetermined parameter) was conceptually related to the obstacle height. These findings provide new insights into the mechanisms of interfacial transport in the presence of obstacles and on nanotopographically patterned surfaces
Network Connectivity and Long-Range Continuity of Lamellar Morphologies in Block Copolymer Thin Films
The connectivity, and thus long-range continuity of the
domains,
in a lamellar polystyrene-<i>block</i>-polyÂ(methyl
methacrylate) copolymer in thin films depends on the volume fraction
of each block and can be shifted by homopolymer addition to substrate-spanning
continuity of either the polystyrene (PS) or polyÂ(methyl methacrylate)
(PMMA) domains. Essential features of the lamellar morphology were
captured by a simple network analysis that quantified the number of
branch points and end points in the lamellar domains. The transition
in network continuity from the PS to PMMA domain as a function of
copolymer volumetric composition (from <i>f</i><sub>PMMA</sub> = 0.45 to 0.55) was correlated with a 5-fold increase in the PMMA
branch point density and a concomitant 3-fold reduction in the PMMA
end point density. These results indicate that the copolymer’s
composition drastically impacts the self-assembled lamellar morphology
in thin films and is an important design consideration when using
such materials for lithographic applications, including for directed
assembly to generate long-range, defect-free order
Photonic Crystal Kinase Biosensor
We have developed
a novel biosensor for kinases that is based on
a kinase-responsive polymer hydrogel, which enables label-free screening
of kinase activity via changes in optical properties. The hydrogel
is specifically designed to swell reversibly upon phosphorylation
of a target peptide, triggering a change in optical diffraction from
a crystalline colloidal array of particles impregnated into the hydrogel.
Diffraction measurements, and charge staining, confirmed the responsive
nature of the hydrogel. Moreover, the change in diffraction of the
hydrogel upon treatment with kinase exhibited a time- and dose-dependent
response. A theoretical model for ionic polymer networks describes
the observed optical response well and can be used to quantify the
extent of phosphorylation
Polymer Surface Transport Is a Combination of in-Plane Diffusion and Desorption-Mediated Flights
Previous studies of polymer motion
at solid/liquid interfaces described
the transport in the context of a continuous time random walk (CTRW)
process, in which diffusion switches between desorption-mediated “flights”
(i.e., hopping) and surface-adsorbed waiting-time intervals. However,
it has been unclear whether the waiting times represented periods
of complete immobility or times during which molecules engaged in
a different (e.g., slower or confined) mode of interfacial transport.
Here we designed high-throughput, single-molecule tracking measurements
to address this question. Specifically, we studied polymer dynamics
on either chemically homogeneous or nanopatterned surfaces (hexagonal
diblock copolymer films) with chemically distinct domains, where polymers
were essentially excluded from the low-affinity domains, eliminating
the possibility of significant continuous diffusion in the absence
of desorption-mediated flights. Indeed, the step-size distributions
on homogeneous surfaces exhibited an additional diffusive mode that
was missing on the chemically heterogeneous nanopatterned surfaces,
confirming the presence of a slow continuous mode due to 2D in-plane
diffusion. Kinetic Monte Carlo simulations were performed to test
this model and, with the theoretical in-plane diffusion coefficient
of <i>D</i><sub>2D</sub> = 0.20 ÎĽm<sup>2</sup>/s,
we found a good agreement between simulations and experimental data
on both chemically homogeneous and nanopatterned surfaces
Nanostructured Silicon Photocathodes for Solar Water Splitting Patterned by the Self-Assembly of Lamellar Block Copolymers
We studied a type of nanostructured
silicon photocathode for solar
water splitting, where one-dimensionally periodic lamellar nanopatterns
derived from the self-assembly of symmetric polyÂ(styrene-<i>block</i>-methyl methacrylate) block copolymers were incorporated on the surface
of single-crystalline silicon in configurations with and without a
buried metallurgical junction. The resulting nanostructured silicon
photocathodes with the characteristic lamellar morphology provided
suppressed front-surface reflection and increased surface area, which
collectively contributed to the enhanced photocatalytic performance
in the hydrogen evolution reaction. The augmented light absorption
in the nanostructured silicon directly translated to the increase
of the saturation current density, while the onset potential decreased
with the etching depth because of the increased levels of surface
recombination. The pp<sup>+</sup>-silicon photocathodes, compared
to the n<sup>+</sup>pp<sup>+</sup>-silicon with a buried solid-state
junction, exhibited a more pronounced shift of the current density–potential
curves upon the introduction of the nanostructured surface owing to
the corresponding increase in the liquid/silicon junction area. Systematic
studies on the morphology, optical properties, and photoelectrochemical
characteristics of nanostructured silicon photocathodes, in conjunction
with optical modeling based on the finite-difference time-domain method,
provide quantitative description and optimal design rules of lamellar-patterned
silicon photocathodes for solar water splitting
Amine Induced Retardation of the Radical-Mediated Thiol–Ene Reaction via the Formation of Metastable Disulfide Radical Anions
The
effect of amines on the kinetics and efficacy of radical-mediated
thiol–ene coupling (TEC) reactions was investigated. By varying
the thiol reactant and amine additive, it was shown that amines retard
thiyl radical-mediated reactions when the amine is adequately basic
enough to deprotonate the thiol affording the thiolate anion, e.g.,
when the weakly basic amine tetramethylethylenediamine was incorporated
in the TEC reaction between butyl 2-mercaptoacetate and an allyl ether
at 5 mol %, the final conversion was reduced from quantitative to <40%.
Alternatively, no effect is observed when the less acidic thiol butyl
3-mercaptopropionate is employed. The thiolate anion was established
as the retarding species through the introduction of ammonium and
thiolate salt additives into TEC formulations. The formation of a
two-sulfur three-electron bonded disulfide radical anion (DRA) species
by the reaction of a thiyl radical with a thiolate anion was determined
as the cause for the reduction in catalytic radicals and the TEC rate.
Thermodynamic and kinetic trends in DRA formations were computed using
density functional theory and by modeling the reaction as an associative
electron transfer process. These trends correlate well with the experimental
retardation trends of various thiolate anions in TEC reactions