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>-poly­(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>-poly­(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 poly­(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>_PMMA = 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>_2D = 0.20 μm²/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 poly­(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⁺-silicon photocathodes, compared to the n⁺pp⁺-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