Block Copolymer at Nano-Patterned Surfaces

We present numerical calculations of lamellar phases of block copolymers at patterned surfaces. We model symmetric di-block copolymer films forming lamellar phases and the effect of geometrical and chemical surface patterning on the alignment and orientation of lamellar phases. The calculations are done within self-consistent field theory (SCFT), where the semi-implicit relaxation scheme is used to solve the diffusion equation. Two specific set-ups, motivated by recent experiments, are investigated. In the first, the film is placed on top of a surface imprinted with long chemical stripes. The stripes interact more favorably with one of the two blocks and induce a perpendicular orientation in a large range of system parameters. However, the system is found to be sensitive to its initial conditions, and sometimes gets trapped into a metastable mixed state composed of domains in parallel and perpendicular orientations. In a second set-up, we study the film structure and orientation when it is pressed against a hard grooved mold. The mold surface prefers one of the two components and this set-up is found to be superior for inducing a perfect perpendicular lamellar orientation for a wide range of system parameters

Organization of Block Copolymers using NanoImprint Lithography: Comparison of Theory and Experiments

We present NanoImprint lithography experiments and modeling of thin films of block copolymers (BCP). The NanoImprint lithography is used to align perpendicularly lamellar phases, over distances much larger than the natural lamellar periodicity. The modeling relies on self-consistent field calculations done in two- and three-dimensions. We get a good agreement with the NanoImprint lithography setups. We find that, at thermodynamical equilibrium, the ordered BCP lamellae are much better aligned than when the films are deposited on uniform planar surfaces

Directing Cluster Formation of Au Nanoparticles from Colloidal Solution

Discrete clusters of closely spaced Au nanoparticles can be utilized in devices from photovoltaics to molecular sensors because of the formation of strong local electromagnetic field enhancements when illuminated near their plasmon resonance. In this study, scalable, chemical self-organization methods are shown to produce Au nanoparticle clusters with uniform nanometer interparticle spacing. The performance of two different methods, namely electrophoresis and diffusion, for driving the attachment of Au nanoparticles using a chemical cross-linker on chemically patterned domains of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) thin films are evaluated. Significantly, electrophoresis is found to produce similar surface coverage as diffusion in 1/6th of the processing time with an ~2-fold increase in the number of Au nanoparticles forming clusters. Furthermore, average interparticle spacing within Au nanoparticle clusters was found to decrease from 2-7 nm for diffusion deposition to approximately 1-2 nm for electrophoresis deposition, and the latter method exhibited better uniformity with most clusters appearing to have about 1 nm spacing between nanoparticles. The advantage of such fabrication capability is supported by calculations of local electric field enhancements using electromagnetic full-wave simulations from which we can estimate surface-enhanced Raman scattering (SERS) enhancements. In particular, full-wave results show that the maximum SERS enhancement, as estimated here as the fourth power of the local electric field, increases by a factor of 100 when the gap goes from 2 to 1 nm, reaching values as large as 10(10), strengthening the usage of electrophoresis versus diffusion for the development of molecular sensors

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