Insights into Permanent Encodings of Macroscopic Spike Patterns by Magnetic-Field-Directed Evaporative Self-Assembly from Ferrofluids.

Field-directed assembly has the potential to make large hierarchically ordered structures from nanoscale objects. Shear forces and optical, electric, and magnetic fields have been used for this purpose. Ferrofluids consist of magnetic nanoparticles hosted in mobile liquids. Though they exhibit rich structures and lattice patterns in response to an applied magnetic field, the patterns collapse when the field is removed. Recently, we adapted evaporation-induced self-assembly to obtain permanent encodings of the complex field response of magnetite nanoparticles in alkane media. The encodings are characterized by order that culminates in macrostructures comprising kinetically trapped spike patterns. The present work examines a number of variables that control pattern formation associated with this encoding. Control variables include applied magnetic field strength, magnetic field gradient, nanoparticle concentration, solvent evaporation conditions, and alkane solvent chain length. The pattern formation process is captured in six stages of evolution until the solvent host has evaporated and the pattern is permanently fixed. The macropatterns consist of hexagonal arrays that coexist with different pentagonal and heptagonal defects. The Voronoi entropy is calculated for different patterns that arise due to changes in the control parameters. Insight into order in the lattice patterns is achieved by extracting measurables like peak-to-peak spike wavelength, spike population, spike height, and base diameter from the patterns. The pattern measurables depend nonlinearly on the magnetic field gradient, solvent evaporation rate, and solvent chain length. Nanoparticle concentration does not impact the measurables significantly. Nonetheless, the results agree qualitatively with a linear expression for the critical magnetization and wavelength that explicitly contains the field gradient and surface tension.

Resonance Raman Vibrational Mode Enhancement of Adsorbed Benzenethiols on CdSe Is Predominantly Franck-Condon in Nature and Governed by Symmetry.

Here, we report mode-specific resonance Raman enhancements of ligands covalently bound to the surface of colloidal CdSe nanocrystals (NCs). By the systematic comparison of a set of structural derivatives, the extent of resonance Raman enhancement is shown to be directly related to the molecular symmetry of the bound ligands. The enhancement dependence on molecular symmetry is further discussed in terms of Franck-Condon and Herzberg-Teller contributions and their associated selection rules. We further show that resonance Raman may be used to distinguish between possible surface binding motifs of bidentate ligands under continuous wave excitation. More generally, this work demonstrates the usefulness of resonance Raman as a characterization tool when characterizing adsorbed molecular species on semiconductor NC surfaces.

Visible laser self-focusing in hybrid glass planar waveguides.

We report that self-focusing occurs with simultaneous self-inscription of a cylindrical waveguide when 514.5-nm light from a cw argon-ion laser propagates in a solgel-derived silica methacrylate hybrid glass planar waveguide. Spatially localized free-radical polymerization of methacrylate substituents is initiated in the path of the guided wave. This causes intensity-dependent refractive-index changes that lead to self-lensing and focusing. A channel waveguide evolves in the matrix, which supports fundamental and higher-order optical modes and suppresses diffraction of the beam.