Noncentrosymmetric Nanoparticles Research Articles

Functional Noncentrosymmetric Nanoparticle-Nanofiber Hybrids via Selective Fragmentation.

Noncentrosymmetric nanostructures are an attractive synthetic target as they can exhibit complex interparticle interactions useful for numerous applications. However, generating uniform, colloidally stable, noncentrosymmetric nanoparticles with low aspect ratios is a significant challenge using solution self-assembly approaches. Herein, we outline the synthesis of noncentrosymmetric multiblock co-nanofibers by subsequent living crystallization-driven self-assembly of block co-polymers, spatially confined attachment of nanoparticles, and localized nanofiber fragmentation. Using this strategy, we have fabricated uniform diblock and triblock noncentrosymmetric π-conjugated nanofiber-nanoparticle hybrid structures. Additionally, in contrast to Brownian motion typical of centrosymmetric nanoparticles, we demonstrated that these noncentrosymmetric nanofibers undergo ballistic motion in the presence of H2O2 and thus could be employed as nanomotors in various applications, including drug delivery and environmental remediation.

Symmetry Breaking in Seed-Mediated Silver Nanorod Growth Induced by Dimethyl Sulfoxide

Engineering symmetry breaking in seed-mediated growth is a fundamental challenge to produce colloidal nanocrystals with controlled morphologies and properties. In this work, we show a simple, aqueous approach to breaking the inversion symmetry of silver nanorods by restricting growth to one end of the pentatwinned gold bipyramid seed. Controlled addition of dimethyl sulfoxide (DMSO) allows us to tune both the symmetry and the length and width of the objects. Simulations and experiments demonstrate the adsorption of DMSO, which displaces interfacial water, reduces binding of surfactant and chloride ions at the gold surface, and slows down the deposition kinetics of silver. Besides showing the potential of DMSO for controlling the synthesis of complex nanostructures, this work opens new perspectives for the study of the physical properties of noncentrosymmetric nanoparticles, e.g., by controlling their plasmon modes and their second-harmonic generation efficiency.

Protecting patches in colloidal synthesis of Au semishells.

Protecting groups are commonly applied in multi-step molecular syntheses to protect one or multiple functional groups from reacting. After the reaction, they are removed from the molecule. In full analogy to this concept, we report the practical and scalable colloidal synthesis of Au semishells using polyphenylsiloxane protecting patches to prevent part of the surface of polystyrene nanoparticles from being covered with Au. After Au deposition, the patches are removed yielding Au semishells. We anticipate that this strategy can be extended to the synthesis of other types of non-centrosymmetric nanoparticles.

Evaluation of the nonlinear response of plasmonic metasurfaces: Miller’s rule, nonlinear effective susceptibility method, and full-wave computation

In this article, the second-harmonic generation (SHG) from gold split-ring resonators is investigated using different theoretical methods, namely, Miller’s rule, the nonlinear effective susceptibility method, and full-wave computation based on a surface integral equation method. The results confirm that Miller’s rule is, in general, not well suited for the description of SHG from plasmonic metasurfaces. On the other hand, the comparison of the nonlinear effective susceptibility method with full-wave computations shows that this method permits us to evaluate second-harmonic (SH) emission patterns from noncentrosymmetric nanoparticles with good accuracy. However, the nonlinear effective susceptibility method fails to reproduce the multipolar nature of the SH emission from centrosymmetric nanoparticles. This shortcoming is attributed to the intrinsic nature of the nonlinear effective susceptibility method, which neglects the exact positions of the nonlinear sources. The numerical implementations of these two methods are also discussed in detail, revealing that the main limitation of the nonlinear effective susceptibility method, aside from the inaccuracy observed in specific cases, is its higher numerical requirements when several emitting directions need to be considered. This limitation stands for most of the numerical methods used for solving Maxwell’s equations at the nanoscale. This work provides clear insight into the limitations and advantages of the different methods available for evaluation of SHG from plasmonic metasurfaces.

Evanescent-Field-Induced Second Harmonic Generation by Noncentrosymmetric Nanoparticles

We demonstrate the excitation of second harmonic radiation of noncentrosymmetric nanoparticles dispersed on a planar optical waveguide by the evanescent field of the guided mode. Polarization imaging reveals information on the orientation of the crystal axis of individual nanoparticles. Interference patterns generated from adjacent particles at the second harmonic frequency are--to the authors knowledge--observed for the first time. The actual form of the interference pattern is explained on the basis of a dipole radiation model, taking into account the nanoparticles' orientation, surface effects, and the characteristics of the imaging optics.

Spontaneous symmetry breaking: formation of Janus micelles

We describe the preparation and solution properties of Janus micelles, i.e., non-centrosymmetric nanoparticles with compartmentalized shells, viaco-assembly of two fully water-soluble block copolymers. They consist of a mixed core of poly(N-methyl-2-vinyl pyridinium iodide) (P2MVP) and poly(acrylic acid) (PAA), and a shell segregated into two sides, consisting of poly(ethylene oxide) (PEO) or poly(acryl amide) PAAm. These Janus particles form spontaneously and reversibly, i.e., association, dissocation, and reassociation can be carefully controlled via parameters, such as polymer mixing fraction, solution pH, and ionic strength. Dynamic (polarized and depolarized) and static light scattering, cryogenic transmission electron microscopy, small angle neutron and X-ray scattering, and two-dimensional nuclear magnetic resonance spectroscopy are used to monitor the micelle formation and to characterize the micellar structure. The Janus particles were found to be ellipsoidal, with a cigar-like overall shape and a disc-like core. This peculiar morphology is driven by the delicate interplay between two opposing forces: an attraction between the oppositely charged core blocks and a subtle repulsion between the water-soluble, neutral corona blocks.