Detecting spatial rearrangement of individual gold nanoparticle heterodimers.

The self-assembly of properly surface-modified gold nanorods and spherical gold nanoparticles in aqueous medium results in the formation of heterodimers, which show a unique optical scattering spectrum due to the plasmon coupling between the particles. While for the majority of the heterodimers, both particles are located at the substrate level, occasionally, some spherical particles are found to be located on top of the gold nanorods instead of the supporting substrate. Based on optical measurements on such individual heterodimers, it is shown that in contrast to the plain white-light scattering spectrum, the polarization-resolved spectra allow us to distinguish between the cases when the sphere is located on top or at the side of the nanorods. This finding is utilized to investigate the structure of heterodimers upon formation in situ in aqueous medium. It is demonstrated at the individual heterodimer level that both arrangements can be found upon assembly and that the nanosphere originally located on top of the rod right after assembly can indeed rearrange and move to substrate level during drying. The results underline the importance of low-level in situ characterization approaches in the field of nanoparticle self-assembly and can be utilized to assess the impact of different surface ligands, interfacial layers and liquid environments on the drying of nanoparticle-based systems.

Self-assembly of like-charged nanoparticles into Voronoi diagrams.

The self-assembly of nanoscopic building blocks into higher order macroscopic patterns is one possible approach for the bottom-up fabrication of complex functional systems. Macroscopic pattern formation, in general, is determined by the reaction and diffusion of ions and molecules. In some cases macroscopic patterns emerge from diffusion and interactions existing between nanoscopic or microscopic building blocks. In systems where the distribution of the interaction-determining species is influenced by the presence of a diffusion barrier, the evolving macroscopic patterns will be determined by the spatiotemporal evolution of the building blocks. Here we show that a macroscopic pattern can be generated by the spatiotemporally controlled aggregation of like-charged carboxyl-terminated gold nanoparticles in a hydrogel, where clustering is induced by the screening effect of the sodium ions that diffuse in a hydrogel. Diffusion fronts of the sodium ions and the induced nanoparticle aggregation generate Voronoi diagrams, where the Voronoi cells consist of aggregated nanoparticles and their edges are aggregation-free and nanoparticle-free zones. We also developed a simple aggregation-diffusion model to adequately describe the evolution of the experimentally observed Voronoi patterns.

Preparation and characterization of two-dimensional metallic nanoparticle and void films derived from a colloidal template layer.

A novel and simple bottom-up fabrication method for the realization of metallic nanovoid and metallic film on nanoparticle (dome) array is presented and their optical performance assessed based on experimental and theoretical investigations. The structures are created by a simple, annealing induced replica formation of a template monolayer, which is composed of submicron particles deposited on top of a thin polymer film. Angle and wavelength dependent reflection measurements indicate the possibility to excite Bragg plasmons at the prepared structures. We found an excellent agreement between the measured and simulated reflection curves, but only when the simulated reflection was averaged over several possible azimuthal lattice orientations of the hexagonal unit cell with respect to the plane of incidence.

Identification of Dewetting Stages and Preparation of Single Chain Gold Nanoparticle Rings by Colloidal Lithography.

Massively parallel nanoparticle assembly was carried out by means of colloidal lithographic experiments over a silicon substrate supported (sub)microparticle Langmuir-Blodgett monolayer, using high purity aqueous solution of PEGylated gold nanoparticles. The size of the polystyrene template particles in the monolayer was varied between 608 nm and 2.48 µm, while gold nanoparticles with diameters between 18 and 65 nm were used. Thanks to the PEGylation of the gold nanoparticles, they could be used as tracer objects to follow the drying process. In this way, different dewetting stages could be identified in the confined space between and underneath the template polystyrene spheres. Depending on the concentration of the nanoparticles, the presented approach allows the preparation of single-particle width necklace structures composed of gold particles. At the same time, the high purity of the substrate as well as of the evolved particle rings is preserved and unwanted particle deposition on the substrate surface is minimized.