Understanding and Controlling the Crystallization Process in Reconfigurable Plasmonic Superlattices

The crystallization of nanomaterials is a primary source of solid-state, photonic structures. Thus, a detailed understanding of this process is of paramount importance for the successful application of photonic nanomaterials in emerging optoelectronic technologies. While colloidal crystallization has been thoroughly studied, for example, with advanced in situ electron microscopy methods, the noncolloidal crystallization (freezing) of nanoparticles (NPs) remains so far unexplored. To fill this gap, in this work, we present proof-of-principle experiments decoding a crystallization of reconfigurable assemblies of NPs at a solid state. The chosen material corresponds to an excellent testing bed, as it enables both in situ and ex situ investigation using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atomic force microscopy (AFM), and optical spectroscopy in visible and ultraviolet range (UV-vis) techniques. In particular, ensemble measurements with small-angle XRD highlighted the dependence of the correlation length in the NPs assemblies on the number of heating/cooling cycles and the rate of cooling. Ex situ TEM imaging further supported these results by revealing a dependence of domain size and structure on the sample preparation route and by showing we can control the domain size over 2 orders of magnitude. The application of HAADF-STEM tomography, combined with in situ thermal control, provided three-dimensional single-particle level information on the positional order evolution within assemblies. This combination of real and reciprocal space provides insightful information on the anisotropic, reversibly reconfigurable assemblies of NPs. TEM measurements also highlighted the importance of interfaces in the polydomain structure of nanoparticle solids, allowing us to understand experimentally observed differences in UV-vis extinction spectra of the differently prepared crystallites. Overall, the obtained results show that the combination of in situ heating HAADF-STEM tomography with XRD and ex situ TEM techniques is a powerful approach to study nanoparticle freezing processes and to reveal the crucial impact of disorder in the solid-state aggregates of NPs on their plasmonic properties.

Plasmonic Nanocrystal Arrays on Photonic Crystals with Tailored Optical Resonances

Hierarchical plasmonic-photonic microspheres (PPMs) with high controllability in their structures and optical properties have been explored toward surface-enhanced Raman spectroscopy. The PPMs consist of gold nanocrystal (AuNC) arrays (3rd-tier) anchored on a hexagonal nanopattern (2nd-tier) assembled from silica nanoparticles (SiO₂NPs) where the uniform microsphere backbone is termed the 1st-tier. The PPMs sustain both photonic stop band (PSB) properties, resulting from periodic SiO₂NP arrangements of the 2nd-tier, and a surface plasmon resonance (SPR), resulting from AuNC arrays of the 3rd-tier. Thanks to the synergistic effects of the photonic crystal (PC) structure and the AuNC array, the electromagnetic (EM) field in such a multiscale composite structure can tremendously be enhanced at certain wavelengths. These effects are demonstrated by experimentally evaluating the Raman enhancement of benzenethiol (BT) as a probe molecule and are confirmed via numerical simulations. We achieve a maximum SERS enhancement factor of up to ∼10⁸ when the resonances are tailored to coincide with the excitation wavelength by suitable structural modifications.