Understanding and Controlling the Crystallization Process in Reconfigurable Plasmonic Superlattices

Abstract

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.

Active Plasmonics with Responsive, Binary Assemblies of Gold Nanorods and Nanospheres

“…The observed Δλ max (20 nm blue shift) can be attributed to the increase in the nearest neighbor distance between NPs from ~4.4 to 6.8 nm, which weakens the coupling between NSs ( Figure 1 f). It is worth noting that the intrinsic weakness of NS plasmonic properties translates to wide and relatively weak absorption bands, limiting the Δλ max [ 23 ] and highlighting the need to prepare reconfigurable systems made of anisotropic NPs with stronger plasmonic properties.…”

“…One reason is complex couplings in NRs assemblies observed for transverse and longitudinal plasmonic bands; for example, close side-to-side (S-S) or tip-to-tip (T-T) arrangement of NRs will cause a blue or red shift of the signal, respectively [ 24 ]. The second reason is the possible variation of structural parameters of the assemblies over bulk scale, e.g., at domain boundaries [ 23 ].…”

Recent Notable Approaches to Study Self‐Assembly of Nanoparticles with X‐Ray Scattering and Electron Microscopy

“…[ 189 ] Modern instruments offer temperature control in a wide range and this was used in a recent study to study thermal structure‐switching of supercrystals of Au NPs (core d = 4.1 nm) with HAADF‐STEM and complementary in situ and ex situ electron microscopy and small‐angle XRD. [ 190 ] To allow for the thermoresponsive crystallinity of the superstructures, the Au NPs were functionalized with a liquid‐crystal‐like ligand. A bct phase was observed up to 80 °C that underwent a phase transition to a bcc phase at higher temperatures and another phase transition above 130 °C into an fcc phase that was stable up to 215 °C.…”

Self‐assembly of Gold Nanoparticles into an Adjustable Plasmonic 3D Lattice using Janus‐type Forked Mesogenic Ligands

“…Previous work in this direction has produced a range of 2D and 3D ordered assemblies. [8][9][10][11][12][13][14] Mesogens of rod-like, [15][16][17] disk-like, [18][19][20] dendritic, [9,21,22] as well as chiral ligands, [10,11,23] have been employed. Furthermore, mesogenic ligands can have functions additional to those directing the assembly, such as being photoresponsive.…”

“…One way of controlling spatial arrangement of NPs is to coat them with mesogenic ligands. Previous work in this direction has produced a range of 2D and 3D ordered assemblies [8–14] . Mesogens of rod‐like, [15–17] disk‐like, [18–20] dendritic, [9,21,22] as well as chiral ligands, [10,11,23] have been employed.…”