@phdthesis{Kirner2022Noble-57927,
  year={2022},
  title={From Noble Metal Nanoparticles to Mesocrystals : Tuning Crystallization, Structure, and Properties},
  author={Kirner, Felizitas},
  address={Konstanz},
  school={Universität Konstanz}
}

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    <dcterms:title>From Noble Metal Nanoparticles to Mesocrystals : Tuning Crystallization, Structure, and Properties</dcterms:title>
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    <dcterms:abstract xml:lang="eng">Mesocrystals are an amazing class of materials with hierarchical morphology, in which crystallographically aligned nanocrystals form a new structure with single crystal-like diffraction behavior. Noble metal nanoparticles exhibit outstanding optical properties, but mesocrystals made from such nanoparticles have not been studied in detail for a variety of reasons both on nanoparticle and superstructure level. First, the reproducible synthesis of gold nanoparticles with specific and adaptable shape and size in sufficient quantity is challenging. Second, silver nanoparticles are susceptible to oxidation and quite unstable. Furthermore, the targeted assembly of superstructures is not possible with conventional methods such as lithography, since, while the 2D structure can be influenced in a targeted manner, the quality of the nanoparticle shape, surface quality and distance tuning suffer on small scales. More elaborate ordered 3D structures are hardly accessible with such methods. On the superstructure level, the complex composition of these 3D structures poses a special challenge for analytical techniques. The time scale for the superstructure generation spans from milliseconds (nucleation of nanoparticles) to weeks (mesocrystal self-assembly by gas-phase diffusion). The size range is also tremendous; small, interatomic distances in the nanocrystals are not negligible but neither is the characterization of the mesocrystalline structure that can extend to several tens of micrometers. The results from this thesis pave the way for further work on mesocrystalline structures from noble metal nanoparticles. A wide size and shape range of nanocrystals is now made accessible. A novel, nondestructive method for the 3D investigation of superstructures was applied successfully. The selfpurification behavior during the particle-mediated crystallization/ self-assembly process of nanoparticles in mixtures was investigated, setting the groundwork for larger scale applications. The optical properties of individual noble metal nanoparticles and their dimers were compared to simulations and a novel structure property relationship was found. In Section 3.1, a novel seed-mediated synthesis of single crystalline gold nanoparticles of adaptable size and habit was developed. In contrast to the most common CTAB-based syntheses, the gold nanoparticles were stabilized by CPC to ensure a high reproducibility. The nanoparticle sizes were steplessly tunable in the range between 26–100 nm by varying the amount of seed spheres added to the growth solution. These seed spheres could be prepared and kept as stock solutions to ensure rapid on-demand supply with the desired anisotropic nanoparticles. The habit (cubic and octahedral facets) of these nanoparticles was adjustable independently from the particle size by adapting the ratio of the concentration of KBr and ascorbic acid. Such a steplessly tunable synthesis was, to my knowledge, previously unknown. It was found that higher concentrations of KBr led to a stronger expression of {111} facets while higher amounts of ascorbic acid led to a higher ratio of {100} facets, thus implying a critical influence of the additives on the growth kinetics. However, these results contradicted known literature. Consequently, these kinetics were systematically investigated. It was found that the growth by autocatalytic surface reduction must have been significantly larger than reduction in solution so that the latter could be neglected for further evaluation. The data were thus evaluated using the minimalistic Finke-Watzky autocatalytic two-step mechanism, which allows to estimate the rate constants of "pseudoelementary" reactions. These "pseudoelementary" reactions are the reduction of gold ions and growth process of Au nanoparticles. The experimental data match the model in extraordinary detail. The halides Br– (from KBr) and Cl– (from CPC) form complexes with the gold precursor that vary in their stability with chloride containing compounds being less stable. It was found that, while increasing concentrations of reducing agent (AA) accelerated the reduction process, the growth kinetic for low Br– concentrations was mainly influenced by the adsorption of Br– on the {100} facets, favoring their expression. For higher Br– concentrations, the growth mechanism changed. The now dominant CP[AuBr2] complex had an increased tendency to autocatalytic surface reduction on the cubic facets, thus the octahedral facets grew more slowly and dominated. These results allowed the understanding of the particle growth and the prediction of synthesis conditions for the realization of defined size and shape combinations; an option that was not possible with previously known synthesis procedures. To emphasize the high quality of the nanoparticles and their extraordinary, shape dependent optical properties, the plasmonic properties of single particles of different faceting and their assemblies were investigated by EELS. A good agreement between experiment and simulation was found for the spatially localized surface plasmon in single particles whereby dimers additionally revealed an excitation that is related to the correlative response of their dielectric dipoles. This already hints towards extraordinary optical properties of the nanoparticle superstructures. This work provides the basis for further work on self-assembly of such nanoparticles to mesocrystals. In Section 3.2, the previously developed, reproducible nanoparticle synthesis was used and a mesocrystal of 60 nm-sized gold nanocubes was assembled and nondestructively characterized. Previously, only the average (internal) structure of an intact 3D mesocrystal, or thin cuts of specific regions of the mesocrystal could be observed. The mesocrystal was synthesized using a depletion-based approach, and a grain was cut from the central mesocrystal part to perform a detailed structural characterization. This sample was subsequently mounted on a tungsten micromanipulator tip. Using coherent X-ray diffraction imaging, a 3D diffraction map was obtained by stepwise sample rotation and interpolation. First evaluation of the angular averaged X-ray diffraction profile suggested a simple cubic symmetry of the superlattice with a lattice parameter of a … 62 nm. Refined angular X-ray cross-correlation analysis was specifically modified to perform the analysis of a diffraction map in 3D reciprocal space and was able to reveal angular correlations between Bragg peaks. On this basis, a centered monoclinic lattice wassuggested. The 3D diffraction pattern was inverted to a real space 3D electron density map using iterative phase retrieval reconstruction algorithms in order to provide a comprehensive overview of the internal structure of the mesocrystal. The resolution of this reconstruction was sufficient to resolve the structural heterogeneity of the investigated mesocrystal grain - including defects and local deviations of the lattice parameters. Structural features were found that were consistent with the layer growth mechanism of mesocrystals, this growth is proceeding by particle-by-particle attachment to the facet grown parallel to the substrate. Additional shear stress during the mesocrystal’s drying could have induced the formation of additional cracks and voids, which mainly propagated perpendicular to the substrate. The 3D reconstructed electron density map was analyzed with a blob detection algorithm to extract the positions of the individual nanocrystals within the superlattice, reaching a remarkable precision more accurate than 6 nm. As expected, the shift of nanoparticle positions from their averaged value within the superlattice was most significant close to structural defects. A complete superlattice strain map was constructed and the superlattice strain tensor was defined as the deviation of the actually observed structure of a mesocrystal compared to the defect free lattice model as determined by the AXCCA. The strain distribution showed that the average superlattice and the real, “multidomain” structure of a mesocrystal were very close to each other, with a reasonable deviation of less than 10 %. The investigation and characterization of the mesocrystal grain with this novel methodological approach provide an important contribution to the understanding of the fundamental principles of self-assembly, structuring, and the resulting properties of mesocrystals. This publication is not only relevant for presenting a novel example of a nondestructive characterization technique but can also help to improve structural models and subsequently strengthen computational methods. After the detailed structural characterization of a mesocrystal, the assembly kinetics of nanoparticles moved into the focus of this thesis project. Section 3.3 contains a study about non-classical crystallization as method for the self-purification of noble metal nanoparticles mixtures. While several laboratory techniques can be used to purify particle dispersions and narrow their polydispersity, a particle-mediated crystallization offers a principally different approach to achieve the same goal. So far, particle-mediated crystallization methods were mainly used for assembly and purification of either big colloidal particles with sizes larger than 100 nm or relatively small ones with sizes well below 20 nm. In this publication, the size gap in between was filled by the investigation of five batches of Au@Ag nanocrystals of different size (30–62 nm) and shape. The samples exhibited cubes as dominant species, rods as most distinctive other species, and some less defined morphologies. The self-assembly behavior of these samples and their mixtures were investigated by both a drying-mediated approach (“coffee ring” effect) and a dispersionbased depletion approach. The resulting “coffee ring” structures reveal a clear separation depending on the particle shape for all investigated batches. Cubes were assembled in ordered superlattices showing a p4mm plain symmetry of the basal plane, while side-to-side assembled rods were forming separate domains. Odd-shaped particles with no defined packing behavior were excluded from both crystalline sub-domains. The rodlike particles that were much larger than the corresponding cubes were mainly assembled at the outer rim of the “coffee ring” pattern with a transition to the inclusion of rod-clusters within a superlattice of cubic particles for smaller rods. First, this seemed contradictive for a “coffee ring”, but could be explained by capillary forces, geometric constraints and a changing colloidal stability for different particle sizes. To estimate a separation limit for the proposed drying approach, mixtures of particles with selected differences in size or shape were dried. It was found that the assembly behavior of rods agreed with the one in non-mixed samples. For particles with size differences above 20 %, the cubes separated in two crystalline phases. No phase separation occurred for (cubic) particles with smaller size differences and a single, structurally distorted phase was formed. In a second set of experiments, the crystallization process was induced by depletion to eliminate the effect of evaporation dynamics on the self-assembly process. Single batch experiments exhibited a major separation of the individual particle species into ordered superstructures/faceted supercrystals, built from similarly shaped and sized particles. When investigating nanoparticle batch mixtures, 3D faceted colloidal crystals were not built from cubes with edge lengths larger than 34 nm. This indicated that the smaller particle species played a major role in the depletion and probably acted as a secondary depletant that destabilized the other species. For classical crystallization, a high driving force results in rough surfaces and an adhesive type growth mechanism. Theory predicted and the performed experiments confirmed a transition from normal (i.e., adhesive) growth to a two-dimensional nucleation growth and further to a dislocation spiral growth mechanism due to decreasing the driving forces of crystallization for smaller particles. This highlights similarities between nanoparticle self-assembly and classical crystallization by ions and confirms the possibility to use some common techniques and principles to control crystallization processes. This publication deepens the understanding of particle-mediated crystallization and formation of mesocrystals by providing a more conclusive study of how particle mixtures or polydisperse samples assemble and how this assembly proceeds. The work of this thesis is closed with the investigation of optical properties of noble metal nanoparticles in Section 3.4. Silver nanoparticles have astonishing optical properties but are very prone to oxidation/degradation and thus so far rare in plasmonic applications. Furthermore, the surfaces would need protection to survive investigation by e.g., electron microscopy. This publication aimed for a protection of a Au@Ag nanocube surface by silica shell encapsulation under retention of the cubic particle shape, something that was to the best of my knowledge not yet reported. A Stöber process was employed and combined with the use of !-thiol-terminated polyethylene glycol (PEG-SH) as intermediate stabilizing agent for the particles and anchor for the silica. The thickness of the silica shell could be carefully tuned, and Au@Ag@SiO2 nanoparticles with a Au@Ag edge length of 60 nm and silica layers of 8, 12, 17, and 22 nm were investigated. High-resolution EEL spectra of the particles exhibited peaks corresponding to four different LSP modes and the silver bulk plasmon peak. The experimental spectra were in striking agreement with simulated results and included the reproduction of minute spatial details of the localized surface plasmon excitation on length scales down to 5 nm. This not only confirmed a high quality of the silica shells and the dielectrically well-defined silver-silica interface but also proved that the encapsulated Au@Ag nanoparticles were not affected by environmental degradation. LSP modes were classified by employing a cubic harmonic basis derived from the angular momentum classification l of LSP eigenmodes. For small values of l the excitation energies of the LSP modes differed noticeable from the fundamental bulk mode whereby the energy splitting increased with decreasing value of l. By delicately modulating the silica shell thickness, the single particle surface plasmon excitation energies were tuned in a wide energy range between 2.55 and 3.25 eV. This behavior was explained through an adapted dielectric media approach. Furthermore, it was found that the spatially resolved loss probability maps excellently matched the theoretical predictions. Distinct feature of thin silica shells in both experiment and simulation was a strong field enhancement at the vacuum-silica interface of the encapsulated nanoparticles. It was originating from resonant coupling between localized surface plasmons at the silica-silver interface and a whispering gallery resonance at the vacuum-silica interface. This effect strongly favors the application of such nanocomposites for sensing or in photonic crystals. The magnitude of the effect was demonstrated by creating strongly hybridized localized surface plasmon modes at the example of a nanoparticle dimer with an interparticle distance of 16 nm. The findings demonstrate the very real possibility of the stabilization of silver nanoparticles against environmental degradation. The widely tunable range of single particle surface plasmon excitation energies, the strong field enhancement at the vacuum–silica interface, and the strongly hybridized LSP modes allow a glimpse on the astonishing optical properties that may emerge in plasmonically active nanoparticle arrays. This stabilization technique opens the pathway to new application possibilities in sensor technology and signal transmission. The logically following step on this work is the generation of mesocrystals built from differently sized and shaped gold nanoparticles. Preliminary experiments on gold nanocubes and nanooctahedra show a promising assembly behavior for a gas-phase diffusion approach with toluene as dispersing medium, polystyrene as ligand and ethanol as antisolvent. The distance between the individual particles can thereby be carefully adjusted by varying the polymer weight. The resulting structures should be investigated by in situ experiments revealing the assembly and growth processes of mesocrystals. This could significantly improve the understanding and thus the adaptability of the mesocrystal assembly process to the target structure. A detailed investigation of a mesocrystal sample on both the atomic and particle scale needs to be performed. This will offer the possibility to understand the origin of the superstructure’s properties as well as pave the way for a future predictability of properties of a defined structure. A suitable experimental setup could consist of simultaneous SAXS and WAXS measurements as possible e.g., in the European Synchrotron Radiation Facility (ESRF). Such studies may also then reveal building, frequency, and type of crystal defects that can significantly influence a (meso)crystal’s property. Based on the findings in this thesis, the optical properties of mesocrystals are expected to significantly exceed the properties of the corresponding single particles. Packing/internanoparticle distance-depending measurements of these optical properties can deepen the understanding and potentially improve optically active materials such as sensors or metamaterials. Related, the mechanical properties of mesocrystals need to be investigated to allow a better estimate of the range of realistic applications. Preliminary experiments show successful assembly of binary mesocrystals from gold and silver nanoparticles by a depletion-based approach. These structures may contribute to a significant broadening of properties of photonic crystals. This work will just be the seed of knowledge about mesocrystals built from noble metal nanoparticles. Further work should be carried out in the future to grow the topic to a shining area of research.</dcterms:abstract>
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