Self-assembly of gold nanoparticles into supercrystals, hybrid systems, and binary superstructures

Abstract

This thesis is oriented towards the fabrication of long-range self-assembled nanoparticles (NPs) into supercrystals or superstructures and their transfer to substrates that preserve their structure and properties. The self-assembly of nanoparticles represents one of the most important discoveries of the last few years since it can lead to new controlled collective properties. Control over the size, shape, surface chemistry, and other factors is crucial for developing new technological applications. Gold nanoparticles (Au NPs) represent one example of the most used materials for self-assembly supercrystals. New advances in colloidal chemistry have enabled the manipulation of synthesis parameters to generate Au NPs with different shapes and sizes. In this context, designing, optimizing, and fabricating new supercrystals or hybrid systems with enhanced plasmonic properties could influence various research areas including fundamental studies, plasmonics, catalysis, and sensing. The work described in this thesis can be separated into three parts. In the first part of this thesis, the optimization parameters for synthesizing high concentrations and high-quality spherical gold nanoparticles with sizes ranging from 25 nm to 120 nm were investigated. Their self-assembly into long-range AuNP supercrystals was studied in detail by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It was shown that the quality and length of the supercrystals are highly influenced by the size and concentration of the Au NPs used as building blocks. An extensive SEM analysis revealed the coexistence of two different growth modes: layered growth, and island growth modes. Surprisingly, modifying the polymer coating of large particles resulted in a phase transition from a hexagonal-close packed to a cubic-close packed structure. These results contribute to understanding the self-assembly of Au NPs and open the way for new kinds of fabricating collective plasmonic platforms with future applications in catalysis, photonics, and surface-enhanced Raman spectroscopy. In the second part, the self-assembly of the Au NPs using porous silicon membranes as a host for the self-assembly of 2D and 3D arrays within their pores was investigated. Optimization conditions to control the self-assembly of Au NPs into the pores of porous silicon membranes are explored. Various parameters are key in this process. Especially, particle size and surface chemistry, as well as centrifugation speed and time. Since the two different configurations achieved in this work are homogeneous and reproducible, they could represent a good choice for using them as surface-enhanced Raman (SERS) substrates. Particularly, the SERS spectra obtained from the self-assembled Au NPs arrays display a good homogeneity. This work provides guidelines for the generation of long-range hybrid systems with high quality and tunability, offering an approach for the development of porous silicon hybrid materials with unique characteristics and future applications in catalysis or as SERS active substrates. In the last chapter, the self-assembly of complementary polyhedral nanoparticles (gold nanotetrahedra, and gold nanoctahedra) into 3D binary tessellating superstructures was investigated. The self-assembly was controlled by the polymer ligand length and depended on the concentration of polyhedral NPs. The shape, morphology, and uniformity of the 3D binary superstructures were studied in detail by using scanning electron microscopy (SEM). These results suggest that metallic polyhedral NPs have the potential to serve as fundamental units for a wide range of materials that exhibit intense light-matter interaction and open a new way for creating metamaterials with unprecedented optical properties. As a whole, this thesis presents significant advancement in both the synthesis and self-assembly of Au NP supercrystals, the generation of hybrid systems composed of Au NPs and pSi membranes, and the self-assembly of 3D binary superstructures. The presented result could have important implications in different fields, such as sensing, light-matter coupling, SERS substrates, catalysis, or fundamental studies.