Silver Nanocrystals Differing By Their Coating Agents : Unexpected Behaviors

Over the last few decades, nanotechnology, which is the development of new methods for synthesizing and manipulating discrete nanoparticles and larger nanostructured assemblies, have made great progress. Nanoparticles provide a high surface-to-volume ratio (compared to the bulk phase), representing an important state of condensed matter, hence exhibiting novel physical and chemical properties that are not observed in their bulk state. Self-organization of spherical particles into two-dimensional (2D) and three-dimensional (3D) superlattices is commonly observed in nature, where nanoparticles having similar diameters and very narrow size distributions can self-organize into 2D or in 3D ordered structures, also called supracrystals. Furthermore, binary nanocrystal superlattices, which are co-assembled from two distinct series of nanoparticles, have also emerged. Compared to the limited phase structures formed in the single-component nanoparticle superlattices, a variety of phase structures of binary nanocrystal superlattices can be produced, and the prediction of these phase structures mainly relies on the space-filling principles. However, in addition to the well-known crystal structures formed with hard sphere model, such as AlB2, NaZn13, NaCl, and laves phases in binary superlattices, other phases such as CuAu-type, Cu3Au-type, CaB6-type as well as quasicrystalline ordering were also discovered in binary nanocrystal superlattices, where nanoparticle interactions must be considered.In this manuscript, recent advances in the synthesis and optical properties of Ag nanocrystals and their assemblies into 2D/3D superlattices will be reviewed in Chapter 1. Chapter 2 focuses on the SPR response from 2D self-assemblies of both Ag and Au spherical nanocrystals. The collective SPR bands observed in the measured absorption spectra are described and compared to those calculated using dipole approximation (DDA) method. The similar results provided by these two approaches show that the theoretical DDA methods are suitable for the simulation of linear optical response of 2D superlattices made of such ultrafine metal nanocrystals and can be used to model and predict their optical responses. The influence of the nanocrystals coating agent and its influence on the validity of DDA simulations is also discussed. Chapter 3 describes the optical properties of silver nanocrytals when dispersed in a solvent. The SPR band of those nanocrystals is shown to be dependent not only on the nanaocrystal diameter, but also on the nature of the coating agent. In Chapter 4, the fabrication of thin supracrystalline films made of Ag nanocrystals is explained, and the optical properties of the as-obtained films composed of nanocrystals with varying diameters and coating agents studied. By comparing the SPR spectra of dispersed Ag nanocrystals with their assemblies of supracrystalline films, marked changes are observed. The energy discrepancy (ΔE), determined from the difference in SPR band between dispersed nanocrystals and their corresponding assemblies is shown to be mainly dependent on both the nanocrystals size and the interparticle distances within the supercrystalline films. These results also show a good agreement between simulated optical properties of 2D superlattices with data acquired with 3D thin films assembled in fcc crystalline structure. Furthermore, it is shown that the interparticle distance of nanocrystals, modulated by the alkyl chain length of the coating agents, could effectively tune the SPR band position. Chapter 5 is related to the fabrication of binary nanocrystal superlattices. Colloidal binary nanocrystal mixtures containing two distinct nanocrystals with either the same surface coating agent, called single ligand system, or with two different surface coating agents, refered as multiple ligands system, are used to grow binary nanocrystal superlattices.