Crystallographic investigation of gold nanoparticles embedded in a SrTiO3 thin film for plasmonics applications.

Metallic nanoparticles represent a widespread subject of study in several different fields of research, such as applied physics (nanotechnology), material science, chemistry and biology. In recent years they have been extensively studied by many authors (Kelly et al. [30], Link and El-Sayed [37], Jackson and Halas [27]) particularly with respect to their optical properties. These are characterized by the presence of the so called localized surface plasmon resonance, which earned the study of metal nanoparticles the name of plasmonics. Their properties have been probed with a wide selection of experimental techniques, ranging from microscopy (TEM, AFM, STM . . . ) to optical spectroscopy and X-ray diffraction, and a large variety of fabrication methods (Pelton et al. [44]) have been explored to obtain systems made up of nanoparticles of various metals. Furthermore, they have been proved to be promising for many different scientific and practical applications (Prasad [45] and Bell [3]), for which both the surrounding medium sensitivity of the plasmonic resonance and the associated local field enhancement play a major role. The morphological and structural features of the metallic nanoparticles are strictly related to their properties in most of the applications mentioned above. These features mainly depend on the particular preparation method exploited for the nanoparticles production: a fundamental understanding of the atomic-scale processes involved in the nanoparticles formation is thus of great importance. The samples object of the present work are composed by anisotropic monocrystalline gold (Au) nanoparticles embedded in a strontium titanate (SrTiO3) thin film. They were prepared through a novel two-steps deposition process, whose main parameters can be varied in order to obtain nanoparticles of different size and shape (Christke et al. [13] and Katzer et al. [29]). This process therefore allows to tune the optical properties of the nanoparticles and it constitutes a valid alternative to other traditionally used fabrication processes, particularly for the production of plasmonic active sensors in life sciences. Nanoparticles prepared with an analogous deposition process have been also exploited as flux pinning centers in high temperature superconducting YBCO thin films (Grosse et al. [23] and Katzer et al. [28]) or for the engineering of YBCO grain boundaries in Josephson junctions (Michalowski et al.[40]). The study here presented consists of a crystallographic characterization realized through synchrotron X-ray diffraction. The aim was to determine the preferred crystallographic orientations of the Au nanocrystals and their interaction with the surrounding SrTiO3 matrix. It represents the first step of a research project addressing the nanoparticles features (such as the shape and the dimension) with an impact on their optical properties and the modifications induced in the surrounding matrix. Samples with different amounts of deposited Au were probed with a hard X-ray beam and two different diffraction setups were used, exploiting both a two-dimensional and a zero-dimensional detector. The main vertical (normal to the substrate) growth direction of the nanoparticles was determined for all the samples, along with the crystalline quality of the SrTiO3 layer. For each vertical growth direction the in-plane orientation of the Au crystals was measured, in order to fully determine their crystallographic orientation with respect to the substrate. The correlation between the nanoparticles orientation and the amount of deposited Au was investigated, trying to understand the role of the SrTiO3 thin film in the nanoparticles formation process. Such a characterization is not only important in the light of the potential practical applications of the samples, but it is also valuable in itself. It indeed offers the possibility of a deeper understanding of the fundamental properties regarding the growth of Au (and transition metals in general) on ceramic substrates, which are still not well explored. The present work is organized as explained hereafter. After a general overview of metallic nanoparticles (Section 1.1), Chapter 1 describes the samples experimentally probed, with particular focus on the preparation methods used and their potential practical applications (Section 1.2). Chapter 2 provides the reader with the main theoretical concepts involved in the measurements presented in the following chapters: Sections 2.1 and 2.2 outline the basics of the X-ray diffraction technique and the properties of transition (face centered cubic) metals deposited on ceramic substrate respectively; then, Section 2.3 describes in detail all the mathematical framework necessary for the description of a diffraction experiment performed with a CCD area detector. Following the presentation of the experimental setup of Chapter 3, the main experimental results obtained are discussed. In particular Chapter 4 presents the preliminary textured analysis undertaken and the main vertical growth direction of the Au nanoparticles (along with the modification induced in the SrTiO3 thin film). Chapter 5 deals with the in-plane investigation of the Au crystals, addressing its relation with the presence of the SrTiO3 layer and the amount of Au deposited. Finally the appendices provide a brief overview of the properties of synchrotron radiation (Appendix A) and some details about the program used to performed the CCD diffraction data reduction (Appendix B). In Appendix C the most important Python scripts used for the data analysis are reported.