Structural investigation of nanoparticle superlattices by advanced X-ray scattering methods

Abstract

Colloidal crystals are ordered arrays of colloidal particles forming a superlattice similar to that in conventional crystals. In recent decades, the research on colloidal crystals has blossomed due to their fascinating structure-related properties such as diffraction of light at optical wavelengths, high porosity, etc. Moreover, colloidal crystals are often viewed as a model system for conventional crystals since their larger dimensions allow easier tracing of changes in structure. Mesocrystals are a special subclass of colloidal crystals in which the crystalline nanoparticles constituting the crystal are mutually oriented. Such highly ordered nanocrystal superlattices can display functional collective properties distinct from those of conventional colloidal сrystals and individual nanoparticles. These collective properties are conditioned by the interparticle interactions which are highly dependent on the structural features of the mesocrystals. Even though colloidal crystals and mesocrystals have already found many practical applications, the structure-property relationships are still poorly understood due to the lack of suitable methods of structural investigations. This cumulative Thesis is based on six publications and is devoted to the development of X-ray methods for the structural study of colloidal crystals and mesocrystals. In the first three publications, the structure of colloidal crystals and mesocrystals is revealed by analysis of the measured two-dimensional (2D) scattering patterns. The first publication addresses the structural evolution of a thermoresponsive colloidal crystal consisting of gold–poly(N-isopropylacrylamide) core–shell nanoparticles during cooling and heating. The Bragg peak analysis of the data obtained in Ultra-Small-Angle X-ray Scattering (USAXS) experimental geometry provided a unique insight into the processes of crystallization and melting of such colloidal crystals. The second and third publications deal with mesocrystals consisting of inorganic lead sulfide or lead halide perovskite semiconductive nanoparticles stabilized by organic copper tetraaminophtalocyanine (Cu4APc) or oleic acid (OA) ligands. The experimental geometry allowed the simultaneous registration of both Small-Angle X-ray Scattering (SAXS) from the superlattice structure and Wide-Angle X-ray Scattering (WAXS) from the atomic lattices of the constituting nanoparticles. Analysis of the Bragg peaks registered at both small and wide angles at different spatial points of the sample allowed spatially-resolved structural mapping of the mesocrystals. The extracted superlattice unit cell parameters were combined with the angular orientation of the nanoparticles inside the superlattice to obtain the complete structure of the mesocrystal on both length scales. The revealed structures were then correlated with the measured functional properties of the mesocrystals such as conductivity and photoluminescence. In the other three publications, the structural information is extracted by application of Angular X-ray Cross-Correlation Analysis (AXCCA) to the measured intensity distributions in three-dimensional (3D) reciprocal space. The fourth publication contains the details of adapting this method for application to 3D intensity distributions instead of common 2D scattering patterns. In this work, AXCCA was shown prospective for qualitative structure determination in close-packed colloidal crystals using an exemplary sample consisting of spherical silica particles. In the fifth and sixth publications, AXCCA was successfully applied to the measured 3D scattered intensity distributions to reveal the structure of mesocrystals composed of gold and magnetite nanocubes. The proposed approaches to structural studies of colloidal crystals and mesocrystals can be further extended to other materials. The obtained results on the structure of colloidal crystals and mesocrystals and their correlations with the functional properties are highly novel and are of great interest to the materials science community.