Mesoscopic materials studied with advanced X-ray scattering methods

Abstract

Mesoscopic materials form the bridge between the quantum world of atomic systems and the classical world of macroscopic systems. The characteristic length scale of such structures is in the range of hundreds of nanometers which is comparable to visible light wavelengths. This proximity often leads to unusual optical and conducting properties that are not observed in the macroscopic systems. The possibility of using such unique materials in electronics, photonics, and nanotechnology motivates the growing scientific interest to mesoscopic physics. The physical, mechanical and chemical properties of the mesoscopic materials originate in their nanostructure. In-situ probing of the structure and dynamics of mesoscale systems is a challenging experimental problem. Due to short wavelength and high penetration depth X-rays offer a great opportunity for structural studies of mesoscopic objects. New generation of X-ray sources such as synchrotrons or free-electron lasers offers a variety of powerful tools such as X-ray nanodiffraction, grazing-incidence small-angle X-ray scattering, angular X-ray cross-correlation analysis and time-resolved X-ray diffraction. The present Thesis is focused on applying these methods to mesoscopic systems and includes three separate projects. In the first project, X-ray nanodiffraction is used to study domains and domain boundaries in mesocrystalline superlattices of PbS nanocrystals. This method was complemented with novel angular X-ray cross-correlation analysis which unraveled the orientational order inside the domains and near the domain boundaries. The second project focuses on the structural evolution of the polystyrene colloidal crystals under dry sintering conditions studied using in-situ grazing incidence X-ray scattering. The detailed analysis of the Bragg peaks from the colloidal crystal allowed to reveal the colloidal particle shape transformation during heating of the sample. These two experiments were performed at P10 beamline at PETRA III synchrotron radiation source. The third project is devoted to the studies of the infrared laser-induced dynamics in the colloidal crystal using a pump-probe setup at an X-ray free-electron laser. Colloidal crystals were pumped with infrared laser of varying power and the subsequent dynamics was measured with picosecond time resolution. Depending on the pump laser intensity two regimes of laser-matter interaction were accessed. For low pump laser intensity, the vibrational modes excited in the colloidal crystal were analyzed using Lamb theory. Higher pump laser intensities resulted in the generation of a unique periodic plasma in the sample. Combined simulation of the femtosecond plasma dynamics and a hydrodynamic shock wave were performed to explain the experimental data.