Advanced optical characterization of nanostructures and nanomaterials in the X-ray range

The electromagnetic radiation has given rise to a number of diagnostics techniques covering a broad range of the wave lengths that are of vital importance for materials science. In particular, their non-destructive character and possibility to perform time-resolved studies under various external conditions (temperature, ambient, electrical and/or magnetic fields, etc.) render these techniques very attractive. Reduction of the typical size of inspected nanoobjects down to nanometers and even below triggered by the development of nanotechnology and nanoscience favours application of the techniques confined to X-ray range, in particular to hard X-rays of the wavelength of the order of 0.1 nm. Here, the grazing-incidence small-angle X-ray scattering (GISAXS) is one of the most important methods allowing to obtain statistically relevant parameters in terms of the position, correlations and size distribution of nanoobjects such as supported nanoparticle assemblies or fractal and correlation properties of the rough surfaces and interfaces. The GISAXS technique probes the nanostructure in a selected part of the reciprocal space defined by the experimental parameters that is projected onto a 2D detector as GISAXS pattern. The GISAXS information is thus complementary to that provided by the real-space investigation methods such electron microscopy or scanning probe techniques which are local by nature. On the other hand, the transformation from the reciprocal to real space and adoption of an appropriate model is necessary for GISAXS. As the GISAXS signal scales with the amount of the probed material and is inherently weak for nanostructures, the X-ray beam flux is an important issue. Recent development of the high-intense liquid-jet Ga X-ray sources has allowed to reach a total flux of 10⁹-10¹⁰ ph/sin laboratory. However, the synchrotron radiation is necessary in particular cases. In this contribution, we exemplify applications of GISAXS on in-situ studies of the cluster formation by the nucleation and growth as well as on 2D and 3D self-assembly and re-assembly phenomena of chemically synthesized nanoparticles. Such large-area ordered 2D and 3D nanoparticle assemblies of macroscopic dimensions built on the simple and effective “bottom-up” approach have potential applications, e.g., in plasmonics and smart sensors. We will illustrate how the time-resolved in-situ GISAXS technique helps to clarify physical and chemical processes behind the nanoparticle self-assembly. We utilized metallic nanoparticles with the size 7-8 nm and covered by an organic surfactant. We applied the GISAXS technique in the fast-tracking mode with ms temporal resolution to analyze the immediate response of the self-assembled layer of the silver nanoparticles confined at the air/water interface to the compression in a Langmuir-Blodgett trough. In this way we could identify all relevant intermediate stages during the nanoparticle layer compression and expansion including those far from the equilibrium. A new non-equilibrium phase preceding the monolayer-bilayer transition was found that was inaccessible by the competing direct space techniques (SEM, TEM) due to the high water vapour pressure and surface tension. The studies were completed by the UV-VIS reflectometry, Brewster microscopy and imaging ellipsometry. In the second example, the in-situ GISAXS was applied to study the copper clusters growth on a graphene monolayer in real time. Such a metal growth on graphene is one of the hot topics aiming at improving and manipulating the electronic and magnetic properties of graphene via metal atom adsorption or doping. In this way it is possible to prepare hybrid materials with possible applications in catalysis, nanoelectronics, optics and nanobiotechnology. We thermally evaporated copper on to a graphene monolayer on a silicon substrate in a dedicated UHV vacuum chamber equipped with the micro focus X-ray source and 2D X-ray detector assembled in GISAXS configuration. A fast repeated collection of 2D GISAXS patterns in the form of a movie enabled us to inspect the growth of Cu clusters from the very early stage of cluster formation up to the full coverage of the graphene layer. The ex-situ AFM measurements of Cu cluster morphology at different growth stages completed the GISAXS study while Raman measurements showed insensitivity of the graphene monolayer to the cluster growth, suggesting weak interaction between graphene and copper.