Sunil K. Sinha

Theory of inelastic x-ray scattering from condensed matter.

We review the basic formation for the inelastic scattering of x-rays by matter. The differential cross sections are related to the scattering function S(q,ω), which in turn can be expressed in terms of various correlation functions and response functions of the electron system. We show how the scattering by electronic excitations may be related to the dielectric function of the electron gas at high energy transfers, while at low energy transfers we derive the cross section for scattering by phonon excitations.

The impact of synchrotron radiation on nanoscience

We shall review the various methods, which are currently in use or planned for the characterization of the structure and dynamics of nanostructured materials. These include X-ray microscopy, microdiffraction, imaging and X-ray scattering. Examples will be presented of nanostructured systems of interest where such studies are being used to study the effects of nanoconfinement in one, two and three dimensions. These include magnetic films, magnetic dots, quantum wells, nanowires, etc.

Surface structure reflectometry with x-rays.

Abstract X-ray surface scattering is rapidly emerging as a popular technique for rapid in situ nondestructive characterization of surfaces and interfaces. Applications have been very widespread and have ranged from studies of film growth and morphology to molecular ordering at solid/liquid and free fluid interfaces. There has also been increased emphasis on the analysis of off-specular surface X-ray scattering to characterize interface roughness and growth morphology.

6. Small-Angle Scattering from Porous Materials

Abstract We review the basic theoretical methods used to treat small-angle scattering from porous materials, treated as general two-phase systems, and also the basic experimental techniques for carrying out such experiments. We discuss the special forms of the scattering when the materials exhibit mass or surface fractal behavior and review the results of recent experiments on several types of porous media. Finally, we discuss SANS experiments probing the phase behavior of binary fluid mixtures or polymer solutions confined in porous materials.

Investigation of surface dynamics on micro- and nanometer scales

A kinematical scattering theory for the interpretation of X-ray photon correlation spectroscopy (XPCS) measurements from fluctuating surfaces is discussed in detail. First a general formulation without approximations is given. Then as an example XPCS measurements of capillary wave dynamics on glycerol surfaces will be presented and compared to the calculations applying the usual approximations. It turns out that these approximations work well if the perpendicular wave-vector transfer is small. The measurements confirm on micrometer length scales the prediction from classical hydrodynamics that all waves are overdamped due to the high viscosity of glycerol. In addition, the measurements show that thermally activated capillary wave motion is slowed down exponentially when the sample is cooled below 273 K. Further, the extension of XPCS measurements down to nanometer scales is discussed.

DIFFUSE X-RAY AND NEUTRON REFLECTION FROM SURFACES AND INTERFACES

Neutron and X-ray reflectometry were initially conceived as tools for studying the average structure of materials in a direction perpendicular to planar surfaces or interfaces. As such, they probed the variation of the intensity of specular scattering at grazing incidence as the momentum transfer perpendicular to the surface was changed. Any offspecular or diffuse scattering observed in these experiments was regarded as a nuisance, i.e. background that simply had to be subtracted to obtain reliable values for the specular reflectivity. More recently, both neutrons and X-rays have been used to study diffuse scattering from surfaces and interfaces in order to learn about lateral inhomogeneities within the reflecting surface. A variety of phenomena ranging from capillary waves on liquids to roughness correlations in the growth of thin films have been studied in this way. Because diffuse scattering is generally weak, the intense beams of X-rays available from modern synchrotron sources have been more widely used in the study of diffuse surface scattering than neutrons. Nevertheless, in certain cases, such as the study of magnetic systems or certain polymer problems, neutrons still have a role to play. In all cases, diffuse scattering observed in reflectometry experiments arises from inhomogeneities in the reflecting medium, and by far the most common cause of such inhomogeneity is surface roughness. Rough surfaces give rise to diffuse scattering when waves (i.e. neutrons or x-rays) scattered from “valleys” and “peaks” of the surface are significantly out of phase with one another. A calculation of the difference in path length for such waves quickly leads to the conclusion that a surface will appear smooth when

Synchrotron X-ray Studies of Molecular Ordering in Confined Liquids

X-ray specular and off-specular reflectivity studies have been carried out to study the density modulations in liquids confined between two smooth silicon mirrors. The special technique as well as the advantages of using high energy and high brilliance synchrotron x-ray beams for carrying out such experiments will be discussed. Results will be presented on the ordering of octamethyl-cyclotetrasiloxane (OMCTS) as a function of the confining pressure, where we find evidence of layering as the gap is decreased from macroscopic down to a few nanometers.