S.K. Sinha

X-ray scattering studies of surface roughness of GaAs/A1As multilayers

We discuss the theory of X-ray scattering from multilayers with conformal roughness of the interfaces, and illustrate with an analysis of specular, diffuse and wide-angle scattering from a GaAs/A1As multilayer. This is a highly coherent multilayer structure deposited on a stepped, but otherwise smooth surface. The roughness due to the steps propagates through the layers and a distinct anisotropy is observed in the diffuse scattering. We discuss a method to treat diffuse scattering from such surfaces with slightly irregular steps.

OBSERVATION OF POWER-LAW CORRELATIONS IN SILICA-PARTICLE AGGREGATES BY SMALL-ANGLE NEUTRON SCATTERING

We have studied small angle neutron scattering on normal, compressed and water-suspended powders of aggregates of fine silica particles. The samples studied possessed average densities ranging from 0.009 gm/cm3 to 0.45 gm/cm3. A universal function S(k) assuming power law correlations between particles and a finite correlation length ξ was found to provide a satisfactory fit to the data for all the samples studied. The fractal dimension D corresponding to the power law correlation was 2.52 ± 0.05 for all samples studied. The normalized scattering per particle scales as ξD as required by the model.

Self-consistent density response function method for dynamics of light interstitials in crystals

Abstract We present a method for calculating the coherent scattering function from a collection of diffusing light interstitial particles on a crystal lattice, which interact with each other and with the lattice. The method relies on a self-consistent field calculation of the particle density response function, and is equivalent to an RPA treatment.

Thin polymer films on rough surfaces

Abstract X-ray diffraction measurements in the region of small incidence and exit angles on thin polystyrene (PS) films deposited on laterally structured surfaces are performed. From fits of the data we obtain how the Fourier-components of the substrates are damped by the adsorbed films. The results are compared with theoretical predictions for liquid films. It turns out that PS-films of low molecular weight (⩽300k) behave like frozen liquids. Within films of a molecular weight of 1000k viscoelastic forces dominate over the substrate—adsorbate van der Waals-interactions so that even a 680 A thick film is not totally flat on top of a grating with a height of 130 A.

X-ray off-specular reflectivity studies of electrochemical pitting of Cu surfaces in sodium bicarbonate solution

Abstract We have studied the electrochemically-induced pitting process on a Cu electrode in NaHCO3 solution using in-situ X-ray off-specular reflectivity measurements. The morphology and growth dynamics of the localized corrosion sites or pits were studied as the applied potential was varied from the cathodic region where the Cu surface is relatively free of oxide films to the anodic region where surface roughening occurs by general corrosion with concomitant formation of an oxide film. Quantitative analysis of the experimental results indicates that early pitting proceeds in favor of nucleation of pit clusters over individual pit growth. It was found that the lateral distribution of the pits is not random but exhibits a short-range order as evidenced by the appearance of a side peak in the transverse off-specular reflectivity. The position, height, and width of the peak was modeled to yield the average size, nearest-neighbor distance (within any one of the clusters), and over-all density of the pits averaged over the entire illuminated surface. In addition, measurements of the longitudinal off-specular reflectivity indicate a bimodal depth distribution for the pits, suggesting a “film breaking” type of pitting mechanism.

Neutron scattering from fractals

We describe briefly the concept of fractal dimension as applied to both mathematical and statistical fractals. We then discuss the scattering of radiation from fractals and describe the results of small angle neutron scattering studies of the aggregation of small particles in fractal clusters.

Complementarity of neutrons and X-rays as probes of surfaces and interfaces

Abstract The use of neutron and X-ray reflectivity and diffuse scattering experiments to study the structure of surfaces and interfaces is reviewed. A new method of obtaining model-independent surface density profiles from reflectivity data using anomalous X-ray reflectivity is presented, and some examples are given of diffuse scattering studies of rough solid surfaces and capillary wave fluctuations on liquid surfaces. Finally, the relative advantages and disadvantages of neutrons and X-rays for such measurements are discussed.

Si/Ge films on laterally structured surfaces: An x‐ray study of conformal roughness

X‐ray diffraction measurements in the region of small incidence and exit angles on thin amorphous silicon/germanium films on laterally structured surfaces are performed. From fits of the data we obtain directly how the Fourier components of the substrates propagate through the evaporated films without being influenced by the intrinsic statistical roughness of the interfaces. The results show that a replication factor extracted from a given model can be quantitatively tested with our measurements.

A small angle neutron scattering study of worm-like micelles

The structure of a cationic worm-like micelle at two concentrations, dilute and semi-dilute in aqueous solution, was studied by a combination of small-angle neutron scattering (SANS) and light scattering. It is found that the shape of the micelles can be best described as a rod with a large aspect ratio. SANS data were determined for the systems under shear. For the semi-dilute solution, we observed that the scattering pattern became anisotropic, as expected. Anisotropy was observed in the dilute solution, but only after a threshold shear rate was exceeded. Details of the anisotropic patterns, moreover, depended on the micelle concentration.

Study of phase separation of a critical binary fluid in gel

We have employed small angle neutron scattering to study the phase separation process of a binary fluid in a permanently cross-linked polymer gel. The fluid trapped in a 7 wt. % polyacrylamide network is a mixture of heavy water 2-6 lutidine near its critical composition, This single phase homogeneous mixture at room temperature exhibits a cloud point transition upon heating. Due to the difference in the affinity of polymer to heavy water and 2-6 lutidine, the polymer network may be considered to exert a type of chemical random field on the trapped binary fluid. Our results were analyzed in terms of the random field Ising model and a more recent theory of de Gennes.