Seminars Archive

Specular and off-specular neutron reflectivity of soft buried interfaces: progress towards full 3D quantitative analysis

Abstract
Specular (SNR) and off-specular neutron reflectometry (OSNR) are widely used in investigations of soft systems such as thin polymer films. Additionally to neutrons being a non-destructive probe, one can use isotopic exchange (deuteration) to achieve contrast even between chemically identical species. By combined analysis of SNR and OSNR, the former yielding the density profile in perpendicular direction and the latter the in-plane structure of the interface, one can extract a full 3D morphology of a buried interface. Although OSNR has existed for some time, it has not been explored in soft matter mainly due to the intrinsic limitation of the neutron beam intensity, as compared to X-rays, and to the lack of OSNR analysis tools. The distorted wave Born approximation (DWBA) has been widely used, for X-ray and neutrons, to calculate the intensity scattered in the off-specular direction, both from a single interface [1] as well as multiple interfaces [2, 3, 4]. In such case, off-specular scattering is treated as a small perturbation to the ideal potential, described by a stratified medium with smooth interfaces. Perturbation itself then contains information about the in-plane scattering length density (SLD) deviations from the mean potential and the morphology of the interface, in its simplest form represented by a correlation function. However, an important contribution in the OSNR comes from the interference of neutrons inside the sample arising from the terms describing the SLD deviations, while the in-plane morphology (the shape of the interface) is playing only a minor but nonetheless crucial role in modulating the measured spectrum. As I will discuss in the talk, in order to correctly describe the OSNR of real systems which usually have several interfaces, one must not only fit the off-specular part, but rather both specular and off-specular together, scaling them correctly to one another. This not only allows for separation of contributions from different interfaces (in intensity and shape), but also gives the ability to predict the OSNR in advance based on sample and instrument properties. The simulations will be backed up by quantitative comparison to measurements of polymer bilayer systems performed at ILL. References [1] Sinha, S. K. et al. PRB 38, 4, p. 2297 (1988) [2] Daillant, J. et al. J Chem Phys 97, 8, p. 5824 (1992) [3] Holy, V. et al. PRB 47, 23, p. 15896 (1993) [4] Lauter, V. et al. Reference Module in Materials Science and Materials Engineering (2016)