Combined specular and off-specular reflectometry: elucidating the complex structure of soft buried interfaces

Abstract

Neutron specular reflectometry (SR) and off-specular scattering (OSS) are nondestructive techniques which, through deuteration, give a high contrast even among chemically identical species and are therefore highly suitable for investigations of soft-matter thin films. Through a combination of these two techniques, the former yielding a density profile in the direction normal to the sample surface and the latter yielding a depth-resolved in-plane lateral structure, one can obtain quite detailed information on buried morphology on length scales ranging from the order of ångströms to ∼10 µm. This is illustrated via quantitative evaluation of data on SR and OSS collected in time-of-flight (ToF) measurements of a set of films composed of immiscible polymer layers, protonated poly(methyl methacrylate) and deuterated polystyrene, undergoing a decomposition process upon annealing. Joint SR and OSS data analysis was performed by the use of a quick and robust originally developed algorithm including a common absolute-scale normalization of both types of scattering, which are intricately linked, constraining the model to a high degree. This, particularly, makes it possible to distinguish readily between different dewetting scenarios driven either by the nucleation and growth of defects (holes, protrusions etc.) or by thermal fluctuations in the buried interface between layers. Finally, the 2D OSS maps of particular cases are presented in different spaces and qualitative differences are explained, allowing also the qualitative differentiation of the in-plane structure of long-range order, the correlated roughness and bulk defects by a simple inspection of the scattering maps prior to quantitative fits.

Structural Characterisation of Nanoparticle-Supported Lipid Bilayers by Grazing Incidence X-ray and Neutron Scattering

“…The addition of hPOPC caused both an increase in the intensity of the off-specular scattering and a shift in the Qz position of the resonances (Figure 4A). Simulations of the off-specular data (27) reproduced qualitatively the signal change observed for the NP monolayer before and after addition of lipids (Figure 4B) confirming the validity of the specular reflectivity model used to fit the data. Moreover, the off-specular scattering simulations supported the GISAXS results indicating better ordering in the larger NP arrays.…”

2D reflectometry for the investigation of polymer interfaces: off-specular neutron scattering

“…This could potentially be improved by spin coating on mica, which does not need to be be polished. Actually, concerning polar forces the more stable system would be the inverted system with the PMMA in contact with the silicon support due to the higher polar forces of PMMA as explained in our previous work [20]. Therefore in the current system any defect would tend to grow.…”

“…SNR data was normalized to the incident beam spectrum by using COSMOS [17] and OSS data was normalized by using LAMP [18] and transformed into momentum space by using Överlåteren [19], if needed for visualization. OSS fitting was done in λ vs. 2θ space using the here described algorithm [20]. SNR fitting was done in q-space with the algorithm described herein.…”

“…In-plane correlation lengths can be extracted with the use of the distorted-wave Born approximation (DWBA) [20], assuming in-plane exponentially decaying correlations of scattering length density (SLD) deviations at distances greater than the correlation length ξ. The consequence of this correlation length in real space is a cut-off at a given wavevector q min = 2π/ξ [3].…”

“…We refer the interested reader to Ref. [20] for a detailed description of this form factor. It is mainly composed of the Fresnel transmission and reflection amplitudes at all interfaces and oscillating Laue functions depending on all layer thicknesses.…”