Abstract
Specular neutron reflectivity is a neutron diffraction technique that provides information about the structure of surfaces or thin films. It enables the measurement of the neutron scattering length density profile perpendicular to the plane of a surface or an interface, and thereby gives access to the profile of the chemical composition of the film. The wave-particle duality allows to describe neutrons as waves; at an interface between two media of different refractive indexes, neutrons are partially reflected and refracted by the interface. Interferences can occur between waves reflected at the top and at the bottom of a thin film at an interface, which gives rise to interference fringes in the reflectivity profile directly related to its thickness. The characteristic sizes that can be probed range from 5Å to 2000 Å. Neutron-matter interaction directly occurs between neutron and the atom nuclei, which enable to tune the contrast by isotopic substitution. This makes it particularly interesting in the fields of soft matter and biophysics. This course is composed of two parts describing respectively its principle and the experimental aspects of the method (instruments, samples). Examples of applications of neutron reflectometry in the biological domain are presented by Y. Gerelli in the book section “Applications of neutron reflectometry in biology”.
Highlights
Neutron reflectometry (NR) is a surface technique that enables to measure the thickness and the chemical composition of one or several thin layers at a surface or an interface
We will only focus on the specular reflectivity that corresponds to the case where the incident and reflected neutron beams are symmetric with respect to the surface normal
The inter-fringes distance is linked to the thickness of the layer and their amplitude comes from the difference of scattering length density (SLD) between the monolayer and both air and substrate
Summary
Neutron reflectometry (NR) is a surface technique that enables to measure the thickness and the chemical composition of one or several thin layers at a surface or an interface. This is due to some features of the neutron-matter interaction, as detailed in the following. Neutrons are very weakly absorbed by matter, except for few atoms of the periodic table that have a huge neutron absorption cross-section (for instance boron, gadolinium, or lithium) This weak interaction makes possible the study of buried interfaces or to achieve in situ experiments in various sample environments. It is important to keep in mind that it is advantageous to combine NR with other surface techniques like X-ray reflectivity, ellipsometry, and with techniques enabling to determine in-plane structure information like either scattering methods (GISAS, surface diffraction, off-specular measurements), or microcopy techniques (AFM, Brewster microscopy) This course is divided into two parts.
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