Applications of Neutron Reflectivity Measurements to Nanoscience: Thin Films and Interfaces
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Applications of
Neutron Reflectivity Measurements to Nanoscience: Thin Films and Interfaces John F. Ankner and Hartmut Zabel
Abstract Neutron reflectivity has matured in recent years from an exotic method used only by a few experts to an essential tool for the investigation of thin films and interfaces on the nanoscale. In contrast to x-ray reflectivity, which provides electron density profiles, neutron reflectivity reveals the nuclear density profile. This is an essential difference when exploring hydrogenous materials such as polymers, Langmuir–Blodgett films, and membranes. Furthermore, neutrons carry a magnetic moment that interacts with the magnetic induction of the film, revealing, in addition to the nuclear density profile, the magnetic density profile in layers and superlattices. Recent developments in the analysis of off-specular neutron reflectivity data enable the characterization of chemical and magnetic correlations within the film plane on nanometer to micron length scales. A new generation of pulsed neutron sources, featuring flux enhancements of factors of 10–100 over existing sources, will make these types of measurements even more exciting, while kinetic studies, pump-probe, and small-sample experiments will become feasible, opening new windows onto nanoscale materials science. Keywords: nanoscale, neutron reflectivity, neutron scattering, thin films.
Neutrons and Matter Thin films surround us: abrasive, corrosion-resistant, and antireflective coatings; magnetic-storage sensors and disks; the transistors in semiconductor chips; paints. Their functionality generally requires multiple layer deposition. There are a number of experimental methods available for film thickness determination, but for detailed structural analysis of density profiles, interface structures, layer roughnesses, and depth-resolved magnetization profiles, neutron and x-ray reflectivity provide unique capabilities. Reflectivity measurements imply small-angle scatter-
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ing with glancing incident and reflected angles and correspondingly small wave vector transfers, Qz 5 nm–1, in the direction normal to the surface (specular reflectivity). In-plane structure is probed by specifying, in addition, a small Qx component parallel to the surface (offspecular scattering). Reflectivity measurements are sensitive to density variations in the layered structure and not to crystalline periodicity. For reflectivity measurements, the crystalline, amorphous, glassy, or liquid state of the sample does not matter.
Neutron specular reflectivity can be understood in terms of a refractive index n, related to the quantum mechanical potential step U that the neutron encounters upon entering a surface, and the neutron’s kinetic energy E: n (1 – U/E)1/2. The neutron–nuclear potential Un is proportional to the number density (N) and average coherent scattering length (b) of a material’s constituent nuclei: Un Nb. One can then use the Schrödinger equation and conventional optics to quantify the neutron reflection process.1 Neutron sensitivity
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