Neutron Reflectometry: A Tool to Investigate Diffusion Processes in Solids on the Nanometer Scale

Abstract

The investigation of self‐diffusion for the characterization of kinetic process in solids is one of the most fundamental tasks in materials science. We present the method of neutron reflectometry (NR), which allows the detection of extremely short diffusion lengths in the order of 1 nm and below at corresponding low self‐diffusivities between 10−25 and 10−20 m2 s−1. Such a combination of values cannot be achieved by conventional methods of diffusivity determination, like the radiotracer method, secondary ion mass spectrometry, quasielastic neutron scattering, or nuclear magnetic resonance. Using our method, the extensive characterization of materials which are in a non‐equilibrium state, like amorphous or nanocrystalline solids becomes possible. Due to the small experimentally accessible diffusion length microstructural changes (grain growth and crystallization) taking place simultaneously during the actual diffusion experiment can be avoided. For diffusion experiments with NR isotope multilayers are necessary, which are chemical homogeneous but isotope modulated films. We illustrate the basic aspects and potential of this technique using model systems of different classes of materials: single crystalline germanium, amorphous silicon nitride, and nanocrystalline iron.

Evidence for the Formation of Nitrogen-Rich Platinum and Palladium Nitride Nanoparticles

“…The resulting profile is proportional to the thickness, density, and composition of layered structures making up the film. NR is analogous to ellipsometry except that it is applicable to metallic films such as the ones investigated in this study; moreover it is sensitive to different isotopes such as 14 N/ 15 N. 52,53 Nitrogen has a significantly larger neutron scattering length ( 14 N = 9.36 × 10 −4 nm) than Pd (5.9 × 10 −4 nm) while being comparable to Pt (9.6 × 10 −4 nm). This large contrast with Pd and difference with N and the likely differences in density of N-rich vs metal-rich films should enable us to identify gradients in density of nitrogen atoms toward the growth surface relative to the bulk of the film.…”

Solid Electrolyte Interphase Architecture Determined through In Situ Neutron Scattering

“…In this letter, we detail studies to explore the porosity and concentration of species within an SEI layer grown on amorphous silicon and probed, in situ, using neutron reflectometry. [7][8][9] Neutron reflectometry (NR) measures the specular reflection of neutrons from a thin film heterostructure as an interference of neutron waves reflected from the hidden interfaces down to the substrate as a function of the wave vector transfer, Q = 4πsin(θ)/λ, perpendicular to the sample surface. The angle of incidence θ is between the incoming neutron beam and the sample surface, and λ is the wavelength of the neutron.…”

Observation of iron diffusion in the near-surface region of magnetite at 470 K

“…Making use of the specific neutron scattering lengths of different isotopes near-surface diffusion can be studied at the nanometer scale using neutron reflectivity. The determination of diffusion lengths below 1 nm was reported for isotopic Si 14 N/Si 15 N and 57 Fe/ 56 Fe multilayer systems [34][35][36][37]. In this kind of experiment, the depth resolution depends on the quality of the interface between the substrate and the isotope film.…”