Resonant neutron reflectometry for hydrogen detection

May 02, 2022

New neutron scattering method enables fast and precise determination of the hydrogen content in thin-film structures and electronic devices.

Hydrogen as a green fuel will play a central role in future energy management. The production of hydrogen by electrolysis, the efficient storage in solid materials, and the conversion into electrical energy in fuel cells is based on the interaction of hydrogen at the surface of electrodes and storage materials. Understanding and optimizing these technologies requires quantitative information about the hydrogen concentration inside materials on nanometer scales, in particular close to the surface and in thin films.

A second rapidly developing research frontier is taking advantage of hydrogen intercalation to modify the electronic properties of solids and solid-state devices. Prominent examples include targeted modification of the lattice architecture and doping level of quantum materials, modulation of the exchange coupling and magnetic anisotropy of magnetic multilayers and devices, and solid state gas sensors. Artificial neural networks, the key component for machine learning algorithms, might gain efficiency by a synapse design with ultra low power consumption. The synapses are programmed by a gating voltage charging or discharging a thin conductive layer with hydrogen to modify its resistivity.

Neutron reflectometry (NR) is a distinguished method for the analysis of hydrogen distributions in thin films. By analogy with the optics of light, NR measures the neutron intensity reflected from the surface of a thin film as a function of the angle of incidence. Experimental reflectivity curves are then modeled to extract the depth dependence of the “neutron optical potential” ρ(z). Injection of hydrogen in the studied sample leads to a modification of ρ(z), which can be traced via the altered reflectivity. Hydrogen concentrations of 5 at.% with a depth resolution of one nanometer can be reliably measured by conventional NR, but real-time experiments remain limited by the required exposition times to slow processes on the scale of minutes to hours.

In this work, the sensitivity of neutron reflectometry for real-time hydrogenation experiments in thin films was substantially increased by taking advantage of waveguide resonances resulting from the formation of neutron standing neutron waves. We demonstrate this technique by reporting in-situ NR experiments on Nb films during hydrogen loading, which show that fast hydrogenation at room temperature can be followed with a sensitivity of ≈1 at.% on a time scale of a few seconds.

Resonance enhancement thus expands the application range of NR by increasing the sensitivity, while retaining some of its unique advantages. In particular, neutron methods yield absolute measurements of the hydrogen concentration, in contrast to X-ray reflectometry or diffraction, which monitor the hydrogenation process indirectly via its impact on the layer thickness. Radiation damage is negligible in contrast to nuclear methods for hydrogen detection that require MeV-energy ions. Another advantage of neutron-based techniques is the possibility to perform in situ studies, while nuclear methods often require high vacuum. With its high sensitivity and its ability to study fast kinetics, resonant neutron reflectometry has the potential to develop into a powerful tool for the microscopic understanding and control of hydrogen in solids.

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