Resolving and imaging ultra-low H concentrations in partially protonated Mg:α Al2O3 using FRET and the luminescence lifetime of Cr3+†

Measuring hydrogen concentrations in solid materials, in situ, at the low parts-per-million or parts-per-billion level is extremely challenging, with such concentrations being at or below detection limits of commonly used spectroscopic, ion beam and nuclear methods. Even if hydrogen is detectable, analyses may suffer from poor spatial resolution, precision and accuracy. Rather than measuring hydrogen directly, indirect methods may provide an alternative. Here, we present a novel technique where the luminescence lifetime of Cr³⁺, present as a contaminant at trace levels (sub-ppm) is used to determine hydrogen concentrations in single crystals of magnesium-doped corundum. The crystals are partially diffused with hydrogen, therefore providing a range of hydrogen concentrations to test the method. While we cannot currently fully quantify the data, we can estimate that hydrogen concentrations on the order of tens of parts-per-billion are readily detected with spatial resolution ∼1 μm. Results from lifetime imaging are corroborated and compared with ultraviolet-visible-near infrared and Fourier transform infrared spectroscopy. Overall, the method can be used to achieve significantly higher spatial resolution than widely used absorption techniques, with increased sensitivity. A further advantage is that the method can be used to resolve hydrogen concentration distributions in three dimensions.