Probing the emission properties of color centers in MgAl2O4 wafers using hard X-ray nanoprobes

In this study, we used hard X-ray-excited optical luminescence (XEOL) to investigate the emission properties of MgAl2O4 wafers with a crystal orientation of (100), (110), or (111). MgAl2O4(110) exhibited the lowest XEOL emission intensity because the (110) orientation is thermochemical unstable and therefore unfavorable for generating oxygen vacancies. By fitting XEOL spectra, we identified 13 and 6 clear emission peaks corresponding to the color centers in MgAl2O4 wafers and the 2Eg→4A2g radiative transition of Cr ion impurities, respectively. Using an X-ray nanoprobe, we obtained an X-ray fluorescence spectroscopy (XRF) map of the elemental distribution of Cr with 100 nm spatial resolution. X-ray absorption spectroscopy (XAS) revealed that the valence state of the Cr ion impurities was Cr3+. We thus also clearly demonstrate the advantages of combining XEOL, XRF, and XAS in the X-ray nanoprobe beamline by obtaining emission spectra using XEOL and using XRF and XAS to analyze elemental distributions and valence states to further elucidate the emissions of samples.