Unusual x-ray excited luminescence spectra of NiO suggest self-trapping of the $d$-$d$ charge-transfer exciton

Luminescence spectra of NiO have been investigated under vacuum ultraviolet (VUV) and soft x-ray (XUV) excitation (DESY, Hamburg). Photoluminescence (PL) spectra show broad emission violet and green bands centered at about 3.2 and 2.6 eV, respectively. The PL excitation (PLE) spectral evolution and lifetime measurements reveal that the two mechanisms with short and long decay times, attributed to the $d\left(e_g\right)$-$d\left(e_g\right)$ and $p\left(\pi \right)$-$d$ charge transfer (CT) transitions in the range 4–6 eV, respectively, are responsible for the observed emissions. The XUV excitation makes it possible to avoid the predominant role of the surface effects in luminescence and reveals a bulk violet luminescence with a puzzling well-isolated doublet of very narrow lines. These lines with close energies near 3.3 eV are attributed to recombination transitions in the self-trapped $d$-$d$ CT excitons formed by the coupled Jahn-Teller Ni${}^{+}$ and Ni${}^{3+}$ centers. The conclusion is supported by a comparative analysis of the luminescence spectra for NiO and solid solution Ni${}_x$Zn${}_{1-x}$O and by a comprehensive cluster model assignment of different $p$-$d$ and $d$-$d$ CT transitions and their relaxation channels. Our paper shows that the time-resolved luminescence measurements provide an instructive tool for the elucidation of the $p$-$d$ and $d$-$d$ CT excitations and their relaxation in $3d$ oxides.

Figure 1

Spectra of the $d$-$d$ and $p$-$d$ CT transitions and intracenter crystal-field $d$-$d$ transitions in NiO. Strong dipole-allowed $\sigma$-$\sigma$ $d$-$d$ and $p$-$d$ CT transitions are shown by thick solid up arrows, weak dipole-allowed $\pi$-$\sigma$ $p$-$d$ transitions are shown by thin solid up arrows, and weak dipole-forbidden low-energy transitions are shown by thin dashed up arrows. Dashed down arrows point to different electron-hole relaxation channels, and dotted down arrows point to PL transitions. The spectrum of the crystal field $d$-$d$ transitions is reproduced from Ref. 12. The right-hand side reproduces a fragment of the RIXS spectra for NiO.[13]

Figure 2

The x-ray diffraction patterns of NiO and rs Ni${}_{0.3}$Zn${}_{0.7}$O solid solutions recorded at room temperature.

Figure 3

The time-resolved PL spectra (VUV excitation) of the (a) cellulose-free and (b) and (c) the cellulose-coated NiO samples: 1, a fast window; 2, a slow window; 3, a time-integrated spectrum. We make use of different units in (a) and (b) than in (c) to underline spectral features and strong differences of the PL intensities.

Figure 4

The time-resolved PLE spectra of (top) the cellulose-free and (bottom) the cellulose-coated NiO samples: 1, a fast window; 2, a slow window; 3, a time-integrated spectrum. Vertical arrows point to $d(e_g$)-$d(e_g$) and $p(\pi$)-$d$ ($t_{2u}\left(\pi \right)\to e_g$ and $t_{1u}\left(\pi \right)\to e_g$) CT transitions, which are the most effective in the PL excitation given $E_{\mathrm{exc}}<$ 7 eV. The solid curve in the bottom panel shows the absorption for the cellulose reproduced from Ref. 21.

Figure 5

PL decay kinetics (VUV excitation) of NiO.

Figure 6

Survey luminescence spectra of the NiO sample under XUV excitation with energy $E_{\mathrm{exc}}$ $=$ 130 eV at T $=$ 7.2 K [fast (1) and slow (2) windows, spectral resolution of 10 meV].

Figure 7

XUV excited luminescence spectra of NiO and solid solution Ni${}_x$Zn${}_{1-x}$O (fast window, spectral resolution of 10 meV). (top) Luminescence spectra of the cellulose-coated NiO and Ni${}_{0.3}$Zn${}_{0.7}$O samples under XUV excitation with energy $E_{\mathrm{exc}}$ $=$ 130 eV at $T=7.2$ K. (bottom) Low-temperature ($T=7.5$ K) luminescence spectra of the Ni${}_{0.2}$Zn${}_{0.8}$O and Ni${}_{0.6}$Zn${}_{0.4}$O cellulose-free samples under XUV excitation with energy $E_{\mathrm{exc}}$ $=$ 130 eV (solid line) and $E_{\mathrm{exc}}$ $=$ 450 eV (dashed line). Luminescence spectra of Ni${}_{0.6}$Zn${}_{0.4}$O under XUV excitation with energy $E_{\mathrm{exc}}$ $=$ 130 eV at $T=7.2$ K (solid line) and room temperature (dash-dotted line).

Figure 8

Illustration of the formation of spin-singlet and spin-triplet $S$- and $P$-type $d$-$d$ CT excitons in NiO. Arrows point to the orientation of the electric dipole moment.