NiO的异常X射线激发发光光谱表明d-d电荷转移激子的自陷
Unusual X-ray excited luminescence spectra of NiO suggestive of a self-trapping of the d-d charge transfer exciton
摘要：
研究了NiO在真空紫外（VUV）和软X射线（XUV）激发下的荧光光谱。光致发光（PL）光谱显示宽的发射带集中在约2.3和3.2 eV。PL激发（PLE）光谱演化和寿命测量表明，两种机制具有短和长的衰减时间，归因于d（$e_g$）-d（$e_g$）和p（$π$）-d电荷转移（CT）跃迁分别在4- 6 μ，eV范围内，是所观察到的发射的原因，而最强的p（$σ$）-dCT跃迁在7.eV处似乎是一种弱的PL激发机制。在4- 7.eV范围内记录的PLE谱与RIXS和反射率数据一致。利用XUV激发可以避免表面效应在发光中的主导作用，并揭示了具有令人困惑的隔离良好的非常窄的线的双峰的体发光，其具有接近3.3eV的能量，其特征在于由耦合的Jahn-Teller Ni $^+$和Ni $^{3+}$中心形成的自陷\emph{d} - \emph{d} CT激子中的复合跃迁。这一结论得到了NiO和固溶体Ni $_{x}$ Zn $_{1-x}$ O的发光光谱的比较分析以及不同\emph{p} - \emph{d}和\emph{d} - \emph{d} CT跃迁及其弛豫通道的综合团簇模型指认的支持。据我们所知，这是首次观察到\emph{d} - \emph{d} CT激子的自陷。我们的论文表明，时间分辨发光测量提供了一个指导性的工具，阐明\emph{p} - \emph{d}和\emph{d} - \emph{d} CT激发和弛豫在三维氧化物。
英文摘要：
Luminescence spectra of NiO have been investigated under vacuum ultraviolet (VUV) and soft X-ray (XUV) excitation. Photoluminescence (PL) spectra show broad emission bands centered at about 2.3 and 3.2 eV. The PL excitation (PLE) spectral evolution and lifetime measurements reveal that two mechanisms with short and long decay times, attributed to the d($e_g$)-d($e_g$) and p($π$)-d charge transfer (CT) transitions in the range 4-6\,eV, respectively, are responsible for the observed emissions, while the most intensive p($σ$)-d CT transition at 7\,eV appears to be a weak if any PL excitation mechanism. The PLE spectra recorded in the 4-7\,eV range agree with the RIXS and reflectance data. Making use of the XUV excitation allows us to avoid the predominant role of the surface effects in luminescence and reveal bulk luminescence with puzzling well isolated doublet of very narrow lines with close energies near 3.3\,eV characteristic for recombination transitions in self-trapped \emph{d}-\emph{d} CT excitons formed by coupled Jahn-Teller Ni$^+$ and Ni$^{3+}$ centers. This conclusion is supported both by a comparative analysis of the luminescence spectra for NiO and solid solutions Ni$_{x}$Zn$_{1-x}$O, and by a comprehensive cluster model assignement of different \emph{p}-\emph{d} and \emph{d}-\emph{d} CT transitions, their relaxation channels. To the best of our knowledge it is the first observation of the self-trapping for \emph{d}-\emph{d} CT excitons. Our paper shows the time resolved luminescence measurements provide an instructive tool for elucidation of the \emph{p}-\emph{d} and \emph{d}-\emph{d} CT excitations and their relaxation in 3d oxides.
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