The heterogeneity of electronic states of transition metal oxides produces a variety of properties, which may be discerned by the measurements of their nanocrystals. The structure and optical spectra of chemically synthesized pure ${\mathrm{Fe}}_3{\mathrm{O}}_4$, $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, and $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanocrystals of different sizes have been investigated. Three types of electronic transitions: ligand to metal charge-transfer transitions, strong magnetic coupled ${\mathrm{Fe}}^{3+}$ ligand field transitions, and pair excitations, occur distinctly in the optical spectra of $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ and $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanocrystals except ${\mathrm{Fe}}_3{\mathrm{O}}_4$. The quantum size effect (for itinerant carriers) and finite size effect (for local magnetic moments) were observed in our nanocrystals, although their bulks are strong electron correlation systems. The localized to delocalized transformation of electrons occurs with time delay in the femtosecond transient absorption spectra of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanocrystal sol, which indicates that heteroresponse times for different transitions agree well with the size-dependent indication of the steady-state absorption bands of these iron oxides nanocrystals. These results are helpful in understanding the relationship among $d\text{−}d$ transition, magnetic pairing, and charge-transfer in transition metal oxides.

Perovskite variants have attracted wide interest because of the lead-free nature and strong self-trapped exciton (STE) emission. Divalent Sn(II) in CsSnX₃ perovskites is easily oxidized to tetravalent Sn(IV), and the resulted Cs₂SnCl₆ vacancy-ordered perovskite variant exhibits poor photoluminescence property although it has a direct band gap. Controllable doping is an effective strategy to regulate the optical properties of Cs₂SnX₆. Herein, combining the first principles calculation and spectral analysis, we attempted to understand the luminescence mechanism of Te⁴⁺-doped Cs₂SnCl₆ lead-free perovskite variants. The chemical potential and defect formation energy are calculated to confirm theoretically the feasible substitutability of tetravalent Te⁴⁺ ions in Cs₂SnCl₆ lattices for the Sn-site. Through analysis of the absorption, emission/excitation, and time-resolved photoluminescence (PL) spectroscopy, the intense green-yellow emission in Te⁴⁺:Cs₂SnCl₆ was considered to originate from the triplet Te(IV) ion ³P₁→¹S₀ STE recombination. Temperature-dependent PL spectra demonstrated the strong electron-phonon coupling that inducing an evident lattice distortion to produce STEs. We further calculated the electronic band structure and molecular orbital levels to reveal the underlying photophysical process. These results will shed light on the doping modulated luminescence properties in stable lead-free Cs₂MX₆ vacancy-ordered perovskite variants and be helpful to understand the optical properties and physical processes of doped perovskite variants.