Control of the Emission and Excitation Energies in Pr³⁺-Activated Perovskite Oxide–Oxynitrides by Bandgap Engineering

Abstract

In this paper, we propose a new concept for the material design for near-ultraviolet (UV)-excited narrow-band phosphors with f−f emissions by bandgap engineering. The perovskite oxide−oxynitride solid solutions, namely, CaTa 1−x Zr x O 2+x N 1−x , were used as host materials to demonstrate our design principle. Photoluminescence (PL) excitation and emission control were systematically performed on Pr 3+ -activated CaTa 1−x Zr x O 2+x N 1−x , where x is in the range of 0.0−1.0. Tuning the PL excitation wavelength was archived over a large wavelength range by tailoring the bandgap of CaTa 1−x Zr x O 2+x N 1−x with different Ta/Zr and N/O ratios. Notably, an intense red emission from Pr 3+ was observed at 614 nm under the near-UV irradiation of 375 nm when the bandgap of the host material CaTa 1−x Zr x O 2+x N 1−x (x = 0.75) was approximately 3.0 eV. Such a red emission peak was assigned to the electron transition between the 1 D 2 and 3 H 4 levels of the Pr 3+ ions. In contrast, when the bandgap was above 3.0 eV, the PL emission spectra were systematically varied with the bandgap of the host materials. Some emission peaks from the electron transition between 3 P 0 and 3 H 4 , 3 H 5 , and 3 F 2 levels were observed in the samples with x = 0.90 and x = 0.95. Our results indicate that the PL properties of the phosphors with f−f emission are systematically controlled based on the bandgap engineering for the host materials.

Relationship between the bandgap energy and photoluminescence properties of Pr3+-activated complex perovskite oxides by cation–nitrogen substitution

“…1 The photoluminescence excitation and emission mechanism on these phosphors is strongly related to the charge transfer from the host material to the Ln 3+ activators when the energy levels of the 4f orbitals in Ln 3+ are located between the valence band maximum (VBM) and the conduction band minimum (CBM) in the host materials. 2,3 The charge transfer from the host material to the Ln 3+ activators happens in two ways: (1) charge transfer via the bandgap excitation process 4–8 and (2) metal–metal charge transfer (MMCT) between the CBM in the host material and the Ln 3+ activators (Ln 3+ + B n + → Ln 4+ + B ( n −1)+ ). 4,7–12 Note that this process is conventionally called the intervalence charge transfer (IVCT) when two metal ions differ only in the oxidation state.…”

“…2,3 The charge transfer from the host material to the Ln 3+ activators happens in two ways: (1) charge transfer via the bandgap excitation process 4–8 and (2) metal–metal charge transfer (MMCT) between the CBM in the host material and the Ln 3+ activators (Ln 3+ + B n + → Ln 4+ + B ( n −1)+ ). 4,7–12 Note that this process is conventionally called the intervalence charge transfer (IVCT) when two metal ions differ only in the oxidation state. 13 The photoluminescence properties of these phosphors depend strongly on the electronic structure of the host materials with the bandgap energy level ( E g ).…”

“…14 Based on Dorenbos’ energy diagram of Ln 3+ , a new concept for the material design of near-ultraviolet (UV)-excited narrow-band phosphors with f–f emissions by bandgap engineering was proposed. 8 The PL excitation wavelength tuning was achieved over a large wavelength range by tailoring the E g of CaTa 1– x Zr x O 2+ x N 1– x with different Ta/Zr and N/O ratios. An intense red emission from Pr 3+ was notably observed at 614 nm under the near-UV irradiation of 375 nm when the E g of the host material CaTa 1– x Zr x O 2+ x N 1– x ( x = 0.75) was approximately 3.0 eV.…”

“…These results indicate that the PL properties of phosphors with an f–f emission are systematically controlled based on the bandgap engineering for host materials. 8…”

“…Therefore, based on the bandgap engineering concept through the formation of an oxide–oxynitride solid solution, the tunable E g achieved by the solid solution between complex perovskite oxides was proposed and perovskite oxynitrides can enhance the f–f emission excitation with near-UV light. 8 This is also facilitated by the selection of the 4f levels of activated Ln 3+ due to the overlap among the 4f levels of Ln 3+ .…”