Narrow-band far red-emitting double-perovskite SrGd₂Al₂O₇:Mn⁴⁺ phosphors

Highlights

• Narrow band far-red emission centered at 719nm can be observed in SrGd₂Al₂O₇: Mn⁴⁺ phosphors under exciting at 365nm.

• The emission band matches well with the absorption of phytochrome P_FR and excludes from that of the phytochrome P_R.

• The most intense far-red emission in SrGd₂Al₂O₇:Mn⁴⁺ phosphors occurs upon the situation of 0.1% Mn⁴⁺ dopant.

• SrGd₂Al₂O₇:Mn⁴⁺shows the potential as a light-converter in the application of plant growth LEDs.

Abstract

Accurate light irradiation towards phytochrome has been considered as a key method to improve the efficiency of plant cultivation. In this work, a series of far red-emitting Mn⁴⁺-activated SrGd₂Al₂O₇(SGAO) phosphors were synthesized by high temperature solid-state reaction. Under excitation at 330 nm, the optimized SGAO:0.1%Mn⁴⁺ phosphor exhibits intense far-red emission in narrow range of 701–740 nm, which matches well with the absorption of phytochrome P_FR and excludes from that of the phytochrome P_R. Moreover, the SGAO:0.1%Mn⁴⁺ sample exhibits considerable thermal quenching of far-red emission towards temperature, the emission intensity around 719 nm at 423 K was approximately 33% of that at room temperature. Also, the crystal field strength D_q, the Racah parameters B and C, and the nephelauxetic ratio β₁ of SGAO: Mn⁴⁺ were calculated and discussed. The results provide a reference for developing new high-efficient far-red-emitting phosphor towards indoor plant growth LEDs.

Introduction

In recent years, with the intensification of climate change, high-efficient and stable plant cultivation has increasingly relied on the mild conditions provided by indoor lighting sources to resist harsh environments. Light source has always been an essential condition in the all growth process of plants including branching, flowering, and fruiting [[1], [2], [3]]. As is known to all, phytochrome P_R and phytochrome P_FR are two main plant pigments which can absorb red (600–700 nm) and far-red (700–780 nm) lights. The phytochrome P_R can convert into P_FR with different light stimulation and vice versa, hence purposefully controlling the rhythms of photosynthesis, phototropism, and photo-morphogenesis of the plants [4]. Phosphor-converted light emitting-diode (pc-LED) shows promising trend to replace conventional light sources due to the advantages such as high efficiencies, long lifetimes, low energy consumption and environment friendliness. Therefore, rational design and in-depth investigations on red and far-red emitting phosphors to match well with the absorption spectra of the phytochrome are still highly in demand.

The Mn⁴⁺ ions with a 3 d³ electronic configuration are able to exhibit the broadband absorption and narrowband red emission in octahedral crystal field. Non-rare-earth Mn⁴⁺-activated phosphors have been widely investigated as red components for solid-state white LEDs towards indoor lighting because of their facile synthesis, good chemical stability, and excellent luminous properties [[5], [6], [7]]. Till now, most of the as-reported Mn⁴⁺-activated oxide-based phosphors can emit red light centered in 650–710 nm under the excitation of near ultraviolet or blue light, while the far-red emitting spectra in range of 710–740 nm are much less, which happens to be the exact absorption band of phytochrome P_FR and almost exclude from that of the phytochrome P_R, Thus, it is important to develop new-type phosphor to meet the requirement of full spectra for plant growth LEDs. Recently, a few kinds of Mn⁴⁺-activated titanates, aluminates and tungstates were reported to exhibit intense emission around 720 nm, such as 710 nm of CaLaMgNbO₆ [8], 725 nm of SrLaAlO₄ [9], 715 nm of CaGdAlO₄ [10], 717 nm of Ca₃Al₄ZnO₁₀:Mn⁴⁺ [11], 711 nm of Ca₃La₂W₂O₁₂ [12], and 710 nm of NaLaMgWO₆ [13], etc.

It is known that the red emission of Mn⁴⁺ doped materials can be vigorously influenced by local crystal field environment and crystal structure based on the nephelauxetic effect of Mn⁴⁺. Besides, the lattice rigidity has a critical effect on the performance of the phosphor. perovskite-structure compounds have been proven to be excellent hosts for white LEDs due to their better chemical–physical stability, higher quantum yield and diversity of structure and composition. SrGd₂Al₂O₇ (SGAO) is the prototypical member of layered perovskite-like aluminate compounds with the general formula Ln₂SrAl₂O₇ (Ln = rare-earth elements) [14]. The compound has a high melting point of 1780 °C [15], which is expected to exhibit a high rigidity. Additionally, the crystal structure has only one type of six-coordinated Al³⁺ ion, hence, it would be expected that the Mn⁴⁺ ions may give rise to narrow red emission when it occupies the site in the host. In this work, a series of SrGd₂(Al_1-x)₂O₇:xMn⁴⁺ phosphors were synthesized, the lattice structure and photoluminescence properties of Mn⁴⁺-doped SGAO phosphor were discussed.