The novel Sr₃LiSbO₆:Mn⁴⁺, Ca²⁺ far-red-emitting phosphors with over 95% internal quantum efficiency for indoor plant growth LEDs

Highlights

• A quantum efficiency of 96% was achieved by Sr₃LiSbO₆: Mn⁴⁺, Ca²⁺ phosphor.

• The emission intensity of Sr₃LiSbO₆:0.3%Mn⁴⁺, 0.45%Ca²⁺ phosphor at 423 K retained 87% compared with its value at 298 K.

• The as-fabricated LED device showed an effective promotion of growth in red beans and pakchoi cabbages.

Abstract

Sr₃LiSbO₆:x%Mn⁴⁺, y%Ca²⁺ (x = 0.1, 0.2, 0.3, 0.4, 0.5, x: y = 1: 1.5) phosphors have been successfully synthesized via conventional high-temperature solid-state reaction method. The phosphor samples were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence spectra, ultraviolet–visible (UV–vis) absorption spectra, Raman spectra and decay curves as well as internal quantum efficiency (IQE). The quantum efficiency of Sr₃LiSbO₆:Mn⁴⁺ phosphor can be as high as 96% by co-doping small amount of Ca²⁺ ions. The excitation and emission intensities of Sr₃LiSbO₆:Mn⁴⁺, Ca²⁺ phosphors can be enhanced 1.3–1.6 times. Furthermore, the Sr₃LiSbO₆:0.3%Mn⁴⁺, 0.45%Ca²⁺ phosphor exhibited good thermal stability and the emission intensity at 423 K maintained 87% compared with its initial value at 298 K. Under the excitation of 365 nm, the phosphor exhibits an intense far-red emission band centered at 693 nm which can match well with the absorption band of phytochrome P_FR. Finally, a LED device was fabricated with the Sr₃LiSbO₆:0.3%Mn⁴⁺, 0.45%Ca²⁺ phosphor, which showed an effective promotion of growth in red beans and pakchoi cabbages. All the results show that the synthesized phosphors may have promising applications for use in plant growth LEDs.

Introduction

Plant cultivation plays a significant role in agricultural production. With the world economy rapid development, the natural environment has been greatly damaged by pollution. Meanwhile, conventional outdoor agriculture is frequently affected with some extreme weathers such as frost, hail, droughts, haze and rainstorms, which threaten the crop yield and quality [[1], [2], [3]]. In recent years, indoor plant cultivation (IPC) has drawn considerable attention for constructing a comfortable and stable surrounding for plant growth.

As is well known, light is the main energy source for plants, which can promote plant branching, blooming and fruiting during the process of plant growth. Besides, light at different wavelength has different effect on plant growth. The blue light centered around 450 nm (440–480 nm) is beneficial to the photosynthesis; the red light centered around 660 nm (620–690 nm) is in favor of the phototropism; while the far-red light centered around 730 nm (700–740 nm) can facilitate the photomorphogenesis. These lights are mainly absorbed by four main plant pigments including Chlorophyll a and b, phytochrome P_R and P_FR. Consequently, the additional supplementary of these lights in IPC could accelerate biomass accumulation [[4], [5], [6], [7]]. Compared with the traditional light sources (e.g., fluorescent lamp, incandescent lamp, and metal halide lamp etc.), profit by typical merits of environment friendliness, long lifetime, energy-saving, durability, low cost etc., the phosphor converted light-emitting diode (pc-LED) has been widely used and become gradually the main-stream in IPC [8,9].

Recently, Mn⁴⁺-doped phosphors have become the research hotspot of many scholars because their raw materials are lower-cost than rare-earth-doped materials and they can exhibit red light under the excitation of ultraviolet (UV) or blue light with high efficiency. Mn⁴⁺ ion has a 3 d³ electronic structure, and some octahedron centers are easily occupied by the doped few Mn⁴⁺ ions, which can only be stabilized in the host crystals, such as [SbO₆] [10], [AlO₆] [11], [TaO₆] [12], [NbO₆] [13], [TeO₆] [14], [WO₆] [15], [SnF₆] [16], [TiF₆] [17], [GaF₆] [18] and [SiF₆] [19,20] octahedra. Generally, Mn⁴⁺-doped phosphors have wide absorption band in the range of 200–600 nm, which is attributed to the electron transition of ⁴A_2g → (⁴T_1g, ²T_2g, ⁴T_2g) in Mn⁴⁺ ions [20,21]. Meanwhile, Mn⁴⁺-doped phosphors show red or far-red emission because of the transition of ²E_g → ⁴A_2g [21]. Therefore, they have bright application prospects for indoor plant growth LEDs. At present, the Mn⁴⁺-activated oxide-based red phosphors were widely studied, like SrLaMgSbO₆:Mn⁴⁺ [22], Sr₂LaNbO₆:Mn⁴⁺ [23], LiLaMgWO₆:Mn⁴⁺ [24], GdZnTiO₆:Mn⁴⁺, Yb³⁺ [25], Sr₂ZnMoO₆:Mn⁴⁺ [26] and KMgLaTeO₆:Mn⁴⁺ [27]. Lei et al. [28] also studied Mn⁴⁺-activated Sr₃NaSbO₆ far-red emitting phosphors and obtained an IQE of 56.2%, which had potential application in indoor plant growth as a LED lamp. It was recently reported [29] that the Sr₃LiSbO₆ (SLSO) phosphors with IQE = 52.3% exhibit a far-red emission peak at 698 nm that matches well with the absorption peaks of P_FR. However, the intense excitation peak of Mn⁴⁺ in SLSO was located at around 340 nm, which cannot match the commercially available near-UV chips and the IQE of SLSO:Mn⁴⁺ phosphors needs to be further improved.

Recently, it has been reported that the excitation band moves towards long wavelengths and that the unit cell receives contraction when the Sr²⁺ ions of Sr_2.85Tb_0.1AlO₄F was replaced by Ca²⁺ ions [30]. Sun et al. [7] also reported that the luminescence intensity and the corresponding IQE of La₂LiSbO₆:0.3%Mn⁴⁺ phosphor was significantly enhanced by doping Mg²⁺ ions. Furthermore, the previous work reported by our group showed [31] that Bi³⁺ ions can significantly enhance the photoluminescence intensity of La₂LiSbO₆:Mn⁴⁺ phosphors and that the excitation spectra could be effectively modulated by doping different concentration Bi³⁺ ions. The above results indicate that the excitation spectra of SLSO:Mn⁴⁺ may be tuned that is suitable for commercial LED chips and IQE may be further improved by appropriate cation substitution. Besides, some studies have concerned influence of pc-LED light supplement on indoor plant growth. For instance, Young et al. [31] synthesized dual spectra band emissive Eu²⁺/Mn²⁺ coactivated Na(Sr, Ba)PO₄ and Ca₃Mg₃(PO₄)₄ phosphors and found that the photosynthesis reaction for oats and onions can be enhanced by the as-fabricated LED lamps. However, there are still few studies about the effect of light supplement on plant growth using pc-LED with Mn⁴⁺-based phosphors.

In this work, Mn⁴⁺, Ca²⁺ co-doped SLSO far-red-mitting phosphors have been synthesized via a solid-state reaction and systematically studied. The crystal structure, particle morphologies, luminescent properties, thermal stability and IQE of SLSO:Mn⁴⁺, Ca²⁺ phosphors were minutely investigated, revealing that the excitation evolution mechanisms and the relative intensities of the photoluminescence (PL) of the samples. Finally, the influence of spectral quality on the growth state of red beans and pakchoi cabbages was studied via the LED device fabricated using the optimized SLSO:Mn⁴⁺, Ca²⁺ phosphor.