Synthesis of Deep-Red La2MgGeO6:Mn4+ Phosphors for Indoor Plant Cultivation By Polymerized Complex Method
Authors: Watanabe Mizuki , Yoshizawa Tsubasa , Kim Sun Woog , Seki Kohei , Ishigaki Tadashi , Uematsu Kazuyoshi , Toda Kenji , Sato Mineo 
Published: February 2020

Phosphor converted white light-emitting diodes (LEDs) has been extensively practiced in lamps and displays as a next generation white light source, because it offer more benefits, including high luminescence efficiency, a long lifetime, compactness, and low environmental load. Recently, in the area of agricultural research, the LEDs have been actively studied as the light source for “indoor plant cultivation” [1].

Indoor plant cultivation is to grow plants within a controlled environment, meaning filtered air, steady temperature, and special growth media. The ideal environment permits stable supply of vegetables without the influence from the outside. The stable supply can hold the price increase, resulting from short supply, due to drought, loss caused by cold weather, and other disasters. Therefore, indoor cultivate is expected as a solution approach to the global food problem.

Artificial light used in indoor plant cultivation is one of important factors. The growth of plant tissues requires three emitting colors: (1) blue light around 450 nm affecting the growth of chlorophyll, flower-bud formation, leaf morphogenesis, and phototropism, (2) red light around 660 nm helping budbreak and internodal elongation, and (3) far-red light around 730 nm promoting photosynthesis. Among them, far-red light is especially significant for preparation of higher-value vegetables, because it prevents aggravation of eating quality and the bitterness caused by the excessive growth of vegetables [2].

Convert-type white LEDs built up with blue LED emitting around of 450 nm and yellow Y3Al5O12:Ce3+ (YAG:Ce3+) have been studied as a prospective light source used in indoor plant cultivation, because of their suitable emission for the culture of plants. Currently, the combination of this the LED and the blue light excitable red LED emitting round 660 nm has been reported as a candidate for the next artificial light source redeemed red light [3]. However, the intensity of its far-red emission is insufficient. Novel blue light excitable deeper-red phosphors, therefore, are required in order to develop an artificial light source in indoor plant cultivation, achieving effective growth of plants and preparation of higher-value vegetables.

Mn4+ doped phosphors are excited by blue light and exhibit red emission corresponding to the 4A2 → 4T2 and 2E → 4A2 transitions of Mn4+. La2MgGeO6:Mn4+ [4] is a good candidate for a deep-red phosphor of an artificial light source in indoor plant cultivation, because of their suitable excitation and emission spectra. In addition, it is obtained by easy synthetic condition compared to other Mn4+ doped phosphors such as CaAl12O9:Mn4+ [5], 3.5MgO・0.5MgF2・GeO2:Mn4+ [6]. La2MgGeO6:Mn4+ phosphors are usually synthesized by a conventional solid state reaction method. The method has two drawbacks: the high temperature (1350℃) needed in the heating condition and the concentration quenching, owing to Mn4+ions inhomogeneously distributed.

In this study, overcome the drawbacks, we synthesized La2MgGeO6:Mn4+ phosphors by polymerized complex method. Polymerized complex method exploits polymerization between ethylene glycol and citric acid. They form the soluble metal-citric acid complexes. The complexes are immobilized in a rigid polyester network almost in the molecular level, reflecting the original solution [7]. Owing to this mechanism, this method has the advantage; the each ion is homogeneously distributed. Therefore, it is expected to be obtained the La2MgGeO6:Mn4+ samples homogeneously distributed Mn4+, resulting in high efficiency, at low temperature [8]. The luminescence properties of synthesized samples by polymerized complex method are compared with that prepared by the conventional solid state reaction method.

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