Synthesis of Near-Infrared Mechanoluminescence Material Via Organic Acid-Aided Process
Authors: Fujio Yuki , Xu Chao-Nan , Terasaki Nao 
Published: December 2020

A mechanoluminescence (ML) material is a kind of phosphor. There has been attracting attention as a sensing material capable of two-dimensionally visualizing physical phenomena such as strain, stress, or load [1-5]. In particular, the ML material that emits near-infrared (NIR:0.75~1.0 μm) and short-wave infrared (SWIR:1.0~2.5 μm) luminescence has biological transparency and a wavelength range different from a room illumination and is expected to spread a new applying field [6-7]. To use the ML material for physical sensing technology using an inorganic-organic hybrid film mixed with resin, the diameter of the ML particle needs to be several micrometers or less. However, the ML material is commonly synthesized by a solid-state reaction method at a rather high temperature above 1400°C, resulting in the formation of large particle size. Such a large ML particle is pulverized under a mechanically milling process to reduce the particle size, producing crystal defects. The crystal defects are known to be one of the causes of the charge career trapped sites in the ML material, posing deterioration of the ML intensity as well as a decrease in the reproducibility of sensing characteristics. Thus, to develop an ML material having a high ML intensity, it is necessary to establish a synthetic method capable of lowering the sintering temperature. For a decrease in the sintering temperature of the functional oxides, it has been reported that it is effective to produce a precursor whose particle size and shape are controlled by a solution technique. Although there have been numerous reports regarding the solution technique, an organic acid-aided method is suitable for producing a complicated material with a multi-element composition because it desolvates an aqueous solution and remains all the metal compounds [8]. In the present study, therefore, we tried to synthesize ML material by using the organic acid-aided method and evaluated both optical properties under visible light and near-infrared regions. Among various ML materials reported, we selected SrAl2O4 co-doped with Eu, Cr, and Nd ions (SAOEuCrNd) which emits both NIR and SWIR luminescence [7] and selected malic acid as a representative organic acid. For comparison, SrAl2O4 doped with Eu (SAOEu), SrAl2O4 co-doped with Eu,Cr (SAOEuCr), or Eu,Nd (SAOEuNd) were also synthesized by using the same synthetic method. The synthesized ML materials were evaluated by various analysis instruments.
From x-ray diffraction (XRD) patterns of the synthesized ML materials, all the diffraction peaks coincided with those of the monoclinic SrAl2O4 (PDF no. 34-0379). This indicates that monoclinic SrAl2O4 can be synthesized by the organic acid-aided method even by using various luminescent center ions. The photoluminescence (PL) spectra of all the synthesized ML materials exhibited a broad emission which is centered at approximately 520 nm under excitation at 365 nm. On the other hand, Nd-doped ML materials such as SAOEuNd and SAOEuCrNd showed several peaks at around 900, 1100, and 1400 nm under excitation at 365 nm. The PL intensities under NIR and SWIR regions for SAOEuCrNd were much higher than those for SAOEuNd. Logically, it is reasonable to expect that Cr ion behaved as a sensitizer such as an energy transfer mechanism of YAG:Cr,Nd. Finally, we evaluated the ML intensity under NIR and SWIR regions for the synthesized ML materials by using a lab-built system comprising Si image sensor and optical filter (pass long wavelengths >750 nm). As a result, Nd-doped ML materials showed ML response toward the tensile strain and the ML intensity increased with increasing the tensile strain, although there is no luminescence response toward the tensile strain for SAOEu. Especially, SAOEuCrNd exhibited the highest ML intensity. Thus, the SAOEuCrNd synthesized by the organic acid-aided method using metal nitrates and malic acid is a promising candidate for ML materials which emits both NIR and SWIR luminescence.
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