Quantum Efficiency Evaluation of Single-Particle Phosphor By the Proximity Measurement
Authors: Takahashi Kohsei , Hirosaki Naoto , Takeda Takashi 
Published: December 2024

Powder ceramic phosphors are fundamental materials for solid-state lighting and have contributed to the progress of general lighting and liquid crystal displays. The development of phosphors for high-color rendering illumination, wide-gamut displays, and high-brightness illumination with laser excitation continues. In recent years, the single-particle diagnostic method, which can determine the crystal structure and composition of the particles in a synthesized mixture, without obtaining a single phase has made it possible to dramatically improve the efficiency of searching for new phosphors.

In this study we established a proximity measurement method that can accurately measure the photoluminescence (PL) spectrum of single phosphor particles using a commercially available multichannel photodetector generally used for power evaluation1. In this method, optical fibers were placed at a distance of less than 1 mm from a single particle sample for measurement. The measured intensity was inversely proportional to the square of the distance. PL excitation spectrum can also be measured using a single optical fiber connected to a monochromatized Xe light source. This method allows accurate spectral calibration because no extra optical components are used. Moreover, by focusing the excitation light using an optical fiber with a spherical tip, the measurement efficiency was significantly improved. The measurement intensity was proportional to the cube of the particle diameter, and even particles as small as 1.2 μm could be measured.

Furthermore, we applied the proximity method to the quantum efficiency (QE) evaluation for a single particle phosphor2. Fluorescence spectroscopy by goniometric measurement, which can accurately evaluate the quantum efficiency of powdered phosphors, has been standardized by ISO23946:2020, and thus this method was applied to the evaluation of single particle. The apparatus consisted of a sample holder, an excitation fiber, and a detection fiber. A 150 W xenon lamp was used for excitation, and a multichannel photodetector was used for detection. The sample holder was attached to a 3-axis goniometer head. The samples were rotated using a motor. The excitation fiber was placed above the sample, whereas the detection fiber was driven by a motor to change the detection angle. A single optical fiber with a spherical tip was used for the excitation. Consequently, the diameter of the excitation beam was focused to 18 μm. A single optical fiber with a core diameter of 230 μm was used for the measurement. The small hole with a diameter of 0.5 mm in the sample holder was filled with barium sulfate powder as a white standard. A single phosphor particle of the test sample was then placed on it. The distances between the tips of the excitation/detection fiber and the sample were set to 500 and 700 μm, respectively. The zenith angle and the rotation angle intervals were set so that the measurement hemisphere was evenly covered with the detection fiber core diameter. Two steps were involved in the measurement. First, the particle was aligned with the center of rotation, and the light distributions in the scattering and emission spectra of the phosphor were measured. Next, the center of rotation was moved above the barium sulfate without the phosphor particle, and the scattered light distribution of the excitation light was measured. CaSiAlN3:Eu2+ red phosphor was used as the test sample. The particle picked from the powder sample was measured. The QE was calculated from the spatially integrated spectrum. Good agreement was achieved with the value for the powder sample.

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