Chemistry
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Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors
Chapters
Summary November 15th, 2016
An excellent chemical and luminescence stabilities of (oxy)nitride phosphors present it as an promising alternative to currently used sulfide and oxide phosphors. In this paper, we present the way to investigate its local luminescence properties using low-energy cathodoluminescence (CL).
Transcript
The primary goal of this procedure is to observe the microstructure of oxynitride phosphors using cathodoluminescence spectroscopy. This method can help answer key questions in the luminescent material field such as semiconductor, nanostructures and inorganic phosphors. The main advantage of this technique is to understand the local structure of phosphors and their coexistence with secondary phases from cathodoluminescence images.
Oxynitride phosphors are great candidates for UV and invisible emission applications. The high efficiency of the emission properties is achieved by controlling the centering conditions. By using cathodoluminescencce spectroscopy, we can optimize these conditions and achieve the best properties of oxynitride phosphors.
Under a low oxygen inert atmosphere, in a glove box, place the appropriate initial amount of powders for the desired oxynitride phosphor in an agate mortar. Crush and mix the powders using the agate mortar and pestle until a homogeneous mixture is formed. Pack the mixture into a boron nitride crucible.
Seal the crucible in a plastic container and remove the crucible from the glove box. Place the crucible in a centering furnace. Create a vacuum on the inside of the chamber.
Keep the furnace temperature constant during firing. Once firing is complete, turn off the furnace and allow the sample to cool to room temperature inside the sample chamber. Using an agate mortar and pestle crush the cooled samples to a fine powder.
Next, mix 150 milligrams of the phosphor product with 300 milligrams of resin and 30 milligrams of hardener. Pour the mixture into a silicone mold. Bake the mixture at 60 degrees Celsius under vacuum for 30 minutes.
Then, pour the mixture into a silicone chip mold. Bake the mixture at 100 degrees Celsius in air for 60 minutes to form the chip with phosphor powder collected at the bottom. File the chip to an appropriate size and, using an argon ion beam cross-section polisher, polish the underside of the chip to a mirror surface.
To begin the setup, adjust the scanning electron microscope sample stage adapter so that the top of the sample is aligned with the reference. Insert the sample into the chamber. Carefully place the ellipsoidal mirror between the sample and the objective lens.
Fill the detector coolant reservoir with liquid nitrogen, Turn on the detector and open the SEM software. Set the appropriate beam energy and current for the phosphor to be analyzed. Next, focus the ebeam and correct astigmatism until the image becomes sharp and clear.
Open the CL software, click on real time measurement and select continuous mode. Set the monochromator mirror to front side. While keeping the secondary electron image focused, slowly change the mirror position and sample height to achieve the strongest CL intensity.
To begin the experiment, click on real time measurement and select one shot mode. Configure the grading, slit width and collection time appropriately, then acquire the spectrum. Once the CL spectrum is acquired, click on picture measurement and select PMT detector.
Set the monochromator mirror to back. Select the appropriate slit, set the scan control to the external, adjust the resolution, magnification, wavelength and collection time for the sample. After taking the monochromatic image in the CL software, set the monochromator mirror to front side.
Click on measurement and set the points in the image for local analysis. Collect the local spectrum. Finally, click time dependence measurement and select CCD.
Set the scan control to the internal. Set the number of spectra to acquire and the time between each measurement and then begin data collection. The cross-sectional CL image of a small amount of silicon-doped aluminum nitride powder showed true information compared with the powder CL image.
The cross-sectional CL image shows a 280 nanometer emission mainly localized at the surfaces of the particles. Emissions at 280 nanometers are associated with silicon-related defects suggesting a silicon exist at the surface. Cross-sectional CL images at different wavelengths were taken of the oxynitride phosphor JEM both with and without calcium doping.
Local analysis at luminescence centers within the particles suggested that without calcium doping it is a single JEM phase, and with calcium doping JEM is agglomerated with sialon secondary phases. A more heavily silicon-doped AIN powder showed a 350 nanometer emission from the particles with weak emissions from the interfaces between the particles. Local analysis in tandem with EDS associated the higher silicon content at the interfaces with nonluminescent defects and a 460 nanometer emission shoulder consistent with silicon accommodating defects.
Two phosphors were evaluated by CL microscopy for stability to electron beam irradiation. The intense luminescence of the cerium-doped phosphor rapidly degraded during irradiation. The intensity of the silicon europium codoped phosphor, however, only decreased by 20%After watching this video, you should have a good understanding of how to synthesize oxynitride phosphors and characterize their microstructure by cross-sectional cathodoluminescence technique.
Following this procedure, other methods such as photoluminescence, x-ray diffraction some occipital test can be formed in order to evaluate the quality of phosphors. After developing cross-sectional CL technique, it paved the way for material scientists to explore optical properties related with silicon microstructures.
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