This work was supported in part by Green Network of Excellence (GRENE) project from the Ministry of Education, Culture, Sport, and Technology (MEXT) in Japan. The authors are also grateful to the technicians of the Sialon Unit for their help in the phosphors synthesis, to MANA for its help in EDS measurements and to K. Nakagawa for the help in the CL system.

1. Samples
<ol>
	<li>Phosphor synthesis
	<ol>
		<li>Design the product, determine the starting materials and their weights based on the expected chemical reactions. Precisely weight the raw starting powders14-20.</li>
		<li>Crush and mix them in an agate mortar by hand. Depending on the quality of the raw powders, mix for 15-30 min in order to get a mix as homogenous as possible. Pack the powder mixture into a boron nitride crucible. 
		NOTE: Carry out steps 1.1.1-1.1.2 in a glove box under inert gas, with very low oxygen concentration as the final product or the raw powders are unstable in the air.</li>
		<li>Fire the powder mixture in a gas-pressure sintering furnace with a graphite heater. Heat the samples at a constant heating rate. Introduce into the chamber the pre-determined nitrogen gas (99.999% purity), and simultaneously raise up the temperature to the desired value. 
		NOTE: Keep the same conditions for the duration of the sintering. Heating temperature and duration are different depending on the materials14-20.</li>
		<li>After firing, shut off the power, and let the samples cool down inside the furnace. Crush well the sintered powders in an agate mortar by hand until getting fine particles.</li>
	</ol>
	</li>
	<li>Cross-section
	<ol>
		<li>Mix 150 mg of phosphors with 300 mg of resin and 30 mg of hardener. Pour into a silicone mold and bake at 60 &#176;C for 30 min in vacuum for evaporating oxygen from the mixture.</li>
		<li>Put into a silicon mold and baked again at 100 &#176;C for 60 min in air to fabricate a powder embedded chip. During heating, most of powders are deposited at the bottom due to high material density.</li>
		<li>Polish the facet of underside of the chip by means of handy-lap and Ar-ion cross-section polisher in order to produce a mirror surface. Higher the amount of molded powders allows better observation of the inner structure of powders.</li>
	</ol>
	</li>
</ol>
2. Cathodoluminescence
<ol>
	<li>Sample and set-up preparation 
	NOTE: As CL is a contactless technique, there are no particular requirements for the measurement itself. The preparation will depend on the measurement objectives. Thus, for quantitative measurements, it may be preferred to put a large amount of powders on carbon tape, or made a film. For qualitative measurements, investigation of isolated particles may be favored by putting a small amount of powder on a carbon tape, or dispersing the powder in ethanol and immerging one conventional copper micro grid for TEM in the solution.
	<ol>
		<li>Adjust the height of the sample stage to align the top of the sample with the reference stage (12 mm).</li>
		<li>Insert the sample stage into the chamber. Insert the ellipsoidal mirror between the electron gun and the samples. Make sure that the samples are not touching the ellipsoidal mirror to prevent contaminating the mirror or breaking it.</li>
		<li>Fill the cooling reservoir of the detectors with liquid N2. Turn on the detector. Wait until the detectors temperature stables to start CL measurements. The ideal temperature for measurement is 110 K. NOTE: Temperature can be checked on the software.</li>
	</ol>
	</li>
	<li>CL measurements 
	NOTE: The electron beam conditions, in particular the electron beam energy and the beam current, are selected. These conditions should be chosen in function of the expected luminescence intensity of the samples, the observed charging, the luminescence degradation and the interests in the depth analysis.
	<ol>
		<li>Position the electron-beam where you want to take SEM and/or CL spectrum. Focus the image in order to bring out the defined shape and correct astigmatism to get a clear and sharp image. Adjust stage height and working distance. Correct the astigmatism by using x, y stigmators on the magnified image. Start the software for CL acquisition.</li>
		<li>Optimize the mirror position and the sample height to get the strongest intensity for the CL spectra. To perform it, click on the &#34;real time measurement&#34; and select &#34;continuous mode&#34; on the software. Set the mirror in the monochromator as &#34;front side&#34; to send the dispersed light to the CCD and take the quick CL spectra. Slowly change the mirror position and the sample height while keeping the SE image focused and increasing the CL intensity.</li>
		<li>For CL spectra of a large area, click on the &#34;real-time measurement&#34; icon and select &#34;one-shot mode&#34; on the software. Configure the light detection system in order to send the emitted light to the CCD. Depending on the sample, choose the grating, slit width and measurement collection time in order to get the most appropriate intensity/spectral resolution.</li>
		<li>For CL monochromatic imaging, click on the &#34;picture measurement&#34; icon and select the &#34;PMT detector&#34; on the software. Set the mirror in the monochromator as &#34;back side&#34; to send the dispersed light to the PMT detector, and insert a slit. Depending on the sample, choose the resolution, magnification, desirable collection wavelength and time. Freeze the image and send it to the software.</li>
		<li>For local CL analysis, take SE or CL image first. Click on the &#34;measurement&#34; icon, select the position to take spectra on the image. Configure the light detection system in order to send the emitted light to the CCD, and then take CL spectra.</li>
		<li>For CL time evolution, click on the &#34;time dependence measurement&#34; icon and select the &#34;CCD&#34; on the software. Configure the light detection system in order to send the emitted light to the CCD detector. Depending on the software, select the numbers of spectra and the time between 2 measurements, for example 360 spectra every 10 sec.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

Through these representative examples of low-energy CL characterization on Sialon phosphors, we have shown how powerful and quick technique for phosphors investigation can be. By measuring the local CL measurements and mapping, taking advantage of the flexibility in the sample preparation and combining CL with other techniques, we can attribute more accurately the origins of the luminescence, clarify the growth mechanisms and determine the most suitable phosphors for applications. These results are mainly achievable due to the improvements of the electron microscopes and the light detectors, which enhance the measurement collection time, the sensitivity and the spatial resolution.
Both Sialon phosphors and CL fields are not naturally limited to the aspects presented in this paper. In the following, in order to enlarge the discussion, we are going to discuss a little bit more about them separately.
In case of Sialon phosphors, with their superior luminescence and stability properties, they are being more and more used for lighting applications. However, they also display very interesting mechanical, thermal, magnetic, superconductivity, electrical, electronic, and optical properties, which can be tuned by changing their composition. Thus, they are also found in a wide range of applications such as antireflection coatings, solar absorbers, heat mirrors, colored pigments, visible-light-driven photocatalysts, transparent windows and armors, or fluorescent probes for bio-medical imaging29. We can anticipate that they are going to play crucial roles in many energy and environment-related aspects, such as efficiently harvesting solar energy, realizing the hydrogen economy, reducing the environmental pollutions, saving the natural resources, etc. However, a lot of work is still required to continue improving their properties while reducing their production cost, such as decreasing the sintering temperature or limiting the use of rare-earth ions. It can be achieved by finding novel Sialon phosphors, and clarifying the role of the composition and growth conditions on the properties. We have seen that CL can play an important role to achieve these objectives. But, recent new approaches have also revealed very promising possibilities. Two of these approaches are time-of-flight secondary ion mass spectrometry (TOF-SIMS) and single-particle diagnosis. TOF-SIMS is able to spatially resolve the entire mass spectrum with high-sensitivity, which enables not only the detection of species at trace-level but also the differences in oxidation state31. The single-particle diagnosis consists in the treatment of an individual luminescent particle in a complex mixture as a tiny single crystal, and to investigate the optical and structural properties by means of super-resolution single-crystal X-ray diffraction and single-particle fluorescence31.
As for low-energy CL characterization, in this paper, we have mainly concentrated on the use of CL for Sialon phosphors, while CL can also be used for other materials, such as semiconductors, nanostructures, organic materials, and ceramics. On the other hand, although CL is an invaluable technique for qualitative characterization of optoelectronic materials, it also induces some cautions for quantitative measurements. Indeed, CL results depend not only on the excitation conditions, beam current and electron energy, but also on the amount of investigated materials25. Thus, a small variation of these parameters may significantly change the CL intensity. In addition, electron-beam irradiation may increase the possibility to damage the samples. It may induce a drastic change in the intensity, or induce the creation/activation of new luminescence centers, which may affect the reliability of quantitative CL measurements. The development of CL in materials characterization was and will be strongly related to the improvements in the electron-beam microscopes and the light detectors. Thus, it is now possible to perform TEM. It allows a higher spatial resolution and a direct observation of the luminescence change in-situ observation of luminescence change accompanied with microstructure change caused by electron-beam induced atomic displacement, for instance32-34. Moreover, with the addition of an in-column beam blanker synchronized with the optical detector, it is now available to use electron-beam in pulse mode, which allows performing decay profile measurements into an electron microscope35. It may be also thought that the use of pulsed electron-beam irradiation may reduce the electron-beam induced damages, which will improve the reliability of quantitative measurements and help in the characterization of electron-beam sensitive materials. These 2 examples illustrate how CL analysis may improve in the future.

Recently, more and more attention is devoted to environmental issues, especially energy production and consumption. To answer these society needs, energy production must be &#34;greener&#34; that means, reducing the energy consumption from traditional sources or developing new environmental-friendly materials. Light emitting diodes (LEDs) and field emission displays (FEDs) have got significant attention due to their compactness, improved performance and lower power consumption compared to the actual displays, such as mercury gas-discharge fluorescent lighting or plasma displays1-5. The key factor for light source of LED and FED is a high-efficient phosphor. Rare-earth doped phosphors are inorganic materials consisting of a host lattice and rare-earth dopants, which can emit light under excitation of photons (ultraviolet (UV), blue light), electrons (electron-beam) or electric field. The requirements for the high-efficient phosphors are: 1) high conversion efficiency with the different excitation sources; 2) good stability with low thermal quenching; 3) high color purity with full color-reproducibility. However, only a very limited number of phosphors can presently meet these minimum requirements. Currently used oxide-based phosphors have low absorption in the visible-light spectrum, while sulfide-based ones have low chemical and thermal stabilities. Moreover, they show degradation under electrons or ambient atmosphere, which limits the device lifetime. Since their color purity and efficiency are limited, it makes them difficult to be used for the realization of high color rendering index (CRI) luminescent devices. Consequently, the exploration of new phosphors is required.
Rare-earth doped nitride and oxynitride (Sialon) phosphors are considered as good candidates with outstanding thermal and chemical stability based on their stable chemical bonding structures. Stokes shift becomes smaller in a strong lattice and it leads to a high conversion efficiency and a small thermal quenching of phosphors6-9. In general, the luminescence of divalent rare-earth ions, such as Eu2+ or Yb2+, and Ce3+ is attributed to 5d-4f electronic transitions, and consists of a broad band with peak position varying with the host lattice due to the strong interaction between 5d orbitals and the crystal field. Due to their properties, wavelength-tunable luminescence is obtained by changing the chemical nature of rare-earth ions and their concentration in the host lattice (Fig. 1). Thus, Sialon phosphors can be used for realizing high CRI white-LED using blue-green-red phosphors system and applications in UV-FEDs.
Although Sialon phosphors are promising materials, a lot of work such as finding novel structures and reducing the productions cost are still required. Moreover, due to the difficulties in terms of optimization of sintering conditions, Sialon phosphors often contain secondary phases18-20. Investigation of such localized structures is important to understand the sintering mechanism and optimize the sintering conditions, and so to improve the optical properties of Sialon phosphors. These objectives can be achieved by low-energy cathodoluminescence (CL) technique.
CL is a phenomenon in which electrons irradiating on a luminescent material cause the emission of photons. Contrary to photoluminescence (PL), which is induced by photon excitation, the excitation area is usually in the order of millimeter and selective excitations enhance particular emission processes, electron-beam excites in the nanometer scale and activates all the luminescence mechanisms present in the material, which may allow the detection of different phases with different luminescence properties10-12. Additionally, the incident electrons can generate not only the CL signal but also various signals, such as reflected electron, Auger or X-ray, which provide different information on the materials. Thus, the structural, chemical or electrical properties can also be obtained. The combination of these techniques with CL results in a better understanding of the origin of the localized structures of Sialon phosphors14-20.
CL investigations can be performed by means of different types of electron-beam sources13. Nowadays, scanning electron microscope (SEM) is the most common system to perform CL measurements. In the following, we are going to discuss mainly this system. As seen in Fig. 2, CL measurements are performed by using an electron source (SEM), a light collector (optical fiber and monochromator) and a detection system. Detection system consists of a charge-coupled device (CCD) and a photomultiplier tube (PMT), which are for parallel-detection mode and serial-detection mode, respectively. In general, the collected light from the sample is adjusted by slit and then dispersed by monochromator grating. When the collected light of the sample is dispersed onto the CCD (parallel-detection mode), each emission wavelength is simultaneously detected. When a specific wavelength of the dispersed light is selected by a slit (serial-detection mode), its intensity is recorded by the PMT to form monochromatic images.
In this paper, we mainly highlight the use of a low energy CL for the characterization of the Sialon phosphors, representatively, Si-doped AlN14, 22, Ca-doped (La,Ce)Al(Si6-zAlz)(N10-zOz) (z~1) (JEM)15, Si/Eu-doped AlN16, 17 and Ce-doped La5Si3O12N materials. Cross sectional polishing method using an argon ion beam (CP method) is an useful method to observe layered structures, due to its wider polishing area with less surface damage. It has been performed for an investigation of a local structure of the phosphors. The correlation of CL with other electron-based techniques and the investigation of luminescence stability will be also illustrated.

<table><tbody><tr><td>SEM</td><td>Hitachi</td><td>S4300</td><td></td></tr><tr><td>Triple-grating monochromator</td><td>Horiba Jobin-Yvon</td><td>Triax 320</td><td></td></tr><tr><td>Photomultiplier</td><td>Hamamatsu</td><td>R943-02</td><td></td></tr><tr><td>Charge-coupled device with 2048 channels</td><td>Horiba Jobin-Yvon&amp;nbsp;</td><td>Spectrum One</td><td></td></tr><tr><td>Gas-pressure sintering furnace with a graphite heater&amp;nbsp;</td><td>Fujidempa Kogyo Co. Ltd.</td><td>FVPHR-R-10, FRET-40</td><td></td></tr><tr><td>Silicone mold&amp;nbsp;</td><td>LADD</td><td>21780</td><td></td></tr><tr><td>Ar-ion cross-section polisher</td><td>JEOL</td><td>SM-09010</td><td></td></tr><tr><td>EDS</td><td>BRUKER</td><td>Xflash6/100</td><td></td></tr><tr><td>Resins</td><td>JEOL</td><td>Part No 780028520</td><td></td></tr></tbody></table>

low-energy cathodoluminescence for (oxy)nitride phosphors

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and flexibility of their emission properties can be achieved by controlling their composition and dopants. However, a lot of work is still required to improve their properties and to reduce the production cost. A possible approach is to correlate the luminescence properties of the Sialon particles with their local structural and chemical environment in order to optimize their growth parameters and find novel phosphors. For such a purpose, the low-voltage cathodoluminescence (CL) microscopy is a powerful technique. The use of electron as an excitation source allows detecting most of the luminescence centers, revealing their luminescence distribution spatially and in depth, directly comparing CL results with the other electron-based techniques, and investigating the stability of their luminescence properties under stress. Such advantages for phosphors characterization will be highlighted through examples of investigation on several Sialon phosphors by low-energy CL.

The luminescence is distributed not only laterally but also in-depth. Such core-surface distribution is observable with CL by changing the electron energy, since it varies the penetration depth of the incident electrons21. However, the penetration depth varies for each material and the correspondence between electron energy and penetration depth is not linear, and may introduce some additional effects, such as the reabsorption of higher energy photon from the deeper regions by the material itself. Thus, it may be preferable to observe directly the core-surface distribution via cross-section observation. In case of phosphor powders, such observation can be achieved by trapping the particles into a resin and polish the powder-resin composite by cross-section polisher, for instance. Since the particles are randomly distributed in the resin, the cutting direction is not controllable. However, the high amount of particles allows cutting enough particles to make such an analysis valid.
To illustrate this point, we have investigated the luminescence distribution of Si-doped AlN powder. Figure 5a shows the CL spectra of AlN powders doped with 0.0% and 1.6% of Si. The emission of undoped AlN consists of 2 bands at 350 and 380 nm, while that of AlN doped with 1.6% of a band at 350 nm with a clear shoulder at 280 nm. The 350 and 380 nm bands are attributed to Al vacancy-oxygen complexes (ON-VAl), while the shoulder at 280 nm with the O-purified AlN affected by Si to form SiO vapor22. Figure 5b and 5c show the CL images taken at 280 nm for as-sintered and cross-sectioned AlN powders doped with 1.6% of Si, respectively. The 280 nm emission is nonuniform along the particles from the CL image of the as-sintered sample, the brighter areas seem to be at the edges of the particles, but the morphologies of the particles and their distribution may make such observations not so obvious. However, from the CL image of the cross-sectioned sample, it appears clearly that the 280 nm emission is mainly localized at the surfaces of the particles, suggesting that AlN particles are actually coated by a Si-rich layer and surface purification may proceed.
Local composition changes in Sialon phosphors can drastically affect the luminescence properties. Thus, the same rare-earth ion in different host-lattices or in different sites may give different emissions15,18-20. But, unfortunately, local differences during the sintering, such as a distribution of the temperature or the raw materials proportion, or the partial oxidation of the surface of the particles, are expected, resulting in changes of the composition along the particles and/or in the coexistence of several phases. Such effects may not be directly observable with structural and chemical characterization techniques. Thus, it is important to investigate the local luminescence properties of a phosphor. With the precise control of the size and position of the electron-beam in a SEM, it is possible not only to acquire a CL spectrum from a nanoscale region but also to obtain high-resolution CL images of the luminescence centers.
(La,Ce)Al(Si6-zAlz)(N10-zOz) (z~1) (JEM) is an intense blue phosphor which is suitable for general lighting. It has been found that by replacing (La, Ce) by Ca, a red shift and a broadening of CL peaks occur according to Ca-doping. It was believed that Ca was affecting the crystal field splitting of Ce3+. However, this explanation, based only on the luminescence spectra, is misleading, as revealed by cross sectional local analysis.15 Figure 6 shows the cross-sectional SE (CS-SE), combined CS-CL images at 300 nm (red), 430 nm (blue) and 540 nm (green) and local CL spectra taken at 5 kV for JEM phosphors doped with 0 (a, b, c) and 0.69 (d, e, f) at. % of Ca, respectively. It has to been noted that these wavelengths were selected in order to reduce the bands overlapping in case that several bands exist. For CS-CL image of Ca-undoped sample, JEM particles consist of many particles agglomerated with each other. The luminescence at 430 nm is almost uniformly distributed with some brighter area and some of localized area at 300 nm. On the other hand, the grain boundaries show darker emission. Local CL analysis reveals that the spectral shape is relatively comparable in any positions, with a shift of the bands from 430 to 450 nm and spectral intensity in a good agreement with the pictures. For CS-CL image of Ca-doped, there are significant differences between 430 and 540 nm. Submicron patches clearly appear brighter at 300 and 540 nm along a large portion of the particles with darker grain boundary regions, while the 430 nm emission is localized in another section of the particles. By local analysis, the CL spectrum taken on a 430 nm bright area (point 3) consists a band at 440 nm, comparable at the one observed for Ca-undoped sample. The bright areas at 540 nm, embedded in the same particle, (points 1 and 4) show a band at 480-490 nm. Small bright areas at 300 nm (point 2) and dark grain boundary region (point 5) show a band at 440 nm with shoulder at 480 nm, with occasionally at emission at 310 nm. Based on the literature and XRD analysis, we can attribute the band centered at around 430 nm to Ce3+ in JEM22 and that at 480 nm to Ce3+ in &#945;-SiAlON23. The darker broad emission is originated to Ce3+ in &#946;-SiAlON, and that at 310 nm to Sialon host material. These results prove that the red shift and the broadening of CL peaks according to Ca doping may not be attributed to Ca-induced changes in the crystal field splitting of Ce3+ as initially thought, but more to the coexistence of different phases inside the same particles and the gradual transformation of &#946;-SiAlON to &#945;-SiAlON with Ca doping.
Although the observation of the different emission centers and their distribution is possible by using low-energy CL, it may not be enough to fully understand the nature of the luminescence centers. In such cases, it is necessary to combine the CL measurements with other techniques. Since the incident electrons can generate other signals beside CL, it is possible to directly correlate the light emission with electrical, chemical or structural properties by investigating the same area with the different electron-beam techniques. Thus, the correlation of CL with high-resolution TEM (HRTEM) and EBIC has been used to characterize defects, such as dislocations or stacking faults. As for the variation of concentration/composition, the combination of CL with TEM, EDS or Auger spectroscopy can result in a better understanding of the origin of the luminescence.
Here, we illustrate this aspect by investigating the emission of Si-doped AlN powder. Figure 7 shows the CS-CL and CS-EDS images (a, b) and local spectra (c, d) of AlN particle doped with 4.0% Si doping. The CS-CL image was taken at 350 nm, while the CS-EDS image consists of the superposition of Si and Al distribution. The CS-CL image shows darker elongated structure in the center of the particles. Local CL spectra taken in the bright region consist of a strong peak at 350 nm with shoulders at 280, 380, and 460 nm. However, there are clear changes in the ratios between these different bands with the position. Areas showing a brighter emission at 350 nm (point 1) shows a higher 280 nm emission and smaller 460 nm emission compared to the main band at 350 nm, while the darker elongated patch (point 2) shows a smaller 280 nm emission and higher 460 nm emission compared to the main band at 350 nm. The 460 nm is originated from Si-accommodating defects in AlN24. EDS images and local spectra reveal that the darker elongated area show a smaller Al and higher Si composition compared to the rest of the particles. Compared with the results observed in Figure 5, we can assume that by increasing the amount of Si into AlN, a secondary reaction occurs between Si and AlN, which induces the formation of SiAlON phases.
The two important parameters for a material used in devices are materials high performance and stability under stress. Indeed, a degradation of the material properties under stress will reduce its lifetime, which is not industrially viable. Thus, for electron-beam stimulated devices, such as cathode ray tubes (CRTs) and FEDs, it is necessary to develop electron-beam irradiation resistant phosphors and/or to understand the electron-beam induced mechanisms in order to prevent or reduce such effects. The luminescence degradation can occur via different mechanisms, such adsorption/desorption or charging at the surfaces, creation or activation of defects, etc.25-27. Although these intensity variations complicate the quantitative analysis of CL results, they can be used to investigate the lifetime of optoelectronic devices.
To illustrate this point, we have the CL spectra and evolutions of two blue-emitting phosphors, Ce-doped La5Si3O12N and Si/Eu-doped AlN. Figure 8a shows the CL spectra for Ce-doped La5Si3O12N and Si/Eu-doped AlN after 20 sec of irradiation at 5 kV. Both samples show an intense blue emission: the band position and intensity for Ce-doped La5Si3O12N are 456 nm and 3,270 cps, respectively, while those for Si/Eu-codoped AlN are 466 nm and 3,100 cps. A priori, the main difference between these 2 samples is the broadness of the emission, since the emission for Ce-doped La5Si3O12N is larger due to the coexistence of several bands. Thus, it seems that both materials are suitable as blue-emitting phosphors for FEDs, and that we have to consider criteria as fabrication cost, the compatibility with the other phosphors or the stability of the luminescence properties under electron-beam irradiation, to determine the most suitable. Figure 8b shows the evolutions of CL intensity of Ce-doped La5Si3O12N and Si/Eu-doped AlN during electron-beam irradiation at 5 kV. For Ce-doped La5Si3O12N, the intensity decreases from 3,270 to 450 cps in 5 min and to 95 cps in 60 min. Namely, under 3,600 sec of 5-kV irradiation, the intensity decreases more than 95% of the initial intensity. For Si/Eu-doped AlN, the intensity decreases 3,100 to 2,500 cps in 60 min, namely a decrease of 20% of this initial intensity. These results clearly show that the Si/Eu-doped AlN is much better candidate than Ce-doped La5Si3O12N is due to its higher stability.
<img alt="Figure 1" src="/files/ftp_upload/54249/54249fig1.jpg" /> 
Figure 1: Luminescence of rare-earth doped SiAlON phosphors. Pictures of different phosphors under visible (a) and ultraviolet (b) light. (c) Normalized CL spectra of Eu2+ in different host lattices. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54249/54249fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54249/54249fig2.jpg" /> 
Figure 2: Setup of CL. (a) Photograph of the SEM with CL system, with inset a photograph of the ellipsoidal mirror. (b) Schematic image of light detection system. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54249/54249fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54249/54249fig3.jpg" /> 
Figure 3: Preparation of SiAlON phosphors. (a) Determination of the starting powders, weight and sintering conditions; (b) Mixing of the raw powders; (c) Sintering of the powder mixture; (d) Sintered powders before and after crushing. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54249/54249fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/54249/54249fig4.jpg" /> 
Figure 4: Cross-section preparation. (a) Mixing of the powders with resin and hardener, and evaporating air in the mixture. (b) Pouring into a silicon mold and heating. (c) Polishing of the chips by handy-lap and Ar-ion cross-section polisher. (d) Measuring cross-sectional polished area. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54249/54249fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/54249/54249fig5.jpg" /> 
Figure 5: Core-shell distribution in Si-doped AlN. (a) CL spectra of AlN powders doped with 1.6% of Si at 5 kV. (b-c) CL images taken at 5 kV and 280 nm for as-sintered (b) and cross-sectioned (c) AlN powders doped with 1.6% of Si, respectively. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54249/54249fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/54249/54249fig6.jpg" /> 
Figure 6: Local analysis of Ca-doped JEM phosphor. CS-SE, combined CS-CL images at 300 nm (red), 430 nm (blue) and 540 nm (green) and local CL spectra taken at 5 kV for JEM phosphors doped with 0 (a, b, c) and 0.69 (d, e, f) at. % of Ca, respectively. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54249/54249fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/54249/54249fig7.jpg" /> 
Figure 7: CL and EDS comparison of Si-doped AlN. CS-CL and CS-EDS images (a, c) and local spectra (b, d) of AlN particle doped with 4.0% Si doping. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54249/54249fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" src="/files/ftp_upload/54249/54249fig8.jpg" /> 
Figure 8: Luminescence stability of two blue phosphors. (a) CL spectra of Ce-doped La5Si3O12N and Si/Eu-doped AlN after 20 sec of irradiation at 5 kV. (b) Evolutions of CL intensity of Ce-doped La5Si3O12N and Si/Eu-doped AlN during electron-beam irradiation at 5 kV. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54249/54249fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Low-energy Cathodoluminescence for OxyNitride Phosphors at JoVE.com

Low-energy Cathodoluminescence for OxyNitride Phosphors

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).

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and ...

low-energy-cathodoluminescence-for-oxy-nitride-phosphors

Nanyang Technological University

Research

JoVE Journal

Chemistry

10.6K Views.  University of Tsukuba.  Een uitstekende chemische en luminescentie stabiliteiten van (oxy) nitride fosforen presenteren als een veelbelovend alternatief voor de momenteel gebruikte sulfide en oxide fosforen. In deze paper presenteren we de weg naar de lokale luminescentie eigenschappen met behulp van lage-energie kathodoluminescentie (CL) te onderzoeken. 

Video: Low-energy Cathodoluminescence for OxyNitride Phosphors

Activating Molecules, Ions, and Solid Particles with Acoustic Cavitation

The chemical and physical effects of ultrasound arise not from a direct interaction of molecules with sound waves, but rather from the acoustic cavitation: the nucleation, growth, and implosive collapse of microbubbles in liquids submitted to power ultrasound. The violent implosion of bubbles leads to the formation of chemically reactive species and to the emission of light, named sonoluminescence. In this manuscript, we describe the techniques allowing study of extreme intrabubble conditions and chemical reactivity of acoustic cavitation in solutions. The analysis of sonoluminescence spectra of water sparged with noble gases provides evidence for nonequilibrium plasma formation. The photons and the "hot" particles generated by cavitation bubbles enable to excite the non-volatile species in solutions increasing their chemical reactivity. For example the mechanism of ultrabright sonoluminescence of uranyl ions in acidic solutions varies with uranium concentration: sonophotoluminescence dominates in diluted solutions, and collisional excitation contributes at higher uranium concentration. Secondary sonochemical products may arise from chemically active species that are formed inside the bubble, but then diffuse into the liquid phase and react with solution precursors to form a variety of products. For instance, the sonochemical reduction of Pt(IV) in pure water provides an innovative synthetic route for monodispersed nanoparticles of metallic platinum without any templates or capping agents. Many studies reveal the advantages of ultrasound to activate the divided solids. In general, the mechanical effects of ultrasound strongly contribute in heterogeneous systems in addition to chemical effects. In particular, the sonolysis of PuO2 powder in pure water yields stable colloids of plutonium due to both effects.

Acoustic cavitation in liquids submitted to power ultrasound creates transient extreme conditions inside the collapsing bubbles, which are the origin of unusual chemical reactivity and light emission, known as sonoluminescence. In the presence of noble gases, nonequilibrium plasma is formed. The "hot" particles and the photons generated by collapsing bubbles are able to excite species in solution.

The chemical and physical effects of ultrasound arise not from a direct interaction of molecules with sound waves, but rather from the acoustic ...

In Depth Analyses of LEDs by a Combination of X-ray Computed Tomography (CT) and Light Microscopy (LM) Correlated with Scanning Electron Microscopy (SEM)

In failure analysis, device characterization and reverse engineering of light emitting diodes (LEDs), and similar electronic components of micro-characterization, plays an important role. Commonly, different techniques like X-ray computed tomography (CT), light microscopy (LM) and scanning electron microscopy (SEM) are used separately. Similarly, the results have to be treated for each technique independently. Here a comprehensive study is shown which demonstrates the potentials leveraged by linking CT, LM and SEM. In depth characterization is performed on a white emitting LED, which can be operated throughout all characterization steps. Major advantages are: planned preparation of defined cross sections, correlation of optical properties to structural and compositional information, as well as reliable identification of different functional regions. This results from the breadth of information available from identical regions of interest (ROIs): polarization contrast, bright and dark-field LM images, as well as optical images of the LED cross section in operation. This is supplemented by SEM imaging techniques and micro-analysis using energy dispersive X-ray spectroscopy.

In Depth Analyses of LEDs by a Combination of X-ray Computed Tomography CT and Light Microscopy LM Correlated with Scanning Electron Microscopy SEM

A workflow for comprehensive micro-characterization of active optical devices is outlined. It contains structural as well as functional investigations by means of CT, LM and SEM. The method is demonstrated for a white LED which can be still be operated during characterization.

In failure analysis, device characterization and reverse engineering of light emitting diodes (LEDs), and similar electronic components of ...

Engineering

Time-resolved Photophysical Characterization of Triplet-harvesting Organic Compounds at an Oxygen-free Environment Using an iCCD Camera

Here, we present a sensible method of the acquisition and analysis of time-resolved photoluminescence using an ultrafast iCCD camera. This system enables the acquisition of photoluminescence spectra covering the time regime from nanoseconds up to 0.1 s. This enables us to follow the changes in the intensity (decay) and emission of the spectra over time. Using this method, it is possible to study diverse photophysical phenomena, such as the emission of phosphorescence, and the contributions of prompt and delayed fluorescence in molecules showing thermally activated delayed fluorescence (TADF). Remarkably, all spectra and decays are obtained in a single experiment. This can be done for solids (thin film, powder, crystal) and liquid samples, where the only limitations are the spectral sensitivity of the camera and the excitation wavelength (532 nm, 355 nm, 337 nm, and 266 nm). This technique is, thus, very important when investigating the excited state dynamics in organic emitters for their application in organic light-emitting diodes and other areas where triplet harvesting is of paramount importance. Since triplet states are strongly quenched by oxygen, emitters with efficient TADF luminescence, or those showing room temperature phosphorescence (RTP), must be correctly prepared in order to remove any dissolved oxygen from solutions and films. Otherwise, no long-lived emission will be observed. The method of degassing solid samples as presented in this work is basic and simple, but the degassing of liquid samples creates additional difficulties and is particularly interesting. A method of minimizing solvent loss and changing the sample concentration, while still enabling to remove oxygen in a very efficient and a repeatable manner, is presented in this work.

Here, we present a method of the spectroscopic characterization of organic molecules by means of time-resolved photoluminescence spectroscopy on the nanosecond-to-millisecond timescale in oxygen-free conditions. Methods to efficiently remove oxygen from the samples and, thus, limit luminescence quenching are also described.

Here, we present a sensible method of the acquisition and analysis of time-resolved photoluminescence using an ultrafast iCCD camera. This system enables ...

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation ...

Luminescence Lifetime Imaging of O2 with a Frequency-Domain-Based Camera System

We describe a method to image dissolved oxygen (O2), in 2D at high spatial (&#60; 50-100 &#181;m) and temporal (&#60; 10 s) resolution. The method employs O2 sensitive luminescent sensor foils (planar optodes) in combination with a specialized camera system for imaging luminescence lifetime in the frequency-domain. Planar optodes are prepared by dissolving the O2-sensitive indicator dye in a polymer and spreading the mixture on a solid support in a defined thickness via knife coating. After evaporation of the solvent, the planar optode is placed in close contact with the sample of interest - here demonstrated with the roots of the aquatic plant Littorella uniflora. The O2 concentration-dependent change in the luminescence lifetime of the indicator dye within the planar optode is imaged via the backside of the transparent carrier foil and aquarium wall using a special camera. This camera measures the luminescence lifetime (&#181;s) via a shift in phase angle between a modulated excitation signal and emission signal. This method is superior to luminescence intensity imaging methods, as the signal is independent of the dye concentration or intensity of the excitation source, and solely relies on the luminescence decay time, which is an intrinsically referenced parameter. Consequently, an additional reference dye or other means of referencing are not needed. We demonstrate the use of the system for macroscopic O2 imaging of plant rhizospheres, but the camera system can also easily be coupled to a microscope.

Luminescence Lifetime Imaging of O2 with a Frequency-Domain-Based Camera System

We describe the use of a novel, frequency-domain luminescence lifetime camera for mapping 2D O2 distributions with optical sensor foils. The camera system and image analysis procedures are described along with the preparation, calibration and application of sensor foils for visualizing the O2 microenvironment in the rhizosphere of aquatic plants.

We describe a method to image dissolved oxygen (O2), in 2D at high spatial (&#60; 50-100 &#181;m) and temporal (&#60; 10 s) resolution. The method employs ...

Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals

In this work, we describe a protocol for a novel application of hyperspectral imaging (HSI) in the analysis of luminescent lanthanide (Ln3+)-based molecular single crystals. As representative example, we chose a single crystal of the heterodinuclear Ln-based complex [TbEu(bpm)(tfaa)6] (bpm=2,2&#8217;-bipyrimidine, tfaa&#8211; =1,1,1-trifluoroacetylacetonate) exhibiting bright visible emission under UV excitation. HSI is an emerging technique that combines 2-dimensional spatial imaging of a luminescent structure with spectral information from each pixel of the obtained image. Specifically, HSI on single crystals of the [Tb-Eu] complex provided local spectral information unveiling variation of the luminescence intensity at different points along the studied crystals. These changes were attributed to the optical anisotropy present in the crystal, which results from the different molecular packing of Ln3+ ions in each one of the directions of the crystal structure. The HSI herein described is an example of the suitability of such technique for spectro-spatial investigations of molecular materials. Yet, importantly, this protocol can be easily extended for other types of luminescent materials (such as micron-sized molecular crystals, inorganic microparticles, nanoparticles in biological tissues, or labelled cells, among others), opening many possibilities for deeper investigation of structure-property relationships. Ultimately, such investigations will provide knowledge to be leveraged into the engineering of advanced materials for a wide range of applications from bioimaging to technological applications, such as waveguides or optoelectronic devices.

Here, we present a protocol to obtain luminescent hyperspectral imaging data and to analyze optical anisotropy features of lanthanide-based single crystals using a Hyperspectral Imaging System.

In this work, we describe a protocol for a novel application of hyperspectral imaging (HSI) in the analysis of luminescent lanthanide (Ln3+)-based ...

Synthesis and Calibration of Phosphorescent Nanoprobes for Oxygen Imaging in Biological Systems

Oxygen measurement by phosphorescence quenching [1, 2] consists of the following steps: 1) the probe is delivered into the medium of interest (e.g. blood or interstitial fluid); 2) the object is illuminated with light of appropriate wavelength in order to excite the probe into its triplet state; 3) the emitted phosphorescence is collected, and its time course is analyzed to yield the phosphorescence lifetime, which is converted into the oxygen concentration (or partial pressure, pO2). The probe must not interact with the biological environment and in some cases to be 4) excreted from the medium upon the measurement completion. Each of these steps imposes requirements on the molecular design of the phosphorescent probes, which constitute the only invasive component of the measurement protocol. Here we review the design of dendritic phosphorescent nanosensors for oxygen measurements in biological systems. The probes consist of Pt or Pd porphyrin-based polyarylglycine (AG) dendrimers, modified peripherally with polyethylene glycol (PEG's) residues. For effective two-photon excitation, termini of the dendrimers may be modified with two-photon antenna chromophores, which capture the excitation energy and channel it to the triplet cores of the probes via intramolecular FRET (F&ouml;rster Resonance Energy Transfer). We describe the key photophysical properties of the probes and present detailed calibration protocols.

We present principles of oxygen measurements by phosphorescence quenching and review design of porphyrin-based dendritic nanosensors for oxygen imaging in biological systems.

Oxygen measurement by phosphorescence quenching [1, 2] consists of the following steps: 1) the probe is delivered into the medium of interest (e.g. blood ...

Biology

A Novel Technique for Generating and Observing Chemiluminescence in a Biological Setting

Intraoperative imaging techniques have the potential to make surgical interventions safer and more effective; for these reasons, such techniques are quickly moving into the operating room. Here, we present a new approach that utilizes a technique not yet explored for intraoperative imaging: chemiluminescent imaging. This method employs a ruthenium-based chemiluminescent reporter along with a custom-built nebulizing system to produce ex vivo or in vivo images with high signal-to-noise ratios. The ruthenium-based reporter produces light following exposure to an aqueous oxidizing solution and re-reduction within the surrounding tissue. This method has allowed us to detect reporter concentrations as low as 6.9 pmol/cm2. In this work, we present a visual guide to our proof-of-concept in vivo studies involving subdermal and intravenous injections in mice. The results suggest that this technology is a promising candidate for further preclinical research and might ultimately become a useful tool in the operating room.

This protocol describes a new intraoperative imaging technique that uses a ruthenium complex as a source of chemiluminescent light emission, thereby producing high signal-to-noise ratios during in vivo imaging. Intraoperative imaging is an expanding field that could revolutionize the way that surgical procedures are performed.

Intraoperative imaging techniques have the potential to make surgical interventions safer and more effective; for these reasons, such techniques are ...

Bioengineering

Time-Resolved Photoluminescence Spectroscopy of Semiconductor Nanocrystals and Other Fluorophores

Time-resolved photoluminescence (TRPL) is a key technique for understanding the photophysics of semiconductor nanocrystals and light-emitting materials in general. This work is a primer for setting up and conducting TRPL on nanocrystals and related materials using single-photon-counting (SPC) systems. Basic sources of error in the measurement can be avoided by consideration of the experimental setup and calibration. The detector properties, count rate, the spectral response, reflections in optical setups, and the specific instrumentation settings for single photon counting will be discussed. Attention to these details helps ensure reproducibility and is necessary for obtaining the best possible data from an SPC system. The main aim of the protocol is to help a student of TRPL understand the experimental setup and the key hardware parameters one must generally comprehend in order to gain useful TRPL data in many common single-photon-counting setups. The secondary purpose is to serve as a condensed primer for the student of experimental time-resolved luminescence spectroscopy.

This paper presents an experimental how-to on time-resolved photoluminescence. The hardware used in many single photon-counting setups will be described and a basic how-to will be presented. This is intended to help students and experimenters understand the key system parameters and how to correctly set them in time-resolved photoluminescence setups.

Time-resolved photoluminescence (TRPL) is a key technique for understanding the photophysics of semiconductor nanocrystals and light-emitting materials in ...

Hydrothermal Synthesis of Persistent Luminescent Nanoparticles

Synthesis of Persistent Luminescent Nanoparticles for Rewritable Displays and Illumination Applications

Persistent luminescent nanoparticles (PLNPs) possess the capabilities to maintain extended longevity and robust emission even after the excitation has ceased. PLNPs have been widely used across various domains, including information displays, data encryption, biological imaging, and artistic decoration with sustained and vivid luminosity, providing boundless possibilities for a variety of innovative technology and artistic projects. This protocol focuses on an experimental procedure for the hydrothermal synthesis of PLNPs. The successful synthesis of enduring luminescent nanomaterials with Mn2+ or Cr3+ serving as a luminescent center in Zn2GeO4: Mn (ZGO: Mn) or ZnGa2O4: Cr highlights the universality of this synthetic method. On the other hand, the optical properties of ZGO: Mn can be changed by adjusting the pH of precursor solutions, demonstrating the tunability of the protocol. When charged with ultraviolet (UV) at a wavelength of 365 nm for 3 min and then stopped, PLNPs exhibit the remarkable capacity to generate afterglow efficiently and consistently, which makes them ideal for making two-dimensional rewritable displays and three-dimensional transparent, luminous artworks. This protocol outlined in this paper provides a feasible method for the synthesis of persistent luminescent nanoparticles for further illumination and imaging applications, opening up novel prospects for the fields of science and art.

Author Spotlight: Advancing Bioimaging and Therapy with Functional Nanomaterials

A protocol is presented for the synthesis of persistent luminescent nanomaterials (PLNPs) and their potential applications in rewritable displays and artistic processing utilizing the afterglow effect under ultraviolet light (365 nm) irradiation.

Persistent luminescent nanoparticles (PLNPs) possess the capabilities to maintain extended longevity and robust emission even after the excitation has ...

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors

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