Luminescence
Introduction
A phenomenon in which the electronic state of a substance is excited by some kind of external energy (electromagnetic radiation, electron beam, electric field, etc.) and the excitation energy is given off as light is called luminescence. Phosphors, in narrow sense, are luminescent inorganic substances, usually those in powder form, synthesized for the purpose of practical application.
Nowadays, phosphors have undeniable importance for our everyday life. They are applied in light sources (fluorescent lamps, phosphor-converted white LEDs), TV sets and computer displays (cathode-ray tubes, plasma display panels, vacuum fluorescent displays, field emission displays, electroluminescence displays, etc.), detector systems (X-ray scintillators, UV-Vis radiation converters), and other long-existing or recently developed uses as, for example, luminous paints and biological labeling.
Although phosphors have been systematically studied for more than a century, there is still a need for further research.
Research on phosphors requires the use of number of fields of science and technology. Synthesis and preparation of inorganic phosphors are based on physical and inorganic chemistry. Luminescence mechanisms are interpreted on the basis of solid-state physics. Research and development of the applications of phosphors mainly belong to the fields of illuminating engineering, electronics, and image engineering.
Currently, the prevailing industrial phosphor synthesis method is solid-state firing, which includes several steps: blending of solid raw materials, firing, crushing, particle classification, washing, surface treatment, and final sieving. The obtained phosphor powder must be then deposited onto a screen by a so-called slurry process or, in a simpler case of monochromic screens, by gravitational sedimentation. The latter deposition method offers good reproducibility and low material losses; however, it comes at the cost of low processing speed and deterioration of phosphor properties by necessary binding additives. Figure 1 represents an example of the screen produced by conventional methods.
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| Fig. 1: Phosphor screen obtained by gravitational sedimentation of commercial Y2O3:Eu powder (picture is taken by scanning electron microscopy). |
Current research activities are directed on development of more cost-effective and less time consuming methods of phosphor synthesis and device fabrication and, of course, on improving of application-specific characteristics of phosphors as, for example, phosphor particle size.
Indeed, there are several applications of phosphors that would benefit from decreased size of phosphor particles, starting from fine-pitch displays, where the optimum phosphor particle size is on the order of 1 micrometer, and ending with bio-labeling, where nanosized phosphor particles are absolutely necessary.
One of the fields where submicron phosphor particles can be successfully applied is deep ultraviolet photolithography used in manufacture of state-of-the-art electronic chips. The resolution requirements imposed on patterning processes constantly become stricter and the improvement of resolution of UV-Vis phosphor converters would provide a clear advantage for systems utilized for adjustment of excimer laser beams and inspection of the mask integrity.
It must be pointed out that the development of the phosphor deposition process for submicron phosphor particles imposes a big challenge because neither solid-state firing synthesis nor gravitational sedimentation can be deployed in this case.
Of course, there are several proposed solutions for this problem. Fine phosphor powder can be produced, for example, by chemical co-precipitation and deposited onto the screen by electrophoretic deposition method. Nonetheless, there are several other promising options to be investigated.
Flame-Assisted Spray Pyrolysis is one of such potential methods for deposition of phosphor coatings with submicron phosphor particle size. The basic idea is simple: solution of particular metal salts is put into an ultrasonic nebulizer. The nebulizer produces small droplets, which then are transported into the flame, where droplet-to-particle conversion takes place. The solvent evaporates, metal salts decompose and solid particle of corresponding oxides can be collected downstream with a filter or (as shown in Figure 2) can be directly deposited onto the substrate (fine particles of aerosol firmly stick to any surface they contact).
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| Fig. 2: FASP deposition setup |
FASP was developed in early 1990s and, since then, it has been used for production of a great variety of functional oxides including all most practical oxide-based phosphors, e.g., SrTiO3:Pr, Al, Sr5(PO4)3Cl:Eu, BaMgAl10O17:Eu, Y2O3:Eu, Gd2O3:Eu, Y2SiO5:Eu, Y3Al5O12:Ce. The method has a significant advantage over existing phosphor synthesis methods: it can continuously produce submicron phosphor particles with uniform dopant distribution due to mixing of components at a molecular level. Size of the phosphor particles can easily be controlled by changing the initial concentration of precursor solution.
Direct deposition of phosphor coatings by FASP provides a further advantage in that it combines phosphor synthesis and deposition within one single technological step. Furthermore, FASP deposition does not require utilization of any binding chemicals However, the performance of phosphor coatings directly deposited by flame spray pyrolysis had never been studied before 2002, when the first experiments on FASP deposition of phosphor coatings started in our institute at the TUHH.
Goals
Our aim is, first, to comprehensively describe relations between FSPD process parameters and physical properties of deposited coatings (mean particle size and shape of particle size distribution, coating deposition rate, porosity of the coating, particle morphology, crystallite size, etc.).
The size of phosphor particles affects the achievable resolution of the screens. Flame spray pyrolysis is capable of producing solid particles in the range from several µm down to a few nanometers. Spraying facilitates effective screen deposition of fine and ultrafine particles by thermophoresis. The coating deposition rate is dependent on the temperature and positioning of the substrate, nebulization rate, and air currents in ambience. The temperature of the flame also strongly affects the crystallinity of synthesized particles, particle morphology and substrate temperature. Future industrial application of FSPD will require in-depth knowledge and understanding of these relations.
The second major research goal is thorough optical characterization of FSPD phosphor coatings and determination of general trends in optical performance throughout the available range of phosphor particle sizes and coating thicknesses.
The dependence of screen characteristics on phosphor particle size has never been systematically studied in the sub-micrometer range. FSPD provides flexibility and simple control of the particle size starting from the values comparable with commercial phosphor powders deep into the nano-range.
For the particles with the size comparable to the wavelength of exciting radiation, scattering dominates the overall light extinction. For smaller particles, scattering decreases and becomes negligible for nanoparticles (on the order of several nanometers). Packing density of ultrafine particles in layers deposited by flame spray pyrolysis is relatively low. Porosity can be advantageous for the brightness of photoluminescence but negatively influences the resolution of the phosphor screen. It would be beneficial to find a compromise between the screen resolution and PL-intensity.
Results
Figure 3 demonstrates a FASP-deposited layer of Y2O3:Eu. The average size of particles is below 1 µm (geometric mean for 0.2 M precursor solution, mostly used in experiments, was about 440 nm).
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| Fig. 3: Y2O3:Eu phosphor coating deposited by flame-assisted spray pyrolysis |
Thicknesses ranging from 0.06 to 1.2 mg/cm2 were successfully obtained. The mean phosphor particle size was varied in the range from 180 nm (0.01 M) to 500 nm (0.5 M) for aqueous solutions.
The brightness of photoluminescence measured in transmission mode strongly depends on the thickness of the coating. There is an optimum value, which depends on phosphor particle size. For 0.2 M solutions, the highest PL-intensity was measured for coating densities of about 0.5 µm/cm2 (Figure 4). Samples with optimum Eu3+-content achieve brightness of the best screens produced by sedimentation of commercial phosphor powder.
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| Fig. 4: Dependence of brightness on thickness of the coating (excitation at 254 nm) |
Dependence of optical performance of phosphor plates on thickness of deposited layers can be visualized by absorption (attenuance) spectroscopy. The measured value of absorbance actually consists of two terms: the light absorbed by the particles of phosphor (some part of it is reemitted at some other wavelengths as a result of the photoluminescence) and the light scattered by the surface of the particles.
The scattering strongly depends on the size of the particles, its maximum correlates with the average particle size. One can see it, for example, when comparing the light extinction spectra of the coatings with the same coating density but different particle size (Figure 5: 0.2 M solution - 440 nm (geometric mean particle size), 0.01 M - 180 nm).
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| Fig. 5: Absorption spectra of phosphor coatings of different thickness |
The as-prepared Y2O3:Eu particles had rough surface due to the insufficient temperature of the propane/air premixed flame. One can change the viscosity of precursor solution at the stage of evaporation of the liquid in the flame by addition of polymeric components. A mixture of a carboxylic acid and a glycol polymerizes upon heating and, thereby, the surface condition of the phosphor particles could be significantly improved (Figure 6).
Spherical dense morphology of the particles appears to be essential for optical characteristics of FSP-phosphor coatings in the range of vacuum UV. The measurements of excitation spectra performed with the use of synchrotron radiation demonstrate a significant advantage of Y2O3:Eu particles deposited with optimized content of citric acid and ethylene glycol over no-polymer phosphor coatings (Figure 7).
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| Fig. 6: ASEM picture of phosphor particles synthesized with addition of polymers | Fig. 7: Excitation spectra of FSPD coatings |
For the wavelengths shorter than 190 nm, there is a significant difference between the samples deposited with and without polymeric precursors. The screening density of the sample deposited by polymer-assisted FSP-procedure is at least three times smaller than that of the reference sample deposited by sedimentation
Summarizing the knowledge gained up to now, the FSP-deposited phosphor coatings are exceptionally suited for application in converters of deep UV radiation in visible light:
- High quantum efficiency, brightness of photoluminescence is comparable to commercial phosphors deposited by sedimentation but with significantly lower coating densities;
- High coating uniformity and shock resistance;
- No binding of tackifying agents required;
- Acceptable destruction threshold value of laser fluence (~30 mJ/cm2);
- Very simple and rapid single-step processing;
- Simple control of phosphor particle size and spherical morphology of particles.
However, the current realization of FSPD-process has several disadvantages that can be partly eliminated by the further development of the setup and the coating procedure:
- Oxide-based composition of the final products;
- Relatively high thermal load on the substrate (it is difficult to coat a CCD directly);
- Significant overspraying;
- Difficult control over the thickness of deposited coating.
The first results were published in the Journal of Electrochemical Society, [1], and presented at the International Display Workshops 2006 (Otsu, Japan), [2].
References
- R. Kubrin, W. Bauhofer, A. Ivankov, No-Binder Screening of Fine Phosphor Powders by Flame Spray Pyrolysis, Journal of Electrochemical Society, v. 154 (9), pp. J253-J261, 2007.
- R. Kubrin, W. Bauhofer, A. Ivankov, The Use of Polymeric Precursors in a Low-Temperature Flame Spray Pyrolysis, International Display Workshops 06 , pp. 1247-1250, Otsu, Japan, December 6-8, 2006.