Anal. Chem. 2001, 73, 5472-5476
Characterization of Phosphor Materials for Use In Plasma Display Panel by Time-Resolved Vacuum-Ultraviolet Laser Spectrometry Yasuyuki Hirakawa, Koji Nakamura, and Totaro Imasaka*
Department of Chemical System and Engineering, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-Ku, Fukuoka 812-8581, Japan
Phosphor materials that were manufactured for use in a plasma display panel (PDP) were investigated by employing a newly designed time-resolved vacuum-ultraviolet (VUV) spectrometer, which consists of a pulsed VUV laser and a fast photodetector. The VUV spectrometer was used to collect quantum efficiency data as well as the rise and decay times for the PDP phosphor luminescence. Both the rise and decay times increased with decreasing excitation wavelength in the VUV region. This result can be explained by a change in the mechanisms of photoexcitation and luminescence, that is, from charge-transfer excitation to host-lattice excitation below 200 nm. The present instrument was also used for an evaluation of the phosphor materials (Ba1-xMgAl10O17:Eu2+x) by changing the Eu2+ concentration. The obtained data suggest that the impurities and defects are located inside the host crystal. Thus, the VUV spectrometer constructed in this study has considerable potential for use in investigating the nature of PDP phosphor materials. Flat display panels (FDP) have been developed to replace the cathode-ray tube (CRT) that is currently used for a display in television sets and computers. A plasma display panel (PDP) represents one of the candidates for a use in a large-scale FDP device, especially for the case of wall-mounted television sets. A PDP device has distinct advantages over a CRT in that it is completely flat and requires a low voltage for its operation. Although PDPs are currently available commercially, they have several problems that must be solved before they can be used to replace CRTs. A major problem involves its poor efficiency (∼0.4-1 lm/W) in converting electricity to light, since the efficiency is much less than those of CRT (∼2-5 lm/W) and liquid crystal displays (LCDs) (∼2-3.5 lm/W). In PDP, three primary colors are emitted from three types of inorganic phosphors, which are excited by VUV emission (147 and 173 nm) generated by a discharge of xenon gas. To improve the efficiency of PDPs, several approaches have been proposed thus far.1,2 Because the efficiency of the electronic circuit for a discharge has already been developed * Phone: +81-92-642-3563. Fax: +81-92-632-5209, E-mail: imasaka@ cstf.kyushu-u.ac.jp. (1) Ronda, C. R. J. Lumin. 1997, 72-74, 49. (2) Hirakawa, Y.; Tokunaga, N.; Nakashima, K.; Imasaka, T. Anal. Chim. Acta 1999, 389, 69.
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by many manufacturers, it will be necessary to improve the quantum efficiency of the phosphor materials. However, only trialand-error approaches have been conducted regarding studies on the PDP phosphor material, because no investigations concerning photoexcitation and luminescence mechanisms have yet been reported, and no guidelines for the development of the PDP phosphor materials have been reported, although many studies have been reported for luminescence properties of solid phosphors, such as scintillators.3 This is probably due to the lack of development of an analytical instrument that can be used to examine the dynamic processes that are involved in this area. In a previous study, we reported on the construction of a VUV spectrometer for use in the measurement of the rise and decay times of phosphor luminescence.2 This spectrometer consisted of a pulsed VUV laser generated by stimulated Raman scattering (SRS) and subsequent four-wave Raman mixing (FWRM) in molecular hydrogen. In this approach, many emission lines with spacings of 4155 cm-1 were generated. The laser was line-tunable, and because of this, the excitation wavelength could be adjusted by selecting one of the emission lines. However, a serious problem was encountered in that no appreciable luminescence could be observed from the phosphors above the eighth order of anti-Stokes beam (190 nm) was measured directly by a pyroelectric joule meter (Molectron, J3-05DW). At the prism’s surface, the fundamental and lower-order anti-Stokes beams were strongly scattered and constituted a major source of the stray light, which complicates the luminescence measurement. To remove the stray light, a VUV monochromator (ARC, VM502; 30-600 nm; maximum spectral response, 120 nm) is inserted into the optical beam path between the CaF2 prism and the PDP phosphor. A VUV mirror is located inside the monochromator to change the direction of the beam to another exit slit,where a VUV photomultiplier (Hamamatsu, R1459, 115200 nm) is located. The present configuration allows the measurement of the ratio of the intensities for the high-order anti-Stokes beam and the stray light that arises from the fundamental and low-order anti-Stokes beams. This procedure was used to decrease the stray light to negligible levels. By flipping off the mirror, the VUV laser beam can be directed toward the phosphor. A CaF2
lens (focal length, 300 mm) is placed after the Raman cell to focus the VUV beam between the entrance slit and the concave grating in the monochromator. The VUV beam is subsequently focused by the concave grating onto a phosphor that is attached to the sample holder. This unorthodox optical configuration was used to minimize the number of VUV optics and the optical damage to the VUV monochromator. The luminescence from the phosphor is collected by a glass lens (BK7; focal length, 60 mm) onto the slit of a visible (VIS) monochromator (Jasco, CT-10) equipped with a high-speed microchannel-plate photomultiplier (Hamamatsu, R1294, 1 GHz). The electric signal was measured and averaged by a digital storage oscilloscope (LeCroy, 9360, 600 MHz). The phosphor material used in this study was Ba1-xMgAl10O17:Eu2+x, which is designed for a blue color. All of the beam paths of the 355-nm laser in the air were covered with brass tubes for eye safety and for protection of skin. In addition, because of a regulation in the laboratory, all of the researchers are requested to wear UV-cut glasses during the experiment. RESULTS AND DISCUSSION VUV Laser Source. The emission spectrum of the VUV laser generated by SRS and FWRM is shown in Figure 2. A VUV beam is generated up to the tenth order of the anti-Stokes emission (144 nm). By taking into account the spectral response of the VUV spectrometer, including the monochromator, the prism, and the photomultiplier, the pulse energy of the ninth-order anti-Stokes beam was estimated to be 0.25 µJ. The intensity of the VUV beam observed in the region from 160 to 180 nm is relatively weaker than that of the ninth-order anti-Stokes beam, which is probably Analytical Chemistry, Vol. 73, No. 22, November 15, 2001
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Figure 2. Emission spectrum of the VUV laser generated by SRS/ FWRM. AS, anti-Stokes beam. The spectral responses of the monochromator, the photomultiplier, and the optics are uncorrected. The seventh- and eighth-order anti-Stokes emissions are attenuated as a result of light absorption by molecular hydrogen. See the text for details.
due to the absorption of light by the Lyman band of molecular hydrogen.4 Therefore, the eighth order of the anti-Stokes beam was not used in the present study. The pulse energy of the VUV beam was adjusted to 0.25 µJ throughout this work to avoid any effects that might arise from differences in the exciting intensity. This was achieved by calibrating the intensity using sodium salicylate, because its quantum efficiency is well-known, and it is often used for calibration of emission intensity in the VUV region.5 To accomplish this, a pellet prepared from sodium salicylate powder was placed in one of the sample holders in the turret. Time-Resolution. The time resolution of the VUV spectrometer was estimated from the pulse width of the VUV laser generated using a 3-ns Nd:YAG laser and the response time of the microchannel-plate photomultiplier (rise time, 300 ps) and a digital oscilloscope (rise time, 600 ps). Although the pulse width of the VUV laser was not measured, it was estimated to be much shorter than 1 ns because of the nonlinear nature of SRS/FWRM, since the intensity decreases exponentially with decreasing intensity of the exciting beam in SRS and quadratically to the intensity of the exciting beam, as well as proportionally to the intensity of the generated anti-Stokes beam in FWRM. In fact, the pulse width is reduced by a factor of 7.5 ( ) 15 ns/2 ns) for the eighth-order anti-Stokes beam of the excimer laser6 and 7.3 ( ) 6 ns/0.85 ns) for the eighth-order anti-Stokes beam of a dye laser pumped by a Nd:YAG laser.7 These results suggest that the pulse width of the VUV laser is less than 400 ps, which corresponds to a rise time of ca. 200 ps. Therefore, the rise time of the total system is estimated to be ca. 700 ps. Thus, the VUV spectrometer developed in this study allows measurements of transient phenomena, the rise and decay times of which are of the order of nanosecond. (4) Herzberg, G.; Howe, L. L. Can. J. Phys. 1959, 37, 636. (5) Samson, J. A. J. Opt. Soc. Am. 1964, 54, 6. (6) Spaan, M.; Goehlich, A.; von der Gathen, S.; Do ¨bele, H. F. Appl. Opt. 1994, 33, 3865. (7) Wallmeier, H.; Zacharias, H. Appl. Phys. B 1988, 45, 263.
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Figure 3. Emission spectra of the PDP phosphor (Ba1-xMgAl10O17: Eu2+x; x ) 0.1, 0.01, 0.001). No remarkable change is observed in the profile of the spectrum, even when the value of x is changed.
Spectrum. The effect of europium content, x, on the luminescence spectrum was investigated using the sixth-order anti-Stokes beam. The result is shown in Figure 3. This peak observed in the blue region is attributed to 4f-5d transition of Eu2+. No remarkable change was observed in the profile of the spectrum, although the signal intensity was altered for samples in which x ) 0.1, 0.01, 0.001. The situation was unchanged at different exciting wavelengths. Therefore, no serious change appears to occur in the emission species and the luminescence process, as long as the UV/VUV laser is used as an exciting source in the range from 150 to 280 nm. The wavelength of the VIS monochromator was then fixed at the maximum for the peak, that is, 450 nm, throughout this experiment. Effect of Laser Intensity. The phosphor materials used in a commercial PDP device are coated on a substrate in an area of 0.1 mm2 and are illuminated by a VUV light at power levels of ∼30-40 µW (repetition rate, 24 Hz; pulse width, 1 µs),8 which corresponds to ∼1.1-1.4 × 1019 photons s-1 mm2. In the present study, the VUV laser, which had a pulse energy of 0.25 µJ and an estimated pulse width of 400 ps, illuminated the phosphor over an area of 13 mm2. This corresponds to a light intensity of 3.86.9 × 1019 photons s-1 mm2. Thus, the intensity of the laser beam used in the present study is similar to that used in an actual PDP device. To avoid the effect of the exciting light intensity on the characteristics of the phosphor, the rise and decay times were measured at different intensities. As shown in Figure 4, no appreciable change is observed in the rise time, but the decay time slightly decreases with increasing exciting intensity; therefore, care should be exercised in quantitative discussions of the decay time. These results suggest that the analytical instrument developed in this study has potential for use in investigations of the dynamic processes of phosphor materials used for PDP. Quantum Efficiency. The relative quantum efficiency of the phosphor material was measured by changing the excitation wavelength in the region from 152 to 274 nm. Figure 5 shows the observed quantum efficiency, indicating that an optimum value is obtained in the vicinity of 250 nm. The observation of this (8) Kurashige, M. Private communication.
Figure 4. Effect of photon density of the exciting source on rise and decay times. The hatched regions indicate the photon densities of the exciting sources used in the present study and in the PDP device, respectively. The sixth anti-Stokes emission (188 nm) was used in this experiment.
Figure 5. Effect of excitation wavelength on the relative quantum efficiency of phosphor luminescence. The content of the Eu2+ ion is indicated in the figure.
maximum can be explained as charge-transfer-band (CTB) excitation, in which UV light is more efficiently absorbed than VUV light and is emitted in the form of luminescence in the VIS region.1 On the other hand, the quantum efficiency increases with decreasing wavelengths below 200 nm. This phenomenon can be explained by excitation of the Eu2+ ion through host-lattice excitation, as will be described below. The luminescence intensity obtained by VUV excitation is weaker than that obtained by UV excitation. This suggests that host-lattice excitation is relatively inefficient. This might be explained by the fact that quenching in the photoexcitation process via CTB is either absent or very low, but a quenching process through host-lattice excitation proceeds via impurities and defects in the host crystal, as will be described in the following sections. Decay Time. The decay time was also measured in the investigation of the relaxation process. As shown in Figure 6, the decay time gradually increases with decreasing excitation wave-
Figure 6. Effect of excitation wavelength on rise and decay times of phosphor luminescence. The Eu2+ ion content is indicated in the figure.
length below 200 nm. One possible reason for this is the difference in the relaxation processes for excitations below and above 200 nm: a hole and an electron are induced by the VUV light below 200 nm and reach an activator ion and then relax to a luminescent species. On the other hand, the luminescent species may be directly excited though CTB. The result implies that the rate of relaxation from the activator ion to the Eu2+ ion becomes slower than the decay rate of the luminescence. The decay time becomes slightly shorter for samples that contain high concentrations of Eu2+ ions. This phenomenon may be explained by concentration quenching, in which a different luminescent species (e.g., a dimer) is formed, thus decreasing the decay rate, even though the quantum efficiency increases with a reduction in luminescence quenching by other species, such as impurities and defects that are present in the host crystal. Thus, the decay time provides information concerning the relaxation mechanism and the quenching process. Rise Time. The rise time of the luminescence provides information concerning the photoexcitation passageway from the laser radiation to the luminescent species in the phosphor material. Three types of photoexcitation processes have been proposed for phosphor materials: (1) direct excitation of the luminescent species in the VIS region, (2) CTB excitation in the UV region, and (3) host-lattice excitation in the VUV region.1 When the phosphor material is excited in the VUV region, the exciting beam does not penetrate sufficiently into the phosphor material, because the absorption cross section of the host crystal increases with decreasing wavelength. As a result of this, only the surface of the phosphor is excited, producing a hole and an electron pair that migrate into the host crystal to the activator ion, which is subsequently relaxed to the luminescent species. It is generally assumed that a considerable number of impurities and defects exist in the phosphor and that they act as quenchers. This reduces the quantum efficiency and also the rise time of the luminescence. As shown in Figure 6, the rise time gradually increases with decreasing wavelength below 200 nm, at which point the excitation mechanism might be changed from CTB excitation to host-lattice excitation. The former is thought to have essentially a zero or very short rise time, and the latter has a rise time that corresponds Analytical Chemistry, Vol. 73, No. 22, November 15, 2001
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to the time required for the hole and the electron to migrate to the activator ion: it is also possible to explain this period as a time duration that the hole and the electron are trapped in a shallow potential. Thus, the rise time may provide information concerning the dominant process, that is, CTB excitation vis-a`vis host-lattice excitation. Effect of Eu2+ Concentration. It is known that the excitation energy is dissipated by the trapping of the hole and the electron by impurities and defects in the host-lattice excitation in the VUV region, which decreases the quantum efficiency of the luminescence. Two locations are possible for such impurities and defects, that is, either on the surface of or inside the host crystal. It is important to investigate the location of impurities and defects and to clarify the quenching mechanism for the development of phosphor materials. If no impurities and defects are present, either on the surface or inside the host crystal, the quantum efficiency will be high and the rise time, long. When the impurities and defects are present on the surface of the host crystal, the quantum efficiency becomes low and the rise time remains constant. This is because the hole and the electron, which migrate to the interior of the host crystal, are no longer affected by impurities and the defects. For example, if a hole and an electron would migrate to the surface of the host crystal, they would be trapped by impurities or the defects and lose the energy, and as a result, they would not contribute to the luminescence. When impurities and the defects are present inside the host crystal, the quantum efficiency becomes low, and the rise time becomes shorter, because the migration time is reduced by quenching the hole and the electron inside the host crystal. The quenching process is also affected by the concentration of Eu2+ ions. At high Eu2+ concentrations, the quantum efficiency and the rise time remain unchanged when impurities and defects are present at the surface of the phosphor material, because the excitation energy is lost in the early stages of photoexcitation prior to the interaction with the Eu2+ ion. When they are present in the interior of the crystal, the quantum efficiency increases, since the hole and the electron reach the Eu2+ ion before quenching and the rise time decreases, because the migration time is reduced by the increased concentration of Eu2+ ions. When the concentration of the Eu2+ ion is too high and concentration quenching is not negligible, the quantum efficiency and the rise time may decrease substantially. The direct excitation of the luminescent species through CTB provides a very short (or essentially zero) rise time, and the quantum efficiency is relatively high, as described above. As shown in Figure 6, the quantum efficiency increases with decreasing excitation wavelength, at which host-lattice excitation is dominant. This is explained by the larger photon energy that is produced at shorter wavelengths, generating a hole and electron pair which have sufficient energies to migrate inside the host
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crystal. On the other hand, the rise time increases with decreasing wavelength. This can also be explained by penetration of a hole and electron into the crystal, because they must migrate a long distance before reaching the activator ion. This is in striking contrast to the case of CTB excitation, in which the rise time is very small or essentially zero. As observed in Figure 6, the quantum efficiency increased and the rise time decreased with increasing Eu2+ concentration, in the case of VUV excitation. As described previously, these results suggest that the crystal is not perfect and that impurities and the defects are present in the interior of the crystal. Thus, the observation of the rise time and its dependence on Eu2+ concentration provides valuable information concerning the mechanism of phosphor luminescence. A new type of phosphor material has recently been developed for purposes of improving luminescence efficiency in which two visible photons are generated by a single VUV photon.8 This is based on the concept of reducing the energy of the radiationless transition, because only 1/5-1/3 of the energy is used for the generation of a single VIS photon; however, the luminescence and quenching mechanisms for this process have not yet been investigated. Thus, the instrument described here may be useful for the investigation of the dynamic process of such new types of phosphor materials, with the goal of improving the luminescence properties. CONCLUSION The characteristics of a phosphor material developed for use in PDPs have been investigated using a VUV spectrometer that is capable of measuring the relative quantum efficiency and the rise and decay times of the luminescence. The PDP phosphor was excited through host-lattice excitation in the VUV region, and the rise time, which is determined by a recombination of a hole and an electron, was determined to be a few nanoseconds, depending on the Eu2+ concentration. The rise time decreased with increasing concentrations of Eu2+ ion, suggesting that quenching occurs during the electron-hole migration process in the host crystal. Thus, the VUV spectrometer developed in this study will be useful for evaluating phosphor materials and may provide guidelines for the development of new types of phosphor materials for PDP. ACKNOWLEDGMENT The authors thank Hideki Fujii and Takayuki Ohnishi of Daiden Corporation (Fukuoka, Japan) for their generous gift of PDP phosphors. Received for review February 6, 2001. Accepted August 30, 2001. AC010162T
