- Email: [email protected]
Luminescence properties of nanostructured alumina ceramic
Radiation Measurements 43 (2008) 341 – 344 www.elsevier.com/locate/radmeas
Luminescence properties of nanostructured alumina ceramic V.S. Kortov a , A.E. Ermakov b , A.F. Zatsepin a , S.V. Nikiforov a,∗ a Department of Technical Physics, The Ural State Technical University, 19 Mira Street, UPI, 620002 Ekaterinburg, Russia b Institute of Metal Physics, Ural Branch RAS, Ekaterinburg, Russia
Abstract Photo- and cathodoluminescence spectra of the nanostructured Al2 O3 ceramic and anion-defective -Al2 O3 crystals are studied under comparable conditions. Luminescence bands of the centers produced by oxygen vacancies are detected in samples of both types. The nanostructured ceramic is characterized by a new band at 3.4 eV and the reduction of the luminescence decay time. The specific features of the luminescence of the nanostructured ceramic can be related to the presence of nonequilibrium phases and specificity of relaxation processes. © 2007 Elsevier Ltd. All rights reserved. Keywords: Aluminum oxide; Nanoceramic; Photoluminescence; Oxygen vacancies
1. Introduction Anion-defective -Al2 O3 crystals are efficient phosphors and dosimeters and their properties have being studied intensively (Akselrod et al., 1990; McKeever et al., 1995; Lo et al., 2006). These crystals contain a relatively high concentration of oxygen vacancies forming electron trapping centers similar to F-centers in alkali halide crystals. It is known that nanostructured modifications of Al2 O3 include nonequilibrium phases, in which oxygen nonstoichiometry is possible. In this connection, it is interesting to study luminescence properties of the nanostructured aluminum oxide and compare them with analogous properties of crystalline samples. The present study deals with the analysis of photo- and cathodoluminescence in the nanostructured Al2 O3 ceramic and -Al2 O3 :C single crystals under comparable conditions. 2. Making and certification of samples, experimental technique The aluminum oxide nanopowders were prepared by the gaseous phase method (Yermakov et al., 2005), which can be summarized as follows. A molten aluminum droplet is heated ∗ Corresponding author.
E-mail address: [email protected] (S.V. Nikiforov). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.008
and held by a high-frequency electromagnetic field. A gas mixture of argon and oxygen taken in some ratio flows around the droplet. The metal vapor is carried by the gas to the cold zone where the vapor condenses and is oxidized with oxygen of the gas mixture. The powder accumulates in a filter. The size of the powder particles depends on the droplet temperature, the gas pressure and the gas feed rate. It can be several nanometers to hundreds of nanometers. Two batches of the aluminum oxide powders prepared by the gaseous phase method were used in this study. The synthesis parameters and the specific surface of the powders are given in Table 1. The electron microscopy examination of Al2 O3 nanoparticles in the powders showed that they had a spherically symmetrical shape and a relatively uniform size distribution. The average size of the nanoparticles was about 30–40 nm and less than 10 nm in the nanopowders with the specific surface of 23 and 95 m2 /g, respectively. Samples of the nanostructured ceramic were made by compaction of the powders in a collapsible mold under a load of about 2 t and sintering in air at 500 ◦ C for 1 h. The samples were opaque dense disks 5 mm in diameter and about 1 mm thick. According to the results of the X-ray diffraction analysis, the cubic and tetragonal phases of the aluminum oxide were present in the powders. Lines of the orthorhombic phase and a small amount of pure Al were also observed in the diffraction patterns. Thus, the X-ray diffraction data suggested that the test samples of the Al–O powders in the two batches had different
342
V.S. Kortov et al. / Radiation Measurements 43 (2008) 341 – 344
200
Batch no.
Ar feed rate (l/h)
O2 feed rate (l/h)
Pressure (Torr)
Specific surface (m2 /g)
160
1 2
170 80
11 11
50 150
95 23
3. Results and discussion As is known, main luminescence centers in crystals of the anion-defective aluminum oxide are F-centers produced by oxygen vacancies, which have trapped two electrons (Akselrod et al., 1990; McKeever et al., 1995). Therefore, PL was studied in the excitation (205 nm) and luminescence (410 nm) spectral ranges of these centers. Figs. 1 and 2 present PL spectra and PL excitation spectra of the nanostructured ceramic with the specific surface of 23 m2 /g and the anion-defective crystals of the aluminum oxide. It is seen that PL is excited in both the nanostructured and crystalline samples over the optical absorption range of F-centers, while the PL intensity is maximum in the luminescence band of these centers. Some widening of the excitation and luminescence bands in the nanostructured ceramic samples can be noted. PCL spectra of the test samples are shown in Fig. 3. PCL spectra of the single crystals, which were measured at 80 K (Fig. 3a) and 300 K (Fig. 3b), include known luminescence bands of F-centers (3.0 eV) and F+ -centers (3.8 eV), the latter induced by oxygen vacancies with one trapped electron. Different PCL spectra of anion-defective -Al2 O3 single crystals at 80 and 300 K can be explained as follows. As indicated
1
120
80 2 40
0 390
400
410
420
430
Wavelength, nm Fig. 1. Photoluminescence spectra of anion-defective single crystals of the aluminum oxide (1) and the nanostructured ceramic (2) after excitation in the 205 nm band.
200
160 Photoluminescence, arb. units
sets of nonequilibrium phases and oxygen-nonstoichiometric compositions. Oxygen vacancies in powders were not affected by heating in air at a temperature of 500 ◦ C. Single crystals of -Al2 O3 :C were grown from an oxide melt (by Stepanov method) in the presence of carbon providing reducing conditions (Akselrod et al., 1990). According to the optical absorption data, the concentration of oxygen-vacancyinduced F-centers was about 1017 cm−3 . The single crystals at hand were shaped as disks 1 mm thick and 5 mm in diameter. They were transparent. Photoluminescence (PL) spectra and PL excitation spectra were measured in the optical channel (a deuterium lamp and a monochromator) of a SF-26 spectrometer. A FEU-130 photomultiplier was used for PL registration. PL spectral bands were isolated by a MUM monochromator. Pulse cathodoluminescence (PCL) was excited by a electron beam having the density of 1 A/cm2 , the energy of 180 keV and the pulse length of 3 ns at 80 K and room temperature. A pulsed accelerator type RADAN served as the source of electrons. Cathodoluminescence was recorded without a delay relative to the excitation pulse, making it possible to detect short-lived luminescence bands. The relative error incurred in the PL and PCL measurements was not over 10%.
Photoluminescence, arb. units
Table 1 Synthesis conditions and the specific surface of the aluminum oxide nanopowders
1 120
80
2
40
0 190
200 210 Wavelength, nm
220
Fig. 2. Photoluminescence excitation spectra in the 410 nm band of anion-defective single crystals of the aluminum oxide (1) and the nanostructured ceramic (2).
above, the available PCL spectrometer allowed detecting shortlived luminescence bands. Consequently, the intensive band of F+ -centers at 3.8 eV, which has, as is well known, a short decay time, was recorded at T = 80 and 300 K. It was shown earlier that the band of F-centers near 3.0 eV consists of two components: the known slow band (the triplet–singlet transition in an F-center) with the decay time of (34–36) ms and the fast band at 3.26 eV with = 1.6 ns (Surdo et al., 2005a). The fast
V.S. Kortov et al. / Radiation Measurements 43 (2008) 341 – 344
70 60 50 3.0
40
3.8
30
4.5
20 10 0 Intensity (a. u.)
60 4.5
50 40
3.8
30 20 10 0 12 10
3.8
8 3.0
6
3.4
4.3
4 2 0 2.5
3.0
3.5
4.0
4.5
5.0
5.5
Photon energy, eV Fig. 3. Pulsed cathodoluminescence spectra of anion-defective single crystals of -Al2 O3 at 80 K (a) and 300 K (b) and the nanostructured aluminum oxide ceramic (c) at 300 K.
component is due to singlet–singlet transitions in an F-center and is recorded only at T 80 K. Most likely it is this fast band of F-centers that shows up in the PCL spectrum at 80 K. As can be seen from Fig. 3a, the band is shifted slightly toward higher photon energies. At T = 300 K the fast band of F-centers is absent, while the slow band at 3.0 eV cannot be recorded when the luminescence intensity is measured in the nanosecond range. The luminescence band at 4.5 eV is detected only if the excitation density is high. It is absent in stationary photoluminescence and radioluminescence spectra. Opinions differ on its origin. This band may be related to the decay of excited states localized near impurity centers. The PCL spectrum of the nanoceramic at 300 K is characterized by a wide band over the interval of (2.5–5.0) eV; a set of narrower Gaussian bands can be separated in this band. They include luminescence bands characteristic of oxygen-deficient single crystals: 3.0 eV (F-centers) and 3.8 eV (F+ -centers). The luminescence band in the near UV radiation is displaced by 0.2 eV to the long-wave region (4.3 eV). The main specific feature of the PCL spectra of the Al2 O3 nanoceramic is the presence of a new luminescence band with the maximum at 3.4 eV. It is probably due to luminescence
343
centers induced by defects of nonequilibrium phases in the nanoceramic samples under study. It is also possible that the new luminescence band is related to surface Fs -centers concentrating on nanoparticle boundaries. Remarkably, luminescence bands of F- and F+ -centers are present simultaneously in the PCL spectrum of the nanoceramic at 300 K. This fact suggests a considerable decrease in the decay time of the band of F-centers in the nanostructured samples. In addition to X-ray measurements, the PL and PCL spectra uniquely intimate that the nanoceramic samples are oxygennonstoichiometric. They include F- and F+ -centers induced by oxygen vacancies, which are responsible for the observed PL and luminescence bands in the PCL spectra. Most likely the oxygen nonstoichiometry is characteristic of nonequilibrium Al2 O3 phases and prevails in the ceramic samples. It is also possible that the nonstoichiometry is a specific property of the nanostructured state of multicomponent materials. It is significant that the luminescence bands of F- and F+ -centers have similar positions in both the single crystals and the nanostructured ceramic. These centers are formed by point defects and the short-range order in the nanoparticles provides luminescence mechanisms for the centers similar to those in the single crystals. Of course, the short-range order in the nanoparticles is not as perfect as in the single crystals. A large concentration of surface atoms having a different energy state influences excitation and relaxation of photoactive centers in the nanostructured ceramic, leading to widening of the luminescence bands and impairment of the PCL spectrum resolution. Notice also that a thin near-surface layer luminesces in the opaque nanoceramic, whereas the whole volume of transparent crystals is involved in luminescence of the Al2 O3 single crystals. As already noted, the PL measurements were made under identical conditions and surface areas of the test samples were equal. Since the PL intensity in the nanostructured ceramic is just 2.5–3 times lower than it is in the crystals, the concentration of oxygen-deficient luminescence centers per unit volume may be assumed to be much larger in the ceramic than in the -Al2 O3 crystals. One may think therefore that powders of nanostructured aluminum can serve as the material of thin chips for dosimetry of short-range particles, specifically skindosimetry of -radiation. The specific surface of the initial powders, which determined the average size of nanoparticles, did not influence the number and positions of bands in the PL and PCL spectra of the nanostructured ceramic. However, such a dependence was observed for the luminescence decay time-constant of some bands in the PCL spectrum of the nanoceramic samples (Table 2). The decay time-constant of the 3.8-eV band was difficult to determine at the given length of the electron pulse, because the luminescence time of this band is about 2 ns. Slow bands of F-centers luminescence were recorded as the integration time constant of the measuring circuit increased. The luminescence decay time-constant of F-centers (3.0 eV) in the -Al2 O3 crystals falls within the millisecond interval and corresponds to the available literature data (Surdo et al., 2005b). Single decay of all luminescence bands in the PCL spectrum of
344
V.S. Kortov et al. / Radiation Measurements 43 (2008) 341 – 344
Table 2 Decay time-constants of luminescence bands in the aluminum oxide Sample
−Al2 O3 crystal Nanoceramic ( = 23 m2 /g) Nanoceramic ( = 93 m2 /g)
Luminescence bands (eV) 3.0
3.4
4.3
4.5
32 ms 0.74 s 0.25 s
– 0.62 s 0.22 s
– 0.98 s 1.52 s
0.27 s – –
the nanoceramic samples is faster and their decay time-constant is in the microsecond interval. The luminescence decay timeconstant of the bands at 3.0 and 3.4 eV is reduced with increasing average size of nanoparticles in the initial Al2 O3 powders. The inverse dependence is observed for the UV band at 4.3 eV, which additionally shows a slower signal decay than the bands at 3.0 and 3.4 eV. Notice also that the decay time-constants of all the three luminescence bands in the nanostructured Al2 O3 ceramic do not differ when PCL is excited at 80 and 300 K. 4. Conclusion The revealed specific features of the luminescence properties of the nanostructured aluminum oxide ceramic may be related to the presence of nonequilibrium phases, which are oxygen deficient. An important established fact is that the Al2 O3 nanoceramic has oxygen-vacancy-induced F- and F+ -centers responsible for photo- and cathodoluminescence and appearance of a new luminescence band in the UV region of the
PCL spectrum. Some evidence of a relationship between luminescence centers and oxygen vacancies was obtained. The observed difference in the decay time constant of the luminescence band at 3.0 eV in single crystals and nanostructured Al2 O3 is probably due to different lifetimes of the excited state of the luminescence center caused by differences in the dynamics of the relaxation process. Considering a high concentration of oxygen-deficient luminescence centers in aluminum oxide nanopowders, it would be reasonable to evaluate the prospects of their use as dosimeters of short-range particles. References Akselrod, M.S., Kortov, V.S., Kravetsky, D.J., Gotlib, V.I., 1990. Highly sensitive thermoluminescence anion defective -Al2 O3 :C single crystals detectors. Radiat. Prot. Dosimetry 32, 15–20. Lo, D., Lawless, J.L., Chen, R., 2006. Superlinear dose dependence of high temperature thermoluminescence peaks in Al2 O3 :C. Radiat. Prot. Dosimetry 119, 71–74. McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford, UK, 278pp. Surdo, A.I., Kortov, V.S., Pustovarov, V.A., Yakovlev, V.Yu., 2005a. UV luminescence of F-centers in aluminum oxide. Phys. Status Solid (c) 2, 527–530. Surdo, A., Pustovarov, V., Kortov, V., Kishka, A., Zinin, E., 2005b. Luminescence in anion-defective -Al2 O3 crystals over the nano-, microand millisecond intervals. Nucl. Instrum. Methods Phys. Res. A 543, 234–238. Yermakov, A.E., Uimin, M.A., Galakhov, V.S., Kuopper, K., Robin, S., Neiemann, M., 2005. Structure and surface states of Cu–O based nanocrystalline powder. J. Metastable Nanocryst. Mater. 24–25, 43–48.
Luminescence properties of nanostructured alumina ceramic V.S. Kortov a , A.E. Ermakov b , A.F. Zatsepin a , S.V. Nikiforov a,∗ a Department of Technical Physics, The Ural State Technical University, 19 Mira Street, UPI, 620002 Ekaterinburg, Russia b Institute of Metal Physics, Ural Branch RAS, Ekaterinburg, Russia
Abstract Photo- and cathodoluminescence spectra of the nanostructured Al2 O3 ceramic and anion-defective -Al2 O3 crystals are studied under comparable conditions. Luminescence bands of the centers produced by oxygen vacancies are detected in samples of both types. The nanostructured ceramic is characterized by a new band at 3.4 eV and the reduction of the luminescence decay time. The specific features of the luminescence of the nanostructured ceramic can be related to the presence of nonequilibrium phases and specificity of relaxation processes. © 2007 Elsevier Ltd. All rights reserved. Keywords: Aluminum oxide; Nanoceramic; Photoluminescence; Oxygen vacancies
1. Introduction Anion-defective -Al2 O3 crystals are efficient phosphors and dosimeters and their properties have being studied intensively (Akselrod et al., 1990; McKeever et al., 1995; Lo et al., 2006). These crystals contain a relatively high concentration of oxygen vacancies forming electron trapping centers similar to F-centers in alkali halide crystals. It is known that nanostructured modifications of Al2 O3 include nonequilibrium phases, in which oxygen nonstoichiometry is possible. In this connection, it is interesting to study luminescence properties of the nanostructured aluminum oxide and compare them with analogous properties of crystalline samples. The present study deals with the analysis of photo- and cathodoluminescence in the nanostructured Al2 O3 ceramic and -Al2 O3 :C single crystals under comparable conditions. 2. Making and certification of samples, experimental technique The aluminum oxide nanopowders were prepared by the gaseous phase method (Yermakov et al., 2005), which can be summarized as follows. A molten aluminum droplet is heated ∗ Corresponding author.
E-mail address: [email protected] (S.V. Nikiforov). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.008
and held by a high-frequency electromagnetic field. A gas mixture of argon and oxygen taken in some ratio flows around the droplet. The metal vapor is carried by the gas to the cold zone where the vapor condenses and is oxidized with oxygen of the gas mixture. The powder accumulates in a filter. The size of the powder particles depends on the droplet temperature, the gas pressure and the gas feed rate. It can be several nanometers to hundreds of nanometers. Two batches of the aluminum oxide powders prepared by the gaseous phase method were used in this study. The synthesis parameters and the specific surface of the powders are given in Table 1. The electron microscopy examination of Al2 O3 nanoparticles in the powders showed that they had a spherically symmetrical shape and a relatively uniform size distribution. The average size of the nanoparticles was about 30–40 nm and less than 10 nm in the nanopowders with the specific surface of 23 and 95 m2 /g, respectively. Samples of the nanostructured ceramic were made by compaction of the powders in a collapsible mold under a load of about 2 t and sintering in air at 500 ◦ C for 1 h. The samples were opaque dense disks 5 mm in diameter and about 1 mm thick. According to the results of the X-ray diffraction analysis, the cubic and tetragonal phases of the aluminum oxide were present in the powders. Lines of the orthorhombic phase and a small amount of pure Al were also observed in the diffraction patterns. Thus, the X-ray diffraction data suggested that the test samples of the Al–O powders in the two batches had different
342
V.S. Kortov et al. / Radiation Measurements 43 (2008) 341 – 344
200
Batch no.
Ar feed rate (l/h)
O2 feed rate (l/h)
Pressure (Torr)
Specific surface (m2 /g)
160
1 2
170 80
11 11
50 150
95 23
3. Results and discussion As is known, main luminescence centers in crystals of the anion-defective aluminum oxide are F-centers produced by oxygen vacancies, which have trapped two electrons (Akselrod et al., 1990; McKeever et al., 1995). Therefore, PL was studied in the excitation (205 nm) and luminescence (410 nm) spectral ranges of these centers. Figs. 1 and 2 present PL spectra and PL excitation spectra of the nanostructured ceramic with the specific surface of 23 m2 /g and the anion-defective crystals of the aluminum oxide. It is seen that PL is excited in both the nanostructured and crystalline samples over the optical absorption range of F-centers, while the PL intensity is maximum in the luminescence band of these centers. Some widening of the excitation and luminescence bands in the nanostructured ceramic samples can be noted. PCL spectra of the test samples are shown in Fig. 3. PCL spectra of the single crystals, which were measured at 80 K (Fig. 3a) and 300 K (Fig. 3b), include known luminescence bands of F-centers (3.0 eV) and F+ -centers (3.8 eV), the latter induced by oxygen vacancies with one trapped electron. Different PCL spectra of anion-defective -Al2 O3 single crystals at 80 and 300 K can be explained as follows. As indicated
1
120
80 2 40
0 390
400
410
420
430
Wavelength, nm Fig. 1. Photoluminescence spectra of anion-defective single crystals of the aluminum oxide (1) and the nanostructured ceramic (2) after excitation in the 205 nm band.
200
160 Photoluminescence, arb. units
sets of nonequilibrium phases and oxygen-nonstoichiometric compositions. Oxygen vacancies in powders were not affected by heating in air at a temperature of 500 ◦ C. Single crystals of -Al2 O3 :C were grown from an oxide melt (by Stepanov method) in the presence of carbon providing reducing conditions (Akselrod et al., 1990). According to the optical absorption data, the concentration of oxygen-vacancyinduced F-centers was about 1017 cm−3 . The single crystals at hand were shaped as disks 1 mm thick and 5 mm in diameter. They were transparent. Photoluminescence (PL) spectra and PL excitation spectra were measured in the optical channel (a deuterium lamp and a monochromator) of a SF-26 spectrometer. A FEU-130 photomultiplier was used for PL registration. PL spectral bands were isolated by a MUM monochromator. Pulse cathodoluminescence (PCL) was excited by a electron beam having the density of 1 A/cm2 , the energy of 180 keV and the pulse length of 3 ns at 80 K and room temperature. A pulsed accelerator type RADAN served as the source of electrons. Cathodoluminescence was recorded without a delay relative to the excitation pulse, making it possible to detect short-lived luminescence bands. The relative error incurred in the PL and PCL measurements was not over 10%.
Photoluminescence, arb. units
Table 1 Synthesis conditions and the specific surface of the aluminum oxide nanopowders
1 120
80
2
40
0 190
200 210 Wavelength, nm
220
Fig. 2. Photoluminescence excitation spectra in the 410 nm band of anion-defective single crystals of the aluminum oxide (1) and the nanostructured ceramic (2).
above, the available PCL spectrometer allowed detecting shortlived luminescence bands. Consequently, the intensive band of F+ -centers at 3.8 eV, which has, as is well known, a short decay time, was recorded at T = 80 and 300 K. It was shown earlier that the band of F-centers near 3.0 eV consists of two components: the known slow band (the triplet–singlet transition in an F-center) with the decay time of (34–36) ms and the fast band at 3.26 eV with = 1.6 ns (Surdo et al., 2005a). The fast
V.S. Kortov et al. / Radiation Measurements 43 (2008) 341 – 344
70 60 50 3.0
40
3.8
30
4.5
20 10 0 Intensity (a. u.)
60 4.5
50 40
3.8
30 20 10 0 12 10
3.8
8 3.0
6
3.4
4.3
4 2 0 2.5
3.0
3.5
4.0
4.5
5.0
5.5
Photon energy, eV Fig. 3. Pulsed cathodoluminescence spectra of anion-defective single crystals of -Al2 O3 at 80 K (a) and 300 K (b) and the nanostructured aluminum oxide ceramic (c) at 300 K.
component is due to singlet–singlet transitions in an F-center and is recorded only at T 80 K. Most likely it is this fast band of F-centers that shows up in the PCL spectrum at 80 K. As can be seen from Fig. 3a, the band is shifted slightly toward higher photon energies. At T = 300 K the fast band of F-centers is absent, while the slow band at 3.0 eV cannot be recorded when the luminescence intensity is measured in the nanosecond range. The luminescence band at 4.5 eV is detected only if the excitation density is high. It is absent in stationary photoluminescence and radioluminescence spectra. Opinions differ on its origin. This band may be related to the decay of excited states localized near impurity centers. The PCL spectrum of the nanoceramic at 300 K is characterized by a wide band over the interval of (2.5–5.0) eV; a set of narrower Gaussian bands can be separated in this band. They include luminescence bands characteristic of oxygen-deficient single crystals: 3.0 eV (F-centers) and 3.8 eV (F+ -centers). The luminescence band in the near UV radiation is displaced by 0.2 eV to the long-wave region (4.3 eV). The main specific feature of the PCL spectra of the Al2 O3 nanoceramic is the presence of a new luminescence band with the maximum at 3.4 eV. It is probably due to luminescence
343
centers induced by defects of nonequilibrium phases in the nanoceramic samples under study. It is also possible that the new luminescence band is related to surface Fs -centers concentrating on nanoparticle boundaries. Remarkably, luminescence bands of F- and F+ -centers are present simultaneously in the PCL spectrum of the nanoceramic at 300 K. This fact suggests a considerable decrease in the decay time of the band of F-centers in the nanostructured samples. In addition to X-ray measurements, the PL and PCL spectra uniquely intimate that the nanoceramic samples are oxygennonstoichiometric. They include F- and F+ -centers induced by oxygen vacancies, which are responsible for the observed PL and luminescence bands in the PCL spectra. Most likely the oxygen nonstoichiometry is characteristic of nonequilibrium Al2 O3 phases and prevails in the ceramic samples. It is also possible that the nonstoichiometry is a specific property of the nanostructured state of multicomponent materials. It is significant that the luminescence bands of F- and F+ -centers have similar positions in both the single crystals and the nanostructured ceramic. These centers are formed by point defects and the short-range order in the nanoparticles provides luminescence mechanisms for the centers similar to those in the single crystals. Of course, the short-range order in the nanoparticles is not as perfect as in the single crystals. A large concentration of surface atoms having a different energy state influences excitation and relaxation of photoactive centers in the nanostructured ceramic, leading to widening of the luminescence bands and impairment of the PCL spectrum resolution. Notice also that a thin near-surface layer luminesces in the opaque nanoceramic, whereas the whole volume of transparent crystals is involved in luminescence of the Al2 O3 single crystals. As already noted, the PL measurements were made under identical conditions and surface areas of the test samples were equal. Since the PL intensity in the nanostructured ceramic is just 2.5–3 times lower than it is in the crystals, the concentration of oxygen-deficient luminescence centers per unit volume may be assumed to be much larger in the ceramic than in the -Al2 O3 crystals. One may think therefore that powders of nanostructured aluminum can serve as the material of thin chips for dosimetry of short-range particles, specifically skindosimetry of -radiation. The specific surface of the initial powders, which determined the average size of nanoparticles, did not influence the number and positions of bands in the PL and PCL spectra of the nanostructured ceramic. However, such a dependence was observed for the luminescence decay time-constant of some bands in the PCL spectrum of the nanoceramic samples (Table 2). The decay time-constant of the 3.8-eV band was difficult to determine at the given length of the electron pulse, because the luminescence time of this band is about 2 ns. Slow bands of F-centers luminescence were recorded as the integration time constant of the measuring circuit increased. The luminescence decay time-constant of F-centers (3.0 eV) in the -Al2 O3 crystals falls within the millisecond interval and corresponds to the available literature data (Surdo et al., 2005b). Single decay of all luminescence bands in the PCL spectrum of
344
V.S. Kortov et al. / Radiation Measurements 43 (2008) 341 – 344
Table 2 Decay time-constants of luminescence bands in the aluminum oxide Sample
−Al2 O3 crystal Nanoceramic ( = 23 m2 /g) Nanoceramic ( = 93 m2 /g)
Luminescence bands (eV) 3.0
3.4
4.3
4.5
32 ms 0.74 s 0.25 s
– 0.62 s 0.22 s
– 0.98 s 1.52 s
0.27 s – –
the nanoceramic samples is faster and their decay time-constant is in the microsecond interval. The luminescence decay timeconstant of the bands at 3.0 and 3.4 eV is reduced with increasing average size of nanoparticles in the initial Al2 O3 powders. The inverse dependence is observed for the UV band at 4.3 eV, which additionally shows a slower signal decay than the bands at 3.0 and 3.4 eV. Notice also that the decay time-constants of all the three luminescence bands in the nanostructured Al2 O3 ceramic do not differ when PCL is excited at 80 and 300 K. 4. Conclusion The revealed specific features of the luminescence properties of the nanostructured aluminum oxide ceramic may be related to the presence of nonequilibrium phases, which are oxygen deficient. An important established fact is that the Al2 O3 nanoceramic has oxygen-vacancy-induced F- and F+ -centers responsible for photo- and cathodoluminescence and appearance of a new luminescence band in the UV region of the
PCL spectrum. Some evidence of a relationship between luminescence centers and oxygen vacancies was obtained. The observed difference in the decay time constant of the luminescence band at 3.0 eV in single crystals and nanostructured Al2 O3 is probably due to different lifetimes of the excited state of the luminescence center caused by differences in the dynamics of the relaxation process. Considering a high concentration of oxygen-deficient luminescence centers in aluminum oxide nanopowders, it would be reasonable to evaluate the prospects of their use as dosimeters of short-range particles. References Akselrod, M.S., Kortov, V.S., Kravetsky, D.J., Gotlib, V.I., 1990. Highly sensitive thermoluminescence anion defective -Al2 O3 :C single crystals detectors. Radiat. Prot. Dosimetry 32, 15–20. Lo, D., Lawless, J.L., Chen, R., 2006. Superlinear dose dependence of high temperature thermoluminescence peaks in Al2 O3 :C. Radiat. Prot. Dosimetry 119, 71–74. McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford, UK, 278pp. Surdo, A.I., Kortov, V.S., Pustovarov, V.A., Yakovlev, V.Yu., 2005a. UV luminescence of F-centers in aluminum oxide. Phys. Status Solid (c) 2, 527–530. Surdo, A., Pustovarov, V., Kortov, V., Kishka, A., Zinin, E., 2005b. Luminescence in anion-defective -Al2 O3 crystals over the nano-, microand millisecond intervals. Nucl. Instrum. Methods Phys. Res. A 543, 234–238. Yermakov, A.E., Uimin, M.A., Galakhov, V.S., Kuopper, K., Robin, S., Neiemann, M., 2005. Structure and surface states of Cu–O based nanocrystalline powder. J. Metastable Nanocryst. Mater. 24–25, 43–48.