- Email: [email protected]
Delayed luminescence from ZnO ceramics upon microwave-induced plasma emission
Journal of Physics and Chemistry of Solids 74 (2013) 837–840
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Delayed luminescence from ZnO ceramics upon microwave-induced plasma emission Taro Sonobe a,c,n, Kan Hachiya a, Tomohiko Mitani b, Naoki Shinohara b, Hideaki Ohgaki c a
Graduate School of Energy Science, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan c Institute of Advanced Energy (IAE), Kyoto University, Japan b
a r t i c l e i n f o
abstract
Article history: Received 23 August 2011 Received in revised form 19 January 2013 Accepted 25 January 2013 Available online 5 February 2013
A delayed luminescence from the ZnO ceramics upon microwave-induced plasma emission was observed and its hybridized excitation mechanism was investigated. A phenomenological model of the luminescence is proposed to explain the occurrence of the hybrid excitation process and confirmed that the hybridization is by the photon of luminescence through atomic plasma emission and by the microwave depending on its power. This finding provides a distinctive methodology for studying the physics of the interaction between microwave and matter. & 2013 Elsevier Ltd. All rights reserved.
1. Introduction A sound understanding of the interaction between electromagnetic radiation and matter has renewed interest in such terahertz and microwaves frequencies. Since these frequency regions correspond to molecular rotation and lattice vibration energy, and because electromagnetic waves of these frequencies can penetrate into materials, the development of methodology for studying the interaction between these frequencies and matter is expected to explore a new cross-disciplinary physics such as microwave solid state physics and processing. On the other hand, microwave material processing is attracting interest as a promising candidate for conserving energy and improving efficiency in conventional industrial processes to reduce CO2 emissions. Because of its various advantages over conventional methods, such as rapid and selective heating, as well as its ability to internally heat substances, microwave heating can reduce the time and lower the temperature necessary for material processing [1–10], although the investigations into physical process which enables such properties are limited. Recently, several approaches have been studied for microwave material processing such as the sintering of ceramics [1–3], metal powder [4–6], and metal production [7–9]. In 1999, Roy et al. reported that metal powders can also be heated and sintered by microwave irradiation, while bulk metallic samples reflect microwave [5]. Buchelnikov et al. have reported experimental results and provided a theoretical explanation of heating mechanism for
n Corresponding author at: Institute of Advanced Energy (IAE), Kyoto University, Japan. Tel.: þ81 774 38 3420; fax: þ81 774 38 3426. E-mail address: [email protected] (T. Sonobe).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.01.027
metal powders, whose eddy current loss (in an alternating Hfield) and magnetic reversal loss (in alternating E-field) generate heat upon microwave irradiation [6]. The different effects of the microwave E-field and H-field on microstructure evolution and heating behavior for ceramic powders have also been investigated [3]. Furthermore, a thermal nonequilibrium state during microwave processing has been often reported such as an enhancement of chemical reactions as well as rapid phase mixing at the grain boundaries of iron [7]. Sato et al. have reported that highly pure iron is obtained in a 2.45 GHz multimode microwave reactor and that many ‘‘local hot cores’’ in the order of 100 mm are formed inhomogeneously with huge temperature gradients at temperature of up to 400 1C [7]. They observed strong visiblelight emission over a sample surface under a microwave E-field, whose spectra shows a marked discrepancy in intensity with black body emission profiles. Recently, we have reported a novel feature of luminescence from ZnO ceramics during microwave irradiation under vacuum, which consists of a sharp atomic plasma emission, such as oxygen and zinc atomic plasmas, and a delayed broad visible-light emission from the ceramics [9]. In particular, the latter, the delayed broad emission likely to show a marked discrepancy in the electron energy over the thermal equilibrium state of electrons in the ceramics, and in-depth investigation on mechanism of the delayed broad emission can give rise to exploring a new cross-disciplinary physics such as microwave solid state physics. In this study, the mechanism of delayed luminescence from the ZnO ceramics upon microwave-induced plasma emission is investigated. A phenomenological model of the luminescence is proposed to explain the occurrence of the hybrid excitation process of electrons by the photon energy from luminescence through atomic plasma emission and microwave power. This
838
T. Sonobe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 837–840
finding provides a distinctive methodology for studying the physics of interaction between microwave and matter.
2. Experiment Commercially available ZnO powder (Wako Pure Chemical Industry, 99.9%) was used as the starting material. The powder (1 g) was pressed into a rectangular pellet (27.0 6.7 1.5 mm3) and then sintered in air in a furnace at 700 1C (973 K) for 2 h before being subjected to microwave irradiation. The microwave heating apparatus mainly consists of a microwave generator including a 2.45 GHz magnetron, an automatic stub tuner (Daihen Corporation, SMA-10), a rectangular single-mode cavity with a quartz tube reactor, a power meter, and a charge-coupled-device (CCD) spectrometer. The rectangular pellet of ZnO was placed on a quartz wool base in the quartz tube, the inner part of the reactor was evacuated continuously to 102 Pa using a rotary pump and the samples were irradiated at the maximum point of a microwave electric field with a microwave power of 300 W for 10 min under vacuum. During the microwave irradiation, we measured the optical emission spectra from the chamber using a CCD spectrometer (Ocean Optics, VIS–NIR) coupled with an optical fiber with wavelength ranging from 350 to 1050 nm. The detailed configuration of the microwave irradiation system was reported in our previous study [10]. We obtained ultraviolet–visible-near–infrared (UV–vis–NIR) diffuse reflectance spectra of the pellets using a spectrophotometer (JASCO V-670) for wavelengths ranging from 200 to 2400 nm. The Kubelka–Munk function [11,12] F(R) was used as the equivalent of the absorption coefficient.
Fig. 1. Optical emission spectra of plasma during microwave irradiation of ZnO under vacuum after 30, 60, and 600 s plasma emission.
3. Results and discussion To investigate the plasma species, we measured the optical emission spectra from 350 to 1050 nm. Fig. 1 shows the optical emission spectra at 30, 60, and 600 s after the start of plasma emission. The detailed assignment for each transition was reported in our previous study [9]. In addition, a delayed broad band at approximately 500–1000 nm that gradually increased in intensity was observed after 30 s from the ZnO ceramics. Since the peak positions are not shifted during the luminescence and the photon energy at corresponding peaks around the visiblelight region such as at 660 and 745 nm are unrealistically high compared with thermal radiation spectra, the increase in intensity can be attributed to different luminescence induced by the microwave irradiation [9]. Fig. 2 shows the changes in optical emission intensity over time for the neutral zinc transition at 481 nm and the ceramic luminescence centered at 745 nm during microwave irradiation. The intensity of the neutral zinc plasma spectra is suddenly increased to a peak at approximately 50 s, then is decreased exponentially until the end of microwave irradiation. Following the change in the neutral zinc plasma spectra, the ceramic luminescence shows a definite delay and a rapid increase in intensity until 100 s, and decreases monotonically. As observed, this luminescence from the ceramics is likely to show a correlation with the zinc plasma spectra over time during microwave irradiation. Moreover, as can be seen in Fig. 3, similar luminescence of the ceramics can be partially induced without simultaneous plasma emission when a microwave is irradiated again on the sample immediately after the end of irradiation. The luminescence from the ZnO ceramics at saturated intensity shows a monotonic increase in the peak intensity with increasing applied microwave power. These results can be interpreted as those of a hybrid excitation process of electrons, one of
Fig. 2. Change in optical emission intensity over time for neutral zinc transition at 481 nm and ZnO ceramic luminescence centered at 745 nm during microwave irradiation.
Fig. 3. Microwave power dependence of ZnO ceramic luminescence during microwave irradiation of ZnO under vacuum with powers of 100, 140, 180, 220, and 250 W at saturated intensity.
T. Sonobe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 837–840
which by photon from atomic plasma and the other by microwave power. The former directly determines the photoexcitation process while the latter indirectly through plasma emission rate for example, whereas the excitation mechanism in the case of microwave excitation is yet not clear. Fig. 4 shows the UV–vis–NIR absorption coefficients obtained from the diffuse reflectance spectra using the Kubelka–Munk relationship for the material sintered at 700 1C and subjected to microwave irradiation. As expected from their white color before microwave irradiation, the sintered pellets exhibit little absorption at a wavelength of 410 nm. On the other hand, the sample that emitted zinc plasmas upon microwave irradiation exhibits marked absorption from 400 to 2400 nm. This suggests the emergence of numerous electronic states in the bandgap of ZnO derived from species such as zinc vacancies during the time that zinc plasmas were emitted [9]. Thus, it can be assumed that these electronic states in the bandgap act as a common radiative centers for the hybrid excitation process of electrons by the photon energy provided by luminescence through atomic plasma emission as well as the microwave power. On the basis of above results, we have developed a phenomenological model for the hybrid excitation process of luminescence from ZnO ceramics under microwave irradiation. A schematic diagram of the model is shown in Fig. 5. Microwave power P (W) is proportional to the squared electric field intensity E (V/m), which is homogeneously distributed over sample since the wavelength of a microwave is long enough as compared with the thickness of sample. When the amount of Zn atomic plasma emission is defined as NZn(t) as a function of time, intensity of photon emission from the Zn atomic plasma iZn(t) is expressed as iZn ðtÞ ¼ gNZn ðtÞ,
ð1Þ
839
Fig. 5. Schematic diagram of the phenomenological model for the hybrid excitation process of luminescence from ZnO ceramics under microwave irradiation.
N(t) is expressed as the sum of n(t) and n0(t), NðtÞ ¼ nðtÞ þ n0 ðtÞ:
ð3Þ
I(t) is proportional to the hybrid excitation intensity of photoexcitation by luminescence from atomic plasma emission and microwave power as well as radiative centers. Thus, it is expressed as IðtÞ ¼ s faie ðtÞn þ bg nðtÞ
ð4Þ
where, s is a quantum efficiency, a and b are microwave power parameters, and n is the order of photoexcitation. Assuming that nonradiative centers are created at a constant rate of N0, n0(t) is expressed as Z t N 0 dt 0 : ð5Þ n0 ðtÞ ¼ 0
where g is constant. The intensity of photoexcitation on the ZnO ceramic surface ie(t) is expressed as ie ðtÞ ¼ iZn ðtÞ
Then, from Eqs. (3) and (5), n(t) is expressed as Z 1 t fiZn ðt 0 ÞgN 0 gdt 0 : nðtÞ ¼ NðtÞn0 ðtÞ ¼
g
ð2Þ
The numbers of radiative centers, nonradiative centers, and vacancies in the ZnO ceramics are expressed as n(t), n0(t), and N(t), respectively. In this reaction, N(t) is equal to the total number of Zn atoms extracted from the crystal during plasma emission, and these vacancies can act primarily as radiative centers in the bandgap as well as nonradiative centers. Thus,
Fig. 4. UV–vis–NIR absorption coefficients obtained from diffuse reflectance spectra using the Kubelka–Munk relationship for ZnO ceramics sintered in air at 700 1C and subjected to microwave irradiation in vacuum.
Finally, from Eqs. (4) and (6), I(t) is expressed as Z t s fiZn ðt 0 Þ gN0 gdt 0 : aie ðtÞn þ b IðtÞ ¼
g
ð6Þ
0
ð7Þ
0
Fig. 6 shows a fitting curve for the ceramic luminescence over time using the intensity of the neutral zinc plasma spectra at 481 nm. By adjusting the parameters such as a, b, and n, the experimental and calculated changes in optical emission intensity over time can be made to correspond well to each other. This result suggests that the vacancies created in ZnO ceramics during microwave plasma emission act as radiative centers as well as nonradiative centers in the bandgap, while hybrid excitation possibly proceeds. Microwave-power-dependence of the luminescence after saturation shown in Fig. 3 is plotted in Fig. 7. The logarithmic dependency of luminescence on microwave power is changed linearly with increasing applied microwave power and shows a transition of the slope at above a power of 180 W. Since the intensity of photon emission from the Zn atomic plasma iZn(t) is negligible in this condition, it is indicated that the luminescence from ZnO ceramics under microwave irradiation is ascribed to microwave power partly through plasma emission rate which is integrated and yields the number of radiative centers consisting of atomic vacancies in the integral part in Eq. (7), and partly through the b parameter in the preposed factor for the rest of the dependency, which is interpreted as the excitation process, whereas further investigation on the order in microwave power dependence is necessary. A possible explanation for the threshold is referred to as the excitation-threshold of ion in the ZnO ceramics over microwave electric power through heating. Thus,
840
T. Sonobe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 837–840
materials [9,10]. This finding provides a distinctive methodology for studying the interaction between microwave and matter.
4. Conclusion
Fig. 6. Experimentally and theoretically obtained changes in optical emission intensity over time for ZnO ceramic luminescence during microwave irradiation with a=b ¼ 0:0011, gN 0 ¼ 300 for arbitrary unit, and v ¼0.2.
In this study, we investigated the novel feature of luminescence from the ZnO ceramics during microwave irradiation under vacuum. This luminescence consists of a sharp atomic plasma emission, such as oxygen and zinc atomic plasmas, and a delayed broad visible-light emission from the ceramics. Since the peak positions are not shifted during the luminescence and the photon energies at corresponding peaks around the visible-light region are unrealistically high compared with the thermal radiation spectra, the increase in intensity of the delayed broad emission can be attributed to different luminescence caused by the microwave-induced plasma emission. A phenomenological model of the luminescence was proposed to show the occurrence of the hybrid excitation process of electrons by the photon energy from the luminescence through atomic plasma emission and microwave power in preference to the thermal effect. The experimental and calculated changes in optical emission intensity over time correspond well to each other. This result suggests that the vacancies created in ZnO ceramics during microwave plasma emission act primarily as radiative centers as well as nonradiative centers in the bandgap, while the hybrid excitation possibly proceeds in preference to the thermal effect. This finding provides a distinctive methodology for studying the physics of interaction between microwave and matter.
Acknowledgments The work was supported by the Kansai Research Foundation for technology promotion and a Grant-in-Aid for Scientific Research (C) (23561030) from the Japan Society for the Promotion of Science. References
Fig. 7. Logarithmic plot of microwave-power-dependence of the luminescence after saturation plotted against microwave power (P).
the direct electric energy transfer through microwave power to the electrons in radiative centers as well as nonradiative centers in the bandgap over thermal effects could be suggested in our model. Although the mechanism of Zn plasma emission from ZnO ceramics upon microwave irradiation under vacuum is still unclear, a sound understanding of this hybrid excitation process induced by microwave irradiation can contribute to elucidating the unique interaction between microwaves and semiconducting
[1] S. Jida, T. Suemasu, J. Appl. Phys. 86 (1999) 2089–2094. [2] M. Susaki, Jpn. J. Appl. Phys. 44 (2005) 866–867. [3] S. Li, G. Xie, D.V. Louzguine-Luzgin, Z. Cao, N. Yoshikawa, M. Sato, A. Inoue, J. Alloys Compd. 476 (2009) 482–485. [4] B. Vaidhyanathan, D.K. Agrawal, R. Roy, J. Mater. Res. 15 (2000) 974–980. [5] R. Roy, D. Agrawal, J. Cheng, S. Gedevanishvili, Nature 399 (1999) 668. [6] V.D. Buchelnikov, D.V. Louzquine-Luzgi, G. Xie, S. Li, N. Yoshikawa, M. Sato, A.P. Anzulevich, I.V. Bychkov, A. Inoue, J. Appl. Phys. 104 (2008) 1135051–113505-10. [7] M. Sato, A. Matsubara, S. Takayama, S. Sudo, O. Motojima, K. Nagata, K. Ishizaki, T. Hayashi, D. Agrawal, R. Roy, Proceedings of the 5th Sohn International Symposium on Advanced Processing of Metals and Materials, 2006, pp. 157–170. [8] K. Nagata, K. Kodama, M. Hayashi, Proceedings of the Global Congress on Microwave Energy Applications, 2008, pp. 457–460. [9] T. Sonobe, T. Mitani, K. Hachiya, N. Shinohara, H. Ohgaki, Jpn. J. Appl. Phys. 49 (2010) 080219. [10] T. Sonobe, T. Mitani, N. Shinohara, K. Hachiya, S. Yoshikawa, Jpn. J. Appl. Phys. 48 (2009) 116003. [11] P. Kubelka, F. Munk, Z. Tech. Phys. (Leipzig) 12 (1931) 593–601. (in German). [12] P. Kubelka, J. Opt. Soc. Am. 38 (1948) 448–457.
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Delayed luminescence from ZnO ceramics upon microwave-induced plasma emission Taro Sonobe a,c,n, Kan Hachiya a, Tomohiko Mitani b, Naoki Shinohara b, Hideaki Ohgaki c a
Graduate School of Energy Science, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan c Institute of Advanced Energy (IAE), Kyoto University, Japan b
a r t i c l e i n f o
abstract
Article history: Received 23 August 2011 Received in revised form 19 January 2013 Accepted 25 January 2013 Available online 5 February 2013
A delayed luminescence from the ZnO ceramics upon microwave-induced plasma emission was observed and its hybridized excitation mechanism was investigated. A phenomenological model of the luminescence is proposed to explain the occurrence of the hybrid excitation process and confirmed that the hybridization is by the photon of luminescence through atomic plasma emission and by the microwave depending on its power. This finding provides a distinctive methodology for studying the physics of the interaction between microwave and matter. & 2013 Elsevier Ltd. All rights reserved.
1. Introduction A sound understanding of the interaction between electromagnetic radiation and matter has renewed interest in such terahertz and microwaves frequencies. Since these frequency regions correspond to molecular rotation and lattice vibration energy, and because electromagnetic waves of these frequencies can penetrate into materials, the development of methodology for studying the interaction between these frequencies and matter is expected to explore a new cross-disciplinary physics such as microwave solid state physics and processing. On the other hand, microwave material processing is attracting interest as a promising candidate for conserving energy and improving efficiency in conventional industrial processes to reduce CO2 emissions. Because of its various advantages over conventional methods, such as rapid and selective heating, as well as its ability to internally heat substances, microwave heating can reduce the time and lower the temperature necessary for material processing [1–10], although the investigations into physical process which enables such properties are limited. Recently, several approaches have been studied for microwave material processing such as the sintering of ceramics [1–3], metal powder [4–6], and metal production [7–9]. In 1999, Roy et al. reported that metal powders can also be heated and sintered by microwave irradiation, while bulk metallic samples reflect microwave [5]. Buchelnikov et al. have reported experimental results and provided a theoretical explanation of heating mechanism for
n Corresponding author at: Institute of Advanced Energy (IAE), Kyoto University, Japan. Tel.: þ81 774 38 3420; fax: þ81 774 38 3426. E-mail address: [email protected] (T. Sonobe).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.01.027
metal powders, whose eddy current loss (in an alternating Hfield) and magnetic reversal loss (in alternating E-field) generate heat upon microwave irradiation [6]. The different effects of the microwave E-field and H-field on microstructure evolution and heating behavior for ceramic powders have also been investigated [3]. Furthermore, a thermal nonequilibrium state during microwave processing has been often reported such as an enhancement of chemical reactions as well as rapid phase mixing at the grain boundaries of iron [7]. Sato et al. have reported that highly pure iron is obtained in a 2.45 GHz multimode microwave reactor and that many ‘‘local hot cores’’ in the order of 100 mm are formed inhomogeneously with huge temperature gradients at temperature of up to 400 1C [7]. They observed strong visiblelight emission over a sample surface under a microwave E-field, whose spectra shows a marked discrepancy in intensity with black body emission profiles. Recently, we have reported a novel feature of luminescence from ZnO ceramics during microwave irradiation under vacuum, which consists of a sharp atomic plasma emission, such as oxygen and zinc atomic plasmas, and a delayed broad visible-light emission from the ceramics [9]. In particular, the latter, the delayed broad emission likely to show a marked discrepancy in the electron energy over the thermal equilibrium state of electrons in the ceramics, and in-depth investigation on mechanism of the delayed broad emission can give rise to exploring a new cross-disciplinary physics such as microwave solid state physics. In this study, the mechanism of delayed luminescence from the ZnO ceramics upon microwave-induced plasma emission is investigated. A phenomenological model of the luminescence is proposed to explain the occurrence of the hybrid excitation process of electrons by the photon energy from luminescence through atomic plasma emission and microwave power. This
838
T. Sonobe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 837–840
finding provides a distinctive methodology for studying the physics of interaction between microwave and matter.
2. Experiment Commercially available ZnO powder (Wako Pure Chemical Industry, 99.9%) was used as the starting material. The powder (1 g) was pressed into a rectangular pellet (27.0 6.7 1.5 mm3) and then sintered in air in a furnace at 700 1C (973 K) for 2 h before being subjected to microwave irradiation. The microwave heating apparatus mainly consists of a microwave generator including a 2.45 GHz magnetron, an automatic stub tuner (Daihen Corporation, SMA-10), a rectangular single-mode cavity with a quartz tube reactor, a power meter, and a charge-coupled-device (CCD) spectrometer. The rectangular pellet of ZnO was placed on a quartz wool base in the quartz tube, the inner part of the reactor was evacuated continuously to 102 Pa using a rotary pump and the samples were irradiated at the maximum point of a microwave electric field with a microwave power of 300 W for 10 min under vacuum. During the microwave irradiation, we measured the optical emission spectra from the chamber using a CCD spectrometer (Ocean Optics, VIS–NIR) coupled with an optical fiber with wavelength ranging from 350 to 1050 nm. The detailed configuration of the microwave irradiation system was reported in our previous study [10]. We obtained ultraviolet–visible-near–infrared (UV–vis–NIR) diffuse reflectance spectra of the pellets using a spectrophotometer (JASCO V-670) for wavelengths ranging from 200 to 2400 nm. The Kubelka–Munk function [11,12] F(R) was used as the equivalent of the absorption coefficient.
Fig. 1. Optical emission spectra of plasma during microwave irradiation of ZnO under vacuum after 30, 60, and 600 s plasma emission.
3. Results and discussion To investigate the plasma species, we measured the optical emission spectra from 350 to 1050 nm. Fig. 1 shows the optical emission spectra at 30, 60, and 600 s after the start of plasma emission. The detailed assignment for each transition was reported in our previous study [9]. In addition, a delayed broad band at approximately 500–1000 nm that gradually increased in intensity was observed after 30 s from the ZnO ceramics. Since the peak positions are not shifted during the luminescence and the photon energy at corresponding peaks around the visiblelight region such as at 660 and 745 nm are unrealistically high compared with thermal radiation spectra, the increase in intensity can be attributed to different luminescence induced by the microwave irradiation [9]. Fig. 2 shows the changes in optical emission intensity over time for the neutral zinc transition at 481 nm and the ceramic luminescence centered at 745 nm during microwave irradiation. The intensity of the neutral zinc plasma spectra is suddenly increased to a peak at approximately 50 s, then is decreased exponentially until the end of microwave irradiation. Following the change in the neutral zinc plasma spectra, the ceramic luminescence shows a definite delay and a rapid increase in intensity until 100 s, and decreases monotonically. As observed, this luminescence from the ceramics is likely to show a correlation with the zinc plasma spectra over time during microwave irradiation. Moreover, as can be seen in Fig. 3, similar luminescence of the ceramics can be partially induced without simultaneous plasma emission when a microwave is irradiated again on the sample immediately after the end of irradiation. The luminescence from the ZnO ceramics at saturated intensity shows a monotonic increase in the peak intensity with increasing applied microwave power. These results can be interpreted as those of a hybrid excitation process of electrons, one of
Fig. 2. Change in optical emission intensity over time for neutral zinc transition at 481 nm and ZnO ceramic luminescence centered at 745 nm during microwave irradiation.
Fig. 3. Microwave power dependence of ZnO ceramic luminescence during microwave irradiation of ZnO under vacuum with powers of 100, 140, 180, 220, and 250 W at saturated intensity.
T. Sonobe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 837–840
which by photon from atomic plasma and the other by microwave power. The former directly determines the photoexcitation process while the latter indirectly through plasma emission rate for example, whereas the excitation mechanism in the case of microwave excitation is yet not clear. Fig. 4 shows the UV–vis–NIR absorption coefficients obtained from the diffuse reflectance spectra using the Kubelka–Munk relationship for the material sintered at 700 1C and subjected to microwave irradiation. As expected from their white color before microwave irradiation, the sintered pellets exhibit little absorption at a wavelength of 410 nm. On the other hand, the sample that emitted zinc plasmas upon microwave irradiation exhibits marked absorption from 400 to 2400 nm. This suggests the emergence of numerous electronic states in the bandgap of ZnO derived from species such as zinc vacancies during the time that zinc plasmas were emitted [9]. Thus, it can be assumed that these electronic states in the bandgap act as a common radiative centers for the hybrid excitation process of electrons by the photon energy provided by luminescence through atomic plasma emission as well as the microwave power. On the basis of above results, we have developed a phenomenological model for the hybrid excitation process of luminescence from ZnO ceramics under microwave irradiation. A schematic diagram of the model is shown in Fig. 5. Microwave power P (W) is proportional to the squared electric field intensity E (V/m), which is homogeneously distributed over sample since the wavelength of a microwave is long enough as compared with the thickness of sample. When the amount of Zn atomic plasma emission is defined as NZn(t) as a function of time, intensity of photon emission from the Zn atomic plasma iZn(t) is expressed as iZn ðtÞ ¼ gNZn ðtÞ,
ð1Þ
839
Fig. 5. Schematic diagram of the phenomenological model for the hybrid excitation process of luminescence from ZnO ceramics under microwave irradiation.
N(t) is expressed as the sum of n(t) and n0(t), NðtÞ ¼ nðtÞ þ n0 ðtÞ:
ð3Þ
I(t) is proportional to the hybrid excitation intensity of photoexcitation by luminescence from atomic plasma emission and microwave power as well as radiative centers. Thus, it is expressed as IðtÞ ¼ s faie ðtÞn þ bg nðtÞ
ð4Þ
where, s is a quantum efficiency, a and b are microwave power parameters, and n is the order of photoexcitation. Assuming that nonradiative centers are created at a constant rate of N0, n0(t) is expressed as Z t N 0 dt 0 : ð5Þ n0 ðtÞ ¼ 0
where g is constant. The intensity of photoexcitation on the ZnO ceramic surface ie(t) is expressed as ie ðtÞ ¼ iZn ðtÞ
Then, from Eqs. (3) and (5), n(t) is expressed as Z 1 t fiZn ðt 0 ÞgN 0 gdt 0 : nðtÞ ¼ NðtÞn0 ðtÞ ¼
g
ð2Þ
The numbers of radiative centers, nonradiative centers, and vacancies in the ZnO ceramics are expressed as n(t), n0(t), and N(t), respectively. In this reaction, N(t) is equal to the total number of Zn atoms extracted from the crystal during plasma emission, and these vacancies can act primarily as radiative centers in the bandgap as well as nonradiative centers. Thus,
Fig. 4. UV–vis–NIR absorption coefficients obtained from diffuse reflectance spectra using the Kubelka–Munk relationship for ZnO ceramics sintered in air at 700 1C and subjected to microwave irradiation in vacuum.
Finally, from Eqs. (4) and (6), I(t) is expressed as Z t s fiZn ðt 0 Þ gN0 gdt 0 : aie ðtÞn þ b IðtÞ ¼
g
ð6Þ
0
ð7Þ
0
Fig. 6 shows a fitting curve for the ceramic luminescence over time using the intensity of the neutral zinc plasma spectra at 481 nm. By adjusting the parameters such as a, b, and n, the experimental and calculated changes in optical emission intensity over time can be made to correspond well to each other. This result suggests that the vacancies created in ZnO ceramics during microwave plasma emission act as radiative centers as well as nonradiative centers in the bandgap, while hybrid excitation possibly proceeds. Microwave-power-dependence of the luminescence after saturation shown in Fig. 3 is plotted in Fig. 7. The logarithmic dependency of luminescence on microwave power is changed linearly with increasing applied microwave power and shows a transition of the slope at above a power of 180 W. Since the intensity of photon emission from the Zn atomic plasma iZn(t) is negligible in this condition, it is indicated that the luminescence from ZnO ceramics under microwave irradiation is ascribed to microwave power partly through plasma emission rate which is integrated and yields the number of radiative centers consisting of atomic vacancies in the integral part in Eq. (7), and partly through the b parameter in the preposed factor for the rest of the dependency, which is interpreted as the excitation process, whereas further investigation on the order in microwave power dependence is necessary. A possible explanation for the threshold is referred to as the excitation-threshold of ion in the ZnO ceramics over microwave electric power through heating. Thus,
840
T. Sonobe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 837–840
materials [9,10]. This finding provides a distinctive methodology for studying the interaction between microwave and matter.
4. Conclusion
Fig. 6. Experimentally and theoretically obtained changes in optical emission intensity over time for ZnO ceramic luminescence during microwave irradiation with a=b ¼ 0:0011, gN 0 ¼ 300 for arbitrary unit, and v ¼0.2.
In this study, we investigated the novel feature of luminescence from the ZnO ceramics during microwave irradiation under vacuum. This luminescence consists of a sharp atomic plasma emission, such as oxygen and zinc atomic plasmas, and a delayed broad visible-light emission from the ceramics. Since the peak positions are not shifted during the luminescence and the photon energies at corresponding peaks around the visible-light region are unrealistically high compared with the thermal radiation spectra, the increase in intensity of the delayed broad emission can be attributed to different luminescence caused by the microwave-induced plasma emission. A phenomenological model of the luminescence was proposed to show the occurrence of the hybrid excitation process of electrons by the photon energy from the luminescence through atomic plasma emission and microwave power in preference to the thermal effect. The experimental and calculated changes in optical emission intensity over time correspond well to each other. This result suggests that the vacancies created in ZnO ceramics during microwave plasma emission act primarily as radiative centers as well as nonradiative centers in the bandgap, while the hybrid excitation possibly proceeds in preference to the thermal effect. This finding provides a distinctive methodology for studying the physics of interaction between microwave and matter.
Acknowledgments The work was supported by the Kansai Research Foundation for technology promotion and a Grant-in-Aid for Scientific Research (C) (23561030) from the Japan Society for the Promotion of Science. References
Fig. 7. Logarithmic plot of microwave-power-dependence of the luminescence after saturation plotted against microwave power (P).
the direct electric energy transfer through microwave power to the electrons in radiative centers as well as nonradiative centers in the bandgap over thermal effects could be suggested in our model. Although the mechanism of Zn plasma emission from ZnO ceramics upon microwave irradiation under vacuum is still unclear, a sound understanding of this hybrid excitation process induced by microwave irradiation can contribute to elucidating the unique interaction between microwaves and semiconducting
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