Influence of Thermal Treatments on the Photoluminescence

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J. Phys. Chem. C 2007, 111, 8483-8488

8483

Influence of Thermal Treatments on the Photoluminescence Characteristics of Nanometer-Sized Amorphous Silica Particles Atsuko Aboshi, Naoko Kurumoto, and Tomoko Yamada Department of Chemistry, Graduate School of Science and Technology, Kobe UniVersity, Kobe 657-8501, Japan

Takashi Uchino* Department of Chemistry, Faculty of Science, Kobe UniVersity, Kobe 657-8501, Japan, SORST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ReceiVed: March 7, 2007; In Final Form: April 17, 2007

We have studied the photoluminescence (PL) characteristics of nanometer-sized amorphous silica particles after appropriate annealing in air and in vacuum. A broad visible PL band peaking at ∼450 nm, which is characterized by a nonexponential decay in the order of nanoseconds, has been found to develop when the samples are heat-treated in air. On the other hand, the vacuum-heated samples do not show such an increase in PL in the visible region but instead show a development of PL in the near-ultraviolet region. The different PL characteristics observed between the air- and vacuum-heated samples are discussed in terms of the different dehydroxylation reactions in air and in vacuum. Possible models of the respective emission centers are presented on the basis of the density functional theory calculations.

1. Introduction There has been an increasing interest in the synthesis, structure, and properties of nanometer-sized amorphous silica particles because of their unique physical and chemical characteristics related to their surface. Among other silica-based nanoparticles, fumed silica, which is a nonporous amorphous silicon dioxide produced by the vapor-phase hydrolysis of silicon tetrachloride in an oxygen-hydrogen flame, is one of the most widely studied materials.1-11 It has been shown that fumed silica has specific particle characteristics, such as the extremely small size of primary particles (∼10 to ∼50 nm), very high specific surface area (∼50 to ∼400 m2/g), and stable chainlike structure. Thus far, great attention has been paid especially to its thixotropic behavior in liquid systems in view of its widespread utility as a reinforcing filler in elastomers, rheology control for liquids, and emulsification agent. In addition to the surface-related chemistry, silica-based nanoparticles have attracted recent interest in terms of their intriguing photoluminescence (PL) characteristics. We have recently reported that white light PL emission is obtained from transparent silica glass prepared from solid-state sintering of fumed silica particles at ∼1000 °C.12 This peculiar PL emission from the sintered fumed silica can be attributed to some highly strained configurations at the interface of the particles. As for the PL from fumed silica in its unsintered state, Glinka and co-workers13-15 reported visible PL emissions consisting of red (∼1.9 eV), green (∼2.4 eV), and/or blue (∼2.8 eV) bands, which are characterized by decay times of ∼8, ∼12, and ∼5 µs, respectively, at room temperature.15 They showed that the blue PL band is observed exclusively under the irradiation of ArF laser and results from a radiative transition of self-trapped excitons generated by the two-photon excitation process trig* To whom correspondence should be addressed. E-mail: uchino@ kobe-u.ac.jp.

gered by the ArF laser irradiation.13-15 They also claimed that the surface hydrogen-related species and the bulk nonbridgingoxygen hole centers, which show the green and red PL bands, respectively, can be excited through the radiationless relaxation of free excitons created by the two-photon process.13-15 In measuring the PL spectra from fumed silica, Glinka and coworkers13-15 used high-intensity laser sources (∼0.1 to ∼1 MW/ cm2) from a pulsed ArF laser or the fourth harmonics of a pulsed Nd:YAG (yttrium-aluminum-garnet) laser to induce the expected two-photon excitation process, which is the main concern of the authors. To our knowledge, however, there have been no systematic studies on the PL properties of unsintered fumed silica under low-intensity irradiation that may induce a one-photon excitation process. In this work, we therefore carry out extensive PL measurements of unsintered fumed silica using a monochromatic light from a Xe lamp and a low-power pulsed-laser light. In our previous paper,16 we have preliminary reported a visible blue PL from fumed silica samples under one-photon excitation of ultraviolet (UV) light in the near UV region (300-400 nm). This blue emission is characterized by a broad PL peak centered at ∼450 nm and a nonexponential decay function on a time scale of several nanoseconds. It is also interesting to note that a similar blue PL with a nanosecond decay time can be found in other nanostructured forms of silica and silicon, including oxidized porous silicon,17 mesoporous MCM-41 silica,18 and a natural hydrated form of silica, namely, opal.19 It is hence probable that the blue PL from these Si- and SiO2-based nanostructured materials originates from similar emission centers; however, the structural origin of the blue PL emission still remains to be solved. To get a better insight into the underlying blue PL phenomena from nanostructured silica, we here investigate the effect of heating atmosphere, namely, heating in air or in vacuum, on the resulting PL characteristics.

10.1021/jp0718505 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/24/2007

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We show that the observed PL and PL excitation (PLE) spectra depend strongly on the heating environment. We then present possible formation mechanisms of the PL centers at the fumed silica surface to interpret the observed PL characteristics on the basis of modeling based on the density functional theory (DFT) calculations. 2. Experimental and Calculational Procedures The experiments were all performed on grade AEROSIL 380 (Nippon Aerosil Co., Ltd.), which has a nominal surface area of 380 m2/g and an average diameter of primary particles of 7 nm (product specification). Fumed silica powders were pressed into free-standing pellets using a uniaxial press at a pressure of 530 MPa for the thermal treatment and the subsequent PL measurements. Thermal treatment for the pressed samples was carried out at temperatures ranging from 200 to 800 °C for 2 h in air or ∼10 Pa vacuum. In this temperature range, it has been demonstrated that fumed silica powders do not show sintering or coalescence but show dehydroxylation reactions at the surface or interface of the respective particles.11 Steady-state PL measurements were performed using a Xe lamp with a single monochromator, a photomultiplier tube, and appropriate interference filters. We also carried out time-resolved PL measurements using a synchroscan streak camera (Hamamatsu, C4334) and two different pulsed laser sources: one is the fourth harmonic (λex ) 266 nm) of a Nd:YAG laser (Spectra Physics, INDI-40) with a pulse duration of ∼8 ns operating at 10 Hz and a maximum power density of ∼50 mW/cm2, and the other is the second harmonic (λex ) 350 nm) of a mode lock Ti:sapphire laser (Spectra Physics, Tsunami) with a pulse duration of ∼200 fs operating at 1 MHz and a maximum power density of ∼3 mW/cm2. All the PL measurements were performed at room temperature. We also carried out a series of DFT calculations using clusters of atoms that model the local structure of the expected emission centers. It has already been demonstrated that the DFT cluster calculations are very useful to investigate the structure and energies of defect centers in various silica-based materials.20 All the DFT calculations in this work were performed with the GAUSSIAN0321 code. The singlet-to-singlet excitation energies of the model clusters were obtained by the time-dependent DFT (TD-DFT) method.22 3. Results 3.1. Effect of Aging on the PL Characteristics. While the intrinsic chemical purity of fumed silica is very high, the surfaces of respective particles are normally coated with hydrogen atoms in the form of SiOH groups and H2O molecules1-5,8 and/or other carbon-related contaminations. To evaluate the effect of these physisorbed species on the PL characteristics of non-heat-treated fumed silica, we first compare the PL characteristics of the as-manufactured and aged samples. Figure 1 shows PL spectra of as-manufactured and heavily aged fumed silica samples under excitation of mid-UV light (239 nm). We see from Figure 1 that the as-manufactured fumed silica, which initially shows a weak PL emission in the UV/ visible region, tends to exhibit a strong PL at ∼300 nm after exposure to ambient air for long periods of time. Such aginginduced PL was not observed when we used near-UV light (300-400 nm) as an excitation source. Previously, similar midUV-excited PL emissions have been observed from aged23 and non-heat-treated24,25 porous silica. Although this type of UV PL was often attributed to surface Si-OH groups,23-25 this attribution cannot accept without reservation because Si-OH

Figure 1. PL) spectra of various fumed silica samples: (a) asmanufactured fumed silica, (b) fumed silica after aging for 1 year in ambient environment, and (c) after heating the sample (b) at 120 °C for 2 h. The excitation wavelength is 239 nm.

Figure 2. PL spectra of air heat-treated fumed silica samples under excitation of 340 nm light: (a) pre-heated in air at 120 °C, post-heattreated in air at (b) 200, (c) 300, (d) 400, and (e) 500 °C for 2 h.

groups both in isolated and hydrogen-bonded forms do not absorb UV light below ∼7.3 eV.16,26 We suggest that this increase in PL after aging results from hydrocarbon contaminations and/or other physisorbed species. We also notice from Figure 1 that the aging-induced UV PL almost disappears after heating the sample at 120 °C in a drying oven, indicating that this low-temperature heating treatment is effective to eliminate the aging-induced PL. Thus, in the following experiments, we use fumed silica samples that are preheated at 120 °C. 3.2. Effect of Heating Environment on the PL Characteristics. In our previous paper,16 we reported that the nearUV-excited PL from fumed silica is substantially developed after heating the sample at ∼300 °C in air. For example, as shown in Figure 2, the blue PL emission (∼430 to ∼450 nm) from post-heat-treated fumed silica at 300-400 °C in air shows a substantial PL under excitation of 340 nm light. The observed PL hardly shows degradation even after the samples are stored in an ambient environment for more than several months. On the other hand, when the sample is post-heat-treated in vacuum, as shown in Figure 3, such a substantial increase in PL under excitation of near UV light can hardly be seen. This result indicates that the heat treatment in air is more effective than that in vacuum in developing near-UV-excited PL in the blue region. When heated at temperatures more than ∼600 °C, both the air- and vacuum-heated samples do not show any PL emissions over the entire spectral range investigated. To more clearly show the excitation energy dependence of the PL, we illustrate in Figure 4 the two-dimensional PL-PL excitation (PLE) spectra of the air- and vacuum-heated samples. We see from Figure 4 that the PL-PLE contour plot of the

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Figure 3. PL spectra of vacuum heat-treated fumed silica samples under excitation of 340 nm light: (a) pre-heated in air at 120 °C, and (b) post-heat-treated in vacuum at 400 °C.

Figure 5. Normalized PL decay curves of the compressed fumed silica (P ) 530 MPa) post-heat-treated at 400 °C in air for 2 h obtained with (a) the fourth harmonic (λex ) 266 nm) of a pulsed Nd:YAG laser and (b) the second harmonic (λex ) 400 nm) of a pulsed Ti:sapphire laser. The time-resolved PL signals were detected at 450 nm. The solid lines are the best fits by a stretched exponential function (eq 1).

vacuum-heated sample occurs mainly in the near-UV region (∼350 nm) under excitation of UV light at ∼250 and ∼290 nm. The above results shown in Figures 2-4 demonstrate that the PL characteristics of fumed silica are strongly affected by the heating environment. In particular, the blue PL is more effectively induced when the sample is heated in air rather than in vacuum. 3.3. Time-Resolved PL Measurements. Time-resolved PL measurements were carried out using two types of pulsed lasers (i.e., the fourth harmonic (λex ) 266 nm) of a pulsed Nd:YAG laser and the second harmonic (λex ) 350 nm) of a pulsed Ti: sapphire laser, respectively. The decay profiles observed for the air- and vacuum-heated samples are shown in Figures 5 and 6, respectively. Figures 5 and 6 show that all the decay profiles observed here show a nonexponential decay on a time scale of several tens of nanoseconds. We found that the observed decay dynamics can be described by a stretched exponential function27

I(t) ) I0 exp[-(t/τ)β]

Figure 4. Contour plots of PL spectra as a function of excitation energy of differently heat-treated fumed silica samples: (a) post-heat-treated in air at 300 °C for 2 h, and (b) post-heat-treated in vacuum at 400 °C for 2 h.

air-heated sample is rather different from that of the vacuumheated sample, in agreement with the results shown in Figure 2 and 3. In the air-heated sample, we see the PL emissions in the blue (∼430 to ∼450 nm) and the near-UV (∼350 nm) regions; the former PL is excited at ∼350 and ∼250 nm, and the latter PL at ∼250 nm. On the other hand, the PL emission from the

(1)

where I0 is the PL intensity at t ) 0, τ is a characteristic decay time, and β is a stretching parameter that represents the degree of deviation from a pure exponential decay. As mentioned in the Introduction, a similar stretched exponential decay on a time scale of nanoseconds has been observed from Si- and SiO2based nanostructured materials.17-19 We should also note that the stretched exponential PL decay can generally be found in disordered systems in which the dispersive diffusion of photoexcited electrons and/or holes are possible.27,28 The dispersive diffusion of carriers most likely results from the excitation of carriers from localized to extended states or hopping among localized states.28,29 Because the PL decays shown in Figures 5 and 6 have a characteristic decay time of several nanoseconds, we consider that the present PL processes are ascribed to the allowed singlet-to-singlet transitions and the subsequent diffusion-controlled electron-hole recombination processes.

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Figure 7. Models of the dehydroxylation reaction involved in the two adjacent geminal silanol groups: (a) formation of a defect pair consisting of dSi(O2) and dSi: (scheme 1) that is expected to occur in air environment, and (b) formation of a defect pair consisting of two dSi: (scheme 2) that is expected to occur in vacuum environment.

Figure 6. Normalized PL decay curves of the compressed fumed silica (P ) 530 MPa) post-heat-treated at 400 °C in vacuum for 2 h obtained with (a) the fourth harmonic (λex ) 266 nm) of a pulsed Nd:YAG laser and (b) the second harmonic (λex ) 400 nm) of a pulsed Ti:sapphire laser. The time-resolved PL signals were detected at 450 nm. The solid lines are the best fits by a stretched exponential function (eq 1).

It should be noted that the τ and β values obtained from the air-heated samples are different from the ones obtained from the vacuum-heated samples, suggesting that there are some differences in the relevant PL processes between these two samples. Considering that the β values of the vacuum-heated samples (∼0.35) are smaller than those of the air-heated ones (∼0.5), we suggest that the distribution of release rates, trap energies, and/or localized states in the former samples is wider than those in the latter samples. However, further investigations (e.g., the analysis of the temperature dependence of β)27,28 will be needed to get a more detailed picture of the dispersive diffusion processes related to these samples. 4. Discussion In Section 3, we have shown that the PL intensity of fumed silica increases after heating the samples at 300-400 °C especially in air. This implies that the emission centers are created at the interface of the respective fumed silica particles during heating. As for the emission center that is responsible for the blue PL from air-heated fumed silica, we have recently put forward a structural model consisting of a dioxasilirane, dSi(O2), and a silylene, dSi:, center (see also scheme 1 in Figure 7a).16 We suggested that this defect pair is formed as a result of the dehydroxylation reaction of a pair of geminal silanol groups, in which two OH groups are bonded to one Si atom. Previous 29Si cross polarization magic-angle spinning NMR experiments on fumed silica revealed that a relatively large amount of internal genimal silanol group exist in fumed silica.8,30 In our previous paper,16 we have demonstrated from the DFT calculations that the defect pair consisting of a dioxasilirane and a silylene center has allowed singlet-to-singlet excitations at ∼350 and ∼250 nm, in agreement with the observed PLE spectra. However, as shown in Figures 3 and 4, the intensity of the blue PL is significantly reduced when the sample is post-heattreated in vacuum. It is expected that a similar dehydroxylation

reaction related to a pair of geminal silanol groups will occur under vacuum as well; however, the present results demonstrate that our recently proposed model cannot be applied to the formation reaction of emission centers under vacuum heating as it is. Thus, further considerations will be required to account for the difference in PL properties between the air- and vacuumheated samples. As mentioned above, dehydroxylation of a pair of geminal silanol groups under air will lead to the formation of a dioxasilirane, a silylene, and two H2O molecules (Figure 7a, scheme 1). When the reaction occurs under vacuum, we suggest that the dioxasilirane group formed from scheme 1 in Figure 7a will further decompose into a silylene and an O2 molecule, leading to the formation of a pair of two silylene groups according to the scheme shown in Figure 7b (scheme 2). We hence propose that this simultaneous release of O2 molecules during vacuum heating is responsible for the difference in PL properties between the air- and vacuum-heated samples. If such O2 molecules are indeed created during vacuum heating, these molecules are likely to be trapped in the silica glass network. Because the interstitial O2 molecules in silica glass show a specific near-IR PL band at 1272 nm under excitation of a 765 nm light,31 these molecules can be experimentally detected as long as they are incorporated into the silica network. To confirm the possible formation of O2 molecules based on scheme 2 in Figure 7b, we heat-treated fumed silica at 980 °C for 6 h in air and vacuum and measured the expected PL emission. For this purpose, we used the fundamental wavelength of a continuous wave Ti:sapphire laser (λex ) 765 nm) as an excitation source. Such a high-heating temperature (980 °C) was employed so as to complete the dehydroxylation reaction of the silanol groups, as proposed in scheme 2, at the expense of the thermal quenching of the emission centers. Figure 8 shows the near-IR PL spectra of non-heat-treated, air heat-treated, and vacuum-heated fumed silica samples. We see from Figure 8 that a slight PL band at 1272 nm is observed even from the non-heat-treated fumed silica sample. This signal may result from the O2 molecules that are introduced during the preparation process of fumed silica. It should be noted that the intensity of the signal significantly increases after heating the sample in vacuum rather than in air. The 1272 nm band is not observed from commercial silica glass unless O2 molecules are intentionally incorporated under O2 or air atmosphere at ∼1000 °C for several weeks.32 Thus, the evolution of this nearIR PL band of fumed silica strongly suggests the intrinsic

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Figure 8. Photoluminescence spectra of (a) as-manufactured fumed silica, (b) fumed silica post-heat-treated in air at 980 °C, and (c) fumed silica post-heat-treated in vacuum at 980 °C.

formation of O2 molecules especially during vacuum heating, in harmony with our proposed dehydroxylation reaction of geminal silanol groups (Figure 7b). It is likely that when the heating temperature is low, namely, in the rage of 300400 °C, the formation of O2 molecules occurs mainly by vacuum heating according to Figure 7b, leading to a substantial decrease in the visible PL emission. Unfortunately, however, we did not observe a noticeable difference in the near-IR PL spectra between air- and vacuum-heated samples when the heating temperature is below 300-400 °C. This is probably because the number of O2 molecules created during vacuum heating at such temperatures is too small to be unambiguously detected by near-IR PL measurements. To get the information about the excitation energies of the proposed defects, we next perform a series of DFT calculations on clusters of atoms that model the local structure of the relevant defects. We call the clusters of atoms including the defects proposed in Figure 7a,b as model 1 and model 2, respectively. As for the defect pair proposed in Figure 7a, we have already reported preliminary results using a cluster of atoms consisting of 6 Si atoms.16 In this work, we employ larger clusters including 14 Si atoms to more properly model the proposed defect structures. The dangling bonds of the surface Si atoms in the clusters were terminated by H atoms. The optimized structure of the clusters are obtained at the B3LYP/6-31G(d) level, as shown in Figure 9. Using the 6-31G(d) optimize geometries, we then calculated the electronic excitation energies on the basis of the TD-DFT method22 using the 6-31++G(d) basis set. Figure 9 shows the principal optimized structural parameters of model 1 and model 2, and Table 1 lists their five lowest singlet-to-singlet excitation energies. We see from Table 1 that the excitation energies of model 1 can be divided into two regions; the first excitation wavelength (380.8 nm) is located in the near UV region and the second or higher ones (∼240 to ∼280 nm) in the mid UV region. This is basically in agreement with the previous calculations using a smaller cluster.16 We should also note that the calculated excitation energies agree well with the PLE energies of the air-heated samples (see Figure 4). Thus, as has been proposed previously in ref 16, we consider that the PL characteristics of the air-heated samples are interpreted in terms of the electronic excitation process associated with the defect pair produced via the scheme in Figure 7a. We next turn to the excitation energies of model 2. We see from Table 1 that the first excitation wavelength (307.0 nm) of model 2 is substantially shorter than that of model 1 (∼380 nm). This indicates that such a pair of silylene centers as seen in model 2 will not be excited under the irradiation of

Figure 9. Optimized structures of the model clusters used to model the defect structures shown in (a) Figure 7a (scheme 1) and (b) Figure 7b (scheme 2). Geometry optimizations were performed at the B3LYP/6-31G(d) level.

TABLE 1: The Five Lowest Excitation Energies Calculated for the Model Clusters Shown in Figure 9a excited state

model I (eV/nm)

model II (eV/nm)

1 2 3 4 5

3.26/380.8 4.49/276.1 4.83/256.5 4.92/252.0 5.12/242.4

4.04/307.0 4.76/260.5 5.05/245.5 5.62/220.6 5.76/215.4

a

The Calculations were performed at the TD-B3LYP/6-31++G(d) basis set using the B3LYP/6-31G(d) geometries.

near UV light, in contrast to the case of a pair of dioxasilirane and silylene centers in model 1. The other higher excited states of model 2 are located in the mid UV region (∼220 to ∼260 nm), which is similar to the corresponding excited states of model 1. It is also interesting to note that these calculated excitation wavelengths are in reasonable agreement with the wavelength regions of the PLE spectra (∼290 and ∼250 nm) observed for the vacuum-heated samples shown in Figure 4. Previously, the electronic excitation energies of a single silylene center have been calculated by first-principles quantumchemical techniques.33-35 The first excitation energy of the single silylene center was calculated to be ∼5.1 to ∼5.4 eV (∼240 to ∼230 nm). It has also been demonstrated experimentally that a single silylene center in a SiO2 matrix yields PL bands at ∼280 and ∼440 nm with decay times of 4 ns and 10 ms, respectively.36 We should note that these calculated and observed results on a single silylene center do not account for the PL characteristics observed not only from the vacuum-heated samples but also from the air-heated ones. We consider that a pair of silylene centers have different electronic structures, because of the interaction between the two two-coordinated d Si: atoms, than a single silylene center and is more appropriate

8488 J. Phys. Chem. C, Vol. 111, No. 24, 2007 to explain the observed PL behaviors especially from the vacuum-heated samples. 5. Conclusions We showed that the PL characteristics from heat-treated fumed silica is highly dependent on the heating environment. The blue PL emission (∼430 to ∼450 nm) is enhanced especially when the samples are heat-treated in air, whereas the UV PL at ∼350 nm is increased when the samples are heattreated in vacuum. All the observed PL emissions are characterized by a stretched exponential decay function on a time scale of nanoseconds irrespective of excitation wavelengths. We then presented a possible formation mechanism of the respective emission centers that are created under air and vacuum heating. We suggest that the dehydroxylation reaction of fumed silica under vacuum results in the release of O2 molecules as well as H2O molecules, whereas the dehydroxylation reaction under air results mainly in the release of H2O molecules. This difference in the dehydroxylation processes under air and vacuum probably accounts for the resulting difference in the observed PL behaviors. Acknowledgment. This work was supported by grants from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (No. 14350352) and Toray Science Foundation. We thank Dr. K. Kajihara and Professor H. Hosono (Tokyo Institute of Technology) for the measurements of the PL spectra related to O2 molecules. References and Notes (1) Hair, M. L. J. Non-Cryst. Solids 1975, 19, 299. (2) Ryason, P. R.; Russell, B. G. J. Phys. Chem. 1975, 79, 1276. (3) Morrow, B. A.; McFarlan, A. J. J. Phys. Chem. 1992, 96, 1395. (4) Hurd, A. J.; Schaefer, D. W.; Martin, J. E. Phys. ReV. A 1987, 35, 2361. (5) Bunker, B. C.; Haaland, D. M.; Michalske, T. A.; Smith, W. L. Surf. Sci. 1989, 222, 95. (6) Gun’ko, V. M.; Turov, V. V. Langmuir 1999, 15, 6405. (7) Saito, H.; Hyodo, T. Phys. ReV. B 1999, 60, 11070. (8) Liu, C. C.; Maciel, G. E. J. Am. Chem. Soc. 1996, 118, 5103. (9) Page, J. H.; Buyers, W. J. L.; Dolling, G.; Gerlach, P.; Harrison, J. P. Phys. ReV. B 1989, 39, 6180. (10) Freltoft, T.; Kjems, J. K.; Sinha, S. K. Phys. ReV. B 1986, 33, 269. (11) Uchino, T.; Aboshi, A.; Kohara, S.; Ohishi, Y.; Sakashita, M.; Aoki, K. Phys. ReV. B 2004, 69, 155409. (12) Uchino, T.; Yamada, T. Appl. Phys. Lett. 2004, 85, 1164; Yamada, T.; Uchino, T. Appl. Phys. Lett. 2005, 87, 081904.

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