- Email: [email protected]
Shallow nitrogen acceptor in TiO2 studied by β-NMR spectroscopy
ARTICLE IN PRESS
Physica B 401–402 (2007) 430–432 www.elsevier.com/locate/physb
Shallow nitrogen acceptor in TiO2 studied by b-NMR spectroscopy M. Miharaa,, R. Matsumiyaa, K. Shimomurab, K. Matsutaa, M. Fukudaa, D. Ishikawaa, J. Komurasakia, D. Nishimuraa, T. Nagasawaa, T. Izumikawac, T. Minamisonod a
Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan b Institute of Material Structure Science, KEK, Tsukuba 305-0301, Japan c Radioisotope Center, Niigata University, Niigata 951-8510, Japan d Fukui University of Technology, Fukui 910-8505, Japan
Abstract The local electronic structure of substitutional nitrogen impurity in rutile TiO2 was studied by means of the b-NMR technique. A short-lived b emitter 12N (I ¼ 1, T1/2 ¼ 11 ms) was implanted into a rutile single crystal as a probe nucleus to detect double-quantum NMR spectrum. The result shows the existence of a small hyperfine interaction at low temperatures due to paramagnetic states localized at oxygen substitutional site, which suggests that a substitutional nitrogen atom acts as a shallow acceptor in rutile TiO2. r 2007 Elsevier B.V. All rights reserved. PACS: 76.60.k; 61.72.Ww; 71.20.Nr; 71.55.i Keywords: Shallow acceptor; TiO2; Implantation; b-NMR
1. Introduction Nitrogen impurities are expected to be one of the key dopants in TiO2 to facilitate its use as a visible light responsive photocatalyst, since the photocatalitic activity in nitrogen-doped TiO2 has been shown under visible light irradiation [1,2]. Substitutional nitrogen doping for oxygen in TiO2 is thought to cause the band-gap narrowing, and to shift the optical response from ultraviolet to visible light range [2]. So far, there have not been sufficient experimental studies of the electronic structure of the nitrogendoped TiO2. An isolated substitutional nitrogen atom is expected to form an acceptor level in the band-gap of TiO2. According to the hydrogen-like impurity model, Bohr radius a and binding energy E nb of a hole at an acceptor center is renormalized by the dielectric constant e and the hole effective mass mnh as an ¼ =ðmnh =me Þa0 and E nb ¼ ð13:6 eVÞ ðmnh =me Þ=2 , respectively, where me is free electron mass and a0 is the Bohr radius of hydrogen in Corresponding author. Tel.: +81 6 6850 5536; fax: +81 6 6850 5535.
E-mail address: [email protected] (M. Mihara). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.08.204
vacuum. In case of rutile TiO2, large dielectric constants e ¼ 111 and 257 along a- and c-axis, respectively, at low temperatures [3] might give a significant large a and small E nb , so that quite a small hyperfine constant due to large volume of the paramagnetic acceptor center might be observed at low temperatures. Studies of the electronic structure and the characteristics of such an acceptor center are of importance in understanding the properties of doped samples with various range of impurity concentration. The experimental studies on the electronic structure of paramagnetic shallow acceptor centers in semiconductors have been done by the electron-nuclear-double-resonance (ENDOR) and the negative muon spin rotation (mSR) techniques. The former was applied to a shallow boron acceptor in SiC [4] and the latter to an aluminum acceptor in Si [5]. In addition to these techniques, the b-NMR technique is also useful for the direct observation of such microscopic properties of isolated impurities, because of the extremely high sensitivity of NMR detection. In the present study, we have measured the double-quantum (DQ) NMR spectra of a short lived nucleus 12N (I ¼ 1, T1/2 ¼ 11 ms) implanted into a single crystal of rutile TiO2
ARTICLE IN PRESS M. Mihara et al. / Physica B 401–402 (2007) 430–432
-6
-8
-10 280 K
105 K
18 K 1770
1760
1750
1740
1730
1770
1760
1750
1740
1730
-12 1770
where nL ¼ gnB/2p, the Larmor frequency with the nuclear gyromagnetic ratio gn; nQ ¼ (3/4)eqQ/h with the electric field gradient (EFG) q and the nuclear quadrupole moment Q, and the asymmetry parameter Z of EFG. y and j are Euler angles of the EFG relative to the external field B0. Using known values of eqQ/h and Z for the substitutional site as eqQ/h ¼ 469 kHz and Z ¼ 0.37 at room temperature [7]; nDQ ¼ 1749 kHz is expected at B0 ¼ 0.5 T for the present crystal orientation, cJB0, giving y ¼ j ¼ 901 [8]. For the interstitial site, nDQ ¼ 2040 kHz is estimated from eqQ/h ¼ 2888 kHz and Z ¼ 0.038, which is far from the DQ frequency for the substitutional site.
-4
1760
ð1Þ
-2
1750
2 1 þ Z cos2 j sin4 y þ Z2 ð4 3 cos2 2j sin4 yÞg, 3 4
0
1740
The experimental method is similar to the previous one [6]. The short-lived b-emitter 12N was produced through the 10B(3He,n)12N reaction. The 3He beam of 3.0 MeV with an intensity of 10–15 mA from the 5 MV Van de Graaff accelerator at Osaka University was used to bombard the 90% enriched 10B target which was prepared by the vacuum evaporation on a 0.5-mm-thick Al backing. The 12 N nuclei ejected from the target at angles in between 121 and 281 relative to the 3He-beam direction was selected with a collimator to obtain nuclear spin polarization of 20%. The polarized 12N nuclei were implanted into a single crystal of rutile TiO2 within a static magnetic field B0 ¼ 0.5 T applied parallel to the polarization direction. The crystalline c-axis was set parallel to B0. Two sets of plastic-scintillation-counter telescopes were located above and below the sample relative to the reaction plane to detect the asymmetry in the b-ray angular distribution. A radiofrequency (rf) magnetic field B1 was applied perpendicular to B0 to induce resonance which was detected by polarization change monitored via the b-ray asymmetry. From the previous b-NMR studies on 12N in rutile, two lattice locations with different electric field gradients (EFG) were identified as the oxygen substitutional site and the octahedral interstitial site [7]. So in the present case, the DQ transition between substrates m ¼ 1 and +1 will be observed as a sharp single NMR line in the middle of a pair of single-quantum transition frequencies n120 and n+120 under a strong external magnetic field, which is useful to search a small internal field Bint due to the hyperfine interaction. The frequency for the DQ transition nDQ in case of nuclear spin I ¼ 1 is described as follows, considering the second-order shift due to the quadrupole interaction under a strong magnetic field B ( ¼ B0+Bint): 2! nQ 1 nDQ ¼ nL þ fsin2 yð3 cos2 y þ 1Þ 8 nL
The DQ NMR spectra of 12N in TiO2 at temperatures of 18, 105 and 280 K are shown in Fig. 1. The rf field B1 of 2.5 mT with the frequency modulation of 71 kHz and the duration of 6 ms was applied to induce the DQ transition. The spectrum at 280 K shows a sharp NMR line and its frequency is consistent with the above estimation for the oxygen substitutional site, so that this resonance corresponds to the diamagnetic component of the substitutional nitrogen. As the temperature decreases, the diamagnetic fraction decreases at 18 K and other components than the narrow diamagnetic line emerge with frequencies shifted to both sides by up to 20 kHz from the diamagnetic line. The fraction of this broad component in the spectrum is about 40% of the total fraction deduced by integrating the spectrum over the observed frequency range. The total fraction at 18 K is nearly the same as that at 280 K. However, about 30% of the total fraction is missing at 105 K. In the spectrum at 18 K, resonances are observed even at lower frequencies than the Larmor frequency nL ¼ 1742 kHz at Bint ¼ 0, which cannot be explained without considering the existence of a negative internal field Bint because the second-order shift must be always positive. The chemical shift for the nitrogen case is 400 ppm [8] which is too small to affect the present spectrum. The possibility of the single-quantum transition with tiny quadrupole interaction is ruled out because, if that is true, a distinct double quantum peak with the
1730
2. Experimental
3. Results and discussion
-ray asymmetry change; AP (%)
by means of the b-NMR technique, in order to observe the hyperfine interactions of the substitutional 12N nucleus with the surrounding electronic state.
431
Frequency (kHz) Fig. 1. Double-quantum NMR spectra of 12N in a single crystal of rutile TiO2. The crystalline c-axis was set parallel to the external magnetic field of 0.5 T.
ARTICLE IN PRESS 432
M. Mihara et al. / Physica B 401–402 (2007) 430–432
second order shift n2Q =nL of nearly zero should be observed at around 1742 kHz. This result suggests the existense of paramagnetic states formed by a nitrogen acceptor at low temperatures. If we accept this picture, the broad component at the higher frequency side can also be considered to be due to the paramagnetic states with the opposite direction of electron spin relative to that for the lower frequency component. From the observed NMR line shift, the hyperfine constant is estimated as 104 times smaller than the vacuum hydrogen value. This result is similar to that of mSR studies on some semiconductors showing extremely small hyperfine constants for muonium (Mu) compared to the vacuum Mu value (103 to 104 times) due to formation of shallow donor levels [9]. In summary, the DQ NMR of 12N in a single crystal of rutile TiO2 was performed by means of the b-NMR technique and quite a small hyperfine field was observed at 18 K. This result suggests that the substitutional nitrogen atom for oxygen acts as a shallow acceptor in rutile TiO2. In order to clarify the electronic structure of the nitrogen acceptor center, further experimental studies such as more precise temperature dependence of the DQ spectrum are in progress.
Acknowledgments The present work was partly supported by the Murata Science Foundation and the grant-in-aid for Scientific Research Program of the Japan Society for the Promotion of Science. References [1] S. Sato, Chem. Phys. Lett. 123 (1986) 126. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [3] R.A. Parker, Phys. Rev. 124 (1961) 1719. [4] A. van Duijn-Arnold, J. Mol, R. Verberk, J. Schmidt, Phys. Rev. B 60 (1999) 15829. [5] T.N. Mamedov, V.N. Gorelkin, A.V. Stoikov, Phys. Part. Nucl. 33 (2002) 519. [6] M. Mihara, S. Kumashiro, H. Fujiwara, R. Matsumiya, K. Matsuta, Y. Nakashima, Y.N. Zheng, M. Ogura, T. Sumikama, T. Nagatomo, K. Minamisono, M. Fukuda, T. Izumikawa, T. Minamisono, Physica B 376–377 (2006) 955. [7] T. Minamisono, K. Sato, H. Akai, S. Takeda, Y. Maruyama, K. Matsuta, M. Fukuda, T. Miyake, A. Morishita, T. Izumikawa, Y. Nojiri, Z. Naturforsch 53a (1998) 293. [8] C. Brevard, P. Granger, in: Handbook of High Resolution Multinuclear NMR, Wiley/Interscience, New York, 1981, pp. 90–91. [9] S.F.J. Cox, J. Phys.: Condens. Matter 15 (2003) R1727.
Physica B 401–402 (2007) 430–432 www.elsevier.com/locate/physb
Shallow nitrogen acceptor in TiO2 studied by b-NMR spectroscopy M. Miharaa,, R. Matsumiyaa, K. Shimomurab, K. Matsutaa, M. Fukudaa, D. Ishikawaa, J. Komurasakia, D. Nishimuraa, T. Nagasawaa, T. Izumikawac, T. Minamisonod a
Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan b Institute of Material Structure Science, KEK, Tsukuba 305-0301, Japan c Radioisotope Center, Niigata University, Niigata 951-8510, Japan d Fukui University of Technology, Fukui 910-8505, Japan
Abstract The local electronic structure of substitutional nitrogen impurity in rutile TiO2 was studied by means of the b-NMR technique. A short-lived b emitter 12N (I ¼ 1, T1/2 ¼ 11 ms) was implanted into a rutile single crystal as a probe nucleus to detect double-quantum NMR spectrum. The result shows the existence of a small hyperfine interaction at low temperatures due to paramagnetic states localized at oxygen substitutional site, which suggests that a substitutional nitrogen atom acts as a shallow acceptor in rutile TiO2. r 2007 Elsevier B.V. All rights reserved. PACS: 76.60.k; 61.72.Ww; 71.20.Nr; 71.55.i Keywords: Shallow acceptor; TiO2; Implantation; b-NMR
1. Introduction Nitrogen impurities are expected to be one of the key dopants in TiO2 to facilitate its use as a visible light responsive photocatalyst, since the photocatalitic activity in nitrogen-doped TiO2 has been shown under visible light irradiation [1,2]. Substitutional nitrogen doping for oxygen in TiO2 is thought to cause the band-gap narrowing, and to shift the optical response from ultraviolet to visible light range [2]. So far, there have not been sufficient experimental studies of the electronic structure of the nitrogendoped TiO2. An isolated substitutional nitrogen atom is expected to form an acceptor level in the band-gap of TiO2. According to the hydrogen-like impurity model, Bohr radius a and binding energy E nb of a hole at an acceptor center is renormalized by the dielectric constant e and the hole effective mass mnh as an ¼ =ðmnh =me Þa0 and E nb ¼ ð13:6 eVÞ ðmnh =me Þ=2 , respectively, where me is free electron mass and a0 is the Bohr radius of hydrogen in Corresponding author. Tel.: +81 6 6850 5536; fax: +81 6 6850 5535.
E-mail address: [email protected] (M. Mihara). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.08.204
vacuum. In case of rutile TiO2, large dielectric constants e ¼ 111 and 257 along a- and c-axis, respectively, at low temperatures [3] might give a significant large a and small E nb , so that quite a small hyperfine constant due to large volume of the paramagnetic acceptor center might be observed at low temperatures. Studies of the electronic structure and the characteristics of such an acceptor center are of importance in understanding the properties of doped samples with various range of impurity concentration. The experimental studies on the electronic structure of paramagnetic shallow acceptor centers in semiconductors have been done by the electron-nuclear-double-resonance (ENDOR) and the negative muon spin rotation (mSR) techniques. The former was applied to a shallow boron acceptor in SiC [4] and the latter to an aluminum acceptor in Si [5]. In addition to these techniques, the b-NMR technique is also useful for the direct observation of such microscopic properties of isolated impurities, because of the extremely high sensitivity of NMR detection. In the present study, we have measured the double-quantum (DQ) NMR spectra of a short lived nucleus 12N (I ¼ 1, T1/2 ¼ 11 ms) implanted into a single crystal of rutile TiO2
ARTICLE IN PRESS M. Mihara et al. / Physica B 401–402 (2007) 430–432
-6
-8
-10 280 K
105 K
18 K 1770
1760
1750
1740
1730
1770
1760
1750
1740
1730
-12 1770
where nL ¼ gnB/2p, the Larmor frequency with the nuclear gyromagnetic ratio gn; nQ ¼ (3/4)eqQ/h with the electric field gradient (EFG) q and the nuclear quadrupole moment Q, and the asymmetry parameter Z of EFG. y and j are Euler angles of the EFG relative to the external field B0. Using known values of eqQ/h and Z for the substitutional site as eqQ/h ¼ 469 kHz and Z ¼ 0.37 at room temperature [7]; nDQ ¼ 1749 kHz is expected at B0 ¼ 0.5 T for the present crystal orientation, cJB0, giving y ¼ j ¼ 901 [8]. For the interstitial site, nDQ ¼ 2040 kHz is estimated from eqQ/h ¼ 2888 kHz and Z ¼ 0.038, which is far from the DQ frequency for the substitutional site.
-4
1760
ð1Þ
-2
1750
2 1 þ Z cos2 j sin4 y þ Z2 ð4 3 cos2 2j sin4 yÞg, 3 4
0
1740
The experimental method is similar to the previous one [6]. The short-lived b-emitter 12N was produced through the 10B(3He,n)12N reaction. The 3He beam of 3.0 MeV with an intensity of 10–15 mA from the 5 MV Van de Graaff accelerator at Osaka University was used to bombard the 90% enriched 10B target which was prepared by the vacuum evaporation on a 0.5-mm-thick Al backing. The 12 N nuclei ejected from the target at angles in between 121 and 281 relative to the 3He-beam direction was selected with a collimator to obtain nuclear spin polarization of 20%. The polarized 12N nuclei were implanted into a single crystal of rutile TiO2 within a static magnetic field B0 ¼ 0.5 T applied parallel to the polarization direction. The crystalline c-axis was set parallel to B0. Two sets of plastic-scintillation-counter telescopes were located above and below the sample relative to the reaction plane to detect the asymmetry in the b-ray angular distribution. A radiofrequency (rf) magnetic field B1 was applied perpendicular to B0 to induce resonance which was detected by polarization change monitored via the b-ray asymmetry. From the previous b-NMR studies on 12N in rutile, two lattice locations with different electric field gradients (EFG) were identified as the oxygen substitutional site and the octahedral interstitial site [7]. So in the present case, the DQ transition between substrates m ¼ 1 and +1 will be observed as a sharp single NMR line in the middle of a pair of single-quantum transition frequencies n120 and n+120 under a strong external magnetic field, which is useful to search a small internal field Bint due to the hyperfine interaction. The frequency for the DQ transition nDQ in case of nuclear spin I ¼ 1 is described as follows, considering the second-order shift due to the quadrupole interaction under a strong magnetic field B ( ¼ B0+Bint): 2! nQ 1 nDQ ¼ nL þ fsin2 yð3 cos2 y þ 1Þ 8 nL
The DQ NMR spectra of 12N in TiO2 at temperatures of 18, 105 and 280 K are shown in Fig. 1. The rf field B1 of 2.5 mT with the frequency modulation of 71 kHz and the duration of 6 ms was applied to induce the DQ transition. The spectrum at 280 K shows a sharp NMR line and its frequency is consistent with the above estimation for the oxygen substitutional site, so that this resonance corresponds to the diamagnetic component of the substitutional nitrogen. As the temperature decreases, the diamagnetic fraction decreases at 18 K and other components than the narrow diamagnetic line emerge with frequencies shifted to both sides by up to 20 kHz from the diamagnetic line. The fraction of this broad component in the spectrum is about 40% of the total fraction deduced by integrating the spectrum over the observed frequency range. The total fraction at 18 K is nearly the same as that at 280 K. However, about 30% of the total fraction is missing at 105 K. In the spectrum at 18 K, resonances are observed even at lower frequencies than the Larmor frequency nL ¼ 1742 kHz at Bint ¼ 0, which cannot be explained without considering the existence of a negative internal field Bint because the second-order shift must be always positive. The chemical shift for the nitrogen case is 400 ppm [8] which is too small to affect the present spectrum. The possibility of the single-quantum transition with tiny quadrupole interaction is ruled out because, if that is true, a distinct double quantum peak with the
1730
2. Experimental
3. Results and discussion
-ray asymmetry change; AP (%)
by means of the b-NMR technique, in order to observe the hyperfine interactions of the substitutional 12N nucleus with the surrounding electronic state.
431
Frequency (kHz) Fig. 1. Double-quantum NMR spectra of 12N in a single crystal of rutile TiO2. The crystalline c-axis was set parallel to the external magnetic field of 0.5 T.
ARTICLE IN PRESS 432
M. Mihara et al. / Physica B 401–402 (2007) 430–432
second order shift n2Q =nL of nearly zero should be observed at around 1742 kHz. This result suggests the existense of paramagnetic states formed by a nitrogen acceptor at low temperatures. If we accept this picture, the broad component at the higher frequency side can also be considered to be due to the paramagnetic states with the opposite direction of electron spin relative to that for the lower frequency component. From the observed NMR line shift, the hyperfine constant is estimated as 104 times smaller than the vacuum hydrogen value. This result is similar to that of mSR studies on some semiconductors showing extremely small hyperfine constants for muonium (Mu) compared to the vacuum Mu value (103 to 104 times) due to formation of shallow donor levels [9]. In summary, the DQ NMR of 12N in a single crystal of rutile TiO2 was performed by means of the b-NMR technique and quite a small hyperfine field was observed at 18 K. This result suggests that the substitutional nitrogen atom for oxygen acts as a shallow acceptor in rutile TiO2. In order to clarify the electronic structure of the nitrogen acceptor center, further experimental studies such as more precise temperature dependence of the DQ spectrum are in progress.
Acknowledgments The present work was partly supported by the Murata Science Foundation and the grant-in-aid for Scientific Research Program of the Japan Society for the Promotion of Science. References [1] S. Sato, Chem. Phys. Lett. 123 (1986) 126. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [3] R.A. Parker, Phys. Rev. 124 (1961) 1719. [4] A. van Duijn-Arnold, J. Mol, R. Verberk, J. Schmidt, Phys. Rev. B 60 (1999) 15829. [5] T.N. Mamedov, V.N. Gorelkin, A.V. Stoikov, Phys. Part. Nucl. 33 (2002) 519. [6] M. Mihara, S. Kumashiro, H. Fujiwara, R. Matsumiya, K. Matsuta, Y. Nakashima, Y.N. Zheng, M. Ogura, T. Sumikama, T. Nagatomo, K. Minamisono, M. Fukuda, T. Izumikawa, T. Minamisono, Physica B 376–377 (2006) 955. [7] T. Minamisono, K. Sato, H. Akai, S. Takeda, Y. Maruyama, K. Matsuta, M. Fukuda, T. Miyake, A. Morishita, T. Izumikawa, Y. Nojiri, Z. Naturforsch 53a (1998) 293. [8] C. Brevard, P. Granger, in: Handbook of High Resolution Multinuclear NMR, Wiley/Interscience, New York, 1981, pp. 90–91. [9] S.F.J. Cox, J. Phys.: Condens. Matter 15 (2003) R1727.