- Email: [email protected]
Photoluminescence in nanocrystalline BaTiO3 and SrTiO3
m__
__ B
ELSEYIER
4 September
1995
PHYSICS
LETTERS
A
Physics Letters A 205 (1995) 72-76
Photoluminescence in nanocrystalline BaTiO, and SrTiO, Jinfang Meng a, Yabin Huang a, Weifeng Zhang a, Zuliang Du a, Ziqiang Zhu a, Guangtian Zou b a Physics Department, Henan Universi& Kaifeng 130023, China b State Key Lab for Superhard Materials, Jilin University, Changehun 130023, China Received 11 April 1995; revised manuscript received 30 June 1995; accepted for publication Communicated by J. F’louquet
4 July 1995
Abstract The luminescence spectra of BaTiO, and SrTiO, at small grain sizes and room temperature reveal an apparent single-emission band at 596 and 572 nm respectively. The visible emission band is found to increase in intensity and its peak position is shifted to larger energy as the grain size decreases. The properties of photoluminescence versus grain size are investigated for the first time. PACS: 61.64; 78.55; 78.60
1. Introduction There has long been an interest in the perovskitetype compounds with chemical formula ABO, because of their unusual magnetic, dielectric [l] and luminescence properties. In particular, when pure SrTiO,(ST) and BaTiO,(BT) samples are excited by radiation above their band gaps, deduced from the observed optical absorption edges to range from 3.0 to 3.2 eV for BaTiO, [2,3] and 3.4 eV for SrTiO, [2,4], a broad luminescence band appears in the visible region whose energy is lower than the corresponding gap energy. In order to explain the origin of the visible emission band, many experiments have been done, for example, radio luminescence [5], cathodoluminescence [6,7], X-ray induced visible luminescence [8], time-resolved spectroscopy [9], etc. However, in view of the many possibilities of the luminescence transition, the origin and nature of this visible emission have not been understood satisfactoElsevier Science B.V. SSDIO375-9601(95)00533-l
rily and many questions remain unanswered, e.g., how to explain the “quenching” of the visible emission above 80 K and how to clarify the origin of a large blue-shift with reducing temperature etc. In this paper we present measurements of the broad luminescence for BaTiO, and SrTiO, as a function of grain size, and investigate the dependence of their luminescence properties on grain size.
2. Experimental The BaTiO, and SrTiO, materials were separately prepared, essentially using a sol-gel process in which barium acetate, strontium acetate, titanium butoxide were used as the precursor materials. The process involved dissolving the metal-containing compounds in the solvent, hydrolyzing and polycondensing the resulting solution into a gel, and finally
J. Meng et al. /Physics
heat treating the gel to form the nanocrystalline BaTiO, and SrTiO, powder [lo]. The average grain size was determined from the full width at half maximum (FWHM) of the X-ray diffraction peak using Scherrer’s equation [ 111 D = kh/B
13
Letters A 205 (1995) 72-76
2.3OeV
I .X9&
18500
152.50
I .49eV
cos 8,
where D is the particle diameter, A the X-ray wavelength, B the FWHM of the diffraction peak, 13 the diffraction angle, and k is the Scherrer constant, of the order of unity for usual crystals. The luminescence signal was collected by a SPEX-1403 Raman spectrometer. The 514.5 nm line from an argon-ion laser was used as the laser excitation. All experiments were performed at room temperature.
3. Results and discussion In Figs. la and lb, there appears respectively a very broad and structureless band at 17465 cm- ’ (2.17 eV> for SrTiO, with a grain size of 242 nm and at 16770 cm-’ (2.08 eV) for BaTiO, with a grain size of 157 nm, at room temperature. For SrTiO,, as the grain size decreases to 26 nm, the visible emission band increases in intensity rapidly. In BaTiO,, a similar relation is also found. From Fig. 2, it is more interesting to find that the peak position of the visible emission band has a blue shift respectively to 18400 cm-’ (2.28 eV) for SrTiO, with a grain size of 26 nm and to 17700 cm-’ (2.19 eV) for BaTiO, with a grain size of 16 nm. In order to account for the origin of the visible emission band, we first consider the transition in isolated titanate groups. Many diluted titanate systems in perovskite and perovskite-related compounds have been investigated [12,13], e.g. BaZrO,-Ti (emission at 2.9 eV, excitation at 4.4 eV), CaZrO,-Ti (2.9 eV, 4.8 eV1, La,MgTiO, (2.6 eV, 4.2 eV) and Sr,SnO,-Ti with two-dimensional perovskite layers (2.8 eV, 4.8 eV), etc. Compared with our experimental results, it is found that although the Stokes shift of this emission of nanocrystalline SrTiO, or BaTiO,, i.e., the energy difference between the luminescence maxima and the reported absorption edges [7], is very similar to that of diluted titanate systems (l-2 eV>, the titanate group is not expected to emit at all under excitation at such a low energy (514.5 nm N
12000
WAVE NUMBER(cm-1) 2.3OeV
B
I
18500
I .49eV
I .89eV
1
15250
.
12000
WAVE NUMBER(cm-1) Fig. 1. (a) Visible emission spectra for SrTiO, (a) and BaTiO, with various gain sizes at room temperature.
(b)
J. Meng et al. /Physics Letters A 205 (1995) 72-76
74
2.4 eV> [12,14]. Also, it is considered that this visible emission transition may be attributed to a direct recombination of a conduction electron in the Ti-3d bands localized at the titanium site and a hole in the 0-2~ valence band. However, the excitation line used (514.5 nm) has an energy much lower than the band gap energy (3.0-3.2 eV for bulk BaTiO,, 3.4 eV for bulk SrTiO,) so that this direct recombination is improbable. We are also inclined to ascribe the emission to an electron-hole recombination of a localized exciton Ti3”-O- in a regular octahedron (TiO,). The large energy difference between the excitation (band to band) and the emission indicates a strong relaxation of the exciton, as has been found for other molecular groups such as niobate vanadate, and tungstate [15171. Our experimental results can be well explained by the model in which the electrons in the valence band of BaTiO, or SrTiO,, absorbing photons from the Ar laser, are excited to some localized sensitizing centers in the forbidden gap which may be some surface states. The electron polarons interact with holes, possibly trapped near crystal defects or impurities, and form an intermediate state: self-trapped excitons (ST%), either immediately or after being trapped for a certain time by impurities, as has been shown at low temperature [9] (see Fig. 3). It is well known that in nanocrystalline materials, the atoms on the crystalline surface are in the “naked state”, their coordination status is unsaturated and differs from that of the same kind of atom in the
0 00
f:;:;t, 0
20
,
,
40
60
GRAIN Fig. 2. Variation with grain size.
y
,o,
1
80 100 200 300
SIZE (nm)
of the peak position of the visible emission band
CONDUCTION
AB;Ni-=-
BAND
STT
Traps
0
1
_____---
Ex
4 -b ;5
1 ____________ VALENCE
BAND
Fig. 3. Schematic of the luminescence processes in SrTiO, or BaTiO,. (1) hole capture; (2) electron capture; (3) and (4) smallpolaron formation as well as retarded formation of a STE, (3) and (5) direct formation of a WE.
bulk phase, so that they can form “broken” or “dangling bonds”, the electrons in the bonds being in the unpaired state [18]. Therefore, in nanocrystalline BaTiO, or SrTiO, these “dangling bond atoms” can be considered as defects as described in PbZrO, [19]. According to the characteristics and investigations of the BaTiO, and SrTiO, structure, the face (001) including 02- and Ti4+, is very stable and is easily exposed on interfaces to favourable chemical reactions [20-221. On the other hand, it is also known that the d-orbital surface states formed by Ti3” can be stabilized in the forbidden gap at a certain concentration of oxygen vacancies [20]. Thus, these d-surface states can be assumed to be the highly localized sensitizing centers which trap electrons from the valence band. Obviously, the smaller the grain size of SrTiO, and BaTiO,, the greater the number of the d-surface states and thus of the localized sensitizing centers, so that the recombination via ST% is enhanced. Finally, the emission band increases in intensity with decreasing grain size. The emission caused by “dangling bond atoms” in nanocrystallites has been observed in other analogous experiments, e.g., the luminescence of nanometer sized amorphous silicon nitride solids, in which the appearance of six emission bands is closely related to the formation of energy levels of the defect states in the forbidden gap [23]. In addition, it should be noted that as the grain size decreased, these emission bands did not appear to shift in their peak positions, only to increase in intensity, which is different from our results.
J. Meng et al. /Physics Letters A 205 (1995) 72-76
The increase of the emission band intensity with decreasing grain size recalls the effect of reducing temperature on the emission intensity in bulk SrTiO, [8,24]. It is found that as the temperature is raised above 80 K, the visible emission intensity approaches zero, that is, the visible emission is “quenched”. This phenomenon has not been well explained. Sihvonen has attributed the steep decrease in the intensity (reported by him to occur at 80 K) to the cubic-to-tetragonal phase transition [25]. But this argument does not seem to be convincing for reasons given by Tom Feng [24]. As for nanocrystalline SrTiO,, the cubic to tetragonal phase transition induced by a critical grain size at room temperature has not been determined. However, BaTiO, with a grain size of 157 nm is still in the tetragonal phase because the phase transformation from the tetragonal to cubic structure is found to occur at the critical grain size of 120 nm [26]. At room temperature, this grain size induced phase transition has been also confirmed by our experiments [27]. Therefore, in BaTiO,, whether the visible emission at room temperature disappears or not seems to be unrelated to the cubic to tetragonal phase transition. It was considered that the self-trapping of excitons originated mostly from the interaction between the lattice and the electronic part of the exciton [9], especially the effect of lattice vibrations on the exciton movement obtained by perturbation theory, for which formation of STEs was found [28]. The STE formation was calculated and could be closely correlated with the electron-to-hole mass ratio ( p,/ph)_ It is therefore suggested that the fact that the visible emission is not quenched at room temperature might result from a decrease of electron mass with grain size. This needs to be further clarified. Finally, we discuss the blue shift of the visible emission with reducing grain size. In the case of nanometer grain size, some TiO, octahedra can be confined in a spherical crystallite. Because of interface stress of the spherical crystallite TiO, octahedra will have a large distortion, so that the dimension of the STEs is changed. Then the energy of the ground state of STEs shifts to an energy higher than that of the corresponding bulk materials. Obviously, the smaller the grain size, the larger the interface stress and thus the energy shift. In the case of nanocrystalline CdS, the luminescence spectra showed that as
75
the grain size decreased, the blue shift of the emission is due to the size quantization of the E, level of CdS [29,30]. Unfortunately, the explanation for nanocrystalline BaTiO, and SrTiO, cannot be based on size quantization, because the carrier masses are much higher than those in CdS. In summary, a visible emission band excited by the 514.5 nm line of an Ar laser whose energy is much lower than that of the band gap has been separately observed in nanocrystalline BaTiO, and SrTiO, at room temperature. Its nature and origin are attributed to the recombination of self-trapped excitons. Enhancement of the efficiency and the blue shift of the visible emission with decreasing grain size are closely correlated with interfaces of nanocrystallites, d-surface states in the forbidden gap and distortion of TiO, octahedra.
Acknowledgements This research was supported by the Natural Sciences Founds of the Henan Education Commission as well as the Science and Technology Commission.
References Ill J.B. Goodenough
and J.M. Longo, in: Landolt-Bomstein Tabellen, eds. K.H. Hellwege and A.M. Hellwege (Springer, Berlin, 1970). 121M. Cardona, Phys. Rev. A 140 (1965) 651. 131 C.N. Berhmd and H.J. Braun, Phys. Rev. 164 (1967) 790. [41 G.A. Barbosa, R.S. Katiyar and S.P.S. Porto, J. Opt. Sot. Am. 68 (1978) 610. 151 M. Aguilar, C. Godefroy and G. Godefroy, Solid State Commun. 30 (1979) 525. [61 H. Ihrig and M. Klerk, Appl. Phys. Lett. 35 (1979) 307. [71 H. Ihrig, J.H.T. Hengst and M. Klerk, Z. Phys. B 40 (1981) 301. WI M. Aguilar and F. Agullo-Lopez, J. Appl. Phys. 53 (1982) 9009. 191 R. Leonelli and J.L. Brebner, Solid State Commun. 54 (1985) 505. DOI Jinfang Meng, Guangtian Zou, Qilaing Cui, Xi Li and Muyu Zhao, J. Phys. 12 (1993) 1012 [in Chinese]. [ill K. Ishikawa, K. Yoshikawa and N. Ukada, Phys. Rev. B 37 (1988) 5852. WI A.J.H. Macke, I. Solid State Chem. 18 (19761 337. [131 A.J.H. Macke, Thesis, University of Utrecht (1976) [141 G. Blasse and A. Bril, J. Solid State Chem. 2 (1970) 291.
76
J. Meng et al. /Physics
[15] M.J. Treadaway and R.C. Powell, J. Chem. Phys. 61 (1974) 4003. [16] C. Hsu and R.C. Powell, J. Lumin. 10 (1975) 273. [17] R.C. Powell and E.E. Freed, J. Chem. Phys. 70 (1979) 4681. [18] X. Li, H. Zhang, S.J. Li, Y. Xin and M.Y. Zhan J. Alloys Compounds 190 (1993) 287. [19] J.F. Meng, G.T. Zou, Q.L. Cui, Z.Q. Zhu and Z.L. Du, Solid State Cornmutt. 91 (1994) 519. [20] T. Wolfram, F.J. Morin and R. Hurst, in: NBS special publication, No. 455. Electrocatalysis on non-metallic surfaces, ed. A.D. Franklin (U.S. Government Printing Office, Washington, DC, 1975) p. 21. [21] T. Wolfram, Appl. Phys. 8 (1975) 125.
Letters A 205 (1995) 72-76 [22] S. Ellialtioglu and T. Wolfram, Phys. Rev. B 18 (1978) 4509. [23] CM. MO, L. Zhang, C.Y. Xie and T. Wang, J. Appl. Phys. 73 (1993) 5185. [24] Tom Feng, Phys. Rev. B 25 (1981) 627. [25] Y.T. Sihvonen, J. Appl. Phys. 38 (1967) 4431. (261 K. Uchino, E. Sadanaga and T. Hirose, J. Am. Ceram. Sot., 72 (1989) 1.555. [27] J.F. Meng, G.T. Zou, J.P. Li, X.H. Wang and M.Y. Zhao, Chin. Phys. Lctt. 11 (1994) 345-348. [28] SW. Gu, J. Phys. 28 (1979) 751 [in Chinese]. [29] L.F. Mattheiss, Phys. Rev. B 6 (1972) 4718. [30] R. Rossetti, J.L. Ellison, J.M. Gibson and L.E. Brus, J. Chem. Phys. 80 (1984) 4464.
__ B
ELSEYIER
4 September
1995
PHYSICS
LETTERS
A
Physics Letters A 205 (1995) 72-76
Photoluminescence in nanocrystalline BaTiO, and SrTiO, Jinfang Meng a, Yabin Huang a, Weifeng Zhang a, Zuliang Du a, Ziqiang Zhu a, Guangtian Zou b a Physics Department, Henan Universi& Kaifeng 130023, China b State Key Lab for Superhard Materials, Jilin University, Changehun 130023, China Received 11 April 1995; revised manuscript received 30 June 1995; accepted for publication Communicated by J. F’louquet
4 July 1995
Abstract The luminescence spectra of BaTiO, and SrTiO, at small grain sizes and room temperature reveal an apparent single-emission band at 596 and 572 nm respectively. The visible emission band is found to increase in intensity and its peak position is shifted to larger energy as the grain size decreases. The properties of photoluminescence versus grain size are investigated for the first time. PACS: 61.64; 78.55; 78.60
1. Introduction There has long been an interest in the perovskitetype compounds with chemical formula ABO, because of their unusual magnetic, dielectric [l] and luminescence properties. In particular, when pure SrTiO,(ST) and BaTiO,(BT) samples are excited by radiation above their band gaps, deduced from the observed optical absorption edges to range from 3.0 to 3.2 eV for BaTiO, [2,3] and 3.4 eV for SrTiO, [2,4], a broad luminescence band appears in the visible region whose energy is lower than the corresponding gap energy. In order to explain the origin of the visible emission band, many experiments have been done, for example, radio luminescence [5], cathodoluminescence [6,7], X-ray induced visible luminescence [8], time-resolved spectroscopy [9], etc. However, in view of the many possibilities of the luminescence transition, the origin and nature of this visible emission have not been understood satisfactoElsevier Science B.V. SSDIO375-9601(95)00533-l
rily and many questions remain unanswered, e.g., how to explain the “quenching” of the visible emission above 80 K and how to clarify the origin of a large blue-shift with reducing temperature etc. In this paper we present measurements of the broad luminescence for BaTiO, and SrTiO, as a function of grain size, and investigate the dependence of their luminescence properties on grain size.
2. Experimental The BaTiO, and SrTiO, materials were separately prepared, essentially using a sol-gel process in which barium acetate, strontium acetate, titanium butoxide were used as the precursor materials. The process involved dissolving the metal-containing compounds in the solvent, hydrolyzing and polycondensing the resulting solution into a gel, and finally
J. Meng et al. /Physics
heat treating the gel to form the nanocrystalline BaTiO, and SrTiO, powder [lo]. The average grain size was determined from the full width at half maximum (FWHM) of the X-ray diffraction peak using Scherrer’s equation [ 111 D = kh/B
13
Letters A 205 (1995) 72-76
2.3OeV
I .X9&
18500
152.50
I .49eV
cos 8,
where D is the particle diameter, A the X-ray wavelength, B the FWHM of the diffraction peak, 13 the diffraction angle, and k is the Scherrer constant, of the order of unity for usual crystals. The luminescence signal was collected by a SPEX-1403 Raman spectrometer. The 514.5 nm line from an argon-ion laser was used as the laser excitation. All experiments were performed at room temperature.
3. Results and discussion In Figs. la and lb, there appears respectively a very broad and structureless band at 17465 cm- ’ (2.17 eV> for SrTiO, with a grain size of 242 nm and at 16770 cm-’ (2.08 eV) for BaTiO, with a grain size of 157 nm, at room temperature. For SrTiO,, as the grain size decreases to 26 nm, the visible emission band increases in intensity rapidly. In BaTiO,, a similar relation is also found. From Fig. 2, it is more interesting to find that the peak position of the visible emission band has a blue shift respectively to 18400 cm-’ (2.28 eV) for SrTiO, with a grain size of 26 nm and to 17700 cm-’ (2.19 eV) for BaTiO, with a grain size of 16 nm. In order to account for the origin of the visible emission band, we first consider the transition in isolated titanate groups. Many diluted titanate systems in perovskite and perovskite-related compounds have been investigated [12,13], e.g. BaZrO,-Ti (emission at 2.9 eV, excitation at 4.4 eV), CaZrO,-Ti (2.9 eV, 4.8 eV1, La,MgTiO, (2.6 eV, 4.2 eV) and Sr,SnO,-Ti with two-dimensional perovskite layers (2.8 eV, 4.8 eV), etc. Compared with our experimental results, it is found that although the Stokes shift of this emission of nanocrystalline SrTiO, or BaTiO,, i.e., the energy difference between the luminescence maxima and the reported absorption edges [7], is very similar to that of diluted titanate systems (l-2 eV>, the titanate group is not expected to emit at all under excitation at such a low energy (514.5 nm N
12000
WAVE NUMBER(cm-1) 2.3OeV
B
I
18500
I .49eV
I .89eV
1
15250
.
12000
WAVE NUMBER(cm-1) Fig. 1. (a) Visible emission spectra for SrTiO, (a) and BaTiO, with various gain sizes at room temperature.
(b)
J. Meng et al. /Physics Letters A 205 (1995) 72-76
74
2.4 eV> [12,14]. Also, it is considered that this visible emission transition may be attributed to a direct recombination of a conduction electron in the Ti-3d bands localized at the titanium site and a hole in the 0-2~ valence band. However, the excitation line used (514.5 nm) has an energy much lower than the band gap energy (3.0-3.2 eV for bulk BaTiO,, 3.4 eV for bulk SrTiO,) so that this direct recombination is improbable. We are also inclined to ascribe the emission to an electron-hole recombination of a localized exciton Ti3”-O- in a regular octahedron (TiO,). The large energy difference between the excitation (band to band) and the emission indicates a strong relaxation of the exciton, as has been found for other molecular groups such as niobate vanadate, and tungstate [15171. Our experimental results can be well explained by the model in which the electrons in the valence band of BaTiO, or SrTiO,, absorbing photons from the Ar laser, are excited to some localized sensitizing centers in the forbidden gap which may be some surface states. The electron polarons interact with holes, possibly trapped near crystal defects or impurities, and form an intermediate state: self-trapped excitons (ST%), either immediately or after being trapped for a certain time by impurities, as has been shown at low temperature [9] (see Fig. 3). It is well known that in nanocrystalline materials, the atoms on the crystalline surface are in the “naked state”, their coordination status is unsaturated and differs from that of the same kind of atom in the
0 00
f:;:;t, 0
20
,
,
40
60
GRAIN Fig. 2. Variation with grain size.
y
,o,
1
80 100 200 300
SIZE (nm)
of the peak position of the visible emission band
CONDUCTION
AB;Ni-=-
BAND
STT
Traps
0
1
_____---
Ex
4 -b ;5
1 ____________ VALENCE
BAND
Fig. 3. Schematic of the luminescence processes in SrTiO, or BaTiO,. (1) hole capture; (2) electron capture; (3) and (4) smallpolaron formation as well as retarded formation of a STE, (3) and (5) direct formation of a WE.
bulk phase, so that they can form “broken” or “dangling bonds”, the electrons in the bonds being in the unpaired state [18]. Therefore, in nanocrystalline BaTiO, or SrTiO, these “dangling bond atoms” can be considered as defects as described in PbZrO, [19]. According to the characteristics and investigations of the BaTiO, and SrTiO, structure, the face (001) including 02- and Ti4+, is very stable and is easily exposed on interfaces to favourable chemical reactions [20-221. On the other hand, it is also known that the d-orbital surface states formed by Ti3” can be stabilized in the forbidden gap at a certain concentration of oxygen vacancies [20]. Thus, these d-surface states can be assumed to be the highly localized sensitizing centers which trap electrons from the valence band. Obviously, the smaller the grain size of SrTiO, and BaTiO,, the greater the number of the d-surface states and thus of the localized sensitizing centers, so that the recombination via ST% is enhanced. Finally, the emission band increases in intensity with decreasing grain size. The emission caused by “dangling bond atoms” in nanocrystallites has been observed in other analogous experiments, e.g., the luminescence of nanometer sized amorphous silicon nitride solids, in which the appearance of six emission bands is closely related to the formation of energy levels of the defect states in the forbidden gap [23]. In addition, it should be noted that as the grain size decreased, these emission bands did not appear to shift in their peak positions, only to increase in intensity, which is different from our results.
J. Meng et al. /Physics Letters A 205 (1995) 72-76
The increase of the emission band intensity with decreasing grain size recalls the effect of reducing temperature on the emission intensity in bulk SrTiO, [8,24]. It is found that as the temperature is raised above 80 K, the visible emission intensity approaches zero, that is, the visible emission is “quenched”. This phenomenon has not been well explained. Sihvonen has attributed the steep decrease in the intensity (reported by him to occur at 80 K) to the cubic-to-tetragonal phase transition [25]. But this argument does not seem to be convincing for reasons given by Tom Feng [24]. As for nanocrystalline SrTiO,, the cubic to tetragonal phase transition induced by a critical grain size at room temperature has not been determined. However, BaTiO, with a grain size of 157 nm is still in the tetragonal phase because the phase transformation from the tetragonal to cubic structure is found to occur at the critical grain size of 120 nm [26]. At room temperature, this grain size induced phase transition has been also confirmed by our experiments [27]. Therefore, in BaTiO,, whether the visible emission at room temperature disappears or not seems to be unrelated to the cubic to tetragonal phase transition. It was considered that the self-trapping of excitons originated mostly from the interaction between the lattice and the electronic part of the exciton [9], especially the effect of lattice vibrations on the exciton movement obtained by perturbation theory, for which formation of STEs was found [28]. The STE formation was calculated and could be closely correlated with the electron-to-hole mass ratio ( p,/ph)_ It is therefore suggested that the fact that the visible emission is not quenched at room temperature might result from a decrease of electron mass with grain size. This needs to be further clarified. Finally, we discuss the blue shift of the visible emission with reducing grain size. In the case of nanometer grain size, some TiO, octahedra can be confined in a spherical crystallite. Because of interface stress of the spherical crystallite TiO, octahedra will have a large distortion, so that the dimension of the STEs is changed. Then the energy of the ground state of STEs shifts to an energy higher than that of the corresponding bulk materials. Obviously, the smaller the grain size, the larger the interface stress and thus the energy shift. In the case of nanocrystalline CdS, the luminescence spectra showed that as
75
the grain size decreased, the blue shift of the emission is due to the size quantization of the E, level of CdS [29,30]. Unfortunately, the explanation for nanocrystalline BaTiO, and SrTiO, cannot be based on size quantization, because the carrier masses are much higher than those in CdS. In summary, a visible emission band excited by the 514.5 nm line of an Ar laser whose energy is much lower than that of the band gap has been separately observed in nanocrystalline BaTiO, and SrTiO, at room temperature. Its nature and origin are attributed to the recombination of self-trapped excitons. Enhancement of the efficiency and the blue shift of the visible emission with decreasing grain size are closely correlated with interfaces of nanocrystallites, d-surface states in the forbidden gap and distortion of TiO, octahedra.
Acknowledgements This research was supported by the Natural Sciences Founds of the Henan Education Commission as well as the Science and Technology Commission.
References Ill J.B. Goodenough
and J.M. Longo, in: Landolt-Bomstein Tabellen, eds. K.H. Hellwege and A.M. Hellwege (Springer, Berlin, 1970). 121M. Cardona, Phys. Rev. A 140 (1965) 651. 131 C.N. Berhmd and H.J. Braun, Phys. Rev. 164 (1967) 790. [41 G.A. Barbosa, R.S. Katiyar and S.P.S. Porto, J. Opt. Sot. Am. 68 (1978) 610. 151 M. Aguilar, C. Godefroy and G. Godefroy, Solid State Commun. 30 (1979) 525. [61 H. Ihrig and M. Klerk, Appl. Phys. Lett. 35 (1979) 307. [71 H. Ihrig, J.H.T. Hengst and M. Klerk, Z. Phys. B 40 (1981) 301. WI M. Aguilar and F. Agullo-Lopez, J. Appl. Phys. 53 (1982) 9009. 191 R. Leonelli and J.L. Brebner, Solid State Commun. 54 (1985) 505. DOI Jinfang Meng, Guangtian Zou, Qilaing Cui, Xi Li and Muyu Zhao, J. Phys. 12 (1993) 1012 [in Chinese]. [ill K. Ishikawa, K. Yoshikawa and N. Ukada, Phys. Rev. B 37 (1988) 5852. WI A.J.H. Macke, I. Solid State Chem. 18 (19761 337. [131 A.J.H. Macke, Thesis, University of Utrecht (1976) [141 G. Blasse and A. Bril, J. Solid State Chem. 2 (1970) 291.
76
J. Meng et al. /Physics
[15] M.J. Treadaway and R.C. Powell, J. Chem. Phys. 61 (1974) 4003. [16] C. Hsu and R.C. Powell, J. Lumin. 10 (1975) 273. [17] R.C. Powell and E.E. Freed, J. Chem. Phys. 70 (1979) 4681. [18] X. Li, H. Zhang, S.J. Li, Y. Xin and M.Y. Zhan J. Alloys Compounds 190 (1993) 287. [19] J.F. Meng, G.T. Zou, Q.L. Cui, Z.Q. Zhu and Z.L. Du, Solid State Cornmutt. 91 (1994) 519. [20] T. Wolfram, F.J. Morin and R. Hurst, in: NBS special publication, No. 455. Electrocatalysis on non-metallic surfaces, ed. A.D. Franklin (U.S. Government Printing Office, Washington, DC, 1975) p. 21. [21] T. Wolfram, Appl. Phys. 8 (1975) 125.
Letters A 205 (1995) 72-76 [22] S. Ellialtioglu and T. Wolfram, Phys. Rev. B 18 (1978) 4509. [23] CM. MO, L. Zhang, C.Y. Xie and T. Wang, J. Appl. Phys. 73 (1993) 5185. [24] Tom Feng, Phys. Rev. B 25 (1981) 627. [25] Y.T. Sihvonen, J. Appl. Phys. 38 (1967) 4431. (261 K. Uchino, E. Sadanaga and T. Hirose, J. Am. Ceram. Sot., 72 (1989) 1.555. [27] J.F. Meng, G.T. Zou, J.P. Li, X.H. Wang and M.Y. Zhao, Chin. Phys. Lctt. 11 (1994) 345-348. [28] SW. Gu, J. Phys. 28 (1979) 751 [in Chinese]. [29] L.F. Mattheiss, Phys. Rev. B 6 (1972) 4718. [30] R. Rossetti, J.L. Ellison, J.M. Gibson and L.E. Brus, J. Chem. Phys. 80 (1984) 4464.