- Email: [email protected]
Radioluminescent mechanism of Li2B4O7:Cu crystal
Journal of Luminescence 130 (2010) 2142–2145
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Radioluminescent mechanism of Li2B4O7:Cu crystal B.T. Huy a,n, V.X. Quang a, M. Ishii b a b
Nhatrang Institute of Technology Research and Application, 2 Hung Vuong, Nhatrang, Vietnam Shonan Institute of Technology, 3-27-7 Nangai, Higashiyamato, Tokyo, Japan
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 September 2009 Received in revised form 4 April 2010 Accepted 10 June 2010 Available online 16 June 2010
The structure of the radioluminescent (RL) spectrum of Li2B4O7:Cu crystal is complex, having no normal Gauss peaks. The RL spectrum recorded showed one sharp and strong peak near 365 nm. For the purpose of explaining this phenomenon, we used the self-trapped exciton (STE) model to analyze experimental data of absorption spectrum, emission spectrum, radioluminescence spectrum and decaytime curves. We found a short lifetime component (of the order of ns) from phosphorous signals of Li2B4O7:Cu. We realized that the short lifetime component related to inherent signal of host lattice and the d–p transitions of Cu + . This result is applicable in the detector system with high time resolution. & 2010 Elsevier B.V. All rights reserved.
Keywords: Absorption Polaron STE Smakula Mechanism Crystal
1. Introduction Lithium tetraborate Li2B4O7 (LBO) has been used for the dosimetry. With the effective number of atoms almost equivalent to that of the human tissue, LBO is one of the preferred dosimetric materials in radiotherapy [1,2]. On the other side, LBO consists of boron and lithium so it is also a candidate for the neutron detectors [3,4]. There have been numerous reports on the effect of doping on its luminescent properties [1–6]. However, though the mechanism of the thermoluminescence of LBO is known, that of the radioluminescence still remains unclear, especially in the single crystal [7,8]. The decay time of the radioluminescence in LBO:Cu single crystals is in the range from 1 to 20 ns. Explanations of this fact are rare and yet unsatisfactory. In this work, based on spectral studies of the absorption, emission and decay-time curves we propose some phosphorous mechanisms, which might be responsible for the short lifetime luminescent component of LBO:Cu single crystals.
2. Experimental Li2B4O7:Cu single crystals were grown from the melt by Bridgman technique in graphite crucibles in nitrogen atmosphere. Polycrystalline, 99.99% purity, Li2B4O7 in the disc form is obtained n
Corresponding author. E-mail addresses: [email protected], [email protected] (B.T. Huy).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.06.008
from Tomiyama chemicals was used as a starting material. It is mixed with 0.03% of Cu2O before melting for homogenization. The grown crystal was about 40 mm in length and 20 mm in diameter. Samples of F ¼20 1 mm3 were cut out of this crystal and their surfaces were polished for evaluation of the optical transmittance and luminescence. All measurements were performed at room temperature. The optical absorption spectra were measured using UV–vis–NIR Carry 5000 spectrophotometer with two beams. One LBO:Cu sample is served as a studied sample. One LBO pure sample is served as a reference one. The emission spectrum was measured by Hitachi F-4500 fluorescence spectrophotometer. The luminescence decay-time curve was measured by the light-excited X-ray tube N5084 with Cu target (two spectral lines: Kb 0.139 nm and Ka 0.154 nm), R4998 PMT Hamamatsu.
3. Results and discussion The optical absorption spectrum of Cu in the LBO:Cu crystal is shown in Fig. 1. As can be seen from the figure, the absorption is strong within the range from 220 to 280 nm. It is known that the 3d10-3d94 s energy band around 40,000 cm 1 is almost independent of the host crystal and the 3d10-3d94p energy band is around 51,000 cm 1. Under the effect of the lattice host, the 4s state is split into three separate substates 1T2g, 1Eg and 3Eg, while the 4p state is split into 2 closely located substates 1T1u. The energy level diagram for Cu + is shown in Fig. 2 [9].
B.T. Huy et al. / Journal of Luminescence 130 (2010) 2142–2145
2143
nm and consists of eight Li2O/2B2O3 per unit cell. It consists of two boron atoms in triangular (W) oxygen environment and two boron atoms in tetrahedral (&) oxygen environment. Therefore, the tetraborate complex can be represented as (2W +2&). In the growing process of crystal, the (2W + 2&) complex can be deformed by the temperature so the octahedron can be formed from tetrahedron. Based on the energy level diagram and last reports [8,9,19], the optical absorption spectrum can be fitted to 4 peaks (3 due to the d–s transition and 1 due to the d–p transition). The fitting curves are shown in Fig. 3. The experimental curve completely fit in the ‘‘sum’’ simulation curves. From the results of fitting procedure, oscillator strength and lifetime were determined by Smakula’s formula for electric dipole absorption process [11]. The details are collected in Table 1. Smakula’s formula :
Fig. 1. The optical absorption spectrum of Cu + .
Lifetime tij :
1
tij
¼
fij ¼ 54:1
n ðn2 þ 2Þ2
amax Do
1 N
ð1Þ
e2 fij nðn2 þ2Þ2 2 9 2pe0 mcl
ð2Þ
amax is the absorption coefficient at the peak, N the density of absorbing center [20], n the refractive index and e0 the permittivity of vacuum. Table 1 indicates that the lifetime decreases with the increasing energy. This result expresses the influence of d9p on d9s because of the mixing between p and s states. In accordance with the Laporte selection rule, the d–p transitions are allowed (short lifetime) and the d–s transitions are forbidden (longer
Fig. 3. The peaks were simulated from absorption spectra of Cu + in LBO.
Fig. 2. The energy level diagram for Cu + .
Table 1 Oscillator strength fif, lifetime of d–s transitions of Cu + . Peak
Note that the diagram in Fig. 2 corresponds to Cu + in the octahedron LiCl, but it is a fairly suitable to the tetrahedron Li2B4O7. Lithium tetraborate belongs to the tetragonal crystal system with the lattice parameters a ¼b¼0.9477 nm, c¼1.0286
d–s3 d–s2 d–s1
Wavelength (nm)
Transition
f
t (ms)
265 255 237
1
1.56E 04 1.77E 03 1.92E 03
69 5.6 4.3
3
A1g– Eg A1g–1Eg A1g–1T2g
1 1
2144
B.T. Huy et al. / Journal of Luminescence 130 (2010) 2142–2145
and L (or M) shell is the second (or third) shell. The cross section of the photoelectric effect increases with the atomic number. Therefore, O or Cu may give important contribution. The binding energy (BE) of K and L shell electrons are BE¼543.1 eV (K-shell), 41.6 eV (L-shell) for O and BE¼8979 eV (K-shell), 1097 eV (L-shell) for Cu [14]. Consequently, the characteristic X-ray energy for the transition L-K shell is the difference of the two energies, i.e. 502 eV in O and 7882 eV in Cu. This energy is far larger than the visible/UV luminescence energy. K-electron has a kinetic energy of ‘‘X-ray injected energy–BE (K-shell)’’. This energy will be absorbed in scintillated atoms including Cu + ions, and excites the outermost (weakly bound) electrons (3d10 in the case of Cu + ) to excited levels (3d94s1). When the excited electron drops to the ground state, a photon can be emitted. The RL spectrum of Li2B4O7:Cu crystal is shown in Fig. 6. The structure of the RL spectrum is complex, having no normal Gauss peaks. With the S¼10, this value implies a strong electronphonon interaction, which is a prerequisite condition for the appearance of the polaron and STE in the materials. So RL spectrum can be related to contribution of STE or core–valence transition. According to other report [15,16], the sharp of this spectrum can be related to STE. As mentioned above, in the structure of Li2B4O7, tetrahedron BO4 (&) plays the hole-trap role because it has a linking excess with oxygen so it easily captures a hole. Other sides, the structure of BO3 are not more stable than BO4 so the oxygen of Li2O will easily link with BO3 to form to BO4. The redundant Li + is located in the lattice also in the form of Cu + . When the radiation beam hits to Li2B4O7:Cu, the electron will be captured by X + (with X¼Li, Cu) to form X0 and the hole will be captured by BO4. The holes of the complex BO4 migrate in the lattice with hopping mechanism and move toward X0. The captured electron of X0 will be released with the tunnel mechanism and combine with the polaron to form STE. In such a way, it is possible to explain why the RL spectrum of Li2B4O7:Cu has a special structure and the decay-time curve has a fast component. The STE mechanism has been explained the appearance of sharp and strong peaks of the RL spectrum of the other materials [16]. The decay-time curve of LBO:Cu crystal is shown in Fig.7. The fast component of the decay-time curve was seen to fall within the range from 20 to 25 ns. Making use of the
Fig. 4. The emission spectrum with 260 nm excitation.
Fig. 5. The transitions in the ion Cu + .
lifetime). The peak at 237 nm corresponds to the 1A1g–1T2g transition, in agreement with our emission spectrum in Fig. 4. In the different hosts, the emission of ion Cu + is almost in the range of 365 nm since it lies in the intersite and thus is much less affected than other locations in the crystal lattice [9,10]. Fig. 5 [10] shows the various transitions in the ion Cu + : the 1T2g-1Eg and 1Eg-3Eg transitions are nonradioactive due to the narrow energy gap. From the absorption peak, emission peak and Raman spectrum, the electron–phonon interaction can be calculated by Huyng–Rhys parameter [11]:
DEs ¼ ð2S1Þho
ð3Þ
where DEs is the shift Stockes, S the Huang–Rhys parameter and ho the energy of phonon. With the data of absorption peak at 37,000 cm 1 in Table 1, emission peak at 28,000 cm 1 from the Fig. 4 and ho ¼510 cm 1 [12], one can obtain the Huyng–Rhys parameter S¼10. This result shows that the strength of the electron–phonon coupling is in the strong coupling regime [13]. When X-rays hit to Li2B4O7:Cu, its energy will be absorbed mostly by the K-shell electron (the innermost electrons) in Li, B, O and Cu atoms. The K-electron will be liberated and go out. The vacancy will be refilled typically by an electron in the L (or M) shell. Here K-shell is the first shell, which is closest to the nucleus,
Fig. 6. The radioluminescence of Li2B4O7:Cu crystal.
B.T. Huy et al. / Journal of Luminescence 130 (2010) 2142–2145
2145
4. Conclusion These radioluminescent mechanisms signalling of Li2B4O7:Cu may comprise by:
The inherent signal includes electron–hole recombination,
bound exciton and core–valence luminescence of LBO lattice host. Most important is STE: it is a luminescence mechanism for lifetime as short as of ns that is expected for the materials with strong electron–phonon interaction. The extrinsic signal is almost phosphor of Cu + with long lifetime (in the order of ms) for d–s transitions. The short lifetime components (in the order of ns) are attributed to d–p transitions.
Acknowledgment
Fig. 7. The decay time curve of Li2B4O7:Cu crystal.
above results, we now propose some possible luminescent mechanisms of Li2B4O7:Cu crystal. The fast component of the decay-time curve can be explained with some hypotheses such as below:
Self-trapped excitation mechanism. The value of S ¼10 shows
that the electron–phonon interaction is strong. This favors formation of the polaron and STE after the crystal was bombarded by the ion beams. Core–valence transition mechanism. The model of core–valence can be found in Refs. [17,18]. The core–valence transition is produced by the recombination of an electron in the valence band with a hole created in the outermost core band of an ionic crystal. If the energy difference Evc between the core band and the valence band is less than the band gap Eg, the process involves the emission of a photon, usually in the UV region. A characteristic of this transition is that the decay-time is very short which is of the order of a nanosecond. Unfortunately, however, this hypothesis is unconfirmed because we still do not have good enough results. Electron–hole pair transition. After the material was irradiated, the electron–hole pairs were created. The electron from the valence band moves to the conduction band and immediately immigrates to defect centers under the bottom of the conduction band. The electron–hole recombination occurs at once. Therefore, the decay process is very fast. The luminescence of Cu + . The 3d94p1-3d10 transition is allowed, so they take part in ‘‘fast component’’ of the signal.
We gratefully acknowledge Vietnam Academy of Science and Technology for financial support. We thank Prof. M. Kobayashi (KEK-Japan) and N.B. An (IOP-Vietnam) for the interesting discussions. References [1] M. Prokic, Radiation Protection Dosimetry 100 (2002) 265. [2] B.T. Huy, V.X. Quang, H.T.B. Chau, Journal of Luminescence 128 (2008) 1601. [3] N. Senguttuvan, M. Ishii, M. Shimoyama, M. Kobayashi, N. Tsutsui, M. Nikl, M. Dusek, H.M. Shimizu, T. Oku, T. Adachi, K. Sakai, J. Suzuki, Nuclear Instruments and Methods in Physics Research A 486 (2002) 264. [4] M. Ishii, Y. Kuwano, T. Asai, M. Kawamura, N. Senguttuvan, T. Hayashi, M. Kobayashi, M. Nikl, S. Hosoya, K. Sakai, T. Adachi, T. Oku, H.M. Shimizu, Nuclear Instruments and Methods in Physics Research A 537 (2005) 282. [5] M. Ignatovych, V. Holovey, T. Vidoczy, P. Baranyai, A. Kelemen, V. Latuga, O. Chuiko, Function Materials 12 (2005) 313. [6] M. Ignatovych, V. Holovey, A. Watterich, T. Vidoczy, P. Baranyai, A. Kelemen, O. Chuiko, Radiation Measurements 38 (2004) 567. [7] S. Watanabe, E.F. Chinaglia, M.L.F. Nascimento, M. Matsuoka, Radiation Protection Dosimetry 65 (1–4) (1996) 79. [8] V.T. Adamiv, O.T. Antonyak, Ya.V. Burak, M.S. Pidzyrailo, I.M. Teslyuk, Function Materials 12 (2) (2005) 278. [9] John Simometti, S.Mc. Donal, Physical Review B 16 (9) (1977) 3887. [10] B. Moine, C. Pedrini, Physical Review B 30 (2) (1984) 2301. [11] J. Garcia Sole, L.E. Bausa, D. Jaque, An introduction to the optical spectroscopy of Inorganic solids, John Wiley and Sons Ltd., West Sussex, England, 2005. [12] M.P. Dergachev, V.N. Moiseenko, Y.V. Burak, Optic and Spectroscopy 90 (4) (2001) 534. [13] G. Blasse, B.C. Grabmaier, in: Luminescent materials, Springer Verlag, 1994. [14] R.L. David, in: Handbook of Chemistry and Physics, 88th Ed., CRC Press, 2007. [15] I.N. Ogorodnikov, V.A. Pustovarov, A.V. Kruzhalov, L.I. Isaenko, M. Kirm, G. Zimmerer, Physics of the Solid State 42 (3) (2000) 464. [16] V. Kiisk, I. Sildos, A. Suisalu, J. Aarik, Thin Solid Films 400 (2001) 130. [17] Carel W.E. van Eijk, Nuclear Instruments and Methods in Physics Research A (1977) 285. [18] J. Wilkinson, K.B. Ucer, R.T. Williams, Radiation Measurement 38 (2004) 501. [19] J.C. Zhang, B. Moine, C. Perdin, G. Flem, Journal of Physics and Chemistry of Solids 50 (8) (1990) 933. [20] Emmanuel Desurvire, in: Erbium-doped fiber amplifiers- Principles and Application, John Wiley & Sons, Inc, 1994.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Radioluminescent mechanism of Li2B4O7:Cu crystal B.T. Huy a,n, V.X. Quang a, M. Ishii b a b
Nhatrang Institute of Technology Research and Application, 2 Hung Vuong, Nhatrang, Vietnam Shonan Institute of Technology, 3-27-7 Nangai, Higashiyamato, Tokyo, Japan
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 September 2009 Received in revised form 4 April 2010 Accepted 10 June 2010 Available online 16 June 2010
The structure of the radioluminescent (RL) spectrum of Li2B4O7:Cu crystal is complex, having no normal Gauss peaks. The RL spectrum recorded showed one sharp and strong peak near 365 nm. For the purpose of explaining this phenomenon, we used the self-trapped exciton (STE) model to analyze experimental data of absorption spectrum, emission spectrum, radioluminescence spectrum and decaytime curves. We found a short lifetime component (of the order of ns) from phosphorous signals of Li2B4O7:Cu. We realized that the short lifetime component related to inherent signal of host lattice and the d–p transitions of Cu + . This result is applicable in the detector system with high time resolution. & 2010 Elsevier B.V. All rights reserved.
Keywords: Absorption Polaron STE Smakula Mechanism Crystal
1. Introduction Lithium tetraborate Li2B4O7 (LBO) has been used for the dosimetry. With the effective number of atoms almost equivalent to that of the human tissue, LBO is one of the preferred dosimetric materials in radiotherapy [1,2]. On the other side, LBO consists of boron and lithium so it is also a candidate for the neutron detectors [3,4]. There have been numerous reports on the effect of doping on its luminescent properties [1–6]. However, though the mechanism of the thermoluminescence of LBO is known, that of the radioluminescence still remains unclear, especially in the single crystal [7,8]. The decay time of the radioluminescence in LBO:Cu single crystals is in the range from 1 to 20 ns. Explanations of this fact are rare and yet unsatisfactory. In this work, based on spectral studies of the absorption, emission and decay-time curves we propose some phosphorous mechanisms, which might be responsible for the short lifetime luminescent component of LBO:Cu single crystals.
2. Experimental Li2B4O7:Cu single crystals were grown from the melt by Bridgman technique in graphite crucibles in nitrogen atmosphere. Polycrystalline, 99.99% purity, Li2B4O7 in the disc form is obtained n
Corresponding author. E-mail addresses: [email protected], [email protected] (B.T. Huy).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.06.008
from Tomiyama chemicals was used as a starting material. It is mixed with 0.03% of Cu2O before melting for homogenization. The grown crystal was about 40 mm in length and 20 mm in diameter. Samples of F ¼20 1 mm3 were cut out of this crystal and their surfaces were polished for evaluation of the optical transmittance and luminescence. All measurements were performed at room temperature. The optical absorption spectra were measured using UV–vis–NIR Carry 5000 spectrophotometer with two beams. One LBO:Cu sample is served as a studied sample. One LBO pure sample is served as a reference one. The emission spectrum was measured by Hitachi F-4500 fluorescence spectrophotometer. The luminescence decay-time curve was measured by the light-excited X-ray tube N5084 with Cu target (two spectral lines: Kb 0.139 nm and Ka 0.154 nm), R4998 PMT Hamamatsu.
3. Results and discussion The optical absorption spectrum of Cu in the LBO:Cu crystal is shown in Fig. 1. As can be seen from the figure, the absorption is strong within the range from 220 to 280 nm. It is known that the 3d10-3d94 s energy band around 40,000 cm 1 is almost independent of the host crystal and the 3d10-3d94p energy band is around 51,000 cm 1. Under the effect of the lattice host, the 4s state is split into three separate substates 1T2g, 1Eg and 3Eg, while the 4p state is split into 2 closely located substates 1T1u. The energy level diagram for Cu + is shown in Fig. 2 [9].
B.T. Huy et al. / Journal of Luminescence 130 (2010) 2142–2145
2143
nm and consists of eight Li2O/2B2O3 per unit cell. It consists of two boron atoms in triangular (W) oxygen environment and two boron atoms in tetrahedral (&) oxygen environment. Therefore, the tetraborate complex can be represented as (2W +2&). In the growing process of crystal, the (2W + 2&) complex can be deformed by the temperature so the octahedron can be formed from tetrahedron. Based on the energy level diagram and last reports [8,9,19], the optical absorption spectrum can be fitted to 4 peaks (3 due to the d–s transition and 1 due to the d–p transition). The fitting curves are shown in Fig. 3. The experimental curve completely fit in the ‘‘sum’’ simulation curves. From the results of fitting procedure, oscillator strength and lifetime were determined by Smakula’s formula for electric dipole absorption process [11]. The details are collected in Table 1. Smakula’s formula :
Fig. 1. The optical absorption spectrum of Cu + .
Lifetime tij :
1
tij
¼
fij ¼ 54:1
n ðn2 þ 2Þ2
amax Do
1 N
ð1Þ
e2 fij nðn2 þ2Þ2 2 9 2pe0 mcl
ð2Þ
amax is the absorption coefficient at the peak, N the density of absorbing center [20], n the refractive index and e0 the permittivity of vacuum. Table 1 indicates that the lifetime decreases with the increasing energy. This result expresses the influence of d9p on d9s because of the mixing between p and s states. In accordance with the Laporte selection rule, the d–p transitions are allowed (short lifetime) and the d–s transitions are forbidden (longer
Fig. 3. The peaks were simulated from absorption spectra of Cu + in LBO.
Fig. 2. The energy level diagram for Cu + .
Table 1 Oscillator strength fif, lifetime of d–s transitions of Cu + . Peak
Note that the diagram in Fig. 2 corresponds to Cu + in the octahedron LiCl, but it is a fairly suitable to the tetrahedron Li2B4O7. Lithium tetraborate belongs to the tetragonal crystal system with the lattice parameters a ¼b¼0.9477 nm, c¼1.0286
d–s3 d–s2 d–s1
Wavelength (nm)
Transition
f
t (ms)
265 255 237
1
1.56E 04 1.77E 03 1.92E 03
69 5.6 4.3
3
A1g– Eg A1g–1Eg A1g–1T2g
1 1
2144
B.T. Huy et al. / Journal of Luminescence 130 (2010) 2142–2145
and L (or M) shell is the second (or third) shell. The cross section of the photoelectric effect increases with the atomic number. Therefore, O or Cu may give important contribution. The binding energy (BE) of K and L shell electrons are BE¼543.1 eV (K-shell), 41.6 eV (L-shell) for O and BE¼8979 eV (K-shell), 1097 eV (L-shell) for Cu [14]. Consequently, the characteristic X-ray energy for the transition L-K shell is the difference of the two energies, i.e. 502 eV in O and 7882 eV in Cu. This energy is far larger than the visible/UV luminescence energy. K-electron has a kinetic energy of ‘‘X-ray injected energy–BE (K-shell)’’. This energy will be absorbed in scintillated atoms including Cu + ions, and excites the outermost (weakly bound) electrons (3d10 in the case of Cu + ) to excited levels (3d94s1). When the excited electron drops to the ground state, a photon can be emitted. The RL spectrum of Li2B4O7:Cu crystal is shown in Fig. 6. The structure of the RL spectrum is complex, having no normal Gauss peaks. With the S¼10, this value implies a strong electronphonon interaction, which is a prerequisite condition for the appearance of the polaron and STE in the materials. So RL spectrum can be related to contribution of STE or core–valence transition. According to other report [15,16], the sharp of this spectrum can be related to STE. As mentioned above, in the structure of Li2B4O7, tetrahedron BO4 (&) plays the hole-trap role because it has a linking excess with oxygen so it easily captures a hole. Other sides, the structure of BO3 are not more stable than BO4 so the oxygen of Li2O will easily link with BO3 to form to BO4. The redundant Li + is located in the lattice also in the form of Cu + . When the radiation beam hits to Li2B4O7:Cu, the electron will be captured by X + (with X¼Li, Cu) to form X0 and the hole will be captured by BO4. The holes of the complex BO4 migrate in the lattice with hopping mechanism and move toward X0. The captured electron of X0 will be released with the tunnel mechanism and combine with the polaron to form STE. In such a way, it is possible to explain why the RL spectrum of Li2B4O7:Cu has a special structure and the decay-time curve has a fast component. The STE mechanism has been explained the appearance of sharp and strong peaks of the RL spectrum of the other materials [16]. The decay-time curve of LBO:Cu crystal is shown in Fig.7. The fast component of the decay-time curve was seen to fall within the range from 20 to 25 ns. Making use of the
Fig. 4. The emission spectrum with 260 nm excitation.
Fig. 5. The transitions in the ion Cu + .
lifetime). The peak at 237 nm corresponds to the 1A1g–1T2g transition, in agreement with our emission spectrum in Fig. 4. In the different hosts, the emission of ion Cu + is almost in the range of 365 nm since it lies in the intersite and thus is much less affected than other locations in the crystal lattice [9,10]. Fig. 5 [10] shows the various transitions in the ion Cu + : the 1T2g-1Eg and 1Eg-3Eg transitions are nonradioactive due to the narrow energy gap. From the absorption peak, emission peak and Raman spectrum, the electron–phonon interaction can be calculated by Huyng–Rhys parameter [11]:
DEs ¼ ð2S1Þho
ð3Þ
where DEs is the shift Stockes, S the Huang–Rhys parameter and ho the energy of phonon. With the data of absorption peak at 37,000 cm 1 in Table 1, emission peak at 28,000 cm 1 from the Fig. 4 and ho ¼510 cm 1 [12], one can obtain the Huyng–Rhys parameter S¼10. This result shows that the strength of the electron–phonon coupling is in the strong coupling regime [13]. When X-rays hit to Li2B4O7:Cu, its energy will be absorbed mostly by the K-shell electron (the innermost electrons) in Li, B, O and Cu atoms. The K-electron will be liberated and go out. The vacancy will be refilled typically by an electron in the L (or M) shell. Here K-shell is the first shell, which is closest to the nucleus,
Fig. 6. The radioluminescence of Li2B4O7:Cu crystal.
B.T. Huy et al. / Journal of Luminescence 130 (2010) 2142–2145
2145
4. Conclusion These radioluminescent mechanisms signalling of Li2B4O7:Cu may comprise by:
The inherent signal includes electron–hole recombination,
bound exciton and core–valence luminescence of LBO lattice host. Most important is STE: it is a luminescence mechanism for lifetime as short as of ns that is expected for the materials with strong electron–phonon interaction. The extrinsic signal is almost phosphor of Cu + with long lifetime (in the order of ms) for d–s transitions. The short lifetime components (in the order of ns) are attributed to d–p transitions.
Acknowledgment
Fig. 7. The decay time curve of Li2B4O7:Cu crystal.
above results, we now propose some possible luminescent mechanisms of Li2B4O7:Cu crystal. The fast component of the decay-time curve can be explained with some hypotheses such as below:
Self-trapped excitation mechanism. The value of S ¼10 shows
that the electron–phonon interaction is strong. This favors formation of the polaron and STE after the crystal was bombarded by the ion beams. Core–valence transition mechanism. The model of core–valence can be found in Refs. [17,18]. The core–valence transition is produced by the recombination of an electron in the valence band with a hole created in the outermost core band of an ionic crystal. If the energy difference Evc between the core band and the valence band is less than the band gap Eg, the process involves the emission of a photon, usually in the UV region. A characteristic of this transition is that the decay-time is very short which is of the order of a nanosecond. Unfortunately, however, this hypothesis is unconfirmed because we still do not have good enough results. Electron–hole pair transition. After the material was irradiated, the electron–hole pairs were created. The electron from the valence band moves to the conduction band and immediately immigrates to defect centers under the bottom of the conduction band. The electron–hole recombination occurs at once. Therefore, the decay process is very fast. The luminescence of Cu + . The 3d94p1-3d10 transition is allowed, so they take part in ‘‘fast component’’ of the signal.
We gratefully acknowledge Vietnam Academy of Science and Technology for financial support. We thank Prof. M. Kobayashi (KEK-Japan) and N.B. An (IOP-Vietnam) for the interesting discussions. References [1] M. Prokic, Radiation Protection Dosimetry 100 (2002) 265. [2] B.T. Huy, V.X. Quang, H.T.B. Chau, Journal of Luminescence 128 (2008) 1601. [3] N. Senguttuvan, M. Ishii, M. Shimoyama, M. Kobayashi, N. Tsutsui, M. Nikl, M. Dusek, H.M. Shimizu, T. Oku, T. Adachi, K. Sakai, J. Suzuki, Nuclear Instruments and Methods in Physics Research A 486 (2002) 264. [4] M. Ishii, Y. Kuwano, T. Asai, M. Kawamura, N. Senguttuvan, T. Hayashi, M. Kobayashi, M. Nikl, S. Hosoya, K. Sakai, T. Adachi, T. Oku, H.M. Shimizu, Nuclear Instruments and Methods in Physics Research A 537 (2005) 282. [5] M. Ignatovych, V. Holovey, T. Vidoczy, P. Baranyai, A. Kelemen, V. Latuga, O. Chuiko, Function Materials 12 (2005) 313. [6] M. Ignatovych, V. Holovey, A. Watterich, T. Vidoczy, P. Baranyai, A. Kelemen, O. Chuiko, Radiation Measurements 38 (2004) 567. [7] S. Watanabe, E.F. Chinaglia, M.L.F. Nascimento, M. Matsuoka, Radiation Protection Dosimetry 65 (1–4) (1996) 79. [8] V.T. Adamiv, O.T. Antonyak, Ya.V. Burak, M.S. Pidzyrailo, I.M. Teslyuk, Function Materials 12 (2) (2005) 278. [9] John Simometti, S.Mc. Donal, Physical Review B 16 (9) (1977) 3887. [10] B. Moine, C. Pedrini, Physical Review B 30 (2) (1984) 2301. [11] J. Garcia Sole, L.E. Bausa, D. Jaque, An introduction to the optical spectroscopy of Inorganic solids, John Wiley and Sons Ltd., West Sussex, England, 2005. [12] M.P. Dergachev, V.N. Moiseenko, Y.V. Burak, Optic and Spectroscopy 90 (4) (2001) 534. [13] G. Blasse, B.C. Grabmaier, in: Luminescent materials, Springer Verlag, 1994. [14] R.L. David, in: Handbook of Chemistry and Physics, 88th Ed., CRC Press, 2007. [15] I.N. Ogorodnikov, V.A. Pustovarov, A.V. Kruzhalov, L.I. Isaenko, M. Kirm, G. Zimmerer, Physics of the Solid State 42 (3) (2000) 464. [16] V. Kiisk, I. Sildos, A. Suisalu, J. Aarik, Thin Solid Films 400 (2001) 130. [17] Carel W.E. van Eijk, Nuclear Instruments and Methods in Physics Research A (1977) 285. [18] J. Wilkinson, K.B. Ucer, R.T. Williams, Radiation Measurement 38 (2004) 501. [19] J.C. Zhang, B. Moine, C. Perdin, G. Flem, Journal of Physics and Chemistry of Solids 50 (8) (1990) 933. [20] Emmanuel Desurvire, in: Erbium-doped fiber amplifiers- Principles and Application, John Wiley & Sons, Inc, 1994.