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Effect of swift heavy ion irradiation on bismuth doped BaS nanostructures
Journal of Alloys and Compounds 509 (2011) L81–L84
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Effect of swift heavy ion irradiation on bismuth doped BaS nanostructures Surender Singh a,∗ , Ravi Kumar b , Nafa Singh a a b
Department of Physics, Kurukshetra University, Kurukshetra 136 119, India Centre for Material Science and Engineering, National institute of Technology Hamirpur, Hamirpur, H.P., India
a r t i c l e
i n f o
Article history: Received 16 July 2010 Received in revised form 10 November 2010 Accepted 15 November 2010 Available online 23 November 2010 Keywords: Swift heavy ion Defects Thermal spike
a b s t r a c t We report the use of swift heavy ion irradiation as a means to tailor the luminescence properties of bismuth doped barium sulphide nanostructures. The samples were irradiated with 120 MeV Ni+9 ions at three different fluences of 1 × 1012 , 5 × 1012 , and 1 × 1013 ion/cm2 . Structural and optical properties of pristine and irradiated samples were carried out using X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL) and UV–vis spectroscopy. X-ray diffraction (XRD) studies were used to estimate the average size of nanoparticles. The average size of the crystallites is estimated from the line widths of the diffraction pattern, while the exact size of the crystallites is estimated from the TEM micrographs. After irradiation with a fluence of 1 × 1013 ion/m2 the photoluminescence intensity increases by 42%. The indirect band gap of BaS:Bi is increased after ion irradiation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, rare earth and transition metal ions doped alkaline earth sulphides have attracted the attention of many scientists and researchers due to their wide applications in cathode ray tubes, fluorescent devices, electroluminescence panels and thermo luminescence dosimetry [1–3]. In low dimensional systems, energy level structures and optical properties are different from those in bulk systems. In view of their practical importance, the studies of structural properties of alkaline earth sulphides (AES) under various conditions remain important and new methods to improve these properties are being continually investigated. Swift heavy ion (SHI) irradiation is a very effective tool to induce structural modifications in materials and have been used to tailor the properties of various metals, semiconductors, polymers, thin films and insulators [4–7]. The nature of modifications depends on electrical, thermal and structural properties of the target material, mass of the projectile ion and the irradiation parameters. Also, the incorporation of energetic heavy ions as a processing technique improves material properties [8]. An energy rich ion (so-called swift heavy ion) interacts with solids through nuclear and electronic interactions. At very high energies, the nuclear energy loss is much smaller than the electronic energy loss, and the interaction between the ions and the solid leads to exciting electrons in the solid. A part of the electronic excitation energy is converted into atomic motion, e.g.
∗ Corresponding author. Tel.: +91 1744 238196x2130/238410x2482; fax: +91 1744 238277. E-mail address: [email protected] (S. Singh). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.11.103
via the electron–phonon coupling. The electron–phonon coupling means the ability of the electron to transfer the energy to the lattice. The material along the trajectory of the ion beam is modified, atoms are pushed out of their normal positions, molecules are split into pieces, and ordered structures—such as that of the crystal lattice are destroyed. Several models have been developed to explain this energy transfer, resulting in track formation in crystalline solids. They are, namely, the thermal spike model [9,10], coulomb explosion model [11,12] and material stability at high levels of electronic excitation (lattice relaxation model) [13]. We present the modifications in luminescence properties of bismuth doped BaS nanocrystalline phosphors irradiated with 120 MeV nickel ion. 15-UD Pelletron facility of the Inter University Accelerator Centre (IUAC), New Delhi, has been used for ion irradiation. The ion fluences used for this purpose were ∼1 × 1012 , 5 × 1012 and 1 × 1013 ion/cm2 . The samples have been studied using different techniques such as XRD, TEM, UV–vis and photoluminescence spectroscopy. 2. Experimental Bismuth doped barium sulphide nanostructures have been prepared. Bismuth (0.2 mol%) was used as a dopant The details of phosphors preparation is given elsewhere [14]. XRD of the given samples was obtained using a model D8-Advance of Bruker (Germany), giving Cu K-alpha radiations, with the energy of 8.04 keV ˚ The operating voltage was 40 kV and current 25 mA. The and wavelength 1.54 A. morphology and sizes of the product were determined by transmission electron microscopy (TEM) carried out by a H-7500 (Hitachi Ltd. Tokyo Japan) operated at 120 kV. Diluted nanoparticles suspended in absolute ethanol were introduced on a carbon coated copper grid, and were allowed to dry in air. For photoluminescence (PL), a FluoroMax-3 (Jobin-Yvon), NJ, USA, equipped with a photomultiplier tube and Xenon lamp as exciting source was employed. The optical absorption spectra were recorded on a double beam UV–vis 2500PC spectrophotometer (Shimadzu Corp.),
d
(420)
(311)
(222)
(220)
Intensity(a.u.)
(200)
S. Singh et al. / Journal of Alloys and Compounds 509 (2011) L81–L84
(111)
L82
c
b
a 30
40
50
60
2θ Fig. 1. XRD pattern for the BaS:Bi (0.2 mol%) (a) virgin and at a fluence of (b) 1 × 1012 (c) 5 × 1012 and (d) 1 × 1013 ion/cm2 .
Japan. For ion irradiation, the Pelletron facility of IUAC, New Delhi was used. Samples were mounted on the sample holder which could be moved up and down as well as rotated about the vertical axis. In this way the different samples could be brought into the path of the ion beam for irradiation. Samples of BaS:Bi (0.2 mol%) were irradiated with different ion fluences. During the ion irradiation, a pressure of ∼10−6 Torr was maintained around the samples. After irradiating a sample to the desired fluence, the beam was turned off and a new sample was brought into the position for irradiation.
3. Results and discussions 3.1. XRD and TEM Fig. 1 shows the X-ray diffraction pattern of bismuth doped BaS nanocrystallites irradiated with 120 MeV Ni ions at fluences of 1 × 1012 , 5 × 1012 and 1 × 1013 ion/cm2 . The pattern was compared with the diffraction pattern in JCPDS database with PDF no 75-0896 the pattern confirms BaS with the rock salt type (NaCl) structure without any traces of impurity. Even at high fluences no
new peak has been observed. It is in agreement with the previous report made on CaS:Bi [15] and in contradiction with the reports on SrS:Ce nanostructures [16]. In the present investigation, the irradiated powder shows an increase in full width at half maxima (FWMH) and reduction in the intensity which shows a loss of crystallinity of the samples. Also the XRD peaks shifted towards large angle after irradiation. The average grain size of the particles was calculated by using Debye–Scherrer equation [17]. TEM images were taken by dispersing the nanoparticles in ethanol and then depositing the suspension on carbon coated copper grid and allowed to dry in air. Some TEM images of virgin and radiated samples at fluence of 1 × 1013 ion/cm2 are shown in Fig. 2. This study shows that most of he particles are in the range 35–40 nm before irradiation and 15–20 nm after ion irradiation. The structural parameters such as size of particles (d), dislocation density (ı) and microstrain (ε) before and after irradiation are summarized in Table 1. After ion irradiation dislocation density (ı) and microstrain (ε) were calculated using the following relation [18]: ε=
ˇ cos B 4
(1)
1 d2
(2)
and ı=
where ˇ is the FWHM and B is the Bragg angle. 3.2. Photoluminescence and UV–vis studies Fig. 3 shows PL spectra of pristine and irradiated nanophosphors. PL intensity is found to increase with increase in ion fluence while peak position shifts from 575 nm to 571 nm, 569 nm and 563 nm, respectively, with the ion fluences 1 × 1011 , 5 × 1012 and 1 × 1013 ion/cm2 . We may assign this emission to the transition from 3 P1 (6s6p) excited state to the ground state 1 S0 (6s2 ) of the Bi3+ . The blue shifting of emission wavelength is due to the decreases in grain size of nanocrystallites which is direct signal of quantum size effect. The PL intensity is sensitive to the damage created by SHIs. A strong PL intensity indicates dominant radiative transitions. As the concentration of the color centers increases, the
Fig. 2. TEM image of BaS:Bi (0.2 mol%) (a) virgin and at fluence of (b) 1 × 1013 ion/cm2 .
S. Singh et al. / Journal of Alloys and Compounds 509 (2011) L81–L84
L83
Table 1 Calculated structural parameters for BaS:Bi (0.2 mol%) nanocrystalline phosphors. BaS:Bi
2
FWMH (◦ )
d (nm)
ε (10−5 lin−2 /m4 )
ı (10−4 lin/m2 )
Virgin 1 × 1012 5 × 1012 1 × 1013
27.91 (2 0 0) 27.98 (2 0 0) 28.05 (2 0 0) 28.20 (2 0 0)
0.2313 0.2699 0.3114 0.3682
35 30 26 22
81.6 111.1 147.9 206.6
561.13 654.71 755.30 892.70
rate of radiative transitions also increases, resulting in an increase in the luminescence intensity [18,19]. The PL intensity is 16%, 33% and 42% more than the intensity of virgin phosphors for ion fluences of 1 × 1011 , 5 × 1012 and 1 × 1013 ion/cm2 , respectively. When energy rich ion enters into a material, temperature around the trajectory of the ion increases due to electron–phonon coupling. The impulses received by the ionized atoms are coherent in space and time, so that an efficient coupling of the excitation with the low frequency phonons can take place. The shock waves or pressure waves develop due to this temperature spike which diffuses the heat in the target. It may be possible that sudden explosion of ionized matter leads to fragmentation of the grains [20,21]. Furthermore, grain boundaries acts as color centers; fragmentation caused by swift heavy ions increases the density of these grain boundaries which increases the PL emission intensity. The PL is intensity is directly linked with the size of particles. As from our early report [14] we found that in bulk phosphors the peak is at 608 nm while in their nanoform it found at 575 nm. After irradiation this peak is shifted to 571 nm, 569 nm and 563 nm, respectively, with the ion fluences 1 × 1011 , 5 × 1012 and 1 × 1013 ion/cm2 . The blue shifting of emission wavelengths is due to the different grain sizes of nanocrystallites which is direct signature of quantum size effects. Fig. 4 shows the absorbance vs wavelength spectra of bismuth doped barium sulphide before and after irradiation at a fluence of 1 × 1013 ion/cm2 . The absorption decreases with the irradiation indicating the presence of smaller particles in the sample. The optical energy gap Eg of a semiconductor can be deduced from absorption spectra as shown in Fig. 4, near the fundamental absorption edge by using the following relation [22,23]: (˛h)
1/2
Fig. 4. Optical absorption spectra of (a) virgin and irradiated at a fluence of (b) 1 × 1013 ion/cm2 .
= h − Eg
where h is the incident photon energy and ˛ is the optical absorption coefficient near the fundamental absorption edge. The indirect energy band gap was obtained by extrapolating the linear portion of the graph and making (˛h)−1/2 = 0. It has been observed that
Fig. 5. Plot for (˛h)1/2 as a function of the incident photon energy for the virgin and 120 MeV Ni+9 irradiated nanocrystallites of BaS:Bi.
band gap shifts from 4.25 eV to 4.28 eV with 1 × 1013 ion/cm2 as shown in Fig. 5. 4. Conclusion
Fig. 3. The variation of the PL intensity of BaS:Bi nanophosphors (a) virgin and at a fluence of (b) 1 × 1012 , (c) 5 × 1012 and (d) 1 × 1013 ion/cm2 of 120 MeV Ni+9 ions.
The present study reveals that on swift heavy ion irradiation there are significant modifications in the structural and optical properties of nanophosphors. XRD confirms the cubic structure of BaS. In photoluminescence, the peak is shifted towards blue region. The PL intensity increases by 42% as compared to that of virgin sample for irradiation by 1 × 1013 ion/cm2 . In optical absorption, the band gap increases with the ion fluence. This may be attributed to the fragmentation of grains.
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S. Singh et al. / Journal of Alloys and Compounds 509 (2011) L81–L84
Acknowledgments The authors are thankful to IUAC, New Delhi for providing ion irradiation facility to carry out this work. We are grateful to the Director of SAIF, Punjab University, Chandigarh for providing us the TEM and XRD facilities available at the centre. References [1] A. Vij, S. Singh, R. Kumar, S.P. Lochab, V.V.S. Kumar, N. Singh, J. Phys. D: Appl. Phys. 42 (2009) 105103. [2] P.D. Sahare, N. Salah, S.P. Lochab, T. Mohanty, D. Kanjilal, J. Phys. D: Appl. Phys. 38 (2005) 3995–4002. [3] P. Chawla, S.P. Lochab, N. Singh, J. Alloys Compd. 494 (2010) L20–L24. [4] D.K. Avasthi, current science 78 (2000) 1297–1306. [5] S. Chandramohan, R. Sathyamoorthy, P. Sudhagar, D. Kanjilal, D. Kabiraj, K. Asokan, V. Ganesan, T. Shripathi, U.P. Deshpande, Appl. Phys. A 94 (2009) 703–714. [6] R.R. Ahire, A.A. Sagade, N.G. Deshpande, S.D. Chavhan, R. Sharma, F. Singh, J. Phys. D: Appl. Phys. 40 (2007) 4850–4854. [7] A. Benyagoub, Nucl. Instrum. Methods Phys. Res. B 245 (2006) 225–230. [8] K.R. Nagabhushana, B.N. Lakshminarasappa, F. Singh, Bull. Mater. Sci. 32 (2009) 515–519.
[9] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362– 14369. [10] Z.G. Wang, C. Dufour, E. Paumier, M. Toulemonde, J. Phys.: Condens. Matter 6 (1994) 6733–6750. [11] E.M. Bringa, R.E. Johnson, Phys. Rev. Lett. 88 (2002) 165501. [12] R.L. Fleischer, P.B. Price, R.M. Walker, J. Appl. Phys. 36 (1965) 3645. [13] P. Stampfli, Nucl. Instrum. Methods B 107 (1996) 138–145. [14] S. Singh, A. Vij, R. Kumar, S.P. Lochab, N. Singh, Mater. Res. Bull. 45 (2010) 523–526. [15] V. Kumar, R. Kumar, S.P. Lochab, N. Singh, J. Nanopart. Res. 9 (2007) 661– 667. [16] A. Vij, A.K. Chawla, R. Kumar, S.P. Lochab, R. Chandra, N. Singh, Phys. B: Condens. Matter 405 (2010) 2573–2576. [17] P. Scherrer, NACHR Ges. Wiss. Gottingen (1918) 96–100. [18] R. Sathyamoorthy, S. Chandramohan, P. Sudhagar, D. Kanjilal, D. Kabiraj, K. Ashokan, Sol. Energy Mater. Sol. Cells 90 (2006) 2297. [19] V.A. Skuratov, S.M. Abu AlAzm, V.A. Altynov, Nucl. Instrum. Methods B 191 (2002) 251. [20] A. Berthelot, S. Heˇımon, F. Gourbilleau, C. Dufour, E. Dooryheˇıe, E. Paumier, Nucl. Instrum. Methods B 146 (2002) 437. [21] F. Garrido, A. Benyagoub, A. Chamberod, J.-C. Dran, A. Dunlop, S. Klaumunzer, L. Thome, Nucl. Instrum. Methods B 115 (1996) 430. [22] M.S. Jin, et al., J. Kor. Phys. Soc. 39 (2001) 692–697. [23] S.M. Sze, Physics of Semiconductor Devices, 2nd ed., Wiley, New York, 2004, p. 39.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Effect of swift heavy ion irradiation on bismuth doped BaS nanostructures Surender Singh a,∗ , Ravi Kumar b , Nafa Singh a a b
Department of Physics, Kurukshetra University, Kurukshetra 136 119, India Centre for Material Science and Engineering, National institute of Technology Hamirpur, Hamirpur, H.P., India
a r t i c l e
i n f o
Article history: Received 16 July 2010 Received in revised form 10 November 2010 Accepted 15 November 2010 Available online 23 November 2010 Keywords: Swift heavy ion Defects Thermal spike
a b s t r a c t We report the use of swift heavy ion irradiation as a means to tailor the luminescence properties of bismuth doped barium sulphide nanostructures. The samples were irradiated with 120 MeV Ni+9 ions at three different fluences of 1 × 1012 , 5 × 1012 , and 1 × 1013 ion/cm2 . Structural and optical properties of pristine and irradiated samples were carried out using X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL) and UV–vis spectroscopy. X-ray diffraction (XRD) studies were used to estimate the average size of nanoparticles. The average size of the crystallites is estimated from the line widths of the diffraction pattern, while the exact size of the crystallites is estimated from the TEM micrographs. After irradiation with a fluence of 1 × 1013 ion/m2 the photoluminescence intensity increases by 42%. The indirect band gap of BaS:Bi is increased after ion irradiation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, rare earth and transition metal ions doped alkaline earth sulphides have attracted the attention of many scientists and researchers due to their wide applications in cathode ray tubes, fluorescent devices, electroluminescence panels and thermo luminescence dosimetry [1–3]. In low dimensional systems, energy level structures and optical properties are different from those in bulk systems. In view of their practical importance, the studies of structural properties of alkaline earth sulphides (AES) under various conditions remain important and new methods to improve these properties are being continually investigated. Swift heavy ion (SHI) irradiation is a very effective tool to induce structural modifications in materials and have been used to tailor the properties of various metals, semiconductors, polymers, thin films and insulators [4–7]. The nature of modifications depends on electrical, thermal and structural properties of the target material, mass of the projectile ion and the irradiation parameters. Also, the incorporation of energetic heavy ions as a processing technique improves material properties [8]. An energy rich ion (so-called swift heavy ion) interacts with solids through nuclear and electronic interactions. At very high energies, the nuclear energy loss is much smaller than the electronic energy loss, and the interaction between the ions and the solid leads to exciting electrons in the solid. A part of the electronic excitation energy is converted into atomic motion, e.g.
∗ Corresponding author. Tel.: +91 1744 238196x2130/238410x2482; fax: +91 1744 238277. E-mail address: [email protected] (S. Singh). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.11.103
via the electron–phonon coupling. The electron–phonon coupling means the ability of the electron to transfer the energy to the lattice. The material along the trajectory of the ion beam is modified, atoms are pushed out of their normal positions, molecules are split into pieces, and ordered structures—such as that of the crystal lattice are destroyed. Several models have been developed to explain this energy transfer, resulting in track formation in crystalline solids. They are, namely, the thermal spike model [9,10], coulomb explosion model [11,12] and material stability at high levels of electronic excitation (lattice relaxation model) [13]. We present the modifications in luminescence properties of bismuth doped BaS nanocrystalline phosphors irradiated with 120 MeV nickel ion. 15-UD Pelletron facility of the Inter University Accelerator Centre (IUAC), New Delhi, has been used for ion irradiation. The ion fluences used for this purpose were ∼1 × 1012 , 5 × 1012 and 1 × 1013 ion/cm2 . The samples have been studied using different techniques such as XRD, TEM, UV–vis and photoluminescence spectroscopy. 2. Experimental Bismuth doped barium sulphide nanostructures have been prepared. Bismuth (0.2 mol%) was used as a dopant The details of phosphors preparation is given elsewhere [14]. XRD of the given samples was obtained using a model D8-Advance of Bruker (Germany), giving Cu K-alpha radiations, with the energy of 8.04 keV ˚ The operating voltage was 40 kV and current 25 mA. The and wavelength 1.54 A. morphology and sizes of the product were determined by transmission electron microscopy (TEM) carried out by a H-7500 (Hitachi Ltd. Tokyo Japan) operated at 120 kV. Diluted nanoparticles suspended in absolute ethanol were introduced on a carbon coated copper grid, and were allowed to dry in air. For photoluminescence (PL), a FluoroMax-3 (Jobin-Yvon), NJ, USA, equipped with a photomultiplier tube and Xenon lamp as exciting source was employed. The optical absorption spectra were recorded on a double beam UV–vis 2500PC spectrophotometer (Shimadzu Corp.),
d
(420)
(311)
(222)
(220)
Intensity(a.u.)
(200)
S. Singh et al. / Journal of Alloys and Compounds 509 (2011) L81–L84
(111)
L82
c
b
a 30
40
50
60
2θ Fig. 1. XRD pattern for the BaS:Bi (0.2 mol%) (a) virgin and at a fluence of (b) 1 × 1012 (c) 5 × 1012 and (d) 1 × 1013 ion/cm2 .
Japan. For ion irradiation, the Pelletron facility of IUAC, New Delhi was used. Samples were mounted on the sample holder which could be moved up and down as well as rotated about the vertical axis. In this way the different samples could be brought into the path of the ion beam for irradiation. Samples of BaS:Bi (0.2 mol%) were irradiated with different ion fluences. During the ion irradiation, a pressure of ∼10−6 Torr was maintained around the samples. After irradiating a sample to the desired fluence, the beam was turned off and a new sample was brought into the position for irradiation.
3. Results and discussions 3.1. XRD and TEM Fig. 1 shows the X-ray diffraction pattern of bismuth doped BaS nanocrystallites irradiated with 120 MeV Ni ions at fluences of 1 × 1012 , 5 × 1012 and 1 × 1013 ion/cm2 . The pattern was compared with the diffraction pattern in JCPDS database with PDF no 75-0896 the pattern confirms BaS with the rock salt type (NaCl) structure without any traces of impurity. Even at high fluences no
new peak has been observed. It is in agreement with the previous report made on CaS:Bi [15] and in contradiction with the reports on SrS:Ce nanostructures [16]. In the present investigation, the irradiated powder shows an increase in full width at half maxima (FWMH) and reduction in the intensity which shows a loss of crystallinity of the samples. Also the XRD peaks shifted towards large angle after irradiation. The average grain size of the particles was calculated by using Debye–Scherrer equation [17]. TEM images were taken by dispersing the nanoparticles in ethanol and then depositing the suspension on carbon coated copper grid and allowed to dry in air. Some TEM images of virgin and radiated samples at fluence of 1 × 1013 ion/cm2 are shown in Fig. 2. This study shows that most of he particles are in the range 35–40 nm before irradiation and 15–20 nm after ion irradiation. The structural parameters such as size of particles (d), dislocation density (ı) and microstrain (ε) before and after irradiation are summarized in Table 1. After ion irradiation dislocation density (ı) and microstrain (ε) were calculated using the following relation [18]: ε=
ˇ cos B 4
(1)
1 d2
(2)
and ı=
where ˇ is the FWHM and B is the Bragg angle. 3.2. Photoluminescence and UV–vis studies Fig. 3 shows PL spectra of pristine and irradiated nanophosphors. PL intensity is found to increase with increase in ion fluence while peak position shifts from 575 nm to 571 nm, 569 nm and 563 nm, respectively, with the ion fluences 1 × 1011 , 5 × 1012 and 1 × 1013 ion/cm2 . We may assign this emission to the transition from 3 P1 (6s6p) excited state to the ground state 1 S0 (6s2 ) of the Bi3+ . The blue shifting of emission wavelength is due to the decreases in grain size of nanocrystallites which is direct signal of quantum size effect. The PL intensity is sensitive to the damage created by SHIs. A strong PL intensity indicates dominant radiative transitions. As the concentration of the color centers increases, the
Fig. 2. TEM image of BaS:Bi (0.2 mol%) (a) virgin and at fluence of (b) 1 × 1013 ion/cm2 .
S. Singh et al. / Journal of Alloys and Compounds 509 (2011) L81–L84
L83
Table 1 Calculated structural parameters for BaS:Bi (0.2 mol%) nanocrystalline phosphors. BaS:Bi
2
FWMH (◦ )
d (nm)
ε (10−5 lin−2 /m4 )
ı (10−4 lin/m2 )
Virgin 1 × 1012 5 × 1012 1 × 1013
27.91 (2 0 0) 27.98 (2 0 0) 28.05 (2 0 0) 28.20 (2 0 0)
0.2313 0.2699 0.3114 0.3682
35 30 26 22
81.6 111.1 147.9 206.6
561.13 654.71 755.30 892.70
rate of radiative transitions also increases, resulting in an increase in the luminescence intensity [18,19]. The PL intensity is 16%, 33% and 42% more than the intensity of virgin phosphors for ion fluences of 1 × 1011 , 5 × 1012 and 1 × 1013 ion/cm2 , respectively. When energy rich ion enters into a material, temperature around the trajectory of the ion increases due to electron–phonon coupling. The impulses received by the ionized atoms are coherent in space and time, so that an efficient coupling of the excitation with the low frequency phonons can take place. The shock waves or pressure waves develop due to this temperature spike which diffuses the heat in the target. It may be possible that sudden explosion of ionized matter leads to fragmentation of the grains [20,21]. Furthermore, grain boundaries acts as color centers; fragmentation caused by swift heavy ions increases the density of these grain boundaries which increases the PL emission intensity. The PL is intensity is directly linked with the size of particles. As from our early report [14] we found that in bulk phosphors the peak is at 608 nm while in their nanoform it found at 575 nm. After irradiation this peak is shifted to 571 nm, 569 nm and 563 nm, respectively, with the ion fluences 1 × 1011 , 5 × 1012 and 1 × 1013 ion/cm2 . The blue shifting of emission wavelengths is due to the different grain sizes of nanocrystallites which is direct signature of quantum size effects. Fig. 4 shows the absorbance vs wavelength spectra of bismuth doped barium sulphide before and after irradiation at a fluence of 1 × 1013 ion/cm2 . The absorption decreases with the irradiation indicating the presence of smaller particles in the sample. The optical energy gap Eg of a semiconductor can be deduced from absorption spectra as shown in Fig. 4, near the fundamental absorption edge by using the following relation [22,23]: (˛h)
1/2
Fig. 4. Optical absorption spectra of (a) virgin and irradiated at a fluence of (b) 1 × 1013 ion/cm2 .
= h − Eg
where h is the incident photon energy and ˛ is the optical absorption coefficient near the fundamental absorption edge. The indirect energy band gap was obtained by extrapolating the linear portion of the graph and making (˛h)−1/2 = 0. It has been observed that
Fig. 5. Plot for (˛h)1/2 as a function of the incident photon energy for the virgin and 120 MeV Ni+9 irradiated nanocrystallites of BaS:Bi.
band gap shifts from 4.25 eV to 4.28 eV with 1 × 1013 ion/cm2 as shown in Fig. 5. 4. Conclusion
Fig. 3. The variation of the PL intensity of BaS:Bi nanophosphors (a) virgin and at a fluence of (b) 1 × 1012 , (c) 5 × 1012 and (d) 1 × 1013 ion/cm2 of 120 MeV Ni+9 ions.
The present study reveals that on swift heavy ion irradiation there are significant modifications in the structural and optical properties of nanophosphors. XRD confirms the cubic structure of BaS. In photoluminescence, the peak is shifted towards blue region. The PL intensity increases by 42% as compared to that of virgin sample for irradiation by 1 × 1013 ion/cm2 . In optical absorption, the band gap increases with the ion fluence. This may be attributed to the fragmentation of grains.
L84
S. Singh et al. / Journal of Alloys and Compounds 509 (2011) L81–L84
Acknowledgments The authors are thankful to IUAC, New Delhi for providing ion irradiation facility to carry out this work. We are grateful to the Director of SAIF, Punjab University, Chandigarh for providing us the TEM and XRD facilities available at the centre. References [1] A. Vij, S. Singh, R. Kumar, S.P. Lochab, V.V.S. Kumar, N. Singh, J. Phys. D: Appl. Phys. 42 (2009) 105103. [2] P.D. Sahare, N. Salah, S.P. Lochab, T. Mohanty, D. Kanjilal, J. Phys. D: Appl. Phys. 38 (2005) 3995–4002. [3] P. Chawla, S.P. Lochab, N. Singh, J. Alloys Compd. 494 (2010) L20–L24. [4] D.K. Avasthi, current science 78 (2000) 1297–1306. [5] S. Chandramohan, R. Sathyamoorthy, P. Sudhagar, D. Kanjilal, D. Kabiraj, K. Asokan, V. Ganesan, T. Shripathi, U.P. Deshpande, Appl. Phys. A 94 (2009) 703–714. [6] R.R. Ahire, A.A. Sagade, N.G. Deshpande, S.D. Chavhan, R. Sharma, F. Singh, J. Phys. D: Appl. Phys. 40 (2007) 4850–4854. [7] A. Benyagoub, Nucl. Instrum. Methods Phys. Res. B 245 (2006) 225–230. [8] K.R. Nagabhushana, B.N. Lakshminarasappa, F. Singh, Bull. Mater. Sci. 32 (2009) 515–519.
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