Journal of Alloys and Compounds 509 (2011) L81–L84
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Journal of Alloys and Compounds
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ffect of swift heavy ion irradiation on bismuth doped BaS nanostructures
urender Singha,∗, Ravi Kumarb, Nafa Singha
Department of Physics, Kurukshetra University, Kurukshetra 136 119, IndiaCentre for Material Science and Engineering, National institute of Technology Hamirpur, Hamirpur, H.P., India
r t i c l e i n f o
rticle history:eceived 16 July 2010eceived in revised form
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 ofbismuth doped barium sulphide nanostructures. The samples were irradiated with 120 MeV Ni+9 ionsat three different fluences of 1 × 1012, 5 × 1012, and 1 × 1013 ion/cm2. Structural and optical properties
0 November 2010ccepted 15 November 2010vailable online 23 November 2010
eywords:wift heavy ion
of pristine and irradiated samples were carried out using X-ray diffraction (XRD), transmission electronmicroscopy (TEM), photoluminescence (PL) and UV–vis spectroscopy. X-ray diffraction (XRD) studieswere used to estimate the average size of nanoparticles. The average size of the crystallites is estimatedfrom the line widths of the diffraction pattern, while the exact size of the crystallites is estimated fromthe TEM micrographs. After irradiation with a fluence of 1 × 1013 ion/m2 the photoluminescence intensity
irect
efectshermal spike
increases by 42%. The ind
. Introduction
In recent years, rare earth and transition metal ions doped alka-ine earth sulphides have attracted the attention of many scientistsnd researchers due to their wide applications in cathode ray tubes,uorescent devices, electroluminescence panels and thermo lumi-escence dosimetry [1–3]. In low dimensional systems, energy
evel structures and optical properties are different from those inulk systems. In view of their practical importance, the studies oftructural properties of alkaline earth sulphides (AES) under var-ous conditions remain important and new methods to improvehese properties are being continually investigated. Swift heavy ionSHI) irradiation is a very effective tool to induce structural modifi-ations in materials and have been used to tailor the properties ofarious metals, semiconductors, polymers, thin films and insulators4–7]. The nature of modifications depends on electrical, thermalnd structural properties of the target material, mass of the pro-ectile ion and the irradiation parameters. Also, the incorporationf energetic heavy ions as a processing technique improves mate-ial properties [8]. An energy rich ion (so-called swift heavy ion)nteracts with solids through nuclear and electronic interactions.
t very high energies, the nuclear energy loss is much smaller than
he electronic energy loss, and the interaction between the ionsnd the solid leads to exciting electrons in the solid. A part of thelectronic excitation energy is converted into atomic motion, e.g.
via the electron–phonon coupling. The electron–phonon couplingmeans the ability of the electron to transfer the energy to the lattice.The material along the trajectory of the ion beam is modified, atomsare pushed out of their normal positions, molecules are split intopieces, and ordered structures—such as that of the crystal latticeare destroyed. Several models have been developed to explain thisenergy transfer, resulting in track formation in crystalline solids.They are, namely, the thermal spike model [9,10], coulomb explo-sion model [11,12] and material stability at high levels of electronicexcitation (lattice relaxation model) [13].
We present the modifications in luminescence properties ofbismuth doped BaS nanocrystalline phosphors irradiated with120 MeV nickel ion. 15-UD Pelletron facility of the Inter UniversityAccelerator Centre (IUAC), New Delhi, has been used for ion irradia-tion. The ion fluences used for this purpose were ∼1 × 1012, 5 × 1012
and 1 × 1013 ion/cm2. The samples have been studied using differ-ent techniques such as XRD, TEM, UV–vis and photoluminescencespectroscopy.
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 else-where [14]. XRD of the given samples was obtained using a model D8-Advanceof Bruker (Germany), giving Cu K-alpha radiations, with the energy of 8.04 keVand wavelength 1.54 A. The operating voltage was 40 kV and current 25 mA. Themorphology and sizes of the product were determined by transmission electron
microscopy (TEM) carried out by a H-7500 (Hitachi Ltd. Tokyo Japan) operated at120 kV. Diluted nanoparticles suspended in absolute ethanol were introduced on acarbon 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 andXenon lamp as exciting source was employed. The optical absorption spectra wererecorded on a double beam UV–vis 2500PC spectrophotometer (Shimadzu Corp.),
the Bi3+. The blue shifting of emission wavelength is due to the
ig. 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.
apan. For ion irradiation, the Pelletron facility of IUAC, New Delhi was used. Sam-les were mounted on the sample holder which could be moved up and down asell as rotated about the vertical axis. In this way the different samples could be
rought into the path of the ion beam for irradiation. Samples of BaS:Bi (0.2 mol%)ere irradiated with different ion fluences. During the ion irradiation, a pressure of10−6 Torr was maintained around the samples. After irradiating a sample to theesired fluence, the beam was turned off and a new sample was brought into theosition for irradiation.
. Results and discussions
.1. XRD and TEM
Fig. 1 shows the X-ray diffraction pattern of bismuth dopedaS nanocrystallites irradiated with 120 MeV Ni ions at fluences
f 1 × 1012, 5 × 1012 and 1 × 1013 ion/cm2. The pattern was com-ared with the diffraction pattern in JCPDS database with PDF no5-0896 the pattern confirms BaS with the rock salt type (NaCl)tructure without any traces of impurity. Even at high fluences no
Fig. 2. TEM image of BaS:Bi (0.2 mol%) (a) virg
mpounds 509 (2011) L81–L84
new peak has been observed. It is in agreement with the previousreport made on CaS:Bi [15] and in contradiction with the reportson SrS:Ce nanostructures [16]. In the present investigation, theirradiated powder shows an increase in full width at half max-ima (FWMH) and reduction in the intensity which shows a lossof crystallinity of the samples. Also the XRD peaks shifted towardslarge angle after irradiation. The average grain size of the parti-cles was calculated by using Debye–Scherrer equation [17]. TEMimages were taken by dispersing the nanoparticles in ethanol andthen depositing the suspension on carbon coated copper grid andallowed to dry in air. Some TEM images of virgin and radiated sam-ples at fluence of 1 × 1013 ion/cm2 are shown in Fig. 2. This studyshows that most of he particles are in the range 35–40 nm beforeirradiation and 15–20 nm after ion irradiation.
The structural parameters such as size of particles (d), disloca-tion density (ı) and microstrain (ε) before and after irradiation aresummarized 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)
and
ı = 1d2
(2)
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 nanophos-phors. PL intensity is found to increase with increase in ion fluencewhile peak position shifts from 575 nm to 571 nm, 569 nm and563 nm, respectively, with the ion fluences 1 × 1011, 5 × 1012 and1 × 1013 ion/cm2. We may assign this emission to the transitionfrom 3P1 (6s6p) excited state to the ground state 1S0 (6s2) of
decreases in grain size of nanocrystallites which is direct signal ofquantum size effect. The PL intensity is sensitive to the damagecreated by SHIs. A strong PL intensity indicates dominant radiativetransitions. As the concentration of the color centers increases, the
in and at fluence of (b) 1 × 1013 ion/cm2.
S. Singh et al. / Journal of Alloys and Compounds 509 (2011) L81–L84 L83
Table 1Calculated structural parameters for BaS:Bi (0.2 mol%) nanocrystalline phosphors.
ate of radiative transitions also increases, resulting in an increasen the luminescence intensity [18,19]. The PL intensity is 16%,3% and 42% more than the intensity of virgin phosphors for ionuences of 1 × 1011, 5 × 1012 and 1 × 1013 ion/cm2, respectively.hen energy rich ion enters into a material, temperature around
he trajectory of the ion increases due to electron–phonon coupling.he impulses received by the ionized atoms are coherent in spacend time, so that an efficient coupling of the excitation with theow frequency phonons can take place. The shock waves or pres-ure waves develop due to this temperature spike which diffuseshe heat in the target. It may be possible that sudden explosion ofonized matter leads to fragmentation of the grains [20,21]. Further-
ore, grain boundaries acts as color centers; fragmentation causedy swift heavy ions increases the density of these grain boundarieshich increases the PL emission intensity. The PL is intensity isirectly linked with the size of particles. As from our early report14] we found that in bulk phosphors the peak is at 608 nm whilen their nanoform it found at 575 nm. After irradiation this peaks shifted to 571 nm, 569 nm and 563 nm, respectively, with theon fluences 1 × 1011, 5 × 1012 and 1 × 1013 ion/cm2. The blue shift-ng of emission wavelengths is due to the different grain sizes ofanocrystallites which is direct signature of quantum size effects.
Fig. 4 shows the absorbance vs wavelength spectra of bismuthoped barium sulphide before and after irradiation at a fluencef 1 × 1013 ion/cm2. The absorption decreases with the irradiationndicating the presence of smaller particles in the sample. Theptical energy gap Eg of a semiconductor can be deduced frombsorption spectra as shown in Fig. 4, near the fundamental absorp-ion edge by using the following relation [22,23]:
˛h�)1/2 = h� − Eg
here h� is the incident photon energy and ˛ is the optical absorp-ion coefficient near the fundamental absorption edge. The indirectnergy band gap was obtained by extrapolating the linear portionf the graph and making (˛h�)−1/2 = 0. It has been observed that
ig. 3. The variation of the PL intensity of BaS:Bi nanophosphors (a) virgin and at auence of (b) 1 × 1012, (c) 5 × 1012 and (d) 1 × 1013 ion/cm2 of 120 MeV Ni+9 ions.
Fig. 5. Plot for (˛h�)1/2 as a function of the incident photon energy for the virginand 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 asshown in Fig. 5.
4. Conclusion
The present study reveals that on swift heavy ion irradiationthere are significant modifications in the structural and opticalproperties of nanophosphors. XRD confirms the cubic structure ofBaS. In photoluminescence, the peak is shifted towards blue region.
The PL intensity increases by 42% as compared to that of virgin sam-ple for irradiation by 1 × 1013 ion/cm2. In optical absorption, theband gap increases with the ion fluence. This may be attributed tothe fragmentation of grains.
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84 S. Singh et al. / Journal of Alloys
cknowledgments
The authors are thankful to IUAC, New Delhi for providing ionrradiation facility to carry out this work. We are grateful to theirector of SAIF, Punjab University, Chandigarh for providing us
he TEM and XRD facilities available at the centre.
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