Assam, , India b Inter University Accelerator Centre, New Delhi, , India

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1 This article was downloaded by: [N. Choudhury] On: 16 July 2013, At: 11:21 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Radiation Effects and Defects in Solids: Incorporating Plasma Science and Plasma Technology Publication details, including instructions for authors and subscription information: Effect of swift heavy ion irradiation on lead sulfide quantum dots embedded in polyvinyl alcohol N. Choudhury a, F. Singh b & B. K. Sarma c a Department of Physics, Pub Kamrup College, Baihata Chariali, Assam, , India b Inter University Accelerator Centre, New Delhi, , India c Physics Academy of the North East (PANE), Gauhati University Campus, Post Box No. 45, Guwahati, , India Published online: 15 Jan To cite this article: N. Choudhury, F. Singh & B. K. Sarma (2013) Effect of swift heavy ion irradiation on lead sulfide quantum dots embedded in polyvinyl alcohol, Radiation Effects and Defects in Solids: Incorporating Plasma Science and Plasma Technology, 168:7-8, , DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,

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3 Radiation Effects & Defects in Solids, 2013 Vol. 168, Nos. 7 8, , Effect of swift heavy ion irradiation on lead sulfide quantum dots embedded in polyvinyl alcohol N. Choudhury a *, F. Singh b and B.K. Sarma c a Department of Physics, Pub Kamrup College, Baihata Chariali, Assam , India; b Inter University Accelerator Centre, New Delhi , India; c Physics Academy of the North East (PANE), Gauhati University Campus, Post Box No. 45, Guwahati , India (Received 16 October 2012; final version received 20 December 2012) The present study compares structural and optical modifications of lead sulfide (PbS) quantum dots under swift heavy ion (SHI) irradiation. PbS quantum dots are prepared following an inexpensive chemical route using polyvinyl alcohol as the dielectric host matrix. SHI irradiation of the samples is carried out with 100 MeV Si 7+ ion beam with fluences in the range from to ions cm 2 and their structural and optical properties, before and after irradiation, are compared by X-ray diffraction (XRD), photoluminescence (PL) and UV Vis spectroscopy. The structural parameters of PbS quantum dots are studied by X-ray line profile analysis using the Williamson Hall plot. The values of average crystallite sizes are found to vary from 8 to 18 nm. XRD studies confirmed the formation of the cubic nanocrystalline PbS. Improvement of crystalline quality for lower fluences is exhibited by an increase in the X-ray intensities of the films. The UV Vis absorption spectra reveal blue shift relative to the bulk material. Size enhancement of PbS quantum dots after irradiation has been indicated in XRD line profiles of the samples which has also been supported by optical absorption spectra. PL studies of all the samples are carried out with excitation wavelength 350 nm. A broad PL emission with peak centered at 473 nm is observed in pristine as well as all the irradiated samples. PL study also shows that PL intensity increases with ion fluences. Keywords: quantum dots; SHI; XRD; W H plot; PL 1. Introduction The synthesis and characterization of many binary metal chalcogenides of group IV VI in nanocrystalline form have attracted considerable attention because of their unique opto-electronic properties (1, 2). Among these chalcogenides, lead sulfide (PbS) is an important semiconductor material with a small bulk band gap energy of 0.41 ev (3) and a large exciton Bohr radius (18 nm). Many workers synthesized nanoparticles of PbS embedded in polymer matrix and found many of their optical and electrical properties deviated from their bulk values. swift heavy ion (SHI) irradiation is a unique post deposition material engineering technique for the modification of properties of materials. The irradiation of materials with energetic ions leads to the creation of a wide variety of defect states in the material system, which changes the physical and the chemical properties such as structural and optical properties of the material. The changes are strongly dependent on the mass of the incident ion, the irradiation energy and the fluence used. When an SHI passes *Corresponding author. c.navajyoti@gmail.com, cnavajyoti@yahoo.com 2013 Taylor & Francis

4 Radiation Effects & Defects in Solids 499 through a medium, its energy dissipates in the lattice through electronic excitation and ionization. During the electronic excitation, energy of the ions is transferred rapidly and can end up in the production of effects such as creation of lattice defects, annealing of defects, phase transformation, crystallization, amorphization, etc. in the material (4). The energy-relaxation process differs from material to material and can be studied from the physical changes produced in the material because of SHI irradiation. Numerous works have been reported on the effects of SHI irradiation in solids including nanocrystalline materials. Most of the works consider electronic energy loss as the major parameter that determines energy-relaxation process and the resultant effects in the material. Some workers (5 7) have also shown that the growth conditions of the materials in thin film form also decide the ion beam-induced effect in materials. Hence, more work is needed to understand the response of the nanomaterial to heavy ion beams. In this paper, results of the study of the effect of SHI irradiation on PbS quantum dots by X-ray diffraction (XRD), UV Vis optical absorption spectroscopy and photoluminescence (PL) are reported. 2. Experimental 2.1. Sample preparation PbS quantum dots of thickness of 210 nm were deposited on suitably cleaned glass substrates. The deposition was carried out in a mixture of matrix solution and thiourea. The matrix solution was prepared by adding lead acetate to aqueous solution (2% v/v) of polyvinyl alcohol with constant stirring at 60 C for 3 h. The solution was then kept for 24 h to allow complete dissolution of lead acetate indicated by the formation of a transparent solution. The ph of the solution was maintained at around 10.5 by adding NH 4 OH. Four glass substrates were introduced vertically in the solution. Then equimolar (1 M) solution of thiourea was added to the mixer solution and in 15 min the color of the solution changes to dark brown. The substrates were kept in the solution for 24 h at room temperature. Free S 2 ions were produced in the solution due to hydrolization of thiourea and these reacted with Pb 2+ ions released from lead acetate to produce PbS. After the deposition of PbS, the substrates were taken out of the bath and washed with running distilled water and dried SHI irradiation For irradiation purpose, nanocrystalline PbS was deposited on mm 2 glass plates. The samples were then mounted on a vacuum-shielded vertical sliding ladder having four rectangular faces. They were irradiated in the general purpose scattering chamber under high vacuum ( Torr) by using the 100 MeV Si 7+ beam with approximate beam current of 1.0 pna (particle nanoampere), available from the 15 UD tandem Pelletron accelerator at Inter University Accelerator Centre (IUAC), New Delhi. The ion beam fluence was measured by integrating the ion charge on the sample ladder, which was insulated from the chamber. In order to expose the whole target area, the beam was scanned in the x y plane over the sample. The samples were irradiated with fluences , and ions cm 2, respectively Characterization The pristine and SHI irradiated samples were characterized by using XRD, optical absorption and PL studies. A Bruker (AXS D8 Advance) glancing angle X-ray diffractometer with CuKα

5 500 N. Choudhury et al. radiation having a wavelength of nm was used for structural characterization. Optical absorption spectra of the samples were studied using a Hitachi UV Vis spectrophotometer. The PL spectrum of the samples was recorded using a Fluoromax-4 spectrofluorometer (HORIBA JOBIN YVON). 3. Results and discussion 3.1. XRD studies The XRD patterns of both pristine and irradiated PbS quantum dots are shown in Figure 1. The diffraction peaks are recorded at 2θ = 25.92, 30.04, and The peaks are assigned to (111), (200), (220) and (311) planes corresponding to the f.c.c. structure of PbS. We observed an increase in the XRD peak intensity and a decrease in the peak width up to the fluence of ions cm 2. The initial increase in the peak intensity and the decrease in the width are indicative of the improvement in crystallinity due to annealing of defects at lower fluences (8). On increasing the ion fluence further, i.e. at ions cm 2, the peak intensity is decreased and the peak width is increased due to the formation of defect clusters and creation of additional grain boundaries (9, 10). The decrease in intensity also indicates that the sample starts to amorphize after SHI irradiation at fluence of ions cm 2. The average grain size of the pristine and irradiated samples is calculated using Scherrer s relation Kλ D = β 2θ Cos θ, (1) where K is a constant taken to be 0.9, λ is the wavelength of the X-ray used, and β 2θ is the full-width at half-maximum (FWHM) of the diffraction peak in radians and θ is Bragg s angle. The average grain size increases up to the fluence of ions cm 2 and then decreases significantly for irradiation at the higher fluence. The value increases from 11 to 16 nm at a fluence of ions cm 2 and reduces to 8 nm at the fluence of ions cm 2. If the size and strain broadening is present simultaneously then the crystallite size and strain may be obtained from the Williamson Hall (W H) plot. Assuming both size and strain broadened profiles are of similar nature, Williamson and Hall used the following equation to determine crystallite size and strain (11) δ(2θ)cos θ 0.9λ + 4ε Sin θ. (2) D WH Figure 1. XRD patterns for the pristine and irradiated PbS quantum dots.

6 Radiation Effects & Defects in Solids 501 Figure 2. W H plots of the pristine and irradiated PbS quantum dots. Figure 3. UV Vis absorption spectra of PbS quantum dots irradiated with fluences (a) ions cm 2, (b) ions cm 2, (c) ions cm 2 and (d) pristine. Here, D WH is the average crystallite size, δ (2θ) is the FWHM measured in radians, θ is the Bragg angle of a reflection, λ is the wavelength of X-ray used and ε is the strain. All PbS reflections are used to construct a linear plot of δ (2θ) Cosθ versus Sinθ (Figure 2). The plot is a straight line with an intercept on the δ(2θ) Cosθ axis. The crystallite size is then obtained from this intercept and strain from the slope of the straight line. From the W H plot of the samples, it has been confirmed that the X-ray line broadening in nanocrystalline PbS is due to the presence of both size effect and strain effect. The average crystallite size of the samples is found to vary from 13 to 18 nm at a fluence of ions cm 2 and reduces to 10 nm at the fluence of ions cm 2. The crystallites are found to be under strain which varies between to Optical absorption studies Optical absorption spectroscopy has become an important tool to observe size quantization effect in terms of blue shift and excitonic absorption (12). The UV Vis absorption spectra of PbS quantum dots before and after irradiation are shown in Figure 3. The band gap of PbS quantum dots is calculated by using relation E gn = hc λ. (3) It can be seen that the band gap decreases from 2.50 ev for the pristine sample to 1.66 ev for the sample irradiated at a fluence of ions cm 2. It is possibly due to the combined effect of increase in grain size or agglomeration of quantum dots due to the interaction of heavy ion with

7 502 N. Choudhury et al. Table 1. Structural and optical properties of PbS quantum dots at different fluences. Fluence (ions/cm 2 ) D (nm) D WH (nm) Strain (ε 10 3 ) Band gap (ev) PL peak position (nm) Figure 4. PL spectra for the pristine and irradiated PbS quantum dots. nanocomposite polymer film. On an average scale, it is known that SHI on their way through solids (metals, semiconductors, superconductors, etc.) lead to heat deposition, followed by a maximum rise in electronic temperature ( 10 5 K) within a time scale of s and lattice temperature ( 10 3 K) within a time scale of s(6). Hence, under energetic ion irradiation, nanoparticles come closer due to the C C bond damage of the polymer and, finally, they melt into bigger cluster (13). Practically, it has been found that the higher the ion fluence, more is the heat generated, and hence bigger the particle size. The data are presented in Table 1. From this table it is observed that the band gap is much higher than that of the bulk value of PbS (0.41 ev) because of the quantum confinement effect in PbS nanocrystals PL studies PL study allows the investigation of energy levels in the semiconductor structures. In general, the emission spectra from quantum dots are characterized by broad emission peaks, arising from allowed transitions between electronic states defined by the quantum confinement. Figure 4 shows the PL spectra of PbS quantum dots before and after SHI irradiation. All the samples are excited with light of 350 nm in wavelength. The PL spectrum of PbS quantum dots (Figure 4) shows that the samples possess luminescence peak around 473 nm, corresponding to energy of 2.62 ev, which is greater than the bulk band gap energy. This result shows that the luminescence peak is largely blue-shifted from the bulk band gap of 0.41 ev, indicating strong quantum confinements. Though the UV Vis spectra indicate size enhancement of the particles after irradiation, no shift in PL peak position is noticed in the PL spectra. Such size-independent luminescence, in nanoparticle system, is reported earlier by Nanda et al. (14). The PL intensity increases significantly after irradiation. This is obvious as the defect concentration is expected to increase after irradiation (15). Accordingly, as the number of defects increase with ion fluence, PL intensity would also increase with fluence. The increase in PL intensity may be due to the combined effect of surface

8 Radiation Effects & Defects in Solids 503 state emission and emission from the large defects generated during SHI irradiation. The position of PL peaks before and after SHI irradiation has been recorded in Table Conclusion We have discussed the effect of SHI irradiation on PbS quantum dots. Some variations of average grain size with ion fluences are found. The samples exhibit an annealing of defects at lower fluences indicated by an increase in the peak intensity and the grain size. At higher fluences, as the energy deposited is very high, a partial amorphization is found to occur in the samples. The optical absorption band gap decreases with the lower fluences. The PL intensity significantly increases with ion fluence. This indicates increase in defect concentrations in the samples after irradiation. Acknowledgements The authors are thankful to IUAC, New Delhi, for providing the ion irradiation facility to carry out this work and Dr. P.K. Kulriya for experimental help during characterization. N.C. thanks the University Grants Commission, New Delhi, India, for awarding the FDP fellowship. References (1) Ozin, G.A. Adv. Mater. 1992, 4, (2) Dashevsky, Z.; Shusterman, S.; Dariel, M.P. J. Appl. Phys. 2002, 92, (3) Cardona, M.; Greenaway, D.L. Phys. Rev. A 1964, 133, (4) Kanjilal, D. Curr. Sci. 2001, 80, (5) Avasthi, D.K. Curr. Sci. 2000, 78, (6) Mehta, G.K. Nucl. Instrum. Methods A 1996, 382, (7) Bolse, W.; Schattat, B.; Feyh, A.; Renz, T. Nucl. Instrum. Methods B 2004, 218, (8) Mikou, M.; Carin, R.; Bogdanski, P.; Madelon, R. Nucl. Instrum. Methods B 1996, 107, (9) Ison, V.V.; Ranga, R.A.; Dutta, V.; Kulriya, P.K.; Avasthi, D.K.; Tripathi, S.K. J. Phys. D 2008, 41, (10) Wesch, W.; Kamarou, A.; Wendler, E. Nucl. Instrum. Methods B 2004, 225, (11) Williamson, G.K.; Hall, W.H. Acta Metall. 1953, 1, 22. (12) Brus, L.E. Nanostruct. Mater. 1992, 1, (13) Chowdhury, S.; Hussain, A.M.P.; Ahmed, G.A.; Singh, F.; Avasthi, D.K.; Choudhury, A. Mater. Res. Bull. 2008, 43, (14) Nanda, J.; Sarma, D.D. J. Appl. Phys. 2001, 90, (15) Schrawat, K.; Singh, F.; Singh, B.P.; Mehra, R.M. J. Luminesc. 2004, 106,