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Photoluminescence properties of monodispersed Mn2+ doped ZnS nanoparticles prepared in high temperature
Journal of Molecular Structure 991 (2011) 202–206
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Photoluminescence properties of monodispersed Mn2+ doped ZnS nanoparticles prepared in high temperature G. Murugadoss ⇑, B. Rajamannan, V. Ramasamy Department of Physics, Annamalai University, Annamalainagar 608 002, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 23 December 2010 Received in revised form 10 February 2011 Accepted 12 February 2011 Available online 17 February 2011 Keywords: Semiconductor materials TOPO Photoluminescence TEM Particle size distribution
a b s t r a c t Photoluminescence properties of trioctylphosphine oxide (TOPO) capped ZnS:Mn2+ nanoparticles were investigated. The particles were synthesized using chemical precipitation method at 160 °C. The nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), particles size analyzer (PSA), Fourier transform infra-red spectrometer (FT-IR), ultraviolet–visible (UV–Vis) and photoluminescence (PL). The PL results showed that the yellow-orange emission peak centered at 580 nm. XRD and TEM analysis results reveal the formation of cubic structure ZnS:Mn2+ particles with an average size of 3.5 nm. The TOPO capped ZnS:Mn2+ nanoparticles showed enhanced luminescence property compared with that of the uncapped particles. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanostructures have been of immense interest due to their potential applications in miniaturized circuits and devices. As an important II–VI group semiconductor, ZnS (Eg = 3.6 eV) [1] has been found diverse applications in flat-panel displays, electroluminescence devices, photonic crystal devices, lasers sensors and DNA detectors [2–5]. In addition, ZnS doped with various transition metal ions such as manganese is an efficient light-emitting applications [6], when such dopants are interest in the nanometer-sized ZnS matrix, they exhibit interesting magnetooptical properties [7,8]. A variety of synthetic methods exist for controlling the size, shape and composition of these semiconductor nanoparticles, allowing for the tailoring of properties specific to the desired application [9,10]. For nanoparticles prepared by solution-based chemical methods, a capping agent, which adsorbs to the particles surface, is generally added both to control the size of the nanoparticles and to prevent agglomeration of the synthesized particles. These adsorbates have been shown to alter the electronic structure of the nanocrystals [11,12]. Nevertheless, semiconductor nanoparticles of II–VI are themselves highly unstable, and in the absence of a trapping media or some other form of encapsulation, they agglomerate or coalesce extreme quickly [13]. For this reason using capping agents to nanoparticles is necessary to provide surface passivation, and also to improve the surface states which significantly influence the opto⇑ Corresponding author. Tel.: +91 9894424828. E-mail address: [email protected] (G. Murugadoss). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.02.026
electronics properties of nanoparticles [14,15]. In general, the agglomeration can be arrested by the following two factors: first one is stabilizing electrostatically and another one is inducing steric hindrances. Consider the former stabilizing factor such as electrostatic stabilization; it involves the creation of an electrical double layer arising from ions adsorbed on the surface and associated counter ions that surround the particle in the dispersing media. Thus, if the electric potential associated with the double layer is sufficiently high, columbic repulsion between the particles will prevent their agglomeration. Consider another factor such as steric hindrance; which can be achieved by the adsorption of large molecule such as polymers on the surface of the particles. When a polymer is adsorbed on a surface, it generally does not lie flat. Rather, some parts of the polymer are adsorbed on the surface, while other portions of the chains extend away from the surface into the medium. When two such polymer layers overlap at a collision distance between particles, repulsion occurs—the result of increasing concentration of polymer molecules in the gap, giving rise to an osmotic pressure, as the two polymer layers overlap. Thus, for a polymer to provide a strong repulsion between two approaching surfaces, a small portion of the molecule must be tightly adsorbed to the underlying nanoparticles surface while most of the polymer chain should extend away from the surface, since the growth particles are isolated [16]. Trioctylphosphine oxide (TOPO) molecules provide good surface coverage and passivation of II–VI semiconductor nanoparticles and afford the nanoparticles high PL yield and stability. Many researchers were used the TOPO to stabilize nanoparticles [17–19]. Here it is reported the preparation of trioctylphosphine
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oxide (TOPO)-capped ZnS:Mn2+ nanoparticles with Na2S, Zn(CH3COO)2 as the sulfur and zinc sources through chemical precipitation method. The water soluble TOPO molecule was used to stabilize the nanocrystals and prevents agglomeration. Through the surface passivation by TOPO, monodisperse and small grain size particles were prepared. It exhibited chemical stability, as the addition of TOPO can greatly improve the efficiency and reproducibility of the composite nanoparticles. An enhanced luminescence is obtained with respect to concentration of TOPO is discussed. The objective of the present study is to synthesize non toxic inorganic luminescent nanoparticles of ZnS doped with transition metal Mn and create bioorganic interface with bio-compatible inorganic/organic material for possible use as nanoscale fluorescent probes for potential pharmaceutical, biological and medical applications such as targeted drug delivery, ultra-sensitive disease detection and labeling of biological cells. 2. Experimental details Fig. 1. XRD patterns of uncapped and capped (TOPO) ZnS:Mn2+ nanoparticles. 2+
ZnS nanoparticles doped with Mn ions were prepared by the chemical precipitation method. ZnSO47H2O, MnCl24H2O, Na2S and trioctylposphine oxide (TOPO) all are analytical grade. Ultra pure de-ionized water was used as solvent. In a typical procedure, aqueous stock solutions of 50 ml of 1 M ZnSO47H2O, and a 0.5 g of surfactant (TOPO) were dissolved in 50 ml of de-ionized water under vigorously stirring, then 0.3 g (4%) of 25 ml MnCl24H2O was added to the above solution. Finally, 50 ml of 0.5 M Na2S solution was introduced into the above solution under continuous stirring. During the whole reaction process, the reactants were vigorously stirred at 160 °C for 1 h. Then cooled to room temperature, the solution with white flocculent precipitate was appeared. The precipitate was cleaned repeatedly with de-ionized water and ethanol. Finally, the powder is dried at 120 °C under vacuum for two hour. The same procedure was followed for all the other capping concentrations (1.0, 1.5 and 2.0 g). Moreover, the volume of the de-ionized water (50 ml) is same for all the TOPO concentrations. The reactions take place as followed:
nðZnSOÞ4 7H2 OÞ þ TOPO þ nðMnCl2 4H2 OÞ ! TOPO-ðZn2þ þ Mn2þ Þn TOPO-ðZn2þ þ Mn2þ Þn þ nðNa2 SÞ ! TOPO-ðZnS : Mn2þ Þn XRD patterns of the ZnS:Mn2+ samples were recorded using 0 XPERT-PRO diffractometer with a Cu Ka radiation (k = 1.54060 Å A). The particles size was estimated using the Scherer equation (kk)/ (b cos h) at the full-width at half-maximum of the XRD peaks. The size and morphology of the particles were determined using TEM (Technai 20G2, FEI) and particles size analyzer (Nanotrac Specifications Model: Nanotrac NPA 150). The FT-IR spectra were obtained on an AVATOR 360 spectrometer. The optical transmission/absorption spectra of the same particles in de-ionized water were recorded using UV-1650PC SHIMADZU spectrometer. Fluorescence measurements were performed on a RF-5301PC spectrophotometer. Emission (350–700 nm) spectra were recorded under 320 and 294 nm for uncapped and TOPO capped ZnS:Mn2+ nanoparticles.
and (3 1 1) planes of cubic ZnS (JCPDS No. 5-566). The peak broadening in the XRD patterns clearly indicated the nature of the very small nanoparticles. The appearance of diffraction peaks demonstrates that particles are not amorphous. From the width of the XRD peak, the mean particles size can be calculated using Scherrer’s equation: D = kk/b cos h, where k is particle shape factor (taken as 0.9), k is the X-ray wavelength used (0.1542 nm), b is the full-width at half-maximum (FWHM), h is the Bragg angle in degrees (half of the peak position angle). According to the calculation, the grain size of uncapped and capped ZnS:Mn2+ nanoparticles is 5 and 3.5 nm, respectively. No other peaks were observed by adding surfactant. It indicates the cubic phase is no disturbed by adding surfactant; it is only physical adsorbed (Van der Waals force) on the surface of particles. It is necessary to obtain the particle size and the information about the nanostructure by direct measurement, such as transmission electron microscope (TEM), which can reveal the size and morphology of the particles. Fig. 2 shows the TEM images of TOPO capped ZnS:Mn2+ nanoparticles with the corresponding diffraction pattern. Presence of fine ZnS:Mn2+ nanoparticles are clearly visible in the TEM picture (Fig. 2a and b). As shown in the figures, the obtained particles are monodispersed with less aggregation. From the TEM images, the measured particles size is in the range 3–4 nm. The diffraction pattern (Fig. 2c) of the sample consists of a central halo with concentric broad rings. The rings correspond to the reflections from (1 1 1), (2 2 0) and (3 1 1) planes confirming the cubic structure of the bulk ZnS. The SAED pattern consists of broad diffuse rings due to small ZnS:Mn2+ particle. The discrete bright spots in selected area electron diffraction pattern reveal well-crystallized cubic from in the ZnS:Mn2+ nanoparticles. The average size of the nanoparticles were determined from TEM is around 3.5 nm. The TEM images of the particles clearly show that the particles are uniform in sizes. Fig. 3 shows that the particle size analyzer (PSA) curve of TOPO capped ZnS:Mn2+ nanoparticles were narrow and the mean particle size is about 3.5 nm. This is in good agreement with the XRD and TEM results. Furthermore, the PSA curve indicates the synthesized particles were monodispersed.
3. Results and discussion 3.1. Structural and morphology study
3.2. FT-IR study
The structure of the obtained uncapped and TOPO capped ZnS:Mn2+ nanoparticles were characterized by X-ray powder diffraction and are shown in Fig. 1. The diffraction peaks at 2h values of 28.6, 47.6, and 56.48 matching perfectly with the (1 1 1), (2 2 0)
FT-IR spectra of the uncapped and capped with TOPO ZnS:Mn2+ nanoparticles were recorded in the range 4000–400 cm1 and are shown in Fig. 4. The peaks appearing at 1110, 618 and 491 cm1 are due to ZnAS vibration, 991 and 668 cm1 are due to MnAS
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Fig. 2. (a and b) TEM micrographs of TOPO capped ZnS:Mn2+ nanoparticles. (c) Shows the SAED pattern of TOPO capped ZnS:Mn2+ nanoparticles.
vibration and 2924, 2364 and 1635 cm1 are due to microstructure formation of the sample. The obtained peak values are in good agreement with the reported values [20,21]. However, the presence of strong IR peak at 1490 cm1 with a shoulder at 1467 cm1 corresponding to P@O stretching vibrational modes indicates the signatures of capping agent, i.e., TOPO bounded to ZnS:Mn2+ nanoparticles. The broad absorption peaks in the range of 3410–3465 cm1 corresponds to AOH group indicates the existence of water absorbed in the surface of nanoparticles. The presence of this band can be clearly attributed to the adsorption of same atmospheric water during FT-IR measurements. The bands at 1500–1650 and at 2370 cm1 are due to the C@O stretching mode arising from the absorption of atmospheric CO2 on the surface of the nanoparticles [22].
3.3. UV–visible study The UV–visible absorbance spectra of freshly-prepared uncapped and TOPO capped ZnS:Mn2+ nanoparticles are shown in Fig. 5. As observed in the figure, the sample exhibits a strong excitonic peak at 294 nm for capped and 320 nm for uncapped ZnS:Mn2+ nanoparticles. These two excitonic peaks are fairly blue shifted from bulk (345 nm) ZnS:Mn2+ [7]. This blue shift appears to be caused by the small size of the particles. The excitonic peaks show, capped (TOPO) ZnS:Mn2+ nanoparticles is 26 nm blue shifted compared the uncapped particles. It is due the quantum confinement effect of the small size. This absorption spectrum has been used to calculate an approximate size for these nanoparticles using the Brus equation [23]. Based on the peak position in the absorption spectra, the size of these nanoparticles have been calculated to be 5 nm for uncapped and 3.6 nm for capped ZnS:Mn2+.
Fig. 3. Particles size distribution of TOPO-capped (1 g) ZnS:Mn2+ nanoparticles.
These particles size are comparable to the Bohr exciton radius of the bulk ZnS i.e., 2.5 nm [22] (when the particles size is smaller than Bohr exciton radius, a large percentage of the atoms are on the particles surface, thus modify the optical properties of the particles). In addition, this result is same as that obtained from XRD, TEM and PSA result as described above.
3.4. Photoluminescence study Fig. 6a shows the PL emission spectra of uncapped and TOPO capped ZnS:Mn2+ nanoparticles. The concentrations of the TOPO on ZnS:Mn2+ nanoparticles were varied from 0.5 to 2 g. There are two broad emission bands; the orange band centered at 580 nm arises from the 4T1–6A1 transition of Mn2+ ions [24], indicating that the Mn2+ ions have been successfully incorporated into the ZnS host lattice [25] and the blue band centered at 445 nm originate
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Fig. 4. FT-IR spectra of uncapped and TOPO capped ZnS:Mn2+ nanoparticles.
Fig. 5. Absorption spectra of uncapped and capped ZnS:Mn2+ nanoparticles.
from the self-activated emission of Zn vacancies [26,27]. Comparison of the orange peak position of capped nanoparticles; it is noticeably enhanced than the uncapped particles. The PL intensity of capped ZnS:Mn2+ is two times than that of the uncapped ZnS:Mn2+ nanoparticles. Generally, in the uncapped nanoparticles, some energy will transfer from excited ZnS host lattice to surface defects, which decreases the energy transferred to Mn2+ ions. On the other hand, for uncapped ZnS:Mn2+ nanoparticles, some Mn2+ ions distribute on the surface and form some non-radiative recombination routes from surface Mn2+ ions to surface quenching centers, which will bring reduction in the PL intensity. The position of the emission band has no change by the addition of surfactant. But, the efficiency (ratio of the intensity between the yellow-orange to blue
Fig. 6. Shows the PL emission spectra of uncapped and TOPO capped ZnS:Mn2+ nanoparticles (a), concentration of TOPO versus intensity (b) and concentration of TOPO versus efficiency.
emission) of capped nanoparticles is increased obviously. Fig. 6b shows concentrations of TOPO versus intensity. The emission intensity was increased while increasing of the capping concentra-
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tion from 0.5 to 1 g. After that, the PL intensity was decreased by the increasing of the concentration from 1 to 2 g. The maximum intensity at 1 g of the TOPO indicates, increasing of the homogeneous particle growth. Fig. 6c shows concentration of TOPO versus efficiency (the ratio of the intensity between yellow-orange to blue emission). A maximum PL efficiency was obtained at 1.5 g of TOPO concentration used as a surfactant. The maximum efficiency shows, reduction of the surface defect by the higher concentration of TOPO.
4. Conclusions Mn-doped ZnS nanoparticles were synthesized through chemical precipitation method using TOPO as a surfactant in high temperature at 160 °C. Particle size was reduced less then exciton Bohr radius using the surfactant. The XRD patterns exhibited a cubic structure of the both (uncapped and capped) samples. The TOPO capped ZnS:Mn2+ nanoparticles show better dispersion as they were adsorbed on the surface of nanoparticles so as to fulfill the steric hindrance between nanoparticles and prevent agglomeration. Two PL emission bands were observed for ZnS:Mn2+ nanoparticles and attributed to the ZnS emission and the Mn2+ emission, respectively. It is also found that the TOPO capped (1.5 g) sample exhibits, the PL efficiency is four times than the uncapped sample. The peak position of the Mn2+ emission was appeared at 580 nm with enhancement intensity. The IR peak at 1490 cm1 with a shoulder at 1467 cm1 corresponding to P@O stretching vibrational modes are identified the signatures of capping agent on ZnS:Mn2+ nanoparticles. The water soluble nanoparticles show an improved PL property, indicating their potential application in sensors and biological labeling. Acknowledgements The authors would like to thank Dr. N. Rajendran, Department of chemistry, Annamalai University for providing PL facility. The authors also like to acknowledge the service rendered by scientific
officers CECRI, Karaikudi for providing XRD, Particles size analysis and TEM facility. References [1] S.H. Yu, M. Yoshimura, Adv. Mater. 14 (2002) 296–300. [2] Z.G. Chen, J. Zou, D.W. Wang, L.C. Yin, G. Liu, Q. Liu, C.H. Sun, X. Yao, F. Li, X.L. Yuan, T. Sekiguchi, G.Q. Lu, H.M. Cheng, Adv. Funct. Mater. 19 (2009) 484–490. [3] L. Yu, H. Matsui, J. Am. Chem. Soc. 127 (2005) 16002–16003. [4] Y. Li, J. Chen, C. Zhu, L. Wang, D. Zhao, S. Zhuo, Y. Wu, Spectrochim. Acta A 60 (2004) 1719–1724. [5] X. Fang, Y. Bando, M. Liao, U.K. Gautam, C. Zhi, B. Dierre, B. Liu, T. Zhai, T. Sekiguchi, Y. Koide, D. Golberg, Adv. Mater. 21 (2009) 2034–2039. [6] M. Chring, The Material Science of Thin Film, Academic Press, San Diego, CA, 1992. [7] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416–419. [8] T.A. Kennedy, E.R. Glaser, P.B. Klien, R.N. Bhargava, Phys. Rev. B 52 (1995) 14356–14359. [9] C.B. Murray, C.R. Kagan, M.G. Bawendi, Annu. Rev. Mater. Sci. 30 (2000) 545– 610. [10] Y. Jun, J.H. Lee, J. Choi, J. Cheon, J. Phys. Chem. B 109 (2005) 14795–14806. [11] F. Seker, K. Meeker, T.F. Kuech, A.B. Ellis, Chem. Rev. 100 (2000) 2505–2536. [12] M.A. El-Sayed, Acc. Chem. Res. 37 (2004) 326–333. [13] J.G. Winiarz, L. Zhang, M. Lal, C.S. Friend, P.N. Prasad, Chem. Phys. 245 (1999) 417–428. [14] L. Qi, H. Colfen, M. Antonietti, Nano. Lett. 1 (2001) 61–65. [15] Y.J. Lee, T.G. Kim, Y.M. Sung, Nanotechnology 17 (2006) 3539–3542. [16] J. Dutta, H. Hofmann, Self-organization of colloidal nanoparticles in: H. S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, American Scientific Publishers, California, 2004, pp. 617. [17] M. Azad Malik, Paul O’Brien, N. Revaprasadu, J. Mater. Chem. 11 (2001) 2382– 2386. [18] T. Trindade, P. O’Brien, Chem. Mater. 9 (1997) 523. [19] B. Ludolph, M.A. Malik, P. O’Brien, N. Revaprasadu, J. Chem. Soc., Chem. Commun. 1998 (1998) 1849. [20] B.S. Rema Devi, R. Raveendran, A.V. Vaidyan, Indian Acad. Sci. 68 (2007) 679– 687. [21] Y. Tong, Z. Jiang, C. Wang, Z. Yi Xin, S. Hong, Liu, C. Li, Mater. Lett. 62 (2008) 3385–3387. [22] S.B. Qadri, E.F. Skelton, D. Hsu, A.D. Dinsmore, J. Yang, H.F. Gray, B.R. Ratna, Phys. Rev. B 60 (1999) 9191–9193. [23] L.E. Brus, J. Phys. Chem. 90 (1986) 2555–2560. [24] B. Xia, I.W. Lenggoro, K. Okuyama, Chem. Mater. 14 (2002) 4969–4974. [25] H. Hu, W.H. Zhang, Opt. Mater. 28 (2006) 536–550. [26] W.G. Becker, A.J. Bard, J. Phys. Chem. 87 (1983) 4888–4893. [27] A.A. Khosrari, M. Kundu, L. Jatwa, S.K. Deshpande, U.A. Bhagwat, Murali Sastry, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2702–2704.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Photoluminescence properties of monodispersed Mn2+ doped ZnS nanoparticles prepared in high temperature G. Murugadoss ⇑, B. Rajamannan, V. Ramasamy Department of Physics, Annamalai University, Annamalainagar 608 002, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 23 December 2010 Received in revised form 10 February 2011 Accepted 12 February 2011 Available online 17 February 2011 Keywords: Semiconductor materials TOPO Photoluminescence TEM Particle size distribution
a b s t r a c t Photoluminescence properties of trioctylphosphine oxide (TOPO) capped ZnS:Mn2+ nanoparticles were investigated. The particles were synthesized using chemical precipitation method at 160 °C. The nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), particles size analyzer (PSA), Fourier transform infra-red spectrometer (FT-IR), ultraviolet–visible (UV–Vis) and photoluminescence (PL). The PL results showed that the yellow-orange emission peak centered at 580 nm. XRD and TEM analysis results reveal the formation of cubic structure ZnS:Mn2+ particles with an average size of 3.5 nm. The TOPO capped ZnS:Mn2+ nanoparticles showed enhanced luminescence property compared with that of the uncapped particles. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanostructures have been of immense interest due to their potential applications in miniaturized circuits and devices. As an important II–VI group semiconductor, ZnS (Eg = 3.6 eV) [1] has been found diverse applications in flat-panel displays, electroluminescence devices, photonic crystal devices, lasers sensors and DNA detectors [2–5]. In addition, ZnS doped with various transition metal ions such as manganese is an efficient light-emitting applications [6], when such dopants are interest in the nanometer-sized ZnS matrix, they exhibit interesting magnetooptical properties [7,8]. A variety of synthetic methods exist for controlling the size, shape and composition of these semiconductor nanoparticles, allowing for the tailoring of properties specific to the desired application [9,10]. For nanoparticles prepared by solution-based chemical methods, a capping agent, which adsorbs to the particles surface, is generally added both to control the size of the nanoparticles and to prevent agglomeration of the synthesized particles. These adsorbates have been shown to alter the electronic structure of the nanocrystals [11,12]. Nevertheless, semiconductor nanoparticles of II–VI are themselves highly unstable, and in the absence of a trapping media or some other form of encapsulation, they agglomerate or coalesce extreme quickly [13]. For this reason using capping agents to nanoparticles is necessary to provide surface passivation, and also to improve the surface states which significantly influence the opto⇑ Corresponding author. Tel.: +91 9894424828. E-mail address: [email protected] (G. Murugadoss). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.02.026
electronics properties of nanoparticles [14,15]. In general, the agglomeration can be arrested by the following two factors: first one is stabilizing electrostatically and another one is inducing steric hindrances. Consider the former stabilizing factor such as electrostatic stabilization; it involves the creation of an electrical double layer arising from ions adsorbed on the surface and associated counter ions that surround the particle in the dispersing media. Thus, if the electric potential associated with the double layer is sufficiently high, columbic repulsion between the particles will prevent their agglomeration. Consider another factor such as steric hindrance; which can be achieved by the adsorption of large molecule such as polymers on the surface of the particles. When a polymer is adsorbed on a surface, it generally does not lie flat. Rather, some parts of the polymer are adsorbed on the surface, while other portions of the chains extend away from the surface into the medium. When two such polymer layers overlap at a collision distance between particles, repulsion occurs—the result of increasing concentration of polymer molecules in the gap, giving rise to an osmotic pressure, as the two polymer layers overlap. Thus, for a polymer to provide a strong repulsion between two approaching surfaces, a small portion of the molecule must be tightly adsorbed to the underlying nanoparticles surface while most of the polymer chain should extend away from the surface, since the growth particles are isolated [16]. Trioctylphosphine oxide (TOPO) molecules provide good surface coverage and passivation of II–VI semiconductor nanoparticles and afford the nanoparticles high PL yield and stability. Many researchers were used the TOPO to stabilize nanoparticles [17–19]. Here it is reported the preparation of trioctylphosphine
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oxide (TOPO)-capped ZnS:Mn2+ nanoparticles with Na2S, Zn(CH3COO)2 as the sulfur and zinc sources through chemical precipitation method. The water soluble TOPO molecule was used to stabilize the nanocrystals and prevents agglomeration. Through the surface passivation by TOPO, monodisperse and small grain size particles were prepared. It exhibited chemical stability, as the addition of TOPO can greatly improve the efficiency and reproducibility of the composite nanoparticles. An enhanced luminescence is obtained with respect to concentration of TOPO is discussed. The objective of the present study is to synthesize non toxic inorganic luminescent nanoparticles of ZnS doped with transition metal Mn and create bioorganic interface with bio-compatible inorganic/organic material for possible use as nanoscale fluorescent probes for potential pharmaceutical, biological and medical applications such as targeted drug delivery, ultra-sensitive disease detection and labeling of biological cells. 2. Experimental details Fig. 1. XRD patterns of uncapped and capped (TOPO) ZnS:Mn2+ nanoparticles. 2+
ZnS nanoparticles doped with Mn ions were prepared by the chemical precipitation method. ZnSO47H2O, MnCl24H2O, Na2S and trioctylposphine oxide (TOPO) all are analytical grade. Ultra pure de-ionized water was used as solvent. In a typical procedure, aqueous stock solutions of 50 ml of 1 M ZnSO47H2O, and a 0.5 g of surfactant (TOPO) were dissolved in 50 ml of de-ionized water under vigorously stirring, then 0.3 g (4%) of 25 ml MnCl24H2O was added to the above solution. Finally, 50 ml of 0.5 M Na2S solution was introduced into the above solution under continuous stirring. During the whole reaction process, the reactants were vigorously stirred at 160 °C for 1 h. Then cooled to room temperature, the solution with white flocculent precipitate was appeared. The precipitate was cleaned repeatedly with de-ionized water and ethanol. Finally, the powder is dried at 120 °C under vacuum for two hour. The same procedure was followed for all the other capping concentrations (1.0, 1.5 and 2.0 g). Moreover, the volume of the de-ionized water (50 ml) is same for all the TOPO concentrations. The reactions take place as followed:
nðZnSOÞ4 7H2 OÞ þ TOPO þ nðMnCl2 4H2 OÞ ! TOPO-ðZn2þ þ Mn2þ Þn TOPO-ðZn2þ þ Mn2þ Þn þ nðNa2 SÞ ! TOPO-ðZnS : Mn2þ Þn XRD patterns of the ZnS:Mn2+ samples were recorded using 0 XPERT-PRO diffractometer with a Cu Ka radiation (k = 1.54060 Å A). The particles size was estimated using the Scherer equation (kk)/ (b cos h) at the full-width at half-maximum of the XRD peaks. The size and morphology of the particles were determined using TEM (Technai 20G2, FEI) and particles size analyzer (Nanotrac Specifications Model: Nanotrac NPA 150). The FT-IR spectra were obtained on an AVATOR 360 spectrometer. The optical transmission/absorption spectra of the same particles in de-ionized water were recorded using UV-1650PC SHIMADZU spectrometer. Fluorescence measurements were performed on a RF-5301PC spectrophotometer. Emission (350–700 nm) spectra were recorded under 320 and 294 nm for uncapped and TOPO capped ZnS:Mn2+ nanoparticles.
and (3 1 1) planes of cubic ZnS (JCPDS No. 5-566). The peak broadening in the XRD patterns clearly indicated the nature of the very small nanoparticles. The appearance of diffraction peaks demonstrates that particles are not amorphous. From the width of the XRD peak, the mean particles size can be calculated using Scherrer’s equation: D = kk/b cos h, where k is particle shape factor (taken as 0.9), k is the X-ray wavelength used (0.1542 nm), b is the full-width at half-maximum (FWHM), h is the Bragg angle in degrees (half of the peak position angle). According to the calculation, the grain size of uncapped and capped ZnS:Mn2+ nanoparticles is 5 and 3.5 nm, respectively. No other peaks were observed by adding surfactant. It indicates the cubic phase is no disturbed by adding surfactant; it is only physical adsorbed (Van der Waals force) on the surface of particles. It is necessary to obtain the particle size and the information about the nanostructure by direct measurement, such as transmission electron microscope (TEM), which can reveal the size and morphology of the particles. Fig. 2 shows the TEM images of TOPO capped ZnS:Mn2+ nanoparticles with the corresponding diffraction pattern. Presence of fine ZnS:Mn2+ nanoparticles are clearly visible in the TEM picture (Fig. 2a and b). As shown in the figures, the obtained particles are monodispersed with less aggregation. From the TEM images, the measured particles size is in the range 3–4 nm. The diffraction pattern (Fig. 2c) of the sample consists of a central halo with concentric broad rings. The rings correspond to the reflections from (1 1 1), (2 2 0) and (3 1 1) planes confirming the cubic structure of the bulk ZnS. The SAED pattern consists of broad diffuse rings due to small ZnS:Mn2+ particle. The discrete bright spots in selected area electron diffraction pattern reveal well-crystallized cubic from in the ZnS:Mn2+ nanoparticles. The average size of the nanoparticles were determined from TEM is around 3.5 nm. The TEM images of the particles clearly show that the particles are uniform in sizes. Fig. 3 shows that the particle size analyzer (PSA) curve of TOPO capped ZnS:Mn2+ nanoparticles were narrow and the mean particle size is about 3.5 nm. This is in good agreement with the XRD and TEM results. Furthermore, the PSA curve indicates the synthesized particles were monodispersed.
3. Results and discussion 3.1. Structural and morphology study
3.2. FT-IR study
The structure of the obtained uncapped and TOPO capped ZnS:Mn2+ nanoparticles were characterized by X-ray powder diffraction and are shown in Fig. 1. The diffraction peaks at 2h values of 28.6, 47.6, and 56.48 matching perfectly with the (1 1 1), (2 2 0)
FT-IR spectra of the uncapped and capped with TOPO ZnS:Mn2+ nanoparticles were recorded in the range 4000–400 cm1 and are shown in Fig. 4. The peaks appearing at 1110, 618 and 491 cm1 are due to ZnAS vibration, 991 and 668 cm1 are due to MnAS
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Fig. 2. (a and b) TEM micrographs of TOPO capped ZnS:Mn2+ nanoparticles. (c) Shows the SAED pattern of TOPO capped ZnS:Mn2+ nanoparticles.
vibration and 2924, 2364 and 1635 cm1 are due to microstructure formation of the sample. The obtained peak values are in good agreement with the reported values [20,21]. However, the presence of strong IR peak at 1490 cm1 with a shoulder at 1467 cm1 corresponding to P@O stretching vibrational modes indicates the signatures of capping agent, i.e., TOPO bounded to ZnS:Mn2+ nanoparticles. The broad absorption peaks in the range of 3410–3465 cm1 corresponds to AOH group indicates the existence of water absorbed in the surface of nanoparticles. The presence of this band can be clearly attributed to the adsorption of same atmospheric water during FT-IR measurements. The bands at 1500–1650 and at 2370 cm1 are due to the C@O stretching mode arising from the absorption of atmospheric CO2 on the surface of the nanoparticles [22].
3.3. UV–visible study The UV–visible absorbance spectra of freshly-prepared uncapped and TOPO capped ZnS:Mn2+ nanoparticles are shown in Fig. 5. As observed in the figure, the sample exhibits a strong excitonic peak at 294 nm for capped and 320 nm for uncapped ZnS:Mn2+ nanoparticles. These two excitonic peaks are fairly blue shifted from bulk (345 nm) ZnS:Mn2+ [7]. This blue shift appears to be caused by the small size of the particles. The excitonic peaks show, capped (TOPO) ZnS:Mn2+ nanoparticles is 26 nm blue shifted compared the uncapped particles. It is due the quantum confinement effect of the small size. This absorption spectrum has been used to calculate an approximate size for these nanoparticles using the Brus equation [23]. Based on the peak position in the absorption spectra, the size of these nanoparticles have been calculated to be 5 nm for uncapped and 3.6 nm for capped ZnS:Mn2+.
Fig. 3. Particles size distribution of TOPO-capped (1 g) ZnS:Mn2+ nanoparticles.
These particles size are comparable to the Bohr exciton radius of the bulk ZnS i.e., 2.5 nm [22] (when the particles size is smaller than Bohr exciton radius, a large percentage of the atoms are on the particles surface, thus modify the optical properties of the particles). In addition, this result is same as that obtained from XRD, TEM and PSA result as described above.
3.4. Photoluminescence study Fig. 6a shows the PL emission spectra of uncapped and TOPO capped ZnS:Mn2+ nanoparticles. The concentrations of the TOPO on ZnS:Mn2+ nanoparticles were varied from 0.5 to 2 g. There are two broad emission bands; the orange band centered at 580 nm arises from the 4T1–6A1 transition of Mn2+ ions [24], indicating that the Mn2+ ions have been successfully incorporated into the ZnS host lattice [25] and the blue band centered at 445 nm originate
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Fig. 4. FT-IR spectra of uncapped and TOPO capped ZnS:Mn2+ nanoparticles.
Fig. 5. Absorption spectra of uncapped and capped ZnS:Mn2+ nanoparticles.
from the self-activated emission of Zn vacancies [26,27]. Comparison of the orange peak position of capped nanoparticles; it is noticeably enhanced than the uncapped particles. The PL intensity of capped ZnS:Mn2+ is two times than that of the uncapped ZnS:Mn2+ nanoparticles. Generally, in the uncapped nanoparticles, some energy will transfer from excited ZnS host lattice to surface defects, which decreases the energy transferred to Mn2+ ions. On the other hand, for uncapped ZnS:Mn2+ nanoparticles, some Mn2+ ions distribute on the surface and form some non-radiative recombination routes from surface Mn2+ ions to surface quenching centers, which will bring reduction in the PL intensity. The position of the emission band has no change by the addition of surfactant. But, the efficiency (ratio of the intensity between the yellow-orange to blue
Fig. 6. Shows the PL emission spectra of uncapped and TOPO capped ZnS:Mn2+ nanoparticles (a), concentration of TOPO versus intensity (b) and concentration of TOPO versus efficiency.
emission) of capped nanoparticles is increased obviously. Fig. 6b shows concentrations of TOPO versus intensity. The emission intensity was increased while increasing of the capping concentra-
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tion from 0.5 to 1 g. After that, the PL intensity was decreased by the increasing of the concentration from 1 to 2 g. The maximum intensity at 1 g of the TOPO indicates, increasing of the homogeneous particle growth. Fig. 6c shows concentration of TOPO versus efficiency (the ratio of the intensity between yellow-orange to blue emission). A maximum PL efficiency was obtained at 1.5 g of TOPO concentration used as a surfactant. The maximum efficiency shows, reduction of the surface defect by the higher concentration of TOPO.
4. Conclusions Mn-doped ZnS nanoparticles were synthesized through chemical precipitation method using TOPO as a surfactant in high temperature at 160 °C. Particle size was reduced less then exciton Bohr radius using the surfactant. The XRD patterns exhibited a cubic structure of the both (uncapped and capped) samples. The TOPO capped ZnS:Mn2+ nanoparticles show better dispersion as they were adsorbed on the surface of nanoparticles so as to fulfill the steric hindrance between nanoparticles and prevent agglomeration. Two PL emission bands were observed for ZnS:Mn2+ nanoparticles and attributed to the ZnS emission and the Mn2+ emission, respectively. It is also found that the TOPO capped (1.5 g) sample exhibits, the PL efficiency is four times than the uncapped sample. The peak position of the Mn2+ emission was appeared at 580 nm with enhancement intensity. The IR peak at 1490 cm1 with a shoulder at 1467 cm1 corresponding to P@O stretching vibrational modes are identified the signatures of capping agent on ZnS:Mn2+ nanoparticles. The water soluble nanoparticles show an improved PL property, indicating their potential application in sensors and biological labeling. Acknowledgements The authors would like to thank Dr. N. Rajendran, Department of chemistry, Annamalai University for providing PL facility. The authors also like to acknowledge the service rendered by scientific
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