- Email: [email protected]
Optical measurements of ZnS nanoparticles aqueous solution
Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 1792–1795
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
Optical measurements of ZnS nanoparticles aqueous solution M. Moussaoui a,n, R. Saoudi a, I.V. Lesnichiy b, A.V. Tishchenko a a b
Universite´ de Lyon, Universite´ Jean Monnet, Laboratoire Hubert Curien CNRS UMR5516, 18 rue du Professeur Benoˆıt Lauras, 42000 Saint-Etienne, France Moscow Institute of Physics and Technology, Institutsky Str. 9, 141701 Dolgoprudny, Russia
a r t i c l e in f o
abstract
Available online 20 February 2011
We synthesized zinc sulfide (ZnS) nanopowders with size ranging from 2 to 100 nm by a simple, low-cost, and mass production chemical method. The nanoparticles (NPs) were characterized by X-ray powder diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and UV–vis absorption spectroscopy. Our study concerns also the change in the refractive index of deionized water in presence of ZnS nanospheres. We present experimental results on effective index variation of water dispersed ZnS NPs at different wavelengths in visible spectrum. & 2011 Elsevier Ltd. All rights reserved.
Keywords: ZnS nanoparticles Nanopowder water suspension Refractive index measurement
1. Introduction Optical properties of semiconductor nanoparticles (NPs) are of fundamental and practical interest and many researches have been performed on them [1,2]. Their studies are mainly concerned with the quantum size effect. When the size of semiconductor particles is reduced to nanometer scale their physical properties differ noticeably from those of the corresponding bulk material and depend on the size and the morphology of the studied structures [3,4]. The extremely small size of the NPs results in quantum confinement of the photogenerated electron–hole pair. When the radius of the particle approaches the Bohr radius of the exciton, the quantum size effect becomes apparent: the energy gap increases with decreasing the grain size, which leads to a blueshift of the optical absorption edge with respect to the bulk material [5]. In order to exploit such sizetuneable properties, many works have been devoted to the development of simple methods for synthesizing semiconductor particles of various sizes in a controllable manner [6]. Optical properties such as optical absorption and refractive index changes have great potential for
device applications in photodetectors, optical modulators, and semiconductor optical amplifiers [7,8]. Among the II–VI materials, zinc sulfide (ZnS) is a nontoxic semiconductor with an important direct wide band gap (Eg =3.6 eV at 300 K), and relatively high refractive index [9]. It is a suitable material for numerous applications such as ultraviolet-light-emitting diodes, electroluminescent devices, flat-panel displays, sensors and injection lasers [10]. In this work, ZnS nanoparticles with size ranging from 2 to 100 nm were prepared by chemical process [11]. The synthesis of ZnS NPs was based on the reaction of zinc acetate and thioacetamide as starting materials. The resulting mixture was treated thermally at different temperatures. The resulting product was collected by centrifuging to select the size distribution. The NPs were characterized by X-ray powder diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and UV–vis absorption spectroscopy. We also investigate the refractive index of ZnS NPs dispersed in deionized water. The critical angle measurement [12] is employed because of its convenience and high precision. 2. Material and methods
n Corresponding author. Tel.: + 330477915824; fax: + 330477915781. E-mail address: [email protected] (M. Moussaoui).
0022-4073/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2011.02.006
All chemicals used in this work were obtained from Sigma Aldrich company commercial sources with no additional purification.
M. Moussaoui et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 1792–1795
XRD patterns were acquired on a Bruker D8 Advance ˚ instrument using Cu Ka radiation (l = 1.5418 A). The nanocrystallite powder was pressed inside the sample holder. The TEM images are taken with a TOPCON EM002B transmission electron microscope operating at 200 kV. Samples for the TEM measurement are supported on a carbon-coated copper grid. ZnS nanopowders were also examined by atomic force microscopy with an Agilent 5500 AFM system in tapping mode. The absorption spectra in the UV and visible ranges were recorded with a lambda 900 Perkin-Elmer spectrophotometer in between wavelength range 300–600 nm.
following relation: ðahnÞ2 ¼ CðhnEg Þ
Fig. 2. Typical AFM image of the prepared ZnS NPs.
Fig. 3. TEM image of the prepared ZnS NPs.
900 800
(111)
700
Intensity (a.u.)
600
(220)
500
(311)
400 300 200
(200)
100 0 20
30
40
50
60
2θ (degrees)
Fig. 1. X-ray diffraction pattern of ZnS NPs.
ð1Þ
where a, n, C, and Eg are the molar absorption coefficient, light frequency, an arbitrary constant, and the band gap of the nanoparticles, respectively. As shown in the inset of
3. Results and discussion The XRD pattern of the ZnS NP is shown in Fig. 1. The spectrum demonstrates that ZnS is in pure cubic phase. The diffraction peaks at 28.31, 33.21, 47.51, 56.61 correspond to crystal planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively. An average crystallite size of about 3 nm was estimated according to the line width analysis of the (1 1 1) diffraction peak based on the Scherrer formula [13]. From the AFM picture (Fig. 2) we observe for the selected scan area that the ZnS NPs are almost spherical and have diameter ranging from 20 to 100 nm. Fig. 3 shows the TEM image of the prepared nanopowders. It can be seen that ZnS NPs have almost spherical shape and an average size of less than 10 nm (about 3–6 nm in diameter) which is consistent with the particle size determined by using XRD analysis. Formation of ZnS NPs has been also confirmed using UV–visible spectroscopy. Absorption spectra were measured in matched quartz cells of 1 cm path length between 300 and 600 nm. Fig. 4 presents the UV–vis absorption spectrum of the prepared ZnS NPs which was recorded after the powder sample being dispersed in deionized water. It shows an absorption peak at 323 nm (E= 3.84 eV) which is considerably blue-shifted from 340 nm (E= 3.65 eV) for bulk zinc-blende ZnS because of quantum size effect [14]. The direct allowed optical band gap of the ZnS NPs was estimated from the Tauc plot [15] according to the
1793
Fig. 4. UV–vis absorption spectrum of the ZnS NPs.
1794
M. Moussaoui et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 1792–1795
Fig. 5. Experimental setup for refractive index measurements.
demonstrate the significance of the quantum and dielectric confinements effects on semiconductor NPs. From Takagahara work [17], it seems that these two confinements are more pronounced in the smaller NPs. Then more experiments have to be done to investigate and understand the contribution of these effects on the refractive index change of ZnS NPs and aqueous mixture. Particularly, the dependence of the dielectric permittivity on NPs size has to be clarified.
6.8x10-4 6.6x10-4 6.4x10-4
Δn
6.2x10-4 6.0x10-4 5.8x10-4 5.6x10-4 5.4x10-4 5.2x10-4
4. Conclusions 460 480 500 520 540 560 580 600 620 640 660 680 λ(nm)
Fig. 6. Spectral dependence of refractive index change between water with and without ZnS NPs.
Fig. 4, the obtained Eg value was fairly large (5.5–3.84 eV) in comparison with the bulk ZnS (3.68 eV) corresponding to the ZnS NPs diameters of 2.7–10 nm, respectively. The experimental setup of refractive index measurement is depicted in Fig. 5. A collimated Ar/Kr laser beam was used to measure the critical angle of total reflection at the prism/liquid interface. A thin layer (about 10 mm) of a measured liquid solution was maintained between two glass prisms (SF11). This block was placed on the rotating table of theodolite Zeiss THEO-010 A permitting angular measurements with precision of about 2 arcseconds. This allows for relative refractive index measurements with precision better than 5 10 5. In our experiments, we use pure deionized water as the solvent to disperse ZnS NPs. The refractive index of the ZnS NPs aqueous solution with 1 10 2 mol/L concentration was measured at different visible wavelengths. All measurements were performed at temperature 24 1C. Fig. 6 shows the spectral dependence of the refractive index change Dn between the pure solvent (deionized water) and the ZnS aqueous solution. The increase of refractive index change of ZnS NPs solution with the wavelength is certainly due to the presence of ZnS NPs but it cannot be explained by existing conventional models like the Maxwell–Garnett formula, for example. In semiconductor NPs, the confinement produces a size dependence of the dielectric function and consequently on the refractive index. Many studies [16–19]
We have synthesized ZnS NPs using an easy and lowcost chemical method. These NPs have polydispersed size distribution with size ranging from 2 to 100 nm. The structural and optical characterization confirmed nanocrystalline nature of ZnS NPs. We use ZnS NPs with 5 nm diameter to investigate their absorption and refractive index behavior in the visible range. As expected the refractive index of the ZnS NPs aqueous solution is higher than that of deionized water. Work is in progress to study the optical index modifications with different parameters like ZnS NPs size and concentration, temperature and the wavelength of the incident light.
Acknowledgements M. Moussaoui was supported by Re´gion Rhone Alpes (bourse MIRA). I. Lesnichiy is supported by the Ministry of Education and Science of the Russian Federation. References [1] Turco Liveri V. Controlled synthesis of nanoparticles in microheterogeneous systems. Springer; 2006. [2] Richards RM, Klabunde KJ. In: Nanoscale materials in chemistry. 2nd ed. New York: John Wiley and Sons; 2009. [3] Bandyopadhyay AK. Nano materials. NewAge International (P) Ltd. Publishers; 2008. [4] Rogach AL. Semiconductor nanocrystal quantum dots: synthesis, assembly, spectroscopy and applications. Springer-Verlag/Wien; 2008. [5] Burda C, Chen X, Narayanan R, El-Sayed MA. Chemistry and properties of nanocrystals of different shapes. Chem Rev 2005;105: 1025–102. [6] Schmid G. Nanoparticles from theory to application. WILEY-VCH Verlag GmbH Co.KGaA; 2004.
M. Moussaoui et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 1792–1795
[7] Knoss RW. Quantum Dots—research, technology and applications. Nova Science Publishers, Inc; 2009. [8] Klimov VI. Nanocrystal quantum dots second. Taylor and Francis Group, LLC; 2010. [9] Palik ED. In: Handbook of optical constants of solids. Orlando, FL: Academic Press; 1985. [10] Cao G. In: Nanostructures and nanomaterials. Imperial College Press; 2004. [11] Lu HY, Chu SY, Tan SS. The characteristics of low-temperaturesynthesized ZnS and ZnO nanoparticles. J Cryst Growth 2004;269: 385–91. [12] Scott RPW. Liquid chromatography detectors. Elsevier Scientific Publishing Company; 1977. [13] Cullity BD. Element of X-ray diffraction. Massachchusetts: A.W.R.C.Inc.; 1967.
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[14] Yoffe AD. Semiconductor quantum dots and related systems: electronic, optical, luminescence and related properties of low dimensional systems. Adv Phys 2001;50:1–208. [15] Tauc J, Menth A. States in the gap. J Non-Cryst Solids 1972;8: 569–85. [16] Hanamura E. Very large optical nonlinearity of semiconductor microcrystallites. Phys Rev B Condens Matter 1988;37:1273–9. [17] Takagahara T. Effects of dielectric confinement and electron–hole exchange interaction on excitonic states in semiconductor quantum dots. Phys Rev B Condens Matter 1993;47:4569–84. [18] Peng YY, Hsieh TE, Hsu CH. Dielectric confinement effect in ZnO quantum dots embedded in amorphous SiO2 matrix. J Phys D: Appl Phys 2007;40:6071–5. [19] Franceschetti A, Zunger A. Addition energies and quasiparticle gap of CdSe nanocrystals. Appl Phys Lett 2000;76:1731–3.
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
Optical measurements of ZnS nanoparticles aqueous solution M. Moussaoui a,n, R. Saoudi a, I.V. Lesnichiy b, A.V. Tishchenko a a b
Universite´ de Lyon, Universite´ Jean Monnet, Laboratoire Hubert Curien CNRS UMR5516, 18 rue du Professeur Benoˆıt Lauras, 42000 Saint-Etienne, France Moscow Institute of Physics and Technology, Institutsky Str. 9, 141701 Dolgoprudny, Russia
a r t i c l e in f o
abstract
Available online 20 February 2011
We synthesized zinc sulfide (ZnS) nanopowders with size ranging from 2 to 100 nm by a simple, low-cost, and mass production chemical method. The nanoparticles (NPs) were characterized by X-ray powder diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and UV–vis absorption spectroscopy. Our study concerns also the change in the refractive index of deionized water in presence of ZnS nanospheres. We present experimental results on effective index variation of water dispersed ZnS NPs at different wavelengths in visible spectrum. & 2011 Elsevier Ltd. All rights reserved.
Keywords: ZnS nanoparticles Nanopowder water suspension Refractive index measurement
1. Introduction Optical properties of semiconductor nanoparticles (NPs) are of fundamental and practical interest and many researches have been performed on them [1,2]. Their studies are mainly concerned with the quantum size effect. When the size of semiconductor particles is reduced to nanometer scale their physical properties differ noticeably from those of the corresponding bulk material and depend on the size and the morphology of the studied structures [3,4]. The extremely small size of the NPs results in quantum confinement of the photogenerated electron–hole pair. When the radius of the particle approaches the Bohr radius of the exciton, the quantum size effect becomes apparent: the energy gap increases with decreasing the grain size, which leads to a blueshift of the optical absorption edge with respect to the bulk material [5]. In order to exploit such sizetuneable properties, many works have been devoted to the development of simple methods for synthesizing semiconductor particles of various sizes in a controllable manner [6]. Optical properties such as optical absorption and refractive index changes have great potential for
device applications in photodetectors, optical modulators, and semiconductor optical amplifiers [7,8]. Among the II–VI materials, zinc sulfide (ZnS) is a nontoxic semiconductor with an important direct wide band gap (Eg =3.6 eV at 300 K), and relatively high refractive index [9]. It is a suitable material for numerous applications such as ultraviolet-light-emitting diodes, electroluminescent devices, flat-panel displays, sensors and injection lasers [10]. In this work, ZnS nanoparticles with size ranging from 2 to 100 nm were prepared by chemical process [11]. The synthesis of ZnS NPs was based on the reaction of zinc acetate and thioacetamide as starting materials. The resulting mixture was treated thermally at different temperatures. The resulting product was collected by centrifuging to select the size distribution. The NPs were characterized by X-ray powder diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and UV–vis absorption spectroscopy. We also investigate the refractive index of ZnS NPs dispersed in deionized water. The critical angle measurement [12] is employed because of its convenience and high precision. 2. Material and methods
n Corresponding author. Tel.: + 330477915824; fax: + 330477915781. E-mail address: [email protected] (M. Moussaoui).
0022-4073/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2011.02.006
All chemicals used in this work were obtained from Sigma Aldrich company commercial sources with no additional purification.
M. Moussaoui et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 1792–1795
XRD patterns were acquired on a Bruker D8 Advance ˚ instrument using Cu Ka radiation (l = 1.5418 A). The nanocrystallite powder was pressed inside the sample holder. The TEM images are taken with a TOPCON EM002B transmission electron microscope operating at 200 kV. Samples for the TEM measurement are supported on a carbon-coated copper grid. ZnS nanopowders were also examined by atomic force microscopy with an Agilent 5500 AFM system in tapping mode. The absorption spectra in the UV and visible ranges were recorded with a lambda 900 Perkin-Elmer spectrophotometer in between wavelength range 300–600 nm.
following relation: ðahnÞ2 ¼ CðhnEg Þ
Fig. 2. Typical AFM image of the prepared ZnS NPs.
Fig. 3. TEM image of the prepared ZnS NPs.
900 800
(111)
700
Intensity (a.u.)
600
(220)
500
(311)
400 300 200
(200)
100 0 20
30
40
50
60
2θ (degrees)
Fig. 1. X-ray diffraction pattern of ZnS NPs.
ð1Þ
where a, n, C, and Eg are the molar absorption coefficient, light frequency, an arbitrary constant, and the band gap of the nanoparticles, respectively. As shown in the inset of
3. Results and discussion The XRD pattern of the ZnS NP is shown in Fig. 1. The spectrum demonstrates that ZnS is in pure cubic phase. The diffraction peaks at 28.31, 33.21, 47.51, 56.61 correspond to crystal planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively. An average crystallite size of about 3 nm was estimated according to the line width analysis of the (1 1 1) diffraction peak based on the Scherrer formula [13]. From the AFM picture (Fig. 2) we observe for the selected scan area that the ZnS NPs are almost spherical and have diameter ranging from 20 to 100 nm. Fig. 3 shows the TEM image of the prepared nanopowders. It can be seen that ZnS NPs have almost spherical shape and an average size of less than 10 nm (about 3–6 nm in diameter) which is consistent with the particle size determined by using XRD analysis. Formation of ZnS NPs has been also confirmed using UV–visible spectroscopy. Absorption spectra were measured in matched quartz cells of 1 cm path length between 300 and 600 nm. Fig. 4 presents the UV–vis absorption spectrum of the prepared ZnS NPs which was recorded after the powder sample being dispersed in deionized water. It shows an absorption peak at 323 nm (E= 3.84 eV) which is considerably blue-shifted from 340 nm (E= 3.65 eV) for bulk zinc-blende ZnS because of quantum size effect [14]. The direct allowed optical band gap of the ZnS NPs was estimated from the Tauc plot [15] according to the
1793
Fig. 4. UV–vis absorption spectrum of the ZnS NPs.
1794
M. Moussaoui et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 1792–1795
Fig. 5. Experimental setup for refractive index measurements.
demonstrate the significance of the quantum and dielectric confinements effects on semiconductor NPs. From Takagahara work [17], it seems that these two confinements are more pronounced in the smaller NPs. Then more experiments have to be done to investigate and understand the contribution of these effects on the refractive index change of ZnS NPs and aqueous mixture. Particularly, the dependence of the dielectric permittivity on NPs size has to be clarified.
6.8x10-4 6.6x10-4 6.4x10-4
Δn
6.2x10-4 6.0x10-4 5.8x10-4 5.6x10-4 5.4x10-4 5.2x10-4
4. Conclusions 460 480 500 520 540 560 580 600 620 640 660 680 λ(nm)
Fig. 6. Spectral dependence of refractive index change between water with and without ZnS NPs.
Fig. 4, the obtained Eg value was fairly large (5.5–3.84 eV) in comparison with the bulk ZnS (3.68 eV) corresponding to the ZnS NPs diameters of 2.7–10 nm, respectively. The experimental setup of refractive index measurement is depicted in Fig. 5. A collimated Ar/Kr laser beam was used to measure the critical angle of total reflection at the prism/liquid interface. A thin layer (about 10 mm) of a measured liquid solution was maintained between two glass prisms (SF11). This block was placed on the rotating table of theodolite Zeiss THEO-010 A permitting angular measurements with precision of about 2 arcseconds. This allows for relative refractive index measurements with precision better than 5 10 5. In our experiments, we use pure deionized water as the solvent to disperse ZnS NPs. The refractive index of the ZnS NPs aqueous solution with 1 10 2 mol/L concentration was measured at different visible wavelengths. All measurements were performed at temperature 24 1C. Fig. 6 shows the spectral dependence of the refractive index change Dn between the pure solvent (deionized water) and the ZnS aqueous solution. The increase of refractive index change of ZnS NPs solution with the wavelength is certainly due to the presence of ZnS NPs but it cannot be explained by existing conventional models like the Maxwell–Garnett formula, for example. In semiconductor NPs, the confinement produces a size dependence of the dielectric function and consequently on the refractive index. Many studies [16–19]
We have synthesized ZnS NPs using an easy and lowcost chemical method. These NPs have polydispersed size distribution with size ranging from 2 to 100 nm. The structural and optical characterization confirmed nanocrystalline nature of ZnS NPs. We use ZnS NPs with 5 nm diameter to investigate their absorption and refractive index behavior in the visible range. As expected the refractive index of the ZnS NPs aqueous solution is higher than that of deionized water. Work is in progress to study the optical index modifications with different parameters like ZnS NPs size and concentration, temperature and the wavelength of the incident light.
Acknowledgements M. Moussaoui was supported by Re´gion Rhone Alpes (bourse MIRA). I. Lesnichiy is supported by the Ministry of Education and Science of the Russian Federation. References [1] Turco Liveri V. Controlled synthesis of nanoparticles in microheterogeneous systems. Springer; 2006. [2] Richards RM, Klabunde KJ. In: Nanoscale materials in chemistry. 2nd ed. New York: John Wiley and Sons; 2009. [3] Bandyopadhyay AK. Nano materials. NewAge International (P) Ltd. Publishers; 2008. [4] Rogach AL. Semiconductor nanocrystal quantum dots: synthesis, assembly, spectroscopy and applications. Springer-Verlag/Wien; 2008. [5] Burda C, Chen X, Narayanan R, El-Sayed MA. Chemistry and properties of nanocrystals of different shapes. Chem Rev 2005;105: 1025–102. [6] Schmid G. Nanoparticles from theory to application. WILEY-VCH Verlag GmbH Co.KGaA; 2004.
M. Moussaoui et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 1792–1795
[7] Knoss RW. Quantum Dots—research, technology and applications. Nova Science Publishers, Inc; 2009. [8] Klimov VI. Nanocrystal quantum dots second. Taylor and Francis Group, LLC; 2010. [9] Palik ED. In: Handbook of optical constants of solids. Orlando, FL: Academic Press; 1985. [10] Cao G. In: Nanostructures and nanomaterials. Imperial College Press; 2004. [11] Lu HY, Chu SY, Tan SS. The characteristics of low-temperaturesynthesized ZnS and ZnO nanoparticles. J Cryst Growth 2004;269: 385–91. [12] Scott RPW. Liquid chromatography detectors. Elsevier Scientific Publishing Company; 1977. [13] Cullity BD. Element of X-ray diffraction. Massachchusetts: A.W.R.C.Inc.; 1967.
1795
[14] Yoffe AD. Semiconductor quantum dots and related systems: electronic, optical, luminescence and related properties of low dimensional systems. Adv Phys 2001;50:1–208. [15] Tauc J, Menth A. States in the gap. J Non-Cryst Solids 1972;8: 569–85. [16] Hanamura E. Very large optical nonlinearity of semiconductor microcrystallites. Phys Rev B Condens Matter 1988;37:1273–9. [17] Takagahara T. Effects of dielectric confinement and electron–hole exchange interaction on excitonic states in semiconductor quantum dots. Phys Rev B Condens Matter 1993;47:4569–84. [18] Peng YY, Hsieh TE, Hsu CH. Dielectric confinement effect in ZnO quantum dots embedded in amorphous SiO2 matrix. J Phys D: Appl Phys 2007;40:6071–5. [19] Franceschetti A, Zunger A. Addition energies and quasiparticle gap of CdSe nanocrystals. Appl Phys Lett 2000;76:1731–3.