August 7, S021886351000525X 2010 9:5 WSPC/S0218-8635 145-JNOPM Journal of Nonlinear Optical Physics & Materials Vol 19, No (2010) 237–245 c World Scientific Publishing Company DOI: 10.1142/S021886351000525X J Nonlinear Optic Phys Mat 2010.19:237-245 Downloaded from www.worldscientific.com by UNIVERSITY OF NEW ENGLAND LIBRARIES on 01/17/15 For personal use only STUDY OF MICROSTRUCTURE AND OPTICAL PROPERTIES OF PVA-CAPPED ZnS : Cu NANOCRYSTALLINE THIN FILMS TRAN MINH THI∗,‡ , BUI HONG VAN† and PHAM VAN BEN† ∗Faculty of Physics, Hanoi National University of Education, 136 Xuan Thuy, Cau Giay District, Hanoi, Vietnam †Faculty of Physics, College of Science, Hanoi National University, 334 Nguyen Trai, Hanoi, Vietnam ‡tranminhthi@hnue.edu.vn Received 19 May 2010 A study has been carried out on the Cu doping and PVA capping induced optical property changes in ZnS : Cu nanocrystalline powders and thin film For this study, ZnS : Cu nanopowders with Cu concentrations of 0.1%, 0.15%, 0.2%, 0.3% and 0.4% are synthesized by the wet chemical method The polyvinyl alcohol (PVA)-capped ZnS thin film with 0.2% Cu concentration and various PVA concentrations are prepared by the spin-coating method The microstructures of the samples are investigated by the X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) The results show that the prepared samples belong to the wurtzite structure with the average particle size of about 3–7 nm The optical properties of samples are studied by measuring absorption and photoluminescence (PL) spectra in the wavelength range from 300 nm to 900 nm at 300 K It is shown that the luminescent intensity of ZnS : Cu nanopowders reaches the highest intensity for optimal Cu concentration of 0.2% with the corresponding values of its direct band gap estimated to be about 3.90 eV While the PVA coating does not affect the microstructure of ZnS nanometerials, the PL spectra of the samples are found to be affected by the PVA concentration as well as the exciting power density The influence of the polymer coating on the optical properties can be explained by the quantum confinement effect of ZnS nanoparticles in the PVA matrix Keywords: PVA-capped ZnS : Cu nanocrystalline thin film; photoluminescence spectra; absorption spectra Introduction The ZnS nanomaterials are semiconducting material with direct and large band gap The direct band gap of ZnS is 3.60 eV for bulk ZnS material and 3.98 eV for ZnS nanomaterial at 300 K1 in the wurzite structure The direct band gap of nanomaterials may be controlled by doping, polymer coating and changing the preparation condition.2–5 The partial substitution of Zn by Cu was shown to have considerable influence on the optical properties of the samples.9 Besides, the polymer coating used to protect ZnS nanoparticles from the environment influences 237 August 7, S021886351000525X J Nonlinear Optic Phys Mat 2010.19:237-245 Downloaded from www.worldscientific.com by UNIVERSITY OF NEW ENGLAND LIBRARIES on 01/17/15 For personal use only 238 2010 9:5 WSPC/S0218-8635 145-JNOPM T M Thi, B H Van & P V Ben is also expected to affect the optical properties of ZnS nanoparticles considerably when the particles diffuse into the polymer matrix at a certain concentration The resulting changes are related to the quantum confinement or the quantum size effect and the surface effect induced in the polymer-capped nanoparticles.1,6 The PVA (polyvinyl alcohol) capped ZnS nanoparticle composites are applicable for a variety of applications such as electro-luminescent devices, solar energy, and many other optoelectronic devices.1,6,9 Recently, some authors have investigated the effects of PVA1 and PVP6 (polyvinyl pyrrolidone) capping polymers of ZnS nanopowder and thin films on the resulted optical properties of those samples In this paper, we present the research results on the role of Cu doping and PVA-capping of ZnS nanoparticles and thin films We studied firstly the variations of optical properties of the ZnS : Cu nanopowder doped with various Cu concentrations of 0.1%, 0.15%, 0.2%, 0.3% and 0.4% (denoted by P1 , P2 , P3 , P4 , P5 ) which allow us to determine the optimal concentration of Cu giving rise to the maximum PL intensity in the visible spectral range Secondly, we also study the influence of the concentration of PVA (polyvinyl alcohol) capping polymer on the optical properties of the PVA capped ZnS : Cu nanopowder and the PVA coated nanocrystalline thin films doped with the optimal Cu concentration Furthermore, the influences of Cu dopant and PVA concentration on the general features of the PL spectra, as well as the optical band gap variation are also discussed Experimental Details The ZnS : Cu nanopowder was prepared by the standard wet chemical method from three separately prepared highly pure initial solutions The first solution was the Zn(CH3 COO)2 ·2H2 O of 0.1 M, the second solution was the Cu(CH3 COO)2 ·H2 O of 0.1 M and the third solution was the Na2 S·9H2 O of 0.1 M The catalyst CH3 OH : H2 O was used for first and second solutions in 1:1 volume ratio The first solution and the second solution were mixed with appropriate ratio in order to produce the P1 , P2 , P3 , P4 , P5 powder samples The water was the solvent used for the third solution The third solution was prepared with different amounts (1g, 2g, 3g and 4g) of PVA, then added drop by drop into the reaction vessel containing the initially mixed solution of 100 ml The ZnS : Cu precipitates were separated by centrifuge at spinning speed of about 3000 rpm and finally dried at 80◦ C The reactions taking place during the final mixing process are described as follows: Zn(CH3 COO)2 + Na2 S = ZnS ↓ + 2CH3 COONa, Cu(CH3 COO)2 + Na2 S = CuS ↓ + 2CH3 COONa These powder samples were designated by P-ZnS : Cu, P-ZnS : Cu-PVA1, P-ZnS : Cu-PVA2, P-ZnS : Cu-PVA3, P-ZnS : Cu-PVA4 (with P standing for the powder samples), corresponding to the optimally Cu doped ZnS:Cu nanopowder samples with different concentrations of PVA capping material Additionally, the PVAcoated Cu doped ZnS thin films were produced by spin-coating the final mixed August 7, S021886351000525X 2010 9:5 WSPC/S0218-8635 145-JNOPM 239 solution on the glass substrates These films were denoted by F-ZnS : Cu-PVA1, F-ZnS : Cu-PVA2, F-ZnS : Cu-PVA3 and F-ZnS : Cu-PVA4, respectively (with F standing for film samples) PL spectra of all the samples were firstly measured at 300 K by the fluorescence spectrophotometer HP340–LP370 using a He-Cd laser source with the excitation wavelength of 325 nm The optimal dopant concentration giving rise to maximum emission intensity was determined on the basis of this observation The nanopowder with this particular Cu dopant concentration as well as its PVA-capped samples were to become the focus of the ensuing measurements The microstructure of these samples were investigated by X-ray diffraction patterns by means of the XD8 A The partiAdvance Bukerding machine using the Cu-Kα radiation of λ = 1.5406 ˚ cle size was measured by means of transmission electron microscope TEM-HITACHI H6000 The ultraviolet absorption spectra of the thin film samples were measured by spectrophotometer JASCO-V670 The dependence of the photoluminescence spectra on the exciting laser power density was also investigated Results and Discussion Figure shows the PL spectra of P1, P2 , P3 , P4 , P5 powder samples, where the PL peaks are found at practically the same wavelength of about 500 nm and not appear to be effected by the Cu-doped concentration But the PL peak intensity exhibits perceptible and non-monotonous changes with increasing Cu concentration It is seen from the figure that the maximum peak intensity comes from the sample with the optimal Cu concentration of about 0.2% We shall henceforth focus our presentation and discussion on the measurement results of the optimally Cu-doped samples (P-ZnS : Cu) of PVA-capped nanopowder and thin films Figure shows the XRD patterns of P-ZnS : Cu, P-ZnS : Cu-PVA2 and P-ZnS : Cu-PVA4 powders The XRD patterns of the uncapped powder and the powder samples with PVA capping at different concentrations show that the crystal Intensity (a.u.) J Nonlinear Optic Phys Mat 2010.19:237-245 Downloaded from www.worldscientific.com by UNIVERSITY OF NEW ENGLAND LIBRARIES on 01/17/15 For personal use only Study of Microstructure and Optical Properties of PVA-Capped 50000 45000 40000 35000 30000 25000 20000 15000 10000 5000 -5000 300 500 nm P2 P4 P3 P5 P1 400 500 600 700 800 900 Wavelength (nm) Fig The PL spectra of the P1 , P2 , P3 , P4 and P5 ZnS : Cu nano powder samples with the 0.1%, 0.15%, 0.2%, 0.3%, 0.4% Cu-doped concentrations, respectively August 7, S021886351000525X 240 2010 9:5 WSPC/S0218-8635 145-JNOPM T M Thi, B H Van & P V Ben 1000 a.P-ZnS:Cu b.P-ZnS:Cu-PVA2 c P-ZnS:Cu-PVA4 900 J Nonlinear Optic Phys Mat 2010.19:237-245 Downloaded from www.worldscientific.com by UNIVERSITY OF NEW ENGLAND LIBRARIES on 01/17/15 For personal use only Intensity (a.u.) 800 (1 1) 700 (2 0) 600 (3 1) 500 400 c 300 a 200 b 100 20 Fig 30 40 50 theta 60 70 The XRD spectra of P-ZnS : Cu, P-ZnS : Cu-PVA2 and P-ZnS : Cu-PVA4 powder sample structure of the ZnS : Cu nanomaterials is of the wurtzite phase with the diffraction peaks (1 1), (2 0), (3 1) in agreement with the previously reported result.1 It is also to be noted that the PVA capping does not affect the crystal structure of the ZnS : Cu nanomaterials The average diameter of grains as calculated by the Scherrer formular varies between 2.70 nm and 2.90 nm for uncapped powder, while it is about 3.20 nm for the powder capped by PVA of highest concentration P-ZnS : Cu-PVA4 The morphology of the F-ZnS : Cu-PVA2 thin film was observed by TEM image which is presented in Fig One observes the grain in sphere form were embedded Fig TEM image of F-ZnS : Cu-PVA2 thin film August 7, S021886351000525X 2010 9:5 WSPC/S0218-8635 145-JNOPM Study of Microstructure and Optical Properties of PVA-Capped 2.5 Absorption (a.u.) a 2.0 c a b c d 241 F-ZnS:Cu-PVA1 F-ZnS:Cu-PVA2 F-ZnS:Cu-PVA3 F-ZnS:Cu-PVA4 d 1.5 b 1.0 0.5 300 350 400 450 500 550 600 J Nonlinear Optic Phys Mat 2010.19:237-245 Downloaded from www.worldscientific.com by UNIVERSITY OF NEW ENGLAND LIBRARIES on 01/17/15 For personal use only Wavelength (nm) Fig The absorption spectra of the thin films with different PVA concentrations in the PVA matrix The average size of the grain is about nm which is in agreement with the above calculated results from the XRD data which is slightly larger than the particle sizes in the uncapped samples Figure presents the absorption spectra of the F-ZnS : Cu-PVA1, F-ZnS : CuPVA2, F-ZnS : Cu-PVA3 and F-ZnS : Cu-PVA4 thin films The relation between the absorption coefficient α and the exciting photon energy can be calculated by the following equation1,6 : K(hν − Eg ) /2 (1) hν Here, K is a constant depending on the effective mass of the hole, the electron and refractive index, and h is the Planck constant, ν the exciting photon frequency and Eg the direct band gap From Eq (1) and the absorption spectra in Fig 4, the direct band gap can be calculated yielding the values of about 3.90 eV, 3.86 eV, 3.77 eV and 3.73 eV for the F-ZnS : Cu-PVA1, F-ZnS : Cu-PVA2, F-ZnS : Cu-PVA3 and F-ZnS : Cu-PVA4 thin films, respectively Apparently, these Eg values are all larger than that of the bulk ZnS (3.60 eV) One can further deduce the crystallite radius r according to the following formula1 : α= ∆Eg = Eg (film) − Eg (bulk) = h2 1.8e2 + − 8r2 m∗e m∗h εr (2) Here, Eg (bulk) is the band gap energy of the bulk sample, ε = 8.76, m∗e = 0.34 m0 , m∗h = 0.24 m0 , where m0 is the mass of the free electron The calculated results of crystallite radius r in these thin films are given in Table along with the corresponding values of Eg The results show that Eg decreases while the crystallite size increases with increasing PVA concentration The table also shows that F-ZnS : CuPVA1 thin film has the largest band gap and smallest grain size with Eg = 3.90 eV and r = 3.60 nm The PL spectra of the thin films with different PVA concentration are presented in Fig These spectra include the blue luminescence band at the left shoulder of spectra and the green luminescence band at about 500 nm wavelength It is clear that the positions of luminescence peak remains more or less unchanged at about August 7, S021886351000525X 242 2010 9:5 WSPC/S0218-8635 145-JNOPM T M Thi, B H Van & P V Ben Table The direct energy gaps and the radii r of the nanocrystallites in the thin films Thin films F-ZnS F-ZnS F-ZnS F-ZnS : : : : Intensity (a.u.) r (nm) 3.90 3.86 3.77 3.73 3.60 4.20 5.60 7.50 Cu-PVA1 Cu-PVA2 Cu-PVA3 Cu-PVA4 a F-ZnS : Cu-PVA1 b F-ZnS : Cu-PVA2 c F-ZnS : Cu-PVA3 d F-ZnS : Cu-PVA4 e F-ZnS : Cu 500 nm 30000 J Nonlinear Optic Phys Mat 2010.19:237-245 Downloaded from www.worldscientific.com by UNIVERSITY OF NEW ENGLAND LIBRARIES on 01/17/15 For personal use only Eg (eV) 25000 20000 b e 15000 10000 a c 5000 d 300 400 500 600 700 800 900 1000 Wavelength (nm) Fig The PL spectra of the thin films with different PVA concentrations 500 nm, implying that it was not effected by PVA concentration However, the intensity of luminescence peak changes non-monotonously with the PVA concentration, with the F-ZnS : Cu-PVA1, F-ZnS : Cu-PVA2 samples showing equally highest PL intensities We further investigated the dependence of PL spectra on the exciting power density Figure presents the PL spectra of the F-ZnS : Cu-PVA1 thin film attained with the exciting wavelength of 325 nm and different exciting power densities As expected, the luminescence peak position is observed to remain practically unchanged when the power density varies from 0.20 W/cm2 to 0.45 W/cm2 But the associated intensity does change perceptibly and monotonously It was found that the variation of the peak intensity can be well-fitted by the power law of the form IP L = A(IEX )n with n = 0.8 This result shows that the Cu is the emission center of the luminescence band at around 500 nm.10 In addition, we compare the shift of PL peak of thin film sample with the ZnS : Cu powder, prepared by solid-state reaction method with the same Cu dopant but without PVA capping The average diameter of the grain of this sample is about 8µm As presented in the Fig 7, the luminescence peak of the thin film sample is shifted towards shorter wavelength with respect to the luminescence peak of the powder sample This wavelength is observed to be shifted downwards by about 30 nm, which corresponds to an energy of 140 meV Meanwhile, the blue August 7, S021886351000525X 2010 9:5 WSPC/S0218-8635 145-JNOPM Study of Microstructure and Optical Properties of PVA-Capped 1600 1400 1200 1000 800 600 400 200 J Nonlinear Optic Phys Mat 2010.19:237-245 Downloaded from www.worldscientific.com by UNIVERSITY OF NEW ENGLAND LIBRARIES on 01/17/15 For personal use only 300 400 500 600 700 800 243 August 7, S021886351000525X 244 2010 9:5 WSPC/S0218-8635 145-JNOPM T M Thi, B H Van & P V Ben This change is explained as follows It is known that the Bohr exciton radius can be determined approximately by the following formula: J Nonlinear Optic Phys Mat 2010.19:237-245 Downloaded from www.worldscientific.com by UNIVERSITY OF NEW ENGLAND LIBRARIES on 01/17/15 For personal use only rB = h2 ε 1 + ∗ , πe2 m∗e mh (3) with the dielectric constant ε = 8.76 for the material The Bohr radius found by this formula is about 2.5 nm Since the size of the nanoparticles becomes comparable to the Bohr-excitonic radius, the properties of nanocrystalline materials is expected to change significantly as a result of quantum size effects, namely the band gap energy increases with decreasing particle size On the other hand, formula (2) shows that the shift of the band gap energy is caused by the shift of the conduction band to higher energy and the shift of the valence band to lower energy However, the energy shift of the conduction band is larger than the energy shift of the valence band because the effective mass of the hole is smaller than the effective mass of the electron in ZnS : Cu Conclusion We produced successfully the ZnS : Cu nanopowders with different Cu concentrations and the F-ZnS : Cu-PVA1, F-ZnS : Cu-PVA2, F-ZnS : Cu-PVA3 and F-ZnS : Cu-PVA4 thin films with the 0.2% optimal Cu dopant and different amounts of PVA capping by the wet chemical method and the spin-coating method on glass substrate While the crystalline structure appears unaffected by the PVA, perceptible changes were observed in the grain size and optical energy gaps as well as the PL intensity It was also observed that the nanocrystallites became embedded in the PVA matrix leading to reduced grain size and thereby induced the quantum size effect which may explain the above-mentioned changes Acknowledgments All authors of this paper would like to thank the organizing committee of ISMOA 2009 The paper is completed by the support of the Ministerial-level project on the topic synthesized and optical properties of the 3d transition metal doped ZnS/polymer composite materials, code B2010-17-234 References P K Ghosh, S Jana, S Nandy and K K 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Chen, A G Joly, J.-O Malm and J.-O Bovin, J Appl Phys 95(2) (2004) 667– 672 ... mass of the electron in ZnS : Cu Conclusion We produced successfully the ZnS : Cu nanopowders with different Cu concentrations and the F -ZnS : Cu- PVA1, F -ZnS : Cu- PVA2, F -ZnS : Cu- PVA3 and F -ZnS :. .. discussion on the measurement results of the optimally Cu- doped samples (P -ZnS : Cu) of PVA-capped nanopowder and thin films Figure shows the XRD patterns of P -ZnS : Cu, P -ZnS : Cu- PVA2 and P -ZnS. .. the direct band gap can be calculated yielding the values of about 3.90 eV, 3.86 eV, 3.77 eV and 3.73 eV for the F -ZnS : Cu- PVA1, F -ZnS : Cu- PVA2, F -ZnS : Cu- PVA3 and F -ZnS : Cu- PVA4 thin films,