DSpace at VNU: The optical properties and energy transition process in nanocomposite of polyvinyl-pyrrolidone polymer and Mn-doped ZnS

DSpace at VNU: The optical properties and energy transition process in nanocomposite of polyvinyl-pyrrolidone polymer an...

Opt Quant Electron (2013) 45:147–159 DOI 10.1007/s11082-012-9611-y The optical properties and energy transition process in nanocomposite of polyvinyl-pyrrolidone polymer and Mn-doped ZnS Thi Tran Minh · Ben Pham Van · Thai Dang Van · Hien Nguyen Thi Received: 22 February 2012 / Accepted: August 2012 / Published online: 15 August 2012 © Springer Science+Business Media, LLC 2012 Abstract This study has been carried out on the optical properties of polyvinyl-pyrrolidone (PVP), the energy transition process in nanocomposite of PVP capped ZnS:Mn nanocrystalline and the influence of the PVP concentration on the optical properties of the PVP capped ZnS:Mn nanocrystalline thin films synthesized by the wet chemical method The microstructures of the samples were investigated by X-ray diffraction, the atomic absorption spectroscopy, and transmission electron microscopy The results showed that the prepared samples belonged to the sphalerite structure with the average particle size of about 2–3 nm The optical properties of samples are studied by measuring absorption, photoluminescence (PL) spectra and time-resolved PL spectra in the wavelength range from 200 to 700 nm at 300 K From data of the absorption spectra, the absorption edge of PVP polymer was found about of 230 nm The absorption edge of PVP capped ZnS:Mn nanoparticles shifted from 322 to 305 nm when the PVP concentration increases The luminescence spectra of PVP showed a blue emission with peak maximum at 394 nm The luminescence spectra of ZnS:Mn–PVP exhibits a blue emission with peak maximum at 437 nm and an orange–yellow emission of ion Mn2+ with peak maximum at 600 nm While the PVP coating did not affect the microstructure of ZnS:Mn nanomaterial, the PL spectra of the PVP capped ZnS:Mn samples were found to be affected strongly by the PVP concentration Keywords Nanocomposite · Time-resoled PL spectra · Absorption spectra · PVP Introduction Despite intensive research on conductivity, local domain orientation and molecular order in organic semiconductor thin films (McNeill 2011), the relationship between morphology, T Tran Minh (B) · H Nguyen Thi Faculty of Physics, Hanoi National University of Education, Hanoi, Vietnam e-mail: tranminhthi@hnue.edu.vn B Pham Van · T Dang Van Faculty of Physics, College of Science, Hanoi National University, Hanoi, Vietnam 123 148 T Tran Minh et al chain structure and conductivity of the polymer is still poorly understood Recently, researchers over the world have worked on the improvement of electrical conductivity, investigated the charge transport and the energy band of a variety of polymers (polyazomethine, aliphatic– aromatic co-polyimides) All determined parameters of the electrical conductivity and the energy band have been found to be related to the influence of the polymer chain structure (Jarzabek et al 2002, 2008) During the last few years, extensive experimental and theoretical studies of the luminescence, non-linear optical and electrical properties of a variety of polymers have been performed (Jarzabek et al 2006, 2008) directed towards understanding the polymers’ material science for use in electronic devices and displays (Hajduk et al 2008; Cihaner and Algi 2009) New progress has been made in the area of thermoelectric (TE) applications of conducting polymers and related organic–inorganic composites (Dubey and Leclerc 2011; Sparavigna et al 2011) Others research efforts aimed to identify the role of additives in optimizing the morphology of organic solar cells and discuss the role of bimolecular recombination in limiting the efficiency of solar cells based on a small optical gap polymer (Agostinelli et al 2011; Miller et al 2011) Recently, methods have been developed to cap the surfaces of the nanoparticles with organic or inorganic groups, so that the nanoparticles are stable against agglomeration Among the inorganic semiconductor nanoparticles, zinc sulfide ZnS is an important II– VI semiconductor, which has been researched extensively because of its broad spectrum of potential applications, such as in catalysis and electronic and optoelectronic nanodevices Furthermore, luminescent properties of ZnS can be controlled using various dopants such as Ni, Fe, Mn, Cu etc (Yang et al 2002, 2003; Hattori et al 2005; Soni et al 2009; Sharma et al 2009; Huang et al 2009; Pouretedal et al 2009) They not only give luminescence in various spectral regions, but also enhance the excellent properties of ZnS In order to cap the ZnS nanoparticles, some particular passivators of ZnS have been used, such as polyvinylalcohol (PVA) (Sharif et al 2010), and polyvinyl-pyrrolidone (PVP) (Wang et al 2006; Maity et al 2006; Ghosh et al 2006; Panda et al 2007; Pattabi et al 2007) Understanding the effect of capping on nanoparticles is one of the most important topics now-a-days The influence of surface passivation on luminescence quantum efficiency of ZnS:Mn2+ , ZnS:Cu2+ nanoparticles has been discussed when using sodium hexametaphosphate (SHMP), PVP, PVA as capping agents (Murugadoss 2010; Manzoor et al 2003; Murugadoss et al 2010) The capping agents of PVP and prevention of agglomeration for the ZnS:Mn nanoparticles were shown clearly not only with low Mn concentration from 0.1 to % (Porambo and Marsh 2009), but also at high Mn concentration from 10 to 40 % (Karar et al 2004) But, the optical properties and influence of PVP on the PL spectra of ZnS:Mn nanoparticles still were not interested appropriately in these papers Despite this, there are only a few papers reporting the optical properties of PVP-capped ZnS nanocomposite materials, and the energy transfer process from an organic surface adsorbate such as PVP to dopant ions such as Cu+ or Mn+ (Manzoor et al 2003, 2004) Furthermore, the increase in optical intensity with PVP capping of ZnS:Mn nanoparticles has still not yet been systematically investigated Thus, in this work we report the optical properties of PVP and the influence of the PVP concentration on the optical properties of the PVP capped ZnS:Mn nanocrystalline thin films synthesized by the wet chemical method with the optimal nominal Mn concentration (Thuy et al 2008) Further, the influences of PVP concentration on the general features of the PL spectra and the process of energy transfer from the PVP to the Mn2+ luminescent centers in doped ZnS as well as the optical band gap variation are also discussed 123 The optical properties and energy transition process 149 Experiments The previous researching results on the optical properties of ZnS:Mn showed that the luminescence intensity increased considerably with the optimal nominal Mn concentration about 9–10 % (Thuy et al 2008) So, in order to research a role and an effect of PVP on the optical properties of ZnS:Mn, the PVP caped ZnS:Mn nanoparticles were synthesized with a constant nominal Mn concentration of % atom, but different polymer-capped concentrations The initial chemical substance with high purity (99.9 %) was prepared as follows: – – – – Polymer polyvinyl-pyrrolidone Solution I: Zn(CH3 COO)2 2H2 O 0.1M; Solution II: Mn(CH3 COO)2 4H2 O 0.1M and Solution III: Na2 S.9H2 O 0.1M The solvent in both solutions I and II was a CH3 OH:H2 O mixture (1:1 by volume) whereas water was used as the solvent in solution III 2.1 Preparation of thin films from polymer capped ZnS:Mn nanocrystals Firstly, ZnS nanoparticles were synthesized by the wet chemical method Solutions I, II and III were mixed at an optimal pH level and in an appropriate ratio in order to create the ZnS:Mn material with % nominal Mn concentration The pH level being crucial to the formation of ZnS:Mn precipitates, we derived and applied, by theoretical calculation, the optimal value of pH = where ZnS:Mn precipitates in mixed solutions while Zn(OH)2 does not The reactions are as follows: Zn(CH3 COO)2 + Na2 S → ZnS + 2CH3 COONa Mn(CH3 COO)2 + Na2 S → MnS + 2CH3 COONa This precipitated ZnS, MnS nanoparticles was filtered by filtering system, then washed in distilled water and ethanol several times After washing, 0.5 g formed ZnS:Mn precipitates were dispersed into ml of CH3 OH:H2 O (1:1) solvent This mixture was called solution A Similarly, 0.5 g of PVP was dissolved in ml of C2 H5 OH:H2 O (1:1) solvent, and was called solution B After that these two solutions A and B were mixed with each other at various volume ratios of (5:0), (5:1), (5:2), (5:3) and (5:4) under continuous stirring for h at speed of 3,000 rpm The PVP-capped Mn doped ZnS thin films were produced by spin-coating method on glass substrate at a centrifugation speed of 3,000 rpm, then was heated at 80◦ C and cooled to room temperature By this way, the PVP-capped ZnS:Mn thin films with the difference PVP concentrations were named respectively as ZnS:Mn–PVP (5:0), ZnS:Mn–PVP (5:1), ZnS:Mn–PVP (5:2), ZnS:Mn–PVP (5:3) and ZnS:Mn–PVP (5:4) 2.2 Research methods The real Mn2+ concentration in the ZnS:Mn was determined using the technique atomic absorption spectroscopy AAS-600 The microstructure of these samples was investigated by X-ray diffraction (XRD) using XD8 Advance Brukeding Diffractometer with CuKα radiation of λ = 1.5406 Å and transmission Electron Microscopy (TEM) JEM 1010 Photoluminescence (PL) spectra, photoluminescence exciting (PLE) spectra and the absorption spectra of these samples at room temperature were recorded by Fluorolog FL3-22, HP340-LP370 Fluorescence Spectrophotometer with an excitation wavelength of 325, 337 nm, xenon lamp 123 T Tran Minh et al 260 240 220 200 180 160 140 120 100 80 60 40 20 -20 -40 111 (a) (b) 111 Lin (Cps) Relative Intensity 150 ZnS ZnS:Mn-1% 220 311 (b) (a) 20 30 220 40 50 θ (degree) 60 70 80 311 b a a ZnS:Mn-PVP(5:2) b ZnS:Mn-PVP(5:4) 20 30 40 50 θ (degree) 60 70 Fig The X-ray diffraction spectra of samples a ZnS:Mn–PVP (5:2) and ZnS:Mn–PVP (5:4); b the ZnS:Mn, pure ZnS nano-powders (inset) XFOR-450 and JASCO-V670 spectrophotometer, respectively The time-resoled PL spectra of samples were measured by GDM-100 spectrophotometer using Boxca technique Results and discussion 3.1 Analysis of microstructure by XRD patterns, atomic absorption spectroscopy, TEM Using the technique of atomic absorption spectroscopy, the real Mn2+ concentration was determined about 0.94 % atom and much smaller than the initial nominal Mn2+ concentration This issue can be explained by the small amount of Mn2+ ions taking part in the reaction to create precipitates Opposite of this, other large amount of Mn2+ can be lost in the centrifuging and washing process to receive ZnS and MnS precipitates Figure shows X-ray diffraction spectra of the ZnS:Mn, pure ZnS nano-powders (inset), ZnS:Mn–PVP (5:2) and ZnS:Mn–PVP (5:4) The analyzed results show that all samples have a sphalerite structure The three peaks with strong intensity correspond to the diffraction peaks of (111), (220) and (311) The quality of the samples is good with the lattice constant a = 5.4 Å The average size of the Mn-doped ZnS grains of about nm was calculated by Scherrer formula Alternatively, the average particle size in ZnS:Mn–PVP (5:4) sample is about nm, as measured in TEM (Fig 2a) Figure 2a shows that these grains are ZnS:Mn nanoparticles coated by polymer covers Figure 2b gives the molecule structure and formula of PVP with both N and C=O groups In PVP, nitrogen is conjugated with adjacent carbonyl groups Thus, the role of PVP consists of (a) controlling the size of the particles by forming passivating layers around the ZnS:Mn core due to coordination bond formation between the nitrogen atom of PVP and Zn2+ , and (b) preventing agglomeration of the particles by the repulsive force acting among the polyvinyl groups (Ghosh et al 2006) 123 The optical properties and energy transition process 151 Fig a TEM image of ZnS:Mn–PVP (5:4) b The structure and formula of PVP 3.2 Photoluminescence spectra, absorption spectra of ZnS:Mn–PVP For the ZnS:Mn nanoparticles with low nominal Mn concentration from to 15 %, two PL bands were observed and attributed to the defect-related emission of ZnS host and the Mn2+ emission (Peng et al 2005; Thuy et al 2008) Both the blue emission of ZnS host and the orange emission of Mn2+ ions increase with the increase of Mn concentration, but the PL intensity of Mn2+ centers has a substantial enhancement with Mn2+ ions as the effective luminescence centers while the PL intensity of ZnS host only shows a slow increase The observations of these samples suggests that the PL spectra of Mn2+ centers is related to the d-d excitation transition of Mn2+ ions in ZnS host and the energy transfer from ZnS host (Peng et al 2005) However, in Fig 3, the luminescence peak maximum positions of PVP uncapped ZnS:Mn sample (curve a) are at 437 and 601 nm which are the same as in PVP-capped ZnS:Mn samples excited by excitation wavelength of 325 nm This clearly shows that the luminescence peak maximum positions are unchanged, but their intensities increase rather strongly with increasing of PVP concentrations The orange emission band is attributable to T1 –6 A1 or A2 –A1 transitions of Mn2+ ions in the crystal field of the ZnS nanoparticles The blue emission band is attributable to the intrinsic emission of defects, vacancy and an incorporation of trapped electron by defects at donor level under conduction range when doped Mn was added into hot ZnS semiconductor Both the blue emission and the orange one increase with the increase of PVP concentration, which suggests that the increase of Mn2+ emission related to the hot lattice emission But noticeably, the base difference in comparision with the above ZnS:Mn samples shows that the blue emission of PVP-capped ZnS:Mn samples has the stronger enhancement than the orange emission with the PVP concentration increase while the constant Mn concentration Table shows the intensity of the peaks at 437 and 601 nm for samples with different PVP concentrations It can be seen clearly from Table that the intensity of PL peak at 437 nm increases stronger than that of the peak at 601 nm, when the PVP concentration increases 123 152 T Tran Minh et al Photoluminescence intensity (a.u) 40000 35000 a ZnS:Mn-PVP(5:0) b ZnS:Mn-PVP(5:1) c ZnS:Mn-PVP(5:2) d ZnS:Mn-PVP(5:3) e ZnS:Mn-PVP(5:4) 437 nm 30000 25000 601 nm e 20000 d 15000 c 10000 b 5000 a 350 400 450 500 550 600 650 700 750 Wavelength (nm) Fig The PL spectra of the ZnS:Mn–PVP excited by excitation wavelength of 325 nm with different content of PVP Table The intensities for the 437 nm peak, 601 nm peak and their intensity ratio Sample ZnS:Mn–PVP (5:0) ZnS:Mn–PVP (5:1) ZnS:Mn–PVP (5:2) ZnS:Mn–PVP (5:3) ZnS:Mn–PVP (5:4) I437 nm (a.u.) 1,443 6,277 13,517 17,313 32,170 I601 nm (a.u.) 5,687 9,838 15,026 17,667 24,105 I437 /I601 0.2537 0.638 0.8995 0.9799 1.335 The absorption spectra of PVP, ZnS:Mn and ZnS:Mn–PVP with different PVP concentrations are shown in Fig The absorption edges are at 230, 322 and 305 nm for PVP (curve a), uncapped ZnS:Mn (curve b) and ZnS:Mn–PVP (5:4) (curve f) samples, respectively For the curvers b, c, d, e, f, their right shoulders in range about of 350–400 nm heaped up by absorption lines that characterized by donor-acceptor absorption transition of vacancies or defects in ZnS when doped Mn was added into hot ZnS semiconductor (Wang et al 2008) The decreasing of the band gap of ZnS:Mn in comparison with that of pure ZnS is possible attributed to the band-edge tail constitution of state density in band gap, by the s-d exchange interaction between 3d5 electrons of Mn2+ and s conduction electrons in ZnS crystal (Twardowski et al 1983; Levy et al 1996) On the contrary to the decreasing of the band gap of ZnS:Mn (in comparison with that of pure ZnS), the absorption edge of PVP capped ZnS:Mn is shifted toward to shorter wavelength from 322 to 305 nm when the PVP concentration increases Because of ZnS:Mn nanoparticles were formed in produced process before they dispersed into PVP matrix, therefore, PVP polymer not effect to size of nanoparticles However the PVP play an important role as the protective layer, against agglomeration ZnS:Mn nanoparticles and contribute to increasing optical properties of material The absorption edge and right shoulder of PVP (in Fig 4) showed in range from 230 to 400 nm, while the absorption edges and right shoulders of PVP coated ZnS:Mn samples in range about of 305–450 nm showed clearly the shift toward to short wavelength when increasing of PVP concentration Due to the PVP absorb the photons in wavelength range from 230 to 400 nm, thus the blue shift of the absorption edge in range 320–400 nm can be explained by increasing of PVP concentration 123 The optical properties and energy transition process 0.8 Absorption (a.u) 0.7 153 322 nm b 305 nm c 230 nm d 0.6 e 0.5 f 0.4 a PVP b ZnS:Mn c ZnS:Mn-PVP(5:1) d ZnS:Mn-PVP(5:2) e ZnS:Mn-PVP(5:3) f ZnS:Mn-PVP(5:4) a 0.3 0.2 0.1 0.0 200 300 400 500 600 Wavelength (nm) Fig The absorption spectra of PVP and ZnS:Mn–PVP with different contents of the PVP coated ZnS:Mn samples These similar results for PVP coated ZnS:Ni samples also received in our paper (Thi et al 2012) The above discussion shows that PVP does not affect the microstructure of ZnS:Mn PL, but plays an important role to improve the optical properties of ZnS:Mn nanoparticles 3.3 Photoluminescence excitation (PLE) spectra of PVP and ZnS:Mn–PVP In order to examine the process of energy transfer in the PVP capped ZnS:Mn nanoparticles, Fig shows the photoluminescence PL spectra of the PVP and typical ZnS:Mn–PVP (5:4) samples It is interesting to see that the PVP is emissive with peak maximum at 394 nm when using the exciting wavelength of 325 nm Beside that, the PL spectrum of PVP is unsymmetrical, thus its right shoulder heaped up by luminescence lines in range about of 390–470 nm The PLE spectrum recorded at 394 nm emission of PVP shows peak maximum at 332 nm in Fig 5(inset) This excitation band is attributed to the electronic transitions in PVP molecular orbitals Alternatively the blue emission band of PVP at 394 nm is attributed to the radiative relaxation of electrons from the lowest energy unoccupied molecular orbital (LUMO) to the highest energy occupied molecular orbital (HOMO) levels in PVP (Manzoor et al 2004) The PL spectrum of ZnS:Mn–PVP (5:4) sample under excitation wavelength of 338 nm shows two peak maxima at 434 and 598 nm These peak maxima are rather close to the corresponding peaks (two peak maxima at 437 and 601 nm) of other ZnS:Mn–PVP samples excited by excitation wavelength of 325 nm in Fig These issues express the effect of PVP to the optical properties of ZnS:Mn nanoparticles in polymer matrix The PLE spectra of samples were measured in Fig As seen in Fig 6, the PLE band of PVP monitored at 394 nm (Fig 6a) has a peak maximum at 332 nm, while the PLE band of ZnS:Mn–PVP (5:4) monitored at 430 nm shows a peak maximum at 340 nm The distance between these two PLE peak is about nm But the distance between the blue emission peak maxima (394 and 430 nm) of the PVP and ZnS:Mn–PVP (5:4) samples is rather large (about 46 nm) On the other hand, the PLE band of ZnS:Mn–PVP (5:4) sample monitored at 598 nm (orange emission of Mn2+ ) shows a peak maximum at 338 nm, which is rather close with the peaks of curves (a) and (b) in Fig From the above results, it is reasonable to suppose 123 154 T Tran Minh et al PL intensity (a.u.) PL intensity (a.u) PL of ZnS:Mn-PVP(5:4) 598 nm Exc 338nm 332 nm 30.0M 800.0k 25.0M 20.0M 15.0M 10.0M PLE spectrum of PVP Monitored at 394nm 5.0M 0.0 600.0k 240 260 280 300 320 340 360 380 Excitation wavelength (nm) 394 nm 434 nm 400.0k 200.0k PL of PVP polymer, Exc 325nm 0.0 250 300 350 400 450 500 550 600 650 700 Wavelength (nm) PL intensity (a.u.) Fig The PL spectra of the PVP and ZnS:Mn–PVP (5:4) samples 4.5x10 4.0x10 3.5x10 3.0x10 2.5x10 2.0x10 1.5x10 1.0x10 5.0x10 340 nm 332 nm PLE spectrum of PVP monitored at 394nm PLE spectrum of ZnS:Mn-PVP(5:4) monitored at 430 nm 338 nm a b c PLE spectrum of ZnS:Mn-PVP(5:4) monitored at 598 nm 240 260 280 300 320 340 360 380 400 420 440 Excitation wavelength (nm) Fig The PLE spectra of the PVP and ZnS:Mn–PVP (5:4) samples that: (i) the high energy band in the PLE spectrum of ZnS:Mn–PVP arises from the surface PVP molecules; (ii) the energy transfer occurs between the energy levels of surface PVP molecular orbitals and the luminescence centers of ZnS:Mn; (iii) the energy transition from surface PVP molecules to the Mn2+ centers occurs via hot ZnS Thus, the intensity of the blue luminescence increases stronger than the intensity of the orange luminescence when increasing of PVP concentration 3.4 Time-resolved PL spectra and luminescence decay curves The investigation of the kinetic decay process of electrons in energy bands is very important to the study of luminescence It can provide a scientific basis for the improvement of the luminescence efficiency of optical materials Figure shows the time-resolved PL spectra of PVP at 300 K excited by pulse N2 laser with 337 nm wavelength, pulse width of ns, frequency of 10 Hz 123 The optical properties and energy transition process 155 Fig The time-resolved PL spectra of PVP at 300 K excited by pulse N2 laser with 337 nm wavelength, pulse width of ns, frequency of 10 Hz The delay times after the excitation pulse are 33, 37, 40, 44 and 50 ns, respectively 400 436 nm 350 60 ns 64 ns 67 ns 73 ns 80 ns Intensity (a.u.) 300 250 200 150 100 50 452 nm 350 400 450 500 550 Wavelength (nm) Fig The time-resolved PL spectra of ZnS:Mn–PVP (5:3) at 300 K excited by pulse N2 laser with 337 nm wavelength, pulse width of ns, frequency of 10 Hz The delay times after the excitation pulse are 60, 64, 67, 73 and 80 ns, respectively The peaks of these spectra are shifted toward longer wavelength from 428 to 437 nm with increasing of the delay time from 33 to 50 ns Thus, the peaks of these spectra belong to the broad PL bands about of 390–470 nm of PVP excited by laser wavelength of 325 nm (in Fig 5) Beside that, the PL peak intensity decrease, while the spectral width of the PL band (full-width at half-maximum) decrease with increasing of the delay time These PL properties are attributed to electron transition from LUMO to HOMO levels in PVP molecules Figure shows the time-resolved PL spectra of ZnS:Mn–PVP (5:3) at 300 K excited by pulse N2 laser with 337 nm wavelength It is clearly that the peaks of PL spectra of ZnS:Mn– 123 156 T Tran Minh et al Intensity (a.u.) a 400 b 350 The PL decay curve 300 ZnS:Mn-PVP(5:3) at 450 nm λ ex = 337 nm 250 200 150 100 50 0 20 40 60 80 100 Time (ns) Fig a The PL decay curve of ZnS:Mn–PVP (5:3) b The PL decay curve of PVP PVP (5:3) are shifted toward longer wavelength from 436 to 452 nm with increasing of the delay time from 60 to 80 ns (in Fig 8) Furthermore the PL peak intensity decrease, while the spectral width of the PL band (full-width at half-maximum) also decrease with increasing of the delay time The PL peak shift with delay time of ZnS:Mn–PVP (5:3) sample is one of typical characteristics of donor-acceptor recombination in semiconductor material (Ishi zumi et al 2005) Thus this blue emission band is attributable to the intrinsic emission of defects, vacancy of sulphur VS , trapped electron by defects at donor level under conduction range and vacancy of zinc VZn at acceptor level up valence range Figure 9a, b show the PL decay curves of ZnS:Mn–PVP (5:3) at 450 nm and PVP at 428 nm when using exciting wavelength 337 nm, respectively The decay curves show that the number of free photoelectrons in exciting energy bands (corresponding to 450, 428 nm wavelengths) showed exponential attenuation and is given by: n ∝ e−t/τ , where τ is the lifetime of electrons in exciting energy band From those PL decay curves, the lifetime of free photoelectrons calculated about of τ1 = 21.5 ns for ZnS:Mn–PVP (5:3) at 450 nm and τ2 = 15.5 ns for PVP at 428 nm The lifetime τ1 is more smaller than that in ZnS:Mn,Cu samples sintered at high temperatures (Guoyi et al 2003) On the other hand, the lifetime τ2 is very short, thus it is characteristic of the radiative relaxation of electrons from the lowest energy unoccupied molecular orbital (LUMO) to the highest energy occupied molecular orbital (HOMO) levels in PVP From above optical results of PVP, the blue luminescence of PVP may be attributed to the radiative relaxation of electron from LUMO to HOMO levels as in Fig 11a 3.5 On the energy transfer from surface PVP molecules to the Mn2+ centers The PVP is a conjugated polymer with both N and C=O groups So with the ZnS:M-PVP samples (M: transition metal ion), it is believed that the bond between metal ions and PVP can give rise to overlapping of molecular orbitals of PVP with atomic orbitals of metal ions in surface regions (Ghosh et al 2006; Manzoor et al 2004) Thus, from the above results, we believe that the PVP passivating layers around the ZnS:Mn core described in Fig 10 are formed by coordination bond between the nitrogen atom of PVP and Zn2+ (Manzoor et al 2004) Figure 10 shows the incomplete coverage with low concentration of PVP (Fig 10a) and the complete coverage with higher concentrations of PVP (Fig 10b) From above analyzed results on the PL peaks of 437, 601 nm (in Table 1, Fig 3), the PLE spectra, the time-resolved PL spectra and the luminescence decay curves of PVP and 123 The optical properties and energy transition process Fig 10 The PVP coverage of ZnS:Mn grains a Incomplete coverage b Complete coverage Fig 11 Schematic illustration of various electronic transition and energy transfer process in ZnS:Mn–PVP a (PVP), b ZnS:Mn 157 a a b b LUMO Conduction band Green emission Mn 2+ T1 Orange emission A1 Valence band HOMO ZnS:Mn–PVP samples, the energy transition process from surface PVP molecules to the Mn2+ centers occurs via hot ZnS illustrated as in Fig 11a, b Conclusion The above experimental results show that the absorption edge of PVP is about of 230 nm The absorption edge of PVP capped ZnS:Mn nanoparticles shifted from 322 to 305 nm when the PVP concentration increases The luminescence spectra of PVP showed a blue emission with peak maximum at 394 nm While the PVP coating did not affect the microstructure of ZnS:Mn nanomaterial, the PL spectra of PVP capped ZnS:Mn nanoparticles samples were found to be affected strongly by the PVP concentration The mechanism of energy transition process from the surface PVP molecules to the Mn2+ centers in ZnS:Mn nanoparticles was explained Acknowledgments This work was supported by Vietnam’s National Foundation for Science and Technology Development (NAFOSTED) (Code 103.02.2010.20) References Agostinelli, T., Ferenczi, T.A.M., Pires, E., Foster, S., Maurano, A., Muller, C., Ballantyne, A., Hampton, M., Lilliu, S., Campoy-Quiles, M., Azimi, H., Morana, M., Bradley, D.D.C., Durrant, J., Macdonald, J.E., Stingelin, N., Nelson, J.: The role of alkane dithiols in controlling polymer crystallization in small band gap polymer:fullerene solar sells J Polym Sci Part B Polym Phys 49, 717–724 (2011) Cihaner, A., Algi, F.: Electrochemical and optical properties of an azo dye based conducting copolymer Turk J Chem 33, 759–767 (2009) Dubey, N., Leclerc, M.: Conducting polymers: efficient thermoelectric materials J Polym Sci Part B Polym Phys 49, 467–475 (2011) Ghosh, G., Naskar, M.K., Patra, A., Chatterjee, M.: Synthesis and characterization of PVP-encapsulated ZnS nanoparticles Opt Mater 28, 1047–1053 (2006) Guoyi, D., Xiaowei, L., Zhiren, W., Shaopeng, Y., Guangsheng, F.: Measurement of the time-resolved spectrum of photoelectrons from ZnS:Mn, Cu luminescent material J Phys Condens Matter 15, 1495– 1503 (2003) 123 158 T Tran Minh et al Hajduk, B., Weszka, J., Cozan, V., Kaczmarczyk, B., Jarzabek, B., Domanski, M.: Optical properties of polyazomethine with oxygen atom in the backbon Arch Mater Sci Eng 32(2), 85–88 (2008) Hattori, Y., Isobe, T., Takahashi, H., Itoh, S.: Luminescent properties of ZnS:Mn2+ nanocrystals/SiO2 hybrid phosphor synthesized by in situ surface modificationco-precipitation J Lumin 113, 69–78 (2005) Huang, C.M., Chen, L.C., Pan, G.-T., Yang, T.C.K., Chang, W.S., Cheng, K.W.: Effect of Ni on the growth and photoelectrochemical properties of ZnS thin films Mater Chem Phys 117, 156–162 (2009) Ishi zumi, A., White, C.W., Kanemitsu, Y.: Photoluminescence properties of impurity-doped ZnS nanocrystals fabricated by sequential ion implantation Physica E 26, 24–27 (2005) Jarzabek, B., Schab-Balcerzak, E., Chamenko, T., Sek, D., Cisowski, J., Volozhin, A.: Optical properties of new aliphatic-aromatic co-polyimides J Non-Cryst Solids 299(302), 1057–1061 (2002) Jarzabek, B., Weszka, J., Domanski, M., Jurusik, J., Cisowski, J.: Optical properties of amorphous polyazomethine thin films J Non-Cryst Solids 352, 1660–1662 (2006) Jarzabek, B., Weszka, J., Domanski, M., Jurusik, K., Cisowski, J.: Optical studies of aromatic polyazomethine thin films J Non-Cryst Solids 354, 856–862 (2008) Karar, N., Singh, F., Mehta, B.R.: Structure and photoluminescence studies on ZnS:Mn nanoparticles J Appl Phys 95(2), 656–660 (2004) Levy, L., Hochepied, J.F., Pileni, M.P.: Control of the size and composition of three dimensionally diluted magnetic semiconductor clusters J Phys Chem 100(47), 18322–18326 (1996) Maity, R., Maiti, U.N., Mitra, M.K., Chattopadhyay, K.K.: Synthesis and optical characterization of polymercapped nanocrystalline ZnS thin films by chemical process Physica E 33, 104–109 (2006) Manzoor, K., Vadera, S.R., Kumar, N., Kutty, T.R.N.: Synthesis and photoluminescent properties of ZnS nanocrystals doped with copper and halogen Mater Chem Phys 82, 718–725 (2003) Manzoor, K., Vadera, S.R, Kumar, N., Kutty, T.R.M.: Energy transfer from organic surface adsorbate-polyvinyl pyrrolidone molecules to luminescent centrers in ZnS nanocrystals Solid State Commun 129, 469– 473 (2004) McNeill, C.: Imaging the domain structure of organic semiconductor films J Polym Sci Part B Polym Phys 49, 909–919 (2011) Miller, N.C., Gysel, R., Miller, C.E., Verploegen, E., Beiley, Z., Heeney, M., McCulloch, I., Bao, Z., Toney, M.F., McGehee, M.D.: The phase behavior of a polymer-fullerene bulk heterojunction system that contains bimolecular crystals J Polym Sci Part B Polym Phys 49, 499–503 (2011) Murugadoss, G.: Synthesis and optical characterization of PVP and SHMP-encapsulated Mn2+ -doped ZnS nanocrystals J Lumin 130, 2207–2214 (2010) Murugadoss, G., Rajamannan, B., Ramasamy, V.: Synthesis and photoluminescence study of PVA-capped ZnS:Mn2+ nanoparticles Dig J Nanomater Biostruct 5(2), 339–345 (2010) Panda, S.K., Datta, A., Chaudhuri, S.: Nearly monodispersed ZnS nanospheres: synthesis and optical properties Chem Phys Lett 440, 235–238 (2007) Pattabi, M., Saraswathi Amma, B., Manzoor, K., Sanjeev, G.: Effect of MeV electron irradiation on the optical properties of PVP capped CdS nanoparticles in PVA matrix Sol Energy Mater Solar Cells 91, 1403– 1407 (2007) Peng, W.Q., Qu, S.C., Cong, G.W., Zhang, X.Q., Wang, Z.G.: Optical and magnetic properties of ZnS nanoparticles doped with Mn2+ J Cryst Growth 282, 179–185 (2005) Porambo, M.W., Marsh, A.L.: Synthesis and photoluminescent properties of doped ZnS nanocrystals capped by poly(vinylpyrrolidon) Opt Mater 31, 1631–1639 (2009) Pouretedal, H.R., Norozi, A., Keshavarz, M.H., Semnani, A.: Nanoparticles of zinc sulfide doped with manganese, nickel and copper as nanophotocatalyst in the degradation of organic dyes J Hazard Mater 162, 674–681 (2009) Sharif, N.S.A., Ridhuan, N.S., Ahmad, Z.A., Hutagalung, S.D.: Structural analysis of zinc sulphide nanopartiles synthesized via wet chemical route Solid State Sci Technol 18(1), 91–95 (2010) Sharma, R., Bhatti, H.S., Kyhm, K.: Enhanced oscillator strengths and optical parameters of doped ZnS bulk and nanophosphors Appl Phys B 97, 145–155 (2009) Soni, H., Chawda, M., Bodas, D.: Electrical and optical characteristics of Ni doped ZnS clusters Mater Lett 63, 767–769 (2009) Sparavigna, A.C., Florio, L., Avloni, J., Henn, A.: Polypyrrole coated PET fabrics for thermal applications Mater Sci Appl 1, 253–259 (2011) Thi, T.M., Tinh, L.V., Van, B.H., Ben, P.V., Trung, V.Q.: The effect of polyvinylpyrrolidone on the optical properties of the Ni-doped ZnS nanocrystalline thin films synthesized by chemical method J Nanomater vol 2012, Article ID 528047, p (2012) doi:10.1155/2012/528047 Thuy, N.M., Sam, D.T., Thi, T.M., Khoi, N.T.: Synthesis of Mn-Doped ZnS and the dopant-induced photoluminescence properties J Nonlinear Opt Phys Mater 17(2), 1–8 (2008) 123 The optical properties and energy transition process 159 Twardowski, A., Dietl, T., Demianiuk, M.: The study of the s-d type exchange interaction in Zn1−x Mnx Se mixed crystals Solid State Commun 48(10), 845–848 (1983) Wang, L., Tao, X.-T., Yang, J.-X., Ren, Y., Liu, Z., Jiang, M.-H.: Preparation and characterization of the ZnS nanospheres with narrow size distribution Opt Mater 28, 1080–1083 (2006) Wang, W., Huang, F., Xia, Y., Wang, A.: Photophysical and photoluminescence properties of co-activated ZnS:Cu,Mn phosphors J Lumin 128, 610–614 (2008) Yang, P., Lü, M., Xü, D., Yuan, D., Chang, J., Zhou, G., Pan, M.: Strong green luminescence of Ni2+ -doped ZnS nanocrystals Appl Phys A 74, 257–259 (2002) Yang, P., Lu, M., Xu, D., Yuan, D., Song, C., Liu, S., Cheng, X.: Luminescence characteristics of ZnS nanoparticlesco-doped with Ni2+ and Mn2+ Opt Mater 24, 497–502 (2003) 123 ... improve the optical properties of ZnS: Mn nanoparticles 3.3 Photoluminescence excitation (PLE) spectra of PVP and ZnS: Mn–PVP In order to examine the process of energy transfer in the PVP capped ZnS: Mn... concentration (Thuy et al 2008) Further, the influences of PVP concentration on the general features of the PL spectra and the process of energy transfer from the PVP to the Mn2+ luminescent... appropriate ratio in order to create the ZnS: Mn material with % nominal Mn concentration The pH level being crucial to the formation of ZnS: Mn precipitates, we derived and applied, by theoretical