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...

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: 4 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 nanocrys-talline 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 micro-structure 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

1 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

chain structure and conductivity of the polymer is still poorly understood Recently, research-ers 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 lumines-cence, non-linear optical and electrical properties of a variety of polymers have been per-formed (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+ nano-particles 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 1 % (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 adsor-bate such as PVP to dopant ions such as Cu+or Mn+(Manzoor et al 2003,2004) Further-more, 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 con-centration (Thuy et al 2008) Further, the influences of PVP concentration on the gen-eral 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

2 Experiments

The previous researching results on the optical properties of ZnS:Mn showed that the lumi-nescence 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 con-stant nominal Mn concentration of 9 % 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(CH3COO)2.2H2O 0.1M;

– Solution II: Mn(CH3COO)2.4H2O 0.1M and

– Solution III: Na2S.9H2O 0.1M.

The solvent in both solutions I and II was a CH3OH:H2O 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 9 % 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 = 5 where ZnS:Mn precipitates in mixed solutions while Zn(OH)2does not The reactions are as follows:

Zn(CH3COO)2+ Na2S→ ZnS + 2CH3COONa

Mn(CH3COO)2+ Na2S→ MnS + 2CH3COONa 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 5 ml of CH3OH:H2O (1:1) solvent This mixture was called solution A Similarly, 0.5 g of PVP was dissolved in 5 ml of C2H5OH:H2O (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 1 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 CuKαradiation

ofλ = 1.5406 Å and transmission Electron Microscopy (TEM) JEM 1010

Photolumines-cence (PL) spectra, photoluminesPhotolumines-cence 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

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a ZnS:Mn-PVP(5:2)

b ZnS:Mn-PVP(5:4)

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311 220 111

(b)

2θ(degree)

(a) ZnS (b) ZnS:Mn-1%

2θ(degree)

111

220

311

a b

Fig 1 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

3 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 tak-ing 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

Figure1shows 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 3nm was calculated by

Scherrer formula

Alternatively, the average particle size in ZnS:Mn–PVP (5:4) sample is about 3 nm, as measured in TEM (Fig.2a) Figure2a shows that these grains are ZnS:Mn nanoparticles coated by polymer covers Figure2b 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)

Fig 2 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 0 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 lumines-cence peak maximum positions are unchanged, but their intensities increase rather strongly with increasing of PVP concentrations The orange emission band is attributable to4T1–6A1

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 incorpora-tion of trapped electron by defects at donor level under conducincorpora-tion 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 Table1shows the intensity of the peaks at 437 and

601 nm for samples with different PVP concentrations

It can be seen clearly from Table1that the intensity of PL peak at 437 nm increases stronger than that of the peak at 601 nm, when the PVP concentration increases

350 400 450 500 550 600 650 700 750 0

5000 10000 15000 20000 25000 30000 35000 40000

601 nm

437 nm

Wavelength (nm)

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)

a b c d e

Fig 3 The PL spectra of the ZnS:Mn–PVP excited by excitation wavelength of 325 nm with different content

of PVP

Table 1 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)

The absorption spectra of PVP, ZnS:Mn and ZnS:Mn–PVP with different PVP concentra-tions are shown in Fig.4 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 absorp-tion lines that characterized by donor-acceptor absorpabsorp-tion transiabsorp-tion of vacancies or defects in ZnS when doped Mn was added into hot ZnS semiconductor (Wang et al 2008) The decreas-ing 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 3d5electrons 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 do not effect to size of nanoparticles However the PVP play an important role as the protective layer, against agglomeration ZnS:Mn nanoparti-cles 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

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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)

Wavelength (nm)

Fig 4 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.5shows 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.3 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.6 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 8 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 338nm, which is rather close with

the peaks of curves (a) and (b) in Fig.6 From the above results, it is reasonable to suppose

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332 nm

PLE spectrum of PVP Monitored at 394nm

Excitation wavelength (nm)

PL of PVP polymer, Exc 325nm

394 nm

PL of ZnS:Mn-PVP(5:4) Exc 338nm

434 nm

598 nm

Wavelength (nm)

Fig 5 The PL spectra of the PVP and ZnS:Mn–PVP (5:4) samples

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PLE spectrum

of PVP monitored

at 394nm

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PLE spectrum of ZnS:Mn-PVP(5:4) monitored at 598 nm

PLE spectrum

of ZnS:Mn-PVP(5:4) monitored at 430 nm

340 nm

Excitation wavelength (nm)

Fig 6 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 Figure7shows the time-resolved PL spectra

of PVP at 300 K excited by pulse N2 laser with 337 nm wavelength, pulse width of 7 ns, frequency of 10 Hz

Fig 7 The time-resolved PL spectra of PVP at 300 K excited by pulse N2laser with 337 nm wavelength, pulse width of 7 ns, frequency of 10 Hz The delay times after the excitation pulse are 33, 37, 40, 44 and 50 ns, respectively

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452 nm

436 nm

Wavelength (nm)

wavelength, pulse width of 7 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 Figure8shows the time-resolved PL spectra of ZnS:Mn–PVP (5:3) at 300 K excited by pulse N2laser with 337 nm wavelength It is clearly that the peaks of PL spectra of ZnS:Mn–

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The PL decay curve ZnS:Mn-PVP(5:3)

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Time (ns)

Fig 9 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 VZnat acceptor level up valence range

Figure9a, 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, whereτ is the

lifetime of electrons in exciting energy band From those PL decay curves, the lifetime of free photoelectrons calculated about ofτ1 = 21.5 ns for ZnS:Mn–PVP (5:3) at 450nm and

τ2 = 15.5 ns for PVP at 428nm 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 lifetimeτ2

is very short, thus it is characteristic of the radiative relaxation of electrons from the low-est energy unoccupied molecular orbital (LUMO) to the highlow-est 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.10are formed by coordination bond between the nitrogen atom of PVP and Zn2+(Manzoor et al.

2004) Figure10shows 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 Table1, Fig.3), the PLE spectra, the time-resolved PL spectra and the luminescence decay curves of PVP and