DSpace at VNU: STUDY OF MICROSTRUCTURE AND OPTICAL PROPERTIES OF PVA-CAPPED ZnS : Cu NANOCRYSTALLINE THIN FILMS

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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.

1 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.2–5The partial substitution of Zn by Cu was shown to have considerable inﬂuence on the optical properties of the samples.9 Besides, the poly-mer coating used to protect ZnS nanoparticles from the environment inﬂuences

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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

PVA1and 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 concentra-tions 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 prop-erties 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

2 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(CH3COO)2·2H2O of 0.1 M, the second solution was the Cu(CH3COO)2·H2O of 0.1 M and the third solution was the Na2S·9H2O of 0.1 M The catalyst CH3OH :

H2O 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(CH3COO)2+ Na2S = ZnS↓ + 2CH3COONa,

Cu(CH3COO)2+ Na2S = CuS↓ + 2CH3COONa.

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 PVA-coated Cu doped ZnS thin films were produced by spin-coating the final mixed

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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 Advance Bukerding machine using the Cu-Kα radiation of λ = 1.5406 ˚A The parti-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

3 Results and Discussion

Figure 1 shows the PL spectra of P1,P2, P3, P4, P5powder samples, where the PL peaks are found at practically the same wavelength of about 500 nm and do 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 2 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 pow-der samples with PVA capping at diﬀerent concentrations show that the crystal

300 400 500 600 700 800 900 -5000

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

P1 P5 P4

P2

P3

Wavelength (nm)

Fig 1 The PL spectra of the P 1 , P 2 , P 3 , P 4 and P 5 ZnS : Cu nano powder samples with the 0.1%, 0.15%, 0.2%, 0.3%, 0.4% Cu-doped concentrations, respectively.

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20 30 40 50 60 70 100

200 300 400 500 600 700 800 900 1000

c

(3 1 1) (2 2 0) (1 1 1)

b a

c P-ZnS:Cu-PVA4 b.P-ZnS:Cu-PVA2 a.P-ZnS:Cu

2 theta

Fig 2 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 1), (2 2 0), (3 1 1) in agreement with the previously reported result.1It

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 3 One observes the grain in sphere form were embedded

Fig 3 TEM image of F-ZnS : Cu-PVA2 thin ﬁlm.

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300 350 400 450 500 550 600 0.5

1.0 1.5 2.0 2.5

b

c d a

a F-ZnS:Cu-PVA1

b F-ZnS:Cu-PVA2

c F-ZnS:Cu-PVA3

d F-ZnS:Cu-PVA4

Wavelength (nm)

Fig 4 The absorption spectra of the thin ﬁlms with diﬀerent PVA concentrations.

in the PVA matrix The average size of the grain is about 3 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 4 presents the absorption spectra of the F-ZnS : PVA1, F-ZnS : Cu-PVA2, 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:

α = K(hν − E g)

1/2

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 E g 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 E g values are all larger than that of the bulk ZnS (3.60 eV) One can further deduce the crystallite radiusr according to the following formula1:

∆E g = E g(ﬁlm)− E g(bulk) = h2

8r2

1

m ∗ e

+ 1

m ∗ h

− 1.8e2

Here, E g (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 1 along with the corre-sponding values of E g The results show that E g decreases while the crystallite size increases with increasing PVA concentration The table also shows that F-ZnS :

Cu-PVA1 thin ﬁlm has the largest band gap and smallest grain size with E g = 3.90 eV and r = 3.60 nm.

The PL spectra of the thin ﬁlms with diﬀerent PVA concentration are presented

in Fig 5 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

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Table 1 The direct energy gaps and the radii r of the

nanocrystallites in the thin ﬁlms.

300 400 500 600 700 800 900 1000 0

5000 10000 15000 20000 25000

c F-ZnS : Cu-PVA3

e F-ZnS : Cu

d F-ZnS : Cu-PVA4

b F-ZnS : Cu-PVA2

a F-ZnS : Cu-PVA1

e

c d

b

a

Wavelength (nm)

Fig 5 The PL spectra of the thin ﬁlms with diﬀerent PVA concentrations.

500 nm, implying that it was not eﬀected by PVA concentration However, the inten-sity 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 6 presents the PL spectra of the F-ZnS : Cu-PVA1 thin film attained with the exciting wavelength of 325 nm and different exciting power densi-ties 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 I P L = A(I EX)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

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300 400 500 600 700 800 0

200 400 600 800 1000 1200 1400

1600

505 nm e

d c b a

Wavelength(nm)

Fig 6 The PL of the F-ZnS : Cu-PVA1 thin ﬁlms with exciting wavelength of 325 nm and diﬀerent power densities.

0 5000 10000 15000 20000

ZnS:Cu

PVA-ZnS:Cu nano-thin film

500 nm

530 nm

Wavelength λ (nm)

Fig 7 The luminescence peak of F-ZnS : Cu-PVA2 thin ﬁlm is shifted towards the shorter wavelength in comparison with the powder sample prepared by the solid-state reaction method.

luminescence band of the thin ﬁlm sample is also shifted towards shorter wavelength From the schematic energy level diagram of ZnS : Cu given in Ref 7 and as proposed previously in Ref 8, the blue luminescence band is attributed to the electronic

transitions from the conduction band to the acceptor levels of the zinc vacancy V Zn,

and the green luminescence band at about 500 nm is due to a transition from the V S donor levels to the t2level of excited Cu2+ ion (in d9outer electron conﬁguration)

which is located in the ZnS band gap Here, the V S states are related to the sulfur vacancy and the doped Cu2+ions become the emission center in the Cu-doped-ZnS. Thus, the relative shift of luminescence peak of the thin ﬁlm sample is an indication

of the increase of band gap in the ﬁlm sample

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This change is explained as follows It is known that the Bohr exciton radius can be determined approximately by the following formula:

r B= h2ε

πe2

1

m ∗ e

+ 1

m ∗ h

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

4 Conclusion

We produced successfully the ZnS : Cu nanopowders with different Cu concentra-tions 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, percep-tible 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

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