The ZnS:Cu nanopowders were synthesized by the wet chemical method with Cu  concentrations of 0.1, 0.15, 0.2, 0.3 and 0.4{\%}. The microstructure of  samples was investigated by the X-ray diffraction (XRD) measurement. The  results show that the prepared samples belong to the Wurtzite structure with  the average particle size of about 3--7 nm. The highest luminescence  intensity of ZnS:Cu nanopowders corresponds to sample with Cu concentration  of 0.2{\%}. To investigate the effect of polyvinylacohol (PVA) on the  structure of ZnS:Cu, we have prepared the polyvinylacohol (PVA)-capped  ZnS:Cu thin films with a Cu concentration of 0.2{\%} by dip-coating method.  The PVA did not affect the microstructure of ZnS nanomaterials. The optical  properties of samples were studied by measuring the absorption and the  photoluminescence spectra in the wavelength range from 300 nm to 900 nm at  room temperature. The value of direct band gap is about 3.8 eV. The  dependence of the photoluminescence (PL) spectra of samples on the exciting  power density, their time-resolved-luminescence spectra were also  investigated

Thi, Tran Minh

Vuong, Nguyen Minh

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 20, No. 1 (2010), pp. 59-65
INFLUENCE OF THE CAPPED POLYMER ON THE OPTICAL
OF ZnS:Cu NANOCRYSTALLINE THIN FILMS
TRAN MINH THI
Faculty of Physics, Hanoi National University of Education
NGUYEN MINH VUONG
Quy Nhon University
Abstract. The ZnS:Cu nanopowders were synthesized by the wet chemical method with Cu con-
centrations of 0.1, 0.15, 0.2, 0.3 and 0.4%. The microstructure of samples was investigated by
the X-ray diffraction (XRD) measurement. The results show that the prepared samples belong to
the Wurtzite structure with the average particle size of about 3–7 nm. The highest luminescence
intensity of ZnS:Cu nanopowders corresponds to sample with Cu concentration of 0.2%. To in-
vestigate the effect of polyvinylacohol (PVA) on the structure of ZnS:Cu, we have prepared the
polyvinylacohol (PVA)-capped ZnS:Cu thin films with a Cu concentration of 0.2% by dip-coating
method. The PVA did not affect the microstructure of ZnS nanomaterials. The optical properties
of samples were studied by measuring the absorption and the photoluminescence spectra in the
wavelength range from 300 nm to 900 nm at room temperature. The value of direct band gap is
about 3.8 eV. The dependence of the photoluminescence (PL) spectra of samples on the exciting
power density, their time-resolved-luminescence spectra were also investigated.
I. INTRODUCTION
The ZnS nano material is a semiconducting material with direct and larger band
gap. The direct band gap of ZnS is about 3.6 eV for bulk form and 3.98 eV for ZnS
nano-form at 300 K [5] in the Wurzite structure. The direct band gap of nano-materials
may be controlled by doping, polymer coating and change of preparing conditions [1-4].
The doping of Cu has considerable influence to the optical properties of samples. However,
the polymer has a role as the protected environment for ZnS nanoparticles. The optical
properties of ZnS nanoparticles change considerably when diffusing them into polymer
matrix with optimal concentration. These properties can be explained by the quantum
confinement effect, the quantum size changed effect and the surface effect of polymer-
capped nanoparticles [5,6].
In this paper, we present some results on the nanostructure and optical properties of
the ZnS:Cu nanopowder with Cu concentrations of 0.1, 0.15, 0.2, 0.3 and 0.4%. In order to
study the influence of polymer PVA (polyvinylacohol) concentration on the optical ZnS:Cu
nano-materials, we have synthesized the PVA capped ZnS:Cu nanocrystalline thin films
with the constant Cu concentration of 0.2%, but different PVA concentration. In addition,
the influence of Cu and PVA concentration to luminescence spectra, grain size and direct
band gap of this film is also presented.
60 TRAN MINH THI AND NGUYEN MINH VUONG
II. EXPERIMENT
The ZnS:Cu nano-powder was prepared by the wet chemical method. Zn(CH3
COO)2.2H2O 0.1 M, Cu(CH3COO)2.H2O 0.1 M and Na2S.9H2O 0.1 M were used as the
initial solutions. For simplicity, we call them first, second, and third solution, respectively.
The catalysis is CH3OH:H2O for first and second solutions in term of ratio 1:1. The water
is the solvent for the third solution.
II.1. The synthetic of the 0.1%, 0.15%, 0.2%, 0.3% and 0.4% Cu doped ZnS
nano-powder (denoted as B1, B2, B3, B4, and B5) and the PVA doped ZnS
nano-powder with Cu concentration of 0.2%
The first and second solution are mixed with appropriate ratio in order to create
0.1%, 0.15%, 0.2%, 0.3% and 0.4% Cu doped ZnS material. The pH has an important
role in forming ZnS:Cu precipitates in this process. The third solution was added drop
by drop to a reaction vessel. The ZnS:Cu precipitates were separated by centrifuge and
drying at 80 C˚.
In order to receive the PVA doped ZnS:Cu nano-powder with Cu concentration of
0.2% and the different PVA concentrations, the PVA amounts of 1 g, 2 g, 3 g and 4 g are
dissolved in 100 ml of first and second mixed solutions with the constant Cu concentration
of 0.2% . The mixed solution with PVA is heated at 80˚ C and then decreased to room
temperature.
II.2. The production of the PVA-capped 0.2% Cu doped ZnS nanocrystalline
thin films
The first and second solutions with optimal concentration of 0.2 % Cu are mixed
in the same way as above. Then PVA amounts of 1 g, 2 g, 3 g and 4 g are dissolved in
100 ml of first and second mixed solution. The PVA-capped Cu doped ZnS thin films are
produced by the dip-coating method in the glass substrate.
Photoluminescence spectra at 300K of the samples are measured by the fluores-
cence spectrophotometer HP340-LP370 using a He-Cd laser source with the excitation
wavelength of 325 nm. The ultraviolet absorption spectra of thin samples are measured
by the spectrometer JASCO -V670. The X-ray diffraction patterns were recordered by
the XD8 Advance Bukerding machine using the CuKα radiation of λ = 1.5406A˚ and the
particle size is measured by TEM The dependence of the photoluminescence spectra (PL)
of samples on the exciting power density, their time-resolved-luminescence spectra are also
investigated.
III. RESULTS AND DISCUSSION
III.1. The 0.1, 0.15, 0.2, 0.3 and 0.4% Cu doped ZnS nano-powder and the
PVA doped ZnS nano-powder with Cu concentration of 0.2%
Fig. 1 shows the typical PL spectra of B1, B3 and B5 nano-powders. It is clear
that, the maximum position of PL peaks is about 500 nm and do not depend on the Cu
doped concentration. However, the intensity of PL peaks changed with the Cu concen-
tration. The optimal Cu concentration in ZnS:Cu nano materials is about 0.2%. For this
concentration, the PL intensity of sample reaches to the highest.
INFLUENCE OF THE CAPPED POLYMER ON THE OPTICAL OF ZnS:Cu ... 61
Figure 2 shows the XRD patterns of B-ZnS:Cu, B-ZnS:Cu-PVA2 and B-ZnS:Cu-
PVA4 powders. The positions of the peaks seem to be not changed in all samples. This
means that the polymer PVA does not affect the crystal structure of the ZnS:Cu nanoma-
terials.
300 400 500 600 700 800 900
-5000
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
B
5
ZnS-0.4%Cu
B
3
ZnS-0.2%Cu
B
1
ZnS-0.1%Cu
B
5
B
1
B
3
In
te
ns
ity
(a
.u
.)
Wavelength(nm)
Fig. 1. The PL spectra of B1, B3 and B5 nano-powder
20 30 40 50 60 70
c
c
b
b
a
a
B-ZnS:Cu-0.2%-PVA4
B-ZnS:Cu-0.2%-PVA2
B-ZnS:Cu-0.2%
I(
a
.u
.)
2 q
Fig. 2. The XRD spectra of, B-ZnS:Cu-PVA, B-ZnS:Cu and B-ZnS:Cu-PVA4
powder samples
Although the form of these patterns is very closed to the Sphalerite phase, but the
peak positions are shifted in comparison with the Sphalerite phase. Thus, we think that
ZnS:Cu nanoparticles present the Wurtzite phase with good crystallization. The positions,
the widths as well as the intensities of XRD peaks are nearly the same for all samples.
The average size of grains is about of 2.7 and 3.2 nm following the Scherrer formula as
shown in Table 1.
Table 1. The average size of grains
Samples B-ZnS:Cu B-ZnS:Cu-PVA2 B-ZnS:Cu-PVA4
D (nm) 2.7 2.9 3.2
62 TRAN MINH THI AND NGUYEN MINH VUONG
III.2. The PVA-capped 0.2% Cu doped ZnS nanocrystalline thin films
As discussed above, the PVA does not affect the structure of ZnS:Cu nanomaterials.
However, their PL intensity achieved maximum with the concentration of 0.2% Cu. Thus,
we have made the PVA capped 0.2%Cu doped ZnS nanocrystalline thin films with different
concentrations of PVA.
The morphology of the M-ZnS:Cu-PVA2 thin film was observed by TEM image in
Fig. 3. The polymer grains are in sphere form which coated outside the ZnS:Mn nano-
particles. The average size of grains is smaller than 5 nm in the polymer PVA matrix.
Thus, the calculated results by the Scherrer formular are in agreement with that of TEM
image.
300 350 400 450 500 550 600
0.5
1.0
1.5
2.0
2.5
b
c
d
a
a M-ZnS:Cu-PVA1
b M-ZnS:Cu-PVA2
c M-ZnS:Cu-PVA3
d M-ZnS:Cu-PVA4
A
b
s
o
rp
ti
o
n
c
o
e
ff
ic
e
n
t
(a
.u
.)
Wavelength(nm)
Fig. 3. TEM image of M-ZnS:Cu-
PVA2 thin film
Fig. 4. The absorption spectra of the thin films with
different polymer concentrations
Figure 4 presents the absorption spectra of the M-ZnS:Cu-PVA1, M-ZnS:Cu-PVA2,
M-ZnS:Cu-PVA3 and M-ZnS:Cu-PVA4 thin films.
The absorption band of M-ZnS:Cu-PVA3 and M-ZnS:Cu-PVA4 thin films shifted
towards the lower energy when the PVA increased. The relation between the absorption
coefficient α and the energy of photon can be calculated by the following equation [5, 6]:
α =
K(hν − Eg)
n/2
hν
· (1)
Here, K is constant, Eg is the direct band gap and n = 1 for direct band gap.
From equation (1) and the absorption spectra in Fig. 4, we calculated the direct band
gap of about 3.89 eV, 3.86 eV, 3.77 eV and 3.73 eV for the M-ZnS:Cu-PVA1, M-ZnS:Cu-
PVA2, M-ZnS:Cu-PVA3 and M-ZnS:Cu-PVA4 thin films, respectively. These calculated
results are given in Table 2. It is clear that the values of Eg decrease with increasing the
PVA concentration. Nevertheless, these values of Eg are still larger than that of ZnS bulk
samples (3.6 eV). If we assume the crystal is in sphere form, the crystallite radius r can
INFLUENCE OF THE CAPPED POLYMER ON THE OPTICAL OF ZnS:Cu ... 63
be determined by formula (see [5]):
∆Eg = Eg(film)− Eg(bulk) =
h2
8µr2
−
1.8e2
εr
(2)
Here, Eg (bulk) = 3.6 (eV), µ =
m∗em
∗
h
m∗e +m
∗
h
with m∗e = 0.34m0, m
∗
h = 0.24m0, m0 is the
mass of free electron. The calculated results of crystallite radius r for these thin films are
given in Table 2.
The results show that the values of Eg decrease and the crystallite size increases
when the PVA concentration increases. The results show that M-ZnS:Cu-PVA1 thin
film gives the largest band gap and the smallest crystallite size with Eg = 3.89 eV and
r = 3.6 nm.
Table 2. The direct band gap and the crystallite radius r of the thin films
Thin films Eg (eV) r (nm)
M-ZnS:Cu-PVA1 3.89 3.6
M-ZnS:Cu-PVA2 3.86 3.9
M-ZnS:Cu-PVA4 3.77 5.5
M-ZnS:Cu-PVA5 3.73 7.4
The above results can be explained as following: In the preparing process, the
viscosity of solutions increases when the PVA concentration increases. Thus, the particle
size increases due to the coalescence of particle crowd. From the preparing conditions, we
see that ultrasonic ring frequency and time affect the particle size of samples and then
affect Eg of samples.
III.3. The photoluminescence spectra, the luminescence dependence on the
laser excitation power density, the time-resolved-luminescence spectra
Figure 5 presents the photoluminescence spectra of the thin films with different
polymer concentration.
The maximum positions of PL peaks are about 500 nm and were not effected by
PVA doped concentration. However, the intensity of PL peaks changed with the PVA
concentration. The PL intensity of the M-ZnS:Cu-PVA1, M-ZnS:Cu-PVA2 samples are
higher than that of other samples.
Figure 6 presents the PL spectra of the M-ZnS:Cu-PVA1 thin film with the exciting
wavelength of 325nm and different exciting power densities. From these PL spectra, it
can be seen that the maximum positions of the peak at 517 nm are not changed when
the power density increases from 0.19W/cm2 to 0.45W/cm2. However, their intensity was
changed in the law IPL = A (IEX)
n with n ≈ 0.54.
Figure 7 presents results of the time-resolved-luminescence spectra of the green band
at 300 K with the exciting wavelength of 337 nm for M-ZnS:Cu-PVA1. It is clearly that
the peaks of the band shift towards the lower energy and their intensity decreases about
5 times when the delay time increases from 60 ns to 77 ns. This is a typical characteristic
of the donor-acceptor emission recombination.
64 TRAN MINH THI AND NGUYEN MINH VUONG
300 400 500 600 700 800 900 1000
0
5000
10000
15000
20000
25000
30000
c
e
d
b
a
e
c
d
b
a
ZnS:Cu_PVA1
ZnS:Cu_PVA2
ZnS:Cu_PVA3
ZnS:Cu_PVA4
ZnS:Cu
In
te
n
s
it
y
(a
.u
.)
Wavelength(nm)
300 400 500 600 700 800
0
200
400
600
800
1000
1200
1400
1600
517
e
d
c
b
a
a 0.19W/cm
2
b 0.25W/cm2
c 0.32W/cm
2
d 0.38W/cm
2
e 0.45W/cm
2
In
te
n
s
it
y
(a
.u
.)
Wavelength(nm)
Fig. 5. The PL spectra of the thin films with
different polymer concentrations
Fig. 6. The PL of the M-ZnS:Cu-PVA1 thin
films at 300K with exciting wavelength of
325nm and power density 0.32W/cm2
360 400 440 480 520 560
0
40
80
120
160
474 nm
450 nm M-ZnS:Cu-PVA1
e
d
c
b
a
I
(a
.u
)
Wavelength (nm)
a 60 ns
b 63 ns
c 66 ns
d 71 ns
e 77 ns
0 5 10 15 20 25 30 35 40 45
0
20
40
60
80
100
120
140
160
180
200
M-ZnS:Cu-0.2%-PVA1
l
ra
= 450 nm
l
ex
= 337 nm
I
(a
.u
)
t (ns)
Fig. 7. The time-resolved-luminescence spectra
and the fluorescence lifetime with the exciting
wavelength of 325 nm for the M-ZnS:Cu-PVA1
thin film
Fig. 8. The decrease of photoluminescence
at wavelength 450 nm at 300 K, the excit-
ing wavelength of 337 nm for M-ZnS:Cu-
PVA1 sample
Fig. 8 describes the PL decrease of M-ZnS:Cu-PVA1 thin film at the wavelength
of 450 nm at 300K with the exciting wavelength of 337 nm. From the above plots, we
calculated the luminescence lifetime of about 11 ns at the wavelength of 450 nm.
From the asymmetry of the time-resolved-photoluminescence spectrum as presented
in Fig. 8 we suggest that the green luminescence band from 450 nm to 473 nm is the overlap
of many photoluminescence lines due to the emission recombination of donor–acceptor
pairs, where vacancies of S2− and Zn2+ play as donor and acceptor centers, respectively.
IV. CONCLUSION
We have successfully prepared the M-ZnS:Cu-PVA1, M-ZnS:Cu-PVA2, M-ZnS:Cu-
PVA3 and M-ZnS:Cu-PVA4 thin films by the wet chemical method and the dip-coating
method on the glass substrate. The above obtained results allow us to conclude that
INFLUENCE OF THE CAPPED POLYMER ON THE OPTICAL OF ZnS:Cu ... 65
these thin films have good quality. The optical properties of ZnS nanoparticles change
considerably when they are diffused into the polymer matrix.
ACKNOWLEDGEMENTS
The paper is completed by the support of the Ministerial-level project on the science
and technology under the code B2010-17-234.
REFERENCES
[1] A.A. Bol, J.Ferwerda, J.A.Bergweff, and A.Meijerink, Jounal of Luminescence 99 (2000) 325-334.
[2] A.Ishizumi, C.W.White, and Y.Kanemitsu, Applied Physics Letters 84 (2004) 13.
[3] M.Wang, K. Sun, X. Fu, and C. L. C. Yan, Solid State Communication 115 (2000) 492-496.
[4] S. Lee, D. Song, D. Kim, J. Lee, S. Kim, I. Y. Park, and Y. D. Choi, Materials Letters 58 (2004)
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[5] P.K. Ghosh, S. Jana, S. Nandy, and K.K. Chattopadhyay, Materials Research Bulletin 42 (2007)
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[6] R.Maity, U.N. Maiti, M.K. Mitra, and K.K. Chattopadhyay, Physica E33 (2006) 104–109.
Received 05 May 2009.


Influence of the Capped Polymer on the Optical of ZnS:Cu Nanocrystalline Thin Films

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Influence of the Capped Polymer on the Optical of ZnS:Cu Nanocrystalline Thin Films

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