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International Journal of Chemical Science and Technology
Universal Research Publications. All rights reserved
ISSN 2249-8532
Original Article
Chemical synthesis of metal doped copper sulphide nanoparticles in PVA matrix
Alaric D. Sangma1 and P. K. Kalita2
1
Department of Physics, Diphu Govt. College, Diphu, Assam, India
Nano Science Research Laboratory, Guwahati College, Guwahati, India
Author for correspondence: Alaric D. Sangma, Email: [email protected]
2
Received 08 November 2012; accepted 05 December 2012
Abstract
Copper sulphide nanoparticles prepared through simple chemical bath deposition show quantum confinement. Polyvinyl
Alcohol (PVA), used as capping agent, was found to play a key role in the confinement process. Both the phases,
chalcocite and covellite, were existing in the synthesised CuS at room temperature. HRTEM (high resolution transmission
electron microscopy) showed well defined hexagonal faced spherical nanoparticles of size 10nm for undoped CuS. The
particle size increased in the range 10nm-32nm on Mn and Zn doping which was also clearly reflected in red shift of PL
peak in PL measurements. The prime UV absorption at 360nm confirmed the formation of more Cu 2S nanoparticles with
an enhanced band gap 3.6 eV. The PL intensity was found to increase on doping. The co-doped copper sulphide exhibited
the maximum PL intensity and that may be significant for using co-doped copper sulphide as blue emitter in fabrication of
LED (light emitting diode).
© 2012 Universal Research Publications. All rights reserved
Keywords: CuS; Quantum confinement; photoluminescence; doping.
1. Introduction
Copper sulphides belonging to the I-IV compound
semiconductor material [1,.2] and are among the
chalcogenides compounds with several application in
Nanosciences. They are found very useful in coating solar
energy conversion systems and solar controlled devices.
The chalcogenides are also used in the fabrication of
microelectronic devices, optical filters as well as in low
temperature gas sensor applications [2]. Moreover, ternary
copper chalcogenides like copper indium diselenide and
copper indium gallium selenides, in recent times, are now
widely used in the fabrication of solar photovoltaic cells
[3].Special attention is given to the study of copper
sulphide thin films due to the discovery of the CdS/CuxS
heterojunction solar cell [2]. Synthesis and characterization
of nanocrystals of semiconducting metal sulphides have
been an intense field of research due to their interesting
properties and potential applications [4-5]. When the size
of the nano crystal is smaller than the Bohr excitation
radius of the material, they exhibit properties which are size
dependent and distinct from the bulk [6]. Such a material is
a very promising one in electronics, data storage, energy
storage, catalysis and sensors [7]. It is, therefore, important
to develop techniques which are simple, cost-effective,
environment friendly, easily scalable and at the same time
with parameters to control size and shape of the materials.
Doping on ZnS with transitional metal like Zn2+ and Mn2+
57
is an important aspect to yield different nanostructures.
Formation of nanostructures depends not only on the metal
ions but also on its concentration [8]. Doping type as well
the sulphur containing ligands play key role for particular
luminescence emission with enhanced efficiency [8-9].
ZnS:Cu have the ability to produce multicolor emission
such as blue, green and even red depending upon the Cu
ions lying on the interstices or substitute Zn2+ ions in the
lattice. However, less work has been done till date on metal
doping into CuS. Therefore, it is desirable to dope CuS
nanoparticles with metals like Zn and Mn, singly and codope, and to study their structural and optical properties. In
this paper the chemical synthesis of CuS, CuS:Zn, CuS:Mn
and CuS:Zn,Mn nanoparticles and their structural and
optical properties has been presented.
2. Materials and Methods
2.1 Synthesis
CuS nanoparticles were grown on glass substrate as their
films as well as colloidal particles through chemical bath
deposition (CBD) at room temperature. Equal volumes and
equimolar (0.5M) solutions of copper acetate monohydrate
(Cu (ac)2) and thiourea (H2NCSNH2) were prepared by
dissolving in deionised water. A 40ml of copper acetate
monohydrate is mixed with 20ml of 3% poly-vinyl alcohol
(PVA) and stirred. Ammonia solution was added to it till
formation of clear metallic complex and the pH was kept at
10. Then 40ml thiourea solution was allowed to mix drop
International Journal of Chemical Science and Technology 2012; 2(4): 57-60
Figure 1 : HRTEM images of undoped CuS (a1 and a2),
CuS:Mn (b1 and b2) and CuS:Zn (c1 and c2).
by drop with the complex solution which yields undoped
CuS nanoparticles under constant stirring. For Zn doping,
zinc chloride weighing 10% of the weight of copper acetate
monohydrate host was mixed to the host solution. To this
mixture 3% solution of PVA was mixed and stirred prior to
deposition. Similar procedure was also followed for Mn
doping. Ammonia was added to that mixture solution to
form clear metallic complex, maintaining the pH in
between 10 and 11. Then thiourea solution was allowed to
mix with the complex solution which yield CuS: Zn and
CuS: Mn nanoparticles. For co-doping, both zinc chloride
and manganese chloride weighing 10% of weight of host
copper acetate monohydrate were mixed with the 0.5M
copper acetate monohydrate host solution. Following the
afterwards procedure finally CuS: Zn, Mn nanoparticles
were synthesised. The glass substrates were cleaned and
dried prior to the synthesis and dipped in the final matrix
solutions to cast films. The colloidol solutions were taken
for structural and optical measurements.
2.2 Characterization
Chemically synthesised undoped and doped CuS samples
were characterized using HRTEM (high resolution
transmission electron microscopy), UV-Visible absorption
and PL (Photoluminescence) spectroscopy to investigate
their structural and optical properties. Sonicated colloidal
solutions of the samples were taken for structural
morphology study using a HRTEM machine [Model: JEM
58
2100, 200kV, Jeol]. The UV-Visible absorption of the
samples was recorded using an automated spectrometer
(Model: HITACI 113210) in the wavelength range 200nm800nm. PL investigations were done using a He–Cd laser, a
1m Cerny–Turner spectrograph, and a photomultiplier. The
excitation wavelength was 325nm for all the samples.
3. Results and Discussion
3.1 HRTEM analysis
The structural morphology of CuS nanoparticles prepared
through CBD technique was studied through HRTEM
analysis. The synthesised samples at room temperature
reveal that the CuS were of polycrystalline and exhibit
uniform distribution of nanocrystals in large area. The
particle sizes of chemically synthesized nanoparticles
usually differ and depend on the dominant growth
parameters viz., bath temperature, molarity and pH [10].
The SAED (selected area electron diffraction) and the
average particle size of undoped CuS is shown in Figure 1:
(a1) and (a2) respectively. It exhibits both hexagonal
chalcocite phase along with covellite at room temperature.
The existence of both phases indicates the formation of
both CuS and Cu2S. The HRTEM shows the hexagonal
shaped nanoparticles formed in the samples are well
defined and distributed over a large area. Hexagonal type
particles suggest the formation of Cu2S chalcocite in the
synthesised material. The nanoplate based architectures of
hexagonal type copper sulfide nanoparticle was reported in
the literatures. The average particle size of undoped CuS
deposited at room temperature was found to be around 10
nm. Doping can change the structural morphology of the
material. In the present work the shape of the particles on
Mn and Zn doping was not found to be changed
significantly as shown in Figure 1: (b1) & (b2) and Figure
1: (c1) & (c2) respectively. The sizes of overall spherical
shaped particles were found to increase on doping. The
HRTEM images show an average size 32nm on Mn doping
and 14nm on Zn doping. In fact the particles show a
tendency to agglomerate to have bigger structures on
doping. The chemical interaction between CuS and –OH
group of PVA can prevent the nanoparticles from
agglomeration. The particles were well separated and
coated with polyvinyl alcohol. This HRTEM morphology
quite suggests that PVA acts as a good stabilizer where the
particles are nicely embedded.
3.2 UV-VIS Spectroscopy
The optical absorption spectra of the nanoparticles were
measured using USB-2000 UV-vis spectrometer. The
nanocrystallites thin film was suspended in glycerol using
magnetic Stirrer and their optical absorption spectra were
recorded at room temperature over the range 350nm to
800nm. Figure 2 shows the absorption spectra of CuS
undoped and doped with Mn and Zn. The effect of codoping was also studied. The UV-VIS spectra depend on
Cu/S composition of copper sulphide nanocrystals as well
as nature of metal dopants and their concentration.
Generally CuS shows a characteristics absorption band near
IR region. Cu2S shows weaker absorption peak and exhibit
a large red shift with respect to CuS. The UV-vis
measurements exhibit a series of absorption peaks at
330nm, 360nm, and at 380nm in UV region whereas small
International Journal of Chemical Science and Technology 2012; 2(4): 57-60
absorption at 360nm become prominent suppressing the
other long wavelength absorption. A small peak around
620nm may indicate the absorption corresponding
manganese impurity. The symmetric nature of the UV peak
reveals the quite uniform distribution of particles. The
absorption spectrum of Zn doped CuS exhibits similar
identical absorptions at 330nm, 360nm and at 370nm.
When the sample was co-doped with Mn and Zn more UV
absorptions around 320nm and 340nm with higher intensity
were observed. Hence it was inferred that the absorption
was mainly confined in the UV region on doping.
Figure 3: PL spectra of (a) undoped CuS, (b) singly doped
CuS:Mn, (c) CuS:Zn and (d) co-doped CuS:Mn,Zn.
Figure 2 : UV-VIS absorption spectra of undoped CuS,
singly doped CuS:Mn, CuS:Zn and co-doped CuS:Mn,Zn.
shoulders around 570nm, 625nm, 650nm and further rise of
absorption at 800nm afterwards for the undoped CuS. The
absorption maximum at 360nm may be assigned to the
band excitonic absorption while the others are expected
because of native defects originated from the structural
deformation. The broad absorption in the short wavelength
side at 360nm corresponds to that of chalcocite phase [11]
and that of absorptions at 625nm onwards are owing to that
of the covellite phase [12] of copper sulphide. This clearly
shows that the absorption edges are blue shifted with
respect to the bulk Cu2S (1.5eV) and CuS (1.27eV)
respectively. Similar results were also reported by other
workers for the chemical bath deposited copper sulphide
nanoparticles. The optical absorption characteristics of the
present work therefore suggest that the synthesised samples
are predominantly are of Cu2S which is also confirmed by
HRTEM measurement. The band gap energy of the
synthesised Cu2S calculated as 3.6eV which is enhanced
with respect to those bulks. The observed blue shift and
corresponding increase in band gap confirms the quantum
confinement amongst the nanoparticles. This clearly
indicates the presence of both CuS and Cu2S in the
prepared samples [11,13]. On Mn doping the UV
59
3.3 PL Spectroscopy
Photoluminescence involves using a beam of light, usually
UV light that excites the electrons in the molecules of
certain compounds and causes them to emit light of a lower
energy, typically but not necessarily visible light. In the
present work the PL spectra of undoped and doped CuS
were taken using excitation wavelength 325nm and those
were depicted in Figure 3. Photoluminescence spectra of
undoped copper sulphide show emission at 423nm. The
copper sulphide usually shows emission in a near band–
edge region corresponds to the compositional variation of
copper and sulphur such as 440 nm for Cu7S4 and 443 nm
for Cu9S8. The present observed blue emission is expected
due to the near band-edge emission of Cu2S nanoparticles.
The PL peak is quite broad having higher FWHM (full
width half maximum) and is red shifted with respect to that
of absorption edge. There may be different type inherent
defects on the surface of the undoped copper sulphide
nanoparticles that can emit band–trap emissions. Hence the
surface states modulate the PL emission along the bandedge emission. Similar PL properties of copper sulphide
are also reported by other workers [14, 15]. It is observed
that the PL peak gradually red shifted to long wavelength
side on doping. This is obviously due to impurity
incorporation in the material. The PL maximum shifted to
496nm on Mn doping along with a shoulder at 454nm. On
Zn doping into the copper sulphide the spectra exhibits
three peaks, the maximum PL peak at 517nm and other two
International Journal of Chemical Science and Technology 2012; 2(4): 57-60
around at 497nm and at 492nm. When copper sulphide was
co-doped with Mn and Zn, the PL spectrum shows
emission at 397nm and 453nm with higher order intensity
over those single doping. The result of co-doped copper
sulphide may be quite significant as it exhibits the
possibility of using it as an effective blue emitter in
fabrication of LED.
4. Conclusion
In the present study CuS nanoparticles, Zn doped CuS
nanoparticles, Mn doped CuS nanoparticles and, Zn and
Mn doped CuS nanoparticles, were synthesized
successfully through chemical bath deposition (CBD)
route. The copper acetate monohydrate (Cu (ac) 2) was used
as copper source while thiourea (H2NCSNH2) as sulphur
source. Polyvinyl Alcohol (PVA) was used as capping
agents. Equal volumes and equimolar (0.5M) solution of
copper acetate and thiourea was used for the synthesis. The
high resolution transmission electron microscopy
(HRTEM) images of CuS nanocrystals showed that the
spherical size of nanoparticles varied from 10-32 nm at
room temperature. The formation of nanoparticles was
confirmed by using UV-visible absorption measurements.
The absorption spectra exhibit the peaks in the range 330370nm as well as 620-750nm which are expected owing to
the existence of both the covellite and the chalcocite phase.
The photoluminescence (PL) spectra show blue emission at
423nm for undoped CuS whereas the peaks are red shifted
up to 454-517nm on doping. It is observed that doping
induced clear blue-green emisssion and enhance PL
intensity which implies the possible LED application.
Acknowledgement
The authors thank the Department of Chemistry and SAIF,
NEHU, Shillong for providing the characterization
facilities regarding TEM, UV-VIS and PL measurements.
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Source of support: Nil; Conflict of interest: None declared
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International Journal of Chemical Science and Technology 2012; 2(4): 57-60