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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]

1369-80

doi:10.1

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journal homepage: www.elsevier.com/locate/mssp

Enhanced photoluminescence properties of ZnS:Cu2þ nanoparticlesusing PMMA and CTAB surfactants

M. Kuppayee a, G.K. Vanathi Nachiyar a, V. Ramasamy b,n

a Department of Physics, Sri Sarada College for Women, Salem, Tamilnadu, Indiab Department of Physics, Annamalai University, Tamilnadu 608002, India

a r t i c l e i n f o

Keywords:

ZnS:Cu2þ

Capping agent

Quantum confinement

Luminescence

Blue shift

01/$ - see front matter & 2011 Elsevier Ltd. A

016/j.mssp.2011.09.006

esponding author.

ail address: [email protected] (V. R

e cite this article as: Kuppayee M, eA and CTAB surfactants. Materials S

a b s t r a c t

ZnS nanoparticles with Cu2þ doping have been synthesized at 80 1C through a soft

chemical route, namely the chemical co-precipitation method at air atmosphere.

The water soluble PMMA and CTAB were used as capping agents. The nanostructures

of the synthesized nanoparticles have been analyzed using X-ray diffraction (XRD),

scanning electron microscope (SEM), transmission electron microscope (TEM),

Fourier transform infrared spectrometer (FT-IR), UV–vis and fluorescence spectro-

photometer. The sizes of as-prepared nanoparticles are found to be below 3.4–5.2 nm

range. Room temperature photoluminescence (PL) spectrum of the undoped sample

exhibits emission in the blue region with multiple peaks under UV excitation. On the

other hand, in the Cu2þ doped ZnS samples enhanced visible light emissions with

emission intensities of �2 times larger than that of the undoped sample are observed

for CTAB capped sample. The phase changes were observed in different temperatures by

TG-DTA.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Nanotechnology is a broad interdisciplinary area ofresearch, development and industrial activity, which hasbeen growing rapidly world wide for the past decade.Nanoparticles are the end products of a wide variety ofphysical, chemical and biological processes, some of whichare novel and radically different, others are quite common[1,2]. Due to their unique optical properties such as broadexcitation band, narrow emission spectrum, size and compo-sition-tunable emission wavelength the nanoparticles wereused as an excellent antiphotobleaching [4,5]. The nanoma-terials are known for their unique mechanical, chemical,physical, thermal, electrical, optical, magnetic, biological andalso specific surface area properties, which in turn definethem as nanostructures, nanoelectronics, nanophotonics,

ll rights reserved.

amasamy).

t al. Enhanced photolcience in Semicondu

nanobiomaterials, nanobioactivators, nanobiolabels, etc.In the last one decade, a large variety of nanomaterials anddevices with new capabilities have been generated byemploying nanoparticles based on metals, metal oxides,ceramics (both oxide and non-oxide), silicates, organics,polymers, etc. The considerable progress has been achievedin this field to understand the size related properties inmaterials. As the size of semiconductor nanoparticlesdecreases, a threshold value characteristic of each type ofsemiconductors is reached. At smaller nanoparticle sizes, theenergy gap (band gap) increases, and the optical spectrum isshifted toward the short-wavelength region [6].

As a nontoxic II–VI semiconductor material, ZnS ischemically more stable and technologically better than othersemiconductor materials (such as ZnSe), it is considered to bea promising host material. Both transition metal ions(e.g. Mn2þ [7,8] and Cu2þ [9]) have been incorporated intoZnS nanostructures by various physical and chemical meth-ods. These doped ZnS semiconductor materials have a widerange of applications in electroluminescence devices,

uminescence properties of ZnS:Cu2þ nanoparticles usingctor Processing (2011), doi:10.1016/j.mssp.2011.09.006

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dx.doi.org/10.1016/j.mssp.2011.09.006

dx.doi.org/10.1016/j.mssp.2011.09.006

Fig. 1. XRD spectra of ZnS, ZnS:Cu2þ (0.2–0.8%) and different surfac-

tants capped (PMMA and CTAB) capped ZnS:Cu2þ nanoparticles.

M. Kuppayee et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]2

phosphors, light emitting displays, optical sensors, etc.Among these nanostructured materials, zinc sulfide nanopar-ticles doped with Cu2þ ions (ZnS:Cu2þ) are of special interestdue to the highly effective luminescence [10].

Semiconductor nanocrystals capped with organicmolecules can have a relatively large number of unpassi-vated surface sites as it is difficult to passivate bothanionic and cationic surface sites simultaneously by thesecapping groups [11]. These unpassivated surface sitesmay act as non-radiative recombination centers, whichsuppress their luminescence. In addition, organicallycapped nanocrystals have very long emission lifetimesand large Stoke’s shifts [12] whereas inorganically cappednanoparticles exhibit enhanced luminescence efficiencies[13] and shorter life-times [14]. The epitaxial growth ofinorganic cap on the nanocrystals can eliminate both theanionic and cationic surface dangling bonds [11]. Inor-ganic passivation is, therefore, more effective than organicpassivation for eliminating surface defect sites. It isreported that fluorescence intensity can be increased bycovering the nanoparticle surface with Cd(OH)2 [15].Zou et al. [12] have studied the effectiveness of variousinorganic capping agents having different band gaps onthe surface passivation of cadmium sulphide (CdS)nanoparticles.

In this work, PL emission properties of the ZnS samplesdoped with Cu2þ and surfactants capped particles havebeen presented. The size of the synthesized ZnS:Cu2þ

samples are in the 3.4–5.2 nm range as has been con-firmed from X-ray diffraction (XRD) peak broadening,transmission electron microscope (TEM) micrograph ana-lysis and UV–vis absorption. The optical absorptionproperties show a blue shift of the absorption edge dueto the quantum confinement effect. In the preparedZnS:Cu2þ samples, PL emission takes place coveringthe whole visible region with the peaks appearing at472, 484 and 496 nm. The enhancement in the PL emis-sion is observed as compared to that of uncapped ZnSnanoparticles.

2. Experimental

All steps of the synthesis were carried out at roomtemperature and all chemicals used were AR grade.The water was used as a solvent in the experiments. First,a desired molar proportion of Zn(CH3COO)2 �2H2O in50 ml de-ionized water and Cu II (CH3COO)2 �4H2O(wt% in Zn¼0.2%, 0.4%, 0.6% and 0.8%) in 50 ml de-ionizedwater were dissolved and surfactant was added tocontrol the growth of the nanocrystals during the reac-tion. Subsequently, the Na2S (50 ml) was added drop wiseby an additional funnel to the above mixture. For eachexperiment, the molar amounts of Zn(CH3COO)2 and Na2Sused were equal. Undoped ZnS nanoparticles weresynthesized by following the same procedure withoutdoping and capping agents (PMMA and CTAB). For thesynthesis of surfactants capped ZnS:Cu2þ nanoparticles,the surfactant should be added. During the whole reactionprocess, the reactants were vigorously stirred under airatmosphere at 80 1C. The formed nanocrystals were sepa-rated from the solution by centrifuging. After washing

Please cite this article as: Kuppayee M, et al. Enhanced photolPMMA and CTAB surfactants. Materials Science in Semicondu

repeatedly with deionized water and then drying at 60 1Cunder vacuum, the powder samples of ZnS:Cu2þ nano-particles were obtained.

The X-ray diffraction (XRD) patterns of the powderedsamples were recorded using X’PERT-PRO diffractometerwith a Cu Ka radiation (l¼1.54060 A) under the sameconditions. The crystallite size was estimated usingScherrer’s equation (0.9l)/(b cos y) at full width at halfmaximum of the major XRD peak. The size and morphologyof the nanoparticles were determined using TEM (PHILIPS-CM200; 20–200 kV). For sample preparation, dilute dropsof suspension were allowed to dry slowly on carbon-coatedcopper grids for TEM measurement. The FT-IR spectra wererecorded on an AVATOR 360 spectrometer. The opticaltransmission/absorption spectra of the same particlesin de-ionized water were recorded using UV-1650PC


dx.doi.org/10.1016/j.mssp.2011.09.006

M. Kuppayee et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]] 3

SHIMADZU spectrometer. Fluorescence measurementswere performed on an RF-5301PC spectrophotometer.Emission (350–650 nm) spectra were recorded under 315,302 and 294 nm for uncapped, TOPO and SHMP cappedZnS:Cu2þ nanoparticles, respectively. The thermal analysiswas carried out with SDT Q600 20 thermometer.

3. Results and discussion

3.1. Structure and morphology

Fig. 1 illustrates the XRD patterns of the ZnS, ZnS:Cu2þ and surfactants

capped ZnS:Cu2þ nanoparticles. It shows very broad diffraction peaks,

which are the characteristic of nanosized materials. For all the samples

three diffraction peak positions correspond to the lattice planes of (1 1 1),

Fig. 2. (a) SEM image of ZnS nanoparticle and (b) corr


(2 2 0) and (3 1 1), which matches very well with the cubic zinc

blende structure (JCPDS No. 05-0566). No diffraction peaks from copper

impurities were detected. Thus it is observed that the Cu2þ ions are

dispersed into the ZnS matrix. According to the Debye–Scherrer formula

[16], the mean crystalline sizes calculated from the full-width at half-

maximum (FWHM) of these lines are about 4.8 and 5.2 nm for

uncapped ZnS and ZnS:Cu2þ nanoparticles. Further, relatively larger line

broadening in surfactant (PMMA and CTAB) capped samples indicates

their smaller particle size as compared to uncapped samples. From the

XRD line width, the mean crystalline domain sizes have been estimated

to be around 3.5 and 4 nm for PMMA and CTAB capped ZnS:Cu2þ

particles, respectively. This result indicates necessity of the capping

agent for the reduction of particles’ size. By comparing the effect of two

surfactants, the CTAB is used as an effective surfactant to reduce the

particles’ size, which may be due to the presence of the sodium ions in this

surfactant.

A typical SEM microphotographs of ZnS and ZnS:Cu2þ nanoparticles

(Figs. 2 and 3) show that the particles have smooth surface and an

esponding elemental dispersive X-ray spectrum.


dx.doi.org/10.1016/j.mssp.2011.09.006

Fig. 3. (a) SEM image of ZnS:Cu2þ nanoparticles and (b) corresponding elemental dispersive X-ray spectrum.

Fig. 4. TEM image of PMMA capped ZnS:Cu2þ nanoparticles.


average agglomerate size of around 30 nm. But the actual size of the

nanoparticles cannot be determined from the SEM images, as it is

limited by the resolution of the used SEM instrument (The actual

particle sizes calculated through XRD are diffraction as 4.8 and

5.2 nm). The elemental composition determined through EDX attached

with SEM instrument is shown in Figs. 2b and 3b, for ZnS and ZnS:Cu2þ

nanoparticles, respectively. This (Fig. 3b) reveals that the Cu2þ ions are

incorporated in the Zn2þ lattice sites.

It is necessary to obtain the particle size and information about

nanostructures by direct measurement, such as transmission electron

microscope (TEM), which can reveal the size and morphology of the

particles. Figs. 4 and 5 show TEM images of the PMMA and CTAB capped

ZnS:Cu2þ (0.4%) nanoparticles, respectively. The TEM images indicate,

the synthesized particles are spherical in shape and homogeneous

distribution. The TEM images of PMMA capped particles reveal the

distinct grain boundaries (Fig. 4) with an average crystallite size about

3.5 nm and 4 nm for CTAB capped ZnS:Cu2þ nanoparticles (Fig. 5). This

supports, that the size of the synthesized ZnS:Cu2þ nanoparticles are in

consistent with XRD result.

Please cite this article as: Kuppayee M, et al. Enhanced photoluminescence properties of ZnS:Cu2þ nanoparticles usingPMMA and CTAB surfactants. Materials Science in Semiconductor Processing (2011), doi:10.1016/j.mssp.2011.09.006

dx.doi.org/10.1016/j.mssp.2011.09.006

Fig. 6. (a–d) FT-IR spectra of ZnS (a), ZnS:Cu2þ (b) and surfactants

(PMMA (c) and CTAB (d)) capped ZnS:Cu2þ nanoparticles.

Fig. 7. UV–vis absorption of ZnS, ZnS:Cu2þ (0.4%), and surfactants

(PMMA and CTAB) capped ZnS:Cu2þ nanoparticles.

Fig. 5. TEM image of CTAB capped ZnS:Cu2þ nanoparticles.


3.2. FT-IR study

Fig. 6(a–d) shows FT-IR spectra of the ZnS, ZnS:Cu2þ , PMMA and

CTAB capped ZnS:Cu2þ (0.4%) nanoparticles. From the FT-IR analysis

of uncapped ZnS nanoparticles (Fig. 6a), the peaks appearing at 1110,

618 and 491 cm�1 are due to Zn–S vibrations. The obtained peak

values are in good agreement with the reported literatures [16,17].

The peaks at 1133 and 1112 cm�1 of Fig. 6(b) show the presence of

the doping ions (Cu2þ) in ZnS. Moreover, the transmittance peaks of

the ZnS:Cu2þ are slightly shifted towards lower wave number side

from the uncapped ZnS, which may be due to the presence of the

doping ions. These peaks are slightly shifted to higher wave number

side with decrease in intensity when ZnS:Cu2þ nanoparticles are

capped with surfactants. In the FT-IR spectrum of the composite

(Fig. 6c), the absorption bands at 1700–1750 cm�1 are the charac-

teristics of CQO stretching vibration of PMMA. In addition, the

absorption bands at 1150 cm�1, 1191 cm�1, 1240 cm�1 and

1269 cm�1also represent C–O–C stretching vibration of PMMA


[18]. The spectrum for CTAB is presented in Fig. 6d. The strong peaks

observed at 1610 and 1402 cm�1 are shifted to long wavenumber

side from the uncapped particles (Fig. 6b), which indicates the

presence of CTAB on the ZnS:Cu2þ surface. The broad absorption

peaks for all the samples in the range of 3410–3465 cm�1 corre-

sponds to –OH group, which indicate the existence of water

absorbed in the surface of nanocrystals. The presence of this band

can be clearly attributed to the adsorption of same atmosphere

water during FTIR measurements. The bands at 1500–1650 and

2360–2343 cm�1 are due to the CQO stretching mode arising from

the absorption of atmospheric CO2 on the surface of the nanoparti-

cles [19].

3.3. UV–visible study

The room-temperature UV–visible absorption spectra of freshly-

prepared undoped and Cu2þ doped ZnS, PMMA and CTAB—capped

ZnS: Cu2þ nanoparticles (Fig. 7) shows the absorption peak position in

the ultraviolet range. The absorption peaks of different concentrations

of Cu2þ doped ZnS nanoparticles has no change in the peak position

(not shown). However, the relative intensity is changed. Among the

four different concentrations (0.2–0.8%), the 0.4% of Cu2þ doped ZnS

gives strong absorption. For this reason, the 0.4% of Cu2þ was selected

as an optimum concentration for the other study. In addition, the

absorption peaks of the surfactant capped particles have no change

with respect to the capping concentrations (not shown). But the

shifting was observed. As observed in Fig. 7, the samples exhibit the

absorption peak at around 310 (4 eV), 315 (3.94 eV), 288 (4.31 eV) and

298 (4.16 eV) nm for ZnS, ZnS:Cu2þ , PMMA and CTAB capped ZnS:Cu2þ

nanoparticles, respectively. The absorption peaks of all samples

were strong by blue shifted as compared with 345 nm (3.6 eV) of bulk

ZnS. This blue shift of the bandgap takes place because of the quantum

confinement effect [10]. Comparison of the four absorption values, the

PMMA capped ZnS:Cu2þ is more blue shifted than the others. It may be

due to reduction of the particles’ size. According to Kumbhojkar et al.,

[20], the properties of nanocrystalline materials show deviation from

the corresponding bulk properties when the sizes of the crystallites

become comparable to the Bohr excitonic radius. The reported value of

Bohr radius of the ZnS is �2.5 nm [3]. The values of grain size in the

nanocrystalline for capped ZnS:Cu2þ are almost same as determined


dx.doi.org/10.1016/j.mssp.2011.09.006

Fig. 8. (a) PL spectra of different concentration of Cu2þ (0.2–0.4%) doped ZnS nanoparticles and (b) corresponding concentrations versus PL intensity.


from XRD and TEM studies. It is comparable to excitonic Bohr radius

(aB) supporting the quantum–size effect. Further, the grain sizes were

calculated using Brus equation. Based on the peak position in the

absorption spectrum, the sizes of the nanocrystals have been calculated

as 4.9, 5.2, 3.4 and 3.9 nm for ZnS, ZnS:Cu2þ , PMMA and CTAB capped

ZnS:Cu2þ nanoparticles, respectively. The obtained values are in

good agreement with XRD, TEM and also excitonic Bohr radius of

bulk ZnS.

3.4. Photoluminescence study

The PL spectra of undoped and various concentrations of Cu2þ

doped ZnS nanoparticles recorded at room temperature were shown

in Fig. 8a. The excitation of the observed emission peaks under 310

and 315 nm. For the undoped sample (inset in Fig. 8a) a broad

emission peak was observed at 445 nm due to donor–acceptor pair

[21]. When Cu2þ ions were doped into ZnS nanoparticles, more

defect states will be introduced. Therefore, it is reasonable some new

peaks had appeared in the longer wavelength side. Comparison of

the two spectra, the impurity (Cu2þ) doped ZnS spectra was more

red shifted from the undoped ZnS. The spectra show a broad

emission spectrum around 400–600 nm. It is in agreement with

earlier results in ZnS:Cu2þ nanoparticles [22–25]. Three emission

peaks were observed in the blue region, at 472 [26], 484 [27] and

496 nm[22], arises from the recombination between the shallow

donor level (sulfur vacancy) and the t2 level of Cu2þ [27]. With an

increase of the Cu2þ concentration (from 0.4 to 0.6%), the green light

position is systematically shifted to longer wavelength (from blue to

green (524 nm)) [28]. Moreover, the intensity of the ZnS:Cu2þ

emission was decreased by higher concentration of the Cu2þ .

Fig. 8b shows concentration versus intensity. This illustrates an


optimum concentration (0.4%) of doping Cu2þ ions for enhanced PL

emission. When Cu2þ ions were incorporated in ZnS host lattice, the

luminescence centers of Cu2þ ions are formed. Hence from the PL

emission spectra, it can be concluded that the Cu2þ ions are

incorporated successfully into the ZnS host lattice.

The enhanced luminescence properties of the capped ZnS:Cu2þ

nanoparticles are attributed to the surface passivation of the nano-

particles by PMMA and CTAB capped, which minimize the surface

defects and enhance the electron–hole recombination. The emission

spectra for both samples appeared in the visible range. Fig. 9a shows

a PL spectra of different concentrations of PMMA capped ZnS:Cu2þ

nanoparticles. The PL emission contains three peaks at 472, 484 and

496 nm. The peak position has no change with respect to the capping

concentrations. However, the relative intensity varies. Moreover, the

observed peak position in the PL emission for PMMA capped

particles has no change from the uncapped ZnS:Cu2þ (Fig. 8a), but

the emission intensity is increased. The PL improvement may be due

to the presence of carboxyl group in PMMA. Fig. 9b shows a plot of

concentration of PMMA and intensity. As shown in the figure, the

emission intensity is increased with respect to concentration of the

surfactant (PMMA). It indicates, increase in particles’ growth with

respect to the concentration of the PMMA. Fig. 10a shows room

temperature photoluminescence of the CTAB capped ZnS:Cu2þ

nanoparticles. It is observed that there is no shifting in PL emission

with respect to uncapped and PMMA capped ZnS:Cu2þ nanoparti-

cles. However, the PL intensity is enhanced more than two times,

this enhancement may be due to the presence the halogen groups

(Br) in CTAB. It indicates the improved particles’ growth by adding

the CTAB as a surfactant. The PL intensity is increased by increasing

the CTAB concentrations (Fig. 10b) up to 1 g after that the intensity is

decreased. This decrease may be due to reduction of particles’

growth in colloidal solution. Thus from the overall point of view


dx.doi.org/10.1016/j.mssp.2011.09.006

Fig. 9. (a) PL spectra of different concentrations of PMMA (0.5–1.5 g) capped ZnS:Cu2þ nanoparticles and (b) corresponding concentration versus PL

intensity.


the role of the PMMA (�3.5 nm) in this work controls the particles’

size and growth. However, the role of CTAB in controlling the size is

seldom than the PMMA, but it excites the particles’ growth, resulting

in enhanced PL emission.

3.5. Thermal study

The TG-DTA thermograms were recorded for ZnS:Cu2þ nanoparti-

cles in the temperature range from room temperature (RT) to 1000 1C

with an increase by 10 1C/min in air atmosphere. Fig. 11 shows

combined plots of TG and DTA. From the TGA data plot, it is noticed

that the weight loss of the nanoparticles are found to take place upto

700 1C. For DTA curve, the first endothermic peak was found at 65 1C.

This peak is attributed to the evaporation of the water. The endothermic

peak around 230 1C probably corresponds to the evaporation of organic

and lattice deformation of ZnS:Cu2þ . The composition does not vary in

the annealing range from 1001 to 200 1C, whereas, as can be seen,

beyond 230 1C, most Cu2þ ions are released from the ZnS matrix. The

observed endothermic peak at 330 1C was believed to be the beginning

of phase transition. A broad exothermic peak at 400 1C implies the

improvement of the crystallinity of the sample. Additionally, above

500 1C, there is a smooth downward trend in DTA curve with significant


weight loss. This may be due to release of residual sulfur ions from the

sample.

4. Conclusion

The ZnS, ZnS:Cu2þ , PMMA and CTAB capped ZnS:Cu2þ

nanoparticles were successfully synthesized in aqueousmedium at air atmosphere. The particles were stabilizedby the steric hindrance to reduce agglomeration. The XRDpatterns of the surfactants capped nanoparticles showedthe materials were of nanometer size regime with a cubicphase. The particle structure has no change with respectto the concentration of Cu2þ and capping agents. How-ever, size of the particles is changed. The TEM imagesindicated that the growth particles are spherical in shapeand homogeneous dispersion. Prominent IR peaks areidentified on ZnS:Cu2þ surface. The particles size wascontrolled by PMMA and PL intensity was improved byCTAB. The stability of the ZnS:Cu2þ nanoparticles werestudied by TG-DTA. The findings demonstrate the


dx.doi.org/10.1016/j.mssp.2011.09.006

Fig. 10. (a) PL spectra of different concentrations of CTAB (0.5–1.5 g) capped ZnS:Cu2þ nanoparticles and (b) corresponding concentration versus PL

intensity.

Fig. 11. TG-DTA spectra of ZnS:Cu2þ (0.4%) nanoparticles.


preparation of the monodispersed ZnS:Cu2þ nanoparti-cles, which is a potential application for biological label-ing and sensor devices.


Acknowledgment

The authors would like to thank SAIF, IIT, Bombay forproviding TEM facility.

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