Photo- and Electro-Luminescence in Copper doped Zinc Sulphide Nanoparticles and Nanocomposites

Meera Ramrakhiania and Sakshi Sahareb

Department of Post Graduate Studies and Research in Physics and Electronics,

Rani Durgawati University, Jabalpur 482001 (M.P.) India

amramrakhiani@hotmail.com, bsakshisahare@gmail.com

Keywords: ZnS:Cu Nanocrystals, XRD, Absorption Spectra, Photoluminescence, Electroluminescence

Abstract. Copper doped Zinc Sulfide (ZnS:Cu)) is a known green light emitter. Present paper

reports luminescence of ZnS:Cu nanoparticles and nanocomposites. Three different nanostructures:

mercaptoethanol capped ZnS:Cu nanoparticles, polyvinyl alcohol (PVA) capped ZnS:Cu

nanoparticles and ZnS:Cu/PVA nanocomposites have been prepared by chemical route. X-ray

diffraction (XRD) revealed cubic zinc blende structure of ZnS:Cu nanocrystals of size below 20

nm. The particle size is found to decrease with increasing capping agent concentration or ZnS

loading in PVA matrix. Optical absorption spectra show blue shift in the absorption edge indicating

quantum size effect. Photoluminescence (PL) of all the samples was studied by exciting with 212

nm light. The PL spectra of ZnS:Cu/ PVA nanocomposite films show quite broad emission peak at

415 nm where as the PL spectra of mercaptoethanol capped and PVA capped nanoparticles show a

very narrow peak at 426 nm and 403 nm respectively. It seems that the nature of passivation of

surface states affects the position of surface states. Electroluminescence (EL) studies have shown

that light emission starts at a threshold and then increases with voltage. Higher EL intensity and

lower threshold voltage is obtained in case of smaller particles. The EL spectra of all the samples

are found to be broad with peak at about 420 nm. The EL intensity of ZnS:Cu/PVA nanocomposites

is much larger than the ZnS:Cu nanoparticles. The high efficiency EL devices for display and

lighting can be fabricated using ZnS:Cu nanocomposites with PVA matrix giving violet emission.

1. Introduction

Quantum dots or nanostructures having sizes comparable that of the bulk Bohr exciton radius [1-2]

exhibit discrete electron energy levels with large oscillator strength and strong luminescence. These

systems have a very high surface to volume ratio and hence surface defects play an important role

in their characteristics. A poorly passivated surface of a quantum dot characterized by the presence

of electron or hole traps, play a significant role in the luminescence properties. It is therefore

essential to carefully control the surface defects along with the size of a quantum dot in order to

obtain the desired radiative properties.

Luminescence of II-VI materials has been, and still is, an extremely active area of research. ZnS is

promising host material for development of phosphors in different visible emission bands.

Transition metal or rare earth doped zinc sulfide (ZnS) crystals have been used as phosphors for a

long time. Photoluminescence (PL) of bulk and nanocrystalline ZnS is given special attention due to

its potential technological applications. Photoluminescence can also be very effectively used to

determine the size dependent electronic structure.

Doped ZnS semiconductor material also has a wide range of applications in electroluminescence

devices; TV phosphors and optical sensors. Type and concentration of dopants play key roles in

luminescence efficiency and the position of emission bands of semiconductor nanoparticles [3].

Recently quantum dots are being explored as alternative emissive materials in view of their

efficient, stable and tunable emission from the doped transition-metal ions [4-8]. Among this

category, doping of Mn in ZnS is widely studied and reported. Many groups have synthesized Mn

doped ZnS by different approaches and used them as candidates for many applications such as

Solid State Phenomena Vol. 201 (2013) pp 181-196Online available since 2013/May/14 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.201.181

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electroluminescence devices [9], field emission displays [10] and sensors [11-12]. Thus considering

their practical applications, it is very important to investigate how the dopant concentration affects

optical property of semiconductor nanoparticles from the viewpoints of understanding trapping and

recombination process. ZnS:Cu was the first formulation successfully displaying

electroluminescence, tested in 1936 by Georges Destriau in Madame Marie Curie laboratories in

Paris [13]. The band gap of the nanoparticles increases with increasing copper concentrations.

Green, blue and violet emission band located at 420 – 510 nm was observed by UV excitation of the

ZnS host. Cu2+

ions in the lattice act as luminescent center [14].

The main objective of the present work is to investigate the photoluminescence and

electroluminescence (EL) properties of ZnS:Cu nanocrystals and ZnS:Cu/PVA composite by

varying size of nanocrystals. The experimental work consists of preparation of three different

nanostructures: mercaptoethanol capped ZnS:Cu nanoparticles, polyvinyl alcohol (PVA) capped

ZnS:Cu nanoparticles and ZnS:Cu/PVA nanocomposites by chemical route. The samples were

characterized by X-Ray diffraction (XRD) and UV-Vis optical absorption and their

photoluminescence as well as electroluminescence investigated.

2. Experimental Technique

Nanostuctures of copper doped zinc sulfide have been synthesized and characterized by XRD,

absorption and luminescence investigations.

2.1 Preparation of Samples. In the present work, three different types of ZnS:Cu nanostructures

have been synthesized:

i. Mercaptoethanol capped copper doped zinc sulfide nanoparticles,

ii. Polyvinyl alcohol capped copper doped zinc sulfide nanoparticles, and

iii. Copper doped zinc sulfide nanocrystals embedded in polyvinyl alcohol matrix.

All the samples were prepared by chemical route since it is a cost effective technique and expected

to give low size distribution. In all the samples, the Cu doping was kept at the level of 0.01%

because it is found to show maximum luminescence [15].

2.1.1 ZnS:Cu Nanoparticles capped by Mercaptoethanol. For preparing ZnS:Cu nanoparticles

dilute solution (0.01 M) of zinc chloride (ZnCl2) and sodium sulfide (Na2S) were mixed in presence

of capping agent mercaptoethanol (C2H5OSH). First aqueous solution of mercaptoethanol was

added to zinc chloride solution drop wise at the rate of 1ml per minute with the help of a burette,

while stirring the solution continuously. In the solution of ZnCl2, a small amount of CuCl2 (0.01 %)

was also mixed for doping. Magnetic stirrer was used for stirring the solution. Thereafter 0.01 M

solution of Na2S was added drop wise into the solution. Subsequently, a milky color solution was

obtained. This solution was kept for 24 hour; white precipitate settled down in the bottom of the

flask. This precipitate was removed and washed several times with the double distilled water. The

mercaptoethanol concentration controls the size of nanoparticles [16]. Five different samples,

ZnS:Cu I, ZnS:Cu II, ZnS:Cu III, ZnS:Cu IV and ZnS:Cu V were prepared by varying

mercaptoethanol concentration as 0M, 0.005 M, 0.010 M, 0.015 M and 0.020 M, respectively.

2.1.2 ZnS:Cu Nanoparticles capped by PVA. For the synthesis of PVA capped ZnS:Cu

nanoparticles, the chemicals used were ZnSO4.7H2O, CuCl2.4H2O, polyvinyl alcohol (PVA) and

Na2S. In a typical synthesis procedure, aqueous stock solutions of 1 M of ZnSO4.7H2O and

different concentrations of PVA were dissolved in 50 ml double distilled water under stirring, and

then 0.01% CuCl2.4H2O solution was added to the above solution. Finally, 50 ml of 1 M Na2S

solution was introduced into the above solution under continuous stirring. During the whole

reaction process, the reactants were vigorously stirred at 800C for 1h. Then the precipitate was

cleaned repeatedly with deionized water [17]. The PVA concentration controls the size of

nanoparticles. Five different samples, ZnS:Cu-i, ZnS:Cu-ii, ZnS:Cu-iii, ZnS:Cu-iv, and ZnS:Cu-v

were prepared by varying PVA concentration at the level of 0 gm, 0.5 gm, 1.0 gm, 1.5 gm and 2.0

gm in 50 ml respectively.

182 Functional Nanomaterials and their Applications

2.1.3 ZnS:Cu/PVA Nanocomposites. The ZnS:Cu nanocomposites were also prepared by the

chemical method. For the synthesis of composite films, 400 mg poly vinyl alcohol (PVA) was

dissolved in dimethylformamide (DMF) by constant stirring and heating up to 70oC. Zinc acetate

was added to it in appropriate quantity. For doping, 0.01% copper acetate was mixed in the initial

solution. The resulting solution was stirred for 30 minutes. The solution was refluxed by applying

nitrogen and then H2S gas was passed for 30 seconds. The solution immediately turned milky white.

Now again the solution was stirred for a few seconds, then caste over glass slides and conducting

glass plate also (for EL measurements) and dried to obtain uniform film of ZnS:Cu/PVA

nanocomposite [18-19]. Five different samples ZnS:Cu-1, ZnS:Cu-2, ZnS:Cu-3, ZnS:Cu-4, and

ZnS:Cu-5 were prepared by varying loading percentage of ZnS:Cu as 5, 10, 20, 30 and 40 % in

PVA by weight, respectively.

2.2 Characterization of Samples. The crystal structures and regularity of atomic arrangements in

all the samples were studied by X-ray diffraction technique using Rigaku Rotating Anode (H-3R)

diffractometer at UGC-DAE Consortium for Scientific Research Indore. The UV-Vis absorption

studies were carried out by Perkin Elimer Lambda-12 spectrometer in the range 200-600 nm.

2.3 Luminescence Studies. ZnS:Cu is a well known luminescent material having a band gap of 3.7

eV at 300 K. This corresponds to ultraviolet (UV) radiation for optical inter-band transition with a

wavelength of 335 nm.

2.3.1 Photoluminescence. The photoluminescence spectra of ZnS:Cu were recorded with F-7000

fluorophotometer. The PL was excited with 212 nm from a UV lamp and the PL spectra for

ZnS:Cu nanoparticles and ZnS:Cu nanocomposites in PVA matrix have been studied in the range of

300 nm to 600 nm.

2.3.2 Electroluminescence. For study of electroluminescence of ZnS:Cu nanostructures, the EL

cells were prepared by placing ZnS:Cu nanoparticles/composites between SnO2 coated conducting

glass plate and aluminium foil. The EL cell was connected with low distortion frequency generator

coupled with power supply (wide band amplifier). The AC voltage of different frequencies was

applied and EL brightness at different voltages was measured at each frequency, with the help of

photomultiplier tube connected to a picoammeter. The corresponding current was recorded by a

microammeter, which is connected in series with the EL cell.

3. Results and Discussion

The samples prepared with mercaptoethanol or polyvinyl alcohol (PVA) as capping agent were in

powder form where as ZnS:Cu/PVA nanocomposites were films on glass substrate. All the samples

were milky white in color.

3.1 XRD analysis. The XRD patterns give information about the crystal structure. Figures 1, 2 and

3 show the XRD patterns of mercaptoethanol capped ZnS:Cu nanoparticles, polyvinyl alcohol

(PVA) capped ZnS:Cu nanoparticles and ZnS:Cu/PVA nanocomposites, respectively.

Solid State Phenomena Vol. 201 183

0

2000

4000

6000

8000

15 25 35 45 55 65

Inte

nsi

ty (ar

b. unit

s)

Angle 2 theta (in degree)

ZnS:Cu-I

ZnS:Cu-III

ZnS:Cu-IV

ZnS:Cu-V

Fig. 1 X-ray diffraction patterns of ZnS:Cu nanocrystals capped by mercaptoethanol

0

1000

2000

3000

4000

5000

6000

7000

20 30 40 50 60 70

Angle 2 theta (in degree)

Inte

nsi

ty (

arb

.unit

s)

ZnS:Cu-i

ZnS:Cu-ii

ZnS:Cu-iii

ZnS:Cu-iv

Fig. 2 X-ray diffraction patterns of ZnS:Cu nanocrystals capped by PVA

184 Functional Nanomaterials and their Applications

θβλ

Cos

KR =

0

1000

2000

3000

4000

5000

6000

7000

8000

15 25 35 45 55 65

Inte

nsi

ty (

in a

rb u

nit

s)

Angle 2 theta (in degree)

ZnS:Cu-2

ZnS:Cu-3

ZnS:Cu-4

Fig. 3 X-ray diffraction patterns of ZnS:Cu/PVA nanocomposites

In case of nanocompsites the peaks due to ZnS are superimposed over the pattern due to PVA with

major peak at ~ 180 (2θ). In all cases XRD reveals that ZnS:Cu nanocrystals have zinc–bland crystal

structure. For all the samples three peaks are observed at about 290, 48

0 and 57

0 (2θ values)

corresponding to diffraction from (111), (220) and (311) planes. Due to the size effect, the XRD

peak tends to broaden and their width increases as the size of crystals decreases. The crystalline size

has been estimated from the broadening of the first diffraction peak using Debye-Scherrer formula

[20]

(1)

where R is crystallite size, K is instrumental Scherrer constant, θ is Bragg angle, λ is wave length

and β is full-width at half-maximum (FWHM) of peak.

The crystalline size of mercaptoethanol capped ZnS:Cu nanoparticles is obtained in the range of 10

nm to 3 nm. In the same way the crystalline size of PVA capped ZnS:Cu nanoparticles is obtained

in the range of 20 nm to 4 nm and the particle size of ZnS:Cu in nanocomposites is in the range of 8

nm to 3 nm. The variations of crystalline size with concentration of capping agent or loading

percentage of ZnS:Cu are shown in Table I. It can be seen that the crystalline size decreases with

increasing capping agent concentration or ZnS:Cu loading percentage.

3.2 Absorption spectra. The optical absorption spectra of ZnS:Cu samples prepared by different

methods are shown in figures 6, 7 and 8. It can be seen from the spectra that there is practically

uniform absorption in the visible range (800 nm-390 nm). Absorption increases suddenly in the UV

region. No absorption peaks are found. The gradual shift in absorption edge to the shorter

wavelength side (blue shift) indicates increased band gap with reducing particle size because of

quantum confinement effects.

Solid State Phenomena Vol. 201 185

The most direct way of extracting the optical band gap is to simply determine the photon energy at

which there is a sudden increase in absorption. The optical band gap of the nanocrystalline samples

were obtained from the absorption edge. It is found that the method of preparation of samples does

not influence to the nature of absorption spectra.

0

2

4

6

8

10

12

200 300 400 500 600

Wavelength (in nm)

Ab

sorp

tio

n (

in a

rb.

un

its)

ZnS:Cu-IZnS:Cu-IIZnS:Cu-IIIZnS:Cu-IVZnS:Cu-V

Fig. 4 Optical absorption spectra of ZnS:Cu Nanocrystals capped by mercaptoethanol

0

1

2

3

4

5

6

7

8

9

200 300 400 500 600 700 800

Wavelength (in nm)

Ab

sorp

tio

n (

arb

. u

nit

)

ZnS:Cu -iZnS:Cu -iiZnS:Cu -iiiZnS:Cu-ivZnS:Cu-v

Fig. 5 Optical absorption spectra of ZnS:Cu nanocrystals capped by PVA

186 Functional Nanomaterials and their Applications

Fig. 6 Optical absorption spectra of ZnS:Cu/PVA nanocomposites

It is known that as the particle size decreases there is increase in effective band gap of

semiconductors due to confinement of charge carriers in a very small dimension [21]. The increase

in the band gap according to the effective mass approximation model is given by

(2)

where me* and mh* are the effective masses of electron and hole respectively, r is radius of the

particle and ε is the dielectric constant of the semiconductor. The second and third terms are much

smaller than the first term, therefore may be neglected and the expression reduces to

(3)

The values of the effective mass of electrons and holes for ZnS are me=0.25mo and mh = 0.59mo.

Substituting all the values and rearranging we get

(4)

Table 1 Particle size of samples by XRD and absorption spectra S.No. Name of

Sample

Name of capping

agent/polymer

composites

Capping agent

conc./loading %

Particle size

by XRD

(nm)

Particle size by

absorption

spectra (nm)

1 ZnS:Cu-I Mercaptoethanol 0.00 M 09.136 10.22

2 ZnS:Cu-II Mercaptoethanol 0.005 M 05.601 04.30

3 ZnS:Cu-III Mercaptoethanol 0.010 M 04.030 03.96

4 ZnS:Cu-IV Mercaptoethanol 0.015 M 03.800 03.54

5 ZnS:Cu-V Mercaptoethanol 0.020 M - 02.60

6 ZnS:Cu-i PVA 0.0 mg 20.000 18.00

7 ZnS:Cu-ii PVA 0.5 mg 16.960 11.00

8 ZnS:Cu-iii PVA 1.0 mg 15.990 04.08

1

**22

42

**2

22 11124.0786.111

2

−

+−−

+=∆

hehemm

e

r

e

mmrE

εεπ

�

�

+=∆

**2

22 11

2hemmr

Eπ�

2/1

1209.2

Er

∆=

Solid State Phenomena Vol. 201 187

9 ZnS:Cu-iv PVA 1.5 mg 04.510 03.76

10 ZnS:Cu-v PVA 2.0 mg - 02.56

11 ZnS:Cu/PVA-1 PVA composites 05 % - 07.92

12 ZnS:Cu/PVA-2 PVA composites 10 % 05.000 07.08

13 ZnS:Cu/PVA-3 PVA composites 20 % 04.590 06.20

14 ZnS:Cu/PVA-4 PVA composites 30 % 03.670 04.66

15 ZnS:Cu/PVA-5 PVA composites 40 % - 03.89

The effective band gap of the nanocrystalline samples is obtained from the absorption spectra and

increase in band gap is determined by comparing it with the standard value of band gap of ZnS as

3.68 eV at 300 K. The particle size estimated using the above relation is found to be decreases with

increasing capping agent concentration or loading percentage of ZnS:Cu in PVA. The estimated

particle size is nearly same as obtained by XRD (Table 1).

It is observed that the ZnS:Cu nanoparticle size is decreased by increasing capping agent

concentration, mercaptoethanol or PVA. Similarly for ZnS:Cu/PVA nancomposites, the ZnS

particle size is reduced by increasing loading percentage of zinc sulfide.

3.3 Photoluminescence. Luminescence studies provide information regarding defect states, which

take part in radiative de-excitation of samples. In nanocrystals the defect states may shift or their

density may change, which is revealed in luminescence studies. The band gap of ZnS:Cu is 3.68 eV

at 300 K, which corresponds to ultraviolet (UV) radiation for optical inter-band transition with a

wavelength of 335 nm. Therefore in present studies the PL was excited by 212 nm.

0

20

40

60

80

100

120

140

160

180

200

410 420 430 440 450

Wavelength (in nm)

Inte

nci

ty (

in a

rb. units)

ZnS:Cu-I

ZnS:Cu-III

ZnS:Cu-IV

Fig. 7 Photoluminescence spectra of ZnS:Cu nanocrystals capped by mercaptoethanol

188 Functional Nanomaterials and their Applications

0

200

400

600

800

1000

1200

380 390 400 410 420

Wavelength (in nm)

Inte

nci

ty (

in a

rb.

un

it)

ZnS:Cu-iii

ZnS:Cu-i

ZnS:Cu-v

Fig. 8 Photoluminescence spectra of ZnS:Cu nanocrystals capped by PVA

0

200

400

600

800

1000

1200

1400

350 400 450 500

Inte

nsity

(in

Arb

. Uni

t)

wavelength (in nm)

ZnS:Cu-2ZnS:Cu-3ZnS:Cu-4ZnS:Cu-5

Fig. 9 Photoluminescence spectra of ZnS:Cu/PVA nanocomposites

Photoluminescence spectra ranging from 300 nm to 500 nm were measured at room temperature

(300 K) for the nanocrystalline samples are shown in figures 7, 8 and 9. In all samples, single peak

was obtained. For mercaptoethanol capped ZnS:Cu, the PL peak was at 426 nm, where as PVA

capped ZnS:Cu give peak at 403 nm. The ZnS:Cu/PVA nanocomposites show broad emission

peaked at 415 nm. It is seen that with an optimum Cu doping of 0.01%, the PL intensity increases

with increasing capping agent concentration or loading % of ZnS:Cu, indicating that the dangling

bonds are better passivated with higher concentration of capping agent or with higher loading of

ZnS:Cu, which also reduces the size of ZnS:Cu nanoparticles.

Solid State Phenomena Vol. 201 189

Peng et al [22] have also reported PL of undoped ZnS nanoparticles with a peak at about 415 nm

which, after de-convolution, indicates two peaks at 411 and 455 nm. The PL peak at 410 nm has

been known due to the recombination between the sulfur-vacancy related donor and the valance

band [23] and the peak position of this blue emission does not change with the increase of the low

concentration of Cu++

but for higher concentration above 0.5% the peak shifts towards longer

wavelength. Jayanthi et al [24] have also reported the same thing when lower concentration of Cu in

ZnS is increased. The intensity of PL peak at particular concentration of Cu (0.01%) is maximum

and further, the overall PL intensity decreased at the higher Cu++

concentration of 0.1%. The

concentration quenching of the luminescence may be caused by the formation of CuS compound.

Similar results are obtained in our earlier work for electroluminescence [15]. It can be explained by

the effect of doping. As explained above, these two peaks are related with native defects (e.g. sulfur

vacancy). When Cu++

ions are doped into ZnS nanoparticles, more defect states are introduced.

Therefore, it is reasonable that these two defect-related peak intensities are enhanced for the doped

samples compared with the undoped samples.

The PL spectra of ZnS:Cu nanoparticles (about same size ~ 4nm) prepared by the three different

methods are shown in figure 10. It is clearly observed that ZnS:Cu nanoparticles give quite narrow

peak at 426 and 403 nm for mercaptoethanol and PVA capping respectively, but the emission from

the ZnS:Cu/PVA nanocomposite is very broad with peak at 415 nm.

0

20

40

60

80

100

120

300 350 400 450 500 550 600

Wavelength (in nm)

Inte

nci

ty (

in a

rb.

un

it) 0.015M

1.0gm

30%PVA

Fig. 10 PL spectra of three different types of ZnS:Cu nanostructures

It seems that the nature of passivation of surface states affects the position of surface states. In PVA

capping, it gives rise to shallow traps where as mercaptoethanol capping produces deep traps. The

position of the traps or surface states in ZnS:Cu/PVA nanocomposites appears to the distributed in

the energy levels with a maximum density in between the above mentioned deep and shallow traps

due to mercaptoethanol and PVA. Because of distributed traps the PL spectra of ZnS:Cu/PVA

nanocomposites is quite broad having a peak at 415 nm where as the PL spectra of mercaptoethanol

capped and PVA capped nanopartcles show quite narrow peak at 426 nm and 403 nm respectively.

Schematic diagram for PL mechanism is shown in figure 11.

190 Functional Nanomaterials and their Applications

Fig. 11 Schematic diagram for PL mechanism

3.4 Electroluminescence. In electroluminescence, a high electric field enables injection of charge

carriers at the interface of emission layer and electrodes. The injected carriers (say electrons) are

accelerated and cause impact ionization and excitation under the influence of the strong interfacial

electric field. . They move to the other end of the crystalline particle and recombine with opposite

sign carriers at the luminescence centers giving rise to the light emission.

The electroluminescence has been investigated for all three types of nanostructures. Voltage-

brightness characteristic shows that, on increasing the input voltage, brightness increases for all

samples. Figure 12, 13 and 14 show EL brightness versus voltage curves at 1000 Hz frequency, for

various samples. It is observed that EL starts at threshold voltage and then increases with increasing

voltage, and lower threshold voltage is obtained for smaller particles. It can be seen from the figures

that in case of mercaptoethanol capped ZnS:Cu nanoparticles, the increase in EL intensity is not so

fast, where as for PVA capped samples, the threshold voltage is lower and EL intensity increases

rapidly with voltage. Similar effect is seen in case of ZnS:Cu/PVA nanocomposites also, but here

EL intensity is quite high and increase with voltage is also very fast.

0

50

100

150

200

300 400 500 600 700

Voltage (in Volts)

Brightn

ess

(in a

rb. units)

ZnS:Cu-I

ZnS:Cu-II

ZnS:Cu-III

ZnS:Cu-IV

ZnS:Cu-V

Fig. 12 Brightness-Voltage characteristics of ZnS:Cu nanocrystals capped by mercaptoethanol

(at 1000Hz)

Solid State Phenomena Vol. 201 191

0

20

40

60

80

100

120

200 300 400 500 600Voltage (in Volt)

Bri

gh

tnes

s (i

n a

rb.

Un

it)

ZnS:Cu-iZnS:Cu-iiZnS:Cu-iiiZnS:Cu-ivZnS:Cu-v

Fig. 13 Brightness-Voltage characteristics of ZnS:Cu nanocrystals capped by PVA (at 1000Hz)

0

50

100

150

200

250

300

350

100 200 300 400voltage (in volt)

Bri

ghtn

ess

(in a

rb. unit

)

ZnS:Cu-5

ZnS:Cu-4

ZnS:Cu-3

ZnS:Cu-2

ZnS:Cu-1

Fig. 14 Brightness-Voltage characteristics of ZnS:Cu/PVA nanocomposites (at 1000Hz)

In the EL mechanism of semiconductor nanocrystal, the electrons and holes injected from

respective electrodes would recombine inside the semiconductor nanocrystals, and characteristics

light would be emitted as a result. As voltage is increased, more electrons and holes are injected

into the emission layer and their subsequent recombination increases the EL brightness. Smaller

ZnS:Cu nanoparticles have increased oscillator strength, which improves the electron-hole radiative

recombination and enhance the electroluminescence. Similar results have been reported for CdS,

CdSe and ZnS nanostructures also [25, 26, 27].

Figure 15 shows the voltage-brightness curve for ZnS:Cu nanoparticles of same size (about 4 nm

size) synthesized by different capping agents and loading of zinc sulfide in PVA matrix at 1200 Hz

frequency. It is observed that threshold voltage is nearly same for all three samples but the increase

192 Functional Nanomaterials and their Applications

in EL brightness with voltage is slow in case of mercaptoethanol capped nanoparticles, it is higher

for PVA capping and there is very fast increase in case of nanocomposites. It appears that in case of

nanocomposites the charge carrier transportation is easy and hence electron and hole can easily

move towards each other and recombine giving high emission.

0

50

100

150

200

250

300

0 200 400 600 800Voltage (in volts)

Bri

gh

tnes

s (i

n a

rb.

un

it)

0.01M1.5 PVA40% PVA

Fig. 15 Brightness-Voltage curve for three different types of ZnS:Cu nanostructures

0

0.005

0.01

0.015

0.02

0.025

0 100 200 300 400 500 600

Voltage (in Volt)

Cu

rren

t (i

n m

A)

40% PVA

0.01M

1.5mg PVA

Fig. 16 Current-Voltage characteristic for different types of ZnS:Cu nanostructures

Solid State Phenomena Vol. 201 193

In all the cases, voltage-current (V-I) characteristic shows that, on increasing the voltage, current

increases continuously and linear relation is found between them, which indicates ohmic nature.

From the slope of line, impedance can be estimated. Lower impedance has been obtained for

smaller particles. Comparing the V-I characteristics for the three types of samples it is found that

impedance is minimum for nanocomposites and maximum for mercaptoethanol capped

nanoparticles indicating easy charge carrier transportation in the composites (Fig. 16).

0

50

100

150

200

250

300

350

400

450

300 400 500 600

Wavelength (in nm)

Bri

gh

tnes

s (i

n a

rb.

un

it)

0.015M

1.5gm

40%PVA

Fig. 17 Electroluminescence spectra of three different types of ZnS:Cu nanostructures

The EL spectra of ZnS:Cu nanoparticles of same size (about 4 nm size) prepared by the three

different methods are shown in figure17. A single broad peak is obtained for all the samples at

about 420 nm but EL intensity of ZnS:Cu/PVA nanocomposites is much larger than the ZnS:Cu

nanoparticles. This shows that the surface states are well passivated in nanocomposites causing the

transition to be radiative.

4. Summary

The studies have shown that smaller ZnS:Cu nanocrystals can be prepared by increasing capping

agent concentration and smaller size ZnS:Cu nanocomposites with PVA matrix can be prepared by

increasing loading percentage of ZnS:Cu. X-ray diffraction (XRD) revealed cubic zinc blende

structure of ZnS:Cu nanocrystals of size below 20 nm. Optical absorption spectra show blue shift in

the absorption edge indicating increase in effective band gap due to quantum size effect. The PL

spectra of ZnS:Cu/ PVA nanocomposite films show quite broad emission peak at 415 nm where as

the PL spectra of mercaptoethanol capped and PVA capped nanoparticles show a narrow peak at

426 nm and 403 nm respectively. It seems that the nature of passivation of surface states affects the

position of surface states.

Electroluminescence studies have shown that light emission starts at a threshold and then increases

rapidly with voltage. Higher EL intensity and lower threshold voltage is obtained in case of smaller

particles. The EL spectra of all the sample is found to be broad with peak at about 420 nm. The EL

intensity of ZnS:Cu/PVA nanocomposites is much larger than the ZnS:Cu nanoparticles. The high

efficiency EL devices for display and lighting can be fabricated using ZnS:Cu nanocomposites with

PVA matrix giving violet emission.

194 Functional Nanomaterials and their Applications

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196 Functional Nanomaterials and their Applications

Functional Nanomaterials and their Applications 10.4028/www.scientific.net/SSP.201 Photo-and Electro-Luminescence in Copper Doped Zinc Sulphide Nanoparticles and Nanocomposites 10.4028/www.scientific.net/SSP.201.181