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Chalcogenide Letters Vol. 5, No. 12, December 2008, p. 387 394

CHEMICAL SYNTHESIS AND VISIBLE PHOTOLUMINESCENCE EMISSION FROM

MONODISPERSED ZnO NANOPARTICLES

P. Kumbhakar*, D. Singh, C. S. Tiwary, and A. K. Mitra

Department of Physics, National Institute of Technology-Durgapur, 713209, India

Monodispersed PVP capped nanoparticles of ZnO have been prepared in double distilled

water through chemical technique. Nanostructures of the prepared ZnO particles have been

confirmed through XRD, TEM, and UV-VIS absorption characterizations. The averagesize of the prepared ZnO nanoparticles is as small as 1.9 nm. A strong green

photoluminescence emission under UV excitation is reported from the prepared ZnOnanoparticles due to the recombination of a photo-generated hole with a singly ionized

charge state.

(Received December 6, 2008; accepted December 15, 2008)

Keywords:ZnO nanoparticle; chemical synthesis; photoluminescence emission

1. Introduction

Semiconductor compounds have drawn much attention during the last few years becauseof their novel optical and transport properties which have great potential for many optoelectronic

applications [128]. To fit the need of different temperatures, scientists are looking for large

intrinsic bandgap semiconductor materials so as to expand the exhaustion regions. We know the

extremely small size of these particles will result in quantum confinement of the photo-generated

electronhole pair, leading to a blue shift in

the absorption spectrum. Research studies are done on the doped nanocrystaline II-VI

semiconductors to improve their emission activity [27-28].Bulk ZnO is a well-known wide direct

band gap (3.2 eV at room temperature) and high exciton binding energy (~ 60 meV)

semiconducting material. ZnO is an IIVI semiconductor with properties similar to GaN. It is

therefore a potential candidate for optoelectronic applications in the short wavelength range

(green, blue, UV), information storage and sensors [1-7]. The high exciton binding energy of ZnO

would allow for excitonic transitions even at room temperature, which could mean high radiativerecombination efficiency for spontaneous emission as well as a lower threshold voltage for laser

emission. Studies have been carried out to fine-tune the properties of ZnO to adopt it for different

applications; for example, the band gap of ZnO is modified to use as UV detectors and emitters.

The optical and electronic properties of semiconductors can be further tuned by varying the size of

the particles in the range below 10 nm. Nanoparticles of ZnO attract an increasing interest, due to

their possible use in a range of new nanodevices. Applications in optoelectronics, as blue colour

light emitting phosphors [4, 5], as nanorod UV light emitters [6], as fluorescence labels in

medicine and biology, in controlling units as UV photodetectors and as high-flame detectors [7],

as nanosensors of various gases, but also in cosmetic industry, as a component of sun screens, are

envisioned [1-3].

*Corresponding author: [email protected], [email protected]
mailto:[email protected]:[email protected]


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Here, we report the synthesis of monodispersed ZnO nanoparticle through chemical

method, using PVP as capping agent. The nanostructures of the prepared ZnO particles have been

confirmed using UV-VIS absorbance, XRD, and TEM analysis. A model equation using the

effective mass model [8] has been developed in this work to calculate the particle size as a

function of the peak absorbance wavelength. The value of the average particle size obtained fromTEM micrograph analysis is ~1.9 nm and the value of the same is 2.1 nm as obtained from the

derived model equation. Photoluminescence (PL) emissions from the prepared ZnO nanoparticles

are observed at two different excitation wavelengths of 300 and 320 nm. The maximum PL

emission takes place in 400-700 nm wavelength range, covering the whole visible region of the

electromagnetic spectrum, from the prepared ZnO nanoparticles at 320 nm excitation.

2. Experiment

ZnO nanoparticles have been synthesized in distilled water. The alkali solution of zinc was

prepared by dissolving zinc nitrate [Zn (NO3)26H2O] (10-1 M, 20 ml) and KOH in distilled water

to form a 100 ml solution [Zn2+ =0.5M, OH =1.0 M]. Then the KOH solution was heated to 50 oCtemperature. Under constant stirring, the zinc nitrate solution was added slowly drop wise up to

pH 8. For stopping agglomeration of nanoparticles we have used (polyvinylpyrrollidone) PVP as

capping agent. After few hours of reaction, the white precipitate deposited in the bottom of the

flask was collected and washed it. Finally, the precipitate was centrifuged and dried at room

temperature for 30 hours. The samples were stored at room temperature for measurements of

optical absorption, steady-state photoluminescence, and other structural properties.

The prepared ZnO nanoparticles were characterized for their optical, and nanostructural

properties. The optical transmission/absorption spectra of ZnO dispersed in water were recorded

using a UV-VIS spectrophotometer (Hitachi, U-3010). The formation of ZnO nanoparticle was

confirmed using transmission electron microscope (TEM, JEOL 2000 FX 11). X-ray diffraction

pattern was recorded using an X-ray diffractometer (PANLYTICAL) using Cu K radiation of

wavelength = 0.15406 nm in the scan range 2 = 20-90o. Scanning electron microscope (SEM

with EDXA, Sirion) has been used for compositional analysis of the prepared ZnO nanoparticles.

The photoluminescence (PL) spectrum of the ZnO nanoparticles dispersed in water has been

measured using a spectrofluorimeter (F-2500 FL Spectrophotometer, Hitachi).

3. Results and discussion

The prepared nanopowders were first dispersed in water and then the UV-VIS optical

absorption characteristics of the ZnO nanoparticles, along-with that of the reactants, Zn(NO 3)2 and

PVP were measured. The measured absorption characteristics are shown in Fig. 1a. The synthesis

of ZnO nanoparticles is clearly evident from the Fig. 1a. The excitonic absorption peak is observeddue to the ZnO nanoparticles at 262 nm, which lies much below the bandgap wavelength (shown

as dotted line in Fig. 1a) of 388 nm (Eg = 3.2 eV) of bulk ZnO. Also it is observed that absorption

of ZnO is very sharp, which indicates the monodispersed nature of the nanoparticle distribution [1-

7]. The monodispresed nature of particle distribution has also been confirmed by TEM

measurement which is presented later. For obtaining the absorption characteristics of all the

samples, at first the transmittance (T) at different wavelengths () are measured and then

absorbance () at the corresponding wavelengths are calculated using the Beer-Lamberts

relation.

=

Td

1ln

1 , (1)

where, d is the path length. The absorption edge for single crystal ZnO is very sharp and is

determined by the nature of the electronic transition between the valence band and conduction

band. The absorption edge for a suspension of nanoparticles is much broader and is determined by


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the distribution of particle size [9]. At the absorption edge, only the largest particles contribute to

the absorbance. In the smaller wavelength range particles with smaller sizes contribute more and at

the region of absorbance maximum, all particles contribute to the absorbance. Thus the average

particle size present in a nanocolloid can be determined from the inflection point in the absorption

vs. wavelength spectrum [9]. The following Eqn. (2) derived using the effective mass model [8]describe the particle size (r, radius) as a function of peak absorbance wavelength (p) for

monodispersed ZnO nanocolloid,

)(

2.24833829.6

)(

72.1024023012.263049.0

)(

nm

nmnmr

p

p

+

++

= . (2)

250 300 350 400 450 500 550

Abs.(a.u.)

Wavelength (nm)

1

23 1--PVP

2--ZnO

3--Zinc nitrite

Bulk bandgap wavelength

250 300 350 400

2

4

6

8

10

12

14

Particlediameter(nm)

Peak absorbance wavelength (nm)

Fig. 1. (a) UV-VIS absorption characteristics of ZnO nanoparticles dispersed in double

distilled water along-with the reactants. Curves 1, 2, and 3 correspond to PVP, ZnO, and

Zinc nitrite, respectively. (b) Demonstrates the variation of the peak absorbance

wavelength vs. particle diameter of ZnO calculated using the Eqn. (2). Circle is the

particle diameter obtained from TEM data.

During the derivation of Eqn. (2), we have used me = 0.26 mo, mh = 0.59mo, mo is the free

electron mass, = 8.5, and Egbulk = 3.2 eV [10-11]. Figure 1b shows the variation of the particle

(a)

(b)


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diameter (2r) with the variation of the peak absorbance wavelength (p) of ZnO nanocolloid,

obtained using Eqn. (2). Our prepared ZnO nanoparticles show peak absorbance at ~262 nm which

corresponds to average particle size of 2.1 nm. The circular point as shown in the Fig. 1b is the

value of the average particle size as obtained from TEM analysis which has been described later.

From Fig. 1b, it is also clear that ZnO

Fig. 2 SEM photograph and the corresponding EDAX spectra of the prepared ZnOnanoparticles are shown in (a) and (b), respectively.

nanoparticles exhibit significant confinement effects for a particle diameter less than ~8 nm.

Scanning electron microscope (SEM) micrograph is shown in Fig. 2a, which demonstrates clearly

the formation of ZnO nanoparticles. The corresponding EDAX spectrum is shown in Fig. 2b. Fig.

3 shows the XRD patterns of prepared ZnO nanoparticles. All the XRD peaks are indexed by

hexagonal wurtzite phase of ZnO (JCPDS Card No. 80-0075) as shown in Fig. 3. XRD pattern

indicates the formation of hexagonal wurtzite phase of ZnO which is in agreement with the

electron diffraction results. The peak broadening in the XRD pattern clearly indicates that very

small nanocrystals are present in the samples. The interplanner spacing (dh k l) as calculated from

TEM, XRD and JCPDS data card and corresponding (h k l) values are summarized in Table 1, forsome major XRD peaks.

(b)

(a)


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Table 1. Interplanner spacing (dhkl) from TEM, XRD and JCPDS data card and

corresponding (h k l) values

dTEM(A0) dXRD (A

0) dJCPDF(A0) (hkl)

2.84 2.80 2.82 (100)

2.58 2.63 2.60 (002)

2.45 2.51 2.48 (101)

1.88 1.92 1.91 (102)

1.6 1.61 1.63 (110)

1.42 1.39 1.38 (112)

In order to estimate the size of these small particles with PVP capping and to characterize

them, we investigated the sample by TEM. The bright field image and the corresponding selected

area electron diffraction (SAED) are shown in Fig. 4a and in the inset of Fig. 4a, respectively. Thenanoparticles are clearly well separated and essentially no aggregation can be found, indicating

effective capping of PVP on the nanoparticle surfaces. The SAED pattern, exhibiting several

uniform bright rings suggests that the nanocrystals have preferential instead of random orientation.

The particle size distribution as obtained from the TEM micrograph of ZnO nanoparticles is

demonstrated

20 30 40 50 60 70 80 900

50

100

150

200

250

300

350

400

(203

)

(1

0

4)

(2

02

)

(004

)

(201

)(112

)

(20

0)

(103

)(1

10

)

(10

2)

(101

)

(002

)

(1

00

)

2 (degree)

Intensi

ty(a.u)

Fig-3X-ray diffraction (XRD) patterns of the prepared ZnO nanoparticles.

1 .0 1.5 2.0 2.5 3.0

0

20

40

60

Noofparticles

Particle size (nm)

(a) (b)


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Fig. 4 Transmission electron micrographs (TEM) picture of ZnO is shown in (a) and the

corresponding electron diffraction patterns are shown in the inset. The particle

distribution as obtained from TEM micrograph is shown in (b).

in Fig. 4b. The particle sizes (diameter) are lying in 1-3 nm range. By log-normal fitting, the

average size of the particle obtained is ~ 1.9 nm which is in close agreement to average particle

diameter of 2.1 nm as obtained from UV-VIS absorbance measurement and our model Eqn. (2).

400 500 600 7000

50

100

150

200

PLintensity(a.u.)

Wavelength (nm)

ZnO in water (320 nm)

ZnO in water (300 nm)

Fig- 5 Photoluminescence (PL) spectra of ZnS nano-particles dispersed in double distilled

water under 300 and 320 nm UV excitation wavelengths.

Fig. 5 shows the photoluminescence (PL) emission from our prepared ZnO nanoparticles

dispersed in double distilled water. We have collected PL spectra for two different excitation

wavelengths, namely at 300 nm and 320 nm. However, the maximum emission was observed

under excitation of 320 nm wavelength. As demonstrated in Fig. 5 that PL emission intensity

increased by ~ 7 times under 320 nm excitation in comparison to that of 300 nm excitation. A

strong maximum at 558 nm green wavelength and a weak band at ~410 nm wavelength appeared

in the PL spectrum, under the excitation at 320 nm. However, the whole PL emission spectrum

covers the 400-700 nm of the visible region of the electromagnetic spectrum. The broad visible

photolumunescence band as shown in Fig. 5 has actually been reported earlier in bulk ZnO as well

as in ZnO quantum dots [12-26].

It has been shown in [24] that the visible emission from nanocrystalline ZnO particles is due to

a transition of a photo-generated electron from the conduction band to a deeply trapped hole. InZnO nanoparticles along-with the visible green emission, UV excitonic emission is also reported

[24]. Therefore, for efficient visible emission a step must be involved in which the photo-

generated hole is trapped efficiently somewhere in the particle. The rate of this hole trapping must

be much faster than the radiative recombination rate of the exciton emission. Because of the large

surface-to-volume ratio of our ZnO particles, efficient and fast trapping of photo-generated holes

at surface sites can be expected. A probable candidate for the trapping of holes are O 2~ ions at the

surface [24-26]. Trapping of a photo-generated hole at the surface is also in agreement with the

size-dependence of the emission intensities [24-26]. The rate for a surface trapping process

increases as the particle size decreases since the surface-to-volume ratio increases and thus the

green emission is observed in our sample with reduced particle size of ~ 1.9 nm.

4. Conclusions


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In this work, we have reported the synthesis of monodispersed ZnO nanoparticles with average

size of 1.9 nm by chemical method with PVP as capping agent and double distilled water as

solvent. The nanostructures of the prepared ZnO nanoparticles have been confirmed using UV-

VIS absorption, XRD, and TEM micrograph analysis. The average size of the prepared ZnO

nanoparticles have also been determined to be 2.1 nm from the peak absorbance wavelength andusing the equation derived from the effective mass model [8]. The sharpness of the UV peak also

shows that the ZnO nanoparticles size in these samples are nearly monodisperse, as supported by

TEM analysis. The PL emissions from the ZnO nanoparticles dispersed in water have been

measured under 300 and 320 nm UV light excitations. The maximum PL emission has been

observed covering the whole 400-700 nm visible region of the electromagnetic spectrum, when

320 nm wavelength is used for excitation. The strong green luminescence we observed,

demonstrates the good quality of the prepared ZnO nanoparticles. As suggested and confirmed by

others [12-26] that such low energy emissions are assigned to the surface states. The density of

surface states in the nanocrystals would increase with a decrease in the size of crystallites of the

prepared nanocrystals, due to the increased surface-to-volume ratio having smaller crystallites.

This would reduce the probability of excitonic emission via non-radiative surface recombination

[12-26]. The green emission is related to the singly ionized oxygen vacancy, and this emissionresults from the recombination of a photo-generated hole with a singly ionized charge state of the

specific defect [24-26].

Acknowledgements

Authors are grateful to Technical Education Quality Improvement Programme (TEQIP), NIT

Durgapur, Govt. of India, for providing the financial support for carrying out this research work.

Authors are grateful to Prof. K. Chattopadhyay, IISc Bangalore for allowing us to use SEM and

TEM facility. Authors are also grateful to Sri R. Sarkar of Physics Department for his help during

the experiment and Dr. D. Sukul of the Chemistry Department for providing the instrumental

facility for PL measurements.

References

[1] J. E. Nause, III-Vs Rev. 12, 28 (1999).

[2] Y. Chen, D. Bagnall, T. Yao, Mater. Sci. Eng. B 75, 190 (2000).

[3] D. C. Look, Mater. Sci. Eng. B80, 383 (2001).

[4] D. H. Zhang, Z. Y. Xue, Q. P. Wang, J. Phys. D Appl. Phys. 35, 2837 (2002).

[5] H. Hayashi, A. Ishizaka, M. Haemori, H. Koinuma, Appl. Phys. Lett. 82,

1365 (2003).

[6] H. T. Ng, B. Chen, J. Li, J. Han, M. Meyyappan, J. Wu, S. X. Li, E. E. Haller, Appl.

Phys. Lett.82, 2023 (2003).

[7] P. Sharma, K. Sreenivas, K. V. Rao, J. Appl. Phys. 93, 3963 (2003).

[8] L. E. Brus, J. Phys. Chem. 90, 2555 (1986).

[9] N. S. Pesika, K. J. Stebe, P. C. Searson, Adv. Matter. 15, 1289 (2003).

[10] L. I. Berger, Semiconductor Materials, CRC Press, Boca Raton, FL (1997).

[11] S. Shionoya, W. M. Yen (Eds.), Phosphor Handbook, CRC Press, Boca Raton,

FL (1998).

[12] U. Koch, A. Fojtik, H. Weller, and A. Henglein, Chem. Phys. Lett. 122, 507 (1985).

[13] M. Haase, H. Weller, and A. Henglein, J. Phys. Chem. 92, 482 (1988).

[14] D.W. Bahnemann, C. Karmann, and M. R. Hoffmann, J. Phys. Chem. 91, 3789 (1987).

[15] L. Spanhel and M. A. Anderson, J. Am. Chem. Soc. 113, 2826 (1991).

[16] M. E. Wong, M. E. Bonevich, and P. C. Searson, J. Phys. Chem. B 102, 7770 (1998).[17] S. Sakohara, M. Ishida, and M. A. Anderson, J. Phys. Chem. B 102, 10169 (1998).

[18] S. Monticone, R. Tufeu, and A. V. Kanaev, J. Phys. Chem. B 102, 2854 (1998).

[19] M. Anpo, Y. Kubokawa, J. Phys. Chem. 88, 5556 (1984).


8/8

138

[20] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, B. E. Gnade,

J. Appl. Phys. 79, 7983 (1996).

[21] A. Moballegh , H. R. Shahverdi , R. Aghababazadeh and A. R. Mirhabibi, Surface

Science 601, 2850 (2007).[22] M. Vafaee and M. S Ghamsari, Materials Lett. 61, 3265 (2007).

[23] S. Sapra, D. D. Sarma, S. Sanvito, N. A. Hill, Nano. Lett.2, 605 (2002).

[24] A. V. Dijken, E. A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, Journal of

Luminescence 87, 454 (2000).

[25] G. H. Schoenmakers, D. Vanmaekelbergh, J. J. Kelly, J. Phys. Chem. 100, 3215

(1996).

[26] M. Wang, E. K. Na, J. S. Kim, E. J. Kim, S. H. Hahn, C. Park, and K. Koo,

Materials Lett. 61, 4094 (2007).

[27] R. Sarkar, C. S. Tiwary, P. Kumbhakar, S. Basu, and A. K. Mitra, Physica-E

40, 3115 (2008).

[28] C. S. Tiwary, R. Sarkar P. Kumbhakar, and A. K. Mitra, Physics Letters A

372, 5825 (2008).

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