Optical properties of Zinc Oxide/Polymer Nanocomposite

S.C. Singh, R.K. Swarnkar and R. Gopal

Laser Spectroscopy & Nanomaterials Lab., Department of Physics,

University of Allahabad, Allahabad-211002, INDIA

spectra2@rediffmail.com, subhash_laserlab@yahoo.co.in

ABSTRACT

ZnO/ PEG Nanocomposite material is synthesized by laser

ablation of pure Zn target in the mix solution of PEG and of

water in different compositions. Synthesized Nanocomposite

materials are characterized by using UV-visible absorption, Transmission electron microscope (TEM) and

Photoluminescence (PL) spectroscopic techniques. TEM

investigation illustrates synthesis of few 2-5 nm quantum

dots with very narrow size distribution. UV-visible

absorption spectrum of quantum dots (QDs) synthesized in

30 ml water and 10 ml PEG (3:1 v/v) have small intensity of

SPR peak corresponding to ZnO as compared to the peak

corresponding to core transition electron. As the PEG

concentration increases SPR peak corresponding to ZnO

raises. It insures that PEG helps in the oxidation of Zn

clusters to make ZnO. Possible synthesis mechanism is also discussed.

1. INTRODUCTION

Metal oxides have been incorporated into polymers as a

means of altering electrical and other properties while

attempting to maintain high optical transmission [1]. Metal oxide/ polymer Nanocomposite materials have wide range of

applications in optoelectronic industry in the fabrication of

LEDs, Photo detectors, Displays etc due to their

composition dependent electroluminescence (EL) and

photoluminescence (PL) properties. As a wide band gap

semiconductor, wurtzite ZnO with band gap energy of 3.37

eV at room temperature and exciton binding energy of 60

meV is one of the best florescent nanomaterial with photo

stable, and solution processable behavior [2,3]. The optical

properties of different structured ZnO have extensively been

studied for several decades. With the rapid developments in nanotechnology, many interesting nanostructures of ZnO

have been fabricated by different synthetic methods, such as

chemical vapor deposition (CVD), thermal evaporation of

oxide powders, polymerization, ion implantation and

template assisted growth [4-6].

In the present study, Zinc oxide quantum dots are

synthesized in the solution of PEG and water in different

compositions by pulsed laser ablation of Zn rod in the

aqueous solution of Polyethylglycol (PEG) with different

PEG water ratio. UV-visible absorption, TEM and PL

characterizations of synthesized semiconductor polymer

nanocomposite are done. The possible mechanism of the

synthesis of the Nanocomposite materials is also discussed.

2. EXPERIMENTAL

Experimental arrangement for the synthesis of colloidal

solution of Nanocomposite using pulsed laser ablation in

aqueous media is described elsewhere [7], briefly high

purity zinc target (99.99 %, Johnston Mathey, U.K.), placed

on the bottom of glass vessel containing aqueous media of

PEG with water in different ratio, was allowed to irradiate

with focused output of 1064 nm from pulsed Nd:YAG laser

(Spectra Phys., Quanta Ray, USA) operating at 40 mJ/pulse

energy, 10 ns pulse width and 10 Hz repetition rate for 2

hours. UV-VIS absorption spectra of as synthesized colloidal

solutions were recorded by PerkinElmer, Lambda-35. One

drop of colloidal solution of nanoparticles were placed on

the carbon coated copper grid and dried. TEM images of the

composite material on the grid were recorded using Technai

G20 Transmission electron microscope working at 200kV.

Photoluminescence spectrum of as synthesized colloidal

nanoparticles was recorded by PerkinElmer, LS-55.

3. RESULTS

3.1 UV-VIS Absorption

UV-visible absorption spectra of Nanocomposite

synthesized mix solution of water and PEG with different

ratio have shown in figure 1. Composite material

synthesized in solution of 10 ml PEG with 30 ml of water

has small intensity of SPR peak corresponding to ZnO as

compared to the peak corresponding to core transition

electron. As the PEG concentration increases SPR peak

corresponding to ZnO raises. It insures that PEG helps in the

oxidation of Zn clusters to make ZnO. For the equal volume

of water and PEG ratio of SPR to intra-band transition peak

is maximum. Position of SPR peak also shifted towards

right, which shows that particle size increases with the addition of PEG.

NSTI-Nanotech 2009, www.nsti.org, ISBN 978-1-4398-1782-7 Vol. 1, 2009 45

200 300 400 500 600 700 800

0.00

0.05

0.10

0.15

0.20

0.25

Ab

so

rba

nce

(a

rb.)

Wavelength (nm)

30 ml Water+ 10 ml PEG 20 ml water + 20 mlPEG

10 ml water +30 ml PEG

0 ml Water + 40 ml PEG

a

b

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4

0.0

0.2

0.4

0.6

0.8

(αh

ν)2

hν (eV)

30ml water + 10ml PEG 20ml water + 20ml PEG

10ml water + 30ml PEG

0ml water + 40ml PEG

Fig.1: UV-visible absorption spectra of colloidal solution of

nanoparticles synthesized in different composition of water

and Polyethylene Glycol (PEG)

The optical band gap, Eg, is determined from the

absorbance spectra [8], where a steep increase in the

absorption is observed because of band – band transition,

from the general relation (αhν)n = B(E-Eg), where B is the constant related to the effective masses of charge carriers

associated with valance and conduction bands, Eg is the

band gap energy, E = hν is the photon energy, and n= ½ or 2, depending on whether the transition is indirect or direct

respectively. The intersection of the slope of (αhν)2 verses

hν curve on the x- axis provide band gap energy of the sample.

Figure 2: Toue’s plot for determining optical band gap

of the ZnO/ polymer nanocomposite.

Taue’s plot of ZnO/polymer nanocomposite material is

displayed in figure 2. Nature of the plot is in such a way that

there are two intercepts at α = 0, which cut X-axis 3.45 and 3.88eV, which corresponds for band gap of ZnO shell and

combination of Zn core and ZnO shell layer. Full scale of

the Taue’s plot for UV-visible data is displayed in the inset.

3.2 TEM Image

TEM images of the ZnO/ polymer composite material

synthesized in (a) 10ml PEG with 30ml water and (b) 20 ml

PEG and 20ml water have shown in figure 3. The ZnO/

polymer composite synthesized in 10ml PEG has 4-5 nm

average size while in that of 20ml PEG has particles lying in

the range of 6-8 nm with 7 nm average size and 2 nm dispersion. Most of the particles are spherical in shape along

Figure 3: TEM image of nanocomposite synthesized in

(a) 10ml PEG with 30ml water and (b) 20ml of PEG in 0ml

of water.

with some of the elongated particles with low aspect ratios. Size of the particles is larger as compared to those

synthesized in the solution of 10 ml PEG in 30 ml water.

HRTEM image shows that nanocomposite contains different

planes of ZnO Nanocrystals. It is evident that particle size

increases with the increase of PEG concentration. Therefore

according to the band gap theory, band-gap of materials

synthesized in low PEG concentration should be higher than

that of synthesized in higher PEG concentration.

NSTI-Nanotech 2009, www.nsti.org, ISBN 978-1-4398-1782-7 Vol. 1, 200946

300 400 500 600 700 800

0

50

100

150

200

250

300

350

400

450

500

550

600

Photo

lum

ine

scence Inte

nsity

(a.u

.)

Wavelength (nm)

225 nm

250 nm

275 nm

325 nm

3.3 Photoluminescence (PL)

Figure 4 shows the photoluminescence spectra of the

ZnO/ polymer nanocomposite synthesized in the solution of

20ml water and 20ml PEG by different excitation. There is

low absorbance at 225 and 250 nm, therefore it shows low

emission efficiency corresponding to these excitations. With the variation of excitation wavelength one can determine

bandgap energy and presence of defect levels. Excitation of

the ZnO/Polymer composite material with 225 and 250 nm

light emits PL in the range of 370-580 with bands at 320 and

400 nm. The band intensity and width at 400 nm produced

by excitation of 250 nm light, is stronger as compared to that

originates by excitation of sample by 225 nm. There is also

emission band in the red region at 615 nm for 225 nm

excitation, which shifted towards longer wavelength, with

slightly decreased intensity, and falls at 630 nm for

excitation by 250 nm light. Excitation of the sample with

275 and 325 nm have enhanced violet (342-600 nm) and

near IR (λ>700 nm) emissions, which may have profitable applications in the fabrication of visible and IR LEDs.

4. DISCUSSIONS

We have observed blue emission as well as red emission

from the composite nanostructures. Weak blue emission

from ZnO nanostructures including their different shape and size are reported by several researchers. Recently Zeng et al.

have reported blue emission from ZnO nanoparticles is due

to interstitial Zn present in ZnO nanoparticles i.e. Zn/ZnO

core/shell synthesized by pulsed laser ablation of Zn in

liquid media. Emission peak at 300 nm corresponds for the

wide band-gap of the polymer semiconductor Nano-

composite particles having 4.0 eV gap. Size of the quantum

dots are below 5 nm in diameter, therefore they are

exhibiting quantum confinement effect.

Laser ablation in active liquid media, causes high

non-equilibrium processing, which allows synthesis of novel phases of materials and, is of particular interest. Synthesis of

nanomaterials by laser ablation in liquid media is under

developing stage. Laser ablation at solid liquid interface,

interaction between laser light and target creates local high

temperature and pressure plasma plumes above target

surface. It is reported that laser ablation of graphite48 target

in water using 70 mJ/pulse energy of laser and detecting

emission spectra and simulation, it was found that pressure and temperature inside plasma plume was 1Gpa and 6000 K

respectively. At this high temperature and pressure several

chemical reactions and physical processes, that are not

possible at normal conditions, will take place among ablated

species, solvent and surfactant molecules, which induces

formation of particles in the solution. The structure,

morphology, size, distribution and hence properties of

nanoparticles are highly dependent on laser wavelength,

irradiance, surfactant concentration, nature of solvent and

other ablation parameters. Therefore properties of materials

can be tuned by tuning ablation parameters.

Production of active metallic oxide polymer nanocomposite involves following processes in their

formation (a) Generation of high temperature and high-

pressure plasma at solid-liquid interface, after interaction

between pulsed laser light and solid target. (b) Cluster

formation by adiabatic and supersonic expansion leads to

cooling of plasma plume. (c) Interaction of theses reactive

clusters with species in aqueous media and reactive oxygen.

Polymer is highly viscous therefore it strongly confines laser

produced zinc plasma expansion. The more the confinement,

more will be the pressure and temperature of ablated species

inside the plasma. High pressure and high temperature induces and induces several reactions between laser induced

plasma plume and ablating media. With the increase of

Polymer/water ratio confinement of the laser ablated plasma

plume increases, which induces more oxidation and reaction

of laser ablated zinc plasma and aqueous media, therefore

oxidation of ablated particles increases with the polymer

concentration.

CONCLUSIONS Size of synthesized polymer /ZnO Nanocomposite

material increases with the increase of polymer water ratio. PL spectra have Strong UV-visble and near IR emissions

applicable to LEDs and lasing applications. Size of the

synthesized quantum dots are below 5 nm in diameter.

4. ACKNOWLEDGEMENTS

We are thankful to CSIR and DRDO New Delhi for

financial support and Prof. B.R. Mehta, IIT New Delhi

for providing TEM facilities.

NSTI-Nanotech 2009, www.nsti.org, ISBN 978-1-4398-1782-7 Vol. 1, 2009 47

REFERENCES

[1] N. Al-Dahoudi, H. Bisht, C. Gobbert, T. Krajewski,

M.A. Aegerter, “Transparent conducting, anti-static and

anti-static-anti-glare coatings on plastic substrates,”

Thin Solid Films,392,299, 2001.

[2] S.-H. Yoon, H. Yang, Y.-S. Kim, “Ordered growth of

ZnO nanorods for fabrication of a hybrid plasma display panel,” Nanotechnology 18, 205608, 2007.

[3] H.-J. Kim, C.-H. Lee, D.-W. Kim, G.-C. Yi, “Fabrication

and electrical characteristics of dual-gate ZnO nanorod

metal-oxide semiconductor field-effect transistors,”

Nanotechnology 17,327, 2006.

[4] C. Guo, Z. Zheng, Q. Zhu, X. Wang, “Preparation and

Characterization of Polyurethane/ZnO Nanoparticle

Composites,” Poly.-Plastics Tech. Eng., 46, 1161, 2007.

[5] C. M. Thompson, H. M. Herring, T. S. Gates, J. W.

Connell, “Preparation and characterization of metal

oxide/polyimide nanocomposites,” Composites Sci.

Tech. 63, 1591, 2003.

[6] J. Pyun and K. Matyjaszewski, “Synthesis of

Nanocomposite Organic/Inorganic Hybrid Materials Using Controlled/“Living” Radical Polymerization,”

Chem. Mater., 13, 3436, 2001.

[7] S.C. Singh and R. Gopal, “Laser Irradiance and

Wavelength-Dependent Compositional Evolution of

Inorganic ZnO and ZnOOH/Organic SDS

Nanocomposite Material,” J. Phys. Chem. C, 112, 2812,

2008.

[8] S.C. Singh, R.K. Swarnkar and R. Gopal, “Synthesis of

Titanium Oxide Nanomaterials by Pulsed Laser

Ablation in Aqueous Media,” J. Nanosci. Nanotech., 9,

1, 2009.

NSTI-Nanotech 2009, www.nsti.org, ISBN 978-1-4398-1782-7 Vol. 1, 200948