Optical properties of Zinc Oxide - TechConnect
Transcript of Optical properties of Zinc Oxide - TechConnect
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
[email protected], [email protected]
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