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Luminescence from Surfactant-Free ZnO Quantum Dots

Luminescence from Surfactant-Free ZnO Quantum Dots
Prepared by Laser Ablation in Liquid
R. S. Ajimsha, G. Anoop,
Arun Aravind, and M. K. Jayaraj
Optoelectronic Devices Laboratory, Department of Physics, Cochin University of Science and Technology,
Kochi-682 022, Kerala, India
Highly transparent, luminescent and biocompatible ZnO quantum dots were prepared in water, methanol, and ethanol using
liquid-phase pulsed laser ablation technique without using any surfactant. Transmission electron microscopy analysis confirmed
the formation of good crystalline ZnO quantum dots with a uniform size distribution of 7 nm. The emission wavelength could be
varied by varying the native defect chemistry of ZnO quantum dots and the laser fluence. Highly luminescent nontoxic ZnO
quantum dots have exciting application potential as florescent probes in biomedical applications.
2007 The Electrochemical Society.
DOI: 10.1149/1.2820767 All rights reserved.
Manuscript submitted September 4, 2007; revised manuscript received November 6, 2007.
Available electronically December 17, 2007.
Synthesis of nanoparticles has been the focus of an ever-
increasing number of researchers worldwide, mainly due to
their unique optical and electronic properties,
which make
them ideal for a wide spectrum of applications ranging from
and lasers
to in vivo biological imaging and therapeutic
A large number of preparation methods are reported to pro-
duce nanoparticles, such as magnetic liquids,
and colloidal systems.
the past decade a technique known as liquid-phase pulsed laser ab-
lation LP-PLA has aroused immense interest.
It involves the
firing of laser pulses through liquids transparent to that wavelength
on to the target surface. The ablation plume interacts with the sur-
rounding liquid particles, creating cavitation bubbles which, upon
their collapse, give rise to extremely high pressures and tempera-
tures. These conditions are, however, localized and exist across the
nanometer scale. LP-PLA has proven to be an effective method for
preparation of many nanostructured materials, including nanocrys-
talline diamond,
cubic boron nitride,
and nanometer-sized par-
ticles of Ti,
and TiC.
Wurtzite ZnO with a wide
bandgap and excitonic energy of 60 meV has many important appli-
cations in UV light-emitting diodes, diode lasers, sensors, etc. Be-
cause zinc is a important trace element of humans,
ZnO is envi-
ronmentally friendly and suitable for in vivo bioimaging and cancer
detection. There have been recent reports on the synthesis of ZnO
nanoparticles using LP-PLA from Zn target in an aqueous solution
containing different surfactants.
Zeng et al.
used 1064 nm for
ablation, which had greater penetration into the target, ablating more
Furthermore, without any surfactant these particles do not stand
isolated. The use of metallic Zn target along with surfactants like
sodium dodecyl sulfate gives rise to the formation of several by-
products like Zn OH
, and the quantum dots QDs were Zn/ZnO
core shell structure. The present investigation is on surfactant-free
pure ZnO QDs without any by-products using LP-PLA. To the best
of our knowledge, LP-PLA has not been used for the synthesis of
pure ZnO QDs without the use of surfactants. The literature survey
shows that not much work has been done on the synthesis of ZnO
QDs using methods without any surfactants. Recently we reported
the growth of luminescent, biocompatible ZnO QDs using the wet-
chemical method without any surfactant. The preparation of high-
quality ZnO QDs with specific interest in their luminescence prop-
erties and surface functionality with the aim of biological
applications has not been studied widely.
In this work, we report the preparation of highly luminescent
visible to the naked eye on UV illumination , transparent, chemi-
cally pure, and crystalline ZnO QDs using LP-PLA without the aid
of any surfactant. Clear, deep yellow, and bluish-violet emitting ZnO
QDs fully dispersed in water, ethanol, and methanol were prepared
directly from the ZnO targets by this technique without any by-
product. Thus obtained biofriendly ZnO QDs can be used as fluo-
rescent probes in various biomedical applications by easily attaching
biomolecules to the bare surface of these ZnO QDs.
A sintered ZnO 99.99% mosaic target was used for the fabri-
cation of ZnO QDs. ZnO target immersed in 20 mL of different
liquid media like deionized water, methanol, and ethanol was ab-
lated at room temperature by the third harmonic of a Nd:yittrium
aluminum garnet laser 355 nm, repetition frequency of 10 Hz,
pulse duration 9 ns . The spot size of the laser beam was 2 mm after
focusing using a lens, and the ablation was done at laser fluences of
25, 35, and 45 mJ/pulse. The duration of laser ablation was 1 h in all
the liquids. In water, ablation was also carried out for different du-
rations of 1, 2, and 3 h by keeping the laser fluence at 45 mJ/pulse.
This simple room-temperature technique produced highly transpar-
ent ZnO QDs well dispersed in respective liquid media. Formation
of nanoparticles of ZnO was confirmed by transmission electron
microscopy TEM, JEOL operating at an accelerating voltage of
200 kV. A small droplet of the liquid obtained after ablation was
deposited onto a copper grid with carbon film for TEM analysis.
Photoluminescence emission PL and excitation spectra were re-
corded using Jobin Yvon Fluoromax-3 spectrometer equipped with
150 W xenon lamp.
Results and Discussion
TEM analysis revealed that the resulting product after laser ab-
lation with an energy of 25 mJ/pulse in water consisted of particles
in the nanoregime, as shown in Fig. 1a. Statistical size analysis Fig.
1b shows almost uniform particle-size distribution with a particle
size of 7 nm. The selective area electron diffraction SAED Fig.
1c exhibits a well-distinguishable concentric ring pattern represent-
ing the 100 , 002 , 102 , 110 , and 103 planes of hexagonal
ZnO. This clearly shows the growth of crystalline ZnO QDs with
random orientation. ZnO QDs were arranged in hexagonal shape as
observed from a high-resolution TEM HRTEM image Fig. 1d .
The stacking of about 85 hexagonal unit cells makes a 7 nm sized
hexagonal-shaped QD. The inset of Fig. 1d shows the arrangement
of individual unit cells, which again demonstrates the crystalline
quality of ZnO QDs. The Zn/ZnO composite nanoparticles grown
by Zeng et al.
have an average particle size of 18 nm and are
colored due to turbidity. An atomic-scale image shows the parallel
lines of ions at intervals of 0.26 Fig. 1e and 0.28 nm Fig. 1f
which correspond to 002 and 100 planes of ZnO, respectively.
From TEM analysis, the formation of other molecules like Zn OH
or ZnO/Zn core shell formation is not found. Because the ejected
molten material from the target normally reacts with medium only at
Electrochemical Society Student Member.
E-mail: [email protected]
Electrochemical and Solid-State Letters, 11 2 K14-K17 2008
1099-0062/2007/11 2 /K14/4/$23.00 The Electrochemical Society
K14Page 2

the outer surface,
the ejected plasma readily cools, thereby reform-
ing ZnO itself. Because there are many surface defects, mainly due
to surface oxygen deficiency discussed later , these nanoparticles
are charged. This surface charge provides a shield, preventing fur-
ther agglomeration and thereby forming self-stabilized particles
even in the absence of surfactant.
Figures 2a, c, and e show the TEM images, and b, d, and f
represents the particle-size distribution pattern of the ZnO QDs pre-
pared in methanol at laser fluences of 25, 35, and 45 mJ/pulse, re-
spectively. It explicitly demonstrates the increase of both particle
size and particle density with laser fluence. Particle sizes as ob-
served from the size distribution Fig. 2b, d, and f are 7.1, 8, and
9.1 nm for ZnO QDs prepared at laser fluence of 25, 35, and
45 mJ/pulse, respectively. TEM shows a similar size distribution for
those prepared in ethanol, whereas the QDs prepared in water by
LP-PLA of ZnO do not show any variation in size when varying the
fluence from 25 to 45 mJ/pulse. The thermodynamic conditions cre-
ated by the laser-ablation plume in the liquid are localized to a
nanometer scale which varies with laser fluence. This is the reason
for the variation of particle size of ZnO QDs with laser fluence.
Figure 3 shows the absorption spectra of ZnO QDs of sizes 7,
8.1, and 9 nm dispersed in methanol prepared at a laser energy of
25, 35, and 45 mJ/pulse. The increase in laser energy increase in
particle size resulted in red shift of the excitonic peak from
3.67 to 3.57 eV and a slightly broadened peak due to quantum size
PL measurement was performed on the QDs dispersed in water,
ethanol, and methanol at an excitation wavelength of 345 nm. Deep-
yellow luminescence was observed from the ZnO QDs dispersed in
water. Figure 3a shows the photograph of highly transparent ZnO
QDs left dispersed in water and their yellow emission under UV
excitation. This yellow luminescence originates from the native oxy-
gen defects of the prepared ZnO QDs discussed later . Figures 4b-d
show the PL spectra of ZnO QDs dispersed in water, ethanol, and
methanol, respectively. Pure water, ethanol, and methanol do not
show any emission under UV excitation. Each figure depicts the
variation of PL intensity with the laser fluence at which these QDs
were prepared in the liquid. A considerable increase in PL intensity
with laser fluence is observed for all the samples. The increase in PL
intensity with laser fluence cannot be attributed to an increase in
particle density alone. There is a possibility of formation of more
defect states at higher fluence which is not clear in the present
A blue shift in PL maximum was observed with a decrease of
laser fluence in the case of QDs grown in methanol from 2.41 eV at
45 mJ/pulse to 2.6 eV at 25 mJ/pulse and ethanol from 2.27 eV at
45 mJ/pulse to 2.35 eV at 25 mJ/pulse . However, the PL peak po-
sition remains unchanged for QDs grown in water.
The origin of yellow luminescence due to oxygen vacancy was
further supported by the experiment done with oxygen bubbled into
Figure 1. a TEM image, b particle-size distribution, and c SAED pat-
tern of ZnO QDs obtained by laser ablation with a fluence of 25 mJ/pulse in
water. d HRTEM image for a single QD and inset arrangement in the
hexagonal close-packed mode. e and f HRTEM showing 002 and 100
planes, respectively.
Figure 2. a, c, and e TEM and b, d, and f particle-size distribution of
ZnO QDs dispersed in methanol prepared at laser fluences of 25, 35, and
45 mJ/pulse, respectively.
Electrochemical and Solid-State Letters, 11 2 K14-K17 2008
K15Page 3

the water during laser ablation of ZnO targets. Interestingly, PL
spectrum shows an emission peaking at 408 and 427 nm in the
violet blue region, suppressing the yellow emission Fig. 5a when
oxygen was bubbled through liquid during the ablation. This emer-
gence of deep bluish-violet emission opens the possibility of tuning
emission color for different biomedical applications. The inset of
Fig. 5a shows the photograph of deep bluish-violet emission. During
bubbling the oxygen-defect density was considerably reduced, tend-
ing toward more stoichiometric ZnO QDs. This further supports that
yellow luminescence originates from oxygen vacancies. Emission at
408 nm is due to the transition of electrons from shallow donor
levels to the valance band.
According to Lin et al.,
the energy
gap between the valance band and energy level of interstitial zinc is
2.9 eV. This is consistent with PL emission at 427 nm in the present
study. Future application potential of ZnO QDs resides in the bio-
medical field, where growth of QDs in a biofriendly medium like
water and its luminescent emission has been studied for various
ablation times. Figure 5b shows the PL of QDs dispersed in water
prepared at different ablation times, 1, 2, and 3 h, keeping the laser
fluence at 45 mJ. It is found that PL intensity increases with duration
of laser ablation without any shift in PL peak position. The increase
in PL intensity is due to the increased density of QDs of the same
size. The transparency of prepared ZnO QDs remained as such even
though the duration of ablation was 3 h. The maximum concentra-
tion of ZnO QDs that was achieved while maintaining transparency
was 17.5 g/mL.
Semiconductor QDs have been covalently linked in vivo to
biorecognition molecules such as peptides, antibodies, and nucleic
acids for application as fluorescent probes.
The ZnO QDs pre-
pared in the present study can be used in various biomedical appli-
cations by conjugating with ligands like poly ethylene glycol that
are soluble in both mediums. Then they can be used as fluorescent
probes in cancer targeting and imaging by attaching the correspond-
ing antibodies to the bare surface of ZnO QDs.
Figure 3. Absorption spectra of ZnO QDs in methanol prepared at a 25,
b 35, and c 45 mJ/pulse.
Figure 4. Color online a Photograph of transparent ZnO QDs obtained by
laser ablation in water with a fluence of 25 mJ/pulse left and its yellow
emission right . b PL spectra of ZnO QDs in water, c ethanol, and d
methanol. In each figure, curve I 25 mJ/pulse , curve II 35 mJ/pulse , and
curve II 45 mJ/pulse represents the laser fluence.
Figure 5. Color online a PL spectra of ZnO QDs prepared without curve
I and with curve II oxygen atmosphere. Inset Photo of bluish-violet
luminescence. b PL spectra of ZnO QDs in water for various durations of
Electrochemical and Solid-State Letters, 11 2 K14-K17 2008
K16Page 4

In conclusion, highly transparent, deep-yellow and bluish-violet
emitting, biocompatible 7 nm sized ZnO QDs were prepared in vari-
ous liquid media using LP-PLA without using any surfactant. The
emission wavelength was tuned by playing the defect chemistry and
varying the laser fluence. The origin of yellow luminescence is due
to oxygen vacancies. Highly luminescent biofriendly ZnO QDs can
be used as fluorescent probes in cancer diagnosis and therapy.
This work is supported by the Department of Science and Tech-
nology, Government of India under the Nanoscience and Technol-
ogy Initiative program. The authors thank the Sophisticated Analysis
Instrument Facility Centre, IIT Chennai for TEM measurements.
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