8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
EFFECT OF ZINC OXIDE SEED LAYER ON THE GROWTH OF
WELL ALIGNED ZINC OXIDE NANORODS BY VAPOR LIQUID
SOLID METHOD
Prabhakar Palni1,*, Satchi Kumari1, P. K. Giri1,2
1Department of Physics, Indian Institute of Technology, Guwahati 781039, India.
2Centre for Nanotechnology, Indian Institute of Technology, Guwahati 781039, India.
ABSTRACT
Recently the growth of high quality and defect free ZnO nanostructures have drawn
considerable attention due to its wide applications in various fields like
optoelectronics, microelectronics, gas sensors, and surface acoustic wave devices. In
the present work, we have grown well aligned ZnO nanorods and microrods by a
three step process: first depositing thin layer of zinc oxide seeds onto the silicon
substrate by radio frequency magnetron sputtering followed by deposition of thin
layer of gold catalyst and subsequently growing ZnO nanorods by the vapour liquid
solid (VLS) growth process using a furnace. The resulting nanostructures are
characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM),
photoluminescence (PL) and Fourier transform infrared spectroscopies (FTIR). The
structural analysis on these nanostructures shows C-axis oriented aligned growth of
ZnO nanorods. The effect of the growth temperature on the morphology and PL
properties of the ZnO nanorods are also studied. These ZnO nanorods exhibit bandedge
related UV photoluminescence in the range 374-388 nm, and a defect-emission
band at ~494 nm in some cases. Structure, morphology and PL properties of aligned
ZnO nanorods have been studied in details and compared with the nanorods grown
without the seed layer.
I. Introduction
Zinc oxide nanostructures have attracted lots of interest due to its unique features such
as wide band-gap (3.37 eV) and large exciton binding energy (60 meV)1,2 and wide
applications in nanosize electronic, optics, sensors and optoelectronic devices.3-5 ZnO
is one of the key materials in nanotechnology and nanosystems. Several techniques
have been used to grow ZnO nanowires. For example, thermal evaporation and
condensation, metalorganic chemical vapor deposition (MOCVD) and laser molecular
beam epitaxy (laser-MBE) have been employed. Among them, the vapor Liquid Solid
(VLS) deposition method involving the vapor-transfer process and thermal
evaporation is the most frequently used method. For the production of high quality
ZnO nanowires, catalysts, such as Au6, Cu and NiO etc have been most commonly
used. The catalysts can improve the controllable growth of ZnO nanostructures. Here,
we report the effect of pre-depositing ZnO seed layer on the morphology and optical
properties. The as synthesized ZnO nanostructures are randomly oriented and hence
have limited applications in nanosize electronic and optoelectronic devices.
Therefore, it is crucial to have controlled and well aligned growth of ZnO nanorod
arrays.
* Corresponding author, email: p.palni@iitg.ernet.in
In this work, we have grown we-aligned ZnO nanorods and microrods by a seeded
layer growth techniques, using the VLS process. The effect of ZnO seed layer and the
growth temperature in the catalytic growth of ZnO nanorods are studied using
structural and optical tools.
II. Experimental Details
Silicon (100) n-type substrate is used which is cleaned in trichloroethylene, acetone,
and methanol under ultrasonic bath for 15 minutes each and then cleaned with buffer
HF solution to etch out oxide layer and finally washed with deionised water and dried
under the nitrogen gas flow. This substrate is used for VLS growth of well aligned
ZnO nanorods by three steps process. In first step, the Zinc Oxide seed layer is
deposited by rf magnetron sputtering system. Sputtering is carried out for 30 minutes
at incident power of 100 watts and the substrate temperature is maintained at 3000C
during the deposition. The Zinc Oxide target and substrate distance was maintained to
optimum distance of 7 cm. Argon and oxygen are used as reacting gases. In next step,
ultra thin layer of gold has been deposited by a Mini sputtering system. Finally high
quality ZnO well aligned nanorods/microrods arrays have been grown by VLS
process. In this process high purity ZnO powders and high purity graphite powders
are grounded well at a weight ratio of 1:1 and used as a source. A quartz boat
containing powder mixture is placed in the central hot zone of 10000C inside a
horizontal quartz tube furnace and substrates are placed away from the powder
mixture in a various temperature zones. The unilateral flow of Ar gas with high flow
rate was introduced at the beginning of the experiment in order to flush out gas
impurities and moisture content. Hot furnace is ramped to a temperature of 10000 C,
and Argon gas flow rate of 70 sccm (standard cubic centimetre mass) is maintained
throughout the process as a carrier gas. The deposition is carried out for 15 minutes.
The entire system is then cooled to a room temperature and the synthesized product is
taken for characterization. Same experiment is carried out for the nanostructure
growth by VLS process with a silicon substrate prepared with only gold catalyst
deposition i.e. without predepositing ZnO nanocrystalline seed layer.
The ZnO nanorods/microrods were characterized with XRD, SEM, PL spectroscopy
and FTIR spectroscopy. Structure and morphology of these nanorods/microrods are
studied by X-ray diffractometer (Bruker, Advance D8) and digital scanning electron
microscope (LEO 1430 VP). Room temperature fluorescence measurements were
performed with 350 nm excitation sources using a commercial Fluorimeter (Thermo
Electron, FA-357). FTIR measurements were performed in the range 400-4000cm-1
using FTIR spectrometer (Perkin-Elmer spectrum one).
III. Results and discussion
Fig.1 (a), (b), and (c) shows the XRD pattern of the ZnO nanorods/microrods grown
on a ZnO seed layer at various substrate temperature, whereas Fig. 1(d) shows the
XRD pattern of randomly oriented ZnO Nanorods grown without the seed layer. Figs.
(a-d) shows the strong peak due to the ZnO (002) plane indicating that the growth
direction along C-axis of ZnO and is normal to the substrate plane. The small FWHM
value from the XRD patterns from Figs.1 (a-c) indicates that the C-axis of the ZnO
nanorods/microrods is well aligned and the growth direction is perpendicular to the
base surface. Additional peak C at 43.10 in Fig. 1(b) is related to the carbon which is
used during the VLS growth as a reducing agent.
Fig. 2 (a), (b), and (c) shows the SEM micrographs of ZnO nanorods/microrods
grown at 9000C, 7000C and 8500C, respectively using the ZnO nanocrystal seed layer.
Fig. 1: XRD pattern of seeded layer grown aligned ZnO nanorods grown at different substrate
temperatures (Ts): (a) 900°C, (b) 700°C, (c) 850°C. (d) XRD pattern of non-aligned ZnO
nanorods grown at Ts=600°C without any ZnO seed layer.
Fig. 2: SEM images of seeded layer grown aligned ZnO nanorods grown at different
substrate temperatures (Ts): (a) 900°C, (b) 700°C, (c) 850°C. (d) SEM images of nonaligned
ZnO nanorods grown at Ts=600°C without any ZnO seed layer.
The size of the nanorods ranges from several hundreds of nanometer to a few
micrometer due to the various sizes of ZnO seeds pre-deposited using rf magnetron
sputtering on the silicon substrates, ZnO seeds acts as a nucleation sites for the
nanorods/microrods growth and importantly offers very negligible lattice mismatch or
almost mismatch free interface between seed layer and nanorods/microrods, which
results in the high quality well aligned growth of ZnO nanorods /microrods arrays.
Moreover, the nanorods/microrods grown have large diameter because of very high
substrate temperature i.e. at or above 7000c during growth process. Size control of
these nanorods/microrods can be accomplished by lowering the growth temperature
and depositing small size ZnO seed layer11. Fig. 1(d) shows the randomly oriented
ZnO nanorods which are grown without ZnO seed layer, the diameter of these
nanorods ranges from 30-50 nm.
Fig. 3 (a) and (b) shows the Photoluminescence measured at room temperature
excited by Xe lamp it shows weak near band edge (NBE) emission peak due to free
excitonic recombination and defect related emission so called green emission band
which is attributed due to the recombination of photogenerated holes with the singly
ionized oxygen vacancies7. Grabowska et al. and R. Dingle et al.8,9 has also reported
weak UV emission from aligned ZnO nanorods when PL spectra was measured at
room temperature i.e. (300K). However, the UV emission dominated green emission
for PL spectra recorded at low temperature i.e. (7K). Fig. 3 (c) shows the UV
emission peak along with the peaks due to Xenon lamp at 395 nm, 420 nm, 450 nm,
467 nm etc. Whereas green emission band is absent in this PL spectra. Fig. 3 (d)
shows the PL spectra of as-deposited non-aligned ZnO nanorods grown without seed
layer which shows UV emission peak at 380 nm along with the green emission band.
The inset of Fig. 3 (d) shows the post annealing effect of this sample at 8000C which
shows the improved optical quality thereby suppressing the green emission due to
singly ionized oxygen vacancies and other defects.
Fig. 3: PL spectra of seeded layer grown aligned ZnO nanorods grown at different
substrate temperatures (Ts): (a) 900°C, (b) 700°C, (c) 850°C. (d) PL spectra of nonaligned
ZnO nanorods grown at Ts=600°C without any ZnO seed layer.
Fig. 4(a) and 4 (b) shows the structural information obtained from the FTIR spectra of
without seed layer growth and with seed layer growth of ZnO nanorods and ZnO
nanorod/microrods respectively. Fig. 4(a) clearly shows the absorption bands at 479
cm-1, 432 cm-1 which correspond to the ZnO bending mode. 406 cm-1 and 572 cm-1
are also the characteristic ZnO absorption peaks due to the nanocrystalline structure of
ZnO, as Andres-Verges et al.10 reported theoretically. Absorption peak at 523 cm-1
corresponds to the ZnO stretching mode Fig. 4(b) also shows similar characteristic
peaks due to ZnO at 408 cm-1 ,417 cm-1 and 550 cm-1, whereas 471 cm-1and 525 cm-1
are due to ZnO bending mode and stretching mode, respectively.
IV. Conclusions
We illustrated that the three step process to grow high quality ZnO nanorod/microrods
which involves seed layer deposition of ZnO nanocrystal followed by ultra thin layer
of gold catalyst and finally growing ZnO nanorods/microrods arrays by VLS process
is extremely important to grow the aligned nanostructures. These ZnO
nanorod/microrods arrays and nanorods are highly crystalline and possessing good
optical quality which is corroborated from XRD, PL and FTIR data. The capability of
growing high quality and well aligned growth of ZnO nanorod/microrods arrays by
this simple three step process of using pre-deposited seed layer gives the breakthrough
towards the development of ZnO based micro-electronics and optoelectronic devices.
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