Copyright © 2010 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 10, 4786–4791, 2010
Growth of Well-Aligned ZnO Nanorod Arrays on
Si Substrates by Thermal Evaporation of
Cu–Zn Alloy Powders
Jijiang Fu1 2 , Biao Gao1 , Kaifu Huo1 2 ∗ , Xuming Zhang1 ,
Liangsheng Hu1 , and Paul K. Chu2 ∗
1
RESEARCH ARTICLE
The Hubei Province Key Laboratory of Refractories and Ceramics Ministry—Province Jointly-Constructed
Cultivation Base for State Key Laboratory, College of Materials and Metallurgy,
Wuhan University of Science and Technology, Wuhan 430081, China
2
Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Well-aligned ZnO nanorod arrays with uniform diameters and lengths have been fabricated on a Si
substrate by simple thermal evaporation of Cu–Zn alloy powders in the presence of oxygen without
using a template, catalyst, or pre-deposited ZnO seed layer. The ZnO nanorods are characterized
by X-ray diffraction, electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy and the growth mechanism is suggested. The nanorods have a single-crystal
hexagonal structure and grow along the 0001 direction. Their diameters range from 200 to 400 nm
and the lengths are up to several micrometers. The photoluminescence (PL) and Raman spectra
disclose the optical properties of the products. The PL spectra show intense near-band ultraviolet
emission at 378 nm from the nanorod arrays. The well-aligned ZnO nanorod arrays have a low turnon field of 6.1 V/m, suggesting good field emission properties. The simple synthesis methodology
in conjunction with the good field emission and optical properties make the study both scientifically
and technologically interesting.
Keywords: ZnO Nanorod Arrays, Cu–Zn Alloy, Raman Spectroscopy, Photoluminescence, Field
Emission.
1. INTRODUCTION
Zinc oxide is an important semiconductor having a wide
direct band gap (3.37 eV), large exciton binding energy
(60 meV), as well as good piezoelectrical properties.1–2
In the past several years, one-dimensional (1D) ZnO
nanostructures such as nanowires, nanotubes, nanobelts,
nanocombs, and their three-dimensional assembly have
attracted increasing attention due to their potential applications in nanolasers, field effect transistors, field emitters,
nanoresonators, and high-efficiency photonic devices.3–10
It has been experimentally confirmed that the performance
and device applications of 1D ZnO nanostructures can
be enhanced and broadened if they are synthesized controllably on suitable substrates with good alignment and
uniform morphology.11 Hence, synthesis of aligned 1D
ZnO nanowire or nanorod arrays attracts increasing attention. Preparation of aligned ZnO nanowires or nanorods
∗
Authors to whom correspondence should be addressed.
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J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 7
has been demonstrated by template-confined growth,12
catalyst-assisted vapor–liquid–solid (VLS) growth,13–15
and ZnO seed layers-assisted solution-phase growth.4 16–17
However, the use of a template, metal catalyst, or ZnO
seed layer increases the complexity of the synthesis procedures and introduce adventitious impurities, consequently
degrading the performance of the products and limiting
their applications in some electronic and optoelectronic
devices. Thermal evaporation and condensation of Zn or
ZnO vapor species can be used to fabricate heteroepitaxially ZnO nanostructures on suitable substrates,18 in
which exotic metals catalysts or ZnO seed particles are not
necessary. This vapor–solid growth method, albeit technologically simple, often yields mixtures of ZnO nanostructures with different morphologies due to sporadic
supply and variable vapor pressure of Zn at the product
zone.19 It has been disclosed that the Zn/O2 vapor pressure ratio is the major factor determining the morphology
of ZnO nanostructures.19–22 Though the importance of the
Zn vapor pressure has been indicated, it is actually hard
1533-4880/2010/10/4786/006
doi:10.1166/jnn.2010.1699
Fu et al.
Growth of Well-Aligned ZnO Nanorod Arrays on Si Substrates by Thermal Evaporation of Cu–Zn Alloy Powders
2. EXPERIMENTAL PROCEDURES
The well-aligned ZnO nanorod arrays were prepared by
direct thermal evaporation and oxidation of Cu–Zn precursors under Ar/O2 at 700–900 C. The Cu–Zn alloy powders (70/30 atm%, 60 mesh, Aldrich) were loaded on a
ceramic boat in a quartz tube 3 cm in diameter which was
located in the center of a horizontal tube furnace. A Si
(100) wafer (1 × 2 cm2 was ultrasonically cleaned in acetone, ethanol, and distilled water sequentially, and then
dried under flowing nitrogen. The Si substrate was placed
1 cm above the surface of the Cu–Zn alloy powers. The
system was first flushed with Ar several times to remove
oxygen and moisture and then heated to the desired temperature (700–900 C). Afterwards, argon was replaced
by an Ar/O2 (4% O2 mixture at a flow rate of 50 sccm
J. Nanosci. Nanotechnol. 10, 4786–4791, 2010
and the system was kept at this temperature for 60 min.
The reactor was then cooled down to room temperature in
argon and a white homogeneous layer was formed on the
substrate.
Field emission scanning electron microscopy (FE-SEM,
FEI, NOVA NANOSEM 400), X-ray diffraction (XRD,
Philips X’ Pert Pro), energy-dispersive X-ray spectrometry
(EDS, Oxford INCA 200), as well as X-ray photoelectron spectroscopy (XPS, ESCALB MK-II) were employed
to characterize the products. Room temperature photoluminescence (PL) was conducted on an Amino Bowman
Series-2 spectrometer equipped with a 325 nm He–Cd
laser. The Raman spectra were acquired on a Renishaw
micro-Raman system 2000 at room temperature. The field
emission measurements were carried out using a parallelplate diode configuration in a test chamber maintained at
15 × 10−6 Torr. The aligned ZnO nanowire arrays on the
Si substrate were directly used as the FE cathode. Another
plate-shaped stainless steel electrode was used as an anode
with a sample-anode distance of 200 m. The high voltage
was supplied by a power source (Keithley 248), and the
emission currents at different applied voltages were measured using a Keithley 6514 electrometer with an accuracy
of 10–11 A.
3. RESULTS AND DISCUSSION
Figures 1(a) and (b) which show the SEM images of
the product synthesized at 750 and 850 C, respectively
indicate large amounts of well-aligned nanorods with a uniform length grown on the entire Si substrate. The hexagonal
facet can be obviously observed from the large magnification images in the inset. The nanorods have lengths of several micrometers. Their diameters increase from 200–300
to 300–400 nm when the reaction temperature is raised
from 750 to 850 C and the nanorods coalesce at their tips
at a high reaction temperature. A typical EDS spectrum
is shown in Figure 1(c). Only Zn and O signals can be
observed, suggesting that the nanorods are ZnO. In contrast, if the Cu–Zn powders are replaced by pure Zn while
keeping other experimental parameters the same, nanorods
cannot be found, implying that the Cu–Zn alloy is crucial to the growth of uniform and aligned ZnO nanorod
arrays on the Si substrate. The detailed growth mechanism will be discussed later in this section. Because the
nanorods produced at 750 C have smaller diameters, the
nanorod arrays synthesized at this temperature are selected
for further investigation.
Figure 2(a) depicts the typical XRD patterns of the ZnO
nanorod arrays. All the strong peaks in these patterns can
be readily indexed to hexagonal wurtzite ZnO with cell
constants comparable to the reported data (JCPDS Card
No. 75-1526). No copper and copper oxide peaks can be
observed. In the XRD patterns, the ZnO (002) peak is
the dominant one and its intensity is much higher than
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to design a simple and effective method to control the
vapor generation. Very recently, a convenient method has
been developed by our group to achieve direct and largearea growth of 1-D ZnO nanostructures on a conductive
brass substrate. This process consists of thermal oxidation
of a Cu066 Zn034 alloy foil in the presence of oxygen.11
Various 1-D nanostructures such as nanowires, nanobelts,
nanocombs, and nanosheets have been in situ grown on
the Cu–Zn alloy substrate under different reaction temperatures. Since the Zn species in the ZnO product comes
from the brass substrate, the reaction temperature has an
important influence on the segregation and diffusion rates
of the Zn atoms as well as the Zn vapor pressure. Thus,
this provides a simple way to control the release of Zn
vapor and adjust the morphology of the final products by
varying the reaction temperature.
In this work, we demonstrate that well-aligned ZnO
nanorod arrays with uniform diameters and lengths can be
fabricated on a Si substrate by a simple thermal evaporation of Cu–Zn alloy powders (30 atom% Zn) in Ar/O2 . The
synthesis of ZnO nanorods presented here is not the same
as conventional thermal evaporation and oxidation of Zn
powders. Addition of Cu increases the boiling point of the
Cu–Zn alloy and consequently, the Zn vapor is released
from the Cu–Zn alloy slowly and continuously at the reaction temperature.23–24 In addition, the CuO coming from
the oxidation of Cu on the surface can be reduced to pure
Cu by the Zn vapor before re-oxidation, thereby absorbing
a portion of the Zn vapor and mitigating the release of
Zn vapor. The added Cu effectively controls the Zn vapor
instead of a catalyst finally yielding uniform and aligned
ZnO nanorod arrays on a large-area Si substrate. These
well-aligned ZnO nanorod arrays possess good and stable field emission properties with a low turn-on field of
6.1 V/m. Raman and photoluminescence measurements
reveal that the nanorod arrays have good crystallinity and
optical properties.
Growth of Well-Aligned ZnO Nanorod Arrays on Si Substrates by Thermal Evaporation of Cu–Zn Alloy Powders
Fu et al.
(a)
(a)
400 nm
1 µm
(b)
(b)
400 nm
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1 µm
(c)
Fig. 2. (a) XRD patterns and (b) XPS spectrum of ZnO nanorod arrays
produced at 750 C.
properties of the ZnO nanorods and the results are displayed in Figures 3 and 4, respectively. It is well known
that the wurtzite structure of ZnO belongs to the C46v
(P 63 mc space group. The primitive cell includes two units
Fig. 1. SEM images of the products synthesized at (a) 750 C and
(b) 850 C; (c) Typical EDS spectrum of the product. The insets in (a)
and (b) are the corresponding large magnification SEM images.
those of the other peaks, implying that the as-prepared
ZnO nanorods grow preferentially along the (001) direction. The XPS results in Figure 2(b) suggest that the products are composed of Zn and O with the atomic ratio close
to 1:1, further corroborating the high purity of the ZnO
nanorods.
Room temperature micro-Raman scattering and photoluminescence (PL) are employed to evaluate the optical
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Fig. 3. Raman spectrum of ZnO nanorod arrays produced at 750 C.
J. Nanosci. Nanotechnol. 10, 4786–4791, 2010
Fu et al.
Fig. 4.
Growth of Well-Aligned ZnO Nanorod Arrays on Si Substrates by Thermal Evaporation of Cu–Zn Alloy Powders
PL spectrum of ZnO nanorod arrays produced at 750 C.
J. Nanosci. Nanotechnol. 10, 4786–4791, 2010
Fig. 5. Field emission current density (J as a function of applied electric field (E measured from the nanorod arrays together with the corresponding F-N plot (inset).
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with all atoms occupying the 2b sites of the symmetry
group C3v .1 25 Group theories predict that the Raman active
modes of ZnO are A1 + E1 + 2E2 , in which the A1 and E1
modes are polar and split into two transverse optical (TO)
and longitudinal optical (LO) components.1 25 The peaks
at 381, 438, and 582 cm−1 in the Raman spectrum (Fig. 3)
can be assigned to the A1 (TO), E2 (high) and E1 (LO)
modes of wurtzite hexagonal ZnO, respectively.1 Second
order Raman scattering is observed at 332 cm−1 .26–27 The
Si Raman signals are also observed at 520 cm−1 .8 The
intense, sharp, and dominant E2 mode at 439 cm−1 indicates that the ZnO nanorods are highly crystalline with
a wurtzite hexagonal phase, in agreement with the XRD
results. The PL spectrum in Figure 4 discloses intense
near-band-edge ultraviolet (UV) emission at 378 nm which
matches the band gap of bulk ZnO. It is commonly
attributed to the radioactive recombination of free exciton via the exciton–exciton collision process.8 No visible
green band emission is observed, indicating that the ZnO
nanorods are highly crystalline with few oxygen vacancies.
These results indicate that well-aligned ZnO nanorod
arrays have been successfully produced on the Si substrate
by simple thermal evaporation of Cu–Zn alloy powders.
At high temperatures of 700–900 C, Cu07 Zn03 alloys and
Cu are in the solid state. Solid Cu has negligibly low vapor
pressure, whereas Zn is in the liquid state and its vapor
pressure which is very high increases rapidly with temperature. When the Cu–Zn alloy is heated, the Zn species segregate, diffuse towards the surface of the Cu–Zn alloy, and
form Zn vapor. Upon exposure to oxygen, the Zn vapor
reacts with O2 to form ZnO species which subsequently
deposit on the Si substrate to form a thin ZnO nanoparticle
film. As the reaction proceeds with continuous supply of
Zn and O species, aligned ZnO nanorods gradually grow
epitaxially on the ZnO seeds. This is similar to the synthesis of 1D ZnO nanowires or nanorods using ZnO seeds or
buffer layers as reported in the literature.28–29 Thus, despite
the large lattice mismatch and the difference in the crystal structure between ZnO and Si, aligned ZnO nanowire
arrays can be fabricated directly on the Si substrate by
heating and oxidation of Cu–Zn alloy powders without
using a template or catalyst because the ZnO nanoparticle seed layers is formed in situ on the Si substrate when
the Cu–Zn powders are heated. When most of the Zn
species in the Cu–Zn alloy are consumed, growth of the
ZnO nanorods ceases. As a result, well-aligned 1D ZnO
nanorod arrays are produced on the Si substrate. Because
the Zn species in the ZnO comes from the Cu–Zn alloy
only, the reaction temperature has a large influence on
the segregation and diffusion rate of the Zn atoms in the
Cu–Zn alloy as well as the Zn vapor pressure. The higher
the reaction temperature, the faster the Zn atoms segregate and diffuse, and the higher is the Zn vapor pressure.
Therefore, the ZnO nanorods produced at higher temperatures have larger diameters due to the faster growth rates,
as suggested by Figure 1.
In recent years, the electron field emission (FE) properties of 1D vertically aligned nanostructures have attracted
a great deal of attention because of their high electron
emission efficiency.30 It is well known that the FE properties depend largely on the work function ( of the
emitter materials and emitter geometry which determines
the field enhancement factor ().30 Research on FE in the
past ten years has mainly focused on carbon nanotubes
(CNTs) because of their high aspect ratio, small curvature
radius, i.e., high , as well as high mechanical stability
and conductivity.31–33 Similar to CNTs, ZnO nanowires or
nanorods have small curvature radii, high aspect ratios,
and high . Furthermore, 1D ZnO nanostructures have better thermal stability and oxidation resistance compared to
CNTs,34 enabling the materials to tolerate a higher oxygen
partial pressure and poorer vacuum in FE applications.35
Therefore, 1D ZnO nanostructures are promising cathodic
Growth of Well-Aligned ZnO Nanorod Arrays on Si Substrates by Thermal Evaporation of Cu–Zn Alloy Powders
materials in FE devices. Here, we investigate the FE properties of the aligned ZnO nanorod arrays produced at
750 C. The experimental procedures to measure the FE
properties can be found elsewhere.11 Figure 5 shows the
measured field emission current density (J as a function of the applied electric field (E measured from the
quasi-aligned ZnO nanorods at a sample to cathode distance of 200 m. J is calculated by dividing the measured emission current by the area of the ZnO nanorods
assuming homogeneous electron emission from the sample. It is found that the turn-on field (Eto , which is usually
defined as the electric field that produces a current density
of 10 A/cm2 , is 6.1 V/m. This value is lower than or
comparable to the Eto reported for 1D ZnO nanostructures
in the literature.8 34 36–37
The field emission current–voltage characteristics are
further analyzed by using the Fowler-Nordheim (F-N)
equation:11
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J = A2 E 2 / exp−B3/2 /E
where J is the current density, E is the applied field,
is the work function of the emitting materials, is
field enhancement factor, and A and B are constants
with values of 156 × 10−10 (A eV V−2 and 6.83 ×
103 (V eV−3/2 m−1 , respectively. The F-N plots of the
ln(J /E 2 versus 1/E are shown in the inset of Figure 5(a).
The good linearity within the measurement range suggests
that electron emission from the ZnO nanorods exhibits
the F-N behavior. Taking the work function of ZnO as
5.3 eV, the value of the nanorods is estimated to be about
2183. The good FE properties observed from the ZnO
nanorods are believed to stem from the sharp tips, high
aspect ratio, good crystallinity, as well as quasi-aligned
configuration.
4. CONCLUSION
Vertically aligned ZnO nanorod arrays have been synthesized on silicon substrates by simple thermal evaporation
of Cu–Zn alloy powders in Ar/O2 without any catalysts or
predeposited buffer layers. The diameters of the nanorods
are in the range of 200 to 400 nm and their lengths
are up to micrometers. The ZnO nanorod arrays possess
good electron emission properties. The turn-on field is
about 6.1 V/m and field enhancement factor is 2183.
Raman and photoluminescence spectra suggest that the
ZnO nanorod arrays with high crystallinity and few oxygen vacancies have an excellent optical quality. The good
field emission and optical properties bode well for their
use in light- and electron-emitting nanodevices.
Acknowledgments: This work was financially supported by Key Project of Chinese Ministry of Education
(No. 208087), Hubei Province Natural Science Foundation (No. ZRY0087), Key grant Project and midlife and
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Fu et al.
youth talent Project of Educational Commission of Hubei
Province (Z200711001 and Q20081105), and City University of Hong Kong Strategic Research Grant (SRG) No.
7002138.
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