Field emission from well-aligned zinc oxide nanowires grown at low temperature

Article abstract of DOI:10.1063/1.1518810

Field electron emission from vertically well-aligned zinc oxide (ZnO) nanowires, which were grown by the vapor deposition method at a low temperature of 550°C, was investigated. The high-purity ZnO nanowires showed a single crystalline wurtzite structure. The turn-on voltage for the ZnO nanowires was found to be about 6.0 V/μm at current density of 0.1μA/cm². The emission current density from the ZnO nanowires reached 1mA/cm² at a bias field of 11.0 V/μm, which could give sufficient brightness as a field emitter in a flat panel display. Therefore, the well-aligned ZnO nanowires grown at such low temperature can promise the application of a glass-sealed flat panel display in a near future.

Full text of DOI:10.1063/1.1518810

Field emission from well-aligned zinc oxide nanowires grown at low
temperature
C. J. Lee, T. J. Lee, S. C. Lyu, Y. Zhang, H. Ruh et al.
Citation: Appl. Phys. Lett. 81, 3648 (2002); doi: 10.1063/1.1518810
View online: http://dx.doi.org/10.1063/1.1518810
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 19
4 NOVEMBER 2002
Field emission from well-aligned zinc oxide nanowires grown
at low temperature
C. J. Lee,a) T. J. Lee, S. C. Lyu, and Y. Zhang
Department of Nanotechnology, Hanyang University, Seoul 133-791, Korea
H. Ruh and H. J. Lee
Microstructure Analysis Laboratory, Korea Research Institute of Standards and Science,
Daejon 305-600, Korea
͑Received 23 April 2002; accepted 13 September 2002͒
Field electron emission from vertically well-aligned zinc oxide ͑ZnO͒ nanowires, which were grown
by the vapor deposition method at a low temperature of 550 °C, was investigated. The high-purity
ZnO nanowires showed a single crystalline wurtzite structure. The turn-on voltage for the ZnO
nanowires was found to be about 6.0 V/␮m at current density of 0.1 ␮A/cm2. The emission current
density from the ZnO nanowires reached 1 mA/cm2 at a bias field of 11.0 V/␮m, which could give
sufficient brightness as a field emitter in a flat panel display. Therefore, the well-aligned ZnO
nanowires grown at such low temperature can promise the application of a glass-sealed flat panel
display in a near future. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1518810͔
One-dimensional wide band-gap nanowires have at-
tracted much interest because of their remarkable physical
and chemical properties.1,2 Among those wide band-gap
nanowires, the zinc oxide ͑ZnO͒ nanowire, which has a wide
band-gap of 3.37 eV and a large exciton binding energy of
60 meV presents substantial interest for practical applica-
tions such as ultraviolet/blue emission devices, piezoelectric
devices, acousto-optical devices, and chemical sensors.3 The
ZnO nanowire has been synthesized by various techniques
using arc discharge, laser vaporization, pyrolysis, elec-
trodeposition, physical vapor deposition, and chemical vapor
deposition.4–7 Recently, electronic and optical devices fabri-
cated by various semiconductor nanowires were reported.8–10
Moreover, ultraviolet lasing action was observed in well-
aligned ZnO nanowire arrays synthesized by the carbon ther-
mal reduction vapor transport method.3 However, there were
rare reports on field emission properties for wide band-gap
semiconductor nanowires.11,12 Carbon nanotubes ͑CNTs͒
have been proved as promising material for a field emitter
due to their high mechanical stability, high aspect ratio, and
high conductivity.13–16 In addition, some wide band-gap
semiconductors, such as gallium nitride ͑GaN͒, aluminum
nitride, and ZnO nanowires were also investigated as elec-
tron field emitters because they have negative electron affin-
ity, high mechanical strength, and chemical stability. The
field emission from hexagonal GaN pyramids or surface
plasma-etched GaN films was studied by some research
groups.17,18 However, the field emission from semiconductor
nanowires is still not sufficiently studied. Therefore, it was
necessary to evaluate the field emission properties for semi-
conductor nanowires systematically.
emission from as-grown ZnO nanowires reveals high emis-
sion current density of ϳ1 mA/cm2 at an applied field of less
than 11 V/␮m. It is sufficient for the application of flat panel
displays.
Our synthesis was based on nanosized cobalt ͑Co͒ cata-
lyst particles dispersed on an n-type Si substrate. In order to
form nanosized Co particles, we employed didodecyldim-
ethylammonium bromide ͑DDAB͒ and sodium borohydride
(NaBH4) as a cationic surfactant and a reducing agent,
respectively.19 In a typical process, the DDAB was dissolved
in toluene with concentration of 10 wt %, and then dissolved
cobalt chloride (CoCl2•6H2O) to the DDAB/toluene solu-
tion ͑concentration of Co2ϩ:0.005 M). With strong stirring,
the NaBH4 aqueous solution ͑5 M͒ was added to the toluene
solution to a molar ratio of BH / Co2ϩ ϭ3:1. The aver-
͓
͔ ͓
͔
4
age diameter of Co nanoparticles was about 6–8 nm. The
nanometer-sized Co particles in toluene were dispersed onto
the Si substrate of 10ϫ10 mm2 in size and then dried at
room temperature. Before the ZnO nanowire growth, the Co
nanoparticles were sulfurized in H2S ambient ͑being diluted
by nitrogen gas͒ at 400 °C for 60 min. The Co nanoparticles
deposited on the Si substrate were put with the face down
direction on a quartz boat filled with a metal zinc powder
͑100 mesh, 99.998%, SIGAMA-ALDRICH͒. The quartz boat
was then loaded in a horizontal quartz tube with an inner
diameter of 20 mm. After loading the quartz boat, the quartz
tube was heated up to 550 °C under a constant flow of argon
͑flow rate: 500 sccm, heating zone: 200 mm͒. The reaction
time was 60 min, and a vertical distance between the zinc
source and the catalyzed Si substrate was about 3–5 mm.
After the reaction, a white wax-like material was deposited
on the surface of Si substrate. The as-deposited product was
characterized by scanning electron microscopy ͑SEM͒ ͓Hita-
chi S-4700͔, transmission electron microscopy ͑TEM͒ ͓Hita-
chi H-9000 NAR͔, energy-dispersive x-ray spectroscopy
͑EDX͒ ͓Hitachi S-4700͔, and x-ray diffraction ͑XRD͒
͓Rigaku DMAX PSPC MDG 2000͔.
In this letter, we report field electron emission properties
from well-aligned single crystalline ZnO nanowires grown
on silicon ͑Si͒ substrate using a metal vapor deposition
method at a low growth temperature of 550 °C. The electron
a͒
Author to whom correspondence should be addressed; electronic mail:
cjlee@hanyang.ac.kr
Figure 1 shows SEM images of the nanowires synthe-
0003-6951/2002/81(19)/3648/3/$19.00
3648
© 2002 American Institute of Physics
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Appl. Phys. Lett., Vol. 81, No. 19, 4 November 2002
Lee et al.
3649
FIG. 1. SEM image of well-aligned nanowires grown on n-type silicon
substrate ͑a͒ and a magnified SEM image of well-aligned nanowires ͑b͒.
sized at 550 °C for 60 min. Vertically aligned nanowires with
a high density were grown on the Si substrate, as shown in
Fig. 1͑a͒. A magnified top view of nanowire array reveals
that a high-purity nanowire has a sharp tip, as shown in Fig.
1͑b͒. The synthesized ZnO nanowire indicates the average
length of 13 ␮m and the typical diameter of 50 nm, revealing
that the average aspect ratio is estimated to be larger than
250. In our experiment, we have also successfully synthe-
sized high-crystalline ZnO nanowires at higher growth tem-
perature (600–750 °C). Higher growth temperature leads to
the higher crystalline ZnO nanowire that possesses higher
growth rate and larger diameter.
XRD peaks demonstrate that the produced nanowires
show a high-quality wurtzite ZnO structure, as shown in Fig.
2͑a͒. EDX analysis also indicates that the nanowires consist
of the ZnO composition clearly, as shown in Fig. 2͑b͒.
The crystallinity of ZnO nanowire was also evidenced
by high-resolution TEM and electron diffraction character-
ization, as shown in Fig. 3. The growth of ZnO nanowire
follows c-axial direction (cϭ5.13 Å). The inset shows a
low-dimensional TEM image of a ZnO nanowire tip that has
sharp morphology. It results in the advantage of acquiring a
large field enhancement in field emission process.
FIG. 3. High-resolution TEM image of a single crystalline ZnO nanowire
that has a crystal face direction of ͓2110͔. Inset shows that the ZnO nano-
wire has a sharp morphology.
factor, ␾ is the work function, Eϭ(V/d) is the applied field,
d is a distance between the anode and the cathode, and V is
the applied voltage.20
Figure 4 illustrates the emission current density from
well-aligned ZnO nanowire array grown on the Si substrate
as shown in Fig. 1͑a͒. The bias voltage sweeps were con-
ducted several times and the plot shows the initiatory four
sweeps. After measuring four times, the current was well
stabilized due to the field annealing effect. After a few
sweeps, the structure of ZnO nanowire tip seemed to reach a
stable configuration and the current intensity was kept con-
stant for each given applied field. The turn-on voltage was
about 6.0 V/␮m at current density of 0.1 ␮A/cm2. The emis-
sion current density reached about 1 mA/cm2 at an applied
field of about 11.0 V/␮m ͑so-called threshold field͒. This
threshold voltage was higher than that of CNTs. Neverthe-
less, the emission current value from ZnO nanowires could
produce sufficient brightness for flat panel displays.21 The
FN plot usually presented in the literature is also shown in
the inset of Fig. 4. It exhibits a linear behavior in a measure-
ment range, and thus the emission is indeed caused by a
vacuum tunneling. The field enhancement factor ␤ can be
calculated from the slope of the FN plot if the work function
of the emitter is known. The measurement value for the work
function of ZnO nanowire was about 5.3 eV.22 From the inset
The n-type Si substrate was used as a cathode-
conducting layer. Field emission experiments were prepared
in a vacuum chamber with a 2ϫ10Ϫ7 Torr at room tempera-
ture. The distance between an indium tin oxide anode and a
tip of ZnO nanowire was 230 ␮m. The measured emission
area was 28 mm2. Emission current was monitored with a
Keithley 6517A and recorded at intervals of 0.5 s. The emis-
sion current-voltage characteristics were analyzed by using
the Fowler–Nordheim ͑FN͒ equation for the field emission:
JϭA ␤2V2/␾d2͒exp ϪB␾3/2d/␤V͒,
͑
͑
where J is the current density, Aϭ1.56ϫ10Ϫ10 (AVϪ2 eV),
Bϭ6.83ϫ109 (VeVϪ3/2 VmϪ1), ␤ is a field enhancement
FIG. 4. Emission current density from ZnO nanowires grown on silicon
substrate at 550 °C. The inset reveals that the field emission follows FN
behavior.
FIG. 2. XRD diffraction ͑a͒ and EDX spectra ͑b͒ of ZnO nanowires on the
substrate.
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3650
Appl. Phys. Lett., Vol. 81, No. 19, 4 November 2002
Lee et al.
of Fig. 4, the calculated ␤ value was about 847, which is
enough for various applications of field emission, even
though it was much lower than that of CNTs. The average ␤
of ZnO nanowire is related to the geometry, structure, and
density of nanowires grown on the substrate. The ZnO
nanowires grown at higher temperatures exhibit a higher
crystalline structure and lower density, resulting in a high
field enhancement factor. We could find a higher emission
current density from ZnO nanowires grown at higher growth
temperatures.
In summary, vertically well-aligned ZnO nanowires,
which have a single crystalline wurtzite structure, were
grown at a low temperature of 550 °C. The turn-on voltage
for ZnO nanowires was about 6.0 V/␮m and the emission
current density was about 1 mA/cm2 at an applied field of
about 11.0 V/␮m. This is a sufficient emission current as a
field emitter in flat panel displays, even though the threshold
voltage for field emission is still higher than that of CNTs. It
is suggested that well-aligned ZnO nanowires grown at a low
temperature can promise the applicaton of glass-sealed field
emission displays.
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