Field emission from crystalline copper sulphide nanowire arrays

Article abstract of DOI:10.1063/1.1478149

Straight crystalline copper sulphide (Cu₂S) nanowire arrays have been grown by using a simple gas-solid reaction at room temperature. These were demonstrated to exhibit semiconductor properties. Field emission was observed at a field of ～6MV/m, and its current-field characteristics deviate from Fowler-Nordheim theory, i.e., showing a nonlinear Fowler-Nordheim plot. The uniform emission from the whole arrays was observed using transparent anode technique, and their variation with applied field was recorded. The emission from individual nanowires was also studied using a field emission microscope, and was found to consist of a number of spatially resolved diffuse spots. Finally, stable emission current at different levels and over time was recorded. These findings indicate that semiconductor nanowires as cold cathode have a potential future, worthy of further comprehensive investigation. The technical importance of using semiconductor nanowires as cold cathode emitter is given.

Full text of DOI:10.1063/1.1478149

Field emission from crystalline copper sulphide nanowire arrays
Jun Chen, S. Z. Deng, N. S. Xu, Suhua Wang, Xiaogang Wen, Shihe Yang, Chunlei Yang, Jiannong Wang, and
Weikun Ge
Citation: Applied Physics Letters 80, 3620 (2002); doi: 10.1063/1.1478149
View online: http://dx.doi.org/10.1063/1.1478149
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APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 19
13 MAY 2002
Field emission from crystalline copper sulphide nanowire arrays
Jun Chen, S. Z. Deng, and N. S. Xua)
Guangdong Province Key Laboratory of Display Material and Technology, and State Key Laboratory
of Optoelectronic Materials and Technologies, Zhongshan University, Guangzhou 510275,
People’s Republic of China
Suhua Wang, Xiaogang Wen, and Shihe Yanga)
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay,
Kowloon, Hong Kong, People’s Republic of China
Chunlei Yang, Jiannong Wang, and Weikun Ge
Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong, People’s Republic of China
͑Received 21 November 2001; accepted for publication 25 March 2002͒
Straight crystalline copper sulphide (Cu2S) nanowire arrays have been grown by using a simple
gas–solid reaction at room temperature. These were demonstrated to exhibit semiconductor
properties. Field emission was observed at a field of ϳ6 MV/m, and its current-field characteristics
deviate from Fowler–Nordheim theory, i.e., showing a nonlinear Fowler–Nordheim plot. The
uniform emission from the whole arrays was observed using transparent anode technique, and their
variation with applied field was recorded. The emission from individual nanowires was also studied
using a field emission microscope, and was found to consist of a number of spatially resolved
diffuse spots. Finally, stable emission current at different levels and over time was recorded. These
findings indicate that semiconductor nanowires as cold cathode have a potential future, worthy of
further comprehensive investigation. The technical importance of using semiconductor nanowires as
cold cathode emitter is given. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1478149͔
There has been increased interest in the study of field
emission from carbon nanotubes ͑CNTs͒,1–4 in particular for
applications in field emission flat panel display.5 Some early
reports suggested that the emission depends on the electronic
property of the CNTs. We know that CNTs may have metal-
lic or semiconducting properties, depending on their struc-
ture. However, one is still not able to control these proper-
ties. Some have tried to modify the CNTs’ electronic
property with success.6,7 Here, we report a type of nanowire
array that exhibits semiconductor property, and its field emis-
sion properties. As a semiconductor, it will have the advan-
tage of a lower surface potential barrier than that of metals,
which is important to field electron emission.
place. The copper foils (0.5 cmϫ0.5 cm) were placed in the
reactor. A gas flow was directed to the reactor which consists
of a mixture of oxygen ͑99.8%͒ and hydrogen sulfide
͑99.8%, Aldrich͒ at a given molar ratio. The total pressure in
the reactor was kept at ϳ1.05–1.08 atm, and the reactor was
kept at room temperature. The reaction time was set at 10 h
based on our previous experience on the synthesis of copper
sulfide nanowires. During reaction, the reactor was kept in
the dark. When exposed to the mixture of H2S and O2 , the
copper surface became dark red immediately, and then shin-
ing cyan and gray in a short time span. After 10 h reaction,
the copper surface became black and fluffy, indicating the
formation of dense Cu2S nanowire arrays.
Our Cu2S nanowire samples can be prepared under am-
bient conditions by using a very simple procedure, which can
be easily scaled up. The details of the growth technique have
been described elsewhere,8,9 only a brief description relevant
to the present work is given below. Before use, copper foils
͑99.98%, Aldrich͒ with a thickness of 0.25 mm were care-
fully cleaned for ϳ5 min in an ultrasonic bath of absolute
ethanol. The gas–solid reaction was carried out in a home-
made reactor. The reactor is essentially a glass cylinder with
two inlets on one end of the cylinder and one outlet on the
other end. One of the inlets was used to control the flow of
the O2 /H2S gas mixture, and the other was connected to an
inert gas ͑Ar or N2͒. The outlet was used to control the
reactor pressure. The glass cylinder has a volume of 255 ml,
where gas–solid reactions and Cu2S nanowire growth took
The arrays consist of straight Cu2S nanowires ͑Fig. 1͒
having a diameter of ϳ50–70 nm. These nanowires are
crystalline, and some of them have a thin layer of oxide
coating ͑fcc Cu2O͒ at the surface ͓Fig. 1͑b͔͒. Bulk Cu2S is
known to be a semiconductor,10 and, indeed, our photolumi-
nescence spectrum of the Cu2S nanowires at low temperature
shows that it has a band gap of about 1.26 eV, which is
slightly larger than the corresponding bulk value.
The electrical conductivity of the Cu2S nanowires was
measured using a HP-4115A semiconductor parameter ana-
lyzer. Copper substrate serves as the lower electrode and Au
pattern with a diameter of about 0.3 mm is successfully fab-
ricated on the other end of the nanowire arrays by inserting
polyimide between nanowires as backbones. Figure 2 shows
the I–V curve of the Cu2S nanowires at various tempera-
tures. It clearly shows that metal-semiconductor Schottky
junction was formed at the contact points between the
nanowires and the metals. At high voltage, the Schottky
a͒
Authors to whom correspondence should be addressed; electronic mail:
stsxns@zsu.edu.cn ͑N. S. Xu͒; chsyang@ust.hk ͑Shihe Yang͒.
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Appl. Phys. Lett., Vol. 80, No. 19, 13 May 2002
Chen et al.
3621
FIG. 3. Emission uniformity of nanowire arrays and their variation with
field: ͑a͒ 10 MV/m ͑inset shows the sample͒, ͑b͒ 11 MV/m, ͑c͒ 12 MV/m,
and ͑d͒ the field emission microscopic image.
The measurement of field emission characteristics was
carried out in a vacuum chamber having vacuum better than
1ϫ10Ϫ7 Pa. A sample film was first adhered to the surface of
an oxygen free high conductivity copper electrode using sil-
ver paint ͓inset of Fig. 3͑a͔͒. The uniformity of emission
from the film was examined using the transparent anode
technique,11 in which a 0.25 mm vacuum gap was used. The
spatial distribution of emission sites and its variation with
field ͑10, 11, and 12 MV/m͒ were recorded ͓Figs. 3͑a͒, 3͑b͒,
and 3͑c͔͒. They consist of points of light generated by the
impacting electrons onto the tin oxide film of the transparent
anode; many are tiny dots with a few large light spots. With
increasing field, the emission became stronger resulting in an
increasing number of bright spots. Also, emission came from
most areas of the film but more emission sites appeared on
the area near the edge of the film.
FIG. 1. ͑a͒ Transmission electron micrograph of Cu2S nanowire arrays. ͑b͒
High resolution transmission electron micrograph of a bulk section of a
Cu2S nanowire.
junction will break through and the voltage will start to fall
on the nanowires. From the slope of the I–V curve at high
voltage, we can see that the conductivity of the nanowires
increases dramatically with the increase of temperature. This
is due to the thermal ionization of the donor or acceptor to
give out free carriers to conduction or valence band of the
nanowires to form the current. Our experiment has shown
that the Cu2S nanowires are highly conductive at high tem-
perature. The resistivity of the nanowires is estimated to
be less than 10 ⍀ cm at room temperature by taking into
consideration the size and density of the wires.
The information about emitting surface was obtained us-
ing a field emission microscope. An anode probe hole of 0.5
mm in diameter is placed in front of the emitting surface
with a vacuum gap of 250 ␮m. Electrons passing the hole are
projected to form an emission image on the phosphor screen.
The typical images consist of a number of bright diffused
FIG. 2. The direct-current I–V characteristic showing that the nanowires
FIG. 4. Field emission current applied field plot and its corresponding FN
plot ͑inset͒.
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have the semiconductor property.
137.30.242.61 On: Tue, 09 Dec 2014 02:53:02

3622
Appl. Phys. Lett., Vol. 80, No. 19, 13 May 2002
Chen et al.
for different nanowires are varied, which would result in
different turn-on fields for the individual nanowires. This
conforms to the result of emission site observation ͓Figs.
3͑a͒, 3͑b͒, and 3͑c͔͒, i.e., more emission sites are produced at
high field.
The emission stability of Cu2S nanowires was recorded
by measuring the current fluctuation with time at a fixed
voltage ͑dc mode͒. Figure 5͑a͒ shows the short-term stability.
In addition, Fig. 5͑b͒ shows the fluctuation during 16 h of
continuous operation at the current of about 100 ␮A ͑density
of about 0.4 mA/cm2͒. The current fluctuation was as low as
Ϯ2%.
In conclusion, semiconductor Cu2S nanowire arrays
have been grown based on a versatile technique at room
temperature. Stable and uniform field emission was observed
from Cu2S nanowire arrays. Nonlinearity in Fowler–
Nordheim plot was observed. These findings indicate that
semiconductor nanowires as cold cathode have a potential
future, worthy a further comprehensive investigation.
J.C., S.Z.D., and N.S.X. gratefully acknowledge the fi-
nancial support from the National Natural Science Founda-
tion of China, Education Ministry of China, the Higher Edu-
cation Bureau and the Science and Technology Department
of Guangdong Province. S.H.Y. gratefully acknowledges the
financial support from research Grant No. HKUST6190/99P
administered by the UGC of Hong Kong.
FIG. 5. Showing the current stability at different levels ͑a͒ and over time
͑b͒.
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