Field emission from in situ-grown vertically aligned SnO2 nanowire arrays.

Abstract

Vertically aligned SnO2 nanowire arrays have been in situ fabricated on a silicon substrate via thermal evaporation method in the presence of a Pt catalyst. The field emission properties of the SnO2 nanowire arrays have been investigated. Low turn-on fields of 1.6 to 2.8 V/μm were obtained at anode-cathode separations of 100 to 200 μm. The current density fluctuation was lower than 5% during a 120-min stability test measured at a fixed applied electric field of 5 V/μm. The favorable field-emission performance indicates that the fabricated SnO2 nanowire arrays are promising candidates as field emitters.

Full text

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ORIGINAL PAPEROpen Access
Field emission from in situ-grown vertically
aligned SnO2nanowire arrays
Zhihua Zhou, Jiang Wu, Handong Li and Zhiming Wang*
Abstract
Vertically aligned SnO2nanowire arrays have been in situ fabricated on a silicon substrate via thermal evaporation
method in the presence of a Pt catalyst. The field emission properties of the SnO2nanowire arrays have been
investigated. Low turn-on fields of 1.6 to 2.8 V/μm were obtained at anode-cathode separations of 100 to 200 μm.
The current density fluctuation was lower than 5% during a 120-min stability test measured at a fixed applied
electric field of 5 V/μm. The favorable field-emission performance indicates that the fabricated SnO2nanowire
arrays are promising candidates as field emitters.
Introduction
SnO2is a wide bandgap semiconductor (Eg= 3.6 eV,
300 K) which has been widely applied in gas sensors,
solar cells, lithium-ion batteries, and nanoelectronic
devices [1-4] due to its outstanding optical and electrical
properties [5]. Nanoscaled SnO2presenting peculiar
properties superior to its bulk counterpart because of
the quantum effects has attracted much interest in
recent years. Various methods have been developed to
fabricate a SnO2nanostructure, including thermal eva-
poration of the metal tin (Sn) [6], sonochemical method
[7], carbothermal reduction [8], hydrothermal method
[9], electrodeposition method [10], and so on. The field-
emission [FE] properties of SnO2nanobelts, nano-
flowers, [11] and nanowires [12] were also studied
considering the potential applications of FE flat displays,
X-ray sources, and microwave devices. It was found that
the nanowires and long nanobelts of SnO2exhibited
outstanding FE properties [13-15]. In general, vertically
aligned nanowire arrays are the best candidates for FE
sources because the efficiency of the field emitters is
based on the extremely small radii of the tips, and the
diameter of the nanowire, the precise position, and the
alignment can be well controlled [16-18]. Therefore, it is
necessary to develop a method to fabricate well-aligned
SnO2 nanowire arrays to further improve the FE
performance.
In this work, an in situ catalytic thermal evaporation
method was developed to fabricate vertically aligned
SnO2nanowire arrays on a silicon substrate. The FE
characteristics of the SnO2nanowire arrays were studied.
The in situ-grown SnO2nanowire arrays, benefiting from
the well-aligned structure and the in situ-grown fabrica-
tion method, demonstrated favorable low turn-on electric
fields and a relatively stable emission behavior.
Experimental details
For preparing the SnO2nanowire arrays, 2 g of tin pow-
der (Sinopharm Chemical Reagent Co., Ltd., Shanghai,
China) was put in a ceramic boat. A silicon (100) sub-
strate sputtered with a 5-nm-thick Pt film was placed on
the top of the ceramic boat. The distance between the tin
powder and the substrate was about 0.5 cm. The ceramic
boat was placed in the middle of an electric resistance
tube furnace. The electric resistance tube furnace was
purged with a continuous 100-sccm high-purity nitrogen
gas for 15 min beforehand. Then, it was heated up to
850°C at a rate of approximately 30°C/min and kept at
850°C for 10 min. Lastly, it was cooled down to room
temperature naturally, and a white layer of product was
found on the silicon substrate.
The surface morphology and crystal structure of the in
situ-grown SnO2nanowire arrays were investigated by
scanning electron microscopy [SEM] (JEOL JEM-6320F,
JEOL Ltd., Akishima, Tokyo, Japan), high-resolution
transmission electron microscopy [HRTEM] (JEOL
JEM-2100, JEOL Ltd., Akishima, Tokyo, Japan), and
X-ray diffraction [XRD] (Bruker-AXS D8, Bruker Optik
* Correspondence: zhmwang@gmail.com
State Key Laboratory of Electronic Thin Film and Integrated Devices, School
of Microelectronics and Solid-State Electronics, University of Electronic
Science and Technology of China, Chengdu 610054, China
Zhou et al. Nanoscale Research Letters 2012, 7:117
http://www.nanoscalereslett.com/content/7/1/117
© 2012 Zhou et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution
License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.

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Gmbh, Ettlingen, Germany). The optical properties of
the in situ-grown nanowire arrays were studied by
Raman spectroscopy (French Labrum-HR cofocal laser
micro-Raman spectrometer (Dilor S.A, Villeneuve
d’Ascq, France) using an argon-ion laser at 514.5 nm).
The FE measurements were performed in a vacuum
chamber at a pressure of 3 × 10-5Pa at room tempera-
ture. The silicon substrate with the in situ-grown SnO2
nanowire arrays served as the cathode, and a fluorine-
tin-oxide coated glass served as the anode. The cathode
and anode were separated with mica spacers. The
applied electric field (E) was determined by dividing the
applied voltage (V) by the anode-cathode separation (d).
The emission density (J) was evaluated from the quoti-
ent of the obtained emission current divided by the
cathode surface area (0.25 cm2).
Results and discussion
Figure 1a shows a top view SEM image of the in situ-
grown SnO2nanowire arrays. It can be seen that the
nanowire was vertically aligned on the silicon substrate
with diameters between 40 to 60 nm. Noticeably, on the
top of each nanowire, there was a globule, which
strongly indicates that the nanowire grew via a vapor-
liquid-solid mechanism [19]. The density of the SnO2
nanowire arrays, determined by counting nanowires in a
representative area of a SEM image, was estimated to be
3 × 107/mm2. Figure 1b presents a high-resolution TEM
image of a single SnO2nanowire. It shows that the
SnO2nanowire is a single crystalline nanowire with an
interplanar spacing of 0.34 nm corresponding to the
(110) plane of a rutile crystalline SnO2.
XRD characterization was employed to investigate the
crystal structure of the in situ-grown SnO2nanowire
arrays. Figure 2a shows a typical XRD pattern. The dif-
fraction peaks can be well indexed to the standard
values of bulk SnO2(JCPDS card: 41-1445), Si (JCPDS
card: 27-1402), and Pt (JCPDS card: 88-2343). The
peaks attributed to SnO2demonstrate that the in situ-
grown sample crystallized with the tetragonal rutile
structure with lattice constants of a = 4.738 Å and c =
3.187 Å.
A typical room temperature Raman spectrum of the in
situ-grown SnO2nanowire arrays is shown in Figure 2b.
It can be seen that there were three fundamental Raman
peaks located at 475, 633, and 774 cm-1, which corre-
spond to Eg, A1g, and B2gvibration modes, respectively.
The results are in good agreement with those of the
rutile single crystal SnO2nanowire reported previously
[20]. Besides the fundamental Raman peaks, the other
two Raman peaks located at about 320 and 695 cm-1
were also observed, which correspond to IR-active
Eu3TO and A2uLO (TO is the mode of transverse optical
phonons; LO is the mode of longitudinal optical
phonons) modes, respectively [21]. The strong and
sharp peak located at about 522 cm-1corresponds to
the characteristic peak of the silicon substrate [22,23]. It
is believed that the broadening of the peaks in the
Raman scattering results is attributed to the quantum
confinement effect of the sample [24].
To investigate FE properties of the in situ-grown SnO2
nanowire arrays, FE measurements were performed at
various anode-cathode separations. Figure 3 presents the
FE current density (J) of the in situ-grown SnO2nano-
wire arrays as a function of the applied electric field (E)
measured at anode-cathode separations of 100, 150, and
200 μm. The turn-on field is defined as the applied elec-
tric field which produces a current density distinguished
from the background noise (here, defined as 0.01 mA
cm-2) [25]. It can be seen from the figure that the turn-
on fields were dependent on the anode-cathode distance:
their value decreased as the anode-cathode distance
increased, and the turn-on fields were measured to be
2.8, 2.0, and 1.6 V μm-1, respectively. These values are
lower than those reported by He et al. [26] (5.8 V μm-1)
and Wang et al. [27] (3.77 V μm-1) for the SnO2nano-
wire. The lower turn-on fields may be attributed to the
good alignment of the SnO2nanowire. Additionally, we
believe that the in situ fabrication method, which made
good electrical contacts between the SnO2nanowire and
the silicon substrate, contributed greatly to the lower
turn-on fields. Moreover, the emitter radius of the SnO2
nanowire among the arrays was approximately 50 nm,
which is small enough to make the FE performance
excellent [28].
In order to understand the emission characteristics, FE
properties were also analyzed by applying the classic
Fowler-Nordheim [FN] law using the following equation
[29]:
J =aE2
loc
ϕ
exp(−bϕ3/2
Eloc
),
(1)
where J is the FE current density, F is the barrier
height of the emission tip surface, and Elocis the local
microscopic electric field at the emission sites. The a
and b in the equation are constants with value of 1.54 ×
10-10(A V-2eV) and 6.83 × 109(V eV-3/2μm-1), respec-
tively. Eloc, which could be up to a hundred or thousand
times of the macroscopic electric field between the cath-
ode and anode, can be calculated using the following
equation:
Eloc= βV
d,
(2)
where b is the field enhancement factor, V is the
applied voltage, and d is the anode-cathode separation.
The value of b, which is related to the spatial distribution
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of emitting centers, the crystal structure, and the geome-
try morphology of emitters, reflects the ability of the
emitters to enhance the applied local electric field around
the probe compared to the macroscopic electric field.
The FN emission behavior can be evaluated from the lin-
earity of the curves plotting ln(J/E2) versus 1/E. Figure 4
shows the corresponding FN plots. It can be seen that,
besides the noise districts, the three plots go near to a
Figure 1 Morphology characterization of the prepared SnO2nanowire arrays. A (a) typical SEM image and (b) HRTEM image.
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straight line. It indicates that the field emission from the
in situ-grown SnO2nanowire arrays follows the FN rela-
tionship well, and the field emission process is a barrier
tunneling quantum mechanical process [30,31].
In general, FE characteristics depend on the work
function and field enhancement factor (b) of emitters
[32]. Both density and tip morphology influence the b
value of emitters. Ordinarily, emitters with high aspect
Figure 2 X-ray diffraction pattern (a) and Raman spectrum (b) of the prepared SnO2nanowire arrays.
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Figure 3 Field-emission current density as a function of the applied electric field. The measurements were performed at various anode-
cathode separations of 100, 150, and 200 μm.
Figure 4 Fowler-Nordheim plots of the field emission current densities of the in situ-grown SnO2nanowire arrays.
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ratios exhibit a favorable FE performance due to their
much higher b value. The value of b can be calculated
from the slope of a FN plot (kFN) according to the
following equation:
k = bϕ3/2d
β,
(3)
where ? is the work function of SnO2(4.3 eV) [33].
By analyzing the data in Figure 4, the values of b are
estimated to be 1,082, 1,378, and 1,638 as d is 100, 150,
and 200 μm, respectively. It can be seen that the values
of b are high enough for practical application as field
emitters.
The temporal FE current stability was measured over
120 min at an anode-cathode separation of 100 μm at a
fixed voltage of 500 V. The current density fluctuation
is lower than 5%, as shown in Figure 5. The stability test
results confirm that the in situ-grown SnO2nanowire
arrays are competent for being high-performance field
emitters [34].
To fabricate the vertically aligned SnO2nanowire
arrays, the thickness of a Pt catalyst is of vital impor-
tance. From many experiment results, it was found that
the suitable thickness of the Pt catalyst was about 2 to
10 nm. The average length of a single SnO2nanowire in
the arrays can be controlled by adjusting the reaction
time. It should be noted that the SnO2nanowire will
bend if their lengths exceeded ca. 100 μm due to the
force of their own gravity. Additionally, it was found
that no nanowire grew on a blank silicon substrate
(without Pt catalyst). This feature makes the in situ-
grown method meaningful because the emitter patterns
can be well controlled and designed by selective sputter-
ing of the Pt catalyst using traditional lithography mask
technology.
Conclusions
In summary, vertically aligned SnO2nanowire arrays
were deposited on a silicon substrate by an in situ-
grown method. The FE properties of the SnO2nano-
wire arrays were systematically studied. The FE mea-
surement results showed that the SnO2 nanowire
arrays had a low turn-on field of 1.6 to 2.8 V μm-1at
anode-cathode separations of 100 to 200 μm. The low
turn-on fields can be attributed to the vertically
aligned structure and the high aspect ratio of the SnO2
nanowire. Moreover, the in situ-grown method, which
makes good electrical contacts between the SnO2
nanowire and the silicon substrate, improved the FE
performance greatly.
Figure 5 Time dependence of the emission current of the in situ-grown SnO2nanowire arrays. The characterization was measured at a
fixed applied voltage (500 V) with an anode-cathode separation of 100 μm.
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Acknowledgements
This work was primarily supported by the Scientific Research Starting
Foundation for Outstanding talent, University of Electronic Science and
Technology of China.
Authors’ contributions
ZHZ conducted all the experiments and drafted the manuscript. JW
provided helpful guidance and suggestions. HDL helped in drafting the
manuscript. ZMW supervised all of the study. All authors read and approved
the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 11 December 2011 Accepted: 13 February 2012
Published: 13 February 2012
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doi:10.1186/1556-276X-7-117
Cite this article as: Zhou et al.: Field emission from in situ-grown
vertically aligned SnO2nanowire arrays. Nanoscale Research Letters 2012
7:117.
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