Effect of BN coatings on oxidation resistance and field emission of SiC nanowires

Article abstract of DOI:10.1063/1.1595721

The effect of BN coatings on oxidation resistance and field emission of SiC nanowires was investigated. The antioxidation ability of SiC nanowires with BN coatings improved due to the chemical stability of BN at high temperature. Results showed that the B

Full text of DOI:10.1063/1.1595721

Effect of BN coatings on oxidation resistance and field emission of SiC
nanowires
Chengchun Tang and Yoshio Bando
Citation: Appl. Phys. Lett. 83, 659 (2003); doi: 10.1063/1.1595721
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APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 4
28 JULY 2003
Effect of BN coatings on oxidation resistance and field emission
of SiC nanowires
Chengchun Tanga) and Yoshio Bando
Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba, Japan
͑Received 13 March 2003; accepted 6 June 2003͒
We compare the effects of BN coatings on antioxidation ability and field emission properties of SiC
nanowires. Under oxidizing condition, SiC nanowires without BN coatings are cracked into
nanoparticles or almost fully converted into SiO2 nanowires at the temperature above 800 K,
depending on the crystallization degree of SiC nanowires. The BN coatings effectively improve the
antioxidation ability of SiC nanowires due to the excellent chemical stability of BN at high
temperature. At a temperature higher than 1273 K, the coated SiC nanowires still exhibit strong
oxidation resistance. For the effect on electron emission, the BN coatings also reduce the turn-on
field of SiC nanowires from larger than 10 V/␮m to lower than 6 V/␮m. The explanation for the
improvement of field emission characteristics has been presented. © 2003 American Institute of
Physics. ͓DOI: 10.1063/1.1595721͔
It is well-known that refractory bulk SiC exhibits an ex-
cellent oxidation resistance at high temperature, because
silica films formed on SiC serve as protecting against further
oxidation.1–3 The mechanism of the passive SiC oxidation is
correlated with the reaction of 2SiCϩ3O2↔2SiO2ϩ2CO.
The oxidation rate depends on the diffusion of oxygen
through the formed SiO2 layer followed by the reaction of
SiC at the SiC–SiO2 interface. The formed SiO2 layer is
considerably dense above 800 °C for bulk materials, and the
oxidation basically follows a quadratic rate law.4 However,
the investigation5 of the oxidation behavior of SiC fiber re-
inforced SiC matrix indicates that one-dimensional SiC ex-
hibits strong oxidization with the rate constant of one order
of magnitude higher than that of two-dimensional SiC ma-
trix, due to the higher porosity, the larger specific surface,
and the higher fraction of unprotected SiC fibers. Therefore,
for downing to a scale of nanometer dimension, SiC nanow-
ires are even more fragile to oxidation. In fact, the extent of
the oxidation interaction between some metallic or semicon-
ducting one-dimensional nanomaterials and oxidizing condi-
tions is an important parameter in practically using them in
the field of nanoelectronics and nanomechanics. However,
few studies have been paid to the chemistry and physics
degradation to date.6,7
dicate that BN coated one-dimensional materials are a very
attractive system for reducing the applied field in cold elec-
tronic emission application.14 Therefore, the BN coated SiC
nanowires should also be valuable for studying the mecha-
nism of the promoting emission.
In this letter, we comparatively study the chemical sta-
bility and field emission of the SiC nanowires with or with-
out BN coating. It is demonstrated that BN coating is effec-
tive in improving the antioxidation ability and the field
emission characteristics of SiC nanowires.
SiC nanowires without BN coatings used in this study
were synthesized by a shape memory synthesis process pre-
viously reported,15 using a reaction between SiO gas and
carbon nanotubes. Two kinds of nanowires were chosen to
investigate their oxidations: curved polycrystalline SiC
nanowires ͓Fig. 1͑a͔͒ and highly crystallized straight SiC
nanowires ͓Fig. 1͑b͔͒. The former was synthesized at
1400 °C and kept the shape of the starting carbon nanotubes
due to the carbon nanotubes space-confined effect; the latter
was further crystallized at 1600 °C and its morphology was
dominated by oriented single crystal growth mechanism. The
The oxidation degradation can be weakened by coating
nanomaterials with some chemically stable materials.8–10
Considering that BN has unique chemical and physical prop-
erties such as low density, high melting point, and chemical
inertness, recently we have successfully coated SiC nanow-
ires with uniform BN layers.11 It is also worthy of note that
this coating should have a strong influence on the electronic
properties of SiC nanowires, for example, in the application
of field emission. It has been reported that SiC nanowires
͑nanorods͒ exhibit good field emission properties in emission
stability, but usually have a relatively high turn-on field typi-
cally higher than 10 V/␮m.12,13 Theoretical investigations in-
FIG. 1. The TEM image of the investigated nanowires: ͑a͒ curved polycrys-
talline SiC nanowires; ͑b͒ straight single-crystal SiC nanowires; ͑c͒ BN
uniformly coated SiC nanowires with a great amount of stacking faults and
microtwins, and ͑d͒ its high-resolution TEM image. The corresponding se-
lected area electron diffraction patterns from a single nanowires were shown
in ͑a͒, ͑b͒, and ͑c͒, respectively.
a͒
Electronic mail: tang.chengchun@nims.go.jp
0003-6951/2003/83(4)/659/3/$20.00
659
© 2003 American Institute of Physics
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660
Appl. Phys. Lett., Vol. 83, No. 4, 28 July 2003
C. Tang and Y. Bando
tallized SiC nanowires do not contain free carbon. From 200
to ϳ800 K, weight gradually increases which implies a slow
oxidization of SiC nanowires. From 800 to ϳ1000 K, the
apparent weight loss and exothermal process can be ob-
served, indicating that the similar volatilization as polycrys-
talline system occurs due to the lack of formation of SiO2
protective layer.
The volatilization process does not occur during the oxi-
dation of SiC nanowires with the BN coatings. From 200 to
ϳ1100 K, there is almost no weight loss, implying the oxi-
dation of BN and SiC has not yet taken place. From ϳ1100
to ϳ1210 K, a ϳ2% weight increase, accompanied with an
exothermal process, can be observed from the DTA–TG
curves. This could be explained according to the slow oxida-
tion reaction of 4BNϩ3O2↔2B2O3ϩ2N2 .18,19 Considering
that within this temperature range boron oxide tends to form
a vitreous structure with high viscosity, the volatilization of
boron oxide should be considerably slow.20 Moreover, the
formed boron oxide can dissolve in the SiC nanowires matrix
to generate a stable silicon borate layer.5 Above ϳ1400 K,
the protection of BN layer is destroyed, and the oxidation of
SiC drastically occurs.
To further determine the oxidation consequence, TEM
examinations were carried out for the three samples, which
all were oxidized in air at 1273 K for 2 h. Polycrystalline SiC
nanowires cracked into nanoparticles with the size of several
tens of nanometers ͓Fig. 2͑b͔͒. Energy dispersive spectros-
copy ͑EDS͒ measurements indicate that the nanoparticles
mainly consist of SiO2 . Single-crystal SiC nanowires kept
the starting one-dimensional morphology, however, the SiC
was almost fully converted to SiO2 although we can also
observe some fractured SiC nanoparticles that are buried
deep within the nanowires ͓Fig. 2͑d͔͒. However, BN coated
SiC nanowires exhibit an excellent antioxidation behavior
under the same oxidizing condition. Figure 2͑f͒ shows the
typical morphology after oxidation. The oxidation has no
apparent influence on the morphology and chemical structure
of the inner SiC nanowires, although the coating BN became
a dense amorphous layer. EDS analyses indicate that the BN
layer contains O and Si, implying the existence of silicon
borate product.
FIG. 2. DTA–TG curves of SiC nanowires and TEM images after oxidizing
at 1273 K for 2 h for ͑a͒ and ͑b͒ polycrystalline SiC nanowires; ͑c͒ and ͑d͒
single-crystal SiC nanowires; ͑e͒ and ͑f͒ BN coated SiC nanowires.
selected area electron diffractions ͑SAED͒ shown in the in-
sets clearly exhibit the crystalline structure. Transmission
electron microcopy ͑TEM͒ examinations indicate that the
some incompletely reacted carbon distributes disorderedly in
the polycrystalline nanowires, while no remnant carbon can
be detected from the single-crystal nanowires. Figure 1͑c͒
shows a TEM image of SiC nanowires with uniform BN
coatings, which were synthesized within the framework of a
vapor-liquid-solid growth mechanism.16 SAED indicates that
the inner SiC nanowires contains a great amount of stacking
faults and microtwins, however, the coated BN with the
thickness of 2–4 nm exhibits highly crystallized layers with
the same interlayer distance as one of BN nanotubes ͓Fig.
1͑d͔͒.
The oxidation behavior of SiC nanowires was investi-
gated by differential thermal ͑DTA͒ and thermogravimetric
͑TG͒ analyses. The measurements were conducted in air at-
mosphere using a Thermo plus DTA–TG-8120 apparatus.
The scan rate of temperature is fixed as 5 °C/min for three
samples. Figures 2͑a͒, 2͑c͒, and 2͑e͒ show the DTA and TG
curves of polycrystalline, single-crystal and BN uniformly
coated SiC nanowires, respectively.
The field emission I–V characteristics of the SiC
nanowires with and without BN coatings were measured in a
For oxidation of polycrystalline SiC nanowires, there is
weight loss of ϳ1% from 520 to ϳ640 K accompanied with
an exothermal process. This is considered to correspond to
the oxidation of the free carbon according to the reaction C
ϩO2↔CO2 .5 The sequent exothermal peak occurs from
ϳ860 to ϳ1120 K with a considerable weight loss of
ϳ20%, indicating an occurrence of constituent volatilization
in oxidizing conditions. It has been reported that,5,17 when
SiO2 layer formation is suppressed, a volatilization process
will take place according to the reaction SiC
ϩO2↔(SiO)gasϩCO. Therefore, the observed weight loss
for polycrystalline SiC nanowires indicates that the protec-
tive SiO2 layer on SiC nanowires has not yet been formed.
When the environmental temperature is higher than 1150 K,
the SiC gradually forms SiO2 with the occurrence of the
reaction 2SiCϩ3O2↔2SiO2ϩ2CO.5,18
vacuum chamber at
a
pressure of 10Ϫ8 Torr.
A
hemispherical-shaped stainless-steel probe of 0.8 mm in di-
ameter (2R) was use as the anode. The anode was positioned
by a linear-motion step controller. A voltage up to 1100 V
was applied to the probe to collect emitted electrons from the
ground cathode samples. The separation (Z) between anode
and sample holder was fixed at 80 ␮m for all experiments.
The effective emission area (A) used to evaluate the current
density and hopping field, was approximately calculated21 by
Aϭ2␲RZ(21/nϪ1), where nϭd ln I/d ln V. Selected repli-
cate experiments were conducted to determine the reproduc-
ibility of the measurements. Although the maximum acces-
sible emission currents slightly vary with the number of the
repeated measurements and with the voltage sweep rate, the
maximum error of currents was less than 5%. The field emis-
sion from the free-standing BN nanotubes without carbon or
metals contaminations, synthesized by MgO-promoted cata-
The oxidation of single-crystal SiC nanowires exhibits
the similar behavior as polycrystalline nanowires, except for
the absence of the oxidation of carbon because highly crys-
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Appl. Phys. Lett., Vol. 83, No. 4, 28 July 2003
C. Tang and Y. Bando
661
eV for single-crystal SiC nanowires, 1.5–4.7 eV for pure BN
nanotubes, and 0.5–1.0 eV for BN coated SiC nanowires.
The values mean that the effective potential height of SiC
nanowires is reduced at least to 1/3 due to the BN coatings.
It is thus conjectured that the reduced barrier heights are the
key factors that account for the reduction in turn-on field of
the BN–SiC nanowires.
We believe that the low effective potential height of
BN–SiC composite system is correlated with the electron
behavior near the interface between wide-band semiconduc-
tor BN and SiC one-dimensional structures. It has been dem-
onstrated by Sugina et al.26,27 that the deposition of BN
nanofilms on some semiconducting substrates is effective in
reducing the effective barrier height of the substrates.
In summary, BN coatings effective improve the antioxi-
dation ability and reduce the turn-on field of SiC nanowires.
The excellent chemical stability of BN is responsible for the
improvement of antioxidation. The low turn-on field is due to
a decrease of the effective potential barrier height, which is
considered to result from the existence of a defect-induced
positive space charge.
FIG. 3. Field emission plots for single-crystal SiC nanowires, BN nanotubes
and BN coated SiC nanowires. The insets show the corresponding FN plots.
lytic growth methods,22 was also measured under same con-
dition as comparison.
The average turn-on field was used as a figure of merit to
compare various samples, which is defined as the applied
electric field needed to produce a current density of
0.01 mA/cm2. Although the lowest turn-on field of about 3
V/␮m has been reported for oriented SiC nanowires synthe-
sized by reacting aligned carbon nanotubes with SiO to
date,23 the single-crystal SiC nanowires discussed here
showed a high turn-on field at 13 V/␮m. The turn-on field of
pure BN nanotubes is also larger than 14 V/␮m ͑Fig. 3͒.
However, the pattern of field emission of BN–SiC nanowires
clearly showed a low turn-on field at 6, and ϳ20 V/␮m of
threshold field providing 10 mA/cm2 of current density,
which is typically required for effectively exciting a phos-
phor pixel in flat display. The values are comparable with the
turn-on and threshold fields of carbon nanotubes, demon-
strating that BN–SiC one-dimensional composite system is a
promising emitting material for applications in flat display, if
considering its excellent oxidation-resistance properties.
In order to understand the effect of the BN coatings on
field emission of SiC nanowires, the obtained I–V curves
were further analyzed using the classical Fowler–Nordheim
͑FN͒ theory by taking into account variations of geometric
͑field enhancement factor ␤͒ and electronic ͑effective poten-
tial barrier height ␾͒ properties, although the theory usually
results in an underestimation of barrier height especially for
wide-band semiconductors.24 According to this theory, the
slope of ln(I/V2)–1/V curve is given by Ϫ6.44ϫ107␾3/2/␤.
The FN plots for the three investigated specimen were shown
in the inset of Fig. 3. The straight line relationships between
ln(I/V2) and 1/V are followed extremely well when the ap-
plied field is higher 300 V, suggesting that a quantum me-
chanical tunneling process is responsible for the electron
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