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Appl. Phys. Lett., Vol. 85, No. 1, 5 July 2004
Kim et al.
FIG. 2. A SEM image showing NCWs (a) with high density, and (b) low
density. (c) TEM image of NCW, (d) EDS spectrum, and (e) XRD patterns.
strate. Under an initial homogeneous magnetic field ͑H͒, ini-
tial NCs aggregation sites are formed on the ITO substrate. If
no magnetic field is present, these sites could evolve into a
homogeneous film. The random thermal motion of particles
has no significant effect on the formation of vertically
aligned NCWs in view of the typical mean-free path in the
FIG. 3. (a) The measured current density as a function of the macroscopic
electric field for H-NCWs and L-NCWs. (b) Fowler–Nordheim plots. The
inset shows the geometric enhancement factor ␥ as a function of interNCW
distance s for h=30 m and r=0.01 m.
ͱ
gas given by g=kBT/͑ 2P͒Ϸ40 m–20 m for reactor
pressure P=10–20 Torr, T=600 K and Х10−19m2. In the
absence of a magnetic field, it was verified that a homoge-
neous NC film was formed without vertical alignment of
individual NCWs. Therefore, the NCWs are the result of the
effect of the magnetic field. The initial protrusion structure of
NCWs in the presence of a magnetic field can enhance the
total magnetic field strength ͑BNCW͒ at the tip of the NCWs
as the relation BNCW=H+4Mind, where Mind is the induced
magnetization of NCWs at their tip. The induced magnetiza-
tion at the tip of a NCW will induce an inhomogeneous
magnetic field and enhance the pile-up of NCs at the NCWs
tip in the linear chain structure, resulting in the relatively
uniform diameter of NCWs [Fig. 2(a)] and even reduces the
diameter [Fig. 2(b)] as they become deposited. Density con-
trol was achieved by tuning the flow of carrier gas ͑Ar͒ from
80 to 10 sccm. Structural details of the NCW were analyzed
using high-resolution TEM (HRTEM), as shown in Fig 2(c).
The NCW arrays consisted of nanoparticles with a diameter
size of 8–12 nm. The HRTEM image of a NCW showed that
individual nanoclusters of the formed NCW were difficult to
distinguish, indicating that firm aggregation between them
occurs during their formation. Fig 2(d) shows the chemical
composition of a typical iron NCW. Fe and O atoms can be
seen in the energy dispersive spectrum (EDS) (Cu and C
peaks in the spectrum arise from the Cu-grids with carbon
film). The NCWs consist of both ␣-Fe and iron oxide phases,
as evidence by x-ray diffraction (XRD), as show in Fig. 2(e).
Figure 3(a) shows the FE characteristics measured for
two groups of NCWs with different densities. The electric
field required to obtain a current density of 1 mA/cm2 [see
the dotted line in Fig. 3(a)] is about 6.5 and 7.6 V/cm for
samples with different NCW densities, respectively. The den-
sity of NCWs has an influence on the current density of FE
in NCWs. The screen effect in FE has been demonstrated in
nanotubes and nanowires by the systematic control of their
growth density.1,12 The field emission from the NCWs suc-
cessfully follows Fowler-Nordheim (FN) behavior, showing
tional field emission of electrons can be described by FN
theory.13 The current density ͑J͒, which is related to the local
field ͑E͒ at the emitter surface, can be expressed as J
=aE2 exp͑−b3/2/E͒, where a and b are constants used to fit
the data. The E͓V/m͔ is the local field at the field emitter
surface and is generally related to the applied voltage, such
as E=␥V/d, where ␥ (geometric enhancement factor) is di-
mensionless and can be determined from the slope ͑−6.44
ϫ1093/2d/␥͒ of a FN plot. The average ␥ values of
H-NCW and L-NCW were found to be about 1200±50 and
1600±50, respectively, assuming the work function of the
iron NCW to be 4.7 eV, similar to bulk iron.14 The ratio
between H-NCW and L-NCW, ␥L-NCW/␥H-NCW was found to
be 1.33 and could reflect the screen effect related to the
interwire distance assuming the same h and r values. The ␥
factor of the diode-type field emitter can be approximated by
the formula of ␥=1.2͑h/r+2.15͒0.9ϫ͓1−exp͑−2.3172s/h͔͒
in the range of 4ഛh/rഛ3000 by taking into consideration
the effect of the interNCW distance and length of the
NCW.15–17 The h, r, and s symbols denote the length, radius
of NCW, and interNCW distance between NCWs, respec-
tively. To achieve the measured ␥ value, the interNCW dis-
tance has a value of ͑s1ജ16 m͒ for H-NCWs and ͑s2
ജ32 m͒ for L-NCWs, assuming h=30 m, r=0.01 m,
as shown in the inset of Fig. 3(b). The s2 of an L-NCW could
have the calculated values if short NCWs are not considered.
The efficient emitting H-NCWs could also have the calcu-
lated s1 values, because NCWs in the top region were less
dense than those in the bottom region. However, an exact
comparison between SEM observation and calculation is dif-
ficult due to the nonuniformity of the length, diameter, and
spacing of NCWs. The emission stability of the sample was
tested at a constant anode voltage of 1500 V, as shown in
Fig. 4. The average current density of 1.4 mA/cm2 was de-
termined and its fluctuation was about 28%. No obvious deg-
radation in current density was observed over approximately
a single linear slope in the FN plot [Fig 3(b)]. The conven-
a 6 h period. The field emission performance of the NCWs is
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