Investigation on the Plasma-Induced Emission Properties of Large Area Carbon Nanotube Array Cathodes with Different Morphologies
Nanoscale Res Lett
Investigation on the Plasma-Induced Emission Properties of Large Area Carbon Nanotube Array Cathodes with Different Morphologies
Qingliang Liao 0
Zi Qin 0
Zheng Zhang 0
Junjie Qi 0
Yue Zhang 0
Yunhua Huang 0
Liang Liu 1
0 State Key Laboratory for Advanced Metals and Materials, Department of Materials Physics, University of Science and Technology Beijing , 100083, Beijing , China
1 Department of Physics, Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University , 100084, Beijing , China
Large area well-aligned carbon nanotube (CNT) arrays with different morphologies were synthesized by using a chemical vapor deposition. The plasma-induced emission properties of CNT array cathodes with different morphologies were investigated. The ratio of CNT height to CNT-to-CNT distance has considerable effects on their plasma-induced emission properties. As the ratio increases, emission currents of CNT array cathodes decrease due to screening effects. Under the pulse electric field of about 6 V/μm, high-intensity electron beams of 170-180 A/ cm2 were emitted from the surface plasma. The production mechanism of the high-intensity electron beams emitted from the CNT arrays was plasma-induced emission. Moreover, the distribution of the electron beams was in situ characterized by the light emission from the surface plasma.
In the past few years, carbon nanotubes (CNTs) have
been extensively investigated due to their remarkable
structures and excellent properties [
]. They have also
been identified as potential materials for a broad range
of useful devices [
], especially in the area of field
emission devices [
]. CNT arrays always have attracted
considerable attentions as ideal electron emitters for
their excellent field emission properties [
]. Many new
field emission devices based on CNT arrays were
fabricated successfully. In the previous CNT-based devices
studies, CNT arrays mainly were applied to the weak
current devices under direct current (DC) electric fields.
It is well known that plasma-flashover cathodes can
generate intense-current electron beams under pulse
electric fields and have been used extensively in high-power
microwave tubes and accelerators [
]. As is
mentioned above, CNT arrays have great potentials for the
applications of plasma-flashover cathodes due to their
excellent field emission properties [
]. Whereas the
reports that focus on the plasma-induced emission
properties of CNT arrays under the high-voltage pulse
electric field are very few. Therefore, the studies on the
field emission properties of CNTs under the pulse
electric field are very important as well as under the DC
Here, we report the plasma-induced emission
characteristics of CNT arrays under the high-voltage pulse
electric field. The effects of the ratio of CNT height to
CNT-to-CNT distance on the electron emission
properties of the CNT arrays were investigated. Moreover, the
distribution of electron beams was in situ characterized
by light emissions from the plasmas. The production
mechanism of the electron beams emitted from the
CNT arrays was studied and explained.
Large area CNT arrays have been grown on substrates
by a chemical vapor deposition method [
], and 2-in.
silicon wafers were used as the substrates. Briefly, a
10-nm Al2O3 layer acting as barrier layer was formed
on the substrate surface by evaporation. Then, a
5-nmthick Fe catalyst layer was e-beam evaporated onto the
substrate surface. Finally, the substrates were inserted
into the center of a quartz tube furnace. The furnace
was heated to about 700°C in the mixed flow of the
acetylene and hydrogen. Uniform well-aligned CNT
arrays on the 2-in. silicon wafers can be obtained, and
the heights of the arrays can be controlled by tuning
growth conditions. The height of the as-grown CNT
arrays depends on growth time. The growth times of
different CNT arrays range from 10 to 80 min. Four
kinds of arrays with different CNT heights in the range
of 4–16 μm were employed in our experiment. The
surface morphologies of the CNT arrays were analyzed by
a field emission scanning electron microscopy (SEM).
A high-resolution transmission electron microscope
(HRTEM) was used to further characterize the
The fabricated samples were placed on copper stages
by electrically conductive glue and fixed by copper
rings. The CNT arrays were adhere onto the copper
stages and assembled into cathodes. Then, the CNT
array cathodes were used to next plasma-induced
emission tests under the pulse electric field. The high-voltage
pulse emission experiments were performed in a diode
powered by a pulse-forming network generator at
background pressure of 5×10-4 Pa [
]. The generator has
an output double-pulse with about 100-ns duration, and
the interval between two pulses was about 400 ns. The
anode–cathode gap was 98 mm. During the emission
process, the light emission from the CNT array cathode
was in situ observed by a charge-coupled device (CCD)
Results and discussions
The assembled CNT array cathode is shown in Figure 1a.
The black disk inside the exterior ring is the CNT array
film. The emission surface of the array cathode is a disk
with 50 mm in diameter. As a whole, the CNT arrays are
distribute uniformly on the silicon substrate. Figure 1b
shows the CCD image of the cathodes that are not emit,
and the middle ellipse of the image is the cathode
surface. The only different growth condition among the
four kinds of CNT arrays is the growth time, and
the CNTs of the four samples have similar diameters.
The low-resolution TEM micrograph of the CNTs is
shown in Figure 1c, which shows that the CNTs are held
together by van der Waals interactions and the CNTs
form tight bundles. The diameter of the nanotubes is
about 10 nm based on the high-resolution TEM image
(shown in the Figure 1d). The high-resolution TEM
image reveals that the CNTs are multi-walled. The
multiwalled CNTs are relatively clean, and the walls have low
Figure 2 shows the side view SEM image of four kinds
of CNT arrays grown at different growth times. The
obvious difference among the four kinds of arrays is the
CNT height. The CNT heights of four kinds of arrays
are 4, 7, 14 and 16 μm, respectively. The CNTs of the
four samples are oriented in a perpendicular fashion
and arrange very close with a high density. Besides the
heights of the four CNT arrays are different, the CNTs
of the four samples have different orientations to the
substrates. Most CNTs of the 4-μm height sample are
flexural and not perpendicular to the substrate. There
are many very long CNTs extruding from the array
surface, and the CNTs of the sample are randomly ordered.
The CNT arrays of Figure 2a have the shortest growth
time among the four kinds of arrays. During the short
growth process, the growth temperature rising and
falling rapidly and the CNTs have different growth velocity.
A lot of CNTs grow at unstable high velocity during the
short grown process, but most grown slowly and
uniformly. Therefore, there are a lot of long CNTs
appeared in the Figure 2a, and the arrays lose the
uniformity. If the growth time is long enough, the growth
of arrays would reaches a steady state and the arrays
would grow very uniform. The CNT arrays of the 7-μm
height sample arranges more orderly than that of the
4-μm height sample. A few nanotubes of the 7-μm
height sample are flexural at the root. The distributions
of the CNTs become regular with the height of the
CNT arrays increasing. The CNTs of the 14- and 16-μm
height samples are more uniform than that of the
4- and 7-μm height samples. The CNTs have uniform
diameters and heights, and they are aligned regularly
one by one. CNT arrays with four different heights in
the range of 4–16 μm have been fabricated. The
nanotubes of the four kinds of arrays have different CNT
heights and similar CNT densities. Therefore, the four
samples have different ratios of CNT height to
CNT-toCNT distance. The intertube distance is about
130~150 nm, and the ratios of CNT height to
CNT-toCNT distance of four kinds of arrays are 31, 54, 108 and
123, respectively. As the growth times increase, the
heights of the CNT arrays increase and the ratios of CNT
height to CNT-to-CNT distance increase simultaneously.
An application of the pulse electric field caused the
appearance of an intense current electron emission from
the CNT array cathodes. Figure 3 gives the emission
current waveforms and the emission stability curves
from the four kinds of array cathodes with different
CNT heights. During one double-pulse, the highest
voltages of two pulses are about 0.75 and 0.58 MV,
respectively. Under the same diode voltage, the emission
current waveforms of four kinds of array cathodes have
same characteristics. The emission currents have big
differences between the first pulse and the second pulse.
The big difference of emission currents attributes to the
formation and expansion of the surface plasma [
For the four kinds of arrays with different CNT heights
of 4, 7, 14 and 16 μm, the highest emission currents are
3474, 2115, 2056 and 1073 A, respectively. The CNT
arrays of four kinds of cathodes have different ratios of
CNT height to CNT-to-CNT distance. The difference in
the emission currents of the CNT arrays can be caused
Figure 2 The side view FESEM images of four kinds of arrays with different CNT heights: a 4 μm, b 7 μm, c 14 μm, d 16 μm.
by only the ratios of CNT height to CNT-to-CNT
distance. With increase in the ratios of CNT height to
CNT-to-CNT distance, the emission currents decrease
gradually. The 4-μm height CNT array has the highest
emission current among the four kinds of CNT arrays.
The average electric field of the second pulse is about
6 V/μm, and the corresponding highest emission current
density of 4-μm height CNT array is about 170–
180 A/cm2. The relationship between the emission
currents and the number of pulses for the four kinds of arrays
is presented in Figure 3b. Along with the continuance of
the pulse emission, the CNT arrays would lose the
emission ability gradually [
]. The results show that after
80 pulses, the emission currents of the four kinds of CNT
array have about from 11.3 to 12.9% reductions.
Figure 4 shows the CCD images of the four kinds of
CNT array cathodes that are emitting. Compared with
the CCD image of the cathodes that are not emit
(shown in Figure 1b), it can be found that the CCD
images of Figure 4 have bright light. The bright light
captured by the CCD camera was considered as the
light emission from plasmas on the cathode surface
]. The luminescent zones on the cathode surface
are emission sites. The CCD images show the spatial
distribution of the emission sites and the plasmas. The
distribution of the emission sites on the CNT arrays is
very similar to that of the coated CNT cathode [
Figure 4a is the CCD image of the 4-μm height cathode.
The luminescence on the cathode surface is very intense
and uniform. The CCD image shows that the emission
current is very intense and almost whole cathode
surface can emit electrons. The luminescence of the 7-μm
height cathode becomes weak obviously relative to the
4-μm height cathode, as shown in Figure 4b. It can
be seen that many separate emission sites distribute on
the cathode surface. The brightness and area of the
luminescence are less than these of the 4-μm height
cathode. It has been known that the 4-μm height CNT arrays
have the higher emission current. Therefore, the CCD
images can reflect the intensity of emission currents. The
CCD images of the 14- and 16-μm height cathodes are
shown in Figure 4c and Figure 4d, respectively. The
brightness and area of luminescence reduce in contrast
with that of the previous short CNT array cathodes. The
emission area of the 14-μm height cathode is larger than
that of the 16-μm height cathode. The emission current
is in direct proportion with the brightness and area of
light emission from the plasma. The CCD images
reconfirmed that the emission currents of the CNT array
cathodes decrease with the increase in the ratios of CNT
height to CNT-to-CNT distance.
Many studies were carried out on the influential factors
on the electron emission properties of CNT arrays
]. For high-density nanotube arrays, field-screening
effects of neighboring tubes reduce the field
enhancement, and thus the emission current decreases. When
the nanotube height is longer than the intertube distance,
the field emission is decreased with the increase in the
nanotube height [
]. All the CNT arrays in this study
have very high densities, and the field-screening effects
become the dominant disadvantageous factor to the
electron emission. Therefore, the screening effects of the
short CNT arrays are less than that of these long CNT
arrays. The short CNT arrays have better emission
properties than these long CNT arrays. With the increase in
the ratio of CNT height to CNT-to-CNT distance, the
emission currents of CNT arrays decrease reversely due
to the screening effects. Moreover, the plasma forms on
the cathode surface and influences the emission currents
of cathodes. The effect cathode radius can be calculated
by Child-Langmuir Law [
]. A series of results on the
plasma-induced emission properties of the CNT arrays
are shown in Table 1. Along with the increase in CNT
array height, the effect cathode radiuses and emission
areas decrease. Based on the above analysis, the areas of
plasma layer decrease with the increase in the array
height. Therefore, the emission area is proportional to
the plasma area. The plasma layer is beneficial to the
increase in emission currents. The results show that the
screening effects are diminished due to the presence of a
A tube configuration of CNTs enables them to absorb
gas, and the dense CNTs can adsorb a large amount of
gas molecules [
]. The CNTs can emit high-intensity
electron beams under the high-voltage pulse electric
Ic is the cathode current, Pi is the diode perveance, deff is the effective diode
gap and reff is the cathode radius
field. Under the effect of the high-intensity electron
beams, the adsorbent gas molecules are easy to become
]. The CCD camera has captured the
light emission from the CNT arrays. This demonstrates
that plasmas formed on the array surface during the
emission process. The electron emissions of CNT arrays
under the pulse electric field are not pure field emission.
The production mechanism of the high-intensity electron
beams from the CNT arrays is plasma-induced emission.
The emission model of the CNT arrays under the
highvoltage pulse electric field is shown in Figure 5. Above
all, the plasma layer forms on the cathode surface under
the effect of high-intensity electron beams. Subsequently,
the cathode surface is covered by plasma, and the
electron beams are extracted from the surface plasma. The
results demonstrate that the CNT arrays have the ability
of emitting high-intensity electron beams under the pulse
electric field. The CNT array cathodes are expected to be
applied to high-power vacuum electronic devices as
electron beam sources.
In this study, large area well-aligned CNT arrays with
different morphologies were fabricated. The
plasmainduced emission properties of the CNT arrays with
different CNT heights under the pulse electric field have
been investigated. The ratios of CNT height to
CNT-toCNT distance have considerable effects on their electron
emission properties. As the ratios increase, the emission
currents of the CNT arrays decrease due to the screening
effects. Plasmas formed on the array surface during the
emission process, and high-intensity electron beams of
about 170–180 A/cm2 were obtained from the CNT
arrays. CNT arrays are excellent candidate as
intensecurrent electron beam sources and can be applied to
high-power vacuum electronic devices in the near future.
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