Self-organized carbon nanotips

Full text of DOI:10.1063/1.1401777

Self-organized carbon nanotips
Jin Jang, Suk Jae Chung, Hong Sik Kim, Sung Hoon Lim, and Choong Hun Lee
Citation: Applied Physics Letters 79, 1682 (2001); doi: 10.1063/1.1401777
View online: http://dx.doi.org/10.1063/1.1401777
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APPLIED PHYSICS LETTERS
VOLUME 79, NUMBER 11
10 SEPTEMBER 2001
Self-organized carbon nanotips
Jin Jang,a) Suk Jae Chung, and Hong Sik Kim
Department of Physics and TFT-LCD National Laboratory, Kyung Hee University, Dongdaemoon-ku,
Seoul 130-701, Korea
Sung Hoon Lim
Department of Information Display, Kyung Hee University, Dongdaemoon-ku Seoul, 130-701, Korea
Choong Hun Lee
Division of Physics and Semiconductor Science, Wonkwang University, Iksan 570-749, Korea
͑Received 17 April 2001; accepted for publication 19 July 2001͒
We have developed a carbon nanostructure, which is comprised of high-density carbon nanotips on
a graphite layer. These carbon nanotips, with tip diameters of ϳ10 nm, are grown by high-density
plasma chemical vapor deposition onto Ni-coated Si using an inductively coupled plasma. The Ni
on Si changes into NiSi2 by substrate heating. First, a carbon buffer layer and then a graphene sheet
are formed on the NiSi2. Then, the carbon nanotips are grown by a C2H2 /H2 plasma on the graphene
sheet. The carbon nanotips show good adhesion to the substrate and are almost aligned, with an
average length of 110 nm. They exhibit a turn-on field of 0.1 V/␮m, a field amplification factor of
ϳ13 000, a current density of 2 mA/cm2 at a field of 2 V/␮m, and uniform electron emission.
© 2001 American Institute of Physics. ͓DOI: 10.1063/1.1401777͔
Carbon materials are found in various forms such as
graphite, diamond, carbon fiber, graphite whiskers,1
fullerenes, carbon nanotubes ͑CNTs͒, amorphous carbon,
diamond-like carbon ͑DLC͒, and polymer-like carbon.
Graphite is a hexagonal plate-like crystal with a very weak
bond between graphene sheets. On the other hand, CNTs,
carbon fibers and carbon whiskers have an unusual coaxial
tubular structure consisting of rolled graphene sheets. Since
Iijima first found carbon nanotubes in 1991,2 carbon compos-
ites of nanometer size have become of increasing interest due
to their unique structural and electrical properties. CNTs es-
pecially have been considered the most promising material
for field emission display ͑FED͒ field emitters3 due to their
low turn-on field and high emission current. In order to have
CNTs as field emitters on a substrate, purifying and aligning
of the CNTs have generally been carried out.3 An advanced
method to make CNT field emitters is to directly deposit
them on the substrate by chemical vapor deposition ͑CVD͒,
plasma CVD4 or other deposition methods.5 However, CNTs
directly deposited on the substrate show poor adhesion to the
substrate and the substrate temperature must be as high as
ϳ800 °C to exhibit good field emission properties.6
tion chamber for formation of the tips. The C2H2 and H2 flow
rates were fixed at 25 and 50 sccm, respectively. The depo-
sition temperature was ϳ700 °C. A Ni layer of 70 nm was
deposited onto the Si using a sputtering system. The Ni was
used to form the catalyst for growth of the carbon nanotips.
The Ni was exposed to a NH3 plasma to modify its surface
morphology prior to growth of the nanotips. NiSi2 formed
during substrate heating. Note that the Ni can be also used as
a catalyst for the growth of CNTs.5,6 However, in this work,
the Ni silicide acts as a catalyst for growth of graphite sheets
and subsequently the carbon nanotips on them. The growth
conditions for carbon nanotips are similar to those for the
CNTs except for the formation of the Ni silicide. However,
we have produced carbon nanotips, a form of carbon nano-
composite, instead of CNTs. This may be due to the forma-
tion of Ni silicide before growth of the carbon nanotips.
Figure 1 shows field emission scanning electron micros-
copy ͑SEM͒ images of the carbon nanotips deposited by
high-density plasma CVD for 25 min. The carbon nanotips
were uniformly grown on the substrate with a tip density of
ϳ400 ea/␮m2. The carbon nanotips have an average length
In this work, we have developed self-organized carbon
nanotips, a form of carbon nanostructure on Si. The carbon
nanotips show good adhesion to the substrate and exhibit a
turn-on field of 0.1 V/␮m and uniform light emission from
an anode phosphor screen above the tips. The self-organized
carbon nanotips exhibit the lowest turn-on field among all
electron-emitting materials reported so far.
To grow carbon nanotips, we used high-density plasma
CVD using an inductively coupled plasma. The rf power and
gas pressure for the plasma were fixed at 1.1 kW and 1 Torr,
respectively. A C2H2 /H2 mixture was introduced into a reac-
a͒
Electronic mail: jjang@khu.ac.kr
FIG. 1. Plane SEM image of the carbon nanotips.
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0003-6951/2001/79(11)/1682/3/$18.00 1682 © 2001 American Institute of Physics
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Appl. Phys. Lett., Vol. 79, No. 11, 10 September 2001
Jang et al.
1683
FIG. 2. Cross-sectional TEM dark-field image of the carbon nanotips.
of 110 nm and are roughly aligned to the substrate. The
diameters of nanotips, the full width at half maximum
͑FWHM͒ of the perimeter for the tips, are not uniform. They
are distributed mostly in the range of 10–20 nm. But the
minimum diameter of the tip end is smaller than 5 nm.
Figure 2 shows a cross-sectional transmission electron
microscopy ͑TEM͒ dark-field image of carbon nanotips
nearly aligned to the substrate. Region A is composed of the
carbon nanotips and epoxy. Note that epoxy was used for the
TEM measurement and this does not exist in the real
samples. The electron diffraction pattern of region B was
measured together with TEM and we found its structure is
the same as that of the bulk Si and this means the silicide is
NiSi2.7
FIG. 4. ͑a͒ Current–voltage characteristics plotted on linear and log ͑in the
box͒ scales of the carbon nanotips deposited for 10 min at a temperature of
700 °C and ͑b͒ FN plot of the emission currents for the carbon nanotips.
in the formation of a sharp tip. The spacing between the
carbon sheets is 0.34 nm which corresponds to the spacing
between carbon layers in graphite. The tips are mostly com-
posed of graphene sheets, but some amorphous phase may be
included.
For growth of CNTs, the Ni layer turns into small grains
during substrate heating and pre-plasma treatment on Ni and
these Ni grains play important roles as catalysts and nucle-
ation sites for the growth of CNTs. The structure of the CNTs
depends on the Ni seeds. The graphene grains were nucleated
around the Ni seeds and became the walls of the CNTs.
These Ni grains can remain at the bottom for a bottom based
growth model of CNTs8–10 or rise to the top for a top based
growth model.8,11,12 In both cases, Ni seeds can be catalysts
by themselves and are encapsulated by the tube walls. Car-
bon atoms or molecules are absorbed onto these Ni seeds and
form the walls of CNTs.
Figure 3 shows high-resolution transmission electron mi-
croscopy ͑HRTEM͒ images of a carbon nanotip. Note that
the carbon nanotip consists of carbon atoms with threefold
covalent bonding-like graphite. The number of sheets de-
creases with the growth of the carbon nanotip and this results
For growth of the carbon nanotips, on the other hand, the
Ni was changed into NiSi2 by a rise in temperature and NH3
plasma treatment. For this, the thickness of Ni must be thin
enough not to remain on the silicide surface after silicide
formation. When the Ni remained on the silicide, multi-
walled CNTs grew. Carbon atoms or molecules were added
to the Ni silicide surface and nucleated, so the carbon layers
form on the whole Ni silicide layer, parallel to the silicide
surface. This is the main reason for the growth of carbon
nanotips instead of carbon nanotubes despite using similar
deposition conditions. Between the Ni silicide and graphene
sheets, an amorphous carbon layer of ϳ10 nm is formed as a
buffer layer due to the difference in crystal structure between
graphite and NiSi2. The graphite has a hexagonal structure
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FIG. 3. HRTEM images of a carbon nanotip.
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with a lattice constant of 0.34 nm to the c axis, normal to the

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Appl. Phys. Lett., Vol. 79, No. 11, 10 September 2001
Jang et al.
CNT emitter.14–17 Note that the film-type CNT emitters show
␤ of 1000–10 000 regardless of the fact that single walled or
multiwalled CNTs are mostly lower than 8000.15–17,21 This
also confirms that the carbon nanotips developed in this work
are better than CNTs as field emitters.
Figure 5 shows a light emission pattern using the carbon
nanotips at a field of 2.3 V/␮m. The gap between the cathode
and the anode was 1 mm and was maintained by glass spac-
ers. ITO glass with a printed ZnO:Mn blue phosphor was
used as an anode plate. Uniform bright light emission can be
seen. The adhesion of nanotips to the substrate is very firm
so they could not be peeled off even by scratching.
In summary, we have developed carbon nanotips on Si
with a diameter of less than 10 nm. Ni silicide formation has
an important role in allowing the growth of carbon nanotips.
The carbon nanotips show good adhesion to the substrate and
good field emission properties because they formed on a
graphene sheet grown on Ni silicide with an amorphous car-
bon buffer layer between the graphite and Ni silicide. The
carbon nanotips exhibited a turn-on field of 0.1 V/␮m, a field
amplification factor of 13 000 and uniform light emission
from a phosphor screen. These nanotips are promising for
use in electron sources.
FIG. 5. Photoimage of light emission from a phosphor screen using the
carbon nanotips.
graphene sheet, and NiSi2 has a diamond structure with a
lattice constant of 0.54 nm. After growth of about tens of
graphene sheets, carbon nanotips were grown on these
sheets. During growth of the graphene sheets, amorphous
carbon or defective graphite nanoparticles can form on the
sheets.
Figure 4͑a͒ shows current–voltage characteristics plotted
on linear and log ͑in the box͒ scales for carbon nanotips
deposited for 10 min at a temperature of 700 °C. The elec-
tron emission performance for carbon nanotips was investi-
gated using a diode structure. The gap between the anode
and the carbon nanotips was 1 mm and the anode area was
2 cm2. The turn-on field of the carbon nanotips is
ϳ0.1 V/␮m. This, we believe, is the lowest value reported
so far. Note that the CNTs show turn-on field of
Ͼ0.5 V/␮m.13 The emission current of the carbon nanotips
at a field of 1.25 V/␮m is about 186 ␮A/cm2. This current
density at a field of 1.25 V/␮m is much higher than the
values of film-type CNTs emitters reported so far.9,14–17
These low turn-on fields and high emission currents are
mainly due to the sharp tip structure and high tip density of
400 tips/␮m2. Figure 4͑b͒ shows a Fowler–Nordheim ͑FN͒
plot of the emission currents for the carbon nanotips. From
Fowler–Nordheim theory,18 the field emission current I can
be expressed as
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