Uniform carbon nanoflake films and their field emissions

Article abstract of DOI:10.1016/S0009-2614(02)00430-X

Films of uniformly distributed carbon nanoflakes have been prepared by using hot filament chemical vapor deposition. A large amount of carbon flakes with a thickness of less than 20 nm interlaced together to form a layer of carbon nest-like film with their sharp edges almost perpendicular to the Si substrate. Raman spectroscopy showed that the films were characteristic of pyrolytic graphite and became more disordered with the increase of substrate temperature and acetylene concentration. The growth mechanism of the carbon nanoflake films was discussed. The field emission performance has been done, showing a turn-on field of about 17 V/μm. Considering the ease of large-area preparation, the carbon nanoflake films might have a potential application in vacuum electronic devices.

Full text of DOI:10.1016/S0009-2614(02)00430-X

31 May 2002
Chemical Physics Letters 358 (2002) 187–191
www.elsevier.com/locate/cplett
Uniform carbon nanoflake films and their field emissions
*
N.G. Shang, F.C.K. Au, X.M. Meng, C.S. Lee, I. Bello, S.T. Lee
Department of Physics and Materials Science, Center Of Super-Diamond and Advanced Films (COSDAF),
City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, PR China
Received 28 January 2002
Abstract
Films of uniformly distributed carbon nanoflakes have been prepared by using hot filament chemical vapor depo-
sition. A large amount of carbon flakes with a thickness of less than 20 nm interlaced together to form a layer of carbon
nest-like film with their sharp edges almost perpendicular to the Si substrate. Raman spectroscopy showed that the films
were characteristic of pyrolytic graphite and became more disordered with the increase of substrate temperature and
acetylene concentration. The growth mechanism of the carbon nanoflake films was discussed. The field emission per-
formance has been done, showing a turn-on field of about 17 V/lm. Considering the ease of large-area preparation, the
carbon nanoflake films might have a potential application in vacuum electronic devices. Ó 2002 Elsevier Science B.V.
All rights reserved.
Carbon has a unique state of affairs in nature,
since it is capable of constructing a variety of novel
materials such as diamond, graphite, fullerenes
and nanotubes. The various structures have shown
remarkable differences in properties and potential
applications [1,2]. Moreover, carbon also consti-
tutes many nanoscaled materials with novel
properties not observed in the bulk materials due
to the quantum size effect [3,4]. As a new carbon
allotropy and nanoscaled material, the carbon
nanoflake film, which consists of petal-like
graphite flakes with a thickness of less than about
20 nm, is of much interest [5]. Each carbon
nanoflake can be considered a quasi-two-dimen-
sional structure with strong anisotropy in physical
properties. It is interesting that almost all the
carbon nanoflakes are vertical to the substrate.
This can lead to a large surface-to-volume ratio for
hydrogen adsorption, as in the case of carbon
nanotubes and herringbone-like carbon nanofibers
[6,7]. Their very sharp and thin edges perpendic-
ular to substrates are potentially good electron
field emission sites. In this Letter, the hot filament
chemical vapor deposition (HFCVD) method is
first used to prepare these carbon nanoflake films.
As compared with the previously reported arc
discharge techniques, the current deposition pro-
cess has the advantages of low synthesis tempera-
ture, better controllability, large-area deposition,
compatibility with current semiconductor tech-
niques and neither catalyst nor special substrate
treatment are needed.
* Corresponding author. Fax: +852-2784-4696.
E-mail address: apannale@cityu.edu.hk (S.T. Lee).
The carbon nanoflake films were fabricated in a
typical HFCVD system previously used for
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)00430-X

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N.G. Shang et al. / Chemical Physics Letters 358 (2002) 187–191
growing diamond films [8]. Two parallel tungsten
filaments of 0.75 mm in diameter were heated to
2100 °C to decompose a C2H2 (3–15%) and H2
mixture fed at a rate of 200 sccm into the system.
After a series of cleaning procedures using acetone,
ethanol, HF acid and de-ionized water, Si(1 0 0)
wafers were put at about 4–7 mm below the fila-
ments, where the substrate temperature can be
controlled between 400 and 700 °C as measured by
a thermocouple under the substrate. The surface
morphology, structure and chemical composition
of the as-deposited samples were analyzed with
scanning electron microscopy (SEM), Raman
scattering spectroscopy and X-ray photoelectron
spectroscopy (XPS).
Fig. 1 shows typical SEMimages of the carbon
nanoflake films deposited for 4 h at different sub-
strate temperatures from 400 to 700 °C. It can be
seen that all films consist of a large amount of
small clusters with the shape of leaves or petals.
They perpendicularly interlace together to con-
struct a layer of the nest-like film with a large
surface-to-volume ratio and very sharp edges. The
carbon nanoflakes deposited at 400–600 °C have a
thickness of less than about 10–20 nm and a size of
300–600 nm in the other two dimensions. In this
temperature range, the size and the density of the
nanoflake gradually decreases and increases re-
spectively as the temperature increases. When the
deposition temperature was increased to 700 °C,
the size of the carbon flakes abruptly decreases to
about 5 nm in thickness and 80 nm in width.
Fig. 2 shows the Raman spectra of the samples
deposited at different temperatures. In these spec-
tra, the two strong peaks at about 1350 (ꢀ5) and
1590 (ꢀ5) cmꢁ1 are the typical D and G bands of
microcrystalline graphite, which originate from the
A1g zone-edge phonon induced by the disorder due
to the finite-crystallite size and the zone-center E2g
first-order mode, respectively. Another strong
peak at 2700 (ꢀ8) cmꢁ1 is the overtone of the D
band, and the weak broad peak at about 2930 cm1
is attributed to the sum of the D and G bands. A
small peak at about 1625 cmꢁ1 and its overtone
(3250 cmꢁ1) were also observed. They correspond
to a maximum in the density of state of the
midzone phonon and they also reveal a disorder-
induced behavior similar to that of the 1350 cmꢁ1
.
The Raman spectra are basically similar to that of
carbon nanotubes and quite different from that of
Fig. 1. SEMimages of the carbon nanoflake films deposited at the substrate temperature of 400 (a), 500 (b), 600 (c) and 700 °C (d).

N.G. Shang et al. / Chemical Physics Letters 358 (2002) 187–191
189
Fig. 3. Relationship between the intensity ratio of D band to G
band and the FWHMof D band versus the acetylene concen-
tration for the samples deposited at the substrate temperatures
of 600 °C.
increase as the acetylene concentration increases.
This reveals that the films become more disordered
as acetylene concentration increases.
To investigate the growth mechanism of the
carbon nanoflake films, chemical compositions of
the deposited films were characterized by XPS. It
was found that the films are composed mainly of
carbon and with less than 3% of tungsten metal
contamination from the hot filaments and 2% F
originated from substrate cleaning processing with
HF acid. The time evolution of the film deposited
at 600 °C and 5% C2H2 is shown in Fig. 4. Fig. 4a–
c show the images of the carbon nanoflake film
after deposition for 7 min, 20 min and 24 h, re-
spectively. The film deposited for 7 min consists of
a large amount of worm-like short carbon rods or
small carbon sheets with a uniform size estimated
about 100 nm long and 5–10 nm wide. After 20
min of deposition, almost all the carbon nano-
flakes (Fig. 4b) have approached a similar size and
thickness to those in the sample after 24 h depo-
sition (Fig. 4c).
From simple surface energy consideration, the
formation of the nanorod and nanoflake rather
than nanosphere structure suggests that an aniso-
tropic process occurs during the growth of carbon
nanoflake films in the current HFCVD system.
The carbon clusters from the vapor could first
condense along one direction to form a rod, and
then grow along another direction almost normal
to the substrate to form a sheet. On the other
hand, the carbon nanoflakes may follow a two-
dimensional growth model with a defined ratio of
Fig. 2. Raman spectra of the carbon nanoflake films deposited
at different substrate temperatures, revealing the carbon films
becoming disordered with the increase of the substrate tem-
perature.
high-oriented pyrolytic graphite [9,10]. As the
substrate temperature increased, the distinct dif-
ferences in these spectra were observed in the ratio
of the integrated intensities of the D band to G
band ðR ¼ I1350=I1590Þ, the peak frequencies and
full width at half maximum (FWHM) of the D and
G bands, and the intensity and position of the
peak at 1625 cmꢁ1. It was found that the ratio R,
FWHMof the D and G bands as well as the in-
tensity of the peak at 1625 cmꢁ1 increased con-
siderably with the substrate temperature. This
suggests that the deposited graphitic carbon flakes
become more disordered as the substrate temper-
ature increases. This is consistent with the SEM
observation that the size of the carbon flakes de-
creases as the substrate temperature decreases.
In addition to the substrate temperature, the
acetylene concentration was also found to have a
significant effect on the deposited films. Fig. 3
shows the relationship between the D to G band
intensity ratio and the FWHMof D band versus
the acetylene concentration from 3% to 15% at a
substrate temperature of 600 °C. It is clear that
both the intensity ratio and D band peak width

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N.G. Shang et al. / Chemical Physics Letters 358 (2002) 187–191
thermal plasma produced by heated filaments.
However, a thin boundary layer of carbon clusters
with a non-Maxwellian distribution could be
formed just above the substrate surface due to the
interaction between the reflected particles from the
surface and the newly arrived ones. This is possibly
favorable for the formation of large carbon clus-
ters and subsequent asymmetric condensation.
When the substrate temperature and acetylene
concentration increase, condensed carbon clusters
in the critical size and mobility on the surface will
change, leading to the formation of more disor-
dered carbon nanoflake film of a smaller size. In
addition, at the temperature of several hundreds
Kelvin, the space structure of the orbitals (or
dangling bonds) of a free carbon atom could be
considered asymmetric, with three in-plane sp2
bonds and one pz orbital that perpendicular to this
plane. Such a configuration has the potential to
form a plane structure and is native for the
asymmetric surface energy of graphite. This
asymmetric factor has proven to be important for
the catalytic method of nanotube fabrication, and
may play a similar role in the formation of carbon
nanoflake films. In previous reports, carbon
nanoflakes were produced outside the flame of arc
discharge and on the graphite wall surrounding the
anode and cathode as a byproduct of carbon na-
notubes. In such a case, the growth radicals for the
carbon nanotubes and carbon nanoflakes must be
quite different, although their formation mecha-
nisms are not yet clear. The arc discharge deposi-
tion method of carbon nanoflake films generally
requires a higher substrate temperature (e.g. about
800 °C) and is less controllable compared with the
present HFCVD method. Using the present ap-
proach, we can easily control the morphology,
structure, and thickness of films on Si substrates.
The present method also has the advantage that
uniform films can be deposited over a relatively
large area (about 30 ꢂ 50 mm2 in the present sys-
tem that uses two filaments) and can be further
scaled-up, as in other HFCVD processes, with a
filament array.
Fig. 4. SEMimages of the samples deposited at the substrate
temperature of 600 °C for 7 min (a), 20 min (b) and 24 h (c).
growth rate along two directions. During the
growth of carbon nanotubes, the asymmetry fac-
tors are the directed flux of carbon (the ion current
in arc discharge, or the flow of carbon vapor in
vapor deposition methods) and the presence of a
metal particle in the catalytic method, which acts
as seed particles for precipitation of carbon from
gas phase into the form of a hollow tube [11,12]. In
the present synthesis, no bias voltage, metal cata-
lysts or other auxiliary techniques that may pro-
duce asymmetric distribution of condensed carbon
clusters were applied to our deposition system.
Instead, just a Maxwellian distribution of carbon
clusters was presented in the low temperature
Since a high density of very sharp carbon
nanoflakes in the film are perpendicular to the Si
substrates and quiet uniform over the large area,
they are expected to have a good field emission.

N.G. Shang et al. / Chemical Physics Letters 358 (2002) 187–191
191
attributed to the asymmetric factor in the deposi-
tion system and the bonding nature of free carbon
atoms. Considering the simple controllable depo-
sition process for a large area and compatible with
the current semiconductor industry, the carbon
nanoflake films may be good candidates for large-
scale applications as nanostructured templates and
electron vacuum emitters.
Acknowledgements
This work was financially supported by the
Research Grants Council of Hong Kong SAR, PR
China (Project No. 9040637).
Fig. 5. Field emission J–E data taken from the carbon nano-
flake films. The inset is the corresponding F–N plot.
The measurement process and apparatus has been
described in detail in our previous paper [13]. The
relations of emission current density (J) versus
applied electric field (E) shown in Fig. 5 were taken
from the sample deposited at the substrate of
about 600 °C and acetylene concentration of 5%.
The turn-on field defined as 0.01 mA/cm2 were
obtained to be about 17 V/lm. This value is
smaller than that of curvy SiC nanorods our pre-
viously reported. For the other samples a turn-on
field range from 20 to 35 V/lm were obtained. The
curve of Fowler (F)–Nordheim (N) shown in the
inset of Fig. 5 shows the emission process of the
carbon nanoflake films followed the traditional
mechanism of field emissions.
In summary, carbon nanoflake films have been
prepared by conventional HFCVD without using
any catalysts or surface pretreatment. The surface
morphology and structure of carbon nanoflake
films depended strongly on the acetylene concen-
tration and substrate temperature. The growth
mechanism of the carbon nanoflake films may be
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