World Journal of Nano Science and Engineering, 2012, 2, 126-133 Published Online September 2012 (
Electron Emission of Graphene-Diamond Hybrid Films
Using Paraffin Wax as Diamond Seeding Source
Deepak Varshney1,2*, Chitturi Venkateswara Rao3, Fr a nk Men do za1,2, Kenneth Perez1,2,
Maxime J.-F. Guinel1,2,3, Yasuyuki Ishikawa2,3, Brad R. Weiner2,3, Gerardo Morell1,2
1Department of Physics, University of Puerto Rico, San Juan, USA
2Institute of Functional Nanomaterials, University of Puerto Rico, San Juan, USA
3Department of Chemistry, University of Puerto Rico, San Juan, USA
Email: *
Received June 6, 2012; revised July 3, 2012; accepted July 20, 2012
We present a scalable, reproducible and economic process for the fabrication of diamond and diamond-graphene hybrid
films using paraffin wax as a seeding source for diamond. The films were characterized using Raman spectroscopy,
scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron energy loss spectroscopy
(EELS). Raman spectra show the characteristic band of diamond at 1332 cm1 and the D, G, and 2D bands of graphene
at 1360, 1582 and 2709 cm1, respectively. Electron microscopy confirms the microcrystalline nature of the diamond
films with crystal size in the range of 0.5 μm to 1.0 μm, and the hybrid film consists of microcrystalline diamond at-
tached to thin, semi-transparent graphene flakes. The graphene-diamond hybrid films exhibit a turn-on field of about 3.6
V/μm with a prolonged current stability of at least 135 h.
Keywords: Graphene; Chemical Vapor Deposition; Diamond Films; Electron Field Emission
1. Introduction
The exceptional properties (e.g. high thermal conduc-
tivity, high strength, and lowest compressibility) of dia-
mond make it an ideal material for many applications,
such as in cutting tools, coatings for magnetic disks, op-
tical switches [1], electronic devices [2], spintronics de-
vices, and quantum computational components [3] and
more. Various surface pre-treatment methods have been
used to enhance the nucleation density of diamond [4-8].
All these reported methods lead to surface alteration or
damage (usually by the formation of nano-scale pits,
scratches and defects concentration) and even contami-
nate the substrate. Furthermore, surface pre-treatment
cannot be easily applied to substrates with complex ge-
ometries, and is often incompatible with industrial appli-
cations because of the increased cost. It is therefore im-
portant to nucleate diamond on an electrically conducting
surface without the need for surface pre-treatment while
maintaining high nucleation density and good adhesion.
Isolated graphene, a one-atom-thick layer of graphitic
carbon, was first reported in 2004 [9], and has been
dubbed as the wonder material. It opens new opportuni-
ties because of its unique electrical and mechanical
properties. Its potential applications are many, including
solar cells, field-emission devices and batteries [10,11].
The field emission properties of diamond [12], amor-
phous carbon [13], and vertically aligned multi- and sin-
gle-walled carbon nanotubes [14,15] for cold cathode
applications have been extensively investigated but the
field emission properties of graphene films have only
been reported very recently [16,17]. These studies on the
field emission from various carbon materials focused on
the importance of the field enhancement factor according
to the morphological and topographical structure of the
cathode surface. The excellent electrical and thermal
properties of graphene and diamond respectively can be
combined into one hybrid material for advanced applica-
We have recently reported the direct nucleation of
diamond from kitchen wrap-polyethylene [18]. In the
present work, we report a new method for diamond nu-
cleation using paraffin wax as the seeding material. Gra-
phenediamond hybrid films were also fabricated using
the same seeding source for diamond. The method here-
by described is simple, cost effective and handy. The
graphene-diamond hybrid films show excellent field
emission characteristics with a low turn-on field and are
able to withstand for longer times compared to graphene
*Corresponding author.
opyright © 2012 SciRes. WJNSE
2. Experimental
For the synthesis of diamond films, paraffin wax (ca. 5 g)
was melted on a hot plate and a small portion of the melt
was transferred onto a copper disk substrate (14 mm di-
ameter) with the help of a glass dropper and allowed to
cool to room temperature. It was then introduced into the
HFCVD chamber. This wax coated copper substrate was
exposed to a gas mixture consisting of 0.3% methane in
hydrogen with 10% methane and 99.7% of hydrogen for
4 h, at a constant pressure of 20 Torr and a total gas flow
of 100 sccm. This gaseous mixture was activated by a
rhenium filament (8 cm in length and 0.5 mm in diameter)
positioned 9 mm above the substrate. The temperature of
the substrate and the filament was kept at approximately
450˚C and 2500˚C, respectively.
The graphene-diamond hybrid films were fabricated
using a similar procedure. An amount of 0.02 g of pre-
synthesized graphene powder [19] was mixed with the
paraffin melt to obtain a homogeneous suspension which
was then transferred onto a copper disk substrate and
subjected to the same reaction conditions as those used in
the fabrication of pure diamond films.
Method of Characterization
The Raman scattering spectra were obtained on a triple
monochromator (ISAJ-Y Model T 64000) with appro-
ximately 1 cm1 resolution using the 514.5 nm line of Ar
laser. Scanning electron microscopy (SEM) images were
recorded using a JEOL JSM-7500F SEM also equipped
with a transmission electron detector. The samples were
analyzed using a Carl Zeiss LEO 922 energy filtered
transmission electron microscope (TEM) operated at 200
The field emission I-V characteristics of the fabricated
films were measured using a custom built system [20, 21].
3. Results and Discussion
3.1. Raman Analysis
The First, The paraffin wax crystallites act as nucleation
sites for diamond growth in the presence of hydro-carbon
radicals and atomic hydrogen in the CVD system. When
the melted paraffin wax cools below its equilibrium
melting temperature, its crystallization via self-nuclea-
tion becomes thermodynamically favorable and it acts as
a seeding source for diamond nucleation. Also, in the
present case, the use of non-carbide forming substrate is
important as it provides a surface with minimal surface
interactions, facilitating the formation of crystallites in
the lowest energy diamond shape [22,23].
The first order Raman spectrum of the wax-coated
substrate is shown in the inset of Figure 1(a). It clearly
shows distinctive Raman contributions at 1063 cm1,
1133 cm1, 1296 cm1, and 1441 cm1. These are attri-
buted to C-C (carbon-carbon) stretching and CH2 and
CH3 deformation [24], as would be expected given the
straight-chain hydrocarbon structure of wax. Figure 1(a)
shows the Raman spectrum recorded from the film
obtained from the CVD process of wax coated copper
substrate. The intense band at 1332.6 cm1 is charac-
teristic of diamond [25]. The disappearance of the typical
vibrational modes of paraffin wax and the appearance of
band at 1332.6 cm1 clearly indicates the growth of
diamond films from paraffin wax. The fabrication of
diamond film using paraffin wax as a seeding source
follows the same mechanism as reported in case of
kitchen wrap polyethylene [18].
The Raman spectrum of the pre-synthesized graphene
is shown in inset of Figure 1(b) showing the presence of
D band corresponding to the disorder induced in sp2 car-
bon [26] and a band around 1582 cm1 corresponds to
G-band [27]. The position and shape of the prominent 2D
peak in the Raman spectrum can be used to clearly dis-
tinguish the number of layers in the graphene sample
[28]. Broad 2D band observed at 2685 ± 5 cm1 in dif-
ferent regions indicate that the material is few layered (5
- 10 layers) graphene. The Raman spectrum of the gra-
phene-diamond hybrid material in the range 1000 - 3000
cm1 is shown in Figure 1(b). The band at 1582 cm1
corresponds to the G band arising from the in-plane vi-
brations of sp2 carbon atoms of graphene, or the doubly
degenerate (TO and LO) phonon mode (E2 g symmetry)
at the Brillouin zone center. The band at 1332.6 cm1
remains present and is that of diamond (sp3 C). A shoul-
der at 1360 cm1 is identified as the D-band which is
attributed to the disorder-induced in sp2-bonded carbon
[29]. The 2D band at 2709 cm1 originates from a two
phonon double resonance Raman process and is closely
related to the band structure of graphene and is used to
confirm the presence of graphene [30]. The Raman spec-
trum in the range of 2600 - 2900 cm1 is shown in the
inset of Figure 1(b). It clearly depicts two bands at 2709
and 2845 cm1 corresponding to the 2D and S3 peaks for
graphene [31]. The 2D band, which is a characteristic of
graphene, is used to determine the number of layers of
graphene in the sample [32]. The broad and less intense
2D band seen here indicates that the hybrid material con-
sists of few layered graphene.
3.2. Scanning Electron Microscopy
The morphology of the diamond and graphene-diamond
hybrid films was ascertained by scanning electron mi-
croscopy (SEM). Figure 2(a) shows an overview SEM
image of the diamond film composed of microcrystalline
diamond. The inset of Figure 2(a) shows a diamond
rystal with well-defined facets having a size of c
Copyright © 2012 SciRes. WJNSE
Copyright © 2012 SciRes. WJNSE
Figure 1. Raman spectra of (a) diamond film and (b) diamond-graphene film. Inset of Figure 1(a) shows the Raman spectrum
of paraffin wax. Inset of Figure 1(b) shows the Raman spectrum of pre-synthesi zed graphene.
approximately 0.5 - 1.5 μm. Figure 2(b) is a SEM image
of the pre-synthesized graphene used as a precursor in
the fabrication of the hybrid film. Figure 2(c) shows a
SEM image of the hybrid material revealing randomly
nucleating diamond microcrystals (sizes ranging from 0.5
to 2.0 μm) embedded onto the graphene flakes. The small
particles residing on the matrix material are the sub mi-
crometer diamond crystallites resulting from the seeding
source (paraffin wax) that are in the intermediate stage of
diamond growth.
3.3. Transmission Electron Microscopy
The diamond and graphene-diamond hybrid films were
further analyzed using TEM. Figure 3(a) shows a TEM
image of the diamond film synthesized using paraffin
wax as a seeding source. The corresponding selected area
electron diffraction (SAED) pattern (inset of Figure 3(a))
confirms the structure of diamond with its {111}, {220}
and {311} reflections indexed (approximately 2.06 Ǻ,
1.26 Ǻ and 1.08 Ǻ, respectively). Figure 3(b) shows a
TEM image recorded at 30 kV, of the pre-synthesized
graphene freely suspended on grid. The graphene sheets
are thin and typically contain wrinkles and rolled edges.
Figure 2. (a) Overview SEM image of the microcrystalline
diamond film. The inset shows a diamond crystal with
well-defined facets; (b) SEM image of the pre-synthesized
graphene used as a precursor in the fabrication of the dia-
mond-graphene hybrid material; (c) SEM image of the
diamond-graphene hybrid material. The diamond micro-
crystals randomly nucleate onto the graphe ne flakes.
Figure 3(c) shows a TEM image of the hybrid film, in-
dicating the presence of layered graphene together with
the diamond microcrystals adhered to the graphene sur-
face. In order to clearly establish the nature of carbon
material grown on the copper substrate, electron energy
loss spectroscopy (EELS) spectra were acquired and are
shown in Figure 4. The spectra were recorded on differ-
ent regions of graphene-diamond hybrid films compris-
ing of diamond crystals, graphene flakes and also the
pure amorphous carbon support shown in the inset of
Figure 4.
3.4. Electron Energy Loss Spectroscopy
EELS can be used to unambiguously distinguish between
the different carbon materials [21] and was used to detect
the elemental composition of the hybrid films. All the
allotropes of carbon generally show K-absorption near
edge structure between 280 eV-320 eV. The EELS spec-
trum for the denser regions (as shown in inset of Figure
blue line)) shows the main band around 292 eV due to 4 (
Figure 3. (a) TEM image showing the diamond crystals obtained from the diamond film synthesized using paraffin wax as a
seeding source. The inset shows the corresponding electron diffraction pattern indexed to diamond with its {111}, {220} and
{311} reflections. (b) TEM image of the pre-synthesized graphene freely suspended on grid (note the carbon support belong-
ing to the grid). (c) TEM image of the hybrid material.
Figure 4. Electron energy loss spectrum on the region shown in the inset (red line for graphene, blue line for diamond and
black line for carbon grid).
the 1s-σ* transition [33] and a dip around 302 eV, which
is a characteristic feature of crystalline diamond [34]. The
small shoulder ca. 285 eV in the diamond spectrum may
be arising due to the neighboring graphene flakes or due
to the presence of sp2 graphitic carbon at the grain
boundaries [35]. The EELS spectra of the semi-trans-
parent zones (red line) exhibits the 1s-π* and 1s-σ* peaks
at 285 and 292 eV confirming the presence of graphene
layers. In order to differentiate the obtained graphene
spectrum from a-C, we took the EELS of the lacey car-
bon grid (a-C) shown in Figure 4 (black line) revealing a
broad and structure less peak, owing to the fact that the
local six fold symmetry is lost in the amorphous state
[36]. Thus, the fabrication of the present hybrid material
is evident from the TEM and EELS analyses.
3.5. Electron Field Emission
3.5.1. J-E Curve
The plot of the measured current density as a function of
the macroscopic electrical field is shown in Figure 5.
The field emission properties of the fabricated diamond
films were reported previously by our group [18,20]. The
field emission measurements reveal a threshold field of
3.6 V/μm with a current density approaching 1 mA/cm2
at 17 - 18 V/μm for graphene-diamond hybrid film (Fig-
ure 5). A low threshold is observed due to the high geo-
metrical factor of graphene flakes that cause local field
enhancement and availability of conducting electrons in
graphene, which is absent in case of diamond at room
temperature. The plateau in emission current occurs at
about 18 V/μm for a current of about 105 A (0.01 mA).
3.5.2. F- N Plot
The results of the electron emission studies can be ana-
lyzed in terms of the Fowler-Nordheim (F-N) theory [37].
In Figure 6, the F-N plot shows a bend in the downward
direction that is often a feature of the carbon-based mate-
rials which can be attributed to the quantum tunneling of
Copyright © 2012 SciRes. WJNSE
electrons through multiple barriers caused by the pres-
ence of different materials [38,39]. The Fowler-Nord-
heim plot for graphene-diamond hybrid has two slopes
representing electron emission from two different regions.
In region 20 < E < 4.5 V/μm the major contribution is
due to electrons that are present in the conduction min-
ima of graphene-diamond hybrid film and in the other,
4.5 < E < 2.7 V/μm, the major contribution is made by
the low occupancy states. An effective field enhancement
factor (β) calculated from the slopes of FN plot for
Figure 5. Plot of the measured field emission current as a
function of the electric field for the graphene-diamond hy-
brid films.
graphene-diamond hybrid (Figure 6) is calculated to be
7.05 × 102 and 2.99 × 103 for the two slopes. An en-
hancement of the field for hybrid films is brought about
by dielectric inhomogeneities originating from the dif-
ferences between conductive, spatially localized sp2 C
clusters surrounded by a more insulating sp3 C matrix
3.5.3. Current Stability Test
To evaluate the stability of the emission from the hybrid
film, the current was monitored over a period of 135 h (7
days) with an initial current of the order of 0.1 mA at a
field of 15 V/μm. The result obtained is shown in Figure
7. Superior emission stability was observed that can be
explained on the basis of theoretical models proposed for
emission from materials having both sp2 and sp3 hybrid-
ized carbon [41]. These models highlight the compli-
mentary role of graphite-like (sp2) region as emission
center that is heated by the ohmic current passing
through it and diamond-like (sp3) region as heat sink that
removes heat from the emission center. Although gra-
phene has very high thermal conductivity, in the absence
of better heat sink, most of the heat gets dumped at the
graphene/substrate interface leading to the destruction of
emission sites. However, in the presence of diamond, this
excess heat gets removed very efficiently maintaining the
integrity of the emission sites. The net result is a robust
cold cathode material.
Figure 6. Fowler-Nordheim (ln(I/V2 ) vs V–1) plot for the graphene-diamond hybrid films.
Copyright © 2012 SciRes. WJNSE
Figure 7. Plot of the emission current density versus time for a period of 135 h recorded from the graphene-diamond hybrid
The diamond-graphene hybrid film of this study shows
better emission stability as compared to that of pure gra-
phene films or graphene composite thin films (Table 1).
In order to confirm the reproducibility of field emission
characteristics, I-V measurements were performed at
several locations. The observed field emission character-
istics were almost independent of the locations, due to
nearly uniform fabrication over the whole substrate sur-
4. Conclusion
Diamond and graphene-diamond hybrid films were fab-
ricated by the HFCVD technique using paraffin wax as a
seeding material for diamond. The characteristic bands of
diamond and graphene were evident from Raman spec-
troscopy. Microscopic images reveal the presence of mi-
cron-sized diamond crystals in the fabricated films. The
graphene-diamond hybrid films exhibits a turn-on field
of 3.8 V/μm with an emission current density approach-
ing 0.3 mA/cm2 at a field of 20 V/μm. The hybrid film
exhibits good emission current stability of 135 h. The
present study provides an inexpensive fabrication ap-
proach towards a graphene-diamond hybrid films that
exhibit excellent field emission properties and can be a
competitive candidate for efficient field emitter material.
5. Acknowledgements
This research was made possible by funds from the In-
stitute for Functional Nanomaterials (NSF Grant #
1002410), PR NASA EPSCoR (NASA Cooperative
Agreement # NNX07AO30A and NNX08BA48A), and
Table 1. Comparison of emission characteristics of present graphene-diamond hybrid film with existing literature.
Field ehan cment
Current stability
(minutes) Reference
Graphene N/A 1.35 - 6.20 5500 [21]
Graphene paper 3.2 × 103 - 4.1 × 103 1.6 - 2.4 1500 - 1700 [42]
Graphene on
polymer film 1.0 × 103 1.8 >180 [43]
Hybrid material 5.9 × 102 - 1.1 × 104 3.8 >8100 Present work
Copyright © 2012 SciRes. WJNSE
PR DOE EPSCoR (DOE Grant # DEFG02-08ER46526).
D.V would like to acknowledge the help of Mr. W. Pérez
for the Raman spectroscopy measurements. Microscopes
are operated at the Nanoscopy facility at UPR.
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