1. 1 23
Journal of Materials Science:
Materials in Electronics
ISSN 0957-4522
Volume 26
Number 8
J Mater Sci: Mater Electron (2015)
26:5964-5969
DOI 10.1007/s10854-015-3170-5
Microstructure and optical properties of
cobalt–carbon nanocomposites prepared by
RF-sputtering
Mehrdad Molamohammadi, Ali Arman,
Amine Achour, Bandar Astinchap, Azin
Ahmadpourian, Arash Boochani, Sirvan
Naderi & Arman Ahmadpourian

2. 1 23
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3. Microstructure and optical properties of cobalt–carbon
nanocomposites prepared by RF-sputtering
Mehrdad Molamohammadi1 • Ali Arman1 • Amine Achour2,3 • Bandar Astinchap4 •
Azin Ahmadpourian1 • Arash Boochani1 • Sirvan Naderi5 • Arman Ahmadpourian1
Received: 1 April 2015 / Accepted: 5 May 2015 / Published online: 10 May 2015
Ó Springer Science+Business Media New York 2015
Abstract Cobalt/carbon nanocomposite coating (Co NPs
@ a-C: H), which consist of cobalt nanoparticles buried in
hydrogenated amorphous carbon are prepared by RF-
sputtering and RF-plasma enhanced chemical vapor depo-
sition on silicon substrates. In these processes, the coatings
are produced from a cobalt sputtered target and acetylene
reactant gas. The crystalline structure and surface topog-
raphy of the deposited films are characterized by X-ray
diffraction (XRD) and atomic force microscopy (AFM),
respectively. The AFM shows that the average size distri-
bution depends on the deposition conditions and RMS
varies from 3.5 up to 6 nm. The XRD analyses indicate the
presence of cobalt nanostructure as centered face cubic
phase and its oxide, but with no evidence of carbide
structure. The energy-dispersive X-ray spectroscopy ana-
lysis was used to identify the elements composition in the
films and the ultraviolet–visible spectrophotometry is used
to study surface plasmon resonance bands of Co
nanoparticles.
1 Introduction
There is an increasing interest in magnetic behavior of
nanoparticles, such as Co, Fe and Ni [1–3], because of their
technological applications for ultrahigh-density magnetic
storage devices [4–6], magnetic sensors [7, 8] and hetero-
geneous catalysts [9]. Moreover, due to possible combi-
nation of magnetic and optical properties of transition
metals like Co and Ni, reporting the magnetic and optical
properties of such nano-metals became very important [10–
14]. In fact, the properties of nanoparticles and their in-
teraction with the hosted matrix determine the unique be-
havior of the whole nanostructures systems [15]. Therefore,
it is vital to have ability of controlling the properties of
nanoparticles such as size, shape, surface etc., which
strongly affect the magnetic properties [16, 17]. Hence,
various methods are used to fabricate nanoparticles with
different experimental conditions to obtain desired
nanoparticles with tunable characteristics. The study of
cobalt nanoparticles and its oxide have attracted many in-
terests, for example, Li et al. [18] have prepared Co films
by thermal evaporation and have studied the effect of
in situ high magnetic field on the films. Gholivand et al.
[19] have synthesized cobalt oxide nanoparticles by che-
mical bath deposition on a Pt wire in order to fabricate a
novel solid phase micro-extraction fiber coating. The syn-
thesis of mono-dispersed cobalt nanocrystals by chemical
method was also reported by Bao et al. [20]. Hongzhen
et al. [21] have also synthesized Co nanoparticles using DC
sputtering for catalytic applications. In this work, we pre-
sent results on the microstructure and optical properties of
cobalt–carbon nanocomposite prepared by co-deposition of
RF-sputtering and RF-PECVD.
The deposition was performed in the presence of acet-
ylene gas, which is the source of carbon matrix that should
& Ali Arman
ali.arman173@gmail.com
1
Department of Physics, Kermanshah Branch, Islamic Azad
University, Kermanshah, Iran
2
Institut des Mate´riaux Jean Rouxel, Universite´ de Nantes,
CNRS, 2 rue de la Houssinie`re, BP 32229,
44322 Nantes Cedex 3, France
3
Ecole nationale polytechnique de Constantine BP 75, A,
Nouvelle ville RP, Constantine, Algeria
4
Physics Department, Faculty of Science, University of
Kurdistan, Sanandaj, Iran
5
Young Researchers and Elite Club, Kermanshah Branch,
Islamic Azad University, Kermanshah, Iran
123
J Mater Sci: Mater Electron (2015) 26:5964–5969
DOI 10.1007/s10854-015-3170-5
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4. host the Co nanoparticles. In this case, an a-C:H layer,
which has interesting properties, such as low coefficient of
friction, chemical neutralization, good insulation, and
compatibility with body tissues [22, 23] could form.
Therefore, there is a need to study the effect of deposition
conditions on the properties of these types of nanocom-
posites. In this work, we report the effect of deposition
pressure on surface morphology, structure and optical ab-
sorbance of the prepared films.
2 Experimental details
An RF-reactor was used to deposit the cobalt nanoparticles
with hydrogenated amorphous carbon, on silicon sub-
strates, using combination of two methods; RF sputtering
and chemical vapor deposition. In this system rotary and
diffusion pumps are used to pump the stainless steel
chamber. The chamber consists of two electrodes with
different areas. The smaller electrode (cobalt electrode) is
connected to the RF source and the other one is connected
to earth via chamber body. The source frequency
(13.56 mHz) is applied to the electrodes in capacitive
configuration. The distance between the two electrodes is
set at 5 cm. The residual pressure before deposition was
10-5
mbar, and depositions were performed without in-
tentional heating.
The chamber pressure after acetylene gas entrance
was set to reach a deposition pressure of
0.021–0.029 mbar. The deposition is performed under
conditions mentioned in Table 1. The power and pres-
sure were chosen so that both cobalt and carbon can be
sputtered. In fact, at small power and increasing pres-
sure, the energy of ions bombardment should decrease
reducing Co sputtering which leads to, more carbon
bed. The schematic of deposition chamber is shown in
Fig. 1.
The non-contact atomic force microscopy (AFM) was
used to obtain the topography of the surface layer and the
average particle size. Also the X-ray diffraction and EDX
were used to determine the crystal structure and composi-
tion of the cobalt/carbon nanocomposite films. The Surface
Plasmon Resonance (SPR) bands of Co nanoparticles
were studied by UltraViolet–Visible spectrophotometry
(UV–Vis).
3 Result and discussion
The AFM image and topographic graph of cobalt–carbon
nanocomposites are shown in Fig. 2. The topographic
graphs of size distribution of nanoparticles along Z axis
(perpendicular to the surface), which represents the distri-
bution of particles heights (not particle size) when they are
stacked together, are shown in Fig. 2(i). It should be no-
ticed that crystal size obtaining from XRD pattern is more
reliable.
The graph maximum gives the average heights hills and
its width represents the heights variance. In this Figure, the
small width indicates uniformity of sort particles and par-
ticle size distribution. According to the topographic graph,
it can be argued that graph of each sample which has less
width, should represent a surface that has more points with
the same height, and so corresponds to have smoother
surface than other samples which is fully consistent with
the surface roughness diagram of the samples (Fig. 3).
The XRD results, shown in Fig. 4, also, demonstrate the
formation of cobalt nanoparticles in these layers. Known
phases for cobalt are FCC and HCP, and the existence of
these phases depend on the size of the formed particles
[24–26].The stable phase for metallic Co crystallites larger
than 40 nm, between 20 and 40 nm, and below 20 nm are
the HCP phase, mixed phase of HCP and FCC, and FCC
phase, respectively. According to the above matters and as
it can be deduced from Fig. 3, the reflections of FCC phase
of Co nanoparticles and its oxide have been formed that
Table 1 Sputtering conditions
Sample (a) (b) (c)
Power (W) 280 280 280
Pressure (mbar) 0.029 0.027 0.021
Fig. 1 The scheme of experimental arrangement
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5. can be observed in the range of near the angles of 42°, 43°
and 50° [26–28]. Furthermore, it can be noticed that the
XRD intensity of the Co phase increases with the initial gas
pressure decreasing, while the cobalt oxide decreases. The
peaks of Co and its oxide are encountered with the increase
and the decrease of growth intensifies.
From the XRD pattern in Fig. 4, also, we can notice that
the intensity and the width of Co diffraction peaks are
changed by decreasing of the working pressure. The above
observations show that both crystallization and grain size
are enhanced by the decrease of working pressure. Which
shows indicates that both crystallization and the green
grain size of Co nanoparticles (crystal plane size) increase
Fig. 2 AFM images of the Co
NPs@a-C:H at different
working pressures, number of
events for topography (i) of
samples a, b and c
Fig. 3 RMS roughness of the samples
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6. may be enhanced by decreasing the chamber pressure. For
better investigation and comparison, the crystal grain sizes
for two peaks (111) and (200) apparent in XRD pattern for
samples of Co nanoparticles were calculated by Scherrer’s
equation (Table 2).
As can be observed, the peak related to cobalt oxide
phase appears more enhanced at higher working pressures
was increased by decreasing pressure, which means that the
formation of cobalt oxide phase can be favored by in-
creasing the working pressure, Depends on particles size.
Therefore, when the particles size decrease, the cobalt
oxide is formed more, due to surface to volume ratio in-
crease. This is cause for more reacted cobalt nanoparticles
with oxygen to form cobalt oxide phase.
The slight shift in peak positions is due in the fact to a
kind of decay and negative stress in the layers because of
the formation of clusters and islands which have an out-
ward force on each other to move towards more stable
states [29].
The optical absorbance curves of the prepared films are
shown in Fig. 5. The broad peak in the range of 315–339 nm
is attributed to Surface Plasmon Resonance (SPR) bands of
Co nanoparticles. The SPR band shifts position is size-de-
pendent for metals and Co nanoparticles. It is known that the
blue shift is related to nanoparticles size decrease, while the
red shift is related to nanoparticles size increase [30]. It can
be seen from the absorbance curves in of Fig. 5 that the SPR
bands positions underwent have blue shift for the films de-
posited at higher working pressure, which indicates that the
nanoparticles sizes decrease with the pressure increase in
accordance with XRD and AFM results.
Figure 6, Shows the energy dispersive X-ray spec-
troscopy (EDX) analysis of sample (a). The EDX shows
the existence of Co, O, and especially C phase which is
belonging to carbon-matrix in the sample. In EDX graph,
In addition to the peaks of the fabricated sample prepared
film, the phases related to the structure of glass the silicon
substrate and gold had been is also detected, due to the
lower film thickness. Preventing screening effect is not
related to the corresponding sample.
Fig. 4 X-ray diffraction profile of Co NPs @ a-C:H at different
pressures
Table 2 The samples’ crystal size
Samples (a) (b) (c)
2h (111) 43.69 43.52 43.58
FWHM (rad) 0.0078 0.0059 0.0047
D (nm) 24.6 32 40.8
2h (200) 50.80 50.74 50.68
FWHM (rad) 0.0098 0.0059 0.0038
D (nm) 22.4 37.3 58
Fig. 5 UV–Visible absorption spectra of the samples
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7. 4 Conclusion
Cobalt nanoparticles, in the FCC phase, and its oxide
were formed by combining sputtering and PECVD at
room temperature within the carbon-matrix and without
carbide contamination. The AFM analysis showed that
surface roughness and particle size were found to in-
crease with gas pressure decrease. The blue shifts of the
SPR bands positions confirm the results of XRD and
AFM analysis for nanoparticles size behavior distribution.
The obtained results show that the formation of cobalt
oxide phase on the Co nanoparticles is pressure-depen-
dent, thus, according to the analyses of samples, it can be
concluded that high working pressures, with a fixed
power are appropriate to grow uniform Co nanoparticles.
That is Co nanoparticles formation is favored at low
working pressure while Co oxide can form at high
pressure.
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