Advances in Nanoparticles, 2013, 2, 236-240
http://dx.doi.org/10.4236/anp.2013.23033 Published Online August 2013 (http://www.scirp.org/journal/anp)
Isolated Cobalt Nanoparticles Prepared on HOPG in
Ultrahigh Vacuum Using Thermal Annealing*
Denis Lebedev, Niyaz Nurgazizov, Anton Chuklanov, Anastas Bukharaev
Zavoisky Physical-Technical Institute (ZPhTI), Kazan, Russia
Received March 5, 2013; revised April 5, 2013; accepted April 12, 2013
Copyright © 2013 Denis Lebedev et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Cobalt nanoparticles on the surface of highly oriented pyrolytic graphite have been studied by atomic force microscopy.
Thermal annealing in ultrahigh vacuum was used to change the size of cobalt nanoparticles and their surface distribu-
tion. The effect of two key parameters, annealing time and temperature, on the size and the surface distribution of
nanoparticles has been studied. The dependence of the particle size on these parameters has been obtained. It has been
shown that the main mechanism of the nanoparticle growth is Ostwald ripening.
Keywords: Nanoparticles; Thermal Annealing; Ultrahigh Vacuum; HOPG
1. Introduction
Recently, a great deal of attention was paid to developing
methods of preparing nanoparticles of different metals
with the given size distribution and location on the sub-
strate surface using chemical [1-4] and physical methods
[5,6]. For example, these nanoparticles are applied as
catalysts for the electrochemical reactions [7]. Ferromag-
netic nanoparticles can be used for the information stor-
age devices [8,9].
Condensation methods are often used for the prepara-
tion of nanoparticles. In this case, the particles are formed
on the surface as a result of the deposition of atoms from
the supersaturated metal vapors. Usually the deposition is
performed under high or ultrahigh vacuum conditions.
Different ways of evaporation are used to create the nec-
essary concentration of metal vapors: laser ablation [10],
thermal evaporation [11], arc discharge, and evaporation
in the plasma [12]. However, the use of condensation
methods has some limitations associated with the control
of the metal deposition conditions. In particular, it is rather
difficult to prepare particles with the given shape and the
narrow size distribution histogram. The size range of the
produced particles and their surface distribution may be
significantly expanded by the subsequent thermal anneal-
ing of samples.
The process of formation of nanoparticles on the sur-
face conventionally can be divided into three stages [13].
The first stage is the formation of stable clusters from the
deposited atoms, it may include the nucleation and spi-
nodal decomposition. The next step in the formation of
nanoparticles can be called “the early stages of growth of
nanoparticles.” At this stage, the clusters grow mainly
due to capturing of the atoms that condensed on the sur-
face, and also due to the diffusion of adatoms along the
surface. Adatom concentration reaches the equilibrium
value at the later stage growth of the particle. These proc-
esses include the Ostwald ripening and coalescence. Ost-
wald ripening was first observed in 1900, during the
growth of mercury oxide particles in solution [14]. How-
ever, the strict formulation of a problem of the evolution
of adatoms systems in the stage of Ostwald ripening, is
given only in a series of papers by Lifshitz and Slezov
[15], later this approach was independently developed by
Wagner [16]. This model is called the Lifshitz-Slezov-
Wagner theory (theory LSV).
In this work, thermal annealing in ultrahigh vacuum
was used to form isolated cobalt nanoparticles and to vary
their size and distribution on the highly oriented pyrolytic
graphite (HOPG). It is important to note that the effect of
two key parameters, annealing time and temperature, on
the size and the surface distribution of nanoparticles was
studied.
2. Experiment
*The work was supported in part by the Physical Department of the
Russian Academy of Sciences. Samples under study were HOPG plates, on the surface
C
opyright © 2013 SciRes. ANP

D. LEBEDEV ET AL. 237
of which nanoparticles were formed. The choice of such
a substrate for the formation of particles was due to the
fact that the HOPG structure is studied well, and its sur-
face is atomically smooth terraces. The atomically smooth
surface of the substrate strongly simplifies the identifica-
tion of the particles on the surface and the further mathe-
matical processing of images of nanoparticles obtained
using an atomic force microscope (AFM).
Cobalt nanoparticles on the HOPG surface were formed
and studied on a Multiprobe P ultrahigh vacuum (UHV)
system (Omicron). This system is composed of an elec-
tron-beam evaporator (EFM 3), a scanning probe micro-
scope (SPM) with a scan field of 9 × 9 mm, which can
work in the tunnel and atomic force mode; a thermotable
with the possibility of annealing samples up to the tem-
perature of 727˚C.
A scheme of the sample preparation is shown in Fig-
ure 1. The substrate was a HOPG plate (size, 3 × 8 mm;
thickness, 0.5 mm). The upper HOPG layer was sepa-
rated with an adhesive tape immediately before loading
into the UHV chamber. Then HOPG was annealed at the
temperature of 727˚C for one hour to remove adsorbates
from the surface and to degas the whole sample. All fur-
ther operations with the sample were carried out in vac-
uum of better than 10−8 mbar.
Cobalt was deposited on the HOPG surface by evapo-
rating the Co target (purity, 99.95%) with an electron
beam having the following parameters: current emission
of 40 ÷ 50 mA, accelerating voltage of 800 V, and vac-
uum of 6 × 10−9 mbar. The deposition time was varied in
order to change the amount of cobalt deposited on the
substrate. The deposition time did not exceed 10 min for
all prepared samples.
We used the following technique to control the amount
of material deposited on the HOPG surface. Cobalt was
deposited on the substrate surface for two hours (which is
substantially higher than the deposition time used to cre-
ate the sample) through a mask at constant parameters of
the electron-beam evaporation of the target. The mask
was used to create a clear edge (step) between the region
where cobalt was deposited and the region without cobalt.
Then the mask was removed and the height of the result-
ing step was measured on an AFM which actually showed
the thickness of the deposited cobalt layer. The measured
height of the step was averaged taking into account rough-
nesses on the cobalt surface. The cobalt deposition rate
was calculated on the basis on the film thickness and
deposition time. It was 0.13 nm per minute.
The samples were studied in an analytical chamber
using an SPM operating in the non-contact mode. In this
case, the sample did not contact with the atmosphere and
the size and shape distortions of nanoparticles associated
with their oxidation were excluded. Ultrasharp AFM can-
tilevers NSG 01DLC (NT-MDT) with a whisker grown
on the tip were used in the measurements. The curvature
radius of the whisker apex was 1 - 3 nm that made it pos-
sible to significantly reduce the effect of the convolution
on the resultant image. An example of AFM images of
cobalt nanoparticles obtained with this probe is shown in
Figure 2(a).
It was established that cobalt nanoparticles with sizes
less than 10 nm have poor adhesion to the HOPG surface,
and the AFM probe working in the contact mode shifts
them that makes it impossible to perform the measure-
ments in this mode.
Figure 1. Stages of the formation of metal nanoparticles on
the HOPG. Line outlined the stages that are implemented in
ultrahigh vacuum without contact with the atmosphere.
Figure 2. AFM images of Co nanoparticles on the HOPG surface obtained for the sample (a) without annealing; (b) after 30
and (c) after 200 min of thermal annealing at T = 450˚C.

D. LEBEDEV ET AL.
238
Since the shape of produced nanoparticles may differ
from the spherical one, we used two main parameters for
their characterization: the particle height and its apparent
radius in the sample plane. It is known that the measured
height is the most reliable size parameter for isolated
particles in atomic force microscopy. However, the parti-
cle radius is one of key parameters in theories of nuclea-
tion and growth, therefore its value was important for the
further interpretation of the obtained results. In order to
estimate the error in the determination of the lateral sizes
of the particles due to the convolution effect of the probe
and surface profiles, we used the deconvolution method
described in [17]. In our case, the use of probes with
whiskers gave an error of 5% - 10%. Therefore we used
the values of the apparent radius measured from the ex-
perimental AFM images directly.
The statistical analysis of the AFM images of particles
was carried out using the original computer program elabo-
rated earlier, which makes it possible to calculate the geo-
metric parameters of closely spaced/agglomerated parti-
cles on the surface with large-scale irregularities [18].
This program is used to calculate the radius and height of
the particles. Histograms of the particle height and radius
distributions, as well as graphs of the particle average
height and radius as functions of annealing time and tem-
perature were plotted after processing.
3. Results and Discussion
3.1. Effect of the Annealing Temperature on the
Sizes of Co Nanoparticles
The following experiment was performed to study the
dependence of the size of nanoparticles and their surface
distribution on the thermal annealing temperature. The
cobalt film was deposited for 3 min on the prepared
HOPG surface. The amount of the material was equiva-
lent to a continuous cobalt film with the thickness of 0.4
nm. Thermal annealing was performed consequently at
different temperatures (550˚C, 650˚C, and 750˚C).
The interval of temperatures in this case was chosen
by analysis of literary data [19,20]. In order to eliminate
the error associated with the time of heating of the sam-
ple, the annealing time was one hour in all cases. The
sample surface was studied on the AFM without contact
with the atmosphere after each thermal annealing.
According to the AFM images (Figure 3), the average
size of nanoparticles increases and their density decreases
with increasing annealing temperature. On the basis of
the statistical analysis results for the AFM images (Fig-
ure 4), the average height of the particles increases from
3.3 to 5.4 nm with the mean radius varying from 7.2 to
10.4 nm with increasing annealing temperature. The den-
sity of the particles decreases from 21 × 104 to 6 × 104
mkm −2.
3.2. Effect of the Annealing Time on the Sizes of
Co Nanoparticles
The following experiment was performed to study the
effect of the annealing time on the sizes and the surface
distribution of nanoparticles. Using the technique de-
scribed above, cobalt was deposited on the HOPG sur-
face for 30 s. The amount of the deposited material was
equivalent to a continuous cobalt film with the thickness
of 0.1 nm. According to the AFM results, nanoparticles
were formed on the HOPG surface (Figure 2) with an
average height of 1.4 nm and an average radius of 5.3 nm
(Figures 5(b) and (c)). The density of the nanoparticles
was 54 × 104 mkm−2 (Figure 5(d)).
Then the sample was consecutively annealed at the
temperature of 470˚C during 15, 30, 45, 75, 135, 200,
and 500 min. The heating of the sample up to 470˚C took
about 1 - 2 minutes therefore the minimum interval of
annealing time was chosen to be 15 minutes. The AFM
measurement of the sample was performed after each
thermal annealing. Figures 2(b) and (c) show typical
AFM images of the sample obtained after 30 and 200
min of annealing, respectively. Histograms of the particle
size (particle height and radius) were plotted for all im-
ages. Figure 5(a) shows histograms for the images shown
Figure 3. AFM images of Co nanoparticles on the HOPG surface obtained after thermal annealing at different temperatures:
(a) 550˚C; (b) 650˚C; (c) 750˚C.

D. LEBEDEV ET AL. 239
Figure 4. (a) Histograms of the height distribution of Co
particles for different annealing temperatures, (b)-(d) de-
pendences of the height, diameter and density of the par-
ticles on the annealing temperature, respectively.
Figure 5. (a) Histograms of the height distribution of Co
particles for different annealing times; annealing time de-
pendences of: (b) the average particle height; (c) the aver-
age particle radius (solid line shows the result of the ap-
proximation by Equation (2)); (d) the density of particles
per 104 mkm−2.
in Figure 2. The dependence of the average sizes of the
cobalt particle on the annealing time is shown in Figures
4(b) and (с).
The particle radius increases monotonically. The parti-
cle height increases during the first hour of annealing,
and then this parameter remains constant for three-four
hours. After six hours, the particle height gradually de-
creases. The particle density decreases with time. All these
facts make it possible to assume that the atoms diffuse
from the surface of small particles to that of larger parti-
cles. As a result, the number of small particles decreases,
and the size of the large particles increases.
This mechanism of the growth of nanoparticles corre-
sponds to Ostwald ripening. It was theoretically described
by Lifshitz and Slezov in [15]. It is known [21,22] that
the following conditions are necessary for Ostwald rip-
ening to occur:
1
2
π()(π)RN 



 21
R

(1)
where R is the average particle radius, λ is the mean free
path of atoms on the substrate, N is two-dimensional par-
ticle density. According to our estimates, this inequality
is satisfied for our samples. This makes it possible to use
the theory of Ostwald ripening of the nanoparticles [21]
for describing our results.
In the theoretical model [21], the average particle ra-
dius depends on the annealing time as follows:


1
0
p
p
RtR At (2)
where R is the average particle radius, R0 is the average
radius at the initial time, A is the constant determined by
the mechanism of the mass transfer at the current tem-
perature, p is the coefficient depending on the shape of
the particles and the mechanism of the mass transfer and
its value is 2, 3 or 4.
In the case of the three-dimensional growth of nanopar-
ticles on the two-dimensional surface, the coefficient p =
4 as a result of the redistribution of the material between
particles [21]. After approximating the experimental time
dependence of the mean particle radius (R(t)) (Figure
5(c)) by Equation (2), the following parameter values
were obtained: А = 0.251 nm4/s; R0 = 6.843 s. It should
be noted that the calculated R0 value is in rather good
agreement with the experimental value (R0exp = 5.3 ± 2.4
nm).
Thus, the nanoparticles of the given size can be pro-
duced by selecting optimum annealing time values using
Equation (2) and the A value at the current temperature.
4. Conclusion
The evolution of the size and distribution of Co nanopar-
ticles on the HOPG surface in ultrahigh vacuum depend-
ing on the annealing time and temperature was studied
using AFM. It was shown that the size of nanoparticles
and their surface distribution can be controlled by vary-
ing these parameters. The value of the parameter, which
characterizes the mass transfer, was found assuming that
the main mechanism of the particle size variation is Ost-
wald ripening. The parameter value makes it possible to
predict the average size of cobalt nanoparticles formed
on the HOPG surface as a function of the initial particle
size, annealing time and temperature.
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