Isolated Cobalt Nanoparticles Prepared on HOPG in Ultrahigh Vacuum Using Thermal Annealing

doi:10.4236/anp.2013.23033

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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

Email: [email protected]

Received March 5, 2013; revised April 5, 2013; accepted April 12, 2013

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

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:

π()(π)RN









 21



(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:





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.

REFERENCES

[1] P. Shen, N. Chi, K.-Y. Chan, et al., “Platinum Nanoparti-

D. LEBEDEV ET AL.

240

cles Spontaneously Formed on HOPG,” Applied Surface

Science, Vol. 172, No. 1-2, 2001, pp. 159-166.

doi:10.1016/S0169-4332(00)00848-5

[2] L. Tamašauskaitė, R. Tarozaitė and A. Vaškelis, “Elec-

trocatalytic Properties of Gold Nanoparticles in Oxidation

Reactions of Reducing Agents,” Chemija, Vol. 17, No. 4,

2006, pp. 13-19.

[3] L.P. Bicelli, B. Bozzini, C. Mele and L. D’Urzo, “A Re-

view of Nanostructural Aspects of Metal Electrodeposi-

tion,” International Journal of Electrochemical Science,

Vol. 3, No. 4, 2008, pp. 356-408.

[4] Z. Tang, S. Liu, S. Dong and E. Wang, “Electrochemical

Synthesis of Ag Nanoparticles on Functional Carbon Sur-

faces,” Journal of Electroanalytical Chemistry, Vol. 502,

No. 1-2, 2001, pp. 146-151.

doi:10.1016/S0022-0728(01)00344-8

[5] R. G. Song, M. Yamaguchi, O. Nishimura, et al., “Inves-

tigation of Metal Nanoparticles Produced by Laser Abla-

tion and Their Catalytic Activity,” Applied Surface Sci-

ence, Vol. 253, No. 6, 2007, pp. 3093-3097.

doi:10.1016/j.apsusc.2006.06.059

[6] B.-X. Chung and C.-P. Liu, “Synthesis of Cobalt Nano-

particles by DC Magnetron Sputtering and the Effects of

Electron Bombardment,” Materials Letters, Vol. 58, No.

9, 2004, pp. 1437-1440. doi:10.1016/j.matlet.2003.06.018

[7] F. W. Campbell and R. G. Compton, “The Use of Nano-

particles in Electroanalysis: An Updated Review,” Ana-

lytical and Bioanalytical Chemistry, Vol. 396, No. 1,

2010, pp. 241-259. doi:10.1007/s00216-009-3063-7

[8] Y. Inaba H. Nakata and D. Inoue, “New High Density

Recording Technology: Energy Assisted Recording Me-

dia,” Fuji Electric Review, Vol. 57, No. 2, 2011, pp. 42-

45.

[9] K. Şendur and W. Challener, “Patterned Medium for Heat

Assisted Magnetic Recording,” Applied Physics Letters,

Vol. 94, No. 3, 2009, Article ID: 032503.

[10] S. Noel, J. Hermann and T. Itina “Investigation of Na-

noparticle Generation during Femtosecond Laser Abla-

tion of Metals,” Applied Surface Science, Vol. 253, No.

15, 2007, pp. 6310-6315.

doi:10.1016/j.apsusc.2007.01.081

[11] S.A. Nepijko, D. Kutnyakhov, et al., “Structure and Mag-

netic Properties of One-Dimensional Chains of Ferro-

magnetic Nanoparticles,” Applied Physics A, Vol. 109,

No. 3, 2012, pp. 699-702.

doi:10.1007/s00339-012-7103-3

[12] F. Fendrych, L. Kraus, O. Chayka, et al., “Preparation of

Nanostructured Magnetic Films by the Plasma Jet Tech-

nique,” Monatshefte Fur Chemie, Vol. 133, No. 6, 2002,

pp. 773-784. doi:10.1007/s007060200049

[13] M. Zinke-Allmang, L. C. Feldman and M. H. Grabow,

“Clustering on Surfaces,” Surface Science Reports, Vol.

16, No. 8, 1992, pp. 377-463.

doi:10.1016/0167-5729(92)90006-W

[14] W. Ostwald, “On the Supposed Isomerism of Red and

Yellow Mercury Oxide and the Surface Tension of Solid

Bodies,” Journal of Physical Chemistry B, Vol. 34, 1900,

p. 495.

[15] I. M. Lifshitz and V. V. Slyozov, “The Kinetics of Pre-

cipitation from Supersaturated Solid Solutions,” Journal

of Physics and Chemistry of Solids, Vol. 19, No. 1-2,

1961, pp. 35-50. doi:10.1016/0022-3697(61)90054-3

[16] C. Wagner, “Theorie der Alterung von Niederschlagen

durch Umlösen (Ostwald-Reifung),” Z. Elektrochem, Vol.

65, No. 7-8, 1961, p. 581-591.

[17] A. A. Bukharaev, N. V. Berdunov, D. V. Ovchinnikov, et

al., “Atomic Force Microscopy for Metrology of Micro-

and Nanostructures,” Russian Microelectronics, Vol. 26,

No. 3, 1997, pp. 137-148.

[18] A. P. Chuklanov, A. A. Bukharaev and S. A. Ziganshina,

“Computer Program for the Grain Analysis of AFM Im-

ages of Nanoparticles Placed on a Rough Surface,” Sur-

face and Interface Analysis, Vol. 38, No. 4, 2006, pp.

679-681. doi:10.1002/sia.2294

[19] I. Beszeda, E. G. Gontier-moya and A. W. Imre, “Surface

Ostwald-Ripening and Evaporation of Gold Beaded Films

on Sapphire,” Applied Physics A, Vol. 81, No. 4, 2005, pp.

673-677. doi:10.1007/s00339-005-3254-9

[20] F. Fillot, Zs. Toke and G. P. Beyer, “Surface Diffusion of

Copper on Tantalum Substrates by Ostwald Ripening,”

Surface Science, Vol. 601, No. 4, 2007, pp. 986-993.

doi:10.1016/j.susc.2006.11.037

[21] S. A. Kukushkin and A. V. Osipov, “New Phase Forma-

tion on Solid Surfaces and Thin Film Condensation,”

Progress in Surface Science, Vol. 151, No. 1, 1996, pp.

1-107. doi:10.1016/0079-6816(96)82931-5

[22] S. A. Kukushkin and A. V. Osipov, “Thin-Film Conden-

sation Processes,” Physics Uspekhi, Vol. 41, No. 10, 1998,

pp. 983-1014.

doi:10.1070/PU1998v041n10ABEH000461