RESEARCH PAPER

Temperature-dependent morphological evolutionof clustered gold surface

Mukesh Kumar • Govind

Received: 21 September 2011 / Accepted: 28 May 2012

� Springer Science+Business Media B.V. 2012

Abstract The present pragmatic deals with the

surface morphology and the temperature induced

modifications of gold surface. The gold surface

consists of three dimensional (3D) large nanoclusters

and the shape of these nanoclusters was identified as

cap like structure with approximately circular periph-

ery. The effect of temperature on the gold surface has

been characterized by Scanning Tunnelling Micros-

copy technique. Annealing the gold surface at 473 K

induce inter-diffusion of the 3D-nanoclusters, while

the formation of nanoscale step and terrace morphol-

ogy near the cluster boundary has been detected at

573 K. This study also reveals that the clusters size

and roughness of gold surface varies differently in

different range of annealing temperature.

Keywords Gold surface � Nanoclusters � STM

Introduction

Many physical, chemical and biological phenomena

are manifestations of self organization of matter, such

as crystal growth or formation of galaxy of clusters or

nanostructures (Snijders et al. 2007; Govind et al.

2009, 2010). The production of clusters is an exciting

and growing area enabling investigations of the

physics and chemistry of truly nanometre scale

systems and the melting of gold cluster is also one of

the essential properties of nanoparticles. Owing to their

novel electronic properties arising from metal nanocl-

usters are now considered as promising candidates for

basic block units for future nanoscale functional

materials and nanoelectronic devices. Gold, with its

peculiar property of non reactive and stable nature and

large number of industrial applications such as bio-

medical, catalysis, electronics, fuel cells and nano-

technology applications (Corti et al. 2002) put forward

this material to used as a potential template for further

growth. Experimental and theoretical studies revealed

that the size of the clusters is governed by the growth

kinetics of the metal on the respective substrate (Hovel

and Barke 2006) and most of the properties of

nanostructures are size dependent (De Heer 1993).

However, the size and size-dependent properties of the

clusters can also be control by thermal annealing

process. The study of these clusters morphology can be

probed using scanning tunnelling microscopy (STM)

technique. The STM can directly image clusters on

weakly interacting substrates and therefore quasi-free

clusters can be investigated. In the present study, we

studied the surface morphology of the thermally

evaporated gold film on the substrate like mica/glass

and the effect of annealing temperature and conse-

quently explained the thermally driven phenomena on

the gold surface.

M. Kumar � Govind (&)

Physics of Energy Harvesting Division, National Physical

Laboratory (CSIR), Dr. K.S. Krishnan Road, New Delhi

110012, India

e-mail: govindnpl@gmail.com

123

J Nanopart Res (2012) 14:963

DOI 10.1007/s11051-012-0963-9

Experimental

The gold film has been grown at room temperature (RT)

in Hind Hi-vac thermal evaporation unit on the substrate

glass/mica with a deposition rate of 3–5 A/s and vacuum

of 1.1–1.2 9 10-5 torr (*1.5–1.6 9 10-3 Pa). To

achieve preliminary deposited interfacial gold surface

on glass substrate, the glass slide was cut into desired

area and glued on the gold/mica system using epoxy

glue. Then, the gold surface was peeled off from mica to

the glass substrate. The annealing of thermally evapo-

rated gold film was done with furnace annealing at

different temperature by putting the sample in the

furnace at room temperature (RT). The temperature was

increased from RT (300 K) to desired temperature at

moderate annealing ramp rate of 3.2 K/min. The sample

was hold at desired temperature for 300 s followed by

cooling to RT in atmospheric conditions before STM

measurements.

The morphology of the gold surface with and

without annealing was characterized by means of the

NanoRev STM by Quazar Tech. The sample was

mounted on a sample plate in vertical position and

screwed it into the sample holder of shroud (which is a

protective jacket for the sensitive piezoelectric assem-

bly housed inside it). The STM tip was prepared by

cutting (diagonal cutters) a tungsten wire. The STM

images were collected with a bias voltage of -80 to

-100 mV and a tunnelling current of 150–200 pA

using topographic constant current mode.

Result and discussion

Figure 1a shows the STM image of thermally evap-

orated gold film on a glass substrate where the surface

morphology of gold surface consists of large number

of nanoscale 3D-clusters. The shape of these gold

nanoclusters has been understood by assuming iso-

lated cluster model where the calculations of surface

energies at interfaces suggest the possible shape of the

cluster as a cap like structure with approximately

circular periphery (Gupta et al. 2009). The size of

these nanoscale 3D-clusters (Fig. 1a) estimated by

calculating the average cluster size \ l [ which is

defined as

Fig. 1 a STM images of

thermally evaporated goldsurface (scan

size = 637.4 nm,

I = 200 pA, and

V = -85 mV). b Process of

mechanical peel off method.

c STM images of peeled

interfacial gold surface

(scan size = 318.7 nm,

I = 160 pA, and

V = -100 mV). (Color

figure online)

Page 2 of 6 J Nanopart Res (2012) 14:963

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\l [ ¼ 1=n1=2 ð1Þ

where n is the total number of clusters per unit area

(Ermanoski et al. 2005). The average size of the cluster

\ l [ is found to be *110 nm which indicate that

these large 3D-nanoclusters may not be originated

directly from the gas phase but resulted due to the

agglomeration of the small nanoclusters into the large

3D-nanoclusters (Barnes et al. 2000). To understand

the initial growth morphology of deposited gold

clusters, we have employed peel off method (Fig. 1b),

where flexible and flat mica substrate has been used.

The mica substrate provides uniform interfacial sur-

face without any loss of surface morphology during

peel off process. The STM images (Fig. 1c) revealed

that the mechanical peeled interfacial gold surface

contains small nanoclusters of the size *25 nm with

higher cluster density. These small nanoclusters are

supposed to grow into bigger clusters due to agglom-

eration of the smaller clusters on further deposition.

The effect of annealing temperature on the mor-

phology of the gold surface with 3D-nanocluster was

analyzed by furnace annealing within the temperature

range RT to 573 K, far below the bulk melting points

temperature of gold (1337 K). It should also be noted

that the melting temperature of free standing 249-atom

Au cluster is about 650 K and the diffusion of gold

nanoclusters on gold surface is very low (Jensen et al.

2004). Furnace annealing provides more control for

thermal conditions than flame annealing and the

heating rate is constant in the furnace annealing. At

each experimental step the sample was hold to the

desired annealing temperature for 300 s as described in

the experimental section.

For the moderate annealing ramp rate, the annealing

history (annealing temperature and annealing time)

seems to be more important for the final shape of

the clusters rather the annealing ramp rate. Santos et al.

(2009) have studied the case of slow and rapid

annealing, correspond to furnace and flame annealing

respectively, on the gold surface and reported that the

crystallization of gold strongly depends on the temper-

ature even if the sample is rapidly annealed, while the

cooling rate (after annealing) also affect the cluster

morphology. Cooling the sample causes the formation

of densely packed clusters. A slow cooling produces an

ordered distribution of cluster walls, while quenching

the surface produces disordered arrangements of cluster

walls. Moreover, Ferralis et al. (2007) corroborated that

the cooling rate is responsible for the formation of

different ordered or disordered phases within the

clusters.

Figure 2a shows the surface morphology at RT

while Fig. 2b demonstrates the cluster morphology of

the surface annealed at 373 K and scanned at RT. The

clusters at RT have the sharp edges and kinks, whereas

at 373 K the kink and the edges of the clusters roughen

out. Since the energy required to mobile or remove an

atom from the surface depends on the number of

formed and broken bonds to neighbouring atoms, the

kink atoms have less number of formed bonds than the

edge atoms, terrace atoms and the bulk atoms and

require low energy to mobile or evaporate. Thus, these

atoms are the first, which subjected to mobile or

evaporate at initial stage of annealing and then the

edge atoms are the next that result in the roughening of

edges of the clusters. The surface roughness (Rz) was

calculated from STM image data to *3.3 nm which is

less than the roughness estimated (*4.2 nm) in the

case of gold surface at RT, where Rz defines as root

mean square of z coordinate values of image points

after subtraction of the slope of xy scanning plane

Rz ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

z2D E

� zh i2� �

r� �

: The average cluster size

was found to be *120 nm. The reduction in Rz is

obvious as the clusters are broaden out with reduction

in their height and occupying the nearby empty space

on account of thermal effects.

On further annealing the surface at 473 K, the inter-

diffusion of clusters across the cluster boundary was

observed (Fig. 2c) in addition to the effects like cluster

broadening and shape modification perceived at

373 K. The average cluster size and surface roughness

in this case was found to be *150 nm and 1.9 nm,

respectively. Since the atoms are trapped within the

cluster at low temperature, therefore they experience

the potential barrier at the boundaries which prevent

them to diffuse outside the cluster. During annealing at

high temperature, the high mobility of Au atoms

induces a delocalization of the atoms in excess which

results in the deterioration of cluster boundaries. At

particular high temperature, these thermally induced

mobile atoms gain sufficient energy to overcome the

potential barrier at the cluster boundary and diffuse

into each other. For the gold nanoclusters, the temper-

ature for inter-diffusions was found to be near 473 K.

Since, the gold clusters (at RT) have cap like structure

J Nanopart Res (2012) 14:963 Page 3 of 6

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with approximate circular periphery, thus the inter-

diffusion process of the two clusters touching each

other can be represented as shown in Fig. 2d. Here, the

two spherical cap like nanoclusters have initial radius

R and R/d (d has a small fraction of numeric value that

relate the radius of both sphere), and the local radius h

(z) (has unit of length) of the figure depends on the

position z along the axis of symmetry. Eggers (1998)

reported the asymptotic description of the coalescence

process of two touching metallic spheres, where he

used conservation of mass which demands that

o

oth2� �

þ 2o

ozJhð Þ ¼ 0 ð2Þ

where J(z) (has the unit of rate of mass flow per unit

area) is the projection of the surface mass flux onto the

axis. This equation can be solved for J(z), that gives the

forth order diffusion constant (A) which has the

dimension of (length)4 per unit times and a typical

time scale is given as sR = R4/A. Thus, it can provide

the information about the time required for coales-

cence of metallic clusters through surface diffusion on

heating with considering the scaling and simulation

theory presented by Eggers.

Figure 3a shows the STM image of gold surface

annealed at 573 K, where the clusters were in 2D

forms (discuss later) with the distinct nanoscale steps

and terraces sites (Fig. 3a, b) near the cluster boundary

besides the other annealing effects same as discussed

above. Line scan profile (Fig. 3c) provides the step

height value 0.5–0.8 nm and the terrace width value

3–6 nm, suggested that these nanoscale steps are in

2–3 monolayer in height (the diameter of gold atom

*0.3 nm). The average cluster size in case of 573 K

annealed gold surface was found to be *180 nm and

the surface roughness was *1.7 nm.

At high annealing temperature the surface atoms of

the clusters gain sufficient thermal energy and hence the

enhancement in the mobility drives them to rearrange

and form nanoscale steps and terraces. The evolution of

nanoscale steps and terraces morphology is exhibited by

many semiconductors surfaces such as silicon (Baski

et al. 1997) and considered to be a minimum free energy

configuration. A number of studies also suggest the

greater free energy reduction at elevated temperature by

surface reconstruction, which indicates that melting

behaviour of the cluster is closely associated with

surface reconstruction which is driven by the vibrational

and configurational entropy of the cluster (Noya and

Doye 2006; Doye and Calvo 2002). Thus, the surface

free energy supports the formation of nanoscale steps

and terraces morphology near the cluster boundary.

Fig. 2 STM images of goldsurface (with scan size

255 nm, I = 160 pA, and

V = -100 mV) (a) RT,

(b) annealed at 373 K,

(c) annealed at 473 K,

(d) schematic diagram of

inter-diffusion process.

(Color figure online)

Page 4 of 6 J Nanopart Res (2012) 14:963

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The effect of annealing temperature on the size of

nanoclusters in terms of growth rate curve is plotted in

Fig. 4a. Annealing the gold film below 473 K, the

cluster growth rate (G(T) = dCs/dT) increase as say

approximately parabolic in nature, where dCs is the

change in cluster size. While for further annealing at

573 K, the G(T) remains constant that means that the

cluster size depends linearly on the annealing

temperature. For higher temperatures ([573 K) the

surface of gold film became planner without the

clusters.

Figure 4b shows the dependence of surface rough-

ness (Rz) on the annealing temperature. The value of

Rz decrease rapidly up to 473 K annealing temperature

beyond it gets almost a constant value. Comparison

between G(T) and Rz indicate that the cluster

Fig. 3 a STM images of gold surface (with scan size 212.5 nm, I = 160 pA, and V = -100 mV) annealed at 573 K, b zoomed STM

images (scan size = 110.2 nm), and (c) line scan profile of step and terraces. (Color figure online)

Fig. 4 (a) Plot of change in

clusters size versus

annealing temperature, and

(b) surface roughness versus

annealing temperature

J Nanopart Res (2012) 14:963 Page 5 of 6

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broadening and cluster height reduction are both

dominant up to 473 K whereas the change in cluster

height is negligible for further annealing at 573 K,

where the cluster size change linearly. In this temper-

ature range, it seems that clusters were in 2-D forms

that expanded and enhanced thermodynamic rear-

rangements of the atomic processes (such as the

formation of nanoscale steps and terraces near cluster

boundaries).

Conclusion

In conclusion, the observed cap like 3D nanoclusters

of the gold film exhibit different surface morphology

at different annealing temperature. These 3D nanocl-

usters inter-diffuse into each other at around 473 K

which is far below the bulk melting temperature

(1337 K) of Gold. Near 573 K, nanoscale steps and

terraces sites are formed where the clusters have 2D

form. The cluster size and surface roughness represent

different dependence relation with different annealing

temperature.

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