PHYS Ashutosh Rath - Homi Bhabha National Institute

Introduction 3
configuration without direct use of conventional lithography techniques. A number of
self-assembly techniques have been reported for fabricating nanoscale structures. One
type of self-assembly involves the preparation of semiconductor/metal islands, and it
can be carried out by a technique called heteroepitaxy, which involves the placement
or deposition of the material that forms the island on a supporting substance made
of a different material with a closely matched interface between them. Heteroepitaxy
has been widely used for research, as well as for the fabrication of many semiconductor
devices, so it is a well-developed technique. It involves bringing atoms or
molecules to the surface of the substrate where they do one of three things as mentioned
in the following. They are either adsorbed and diffuse about on the surface
until they join or nucleate with another adatom to form an island attach themselves
to or aggregate into an existing island or desorb and thereby leave the surface. Small
islands can continue to grow, migrate to other positions or evaporate. There is a
critical size at which they become stable and no longer experience much evaporation.
Thus, there is an initial nucleation stage when the number of islands increases with
the coverage. This is followed by an aggregation stage when the number of islands
level off and the existing ones grow in size. Finally, there is the coalescence stage
when the main events that take place involve the merger of existing islands with each
other to form larger islands. There have been numerous interesting studies on the
mechanism of relaxation of strain in such epitaxial layers. Strained epitaxial layers
are inherently unstable and have interesting properties. It has been reported that the
dislocation formation and the shape transition are the important processes by which
relaxation of strain occurs [26, 27, 28, 29]. In recent years, it has been recognized
that shape changes such as island formation constitute a major mechanism for strain
relief [26, 28]. Tersoff and Tromp reported that a strain-induced shape transition may
occur [26]. Below a critical size, islands have a compact symmetric shape. For larger
sizes, they adopt a long thin shape that allows better elastic relaxation of the island’s
stress [26].
Metallic nanoparticles are important for both theoretical and practical view points.
They represent one of the most promising classes of nano materials by virtue of
their interesting optoelectronic, thermal, magnetic, and superior catalytic properties
[30, 31, 33, 32], which result in the preparation of new materials for energy storage,
photonics, communications and sensing application [34, 35, 36, 37]. Unlike the bulk