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  Evolution of Si and Si-Co Clusters on Si(111) Surface



Time: 1:20pm, October 18 (Thursday)

Title: Evolution of Si and Si-Co Clusters on Si(111) Surface

Speaker: Prof. Eng Soon TOK
   (Department of Physics, National University of Singapore)

Place: Science Building III 1F SC157


The self assembly of well defined clusters is an alternative route to creating mono-dispersed
nano-sized structures on surfaces of materials. The unique stability of these structures
allows us to harness them as potential building blocks for nano-scopic devices or as templates
for nano-fabrication. This cluster characteristic is attributed to the collection of a "magic"
number of adatoms which is found in metallic (e.g. Ag/Ag(100) and Pt/Pt(110)), semiconducting
(e.g. Si/Si(111)) as well as mixed metal/semi-conducting materials
(e.g. In/Si(001) and Ga, In, Ag, Mn or Pb/Si(111)).

In this talk, we examine the formation and evolution of Si magic clusters on the Si(111) surface
in terms of its origin, formation, interactions and dynamic behavior on the surface.
We will demonstrate how to grow mono-disperse Si magic clusters of size ~ 13.5±0.5Å which also
exhibit localized spatial ordering at the same time. This is achieved from depositing Si adatoms
on a Si(111)-(7x7) template using a molecular beam epitaxy solid source.
We will show that the Si magic cluster consists of n=12 Si adatoms, and occurs via a step-wise
assembly of building blocks comprising of Si tetra-clusters instead of Si adatoms. By studying
the mechanism and energetics leading to the formation of magic clusters, we address the issues
of control over cluster fabrication. In addition, we will also show that the surface which is
known to undergo a phase transformation from disordered (1x1) to (7x7) reconstruction during
cooling from high temperature is mediated by the formation of Si magic clusters.

Finally, apart from Si clusters, we will also be discussing the dynamics of "Co-Si" magic cluster
diffusion and attachment/detachment behavior during the formation of different 2D cluster
arrangements on a Si(111)-(7x7) surface at elevated temperatures. We will introduce the idea of
a configuration dependent critical 2D nucleus which is directly responsible for the self alignment
and assembly of magic clusters.



Update: 2012/10/12
Contacts: Li-Chun Chen

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