Invitation to attend: NANO Talk on “Si “Magic Clusters” on Si(111) and 6H-SiC(0001) surfaces”
By: Assoc. Prof. Eng Soon Tok, Deputy Director Surface Science Laboratory, Department of Physics, National University Singapore.
When: September 4, 2013 at 10:00 – 11:30
Where: NANOTEC (Meeting room 406)
Abstract:
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 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)-(7×7) template using a molecular beam epitaxy solid source. We will show that the Si cluster which consists of n=12 Si adatoms 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 this (7×7) surface which is known to undergo a phase transformation from disordered “(1×1)” to (7×7) reconstruction during cooling from high temperature is mediated by the formation of Si these clusters. Interestingly, similar cluster like features are also seen during progressive annealing of 6H-SiC(0001) surface in vacuum; starting from the Si-rich (3×3) phase at 850 oC → (6×6) Si clusters at 1000 oC → Si-poor/carbon- rich (6×6) rings (commonly known as the 63×63)R30°) at 1100 oC, and eventually leading to the formation of graphene at 1200o C.
Finally, apart from Si clusters, we will also examine the dynamics of “Co-Si” cluster diffusion, attachment and detachment behavior during the formation of different 2D cluster arrangements on a Si(111)-(7×7) 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.
