Page 103 - Science at the nanoscale

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June 9, 2009
Chapter Five
Surfaces at the Nanoscale ch05

Surfaces play an important role in many aspects and applications
of materials. Specifically, the surface of a material object refers to
its outermost layer of atoms or molecules that comes into direct
contact to its surroundings. In general however, surface proper-
ties often involve the subsequent one or few more layers of atoms
beneath due to surface reconstruction or reorganisation.
Fundamentally, surface properties are not altered when materi-
als are reduced to the nanoscale. Nonetheless, surface behaviour
and reactivity become more significant at the nanoscale and thus
warrant special consideration in this chapter. In the last section of
this chapter, we will also introduce microemulsions and surfactants,
which are often encountered in the chemical synthesis of nano-
structures.

5.1 SURFACE ENERGY

5.1.1 Fraction of Surface Atoms

One of the main changes when materials are reduced to the
nanoscale is the tremendous increase in the fraction of atoms that
reside on the surface in comparison to the total number of atoms
in the material. As a simple illustration, we compare two sizes
of gold fcc crystals in cubic form as shown in Fig. 5.1. It can be
estimated quite easily from the packing efficiency (Chapter 4) that
3
there is a total of ∼5.9 × 10 22 closely packed atoms in a 1 cm gold
cube. In a cube of this size, only ∼2 × 10 −6 % of the atoms are
residing on the six facets of the cube. Hence, any slight defect

Science at the Nanoscale: An Introductory Textbook
by Chin Wee Shong, Sow Chorng Haur & Andrew T S Wee
Copyright c
2010 by Pan Stanford Publishing Pte Ltd
www.panstanford.com
978-981-4241-03-8