Page 151 - Science at the nanoscale

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June 5, 2009
Chapter Seven
Formation and Self-Assembly
at the Nanoscale
We have briefly discussed the bottom-up approach (Fig. 7.1) for the
preparation of quantum dots and quantum wires in Section 6.3. In
this chapter, let us look at some basic details of this approach. This
will require some fundamental concepts in thermodynamics — i.e.
the study of energy exchanges between physical systems.
SOME BASIC THERMODYNAMIC DEFINITIONS
7.1
Gibbs Energy
7.1.1
In Section 5.1, the Gibbs energy (G) was mentioned briefly when
explaining the concept of surface energy. We will now take a
closer look at its properties with reference to this fundamental
thermodynamic relationship:
(7.1)
∆G = ∆H − T∆S
The notation ∆ denotes a change; thus ∆G is the change in Gibbs ch07
energy, ∆H is the change in enthalpy and ∆S is the change in
entropy.
Equation 7.1 is applicable for a process occurring at temperature
T and constant pressure, e.g. the preparation of quantum dots in
an open vessel. Under constant pressure condition, ∆H is equiva-
lent to the heat supplied to the system. When heat is absorbed by a
process, ∆H is positive and the reaction is known as an endothermic
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