Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 46

2.4.3 Quantum Confi nement

Quantum confi nement is an effect which is observed when the electron hole pair,
generated through photonic excitation of a semiconductor, is spatially limited due
to the small size of the particle. This is an exceptionally complex area and has been
reviewed in depth in the academic literature (Bukowski and Simmons, 2002). For
the purposes of this topic, a simplistic explanation of the effect is considered in
order to gain an understanding of how it is related to nanotechnology.
The effect of semiconduction can be thought of as resulting from the bonding in
a material (Figure 2.12). If two atoms are bonded together there exists a set of
bonding and antibonding orbitals. The electrons remain in the bonding orbital and
the antibonding orbital remains unoccupied. If there are only two atoms present
then the bonding and antibonding orbitals would be termed ' molecular orbitals '
and have discrete energies. However, if a large lattice is considered then the result
of having a whole series of bonds which are all similar but slightly different is a
whole range of bonding and antibonding orbitals. In a large lattice there are so
many of these bonding and antibonding orbitals that they begin to overlap and
form a band, a pseudo-continuum of orbitals. The overall result is a valence band
containing all the electrons and a conduction band which is empty. In a semicon-
ductor the process of conduction occurs when suffi cient energy is applied to the
system in order to promote an electron from the valence band to the conduction
band. The energy difference between the valence band and conduction band is
called the band gap.
In confi ned system it can be considered that the number of atoms and bonds in
the particle is insuffi cient to provide all of the bonding and antibonding orbitals

Molecular
Orbitals

Band
Structure

Conduction Band/
Antibonding Orbitals

Band
Gap

Valence Band/
Bonding Orbitals

Cd 2 S 2

Cd 1 S 1 Cd 3 S 3 Cd ∞ S ∞ Cd 500 S 500

Figure 2.12 The bonding and antibonding orbitals in a theoretical 'molecule' of cadmium
sulfi de (CdS) and the gradual formation of conduction and valence bands as the orbitals
overlap with growth of the lattice. The intermediate state of a nanoparticle with a larger than
bulk band gap is shown on the right.