Fundamentals of Hierarchical Nanostructures - Hierarchical Nanostructures for Energy Devices - page 8

become nanometre sized, the percentage of surface atoms cannot be ignored
anymore and the nanomaterial system starts to show characteristics of both
surface atoms and bulk atoms. Depending on the size, the ratio between the
surface atoms and bulk atoms and the resultant characteristics of the
nanomaterials also show size-dependent properties. As the nanomaterial
becomes

d n 3 r 4 n g | 7

smaller, the system shows more surface-atom-dominated

characteristics with huge surface energy.
In nanomaterial systems, we need to decide when the surface atom
becomes dominant and size-dependent phenomena start to show. However,
this is not a simple problem because the length reference size scale to decide
whether a system is small enough to be called a nanosystem depends
on which characteristics you are interested in (e.g. optical, electrical,
mechanical, thermal, chemical etc.) and material type.
In this chapter, we will explore the unique characteristics of nano-
materials and the simple physics behind them. These characteristics include
thermal, electrical, phonon transport, mechanical, optical and magnetic
properties.

2.2 Unique Characteristics of Nanomaterials
In light of the down-sizing trend in microelectronics, nanomaterials have
received tremendous interest from various fields due to their unique
characteristics. 1-8 One may find various examples of miniaturization in-
cluding magnetic and optical storage components with critical dimensions
as small as tens of nanometres, 9 size-dependent excitation or emission, 10-13
quantized (or ballistic) conductance, 14,15 Coulomb blockade (or single-
electron tunneling, SET), 16,17 and metal-insulator transition. 18 It is generally
accepted that quantum confinement of electrons by the potential wells of
nanometre-sized structures may provide one of the most powerful (and
versatile) means to control the electrical, optical, magnetic, and thermo-
electric properties of solid-state functional materials. 19 Additionally, some
remarkable specific properties are related to other origins: for example,
(i) large fraction of surface atoms, (ii) large surface energy, (iii) spatial
confinement, and (iv) reduced imperfections. 20 The following are a few ex-
amples suggested by G. Cao: 20

.

(1) Nanomaterials may have a significantly lower melting point or phase
transition temperature and appreciably reduced lattice constants, due
to a huge fraction of surface atoms in the total amount of atoms.
(2) Mechanical properties of nanomaterials may reach the theoretical
strength, which is one or two orders of magnitude higher than that of
single crystals in the bulk form. The enhancement in mechanical
strength is simply due to the reduced probability of defects.
(3) Optical properties of nanomaterials can be significantly different
from bulk crystals. For example, the optical absorption peak of a
semiconductor nanoparticle shifts to a short wavelength due to an