 | Metal-Polymer Nanocomposites |  |
| | Publisher: John Wiley & Sons | Publication: 2004, English | ISBN: 9780471471318 | Pages: 320 |
A nanocluster is a tiny chunk of the bulk measuring a few nanometers with
a finite number of atoms in it. Nano-sized metals with sizes in the range of 1–
50 nm are considered important and are obtainable as sols—a dispersion of a
solid in a liquid. Metal sols possess fascinating colors and have long been used
as dyes and catalysts. That such dyes indeed consist of tiny metal chunks was
established as early as 1857 by Faraday [1, 2]. Modern techniques of synthesis
enable one to obtain sols of metals that can be dried and redissolved like
water colors. The nano-sized clusters display a remarkable tendency to remain
single-crystalline and hence are also called nanocrystals. In addition, nanocrystals
possess a high surface area: A great fraction of atoms in a nanocrystal is
on its surface [3].
An added dimension to research on nanocrystals is their size-dependent
properties. The electronic, magnetic, and optical properties of a nanocrystal
depend on its size [3]. In small nanocrystals, the electronic energy levels are
not continuous as in the bulk but are discrete, due to the confinement of the
electron wavefunction to the physical dimensions of the particles [4]. This phenomenon
is called quantum confinement; therefore, nanocrystals are also known
as quantum dots. In other words, a small nanocrystal could be a very bad conductor,
although nanocrystals are tiny silhouettes of the conducting bulk. Likewise,
a tiny nanocrystal of a ferromagnet can be paramagnetic in nature. In
several respects, small nanocrystals behave like molecules. The nanocrystals
can be discretely charged with electrons with characteristic charging energies.
This means that a nanocrystal carrying an extra electron can exhibit properties
different from those of a neutral species.
The shrinking dimensions of the current microelectronic devices and the
realization that current lithographic processes cannot extend to the nanoworld
[5] have lent tremendous thrust to research aimed at ordering nanocrystals into
functional networks [6–11]. The nanocrystals akin to covalent systems selfassemble
into ordered arrays in one, two, and three dimensions under the right
conditions. Lattices of nanocrystals consist of interacting nanocrystals and may
exhibit novel properties arising out of such interactions. Thus, the ability to
engineer such assemblies extends the reach of current lithographic techniques
and holds promise for a new generation of electronics of the nanoworld [6].
In this context, synthesis and programmed assembly of nanocrystals assume
significance. | |
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