Environmental Engineering Reference
In-Depth Information
8.2.2.3
Nanowires
Nanowires are small conducting or semi-conducting nanoparticles with a single
crystal structure and a typical diameter of a few nanometres and a large aspect
ratio. They are composed of metals such as silver, gold or iron, or semi-conductors
such as silicon, zinc oxide or germanium. Applications include use as inter-connec-
tors for the transport of electrons in nanoelectronic devices.
Most approaches to the fabrication of nanowires are derived from methods cur-
rently used in the semi-conductor industry for the fabrication of microchips. Van
Zant (2000) provides a comprehensive review of microchip fabrication that makes
useful background reading. Typically, they involve the manufacture of a template
followed by the deposition of a vapour to fi ll the template and grow the nanowire.
Deposition processes include Electrochemical Deposition and Chemical Vapour
Deposition (CVD). The template may be formed by various processes including
etching or the use of other nanoparticles, particularly nanotubes.
8.2.2.4
Quantum Dots
Quantum dots, also known as nanocrystals or qdots, are a special class of semi-
conductor crystal. They may be composed of various types of semi-conductor mate-
rial including zinc sulfi de, lead sulfi de, cadmium selenide and indium phosphide.
The number of atoms in a quantum dot, which range from 1000 to 100 000, makes
it neither an extended solid structure nor a single molecular entity leading to novel
electronic, optical, magnetic and catalytic properties.
Quantum dots, exhibit distinct 'quantum size effects'. The light emitted from a
quantum dot can be tuned to the desired wavelength by altering the particle size
through careful control of the growth steps. Various methods can be employed to
make quantum dots, but the most common is the wet chemical colloidal process.
8.2.2.5
Other Nanoparticles
This catch-all category includes a wide range of primarily spherical or aggregated
dendritic forms of nanoparticles. Dendritic forms are where spherical or other
compact forms of primary particles aggregate together to form chain like or branch-
ing structures. Welding fume is the best known example of this type of nanoparticle,
plus other materials such as ultrafi ne carbon black and fumed silica, which are
synthesised in bulk form through fl ame pyrolysis methods. Nanoparticles of this
type may be formed from many materials including metals, oxides, ceramics, semi-
conductors and organic materials. The particles may be composites having, for
example, a metal core with an oxide shell or alloys in which mixtures of metals are
present.
Many of the production processes involve the direct generation of aerosols
through gas phase synthesis, similar to fl ame pyrolysis, but other production pro-
cesses including wet chemical and attrition methods may be used. This group of
particles may be categorised as being less well defi ned in terms of size and shape,
generally larger (although still within what could be considered nanoparticles), and
likely to be produced in larger bulk quantities than other forms of nanoparticles.
Search WWH ::




Custom Search