Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 47

for the complete band structure associated with the infi nite lattice. The result of
this loss of orbitals is that the particle now exhibits a band gap which is larger than
that for the bulk material. In the case where the infi nite approximation breaks
down, the band gap of the semiconductor becomes dependent on the particle size.
The overall effect of this is that the nanoparticle may exhibit electronic properties
which are different from those of the bulk material from which it was prepared.
Often materials which exhibit this sort of confi nement are termed quantum dots.
Much of the interest in quantum dots lies in their ability to effi ciently harness
light. It is possible to excite an electron from the valence band into the conduction
band by shining light with an energy greater than that of the bandgap of the particle.
This results in the promotion of an electron from the valence band to the conduc-
tion band and the formation of a positively charged hole in the valence band. These
so called excitons rapidly migrate to the surface of the particle and can be used to
either generate electricity by separating the pair or generate light by allowing them
to recombine; so-called photoluminescence. In a nanoparticle, because the electron
hole pairs are confi ned within the nanoparticle, there is a greatly increased prob-
ability that the recombination of the hole and electron will occur as opposed to
some other relaxation process. This means that the quantum effi ciency, or the frac-
tion of light emitted per excitation photon, will be high. Quantum dots have been
shown to have excellent luminescence properties and have narrower emission
spectra compared to organic fl uorophors. For this reason they are becoming impor-
tant in fl uorescence labelling of cells. Probably the most common examples are
those based on cadmium selenide. These materials generally need to be coated with
a layer of a second, large band gap semiconductor, in order to ensure that the
electron and hole are tightly confi ned and therefore only relax via emissive recom-
bination. The synthesis and some of the properties of these materials later is dis-
cussed later. It should be noted that whilst ideally the electron and hole will behave
in a manner expected, there are several other possibilities that may occur depend-
ing on the exact structure of the nanoparticle, the nature of the surface species and
the medium in which it has been placed. These issues, where relevant, are discussed
in greater detail in later chapters.

2.4.4 Mechanical Performance

Carbon nanotubes particularly have received interest as materials with exception-
ally high mechanical performance. The measurement of the mechanical properties
of nanowires is extremely diffi cult to conduct and this demonstrates why there is
a broad range of values for the mechanical properties of carbon nanotubes reported
in the literature (Table 2.3). Some of the fi rst measurements were conducted on
multi-walled carbon nanotubes (see below). It was found that the breaking strain
varied from 1.4 to 2.9 GPa and the Youngs modulus varied from 18 to 68 GPa.
However, these values do not take into account the hollow nature of the fi bres. If
this is included in the calculation, the values rise from 11 to 63 GPa for strength
and 270-950 GPa for modulus. Compared to conventional materials and elements
this is an impressive set of values even if the lower fi gures are considered. (Table
2.3). It therefore seems advantageous to prepare composites from such materials,