4.2.3
Shape Control
Since the properties of nanocrystals follow from the confinement of the electrons
to the physical dimensions of the nanocrystals, it would be interesting to vary the
shape of the nanocrystals and study the effect of confinement of electrons in such
artificial shapes [77]. For example, it is predicted that light emitted from a nanorod
would be linearly polarized along the growth-axis [23]. Such predictions have led to
the revival of interest in synthetic strategies yielding non-spherical nanocrystals.
Conventional methods such as those due to Turkevich [36] yield, in addition to
spherical particles, a mixture of shapes: triangular, teardrop etc., which was then
thought of as undesirable. Today, smarter synthetic schemes have been designed
which selectively yield nanocrystals in the form of rods, elongated spheres, cubes
and hexagons. CdSe nanocrystals in the form of rods, arrows, teardrops and tetrapods have been obtained by careful control of thermolysis conditions such as
ratio of surfactants and injection volumes [23, 78]. In Figure 4.5 are shown, TEM
micrographs of soluble CdSe nanorods of various aspect ratios. Triangular CdS
nanocrystals have been obtained by inverse micelle methods [79]. Large tetrahedral
Si nanocrystals as exclusive products have been obtained by careful control of the
reducing conditions [80]. TEM images of triangular CdS and tetrahedral Si nanoFig. 4.4. Scanning tunneling microscopy
image of polymer-coated Pd 561 nanocrystals.
The nanocrystals are seen as fluffy balls
against the plane background of the graphite
substrate. The inset shows a high-resolution
electron micrograph (HRTEM) of an individual
nanocrystal. We see the characteristic 11 [111]
fringes in the icosahedral shape measuring
2.5 nm. The diameter estimated from STM is
@3.4 nm, the difference being due to the
ligand shell.
4.2 Synthetic Strategies 57
Shape Control
Since the properties of nanocrystals follow from the confinement of the electrons
to the physical dimensions of the nanocrystals, it would be interesting to vary the
shape of the nanocrystals and study the effect of confinement of electrons in such
artificial shapes [77]. For example, it is predicted that light emitted from a nanorod
would be linearly polarized along the growth-axis [23]. Such predictions have led to
the revival of interest in synthetic strategies yielding non-spherical nanocrystals.
Conventional methods such as those due to Turkevich [36] yield, in addition to
spherical particles, a mixture of shapes: triangular, teardrop etc., which was then
thought of as undesirable. Today, smarter synthetic schemes have been designed
which selectively yield nanocrystals in the form of rods, elongated spheres, cubes
and hexagons. CdSe nanocrystals in the form of rods, arrows, teardrops and tetrapods have been obtained by careful control of thermolysis conditions such as
ratio of surfactants and injection volumes [23, 78]. In Figure 4.5 are shown, TEM
micrographs of soluble CdSe nanorods of various aspect ratios. Triangular CdS
nanocrystals have been obtained by inverse micelle methods [79]. Large tetrahedral
Si nanocrystals as exclusive products have been obtained by careful control of the
reducing conditions [80]. TEM images of triangular CdS and tetrahedral Si nanoFig. 4.4. Scanning tunneling microscopy
image of polymer-coated Pd 561 nanocrystals.
The nanocrystals are seen as fluffy balls
against the plane background of the graphite
substrate. The inset shows a high-resolution
electron micrograph (HRTEM) of an individual
nanocrystal. We see the characteristic 11 [111]
fringes in the icosahedral shape measuring
2.5 nm. The diameter estimated from STM is
@3.4 nm, the difference being due to the
ligand shell.
4.2 Synthetic Strategies 57