Stable, cubic phase, PbS nanoparticles were prepared in a polymeric matrix by
exchanging Pb
2þ ions in an ethylene – 15% methacrylic acid copolymer followed
by reaction with H 2 S [91]. The size of the PbS nanoparticles was dependent on the
initial concentration of Pb
2þ ions with diameters ranging from 13 to 125 A ˚ . The
smallest particles (13 A ˚ ) are reported to be molecular in nature and exhibit discrete
absorption bands in their optical spectra. Two theoretical models, which take into
account the effect of nonparabolicity, were proposed in order to explain the observed size-dependent optical shifts for PbS nanocrystallites. The authors reported
that the effective mass approximation fails for PbS nanocrystallites.
Steigerwald et al. prepared capped CdSe, ZnS, ZnS/CdSe and CdSe/ZnS nanocrystallites from inverse micellar solutions [30, 99]. Silylchalcogenide reagents
were added to micro-emulsions containing the appropriate metal ions. The particle
surfaces were subsequently capped; for example with phenyl groups or with other
semiconductor materials such as ZnS. Silylorganochalcogenides react readily with
metal salts or simple metal alkyls to form metal–chalcogenide bonds [100]. Micelle stabilised CdSe nanocrystallites, with Cd
2þ rich surfaces, react similarly with
R[(CH 3 ) 3 Si] 2 Se to give larger CdSe crystallites encapsulated by a layer of organic
ligands (R). These surface passivated crystallites can be isolated as powders, which
are soluble in organic solvents such as pyridine.
77 Se NMR spectra of three size
distributions of organic-capped CdSe were reported, with each giving different
spectra [101], consisting of broad lines corresponding to bulk material along with
additional peaks appearing at higher field and becoming more intense with decreasing particle size.
Several types of nanoparticles prepared from synthesis involving biologically related processes, biomimetic, have been reported [102–104]. For example, using
empty polypeptide cages found in the iron storage protein ferritin; bio-inorganic
nanocomposites of CdS–ferritin can be synthesised [102]. Another approach to
nanocrystallite synthesis in a matrix was developed by Choi and Shea [90, 91], who
report using porous inorganic–organic xerogel (polysilsesquioxanes) to produce
CdS (6 and 9 nm) [90] and chromium particles (1–10 nm) [91]. The chromium
precursor used was a zero-valent arene tricarbonyl chromium complex, introduced
as a component of the xerogel matrix, which after heating under vacuum produced
chromium nanoparticles. By first doping CdS into the starting material two different phases of chromium and CdS are reported to be obtained. Perhaps the most
important use of a biological approach to nanoparticle growth is that taken in the
commercial sector by Nanomagnetics Ltd. They have used the 8 nm cavity of the
iron storage protein ferritin to grow iron oxide for use in magnetic storage devices
and hope to fully commercialise the process.
2.5
The Suitability of Such Methods for Scaling
The vast majority of semiconductor nanoparticles are produced, at present, by
batch methods in research laboratories in processes which essentially depend on
the separation of nucleation and growth. In the semiconductor area high quality
2.5 The Suitability of Such Methods for Scaling 25
exchanging Pb
2þ ions in an ethylene – 15% methacrylic acid copolymer followed
by reaction with H 2 S [91]. The size of the PbS nanoparticles was dependent on the
initial concentration of Pb
2þ ions with diameters ranging from 13 to 125 A ˚ . The
smallest particles (13 A ˚ ) are reported to be molecular in nature and exhibit discrete
absorption bands in their optical spectra. Two theoretical models, which take into
account the effect of nonparabolicity, were proposed in order to explain the observed size-dependent optical shifts for PbS nanocrystallites. The authors reported
that the effective mass approximation fails for PbS nanocrystallites.
Steigerwald et al. prepared capped CdSe, ZnS, ZnS/CdSe and CdSe/ZnS nanocrystallites from inverse micellar solutions [30, 99]. Silylchalcogenide reagents
were added to micro-emulsions containing the appropriate metal ions. The particle
surfaces were subsequently capped; for example with phenyl groups or with other
semiconductor materials such as ZnS. Silylorganochalcogenides react readily with
metal salts or simple metal alkyls to form metal–chalcogenide bonds [100]. Micelle stabilised CdSe nanocrystallites, with Cd
2þ rich surfaces, react similarly with
R[(CH 3 ) 3 Si] 2 Se to give larger CdSe crystallites encapsulated by a layer of organic
ligands (R). These surface passivated crystallites can be isolated as powders, which
are soluble in organic solvents such as pyridine.
77 Se NMR spectra of three size
distributions of organic-capped CdSe were reported, with each giving different
spectra [101], consisting of broad lines corresponding to bulk material along with
additional peaks appearing at higher field and becoming more intense with decreasing particle size.
Several types of nanoparticles prepared from synthesis involving biologically related processes, biomimetic, have been reported [102–104]. For example, using
empty polypeptide cages found in the iron storage protein ferritin; bio-inorganic
nanocomposites of CdS–ferritin can be synthesised [102]. Another approach to
nanocrystallite synthesis in a matrix was developed by Choi and Shea [90, 91], who
report using porous inorganic–organic xerogel (polysilsesquioxanes) to produce
CdS (6 and 9 nm) [90] and chromium particles (1–10 nm) [91]. The chromium
precursor used was a zero-valent arene tricarbonyl chromium complex, introduced
as a component of the xerogel matrix, which after heating under vacuum produced
chromium nanoparticles. By first doping CdS into the starting material two different phases of chromium and CdS are reported to be obtained. Perhaps the most
important use of a biological approach to nanoparticle growth is that taken in the
commercial sector by Nanomagnetics Ltd. They have used the 8 nm cavity of the
iron storage protein ferritin to grow iron oxide for use in magnetic storage devices
and hope to fully commercialise the process.
2.5
The Suitability of Such Methods for Scaling
The vast majority of semiconductor nanoparticles are produced, at present, by
batch methods in research laboratories in processes which essentially depend on
the separation of nucleation and growth. In the semiconductor area high quality
2.5 The Suitability of Such Methods for Scaling 25