posits of ZnS, ZnSe, CdS and CdSe, with the deposits consisting of poorly formed
hexagonal phase micro-crystalline material [77–79]. The addition, into the gas
phase, of small amounts of pyridine greatly improved the crystallinity exhibited by
the particles, while the addition of larger quantities of pyridine retained the improved crystal quality and also led to a decrease in particle size to the range 10–100
nm. Moreover, as the gas phase concentration of pyridine increased particle size
decreased. In the case of CdSe a quantization effect was observed by a change in
color of the deposit, when Me 2 Cd and H 2 Se are mixed in the gas phase in the absence of pyridine the deposit is black, when a high concentration of pyridine is
present the deposit is a yellowish-red color. For ZnSe when using hydrogen as the
carrier gas, instead of helium or argon, with a high gas-phase concentration of
pyridine, a relatively narrow particle size distribution (1–5 nm) could be obtained.
2.4.5
Synthesis in a Structured Medium
The use of a matrix to define the reaction space is an intrinsically attractive
approach to the preparation of large amounts of material which could be deposited
from solution or from the vapor phase. A number of matrices have been used
including: zeolites [80], layered solids [81], molecular sieves [82–84], micelles/
microemulsions [85–89], gels [90–92], polymers [93–97] and glasses [98]. The
matrix provides a mesoscopic reaction chamber in which the crystal can only grow
to a certain size.
The properties of the nanocrystallites are determined, not only by the confinements of the host material, but also by the properties of the system, which can include a range of factors including the internal/external surface properties e.g. of
the zeolite or the lability of a micelle. The particle size is controlled by the system
chosen e.g. in zeolites the nanocrystallite diameter is limited by the pore size of the
zeolite (typically smaller than 2 nm).
Wang and Herron [80] have studied the optical properties of both CdS and PbS
clusters encapsulated in zeolites. The nanocrystallites were prepared in two different zeolites; modernite (unidirectional channels of 7 A ˚ diameter) and zeolite Y
(13 A ˚ diameter channels with cages of tetrahedral symmetry interconnected by 8 A ˚
windows, and 5 A ˚ cages interconnected by 3 A ˚ windows). For the preparation of
CdS in zeolite Y, the sodium cations in the zeolite were first ion-exchanged with
cadmium cations, by treatment with aqueous Cd(NO 3 ) 2 at pH 5. This was followed
by passing hydrogen sulfide (H 2 S) gas over the sample [80]. Depending on the
loading level of cadmium ions within the zeolite, different sizes of CdS clusters
could be obtained. At low loading levels (1:1 metal/sulfide) CdS clusters with an
average size less than 13 A ˚ were obtained. These gave an absorption peak at around
280 nm in their optical spectra. When an excess of cadmium was used the individual clusters aggregated into an extended structure, modulated by the internal cavities of the zeolite. These produced optical spectra showing an excitonic
shoulder near 350 nm corresponding to CdS clusters of approximately 28 A ˚ in diameter. The small dimensions reported in this work are typical of nanoparticles
obtained when zeolites are used as the host structure.
2 Strategies for the Scalable Synthesis of Quantum Dots and Related Nanodimensional Materials
24
hexagonal phase micro-crystalline material [77–79]. The addition, into the gas
phase, of small amounts of pyridine greatly improved the crystallinity exhibited by
the particles, while the addition of larger quantities of pyridine retained the improved crystal quality and also led to a decrease in particle size to the range 10–100
nm. Moreover, as the gas phase concentration of pyridine increased particle size
decreased. In the case of CdSe a quantization effect was observed by a change in
color of the deposit, when Me 2 Cd and H 2 Se are mixed in the gas phase in the absence of pyridine the deposit is black, when a high concentration of pyridine is
present the deposit is a yellowish-red color. For ZnSe when using hydrogen as the
carrier gas, instead of helium or argon, with a high gas-phase concentration of
pyridine, a relatively narrow particle size distribution (1–5 nm) could be obtained.
2.4.5
Synthesis in a Structured Medium
The use of a matrix to define the reaction space is an intrinsically attractive
approach to the preparation of large amounts of material which could be deposited
from solution or from the vapor phase. A number of matrices have been used
including: zeolites [80], layered solids [81], molecular sieves [82–84], micelles/
microemulsions [85–89], gels [90–92], polymers [93–97] and glasses [98]. The
matrix provides a mesoscopic reaction chamber in which the crystal can only grow
to a certain size.
The properties of the nanocrystallites are determined, not only by the confinements of the host material, but also by the properties of the system, which can include a range of factors including the internal/external surface properties e.g. of
the zeolite or the lability of a micelle. The particle size is controlled by the system
chosen e.g. in zeolites the nanocrystallite diameter is limited by the pore size of the
zeolite (typically smaller than 2 nm).
Wang and Herron [80] have studied the optical properties of both CdS and PbS
clusters encapsulated in zeolites. The nanocrystallites were prepared in two different zeolites; modernite (unidirectional channels of 7 A ˚ diameter) and zeolite Y
(13 A ˚ diameter channels with cages of tetrahedral symmetry interconnected by 8 A ˚
windows, and 5 A ˚ cages interconnected by 3 A ˚ windows). For the preparation of
CdS in zeolite Y, the sodium cations in the zeolite were first ion-exchanged with
cadmium cations, by treatment with aqueous Cd(NO 3 ) 2 at pH 5. This was followed
by passing hydrogen sulfide (H 2 S) gas over the sample [80]. Depending on the
loading level of cadmium ions within the zeolite, different sizes of CdS clusters
could be obtained. At low loading levels (1:1 metal/sulfide) CdS clusters with an
average size less than 13 A ˚ were obtained. These gave an absorption peak at around
280 nm in their optical spectra. When an excess of cadmium was used the individual clusters aggregated into an extended structure, modulated by the internal cavities of the zeolite. These produced optical spectra showing an excitonic
shoulder near 350 nm corresponding to CdS clusters of approximately 28 A ˚ in diameter. The small dimensions reported in this work are typical of nanoparticles
obtained when zeolites are used as the host structure.
2 Strategies for the Scalable Synthesis of Quantum Dots and Related Nanodimensional Materials
24