Page 142 - Science at the nanoscale

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June 9, 2009
Low-Dimensional Nanostructures
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Quantum Dots
6.3.1
Semiconductor nanoparticles or quantum dots are normally pre-
pared chemically via solution-based routes, often at elevated
temperatures and sometimes at elevated pressures (hydro- or
solvo-thermal methods). The most commonly studied quantum
dots include metal sulfide or metal selenide compounds such as
CdS, CdSe, InSe, PbS, ZnS, etc.
In the chemical synthesis of binary semiconductors, two precur-
sors each containing one of the two elements are typically used
as the starting reactants. A suitable solvent system is needed for
good mixing of the two reactants for a homogeneous reaction to
occur. In many cases, the injection method is used whereby a
solution of one precursor is injected into a hot solution of the
second precursor. In such cases, a burst of nuclei is generated
instantaneously in the hot solution when the reactants are mixed
together. These nuclei are allowed to grow in the hot solution
(ageing) and the reaction is quenched at the appropriate time to
give the desired nano-sized particles. The basics of the nucleation
and growth processes will be further discussed in Chapter 7.
In order to reduce the tendency to aggregation, organic surface
capping agents (as discussed in Chapter 5) are often added in the
preparation of quantum dots. Hence, surface-capped quantum
dots of varying sizes have been isolated through such colloidal
methods and were found to display interesting colours as shown
in Fig. 6.14. In the quantum confined regime, the control of par-
ticle sizes allows the band gap to be “tuned” to give the desired
electronic and optical properties.
The nature and amount of organic surface capping agents used
have been found to be crucially important in solution-based routes ch06
of nanostructure fabrication. Typically, smaller sized quantum
dots are formed when the amount of capping agent added is
increased. In some cases, the added capping agent also plays a
second role in influencing the kinetics of the chemical processes
concerned. For example, when silver(I) thiobenzoate (AgTB) pre-
cursor was decomposed to produce Ag 2 S nanocrystals, the use
of hexadecylamine (HDA) as capping agent was found to also
5
affect the morphology of the produced nanocrystals. A combined
5 W. P. Lim, Z. Zhang, H. Y. Low and W. S. Chin, Angew. Chem. Int. Edn, 43, 5685
(2004).