Ligand Chemistry - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 177

in both polar and non-polar solvents. 114 Since oleic acid in particular has
been extensively used in magnetic nanoparticle synthesis, and materials
such as oleic acid-capped FePt need to be heated to 560 C to be converted to
the face-centred cubic phase for useful applications, the thermal behaviour
of these ligands at such temperatures has been explored, identifying that
the excess secondary surface ligands desorb from the particle surface at ca.
200 C, and dehydrogenates at 400 C yielding a graphitic layer with oleic acid
alkyl fragments underneath. 115
A particularly useful application of carboxylic acids is in the phosphine-
free synthesis of QDs, as mentioned in Chapter 1. In a typical example, 40 the
synthesis of CdSe was achieved by dissolving the cadmium precursor (CdO)
in a solution of oleic acid and ODE, followed by the injection of ODE/Se. It
was found that TOP was not essential for the production of high-quality
CdSe, although the use of ODE/Se instead of TOPSe resulted in poorer
nucleation, producing approximately half as many particles. It does,
however, allow the study of the nature of the selenium precursor, which is
thought to convert from the polymeric elemental form to various allotropes
over 2 hours of heating in ODE, unlike TOPSe, in which the selenium is
thought to exist as the elemental monomer. Excessive heating of ODE/Se
(>4 hours) results in a polymeric species unsuitable for QD synthesis (the
inclusion of a phosphonic acid in the synthesis allowed the formation of the
wurtzite-structured crystalline form rather than the zinc blende-structured,
oleic acid-capped CdSe). Little di

d n 1 y 4 n g | 6

erence was found in the optical quality
between the QDs prepared using TOP and those prepared using just oleic
acid. It is also worth noting than trap emission was observed in both
synthetic pathways, irrespective of whether TOP was used. Other methods of
producing phosphine-free QDs includes the use of olive oil as a solvent and
oleic acid as a capping agent, as outlined in Chapter 1, giving high-quality
QDs of CdSe. Olive oil is known to contain a high proportion of triglyceride
esters of oleic acid, so might be thought of as a trimer of oleic acid. The
kinetics of QD growth using carboxylic acids have also been reported. 116,117
Related to carboxylic acids is the use of high boiling point esters and ketones,
such as hexadecyl hexadecanoate and benzophenone, as replacements
for TOPO. 118 Notably, the initial nuclei growth and particle growth rate at
high temperatures were hindered in hexadecyl hexadecanoate, allowing
potentially smaller nanoparticles with a narrower size distribution.

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6.6 Surfactant Exchange
The key aspect of most surfactants is the dynamic character associated with
ligands bound to a nanoparticle surface. Most surfactants are interchange-
able, importantly allowing the exchange of pendant functionalities or the
switch from organic to aqueous phases, or vice versa . Most surfactant
exchange reactions are driven by mass action where the exposure of the QDs
to a large excess of the new surfactant drives the exchange. A study of some
typical phase-transfer protocols to prepare QDs for biological applications