The Preparation of II–VI Semiconductor Nanomaterials - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 15

the observation of the precipitate resulted in QD formation, approximately
4 nm in diameter and with emission quantum yields of up to 80%. Injection
of the chalcogen precursor before observation of the precipitate resulted
in CdTe tetrapod formation. The formation of the Cd 0 particles results in
a regulation of monomer supply, avoiding fast growth into the tetrapods.
The use of green chemistry has attracted a great deal of attention, and
numerous adaptation of the routes have been reported, including routes to
PbS, MnS 85 and CdS, 86 the use of two phases 87 - 90 and the use of initiators to
control the size distribution. 91 Although some systems avoid the use of
TOPO, several still use TOPSe as a convenient chalcogen precursor. Probably
the most notable replacement for TOPO as the reaction solvent is ODE,
although the seminal reports of the related synthesis of cobalt nanoparticles
utilised long-chain ethers as non-coordinating solvents. 92 A notable report is
the use of heat transfer

d n 1 y 4 n g | 1

uids, not normally used as reaction solvents, in the
preparation of CdSe and CdSe/CdS QD structures. 93 In this report, TOPSe is
still used as the selenium precursor and oleic acid is used as a capping agent,
and a range of reaction solvents compared. Two heat transfer solvents, DTA
(a mixture of biphenyl and diphenylether) and T66 (a mixture of terphenyls
and polyphenyls), were found to be useful in the controllable preparation of
small QDs relative to the TOPO-based synthesis, and a detailed kinetic study
highlighted di

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d n 4 .

erences in viscosity, surface free energy and CdSe solubility
as the reasons behind the di erences when compared to the standard

preparations. Similarly, the ionic liquid trihexyl(tetradecyl)phosphonium
bis(2,4,4-trimethylpentylphosphinate) has been used as a replacement for
TOPO in the preparation of CdSe, with both the anionic and cationic
constituents coordinating to the particle surface sites. 94,95 A gradual emer-
gence of strong band edge luminescence was observed, although only when
TOPSe was used as a precursor, consistent with previous reports of TOPSe
being a cause of enhanced emission. 96 Other methylimidazolium-based ionic
liquids have also been used as a capping agent and solvent for single-source
precursors. 97 Para

n has also been used as a phosphine-free solvent for the
preparation of oleylamine (OAm)-capped CdSe/ZnS QDs, with quantum
yields of up to 36%. 98,99 Similarly, olive oil has been used as a reaction
solvent, giving the cubic phase of oleic acid-capped CdSe, the diameter
tuneable from 2.3 to 6.0 nm and exhibiting band edge emission with
quantum yields up to 15%. 100 Also, the use of microwaves in synthesis, 101 and
speci cally the use of microwave-active trialkylphosphine-based precursors,

have opened up the use of low-boiling-point, non-microwave-active solvents,
such as pentane, heptane, octane and decane. 102 - 104

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¼

1.4 ZnE (E S, Se, Te)

Bulk zinc chalcogenides exist as wide-bandgap semiconductors (ZnS 3.54 eV,
ZnSe 2.58 eV, ZnTe 2.26 eV) and are therefore ideal materials for emission in
the near UV/blue end of the visible spectrum. The preparation of ZnE (E ¼ S,

Se, Te) nanoparticles was initially brie y reported by Murray, who described

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