Ligand Chemistry - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 169

expected, the presence of phosphine-based acids in technical-grade TOPO
also results in the cadmium enrichment of CdSe QDs. 24
Calculations on the binding strength of phosphonic acid-based ligands
need to take into account that the species is o

en deprotonated (as part of
the cadmium precursor) when used in reactions. Assuming a single charge
increased the binding energy substantially to 3 eV when considering the
interaction of the acid with the (0001



d n 1 y 4 n g | 6

) facet, as compared to 1.12 eV for the

neutral complex. Despite this signi cant increase, the binding is comparable

on all other facets. When calculations were carried out for the double
negative charged acid species, it was found that the ligand bound to all
surfaces tightly and even to subsurface cadmium atoms in the selenium-
terminated (0001) layer. 25,26
Phosphinic acids are usually present as impurities in reagent-grade TOPO.
Whereas TOPO binds through a dative bond, phosphinic/phosphonic acids
bind in an ionic manner to a surface metal site, and such ligands are some-
times referred to as X-ligands. 21 A set of ligand transfer experiments by Owen
et al. con



rmed this, and raised the interesting question of whether dative
bonds are enough to stabilise a particle. 21 Other impurities in TOPO have
been identi



ed and examined (the essential presence of impurities in the role
of growing anisotropic particles is described in Chapter 1), notably by Wang, 27
who identi



ed the main impurities in TOPO and their role in the reproducible
growth of certain anisotropic structures. Di- n -octylphosphine oxide (DOPO)
was found to aid in the growth of spherical QDs, di- n -octylphosphinic acid
(DOPA) and mono- n -octylphosphinic acid (MOPA) were found to aid quantum
rod growth, and DOPA was also found to aid wire growth. A separate study
highlighted the important role played by chain length in phosphinic acid-
capped QDs; shorter phosphonic acids resulted in a more branched and
elongated rod structure. When mixtures were used, more branching and
elongated structures were observed when a high molar fraction of the shorter
ligand was employed. 28 Other studies reported that short-chain phosphonic
acids stabilised CdSe particles in the zinc blende phase, whereas longer-chain
phosphonic acids stabilised CdSe particles in the wurtzite phase. 29 An in-
depth examination of CdSe QDs synthesised in technical-grade TOPO, puri-




.

ed using non-solvents, revealed that almost all the datively bound ligands
were removed, leaving just n -octylphosphonate (OPA), an impurity in tech-
nical-grade TOPO and P 0 - P 0 -(di- n -octyl)pyrophosphonate, a self-condensation
product of OPA, with small amount of stearate molecules from the cadmium
precursor. 30

6.2.2 Tri-n-Octylphosphine (TOP)
It is worth noting that tri- n -octylphosphine (TOP) is routinely used both as
a surfactant and as a chalcogen delivery solvent when in the form of
a trioctylphosphine selenide/sul

de/telluride (TOPSe/TOPS/TOPTe) solution
during the preparation of TOPO-capped nanocrystals. The use of tri-
alkylphosphine chalcogenides in materials synthesis was pioneered by

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