Ligand Chemistry - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 171

without the need for an inorganic shell. 50,51 This has been attributed to the
closer packing of the ligands on the nanoparticle surface and the etching of
surface defects, 52,53 and amines have also been shown to contribute to the
oxygen etching process. 54 The use of amines as capping agents on CdSe
particles has also been shown to result in a surface reconstruction, 55
speci cally a lattice contraction during growth, which may have contributed

to the elevated emission. 56 Further evidence for a ligand-directed recon-
structed surface was obtained by measuring the surface strain of CdSe cap-
ped with di

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d n 1 y 4 n g | 6

ering capping agents, where TOPO-capped CdSe exhibited
tensile stress, whereas amine-capped CdSe exhibited compressive stress. 57
Mulvaney examined the role of primary, secondary and tertiary amines on
the emission quantum yield of CdSe particles, and noted that primary amine
strongly enhanced the emission whereas secondary and tertiary amines had
a negligible e ect. 58 The primary amines have, theoretically, close to 100%

surface coverage when assuming a footprint of 0.25 nm, as compared to
a theoretical maximum of 30% with TOPO. The addition of primary amines
to a solution of CdSe was also found to blue-shi

the emission position,
which was attributed to an electronic contribution from the amine but may
also have an origin in the etching observed by Woo where the absorbance
edge was also found to blue-shi

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and was assigned to a slight reduction in
particle size. 52 Notably, the addition of amides has no e

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ect on the emission
intensity, attributed to the poor electron-donating properties. Even post-
treatment of TOPO-capped CdSe particles with primary amines improved the
emission quantum yield by an order of magnitude to 50%, although this was
accompanied by a slight blue shi

.

in the absorption spectra consistent with
a small decrease in particle size. (This was con

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rmed by Foos et al. who also
reported that secondary amines slowed the growth rate of CdSe QDs. 59 )
Interestingly, other reports state that addition of amines quenches QD
emission, 60,61 although work by Munro et al. has highlighted an interesting
concentration dependence, with a bright point at a certain concentration,
either side of which resulted in quenched emission, thus possibly explaining
the contradictory reports. 62 It was suggested that the enhancement of the
emission was not necessarily based on improved surface passivation, rather
a surface reconstruction, as described earlier. Primary amines have also been
shown to remove surface cadmium adatoms in CdSe QDs, and passivate
cadmium dangling bonds. 43 The role of amines in the synthesis of III

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V QDs
is unclear. Initially, amines were thought to be activators for the reaction,
notably activating the phosphorus precursor; 63,64 however, later results have
questioned that particular assignment, suggesting that amines actually
inhibit the reaction. 65 Yu et al. have suggested that the linear shape of some
capping agents (amines) allows oxygen to reach the particle surface, oxidising
the surface and decreasing the emission quantum yield, whereas capping
agents with branched structures protect the surface in more satisfactory
manner. 66 Notably, oleylamine (OAm) has also been identi

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ed as a strong
reducing agent as well as a capping agent in this synthesis of Fe 3 O 4 ,an
excellent replacement for the usual diol type of synthesis. 67

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