780 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics
rates were fast. They considered this to be due probably to the more controllable nucleation step
initiated by cadmium carboxylates less active than Cd(CH
3
)
2
used in the traditional organometal-
lic approach. This phenomenon implies that the control of the nucleation process may be the key
step towards a fully controllable synthesis. Moreover, the authors state that in practice, the fatty
acid systems are not recommended to synthesize small nanocrystals because of their fast growth
rates.
In the late 1990s two rather different alternative methodologies were also described for the
synthesis of selenides and tellurides. In the fi rst one the chalcogenide precursors were produced
in situ by using reducing agents such as KBH
4
, which converts Se
2
and Te
2
to Se
2
and Te
2
,
respectively [56, 57] . Wang and collaborators proposed a simple solution synthesis for pure
quantum dots of M chalcogenides ( M Bi, Cu, Cd, Sn, Zn; chalcogenide S, Se) by providing
the in situ reduction of S or Se in the presence of KBH
4
and the corresponding metal salt at room
temperature in strong basic solvents. They showed that the solvent signifi cantly infl uenced the
quality of the fi nal product, yielding small uniform nanoparticles (4–6 nm) in the case of ethyl-
enediamine and a mixed metal/chalcogenide precipitate with poor crystallinity and low yield in
the case of pyridine. In fact, pyridine is known to provide stable capping through the N atom, but
its low boiling point suggests limitations as a growth solvent.
Recently, amine-capped PbSe nanoparticles of tunable sizes and shapes were also obtained
with this method [58] . As in the previous methods, the main limitation is the diffi culty to achieve
narrow-size distributions and high crystallinity.
The second methodology, proposed for large-scale production, involves the application of ultra-
sound (formation and implosive localized hot spots induced by acoustic cavitations) on chemical
reactions [59, 60] . Zhu et al. reported the preparation of spherical ZnSe nanoparticles of average
sizes of 3, 4, and 5 nm by reacting Zn(acetate)
2
and selenourea in water followed by sonication
with a high-intensity ultrasonic probe under inert atmosphere for a determined period of time
[60] . Pb and Cu selenide nanoparticles were also obtained by using the corresponding acetates.
A recent search for “ greener ” and simpler procedures that could produce semiconductor
nanocrystals directly in aqueous media, aimed at bioapplications, readapted the thiol stabi-
lized CdTe synthetic methods originally reported by Rogach et al. [33, 34] and the one reported
by Nosaka et al. for the synthesis of CdS [36] . By using different approaches Gaponik et al. [38] ,
Zhang et al. [61] , and Menezes et al. [17] prepared highly luminescent CdSe or CdTe nanoparti-
cles directly in water. Gaponik reported the dissolution of Al
2
Te
3
in an acidic solution to render
the Te
2
precursor ions as H
2
Te(g), while, Zhang et al. [61] and Menezes et al. [17] used NaBH
4
in aqueous solution to reduce Te to Te
2
. This reduction process generates Na–Te–Te–Na a sta-
ble intermediate complex which will be converted to CdTe after injection of the metal precursor
complexed with a thiol molecule in water under inert atmosphere. These colloidal systems render
particles in the 2–6 nm range and show luminescence after a certain period of time suggesting
a slow kinetic surface passivation process [17] . This observation will be discussed in the next
section.
Regarding the inherent toxicity of these systems for in vivo applications, for example, it was
recently reported by Pradhan et al. [62] that the synthesis of pure and doped ZnSe QDs as an
alternative for CdSe nanocrystal aimed at the obtention of a less toxic labelling material which
could be envisioned for a safe use in in vivo experiments and diagnostics.
On the other hand, to avoid the inconvenient autofl uorescence observed in biological systems
when these are excited in the UV blue region of the spectrum, lower band gap II–VI semicon-
ductor QDs are also being converted into biolabels. These systems when quantized present the
onset of the absorption and emission bands in the near-infrared region (700–1300 nm). As an
example, Kumar and Jahkmola reported on the RNA-mediated fl uorescent PbS nanoparticles
as novel tools for biophotonic applications [63] . Also a recent report of DNA-directed semi-
conductor quantum dot synthesis described highly optically emissive PbS nanocrystals [64] .
Furthermore, Hinds et al. [65] were able to synthesize infrared emitting PbS QDs (4 nm) stabi-
lized with Guanine-triphosphates (GTP). The authors systematically investigated how nucleotide
functionalities (base, sugar and phosphate) infl uenced nanoparticle growth. They proposed a set
of rules for using nucleic acids as ligands in order to profi t from the natural biorecognition prop-
erties of DNA and programmable templates for nanoparticle synthesis.
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