Chemistry Reference
In-Depth Information
A detailed study of the dynamics of amines on CdSe nanoparticles iden-
ti
ed several key features; the chain length of the ligand, which determined
ligand
ligand interactions on the particle surface, and notably the boiling
and melting point of the ligand. Reaction temperatures above the boiling
point of the amine resulted in a quasi-gas state in which the reaction pro-
ceeded too quickly, and too low a reaction temperature resulted in growth
that was too slow, meaning shortening of the hydrocarbon chain could result
in the reduction of the reaction temperature while still yielding high-quality
crystals.
68
Modelling of the coordination of amines on to a QD surface has
highlighted that the particle can accommodate more than one ligand per
surface cadmium ion, suggesting what is referred to as indirect adsorption,
where a second ligand would bind to an already occupied cadmium ion or
hydrogen bond with a less favourable site such as selenium.
23
This over-
loading has been suggested to be a cause of the concentration-induced
changes in optical properties.
Ab initio
calculations suggest that the amine mode of bonding is through
the lone pair of electrons on the nitrogen atom, and in the case of CdSe,
the amine binds to both selenium sites and cadmium sites. Surprisingly,
amines are expected to bind to selenium sites preferentially over cadmium
sites (binding energy of 1.05 eV for the selenium-terminated (0001) face as
compared to 0.91 eV for the cadmium-terminated (0001
)face).
23,25
There is
an argument that the acidic protons in the NH
2
group on a primary amine
coordinate to surface metal sites.
69
The use of tertiary amines in nano-
particle synthesis appears to contradict this,
70,71
and it is possible that the
coordination of amines to nanoparticle surfaces may actually occur
through both the lone pair of electrons and protons. Fourier transform
infrared (FTIR) spectra on OAm-capped FePt particles con
-
d
n
1
y
4
n
g
|
6
.
rm the presence
of an N
H feature and OAm appeared to bind through electron donation
from the nitrogen atom.
72
Studies using nuclear magnetic resonance (NMR)
have determined the equilibrium constant for octylamine (OA) and CdSe,
and also the kinetic constants for the desorption and adsorption
processes.
73
Interestingly, although amine-based alternatives to TOPO
are common, there are few alternative to the TOP-based system even
though
-
'
'
routes are becoming more popular. There are
several reasons for this. Primarily, TOP is used as a delivery system for
chalcogenide precursors, and
phosphine-free
nding a liquid alternative that dissolves
elemental chalcogens is di
ective, although it
appears to need heating with the chalcogen, and prolonged heating is
o
cult. Octadecene (ODE) is e
nal
product was not as luminescent as materials prepared with TOP.
74
An
amine-related alternative,
N
,
N
0
-dimethyl-oleyl amide, was found to dissolve
selenium easily and was used in the synthesis of CdSe
75
and ZnSe
76
in
a phosphine-free synthesis. The other notable TOP replacement was hex-
apropylphosphorus triamide used during the synthesis of CdTe particles
77
as described in Chapter 1, although this is clearly less
en detrimental,
40
and ionic liquids have been used although the
'
green
'
than
N
,
N
0
-dimethyl-oleyl amide.
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