Chemistry Reference
In-Depth Information
oleic acid/oleylamine mediated thermal decomposition method to
synthesize cubic α-NaREF
nanocrystals.
It is revealed that short reaction time, relatively low temperature,
and low Na/RE precursor ratio favor the formation of α-phase
nanocrystals. In contrast, β-phase nanocrystals were formed in more
rigorous conditions like high temperature, longer reaction time, and
high Na/RE ratio. However, phase transformation can take place
by addition of extra precursor during the synthesis process. For
example, in the above-mentioned experiment, the authors injected
additional Na(CF
and hexagonal β-NaREF
4
4
COO) precursor into α-NaREF
nanocrystals,
3
4
with longer time. The transformation
can also take place in direct synthesis route but is dependent on the
type of lanthanides [121].
which resulted in β-NaREF
4
. [40] reported an interesting
study to control phase and size simultaneously by doping of
lanthanide ions. Generally, doping has been reported to stabilize
special crystal phase, modifying electronic properties, modulating
magnetism, and tuning emission properties. However, the authors
reported that doping can modulate simultaneously crystal phase,
size, and emission properties in their work. By doping with
lanthanide ions, NaYF
In a very recent report, Liu
et al
nanocrystals can be tuned in size, phase
(cubic and hexagonal), and upconversion emission wavelengths
(green to blue). Figure 2.30 shows the cubic and hexagonal phases of
NaREF
4
structures and general trend of phase transition from cubic
to hexagonal as a function of ionic radius of lanthanide-doped ions.
The authors also demonstrated that size and shape can be controlled
by doping NaYF
4
3+
ions at different concentrations
(0-60%). The TEM images show particles of distinct morphology
and phase. The authors attributed the size evolution of NaYF
:Yb/Er with Gd
4
:Yb/Er
4
3+
nanocrystals to the strong effect of the Gd
dopant ions on crystal
growth rate through surface charge modification. DFT calculation
suggests that the electron charge density of the crystal surface
increases after a Gd
3+
3+
ion in the crystal lattice.
The change of electron charge density on the surface of the small-
sized nanocrystals can substantially slow the diffusion of negatively
charged F
ion substitutes the Y
−
ions to the surface, because of an increase in charge
repulsion, resulting in a tunable reduction of the NaYF
nanocrystal
4
size.
