Inorganic luminescent materials have
practical applications in a variety of devices from conventional TV to
electroluminescent, plasma or field emission displays. Moreover, the explosive growth of optical
communications increased the demand for high performing luminescent materials in the area
of solid state lasers.
Traditional techniques make use of high
temperatures solid state reactions for the obtention of luminescent materials and leads
to agglomerates of 5 to 20 µm in size.
Sol-Gel and colloidal chemistry proves to
be a powerful technique to control the size of the ultimate grains of the material giving
the freedom to the researcher to tailor and better control the final structural parameters
which have an impact on the luminescent efficiency.
Bulk materials with strong luminescent
properties are usually oxides doped with lanthanides or transition elements :
Y3Al5O12:Nd3+ (YAG), Al2O3:Cr3+
Recently an intensive research
effort was directed towards chalcogenides core-shell nanostructures or doped semiconductor
nanoparticles with a narrow particles distribution and high luminescent quantum yield.
CdSe/ZnS (1-3), ZnS/Mn(4), CdS/Mn(5-6)
1. They can replace in most of
the case instable organic chromophores used as luminescent probes in a variety of
2. They can be incorporated in
a solid matrix (hybrid, sol-gel, etc) and used for optical device as bulk or films
3. High transparent material
can be made by the appropriate chose of the nanoparticles size and refractive index.
4. Surface chemistry, doping
and size control should lead to improved luminescent performances.
Doped YVO4 (7-8) exhibits a
high luminescent efficiency and it remains a good example for comparison.
The table below shows the
wavelength of emission in respect to the dopants used:
|Lanthanides||Emission wavelength (nm)||Application|
The structure of Orthovanadates is of Zircon type (Quadratic (a = b = 7,23 Ĺ, c = 6,29 Ĺ))
YVO4: Ln Synthesis
YVO4 was synthesized by aqueous co-precipitation reactions
from Y 3+ Ln 3+ and VO43- salts in water
(Figure1). The details of the procedure can be found in (9)
Figure 1 :Aqueous co-precipitation
The colloidal suspensions of Ln:
YVO4 nanoparticles was obtained by
dispersing the precipitate using sonification and stabilization of the particles with
To achieve purification and improve its stability,
the obtained colloidal suspension is then dialyzed in pure
water for 12 hours.
The Colloidal solutions those prepared remain stable for several months in a large pH range
(4< pH < 12)
High resolution transmission microscopy shows an
ellipsoidal geometry with characteristic dimensions of around 15 and 30 nm.
Under UV excitation, the colloidal solution shows a bright emission at 615 nm which is associated to the 5D0 - 7F2transition of the Eu 3+ ions (figure2)
Figure 2 : Colloidal solution of
Y0.95Eu0.05VO4 (5*10-4 mol.l-1) before and after UV excitation
Measured quantum efficiency (15 %) remains however far below the bulk material . This low
QE is presumably due to the high water presence at the particles surface. OH groups are
well known luminescence quencher centers.
Quantum yield was found to increase with both
D2O<-> H2O exchange and hydrothermal annealing. Both luminescence QE and lifetime
effects are shown below
OH impact on the luminescence
properties of nanoparticles was evaluated by considering a core-shell model where the shell is fully
hydrated and therefore completely quenched ( QE of shell =0). Assuming that the
Core quantum efficiency is similar to the one obtained with Deuterated samples (Core QE =
QE (D2O)) then one can calculate that 50 % of the Eu atoms are in 3 nm
fully hydrated shell and completely
Colloidal solution of Y0.98Nd0.02VO4 (0,2 mol.l-1)
have been made in
collaboration with P. Aschehoug et B. Viana,
of Solid State Applied
Chemistry Laboratory, ENSCP, Paris
Nanocomposites materials can be easily prepared by
incorporating the YVO4:Ln stabilized nanoparticles on a silica sol-gel made precursor.
Both dip or spin coating techniques can be used.
A very simple and efficient chemical route to prepare
high luminescent colloids with controlled composition has been presented. Nanoparticles
those formed can easily assembled as thin films by dip or spin coating techniques. They
are water soluble and biocompatible. In comparison with organic dyes and
semiconductor QD, they are narrower in spectral line width and probably more stable against
Further studies are directed towards better size, crystallinity and surface molecular grafting control.
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