3+
Eu
Doped Y2O3 Nanocrystals
rare earth
sol gel research
Departments of Chemistry & Physics
Hamilton College
Greg Armstrong ‘06, Peter Burke ‘06, Ann Silversmith, & Karen Brewer
Properties of Rare Earth
Ions
Properties of
Nanoparticles
•Shielding by the 5d orbitals lead to
narrow spectral lines
•Typically defined as particles with
diameters less than 100nm
•Relative insensitivity to electronic
environment
•High surface area to volume ratio
Suspensions
•Surface defects significantly change
crystal structure
•Long radiative lifetimes
Emission at 611 nm (a.u.)
Emission at 611 nm (a.u.)
•High percentage of atoms near
surfaces lead to novel properties
•Forbidden f-f transitions become
slightly allowed in asymmetric hosts
Excitation spectra of 1.7:1 glycine fuel
ratio nanoparticles heated at 850C for
18 hour
Excitation spectra of 1:1 glycine fuel
ratio nanoparticles heated at 500C for
1 hour
Suspension
Powder
575
575
577
579
581
583
585
587
Suspension
Powder
577
579
•Synthesize nanoparticles
•Determine the reaction and heat treatment parameters that produce particles with the best
optical properties
•Suspend the particles in ethanol using a surfactant to break up clusters of particles
Excitation of powders
No heating
Heated at 500C for 1 hour
Heated at 850C for 18 hours
580
582
584
586
wavelength (ms)
588
1:1 glycine
1.3:1 glycine
1.7:1 glycine
590
578
580
582
584
586
588
wavelength (ms)
590
1:1 glycine fuel ratio Y2O3:Eu
nanoparticle decay
-1
2
4
6
8
10
12
0
ms)
-3
-3
-4
-4
-5
-5
-7
time (ms)
2
4
6
8
10
3
4
5
time (ms)
Placing a particle in a suspension will affect the phonon energy levels of its surface atoms and
change its non-radiative decay paths. A corresponding change in the lifetime of our particles
was observed. However, all particles suspended had nearly identical lifetimes. One explanation
is that we preferentially excited atoms near the surface of the particles. This would explain the
lack of size dependence, though there is no data that shows that surface atoms have a different
excitation wavelength than interior atoms. Further testing must be done to determine the cause
of the shift in lifetimes.
12
-1
-2
No heating (tau = 2.3 ms)
Heated at 500C for 1 hour (tau = 2.3 ms)
Heated at 850C for 18 hours (tau = 2.3 ms)
When a material is placed in different mediums the electronic environment of the surface
changes while there is no change in the interior of the material. It follows that there will be a
change in the spectra of nanoparticles in different environments while the spectra of bulk
material will remain constant. Nanoparticles synthesized using a 1:1 glycine fuel ratio and
heated at 500C were suspended in ethanol with AOT (a surfactant used to break up clumps of
particles). It was found that the peak widths in the excitation spectra increased. When the
experiment was repeated using particles synthesized with a 1.7:1 glycine fuel ratio and heated
to 850C for 18 hours (these parameters were chosen because they yield the largest particles
possible) there was no change in the spectra. Because the suspensions were cloudy we know
that many particles were grouped together (particles on the order of 10 nm do not scatter
enough light in the visible range to make the suspension appear cloudy). We expect that
breaking up the clusters completely would result in even more broadening
0
-0.5 0
1
2
-1
-1.5
-2
1:1 glycine ratio heated at 500C for 1
hour (tau = 1.5 ms)
-2.5
-3
1.7:1 glycine ratio heated at 850C for 18
-3.5
hours (tau = 1.3 ms)
-4
Laser ablation nanoparticles (tau = 1.4
-4.5
Decay of Y2O3:Eu nanoparticles
heated at 500C for 1 hour
0
-2
-6
589
Nanoparticles suspended in ethanol with AOT
Photoluminescent Decay
0
587
Photoluminescent decay of suspensions
As the size of nanoparticles decreases the percentage of surface defects increases.
Because the crystal structure becomes more inhomogeneous due to defects as the
particles decrease in size, we expect the peak widths of the excitation spectra to
increase. We found that treating the particles at higher temperature and for longer
periods of time resulted in narrower peaks, suggesting that increased heat treatment
resulted in larger particles. We also found that increasing the ratio of fuel to metal
nitrate in the precursor solution resulted in wider peaks. This is consistent with
Shea et. al., who reported that a larger fuel to metal nitrate ratio resulted in a larger
particle size. From these data we determined that the optimal fuel and fuel ratio
was glycine, 1:1. Because the samples without heat treatment were not as
luminescent as the treated samples we determined that 500O C for 1 hour is the
optimal heat treatment.
0
585
Excitation of Y2O3:Eu nanoparticle
heated at 500C for 1 hour
1:1 glycine fuel ratio Y2O3:Eu
nanoparticle excitation
578
583
wavelength (nm)
wavelength (nm)
Goals
581
589
-6
-7
1:1 glycine (tau = 2.3 ms)
1.3:1 glycine (tau = 2.3 ms)
1.7:1 glycine (tau = 2.3 ms)
time (ms)
A shorter decay time indicates that there is an efficient non-radiative decay pathway for
excited electrons. Usually this is phonon emission. As particle size decreases and crystal
structure changes we expect a change in the phonon energy levels and a corresponding
change in the lifetime. Brian Tissue reported that the lifetime of his Y2O3:Eu3+
nanoparticles (produced by laser ablation) has a lifetime of 3.3 ms whereas bulk
Y2O3:Eu3+ has a lifetime of 1.5 ms. We expected to see the lifetime increase as particle
size decreased. However, the lifetimes of all of our samples were almost identical. This
indicates that there is no simple relationship between particle size and lifetime.
Y2O3:Eu3+ nanoparticles (left) and
Y2O3:Tb3+ nanoparticles (right)
The same particles under UV excitation.