A spectroscopic analysis of blue and ultraviolet upconverted emissions from Gd3Ga5O12:Tm3+, Yb3+ nanocrystals

Article abstract of DOI:10.1021/jp052192w

The spectroscopic behavior of gadolinium gallium garnet (Gd ₃Ga₅O₁₂, GGG) nanocrystals codoped with 1% each of Tm³⁺ and Yb³⁺ prepared via a solution combustion synthesis procedure was investigated. Initial excitation of the codoped nanocrystals with 465.8 nm (into the ¹G₄ state) showed a dominant blue-green emission ascribed to the ¹G₄- ³H₆ transition as well as red and NIR emissions from the ¹G₄-³F₄ and ¹G ₄-³H₅/³H₄- ³H₆ transitions, respectively. Excitation at this wavelength (465.8 nm) showed the existence of a Tm³⁺ a?? Yb ³⁺ energy transfer process evidenced by the presence of the ²F_5/2-²F_7/2 Yb³⁺ emission in the NIR emission spectrum. The decay time constants proved that the transfer of energy occurred via the 3H4 state. Following excitation of the Yb ³⁺ ion with 980 nm, intense upconverted emission was observed. Emissions in the UV (¹D₂-³H₆), blue (¹D₂-³F₄), blue-green ( ¹G₄-³H₆), red (¹G ₄-³F₄), and NIR (¹G ₄-³H₅/³H₄- ³H₆) were observed and were the direct result of subsequent transfers of energy from the Yb³⁺ ion to the Tm ³⁺ ion. Power dependence studies showed a deviation from expected values for the number of photons involved in the upconversion thus indicating a saturation of the upconversion process. An energy transfer efficiency of 0.576 was determined experimentally. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp052192w

17400
J. Phys. Chem. B 2005, 109, 17400-17405
A Spectroscopic Analysis of Blue and Ultraviolet Upconverted Emissions from
3+
3+
Gd3Ga5O12:Tm , Yb Nanocrystals
†
†
†
†
Fabiano Pandozzi, Fiorenzo Vetrone, John-Christopher Boyer, Rafik Naccache,
,†
‡
‡
John A. Capobianco,* Adolfo Speghini, and Marco Bettinelli
Department of Chemistry and Biochemistry, Concordia UniVersity, 7141 Sherbrooke Street West, Montreal,
QC, H4B 1R6, Canada, and Dipartimento Scientifico e Tecnologico, UniVersit a` di Verona, and INSTM,
UdR Verona, Ca’ Vignal, Strada Le Grazie 15, I-37134 Verona, Italy
ReceiVed: April 27, 2005; In Final Form: July 11, 2005
3 5
The spectroscopic behavior of gadolinium gallium garnet (Gd Ga O12, GGG) nanocrystals codoped with 1%
each of Tm3 and Yb prepared via a solution combustion synthesis procedure was investigated. Initial
+
3+
1
excitation of the codoped nanocrystals with 465.8 nm (into the G
emission ascribed to the G
4
state) showed a dominant blue-green
1
3
1
3
1
4
- H
6
transition as well as red and NIR emissions from the G
4
4 4
- F and G -
3
3
3
H
5
/ H
4
6
- H transitions, respectively. Excitation at this wavelength (465.8 nm) showed the existence of a
Tm3 f Yb energy transfer process evidenced by the presence of the F5/2- F7/2 Yb emission in the NIR
+
3+
2
2
3+
3
emission spectrum. The decay time constants proved that the transfer of energy occurred via the H
4
state.
3+
Following excitation of the Yb ion with 980 nm, intense upconverted emission was observed. Emissions in
1
3
1
-3F
1
3
1
3
1
3
3
3
the UV ( D
2
- H
6
), blue ( D
2
4
), blue-green ( G
4
- H
6
), red ( G
4
- F
4
), and NIR ( G
4
- H
5
/ H
4
6
- H ) were
3
+
3+
observed and were the direct result of subsequent transfers of energy from the Yb ion to the Tm ion.
Power dependence studies showed a deviation from expected values for the number of photons involved in
the upconversion thus indicating a saturation of the upconversion process. An energy transfer efficiency of
0.576 was determined experimentally.
3
+
1
. Introduction
reasonable efficiency. The process of upconversion in Ln -
doped inorganic materials has been investigated by a number
of researchers in a variety of different hosts (see review 4, for
example) from single crystals to glasses but to a much lesser
Advances in inorganic crystal synthesis have led to the ability
to produce nanocrystalline particles suitable for spectroscopic
studies, which are characterized as having sizes generally below
5
-9
extent in nanomaterials.
100 nm. Synthesis techniques presented in the literature are wide
The Tm3 ion used as a dopant in inorganic nanocrystals has
been shown to readily undergo red-to-blue upconversion.10
However, the drawback with this scheme is that the pump laser
required is expensive and rather cumbersome and thus does not
constitute a very appealing means of obtaining blue (or UV)
upconverted light. On the other hand, certain commercially
available and relatively inexpensive sources of NIR emitting
semiconductor diodes (λem ≈ 980 nm) would offer a more cost-
effective means for any application involving the upconversion
+
ranging from the very complex to the very simple (examples
include combustion synthesis, sol-gel synthesis, wet chemical
synthesis, etc.). These preparation methods capable of producing
luminescent nanomaterials have spawned a renewed interest in
the field. Some inherent differences in the luminescent properties
of these nanocrystalline particles compared to conventional
single crystals (bulk) include changes in the emission lifetime,
intensity, and emission resolution. In fact, these properties
depend strongly on the size of the nanoparticles.1
Lanthanide-doped nanocrystals prove very interesting in that
the trivalent dopant ions have many electronic energy levels,
some of which are suitable for direct pumping by common laser
sources. Additionally, many of these energy levels are quasi-
evenly spaced such that multiple resonant excitation from an
excited state can occur, and in turn leads to upconversion. This
latter phenomenon is basically defined as the process involving
the emission of photons whose energies are superior to those
,2
3+
from NIR to visible light. However, the Tm ion has no energy
level that can be efficiently and directly populated with 980
3
+
nm radiation, so to overcome this, another lanthanide ion, Yb ,
3
+
can be used as a sensitizer. The benefit of codoping with Yb
2
is that it possesses only two energy levels, the F7/2 ground state
and the F5/2 excited state. The latter has a high cross section
of absorption and in turn leads to an increase in the upconversion
efficiency compared with direct pumping. Following the excita-
tion of the Yb ion, multiple energy transfer processes may
2
3
3+
of the incident photons. Moreover, the associated long lifetimes
3
+
3+
of these energy levels are such that they facilitate the upcon-
version process by leading to a significant buildup of the
population in the intermediate excited state (population reser-
voir), which is essential if the mechanism is to proceed with
occur from Yb to Tm which makes upconversion to the
1
1
blue and UV emitting excited states possible.
The development of a nanomaterial that is capable of being
excited in the near-infrared and emitting from the visible (and
in some cases the UV) has potentially many applications. Some
of the major ones include biological markers, and consumer
electronics for use as efficient 3D high-resolution display
*
To whom correspondence should be addressed. Phone: 1-514-848-
12
2
424, ext 3350. Fax: 1-514-848-2868. E-mail: capo@vax2.concordia.ca.
†
Concordia University.
Universit a` di Verona.
‡
13
phosphors. For some of these applications, the nanoparticles
1
0.1021/jp052192w CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/26/2005

3+
3+
Blue/UV Upconverted in Gd3Ga5O12:Tm , Yb Nanocrystals
J. Phys. Chem. B, Vol. 109, No. 37, 2005 17401
must no doubt be dispersed in an appropriate solvent to satisfy
the requirements of the device. Upconversion in colloidal
solutions has been shown by G u¨ del et al. on lanthanide
3
+
3+
3+
phosphate (LuPO4:Tm , Yb and YbPO4:Er ) and fluoride
(
3+
3+
14
NaYF4:RE , Yb ) nanoparticles. However, the study of the
upconversion in the dry material provides invaluable insight to
the spectroscopic behavior of the upconverting material. In this
paper, we present such a study on the upconversion properties
3
+
of Gd3Ga5O12 nanoparticles codoped with 1% both of Tm
3+
and Yb , and discuss the relevant mechanisms associated with
the upconversion process.
2. Experimental Section
2
.1. Sample Preparation. Gadolinium gallium garnet (Gd3-
3
+
Ga5O12, GGG) nanocrystals codoped with 1% each of Tm
and Yb
solution combustion (propellant) synthesis procedure.
3
+
(Gd2.94Tm0.03Yb0.03Ga5O12) were prepared via a
1
5-17
An
aqueous solution containing the appropriate molar ratios of
carbohydrazide (NH2NH)2CO (Aldrich, 98%), Gd(NO3)3‚6H2O
Figure 1. Room temperature visible emission spectrum (λexc ) 465.8
nm) of nanocrystalline Gd Ga O :Tm3+, Yb3+ and Gd Ga O :Tm3+
3
5
3
12
3
5
12
(inset) showing the (i) 1
transitions.
G
4
- H
, (ii) 1
G
4
-3F
3
H
4
- H
3
(
(
9
Aldrich, 99.99%), Ga(NO3)3‚H2O (Aldrich, 99.999%), Tm-
NO3)3‚6H2O (Aldrich, 99.9%), and Yb(NO3)3‚5H2O (Aldrich,
6
4
, and (iii)
6
9.9%) was prepared having an oxidant-to-reductant (carbohy-
drazide-to-metal nitrate) molar ratio of 2.5. This aqueous
precursor solution was heated with a Bunsen flame to evaporate
the water, after which the autocombustion process began with
the evolution of a brown fume. After a very short time (within
a few seconds), a remarkably porous voluminous mass of
powder was formed. The proposed stoichiometric equation for
the synthesis reaction is given below:
Research Systems model SR 400 gated photon counter data
acquisition system was used as an interface between the
spectroscopic equipment and the computer running the SRS SR
465 data acquisition software. The near-infrared emission spectra
3
were recorded using a Jarrell-Ash /4 m Czerny-Turner single
monochromator in second order and the signal was detected
with a liquid nitrogen cooled North Coast EO-817P germanium
detector connected to a computer-controlled Stanford Research
Systems SR 510 lock-in amplifier. Luminescent spectra obtained
were not corrected for instrumental response.
Decay curves for each manifold were obtained by modulation
of the 465.8 and 980 nm excitation wavelengths through the
use of a Stanford Research Systems SR 540 optical chopper.
The emission from the sample was fed into the same data
acquisition system as above.
3
(1 - x - y)Gd(NO ) + 3xTm(NO ) + 3yYb(NO ) +
3 3 3 3 3 3
5
Ga(NO ) + 3(NH NH) CO f
3 3 2 2
(
Gd1-x- yTm Yb ) Ga O + 24NO +
x y 3 5 12 2
6
N + 3CO + 9H O
2 2 2
where in this case x ) 0.01 and y ) 0.01.
All spectroscopic measurements were performed at room
temperature.
Once the synthesis reaction has completed, any unreacted
carbohydrazide and/or nitrate ions were decomposed by heating
the synthesized powders at 500 °C for 1 h. The particles had
an average diameter of approximately 20 nm, when analyzed
by wide-angle powder X-ray diffraction.18
3
. Results and Discussion
3.1. Luminescence Spectroscopy. The room temperature
No other actions were taken to prevent the nanosample from
coming into contact with the ambient atmosphere, as surface
adsorbed components are not significant for the GGG samples.
The nanocrystalline powder was placed in a glass capillary for
all spectroscopic investigations.
emission spectrum (Figure 1) was obtained for nanocrystalline
3
+
3+
Gd3Ga5O12:Tm , Yb
wavelength, which corresponds to excitation into the G4 excited
using 465.8 nm as the excitation
17
1
3
+
energy level of the Tm ion. A dominant blue-green emission
centered at approximately 485 nm was observed and assigned
1
3
2
.2. UV, Visible, and NIR Room-Temperature Emission
to the G4- H6 transition. Red emission between 640 and 680
1
3
Spectroscopy (λexc ) 465.8 and 980 nm). The direct excitation
visible emission spectra were obtained from a Coherent Sabre
Innova, 20 W argon laser, using the 465.8 nm line as the
excitation source. The upconverted emission spectra were
obtained using 980 nm from a Spectra-Physics model 3900 Ti
sapphire laser pumped by the 514.5 nm line of the Coherent
nm was also seen, corresponding to the G4- F4 transition. Last,
emission in the near-infrared was noted from about 760-840
1
3
3
3
nm, which is assigned to the G4- H5/ H4- H6 transitions.
Figure 1 (inset) shows the emission spectrum of singly doped
GGG:Tm (1%) nanocrystals. Clearly, the addition of Yb
3
+
3+
does not affect the visible emission of this material.
+
3+
Sabre Innova Ar laser. UV emissions were recorded with a
Interestingly, the Yb ion possesses only one excited state
1
2
Spex Minimate /4 m monochromator and detected with an Oriel
( F5/2) centered at approximately 1000 nm. This is ideal from
2
7
0680 photomultiplier tube. Visible emissions were collected
the point of view of upconversion where the F5/2 state can act
3+
3+
from the Gd3Ga5O12:Tm , Yb nanocrystals at π/2 from the
incident beam and then dispersed by a 1 m Jarrell-Ash Czerny-
Turner double monochromator. The visible emissions from the
sample exiting the monochromator were detected by a thermo-
electrically cooled Hamamatsu R943-02 photomultiplier tube
and the photomultiplied signals were processed by a Stanford
Research Systems (SRS) model SR440 preamplifier. A Stanford
as the population reservoir in the upconversion process when
pumping with 980 nm radiation. However, following excitation
with 465.8 nm, it is conceivable that only emissions from the
3
+
Tm ion should be observed. Figure 2 presents a portion of
3
+
3+
the NIR emission spectrum of GGG:Tm , Yb following
excitation with 465.8 nm and a strong emission is observed
3+
between 900 and 1120 nm. Since Tm has no emission between

17402 J. Phys. Chem. B, Vol. 109, No. 37, 2005
Pandozzi et al.
Figure 3. Room temperature upconversion spectrum (λexc ) 980 nm)
Figure 2. Room temperature NIR emission spectrum (λexc ) 465.8
3+
3+
1
3
of nanocrystalline Gd
3
Ga
5
O
12:Tm , Yb showing the (i) D
2
- F
4
, (ii)
2
2
3+
3
3
3
3
nm) showing the (i) F5/2- F7/2 (of Yb ), (ii) H
5
- H
6
, (iii) H
4
4
- F ,
1
3
1
3
1
3
3
3
G
4
- H
6
, (iii) G
4
- F
4
, and (iv) G
4
- H
5
/ H
4
- H
6
visible transitions.
3
3
and (iv) F
4
- H
6
transitions. Inset: Room temperature NIR emission
2
2
spectrum (λexc ) 930 nm) showing the (i) F5/2- F7/2 emission in Gd
3
-
1
3
+
the lifetime of the G4 state was determined to be 430 µs while
5
Ga O12:Yb nanocrystals.
3
+
1
in the presence of Yb , the effective lifetime of the G4 state
was approximately 420 µs. In contrast, there is a significant
change in the decay time constant of the manifold between 760
m
TABLE 1: Effective Decay Time Constants, τ , for
3 5
Nanocrystalline Gd Ga O
12:Tm3 (1%) and
+
+
3+
3+
Gd
3
Ga
5
O
12:Tm3 , Yb (1% each of Tm and Yb3+
)
3+
and 840 nm when codoping with 1% Yb and it decreases
from 641 µs in GGG:Tm to 546 µs in GGG:Tm , Yb
nanocrystals, a 100 µs decrease. This clearly indicates that the
Tm to Yb energy transfer occurs from this state. Addition-
ally, the lifetime of the Yb F5/2 state was measured following
Tm excitation (λexc ) 465.8 nm) and an effective lifetime of
900 µs was determined.
Conversely, the lifetime of the Yb
changes considerably in the presence (and absence) of Tm .
In the absence of energy transfer (no Tm codoping), the
3 5
Following excitation with 465.8 nm and Gd Ga O
12:Tm3+,
3
+
3+
3+
3
+
Yb with 980 nm
effective decay time constant, τ
exc ) 465.8 nm exc ) 980 nm
Gd Ga Gd Ga
m
(µs)
3+
3+
λ
λ
3+ 2
3
+
Gd
3
Ga
5
O
12
:
3
3+
5 12
O :
3+
3
5
O
12
:
3+
3
+
3+
transition
Tm
Tm , Yb
Tm , Yb
3
+ 2
1
1
1
1
3
F5/2 excited state
D
2
G
4
G
4
G
4
- F
4
312
536
554
552
3
3+
- H
6
425
442
641
418
426
546
3
3+
- F
4
3
/3H
3
- H
5
4
- H
6
2
3+
lifetime (λexc ) 930 nm) of the F5/2 state in GGG:Yb was
determined to be 1180 µs. However, when 1% Tm was added
to the matrix, a lifetime of 500 µs was obtained for the F5/2
3+
9
Yb
00 and 1120 nm, the band in that region is ascribed to the
2
3+
2
2
F5/2- F7/2 emission, which undoubtedly occurs via a
3+
3+
3+
Yb state of GGG:Tm , Yb nanocrystals. We can calculate
3
+
3+
Tm to Yb energy transfer since there cannot be a direct
the efficiency of the Yb3+ f Tm3+ energy transfer (ητ) from
20
2
3+
population of the upper F5/2 energy state of Yb through the
use of the 465.8 nm excitation wavelength. Furthermore, minor
NIR emissions from the Tm3 ion centered at 1200, 1450, and
τDA
ητ ) 1 - τD
+
(2)
3
3
3
3
1
670 nm are observed and attributed to H5- H6, H4- F4, and
3
3
F4- H6 transitions, respectively. Figure 2 (inset) presents the
3+
2
2
3+
where τDA (500 µs) is the lifetime of the donor (D, Yb ) in
F5/2- F7/2 NIR emission of nanocrystalline GGG:Yb (1%)
following excitation with 930 nm.
3
+
the presence of the acceptor (A, Tm ) and τD (1180 µs) is the
lifetime of the donor in the absence of the acceptor. Thus, the
efficiency of the energy transfer was determined to be 0.576.
3.2. Upconversion Luminescence Spectroscopy. Excitation
Proof for this Tm3 f Yb energy transfer mechanism is
+
3+
3+
found by comparing the decay times of GGG:Tm (1%) and
GGG:Tm , Yb (1% each of Tm and Yb ) following 465.8
nm excitation. The decay curves for GGG:Tm and GGG:
Tm , Yb nanocrystalline samples deviated from exponen-
tiality and thus were fit with the equation proposed by
3+
3+
3+
3+
2
3+
with 500 mW (800 W/cm ) of 980 nm light to directly populate
2
3+
3
+
3+
the F5/2 energy level of Yb subsequently leads to an energy
3
3+
transfer to the H5 energy level of Tm of the following type:
F5/2 (Yb ), H6 (Tm ) f F7/2 (Yb ), H5 (Tm ). Following
2
3+
3
3+
2
3+
3
3+
1
9
Nakazawa:
this, a series of upconversion processes can occur, each with
their own end result, and these will be discussed in further detail
later on.
∞
∫
tI(t) dt
0
τ )
(1)
m
Resulting from the irradiation of the sample with 980 nm,
upconverted emission in the UV, blue, blue-green, red, and near-
infrared was observed (Figures 3 and 4). Ultraviolet upconverted
emission was observed in the region of 360-380 nm and
∞
∫
I(t) dt
0
where τm is the effective decay time constant, and I(t) is the
intensity at time t. Thus, using this method, the effective decay
time constants, τm, were obtained (Table 1). We observe that
1
3
ascribed to the D2- H6 transition. Blue emission was observed
centered at ca. 454 nm and assigned to the energy transition
1
1
3
the effective decay time constants of the G4 emissions (for the
D2- F4. A much more intense blue-green emission resulting
1
3
1
3
1
3
G4 f H6 and G4 f F4 transitions) were not affected by the
from the G4- H6 transition was observed centered at 484 nm
and red upconverted emission resulting from the G4- F4
3+
3+
3+
1
3
insertion of Yb . In the absence of Yb (GGG:Tm , 1%),

3+
3+
Blue/UV Upconverted in Gd3Ga5O12:Tm , Yb Nanocrystals
J. Phys. Chem. B, Vol. 109, No. 37, 2005 17403
Figure 4. Room temperature upconversion spectrum (λexc ) 980 nm)
3+
3+
1
3
of nanocrystalline Gd
transition.
3
Ga
5
O
12:Tm , Yb showing the (i) D
2
6
- H UV
TABLE 2: Number of Photons (n) Involved in the
Upconversion Mechanism(s) Determined Experimentally for
5
Ga O
12:Tm3 , Yb Following Excitation
+
3+
Nanocrystalline Gd
with 980 nm
3
no. of photons (n)
transition
expected
obsd
1
1
1
1
3
D
2
G
4
G
4
G
4
- F
4
4
3
3
2
2.99
2.05
2.09
1.47
3
- H
6
3
- F
4
3
/3H
3
- H
5
4
- H
6
transition was noted between 640 and 680 nm. Finally, intense
NIR emission was observed between 760 and 840 nm corre-
1
3
3
3
sponding to the G4- H5/ H4- H6 transitions. It should be noted
3
+
that cooperative upconversion involving Yb pairs was not
observed in the GGG:Tm , Yb nanocrystals studied. More-
3
+
3+
3
+
over, studies on singly doped nanocrystalline GGG:Yb have
shown that at this particle size (20 nm) and up to a 10 mol %
doping level, no cooperative upconversion was observed.
To better understand the mechanism(s) by which the upper
emitting states are populated, we investigated the dependence
of the upconverted emission intensity on the pump power. For
an unsaturated upconversion process, it is possible to determine
the number of photons (n) required to populate the emitting
state from the slope of the graph of ln(intensity) versus ln(power)
according to
Figure 5. (a) The energy levels diagrams of the Tm3 and Yb3+ dopant
+
ions and possible upconversion processes following excitation with 980
nm. (b) Schematic representation of the Tm3 ( G
+
1
, H
2
state.
3
3
1
4
4 4 2
) f ( F , D )
1
ion pair process responsible for populating the D
1). As can be seen in Table 1, the decay time constants of the
G4 and H4 excited states are lengthened when comparing
upconversion (λexc ) 980 nm) to emission (λexc ) 465.8 nm)
phenomena. This is consistent with the idea of an ETU
mechanism, which is one of the mechanisms proposed for the
population of the excited Tm energy levels through successive
energy transfers from excited Yb ions. On the other hand, if
one examines the data in Table 1 in more detail, it seems that
the observed decay times for the G4- H6, G4- F4, and G4-
H5/ H4- H6 transitions are all similar, which may possibly be
explained by the fact that it is the lifetime of the feeding state
1
3
n
I ∝ P
(3)
where the upconverted luminescence intensity (I) is proportional
21,22
3+
to the nth power of the low-frequency excitation power (P).
3
+
The summary of the power study is given in Table 2. The above
relation holds for low excitation powers only and under high
excitation densities, the upconversion processes will saturate.
Upconversion is a nonlinear process and as a consequence of
the conservation of energy, it cannot maintain its nonlinear
behavior up to infinite excitation. Therefore, the power depen-
dence does not follow the above straightforward relation at
higher pump powers. Thus, we obtain slopes (n) which deviate
from the expected values in cases where the influence of
upconversion is large (i.e., high pump densities).22
1
3
1
3
1
3
3
3
2
3+
( F5/2 of Yb ) being measured and since all excitations must
proceed by ETU, this would be the bottleneck for the upcon-
3
+
version processes. The lifetime of the Yb emission (λexc )
930 nm) in the presence of energy transfer was determined to
be 500 µs.
1
A schematic representation of the Yb3+ to Tm3+ upconversion
processes is depicted in Figure 5a. Following the initial
The effective decay time constants were obtained for the D2,
G4, and H4 emitting states following excitation with 980 nm
1
3
3
+
and were compared to the decay times obtained when directly
populating the upper emitting states (λexc ) 465.8 nm) (Table
absorption of the 980 nm pump photon, the Yb ion is excited
2
2
from the F7/2 ground state to the F5/2 excited state (ground-

17404 J. Phys. Chem. B, Vol. 109, No. 37, 2005
Pandozzi et al.
3
+
1
3
state absorption). The excited Yb ion transfers its energy
Ga5O12 lattice vibrations. This in turn leads to the G4- F4 (red)
3+
3
1
3
nonresonantly to the Tm ion exciting it from the H6 ground
and G4- H6 (blue-green) transitions. Finally, it is possible that
the Tm ion is excited to the D2 state via a fourth nonresonant
transfer of energy from a Yb ion to a Tm ion in the G4
level. The population of the D2 state can lead to the radiative
state to the 3H5 excited state where the excess energy
11
3+
1
-1
3+
3+
1
(
approximately 1600 cm ) can be dissipated by the host lattice
the highest phonon energy of the GGG nanocrystal lattice is
approximately 600 cm ). Multiphonon relaxation in turn
populates the F4 state. The Tm ion is subsequently resonantly
excited to the F2 state via a second transfer of energy from the
excited Yb ion to the Tm ion in the F4 intermediate state.
Alternately, the F2 state may be populated via resonant excited-
state absorption (ESA) of the F4 state to the F2 state. This
process would be more efficient since it is independent of the
Tm -Yb distance and coupling. Thus, following population
of the F2 level, the thulium ion in turn relaxes nonradiatively
via multiphonon relaxation to the H4 state after which the H4-
H6 radiative emission occurs. Upon comparison of the direct
emission (λexc ) 465.8 nm) and upconversion (λexc ) 980 nm)
spectra, it is clear that there is a severe change in the relative
emission intensity of the NIR transition between 760 and 840
nm compared to the intensity of the other manifolds within the
spectrum. This change in the relative intensity is a direct result
of the proposed upconversion mechanism. As stated earlier, the
NIR emission centered at approximately 800 nm is due to the
1
(
-
1
1
3
emission of a UV photon via the D2- H6 transition or a blue
3
3+
1
3
photon via the D2- F4 transition. It should be noted that there
3
3+
is an alternate possibility for the promotion of a Tm ion into
3
+
3+
3
1
the D2 energy level. There exists a resonant mechanism
involving an excited Tm ion in the G4 level and another in
the H4 level (Figure 5, inset). One Tm ion in the H4 state
will interact with another Tm ion in the G4 state undergoing
an ion-pair process of the type ( G4, H4) f ( F4, D2), which
populates the D2 level. However, since the ( G4, H4) f ( F4,
D2) ion-pair process depopulates the H4 state, it is clear that
the influence of this mechanism is small given the relatively
high intensity of the H4- H6 transition. Last, it is rather
interesting to note that in Y2O3:Tm , Yb single crystals, no
emission from excited states higher than G4 was observed.
Experiments are currently well underway to help better
3
3+
1
3
3
3
3+
3
3
+
1
3+
3+
1
3
3
1
3
1
1
3
3
3
3
1
3
3
3
3
3
+
3+
1
23
understand the NIR-to-blue (and UV) upconversion in Gd -
3
3
+
3+
Ga O :Tm , Yb nanocrystals. We are in the process of
5
12
3
+
examining both the effects of temperature and Yb concentra-
tion on the upconversion properties as well as utilizing pulsed
NIR excitation to study the upconversion kinetics. These will
be the subject of a future paper.
1
3
3
3
23
overlapping G4- H5 and H4- H6 transitions. When irradiat-
ing the nanocrystals with 465.8 nm, the G4 state is excited
directly. The energy gap between the G4 and the next lower
lying state, F2, is approximately 5900 cm . The observed rate
of depopulation Wobs of an excited state could be expressed as
the sum of the radiative, WR, and multiphonon transition
probabilities, WMPR. In fact, the rate of multiphonon relaxation
1
1
3
-1
4. Conclusions
The goal of these experiments was to study a nanocrystalline
sample capable of undergoing efficient upconversion into the
visible and UV when being excited with NIR radiation. The
GGG:Tm , Yb sample studied is capable of this upconver-
sion process and for this reason can possibly be used in such
applications as was previously noted: biological markers and
consumer applications such as efficient and high-resolution
display phosphors.
A spectroscopic analysis of the luminescence from GGG:
Tm , Yb nanocrystals (1% each of Tm and Yb ) was
undertaken. Excitation of the Tm G4 excited state with 465.8
nm radiation showed blue-green, red, and NIR emissions from
the G4- H6, G4- F4, and G4- H5/ H4- H6 transitions,
respectively. Examination of the NIR emission spectrum showed
the presence of the F - F Yb transition indicating that a
Tm to Yb energy transfer was present. Upon comparison
of the decay time constants for nanocrystalline singly doped
2
4
is dominated by
3
+
3+
p
(
1 + neff)
(4)
where
-1
neff ) [exp(pω /kT) - 1]
(5)
eff
3
+
3+
3+
3+
is the occupancy of the effective phonon mode of energy (pωeff)
and p is the number of phonons necessary to bridge the energy
gap between the emitting level and the next lower level. Thus,
it would require approximately 10 GGG phonons to bridge the
energy gap between the G4 and F2 states. The result is that
the G4 state will be depopulated either radiatively through the
emission of photons or via a cross-relaxation process. As a
result, since the H4 state is populated mostly via multiphonon
relaxation from the upper states, it is clear that the population
of this state will be low and, consequently, the emission intensity
3+ 1
1
3
1
3
1
3
3
3
1
3
2
2
3+
1
5/2
7/2
3
+
3+
3
3
+
3+
3+
GGG:Tm (1%) and the GGG:Tm , Yb sample under
investigation, it was determined that the energy transfer occurred
3
3+
3
3
4
via the H state of Tm .
of the H4- H6 will be low. On the other hand, when pumping
with 980 nm, two successive transfers of energy from Yb
will populate the F2 state directly. The Tm ion will im-
3
+
2
3+
Following excitation of the F5/2 state of Yb , intense
upconversion was observed in the UV, blue, blue-green, red,
and NIR regions of the spectrum. The upconversion occurred
via successive resonant and nonresonant energy transfers from
the Yb ion to the Tm ion since the Tm ion has no energy
level, which can be directly pumped with 980 nm. The
upconversion spectrum showed an increase in the relative
emission intensity of the NIR transition centered at 800 nm
compared to the direct excitation spectrum obtained using 465.8
nm. This was the result of an increase in the intensity of the
3
3+
3
mediately decay to the F3 state as the energy gap is only ∼640
-
1
cm and requires only one GGG phonon. The energy gap
3
3
3+
3+
3+
between the F3 state and the H4 state is on the order of 1800
-
1
3
cm . Therefore, the probability that the H4 state is populated
via nonradiative decay from this state is very high as it only
requires 3 GGG phonons. This would obviously lead to higher
emission intensity in the upconversion spectrum compared to
the spectrum obtained when utilizing 465.8 nm to directly excite
1
3
3
the G4 state.
H4- H6 emission following efficient multiphonon relaxation
Following this initial process, another excited Yb3 ion in
+
3
3
from the F2 state to the F3 state.
close proximity can also transfer its energy nonresonantly to
The power dependence studies showed a deviation for n, the
expected number of photons partaking in the upconversion
process, indicating a saturation of this process at high pumping
the Tm3 ion in the H4 intermediate state, thus exciting the
+
3
1
ion to the G4 state (third energy transfer). Again, the excess
-
1
3+
3+
energy (approximately 1800 cm ) is dissipated by the Gd3-
intensities. The efficiency of the Yb to Tm energy transfer

3+
3+
Blue/UV Upconverted in Gd3Ga5O12:Tm , Yb Nanocrystals
J. Phys. Chem. B, Vol. 109, No. 37, 2005 17405
was determined from the effective decay times of Yb3+ in the
presence and absence of energy transfer. The energy transfer
efficiency, η, was determined to be 0.576.
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(
Acknowledgment. The authors gratefully thank Erica Vivi-
ani (Universit a` di Verona, Italy) for expert technical assistance.
The authors acknowledge the Natural Science and Engineering
Research Council of Canada and MIUR of Italy (Project
2
93, 22-30.
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996, 273, 1185-1189.
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003035190_001), for financial support.
(
5
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(
1
2
69.
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97-300.
4
(
(
(
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(
(