Strong photoluminescence and cathodoluminescence due to f-f transitions in Eu3+ doped Al2O3 powders prepared by direct combustion synthesis and thin films deposited by laser ablation

Article abstract of DOI:10.1063/1.1592636

Fabrication and luminescent properties of phosphor powders and thin films of Eu³⁺ doped alumina were studied. The powders were fabricated by combustion synthesis process at a low temperature and showed strong photoluminescent and cathodoluminescent. Powders of Eu³⁺ doped alumina of concentration 1.0 mol% were deposited on quartz-glass substrates to form thin films by means of laser ablation.

Full text of DOI:10.1063/1.1592636

Strong photoluminescence and cathodoluminescence due to f–f transitions
in Eu3+ doped Al2O3 powders prepared by direct combustion synthesis
and thin films deposited by laser ablation
Nikifor Rakov, Francisco E. Ramos, Gustavo Hirata, and Mufei Xiao
Citation: Appl. Phys. Lett. 83, 272 (2003); doi: 10.1063/1.1592636
View online: http://dx.doi.org/10.1063/1.1592636
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APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 2
14 JULY 2003
Strong photoluminescence and cathodoluminescence due to f–f
transitions in Eu3¿ doped Al2O3 powders prepared by direct combustion
synthesis and thin films deposited by laser ablation
Nikifor Rakov, Francisco E. Ramos, Gustavo Hirata, and Mufei Xiaoa)
´
´
Centro de Ciencias de la Materia Condensada, Universidad Nacional Autonoma de Mexico,
´
Apartado Postal 2681, Ensenada, CP22800, Baja California, Mexico
͑Received 14 March 2003; accepted 14 May 2003͒
In this letter, we report on fabrication and luminescent properties of phosphor powders and thin
films of Eu3ϩ doped alumina Al2O3 . The powders were fabricated by combustion synthesis process
at a low temperature, 280 °C and showed strong photoluminescent and cathodoluminescent
emissions. Powders of Eu3ϩ doped Al2O3 of concentration 1.0 mol % were deposited on
quartz-glass substrates to form thin films by means of laser ablation. Under ultraviolet excitation and
electron beam excitation, these samples containing microcrystalline structures showed strong
luminescence due to f – f transitions, and the dominant transition was the hypersensitive 5D0
7
→ F2 red emission of Eu3ϩ
. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1592636͔
Luminescence of rare-earth ions in different host matri-
ces has been of great interest for researchers from both the
scientific and technological communities. Much of this inter-
est stems from the unique physical and chemical properties
of the lanthanide oxides that make the materials useful in a
variety of diverse applications, such as, laser materials, opti-
cal amplifiers, phosphors, and photocatalysts.1–4
nique for preparing micro/nanocrystalline materials due to its
low cost, high yield, and good ability to achieve high purity
in making single or multiphase complex oxide powders at
the as-synthesized state.13 Since the mixing of the reagents
occurs at the molecular level and in solution, a high homo-
geneity and purity of the powders can be obtained because
precursor impurities vaporize during the combustion. In the
current project, the combustion synthesis process allows
Eu3ϩ to incorporate into the Al2O3 lattice despite of the
large size difference between Eu3ϩ ͑1.07 Å͒ and the Al3ϩ
͑0.54 Å͒.
Of the many rare-earth ions, Eu3ϩ ions have attracted
significant attention because they have tremendous potential
for applications in some optical fields, such as phosphors,
electroluminescent devices, and optical amplifiers or
lasers.4–6 In addition, the luminescence of Eu3ϩ ions is par-
ticularly interesting because the emission corresponding to
Al2O3 powders doped with Eu3ϩ ͑1.0 mol %͒ were fab-
ricated in the combustion synthesis process where europium
nitrate Eu(NO ) •6H O , aluminum nitrate Al(NO )
7
the 5D0→ F2 transition, centered near 612 nm, is one of the
͓
͔
͓
3
3
2
3 3
three fundamental colors ͑red, blue, and green͒.
•9H O and hydrazine (N H •H O) were used as a reduc-
͔
2
2
4
2
Among the materials investigated to date, a few experi-
ments have been performed on rare-earth ion doped alumi-
num oxide (Al2O3).6–12 The Al2O3 crystal is a material with
a significant technological importance because of the large
optical transparency from ultraviolet to near-infrared, and be-
cause of its excellent mechanical properties and good chemi-
cal stability. Several structural modifications of Al2O3 are
known and it seems ␣-Al2O3 is the only stable phase. These
rare-earth doped Al2O3 materials have only been synthesized
by techniques of sol-gel, ion beam implantation, and
sonochemical preparation.6–11
tive noncarbonaceous fuel that prevents carbon contamina-
tion. The reaction is exothermic and occurred at ϳ280 °C
͑self-ignition temperature͒. The raw material was commer-
cially available. The nitrates were first dissolved in de-
ionized water and then the hydrazine was added in the solu-
tion. The solution was stirred for 20 min at room
temperature. The resulting homogeneous solution was finally
introduced into the reactor. The reactor was then tightly
closed and a flux of 80 sccm of argon was established in
order to create an inert atmosphere inside the reactor. The
reaction was carried out at atmospheric pressure. Further de-
tails of this apparatus and procedures can be found
elsewhere.13 Through the synthesis process, the temperature
as low as 280 °C was required to initiate the reaction. In the
fabricated powders, europium ions are incorporated as the
trivalent state.
In the present communication, we shall present a simple,
low cost and yet highly effective method to prepare the rare-
earth ion Eu3ϩ doped aluminum oxide (Al2O3). We used the
technique of direct combustion synthesis to produce Eu3ϩ
doped Al2O3 powders of various concentrations. By means
of laser ablation, the powders were deposited on a quartz-
glass substrate to form a thin film of a few hundreds nanom-
eters in thickness. The material in both solid and powder
forms has demonstrated strong photoluminescent and
cathodoluminescent properties, which can be attributed to
Once the powders were prepared, we used the laser ab-
lation to form thin films. Laser ablation targets were fabri-
cated from powders of Al2O3 doped with Eu3ϩ ͑1.0 mol %͒.
In order to obtain dense target, the Eu3ϩ doped Al2O3 pellet
was sintered at 1200 °C for 6 h. We selected quartz glass for
the substrate. The substrates were flat and previously de-
greased and cleaned by a standard cleaning process based on
de-ionized water and hydrogen-peroxide solution. Laser ab-
lation was performed by KrF-excimer laser radiation ͑248
the f – f transitions of Eu3ϩ
.
The combustion synthesis method is an excellent tech-
a͒
Electronic mail: mufei@ccmc.unam.mx
0003-6951/2003/83(2)/272/3/$20.00
272
© 2003 American Institute of Physics
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Appl. Phys. Lett., Vol. 83, No. 2, 14 July 2003
Rakov et al.
273
FIG. 1. XRD spectra of Eu3ϩ doped Al2O3 : ͑a͒ as-synthesized powders
doped with 0.0 and 1.0 mol % of Eu3ϩ obtained through combustion syn-
thesis and ͑b͒ Eu3ϩ doped powders ͑1.0 mol %͒ heat treated at 1200 °C and
Eu3ϩ doped Al2O3 thin film heat treated at 1000 °C.
nm wavelength, 25 ns pulse duration, repetition rate 10 Hz,
200 mJ pulse energy͒ by focusing the beam so that a fluence
of 2 J/cm2 can be located at the target. In order to obtain a
homogenous ablation, the targets were rotated and the laser
focus was moved across the target. The substrate-to-target
distance was 2.0 cm. The sample was deposited using the
pressure in the vacuum chamber 10Ϫ5 mbar. For a deposi-
tion time of 2 h, the film thickness was between 650 and 700
nm. After deposition, the films were annealed for 2 h at
1000 °C.
FIG. 2. Scanning electron micrograph of Eu3ϩ doped Al2O3 ͑a͒ precursor
film and ͑b͒–͑c͒ Eu3ϩ doped Al2O3 thin film.
reflections for Eu3ϩ doped Al2O3 film are similar to those of
powders, which indicates that the sample shows the ␣-Al2O3
and ␥-Al2O3 lattice in its content.
The SEM images of Eu3ϩ doped Al2O3 precursor film
and of Eu3ϩ doped Al2O3 thin film were taken to provide
detailed information about the surface morphology and the
homogeneity of the deposited film. Figure 2͑a͒ shows the
SEM images of Eu3ϩ doped Al2O3 powders annealed at
1200 °C for 2 h. It is interesting to note the irregular shape of
agglomerated particles in the photograph of these powders.
Figures 2͑b͒–2͑c͒ show the SEM images of the film. It can
be seen from the images that the film is uniformly composed
of very small particles with regular shapes.
Before the luminescence measurements, the samples in
the form of powders and thin films were examined with
x-ray diffraction ͑XRD͒ for their structure and scanning elec-
tron microscopy ͑SEM͒ for surface topography.
The XRD patterns of the as-synthesized powders doped
at 0.0, and 1.0 mol % of Eu3ϩ, obtained through combustion
synthesis, are shown in Fig. 1͑a͒. For this Eu3ϩ concentra-
tion, the diffraction patterns exhibit reflections characteristics
of the ␣-Al2O3 and ␥-Al2O3 lattice. It is evident from this
figure that the presence of Eu ions inhibits the formation of
the pure ␣ phase in the as-synthesized state. Figure 1͑b͒
shows an XRD pattern of the phosphor powder that was
heated for 2 h in air at 1200 °C and an XRD pattern of the
aluminum film that was heated at 1000 °C for 2 h. It is ob-
served that for thermal treatment in the range of 1200 °C, the
morphology of the powders does not change. In this case, Eu
prevents the complete formation of the ␣-Al2O3 phase at
this temperature because for undoped Al2O3 this phase tran-
sition commonly occurs below 1200 °C.8 The positions of
The luminescence properties were investigated by mea-
suring the photoluminescence ͑PL͒ spectra of as-synthesized
and annealed powders and thin film samples. The PL spectra
were recorded using a monochromator ͑Spex/Triax-180͒ and
detected by a charge coupled device ͑CCD͒ camera. The
light from a 450 W xenon lamp through the monochromator
͑Spex/Triax-180͒ was used for the optical excitation. The
excitation spectra were measured by monitoring the peak in-
tensity at 614 nm. The excitation and emission spectra of
Eu3ϩ doped Al2O3 as-synthesized powders are shown in Fig.
3. The excitation spectrum, performed at room temperature,
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274
Appl. Phys. Lett., Vol. 83, No. 2, 14 July 2003
Rakov et al.
lines represent actually the local environment of the Eu3ϩ
7
ion. Among the 5D0→ FJ transitions, the selection rule
5
7
5
7
makes the D0→ F1 and D0→ F2 transitions of particular
interest. The 5D0→ F1 band at 598 nm is a magnetic dipole
7
one and hardly varies with the crystal field strength around
the Eu3ϩ ion. On the other hand, the hypersensitive transi-
5
7
tion D0→ F2 at 614 nm is electric dipole allowed. Conse-
quently, it depends on the local electric field and, hence, the
local symmetry. Based on the earlier considerations it is clear
that the (5D0→ F2)/(5D0→ F1) intensity ratio, known also
as the asymmetry ratio, gives a measure of the degree of
distortion from the inversion symmetry of the local environ-
ment of the Eu3ϩ ion in the lattice. The asymmetry ratio of
Eu3ϩ doped Al2O3 sample heat treated at 1200 °C is calcu-
lated and has a value of 4. A large value of the asymmetry
ratio obtained for these samples is an indicative of strong
electric fields of low symmetry at the Eu3ϩ ions. This result
suggests that Eu3ϩ ions occupy low symmetry sites as theo-
retically inferred by Verdozzi et al.12
7
7
FIG. 3. PL excitation and emission spectra of as-synthesized powder
samples of Eu3ϩ doped Al2O3 ͑1.0 mol %͒ measured at room temperature.
The cathodoluminescence ͑CL͒ emission spectra of the
three samples were collected using a monochromator ͑Spex/
Triax-180͒ through an optical fiber and detected by a CCD
camera. Figure 4͑b͒ shows the CL spectra, excited with a 5
keV electron, of Eu3ϩ doped Al2O3 thin film ͑1 mol %͒ pre-
pared through ablation laser and annealed at 1000 °C for 2 h.
The largest peak of emission, the red emission line around
614 nm, is assigned to the hypersensitive D0→ F2 transi-
tion working using a forced electric dipole transition mecha-
nism. This is a parity forbidden f – f intraconfigurational
transition. All samples presented an asymmetry ratio of 4,
which indicates that the Eu3ϩ ions occupy a site with no
inversion symmetry. Lack of inversion symmetry at the cat-
ionic site is quite favorable for observing the electric dipole
transition as a forced transition due to the admixture of the
odd parity states.
is observed in the UV region with a maximum at 245 nm.
The strong peak emission was assigned to the transition
7
5D0→ F2 of Eu3ϩ. The as-synthesized powders were ex-
cited at a wavelength of 245 nm. The PL emission lines of
Eu3ϩ doped Al2O3 powders annealed at 1200 °C for 2 h and
excited at a wavelength of 245 nm are shown in Fig. 4͑a͒.
The spectrum consists of a series of well resolved features at
593, 614, and 693 nm, which can be assigned to 5D0
5
7
7
→ FJ , Jϭ1, 2, 4, transitions, respectively. These emission
In summary, we have presented a simple and effective
process to produce Eu3ϩ doped Al2O3 powders and thin
films. The material may have strong fluorescence emission
under light and electron excitation, and the emissions are due
5
7
to the f – f transitions, especially the D0→ F2 .
The authors thank J. McKittrick, I. Gradilla and E.
Aparicio for technical assistance.
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doped Al2O3 powders heated in air at 1200 °C for 2 h and ͑b͒ Eu3ϩ doped
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