Spectroscopic properties of Nd3+ in MgAl2O 4 spinel nanocrystals

Article abstract of DOI:10.1016/j.jallcom.2012.02.065

Nd³⁺ doped MgAl₂O₄ spinel nanocrystals have been prepared by the sol-gel method. Their size decreases from 12 to 7 nm with increasing Nd³⁺ concentration from 0.1 to 5%, respectively. Some crystal field component

Full text of DOI:10.1016/j.jallcom.2012.02.065

Journal of Alloys and Compounds 525 (2012) 39–43
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Spectroscopic properties of Nd3+ in MgAl2O4 spinel nanocrystals
P.J. Deren´ ∗, K. Maleszka-Bagin´ ska, P. Głuchowski, M.A. Małecka
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul.Okólna 2, 50-422 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Nd3+ doped MgAl2O4 spinel nanocrystals have been prepared by the sol–gel method. Their size decreases
from 12 to 7 nm with increasing Nd3+ concentration from 0.1 to 5%, respectively. Some crystal field com-
ponents of the Nd3+ energy levels in MgAl2O4 spinel were found from the 300 K reflectance absorption
Article history:
Received 25 November 2011
Received in revised form 2 February 2012
Accepted 6 February 2012
and the 77 K emission spectra. The 4F3/2
→
4I9/2 transition dominates in the emission spectra and accord-
Available online 16 February 2012
ingly spectroscopic-quality parameter (SQP) is equal to 3.3 for all samples. Decay time of the emission
from the 4F3/2 level in MgAl2O4:Nd3+ (0.1%) nanocrystals is equal to 90 ␮s, and decreases with increasing
neodymium concentration due to very efficient cross-relaxations. The slope of cross-relaxation rate is
close to 2 which indicates fast donor–donor transfer.
Keywords:
Nanocrystallite
MgAl2O4 spinel
Nd3+
© 2012 Elsevier B.V. All rights reserved.
Cross-relaxation
Sol–gel process
1. Introduction
Mg(CH3COOH)2·4H2O (Alfa Aesar 99.99%), AlCl3·6H2O (Alfa Aesar 99.99%), and
Nd2O3 were chosen. Citric acid and ethylene glycol were added as a chelating and
polymerization agents, respectively.
It was shown that change of the crystallites size from bulk
to nano often results in new structural, electronic, and spectro-
scopic properties [1], therefore rare-earth activated nanomaterials
are nowadays investigated extensively. One of the potential appli-
cations of these nanomaterials are phosphors in plasma display
devices (PDP), field emission displays (FED), cathode ray tubes.
Magnesium aluminate belongs to wide spinel family. MgAl2O4
has the cubic crystal system with space group Fd3m [2,3]. In nor-
mal spinel Mg2+ ion occupy tetrahedral sites and Al3+ octahedral
ones. Nanosized MgAl2O4 spinel was synthesized by several meth-
ods [4–7]. Magnesium spinel was doped with Cr3+, Co2+, and Ti3+
[8–10]. The effect of Tb3+ doping on the structure and spectroscopic
properties in nanosized MgAl2O4 was investigated recently [11].
In this study we presented the results of synthesis of magnesium
aluminate spinels doped with neodymium and their luminescent
properties. It was assumed that Nd3+ ions are substituted for Mg2+
ions in MgAl2O4. Mismatch of valence and size of the replaced
ions is manifested by broad excitation and emission bands. Influ-
ence of dopant concentration on spectroscopic properties was also
investigated.
Stoichiometric amounts of substrates were dissolved in deionized water. Nd2O3
was dissolved in dilute HNO3 to transfer them into soluble in water nitrate salt and
the excess of acid was evaporated at high temperature. Afterwards all substrates
were dissolved in deionized water and mixed with the solution of ethylene glycol
and citric acid. The fixed molar ratio of citric acid and ethylene glycol to total chelated
metal cations was 5:1. After that final homogeneous solution was placed into plastic
containers and kept into dryer at 80 ◦C until brown resin was obtained. Subsequently
above resin was sintered at 900 ◦C for 3 h in air atmosphere with 15 ◦C/min temper-
ature rise step. Structure and morphology studies of the obtained nanocrystallite
powder were characterized using XRD powder diffraction and TEM microscopy.
The XRD patterns were measured using powder diffraction method on Panality-
cal X’pert equipped with Cu K␣ lamp. As a continuous excitation source 808 nm laser
diode was used. Pulse excitation source was provided by second harmonic (532 nm)
of Nd:YAG laser line. As a detector InGaAs diode was used. Emission spectra were
measured on a McPherson spectrometer equipped with 150 W Xe lamp. Emission
spectra were corrected for spectrophotometer sensitivity. To measure decay profiles
we used digital oscilloscope LeCroy WaveSurfer 400.
3. Results and discussion
The XRD spectra (see Fig. 1) confirm that obtained powders are
pure phase of MgAl2O4. Samples were annealed for short time, only
3 h and that makes an opportunity to receive nanocrystallites with
an average grain sizes about 11 nm which was estimated using
Scherrer’s formula [13]. With increasing Nd3+ content the lines in
the XRD spectrum become broader, which is due to decreasing of
the mean nanocrystallite size from 12 to 7 nm for 0.1 and 5% of
Nd3+, respectively.
2. Experimental
Samples with varied neodymium concentration (0.1%, 0.5%, 1.0%, 2%, 5%)
were prepared by modified Pechini’s method [12]. As the starting materials
Morphology of nanopowders was characterized with Transmis-
sion Electron Microscopy (TEM). TEM images are presented in Fig. 2.
This investigation revealed range of grains size from 2 to 20 nm for
∗
Corresponding author.
E-mail address: P.Deren@int.pan.wroc.pl (P.J. Deren´ ).
0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.02.065

40
P.J. Deren´ et al. / Journal of Alloys and Compounds 525 (2012) 39–43
Fig. 1. XRD patterns of MgAl2O4:Nd3+
.
Fig. 3. Reflectance absorption spectra of MgAl2O4:1% Nd3+ measured at 300 K.
the sample with the lowest concentration of neodymium (0.1%).
When Nd3+ content increases then growing of nanograins is inhib-
ited and therefore for the sample with 5% of Nd3+ the mean grain
size ranges from 4 to 12 nm. Distribution of the nanocrystallite
diameter decreases with increasing Nd3+ concentrations. Inhibi-
tion of the crystals’ growth is associated with crystallization process
which depends on amount of dopant dissolved in a host lattice. This
phenomenon was observed already by other authors [14].
the Nd3+ site. Similar values were observed in many other hosts like
for example in: NaGdGeO4 for which ꢀE = 215 cm−1 or La2Be2O5;
ꢀE = 215 cm−1 [15].
The emission spectra were measured at room temperature (see
Fig. 4) and at 77 K (see Fig. 5). At 77 K the emission goes only from
4
the lowest component of the F3/2 level, therefore it was possi-
ble to distinguish transitions to the crystal field components of the
Reflectance absorption spectra of MgAl2O4:Nd3+ (5%) (see
Fig. 3) were measured at room temperature. Characteristic bands
for neodymium(III) ions are observed, the most intense is the
4IJ therm. Three characteristic bands centered at 896, 1074, and
4
1352 nm are observed. They are due to transitions from the F3/2
level to the ground, first, and second excited levels, the 4I9/2 4I11/2
, ,
4
4I9/2 → G5/2 hypersensitive transition associated with the lan-
and the 4I13/2, respectively. Transition to the ground 4I9/2 level dom-
4
thanide’s ion surrounding [15]. Second intense band centered at
inates the emission spectra. The 4F9/2 → I11/2 transition has almost
803 nm (12 450 cm−1) assigned the 4I9/2 → F5/2 transition is broad.
the same intensity, third band is much less intense (see Fig. 4), the
4
4
Its full width at half its maximum intensity (FWHM) is equal to
450 cm−1. It is useful feature because the energy of this band will
match energy of wide range of laser diodes and thermal drift of
the output diode emission wavelength will not affect pumping sig-
nificantly. The reflectance absorption spectra and low temperature
emission spectra allowed to obtain Nd3+ energy levels in MgAl2O4
4F9/2 → I15/2 transition was not observed in our setup.
Increased Nd3+ concentration changes the emission spectra, the
most significant changes are observed for spectra recorded at 77 K
(see Fig. 5). With increasing the Nd3+ concentration from 0.1 to 5%
all lines become broader and exhibit red shift of 6 cm−1. For 0.1%
of Nd3+ the most intensive peak in all bands is due to transition to
the ground component of the 4I9/2 level. Comparing the integrated
intensity of this most intense peak to the integrated intensity of the
4
spinel, they are listed in Table 1. The spitting of the F3/2 level is
equal to 210 cm−1, this rather high value is due to low symmetry of
Fig. 2. TEM images of MgAl2O4:Nd3+ (sample 0.1% on left and sample 5.0% on right side) prepared by the Pechini method.

P.J. Deren´ et al. / Journal of Alloys and Compounds 525 (2012) 39–43
41
Table 1
Energy levels and its Stark components found from the 300 K reflectance absorption and the 77 K emission spectra of nanocrystallite MgAl2O4:Nd3+
.
Energy level of Nd3+
Energy (cm−1
)
ꢀE (cm−1
)
Number of levels
Theoretical
Experimental
a
4I9/2
0, 71, 175, 336, 483a
1958, 2099, 2234a
3911, 4005, 4068, 4242a
5835, 6134, 6350, 6625a
11 389 a,b, 11 599 a,b
12 023, 12 386, 12 523b
13 324, 13 555b
483
276
331
790
210
694
231
–
5
6
7
8
2
7
6
5
6
7
16
6
3
11
5
3
4
4
2
3
2
1
1
2
3
2
2
2
a
4I11/2
a
4I13/2
4I15/2
a
4F3/2
a,b
b
4F5/2
4F7/2
4F9/2
+
+
4H9/2
b
4S3/2
b
14 652b
b
2H11/2
15 966b
–
b
4G5/2
4G7/2
+
+
2G7/2
4G9/2
16 669, 17 179b
510
947
706
628
548
b
+
2K13/2
18 957, 19 531, 19 904b
20 990, 21 696b
b
4G11/2
b
2D5/2
23 153, 23 781b
b
4D3/2
+
4D5/2
+
2I11/2
27 909, 28 457b
a
Data collected from the emission spectra measured at 77 K.
Data collected from the reflectance absorption spectra measured at 300 K.
b
4
remaining part of the 4F3/2 → I9/2 band one can notice that the lat-
ter increases with Nd3+ concentration and is 1.8 times more intense
for 5% of Nd3+ than for 0.1% sample. The full width at half maximum
also increases with concentration from 631 for 0.1% to 691 cm−1 for
5% of dopant. This observation is in general valid for all emission
bands. The transition intensity to the lower crystal field component
decreases and the emission bands are broader with increasing Nd3+
concentration.
Since the spectra were corrected for the sensitivity of the spec-
trophotometer we could calculate so called spectroscopic quality
parameter (SQP) ꢂNd (where YNd = 4I11/2/4I13/2 and IJ are integrated
intensities of the relevant emission bands), SQP was calculated
using below equation [15]:
ꢂNd = 0.765YNd − 2.96
(1)
␹
by definition is equal to ꢃ4/ꢃ6 ratio, where ꢃꢁ are
Nd
It indicates that Nd3+ ions are located in many non-equivalent
positions in MgAl2O4 nanocrystals. The number of so many “sites”
is created by the sample morphology. It is evident that volume to
surface ratio is small in nanocrystals comparing to bulk materi-
als. Taking the magnesium spinel lattice constant as the thickness
of sample surface one can easily calculate that for the samples
doped with 5% Nd3+ (nanocrystallite diameter equal 7 nm) almost
1/3 of spinel cells are located on the surface, while for the 0.1% of
Nd3+ (nanocrystallite diameter equal 12 nm) only 1/5. Neodymium
ions located in “surface unit cells” see another surroundings
than ions inside the crystallite. Since with increased neodymium
concentration crystallite size decreased more Nd3+ ions are located
on the surface. Valence mismatch between Mg2+ and Nd3+ and
difference in their size, 86 and 112 pm, respectively, creates addi-
tional defects like vacancies or inversion of spinel structure [3]. As a
consequence with increasing dopant concentration stronger broad-
ening is observed in the emission spectra.
Judd–Ofelt intensity parameters [16,17]. It has values greater than
3, which was reported only for few crystals. According to graph
of branching ratio presented by Kaminski [15] such high value is
Fig. 5. The 4F3/2
sured at 77 K in function of neodymium(III) concentration.
→
4I11/2 transition of MgAl2O4:Nd3+ excited by ꢁexc = 808 nm, mea-
Fig. 4. Emission spectra of MgAl2O4:Nd3+ excited by ꢁexc = 808 nm, measured at
300 K and 77 K for 1% of Nd3+
.

42
P.J. Deren´ et al. / Journal of Alloys and Compounds 525 (2012) 39–43
Table 2
Spectroscopic-quality parameter values obtained from the emission spectra of
nanocrystalline MgAl2O4:Nd3+recorded at 300 K.
Concentration of Nd3+ (%)
SQP
0.1
0.5
1.0
2.0
5.0
3.34
3.02
3.34
3.32
3.32
typical for the host when the 4F3/2 → I9/2 is the most intense tran-
sition. Indeed the first band either at room and 77 K is the strongest
one. We have to stress also that Judd–Ofelt intensity parame-
ters are host depended and do not depend on concentration. In
MgAl2O4:Nd3+ the ꢂNd changes slightly from sample to sample (see
Table 2) and these changes are so small that could be assigned to
experimental error. Therefore, one may conclude that neodymium
concentration in the range 0.1–5% do not change properties of the
MgAl2O4 host.
4
4
Fig. 7. Decay time of the F3/2 emission of Nd3+ ions in MgAl2O4 in function of
4
The luminescence decay profiles of the F3/2 emission were
neodymium concentration measured at room and liquid nitrogen temperature. The
lines serve to guide the eyes.
recorded at 877 nm. They have multiexponential character, which
is seen especially for the sample with 5% of Nd3+ (see Fig. 6) and
decay time was calculated as
ꢀ
I(t)dt
I0
ꢄ =
.
(2)
For 77 K decay times are shorter than at room tempera-
ture. Decrease of the lifetime with temperature decreasing was
also observed by the other authors [18–20]. They explained this
phenomenon by taking into account decay time of the second com-
ponent of the 4F3/2 level. At 77 K the higher component is no more
populated and we can measure decay constant of only lower one.
4
Consequently lifetime of the F3/2 levels at 300 K is described by
the following equation:
1 + e−ꢀE/kT
ꢄrad
=
(3)
(1/ꢄ1) + (1/ꢄ2)e−ꢀE/kT
where ꢄ1 and ꢄ2 are the lifetime of lower and upper crystal field
components of the 4F3/2 level. Energy separation between them is
ꢀE = 210 cm−1 (see Table 1). For calculation we took for ꢄ1 decay
time measured at 77 K. The estimated from Eq. (3) lifetime ꢄ2 is
equal 340 ␮s, which is quite similar to values obtained by the other
authors [18–20]. The calculations were done for sample with the
Fig. 8. Cross relaxation rate of the 4F3/2 level at liquid nitrogen and room tempera-
ture in function of Nd3+ concentration in the MgAl2O4 spinel lattice.
lowest concentration of neodymium, to avoid the errors introduced
by the concentration quenching processes.
Decay times of the 4F3/2 emission decrease with increasing Nd3+
concentration(seeFig. 7) becausenonradiativetransitionsincrease.
The shortening of decay time is significant and for 5% Nd3+ decay
time has only 12% of its value registered for 0.1%. We have measured
decay time of the sample doped only with 0.001% of Nd3+. The decay
profile at 300 K is single exponential (see Fig. 6) and decay time is
equal to 150 ␮s, it is the same value calculated applying Eq. (2) for
the 0.1% sample.
4
Since energy gap between the F3/2 and 4IJ manifold is large,
multiphonon nonradiative transitions could be neglected in this
host. One can argue that in such small nanoparticle effective sur-
face is large and adsorb gases, which may quench emission, but in
function of concentration the main factor which makes emission
shorter is cross-relaxation (CR). There are several possibilities of
almost resonant CR, for example:
(4F3/2
(4F3/2
(4F3/2
(4F3/2
,
,
,
,
4F3/2) → (4I11/2
, )
4G11/2
4F3/2) → (4I13/2, (4G7/2 + 4G9/2 + 2K13/2))
4F3/2) → (4I15/2, (4G5/2 + 2G7/2))
4
Fig. 6. Decay profiles of the F3/2 emission of Nd3+ ions in MgAl2O4 measured at
300 K.
4I9/2) → (4I15/2
, )
4I15/2

P.J. Deren´ et al. / Journal of Alloys and Compounds 525 (2012) 39–43
43
This type of rapid donor–donor transfer was described by Huber
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4
tration from 0.1 to 5%, respectively. For all samples the 4F3/2 → I9/2
transition is the most intense in the emission spectra. It is well
correlated with spectroscopic quality factor equal to 3.3, which
remains the same in the Nd3+ concentration range equal to 0.1–5%.
It indicates that dopant do not changes host properties up to 5%.
Emission intensity as well as decay times of higher doped samples
are affected by cross-relaxation processes which are very effective.
Acknowledgment
This work was financially supported by the Ministry of Science
and Higher Education under Grant no. N N507 372335, which is
greatly acknowledged.