The photoluminescence of Eu3+ions results from the radiative intra-configurational f–f transitions that happen between the5DJJ = 0, 1–3 exited states and the7FJJ = 0,1–4 ground states; th
Trang 1Preparation and optical characterization of Eu 3+ -doped CaTiO 3 perovskite powders
Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai Road, Thanh Xuan District, Hanoi, Viet Nam
a r t i c l e i n f o
Article history:
Received 8 March 2012
Received in revised form 13 May 2012
Accepted 20 May 2012
Available online 29 May 2012
Keywords:
CaTiO 3 :Eu 3+
perovskite
Sol–gel method
Absorption
Photoluminescence
a b s t r a c t
CaTiO3perovskite powders doped with 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0 mol% Eu3+were prepared by sol–gel technique followed by annealing at high temperatures The powders were characterized by X-ray diffrac-tion, scanning electron microscopy, Raman scattering, absorpdiffrac-tion, and photoluminescence spectroscopy The obtained powders possessed orthorhombic crystal structure Raman spectra of the CaTiO3:Eu3+ pow-ders exhibited seven new peaks at 798, 1048, 1188, 1371, 1441, 1601, and 1644 cm1which were assigned to the localized vibrational modes related to the complexes containing Eu3+ It was found that the band edge of the material shifted to the higher-energy side with increasing Eu3+-impurity content The photoluminescence of Eu3+ions results from the radiative intra-configurational f–f transitions that happen between the5DJ(J = 0, 1–3) exited states and the7FJ(J = 0,1–4) ground states; the photolumines-cence excitation of Eu3+ions takes place from the7F0ground state to the5DJ(J = 1–4),5L6, and5G2,6exited states
Ó 2012 Elsevier B.V All rights reserved
1 Introduction
Nanophosphors have been extensively investigated during the
last decade because of their application potential for various
high-performance and novel displays and devices The
lumines-cence of rare-earth metal ions has a large technological importance
in a variety of materials widely used in devices like phosphor
lamps, displays, lasers and optical amplifiers The best host for
these rare-earth ions is inorganic materials like Y3Al5O12, Y2O3,
YVO4, LaF3, CaTiO3, LaPO4etc.[1,2]because these ions generally
show high quantum yields in the above hosts
Calcium titanate (CaTiO3) perovskite phosphor has attracted
considerable attention and represents one of the most important
classes of mixed oxides CaTiO3 doped with rare-earth presents
various applications in the field of optoelectronic devices Recently,
rare-earth doped CaTiO3has attracted significant attention because
of its strong luminescence properties, good chemical stability and
promising applications in field emission displays [3] and white
light-emitting diodes[4] However, to the best of our knowledge,
most of studies[3,5–10]were focused on long afterglow
phospho-rescent materials, for example, CaTiO3doped with praseodymium
(Pr3+) ions Europium (Eu3+) ion is one of the most popular and
important rare-earth dopants because Eu3+-doped phosphors are
well known to be promising materials for electroluminescent
devices, optical amplifiers, and lasers In the existing literature,
there are few studies devoted to Eu3+doped CaTiO3(CaTiO3:Eu3+)
[11–13]
CaTiO3was synthesized by various methods: high temperature solid state reaction [6], co-precipitation [8], spray pyrolysis[9], sol–gel[10]and microwave assisted hydrothermal method[13] Among the above mentioned methods, sol–gel is the simple and widely used one for preparation of CaTiO3
In the present paper, we report on CaTiO3:Eu3+ powders pre-pared by sol–gel technique followed by heating at high tempera-tures The powders were characterized by X-ray diffraction, scanning electron microscopy, Raman scattering, absorption, and photoluminescence spectroscopy It was found that the photolumi-nescence (PL) of Eu3+ions results from the radiative intra-configu-rational f–f transitions that happen between the5DJ (J = 0,1–3) exited states and the7FJ(J = 0,1–4) ground states; the photolumi-nescence excitation (PLE) of Eu3+ ions takes place from the 7F0
ground state to the 5DJ (J = 1–4), 5L6, and 5G2,6exited states It was noted that the photoluminescence intensity was strongest in the samples doped with 3.0 mol% Eu3+
2 Experimental
Ca 1x Eu x TiO 3 with x = 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0% mol ratio of Eu 3+
ions pow-ders were synthesized by sol–gel method using the following precursors: CaCl 2 , TiCl 4 , and Eu(NO 3 ) 3 All chemicals are of analytic grade without further purification.
A mixed aqueous solution contained the above chemicals with the appropriate mol ratio Ca: Eu: Ti = (1x): x: 1 was prepared The mixture was then constantly stirred
to get an opalescent solution Citric acid (CA) was dissolved in the double distilled water to form a 50% CA solution The CA solution was added into the opalescent mixture under constant magnetic stirring at 90 °C After 4 h of stirring, the sol chan-ged into a yellow chrome homogeneous gel The gel was dried at 120 °C for 24 h to remove the water, and was then annealed at 300 °C for 30 min After that, an ash-gray powdered product was obtained In order to support the crystallization 0925-8388/$ - see front matter Ó 2012 Elsevier B.V All rights reserved.
⇑Corresponding author.
E-mail address: longnn@vnu.edu.vn (N.N Long).
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Trang 2of materials, the resultant product was annealed at different temperatures ranging
from 700 to 1000 °C in air for 2 or 4 h After the heat treatment powdered materials
had white color.
Crystal structure of the powders was analyzed by X-ray diffraction (XRD) using
an X-ray diffractometer SIEMENS D5005, Bruker, Germany with Cu Ka1 (k =
0.154056 nm) irradiation The surface morphology of the samples was observed
by using a JSM 5410 LV, JEOL, Japan scanning electron microscope (SEM) The
com-position of the samples was determined by an energy-dispersive X-ray
spectrome-ter (EDS) OXFORD ISIS 300 attached to the JEOL-JSM5410 LV scanning electron
microscope Raman scatting spectra measurements were carried out by using
LabRam HR800, Horiba spectrometer with 632.8 nm excitation Diffuse reflection
spectroscopy measurements were carried out on a VARIAN UV–VIS–NIR
Cary-5000 spectrophotometer The spectra were recorded at room temperature in the
wavelength region of 200–900 nm Absorption spectra of the samples were
ob-tained from the diffuse reflectance data by using the Kubelka–Munk function [14] :
FðRÞ ¼ð1 RÞ
2
2R ¼
K
where R, K and S are the reflection, the absorption and the scattering coefficient,
respectively The PL and the PLE spectra measured at room temperature were carried
out on a spectrofluorometer Fluorolog FL 3-22 Jobin–Yvon–Spex, USA with a 450 W
xenon lamp as an excitation source.
3 Results and discussion
3.1 Structure characterization and morphology
Fig 1(a) shows XRD patterns of the powders CaTiO3doped with
3.0 mol% Eu3+annealed at different temperatures ranging from 300
to 1000 °C for 2 h As can be seen from the figure, the samples
an-nealed at 300 °C exhibited a bad crystallinity: The characteristic
peaks of CaTiO3appeared with very weak intensity The samples exhibited better cystallinity with increasing annealing tempera-ture At calcinating temperatures of 800, 900, and 1000 °C the sam-ples displayed a good crystallization
XRD patterns of the powders CaTiO3undoped and doped with 1.5, 2.0, 3.0, and 5.0 mol% Eu3+annealed at 1000 °C for 2 h in air are shown in Fig 1(b) All the peaks in the XRD patterns clearly indicate that the CaTiO3:Eu3+samples possess orthorhombic crys-tal structure No other diffraction peaks are detected except for the CaTiO3related peaks
The lattice constants determined from the XRD patterns are
a = 5.432 Å, b = 7.643 Å and c = 5.390 Å, which are in good agree-ment with the standard values (a = 5.440 Å, b = 7.643 Å and
c = 5.381 Å, JCPDS card No 22-0153) The average size of the crys-tallites was estimated by Debye–Scherrer’s formula[15]:
where b is the full width at half maximum (FWHM) in radians of the diffraction peaks, h is the Bragg’s diffraction angle and k = 0.154056 nm The calculated size of the CaTiO3 nanocrystallites was estimated to be 24 nm
with 3.0 mol% Eu3+calcined at 1000 °C for 2 h under atmospheric condition are shown inFig 2 From the figure it can be noted that the fine crystallites agglomerated into big slabs with the size of several micrometers
Representative EDS spectra of the CaTiO3powders are shown in
Fig 3 The EDS spectrum of the CaTiO3sample doped with 5.0 mol%
Eu3+exhibits the peaks related to element Eu It is noted that the gold peaks observed in the EDS spectra originated from the gold
Fig 1 XRD patterns of (a) the powders CaTiO 3 doped with 3.0 mol% Eu 3+
annealed
at different temperatures ranging from 300 to 1000 °C for 2 h, (b) the powders
CaTiO 3 undoped and doped with 1.5, 2.0, 3.0, and 5.0 mol% Eu 3+
annealed at 1000 °C Fig 2 Typical SEM images of CaTiO 3 (a) undoped and (b) doped with 3.0 mol% Eu 3+
Trang 3layer deposited on silicon substrate for enhancement of
conductiv-ity in the EDS measurement
It is known that effective radii of Ca2+, Eu3+, and Ti4+ ions in
octahedral sites are 1.00, 0.947, and 0.605 Å, respectively[16] It
is expected that the Eu3+ions can substitute for the Ca2+ions more
easily than for the Ti4+ions in CaTiO3:Eu3+lattice because ionic
ra-dii for Ca2+and Eu3+are close In addition, Mazzo et al.[13]showed
a simulated orthorhombic lattice of CaTiO3:Eu3+, which illustrated
the substitution of Eu3+ions for Ca2+ions in octahedral sites
3.2 Raman scattering spectra
Raman spectroscopy is an important and useful tool for
obtain-ing information about the vibrational modes of the materials and it
is well known that a small concentration of impurities introduced
into a perfect crystal will have little effect on vibrational modes, in
some case even there may appear vibrational modes lying outside
of the allowed frequency range of the perfect crystal[17] These are
called localized vibrational modes (LVMs)
Typical room temperature Raman spectra of the undoped
CaTiO3and the CaTiO3:Eu3+powders with various contents of Eu
are shown inFig 4 As seen from the figure, the spectra can be
divided into two regions In the low-energy region we found 10 peaks at 154, 177, 222, 245, 287, 337, 471, 494, 530, and
640 cm1, which are in good agreement with the existing literature and are usually attributed to the Raman modes of the orthorhom-bic crystal structure CaTiO3[18–24] The peak at 154 cm1is re-lated to the CaTiO3lattice mode The peaks at 177, 222, 245, 287,
at 471, 494, and 530 cm1 are related to Ti–O3torsional modes and the 640 cm1peak is characteristic of Ti–O symmetric stretch-ing mode It is intereststretch-ing to note that when europium was intro-duced into the CaTiO3powders, in the high-energy region we first time observed seven new peaks at 798, 1048, 1188, 1371, 1441,
1601, and 1644 cm1(lines b–e inFig 4) These vibrational modes may be related to LVMs of the Eu3+-containing complexes with dif-ferent configurations
3.3 Absorption and photoluminescence spectra
Fig 5depicts diffuse reflection spectra measured at room tem-perature of the undoped CaTiO3and the Eu3+-doped CaTiO3 pow-ders with various dopant contents Can be seen that in addition
to the strong absorption in the energy region higher than 3.75 eV, four weak absorption peaks located at 3.12 eV (397 nm), 2.66 eV (466 nm), 2.07 eV (599 nm), and 1.92 eV (646 nm) were clearly observed from the reflection spectra of the 2.0, 3.0 and 5.0 mol% Eu-doped CaTiO3samples, in which two absorption peaks
at 3.12 and 2.66 eV were found by previous work[13] These four absorption peaks can be assigned to the transitions7F0?5L6,5D2,
5D0, and7F3?5D0of the Eu3+ion, respectively, because their ener-gies are in good agreement with those of the basic and excited states of the Eu3+ion[2]
Absorption spectra of the CaTiO3:Eu3+ samples obtained from the diffuse reflectance data by using the Kubelka–Munk function F(R) are shown inFig 6 All the spectra exhibit a sharp absorption edge and an onset of absorption at 3.5–3.6 eV The inset ofFig 6
obviously shows the mentioned above four of absorption peaks re-lated to the optical transitions within Eu3+ion in the CaTiO3 sam-ples doped with 2.0, 3.0, and 5.0 mol% Eu3+
It is known that the band structure of the CaTiO3 displays a direct band gap atCpoint[25] The relation between the absorp-tion coefficients (a) and the incident photon energy (hm) for the case of allowed direct transition is written as follows[26]:
Fig 3 The EDS spectra of the undoped CaTiO 3 and Eu 3+
-doped CaTiO 3 powders with 5.0 mol% Eu 3+
Fig 4 Typical Raman spectra of the CaTiO 3 powders undoped and doped with 1.0,
3+
Fig 5 Diffuse reflection spectra at room temperature of the undoped CaTiO 3 and the Eu 3+ -doped CaTiO 3 powders Four absorption peaks related to the optical transitions within Eu 3+
ion are clearly observed in the spectra of the 2.0, 3.0, and 3+
Trang 4ahm¼ Aðhm EgÞ1=2 ð3Þ
where A is a constant and Eg is the band gap of the material The
plots of ½FðRÞ hm2versus hmfor the undoped and the Eu3+-doped
CaTiO3 powders are represented in Fig 7 By extrapolating the
straight portion of the graph on hmaxis ata= 0, we found the band
gaps of the CaTiO3powders doped with the concentration of 0, 1.5,
2.0, 3.0, and 5.0 mol% Eu3+ to be 3.670, 3.670, 3.687, 3.695, and
3.719eV, respectively Thus, with increasing Eu3+-dopant content
from 0 to 5.0 mol%, the optical band gap is gradually increased from
3.670to 3.719eV The similar phenomenon was also observed for
ZnO doped with any of the group III elements (B, Al, Ga, In) and
for many various semiconductors (see, for example, Ref.[27])
This phenomenon can be explained as follows When the Eu
impurity atoms of valence 3 substitute for the Ca atoms of valence
2 in CaTiO3:Eu3+lattice, the Eu atoms become donors, which can
give up conduction electrons If we introduce a lot of the Eu donors,
the conduction electron concentration is increased; the Fermi level
will rise more and more towards the conduction band Since the
states below the Fermi level are already filled, according to the
Pauli Exclusion Principle, the fundamental transitions to the states
below the Fermi level are forbidden; hence the optical absorption
edge should shift to higher energy side This is the well-known
Burstein–Moss effect [28–30] According to the Burstein–Moss
effect, the broadening of the optical band gapDEgis:
DEg¼ h 2m eh
where h is the reduced Planck’s constant, m
ehis the reduced effec-tive mass of electron and hole (1
m eh
m þ 1
m ;m and m are the effective masses of electron and hole, respectively), and n is the car-rier concentration Therefore, the increase of Eu3+impurity content making carrier concentration increase, leads to the high-energy shift of the band gap, as observed in our experiment
In order to confirm the mentioned explanation, we measured the resistivity of some CaTiO3samples with various Eu3+contents The powders were pressed in tablet form with the size of 0.5 mm
in thickness and 0.6 cm in diameter by a pressure of 4.3 108Pa The impedance of the tablets was measured at room temperature The resistivities (q) were found to be >5.0 108, 2.0 108, and 8.5 107Xcm for undoped, 3 mol% Eu3+, and 5 mol% Eu3+doped CaTiO3powders, respectively Thus, with increasing the Eu3+ dop-ant content the conduction electron concentration increases, which decreases the samples resistivity
Fig 8shows the room temperature PL spectra under excitation wavelength of 398 nm of CaTiO3powders doped with various con-centrations of Eu3+ It is noted from the inset of theFig 8that the
PL intensity was strongest in the samples doped with 3.0 mol%
Eu3+ When increasing Eu3+ concentration higher than 3.0 mol% the PL intensity decreased Recently, Fu et al reported that the optimal concentrations for obtaining the highest PL intensity of CaTiO3:Eu3+ were 28 mol% of Eu3+ in the samples prepared by solid-state reaction [11] and 16 mol% of Eu3+ in those prepared
by sol–gel method[12], while Mazzo et al reported that the opti-mal concentration of Eu3+is 1 mol%[13] When the concentration
of an activator is higher than an appropriate value, the lumines-cence of the phosphor is usually lowered This effect is called con-centration quenching The origin of this effect is known to be one of the following: the cross-relaxation between the activators, excita-tion energy migraexcita-tion to quenching centers or the surface states acting as quenching centers, the pairing or coagulation of activator ions and their change to quenching centers As mentioned above, the concentration quenching occurs at different concentrations maybe because the samples were prepared by different methods
In fact, under various technological conditions Eu3+ions were dif-ferently incorporated into the samples
In order to interpret the origin of the emission lines, the room temperature PL spectrum under 398 nm excitation wavelength of CaTiO3powder doped with 3.0 mol% of Eu3+is illustrated inFig 9 The groups of emission lines located in the range of wavelength from 590 to 725 nm are attributed to the radiative transitions from
Fig 6 Plots of Kubelka–Munk F(R) versus photon energy hmfor the undoped CaTiO 3
and the Eu 3+
-doped CaTiO 3 powders The inset shows four absorption peaks related
to the optical transitions within Eu 3+
ion in the spectra of the 2.0, 3.0, and 5.0 mol%
Eu 3+ -doped CaTiO 3 samples.
Fig 7 The plots of ½FðRÞ hm2
versus photon energy hmfor the undoped CaTiO 3 and
3+
Fig 8 Room temperature PL spectra under excitation wavelength of 398 nm of
3+
Trang 5the5D0exited states to the7FJ(J = 1–4) ground states, namely, the
groups of lines at 592, 615, 654, and 695 nm are assigned to the
emission transitions from the5D0excited state to the7F1,7F2,7F3,
and7F4ground states, respectively Some groups of very weak
emis-sion lines at 430, 447, 465, 489, 511, 527, 540, 555, and 580 nm are
assigned to5D3?7F0,7F2; 5D2?7F0,7F2,7F3; 5D1?7F0,7F1,7F2;
and5D0?7F0transitions, respectively (the inset ofFig 9)
It is worth noting that all the emission line groups have the
same excitation spectra, which prove that all these lines possess
the same origin Typical PLE spectrum monitored at 615 nm
emis-sion line of CaTiO3:3.0 mol% Eu3+powders is depicted inFig 10
The groups of excitation lines located around 362, 376, 398, 418,
465, and 526 nm are attributed to the absorption transitions from
the7F0ground state to the5D4,5G2,6,5L6,5D3,5D2, and5D1excited
states, respectively
Fig 11shows the energy level diagram of Eu3+ions and the
ob-served excitation and emission transitions in f–f configuration of
Eu3+ions
Finally, it is noted that contrary to Pr-doped CaTiO3powders,
our CaTiO3:Eu3+samples do not exhibit a long afterglow
lumines-cence The afterglow luminescence (phosphorescence) occurs due
to the thermally stimulated recombination of trapped charged
car-riers.Fig 12depicts the decay behavior of the 615 nm (5D0?7F2
transition) emission line for Eu3+in the CaTiO3:3.0 mol% Eu3+
sam-ples As seen from the figure that the experimental data were very
well fitted using a double-exponential function:
where I(t) is the phosphorescence intensity, A1and A2are the con-stants, ands1ands2are the decay constants (or lifetimes) The re-sults showed that two lifetimes, a fast ones1= 0.194 ms, and a slow ones2= 0.919 ms have been observed for the5D0?7F2emission of
Eu3+ The fact that our CaTiO3:Eu3+samples do not exhibit a long afterglow luminescence indicated there are not the metastable traps in these samples
4 Conclusion CaTiO3:Eu3+ perovskite powders were synthesized by sol–gel method followed by annealing at high temperatures At calcinating temperatures higher than 800 °C the samples displayed a good crystallization The obtained powders possess orthorhombic crys-tal structure with lattice constants a = 5.432 Å, b = 7.643 Å and
c = 5.390 Å The average sizes of the crystallites estimated by De-bye–Scherrer’s formula are 24 nm Raman scattering spectra show
7 new peaks observed at 798, 1048, 1188, 1371, 1441, 1601, and
1644 cm1 These vibrational modes may be related to LVMs of the complexes containing Eu3+with different configurations With increasing Eu3+-dopant content from 0 to 5.0 mol%, the optical band gap is gradually increased from 3.67 to 3.71 eV, which is
Fig 9 Room temperature PL spectrum under 398 nm excitation wavelength of
CaTiO 3 powder doped with 3.0 mol% of Eu 3+
Fig 10 Typical PLE spectrum monitored at 615 nm emission line of CaTiO 3 :3.0
-mol% Eu 3+ powders.
Fig 11 Energy level diagram of Eu 3+
ions and the observed excitation and emission transitions.
Fig 12 Decay curve of the 615 nm ( 5
D 0 ?7F 2 transition) emission line for Eu 3+
in the CaTiO 3 :3.0 mol% Eu 3+
samples.
Trang 6assigned to Burstein–Moss effect The photoluminescence intensity
is strongest in the samples doped with 3.0 mol% Eu3+ The PL of
Eu3+ions results from the radiative intra-configurational f–f
transi-tions that happen between the5DJ(J = 0, 1–3) exited states and the
7FJ(J = 0,1–4) ground states; the PLE of Eu3+ions takes place from
the7F0ground state to the5DJ(J = 1–4),5L6, and5G2,6exited states
Two lifetimess1= 0.194 ms, ands2= 0.919 ms have been observed
for the5D0?7F2emission of Eu3+
Acknowledgments
This work is supported in part by the Grant-in-Aid for Scientific
Research from Ministry of Science and Technology of Vietnam
(NAFOSTED, Project No 103.02.51.09) Authors thank Dr Tran Thi
Kim Chi for the decay time measurement
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