Tm3+ doped barium molybdate: A potential long-lasting blue phosphor

Article abstract of DOI:10.1016/j.jallcom.2017.10.225

Molybdates have applications in various fields such as phosphors, optical fibers, scintillators, magnets, sensors and catalysts. Thulium ion is an excellent blue activator and plays an important role in the design of persistent luminescent materials. This paper reports the investigation of the structural and luminescent properties of Barium Thulium Molybdate (Ba_1?xTm_x)MoO₄ microcrystals (with x = 0, 0.01 or 0.03). The scheelite-type crystalline structure was identified from XRD and Raman studies. Under ultraviolet (359 nm) excitation, photoluminescence (PL) spectra present the characteristic emission bands at 453 nm and 545 nm which are due to the D1₂→F3₄ and D1₂→H3_4,5 transitions, respectively, from Tm³⁺ ions. Thermoluminescence (TL) measurements were performed with powdered samples previously irradiated with beta radiation. The depth of traps, associated with trap levels located inside the band-gap, was determined from TL data using different methods of glow curve analysis. The kinetic parameters, determined from thermoluminescent glow curves, provide evidence about a possible persistent luminescence emission from the (Ba_0.97Tm_0.03)MoO₄ sample. This is a potential blue-light or ultraviolet long-lasting phosphor, with a trapping level lifetime, at room-temperature (300 K), of about 6 days.

Full text of DOI:10.1016/j.jallcom.2017.10.225

Accepted Manuscript
3+
Tm doped barium molybdate: A potential long-lasting blue phosphor
Ana Paula de Azevedo Marques, Roseli Künzel, Nancy K. Umisedo, Renato Mazin
Latini, Elizabeth Mateus Yoshimura, Emico Okuno
PII:
S0925-8388(17)33661-7
10.1016/j.jallcom.2017.10.225
JALCOM 43622
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 4 August 2017
Revised Date: 23 October 2017
Accepted Date: 24 October 2017
Please cite this article as: A.P.d.A. Marques, R. Künzel, N.K. Umisedo, R.M. Latini, E.M. Yoshimura, E.
3+
Okuno, Tm doped barium molybdate: A potential long-lasting blue phosphor, Journal of Alloys and
Compounds (2017), doi: 10.1016/j.jallcom.2017.10.225.
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ACCEPTED MANUSCRIPT
Tm3+ doped barium molybdate: A potential
long-lasting blue phosphor
Ana Paula de Azevedo Marquesa, Roseli Ku¨nzelb, Nancy K. Umisedoc, Renato
Mazin Latinia, Elizabeth Mateus Yoshimurac, Emico Okunoc
aUniversidade Federal de Sa˜o Paulo - UNIFESP, Departamento de Qu´ımica, R. Sa˜o
Nicolau, 210 - Centro, Diadema - SP, CEP 09913-030 Brazil
bUniversidade Federal de Sa˜o Paulo - UNIFESP, Departamento de F´ısica, R. S˜ao Nicolau,
210 - Centro, Diadema - SP, CEP 09913-030 Brazil
cUniversidade de Sa˜o Paulo, Instituto de F´ısica, Cidade Universita´ria, CEP 05508-900,
Sa˜o Paulo, SP, Brazil
Abstract
Molybdates have applications in various fields such as phosphors, optical fibers,
scintillators, magnets, sensors and catalysts. Thulium ion is an excellent blue
activator and plays an important role in the design of persistent luminescent
materials. This paper reports the investigation of the structural and lumines-
cent properties of Barium Thulium Molybdate (Ba1−xTmx)MoO4 microcrystals
(with x = 0, 0.01 or 0.03). The scheelite-type crystalline structure was iden-
tified from XRD and Raman studies. Under ultraviolet (359 nm) excitation,
photoluminescence (PL) spectra present the characteristic emission bands at
3
3
453 nm and 545 nm which are due to the 1D2→ F4 and 1D2→ H4,5 transitions,
respectively, from Tm3+ ions. Thermoluminescence (TL) measurements were
performed with powdered samples previously irradiated with beta radiation.
The depth of traps, associated with trap levels located inside the band-gap,
was determined from TL data using different methods of glow curve analy-
sis. The kinetic parameters, determined from thermoluminescent glow curves,
provide evidence about a possible persistent luminescence emission from the
(Ba0.97Tm0.03)MoO4 sample. This is a potential blue-light or ultraviolet long-
lasting phosphor, with a trapping level lifetime, at room-temperature (300 K),
of about 6 days.
Keywords: Ceramics, Luminescence, Microwave, Rare Earth Alloys and
Preprint submitted to Journal of Alloys and Compounds
October 23, 2017

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Compounds, Optical Properties
1. Introduction
Barium Molybdates (BaMoO4) have attracted increasingly attention as a
host material for designing novel optical devices due to their thermal and chem-
ical stabilities as well as their efficient energy transfer from the tetragonal molyb-
date group (MoO−4 2) to dopant ions [1, 2, 3]. Molybdates have a scheelite-type
crystalline structure and allow doping with trivalent rare-earth ions, which re-
sult in excellent luminescent properties, enabling the light emission of a va-
riety of colors [2, 3, 4, 5, 6]. Among rare earth ions, Tm3+ is employed by
researchers in the design of blue emitting phosphors [2, 7, 8, 9, 10], white light
5
10 emitting diodes [11, 12, 13] as well as field-emission display (FED) devices[14].
Tm3+ ions also play an important role on the design of persistent lumines-
cent materials. The use of Tm3+ as a co-doping ion in Ca2Si5N8:Eu2+ and
Ba2MgSi2O7:Eu2+ phosphors showed an enhancement of the persistent lumi-
nescence emission, increasing both the intensity and duration of light emission
15 [15, 16]. Cho (2016) investigated the light emission from a BaMoO4:Tm3+
phosphor, which occurs in the blue spectral range with a tetragonal crystal
structure[2]. However, to our present knowledge, the influence of Tm3+ doping
concentration on the BaMoO4:Tm3+ emission and the investigation about the
luminescent behavior, regarding the determination of traps parameters, have
20 not yet been reported.
The optical properties of luminescent materials are influenced by defects or
impurity sites called traps located inside the band-gap [17, 18]. These defects
can be caused by chemical reactions during the fabrication process, alio-valent
co-doping of the material, unidentified lattice defects and distortions or defects
25 generated by radiation exposure [19, 20, 21, 22, 23].The exposure of the phos-
phor to ultraviolet or ionizing radiation generates electron-hole pairs which can
get trapped in these states located inside the band gap of the material, remain-
ing in these traps until they acquire sufficient energy from an external heat or
2

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light source to escape [17, 23, 24]. The required energy for releasing these elec-
30 trons, the trap depth, provides information about some luminescence features
[25, 26, 27]. For materials with very shallow traps the observed afterglow is
short, whereas, for those which present deep traps, no charge carriers can es-
cape at room temperature [25, 27]. The light emission resulting from gradual
release of the charge carriers from these trapping centers is called persistent
35 luminescence[15, 28, 29]. Persistent luminescent materials present traps with
depth values ranging around 0.58 to 1.13 eV, associated with a physically ap-
propriated frequency factor value, resulting in light emission in the ultraviolet,
visible or infrared spectral ranges lasting from minutes to days after ceasing the
excitation [27, 30, 31, 32].
40
In this work, we present surface and luminescent investigations of Thulium-
Doped Barium Molybdate (Ba1−xTmx)MoO4 microcrystals (with x = 0, 0.01
or 0.03), synthesized by a coprecipitation method and grown in a microwave-
assisted hydrothermal (MAH) system. Microstructural characterization has
been performed using XRD, Raman and SEM techniques. Photoluminescence
45 and Thermoluminescence measurements have been carried out in order to get
information about the luminescence behavior and trap kinetics in these com-
pounds.
2. Materials and Methods
2.1. Materials
50
All chemical reagents were analytical grade and were used without further
treatments: molybdenum trioxide (MoO3) (Alfa Aesar, 99.95%), barium acetate
(Ba(CH3CO2)2) (Alfa Aesar, 99%), thulium (III) oxide (Tm2O3) (Aldrich, USA,
99.9%) and polyethylene glycol (HOCH2CH2OH) (Mw. 200) (PEG 200) (Sigma-
Aldrich, 99.9%).
55 2.2. Synthesis of Barium molybdate BaMoO4:Tm3+
)
BaMoO4:Tm3+ (with 0, 1 and 3 mol% Tm3+, in relation to barium ions)
were synthesized using the microwave-assisted hydrothermal (MAH) method.
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For this synthesis, molybdenum trioxide (5x10−3 mol) was dissolved in 50 mL
of distilled water with vigorous stirring. The pH of the solution was adjusted
60 to 12 by adding KOH. (Ba(CH3CO2)2) (5x10−3 mol) and thulium nitrate were
added under constant stirring. Thulium nitrate reagent was produced previously
o
by adding nitric acid to thulium oxide (III) at 70 C. Then, 1.5 mL of a cap-
ping reagent, PEG 200, was added. The systems were hydrothermally treated
using the MAH method. Reaction systems were transferred into a sealed teflon
65 autoclave and placed in an adapted domestic microwave (2.45 GHz, maximum
o
power of 800 W), heat treated at 100 C for 16 min with a heating rate fixed
o
at 100 C/min. The pressure in the sealed autoclave was stabilized at around
2.0 atm. The autoclave was cooled to room temperature naturally, and the
resulting white precipitate was collected, washed with water and ethanol and
o
70 dried at 60 C. Hereafter, the (Ba1−xTmx)MoO4 (x = 0; 0.01 or 0.03) samples
will be referred to as BMO:Tm-0 (x=0), BMO:Tm-1 (x=0.01) and BMO:Tm-3
(x=0.03).
2.3. Characterization of BaMoO4 samples
The samples were characterized by XRD using a Rigaku-DMax/2500PC
o
˚
75 (Japan) with Cu − Kα radiation (λ = 1.5406 A) in the 2θ range from 5 to
75o with a 0.02o/min increment. Lattice parameters and unit cell sample vol-
umes were calculated using the least-square refinement REDE93 program. The
average crystallite diameter (Dcrys) of the materials after hydrothermic treat-
ment was determined by XRD results with the (112) diffraction peak of the
80 BaMoO4 phase located around 26.6o according to the procedure described else-
where [4]. Raman spectra were recorded using a Raman Renishaw microscope,
inVia model, with multichannel CCD detector and He-Ne laser (632.8 nm), set
to 0.17 mW at the sample. Automatic cosmic ray removal option was utilized,
with range of Raman spectra from 100 cm−1 to 1000 cm−1. Fourier Transformed
85 Infrared (FTIR) spectra were acquired on a Shimadzu IRPrestige-21 spectropho-
tometer, with 120 scans and spectral range from 400 to 4000 cm−1. Absorbance
measurements were obtained on a Shimadzu UV-2600 spectrophotometer in the
4

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wavelength range from 200 to 800 nm and spectral resolution of 0.1 nm. The
conversion of wavelength to energy scale for quantitative analysis was performed
90 by using a Jacobian Conversion, outlined by Mooney & Kambhampati (2013)
[33]. The band gap energy was determined using Tauc relation [34, 35]. Photo-
luminescence excitation and emission spectra were acquired using a Fluorolog
3-22 spectrometer (HORIBA Jobin-Yvon, Longjumeau, France) coupled with
a 450 W xenon arc lamp, with a fixed emission of 453 nm and fixed excita-
95 tion of 359 nm. The morphology and particle sizes of the prepared samples
were observed using scanning electron microscope, SEM (Jeol JSM-6610LV),
and Field emission scanning electron microscope, FE-SEM (FEI Quanta 250)
for BMO:Tm-0. All measurements were taken at room temperature.
2.4. Thermoluminescence measurements
100
Thermoluminescence (TL) of 6 mg sample of pure and Tm3+-doped (1% or
3%) powder BaMoO4 samples were measured. A Risø TL/OSL reader, model
DA-20, with a built-in 90Sr/90Y beta source with activity of 1.48 GBq was used.
TL signal was detected by a photomultiplier tube EMI 9235QB (Bialkali) with
better detection efficiency between 200 and 600 nm. Readouts were performed
105 with samples exposed to beta radiation with doses ranging from 75 mGy to 750
mGy and non irradiated samples. Thermoluminescent signal was also registered
for heating rates of 1 K/s, 3 K/s, 5 K/s and 10 K/s from 323 K to 773 K for a
sample exposed to 300 mGy.
2.5. TL glow peak analysis
110
TL glow curves were analyzed according to the following methods:
2.5.1. Glow curve deconvolution
Computadorized Curve Deconvolution Analysis (CCDA) has been carried
out by using the single glow-peak equation of general order kinetics [36]:
−b
ꢀ
ꢁ
ꢂ
ꢀ
ꢁ
ꢃ
b−1
b
E T − Tm
T2
Tm2
E T − Tm
b−1
I(T) = Imb exp
× (b − 1)(1 − ∆)
exp
+ Zm
(1)
kT Tm
kT Tm
5

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2kTm
E
where ∆ = 2kT , ∆m
=
and Zm = 1 + (b − 1)∆m
E
In these expressions E is the trap depth, b, is the order of the kinetics, k, the
Boltzmann’s constant and Tm, the temperature corresponding to the maximum
intensity of the peak. Curve fitting has been performed by using the software
package Microsoft Excel, with Solver utility [37]. The goodness of fit was tested
with the Figure of Merit (FOM) [38]:
P
p |TLexp − TLfit
|
P
FOM =
(2)
p TLfit
where TLexp corresponds to the experimental data and TLfit to the fitted data
115 points. The frequency factor (s) for general order kinetic glow peak is obtained
by the relation [36]:
ꢀ
ꢁ
βE
kTm2 Zm
1
E
s =
exp
(3)
kTm
2.5.2. Peak shape method
The trap depth (E) was also calculated using the peak shape method with
the methodology proposed by Chen as follows [39]:
ꢀ
ꢁ
Tm2
α
Eα = Cα
k
− bα(2kTm)
(4)
where α stands for τ,δ or ω. The constants cα and bα are given by [39]:
120
cτ = 1.510 + 3.0(µ − 0.42), bτ = 1.58 + 4.2(µ − 0.42)
cδ = 0.976 + 7.3(µ − 0.42), bδ = 0
cω = 2.52 + 10.2(µ − 0.42), bω = 1
The parameters τ, δ and ω are given by: τ = Tm − T1, δ = T2 − Tm,
ω = T2 − T1
125
where Tm is the peak temperature at maximum height. T1 and T2 are the
temperatures corresponding to half of the maximum intensities on either side
of the glow peak maximum temperature (Tm) [39]. The geometrical shape
parameter µ was calculated as µ = δ/ω. Where µ = 0.42 for first-order kinetics
and µ = 0.52 for second-order kinetics.
6

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130 2.5.3. Variable heating rate method
The variable heating method relies on the analysis of the shift of thermo-
luminescent peak to higher temperatures when the heating rate is increased.
Using the determined values of Tm at various heating rates (β), the trap depth
(E) was determined with the following relation [39]:
ln(Tm2 /β) = ln(E/sk) + E/kTm
(5)
The slope of straight line adjusted to the experimental data of ln(Tm2 )/β vs.
1/kTm provides the trap depth value.
3. Results and Discussions
To investigate the structure of prepared phosphors, X-ray diffraction (XRD)
135 patterns were recorded as depicted in Figure 1. XRD patterns of BaMoO4:Tm3+
synthesized by MAH method exhibit a scheelite-type crystalline structure of
space group I41/a, without additional phases or impurities. All peaks observed
in Figure 1a can be indexed to the tetragonal BaMoO4 phase which is in agree-
ment with standard data from JCPDS card No. 29-0193. Table 1 lists the
140 main 2θ diffraction angles (and corresponding intensities), lattice parameters
and crystallite sizes of BaMoO4:Tm3+ samples and JCPDS 29-0193 standard
data. The diffraction angles data are in good accordance with the JCPDS stan-
dard, and all expected peaks were detected in the experimental results.
From peak positions displayed in XRD, lattice parameters and unit cell sam-
145 ple volumes were calculated using the least-square refinement REDE93 program.
The average sample crystallite diameter (Dcrys) was determined using the (112)
diffraction peak (2θ around 26.6o) according to the Scherrer equation [40, 41]:
κλ
B =
(6)
Dcryscosθ
where B is the full width at half maximum (FWHM), κ is the shape factor,
λ is the wavelength of the Cu Kα radiation and θ is the Bragg angle.
The lattice parameters a and c for BMO:Tm-0, BMO:Tm-1 and BMO:Tm-3
150
7

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Table 1: Crystallite diameter (Dcrys), band gap energy values (Eg), main diffraction angles
and relative intensities of 100% diffraction peak (112), and lattice parameters of BaMoO4
powders
Sample
JCPDS 29-0193
BMO:Tm-0
47 ± 1
3.98
BMO:Tm-1
40 ± 2
3.77
BMO:Tm-3
46 ± 2
3.55
a
Crystalline Size (nm)
–
E
–
g
o
2θ intensity( )(%)
(112)
(004)
(200)
(204)
26.53/100
27.84/21
32.07/30
42.96/48
5.58
26.68/100
27.98/10
32.22/33
43.08/23
5.58
26.46/100
27.78/15
32.00/28
42.88/27
5.59
26.64/100
27.94/13
32.20/35
43.07/23
5.57
˚
a=b(A)
Lattice Parammeters
˚
c(A)
12.82
12.80
12.84
12.80
3
V
(nm
)
0.399
0.398
0.401
0.398
cell
c/a
2.30
2.29
2.30
2.30
a
Calculated using the (112), 100% diffraction peak.
powders synthesized using MAH method are close to the standart bulk material
˚
˚
lattice parameters (a=5.5802 A and c=12.821 A) (JCPDS card no. 29-0193).
Small differences can be related to small lattice distortions caused by residual
stresses induced from preparation conditions, hydrotherm processing times and
155 rare earth presence in the unit cell.
The unit cell volumes were calculated from the product of the lattice param-
eters, as in the scheelite crystalline structure type, a = b and volume = a×a×c.
Unit cell volumes were 0.398 nm3, 0.401 nm3 and 0.398 nm3 for BMO:Tm-0,
BMO:Tm-1 and BMO:Tm-3, respectively, values that are very similar to that of
160 the JCPDS card (0.399 nm3). Crystallite diameter (Dcrys) calculated by using
equation 6 were (47 ± 1) nm, (40 ± 2) nm and (46 ± 2) nm for BMO:Tm-0,
BMO:Tm-1 and BMO:Tm-3, respectively. These results are close to the value
of ≈ 52.2 nm presented by Adhikari et al (2013), and the range of 20-40 nm
calculated by Yoon et al. (2006) [42, 43]. The BaMoO4 primitive cell displays
165 internal and external vibrational mode groups. Internal vibrations correspond
to oscillations inside the [MoO4]2− ionic molecular group with an immovable
mass center and strong covalent Mo-O bonds. External or lattice phonons cor-
respond to the Ba2+ cation motion and the rigid molecular unit [MoO4]2−
motions. The scheelite primitive cell presents 26 different vibration modes:
170 Γ = 3Ag + 5Au + 5Bg + 3Bu + 5Eg + 5Eu, but only Ag, Bg and Eg are Raman
8

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Table 2: Raman frequencies (cm−1) in BaMoO4 powders doped with thulium ions.
−1
Lattice mode
BaMoO (cm
)
Assignments
4
Symmetry
6
C
o
a
C
BMO:Tm-0
BMO:Tm-1
BMO:Tm-3
1100-1200
4h
A
892
838
791
360
346
325
325
189
141
891
837
791
359
345
325
325
189
138
890
838
791
360
345
326
326
190
139
892
837
791
358
345
324
324
188
137
110
76
ν
ν
(A
(F
)
1
g
g
g
g
g
g
1
3
B
E
E
B
B
)
2
ν
ν
(F
)
4
2
2
(E)
A
g
g
g
g
g
g
E
B
E
B
ν
ν
(F
)
free rotation
1
f,r
-
External
ext
2−
]
modes [MoO
4
2+
and Ba
motions
E
74
a
Czocharalski [10]
active modes, while the odd ones (4Au and 4Eu) can be registered only in the
infrared spectra. The three Bu vibrations are silent modes; one Au and one Eu
modes are acoustic vibrations [44].
Characteristic Raman active mode peaks of BaMoO4 scheelite structure are
175 observed in Figure 1b and detailed in Table 2. The peaks correspond to external
modes of [MoO4]2− and Ba2+(Tm3+) motions, ext. (2Eg and 1Bg) are not
observed, probably due to the fluorescence signal overlap of the samples [45, 46].
Three sets of defined peaks between 130 and 190 cm−1, 320 and 360 cm−1
,
and 790 and 900 cm−1 are observed in Raman spectra. The first region (130
180 to 190 cm−1) corresponds to external modes (1Bg) and free rotation (1Eg)
of [MoO4]2− and Ba2+ or Tm3+ ions; the second region (320 to 360 cm−1
)
corresponds to ν2(E) and ν4(F2) (2Bg +1Eg) of [MoO4]2− and Ba2+ (or Tm3+
doped); and the third region (790 to 900 cm−1) to ν3 (F2) and ν1 (A1) internal
Raman modes associated with stretching vibrations of strong covalent Mo-O
185 bonds of [MoO4]2− ionic molecular group with an immovable mass center [44]. A
frequency shifting of some peaks proportional to the concentration of the doped
element (i.e., one-phonon-like behavior) and the splitting of a number of peaks
involving different elements that share the same lattice site (i.e., two-phonon-like
behavior), specially in materials with addition of dopant that modify crystalline
190 lattice substantially, were expected, but weren’t observed in these samples [47,
9

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4].
The full width at half maximum (FWHM) values from Bragg reflections
and Raman bands were calculated to determine a disorder degree at short-
and intermediate-ranges of samples. The FWHM calculated values were very
195 similar, 1 nm for Raman data and 0.03o for XRD data, values close to the
experimental error. These observations demonstrate that these materials have
similar levels of deep defects probably because thermal treatments parameters
were similar. However, these characterization techniques do not help the study
of energy levels of the material.
200
The [MoO4]2− ionic molecular group, characteristic of scheelite-type crys-
talline structures, observed in Raman and XRD data, can be confirmed by FTIR
spectra. These spectra showed stretching in the region around 860 cm−1 and
790 cm−1 assigned to F2(3) antisymmetric and symmetric stretch vibrations,
respectively, which are associated with the covalent Mo-O bonds stretching vi-
205 brations in a [MoO4]2− tetrahedron [48, 49].
Figure 2a presents UV-Vis Absorbance spectra of BaMoO4:Tm3+ samples
obtained between 200 and 800 nm. BMO presents two fundamental absorption
bands at 200-300 nm range, one around 260 nm (4.8 eV) and another band
around 208 nm (6.0 eV). The band around 260 nm (4.8 eV) was attributed
210 to electronic transitions within the [MoO4]2− complex, according to Spassky
et al. 2004 [50] and other results of research already published by our group
[51, 52, 4]; these absorption bands are dislocated to higher wavelength values
with increased concentration of Tm3+ ions in the crystalline lattice, similar to
that observed by Adhikari et. al (2013) in BaMoO4:Er3+ nanoparticles [42].
215
Inset of Figure 2a shows more details of absorbance bands in BMO:Tm-1
and BMO:Tm-3 samples. These absorbance bands around 358.5 nm, 464-472.5
1
1
3
nm, 662-699 nm, 773-794 nm correspond to 3H6→ D2, 3H6→ G4, 3H6→ F3
and H6→ H4 transitions of Tm3+ ion, respectively. BMO:Tm-3 presents one
absorbance band at 523.5 nm, which could be atributed to both H6→ G4 and
3
3
3
1
1
220 3H5→ D2 transitions [53, 54].
The band gap energies of BaMoO4:Tm3+ samples were calculated using Tauc
10

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relation and are summarized in Table 1. The estimated band gap energies
were found to be 3.98 eV, 3.77 eV and 3.55 eV for BMO:Tm-0, BMO:Tm-
1 and BMO:Tm-3, respectively (Figure 2b), related to electronic transitions
225 in [MoO4]−2 complex. The BaMoO4:Tm3+ samples were produced in same
temperature, consequently, a great difference in order-disorder was not expected,
as previously discussed in the XRD and Raman results. However, with the
increase of Tm3+ ions, an additional difference in morphology of particle surface
was observed (Figure 3a-f). The surface atoms have unsatisfied valencies and,
230 to reduce the surface energy, surface reconstruction occurs which leads to energy
levels in the semiconductor’s forbidden gap region. It is expected that band gap
and trap concentration increase with a decrease in the particle size. Therefore
the necessary energetic gap to occur electron-hole pair (e 0 − h·) recombination
can be variable [55].
235
With addition and increase of Tm3+ ion concentration in BaMoO4, it was
also expected an increase of atoms with unsatisfied valencies and, to reduce
the local energy, localized reconstruction occurs, which leads to energy levels
in the forbidden gap of the semiconductor, increasing the amount of traps [55].
3
1
In addition, the electronic transition H6→ D2 of thulium ion, that proved to
240 be dependent on its concentration, occurs around 359 nm, close to the band
gap energy presented by BMO:Tm. The direct band gap by Tauc relation
graph is plotted with hν along the X-axis and (hνα)2 along the Y-axis, where
hν is energy of the photon and α is the absorption coefficient and the band
gap is found by extrapolating the linear region of the curves to the X axis
245 [34, 35]. Therefore, it is clear that the extrapolation of the curves linear region
(hνα)2=0 (obtaining the band gap) is dependent of absorption and affected by
thulium concentration. Consequently, the electron transition from valence band
to conduction band can be favored with increase of Tm3+ ion concentration to
3% in crystalline lattice, shifting the absorption edge to lower energies in the
250 visible electromagnetic spectra.
BMO:Tm samples with predominantly octahedral morphologies are visible
in SEM (Scanning Electron Microscopy) micrographs (Figure 3). In the MAH
11

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system, with a dominant liquid phase, the crystalline unit is free to grow in
any direction (3D), therefore differences among morphologies of BMO:Tm sam-
255 ples are expected [52]. The high pressure during hydrothermal treatment and
the presence of thulium doping in crystalline lattice induce modifications in the
morphology. Figures 3(a,b) show superficial defects and distortions in micro-
octahedra with a shortening and rounding of octahedral shapes, generating a
hollow micro-ellipsoids structured BMO:Tm-0 sample as illustrated in the high
260 resolution images (Figures 3(g,h)). Wang et. al. (2013) observed hollow struc-
tures in SrMoO4 samples [56]. Micrographs of BMO:Tm-1 (Figures 3(c,d)) and
BMO:Tm-3 (Figures 3(e,f)) show dense structure, keeping the characteristic
octahedral morphology of molybdates obtained by MAH method, with aggre-
gates of small particles, associated with the method and presence of thulium
265 [1]. The presence of thulium causes the retention of octahedral morphology or
the new formation of micro-octahedra; the increase in the amount of thulium
in lattice leads to the growth of particles due to the formation of aggregates of
smaller ones. As previously discussed, it is expected a variation on the band
gap and on the amount of traps with presence of Tm3+ alio-valent doping and
270 the variation of morphology of particles. Both smaller particle size and higher
concentration of alio-valent Tm3+ induce formation of atoms with unsatisfied
valences (in surface and in bulk, respectively), leading to an increase in the con-
centration of traps and a decrease in the band gap value. BMO:Tm-3 sample
presents higher amount of alio-valent Tm3+ and larger particle size, therefore,
275 it is expected that this sample presents higher concentration of traps and lower
band gap value.
Figure 4a illustrates the excitation spectrum (PLE) of BaMoO4:Tm3+, λEM =453
nm at room-temperature. In PLE, the main peak observed at 359 nm in
280 BMO:Tm-1 and BMO:Tm-3 is the characteristic excitation band related to the
1
3H6→ D2 transition, confirming the UV-Vis absorbance spectra data previously
discussed [53]. The peak at 359 nm is approximately 6 times more intense in
BMO:Tm-3 than in BMO:Tm-1 samples. This fact supports the results observed
12

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in band gap data and in absorbance spectra.
285
Figure 4b illustrates emission spectrum of BaMoO4:Tm3+, λexc=359 nm
at room temperature. Photoluminescence (PL) spectra of BMO:Tm samples
exhibits broad and low intensity band transitions related to [MoO4]2−, covering
the visible region from 390 to 590 nm. PL spectra of BMO:Tm-1 and BMO:Tm-
3 exhibit additional transitions due to thulium ions present in the molybdate
1
3
290 matrix: the emission at 453 nm is assigned to D2→ F4 transition of Tm3+
,
[53, 57] and the energy corresponding to the peak at 545 nm is similar to the
3
1D2→ H4,5 of Tm3+ transition (Figure 4c). Although the probability of this
transition is modest, the increase of traps in the band gap could facilitate its
appearance [58]. BMO:Tm-0 does not present these emissions assigned to Tm3+
295 ion.
1
Under the same measurement conditions, the 3H6→ D2 transition at 453
nm in BMO:Tm-3 is approximately six times more intense that in BMO:Tm-
1. This relation agrees with our previous discussion, in which the band gap
of BMO:Tm-3 (3.55 eV) is closer to the applied excitation wavelength (3.69
300 eV) than the band gap of BMO:Tm-1 (3.77 eV), favoring electron-hole pair
recombination and consequently increasing the intensity of photoluminescence.
Furthermore, the presence of Tm3+ alio-valent ions induces a local increase
of atoms with unsatisfied valences leading to an increase in the concentration
of traps that act as recombination centers, amplifying the number of electron
305 transition events from valence band to conduction band. As a consequence, the
photoluminescence emission is favored for higher doping concentrations.
Figure 4d illustrates the CIE chromaticity diagram of BaMoO4:Tm3+. CIE
parameters were calculated to estimate the change in the phosphor photomet-
ric characteristics with ion influence. The (x, y) chromaticity coordinates are
310 (0.1657, 0.2137) for BMO:Tm-0; (0.1666, 0.2111) for BMO:Tm-1; and (0.1685,
0.1598) for BMO:Tm-3. The chromaticity diagram shows emission in the blue
region for BMO:Tm-3 and in the greenish blue/blue region for the other two
phosphors.
Thermoluminescence glow curves from samples irradiated with a 300 mGy
13

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315 dose, measured at 5 K/s heating rate, are shown in Figure 5. Results show
that pure BMO sample has a glow curve in the temperature range from 500
to about 700 K. BMO:Tm-1 exhibits a glow curve in the range from 500 to
about 750 K, with maximum TL intensity at around 670 K. BMO:Tm-3 sample
presents a rather well-defined TL glow curve with a main peak (Tmax ≈ 450 K),
320 and a compound band at the 500-750 K range. The registered glow curve for
BMO:Tm-3 is rather broad, extending over a large temperature range, which
suggests the presence of multiple trap depths. The TL signal for the BMO:Tm-3
sample is much more intense than for the other compositions, suggesting the
presence of a larger number of traps. Increasing the concentration of thulium
325 ions from 1% to 3% in the BaMoO4 matrix results in a significant increase
of the TL emission. Additionally, the lower temperature of the glow peak for
BMO:Tm-3 samples indicates that the traping centers are different for different
doping levels.
Figure 6 shows the effect of the heating rate variation on the main TL peak
330 registered for BMO:Tm-1 and BMO:Tm-3 samples. Data show a shift of the
TL glow peak to higher temperatures with increase of heating rate, as expected.
Figure 7 shows TL glow curves measured from non-irradiated and irradiated
BMO:Tm-3 samples. The comparison of the glow curves show that the exposure
to 300 mGy and 750 mGy beta radiation doses results in a subtle variation on
335 the main peak intensity and position. However, the intensity of the second peak
located at temperatures higher than 570 K decreases for the irradiated samples.
These outcomes point to a reduction of deeper traps due to the irradiation
process. Results also show that the intensity of the BMO:Tm-3 glow curve is
not affected by the radiation doses in this range of absorbed doses.
340
Figure 8 illustrates the deconvoluted TL glow curves, registered with a heat-
ing rate of 1 K/s, for both doped samples. Our curve fitting studies have shown
that four isolated peaks are necessary to achieve an acceptable fit to the ex-
perimental glow curves. The fitting parameters are displayed in Table 3. The
Figure of Merit (FOM) values are smaller than 2.4%, showing that experimental
345 and fitted glow curves are in good agreement. The trap levels are in the energy
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Table 3:
Kinetic parameters of deconvoluted thermoluminescent peaks measured for
BMO:Tm-1 and BMO:Tm-3 samples.
Sample
Peak
E
(eV)
E(eV)
Chen
—
Deconvolution (1 K/s)
−1
Arrhenius
—
E(eV)
1.04
1.2
Frequency factor [s(s
)]
b
Lifetime at 300
71 years
K
8
BMO:Tm-1
1
2
3
4
1
2
3
4
1.3 × 10
2.4 × 10
1.8 × 10
1.4 × 10
5.8 × 10
1.1 × 10
7.4 × 10
2.1 × 10
1.2
1.6
1.25
1.2
2
8
4
1.2
1.13
1.25
±
±
0.04
0.04
1.9 × 10 years
8
5
1.28
1.34
1.14
1.16
1.2
1.28
1.33
0.92
1.02
1.14
1.24
5.6 × 10 years
8
6
1.3
±
0.05
4.9 × 10 years
5.7 days
9
BMO:Tm-3
1.04
1.11
1.23
1.30
±
±
±
±
0.04
0.04
0.04
0.05
10
2
140 days
8
2
614 years
5
8
1.3
1.2
1.04 × 10 years
range from about 0.92 to 1.33 eV for both samples. The frequency factors (s)
associated with traps states for BMO:Tm-1 and BMO:Tm-3 are in the range
from 108 s−1 to 1010 −1. Acceptable values of s are in the range from 108 to
s
1014 s−1 [27]. The frequency factor values are higher and have a larger range
350 for the BMO:Tm-3 samples, more than 10-fold variation, contrasting with the
2-fold variation for BMO:Tm-1. This is consistent with the overall fitting re-
sults, as for similar values of activation energies, the TL peak temperatures are
smaller for the BMO:Tm-3 samples than for the BMO:Tm-1 ones. At higher
Tm3+ doping levels, higher trap concentrations are expected; also additional
355 traps may be created, resulting in an increase on the intensity of luminescent
signal. The b values in Table 3, related to the kinetic order of the glow peaks,
correspond mainly to second order for the BMO:Tm-3 sample, indicating that
the retraping probability is nearly the same as the recombination probability
[25, 28].
360
The trap depth (activation energy) can also be determined through the Ar-
rhenius method using various heating rates: a linear plot of ln(Tm2 /β) vs. 1/kTm
is obtained, as shown in Figure 9. The slope of the straight line fitted to the
experimental data corresponds to the trap depth (Table 3). Trap depth values
calculated for all peaks by the glow curve method proposed by Chen are also
365 displayed in Table 3. The activation energy values essentially agree with those
calculated by different heating rate method and also by deconvolution proce-
dure. The trap-depths calculated in this work for barium molybdate are similar
to those calculated for lanthanum molybdate and gadolinium molybdate, both
15

ACCEPTED MANUSCRIPT
doped with Tb3+ [26]. A simple diagram of band-gap values and trap depths
370 for each sample is presented in Figure 10.
Table 3 provides the charge carriers expected lifetime in each trapping level
for BMO:Tm-1 and BMO:Tm-3 at room temperature (T = 300 K). The ex-
pected trapping level lifetime corresponds to the time required for the TL peak
intensity to be reduced by a factor 1/e of its initial value [59]. Using the kinetic
parameters discussed above, the expected lifetime (τ) associated to each TL
peak at room temperature was calculated from the equation [27]:
ꢀ
ꢁ
E
τ = s−1exp
(7)
kT
where s is the frequency factor and k is the Boltzmann constant. Results dis-
played in table 3 show that the mean charge lifetime for all TL peaks from
the BMO:Tm-1 sample are longer than seventy years. Thus the time required
for release and recombination of trapped electrons in the BMO:Tm-1 trapping
375 levels is too large to be useful in persistent luminescence, in accordance with
the glow-peak temperatures observed for this sample [27, 32]. For BMO:Tm-3
sample, the combination of s and E values results in expected lifetimes higher
than 140 days for traps corresponding to the TL peaks 2, 3 and 4, which are also
too long to be useful in persistent luminescence. The results calculated for peak
380 1 from BMO:Tm-3 lie in the zone relevant to persistent luminescence [27]. For
peak 1 (BMO:Tm-3) the expected trapping level lifetime, at room-temperature
(300 K), is around 5.7 days. Table 4 exhibits the kinetic parameters and cal-
culated expected lifetime for peak 1 of BMO:Tm-3 glow curves measured with
all the experimental heating rates and the values are very similar. Column 5
385 (Table 4) presents the FOM values for each heating rate. The lifetime calculated
for peak 1 of BMO:Tm-3 is consistent with a possible persistent luminescence
emission mechanism from this trapping level. However, the observation of per-
sistent luminescence by this material needs further theoretical and experimental
investigation and it’s beyond the scope of the present work.
16

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Table 4: Deconvolved Kinetic parameters for peak 1 from BMO:Tm-3 with four heating rates.
−1
Heating rate (K/s) (β)
E
(eV)
Frequency factor [s(s
)]
b
2
2
2
2
Lifetime at 300
K
FOM
2.3%
2.7%
2.8%
2.6%
9
1
0.92
0.93
0.94
0.98
5.8 × 10
7.8 × 10
1.1 × 10
5.7 days
9
3
6.2 days
10
5
6.5 days
10
10
5.56 × 10
6.0 days
390 4. Conclusions
We have synthesized BaMoO4:Tm3+ powders by the co-precipitation method
and grown in a microwave-assisted hydrothermal system. The structural, optical
and luminescence properties were determined by means of several characteriza-
tion methods such as XRD, Raman spectroscopy, Scanning Electron Microscope
395 (SEM), Fourier Transformed Infrared spectra (FTIR), absorbance spectra, pho-
toluminescence (PL) and thermoluminescence (TL). XRD and Raman studies
of BaMoO4:Tm3+ powders provide evidence of a scheelite-type crystalline struc-
ture. The optical emission for the BMO:Tm-3 sample occurs in the blue region
2
(1D2 → F4 transitions from Tm3+ ions) whereas BMO:Tm-0 and BMO:Tm-1
400 phosphors emit in the greenish blue/blue region. TL glow curves of the phos-
phors provide evidence that the increase of Tm3+ ions in the BaMoO4 matrix
results in a larger concentration of trapping levels inside the band-gap, and it’s
in full agreement with the observed decrease in band gap values and the in-
crease in photoluminescence emission. The analysis of the kinetic parameters,
405 determined from thermoluminescent glow curves, points to a possible persistent
luminescence emission in the BMO:Tm-3 sample. This material can be a poten-
tial blue-light or near UV long-lasting phosphor, although more studies about
the luminescent behavior of BaMoO4:Tm3+ compounds are needed in order to
investigate the luminescence lifetime and the possibility of applications of this
410 compound.
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5. Acknowledgements
The authors would like to thank Prof. Dr. N. S. Gon¸calves and Profa.
Dra. L. C. Courrol from DQ-UNIFESP-Diadema and DF-UNIFESP-Diadema,
respectively, for contributions with Raman and photoluminescence measure-
415 ments, and Profa. Dra. R. S. Coimbra of CEME-UNIFESP-S˜ao Paulo for con-
tributions with FE-SEM measurements. They also acknowledge the S˜ao Paulo
Research Foundation (FAPESP) for grants #2016/20578-5, #2013/07437-5,
#2010/16437-0, #2010/06816-4 and Conselho Nacional de Desenvolvimento
Cient´ıfico e Tecnol´ogico (CNPq).
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Figure 1: (a) XRD pattern and (b) Raman spectra of BMO:Tm-0, BMO:Tm-1 and BMO:Tm-
3.
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Figure 2: (a) UV-Vis diffuse reflectance spectra of BMO:Tm3+ samples and (b) Band gap
energies obtained using the Tauc relation.
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Figure 3: SEM images of the BMO:Tm-0 (a-b), BMO:Tm-1 (c-d) and BMO:Tm-3 (e-f). High
resolution FE-SEM images from BMO:Tm-0 (g,h)
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