10.1007@s11664-019-07198-3

Optical Characteristics of Dy
3+
Ions in Alkali Fluoroborate
Glasses for WLEDs
SUBASH GOPI,
1
P. REMYA MOHAN,
1
E. SREEJA,
1
N.V. UNNNIKRISHNAN,
1
CYRIAC JOSEPH,
1
and P.R. BIJU
1,2
1.—School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam 686560, India.
2.—e-mail: [email protected]
The aim of this study is to synthesize and characterize an economical alkali
fluoroborate glass doped with Dy
3+
ions using melt quenching technique for
white light generation applications. The glasses under investigation are pre-
pared from the precursor mixture keeping the molar composition 10K
2
O+
10BaO + 10ZnF
2
+ (70-x)B
2
O
3
+ xDy
2
O
3
, where x= 0.1 mol.%, 0.5 mol.%,
1.0 mol.%, 1.5 mol.% and 2.0 mol.%. Optical characterization techniques such
as absorption, photoluminescence excitation, emission and decay analysis
were accomplished to validate the use of the prepared glasses for white light
emitting diodes. Optical band gap energy and vital Judd–Ofelt (JO) intensity
parameters were derived using the absorption spectrum. The JO intensity
parameters were used to explore some characteristic radiative parameters of
the present glass system. The photoluminescence spectra of the glasses have
been recorded at an excitation wavelength of 348 nm and the spectra contain
two intense emission bands in the blue (480 nm) and yellow (572 nm) regions
and a weak band in the red region (664 nm). With the increase of dopant ion
concentration, the intensity of all emission bands marked a gradual increase.
The variation of the ratio of integrated intensity of yellow band to blue band
(Y/B ratio) with the concentration of Dy
2
O
3
is also studied. Color coordinates
determined using commission international de l’eclairage (CIE) 1931 suggest
that the prepared glass can be a potential material for white light applica-
tions. The experimental lifetime values marked a significant decrease with
increase in dopant ion concentration and the mechanism responsible for the
quenching is identified. Quantum yield is determined experimentally as well
as using JO theory.
Key words: Fluoroborate, Judd–Ofelt theory, radiative properties, dipole-
quadrupole interaction
INTRODUCTION
White light emitting diodes (WLEDs) became a
hot topic owing to their promising characteristics
such as higher efficiency, better reliability, high
brightness, excellent low-temperature performance,
eco-friendly in nature, low voltage and better qual-
ity light output (low ultraviolet and infrared
radiation).
1–3
They can be fabricated either by
combining the three primary colors (RGB) or by
exciting a yellow phosphor using a blue emitting
source. The former has low luminescence efficiency
because of re-absorption of blue and thus not a wise
choice. The second method, popularly known as
phosphor-converted WLEDs (pc w-LEDs), has
higher luminescence efficiency compared to the first
method.
4,5
But both of these methods use phosphors
which must be encapsulated in the epoxy resin that
faces serious issues as mentioned in literature.
6,7
(Received August 4, 2018; accepted April 5, 2019)
Journal of ELECTRONIC MATERIALS
https://doi.org/10.1007/s11664-019-07198-3
2019 The Minerals, Metals & Materials Society

Nowadays, many researchers strive for improving
the performance, durability, and cost-effectiveness
of solid-state lighting applications and also to
overshoot the problems associated with resin plat-
form. The fascinating material in condensed matter
physics, glass, is a favorable alternative to phos-
phors for the realization of white light emission as it
does not require a resin platform. Attractive fea-
tures like high transparency, simpler manufactur-
ing process, free of halo effect, low cost, high
thermal stability, ease of mass production and the
epoxy resin free assembly make luminescent glasses
more favorable for the preparation of WLEDs.
8
Low-
cost production and ease of manufacturing in dif-
ferent sizes and shapes make glasses containing
rare earths more promising alternatives to single
crystals and ceramic and plastic scintillators. Thus
rare earth ion doped glasses gained immense atten-
tion from the scientific community for realizing
white light.
Rare earth (RE) doped glasses are potential
candidate in a diverse area such as waveguide,
optical fibers, solar concentrators, plasma display
panel, optical amplifiers and many other.
9
Optical
properties of RE-doped glasses strongly depend on
the host matrix. Oxide glasses are very functional
for outstanding optical applications due to their
dominant thermal stability and chemical durability
and in particular, borate based glasses are worthy
due to their unique characteristics like lower melt-
ing point, high transparency, better chemical dura-
bility, thermal stability, and good rare earth
solubility compared to the other glasses. Structural
properties of borate glasses can significantly be
improved by the incorporation of network modifying
oxides such as alkali metal oxides or alkaline earth
metal oxides. These modifier oxides convert BO
3
to
BO
4
unit without the formation of non-bridging
oxygen. When two types of alkali ions are intro-
duced into a glassy network keeping the total alkali
content same, a non-linear variation in many phys-
ical properties of the matrix takes place and the
phenomenon is popularly known as mixed alkali
effect.
10–12
New economical and efficient borate
based glassy systems are still a hot topic due to
the above said properties. Borate glasses doped with
rare earth oxides have significant applications in
many important fields.
13–16
Among RE ions, the trivalent Dysprosium (Dy
3+
)
ions in glasses are more interesting to study because
of its intense emission in the visible spectral regions
470–500 nm (blue) and around 570–600 nm (yel-
low). The exact color from Dy
3+
doped glasses
depends on the relative integrated intensity ratio
of yellow to blue emission (Y/B). Thus Y/B ratio is
very crucial for Dy
3+
doped materials and which is
strongly rely on the host matrix. The yellow emis-
sion arising through the emission transition
4
F
9/2
fi
6
H
13/2
is a forced electric dipole transition
and strongly influenced by the crystal field strength
around the rare earth ion and red emission assigned
to
4
F
9/2
fi
6
H
11/2
is allowed electric dipole transi-
tion. Dy
3+
ions also offer laser emission around
1.3 lm laser emission.
17–19
As mentioned in the
beginning, WLEDs are a hot topic of research and it
has been a great interest in the development of new
materials for their fabrication. The materials for the
fabrication of WLEDs should be simultaneously
effective and economical. Different spectroscopic
studies such as optical absorption and decay anal-
ysis can give insight towards the suitability of a
material for the fabrication of WLED. The present
work is aiming at synthesizing and characterization
of a new economic and chemically stable alkali
fluoroborate glasses doped with dysprosium to
check the suitability of these glasses for WLED
applications.
EXPERIMENTAL
A molar composition 10K
2
O + 10BaO + 10ZnF
2
+
(70-x)B
2
O
3
+ xDy
2
O
3
, where x= 0.1 mol.%,
0.5 mol.%, 1.0 mol.%, 1.5 mol.% and 2.0 mol.% is
used for the synthesis of the tile glass. About 10 g of
the finely crushed homogeneous mixture was
melted at around 950C and the resultant melt
was poured onto a preheated thick brass plate at
350C and annealed at the same temperature for
12 h to remove thermal stress and strains. The as
obtained samples were slowly quenched to room
temperature. Finally, these glass samples were well
polished to achieve smooth surfaces for optical and
spectroscopic measurements.
The UV–Vis-NIR absorption spectrum of the
archetypical glass sample was recorded with a
Varian Cary 5000 with a spectral resolution of
1 nm. Photoluminescence excitation (PLE), emis-
sion spectra (PL) and Quantum yield were taken
using spectrofluorophotometer with xenon arc lamp
(250 W) as an excitation source (Horiafluorolog-3
with an integrating sphere attachment). The lumi-
nescence spectra were also used to determine CIE-
1931 chromaticity co-ordinates. PL lifetime mea-
surements were carried out on Edinburgh UV–Vis-
NIR (FLS-980) spectrometer. The refractive indices
of these glasses have been measured using J.AWool-
lam Co. EC-400 ellipsometer.
RESULTS AND DISCUSSION
An absorption spectrum is an efficient tool for the
appraisal of the band structure and energy gap of
amorphous as well as crystalline materials. Figure 1a
depicts the optical absorption spectrum of a represen-
tative sample containing 2 mol.% of Dy
2
O
3
in the
UV–Vis region and Fig. 1b represents the absorption
spectrum the same sample in the NIR region.
Ten absorption bands are observed in the UV–Vis
region are assigned as
6
H
15/2
fi
4
M
17/2
(324 nm),
6
H
15/2
fi
4
I
9/2
(336 nm),
6
H
15/2
fi
6
P
7/2
(349 nm),
6
H
15/2
fi
6
P
5/2
(364 nm),
6
H
15/2
fi
4
I
13/2
(386 nm),
6
H
15/2
fi
4
G
11/2
(425 nm),
6
H
15/2
fi
4
I1
5/2
(453 nm),
6
H
15/2
fi
4
F
9/2
(472 nm),
6
H
15/2
fi
6
F
3/2
(750 nm)
Gopi, Remya Mohan, Sreeja, Unnnikrishnan, Joseph, and Biju

and
6
H
15/2
fi
6
F
5/2
(798 nm). The four absorption
bands observed in the NIR region are
6
H
15/2
fi
6
F
7/2
(897 nm),
6
H
15/2
fi
6
H
7/2
(1089 nm),
6
H
15/2
fi
6
F
11/2
(1268 nm) and
6
H
15/2
fi
6
H
11/2
(1675 nm).
20
The energy corresponding to the fundamental
absorption edge is usually referred to as optical
band gap energy, as glassy systems exhibit tailing of
localized states into the forbidden energy gap.
Optical band gap energy of the present glassy
system is evaluated for the representative sample
by using the relation given by Davis and Mott.
21,22
The Tauc’s plot for the determination of optical band
gap energy is illustrated in Fig. 2and the as
obtained value 3.83 eV is comparable with early
reports on fluoroborate glassy systems.
23–25
The intensity of an absorption band for an allowed
transition is usually determined by oscillator
strength which is directly proportional to the area
under the absorption band. The experimental and
calculated oscillator strengths are obtained using
expressions available in previous literature.
26,27
The least square fitting approach is carried out in
the experimental and calculated oscillator strengths
to evaluate the phenomenological JO intensity
parameters X
k
. The JO parameters have a vital
role in exploring the local structure and bonding
vicinity of RE ions in the host matrix. The JO
parameter, X
2
is correlated to the local structure of
the RE ions and associated with the asymmetry and
covalency between rare earth (RE) ions and ligand
ions. On the other hand, parameters X
4
and X
6
are
allied to the bulk properties of the host such as
rigidity and viscosity. X
2
is found to be the highest
and which points towards the higher degree of
covalence between Dy
3+
ions and their surrounding
ligands. Table Icompares the JO parameters of the
given glassy host with those obtained for some
reported matrices. The present trend suggests the
better quality of the prepared glasses as the ratio of
X
4
and X
6
gives the value of spectroscopic quality
factor. Thus the prepared glasses are suitable for
optical device fabrication.
The present value is also much closer to our own
previous work in which Eu
3+
ions were doped in the
same host matrix and another approach for the
evaluation of JO parameters was used.
32
Thus the
obtained result is consistent with the well-known
fact that JO parameters are host dependent param-
eters and do not depend explicitly on the rare earth
ion. As the dopant changes, the values of JO
parameter changes only because of the modifica-
tions developed in the matrix. Table II compares f
exp
and f
cal
and the degree of fit between these oscillator
strengths is expressed by the root mean square
(rms) of oscillator strength (s).
Recognizing suitable excitation wavelengths of
Dy
3+
ions in the prepared glasses is highly essential
to explore luminescence characteristics. Figure 3
Fig. 1. Optical absorption spectrum of 10K
2
O + 10BaO + 10ZnF
2
+
68 B
2
O
3
+ 2Dy
2
O
3
glass (a) in the UV–VIS region and (b) in the NIR
region.
Fig. 2. Tauc’s plot of 10K
2
O + 10BaO + 10ZnF
2
+68 B
2
O
3
+
2Dy
2
O
3
glass.
Optical Characteristics of Dy
3+
Ions in Alkali Fluoroborate Glasses for WLEDs

presents the excitation spectrum of the representa-
tive sample by monitoring the yellow emission at
572 nm. Well resolved excitation bands are
observed at 323 nm (
6
H
15/2
fi
4
M
17/2
), 336 nm
(
6
H
15/2
fi
4
I
9/2
), 348 nm (
6
H
15/2
fi
6
P
7/2
), 363 nm
(
6
H
15/2
fi
6
P
5/2
), 385 nm (
6
H
15/2
fi
4
I
13/2
), 424 nm
(
6
H
15/2
fi
4
G
11/2
), 456 nm (
6
H
15/2
fi
4
I
15/2
)and
471 nm (
6
H
15/2
fi
4
F
9/2
).
20
It is clear from the
spectrum that the band through the excitation
transition (
6
H
15/2
fi
6
P
7/2
) has greater intensity
compared to others and thus 348 nm has opted as
an excitation wavelength for luminescence studies. It
can also be noted that intense bands are observed in
the near UV and blue regions for the prepared glassy
system, which is a prerequisite for the development
of white-light-emitting devices by means of commer-
cial blue InGaN/GaN LED chip.
33
The luminescence spectra of the title glasses have
been recorded under excitation at 348 nm. The
excited Dy
3+
ions relax non-radiatively to the
metastable state (
4
F
9/2
) and from this level radia-
tively transfer to various lower levels as this level
has sufficient energy gap of about 8000 cm
1
with
respect to the next lower level
6
F
3/2
. As depicted in
Fig. 4,Dy
3+
ions give blue, yellow and red lumines-
cence in the present glassy system. The spectra
contain two intense emission bands in the blue
region at 480 nm (
4
F
9/2
fi
6
H
15/2
), the yellow
region at 572 nm (
4
F
9/2
fi
6
H
13/2
) and a weak band
in red region at 664 nm (
4
F
9/2
fi
6
H
11/2
). Further,
no change in the shape or peak position of bands is
observed from the emission spectra. The large line
Table I. Comparison of JO intensity parameters of 10K
2
O + 10BaO + 10ZnF
2
+68B
2
O
3
+ 2Dy
2
O
3
glass with
some previous reports
X2(310
220
)cm
2
X4(310
220
)cm
2
X6(310
220
)cm
2
References
3.38 1.27 1.15 Present
3.95 0.91 1.95 28
4.03 1.14 1.65 29
2.68 2.56 0.89 30
3.22 1.35 2.38 31
Table II. Experimental and calculated oscillator strengths of different absorption transitions in
10K
2
O + 10BaO + 10ZnF
2
+68B
2
O
3
+ 2Dy
2
O
3
glass
Sl no. Transition
6
H
15/2
fiBand position (cm
21
)
Oscillator strength
Deviation (310
26
)
f
exp
(310
26
)f
cal
(310
26
)
1
6
H
11/2
5950 0.5400 0.5815 0.0415
2
6
F
11/2
7887 3.1500 3.1455 0.0045
3
6
H
7/2
9201 1.1900 1.3085 0.1185
4
6
F
7/2
11140 1.0600 0.988 0.0720
5
6
F
5/2
12534 0.6300 0.4434 0.1866
6
6
F
3/2
13321 0.2900 0.0833 0.2067
7
4
F
9/2
21166 0.0530 0.0753 0.0223
8
4
I
15/2
22170 0.2230 0.1982 0.0248
9
4
G
11/2
23511 0.0613 0.0423 0.0190
10
4
F
7/2
25811 0.7159 0.1014 0.6145
11
4
P
3/2
27462 1.2100 0.1961 1.0139
12
4
I
11/2
28637 2.0800 1.7372 0.3428
13
4
I
9/2
29691 0.0385 0.0758 0.0373
r= 0.353
Fig. 3. Excitation spectrum of 10K
2
O + 10BaO + 10ZnF
2
+68
B
2
O
3
+ 2Dy
2
O
3
glass.
Gopi, Remya Mohan, Sreeja, Unnnikrishnan, Joseph, and Biju

width observed in the spectra may be due to the
inhomogeneous local fields around the luminescent
centers in the present glassy system.
34
Moreover,
the yellow emission ascribed to the transition
4
F
9/2
fi
6
H
13/2
obeys the selection rule DL=±2;
DJ=±2 is hypersensitive and its intensity is
highly influenced by the environment around the
Dy
3+
ion in the host lattice.
35
It is well known that
yellow emission is often prominent when Dy
3+
is
located in low symmetry sites and becomes zero if
located at sites with inversion center as electric-
dipole transitions are forbidden in those sites.
36
The
intensity of emission bands increases gradually with
the concentration of Dy
3+
ions in the given range of
dopant ion concentration. The yellow emission band
is almost a perfect Gaussian whereas the blue
emission band seems to be split into components
due to the partial lifting of degeneracy by the crystal-
field around the Dy
3+
ion.
37
The (Y/B) intensity ratios
of luminescence spectra have been evaluated since it
is crucial in generating white light. In general, the Y/
B ratio is sensitive to the chemical composition of the
host and not on the dopant concentration and as
expected the ratio changes slightly in the present
investigation. The variation of the integrated inten-
sity of blue and yellow band is given as Fig. 5and
inset of which shows the variation of Y/B ratio with a
concentration of Dy
2
O
3
. The variation in the inten-
sity of the yellow band with different excitation
wavelengths is also studied and given as Fig. 6. It can
be seen that the highest intensity is obtained for the
excitation wavelength 348 nm which is consistent
with the excitation spectrum.
JO theory gives a platform to envisage some
important radiative properties. The required
expressions are well discussed in the literature.
38,39
All the radiative parameters for the emission tran-
sition from metastable state
4
F
9/2
are summarized
in Table III. It can be observed that the crucial
parameters such as branching ratio and emission
cross section are the highest for the emission
transition
4
F
9/2
fi
6
H
13/2
, which is consistent with
the recorded emission spectra. The stimulated
emission cross-section is a measure of the rate of
energy extraction from the optical materials using
an external stimulus and is directly proportional to
the total transition probability (A) of the emission
bands.
40
In order to validate the use of the prepared
glassy samples for optoelectronic applications, the
obtained radiative properties for the strongest yel-
low emission band is compared with some previous
reports and given in the table. The higher stimu-
lated emission cross-section and lower effective
bandwidth of the emission band is a relevant
feature for laser materials and it is noted from
Table IV that the prepared glasses are better than
Fig. 4. Emission spectrum of 10K
2
O + 10BaO + 10ZnF
2
+ (70-x)
B
2
O
3
+ xDy
2
O
3
glasses.
Fig. 5. Variation of integrated intensity of blue and yellow emission
bands 10K
2
O + 10BaO + 10ZnF
2
+ (70-x)B
2
O
3
+ xDy
2
O
3
glasses.
Inset figure shows the variation of Y/B ratio with different
concentrations of Dy
2
O
3
.
Fig. 6. Variation of intensity of yellow emission band of
10K
2
O + 10BaO + 10ZnF
2
+68B
2
O
3
+ 2Dy
2
O
3
glass.
Optical Characteristics of Dy
3+
Ions in Alkali Fluoroborate Glasses for WLEDs