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2019 The Minerals, Metals & Materials Society
Optical Characteristics of Dy3+ Ions in Alkali Fluoroborate
Glasses for WLEDs
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 Dy3+ ions using melt quenching technique for
white light generation applications. The glasses under investigation are prepared from the precursor mixture keeping the molar composition 10K2O +
10BaO + 10ZnF2 + (70-x)B2O3 + xDy2O3, 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 Dy2O3 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 applications. 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, dipolequadrupole interaction
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 quality light output (low ultraviolet and infrared
(Received August 4, 2018; accepted April 5, 2019)
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
Gopi, Remya Mohan, Sreeja, Unnnikrishnan, Joseph, and Biju
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 platform. The fascinating material in condensed matter
physics, glass, is a favorable alternative to phosphors for the realization of white light emission as it
does not require a resin platform. Attractive features like high transparency, simpler manufacturing 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 Lowcost production and ease of manufacturing in different 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 attention 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 melting point, high transparency, better chemical durability, 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 BO3 to
BO4 unit without the formation of non-bridging
oxygen. When two types of alkali ions are introduced into a glassy network keeping the total alkali
content same, a non-linear variation in many physical 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 (Dy3+)
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 (yellow). The exact color from Dy3+ doped glasses
depends on the relative integrated intensity ratio
of yellow to blue emission (Y/B). Thus Y/B ratio is
very crucial for Dy3+ doped materials and which is
strongly rely on the host matrix. The yellow emission arising through the emission transition
F9/2 fi 6H13/2 is a forced electric dipole transition
and strongly influenced by the crystal field strength
around the rare earth ion and red emission assigned
to 4F9/2 fi 6H11/2 is allowed electric dipole transition. Dy3+ 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 analysis 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
A molar composition 10K2O + 10BaO + 10ZnF2 +
x = 0.1 mol.%,
(70-x)B2O3 + xDy2O3,
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 950C and the resultant melt
was poured onto a preheated thick brass plate at
350C 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), emission 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 luminescence spectra were also used to determine CIE1931 chromaticity co-ordinates. PL lifetime measurements were carried out on Edinburgh UV–VisNIR (FLS-980) spectrometer. The refractive indices
of these glasses have been measured using J.AWoollam Co. EC-400 ellipsometer.
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 representative sample containing 2 mol.% of Dy2O3 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 6H15/2 fi 4M17/2 (324 nm),
H15/2 fi 4I9/2 (336 nm), 6H15/2 fi 6P7/2 (349 nm),
H15/2 fi 6P5/2 (364 nm), 6H15/2 fi 4I13/2 (386 nm),
H15/2 fi 4G11/2 (425 nm), 6H15/2 fi 4I15/2 (453 nm),
H15/2 fi 4F9/2 (472 nm), 6H15/2 fi 6F3/2 (750 nm)
Optical Characteristics of Dy3+ Ions in Alkali Fluoroborate Glasses for WLEDs
Fig. 2. Tauc’s plot of 10K2O + 10BaO + 10ZnF2 + 68 B2O3 +
2Dy2O3 glass.
Fig. 1. Optical absorption spectrum of 10K2O + 10BaO + 10ZnF2 +
68 B2O3 + 2Dy2O3 glass (a) in the UV–VIS region and (b) in the NIR
and 6H15/2 fi 6F5/2 (798 nm). The four absorption
bands observed in the NIR region are 6H15/2 fi 6F7/2
(897 nm), 6H15/2 fi 6H7/2 (1089 nm), 6H15/2 fi 6F11/2
(1268 nm) and 6H15/2 fi 6H11/2 (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. 2 and 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 Xk. 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, X2 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 X4 and X6 are
allied to the bulk properties of the host such as
rigidity and viscosity. X2 is found to be the highest
and which points towards the higher degree of
covalence between Dy3+ ions and their surrounding
ligands. Table I compares 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
X4 and X6 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 Eu3+ 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 parameters and do not depend explicitly on the rare earth
ion. As the dopant changes, the values of JO
parameter changes only because of the modifications developed in the matrix. Table II compares fexp
and fcal 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
Dy3+ ions in the prepared glasses is highly essential
to explore luminescence characteristics. Figure 3
Gopi, Remya Mohan, Sreeja, Unnnikrishnan, Joseph, and Biju
Table I. Comparison of JO intensity parameters of 10K2O + 10BaO + 10ZnF2 + 68 B2O3 + 2Dy2O3 glass with
some previous reports
X2 (3 10220) cm2
X4 (3 10220) cm2
X6 (3 10220) cm2
Table II. Experimental and calculated oscillator strengths of different absorption transitions in
10K2O + 10BaO + 10ZnF2 + 68 B2O3 + 2Dy2O3 glass
Oscillator strength
Sl no.
Transition 6H15/2 fi
Fig. 3. Excitation spectrum
B2O3 + 2Dy2O3 glass.
Band position (cm21)
fexp (3 1026)
fcal (3 1026)
Deviation (3 1026)
r = 0.353
10K2O + 10BaO + 10ZnF2 + 68
presents the excitation spectrum of the representative sample by monitoring the yellow emission at
572 nm. Well resolved excitation bands are
observed at 323 nm (6H15/2 fi 4M17/2), 336 nm
(6H15/2 fi 4I9/2), 348 nm (6H15/2 fi 6P7/2), 363 nm
(6H15/2 fi 6P5/2), 385 nm (6H15/2 fi 4I13/2), 424 nm
(6H15/2 fi 4G11/2), 456 nm (6H15/2 fi 4I15/2) and
471 nm (6H15/2 fi 4F9/2).20 It is clear from the
spectrum that the band through the excitation
transition (6H15/2 fi 6P7/2) 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 commercial blue InGaN/GaN LED chip.33
The luminescence spectra of the title glasses have
been recorded under excitation at 348 nm. The
excited Dy3+ ions relax non-radiatively to the
metastable state (4F9/2) and from this level radiatively transfer to various lower levels as this level
has sufficient energy gap of about 8000 cm1 with
respect to the next lower level 6F3/2. As depicted in
Fig. 4, Dy3+ ions give blue, yellow and red luminescence in the present glassy system. The spectra
contain two intense emission bands in the blue
region at 480 nm (4F9/2 fi 6H15/2), the yellow
region at 572 nm (4F9/2 fi 6H13/2) and a weak band
in red region at 664 nm (4F9/2 fi 6H11/2). Further,
no change in the shape or peak position of bands is
observed from the emission spectra. The large line
Optical Characteristics of Dy3+ Ions in Alkali Fluoroborate Glasses for WLEDs
Fig. 4. Emission spectrum of 10K2O + 10BaO + 10ZnF2 + (70-x)
B2O3 + xDy2O3 glasses.
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
F9/2 fi 6H13/2 obeys the selection rule DL = ± 2;
DJ = ± 2 is hypersensitive and its intensity is
highly influenced by the environment around the
Dy3+ ion in the host lattice.35 It is well known that
yellow emission is often prominent when Dy3+ is
located in low symmetry sites and becomes zero if
located at sites with inversion center as electricdipole transitions are forbidden in those sites.36 The
intensity of emission bands increases gradually with
the concentration of Dy3+ 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 crystalfield around the Dy3+ 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 intensity of blue and yellow band is given as Fig. 5 and
inset of which shows the variation of Y/B ratio with a
concentration of Dy2O3. The variation in the intensity 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 transition from metastable state 4F9/2 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 4F9/2 fi 6H13/2, which is consistent with
Fig. 5. Variation of integrated intensity of blue and yellow emission
bands 10K2O + 10BaO + 10ZnF2 + (70-x)B2O3 + xDy2O3 glasses.
Inset figure shows the variation of Y/B ratio with different
concentrations of Dy2O3.
Fig. 6. Variation of intensity of yellow emission
10K2O + 10BaO + 10ZnF2 + 68 B2O3 + 2Dy2O3 glass.
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 yellow emission band is compared with some previous
reports and given in the table. The higher stimulated 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
Gopi, Remya Mohan, Sreeja, Unnnikrishnan, Joseph, and Biju
Table III. Different radiative properties of 10K2O + 10BaO + 10ZnF2 + 68 B2O3 + 2Dy2O3 glass
F9/2 fi
Transition probability (s21)
Emission cross section
(3 10222 cm2)
Gain bandwidth
(3 10230 cm3)
Optical gain
(3 10225 cm2 s)
Table IV. Comparison of radiative properties of 10K2O + 10BaO + 10ZnF2 + 68 B2O3 + 2Dy2O3 glass with
other reports
Emission cross section
(3 10222 cm2)
Gain bandwidth
(3 10230 cm3)
Optical gain (3 10225
cm2 s)
many of the previous reports for yellow laser
The exact color rendered by the prepared glass
under 348 nm excitation can be predicted using CIE
1931 color coordinates which can be calculated
using the equations well discussed in previous
literature.44,45 Figure 7 illustrates the CIE chromaticity chart with color coordinates. The color
coordinates for all glasses fall in the white region
and is much closer to the blackbody locus. Thus the
prepared glassy system can be suggested as a
potential material for white light applications.
McCamy gave an empirical formula for the determination of correlated color temperature (CCT)
which is a vital parameter to monitor the quality
of white light.46 Table V compares the CIE coordinates and CCT values of the present system with
some previous reports. The CCT value of the
present glass is also closer to that of daylight
(5500 K) and commercial white light LED (6400 K)
sources54 and thus the glass under investigation
might be a promising material for WLED under UV
The concentration dependent decay profiles of the
metastable state 4F9/2 of Dy3+ ions in the present
glasses are recorded by monitoring emission at
572 nm under an excitation of 348 nm and displayed as Fig. 8. It can be seen that the profiles
follow almost single exponential up to a concentration of 0.5 mol.% and beyond this concentration,
profiles exhibit non-exponential nature. The
observed non-exponential behavior at higher concentration is attributed to the energy transfer
between an excited Dy3+ and a Dy3+ ion in the
ground state. The experimental lifetimes were
evaluated from the profiles using the expression,35
Fig. 7. CIE chromaticity co-ordinates of 10K2O + 10BaO +
10ZnF2 + (70-x)B2O3 + xDy2O3 glasses. Inset figure shows the
digital photograph of white emission in glass with x = 1.0 of Dy2O3
under UV radiation.
sexp ¼
r tI ðtÞdt
r IðtÞdt
The lifetime values show a significant decrease
from 0.58 ms to 0.08 ms with increasing dopant
Optical Characteristics of Dy3+ Ions in Alkali Fluoroborate Glasses for WLEDs
Table V. Comparison of CIE and CCT of
10K2O + 10BaO + 10ZnF2 + 68 B2O3 + 2Dy2O3 glass
with other reports
CIE (x,y)
Fig. 8. Decay profiles
B2O3 + xDy2O3 glasses.
10K2O + 10BaO + 10ZnF2 + (70-x)
concentration. This quenching in lifetime arises due
to the energy transfer by means of cross-relaxation
and resonant energy transfer.55 The nonradiative
relaxation processes strongly depend on the interionic distance. The concentrations 0.1 mol.% and
0.5 mol.% of Dy2O3 is very small and is much closer.
The interionic distance in such low dopant concentration will be comparatively larger and the nonradiative interactions will be less. The comparatively
greater lifetime of these samples is thus due to this
fact. As concentration increases the interionic distance decreases and favors different inter-ionic
interaction. But the interaction will be almost
saturated after a particular distance. In the present
investigation, the lifetime value of samples with
high concentrations such as 1.5 mol.% and 2 mol.%
of Dy2O3 are very close and it may be due to this
saturation effect. In the sample with 1 mol.% of
Dy2O3, the interactions are moderate and the
lifetime value is well between the above mentioned
concentrations. The partial energy level diagram
given as Fig. 9 illustrates the overall excitation and
emission schemes along with the possible crossrelaxation channels (CR1, CR2 and CR3) and resonant energy transfer (RET) channel in the prepared glasses. The spectral overlap between the
excitation and emission spectra given as Fig. 10
underlines the possibility of resonant energy transfer within the prepared glasses.
The non-exponential behavior at higher Dy2O3
concentrations has been investigated using the
Inokuti–Hirayama (I–H) model according to which
the intensity of luminescence decay with time [I(t)]
is given as56
3=s #
IðtÞ ¼ Ið0Þ exp C 1
where s0 is the lifetime of donor in the absence of
acceptor, which is taken as the lifetime of sample
containing 0.1 mol.% of Dy2O3 where the donor–
acceptor interaction is negligible, ‘s’ is a critical
parameter which can be 6, 8 or 10 for dipole–dipole,
dipole–quadrupole and quadrupole–quadrupole
interactions respectively.
Its value
i can be obtained
st0 versus st0 plot
from the slope of ln ln Ið0Þ
given in Fig. 11. The obtained value approximately
equals to 8 suggests that dipole-quadrupole interaction is the dominant mechanism of energy transfer in the present fluoroborate glassy system.
The total transition probability of the prepared
glass sample with x = 1 mol.% using JO theory is
determined as 501.79 s1 and the corresponding
lifetime is 1.992 ms. The discrepancy in the experimental and calculated lifetime is probably due to
several nonradiative processes within the sample.
Quantum yield of the samples can be estimated as
the ratio of experimental and calculated lifetimes.
In the present investigation, it ranges from 29% to
4%. We have accurately measured the quantum
yield experimentally. Figure 12a depicts the excitation of the sample and the blank (undoped) and
Fig. 12b represents the emission of the same using
an integrating sphere for quantum yield determination. The experimental quantum yield is found to
be 51.34%.
A series of trivalent Dysprosium ion doped alkali
fluoroborate glasses have been successfully synthesized by the conventional melt-quenching technique
and characterized by using different spectroscopic
Gopi, Remya Mohan, Sreeja, Unnnikrishnan, Joseph, and Biju
Fig. 9. Partial energy level diagram and possible cross relaxation an resonant energy transfer channels of Dy3+ ions in alkali fluoroborate
Fig. 10. Spectral overlap of excitation and emission
10K2O + 10BaO + 10ZnF2 + 68 B2O3 + 2Dy2O3 glass.
techniques. The JO intensity parameters followed a
trend X2 > X4 > X6 in the present system. Using
the phenomenological intensity parameters, various
radiative properties were derived and are consistent
with previous reports. The emission spectrum
exhibit a gradual increase in intensity with increase
in the concentration of Dy3+. It is found that the
sample containing 2 mol.% of Dy2O3 has the highest
Fig. 11. The plot of ln ln IIðð0t ÞÞ st0 versus st0 .
intensity. CIE coordinates calculated using emission spectra suggest that the samples are capable of
giving white light. The lifetime values of the
fluorescent level were estimated and interaction
responsible for lifetime quenching was explored.
The experimental quantum yield of 51.34% makes
the prepared glassy samples excellent for many
Optical Characteristics of Dy3+ Ions in Alkali Fluoroborate Glasses for WLEDs
Fig. 12. Quantum yield determination (a) sample and blank
excitation (b) sample and blank emission.
optoelectronic applications, especially W-LEDs. The
obtained results conclude that the prepared glass is
suitable for fabrication various optoelectronics
devices, particularly for the production low cost
white light emitting devices without a resin
The authors are thankful to UGC, India and DST,
India for the financial assistance through SAP-DRS
(No. F.530/12/DRS/2009(SAP-1)) and DST-PURSE
(Grant No. DST-PURSE(SR/S9/Z-23/2010/22(CG)))
programs respectively. The authors acknowledge
MoU-DAE-BRNS Project (No. 2009/34/36/BRNS/
3174), Department of Physics, S.V. University,
Tirupati, India for extending the experimental
facility. Subash Gopi, Remya Mohan P and Sreeja E
are thankful to UGC, India for financial assistance
through BSR scheme.
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