Absolute quantum yield measurements in Yb/Ho doped M2O2S (M ...

Absolute quantum yield measurements in Yb/Ho doped M2O2S
(M¼Y, Gd, La) upconversion phosphor
G.A. Kumar n , M. Pokhrel, D.K. Sardar
Department of Physics and Astronomy, University of Texas at San Antonio, TX 78249, USA
article info
Article history:
Received 21 December 2012
Accepted 1 February 2013
Available online 9 February 2013
Keywords:
Upconversion
Quantum yield
Phosphor
Oxysulfide
1. Introduction
abstract
Rare earth doped luminescent materials attract a great deal of
attention nowadays because of their numerous applications
including display phosphors, security, biomedical imaging, therapy,
solid state lasers, and optical amplifiers [1–3]. Compared to several
other phosphors rare earth doped phosphors have found a big
market in the photonics industry due to their ease of synthesis, low
cost of production, photo-stability, non-toxicity and size independent
optical properties [4]. It should be noted that these superior
properties make them competitors of organic dyes and quantum
dots; both of which have several limitations [4].
It was found that halides and heavy metal combinations are
the best materials for efficient luminescence due to their low
vibration frequencies in the range of 150–450 cm 1 [5]. However,
many of the halides are air sensitive as well as toxic and several of
them could not find large scale industrial applications. Chalcogenides
such as S, Se, Te, etc. are also found to be potential
candidates though the phonon frequency is little higher than
halides [6]. Among chalcogenides, rare earth oxysulfide possesses
several excellent properties such as chemical stability, low
toxicity and can be easily mass produced at low cost. It has
average phonon energy of about 520 cm 1 [7]. For example,
Y2O2S: Yb, Er and Y2O2S: Eu are two of the best mass produced
commercial up and down conversion phosphors for applications
such as authentication technology and lighting. It was found that
the upconversion brightness of Y2O2S: Yb, Er is 6.5 times that of
Y 2O 3: Yb, Er [8]. Yocom et al. [9] demonstrated that Y 2O 2S: Yb, Er
exhibited 82% brighter output than that of fluoride. Our recent
n Corresponding author. Tel.: þ1 2104585748.
E-mail address: akgsh@yahoo.com (G.A. Kumar).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.matlet.2013.02.001
Materials Letters 98 (2013) 63–66
Contents lists available at SciVerse ScienceDirect
Materials Letters
journal homepage: www.elsevier.com/locate/matlet
Using the integrating sphere technique the absolute quantum yield measurements of the visible
emission bands at 550, 650 and 750 nm in Yb/Ho doped M2O2S (M¼Y, Gd, La) upconversion phosphor
have been reported for the first time. Observations show that the 750 nm emission in La2O2S doped
with 1 mol% each of Yb and Ho yield the highest efficiency of 0.12% followed by green emission which is
0.065% with 9.5% Yb and 0.5% Ho in the same host. The red emission in La2O2S, Y2O2S are nearly 6 times
weaker than green emission, whereas in Gd2O2S it is 14 times less efficient making Gd2O2S doped with
4.5 mol% Yb and 0.5 mol% Ho as the most efficient single color green upconvertor among all three host
compositions studied.
& 2013 Elsevier B.V. All rights reserved.
work [10] demonstrated the application of Gd 2O 2S: Yb, Er as
another potential candidate for both up and down conversions. In
all practical phosphor applications, quantification of the light
output is so important that it determines the light output for a
particular excitation. This quantity defined as the quantum yield
can be experimentally measured by integrating the sphere technique
[11]. In this work, we report the absolute quantum yield of
Yb and Ho co doped M2O2S (M¼Y, Gd and La) phosphors.
2. Materials and method
A high temperature solid state flux fusion method was used for
the phosphor syntheses. The starting materials are Y2O3, La2O3,
Gd 2O 3,Yb 2O 3,Ho 2O 3, (Sigma Aldrich, all 99.999%), S (powder) and
Na2CO3, K3PO4 (Sigma Aldrich, 99.99%) as flux. More details regarding
the synthesis was reported in our previous publications [10].
Crystallographic phase and morphology of the phosphor particles
were obtained by XRD and SEM methods. A typical XRD pattern
obtained for Y2O2S: Yb, Er phosphor sample is shown in Fig. 1,which
is in perfect match with standard powder peak positions of Y2O2S
hexagonal phase (JCPDS Card no.26-1422). The XRD results reveal
that the well-crystallized Y2O2S: Yb, Ho sample is in hexagonal
Y2O2S phase with cell parameters a¼b¼0.3852 nm, c¼0.6567 nm.
According to the dynamic light scattering measurements the average
particle size was estimated to be 3.8 mm with a FWHM of
4.9 mm. FE-STEM micrographs obtained from different locations
show that the material mostly crystallized in hexagonal shape.
3. Results and discussions
Fig. 2 shows a comparison of the 980 nm excited upconversion
in M 2O 2S: Yb Ho in the 400–850 nm range with an inset

64
photograph of the naked eye emission under 100 mW laser
excitation in samples Gd2O2S:Yb(4.5)Ho(0.5) [GOS], La2O2S:Yb(8)
Ho(0.5) [LOS] and Y2O2S:Yb(5)Ho(0.5) [YOS].The inset also shows
the schematic of the integrating sphere setup for quantum yield
measurements. In the 500–850 nm range three emission bands
were mainly observed with 3 Stark components for each of them.
The green band is peaked at 541 with Stark components at 545
and 550 nm and is arising from 5 F 4- 5 I 8 transition. The red band
has 3 components at 648, 652 and 660 nm and is far weaker
compared to green. The 756 nm is arising due to the transition
from 5 F4 to 5 I7. The fluorescence branching ratios of the three
bands are 47.1% (541 nm), 6.6% (648 nm) and 35.6% (756 nm). In
GOS the green emission intensity is 22 times that of red whereas
in LOS and YOS it is only 10 times stronger than red emission
making GOS a more efficient green upconversion phosphor.
G.A. Kumar et al. / Materials Letters 98 (2013) 63–66
The accuracy of the quantum yield measurement depends on
various factors such as the excitation power density, temperature
of the sample arising from high power density excitation, sample
quantity and the reflectivity of the sample holder. In order to
measure the quantum yield of the emission bands the upconversion
phosphors are equally weighed in 0.5 g and packed tightly
inside the specially designed powder sample holder which has a
glass window for excitation and emission. The holder was placed
at the sample port of the integrating sphere and excited with a
980 nm laser of 0.5 W/cm 2 power density. Emission spectra were
collected in the 500–1000 nm range with all three phosphor
samples. In order to overcome the heating effect all measurements
are done within a short time period. Further, since the
powder was tightly packed inside the sample holder error from
the scattering was negligible. To measure the absorbed photons
Fig. 1. XRD pattern of the Gd 2O 2S: Yb/Er sample. Vertical lines show the standard peak positions of the Gd 2O 2S (JCPD File no. 26-1422). The left inset shows crystal
structure of Gd2O2S indicating the locations of Gd (Pink), O (Red) and S (Yellow) atoms. Right inset shows the FE-SEM image of the phosphor particles. (For interpretation
of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Comparison of the upconversion emission spectrum of the phosphor under 980 nm laser excitation. Inset shows the green emission at 980 nm excitation with
20 mW of power and the integrating sphere experimental setup for quantum yield measurements.

Quantum Yield (%)
0.04
0.03
0.02
0.01
0.00
GOS
0 2 4 6 8 10 12 14 16 18 20
Yb/Ho Ratio
YOS
LOS
Fig. 3. (a) Quantum yield of green emission as a function of Yb/Ho ratio in the
three phosphor host compositions. (b) Quantum yield of red emission as a
function of Yb/Ho ratio in the three phosphor host compositions studied. (c)
Quantum yield of 756 nm emission as a function of Yb/Ho ratio in the three
phosphor host compositions studied.
the excitation laser profile was collected from a fully reflecting
BaSO4 background which shows no absorption at 980 nm excitation.
The difference between the background and sample at the
excitation wavelength gives the absorbed photons, whereas
the emitted photons can be calculated from the area under the
emission spectra. All spectra are corrected for spectral response
G.A. Kumar et al. / Materials Letters 98 (2013) 63–66 65
of the fluorimeter and integrating sphere. The quantum yield is
obtained as the ratio of the number of photons emitted vs.
number of absorbed photons. The spectra collected using integrating
sphere are shown in Fig. 3. Following this procedure we
obtained the quantum yield for various Yb/Ho ratios and the
results obtained for 541, 652 and 756 nm bands are summarized
in Fig. 3a, b and c. Based on this, the green emission efficiency
shows an increasing tendency in Y 2O 2S and La 2O 2S whereas in
Gd2O2S it decreases with higher Yb/Ho ratio. On the other hand,
the red upconversion efficiency shows a decreasing tendency
with Yb/Ho ratio in all three phosphors.
In all compositions the 756 nm emission quantum yield shows
highest value at Yb/Ho ratio of 1 and thereafter decreases except
in the case of LOS where it shows a slightly increasing tendency
from Yb/Ho ratio of 16. Among all emission bands observed the
756 nm emission in LOS shows the highest value of quantum
yield 0.12% which is nearly 2 times more efficient than the green
emission in LOS. In all three compositions 0.5 mol% Ho is
the optimum concentration for best efficiency and this result is
in agreement with previous observation [12]. Concentration
quenching arising due to the non radiative energy transfer
interactions between the emitting ions can suppress the emission
intensity and hence efficiency decreases considerably.
Recently we explored in detail the quantum yield of various
emission bands in the UV–vis–NIR region as a function of the
excitation power density in M 2O 2S doped with various Yb/Er and
Yb/Ho and compared them with the well know NaYF4:Yb(20%)/
Er(2%) host [13,14]. According to this observation the green
upconversion efficiency at 0.5 W/cm 2 power density was estimated
to be 0.157% and 0.015% respectively in La 2O 2S:Yb(9%)/
Er(1%) and NaYF4:Yb(20%)/Er(2%). Based on this comparison
La 2O 2S:Yb(9%)/Er(1%) is the most efficient green emitter followed
by La2O2S:Yb(9.5%)/Ho(0.5%).
4. Conclusions
In conclusion we have studied the 980 nm excited external
upconversion quantum yield of Yb/Ho doped M2O2S (M¼Gd, Y,
La) as a function of the Yb/Ho ratio and found functional
dependence of the quantum yield of 541, 658 and 756 nm
emission bands with Yb/Ho ratio. Though most of the phosphor
shows intense green upconversion which was visible to the naked
eye, the 756 nm emission quantum yield in La 2O 2S is nearly two
times higher than the green emission.Gd2O2S:Yb(4.5%)Ho(0.5%) is
the brightest single color green upconversion phosphor with
green efficiency 14 times higher than red.
Acknowledgment
This work was supported by the National Science Foundation
Partnerships for Research and Education in Materials (PREM)
Grant no. DMR-0934218.
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