Enhancement of upconversion emission and temperature sensing of paramagnetic Gd2Mo3O9: Er3+/Yb3+ phosphor via Li+/Mg2+ co-doping

Article abstract of DOI:10.1016/j.jallcom.2018.02.162

Herein, enhancement of upconversion emission and temperature sensing in Li⁺/Mg²⁺ co-doped Gd₂Mo₃O₉: Er³⁺/Yb³⁺ is reported. The upconversion emission enhancement of 53 and 21 time

Full text of DOI:10.1016/j.jallcom.2018.02.162

Accepted Manuscript
Enhancement of upconversion emission and temperature sensing of paramagnetic
3+
3+
+
2+
Gd Mo O : Er /Yb phosphor via Li /Mg co-doping
2
3 9
Shriya Sinha, Ashok Mondal, Kaushal Kumar, H.C. Swart
PII:
S0925-8388(18)30607-8
10.1016/j.jallcom.2018.02.162
JALCOM 45038
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 6 November 2017
Revised Date: 12 February 2018
Accepted Date: 13 February 2018
Please cite this article as: S. Sinha, A. Mondal, K. Kumar, H.C. Swart, Enhancement of upconversion
3+
3+
+
2+
emission and temperature sensing of paramagnetic Gd Mo O : Er /Yb phosphor via Li /Mg co-
2
3 9
doping, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.02.162.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Enhancement of Upconversion emission and Temperature Sensing of
3+
3+
+
2+
Paramagnetic Gd
2
Mo
3
O : Er /Yb Phosphor via Li /Mg Co-doping
9
1
1
1*
2
Shriya Sinha , Ashok Mondal , Kaushal Kumar , H. C. Swart
1
Optical Materials & Bio-imaging Research Laboratory, Department of Applied Physics,
Indian Institute of Technology (Indian School of Mines) Dhanbad-826004, India
2
Physics Department, University of Free State, Bloemfontein, South Africa
ABSTRACT
Herein, enhancement of upconversion emission and temperature sensing in Li /Mg
+
2+
3
+
3+
co-doped Gd Mo O : Er /Yb is reported. The upconversion emission enhancement of 53
2
3
9
and 21 times is observed in green (524 nm) and red (660 nm) emission bands via
+
2+
simultaneous co-doping of Li /Mg ions and is explained in terms of radiative and
non-radiative transition rates. High thermal sensitivity of the material is realized upon
+
2+
Li /Mg co-doping by measuring the fluorescence intensity ratio of two thermally coupled
2
4
3+
-1
energy levels ( H1 & S ) of Er ion. The maximum absolute sensitivity of 0.0230 K at
1/2
3/2
5
13 K is found for present material and is among the highest measured sensitivities for
luminescence based thermometers. Additionally, improved magnetization and magnetic mass
+
2+
susceptibility is also observed on co-doping of Li /Mg in the material. The result suggests
+
2+
that simultaneous incorporation of Li /Mg is an effective approach for enhancing
3
+
3+
multifunctionality of Gd Mo O : Er /Yb phosphor and this material has potential
2
3
9
applications in color displays, high temperature sensing and magnetic resonance imaging.
Keywords: Upconversion; Temperatures sensing; Magnetic property; Multifunctional material
*
Corresponding author’s e-mail address: kumar.bhu@gmail.com; Telephone: +91-326-223 5754

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1
. Introduction
The last decade has seen enormous growth in the search of highly efficient upconversion
nanaomaterials capable of converting near infrared (NIR) photons into visible or ultraviolet
(UV) radiation. The upconversion (UC) luminescence is generally achieved in rare earth (RE)
ions due to their abundant intra-4f configurational energy levels with several metastable
levels [1]. The RE based upconversion phosphors have potential applications in biological
imaging, optical temperature sensing, drug delivery, cancer therapy, display devices, security
3
+
3+
3+
3+
applications, solar light conversion etc. [2-7]. The RE ions (such as Ho , Er , Eu , Tb and
3
+
Tm ) doped UC phosphors as optical temperature sensors have received considerable
attention in recent years because they can provide non-contact temperature measurement in
nanometre to submicron scale by probing the temperature dependent fluorescence intensity
3
+
ratio (FIR) of two thermally coupled energy levels [8-13]. Among all the RE ions, Er is
most popular activator for optical temperature sensors due to its two suitable thermally
2
4
-1
coupled energy levels, H1 and S with an energy gap of ~ 770 cm [14-16]. Many
1/2
3/2
researchers are involved in the investigation of optical temperature sensing behavior based on
the FIR technique [16-19]. Recently, Tang et al. [16] have reported temperature sensitivity as
-1
3+
3+
high as 0.0087 K for La O codoped with Yb and Er . Wang et al. [17] have reported
2
3
3
+
3+
+
improvement in temperature sensitivity of CaWO : Er /Yb phosphors by doping with Li ,
4
2
+
2+
3+
Sr and Mg ions. The effect of Yb concentration on temperature sensing behaviour of
3
+
3+
Gd TiO : Er /Yb phosphor has been investigated by Liao et al. [18] and found that sensor
2
5
3
+
sensitivity decreases with increasing Yb concentration.
It is well known that UC emission efficiency is strongly dependent on the host lattice and
its interaction with the dopant ions. Generally, low phonon frequency host materials can
minimize the multiphonon relaxation rate and offer high UC efficiency [16, 19]. Among
various RE doped materials, molybdates with scheelite-type structure such as MMoO (M =
4
Ca, Sr & Ba), RE (MoO ) (RE= Gd, Y & La), Gd Mo O , Y MoO , La MoO etc. have been
2
4 3
2
3
9
4
9
2
6
extensively studied as hosts owing to their high emission efficiency, excellent chemical and
thermal stability. Such materials also have broad optical transparency in the near-UV region
[
20-25]. These features make them suitable for applications in optoelectronics, scintillators,

ACCEPTED MANUSCRIPT
catalysis etc. [22]. The Gd Mo O is recognized as one of the excellent up/down conversion
2
3
9
luminescent material and is used by several researchers over the past few years [23, 26, 27].
3
+
Moreover, paramagnetic Gd ions based host matrices are used as a contrast agents for
magnetic resonance imaging [28-30]. Therefore, Gd Mo O is considered to be an ideal
2
3
9
bi-functional material that exhibits luminescence as well as magnetic properties
simultaneously and such combination could be used as a dual mode probe for biomedical
applications [29, 30].
Improving the luminescence efficiency of RE doped materials is of great importance to
broaden their applications, especially in bio-medical field [31]. The UC luminescence of RE
doped materials depends on the probability of their intra-4f electronic transition, which is
greatly affected by the symmetry of crystal field around RE ions [32]. Thus, tailoring the
local crystal field around RE ions is an effective method to improve the UC luminescence
intensity and hence is the topic of recent research interest. Many researchers have reported
3
+
3+
3+
2+
2+
2+
+
+
2+
the effect of non-RE ions (e.g., Sc , Fe , Bi , Cd , Mg , Zn , Li , K , Mn ) to enhance
the UC luminescence intensity [33-40]. Recently, Li et al. [33] have reported that UC
3
+
3+
3+
3+
luminescence intensity of BiPO : Er /Yb phosphor can be enhanced by co-doping of Sc
4
3
+
3+
ions. The effect of Fe tri-doping on the UC luminescence of NaGdF : Yb , Er
4
nanoparticles has been studied by Ramasamy et al. [34] and have reported an enhancement of
4 and 30 times for green and red emission bands, respectively. Zhu et al. [36] have reported
3
3
+
3+
2+
UC enhancement in LiYF : Yb , Er nanocrystals via co-doping of Cd ions. The effect of
4
2
+
3+
3+
Mg tri-doping on UC of β-NaGdF : Yb , Er nanocrystals has been studied by Zhao et al.
4
[
32]. Other attempts have also been made to enhance the luminescence intensity via
simultaneous co-doping of two non-RE ions or alkali metal ions [41-43]. However, this
strategy has not yet been explored on variety of phosphors and hence more research is
required. Recently, our group has reported the effect of simultaneous incorporation of alkali
+
+
3+
3+
metal ions (K /Na ) into CaMoO : Er /Yb phosphor and has reported 36 and 5 times
4
enhancement in green and red emission bands [41]. Astha et al. [42] have reported an
2
+
2+
enhancement in the blue UC emission intensity by co-doping of Zn -Mg ions in La O :
2
3
3
+
3+
3+
Tm /Yb phosphor. The luminescence enhancement in Gd O : Ln (Ln = Yb/Er, Eu)
2
3
+
2+
nanocrystals by dual co-doping of Li /Zn ions has been studied by Li et al. [43]. All these

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researchers have proved that co-doping of two ions (non-RE or alkali metal ions)
simultaneously into the host lattice is a better approach to achieve high luminescence
intensity as compared to single co-doping. The investigations on the improvement of UC
+
2+
efficiency by simultaneous co-doping of Li /Mg has not yet been carried out to the best of
authors knowledge.
+
2+
3+
3+
Here, authors have tried to introduce Li /Mg ions into Gd Mo O : Er /Yb crystal
2
3
9
lattice to see its effect on UC emission property. Authors have also investigated the
+
2+
magnetization of sample after co-doping of Li /Mg . The temperature dependence of
magnetization properties has studied using zero-field-cooled and field-cooled procedures in
an applied magnetic field. The scope of the present work is to offer an effective way to
+
2+
enhance the multi-functionality of Gd Mo O phosphor via co-doping of Li /Mg with
2
3
9
3
+
3+
Er /Yb ions.
2
. Experimental details
2
.1 Material synthesis
+
2+
+
2+
3+
3+
A series of Li , Mg and Li /Mg ions co-doped Gd Mo O : Er /Yb phosphor was
2
3
9
prepared via co-precipitation method using ethylene glycol (EG) as chelating agent. The
compositions of the samples were chosen as follows:
(
96.7 – x – y) Gd Mo O + 0.3 mol% Er O + 3 mol% Yb O + x Li CO + y MgCO
2 3 9 2 3 2 3 2 3 3
Where,
3+
3+
x = 0 mol%, y = 0 mol% for Gd Mo O : Er /Yb (sample named as M)
2
3
9
3
+
3+
+
x = 2, 4, 6 and 8 mol%, y = 0 mol% for Gd Mo O : Er /Yb /Li (MLi )
2
3
9
6
3+
3+
2+
x = 0 mol%, y = 2, 4, 6 and 8 mol% for Gd Mo O : Er /Yb /Mg (MMg )
2
3
9
4
3
+
3+
+
2+
x = 1, 1.5, 2, 2.5 and 3 mol%, y = 4 mol% for Gd Mo O : Er /Yb /Li /Mg (MLi Mg )
2
3
9
2
4
The starting precursors, Gd O (99.99 %, Merck, India), Er O (99.99 %, Otto, India), Yb O
3
2
3
2
3
2
(99.99 %, Otto, India) Mg CO (99.99 %, Merck, India), Li CO (99.99 %, Merck, India) and
, 2 3 2 3
(
NH ) Mo O .4H O (99 %, Merck, India) were taken into a stoichiometric ratio. In a typical
4
6
7
24
2

ACCEPTED MANUSCRIPT
synthesis process, the nitrates of the chemicals were prepared by dissolving oxides and
carbonates in concentrated nitric acid (HNO ). All the prepared nitrates were mixed together
3
and extra amount of nitric acid was removed by evaporation with addition of water under
continuous stirring on a magnetic stirrer whose hot plate was kept at 80 °C. After that
ammonium heptamolybdatetetrahydrate (NH ) Mo O .4H O solution was mixed into the
4
6
7
24
2
above solution. To prevent the aggregation of particles, 25 ml of ethylene glycol (EG) was
added as a chelating agent into the final solution. The PH of the resultant solution was kept at
9
.0 and left for 12 hours to get the precipitation. After that, the precipitate was filtered and
washed with de-ionized water and ethanol for several times followed by drying in an oven at
8
0 °C for 8 hours. Finally, the obtained powder was annealed at 800 °C for 4 hours.
2
.2. Characterizations
The X-ray diffraction (XRD) patterns were measured on Bruker-D8 Advanced X-ray
diffractometer using CuK (1.5406 Å) radiation source over the angular range 10° ≤ 2θ ≤ 60°.
α
Fourier transform infrared (FTIR) spectra of the samples were recorded on FTIR
-1
spectrometer (Perkin Elmer, spectrum RXI) using KBr pellet technique in the 4000-400 cm
spectral region. Morphologies of the samples were obtained using the ZEISS SUPPRA55
Field emission scanning electron microscopy (FESEM) equipped with the energy dispersive
X-ray (EDX) spectroscopy system. Transmission electron microscopy (TEM) images were
recorded on Hitachi model (H-7500). The electronic absorption spectra were recorded using a
Perkin-Elmer, Lambda 950, UV-Visible-NIR spectrophotometer in the wavelength range of
2
00-1200 nm. The UC emission spectra were recorded on a SP2300 grating spectrograph
(Princeton Instruments, USA) using a 980 nm continuous wave (CW) diode laser as
excitation source. The decay curve analysis was performed by using a Lifetime spectrometer
upon excitation with a Xenon flash lamp. For temperature dependent UC emission study, a
thermocouple was placed close to laser spot on the sample surface and the spectra were
recorded on a CCD spectrometer (Model: ULS2048X64, Avantes, USA). The laser beam
−
2
power was set at 15 Wcm and a chopper was used to chop the CW light to avoid the direct
heating of the material. Magnetic measurements were performed using a Quantum Design
Physical Property Measurement System (PPMS) in the temperature range of 2-300 K.

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. Results and discussion
3
3
.1 Structural studies
3
.1.1 X-ray diffraction analysis
The crystal structure and phase composition of the synthesized samples was determined by
X-ray diffraction analysis. Fig. 1(a) shows the XRD patterns of samples named as M, MMg4,
MLi and MLi Mg (all annealed at 800 °C). The diffraction peaks of all the samples were
6
2
4
well matched with the standard JCPDS file no. 33-0548 with no other impurity phases. It
confirms the formation of pure tetragonal phase of Gd Mo O . It is noticed that peak
2
3
9
positions are slightly shifted towards higher angle side with improved crystallinity for
+
2+
+
2+
samples incorporated with Li , Mg and Li /Mg ions [Fig. 1(b)]. The shifting in diffraction
3
+
peak is related to the lattice contraction caused by the substitution of Gd (93.8 pm) ions by
+
2+
smaller ionic radii Li (76 pm) and Mg (72 pm) [44]. Furthermore, the lattice parameters
and unit cell volume were calculated for M (a = b = 5.18 Å, c = 11.20 Å; unit cell volume:
3
3
3
00.52 Å ) and MLi Mg (a = b = 5.16 Å, c = 11.15 Å; unit cell volume: 296.87 Å ) samples.
2 4
It is found that the lattice parameters and unit cell volume both decrease with the doping of
3
+
3+
+
2+
Er /Yb , Li and Mg ions as compared to the standard JCPDS data. This is because of
3
+
3+
3+
larger size Gd (93.8 pm) ions are replaced by smaller size Er /Yb (89 pm/ 86.8 pm) and
+
2+
Li /Mg (76 pm/72 pm) ions and it leads to shrinkage of unit cell volume. The strain present
in the lattice was determined from the Williamson-Hall relation [27]
β
cos
θ
1
ε
sinθ
=
+
(1)
λ
D
λ
where, D is the crystallite size (in nm),
β
is full width at half maximum,
θ
is Bragg diffraction
angle,
cos
λ
λ
is the wavelength of radiation (1.54 Å). The strain (
ε) is calculated from the slope of
β
θ
/
vs. sin plot for M and MLi Mg sample. The plot is given in supplementary Fig.
θ/λ
2
4
S1. The positive slope indicates tensile strain for M sample. The slope becomes negative for
+
2+
Li /Mg doped sample and strain goes towards compressive strain. The compressive strain is
3
+
+
2+
possible when ions of larger radii (Gd ) are replaced by smaller ionic radii (Li , Mg ).

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3
.1.2 Fourier Transform Infrared (FTIR) study
The FTIR spectra of as-synthesized as well as samples annealed at 800 °C are shown in Fig.
. The as-synthesized sample shows the presence of impurity bands of organic molecules.
2
-1
-1
The bands at 3640 cm and 3410 cm are attributed to the O-H asymmetric and symmetric
-1
stretching vibrations of water molecules, respectively. The band at 1637 cm is assigned to
-1
the H-O-H bending vibration of water absorption while the band around 1390 cm is
attributed to the N-O mode of vibration of HNO used in the sample preparation. The band
3
-1
around 2431 cm is assigned to the C-O vibration of CO adsorbed at the surface of sample
2
+
2+
from the atmosphere [8]. It is clear from the Fig. 2 that when Li and Mg are incorporated
into the sample, the intensity of impurity bands decreases and finally bands are removed
completely from MLi Mg sample due to annealing at 800 °C. Simultaneous co-doping of
2
4
+
2+
Li /Mg can eliminate the organic impurities from the sample which may increase the
luminescence efficiency by reducing non-radiative relaxation rates. The bands ranging from
-1
8
50 to 670 cm are related to the lattice vibrations of Gd Mo O host.
2 3 9
3
.1.3 Microstructure analysis
Fig. 3[a-d] shows the FESEM images of annealed M, MMg , MLi and MLi Mg samples.
4
6
2
4
+
2+
Without doping of Li /Mg , the sample exhibits non-uniform and irregular morphology
+
2+
agglomerated with each other. When Li /Mg ions are incorporated into the sample, the
morphology and size of the particles increases which is evident in Fig. 3(b-d). The TEM
image of MLi Mg sample has shown in Fig. 3(e). The particles have seen connected through
2
4
grain boundaries. The high resolution transmission electron microscopy (HRTEM) image
Fig. 3(f)] clearly shows the resolved lattice fringes and measures interplanar distance of 0.26
[
nm between the two adjacent planes, which corresponds to (200) crystal plane of tetragonal
Gd Mo O . The energy dispersive X-ray spectroscopy (EDX) was further used to investigate
2
3
9
the elemental composition and purity of annealed MLi Mg sample as shown in Fig. 4(a).
2
4
The EDX spectrum confirms the presence of Gd, Mo, O, Er, Yb and Mg elements in expected
stoichiometric proportion in the Gd Mo O host. However, EDX detector could not be able to
2
3
9
+
detect the presence of Li ion due to its smaller atomic weight among all other elements. In
addition, the elemental mapping results that all the constituent elements (Gd, Mo, O, Er, Yb

ACCEPTED MANUSCRIPT
and Mg) are homogeneously distributed throughout the whole region of samples [Fig.
4
(b-g)].
3
3
.2 Optical studies
.2.1 UV-Visible absorption spectroscopy
The UV-Visible absorption spectra of M and MLi Mg samples were recorded in diffuse
2
4
reflectance mode against a reference standard of BaSO compound in the 200-1200 nm
4
wavelength range and the graph has shown in Fig. 5. The spectrum shows a broad absorption
band around 300 nm assigned to the band absorption of the host [27]. The other absorptions
2
4
4
4
bands are observed around 521 and 656 nm due to the H11/2← I15/2 and F9/2← I15/2
3
+
transitions, respectively of Er ion. The band around 978 nm is observed due to the
2
2
3+
F5/2
←
F transition of Yb ion. It is evident from the Fig. 5 that the intensities of these
7/2
+
2+
absorption bands increase on the addition of Li /Mg ions.
The optical band gap energy (E ) of the samples was estimated from the diffuse
g
reflectance spectrum using the equation proposed by Kubelka and Munk [45]. The procedure
is described in our previous report [41]. Molybdates have optical absorption governed by
direct band gap transitions [46] and the Kubelka-Munk (K-M) plot has shown in the inset of
Fig. 5 with best straight line fit. The value of band gap energy (E ) was extracted by
g
2
extrapolating the linear fitted regions to [F(R)h
ν
] = 0. It is found that the value of Eg
+
2+
increases from 3.55 eV to 4.09 eV with the addition of Li /Mg ions into the sample M. This
may be due to the removal of lower lying impurity states in the sample [47].
+
2+
+
2+
3
.2.2 Effect of Li , Mg and Li /Mg incorporation on UC properties
+
2+
The UC emission spectra of samples codoped with various concentrations of Li and Mg
ions were studied at room temperature upon 980 nm laser light excitation with a fixed
2
excitation power density of 25 W/cm . First, the emission intensity was optimized by varying
+
2+
the concentrations of each Li and Mg from 0 to 8 mol%. The resulted emission
enhancement for green and red emission bands are plotted in Fig. 6(a) and (b). It is found that
+
2+
sample containing 6 mol% Li and 4 mol% Mg show maximum enhancement in emission
intensity. Beyond this optimum doping concentration, a drastic decrease in emission intensity

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is observed for each sample. Fig. 6 (c) and (d) shows the comparison of the UC emission
+
2+
spectra of sample M with optimized codoped samples i.e. 6 mol% Li and 4 mol% Mg . All
the samples show green (524 nm and 548 nm) and red (660 nm) emission bands due to the
2
4
4
4
4
4
3+
H11/2
→
I15/2, S
→
I15/2 and F9/2→ I15/2 transitions of Er . It is observed that there is no
3/2
+
change in the shape and peak positions of the emission bands on co-doping with Li and
2
+
+
Mg ions except change in emission intensity. The Li co-doping enhances the green and red
2
+
emission bands by 3.7 and 6.1 times, respectively. On the other hand, Mg co-doping
enhances the green and red emission bands by 3.2 and 5.9 times, respectively as compared to
the sample M.
+
2+
In the next study, both Li and Mg were simultaneously codoped in sample M. Here,
2
+
+
concentration of Mg was fixed at 4 mol % and concentration of Li (1, 1.5, 2, 2.5 and 3
mol %) was varied. Fig. 7 shows the comparison of UC emission spectra of M, MMg , MLi6
4
2
and MLi Mg samples measured at same excitation power of 25 W/cm . It is found that
2
4
+
2+
simultaneous co-doping of Li /Mg hugely enhance the emission intensity as compared to
+
2+
single co-doping of either Li or Mg . Inset of Fig. 7 shows the variation of the green and
+
2+
red band intensities with an increase in Li concentration at a fixed Mg concentration i.e, at
4
mol%. The optimum emission intensity is obtained corresponding to a doping of 2 mol %
+
Li . At this doping concentration, intensities of the green and red emission bands have
increased by 53 and 21 times as compared to the sample M. The enhancement mechanisms of
+
2+
emission intensity with co-doping of Li and Mg ions has been correlated to the distortion
3
+
3+
+
of the local crystal field symmetry around Er and Yb ions [32, 38, 48]. Both Li (76 pm)
2
+
3+
&
Mg (72 pm) ions have smaller ionic radii and can easily substitute the Gd sites causing
the shrinkage of the host lattice. This lattice shrinkage results decrease in lattice parameters
and thus perturbing the symmetry of the local crystal field around the RE ions. According to
previous studies, asymmetry surrounding of RE ions is favorable for radiative transition and
consequently increasing the UC luminescence intensity [38, 49, 50]. Furthermore, beyond the
+
2+
optimum doping concentrations of Li and Mg ions, the UC intensity has seen to quenched
due to the creation of excess defect centers in the Gd Mo O lattice. Moreover, enhancement
2
3
9
+
2+
in UC emission intensity with Li /Mg co-doping may also be due to the reduction in optical
-
quenching centers. It is evident from the FTIR spectra, OH organic impurity is completely

ACCEPTED MANUSCRIPT
+
2+
removed from the Li /Mg codoped samples which helps to increase the UC intensity by
+
2+
diminishing non-radiative relaxation rates. The improvement in crystallinity of Li /Mg
codoped sample as observed from XRD and FESEM analysis may also support the UC
+
2+
enhancement. The tensile lattice strain goes towards compressive strain upon Li /Mg ions
co-doping and this could also be a reason for UC enhancement [51].
4
4
In order to prove the enhancement in UC, the emission decay curves of S3/2→ I15/2
3
+
transition of Er ion for all the samples were measured under 380 nm excitation and graph
has shown in Supplementary Fig. S2. The decay curves were well fitted to bi-exponential
function expressed as: I (t) = A exp (-t/τ1) + A exp (-t/τ2), where I (t) is the luminescence
1
2
intensity at time t. A and A are constants and τ1 and τ2 are the shorter and longer lifetime
1
2
components, respectively. The fitted curves along with corresponding lifetime values for all
4
the samples are mentioned in the Fig. S2. It is found that the lifetime (
τ
) of S level
3
/2
+
2+
increases with the incorporation of Li and Mg ions into sample M. A similar trend in
intensity variation was also observed (Fig. 7). The increase in lifetime suggests the decrease
of non-radiative transitions and increase of radiative emission. As we know the experimental
lifetime of an excited state,
τ, is determined by the theoretical lifetime (τrad), the non-radiative
transition rate (W ), and the energy transfer rate (W ), which can be expressed as 1/
τ
=
NR
ET
1/τrad + WNR + W . This experimental larger lifetime value (τ) could be attributed to the
ET
tailored local environment of activators (τrad) and the decrease of non-radiative decay rates of
excited state (W ) [38].
NR
+
2+
Researchers have reported that addition of Li and Mg ions into the RE doped materials
results in a significant enhancement of the luminescence intensities [48-50]. Singh et al. [48]
+
3+
3+
have reported the effect of Li ion on the Y Ti O : Er /Yb phosphor and found
2
2
7
enhancement in the green and red emission bands by 15 and 8 times, respectively.
Enhancement of about 2 and 3.3 times in green and red emission bands of -NaYF4:
Er /Yb microcrystals due to co-doping of Li ions has been observed by Ding et al. [49].
β
3
+
3+
+
2
+
However, the effect of Mg on the UC enhancement of materials has rarely been found in the
2
+
literature [32, 50, 52]. In this regard, Singh et al. [50] have reported the effect of Mg in
3
+
3+
CaAl O :Er :Yb phosphor and observed enhancement of about 60 and 10 times in the
1
2
19
green and red emission intensities, respectively. Herein, authors have observed maximum

ACCEPTED MANUSCRIPT
enhancement of ~ 53 and ~ 21 times for the green (524 nm) and red (660 nm) emission bands,
+
2+
respectively. Above observations conclude that dual co-doping of Li /Mg
ions
3
+
3+
simultaneously into Gd Mo O : Er /Yb improves the UC intensity significantly as
2
3
9
+
2+
compared to single co-doping of either Li or Mg .
+
2+
To further examine the effect of Li /Mg ions on UC mechanisms, the pump power
dependence of the green and red emission bands was investigated for M and MLi Mg4
2
samples and the plot has shown in Fig. [8(a) & (b)]. For an unsaturated UC process, it is well
known that intensity of UC luminescence (I) is proportional to the nth power of input laser
n
power (P) follows the relation I ∝ P where n is the number of photons involved in the UC
,
process. Value of ‘n’ can be obtained from the line fitting of lnI versus lnP plot [53]. Both
green and red emission bands must originate from two photon absorption processes. However,
the slope ‘n’ values for green (548 nm) and red (660 nm) emission bands were found to
+
2+
decrease with Li /Mg incorporation. The calculated slope values 0.71 (for 548 nm) and 0.66
for 660 nm) are much deviated from the expected two photon values. The reduction in slope
value may be due to the saturation mechanism of intermediate levels, which has been
(
+
2+
theoretically described by Pollnau et al. [53]. Despite the low slope values in Li /Mg doped
+
2+
sample, the emission intensity has found much higher compared to Li /Mg free sample. The
+
2+
2
4
4
Li /Mg codoped sample create more and more populations in H , S and F levels
11/2
3/2
9/2
due to decrease in the non-radiative decay rates [41]. There are several theoretical modeling
in literature where it has been proved that due to saturation mechanism of intermediate levels
the number of photons decreases from the expected values [37, 53, 54]. The possible UC
3+
3+
mechanism in the Er -Yb system upon 980 nm light excitation has schematically shown in
3
+
3+
the energy level diagram [Fig 8(c)]. The dominant UC process between Yb and Er is the
2
3+
4
4
4
energy transfer (ET) from the excited F level of Yb to the I , F and F levels of
5
/2
11/2
9/2
7/2
3
+
Er ions. The whole description of the UC processes can be found in our previous reports [8,
7, 37, 55].
Additionally, the variation of UC emission colour with excitation power density was
2
calculated using CIE (Commission Internationale de I’Eclairage) chromaticity diagram [27].
Fig. 8(d) shows the variation of colour coordinates with excitation power for MLi Mg4
2

ACCEPTED MANUSCRIPT
2
sample. At low pump power (7 W/cm ), the emission colour is in the green region (0.28, 0.70)
which is shifted to pure green region (0.17, 0.79) with increase in power density upto 65
2
3+
2
4
W/cm . This may be due to the increased population of Er ions at H and S levels as
11/2
3/2
4
compared to F level under high excitation power density. The green to red intensity ratio
9
/2
increases as the excitation power increases in the MLi Mg sample, indicating that green
2
4
emission is more intense when the sample is excited under high pump power. The slow
increase in red band correlates the lowest defects/impurities in MLi Mg co-doped sample.
2
4
3
.2.3 Temperature sensing performance
It is clear from the UC emission spectra that MLi Mg sample gives optimum green UC
2
4
emission; therefore authors have chosen it to investigate temperature sensing performance.
The temperature dependent green UC emission spectra of MLi Mg sample were recorded
2
4
within 300-513 K temperature range. In Fig. 9(a), the normalized UC emission spectra in
5
05-575 nm wavelength range are plotted at different temperatures and at fixed excitation
2
power of 15 W/cm . It can be seen from the Fig. 9(a) that peak position of the emission bands
does not shift with temperature, whereas relative intensity of the emission bands changes with
2
4
-1
temperature. Since energy gap between the H and S levels is very small (770 cm ), the
11/2
3/2
2
4
H11/2 level can be populated from the S3/2 level via thermal excitation, resulting in the
2
4
4
4
variation of UC emission intensities of the H
→
I15/2 and S →
I15/2 transitions at
11/2
3/2
different temperatures. According to Boltzmann distribution theory, the fluorescence intensity
2
4
3+
ratio (FIR) of two thermally coupled levels H and S of Er ion can be expressed as [8]:
11/2
3/2
IH W g hν
H
 −∆E 
 −∆E 
H
H
FIR =
=
exp
= Bexp
(2)







IS
W g h
S
S
ν
S
k T
B

k T
B
2
4
where, I and I are the integrated intensities corresponding to H
→
I15/2 (510-540 nm)
H
S
11/2
4
4
and S
→ I15/2 (540-570 nm) transitions, respectively. WH and W are the radiative
S
3/2
2
4
probabilities of the transitions, g and g are the (2J+1) degeneracy of levels H and S
H
S
11/2
3/2
2
4
4
4
respectively and h H
ν
and hνs are the photon energies of the H
→
I15/2 and S
→
3/2
I15/2
11/2
transitions respectively,
ꢀ
E is the energy gap between the two emitting levels, k is the
B
Boltzmann constant and T is the absolute temperature. Equation (2) can be expressed as
follows

ACCEPTED MANUSCRIPT

∆E 
ln(FIR) = ln(B) + −
(3)


k T

B

Inset of Fig. 9(a) shows the plot of ln(FIR) as a function of inverse absolute temperature T in
the range of 300-513 K for MLi Mg sample. The experimental data is well fitted to straight
2
4
-1
line which gives slope (-ꢀE/k) and intercept (lnB) equal to 1084 cm and 3.58, respectively.
-1
The calculated energy gap from the slope is found to be 753 cm .
For temperature sensing application, it is of great importance to understand the rate at which
fluorescence intensity ratio changes with temperature. This parameter is known as sensitivity.
The absolute sensitivity (S ) and relative sensitivity (S ) are defined as [12, 56, 57]:
a
r
dR
dT
∆E
Sa =
= R
(4)
(5)
2
KT
1
dR
∆E
and
Sr =100%×
=100%×
2
R dT
KT
where all the terms have their usual meaning. The absolute and relative sensitivities were
calculated as a function of temperature and the plot has shown in Fig. 9(b). It can be seen
from the Fig. 9(b) that the absolute sensitivity of MLi Mg phosphor increases with
2
4
-3
-1
increasing the temperature and reaches its maximum of about 23.06x10 K at 513 K. The
result suggests that the material is highly stable at elevated temperature and suitable for high
+
2+
temperature thermometry. The sensitivity was further compared with Li /Mg free
3
+
3+
3+
3+
Gd Mo O : Er /Yb phosphor reported in our previous work [27]. In Gd Mo O : Er /Yb
2
3
9
2
3
9
-3
-1
the temperature sensitivity was obtained as 10.57x10 K at 450 K. But due to co-doping of
+
2+
3+
3+
Li /Mg into Gd Mo O : Er /Yb system, the temperature sensitivity increases to
2
3
9
-3
-1
+
2+
2
3.06x10 K at 513 K. The reason behind the enhanced sensitivity in Li /Mg codoped
3
+
sample is expected due to the tailoring effect of crystal field environment around Er ion in
Gd Mo O matrix [17]. The maximum value of relative sensitivity was calculated to be
2
3
9
-1
1
.20 % K at 300 K. It is worth to compare the relative sensitivity of present material with
3
+
3+
other prominent studies of optical temperature sensors based on Er /Yb doped host
materials and comparison is given in Table. 3 [14, 41, 56, 58-63]. From the comparison, it is
found that sensitivity of present material is comparatively high compared to other materials
+
2+
reported previously. The obtained result indicates that simultaneous co-doping of Li /Mg

ACCEPTED MANUSCRIPT
3+
3+
into Gd Mo O : Er /Yb phosphor could be an effective approach to realize high
2
3
9
temperature sensitivity.
3
.2.4 Magnetic property
+
2+
Apart from the UC luminescence properties, the effect of Li /Mg ions on the magnetic
3+
3+
properties of Gd Mo O : Er /Yb phosphor has also investigated. The magnetic field
2
3
9
dependence of magnetization M(H) in MLi Mg sample was measured at three different
2
4
temperatures namely 300 K, 77 K and 2 K and recorded magnetization curves have shown in
Fig. 10(a). From the Fig. 10(a), one can see that material exhibit paramagnetic nature at 300
K and 77 K over a wide range of magnetic field (-90 to 90 kOe). A similar behavior was
+
2+
3+
3+
observed for Li /Mg free Gd Mo O : Er /Yb sample [27]. The paramagnetic properties
2
3
9
3
+
7
of the Gd ions in Gd Mo O come from the seven unpaired inner 4f electrons (4f ), which
2
3
9
are closely bound to the nucleus and effectively shielded by the outer closed shell electrons
2
6
5
s 5p from the crystal field. The straight line crossing the origin indicates paramagnetic
3
+
nature and the magnetic moments associated with the Gd ions are all localized and
3
+
non-interacting, which led to paramagnetism of Gd ions. The magnetic mass susceptibility
-5
of MLi Mg sample at 300 K and 77 K has determined to be 2.50x10 emu/g.Oe and
2
4
-5
8
.93x10 emu/g.Oe, respectively. The magnetization at 300 K and 77 K at 20 kOe has found
to be 0.50 emu/g and 1.79 emu/g, respectively. These obtained values are higher than the
+
2+
3+
3+
Li /Mg free Gd Mo O : Er /Yb sample as reported in our previous work [27]. In present
2
3
9
study sample indicates superparamagnetic behaviour at 2 K temperature with saturation
magnetization (30 emu/g) at 90 kOe magnetic field. In order to confirm paramagnetic nature,
the temperature dependence of the magnetization M(T) in the zero-field-cooled (ZFC) and
4
field-cooled (FC) regimes was done under an applied magnetic field of 10 Oe, and graph has
shown in Fig. 10(b) [left scale]. It is noticed that ZFC and FC curves totally overlapped in the
studied temperature range and do not show any hysteresis loop in the lowest temperature
range. Such behavior confirms the characteristic of paramagnetism and obeys the
Curie-Weiss law [64]. The reciprocal magnetic susceptibility, 1/χ (M/H) as a function of
4
temperature T for fixed H = 10 Oe has also plotted in Fig. 10(b) [right scale]. The
experimental data is linear in the 2-300 K temperature range and can be well fitted to

ACCEPTED MANUSCRIPT
Curie–Weiss formula
χ
= C/ (T-
Θ
), where
χ
is magnetic susceptibility, C is curie constant and
is close to 0 K, confirming paramagnetic
Θ
is Curie temperature. The Curie temperature
Θ
3
+
behavior of our sample [65]. The effective magnetic moment ꢁeff per Gd ion and Curie
constant were determined to be 7.09 ꢁB and 6.28, respectively. The value of ꢁeff is close to the
8
3+
expected value of ground state ( S ) Gd free ion (7.94 ꢁB) [66]. However, the low
7
/2
temperature M(H) curve has indicated superparamagnetism nature but is not confirmed from
ZFC and FC measurements [28, 64-66]. Similar type of observation has been reported by
Szczeszak et al. [64] and Heuvel et al. [67] for Gd Eu VO and Er Pb F nanoparticles,
0
.8
0.2
4
0.3
0.7 2.3
respectively. Thus, it is concluded from the M(H) and M(T) curves the present material is
found in a paramagnetic state. The strong paramagnetic behavior observed in present sample
has potential for external magnetic tracking of the nanophosphor in the biological
applications [29, 30].
4
. Conclusions
3
+
3+
In conclusion, highly luminescent-paramagnetic Gd Mo O : Er /Yb phosphor co-doped
2
3
9
+
2+
+
with Li and Mg ions has been successfully synthesized and characterized. The Li and
2
+
Mg ions have significantly enhanced emission intensity of green and red emission bands by
3 and 21 times, respectively. The enhancement is attributed to the distortion of the local
5
3
+
crystal field around the Er ions which was further verified by an increase in the radiative
3
+
lifetimes of the emitting levels of Er ion. Furthermore, temperature sensing of the sample
+
2+
has found to improve by Li /Mg co-doping with a maximum absolute sensitivity of 0.0230
-1
K at 513 K. Additionally, magnetization and magnetic mass susceptibility has improved by
+
2+
+
2+
co-doping of Li /Mg ions. The result indicates that intense green emitting Li /Mg
3+
3+
co-doped Gd Mo O : Er /Yb phosphor is an interesting thermographic phosphor and could
2
3
9
be considered for various applications including temperature sensing, display device and
magnetic resonance imaging.

ACCEPTED MANUSCRIPT
Acknowledgements
Authors are thankful to Dr. V. K. Rai, Department of Applied Physics, IIT (ISM) Dhanbad for
upconversion measurements. One of the authors, Ms. S. Sinha is thankful to IIT(ISM)
Dhanbad, for providing research fellowship. Dr. K. Kumar acknowledges Council of
Scientific & Industrial Research, New Delhi (project no. 03(1303)/13/EMR-II) for financial
assistance and DST, New Delhi for lifetime facility under DST-FIST (PSI-184-13) scheme.
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Figure captions
Fig. 1 X-ray diffraction patterns of M, MMg , MLi and MLi Mg samples annealed at
4
6
2
4
8
00 °C with their standard JCPDS data; (b) Enlarged XRD patterns of (112) diffraction peak
for all the samples.
Fig. 2 Fourier transform infrared spectra of (a) as-synthesized M (b) M annealed at 800 °C (c)
MMg annealed at 800 °C (d) MLi annealed at 800 °C (e) MLi Mg annealed at 800 °C.
4
6
2
4
Fig. 3 Field emission scanning electron microscopy image of (a) M (b) MMg (c) MLi (d)
4
6
MLi Mg samples annealed at 800 °C; (e) Transmission electron microscopy image of
2
4
annealed MLi Mg sample; (f) High resolution transmission electron microscopy image of
2
4
MLi Mg sample.
2
4
Fig. 4 (a) Energy dispersive X-ray spectrum of MLi Mg sample; [b-g] Elemental mapping of
2
4
Gd, Mo, O, Er, Yb and Mg in MLi Mg sample.
2
4
Fig. 5 UV-Visible absorption spectrum in diffuse reflectance mode of M and MLi Mg ; Inset
2
4
of Fig. 5 shows Kubelka-Munk plots to calculate optical band gap for M and MLi Mg4
2
sample.
+
Fig. 6 (a) Variation of green and red UC emission intensity at different Li concentration (b)
2
+
Variation of green and red UC emission intensity at different Mg concentration (c)
+
Comparison of UC emission spectrum of M sample with 6 mol% Li doped M sample (d)
2
+
Comparison of UC emission spectrum of M sample with 4 mol% Mg doped M sample
under 980 nm laser excitation.
Fig. 7 Comparison of UC emission spectra of optimized M, MMg , MLi and MLi Mg
4
4
6
2
samples. Inset of Fig. 7 shows the variation of UC intensity of green and red emission bands

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+
2+
as a function of Li concentration at a fixed Mg concentration at 4 mol%.
Fig. 8 Pump power dependence of green and red emission bands in (a) M (b) MLi Mg4
2
3
+
3+
phosphors; (c) Schematic energy level diagram of Er and Yb ions and the proposed UC
mechanisms under 980 nm laser light excitation; (d) CIE chromaticity diagram for MLi Mg4
2
sample (variation of emission colour with excitation power densities).
2
4
4
Fig. 9 (a) Temperature dependent green UC emission spectra for H11/2→ I15/2 and S
3/2
4
→
I15/2 transitions of MLi Mg phosphor excited by 980 nm excitation. Inset of Fig. 9(a)
2 4
shows monolog plot of FIR (I /I ) as a function of inverse absolute temperature; (b) variation
H
S
of absolute sensitivity (S ) and relative sensitivity (S ) as a function of temperature.
a
r
Fig. 10 (a) Magnetization versus applied magnetic field (M-H) curves in as-synthesized
MLi Mg sample at 2 K, 77 K and 300 K (b) zero-field-cooled (ZFC) and field-cooled (FC)
2
4
curves versus temperature (left scale) and the reciprocal of magnetic susceptibility (1/χ)
4
versus temperature (right scale) at fixed magnetic field of 10 Oe.

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FIGURES
Fig. 1 X-ray diffraction patterns of M, MMg , MLi and MLi Mg samples annealed at
4
6
2
4
8
00 °C with their standard JCPDS data; (b) Enlarged XRD patterns of (112) diffraction peak
for all the samples.
Fig. 2 Fourier transform infrared spectra of (a) as-synthesized M (b) M annealed at 800 °C (c)
MMg annealed at 800 °C (d) MLi annealed at 800 °C (e) MLi Mg annealed at 800 °C.
4
6
2
4

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Fig. 3 Field emission scanning electron microscopy image of (a) M (b) MMg (c) MLi (d)
4
6
MLi Mg samples annealed at 800 °C; (e) Transmission electron microscopy image of
2
4
annealed MLi Mg sample; (f) High resolution transmission electron microscopy image of
2
4
MLi Mg sample.
2
4

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Fig. 4 (a) Energy dispersive X-ray spectrum of MLi Mg sample; [b-g] Elemental mapping of
2
4
Gd, Mo, O, Er, Yb and Mg in MLi Mg sample.
2
4
Fig. 5 UV-Visible absorption spectrum in diffuse reflectance mode of M and MLi Mg ; Inset
2
4
of Fig.5 shows Kubelka-Munk plots to calculate optical band gap for M and MLi Mg4
2
sample.