Yb3 + doped CaMoO4 upconversion phosphor

    Dualistic temperature sensing in Er 3 + /Yb3 + doped CaMoO 4 upconversion phosphor Shriya Sinha, Manoj Kumar Mahata, Kaushal Kumar, S.P. Tiwari, V.K. Rai PII: DOI: Reference:

S1386-1425(16)30557-1 doi: 10.1016/j.saa.2016.09.039 SAA 14685

To appear in: Received date: Revised date: Accepted date:

3 July 2016 19 September 2016 21 September 2016

Please cite this article as: Shriya Sinha, Manoj Kumar Mahata, Kaushal Kumar, S.P. Tiwari, V.K. Rai, Dualistic temperature sensing in Er3 + /Yb3 + doped CaMoO4 upconversion phosphor, (2016), doi: 10.1016/j.saa.2016.09.039

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ACCEPTED MANUSCRIPT Dualistic Temperature Sensing in Er3+/Yb3+ Doped CaMoO4 Upconversion Phosphor Shriya Sinha, Manoj Kumar Mahata#, Kaushal Kumar*, S. P. Tiwari, and V. K. Rai

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Optical Materials & Bio-imaging Research Laboratory, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad-826004, India

Abstract

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Temperature sensing performance of Er3+/Yb3+ doped CaMoO4 phosphor prepared via polyol method is reported herein. The X-ray diffraction, Fourier transform infrared spectroscopy and

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Field emission scanning electron microscopy are done to confirm the phase, structure and purity of the synthesized phosphor. The infrared to green upconversion emission is investigated using 980 nm diode laser excitation along with its dependence on input pump

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power and external temperature. The temperature dependent fluorescence intensity ratio of

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two upconversion emission bands assigned to 2H11/2→4I15/2 (530 nm) and 4S3//2→4I15/2 (552 nm) transitions has shown two distinct slopes in the studied temperature range - 300 to

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760 K and therefore, dual nature of temperature sensitivity is observed in this phosphor. This phenomenon in rare earth doped materials is either scarcely reported or overlooked. The material has shown higher sensitivity in the high temperature region (535 K < T < 760 K) with a maximum of 7.21x10-3 K-1 at 535 K. The results indicate potential of CaMoO4:

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Er3+/Yb3+ phosphor in high temperature thermometry.

Keywords: 1. Upconversion; 2. Temperature sensing; 3. Fluorescence intensity ratio (FIR); 4. Polyol method.

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Present address: Second Institute of Physics, University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany *

Corresponding author’s e-mail address: [email protected]; Phone no.:+91-326-223 5754

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1. Introduction Rare earth doped upconversion (UC) phosphors are capable of converting low frequency radiation (near infrared) into high frequency radiation (ultraviolet or visible) via multiphoton

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absorption of the excitation radiation. Such UC phosphors have received extensive attention

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because of their potential applications in display systems, optical temperature sensors, solar

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cells, latent fingerprint detection, bio-imaging, etc. [1-5]. The trivalent erbium (Er3+) is one of the widely used rare earth (RE) ions in achieving UC emission. The Er3+ is suitable for this purpose since it has number of energy levels spanning from infrared to UV region and many of them are metastable in character. The Er3+ gives very strong green emission along with red

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and blue emission upon selective excitation in infrared region. However, the Er3+ ions have rather low absorption cross-section at 980 nm excitation wavelength. The Yb3+ ions on the

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other hand have higher absorption cross-section around 980 nm (~7 times higher than the Er3+ ions) with strong energy transfer efficiency to Er3+ ions and thereby, used as sensitizer to enhance the Er3+ emission [6].

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Recently, the molybdate materials have attracted overwhelming attention due to their

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potential applications in various fields including electro-optic devices, acoustic-optic modulators, solid state lasers, scintillator materials, fluorescent lamps and catalysis

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[7, 8]. The CaMoO4 has been recognized as an excellent downconversion luminescence host material for high emission quantum efficiency for doped RE ions due to its luminescent MoO4 tetrahedron cluster [9]. Moreover, the Mo6+ ions in CaMoO4 matrices have strong

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polarization induced by large electric charge and small radius, accounting for the decreased symmetries and enhanced Stark energy split in the crystal field [8, 9]. Reports have also shown that upconversion emission in this host is as good as observed in the rare earth based hosts such as Y2O3, Gd2O3 [10-12]. Hence, it is one of non-rare earth based efficient upconverter and having lower cost than the other rare earth based phosphors. Additionally, it has high density (4.25 g/cm3), high refractive index (1.98), low phonon frequency (~800 cm1), and stable physical and chemical properties compared to other oxide materials [13]. Because of the above mentioned properties, this phosphor material has attracted much scientific interest in recent years [7-9]. Several techniques have been utilized to synthesize the CaMoO4 phosphor. Vidya et al. [8] have used auto-ignition combustion technique for the preparation of CaMoO4 including discussion on its optical properties. Jin et al. [14] have synthesized Eu3+ and Sm3+ co-doped CaMoO4 by hydrothermal method. The effect of RE concentration and temperature on the luminescence properties of Tb3+ doped CaMoO4 is

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investigated by Parchur et al. [15] with a conclusion that ethylene glycol (EG) can be used not only as reaction medium but also as capping ligand to limit the growth of the synthesized particles. Recently, Amini et al. [16] have prepared the calcium molybdate nanostructures

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having octahedral and hierarchical ‘self-assemblies’ morphology by co-precipitation method.

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But, the polyol method has several advantages over other synthesis processes such as very short-reaction time, low cost processing, etc.

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Over the past few years, research on optical temperature sensing based on the UC emission of RE ions has attracted considerable interest [17-19]. Optical temperature sensors are based on two types of methods- fluorescence intensity ratio and fluorescence lifetime

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method. Fluorescence intensity ratio (FIR) method has extensively been employed since this method can reduce the measurement conditions and having good measurement accuracy as

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well as resolution [17, 18]. FIR method is based on the temperature dependent intensity ratio arising from two thermally coupled energy levels of the same rare earth ion and the energy gap between these two levels should be in the range of 200-2000 cm-1 [20- 21]. Among the

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RE ions, Er3+ based optical temperature sensors have been widely studied under 980 nm

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excitation owing to its two suitable thermally coupled energy levels - 2H11/2 and 4S3/2 with an energy separation of ~770 cm-1 [22]. This energy separation allows 2H11/2 level to be

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populated from 4S3/2 level via thermal absorption. Many researchers are involved in the investigation of optical temperature sensing behavior based on the FIR technique using two thermally coupled levels of rare earth ions and reported the obtained sensitivities

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[23-29]. There are only few reports of temperature sensing in molybdate materials [26, 30-32] and hence this group of materials needs further attention. In this work, authors have prepared the Er3+/Yb3+ doped CaMoO4 phosphor via polyol method and studied the UC emission properties along with the structural properties. The pump power dependent UC emission and mechanism involved in the UC processes have been addressed. Moreover, optical sensing performance has been analyzed by considering the FIR of two thermally coupled levels of the green UC emission bands upon 980 nm excitation.

2. Experimental details 2.1. Sample preparation Polyol method [15] was followed to prepare Er3+/Yb3+ doped CaMoO4 phosphor using urea hydrolysis in ethylene glycol (EG) medium at 150 0C. The composition of the sample

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was as follows: 97.7 mol% CaMoO4 + 0.3 mol% Er2O3+ 2 mol% Yb2O3. The optimum doping concentration was taken from our previous reports [33, 34]. The calcium carbonate (CaCO3,

99.99%,

Merck,

India),

ammonium

heptamolybdate

tetrahydrate

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((NH4)6Mo7O24.4H2O, 99%, Merck, India), erbium oxide (Er2O3, 99.99%, Otto, India),

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ytterbium oxide (Yb2O3, 99.99%, Otto, India) were taken as a starting precursors. In a typical synthesis process, the 0.95 g of CaCO3, 0.0112 g of Er2O3 (0.3 mol%) and 0.0771 g of Yb2O3

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(2.0 mol%) were dissolved in concentrated nitric acid (HNO3) under continuous stirring on a magnetic stirrer whose hot plate was kept at 80 0C. Extra amount of nitric acid was removed by evaporation with addition of water. Meanwhile, 1.68 g of (NH4)6Mo7O24.4H2O was de-ionized water and stirred for 1 hour on hot plate (80 0C). The 2 g

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dissolved in 50 ml of

urea was dissolved in 25 ml EG and added to the solution containing molybdenum. These

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two solutions were then mixed drop-wise and kept for continuous stirring under heating at 150 0C for precipitation. The precipitated material was collected and washed several times with water and ethanol and then dried at room temperature. Finally, the as-prepared sample

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2.2. Characterizations

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was annealed at 800 0C for 4 hours for further characterizations.

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The X-ray diffraction pattern (XRD) was recorded on Bruker-D8 Advanced X-ray diffractometer monochromatized with Cu-Kα (1.5406 Å) radiation source over the angular range 150 ≤ 2𝜃 ≤ 750. Infrared absorption spectrum was recorded on FTIR spectrometer (Perkin Elmer spectrum RXI) using KBr pellet technique in the wavelength range of 4000-

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400 cm-1. The surface morphology of the sample was taken using the ZEISS SUPPRA55 field emission scanning electron microscope (FESEM). The electronic absorption spectrum was recorded on UV-Visible-NIR spectrophotometer (Perkin-Elmer, Lambda 950) in the wavelength range of 200-1100 nm. The UC emission spectra were recorded on SP2300 grating spectrograph (Princeton Instruments, USA) using a 980 nm CW diode laser as the excitation source. The lifetime analysis was performed by chopping the CW laser beam at 15 Hz and recording the decay curves with the help of a digital storage oscilloscope. For temperature dependent UC emission study, a thermocouple was placed at ~2 mm to the focal spot on the sample for the detection of temperature and the laser beam was set at 7 W cm−2 and during the measurement time, a chopper was used to chop the CW light to avoid the direct heating of the material, though this power density produces negligible heat to the sample.

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3. Results and discussion 3.1. Structural studies

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3.1.1. X-ray diffraction

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The X-ray diffraction pattern of the sample was recorded to know phase of the prepared

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material. The recorded XRD pattern of the sample annealed at 800 0C is shown in Fig. 1. The diffraction pattern is found in good agreement with the standard pattern of JCPDS card no. 85-1267 with space group I41/a. There is no additional or secondary phase is observed. It indicates the formation of pure tetragonal phase of CaMoO4 phosphor. The prepared powder

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fundamentally maintains the characteristic of scheelite structure, which are not affected by small doping of lanthanide ions. The (hkl) values of the most prominent peaks are assigned in

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the XRD pattern. The experimentally calculated value of lattice parameters area =b = 5.224 Å, c = 11.430 Å and the unit cell volume is found to be V = 311.93 Å3. From JCPDS data, the lattice parameters of unit cell were noted as a = b = 5.223 Å and

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c = 11.42 Å and unit cell volume is V = 311.86 Å3. A little increase in unit cell volume is

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observed due to slight increase in the dimension along c-axis. The crystallite size of the

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synthesized material was calculated using Williamson-Hall equation [23]

 cos  1  sin     D 

(1)

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where, D is the crystallite size (in nm), β is full width at half maximum, θ is Bragg diffraction angle, ε is the microstrain present in the sample and λ is the wavelength of radiation (1.54 Å). The average crystallite size was found to be ~44.6 nm.

3.1.2. Fourier transform infrared spectroscopy Fourier transform infrared (FTIR) spectrum of the sample was analyzed to know the purity and maximum cut off frequency of the synthesized material. As the sample was prepared using the chemical route and hence organic impurities are expected to be present in the sample. The FTIR spectrum of Er3+/Yb3+ doped CaMoO4 phosphor annealed at 800 0C is shown in Fig. 2. The spectrum shows the presence of small quantity of water and nitrate. A broad band around 3436 cm-1 is due to O-H stretching vibration of absorbed water molecules present in the sample. The band observed around 2336 cm-1 is associated with the asymmetric

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stretching mode of CO2 group, adsorbed from the atmosphere [35] while the band around 1388 cm-1 arises due to the N-O mode of vibration of HNO3 used in the sample preparation. The band around 1630 cm-1 is due to O-H bending vibration of water. Also, the weak band

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observed around 2852 and 2924 cm-1 are due to stretching vibrations of CH2 group of

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ethylene glycol (EG) molecule [15]. The strong absorption bands at 800 and 739 cm-1 are due to the presence of antisymmetric stretching vibrations of O-Mo-O modes of [MoO4]2-

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tetrahedron whereas the weak absorption band at 435 cm-1 is due to antisymmetric bending vibration of Mo-O mode [15,36]. The most intense lattice vibration is found at 800 cm-1 and hence it can be considered as maximum phonon frequency of this material. The obtained

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phonon frequency is low enough and hence strong upconversion emission is expected in this

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host matrix.

3.1.3. FESEM and EDX analysis

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In order to investigate the particle size and morphology of the phosphor material, the

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field emission scanning electron microscope (FESEM) image of Er3+/Yb3+ doped CaMoO4 phosphor annealed at 800 0C was recorded and is shown in Fig. 3(a). Morphology of the

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sample shows particles having irregular shape and size with large density of pores. The particle size distribution was estimated by plotting the histogram with Gaussian fitting and is shown in inset of Fig. 3(b). By counting random particles in the FESEM image, the average

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particle size was found to be 583 nm. The energy dispersive X-ray spectroscopy (EDX) was also analyzed to investigate the chemical composition and purity of the annealed CaMoO4 phosphor. The EDX spectrum, as shown in Fig. 3(b), confirms the presence of calcium (Ca), molybdenum (Mo), oxygen (O), erbium (Er) and ytterbium (Yb) elements in the sample. According to EDX analysis the weight percentage of Ca, Mo, O, Er and Yb elements in the sample are noted to 9.86%, 15.94%, 72.59%, 0.28% and 1.34%, respectively. The actual weight percentage of these elements is 19.28%, 47.25%, 31.50%, 0.24% and 1.70%, respectively. It is observed that the percentage of oxygen is increased in the sample, which may be due to the absorption of oxygen from the environment.

3.2. Optical Studies 3.2.1. UV-Visible absorption spectroscopy

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The UV-Visible absorption spectrum of Er3+/Yb3+ doped CaMoO4 phosphor in diffuse reflectance mode was recorded against a reference standard of BaSO4 compound. The spectrum is shown in Fig. 4. The broad absorption band around 286 nm is assigned to the

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band absorption of CaMoO4. Also, the sharp absorption bands at 488, 521 and 656 nm are

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observed due to the 4F7/2←4I15/2, 2H11/2←4I15/2 and 4F9/2←4I15/2 transitions, respectively of Er3+

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ion. The band at 978 nm is observed due to the 2F5/2←2F7/2 transition of Yb3+ ion.

3.2.2. Upconversion emission study

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For upconversion emission measurement, the material was excited with 980 nm laser light and spectra were recorded in 350-800 nm wavelength range. The room temperature

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emission spectrum of the annealed CaMoO4: Er3+/Yb3+ sample is shown in Fig. 5. The UC emission spectrum contains two strong green emission bands at 530 nm and 552 nm which are assigned to the 2H11/2 →4I15/2 and 4S3/2 →4I15/2 transitions of Er3+, respectively. Usually, in

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Er3+/Yb3+ doped phosphors, the red emission due to the 4F9/2→4I15/2 transition is observed

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with enough intensity but here in this material its intensity is found too low as compared to the green band. Similar, observation is also reported by Chung et al. [37]. In the inset (II) of

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Fig. 5, enlarged red band is shown. In addition, low intense emission bands are also observed in the blue and near infrared regions of the spectrum. The blue region is shown in inset (I) of Fig. 5. These emission bands are centered at 383, 410, 475 and 798 nm and correspond to the 4

G11/2→4I15/2, 2H9/2→4I15/2, 4F3/2→4I15/2 and 4I9/2→4I15/2, transitions of Er3+ ion, respectively.

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The possible upconversion mechanisms with excitation and emission pathways are shown in simplified energy level diagram in Fig. 6. The ground state absorption (GSA), excited state absorption (ESA), energy transfer (ET) and non-radiative relaxations (NR) are responsible for the upconversion emission. Since, Yb3+ ion has much higher absorption cross section in comparison to the Er3+ ion and also perfect resonance exist between 2F5/2 level of Yb3+ ion and 4I11/2 level of Er3+ ion, therefore efficient energy transfer can takes place from Yb3+ to Er3+ ion. In the first step, Er3+ ions in ground level, 4I15/2 is excited to 4I11/2 level via GSA or ET from neighboring Yb3+ ions. A part of the excited ions in the 4I11/2 level decay non-radiatively and populate the 4I13/2 level. Rest of the excited ions in 4I11/2 level absorb the excitation radiation through ESA and excited to the 4F7/2 level. The excited ions in 4F7/2 level decays non-radiatively to the 2H11/2 and 4S3/2 levels. The ions excited to the 4I13/2 level also absorb the incident photons and reaches to the 4F9/2 level. The multiphonon emission is required in this process because of small energy mismatch. This may be the cause of weak

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emission from 4F9/2 level. The 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions result strong green emission at 530 nm and 552 nm, respectively. The ion in 4S3/2 level further absorbs another incident photon and gets excited to the 4G7/2 level. The 4G7/2 level populates the intermediate

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excited levels 4G11/2, 2H9/2, 4F3/2, and 4F7/2 via multiphonon relaxation process. Finally, the

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radiative transitions 4G11/2→4I15/2, 2H9/2→4I15/2 and 4F3/2→4I15/2 give emission bands at 383 nm, 410 nm and 475 nm respectively. Also, the radiative transitions 4F9/2→4I15/2 and I9/2→4I15/2 result emission bands at 656 nm and 798 nm, respectively. In Er3+ and Yb3+ co-

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doped system ET plays a dominant role over ESA as ESA requires high pump power to

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occur.

3.2.3. Pump power dependent upconversion emission

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The UC emission intensities at different input pump powers were recorded and spectra are shown in the Fig. 7(a). It is observed that the green UC luminescence intensity increases with increasing the pump power. As pump power increases, intensity of the emission band

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located at 530 nm increases faster than that of the band at 552 nm. Inset of Fig. 7(a) shows

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the CIE chromaticity diagram of Er3+/Yb3+ doped CaMoO4 phosphor upon 980 nm excitation. The color coordinates were calculated at different pump power density. There is a small shift

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in the color coordinates with the pump power and the co-ordinates move little towards the more greenish region. The absence of red emission in this phosphor produces nearly pure green color. At moderate pump powers, UC intensity follows the relation I  P n [38], where

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‘n’ is the number of incident photons involved in the UC process and ‘P’ is the input power. The number of photons absorbed in an UC process can be obtained from the slope of ln(I) versus ln(P) plot for the particular UC emission band. From Fig. 7(b), the slopes for the bands at 383, 410, 475, 530, 552, 656 and 798 nm are found to 2.18, 1.61, 1.45, 1.57, 1.30, 1.28 and 1.64, respectively. If we see the energy level diagram (Fig. 6), it is clear that the bands located at 383, 410 and 475 nm must show three-photon process and the emission bands at 530, 552, 656 and 798 nm must show two photon process. The experimentally calculated slopes for 410 and 475 nm bands are not in agreement with the expected three photon values. The low slope values are expected due to the involvement of non-radiative channels in the system. Also there is possibility of saturation in population density of the involved intermediate energy levels that play role in populating the upper excited levels. As we know, the UC efficiency is mainly governed by the multiphonon nonradiative relaxation rate. The rate of multiphonon relaxation depends on the energy gap separating the

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upper and lower states as well as on phonon energy of the material and can be expressed as [39]: n

(1)

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  

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  h Wnr (T )  Wnr (0) 1  exp   kT 

T= 0 K , given by the expression [40]:

(2)

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E   Wnr (0)  W0 (0) exp    h  

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where, Wnr (T) is the multiphonon decay rate at temperature T, Wnr (0) is the decay rate at

where, W0 (0) is the decay rate at ΔE = 0 and T = 0, α = ln(n/g)-1, n = ΔE/hν is the number of

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phonons required to bridge the energy gap between the emitting and the next lower level, ΔE is the energy gap between the levels involved and hν is the phonon energy, g is the electron phonon coupling strength, k is the Boltzmann constant and T is the absolute temperature. At

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fixed value of ‘n’, multiphonon relaxation rate (Wnr) increases exponentially with

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temperature. When the energy difference ΔE between the two relevant levels is equal to or less than four to five times the phonon energy, the multiphonon nonradiative relaxation

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becomes competitive with the radiative process. The above equation suggests that lower is the phonon energy (hν) and electron-phonon coupling strength (g); lower would be the multiphonon decay rate, which provides high UC emission efficiency.

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The decay curves for 4S3/2 →4I15/2 (552 nm) and 4F9/2→4I15/2 (656 nm) transitions under 980 nm laser light excitation are shown in Fig. 8. The curves were fitted with single exponential formula:

 t  I (t )  I (0) exp    

(3)

where, I (t) and I0 denotes luminescence intensities at time t and 0 respectively, t is the time and τ is the decay time of the emitting level. Due to large lifetimes of intermediate levels of Er3+ ion, the initial emission decay part of upper levels viz. 4S3/2 and 4F9/2 becomes nonexponential and hence deviation is seen in the fitted data. The measured effective decay times for the transitions 4S3/2→4I15/2 and 4F9/2→4I15/2 in upconversion emission mode are ~272 and ~337 μs, respectively. The longer effective decay time of red emitting level (4F9/2), which is just below the green emitting level (4S3/2), is apparent due to involvement of energy transfer

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channels in populating this level [41]. The larger lifetime of 4F9/2 compared to 4S3/2 in Er3+ is also reported in some past reports [42- 44]. Due to involvement of intermediate levels in populating these levels, the rise times in the transients are also observed. The values of rise

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times (τr) are given in the Fig. 8.

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3.2.4. Optical temperature sensing

To study the temperature sensing behavior, the UC emission spectra of Er3+/Yb3+ doped CaMoO4 phosphor were recorded within 300-760 K temperature range. In Fig. 9(a),

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the UC emission spectra in 500-570 nm wavelength range are plotted at different temperatures and at fixed excitation pump power density (7 W/cm2). The fluorescence

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intensity ratio (FIR) values appear to increase with temperature. The fluorescence intensity ratio (FIR) of two thermally coupled levels 2H11/2 and 4S3/2 of Er3+ ion follows Boltzmann-

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type population distribution and can be written using the following formula [28]:

 E  I 530 (2 H11/2 4 I15/2 ) WH g H h H  exp   4 4 I 552 ( S3/2  I15/2 ) WS g S h S  k BT 

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FIR 

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     k BT 

= B exp 

(4)

where, I530 and I552 are the integrated intensities corresponding to the 2H11/2 →4I15/2 (530 nm)

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and 4S3/2 →4I15/2 (552 nm) transitions, respectively. WH and WS are the radiative probabilities of the transitions; gH and gS are (2J+1) degeneracy of the levels 2H11/2 and 4S3/2 respectively; hνH and hνs terms are the photons energies of the 2H11/2 →4I15/2 and 4S3/2 →4I15/2 transitions, respectively; ΔE is the energy gap between the two emitting levels; kB is the Boltzmann constant and T is the absolute temperature. Figure 9(b) shows the monolog plot of FIR against inverse absolute temperature T in the range of 300-760 K. The two linear regions are clearly seen in the experimental plot. But according to equation (4) there should be only one slope. The expressions of two fitted lines were: (1) ln(FIR) = 1.63-542/T (300K < T < 535K) and (2) ln(FIR) = 2.63-1072/T (535 K < T < 760 K). Corresponding to these fittings the experimental energy gap between the two levels are found as 377 cm-1 and 745 cm-1. As per the recorded spectrum estimated energy gap between the 2H11/2 and 4S3/2 levels is around 334 cm-1. The peak positions for energy gap estimation are shown in Fig. 9(a). The FIR is also plotted for the calculated

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energy difference 334 cm-1 and this plot well matches with the first slope. However, second slope is much deviated from the data points obtained from spectral energy gap estimation. Logically, two slope values are not expected to come as energy gap between the two levels is

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fixed and also does not vary much with the temperature. Hence, above observation needs

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proper justification. From DTA/TG measurement (graph not shown) it was found that sample is stable up to 600 K and there is no melting or loosening of the lattice. Hence, for this

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unusual behavior it has to be assume that there may be some additional channels that start working above a certain temperature. As seen from Fig. 9(a) intensities of the 530 and 552 nm bands are decreasing continuously with increasing temperature but rate of decrease of 552

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nm band becomes faster as temperature goes above 535 K. The Boltzmann equation is defined for the dependence of population of upper level with respect to temperature and

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assumes that lower level population also increases exponentially. The Boltzmann equation does not include the non-radiative transitions from the lower level. In this phosphor it may happen that non-radiative transitions from the 4S3/2 to lower 4F9/2 level become prominent

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after a certain temperature (≥ 535 K).

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In order to confirm this, variation in intensity of the red emission band was measured with the temperature. The obtained plot is shown in Fig. 9(c). From the figure it is seen that

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rate of change of intensity abruptly decreases around 535 K and above this temperature intensity almost remains constant. This is the temperature from where second slope of FIR starts. From this observation authors can interpret that there is a correlation between the two

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observations. It supports the idea that 4S3/2 level starts relaxing faster to the lower 4F9/2 level above 535 K. It can also be understood by another way. If we calculate the energy difference between the 4S3/2 and 4F9/2 levels it comes around 2190 cm-1. The maximum phonon vibration of the host is around 800 cm-1. Hence number of phonons required to bridge this gap = 2200/800 = 2.8, which means that three phonons are required to bridge during nonradiative transition from 4S3/2 to 4F9/2 level, which is less probable. But when sample temperature rises (say ≥ 535 K) less number of phonon would require in bridging the gap and hence non-radiative transition would be highly probable. Similar type of observation (two slopes in FIR vs. temp.) has also been reported by Boruc et al. [45] in Dy3+ ion doped Y4Al2O9 phosphor. Authors have found two temperature ranges viz. 296 K–573 K and 573 K – 973 K with different slope values. Authors have mentioned several reasons such as different lattice sites for Dy3+ ion, new CR channels at higher temperatures etc. for this

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anomalous nature. But no definitive reason was given. In the present case, the slope is found higher and is observed at higher temperature. The sensor sensitivity is an important parameter for temperature sensing application. It is

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defined as the rate at which fluorescence intensity ratio changes with temperature and

dR E R dT KT 2

(5)

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S

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absolute sensitivity (S) is expressed by the following equation [28]:

This equation shows that at a given temperature, higher energy gap between the two thermally coupled levels would give higher sensitivity. Fig. 9(d) exhibits the plot of

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sensitivity as a function of temperature in the range of 300 K to 760 K. The sensitivity plot has also been divided into two regions of temperature by taking corresponding calculated

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energy difference ΔE (377 cm-1 for T < 535 K and 745 cm-1 for T > 535 K). An abrupt change in sensor sensitivity is observed because of using two different energy gap values. The material shows high sensitivity in the higher temperature region (535-760 K) with the

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maximum sensor sensitivity of 7.21x10-3 K-1 at 535 K. Table 1 shows that the sensor

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sensitivity of the present phosphor is favorable to other reported works [17, 22- 25, 27, 28, 46,47]. As human sensitivity is highest in green region, this material can also be used in

[48].

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4. Conclusions

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infrared to visible upconverter devices and display systems where pure green light is required

The Er3+/Yb3+ doped CaMoO4 phosphor in pure tetragonal phase is successfully

synthesized via polyol method and characterized by XRD, FTIR and FESEM analysis. Strong green upconversion emission under 980 nm laser excitation is observed in this phosphor. The fluorescence intensity ratio (FIR) between 2H11/2 and 4S3/2 levels for the temperature range 300-760 K was calculated and the plot has shown two slopes in this phosphor. In the higher temperature region (T > 535 K), the maximum sensitivity i.e. 7.21x10-3 K-1 is found at 535 K. This unusual dual slope is not found to correlated with melting or softness of the material and it is thought that second slope (starts at 535 K) in the higher temperature region appears due to increased non-radiative 4S3/2→4F9/2 transition. Obtained CIE color coordinates are found in the pure green region and does not change with pump power density indicating CaMoO4 phosphor can be a promising candidate for display devices and high temperature sensing.

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Acknowledgements One of the authors, Ms. S. Sinha is thankful to IIT(ISM), Dhanbad, for providing doctoral research fellowship. Dr. K. Kumar acknowledges Council of Scientific &Industrial Research,

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New Delhi (project no. 03(1303)/13/EMR-II) for financial assistance.

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Figure captions Fig. 1. X-ray diffraction pattern of 0.3 mol% Er3+ and 2.0 % Yb3+ doped CaMoO4 annealed at

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800 0C.

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Fig. 2. Fourier transform infrared spectrum of CaMoO4: Er3+/Yb3+ phosphor annealed at

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800 0C.

Fig.3. (a) FESEM image (b) EDX spectrum and particle size distribution of CaMoO4: Er3+/Yb3+ phosphor.

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Fig.4. UV-Visible absorption spectrum in diffuse reflectance mode of CaMoO4: Er3+/Yb3+ phosphor.

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Fig. 5. Upconversion emission spectrum of 0.3 mol% Er3+ and 2.0 % Yb3+ doped CaMoO4 (at power density 15 W/cm2) under 980 nm excitation. Inset (I) and (II) shows the enlarged

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spectra in blue (350-480 nm) and red (620-700 nm) regions. The inset in upper- left corner

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shows the photograph of the sample.

Fig. 6. Energy level diagram of Er3+ and Yb3+ ions with possible upconversion mechanism

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under 980 nm laser light excitation.

Fig. 7. (a) Variation of upconversion emission intensity at different pump power density of CaMoO4 : Er3+/Yb3+ phosphor. Inset shows CIE chromaticity diagram at different pump

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power density (b) lnI–lnP between emission intensity and excitation power for upconversion emission bands.

Fig. 8. Fluorescence lifetime curves of 4S3/2 and 4F9/2 levels of Er3+ ion in CaMoO4: Er3+/Yb3+ phosphor with single exponential fitting. The transients also show the rise times. Fig. 9. (a) Temperature dependent upconversion emission spectra for 530 nm (2H11/2 →4I15/2) and 552 nm (4S3/2 →4I15/2) of CaMoO4: Er3+/Yb3+ phosphor excited by 980 nm excitation (b) the monolog plot of the FIR (I530/I552) as a function of inverse absolute temperature. (c) Variation in emission intensity of the 4F9//2→4I15/2 transition (656 nm) with temperature. (d) Variation of sensitivity (S) with temperature.

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FIGURES

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Temperature range

Maximum sensitivity

Er-Yb-Mo : Al2O3

294-973 K

0.0051 K-1at 443 K

Er-Yb : Al2O3

295-973 K

0.0051 K-1 at 495 K

[22]

Er-Yb-Zn : BaTiO3

120-505 K

0.0047 K-1 at 430 K

[23]

Er-Nd : SBN glass ceramic

300-700 K

0.0017 K-1 at 600 K

[24]

Er-Yb : Gd2O3

300-900 K

0.0039 K-1 at 300 K

[25]

Er : PKAZLF glass

298-773 K

0.0079 K-1 at 630 K

[27]

Er-Yb : YVO4

300-485 K

0.0116 K-1 at 380 K

[28]

Er-Yb : (Na, Ba)TiO3

93-613 K

0.0031 K-1 at 400 K

[46]

Er-Yb : Silicate glass

296-723 K

0.0033 K-1 at 296 K

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Er-Yb : CaMoO4

300-760 K

0.0072 K-1 at 535 K

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Materials

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Table 1: The comparison of maximum value of temperature sensitivity in Er3+ based materials by FIR technique. References

[17]

This work

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Graphical abstract

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Research highlights

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CaMoO4: Er3+/Yb3+ phosphor synthesized via polyol method

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 External temperature and input pump power dependent upconversion emission study in

 Dual nature of fluorescence intensity ratio (FIR) and optical temperature sensitivity

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within 300-760 K (300 K < T < 535 K and 535 K < T < 760 K).

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 A potential phosphor to be used in high temperature thermometry.