Influence of Ho³⁺ doping on the temperature sensing behavior of Er³⁺–Yb³⁺ doped La₂CaZnO₅ phosphor

Abstract

In this paper, a series of Er³⁺/Yb³⁺ and Er³⁺/Yb³⁺/Ho³⁺ codoped La₂CaZnO₅ (LCZ) upconversion (UC) phosphors were synthesized by the combustion route. The UC emission from LCZ phosphors, codoped with fixed Er³⁺, Ho³⁺, and various Yb³⁺ concentrations has been investigated. The structural and upconversion properties of the synthesized phosphors were studied in detail. Under 980 nm laser excitation, the codoped samples showed green UC emission that consisted of three well-known emission bands centered at 522, 548 and 672 nm generated by the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. The emission intensities of these bands have been enhanced sufficiently on codoping of Yb³⁺ ions in the LCZ : Er³⁺ system. An effort has been presented to explain the enhancement on the basis of a power dependence study and an energy level diagram. The luminescence lifetimes of the green emission of the LCZ samples with different codoping were also recorded and incorporated to explain the energy transfer mechanism. The strong temperature dependence of the fluorescent intensity ratio between two green emissions makes the material suitable for temperature sensing purposes and it is also suitable up to high temperatures of 500 K. A relatively high-temperature sensor with good sensitivity 0.00625 K⁻¹ was found from the observed results. An increment of 6% for the sensitivity is observed over the existing LCZ phosphor after co-doping with Ho³⁺. These results indicate that the Er³⁺/Ho³⁺/Yb³⁺ codoped LCZ material is an effective UC phosphor and may be a potential candidate for high-temperature sensors.

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Introduction

Rare earth (RE) ion-doped upconversion (UC) materials have been the subject of scientific interest due to their significant applications in a variety of fields such as, display devices, temperature sensors, solar cell, bio-imaging, optoelectronics devices, finger print detection, etc.^1–6 Among all these applications, RE-doped UC phosphor based temperature sensors are drawing much attention from the scientific community.^7,8 Traditional temperature sensors are based on a liquid and metal expansion principle, and measure the temperature during the heat flow using an invasive probe.⁹ Though this strategy suffers from some limitations, including spatial resolution, sensitivity and accuracy of detection. This method is also not applicable in many places, such as corrosive environments, coal mines, refineries, etc.^10–12 To overcome these problems, temperature sensing based on a non-contact temperature measurement technique such as infrared (IR) thermometry, Raman spectroscopy, and luminescence can be implemented.^10,11 One practical approach to measuring the temperature of RE-doped UC materials is the fluorescence intensity ratio (FIR) technique. This method is based on the temperature connection of the FIR between two thermally coupled levels of RE ions as a function of external temperature.^10–12 FIR, in context, is of particular importance for better accuracy, high sensitivity and independence of the measurement conditions.^9–12

In the last decades, attention has been attracted towards the ternary oxides RE₂MZO₅ (RE = rare earth, M = Ba, Ca, Z = Cu, Zn) based UC and down-conversion materials.¹³ RE₂MZO₅ based hosts, in context, are of certain significance due to their excellent structural, physical, chemical, magnetic, optical and superconducting properties. The luminescence properties of La₂BaZnO₅ and Gd₂BaZnO₅ activated with Eu³⁺, Tb³⁺ and Tm³⁺ were reported for the first time in the year of 1985.^13g Thereafter, significant research efforts have been directed towards the luminescent properties of RE doped BaRE₂ZnO₅ phosphors synthesized by various methods.^13h,i The optical and luminescent properties of La₂CaZnO₅ host doped with different RE ions have been investigated mainly for down-conversion.¹⁴

The Yb³⁺ ion was demonstrated to be an attractive sensitizer for RE³⁺ ions in various hosts with not only the improved luminescent intensity but also widened excitation spectrum.² There are some well-known combinations of the photon energies Er³⁺–Yb³⁺, Ho³⁺–Yb³⁺, and Tm³⁺–Yb³⁺. These combinations were extensively studied by various authors in a significant number of hosts due to their potentiality conversion of the NIR light into visible light. The UC properties of the RE doped systems La₂BaZnO₅ and Gd₂BaZnO₅ have been investigated by Birkel and coworkers.¹⁵ Recent results suggest that the sol–gel derived La₂CaZnO₅ : Er³⁺–Yb³⁺ phosphor is an efficient green UC phosphor.¹⁶ They have also studied optical thermometry in the temperature scale of 298–513 K adopting the FIR technique. But, the internal heating in the material and optical heating generated by the laser excitation has only been rarely described.^17a It is worth mentioning that, the interaction between the electron and phonon could be improved by the effect of quantum confinement among phonons, which results in internal heat generation in crystalline phosphors. Therefore, internal heating must be prominent for nanocrystalline solids.^17b However, the optical heating is related to the interaction between phosphors and the excitation source. When any phosphor material is exposed to the laser irradiation, some portion of the absorbed photons in the phosphor is transformed into heat energy via nonradiative processes owing to which the material gets heated optically.^17c The nano thermometry behaviour by using the green emissions from Er³⁺ ions in the Er³⁺–Ho³⁺–Yb³⁺ triply doped La₂CaZnO₅ (LCZ) phosphors upon NIR excitation have not been investigated at this stage. Moreover, recently, there is a rising interest in the development of temperature sensing technique that is based on the utilization of fluorescent nanomaterials or nanoparticles.^18,19 Vetrone et al.²⁰ reported that the fluorescent NaYF₄ : Er³⁺, Yb³⁺ nanoparticles can be utilized as nanothermometers. They have further stated that the ratio between the green emissions bands of the Er³⁺ ion offers an optical system that in terms regulate temperature distributions in liquids using confocal fluorescence. This opens up the possibility of creating RE³⁺ ions doped UC nanomaterials that can act as thermal probes with interesting applications in biosensors, fluorescent imaging, and therapeutic purposes, etc.^18–20

In this work, we have focused on the La₂CaZnO₅ host doped with lanthanide ions (Yb³⁺, Ho³⁺, Er³⁺) for strong green and red UC emission properties. An attempt has been made to increase the UC and optical heating performance of the previously reported Yb³⁺, Er³⁺ codoped material for optical temperature sensing by changing the synthesis condition and the addition of another co-dopant.¹⁶ In this paper, we produced LCZ : Er³⁺/Ho³⁺/Yb³⁺ UC phosphors via solution combustion method. Control of crystallization in the phosphors was studied by X-ray diffraction (XRD). The UC emission characteristics of the synthesized materials have been studied upon 980 nm diode laser excitation at room temperature. The effect of doping and the mechanism involved in the UC process has been studied in detail. The color of the infrared emissions of the codoped sample was also tuned by varying the laser power. Meanwhile, the temperature sensing behavior of the synthesized phosphor has been studied by using FIR of two thermally linked levels of the central UC emission band against 980 nm excitation.

Experimental section

Synthesis of La₂CaZnO₅ : Er³⁺, Ho³⁺, Yb³⁺ UC phosphors

The La₂CaZnO₅ : Er³⁺, Ho³⁺, Yb³⁺ UC phosphors (2 mol% Er³⁺, 2.0–20 mol% Yb³⁺, 1 mol% Ho³⁺, respectively) were synthesized via solution combustion route. Analytical reagent (AR) grade lanthanum acetate (C₆H₉LaO₆, 99.9%), calcium nitrate (Ca(NO₃)₂, >99%), zinc nitrate (Zn(NO₃)₂, 97%), erbium nitrate (Er(NO₃)₃, 99.9%), holmium nitrate (Ho(NO₃)₃, 99.9%), ytterbium nitrate (Yb(NO₃)₃, 99.9%) and urea (CO(NH₂)₂, >90%) obtained from Sigma-Aldrich were used as the starting raw materials. These materials were taken in the required stoichiometric ratios and mixed properly with deionized water under vigorous stirring condition. When the transparent solution was obtained, the proportionate amount of urea was added to the solution and the solution was stirred for 20 min. Then the solution was transferred in an alumina crucible and kept in a preheated furnace (500 °C) for about 10 min. Within this time, the auto combustion process took place, and a very porous voluminous mass of the powder was formed. The resultant powder was then calcined at 800 °C for 1 h.

Characterizations

X-ray diffraction (XRD) patterns of the as-prepared phosphors were recorded using a Bruker D8 Focus X-ray diffractometer. The scanning electron microscope (SEM) analysis was carried out using a Shimadzu SSX-550. The UC emission and lifetime measurements were performed using the home built setup with a 980 nm laser source. The emitted light was dispersed into a monochromator coupled to a photomultiplier tube through the appropriate lens system. The sample was heated in a home-made furnace and the temperature was controlled by varying the voltage. The temperature was measured with the help of a calibrated thermocouple located in contact with the sample for the temperature sensing investigation. The sample holder along with the sample was kept for 5 min at a constant temperature unless its temperature was stabilized for the upconversion. A common thermometry unit based on phosphors has been illustrated in Scheme 1 schematically. An excitation source (such as LEDs, lasers or flash lamps) is used for the excitation of the phosphor which is attached onto the surface of interest. The resultant emission light passes through an optical filter and is collected in a data acquisition system via an emission detector.

Scheme 1 Schematic diagram for temperature sensing.

Results and discussion

Structures of the prepared samples

To examine the structure of the LCZ phosphor, the effect of Er³⁺-doping, Er³⁺/Yb³⁺ and Er³⁺/Yb³⁺/Ho³⁺-codoping in the crystal lattice and to identify the phase, XRD patterns of the UC phosphors were recorded and are shown in Fig. 1. The XRD peak matches well with the LCZ phase as reported elsewhere.¹⁶ The XRD pattern varying dopant and codopant show the same behavior indicating the invariance of the crystal structure after doping and codoping. The crystal system of the prepared samples was identified as a tetragonal structure with space group P4/mmm (no. 123) having lattice parameters a = b = 7.0837 Å, c = 11.884 Å and α = β = γ = 90°. It is evident from the existing literature that the iso-structures such as BaLa₂ZnO₅ and BaNd₂ZnO₅ derived via the sol–gel method follow the same tetragonal behavior.^13a,14a,16

Fig. 1 XRD pattern of LCZ phosphor with varying RE concentration.

Structure formation and unit cell

Rietveld refinement of long scan XRD patterns of undoped La₂CaZnO₅ was carried out and a part is shown here in Fig. 2(a) in the range ∼20–80°. The refinement was carried out by FullProf software using the XRD data. The experimental diffraction data is shown by black line. The “×” marks represent the simulated diffraction data. The blue line indicates how much the calculated data deviates from the original values. If this line will be straight it will be perfect matching. In the present case small noise in the blue line indicates good matching for this refinement. The eminence of refinement can be quantified by some factors obtained during refinement known as reliability factors. Reliability factor includes factor for the weighted pattern (R_wp), factor for the pattern (R_p), and the goodness-of-fit indicator (s). For good refinement, R_p and R_wp must be less than 10% and s must approach to unity. In the present case, the parameters were found to be well within the range of a good fitting as summarized in Table 1. The estimated reliability factors verified that the pure phases were obtained without impurities. The crystal system of the prepared samples was identified as a tetragonal structure with space group P4/mmm (no. 123) having lattice parameters a = b = 7.0837 Å, c = 11.884 Å and α = β = γ = 90°. The structural parameters obtained from the Rietveld analysis of the La₂CaZnO₅ phases are listed in Table 1. The refined structural parameters show good agreement with the previous work.^13a,h,14a,16 The structure of the undoped LCZ was estimated via the VESTA software using the data obtained from the Rietveld refinement as shown in Fig. 2(b). The crystal structure of La₂CaZnO₅ is composed of three polyhedral LaO₈, CaO₁₀ and ZnO₄ and the composition of three different polyhedra is schematically shown in Fig. 2(b).

Fig. 2 (a) Fitted (×) and original (black line) powder XRD pattern of La₂CaZnO₅ phosphors and residuals (blue line in the bottom) for the structure refinement by the Rietveld method using the EXPO Program. (b) Schematic diagram of the La₂CaZnO₅ complex structure drawn by VESTA software.

Table 1 Refinement parameter obtained from FullProf Software

Element	x	y	z	Frac. occup.
La (8p)	0.17363	0.67363	0.0	1.00
Zn (4i)	0.0	0.5	0.25	1.00
Ca (2g)	0.0	0.0	0.25	1.00
O(1) (1a)	0.0	0.0	0.0	1.00
O(2) (16u)	0.3519	0.8519	0.1376	1.00
a (Å)	7.0837
c (Å)	11.8848
Rwp, Rp, χ^2	7.36%, 5.89% and 3.92
Space group	P4/mmm (tetragonal)

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UC emission study

The UC of the as-synthesized LCZ : Er³⁺/Yb³⁺ phosphors were recorded as a function of different Yb³⁺-codoping concentrations and the corresponding results are shown in Fig. 3.

Fig. 3 UC spectra of LCZ phosphor with varying Er/Yb concentration.

As shown in Fig. 3, three sensitized UC emission bands centered at 523, 548 and 672 nm were observed in the case of 2 mol% Er³⁺ doped LCZ phosphor. These bands corresponded to the transitions of Er³⁺ ions: ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, respectively.^16,21,22 With the increase in Yb³⁺ concentration the UC intensity was seen to increase and at the concentration of 2 mol% Er³⁺ and 5 mol% of Yb³⁺ the UC intensity reached a maximum. When the Yb³⁺ concentration was incorporated and increased, the green and red emission of Er³⁺ increased significantly, suggesting the appreciable energy transfer process between sensitizers (Yb³⁺) to the activators (Er³⁺). The Yb³⁺ ion has a pretty long absorption energy level (²F_5/2 → ²F_7/2) at 980 nm excitation wavelength. Therefore, upon excitation with 980 nm, apart from excited state absorption (ESA), an adequate energy transfer (ET) from Yb³⁺ to Er³⁺ could be conceivable due to spectral overlap between the Yb³⁺ transition of ²F_5/2 → ²F_7/2 and that of Er³⁺ absorption energy ⁴I_11/2 → ⁴I_15/2, which significantly increased the UC intensity. This is clearly shown in the energy-level diagram explained later. The digital image of the green colour emission is shown in the inset of Fig. 3.

Fig. 4 shows the UC spectra of LCZ: 2 mol% Er³⁺/5 mol% Yb³⁺, LCZ: 1 mol% Ho³⁺/5 mol% Yb³⁺ and LCZ: 2 mol% Er³⁺/1 mol% Ho³⁺/5 mol% Yb³⁺ phosphors. It is obvious that the green emission was more intense in the triply doped sample than that of other samples, which indicates efficient energy transfer between Yb³⁺, Er³⁺ and Ho³⁺ and the energy transfer mechanisms will be discussed later. The enhancement of UC emission after codoping with Ho³⁺ has been reported in other papers.^23,24 To confirm the energy transfer phenomena the lifetime of the green emission of the LCZ samples with codoping ratio Er : Ho : Yb as 0 : 1 : 0, 0 : 1 : 5, 2 : 0 : 0, 2 : 0 : 5 and 2 : 1 : 5 were recorded and are shown in Fig. 5. The resulting decay curves were found to follow the double exponential nature as per the following equation:^25a

I(t) = I₁e^−t/τ₁ + I₂e^−t/τ₂ (1)

where I(t) represents the intensity of luminescence at a particular time, and τ₁ and τ₂ are the short and long lifetimes that correspond to the intensity coefficients I₁ and I₂, respectively. The average lifetime was then estimated using the formula^25a τ_av = (I₁τ₁² + I₂τ₂²)/(I₁τ₁ + I₂τ₂). The multiexponential decay response designates the inhomogeneous dispersion of the doping ions in the host material pointing to the change in the local concentration or the shift of excitation energy from a contributor to lanthanide activators.^25a However, the green emission lifetime increased after the codoping of Yb³⁺ in both the doping separately. In case of triply doping phosphor, the lifetime was obtained to be a maximum of 137 μs. This phenomenon confirmed the presence of ET from Yb³⁺ to Er³⁺/Ho³⁺ in the present host matrix.

Fig. 4 Comparative UC spectra of LCZ phosphor with varying Er/Ho/Yb concentration.

Fig. 5 Decay intensity as function of time of the green emission (548 nm) of the LCZ samples with different codoping ratios.

In this research, it is proven that Ho³⁺/Yb³⁺/Er³⁺ and Er³⁺/Yb³⁺ codoping samples are both suitable for UC emission by pumping at 980 nm, but the triply doped sample was even better. The close vicinity of the individual energy levels of these three ions make the energy transfer processes possible. To understand the mechanism precisely the UC intensity was recorded of LCZ: 2 mol% Er³⁺/5 mol% Yb³⁺ phosphor by varying the laser power (P). The intensity was seen to increase with the increase in P as shown in Fig. 6(a). Fig. 6(b) displays the similar increment behavior of the UC intensity of the LCZ: 2 mol% Er³⁺/1 mol% Ho³⁺/5 mol% Yb³⁺ phosphor while varying the laser power (P).

Fig. 6 Variation of UC intensity with the increment of the laser power for (a) LCZ : Er/Yb and (b) LCZ : Er/Ho/Yb phosphors.

The variation colour coordinate of LCZ: 2 mol% Er³⁺/5 mol% Yb³⁺ phosphor with P is shown in the Commission Internationale de L'Eclairage (CIE) chromaticity diagram of Fig. 7(a). The similar variation of colour coordinate from the yellowish green region towards the deep green region with the increase in pump power was also observed in the case of the LCZ: 2 mol% Er³⁺/1 mol% Ho³⁺/5 mol% Yb³⁺ phosphor as is shown in Fig. 7(b). It is well accepted that the temperature has been changed by varying the excitation power density, which in turn affect the emission intensities and colors. In this present research, the above possibility has been examined for LCZ : Er/Yb and LCZ : Er/Ho/Yb phosphors by measuring their emission colour and corresponding CIE coordinates as a function of different pump-power densities, as shown in Fig. 7. It shows that pump-power/temperature has a significant effect on the chromaticity coordinates, as a variation of colour coordinate from the yellowish green region towards the deep green region with the increase in pump power was observed in the case of the LCZ: 2 mol% Er³⁺/5 mol% Yb³⁺ and LCZ: 2 mol% Er³⁺/1 mol% Ho³⁺/5 mol% Yb³⁺ phosphors as shown in Fig. 7(a) and (b), respectively. Such variation in the CIE or emission colour may be accounted for due to the different saturation behavior of the green and red emissions of Er³⁺ and Ho³⁺ ions.^25b The colour changing property of the present UC phosphors with the variation of power/temperature directly indicated their suitability for thermometry applications in which the emission of a phosphor need to be modulated very precisely under temperature variations.

Fig. 7 Variation of colour coordinates with the increment of laser power for (a) LCZ : Er/Yb and (b) LCZ : Er/Ho/Yb phosphors.

The intensity of the UC bands (I_emission) follows the relation: I_emission ∝ Pⁿ, where n denotes the number of incident photons involved in the UC emission.^16,21,22 The n value can be determined from the slope of the linear fitting between log I vs. log P as shown in Fig. 8. The straight line plot for three different bands and the slope (n) for LCZ: 2 mol% Er³⁺/5 mol% Yb³⁺ phosphors are found as 1.79 and 2.02 for the green transitions at 522, and 548 nm, and 2.48 for the red transition at 672 nm as shown in Fig. 8(a). In the case of LCZ: 2 mol% Er³⁺/1 mol% Ho³⁺/5 mol% Yb³⁺ phosphor the n value was obtained as 1.51 and 2.05 for the green transitions at 522, and 548 nm, and 1.99 for the red transition at 672 nm as shown in Fig. 8(b). The n values are much larger than 2 and revealed that two and three IR photons are involved in producing one green (548 nm) or red (672 nm) photon in this UC process, and the n values are close to 2 in the case of the 522 nm band, which indicates the involvement of two-photons as clearly explained in the literature.¹⁶

Fig. 8 Dependence of UC intensity with pumping power for (a) LCZ : Er/Yb and (b) LCZ : Er/Ho/Yb phosphors.

The energy level diagram showing the probable mechanism and population processes in LCZ : Er³⁺/Ho³⁺/Yb³⁺ UC phosphor is schematically shown in Fig. 9.^16,21,22 The details of the different energy transfer processes in the present system were reported recently.¹⁶ The involved possible energy transfer mechanisms from Yb³⁺ and Er³⁺ to Ho³⁺ for the UC emission are discussed below based on the energy level diagram showed in Fig. 9. First, the ²F_7/2 level of Yb³⁺ is excited to the ²F_5/2 level by ground state absorption (GSA) when the sample is pumped by 980 nm laser and then can transfer their energy to the Ho³⁺ : ⁵I₆ and Er³⁺ : ⁴I_11/2 levels. GSA can also occur for Er³⁺ : ⁴I_15/2 → ⁴I_11/2 when pumped by 980 nm laser. However, compared with the ground state absorption of Ho³⁺/Er³⁺, Yb³⁺ ions possess a larger absorption cross-section at 980 nm and ion concentration, and energy transfer occurs efficiently as a result of the large spectral overlap between the Yb³⁺ emission ²F_5/2–²F_7/2 and the Er³⁺ absorption ⁴I_15/2–⁴I_11/2 (ET1) or Ho³⁺ absorption ⁵I₈–⁵I₆ (ET2) bands. Because the energy gap (1040 cm⁻¹) of the ET2 process is larger than that (432 cm⁻¹) of the ET1 process, the energy transfer efficiency of ET1 is expected to be higher than that of ET2. Thus, ET1 can promote the Er³⁺ ion from the ⁴I_15/2–⁴I_11/2 state, and if the latter is previously populated, the Er³⁺ ion may transit from the ⁴I_11/2 to the ⁴F_7/2 and from ⁴F_7/2 to ⁴G_11/2 states. Succeeding nonradiative relaxations could fill the ²H_11/2 and ⁴S_3/2 states that are the emitting levels for the green luminescence.

Fig. 9 Energy level diagram of Er³⁺, Ho³⁺ and Yb³⁺ ions for various emissions of the La₂CaZnO₅ phosphor.

The ⁴I_11/2 states of Er³⁺ ions could be eliminated by an alternative relaxation process to ⁴I_13/2 states, which may be further stimulated to the red emitting levels ⁴F_9/2, and the identical transition to the ground state ⁴I_15/2 gives red emission. In case of the Ho³⁺ ion, ET2 populated the ⁵I₆ state of the Ho³⁺ ion which further promote the Ho³⁺ ion from the ⁵I₆–⁵F₄ state and the ⁵F₄ state is the emitting levels for the green luminescence. If the ⁵I₆ state of the Ho³⁺ ion depopulated by a nonradiative relaxation route to the ⁵I₇ state, it can be excited to the ⁵F₅ state of the Ho³⁺ ion which is the red emitting level.

But due to the stronger UC emission of Ho³⁺/Er³⁺/Yb³⁺ triply doped sample compared to other codoped samples, it can be inferred that there may exist some energy transfer process between the Er³⁺ and Ho³⁺ ions. The spectral overlap between the Er³⁺ emission ⁴S_3/2–⁴I_15/2 and the Ho³⁺ absorption ⁵F₄–⁵I₈ supports this conclusion. When Ho³⁺ ion is codoped in the LCZ : Er/Yb phosphor, the density of the Er³⁺ ions increases in the ⁴S_3/2 level and hence the green (∼548 nm) emission increases.^16,23,24 The possible energy migrations between Ho³⁺ and Er³⁺ can be verified via a comparative UC emission spectra of LCZ : Er/Yb, LCZ : Ho/Yb and LCZ : Er/Ho/Yb, as shown in Fig. 4. Substantial increase in the green-red intensity (I_{548 nm}/I_{672 nm}) ratio from 1.78 to 2.36 with the incorporation of Ho³⁺ in LCZ : Er/Yb directly indicates the possible energy transfer between Er³⁺ and Ho³⁺. Now, the energy transfer process from Ho³⁺ to Er³⁺ in LCZ : Er/Ho/Yb phosphor can be clearly confirmed from the lifetime analysis. The decay time for the 548 nm green emission were estimated to be 109, 113 and 137 μs for LCZ : Er/Yb, LCZ : Ho/Yb and LCZ : Er/Ho/Yb, respectively. As illustrated Fig. 5, an increment in the decay time for LCZ : Er/Ho/Yb could be due to the energy transfer process from Ho³⁺ to Er³⁺. Nevertheless, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions (548 and 672 nm, respectively) of Er³⁺ ions are significantly overlapped with that of ⁵S₂, ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions (546 and 661 nm, respectively) of Ho³⁺ ions, as illustrated in Fig. 9. Owing to such overlapping, probabilities of energy transfer from Ho³⁺ and Er³⁺ enhanced. Therefore, it is predicted that such energy transfer phenomena may enhanced the green emission intensity and its life time in LCZ : Er/Ho/Yb.

Temperature sensing ability

The temperature sensing study of UC emission spectra of the green bands from 300 K to 573 K of Er³⁺/Yb³⁺ codoped and Er³⁺/Ho³⁺/Yb³⁺ codoped (triply doped) LCZ phosphor have been performed with 980 nm excitation and are shown in Fig. 10 and 11, respectively. From both the figures, it is observed that on increasing the sample temperature, the band positions remain unchanged, but the integrated intensity of two green bands changed in a reverse way. At room temperature (300 K), the intensity corresponding to the ²H_11/2 → ⁴I_15/2 transition was lower than that of the ⁴S_3/2 → ⁴I_15/2 transition. While at a higher temperature (573 K), the reverse effect was observed.

Fig. 10 Variation of UC spectra with temperature for La₂CaZnO₅ : Er³⁺/Yb³⁺ phosphor.

Fig. 11 Variation of UC spectra with temperature for La₂CaZnO₅ : Er³⁺/Ho³⁺/Yb³⁺ phosphor.

This variation in the intensity of the two bands that increased their intensity ratio (I_{522 nm}/I_{548 nm}) as a function of temperature was caused due to the change in their relative population.¹⁶ The ratio is called the Fluorescent Intensity Ratio (FIR) and the variation of FIR with temperature for both the sample is shown in Fig. 12.

Fig. 12 Variation of FIR value of the green emission in LCZ host with different doping as a function with temperature.

As, the population of these two thermally coupled levels follows the Boltzmann's distribution, the FIR of these transitions can be used for optical thermometry via the relation:

FIR = I₅₂₂/I₅₄₈ = C exp(−ΔE/kT) (2)

where I₅₂₂ and I₅₄₈ are the integrated intensities corresponding to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, respectively; C is a pre-exponential constant; ΔE is the energy difference between the ²H_11/2 and ⁴S_3/2 levels; k is Boltzmann's constant and T is the absolute temperature.¹⁶

The linear conversion of eqn (2) can be written as

ln(I₅₂₂/I₅₄₈) = −(ΔE/k)(1/T) + ln(C) (3)

The corresponding energy difference ΔE can be calculated from the slope (ΔE/k) of the linear fitting of ln(I_{525 nm}/I_{548 nm}) versus 1/T plot as shown in Fig. 13. The fitting of the experimental data gives a slope of about 640.53 and 1168.85 for codoped and triply doped phosphors respectively and resulting in an energy difference ΔE of about 444 and 810 cm⁻¹.¹⁶ The performance of the temperature sensor generally depends on the figure of merit of the sensing behavior. The figure of merit includes different parameters such as absolute sensitivity (S_a), relative sensitivity (S_r) and the resolution. The absolute sensitivity is defined as the variation of the FIR or lifetime (for two approaches) with respect to temperature and is expressed as:²⁶

S = dR/dT = FIR(−ΔE/kT²) (4)

where S is the sensor sensitivity and the other terms have their usual meanings.⁹

Fig. 13 Variation of logarithmic FIR value of the green emission in the LCZ host with inverse temperature.

The absolute sensor sensitivity in the present case was calculated for the FIR measurements and was obtained as 0.0029 and 0.0047 K⁻¹ at 300 K for the codoped and the triply doped phosphors, respectively and then increased with the rise in temperature. The maximum sensitivity was observed as 0.0036 at 348 K for the codoped phosphor whereas 0.0067 at 398 K for the triply doped phosphor. The sensitivity value was higher in the case of the triply doped sample. This is because after codoping of Ho³⁺, the Boltzmann population of the Er³⁺ ion increased in the ⁴S_3/2 state. Therefore, the intensity ratio between the transition from ²H_11/2 and ⁴S_3/2 state to ground state as well as the FIR were modified, which affected the sensitivity.²⁷ As illustrated in the energy diagram (Fig. 9), UC initiates through Yb³⁺ sensitizer excitation and ET to Er³⁺ and Ho³⁺ ions and consequently favors the simultaneous inter-ion ET between Er³⁺ and Ho³⁺ ions. Moreover, excited state absorption (ESA) and cross-relaxation (CR) processes in both Er³⁺ and Ho³⁺ ions significantly improve the UC efficiency. Now, as we know that the temperature sensitivity is depended on the FIR which is defined as the intensity ratio of the emission intensities centered at 525 nm and 548 nm. Due to the incorporation of Ho in the Er/Yb codoped system, this ratio has been changed and it is found that the FIR values of Er/Yb/Ho codoped system changed more systematically owing to which the sensitivity was increased.

With the further increase in temperature the sensitivity decreased for both the cases. The variation of absolute sensitivity with temperature for FIR measurement is shown in Fig. 14. This figure shows that the sensitivity increased after codoping of Ho³⁺ in the LCZ : Er³⁺/Yb³⁺ phosphor. The relative sensor sensitivity is the normalized absolute sensor sensitivity with respect to the measured value. The eqn (5) is used to calculate the relative sensitivity as per the following equation:^28,29

Fig. 14 Variation of absolute sensor sensitivity of the LCZ phosphors with temperature.

Fig. 15 indicates that the relative sensor sensitivity values decreased with temperature in the measured range. However, the values are sensibly higher than the reported over the examined large temperature range from room temperature to the higher temperatures. This behavior indicates the suitability of this material as an attractive host in the operation of electronic devices for temperature sensing purposes. The maximum relative sensor sensitivity value is 0.71% K⁻¹ and 1.52% K⁻¹ at 300 K for the codoped and the triply doped phosphors respectively.

Fig. 15 Variation of relative sensor sensitivity of the LCZ phosphors with temperature.

The effect of Ho³⁺ concentration on the experimentally estimated sensitivity was determined for the 300–573 K temperature range and the variation of the maximum relative sensitivity at 300 K with Ho³⁺ concentration is presented in Fig. 16. The figure shows that with an increase of the Ho³⁺ ions amount from 1 to 9 mol% the relative sensitivity decreased exponentially from 1.52% K⁻¹ to 0.28% K⁻¹. This effect can be explained via the energy transfer phenomena between Ho³⁺ ions. With the increment of Ho³⁺ concentration, the cross relaxation [⁵F₄,⁵I₈] → [⁵I₅,⁵I₇] between Ho³⁺ ions starts which leads to lowering of the ²H_11/2 state population of Er³⁺ and hence ²H_11/2 → ⁴I_15/2 emission intensity. The probability of this energy transfer increases with the reduction of the average distance between these ions connected. Therefore, the cross relaxation process will affect the changes of FIR and hence the relative sensitivity. Marciniak et al.³⁰ has observed the similar behaviour in the case of Er : LiYbP₄O₁₂ luminescent thermometer.

Fig. 16 Variation of relative sensor sensitivity of the LCZ phosphors with Ho³⁺ concentration.

A comparison of relative sensor sensitivity value and the temperature range between the recently developed Ln³⁺ phosphor based inorganic nano-thermometers is summarized in Table 2 and it indicates that the relative sensor sensitivity value in case of the present phosphor is among the highest.

Table 2 Comparative analysis of relative sensitivity and temperature range of the inorganic sensor materials

S. No.	Phosphor	Sr at Tm	ΔT (Tm)	Ref.
1	UC Nps : Er^3+/Yb^3+	2.3	293–318 (318)	31
2	UC Nps : Tm^3+/Yb^3+	0.2	293–318 (315)	31
3	NaYF4 : Er^3+/Yb^3+	1.0	298–318 (298)	20
4	Gd2O3 : Er^3+/Yb^3+	0.2	295–1000 (600)	32
5	Fluoride glass : Er^3+/Yb^3+	1.1	333–375 (342)	33
6	ZnO : Er^3+/Yb^3+	0.6	273–473 (273)	34
7	LCZ : Er/Yb	∼0.51 (s = 0.0059 K^−1)	298–513 (483)	16
8	LCZ : Er/Yb	0.71 (s = 0.0036 K^−1)	300–573 (300)	Present work
9	LCZ : Er/Yb/Ho	1.52 (s = 0.0067 K^−1)	300–573 (300)	Present work

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Temperature resolution is also an important factor to characterize any temperature sensor devices and can be defined to the minimal detectable signal change. The standard deviation data of residuals in the fit of the experimental FIR data points with temperature and the absolute sensitivity were used to estimate resolution as the method described by Brites et al.¹⁸ and the estimated resolutions of FIR temperature sensing is shown in Fig. 17.

Fig. 17 Variation of resolution of the LCZ temperature sensor with temperature.

Both the curve follows similar behavior and it is observed that the FIR measurements for the codoped phosphors provided better resolution compared to the triply doped sample. The resolution values in both the cases are very much lower than 1 K over a large temperature range from 300 K to 573 K. The maximal resolutions were obtained at 325 K for the codoped phosphor as 0.105 K and at 425 K for the triply doped sample 0.115 K. The comparative sensitivity data and resolution behavior of the LCZ : Er/Ho/Yb phosphor with the other phosphor indicates the suitability of this material for temperature sensing application. Similar studies have been reported by Li and co-workers for sol–gel derived Yb³⁺–Er³⁺ co-doped La₂CaZnO₅ phosphors, with a sensitivity of 0.0059 K⁻¹ at 483 K.¹⁶ But the FIR at 483 K is about 1.15. This indicates the Sr value is about 0.51% K⁻¹. Similar studies were also carried out by various authors for different materials.^9,17 In the present case for the sample LCZ : Er/Yb the sensitivity was observed as 0.0036 K⁻¹ at 348 K. This value has increased by above 69% after codoping of Ho³⁺, which is about 7% greater than the reported one. The obtained relative sensitivity is higher than the reported. And also, the resolution is very much lower than 1 K over a large temperature range from 300 K to 573 K. The results imply that the studied UC phosphor is superb for temperature sensing applications.

Conclusions

In summary, a series of Er³⁺/Yb³⁺ and Er³⁺/Ho³⁺/Yb³⁺ codoped La₂CaZnO₅ phosphors were prepared by the combustion synthesis method. The crystal system of the prepared samples was identified as a tetragonal structure and the doping of RE ions did not affect the crystal structure. Under the laser excitation of 980 nm, the Er³⁺/Yb³⁺-codoped sample exhibited strong green and red UC emissions from ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. The enhanced UC emission after codoping of Ho³⁺ was explained on the basis of energy exchange mechanisms between Er³⁺/Ho³⁺ and Yb³⁺ ions. Power dependence studies infer that the UC bands arose through a two/three-photon absorption process. The high corresponding sensor responsiveness over a broad temperature range with large temperature resolution make the phosphors suitable for future applications in thermometry. The FIR technique was used to measure the sensitivity of the synthesized temperature sensors, and it can be concluded that at 400 K the obtained phosphors shows a relatively higher sensitivity and resolution over the existing one. These data indicate the suitability of LCZ : Er³⁺/Ho³⁺/Yb³⁺ phosphors for temperature sensor applications.

Acknowledgements

Dr Vijay Kumar is greatly acknowledged the Science and Engineering Research Board (SERB), New Delhi for financial assistance in the form of Young Scientist Scheme (Fast Track; File No. DST No. SB/FTP/ETA-215/2014). The financial assistance from the University of the Free State is also highly recognized.

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