Enhanced green upconversion photoluminescence from Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor via Mg²⁺ doping

Abstract

This paper reports enhanced green upconversion photoluminescence from Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor via Mg²⁺ doping synthesized through a solid state reaction method. The X-ray diffraction measurements confirm a shift in the peak position due to the presence of Mg²⁺ in the CaZrO₃ phosphor. The scanning electron micrographs reveal an increase in the particle size for doping with Mg²⁺ ions. The Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor gives an intense monochromatic green upconversion emission centered at 543 nm due to ⁵F₄/⁵S₂ → ⁵I₈ transition along with weak UV, blue, red and NIR emissions on excitation at 976 nm. The emission intensity of Ho³⁺ ions was optimum for 3 mol% Yb³⁺. The doping of Mg²⁺ ions slightly changes the band gap of the CaZrO₃ phosphor; thereby enhancing the emission intensity significantly. When Mg²⁺ ions are doped in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor the emission intensity of the green band is enhanced by up to 4 times. This enhancement is due to substitution of Ca²⁺ by the Mg²⁺ ions, which decreased the lattice parameters and increased the crystallinity. The lifetime of the ⁵F₄/⁵S₂ level increases with the increase in the concentration of Mg²⁺ ions. Thus, the Ho³⁺/Yb³⁺/Mg²⁺ co-doped CaZrO₃ phosphor could be a suitable candidate for intense monochromatic green light and optical devices.

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1. Introduction

Rare earth doped photoluminescent materials have been fascinating to researchers in recent years due to their wide applications in various fields such as lasers, solar cells, temperature sensing, optical storage, bio-thermal treatment, bio-imaging, etc.^1–9 Rare earth ions possess a unique property to convert low energy near infrared (NIR) photons into a visible photon with high energy known as upconversion (UC). It is a non-linear optical process and characterized by either successive absorption of two or more low energy photons via intermediate meta-stable levels or cooperative absorption of two photons resulting in a photon of high energy.¹⁰ The photon conversion has also attracted the attention of researchers in the field of energy harvesting by cooperative downconversion energy transfer.⁴ Researchers are still in search of efficient phosphor materials that could enhance the emission intensity for longer performance. The emission intensity of Ho³⁺/Yb³⁺ co-doped phosphors has been extensively studied in which Yb³⁺ transfers its excitation energy to a Ho³⁺ ion and enhances the emission intensity. Thus, it acts as a sensitizer.^5,11–16 Some efforts have been made to enhance the emission intensity of Ho³⁺/Yb³⁺ co-doped phosphors using different impurity ions/elements such as Mg²⁺, Zn²⁺ and Li⁺ ions.^5,15,17 When these ions are incorporated in the host matrices they modify the crystal field in such a way that a large emission intensity could be obtained. However, further attention is needed to synthesize it in a suitable host material with a suitable combination of activator and impurity ion for longer life and wide applications.

In recent years, the impurity elements have proved their suitability to enhance the emission intensity of the phosphors to a great extent.^5,15,17–19 They not only modify the local crystal field but also enhance the emission intensity significantly. Among the various impurity elements, Li⁺ has been studied extensively to improve the optical properties of the activators.^17–19 The effect of Li⁺ on the emission intensity of Ho³⁺/Yb³⁺ co-doped Y₂O₃ phosphor has been studied by Yadav et al.²⁰ They have found that Li⁺ changes the local crystal field, which tailors the emission intensity appreciably. The emission intensity of Ho³⁺/Yb³⁺ co-doped phosphor is also affected by incorporation of Zn²⁺ ions in the Y₂O₃ host.¹⁵ The addition of Zn²⁺ in the phosphor enhances the emission intensity significantly. However, the emission intensity of Ho³⁺/Yb³⁺ co-doped CaMoO₄ phosphor is also enhanced greatly by incorporating Mg²⁺ ions. It has been observed that Mg²⁺ with lower ionic radii substitutes Ca²⁺ from the host and creates shrinkage in the lattice, which gives rise to an enhancement in the emission intensity.⁵ The effect of Mg²⁺ on the emission intensity of Ho³⁺/Yb³⁺ co-doped phosphor is found rarely. However, the optical properties from Ho³⁺/Yb³⁺ in the CaZrO₃ host is not reported to our knowledge and needs further attention. The enhancement in the emission intensity from none of the impurity ion in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor is also not investigated.

The Mg²⁺ ion has been used in different combinations of activators and host matrices to enhance the emission intensity.^21–26 The effect of Mg²⁺ has been studied by Wang et al. in non-rare earth activated BaMgAl_10−2xO₁₇:xMn⁴⁺,xMg²⁺ phosphor. It has been found that Mg²⁺ doping not only affects narrow band red emission of the Mn⁴⁺ ions but also reduces the concentration quenching caused by Mn⁴⁺ dipole–dipole interaction.²⁴ The Mg²⁺ doped materials show good color purity and thermal stability. Similarly, the effect of alkaline earth metal ions (i.e. Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) has been studied in non-rare earth activated ZnO nanoparticles by Hameed et al.²⁵ It has been reported that these ions enhance the emission intensity of ZnO nanoparticles originating from defect levels. In the case of rare earth doped compound, the effect of Mg²⁺ has been studied by Dey et al. in the Ho³⁺/Yb³⁺ co-doped CaMoO₄ phosphor. They have explained an enhancement in the emission intensity due to shrinkage in the lattice dimension by incorporation of Mg²⁺ in the host matrix.⁵ However, it has been also noticed that the Mg²⁺ doping promotes overlapping between CTS and 4f–5d in Eu³⁺ ion, which increases the absorption cross section; thereby gives larger emission intensity.²⁶ The Mg²⁺ doped materials are also useful in biological systems as an efficient fluorescent probe to improve the resolution of bio-imaging.^25,27 Thus, the Mg²⁺ is a potential candidate for enhancing the photoluminescence intensity of different activator doped materials.

The host material also plays an important role to enhance the emission intensity. The host with low phonon frequency reduces the non-radiative relaxation and gives intense emission intensity.^28,29 Calcium zirconate (CaZrO₃) is a low phonon frequency host (∼505 cm⁻¹) and is widely studied for optical properties. It possesses strong chemical, thermal and structural stability and thus has long lifespan.^30–33 The Ho³⁺ and Yb³⁺ ions have been used as co-dopants for CaZrO₃ in which Yb³⁺ ions transfer their excitation energy successively and cooperatively in the different levels of Ho³⁺ ion and enhance the emission intensity appreciably.^11–16 The magnesium (Mg²⁺) ion has been used as an impurity element, which improves the emission intensity of Ho³⁺/Yb³⁺ co-doped phosphor. Addition of Mg²⁺ ion influences the structural as well as the optical properties drastically.

In this paper, the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor has been synthesized through solid state reaction method. The structural and morphological analyses of the synthesized phosphor were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The vibrational features of the synthesized phosphors were studied using Fourier transform infrared (FTIR) measurements. The phosphor samples give very intense and almost monochromatic green emission on excitation with 976 nm radiation from a diode laser via upconversion process (i.e. two photon processes). The emission intensity of the phosphor enhances many times in the presence of Mg²⁺ ion. The lifetime measurement reveals the factor responsible for enhancing the emission intensity. Thus, the Ho³⁺/Yb³⁺/Mg²⁺ co-doped CaZrO₃ phosphor can be a potential candidate for intense monochromatic green light and optical devices.

2. Experimental

2.1 Materials and method

The 0.5 mol% Ho³⁺, x mol% Yb³⁺ (i.e. x = 3, 5, 7, 10, 15 and 20) and 0.5 mol% Ho³⁺, 3 mol% Yb³⁺, y mol% Mg²⁺ (i.e. y = 0, 5, 10, 15, 20 and 25) co-doped CaZrO₃ phosphor samples were prepared by solid state reaction method.⁴ The Ho₂O₃ (99.99%), Yb₂O₃ (99.99%), CaCO₃ (99%), ZrO₃ (99.99%) and MgO (99.9%) were used as starting materials. These materials were crushed rigorously with the help of an agate mortar and pestle for 30 minutes using acetone as mixing solution. The homogeneous product thus obtained was kept in an alumina crucible for annealing in a closed programmable furnace maintained at 1300 °C for 5 hours. The product thus obtained is known as phosphor. We have also prepared phosphor sample by adding different concentration of Mg²⁺ in the phosphor above stoichiometric ratio. The phosphor samples were examined for structural and optical properties using different characterization techniques.

2.2 Characterization

The X-ray diffraction (XRD) technique has been used to identify the crystallite phase and effect of doping element using Cu, K_α radiation (λ = 0.15406 nm) from a RINT/DMAX 2200H/PC (Rigaku, Japan) machine with 2° min⁻¹ scan speed at room temperature. The phase of the XRD patterns was analyzed using International Centre for Diffraction Data (ICDD). The microstructure features of the sample were studied using a scanning electron microscopy (SEM) with JEOL-TM Model JSM 5410 unit. The Fourier transform infrared (FTIR) spectra of the phosphors were recorded using Perkin Elmer IR spectrometer (FT-IR/FIR spectrometer Frontier) to see the presence of different molecular species in the host. The UV-vis-NIR (ultraviolet-visible-near infrared) spectra of the phosphor samples were monitored in diffuse reflectance method from a Perkin Elmer UV-vis-NIR spectrometer (Lambda 750). The upconversion emission spectra of different samples were recorded using 976 nm radiation from a diode laser and a iHR320, Horiba Jobin Yvon, spectrometer attached with PMT (photomultiplier tube) was used to record the spectra. The lifetime of Ho³⁺ ion in the samples for ⁵F₄/⁵S₂ transition in green region were measured by chopping the continuous radiation of 976 nm laser using 150 MHz digital oscilloscope (Model no. HM 1507, Hameg Instruments).

3. Results and discussion

3.1 Structural measurements

The XRD pattern gives clear information about the nature of the material whether it is crystalline or non-crystalline. It facilitates the crystallinity, phase and average crystallite size of the phosphor. The XRD patterns of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ and its Mg²⁺ co-doped phosphor samples have been recorded in the 20–80° region and they are shown in Fig. 1. The XRD patterns match well with the JCPDS file no. 35-0790 having lattice parameters a = 5.755, b = 8.010, c = 5.592. The indexing of the XRD patterns has been done using the JCPDS file (see Fig. 1(a)). The phase of the crystal is orthorhombic with the space group Pbmn. The XRD pattern of Mg²⁺ co-doped phosphor shows a single phase phosphor with improved crystallinity.

Fig. 1 (a) XRD patterns of the Ho³⁺/Yb³⁺ co-doped and its Mg²⁺ co-doped CaZrO₃ phosphor samples (annealed at 1300 °C/5 h). The inset figure shows the variation in FWHM and a shift of the most intense XRD peaks in the two cases. (b) XRD patterns of the Ho³⁺/Yb³⁺ co-doped; its Mg²⁺ co-doped and added CaZrO₃ phosphors. The insets show the variation in FWHM of the XRD peaks in the three cases.

When Mg²⁺ is co-doped in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor the peaks of the XRD patterns are shifted towards higher angle side. As the concentration of Mg²⁺ increases the XRD peaks are regularly shifted towards higher angle side. Since Mg²⁺ has smaller ionic radii (72 pm) than Ca²⁺ ion (100 pm), therefore, it substitutes Ca²⁺ from the CaZrO₃ host. This causes a shift in the XRD peaks towards higher angle side. As a result, Mg²⁺ ion creates shrinkage in the lattice parameters and improves the crystallinity.⁵ It is clear from the figure that all the samples are highly crystalline. On the other hand, when Mg²⁺ is added (15 mol%) above the stoichiometric ratio the XRD peaks are slightly shifted towards lower angle side.³⁴ It may be due to the fact that some of the Mg²⁺ ions may occupy at the substitutional sites and remaining the interstitial sites in the CaZrO₃ host (see Fig. 1(b)).

The Voigt function is a single line profile method used to analyze the crystallite size and the lattice strain. The crystallite size ‘D’ and the lattice strain ‘e’ have been calculated from the integral breadths of the broadened profile of the XRD peaks. The Cauchy and Gaussian components of the XRD peak are obtained from the ratio of full width at half maximum (FWHM) intensity (2ω) and the integral breadths (β). In a single line analysis, the ‘D’ and ‘e’ are related to the Cauchy (β_c) and Gaussian (β_G) widths of the XRD peak at Bragg angle θ;

D = kλ/β_c cos θ (i)

where k = 0.89; is a constant and λ is the wavelength of the X-ray beam and

e = β_G/4 tan θ (ii)

The constituent Cauchy and Gaussian components are given as

β_c = (a₀ + a₁ψ + a₂ψ²)β

β_G = (b₀ + b_1/2(ψ − 2/π)^1/2 + b₁ψ + b₂ψ²)β

where a₀, a₁ and a₂ are Cauchy constants, b₀, b_1/2, b₁ and b₂ are Gaussian constants and ψ = 2ω/β; where β is the integral width obtained from the XRD peak. The values of these constants have been used from Langford table.^18,35

a₀ = 2.0207, a₁ = −0.4803 and a₂ = −1.7756

b₀ = 0.6420, b_1/2 = 1.4187, b₁ = −2.2043 and b₂ = 1.8706

The crystallite size thus calculated are found to be 78, 82, 110 and 264 nm for 0, 5, 15 and 25 mol% concentration of Mg²⁺ ions, respectively. However, on addition of Mg²⁺ ions (15 mol%) above stoichiometric ratio the crystallite size is found to be 88 nm, which is comparatively less than the crystallite size obtained in doped condition (110 nm). It reveals that doping the ions (15 mol%) improves crystallinity in the phosphor compared to adding the ions.

The lattice strains have been also calculated for doping the 0, 5, 15 and 25 mol% concentration of Mg²⁺ and they are found to be 0.51, 0.48, 0.46 and 0.45, respectively.³¹ However, in the case of Mg²⁺ addition (15 mol%) the lattice strain is increased to 0.57. This suggests that the crystal strain reduces regularly on doping whereas addition of the ions creates strain in the phosphor. Thus, an ordered crystal structure is obtained for doping the ions.

We have also analyzed the XRD data for Rietveld refinement. The observed XRD profile matches well with the theoretical profile and their intensity counts are also close to each other. The Rietveld refinement is good if the goodness of fit (GOF) factor is achieved ∼1. In our case, we have obtained the GOF value to 0.64 after refining the XRD data.¹⁸ The Rietveld refinement of XRD data for the annealed Ho³⁺/Yb³⁺/Mg²⁺ co-doped CaZrO₃ phosphor is given in Fig. 2.

Fig. 2 Rietveld refinement of XRD data of the Ho³⁺/Yb³⁺/Mg²⁺ co-doped CaZrO₃ phosphor sample (annealed at 1300 °C/5 h).

The SEM micrographs of the Ho³⁺/Yb³⁺ co-doped (annealed at 1300 °C/5 h) and its Mg²⁺ co-doped CaZrO₃ phosphor samples are shown in Fig. 3(a) and (b). In both the cases, the shape of the particle is almost spherical and agglomerated with each other in different orientations. The particle size is found to be increased on Mg²⁺ doping (see Fig. 3(b)).^5,19

Fig. 3 SEM micrographs of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor samples in absence (a) and presence (b) of Mg²⁺ ions.

3.2 Optical measurements

3.2.1 FTIR measurements. The Fourier transform infrared (FTIR) studies of the synthesized phosphors have been carried out to see the presence of different molecular species. The FTIR spectra of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor (annealed at 1300 °C/5 h) in absence and presence of Mg²⁺ ions recorded in the 400–4000 cm⁻¹ region are shown in Fig. 4. The spectra contain vibrational bands at 435 and 505 cm⁻¹, which are assigned to arise due to stretching vibrations of Ca–O and Ca–O–Zr, respectively.^36,37 It is clear from the figure that when Mg²⁺ ions are co-doped in the phosphor sample the nature of the vibrational bands remains unaffected, however, the absorbance of these bands increases due to the substitution of Ca²⁺ by Mg²⁺ ions in the phosphor samples and it is maximum for 15 mol% of Mg²⁺ ions. Thus, the increase in the absorbance is a consequence of reduction in crystal defect.

Fig. 4 FTIR spectra of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor (annealed at 1300 °C/5 h) in absence and presence of Mg²⁺ ions.

3.2.2 UV-vis-NIR diffuse reflectance measurements. The UV-vis-NIR diffuse reflectance spectra of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor (annealed at 1300 °C/5 h) in absence and presence of Mg²⁺ ions recorded in the 200–1200 nm region are shown in Fig. 5. It is clear from the figure that the intensity of the absorption bands increases on doping the Mg²⁺ ions. This reveals strong absorption of incident NIR photons, which results larger emission intensity. The spectra contain various absorption bands due to charge transfer state (CTS) and 4f–4f transitions of Ho³⁺ ion. It has been also reported that Mg²⁺ helps in enhancing absorption cross section due to spectral overlap of CTS and 4f–4f by improving crystallinity of the phosphor. Therefore, the spectra show large number of bands centered at 362, 384, 422, 450, 488, 541 and 651 nm and they are assigned to arise due to ⁵I₈ → ³H₆, ⁵G₄, ⁵G₅, ⁵G₆, ⁵F₃, ⁵S₂ and ⁵F₅ transitions, respectively.³⁸ The spectra also contain a broad absorption band centered at 238 nm, which is assigned to arise due to CTS of Ho³⁺ → O²⁻ ions. An additional band is observed at 976 nm due to the ²F_7/2 → ²F_5/2 transition of Yb³⁺ ion and the presence of Mg²⁺ in the co-doped phosphor enhances the intensity of this band significantly (see inset figure). This reveals that Yb³⁺ ion can efficiently absorb the incident 976 nm photons and at the same rate they transfer their excitation energy to Ho³⁺ ion; thereby resulting larger emission intensity.

Fig. 5 UV-vis-NIR diffuse reflectance spectra of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor in absence and presence of Mg²⁺ ions.

We have also calculated the optical band gap for the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor in absence and presence of Mg²⁺ ions using Wood and Tauc formula³⁹ as given below

where E_g is the energy of optical band gap, hν is the energy of incident photon and B is a constant known as band tailoring parameter. The value of ‘m’ is used as 1/2 for a direct allowed transition. The (αhν)² versus hν curves has been plotted for the Ho³⁺/Yb³⁺ co-doped phosphor in absence and presence of Mg²⁺ ions and they are shown in Fig. 6. The plots give the direct optical band gap values 4.15 and 4.16 eV in the two cases. This value matches well with earlier reported value for the CaZrO₃ phosphor.⁴⁰ Thus, the Mg²⁺ doping slightly change the optical band gap, which could enhance the photoluminescence intensity of the phosphor.

Fig. 6 The (αhν)² versus hν plots for the Ho³⁺/Yb³⁺ co-doped phosphor in absence and presence of Mg²⁺ ions.

3.2.3 Upconversion emission spectra of Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor. The upconversion (UC) emission intensity of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor samples have been monitored on excitation with 976 nm radiation in the 350–775 nm region and the spectra thus obtained are shown in Fig. 7. The emission spectra contain several peaks centered at 422, 461, 488, 543, 584, 654 and 757 nm and they are assigned to arise due to ⁵G₅ → ⁵I₈, ³K₈ → ⁵I₈, ⁵F₃ → ⁵I₈, (⁵F₄, ⁵S₂) → ⁵I₈, ⁵G₄ → ⁵I₆, ⁵F₅ → ⁵I₈ and ⁵S₂ → ⁵I₇ transitions, respectively.^11–16,38 The phosphor samples have been prepared with different concentrations of Yb³⁺ (i.e. 2, 3, 5, 7 and 10 mol%) and a fixed (0.5 mol%) concentration of Ho³⁺ ions. It is clear from the figure that as the concentration of Yb³⁺ ions increases the emission intensity initially increases and after a certain concentration it starts decreasing. The emission intensity of the phosphor was found to be optimum at 3 mol% concentration of Yb³⁺ ion. Actually, when the concentration of Yb³⁺ ions increases large numbers of Yb³⁺ ions are excited and in the same proportion they transfer their excitation energy to the Ho³⁺ ions. As a result, large numbers of Ho³⁺ ions takes part in the excitation process and the populations of the ions in the excited states become dominant resulting very intense green emission. Since, the intensity of other bands is negligibly small compared to green band, therefore, the emission can be considered as pure (monochromatic) green light. Beyond 3 mol% concentration of Yb³⁺ ions, the concentration quenching takes place for all the bands and a decrease in the emission intensity has been observed for 5, 7 and 10 mol% concentration.^19,20

Fig. 7 UC emission spectra of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphors for different concentration of Yb³⁺ ions on excitation with 976 nm. The inset figure shows emissions in the UV and blue regions.

This is due to the fact that at higher concentrations the inter-nuclear separation between the ions becomes smaller than the critical separation and the excitation energy migrates to the optical quenching centers. This causes a poor emission intensity of the co-doped phosphor. The peaks seen in the emission spectra matches very well for different concentration of Yb³⁺ ions. The inset figure shows a zoomed region of emissions in the UV and blue regions. They also show a variation in the emission intensity for different concentrations.

It well known that the Ho³⁺ ion contains large number of meta-stable states and can be easily excited for intense fluorescence. The Yb³⁺ ions can transfer their excitation energy to the different levels of Ho³⁺ ions and can enhance the emission intensity several times. Since the Yb³⁺ ion has very high absorption cross section for 976 nm, therefore, it efficiently absorbs 976 nm photons, and promoted to its excited (²F_5/2) state. These excited ions transfer their excitation energy to the Ho³⁺ ions in the ground state (⁵I₈). As a result, the Ho³⁺ ions are excited to ⁵I₆ state through ground state absorption (GSA). These ions in ⁵I₆ state absorb other 976 nm photons and are promoted to ⁵F₄/⁵S₂ excited states through excited state absorption (ESA). The ions in this state emit photons of 543 and 754 nm radiations via ⁵F₄/⁵S₂ → ⁵I₈ and ⁵F₄/⁵S₂ → ⁵I₇ transitions, respectively. Furthermore, some ions in this state relax non-radiatively to ⁵F₅ state. The ions in ⁵F₅ state give a transition to ground state with emission of a photon of 654 nm radiation. The ions in the ⁵F₄ state further absorb other photons through excited state absorption (ESA) and are promoted to ³K₇ excited state via energy transfer upconversion (ETU), which populates low lying ⁵G₄, ⁵G₅, ³K₈ and ⁵F₃ states. Finally, the ions in these states relax radiatively to emit different photons in UV and visible regions.^{11–16,19,20,38} The excitation processes involved in these emissions can be easily understood by schematic energy level diagram. The schematic energy level diagram for Ho³⁺ and Yb³⁺ ions are shown in Fig. 8 representing the involvement of different mechanisms such as GSA, ESA, ETU and CET in the excitation processes.

Fig. 8 Schematic energy level diagram of Ho³⁺ and Yb³⁺ ions for different transitions on excitation with 976 nm.

In order to confirm the number of photons involved in the upconversion emission, the power dependent measurements of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor on excitation with 976 nm have been carried out. The UC emission intensity of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor has been recorded at different pump power of 976 nm diode laser for 543 and 654 nm transitions. The dual logarithmic plot of input pump power versus emission intensity is found to be linear for these transitions, which is shown in Fig. 9. The values of the slopes (n) for these transitions have been calculated and are found to be 2.27 and 1.99.^19,20 This confirms that the ⁵F₄/⁵S₂ and ⁵F₅ levels of Ho³⁺ ions are populated by absorption of two NIR photons.

Fig. 9 Dual logarithmic plots for input pump power versus emission intensity of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor sample on excitation with 976 nm.

3.2.4 Effect of Mg²⁺ doping on UC emission of Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor. The emission intensity of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor can be enhanced significantly by doping Mg²⁺ ion. It has been extensively studied that Mg²⁺ ion has been used to enhance the emission intensity of the activator ions.^21–26,41 The doping of Mg²⁺ ion not only modifies the local crystal field but also enhances the optical properties of the materials. It can occupy the substitutional or interstitial sites in the host matrix depending on the ionic size of the metal ion used in the host matrix.³⁴ It creates charge imbalance into rare earth co-doped materials^18,42 and improves the emission intensity significantly.

The emission spectra of the Mg²⁺ activated Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor for different concentration of Mg²⁺ ions (i.e. 0, 5, 10, 15, 20 and 25 mol%) recorded in the range of 350–775 nm on excitation with 976 nm radiation are shown in Fig. 10. The emission intensity varies with the increase in the concentration of Mg²⁺ ions. Initially, the emission intensity increases from 5 to 15 mol% concentration of Mg²⁺ ions and it is optimum for 15 mol%. Finally, the emission intensity starts decreasing after this concentration due to concentration quenching. As is clear from the figure that the emission intensity of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor is found to be enhanced up to 4 times in presence of Mg²⁺ ions. As discussed earlier, the Mg²⁺ ion has smaller ionic radii than Ca²⁺ ion, it substitutes Ca²⁺ ion from the host and creates shrinkage in the lattice dimension; thereby increases the crystallinity. However, the effect of Mg²⁺ ion is reported in Ho³⁺/Yb³⁺ co-doped CaMoO₄ phosphor by Dey et al. These workers have also found an enhancement in the emission intensity due to substitution and crystallinity of the host.⁵ Furthermore, Mg²⁺ doping results a relaxation in the crystal strain and the crystal strain relaxes with the increase in the concentration (i.e. 0.51, 0.48, 0.46 and 0.45 for 0, 5, 15 and 25 mol%). Interestingly, as the crystal strain decreases; the crystal defects present in the phosphor decreases simultaneously. As a result, the more ordered crystal structure is formed, which increases the emission intensity. The rate of decrease in the crystal strain is smaller for higher concentration. After 15 mol% of Mg²⁺ ions, the variation in the crystal strain seems to be saturated (i.e. from 0.46 to 0.45), however, the largest emission intensity is observed for 15 mol% concentration of Mg²⁺ ions. Therefore, the relaxation in the crystal strain is a consequence of large enhancement in the emission intensity.¹⁸

Fig. 10 Emission spectra of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor in absence and presence of different concentration of Mg²⁺ ions on excitation with 976 nm radiation. The insets are emissions in the UV and blue regions and an image of the sample.

It is worth noting that the doping of Mg²⁺ does not influence the positions of emission peaks. A highly intense monochromatic green UC emission could become possible alongwith other emissions in the UV, blue, red and NIR regions. However, the peaks which are not present in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor are obtained with a larger intensity via Mg²⁺ doping (see inset figure). An additional peak centered at 392 nm is also observed in UV region due to ⁵G₄ → ⁵I₈ transition of Ho³⁺ ion, which arises mainly due to an increase in the crystallinity of the CaZrO₃ phosphor. The color emitted by the sample is also shown as inset image in the figure. An enhancement in UC green emission intensity has been observed up to 4 times for Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor via Mg²⁺ doping. Therefore, the Ho³⁺/Yb³⁺/Mg²⁺ co-doped CaZrO₃ phosphor can be suitable candidate for monochromatic green light and optical devices.

On the other hand, when the optimized concentration (15 mol%) of Mg²⁺ ions are added above the stoichiometric ratio in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor, the emission intensity is also found to be enhanced up to 1.35 times. However, this enhancement in the emission intensity is much smaller than that of doping the Mg²⁺ ions (15 mol%). It is expected that when the Mg²⁺ ions are added above the stoichiometric ratio, some of the Mg²⁺ ions occupy at the substitutional sites whereas remaining the interstitial sites.³⁴ As a result, in this case a small shift in the XRD peaks is observed towards the lower angle side, which reflects an expansion in the lattice dimension. However, the crystal strain (0.57) is also higher in this case than the case of doping the Mg²⁺ ions (0.46), which is responsible for smaller emission intensity. A comparison of the emission intensity for 15 mol% doping and addition of Mg²⁺ ions in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphors on excitation with 976 nm radiation is shown in Fig. 11. The inset in the figure is the emissions in the UV and blue regions of the sample. The ratio of the emission intensity of Mg²⁺ doping to its addition is almost 3 : 1 and it is due to change in crystallinity.³⁴ Therefore; one can get larger emission intensity for doping the ions.

Fig. 11 Comparison of the emission intensity for 15 mol% doping and addition of Mg²⁺ ions in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphors on excitation with 976 nm radiation. The inset shows emissions in the UV and blue regions of the sample.

The visual perception realized by human eye is the only way to confirm the color emitted by the phosphor sample. However, it can be also expressed mathematically in two coordinate systems i.e. commission internationale de l'e'clairage (CIE) coordinates. The color emitted by a phosphor varies slightly with the concentration.^18,19 The CIE plots for different concentration of Mg²⁺ in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphors are shown in Fig. 12.

Fig. 12 CIE diagram for different concentration of Mg²⁺ doping in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphors.

The CIE coordinates for different concentration of Mg²⁺ in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphors have been calculated and they are summarized in Table 1. The table shows the color coordinates have a small variation for Mg²⁺ doping and the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphors emit intense green emission in absence as well as presence of Mg²⁺ ions.

Table 1 The CIE coordinates of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor in absence and presence of Mg²⁺ ions on excitation with 976 nm

Concentration of Mg^2+ (mol%)	CIE coordinates (x, y)
0	(0.28, 0.70)
5	(0.28, 0.70)
10	(0.28, 0.70)
15	(0.28, 0.70)
20	(0.28, 0.71)
25	(0.28, 0.71)

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3.3 Lifetime measurements

The lifetime measurements have been carried out in order to understand the factor responsible for an enhancement in the emission intensity via Mg²⁺ doping. The lifetime (τ) of the ⁵F₄/⁵S₂ level of Ho³⁺ ion is recorded on excitation with 976 nm radiation in absence and presence of Mg²⁺ ions and the decay curves thus obtained are shown in Fig. 13. The decay curves fit well with the single exponential profile with the following equation:
where, I₀ and I are the intensity at time 0 and t s, respectively and τ is the lifetime, which give lifetime of the ⁵F₄/⁵S₂ level of Ho³⁺ ion.

Fig. 13 Decay curves of ⁵F₄/⁵S₂ level of the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor in absence (a) and presence of Mg²⁺ ions (b–f) on excitation with 976 nm radiation.

The lifetimes thus obtained are given in the figure. It is found that the lifetime of the ⁵F₄/⁵S₂ level increases monotonically with Mg²⁺ doping up to 15 mol% and then decreases for higher concentration of Mg²⁺ ion.¹⁹ The enhancement in the lifetime of the ⁵F₄/⁵S₂ level of Ho³⁺ ion is a clear evidence for an increase in the emission intensity. The increase in the lifetime of the emitting levels may be due to an improvement in crystallinity, which enhances the rapid excitation, energy transfer and radiative transitions significantly.

4. Conclusions

The Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor has been synthesized through solid state reaction method. The XRD measurement confirms a shift in the peak position due to Mg²⁺ doping in the CaZrO₃ phosphor. The scanning electron micrographs show an increase in the particle size via Mg²⁺ doping. The Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor gives intense monochromatic green upconversion emission at 543 nm due to ⁵F₄/⁵S₂ → ⁵I₈ transition alongwith weak UV, blue, red and NIR emissions on excitation with 976 nm. The emission intensity is optimized with different concentrations of Yb³⁺ and it is optimum at 3 mol%. When Mg²⁺ ions are co-doped in the Ho³⁺/Yb³⁺ co-doped CaZrO₃ phosphor the emission intensity of green band is enhanced up to 4 times. This enhancement is due to shrinkage in the lattice parameters and an increase in the crystallinity. The lifetime of ⁵F₄/⁵S₂ level increases in presence of Mg²⁺ ions, which favors to give large emission intensity. The intensity of other bands is negligibly small compared to green band. Therefore, the Ho³⁺/Yb³⁺/Mg²⁺ co-doped CaZrO₃ phosphor can be a suitable candidate for monochromatic green light and optical devices.

Acknowledgements

One of the author, Ms Akanksha Maurya, is thankful to ‘UGC’ – ‘India’ for providing research fellowship. The authors acknowledge to Prof. O. N. Srivastava, Department of Physics, Banaras Hindu University, Varanasi for XRD and SEM measurement facilities. We also acknowledge to ‘DST’ – ‘India’ for providing the financial assistance (Grant No. SR/S2/LOP-023/2012).

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