Blue and green emission from Ho³⁺ doped zinc titanate phosphor thin films by sol-gel

After dip-coating, and in order to transform these specimens into thin films all of them were annealed at high temperatures ranging-over 600 °C–900 °C. The morphology of these specimens was investigated with the aid of SEM. The SEM images of 2 mol% Ho³⁺ ions doped specimens showed that the morphology of these thin films specimens was strongly affected by the annealing temperature. Figure 1 demonstrate images of specimens annealed over the aforementioned range. When the specimen annealed upon 600 °C, the film became denser with a negligible pore size and composed of the particles with near circles shape (figure 1(a)). These pores were totally disappeared at the highest annealing temperature such as (900 °C). At 700 °C annealing (figure 1(b)), nanoparticles with near cubic shape start grew and were became more crystalline upon annealing at 800 °C (figure 1(c)). The specimen has shown obviously grain boundaries for the near cubic shape when the annealing temperature was 900 °C (figure 1(d)). It should be noted that cracks appeared in (figure 1(d)) due to specimen cutting.

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Effects of dopant concentration on the structure of these thin films were further demonstrated using the energy dispersive x-ray spectroscopy (EDS). The analysis of the EDS elemental mapping for the specimens annealed at 800 °C confirmed the dense structure and the homogeneous distribution of oxygen, titanium, zinc and holmium in the prepared thin films (figure 2). It has to be noted that homogenous distribution of holmium in the thin film specimens presented that these Ho³⁺ ions were incorporated into the zinc titanate nanocrystals without observation of any segregation over all annealed specimens. More so, the EDS spectrum of these specimens (figure 3) revealed the presence of Ho peaks at ∼1.5 KeV and a negligible faint peaks over the range of 6-7 KeV, and also expose evenly distributed Zn, O, and Ti.

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Typical x-ray diffraction patterns for 1, 2, and 3 mol% of Ho³⁺ ions doped zinc titanate luminescence thin films processed using sol-gel dip-coating method and subsequently annealed in temperature ranging-over 600 °C–900 °C were depicted on figure 4. The phase analysis indicates that the doped thin film specimens annealed at 600 °C are a cubic crystal structure. The detected diffraction peaks were found corresponded and matched very well with (220), (311), (400), (422), (511), and (440) planes of cubic crystalline zinc titanium with a chemical formula ZnTiO₃, according to the data given by JCPDS card No.: 39–0190. The miller indices of the prominent peaks are represented in the bottom of (figure 4). When the annealing temperature increased upon 700 °C, the diffraction peaks of the specimens show increases in the intensity which indicates better crystallinity and increases of crystallite size without a change in the phase detected at 600 °C (figure 4(b)). The crystallinity of this phase was became better when annealed at 800 °C (figure 4(c)). Further increasing in annealing temperature upon 900 °C resulted in high crystalline films. The temperature dependence of the crystalline quality of these thin film specimens is checked from temperature dependece of the full width at half maximum (FWHM) using the dominant (311) diffraction peak demonestrated in figure 4. The FWHM of 1 mol% doped specimens was found to be 0.4622, 0.3417, 0.2651 and 0.1730 Å for the films annealed at temperature ranging over 600 °C–900 °C, respectively. This shows that the crystalline quality of the film is improved with increasing the annealing temperature. Furthermore, specimens annealed at 900 °C showed small impuruties existed on the films specimens (figure 4(d)). These impuruties were indexed to titanium dioxide anatase phase (A-TiO₂) at ∼25.6° (indicated by circle) according to JCPDS card No.: 53–0619, while the peak at ∼30.6° was assingned to Ho₂Ti₂O₇ cubic pyrochlore (indicated by asterisks) which was found to be matched with JCPDS card No.: 74—1984. However, the diffraction peak at ∼34° was scribed to be due to ZnO phase (indicated by cross) which was found matched very well with JCPDS card No.: 36—1451. Furthermore, the peak intensity for these impurities was increased with increasing the dopant concentration (figure 4(d)). On the other hand, the x-ray diffraction patterns for dominant phase (ZnTiO₃) were shows slight shift toward small angles (two-theta) with increasing Ho³⁺ ions concentration. This shift may be due to the difference in the radii between the dopant with that of Zn and/or Ti. Considering absent of impurities when annealing at lower annealing temperature in range of 600 °C–800 °C, indicating that Ho³⁺ ions were obviously homogeneously mixed and effectively doped into the host lattice and substituted Zn in Zn²⁺ site. In addition, because of larger size of the Ho³⁺ ions incorporated into the crystal lattice compared to that of Zn²⁺ upon annealing at aforementioned range the shift was occur.

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Debey—Scherrer equation [30] represented with (equation (1)) and the (311) diffraction peak were applied to estimate and investigate the influence of annealing temperature and dopant concentration on the average diameter of zinc titanate nanocrystals formed in the thin films specimens and the calculated results were listed in table 1.

$$\begin{eqnarray}&&D=\displaystyle \frac{k\lambda }{\beta \left(\cos \,\theta \right)}\end{eqnarray} \tag{ 1 }$$

Where D is crystal diameter, k is the Scherrer constant, λ (nm) is the wavelength of x-ray radiation, β (radian) is the full width at half maxima (FWHM) of the XRD peak, and θ (degree) is the diffraction angle. The lattice constant (a) and volume (V) of the nanocrystals were calculated using the following relations [31];

$$\begin{eqnarray}&&a=d\sqrt{{h}^{2}+{k}^{2}+{l}^{2}}\end{eqnarray} \tag{ 2 }$$

And;

$$\begin{eqnarray}&&V={a}^{3}\end{eqnarray} \tag{ 3 }$$

Where d is the interplanar spacing and h, k, and l are miller indices.

The lattice constant (a), and lattice volume (V) as well as interplane spacing that have been calculated from the x-ray diffraction patterns were found to be in a good agreement with that of the JCPDS card data. These results indicate that the specimens were built of highly crystalline thin films. However, the average diameter of ZnTiO₃ nanocrystals was infilunced by the annealing temperature. For example, the average diameter for 1 mol% Ho³⁺ ions doped specimens increased from 18 nm when annealed at 600 °C to 48.2 nm for 900 °C annealing temperature.

The transmission spectra of specimens annealed at different temperature in the range of 600 °C–900 °C were recorded in the range of 250 to 800 nm, and the results were illustrated in figure 5. The spectrum showed that the specimens have been built of highly transparent thin films in the visible region in an average of ∼90% when annealed at 600 °C. In addition, the transmittance of the specimens slightly decreased to reach an average of ∼85% when annealed at higher temperature such as 900 °C (figure 5(d)). However, the spectra showed sharp decreases in the transmission near the ultraviolet region at ∼365 nm. These absorption edges were determined by ZnTiO₃ nanocrystals formed in these thin films specimens. This value gives a possibility of efficient excited for these specimens with a wavelength below 365 nm. Considering the optical band gap of ZnTiO₃ nanoparticles (band gap 3.7 eV) [32], the detected absorption edge has small value. Which this red shift can be explained by (Quantum size effect) the difference in size of the nanocrystals [33]. Moreover, the specimens showed interference fringe in the transmission spectra. These fringes may be due to thicker films or weak absorption of light above the absorption edge [34]. However, no observation for influence of dopant concentration on the spectra.

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To understand the excitation paths of Ho³⁺ ions, the photoluminescence excitation (PLE) spectrum of Ho³⁺ ions doped these ZnTiO₃ phosphor thin films, with the emission monitored at room temperature of 546 nm, the results were illustrated in figure 6. Strong sharp excitation peaks were originated, which are commensurate with the f–f transitions from the ⁵I₈ ground state to the higher excited states of Ho³⁺ ions. The spectrum showed excitation bands which were appeared at ∼309, ∼360, and ∼430 nm and were correspond to the ⁵I₈ → ³D₃, ⁵I₈ → ³F₄, and ⁵I₈ → ³H₆ transitions of Ho³⁺ ions respectively [1]. The ⁵I₈ → ³D₃ transition showed constant intensity and position, which indicates that this transition was not influenced by the dopant concentration and the annealing temperature. However, the specimens annealed at 600 °C, 700 °C, even at 900 °C represented similar behavior. In which the ⁵I₈ → ³F₄ transition dominated the spectrum figures 6(a), (b), and (d)), while the spectra of specimens annealed at 800 °C was dominated by ⁵I₈ → ³H₆ transition figure 6(c).

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Annealing temperatures and dopant concentration induced structure evolution of these prepared thin films were also have a significant effects on the emission properties. Depending on the excitation results, and in order to investigate the effect of annealing and Ho³⁺ ions concentration on the PL properties, emission spectra of thin film specimens were recorded at room temperature under 430 nm light excitation (figure 7). Emission spectra of these specimens showed dependence on dopant concentration and annealing temperature. For specimens with 1 mol% of Ho³⁺ ions, three emission bands were observed ranging-over 500–800 nm. These bands peaking were located at green region ∼547 nm, red region ∼650 nm, and near infrared region ∼755 nm. Appearance of these band emission indicated that Ho³⁺ ions were incorporated into ZnTiO₃ nanocrystals, and these bands were ascribed to ⁵S₂, ⁵F₄ → ⁵I₈, ⁵F₅ → ⁵I₈, and ⁵I₄ → ⁵I₈ transitions of Ho³⁺ ions respectively. Green light and near-infrared emission occurred due to radiative relaxation to the ground state, while red light emission come from the multi-phonon relaxation through radiative relaxation from ⁵F₅ level to the ground state. Similar bands emission were observed in the spectra of specimens with 2, and 3 mol% of Ho³⁺ ions (figures 7(b), and (c)). However, the spectra showed a competitive emission between green light and red light depending on the dopant concentration. Of which, green emission dominated the spectrum, and emission intensity was increased with increasing annealing temperature upon 800 °C, then decreased when specimens annealed upon 900 °C. This decreasing in the intensity may be due to the increasing in the interaction between Ho³⁺ ions, which leads to non-radiative energy transfer from one Ho³⁺ ion to another Ho³⁺ ion. Or because of the fact that at higher annealing temperature the morphology of the thin films has become differ as shown in (figure 1), thus increasing the total internal reflections resulted in decreasing emission intensity of specimens. Moreover, infrared band emission showed approximately constant intensity (figure 7).

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In addition, emission properties of these thin film specimens were further investigated under 360 nm light excitation, and the result was illustrated in (figure 8). Three emission bands were observed covered blue, green, and red regions. Very strong blue emission was observed at around 450 nm attributed to ⁵F₁ → ⁵I₈ transition, green emission at ∼547 nm due to ⁵S₂, ⁵F₄ → ⁵I₈ transition, where a faint hump red emission at ∼645 nm revert to ⁵F₅ → ⁵I₈ transition of Ho³⁺ ions. However, the red light band emission centering at ∼645 (⁵F₅ → ⁵I₈) exhibit constant intensity and blue shift with around 5 nm of that was recorded under 430 nm excitation, while green light transition showed dependence on annealing temperature and Ho³⁺ ions concentration. More so, with increase in annealing temperature upon 800 °C, the ∼450 nm band blue emission intensity was further enhanced compared to that of green band, and finally dominated the whole emission spectra (figure 8). However, the specimen with 2 mol% of Ho³⁺ ions concentration was found has a higher emission intensity, which was considered as the optimal concentration.

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