Photoluminescence, Photoacoustic and Structural Characteristics of Polycrystalline Zn₂TiO₄: Ni²⁺ Semiconductor

Abstract

This work presents the investigation, at room temperature, of Zn₂TiO₄samples containing Ni²⁺ ions as substitutional impurities. Samples with molecular formula Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003, in which Zn²⁺ ions were replaced by Ni²⁺ ions, were prepared through solid-state method, crystalline data were obtained by X-ray diffraction, the elemental composition was determined by X-ray fluorescence and the optical properties were investigated through photoluminescence and photoacoustic techniques. The photoluminescence spectra showed broadbands from 640 nm to 800 nm. The emission of the Zn_2(1-x)Ni_2xTiO₄ samples was assigned to the overlapping of the Ni²⁺and the Zn₂TiO₄ emission. The most intense photoluminescence signal was obtained with excitation radiation at a wavelength of 360 nm for the sample with x = 0.001. The photoacoustic spectra showed absorption bands characteristic of semiconductor materials due to the host Zn₂TiO₄. The energy bandgap of the Zn_2(1-x)Ni_2xTiO₄was obtained from photoacoustic absorption spectra.

Keywords:
Photoluminescence, photoacoustic; Ni²⁺; semiconductor material

1. Introduction

Zn₂TiO₄ is a spinel-type compound, in which the Zn²⁺ ions are randomly distributed in tetrahedral and octahedral sites, while the Ti⁴⁺ ions are in octahedral sites¹. This compound presents interesting characteristics for different applications, such as photocatalytic material, used for degradation of organic pollutants, (for example, pesticides) in water². The basic photocatalytic process consists of the irradiation of Zn₂TiO₄ particles with energy higher than the bandgap, generating electron-hole pairs. This system starts chemical reactions capable of mineralizing the organic pollutants, in a similar way to photocatalytic degradation produced by TiO₂ of the Microcystin-LR (MC-LR), which is a toxin produced by cyanobacteria³. Other examples of Zn₂TiO₄ applications are dielectric resonators in microwave devices^4,5 used in ultra-high-frequency range for wireless communications, since the compound presents a low dielectric constant and high-quality factor⁶. Microwave dielectric properties of Zn₂TiO₄ and Zn₂TiO₄+TiO₂ were discussed in reference⁷, where the dependence of dielectric constant with a quantity of TiO₂ in the sample can be seen. Some works also report the system in use as white color pigment⁸.

Another important characteristic is the photoluminescence observed from

Zn_2(1-x)Ni_2xTiO₄. When the host cations are substituted by transition metal ions, the Zn₂TiO₄ can present broadbands in the visible and near-infrared region at room temperature. For example, the photoluminescence at room temperature of the Zn₂TiO₄ obtained through solid-state method presents a broad and intense band from 680 nm to 800 nm^8,9. Zn₂TiO₄:Co²⁺ shows a broadband in the red-infrared region characteristic of Co²⁺ ions in tetrahedral sites¹⁰, and when the host contains substitutional Mn²⁺ ions the system presents two bands in the green and red regions due to electronic transitions of the Mn²⁺ ions in tetrahedral and octahedral sites, respectively⁹. Zn₂TiO₄ also presents a reddish-orange luminescence when lanthanides like Sm³⁺ ions are inserted in the host¹¹. Apart from the several interesting properties, Zn₂TiO₄ can be easily obtained through the conventional solid-state reaction of 2ZnO-TiO₂ without any additional sintering processes^8-10.

The optical properties of Ni²⁺ have been investigated for many years. For example, the MgO: Ni²⁺ was identified as an excellent tunable solid-state laser system¹², systems MgF₂:Ni²⁺ (emitting from 1610 nm to 1740 nm at 80 K) and Gd₃Ga₅O₁₂:Ni²⁺ (emitting from 1434 nm to 1520 nm at 100 K)¹³ were classified as laser materials, but the lasing action in these materials occur at low temperature. Nowadays Ni²⁺-doped transparent nanoglass ceramics (nano-GC) containing Ga₂O₃ nanocrystals have been investigated by ultra-broadband emission at the near-infrared region due to the addition of Nd³⁺ and Yb³⁺, because energy-transfer processes result in an enhancement of Ni²⁺ emission¹⁴. The ZnGa₂O₄:Ni²⁺ inserted in glass-ceramics showed a strong near-infrared emission, pointing to a promising use as broadband fiber amplifiers¹⁵. Beyond near-infrared emission, materials containing Ni²⁺ impurities also present emission in the visible region. For example, photoluminescence spectra in blue, green and red regions were observed in Y(OH)₃:Ni²⁺ and cubic Y₂O₃:Ni²⁺ phosphors¹⁶, violet and blue emissions were observed in polyvinyl alcohol cadmium telluride nanoparticles containing Ni²⁺¹⁷, and an emission in the red region was observed in MgGa₂O₄:Ni²⁺¹⁸. In addition to the near-infrared emission, the Ni²⁺ ion is very stable, therefore, valence changes during thermal processes are not expected. As a consequence, valence control is not necessary during materials synthesis, which means atmosphere control is expendable during the thermal processes¹⁹.

Besides the achievement of the emission and excitation bands, the determinations of the valence state and of the type of substitutional element are also necessary to understand quantum processes in an emitting sample. Photoluminescence is a very appropriate technique for these, because it is 10⁴ times more sensitive than others techniques that are also capable of revealing this information, as the X-ray photoelectron spectroscopy (XPS), for example^20,21. Therefore, in this paper the photoluminescence was the technique used to confirm both, the type and the valence of the emitting substitutional ion, besides the symmetry of the occupation site.

By the facile production of the semiconductor Zn₂TiO₄, together with photoluminescence possibilities from Ni²⁺ substitution, this work reports on the Zn₂TiO₄:Ni²⁺ produced by the solid-state method and its investigation through X-ray diffraction, photoluminescence, excitation, and photoacoustic spectroscopies at room temperature. This investigation was motivated by the application possibility of the broadband observed in a potential lasing system at room temperature.

2. Materials and Methods

Samples with molecular formula Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002, and 0.003, respectively, were prepared through solid-state method using raw compounds ZnO (Carlo Erba), TiO₂ (B. Herzog), and Ni(OH)₂ (Sigma Aldrich). The components of each sample were weighted in accordance with the desired stoichiometric reaction and mixed into an agate mortar to produce a homogeneous powder. During the next step, each admixture powder was pressed under 4 t into three pellets, with 13 mm diameter and 2 mm thickness, with a mass of about 0.7 g. The pellets were put into alumina crucibles and heated in an atmospheric pressure oven at 1200 ^oC for 6 hours. The oven was turned off and the samples were cooled to room temperature by furnace inertia. The Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, presented a slight yellow color while the Zn_2(1-x)Ni_2xTiO₄, where x = 0.0007, 0.001, 0.0013, 0.002 and 0.003 presented a green color. The concentration of Ni²⁺ in the samples is the fraction of zinc (Zn²⁺) ions which have been replaced with nickel (Ni²⁺) ions.

The sample coloration is formed by the components of visible light, which are not absorbed in the sample surface. The observed color is the “complementary color” of the absorbed radiation. The yellow color of Zn₂TiO₄ pure sample (without Ni²⁺) is due to the absorption at the sample surface, of the wavelengths in the blue region of the spectrum. Since blue and yellow are complementary colors, the yellow color is reflected in sample surface, reaching the detector (eyes)²². The color of Ni²⁺-samples will be explained in more details in the Results section.

The X-ray diffraction was acquired at room temperature by a Bruker D2-PHASER powder-diffractometer (40kV, 40mA) using Cu-Kα₁ radiation with a wavelength of 1.5406 Å. The obtained data were refined by the Rietveld method using the FullProf package²³ and a pseudo-Voigt (pV) profile function to fit the diffracted peaks. The data were compared with the ICSD (Inorganic Crystal Structure Database)²⁴. Lattice parameters and atomic positions were determined from the refinement.

The X-ray fluorescence measurements were obtained by an Artax 200 X-Ray Fluorescence Spectrometer (30 kV, 200 μA, and a molybdenum anode) and used to identify the atomic elements present in the samples.

The measurements of time-resolved photoluminescence and excitation were performed with a spectrofluorometer PTI QuantaMaster 300-Plus equipped with a 75 W pulsed Xenon lamp with a spectral resolution of 2 nm in the visible region. For phase-resolved photoluminescence measurements, an Acton AM 510 spectrometer with a spectral resolution of 1 nm was used. The excitation source was a 532 nm, 50 mW Coherent Compass 215M laser, modulated by a Newport 75160 variable speed chopper. The signal was detected by a Newport Oriel 77348 photomultiplier, amplified and analyzed with a Princeton 5209 lock-in. The scattered excitation light was blocked with Newport optical filters.

The photoacoustic spectra were obtained using a Newport 300 W Xe lamp as the radiation source and an Oriel 77200 monochromator with a spectral resolution of 10 nm to select the excitation wavelength. An Oriel Model 75095 chopper was used for the lamp light chopping at 10 Hz. The signal was detected with a B&K model 4943 microphone in a homemade photoacoustic cell with a quartz window. The signal analysis was done with a Stanford Research Systems Model SR830 lock-in amplifier. All spectra were corrected by the response of the detection system.

3. Results

Figure 1 shows the X-ray diffraction and the Rietveld refinements results for the Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003. The circles represent the measured data and the solid line represents the adjustment obtained from a pseudo-Voight function of Rietveld refinements. The first set of sticks (at the top) indicates the Bragg diffraction positions of Zn₂TiO_4, and the second set of sticks (at the bottom) is related to the Bragg diffraction positions of the ZnO phase. The bottom horizontal solid line is the difference between the measured diffraction and the calculated data obtained from Zn₂TiO₄ PDF 860156 (ICSD 080851)²⁵ and ZnO PDF 800075 (ICSD 067849)²⁶. Results from X-ray data refinements are exhibited in Tables 1 and 2.

Figure 1
Room temperature X-ray diffraction and Rietveld refinement results of Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003. Bragg positions are related to the Zn₂TiO₄ (top) and ZnO (bottom) phases.

Table 1
Structural properties and Rietveld refinement parameters for cubic Zn2(1-x)Ni2xTiO4, where x =0.00, 0.0007, 0.001, 0.0013, 0.002, and 0.003.

x	0.00	0.0007	0.001	0.0013	0.002	0.003
I) Main phase
Zn2TiO4 (%)	98.46	98.78	98.38	100	100	100
Space group	$Fd\overline{3}m$	$Fd\overline{3}m$	$Fd\overline{3}m$	$Fd\overline{3}m$	$Fd\overline{3}m$	$Fd\overline{3}m$
Crystal system	Cubic	Cubic	Cubic	Cubic	Cubic	Cubic
a = b = c (Å)	8.467(1)	8.471(0)	8.474(5)	8.474(7)	8.474(6)	8.475(2)
V(Å3)	607.02	607.86	608.61	608.65	608.63	608.76
(2 θ)311(deg.)	35.17	35.16	35.15	35.18	35.10	35.10
FWHM(311) (deg.)	0.075	0.127	0.087	0.107	0.132	0.148
D(311)(nm)	2139.08	757.56	1412.12	969.74	727.13	624.15
ε x(10-3)	0.53	1.51	0.81	1.18	1.58	1.84
δ x(10-3)	0.46	1.32	0.71	1.03	1.38	1.60
II) Residual phase
ZnO (%)	1.54	1.22	1.62	-	-	-
Space group	P63mc	P63mc	P63mc	-	-	-
Crystal system	Hexagonal	Hexagonal	Hexagonal	-	-	-
a = b (Å) c (Å)	3.249(3) 5.207(3)	3.253(6) 5.190(5)	3.249(3) 5.207(3)	- -	- -	- -
V(Å3)	47.61	47.58	47.61	-	-	-
III) Agreement factors
Rwp	10.2	11.6	10.5	16.0	11.6	10.8
Rp	10.9	11.4	10.6	25.2	9.7	8.93
Rexp	7.15	7.99	7.95	9.0	8.8	9.02
GOF	1.4	1.4	1.3	1.8	1.3	1.2
χ2	2.05	2.1	1.76	3.17	1.71	1.4

Table 2
Atomic positions obtained from Rietveld refinements.

Atom	Ox State	Wyck Symb	x	y	z
Zn2TiO4
Zn1	+2	8 a	0.125	0.125	0.125
Ti1	+4	16 d	0.500	0.500	0.500
Zn2	+2	16 d	0.500	0.500	0.500
O1	-2	32 e	0.2616(7)	0.2616(7)	0.2616(7)
Zn2TiO4: Ni2+ (0.07%)
Zn1	+2	8 a	0.125	0.125	0.125
Ti1	+4	16 d	0.500	0.500	0.500
Zn2	+2	16 d	0.500	0.500	0.500
O1	-2	32 e	0.2572(3)	0.2572(3)	0.2572(3)
Zn2TiO4: Ni2+ (0.10%)
Zn1	+2	8 a	0.125	0.125	0.125
Ti1	+4	16 d	0.500	0.500	0.500
Zn2	+2	16 d	0.500	0.500	0.500
O1	-2	32 e	0.2585(2)	0.2585(2)	0.2585(2)
Zn2TiO4: Ni2+ (0.13%)
Zn1	+2	8 a	0.125	0.125	0.125
Ti1	+4	16 d	0.500	0.500	0.500
Zn2	+2	16 d	0.500	0.500	0.500
O1	-2	32 e	0.2580(9)	0.2580(9)	0.2580(9)
Zn2TiO4: Ni2+ (0.20%)
Zn1	+2	8 a	0.125	0.125	0.125
Ti1	+4	16 d	0.500	0.500	0.500
Zn2	+2	16 d	0.500	0.500	0.500
O1	-2	32 e	0.2581(9)	0.2581(9)	0.2581(9)
Zn2TiO4: Ni2+ (0.30%)
Zn1	+2	8 a	0.125	0.125	0.125
Ti1	+4	16 d	0.500	0.500	0.500
Zn2	+2	16 d	0.500	0.500	0.500
O1	-2	32 e	0.2580(4)	0.2580(4)	0.2580(4)

The GOF-index (Goodness-of-Fit) indicates the discrepancy between the observed and expected values of X-ray diffraction data. The desired value of the GOF-index is 1.0²³. The values for the Zn_2(1-x)Ni_2xTiO₄, where x =0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003, obtained from the Rietveld refinement indicate the correct identification of the sample phase(s) and of the crystallographic structure (space group). Table 1 shows that for the investigated Ni²⁺ concentration interval, the space group, the cell parameter and the cell volume are not significantly affected when the Ni²⁺ ion is introduced in the host. This fact can be explained by the low Ni²⁺ concentration of the sample. The lattice parameters obtained from Rietveld refinements (Table 1) indicate samples with a cubic (face-centered) space group. Due to the small difference between the values shown in Table 1 (< 0.3%), the cell volume of Zn₂TiO₄ and Zn_2(1-x)Ni_2xTiO₄ samples can be considered unchanged when Ni²⁺ ion is inserted in the host²⁷.

Table 1 also shows the values of crystallite size (D), micro-strain (ε) and dislocation density (δ) parameters for all samples, obtained from X-ray diffraction measurements^28,29. It can be seen that the crystallite size decreases when the Ni²⁺ concentration increases while micro-strain and dislocation density increase with the increasing of Ni²⁺ concentration, except for the 0.07% sample. The decrease of the average crystallite size with the increasing of the impurity ion concentration is related with a worsens of the crystallinity of samples²⁸. Besides this, the micro-strain and dislocation density parameters reflect in the same way the crystalline quality of samples³⁰. Table 1 shows that the dislocation density increases with Ni²⁺ concentration, which reflects an increasing of lattice imperfections due to the increase of micro-strains in the samples³¹. Therefore, the increasing of the dislocation density and of the micro-strain in the samples is in accordance with the decreasing of crystallite size, indicating a lower crystalline quality of the Zn₂TiO₄:Ni²⁺samples when compared to the Zn₂TiO₄ sample. The changes in these specific crystal parameters are associated with the ionic radii differences between host ion Zn²⁺ and the substitutional impurity Ni²⁺, which inserts small defects in the host, however without space group, cell parameter or cell volume changes in the studied interval of Ni²⁺concentration, as mentioned above.

Table 2 shows the atomic positions obtained from Rietveld refinements. Zn²⁺ ions are in two non-equivalent atomic positions: Zn1 in 8a-Wyckoff position (tetrahedral symmetry) and Zn2 in 16d-Wyckoff position (octahedral symmetry). Ti⁴⁺ ions have the same atomic coordinates as Zn2 ions, showing that the octahedral position is common to the two ions. Therefore, the molecular system can be represented by [Zn]^tet[Zn,Ti]^octO₄. Ionic compounds as Zn₂TiO₄ are typically neutral. In this way, the charges of the cations balance out the anions charges. The valence states of the elements of the molecular formula are determinate by a condition of maximum stability.

This rule determinates that an ion is more stable when it has eight valence electrons (octet rule). The electronic configuration of the Zn is [Ar]3d¹⁰4s², in which the electron configuration of Argon gas [Ar] has a total of eight electrons (3s² 3p⁶) in the valence shell. When the Zn atom loses the two electrons of the 4s² shell, becomes the more stable Zn²⁺cation. The Ti atom with electronic configuration [Ar]3d² 4s² loses four electrons and became Ti⁴⁺ with stable electronic configuration [Ar]. The oxygen anions has six electrons in the valence shell (electronic distribution 1s² 2s² 2p⁴). Therefore, it gains two electrons for the 2p shell, achieving the stable valence O^2-. In Zn₂TiO₄ formation the raw oxides were, as mentioned in experimental section, $Z{n}^{2+}{O}^{2-}$ and ${\text{Ti}}^{4+}{\text{O}}_2^{2-}$, both contain elements in more stable valence states. Therefore, during the ZnO + TiO₄ →Zn₂TiO₄ processes, the valence is preserved and the electrostatic attraction between cations and oxygen leads to the ionic bond between them. Therefore, as the chemical equilibrium is the stronger condition for ionic solid formation, this fact confirms the valence of the ions Zn²⁺, Ti⁴⁺ and O^2- in the host.

The crystal radii of the ions in octahedral coordination are: $r_{T{i}^{4+}}=$ 0.745 Å, $r_{Z{n}^{2+}}=$0.88 Å, and $r_{N{i}^{2+}}=$ 0.83 Å, while in the tetrahedral coordination the crystal radii are $r_{Z{n}^{2+}}=$ 0.74 Å and $r_{N{i}^{2+}}=$0.69 ų². Because of the same valence and similar crystal radii, it is expected that Ni²⁺ ions occupy Zn²⁺ octahedral or/and tetrahedral sites in Zn₂TiO₄. However, the green color of the Zn₂TiO₄:Ni²⁺ sample indicates the occupation of Ni²⁺ ions in octahedral sites^18,33-35.

The green color can be explained as follows. Since the sample has a green color this means that red color is absorbed in sample surface, because the red and the green are complementary colors. Besides this, the green color also means that the light with wavelength in the blue region of the spectrum is also absorbed at the sample surface. Therefore, the green color is a consequence of the absorption of the red and blue lights by sample surface. The energy absorbed by the Ni²⁺ ion in a coordination complex corresponds to the spacing between the 3d⁸-configuration energy levels involved in the electronic transition. In consequence, the color of the sample also depends upon this spacing. However, this spacing depends on the valence state of the central cation, the cation site symmetry (octahedral or tetrahedral), and the type of ligands (O^-2, F^-1, Cl^-1, for example). When a Ni²⁺ ion is in a strong octahedral crystalline site, the spacing between 3d energy levels (t_2g and e_g) is large³⁵. Therefore, in this transition, the ion Ni²⁺ absorbs a high-energy radiation, and the color of the sample will be yellow. If this cation is in an octahedral site, weaker than the first example, the spacing between 3d energy levels is smaller than for the earlier case³⁵ and the absorbed radiation corresponds to the yellow-red region of the spectrum. In consequence, the sample acquires a green color. When the Ni²⁺ ion is in a tetrahedral site, the electronic configuration corresponds to the 3d² in octahedral coordination³⁴. The energy levels distribution of 3d² configuration indicates that for weak crystalline field, the spacing between 3d (t₂ and e) energy levels is small. Therefore, the absorbed energy is in the yellow-red region and the samples acquire colors as blue and turquoise, for example³⁶. The above considerations allow affirming that the photoluminescence technique is appropriate to identify both, the type and the valence of the emitting substitutional ion, besides the symmetry of the occupation site.

Room temperature X-ray fluorescence (XRF) for (a) Zn₂TiO₄ and (b) Zn₂TiO₄:Ni²⁺ (0.1% of Ni²⁺) samples (Figure 2) only showed peaks related to the atoms Zn and Ti. The measurements were performed in several regions on the samples’ surfaces, with similar results showing the homogeneity of the samples. The lack of Ni²⁺ signal can be explained by the low concentration in the samples.

Figure 2
Room temperature X-ray fluorescence (XRF) for (a) Zn₂TiO₄ and (b) Zn₂TiO₄:Ni²⁺ (0.1% of Ni²⁺).

Figure 3 shows the photoacoustic absorption spectra at room temperature of the Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003. All spectra present a strong absorption below 400 nm that explains the yellow color of the Zn₂TiO₄ sample, because when the color absorbed is the violet, the color seen is yellow. Besides this, all the Zn₂TiO_4:Ni²⁺ samples show an absorption band from 380 nm to 450 nm, and a weak red absorption band. The absorption at red and blue regions of the spectrum explains the green color of samples containing Ni²⁺.

Figure 3
Room temperature photoacoustic absorption spectra of Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003, obtained with an Xe lamp modulated at 10 Hz.

The photoacoustic spectrum shows all absorber centers in the material, therefore, the band from 380 nm to 450 nm can be attributed to the Ni²⁺ absorbing center in the samples. Because of the strong preference for this kind of site, Ni²⁺ ions occupation in octahedral Zn²⁺ sites are to be expected. But the crystal field transitions^34,35,37 from Ni²⁺ in the photoacoustic spectrum were not observed, because they were strongly overlapped by the absorption of the host. A photoacoustic spectrum similar to that shown in Figure 3 was observed for Zn₂TiO₄ thin films obtained by the sol-gel method³⁸.

The semiconducting character of the samples was identified using the photoacoustic technique, by determining the energy gap using the Tauc method³⁹. The method was originally used for analyses of amorphous materials, but recently its applicability to polycrystalline semiconductor materials was verified⁴⁰. The energy gap can be determined from spectra fitting the photoacoustic data through Equation 1 ³⁹:

$${\left(PAS\times hv\right)}^n=\alpha \left(hv-E_G\right)$$ (1)

where PAS is the normalized photoacoustic signal intensity as function of the energy hv of the absorbed photon,his Planck's constant, E_G is the optical gap of the material, α is a proportionality constant and the exponent n characterizes the transition, as such: n = 2 (allowed and direct transitions), 1/2 (allowed and indirect transitions), 2/3 (forbidden and direct transitions) and 1/3 (forbidden and indirect transitions)⁴¹.

Figure 4 shows the plot of the (PAS×hv)ⁿ against the energy in eV units of the absorbed photon, in which the better result n = 2 is fitted.

Figure 4
(PAS×hv)² data (black dots) against the energy of the absorbed photon, for Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003. The energy of the absorption transitions were plotted on the horizontal axis. The lines positions on the horizontal axis indicate the bandgap energy of the samples.

Therefore, the samples obtained in this work are semiconductors of allowed and direct transition. The bandgap energies obtained for Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003 were 3.140 eV, 3.085 eV, 3.077 eV, 3.054 eV, 3.032 eV, and 3.023 eV, respectively. These values correspond to wavelengths: 394 nm, 402 nm, 403 nm, 406 nm, 409 nm, and 410 nm.

The lowering of the energy gap of the doped sample was also observed for the Zn₂TiO₄:Fe³⁺⁴². In that paper, Jang et al. showed that Fe³⁺ substitution reduced the bandgap of Zn₂TiO₄ and leads to a photocatalytic activity in the system under visible light irradiation. The lowering of the band gap investigated in this paper would be associated to changes in photoluminescence, as discussed below.

The decreasing of the gap energy with the increasing of substitutional Ni²⁺ concentration could be due to: a) transfer-charge effects from Ni²⁺ to host; b) enhancement of the interactions between orbitals of the localized Ni²⁺ d-electrons and sp-orbitals of Zn²⁺ ions⁴³; c) changes in sample surface morphology in consequence of the Ni²⁺ substitutional concentration increasing, that can lead to an enhancement of scattering from surface of the higher energy radiation, followed by a decreasing of the absorption⁴⁴, and/or d) creation of a new energy level below the Zn₂TiO₄ conduction band, due to the Ni²⁺ ions. These energy levels lead to absorption of lesser energy photons, which decay through d-d transitions and transitions to the valence band of the host Zn₂TiO₄⁴⁵.

Sometimes when an impurity ion is inserted in a semiconductor, an energy inter-band between the valence and conduction band can arise. In this case, the impurity inserted in the host reduces the energy gap and the optoelectronic properties of the sample are enhanced, as such, there is an improvement to photoluminescence with impurity concentration⁴⁶. In order to verify the way in which the lowering of the band gap in addition with crystallite size (D), micro-strain (ε), and dislocation density (δ) parameters change the emission, photoluminescence spectra are presented in what follows.

Figure 5 shows room temperature phase-resolved emission spectra for the Zn₂TiO₄ sample (open circles) and the Zn₂TiO₄:Ni²⁺ (0.1% Ni²⁺) (full circles). The spectra were obtained with a continuous 532 nm wavelength laser line chopped at 200 Hz as the excitation source. The 200 Hz chopper frequency is equivalent to a temporal window of 5 ms on and 5 ms off samples illumination. The phase of the signal was adjusted in the lock-in to obtain the maximum signal of the luminescence band observed.

Figure 5
Room temperature emission spectra for Zn₂TiO₄ sample (open circles) and for the Zn₂TiO₄:Ni²⁺ (0.1% of Ni²⁺) (full squares) excited with 532 nm continuous laser, modulated at 200 Hz.

The spectrum of the Zn₂TiO₄ consists of an inhomogeneous broadband extending from 630 nm to 800 nm, with a maximum intensity at 718 nm and weak shoulders at 705 and 730 nm. The inhomogeneous broadening of the emission band can be explained by the degree of disorder of the sample⁹.

The spectrum of the Zn₂TiO₄:Ni²⁺ also consists of an inhomogeneous broadband extending from 640 nm to 800 nm with a maximum intensity at 680 nm and weaker shoulders at 690 nm and 718 nm. These shoulders were also observed in the Zn₂TiO₄ sample, therefore, they can be attributed to the host Zn₂TiO₄, while the peak at 680 nm can be assigned to the nickel emission. The band extends beyond 800 nm, outside our experimental limits.

The most common valences of nickel ion are 2+ and 3+. Ni³⁺ ions have a 3d⁷ electronic configuration and their emission is often assigned to bands in the near-infrared region. For example, the ZnO:Ni³⁺ presented a band with maximum intensity at 1.5 µm⁴⁷ and the ZnS:Ni³⁺ showed infrared emission from 1.8 µm to 2.2 µm⁴⁸, both very far from the emission with barycenter at 680 nm as observed in Figure 5. On the other hand, Ni²⁺ emissions are often observed at the red-infrared region, below 700 nm, therefore, the band with barycenter at 680 nm in the near infrared region is assigned to the ¹T₂(¹D) →³T₂(³F) transition of Ni²⁺ ions in octahedral sites^18,34,35,37. Additional nickel transitions are expected at the green and infrared regions, attributed to the ¹T₂(¹D) →³A₂(³F) and³T₂(³F) →³A₂(³F) transitions of Ni²⁺in octahedral sites, respectively. These bands were not observed probably because they are totally overlapped by the host emission, since a broad emission band of Zn₂TiO₄ from 450 nm to 800 nm was observed in reference⁸.

As the concentration of ZnO is about 1.6%, which is much higher than Ni²⁺ concentration in the 0.1% Ni²⁺ sample, the possibility of an emission band due to ZnO or ZnO:Ni²⁺ was considered. The emission of ZnO was referred to the blue⁴⁹ and green region⁵⁰, while emission from ZnO:Ni²⁺ was also reported at blue and green spectral regions⁵¹. Therefore, the highly probable explanation of the optical properties of the sample is the assignment of these to Ni²⁺ centers in the Zn₂TiO₄ host, because the emission is in the red spectral region.

Figure 6 shows the room temperature time-resolved emission spectra of (a) Zn₂TiO₄ and (b) Zn₂TiO₄:Ni²⁺ (0.1% Ni²⁺) with excitation radiation from a pulsed Xenon lamp with a repetition rate at 200 Hz, using 360 nm, 480 nm, and 532 nm as excitation wavelengths. Time-resolved emission was obtained in scans with a lamp flash rate of 200 Hz (temporal window of 5 ms), 600 pulses averaged per wavelength and scan step size of 1 nm. All measurements shown in Figure 6a and b were performed under the same instrumental conditions.

Figure 6
Room temperature time-resolved emission spectra of (a) Zn₂TiO₄ and (b) Zn₂TiO₄:Ni²⁺ (0.1% of Ni²⁺), both with a lamp repetition rate of 200 Hz, obtained with excitation wavelengths of 360 nm, 480 nm, and 532 nm.

Figure 6a shows the most significant spectra among the measurements performed with a wavelength excitation set. The most intense photoluminescence signal was obtained with excitation radiation at a wavelength of 360 nm, where the emission band extends notably further than the measurement limits. The 532 nm wavelength excitation produces an emission band three times less intense than that produced with 360 nm wavelength excitation. These photoluminescence spectra can be attributed to electronic recombination effects, emission from energy levels of structural defects close to the bandgap, and from atomic order-disorder effects⁸.

Figure 6b shows the emission of the Zn₂TiO₄:Ni²⁺ (0.1%) sample excited with 360 nm, 480 nm, and 532 nm. The intensity of the band excited with 360 nm decreases to approximately a half-height of the similar band in the Zn₂TiO₄ sample exhibited in Figure 6a. Moreover, the emission band in the sample Zn₂TiO₄ is broader than the emission of the Zn₂TiO₄:Ni²⁺ (0.1%) sample. This fact indicates that: a) some quantity of absorbed energy by Zn₂TiO₄ is transferred from host to Ni²⁺ ions, decreasing the intensity of the emission band; and/or b) Ni²⁺ ions easily absorb the excitation light, and decays to predominantly lower energy levels by non-radiative transitions, leading to a decrease of emission intensity. In Figure 6b a band at 680 nm can also be seen. This band became more intense with 480 nm excitation. Excitation radiation with 360 nm and 532 nm wavelengths favors the emission at 718 nm (from the host) and 680 nm (from the Ni²⁺ centers), respectively. The excitation with 480 nm, in the same way, favors the emission at 680 nm and 718 nm, and 532 nm is the excitation wavelength that favors less the host emission at 718 nm.

In order to verify the emission intensity dependence with Ni²⁺ concentration, room temperature photoluminescence measurements were performed for samples Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003. These values mean that x = 0.0007 (0.07%) of the number of Zn²⁺ ions were substituted for Ni²⁺ ions, and so on up to the x = 0.003 (0.30%) value.

The results are shown in Figure 7. For the measurements, the excitation radiation used was 480 nm, because this wavelength favors both bands at 680 nm (Ni²⁺) and 718 nm (host), as can be seen in Figure 6b.

Figure 7
Room temperature emission spectra for Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003. These values mean that 0.07% of Zn²⁺ ions were substituted for Ni²⁺, and so on up to the 0.30%.

The results will be compared to those shown in Table 1. The X-ray diffraction data showed a decreasing of the quality of crystallization with the increasing of the Ni²⁺ concentration. At the same time, the luminescence intensity was drastically reduced with the Ni²⁺ quantity increasing. It is well-known that the photoluminescence intensity increases/decreases with the increasing/decreasing of the crystallite size^29,52 Therefore, in this work, the reduction of the photoluminescence intensity with the increasing of the Ni²⁺ concentration is in accordance with structural characteristics of samples, since the Ni²⁺ concentration increasing leads to a lowering of the crystallization quality of the samples. Due to the low Ni²⁺ concentration, the photoluminescence intensity is associated to transitions between d-d states of Ni²⁺ and transitions between Zn₂TiO₄ host and d states of Ni²⁺ ions.

The lowering of the energy of the absorption edge (red shift) of the Ni²⁺ samples, verified in Figure 4, suggests a transfer energy between Zn₂TiO₄ host and d states of Ni²⁺ ions⁵³. Furthermore, a decrease in crystallite size indicates a degree of coupling of d orbitals of Ni²⁺ with energy levels of the host⁵⁴. Therefore, the increasing of the Ni²⁺concentration leads to the lowering of the crystalline quality, which in turn leads to both: a shift of the absorption edge to the red region and a reduction of the photoluminescence intensity.

Figure 7 shows that the higher intensity of photoluminescence emission among the investigated concentrations comes from the 0.1% Ni²⁺ sample excited with 480 nm. The emission of the 0.07% Ni²⁺ sample is lower than the emission of the Zn₂TiO₄ sample. This fact can be explained by taking into consideration that the Ni²⁺ ions are also absorbing centers for the radiation emitted by host Zn₂TiO₄. At the same time, the optimum Ni²⁺ concentration value for photoluminescence was not reached during the interval 0.00%-0.07% of the Ni²⁺ ions. Therefore, the emission intensity of the host was reduced by increasing of Ni²⁺ concentration and the emission from Ni²⁺ transitions is still weak.

However, the emission intensity increase when the Ni²⁺ concentration increases from 0.07% to 0.1% indicates that the radiative relaxation processes of Ni²⁺ become competitive in this concentration interval.

For concentrations higher than 0.1%, the intensity of the whole band decreases, indicating that the weakening of the emission could be due to energy transfer processes between host and Ni²⁺ ions. Due to the low Ni²⁺ concentration, and due to the weak absorption in the spectral region above 500 nm, as can be seen in photoacoustic spectra (Figure 3), the occurrence of Ni²⁺-Ni²⁺ energy transfer is not expected. In other words, emitting radiation by a Ni²⁺ ion being absorbed by Ni²⁺ neighborhoods’ ions is not expected.

Figure 7 also shows that the band at 680 nm (Ni²⁺) decreases quickly when compared to the band at 718 nm (host), and the whole band decreases with the Ni²⁺ concentration increase. This effect is in accord with the earlier hypothesis about Figure 6b, in which some quantity of absorbed energy by Zn₂TiO₄ is transferred from host to Ni²⁺, followed by the Ni²⁺ ions decay by predominantly non-radiative transitions, leading to a decrease of emission band intensity. In Figure 7 the Ni²⁺ concentration of 0.30% of Ni²⁺ can be considered as the concentration quenching value for Zn₂TiO₄:Ni²⁺ among the Ni²⁺ concentration investigated in this work.

In the investigation of luminescent material, an important characteristic is the quantum efficiency of the emission. The quantum efficiency (φ) of photoluminescence is defined in Equation 2 ⁵⁵:

$\phi =\frac{n_r}{n_{abs}}$ (2)

where $n_r$ and $n_{abs}$ are the number of emitted and absorbed photons, respectively. The quantum efficiency of Zn₂TiO₄:Ni²⁺ (0.1%) (higher intensity emitting among the Ni²⁺ samples) can be roughly estimated, considering the intensity of the luminescence at 680 nm in Figure 7 and the intensity of the incident light (excitation radiation) which reaches the sample surface as absorbing radiation. The quantum efficiency of emission excited with 480 nm was φ = 0.6, while for the emission excited with 532 nm wavelength the value was φ = 0.3.

Materials in which the energy transitions occur between energy levels with different spin multiplicity (spin-forbidden transitions) in general have quantum efficiency lower than those materials in which the transitions are between energy levels with the same spin multiplicity (spin-allowed)⁸. Also, it is known that low quantum efficiency indicates strong luminescence quenching due to the non-radiative decay processes⁵⁶. However, some spin-forbidden transitions can have high quantum efficiency if non-radiative processes or the interaction between Ni²⁺ ion and host are not significant⁴⁶.

The difference between the quantum efficiency of emissions obtained with 480 nm and 532 nm wavelength excitation can be justified: the 480 nm wavelength excited the Ni²⁺ ions and host in the same way, while the 532 nm wavelength excited preferably Ni²⁺ ions. The ¹T₂(¹D) → ³T₂(³F) transition assigned to the Ni²⁺ emission in this paper is a spin-forbidden transition. Moreover, the emission spectra analysis indicates the possibility of energy transfer processes between Ni²⁺ and Zn₂TiO₄ hosts. Therefore, a lower quantum efficiency of Zn₂TiO₄:Ni²⁺ emission with an excitation wavelength exciting preferably Ni²⁺ ions rather than the Zn₂TiO₄ host is expected. This fact leads to evident losses by non-radiative energy transfers between Ni²⁺ ions and the host.

The quantum efficiency obtained for the Zn₂TiO₄:Dy³⁺ and Zn₂TiO₄:Cr³⁺ were φ = 0.58⁴⁶ and 0.53⁸, respectively. These values are close to the Zn₂TiO₄:Ni²⁺ value (0.6). This fact indicates that the emission quantum efficiency is due mainly to the host Zn₂TiO₄ radiative processes.

Comparing the quantum efficiency of Zn₂TiO₄:Ni²⁺ (0.1% Ni²⁺) with those of similar samples, and by the broad and intense emission band, this sample can be considered a possible potential candidate as a tunable active medium at room temperature.

Figures 5-7 show that the Zn₂TiO₄:Ni²⁺ photoluminescence is an overlap of the emissions from the Zn₂TiO₄ host and the emission assigned to the ¹T₂(¹D) →³T₂(³F) spin-forbidden transition of Ni²⁺ ions in octahedral sites^18,34,35,37. Figure 8 shows photoluminescence excitation data for (a) Zn₂TiO₄ and (b) Zn₂TiO₄:Ni²⁺ (0.1% of Ni²⁺)-samples. Time-resolved excitation scans were obtained with a lamp rate of 200 Hz (time window of 5 ms), 1 ms signal integration time, 600 pulses averaged per wavelength and a scan step size of 1 nm.

Figure 8
Excitation spectra at room temperature of (a) Zn₂TiO₄ sample monitored at 718 nm and (b) Zn₂TiO₄:Ni²⁺ (0.1% of Ni²⁺) sample monitored at 680 nm (solid line) and 718 nm (black circles)

In Figure 8a the absorption edge of Zn₂TiO_4, around 400 nm, is a charge transfer band assigned to the transition of an electron between O^2- 2p-orbitals and empty Zn²⁺ 4s-orbitals⁴². The second excitation band at 290 nm was ascribed to the interaction between conduction band electrons and valence band in a charge transfer process between Ti⁴⁺ 4d-orbitals and O²⁻ 2p-orbitals^57,58. The excitation spectrum of the Zn₂TiO₄:Ni²⁺ (0.1%) sample (Figure 8b) monitored at 718 nm (black circles) shows similar features to the Zn₂TiO₄ sample monitored at the same wavelength, except for a shoulder in the longer wavelength region. When the emission was monitored at 680 nm, the excitation spectrum showed an additional and intense band of 450 nm to 550 nm, with maximum intensity at 480 nm. This band was assigned to the ³A₂(³F) → ¹T₂(¹D) spin-forbidden transition of Ni²⁺ ion in the octahedral site¹⁸.

Despite the different electronic configurations of Ni²⁺ and Ni³⁺ (d⁸ and d⁷, respectively), both can present an overlapping of bands assigned to the Ni³⁺ octahedral, Ni³⁺ tetrahedral, and Ni²⁺ octahedral coordination^34,35,37 in the spectral region from 400 nm to 500 nm.

In this paper, based on the shape and energy of the observed ¹T₂(¹D) →³T₂(³F) emission band, the excitation band at 480 nm was assigned to ³A₂(³F)→¹T₂(¹D) transition of Ni²⁺ ions in octahedral sites.

The energy diagram of Zn₂TiO₄:Ni²⁺(0.1%) transitions is shown in Figure 9, in which: the absorption energy for band edge (photoacoustic spectra, Figure 4, at 3.08 eV) is 24,738 cm^-1 (404 nm); the excitation in Figure 8 (excitation spectra), shows energy at 20,883 cm^-1 (480 nm); and finally the red emission at 680 nm (14,706 cm^-1).

Figure 9
Energy levels diagram of the Zn₂TiO₄:Ni²⁺ (0.1% of Ni²⁺) transitions.

The comparison of photoluminescence and excitation spectra of Zn₂TiO₄ and Zn₂TiO₄:Ni²⁺ (0.1%) samples leads to the conclusion that in the samples which contain Ni²⁺ as a substitutional impurity, the energy processes are assigned to two reasons: the transitions in the host Zn₂TiO₄ and the d-d transitions of Ni²⁺ ions. The assignment of an emission with a two decay pathway can be confirmed through photoluminescence decay measurements because different energy relaxation processes have characteristic decay times.

The results for the decay time experiments are shown in Figure 10, in which one can see the time dependence of photoluminescence intensity at room temperature for 718 nm wavelength, excited with 360 nm of (a) Zn₂TiO₄ and (b) Zn₂TiO₄:Ni²⁺ (0.1%).

Figure 10
Photoluminescence decay time at room temperature excited with 360 nm and monitored at 718 nm of (a) Zn₂TiO₄ and (b) Zn₂TiO₄:Ni²⁺ (0.1% of Ni²⁺).

Photoluminescence intensity decay of the Zn₂TiO₄ sample (Figure 10a) was fitted by a simple exponential function, while the decay of the Zn₂TiO₄:Ni²⁺(0.1%) sample (Figure 10b) was fitted by a bi-exponential function, described by Equation 3:

$$I\left(t\right)=A_1{e}^{\raisebox{1ex}{$-t$}\!\left/ \!\raisebox{-1ex}{${\tau }_1$}\right.}+A_2{e}^{\raisebox{1ex}{$-t$}\!\left/ \!\raisebox{-1ex}{${\tau }_2$}\right.}$$ (3)

The excitation and emission wavelengths were chosen in order to determine how the Ni²⁺ insertion changes the host decay time.

The bi-exponential decay shows that the Zn₂TiO₄:Ni²⁺ (0.1%) sample has two decay paths⁵⁹. It is important to note that the decay times ${\tau }_1$ and ${\tau }_2$ are both functions of the coupling between the emitting centers, as well as functions of the number of the emitted photons in each decay path, defined by the weights A₁ and A₂. The decay time average $\tau$ of a bi-exponential function can be obtained from Equation 4 ⁶⁰.

$\tau =\frac{A_1{\tau }_1^2+A_2{\tau }_2^2}{A_1{\tau }_1+A_2{\tau }_2}$ (4)

The parameters A₁ and A₂ and decay times ${\tau }_1$, ${\tau }_2$ and $\tau$, are listed in Table 3. The emission decay time of the Zn₂TiO₄ is about 10µs. This short decay time is due to the influence of the shorter electron-hole recombination in the decay process.

Table 3
Table 3. Adjusted parameters and decay time values of Zn₂TiO₄ and Zn₂TiO₄:Ni²⁺(0.1%)..

	A 1	${\tau }_1$(µs)	A 2	${\tau }_2$ (µs)	$\tau$ (µs)
Zn2TiO4	84.53	10.26	-	-	10.26
Zn2TiO4:Ni2+(0.1%)	3.91	147.65	12.47	14.31	116.15

However, the decay of the Zn₂TiO₄:Ni²⁺ (0.1% Ni²⁺) sample is a bi-exponential decay in which the shorter value ${\tau }_2$ can be assigned mainly to the relaxation from the conduction band to the valence band of Zn₂TiO_4, because this value is close to the time decay obtained for the Zn₂TiO₄ sample. Furthermore, the longer decay time ${\tau }_1$ can be attributed mainly to the spin-forbidden transitions between d-d energy levels of Ni²⁺ in octahedral sites^18,34,35,37. Therefore, decay times ${\tau }_1$and ${\tau }_2$ are both functions of the coupling between Zn₂TiO₄ host and Ni²⁺ emissions. The average lifetime $\tau$ of the Zn₂TiO₄:Ni²⁺ (0.1%) sample is close to the longer decay time component because there are large differences between the two decay time components. The decay time of the most intense emission (at 718 nm) of both Zn₂TiO₄ and Zn₂TiO₄:Ni²⁺ (0.1%)-samples, is in accord with the statement of the emission band of the doped sample, having an overlap of transitions of Zn₂TiO₄ with d-d transitions of Ni²⁺ ions in octahedral sites.

4. Conclusions

This work investigated the structural, optical and energy level features of Zn_2(1-x)Ni_2xTiO₄, where x = 0.00, 0.0007, 0.001, 0.0013, 0.002 and 0.003, in which Zn²⁺ ions were replaced by Ni²⁺ ions. The Ni samples present a broad emission band from 600 nm to 800 nm, and the intensity of emission at 800 nm indicates that this band extends to a higher wavelength. The emission band is an overlap of host transitions and d-d transitions of Ni²⁺ in octahedral sites. Because of the strong overlap, the two different contributions were not fully separated by light excitation or phase-signal selection. The photoluminescence intensity of the Zn₂TiO₄ is higher than Zn₂TiO₄:Ni²⁺. Therefore, the excited energy levels of Ni²⁺ are probably partially populated by electrons from the Zn₂TiO₄ conduction band, which decay to excited energy levels of the Ni²⁺ ions. The intensity of photoluminescence decreases with Ni²⁺ concentration and the concentration quenching is reached even at a low Ni²⁺ concentration. The decrease of emission intensity when Ni²⁺ is inserted in Zn₂TiO₄, shows that, in this case, the lowering of the bandgap does not contribute to the efficiency of the emission.

Analyzing the quantum efficiency and the broad, high intensity of the emission band of Zn₂TiO₄:Ni²⁺ (0.1%), it is possible to consider this sample as a potential candidate as radiation tunable active medium at room temperature.

5. Acknowledgments

This study was financed in part by the Coordenação de Aperfeiçamento de Pessoal de Nível Superior – Brasil (CAPES) - Finance Code 001. The authors are also grateful to Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for their financial support; and to the LIETA/IFADT/UERJ for XRD and XRF data. S. S. Pedro and L. P. Sosman thank CNPq for the Research Productivity fellowships. S. S. Pedro gives thanks to the Jovem Cientista do Nosso Estado (JCNE-FAPERJ) fellowship.

6. References

• ¹
  Marin SJ, O’Keeffe M, Partin DE. Structures and crystal chemistry of ordered spinels: LiFe₅O₈, LiZnNbO₄, and Zn₂TiO₄ J Solid State Chem. 1994;113(2):413-9. http://dx.doi.org/10.1006/jssc.1994.1389
  » http://dx.doi.org/10.1006/jssc.1994.1389
• ²
  Fenoll J, Garrido I, Hellín P, Flores P, Vela N, Navarro S. Photocatalytic oxidation of pirimicarb in aqueous slurries containing binary and ternary oxides of zinc and titanium. J Photochem Photobiol Chem. 2015;298:24-32. http://dx.doi.org/10.1016/j.jphotochem.2014.10.014
  » http://dx.doi.org/10.1016/j.jphotochem.2014.10.014
• ³
  Antoniou MG, Shoemaker JA, Cruz AA, Dionysiou DD. Unveiling new degradation intermediates/pathways from the photocatalytic degradation of microcystin-LR. Environ Sci Technol. 2008;42(23):8877-83. http://dx.doi.org/10.1021/es801637z PMid:19192812.
  » http://dx.doi.org/10.1021/es801637z
• ⁴
  Obradovic N, Labus N, Sreckovic T, Stevanovic S. Reaction sintering of the 2ZnO -TiO₂ system. Sci Sinter. 2007;39(2):127-32. http://dx.doi.org/10.2298/SOS0702127O
  » http://dx.doi.org/10.2298/SOS0702127O
• ⁵
  Ghanbarnezhad S, Nemati A, Naghizadeh R. Low temperature synthesis of zinc-titanate ultra fine powders. APCBEE Procedia. 2013;5:6-10. http://dx.doi.org/10.1016/j.apcbee.2013.05.002
  » http://dx.doi.org/10.1016/j.apcbee.2013.05.002
• ⁶
  Ohsato H. Microwave materials with high Q and low dielectric constant for wireless communications. MRS Proceedings. 2004;833:G2.4. http://dx.doi.org/10.1557/PROC-833-G2.4
  » http://dx.doi.org/10.1557/PROC-833-G2.4
• ⁷
  Kim HT, Kim Y, Valant M, Suvorov D. Titanium incorporation in Zn₂TiO₄ spinel ceramics. J Am Ceram Soc. 2001;84(5):1081-6. http://dx.doi.org/10.1111/j.1151-2916.2001.tb00793.x
  » http://dx.doi.org/10.1111/j.1151-2916.2001.tb00793.x
• ⁸
  Chaves AC, Lima SJG, Araújo RCMU, Maurera MAMA, Longo E, Pizani PS, et al. Photoluminescence in disordered Zn₂TiO₄ J Solid State Chem. 2006;179(4):985-92. http://dx.doi.org/10.1016/j.jssc.2005.12.018
  » http://dx.doi.org/10.1016/j.jssc.2005.12.018
• ⁹
  Sosman LP, López A, Camara AR, Pedro SS, Carvalho ICS, Cella N. Optical and structural properties of Zn₂TiO₄:Mn²⁺ J Electron Mater. 2017;46(12):6848-55. http://dx.doi.org/10.1007/s11664-017-5742-z
  » http://dx.doi.org/10.1007/s11664-017-5742-z
• ¹⁰
  Espinoza VAA, López A, Neumann R, Sosman LP, Pedro SS. Photoluminescence of divalent cobalt ions in tetrahedral sites of zinc orthotitanate. J Alloys Compd. 2017;720:417-22. http://dx.doi.org/10.1016/j.jallcom.2017.05.188
  » http://dx.doi.org/10.1016/j.jallcom.2017.05.188
• ¹¹
  Girish KM, Prashantha SC, Naik R, Nagabhushana H. Zn₂TiO₄: a novel host lattice for Sm³⁺ doped reddish orange light emitting photoluminescent material for thermal and fingerprint sensor. Opt Mater. 2017;73:197-205. http://dx.doi.org/10.1016/j.optmat.2017.08.009
  » http://dx.doi.org/10.1016/j.optmat.2017.08.009
• ¹²
  Iverson MV, Windscheif JC, Sibley WA. Optical parameters for the MgO:Ni²⁺ laser system. Appl Phys Lett. 1980;36(3):183-4. http://dx.doi.org/10.1063/1.91439
  » http://dx.doi.org/10.1063/1.91439
• ¹³
  Kück S. Laser-related spectroscopy of ion-doped crystals for tunable solid-state laser. Appl Phys B. 2001;72(5):515-62. http://dx.doi.org/10.1007/s003400100540
  » http://dx.doi.org/10.1007/s003400100540
• ¹⁴
  Zhang Y, Li X, Lai Z, Zhang R, Lewis E, Azmi AI, et al. Largest enhancement of broadband near-infrared emission of Ni²⁺ in transparent nanoglass ceramics: using Nd³⁺ as a sensitizer and Yb³⁺ as an energy-transfer bridge. J Phys Chem C. 2019;123(15):10021-7. http://dx.doi.org/10.1021/acs.jpcc.9b00359
  » http://dx.doi.org/10.1021/acs.jpcc.9b00359
• ¹⁵
  Gao Z, Liu Y, Ren J, Fang Z, Lu X, Lewis E, et al. Selective doping of Ni²⁺ in highly transparent glass-ceramics containing nano-spinels ZnGa₂O₄ and Zn_1+xGa_2−2xGe_xO₄ for broadband near-infrared fiber amplifiers. Sci Rep. 2017;7(1):1783. http://dx.doi.org/10.1038/s41598-017-01676-6 PMid:28496207.
  » http://dx.doi.org/10.1038/s41598-017-01676-6
• ¹⁶
  Hari Krishna R, Nagabhushana BM, Nagabhushana H, Monika DL, Sivaramakrishna R, Shivakumara C, et al. Photoluminescence, thermoluminescence and EPR studies of solvothermally derived Ni²⁺ doped Y(OH)₃ and Y₂O₃ multi-particle-chain microrods. J Lumin. 2014;155:125-34. http://dx.doi.org/10.1016/j.jlumin.2014.06.019
  » http://dx.doi.org/10.1016/j.jlumin.2014.06.019
• ¹⁷
  Koteswara Rao K, Rao MC. Optical and luminescent properties of Ni²⁺ doped PVA capped CdTe nanoparticles. Rasayan J Chem. 2017;10:904-9.
• ¹⁸
  Costa GKB, Sosman LP, López A, Cella N, Barthem RB. Optical and structural properties of Ni²⁺-doped magnesium gallate polycrystalline samples. J Alloys Compd. 2012;534:110-4. http://dx.doi.org/10.1016/j.jallcom.2012.04.039
  » http://dx.doi.org/10.1016/j.jallcom.2012.04.039
• ¹⁹
  Sigaev VN, Golubev NV, Ignateva ES, Savinkov VI, Campione M, Lorenzi R, et al. Nickel-assisted growth and selective doping of spinel-like gallium oxide nanocrystals in germano-silicate glasses for infrared broadband light emission. Nanotechnology. 2012;23(1):015708. http://dx.doi.org/10.1088/0957-4484/23/1/015708 PMid:22155977.
  » http://dx.doi.org/10.1088/0957-4484/23/1/015708
• ²⁰
  Xing YJ, Borguet E. Specificity and sensitivity of fluorescence labeling of surface species. Langmuir. 2007;23(2):684-8. http://dx.doi.org/10.1021/la060994s PMid:17209620.
  » http://dx.doi.org/10.1021/la060994s
• ²¹
  Ivanov VB, Behnisch J, Holländer A, Mehdorn F, Zimmermann H. Determination of functional groups on polymer surfaces using fluorescence labelling. Surf Interface Anal. 1996;24(4):257-62. http://dx.doi.org/10.1002/(SICI)1096-9918(199604)24:4<257::AID-SIA107>3.0.CO;2-1
  » http://dx.doi.org/10.1002/(SICI)1096-9918(199604)24:4<257::AID-SIA107>3.0.CO;2-1
• ²²
  Fernández A, Alonso JR, Flores JL, Ayubi GA, Di Martino JM, Ferrari JA. Optical processing of color images with incoherent illumination: orientation-selective edge enhancement using a modified liquid crystal display. Opt Express. 2011;19(21):21091-7. http://dx.doi.org/10.1364/OE.19.021091 PMid:21997117.
  » http://dx.doi.org/10.1364/OE.19.021091
• ²³
  FullProf Suite. Crystallographics tools for Rietveld, profile matching & integrated intensity refinements of X-ray and/or neutron data [Internet]. 2006 [cited 2019 May 7]. Available from: http://www.ill.eu/sites/fullprof/
  » http://www.ill.eu/sites/fullprof/
• ²⁴
  International Centre for Diffraction Data – ICDD. Card International Centre for Diffraction Data. Newtown Square: ICDD; 2002.
• ²⁵
  Millard RL, Peterson RC, Hunter BK. Study of the cubic to tetragonal transition in Mg₂TiO₄ and Zn₂TiO₄ spinels by 17O MAS NMR and Rietveld refinement of X-ray diffraction data. Am Mineral. 1995;80(9-10):885-96. http://dx.doi.org/10.2138/am-1995-9-1003
  » http://dx.doi.org/10.2138/am-1995-9-1003
• ²⁶
  García-Martínez O, Rojas RM, Vila E, Martín de Vidales JL. Microstructural characterization of nanocrystals of ZnO and CuO obtained from basic salts. Solid State Ion. 1993;63-65:442-9. http://dx.doi.org/10.1016/0167-2738(93)90142-P
  » http://dx.doi.org/10.1016/0167-2738(93)90142-P
• ²⁷
  Lokesh B, Rao NM. Effect of Cu-doping on structural, optical and photoluminescence properties of zinc titanates synthesized by solid state reaction. J Mater Sci Mater Electron. 2016;27(5):4253-8. http://dx.doi.org/10.1007/s10854-016-4290-2
  » http://dx.doi.org/10.1007/s10854-016-4290-2
• ²⁸
  Goktas A. Role of simultaneous substitution of Cu²⁺ and Mn²⁺ in ZnS thin films: defects-induced enhanced room temperature ferromagnetism and photoluminescence. Physica E. 2020;117:113828. http://dx.doi.org/10.1016/j.physe.2019.113828
  » http://dx.doi.org/10.1016/j.physe.2019.113828
• ²⁹
  Shekharam T, Siddarth J, Reddy YV, Nagabhushanam M. Photoluminescence studies of Cd_0.8−xPb_xZn_0.2S semiconductor compounds. J Lumin. 2018;197:56-61. http://dx.doi.org/10.1016/j.jlumin.2017.12.067
  » http://dx.doi.org/10.1016/j.jlumin.2017.12.067
• ³⁰
  Goktas A, Tumbul A, Aba Z, Durgun M. Mg doping levels and annealing temperature induced structural, optical and electrical properties of highly c-axis oriented ZnO:Mg thin films and Al/ZnO:Mg/p-Si/Al heterojunction diode. Thin Solid Films. 2019;680:20-30. http://dx.doi.org/10.1016/j.tsf.2019.04.024
  » http://dx.doi.org/10.1016/j.tsf.2019.04.024
• ³¹
  Joishy S, Antony A, Poornesh P, Choudhary RJ, Rajendra BV. Influence of Cd on structure, surface morphology, optical and electrical properties of nano crystalline ZnS films. Sens Actuators A Phys. 2020;303:111719. http://dx.doi.org/10.1016/j.sna.2019.111719
  » http://dx.doi.org/10.1016/j.sna.2019.111719
• ³²
  Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. 1976;32(5):751-67. http://dx.doi.org/10.1107/S0567739476001551
  » http://dx.doi.org/10.1107/S0567739476001551
• ³³
  Roselina NRN, Azizan A. Ni nanoparticles: study of particles formation and agglomeration. Procedia Eng. 2012;41:1620-6. http://dx.doi.org/10.1016/j.proeng.2012.07.359
  » http://dx.doi.org/10.1016/j.proeng.2012.07.359
• ³⁴
  Tanabe Y, Sugano S. On the absorption spectra of complex ions, I. J Phys Soc Jpn. 1954;9(5):753-66. http://dx.doi.org/10.1143/JPSJ.9.753
  » http://dx.doi.org/10.1143/JPSJ.9.753
• ³⁵
  Tanabe Y, Sugano S. On the absorption spectra of complex ions, II. J Phys Soc Jpn. 1954;9(5):766-79. http://dx.doi.org/10.1143/JPSJ.9.766
  » http://dx.doi.org/10.1143/JPSJ.9.766
• ³⁶
  Bhim A, Laha S, Gopalakrishnan J, Natarajan S. Color tuning in garnet oxides: the role of tetrahedral coordination geometry for 3 D metal ions and ligand-metal charge transfer (band gap manipulation). Chem Asian J. 2017;12(20):2734-43. http://dx.doi.org/10.1002/asia.201701040 PMid:28868809.
  » http://dx.doi.org/10.1002/asia.201701040
• ³⁷
  Tanabe Y, Sugano S. On the absorption spectra of complex ions, III. The calculation of the crystalline field strength. J Phys Soc Jpn. 1956;11(8):864-77. http://dx.doi.org/10.1143/JPSJ.11.864
  » http://dx.doi.org/10.1143/JPSJ.11.864
• ³⁸
  Mayén-Hernández SA, Torres-Delgado G, Castanedo-Pérez R, Villarreal MG, Cruz-Orea A, Alvarez JGM, et al. Optical and structural properties of ZnO + Zn₂TiO₄ thin films prepared by the sol–gel method. J Mater Sci Mater Electron. 2007;18(11):1127-30. http://dx.doi.org/10.1007/s10854-007-9267-8
  » http://dx.doi.org/10.1007/s10854-007-9267-8
• ³⁹
  Tauc J, Grigorovici R, Vancu A. Optical properties and electronic structure of amorphous germanium. Phys Status Solidi, B Basic Res. 1966;15(2):627-37. http://dx.doi.org/10.1002/pssb.19660150224
  » http://dx.doi.org/10.1002/pssb.19660150224
• ⁴⁰
  Viezbicke BD, Patel S, Davis BE, Birnie DP 3rd. Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system. Phys Status Solidi, B Basic Res. 2015;252(8):1700-10. http://dx.doi.org/10.1002/pssb.201552007
  » http://dx.doi.org/10.1002/pssb.201552007
• ⁴¹
  García-Ramírez E, Mondragón-Chaparro M, Zelaya-Angel O. Bandgap coupling in photocatalytic activity in ZnO-TiO₂ thin films. Appl Phys, A Mater Sci Process. 2012;108(2):291-7. http://dx.doi.org/10.1007/s00339-012-6890-x
  » http://dx.doi.org/10.1007/s00339-012-6890-x
• ⁴²
  Jang JS, Borse PH, Lee JS, Lim KT, Jung OS, Jeong ED, et al. Energy band structure and photocatalytic property of Fe-doped Zn₂TiO₄ material. Bull Korean Chem Soc. 2009;30(12):3021-4. http://dx.doi.org/10.5012/bkcs.2009.30.12.3021
  » http://dx.doi.org/10.5012/bkcs.2009.30.12.3021
• ⁴³
  Goktas A. High-quality solution-based Co and Cu co-doped ZnO nanocrystalline thin films: comparison of the effects of air and argon annealing environments. J Alloys Compd. 2018;735:2038-45. http://dx.doi.org/10.1016/j.jallcom.2017.11.391
  » http://dx.doi.org/10.1016/j.jallcom.2017.11.391
• ⁴⁴
  Tumbul A, Aslan F, Demirozu S, Goktas A, Kilic A, Durgun M, et al. Solution processed boron doped ZnO thin films: influence of different boron complexes. Mater Res Express. 2018;6(3):035903. http://dx.doi.org/10.1088/2053-1591/aaf4d8
  » http://dx.doi.org/10.1088/2053-1591/aaf4d8
• ⁴⁵
  Cahyaningsih D, Taufik A, Saleh R. Effect of Ni doping on the structural and optical properties of TiO₂ nanoparticles prepared by co-precipitation method. J Phys Conf Ser. 2020;1442:012017. http://dx.doi.org/10.1088/1742-6596/1442/1/012017
  » http://dx.doi.org/10.1088/1742-6596/1442/1/012017
• ⁴⁶
  Girish KM, Prashantha SC, Nagabhushana H. Facile combustion based engineering of novel white light emitting Zn₂TiO₄:Dy³⁺ nanophosphors for display and forensic applications. J Sci Adv Mater Devices. 2017;2:360-70.
• ⁴⁷
  Thurian P, Heitz R, Hoffmann A, Broser I. Nonlinear Zeeman effect of the Ni³⁺ centre in ZnO. J Cryst Growth. 1992;117(1-4):727-31. http://dx.doi.org/10.1016/0022-0248(92)90845-A
  » http://dx.doi.org/10.1016/0022-0248(92)90845-A
• ⁴⁸
  Goetz G, Schulz HJ. Kinetics of the formation of Ni³⁺(3d⁷) ions in ZnS and their detection by near-infrared emission. J Lumin. 1988;40-41:311-2. http://dx.doi.org/10.1016/0022-2313(88)90208-6
  » http://dx.doi.org/10.1016/0022-2313(88)90208-6
• ⁴⁹
  Ding J, Chen H, Fu H. Defect-related photoluminescence emission from annealed ZnO films deposited on AlN substrates. Mater Res Bull. 2017;95:185-9. http://dx.doi.org/10.1016/j.materresbull.2017.07.042
  » http://dx.doi.org/10.1016/j.materresbull.2017.07.042
• ⁵⁰
  Chunduri LAA, Kurdekar A, Pradeep BE, Haleyurgirisetty MK, Venkataramaniah K, Hewlett IK. Streptavidin conjugated ZnO nanoparticles for early detection of HIV infection. Advanced Materials Letters. 2017;8(4):472-80. http://dx.doi.org/10.5185/amlett.2017.6579
  » http://dx.doi.org/10.5185/amlett.2017.6579
• ⁵¹
  Samanta A, Goswami MN, Mahapatra PK. Magnetic and electric properties of Ni-doped ZnO nanoparticles exhibit diluted magnetic semiconductor in nature. J Alloys Compd. 2018;730:399-407. http://dx.doi.org/10.1016/j.jallcom.2017.09.334
  » http://dx.doi.org/10.1016/j.jallcom.2017.09.334
• ⁵²
  Wang WN, Widiyastuti W, Ogi T, Lenggoro IW, Okuyama K. Correlations between crystallite/particle size and photoluminescence properties of submicrometer phosphors. Chem Mater. 2007;19(7):1723-30. http://dx.doi.org/10.1021/cm062887p
  » http://dx.doi.org/10.1021/cm062887p
• ⁵³
  Sakthivel P, Muthukumaran S, Ashokkumar M. Structural, band gap and photoluminescence behaviour of Mn-doped ZnS quantum dots annealed under Ar atmosphere. J Mater Sci Mater Electron. 2015;26(3):1533-42. http://dx.doi.org/10.1007/s10854-014-2572-0
  » http://dx.doi.org/10.1007/s10854-014-2572-0
• ⁵⁴
  Bhargava RN, Gallagher D, Hong X, Nurmikko A. Optical properties of manganese-doped nanocrystals of ZnS. Phys Rev Lett. 1994;72(3):416-9. http://dx.doi.org/10.1103/PhysRevLett.72.416 PMid:10056425.
  » http://dx.doi.org/10.1103/PhysRevLett.72.416
• ⁵⁵
  Kück S, Petermann K, Pohlmann U, Huber G. Near-infrared emission of Cr⁴⁺-doped garnets: Lifetimes, quantum efficiencies, and emission cross sections. Phys Rev B Condens Matter. 1995;51(24):17323-31. http://dx.doi.org/10.1103/PhysRevB.51.17323 PMid:9978759.
  » http://dx.doi.org/10.1103/PhysRevB.51.17323
• ⁵⁶
  Kuleshov NV, Mikhailov VP, Scherbitsky VG, Prokoshin PV, Yumashev KV. Absorption and luminescence of tetrahedral Co²⁺ ion in MgAl₂O₄ J Lumin. 1993;55(5-6):265-9. http://dx.doi.org/10.1016/0022-2313(93)90021-E
  » http://dx.doi.org/10.1016/0022-2313(93)90021-E
• ⁵⁷
  Andrade LHC, Lima SM, Novatski A, Neto AM, Bento AC, Baesso ML, et al. Spectroscopic assignments of Ti³⁺ and Ti⁴⁺ in titanium-doped OH-free low-silica calcium aluminosilicate glass and role of structural defects on the observed long lifetime and high fluorescence of Ti³⁺ ions. Phys Rev B Condens Matter Mater Phys. 2008;78(22):224202. http://dx.doi.org/10.1103/PhysRevB.78.224202
  » http://dx.doi.org/10.1103/PhysRevB.78.224202
• ⁵⁸
  Yamaga M, Yosida T, Hara S, Kodama N, Henderson B. Optical and electron spin resonance spectroscopy of Ti³⁺ and Ti⁴⁺ in Al₂O₃ J Appl Phys. 1994;75(2):1111-7. http://dx.doi.org/10.1063/1.356494
  » http://dx.doi.org/10.1063/1.356494
• ⁵⁹
  Lyvers DP, Moazzezi M, de Silva VC, Brown DP, Urbas AM, Rostovtsev YV, et al. Cooperative bi-exponential decay of dye emission coupled via plasmons. Sci Rep. 2018;8(1):9508. http://dx.doi.org/10.1038/s41598-018-27901-4 PMid:29934509.
  » http://dx.doi.org/10.1038/s41598-018-27901-4
• ⁶⁰
  Okabe K, Inada N, Gota C, Harada Y, Funatsu T, Uchiyama S. Intracellular temperature mapping with a fluorescent polymeric thermometer and fluorescence lifetime imaging microscopy. Nat Commun. 2012;3(1):705. http://dx.doi.org/10.1038/ncomms1714 PMid:22426226.
  » http://dx.doi.org/10.1038/ncomms1714

Publication Dates

• Publication in this collection
  13 July 2020
• Date of issue
  2020

History

• Received
  12 Feb 2020
• Reviewed
  26 May 2020
• Accepted
  27 May 2020