Optimization of visible photoluminescence emission from Ni-Zn ferrite thin films

Ni-Zn ferrite films with different thicknesses were prepared by the spray method, aiming to study the relationship between the annealing effect in an oxygen rich environment and the structural, optical properties and photoluminescence emission. X-ray diffraction (XRD) analysis used with Rietveld refinement showed that all prepared samples had a single spinel phase structure. Likewise, the Fourier transform infrared (FTIR) spectra confirmed the phase formation of Ni-Zn ferrites by appearing in both of the two characteristic absorption bands which are related to the tetrahedral and octahedral sites. For annealed thin film samples of Ni-Zn ferrite, the atomic force microscope (AFM) surface morphology exhibits pinning structure on the surface in nanoscale height, whereas for un-annealed samples, there are hills and valleys cover a broad region. The different electronic transitions were estimated from the UV-visible transmission spectrum. Strong photoluminescence (PL) intensity in the visible range was observed under the excitation of UV radiation. The intensity of the PL signal was strongest at a film thickness of 750 nm then decreased for higher thicknesses. This could be interpreted by using proposed energy level structures based on the transmission spectrum of the investigated samples. The strong PL intensity introduces the samples as a direct optical detector for UV radiation.


Introduction
Spinel ferrite material is the first solid that comes to mind whenever we discuss magnetic materials.Due to their diverse electro-magnetic properties [1][2][3][4], the spinel ferrite materials are among the most actively researched solid oxide materials in the world.The typical chemical formula for spinel ferrites is AB 2 O 4 , with A 2+ and B 3+ ions occupying tetra (A) and octa [B] sites.Iron ions are located in either tetra (A) or octa [B] sites between the oxygen ions, creating a cubic structure.The magnetic properties of ferrite thin films are extremely sensitive to the growth process and preparation method employed.Because ferrite thin films have a broad variety of technical applications in many different fields, there has been an incredible amount of study on their development and characterization in recent years.The spinel structure of ferrite-based thin films has the potential to be of significant scientific and academic interest.The magnetic and electrical properties, chemical stability, and ease of preparation of ferrite thin films make them highly significant in the production of microwave devices as phase shifters, gyrators, circulators, memory core devices, transformers, sensors, and related applications.This has been documented in various studies [5][6][7][8][9][10].
A particularly interesting and promising choice among the transition metals is non-magnetic zinc (Zn 2+ ).Due to the significance of these materials for high-frequency devices and soft magnetic applications, the magnetic characteristics of Zn 2+ -substituted ferrites have received a great deal of interest [1].
Bulk normal spinel Zn-ferrite (ZnFe 2 O 4 ) has paramagnetic characteristics at RT and a long range antiferromagnetic order below 10 K.As a component of the technologically significant Ni-Zn and Mn-Zn mixed ferrites, it has found extensive usage in a variety of electronic components [11][12][13].However, the magnetic characteristics of nanocrystalline zinc ferrite are of great significance since they vary considerably from those of bulk zinc ferrite [14].Surface magnetic ions play a crucial role in this.Due to their potential use in high-frequency and data storage systems, thin films of ZnFe 2 O 4 have garnered a lot of attention.Nickel ferrite (NiFe 2 O 4 ) is an excellent choice among the spinel ferrites for technological uses, and the special properties of its macroporous films may lead to exciting new possibilities in many different fields of technology [15].Thin-film nickel ferrites are of particular interest because of their improved magnetic, electrical, and dielectric characteristics, as well as their higher chemical stability and reduced magnetic loss factor at high frequencies.
There are several techniques for creating ferrite thin films, including electro deposition, pulsed laser deposition, sputtering, ferrite plating, dip coating, and spray pyrolysis.Compared to alternative methods, the spray pyrolysis technique offers advantages in terms of compositional convenience, low-temperature processing, shorter processing time, and reduced cost.This method enables the cost-effective, uncomplicated, and convenient fabrication of ferrite thin films.The technological importance of nickel ferrite is attributed to its applications in the electrical and magnetic industries.The aforementioned material has been utilized as a reliable agent for gas and humidity sensing applications [16,17].The characteristic feature of nickel ferrite is its inverse spinel arrangement.Nickel ferrite has an inverse spinel structure.When zinc ions are added to nickel ferrite, it could enhance the magnetic, electrical, and optical properties [18].
The results of study on thin films of Ni-Zn ferrite show that they have soft magnetic qualities that are very interesting.Based on current knowledge, it appears that a comprehensive analysis of the structural, optical, and photoluminescence characteristics of thin films comprising zinc-substituted nickel ferrite has yet to be understood.The current study delves into the significance of nickel zinc ferrite thin films synthesized by the spray pyrolysis technique, and the effect of annealing temperature of different film thicknesses was correlated by the structure, optical and photoluminescence properties.

Synthesis and structural characterization
Five different thickness samples of nickel-zinc ferrite thin films (Ni 0.5 Zn 0.5 Fe 2 O 4 ) were deposited on properly cleaned soda lime glass substrates by the chemical spray pyrolysis technique.The precursor solution was made by dissolving high purity Ni-nitrates (Ni(NO 3 ) 2 .9H 2 O), Zn-nitrates (Zn(NO 3 ) 2 .6H 2 O) (99.9%,LOBAChemie) and Fe-nitrates (Fe(NO 3 ) 3 .9H 2 O) (99,9%, Segma Aldrich, Germani) with a proper concentration is distilled water and ethanol solution with a ratio of 1 : 3. The governing chemical reaction can be seen as follow: A Two directional motorized micro nozzle with air as a carrier gas was used.The optimal substrate temperature for synthesizing Ni 0.5 Zn 0.5 Fe 2 O 4 thin films of good quality was found to be 350 ± 10 °C.A Schematic diagram of spray pyrolysis technique and its equipment is shown in figure 1.
The thickness was changed by changing the number of spray cycles.Each spray cycle takes 10 s and waits 10 s to keep the temperature on the substrate around 350 °C.The five samples with different thicknesses divided into two groups: 1-Room temperature 'RT' samples.2-Annealed samples at 500 °C for 3h in oxygen rich atmosphere '500 °C' samples.The thickness of the films was determined using the weight-difference method and a high-precision electronic balance (Citizen, Model: CY 204).For simplest, the samples thicknesses were labeled as follow (T1 = 290 nm, T2 = 597 nm, T3 = 750 nm, T4 = 1107 and T5 = 1395 nm).The last two samples (T4 and T5) were foggy films with poor transmissions.The Grazing incidence x-ray diffraction (GIXRD) (Philips X'Pert PRO MRD triple axis diffractometer (angular resolution 0.01°, precision 0.001°)) was used to identify the phase structure of the samples.Fourier transform infrared spectroscopy (FTIR) was performed at room temperature in transmission mode (Spectrometer JASCO, 6300, Kyoto, Japan) in the range 400-4000 cm −1 .The surface roughness of the films was measured by a NT-MDT Atomic Force Microscopy (AFM) operated in semi-contact mode.The optical properties of the films were measured using UV-vis optical spectroscopy, (Jasco V-670 UV-vis-NIR) Spectrophotometer with working range 200-800 nm.Room temperature photoluminescence (PL) spectra with an excitation wavelength of 325 nm were measured on a (FLUOROMAX-2 spectrofluorometer, JOBIN YVON-SPEX, New Jersey, USA).

X-ray analysis
The grazing incident x-ray diffractometer (GIXRD) of thin film samples of Ni 0.5 Zn 0.5 Fe 2 O 4 are presented in figure 2.
The findings of this study is compared to the information presented in JCPDS No. 74-2081 and 82-1049, wherein a spinel cubic structure phase with a cubic lattice structure of Ni 0.5 Zn 0.5 Fe 2 O 4 thin films was identified through the observation of diffraction peaks at (220), (311), ( 222), (400), ( 422), ( 511) and (440) [7].However, the figure 2 reveals the clear effective incorporation of annealing temperature on the formation of Ni 0.5 Zn 0.5 Fe 2 O 4 spinel phase as well the crystallite size and strain.Through the use of numerical calculations of diffraction pattern data, the lattice parameter 'a' was determined (table 1).Moreover, the micro strain and the average crystallite size are determined from Williamson-Hall (W-H) equation [19]; where ε is the strain produced in the sample, D is average crystallite size and k is the constant.The dislocation density (δ), the length of dislocation lines per crystal lattice unit volume, is calculated from the equation [20,21]; δ = , D 1 2 where D is the crystallite size.Table 1 lists the lattice parameters, unit cell volume, crystallite size, strain, and dislocation parameter for different thickness samples of both RT and 500 °C groups.
Moreover, the MAUD programme was used to refine the XRD data in order to confirm that the synthesized thin films had spinel structure and to look into how the ferromagnetic ions (Fe 3+ , Ni 2+ ) and nonmagnetic transition metal ions (Zn 2+ ) occupy tetrahedral and octahedral positions, respectively and the results of profile fitting are illustrated in figure 3. The initial input parameters for the Rietveld refinement were obtained from the literature [22][23][24] and CIF data files [25].The XRD data refinement revealed that all samples formed a singlephase spinel cubic structure with space group: F 4̅ 3 m.The pattern fitness evaluation depends on a number of factors.The fit goodness (S) is defined as the ratio between the weighted pattern (R wp ), and the expected values (R exp ) (S = R wp / R exp ).The refinement was maintained until a value of S close to one was found to be convergent, which validates the value of refinement [26][27][28].The refinement parameters values as well as the lattice parameter and the crystallite size are shown in table 1, which suggests a rather good precision in synthesizing the spinel thin films without any additional phases.
It is obvious from table 1 that, the lattice parameter of all formed samples is around 8.39 Å which in good agreement with the previous work [5,8,29].Also, the lattice parameters are slightly decreased by annealing for all samples.Furthermore, the crystallite size of the all prepared samples in the nanoscale and annealed samples has crystallite size greater than the RT samples.Also, the crystallite size increases by increasing the thickness.On the other hand, the microstrain has the inverse behavior.The phenomenon of crystallite size growth with rising temperature can be attributed to the coalescence process [30].The positive values of the strain indicate the existence of tensile strain [31].These results are very close to what presented by Prashant Thakur et al. for the comparable annealed samples [31].It is clear from table 1, that the obtained dislocation density is significantly lower in 500 °C samples than RT ones, which infers the effect of annealing in the matrix.
The reduction in microstrain for 500 °C samples results in a decrease in lattice imperfections, thereby facilitating crystal growth and leading to an increase in crystallite size.One can see that, the sample T3 annealed at 500 °C has the minimum strain.The increase of the strain for high thickness samples may be due to the formation of island structure of the films at high thicknesses (greater than 1000 nm).Furthermore, it was reported that at high thickness of ferrite thin films the microstructure increased; this may be due to the decrease in lattice effects among the grain boundary [32,33].

Fourier transform infrared (FTIR) analysis
In order to determine the chemical bands of thin films and carry out structural investigations, the Fourier Transform Infrared (FTIR) of the analyzed materials was performed.FTIR spectra of Ni 0.5 Zn 0.5 Fe 2 O 4 thin film samples as prepared (RT) and annealed at (500 °C) within the wavenumber range of 400-4000 cm −1 are illustrated in figure 4. The Ni-Zn ferrite has a mixed spinel crystal structure and belongs to Oh 7 (Fd3m) space group [34].The spinel ferrites demonstrate the presence of four infrared active bands, namely the (F 1u mode) bands.The absorption bands shown by the Ni-Zn ferrites within the spectral range of 400-775 cm −1 may be categorized into two distinct groups of vibrational modes as reported in literature [35,36].From figure 4, the presence of two absorption bands (ν 1 , ν 2 ) provides confirmation of the creation of a Ni 0.5 Zn 0.5 Fe 2 O 4 spinel Table 1.Lattice parameter (a), cell volume (V), refinement parameters (R wp , R exp , and S) Crystallite size (D), strain (ε) and dislocation density (δ), for Ni 0.5 Zn 0.5 Fe 2 O 4 thin film samples RT and annealed at (500 °C).ferrite structure.Specifically, absorption band ν 1 is detected within the 421 cm -1 -451 cm -1 range, while absorption band ν 2 is observed within the 556 cm -1 -582 cm -1 range.The confirmation of bond formation between metal ions and oxygen ions in the tetrahedral site is supported by the absorption band ν 2 , while the confirmation of bond formation between cations and oxygen ions in the octahedral site is supported by the absorption band ν 1 , as reported elsewhere [9].Furthermore, compared to octahedral sites (ν 2 ), it is apparent that the tetrahedral site (ν 1 ) vibration modes are separated into more shoulder bands.According to Mazen et al [37], the presence of Fe ions at the tetrahedral site (ν 1 ) may divide the IR absorption bands because they induce the John-Teller distortion.One can notice that, one of the vibrational modes of the nitrate group of precursors is responsible for the appearance of a band at a wave number of approximately 830 cm -1 [20,38].Table 2 presents the absorption band values.A shift in the location of the absorption bands (ν 1 , ν 2 ) has been noticed as the sintering temperature rises.Based on the results obtained from the FTIR spectral analysis, it can be inferred that the utilization of the chemical spray pyrolysis technique leads to the formation of single phase cubic spinel nanocrystalline Ni-Zn ferrites thin films with different thicknesses.

Atomic force microscope (AFM)
One of the most promising techniques for surfaces and roughness's study for thin films is the atomic force microscope (AFM) with gold tip in contact mode was carried out.Figure 5 depicts AFM images obtained on a 9 μm 2 area of thin films at RT and 500 °C with different thicknesses, as indicated in the present work.The synthesis conditions did not have a direct correlation with the surface morphology.The AFM surface morphology shows that the pins structure with height in the nanoscale for annealed thin film samples, while for RT samples the morphology almost hills and villages extended over a large area.Upon undergoing annealing at a temperature of 500 °C, the crystalline quality is enhanced as a result of the favorable positioning of atom arrangements, as we can see from GIXRD, leading to a reduction in the average surface roughness (R) [39].
The root mean square (RMS) reflects the roughness of the surface and one of the effects of the annealing temperature is increasing the grain size and subsequently decreasing the roughness which is in good agreement with samples RT and 500 °C.Moreover, the effect of annealing temperature is mainly on the crystallite size which should increase by annealing, i.e. the smoothing of the samples will increase which is the reason of the slight reduction of root mean square [40,41].
As well as, with increasing the thickness in both groups (RT) and (500 °C) samples, the average surface roughness decreases which confirm the effect of thickness dependence and the cubic phase formation of Ni 0.5 Zn 0.5 Fe 2 O 4 spinel thin films.Understanding this tendency requires thinking about how the tip of the AFM probe interacts with the specimen surface.When the particles are distributed across the surface of the substrate, the interaction between the tip and the particles in the horizontal direction causes a deformation in their shape and results in an overestimation of the measured dimensions [6].

Optical properties and band gap
To probe the effect of both thickness and annealing of the synthesized Ni 0.5 Zn 0.5 Fe 2 O 4 thin films, UV-vis absorption spectroscopy was used.Figure 6 shows the transmission spectra for both of RT and annealed samples for different thicknesses.It is obvious that, in the high transmission region (IR) there is no interference fringe, which means that the thickness of the prepared samples is in order of the wavelength in Infrared region or less which is in good agreement with the measured values by the balance method.To get more insight into the energy and the nature of the electronic transitions in the investigated samples the derivative of transmission spectrum fitting method is applied [42].Figure 7 depicts the derivative of the transmission (dT/d(hν)) for RT samples.The transitions are not very clear and there are only two wide transition peaks around 2.4 eV and 2.8eV.On the other hand, the transitions of the annealed samples (figure 8) are well defined and it has many transition states.The poor optical transition for the RT samples could be understood in terms of the poor crystallinity and high strain of the samples as shown in XDR measurements.Also, the band at 2.4 eV can be attributed to a direct O 2− + Fe 3+ → O − + Fe 2+ transition [43].Finally the band at 2.8 eV is due to 6 A 1g → 4 T 2g (D) transitions of the crystal field splitting of O 2− anion [44].For the annealed samples, there are five electronic transitions at 1.6, 1.88, 2.11, 2.42 and 2.72 eV.The last two transitions are due to the direct and 6 A 1g → 4 T 2g (D) transitions.Comparing the direct transition between the RT and annealed samples, one can see that, there is red shift.This shift is due to the annealing effect on the grain size.The first three new transition peaks could be explained in terms of the change of the oxidation states of Ni 2+ to Ni 3+ as a result of annealing in oxygen atmosphere [45].Also, it is known that Zn is a volatile element which causes the formation of Fe 2+ for charge compensation.Therefore, the new transitions could be discussed in terms of the presence of new states in the energy gap between the valance and conduction band.One of these states is near to the valence band (acceptor level) due to the creation of Ni 3+ [46] and the other is near to the conduction band (donor level) due to the presence of Fe 2+ [47].The transitions between the impurity levels and the top of valance band and the bottom of the conduction band could explain the other transition states.Figure 8 shows a schematic diagram for the proposed electronic transitions of the investigated samples after annealing.It is very important to note that, the PL intensity increases with increasing the thickness up to t = 750 nm then it decreases again.By comparing the photon energy of PL emission with the proposed energy band diagram, it could be assumed that the PL transition from the bottom of the conduction band and the acceptor level of  Ni 3+ near to the valance band.As the thickness increases it is expected to increase the PL peak intensity, as samples T1, T2 and T3 but at higher thickness the PL intensity decreases (T4 and T5).This may be attributed to the re-absorption of the emitted photons by the unexcited states which called photo luminous quenching [49].Furthermore, the pinning structure (as shown in AFM images) for the annealed samples enhances the electron confinement at the tips of these pins which in turns increases the recombination rate between electron-hole and as a result enhances the photo luminous mechanism.Also, as the sample T3 has the lowest strain (table 1) this means it is the lowest sample to make electron trapping due to the crystal defects which increases the chance for electron hole recombination mechanism.The PL emission spectrum of the investigated samples introduces a promising thin film as a detector for UV radiations by detecting the strong PL signal in the visible range.Finally the weak and broad peak at (2.9 eV) was observed in Ni-Zn ferrite and is commonly referred to as deep-level or trap-state emission and singly ionized oxygen vacancy [50].

Conclusions
Single phase structure of Ni-Zn ferrite thin film by spray pyrolysis has been prepared with different thicknesses.The Rietveld-refined X-ray diffraction observations confirmed the formation of a single-phase cubic spinel with the space group Fd3m.The values of the crystallite size of synthesized nanocrystal thin films, were fell between  4.6 and 12.8 nm for RT samples and 7.7 and 14.2 nm for annealed samples obtained from Williamson-Hall (W-H) equation.The AFM imaged shows pins structure at the surface for annealed samples.Moreover, the different electronic transitions have been investigated from the UV-vis and one potential topic of discussion is to the appearance of novel transitions which is a rising from introducing new states inside the energy gap between the valence and conduction bands.These transitions are attributed to the charge compensation phenomenon associated with Fe 3+ .Furthermore, a strong PL signal in the visible range has been detected under UV excitation and the maximum PL signal was achieved at thickness (t = 750 nm).The sample T3 annealed at 500 °C could be introduced as promising an optical direct detector for UV radiations.This study introduces Ni-Zn ferrite as an efficient photoluminescence material in the visible range under UV radiation.For future work these samples may be used as diagnostic tool (from optical properties point of view) as well as therapeutic tool (from magnetic properties point of view) for cancer tumors.

Figure 1 .
Figure 1.Schematic diagram of spray pyrolysis technique used for nickel-Zinc ferrite thin films deposition.

Figure 2 .
Figure 2. XRD patterns of Ni 0.5 Zn 0.5 Fe 2 O 4 thin films for both samples groups, RT and annealed at 500 °C.

Figure 3 .
Figure 3. Rietveld refinement profile for Ni 0.5 Zn 0.5 Fe 2 O 4 thin films for both samples groups, RT and annealed at 500 °C.

Figure 4 .
Figure 4. FTIR spectra of Ni 0.5 Zn 0.5 Fe 2 O 4 thin films for both samples groups, RT and annealed at 500 °C.

3. 5 .
Photoluminescence spectra measurementsIn order to look into the spectral properties of a investigated Ni-Zn ferrite, photoluminescence (PL) spectroscopy is often used at room temperature.The PL spectra for the annealed samples are shown in figure9under the excitation wavelength 325 nm (3.8 eV).One can see that, there is a strong sharp PL peak at 658 nm (1.88 eV) and a broad weak band at 425 nm (2.9 eV).The last weak emission bands are attributed to the 3d 5 → 3d 3 4s 2 transitions of Fe 3+ ions.The same results reported for ZnFe 2 O 4 and NiFe 2 O 4 thin films[48].

Figure 6 .
Figure 6.The transmission spectra of Ni 0.5 Zn 0.5 Fe 2 O 4 thin films for both samples groups, RT and annealed at 500 °C.

Figure 7 .
Figure 7.The derivative of T% against the photon energy for thin film samples annealed at 500 °C.

Figure 8 .
Figure 8.A schematic diagram for the proposed electronic transitions of the investigated samples after annealing at 500 °C.

Figure 9 .
Figure 9.The PL spectra of the investigated Ni 0.5 Zn 0.5 Fe 2 O 4 thin films samples after annealing at 500 °C.

Table 2 .
Change of wave numbers (cm −1 ) of IR observed bands with film thicknesses of RT and 500°C for Ni 0.5 Zn 0.5 Fe 2 O 4 thin films.