Preparation and characterization of lanthanum-based perovskite oxides LaMO3 (M=Fe, Cr, Mn) thin films by electrophoretic deposition

In this study, we report the fabrication and characterization of LaMO3 (M=Fe, Cr, Mn) perovskite thin films deposited on FTO substrates via electrophoretic deposition (EPD). The x-ray diffraction (XRD) patterns showed the formation of the orthorhombic structure for LaFeO3 and LaCrO3 films and the trigonal structure for the LaMnO3 film. The field-emission scanning electron microscopy (FE-SEM) micrographs revealed nano-spherical particles with an average grain size of about 50 nm covering the LaMnO3 surface with fewer porosities compared to the other films. Moreover, the LaMnO3 film exhibited a higher transparency in the visible region (∼84%). The estimated bandgaps of LaFeO3, LaCrO3, and LaMnO3 perovskite films were found to be 2.17, 3.18, and 2.92 eV, respectively. In addition, the refractive indices of all deposited films were calculated using five empirical models that relate the refractive index to the bandgap, which consistently revealed a decrease in refractive index with an increase of bandgap energy This study holds significant implications for the field of perovskite thin films as we contribute new insights into the fabrication of lanthanum-based perovskite thin films using the EPD technique. The comprehensive characterization of microstructural, morphological, and optical properties provides valuable information for understanding the structure-property relationship in these materials. Furthermore, the determination of bandgaps and refractive indices enhances our understanding of the optical properties of perovskite films. The findings from this study pave the way for potential applications in optoelectronic devices, where fine-tuning of material properties is critical for optimal device performance.


Introduction
Perovskite-type oxides, exemplified by compounds with the general formula ABO 3 , have attracted considerable attention in recent years due to their fascinating properties making them highly promising for various applications. In the field of solid oxide fuel cells, they have been investigated as electrode materials due to their excellent electrochemical performance [1]. Furthermore, they exhibited a stable resistive switching behavior, which is an important factor in the design of resistive switching memory devices [2][3][4][5]. In gas sensing application, it has been found that perovskite-type oxides are great candidates to be used as liquid petroleum gas sensors [6], formaldehyde gas sensors [7,8], and ethanol gas sensors [9], due to their rapid response, good selectivity and fast response/recovery time. Additionally, perovskite-type oxides have also shown a great potential in photocatalytic applications, owing to their superior photocorrosion resistance and high structural and thermal stability [10]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Lanthanum-based perovskite-type oxide thin films containing transition metals, such as LaMnO 3 ( = M Fe Cr Mn , , ), have garnered significant attention due to their intriguing optical, electrical, and magnetic properties, rendering them suitable for a wide range of applications. For instance, LaFeO 3 films find application as active components in magneto-optic devices [11], gas sensors [12], solid oxide fuel cells, and catalysts [13]. LaCrO 3 has been extensively studied as an electrode material in fuel cells [14], energy storage materials, catalysts, and for air purification methods, including controlling the concentration of pollutant gases like CO and CO 2 [15]. On the other hand, LaMnO 3 films are employed in spintronic devices [16,17].
In fact, the properties of the prepared films can be strongly influenced by the preparation technique. To date, a variety of preparation methods have been used to synthesize LaMO 3 = M Fe Cr Mn , , ( ) perovskite thin films including the sol-gel method [15,18,19], pulsed laser deposition (PLD) [20][21][22], polymer assisted deposition (PAD) [23,24], spray-pyrolysis (SP) [16,25], molecular beam epitaxy (MBE) [26], chemical vapor deposition (CVD) [14], atomic layer deposition (ALD) [27], and RF-magnetron sputtering [28]. The sol-gel method offers compositional control and substrate compatibility but has longer processing times, organic contaminants, and there is often a large volume cracking during drying [29]. PLD provides high-quality films but requires expensive equipment [30]. PAD enhances film adhesion but requires additional processing steps [31]. SP offers fast deposition rates, but it seems not useful due to poor quality of thin film and thermal decomposition [32] MBE allows precise control but has limited substrate compatibility [33]. CVD offers high deposition rates but has complex precursor handling [33]. ALD provides precise control but has slow deposition rates and it cannot be used to the limited choice of effective reaction [34]. RF-magnetron sputtering offers high deposition rates but has equipment complexities [35].
Nevertheless, there is a dEarth of information regarding the study of the physical characteristics of lanthanum-based perovskite-type oxides prepared using the electrophoretic deposition (EPD) technique. The EPD is a good method that enables to produce uniform and dense films, with advantages including simple equipment, short processing time, high-quality microstructures, and adjustable particle packing in the thin film [36,37]. In the EPD process, charged particles in a stable colloidal suspension are mobilized through the liquid under the influence of an electric field and deposit onto a conductive substrate with an opposite charge [38]. Depending on the electrode and charged particles, two types of EPD coatings can be achieved. In anodic EPD, particles with a negative charge are deposited on the anode (positive electrode), while in cathodic EPD, positive particles are deposited on the cathode (negative electrode) [38].
Herein, the EPD method was chosen to synthesize LaMO 3 = M Fe Cr Mn , , ( ) perovskite thin films. In this approach, the powders were initially synthesized using the sol-gel method and subsequently utilized to prepare the suspension used for the films fabrication. The microstructural, morphological, and optical properties of the electrophoretically deposited films were examined using x-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and UV-vis spectroscopy.
Evaluation of the refractive indices of the films is considerably important for the applications in integrated optics and optical devices, where the refractive index serves as a key parameter for device design. High refractive index thin film materials are sought after to enhance the performance of optical devices like waveguide-based optical circuits and photonic crystals [39]. In photovoltaic applications, anti-reflection coatings consisting of high refractive index thin films are employed in solar cells to capture incident light and enhance light absorption within the solar cell [39]. It has been demonstrated that waveguide-based optical sensors exhibit lower limits of detection when a superstrate layer with a high refractive index is present on the waveguide surface [39,40].
In this regard, we will pay particular attention on the calculation of the refractive indices of the as-deposited films relative to the energy gap using empirical models. Each was dissolved in a separate beaker by adding 30 ml of a mixsolvent of deionized water and ethanol in the ratio (DI: Ethanol 1: 2 .

Experimental
) / The solutions were mixed using a magnetic stirrer at room temperature (RT) for 10 min until clear solutions were achieved. Subsequently, 0.010 mol of citric acid (CA) was added as a chelating agent to each solution to prevent aggregation. The solutions were continuously stirred at 80°C for 3 h until viscous solutions were formed. The powders were obtained by drying the viscous solutions at 110°C overnight and then manually milled using an agate mortar. After that, they were calcined at 900°C for 3 h with a heating and cooling rate of 5°C min −1 to enhance their crystallinity. Finally, the perovskite powders were removed from the furnace and re-milled to obtain fine particles. The flow chart represents the procedure used for preparing LaFeO LaCrO , 3 3 and LaMnO 3 crystalline powders, is given in figure 1.

Film preparation
2.3.1. Substrate preparation FTO-glass substrates with dimensions of 2 cm × 1 cm were cleaned using an ultrasonic bath. The cleaning process involved immersing the substrates in liquid detergent, followed by rinsing with deionized water, acetone, and isopropanol, each for 20 min. Finally, the substrates were dried in ambient air.

Suspension preparation
The suspensions used to synthesize the films via the EPD technique were prepared by individually mixing 30 mg of each perovskite powder with 20 mg of iodine I , 2 which served as an active charge carrier. These mixtures were dissolved in 25 ml of acetone in capped glass test tubes. The suspensions were then ultrasonicated for 1 h to homogenize them and ensure good dispersion of the particles. ) perovskite thin films. The electrophoretic cell used had a diameter of 3 cm and consisted of two FTO substrates; one used as an electrode (cathode) and the other as a counter electrode (anode). The cathode and anode were positioned 1 cm apart and parallel to each other. Both electrodes were then connected to a 10 V DC power supply and immersed in the suspension for 4 min at RT, allowing the particles to deposit and form a layer on the cathode substrate. Finally, the as-deposited films were dried at 100°C for 1 h. To prevent particle sedimentation, the suspensions were ultrasonicated for a few minutes between each deposition.

Characterization techniques
The thicknesses of the as-deposited films were estimated using contact profilometry (Dektak-150). The microstructural properties of the prepared films were conducted at room temperature using a Rigaku miniflex Tokyo, Japan) with a scan rate of 2°min −1 and a step size of  0.02 . The surface morphologies of the films were analyzed using a FE-SEM (JEOL JSM-7600 F) with an accelerating voltage of 15 kV. Measurements of the absorbance and transmittance spectra were performed between 300 and 800 nm using a UV-vis spectrophotometer (Shimadzu UV-2600).

Thin film measurement
The thicknesses of LaFeO , 3 LaCrO , 3 and LaMnO 3 perovskite thin films were measured and found to be approximately 2.3 μm, 3.2 μm, and 2.2 μm, respectively. It can be noticed that even though the films were synthesized under the same conditions, their thickness differs. This discrepancy can be attributed to several factors, including the nature of the material, the homogeneity of the suspension, and the fineness of the starting powder. Notably, the manual milling process might have introduced differences in the starting powder, further contributing to the variation in film thickness. Figure 3 depicts the XRD patterns of as-deposited films on FTO substrates carried out at room temperature within the q 2 range of 20°−80°. The pictures of as-deposited perovskite films were inserted in the corresponding XRD pattern. The spectra for all samples demonstrate the presence of a single polycrystalline phase without the presence of any impurity phases, which is a sign of the high purity of the prepared films. In  where l is the x-ray wavelength used (1.5418Å), b is the full width at half maximum (FWHM) of the main intense peak, and q is the diffraction angle at which the main intense peak occurred. All the estimated values are regrouped in table 1. According to this table, the lattice parameters and cell volume of the prepared films were found to be in agreement with the matching cards. Moreover, the LaFeO 3 film exhibited a larger crystalline size and lower values for both dislocation density and micro-strain, confirming a diminution in the lattice imperfections and an improvement in the film crystallinity.

Morphological study
The surface morphology of = LaMO M Fe Cr Mn , , 3 ( ) thin films was examined by FE-SEM. Figure 4 depicts micrographs of the prepared films, along with their corresponding grain size distribution histograms. As can be observed, the micrographs of LaFeO 3 ( figure 4(a)) and LaCrO 3 ( figure 4(b)) films display hemi-spherical particles with average grain sizes of 190 nm and 208 nm, respectively. On the other hand, LaMnO 3 ( figure 4(c)) film exhibits nano-spherical particles with an average grain size of about 50 nm. However, particle agglomeration was observed in some areas of the films, which was primarily caused by the nature of the solvent used to prepare the sol-gel. Furthermore, all samples displayed a porous surface structure, which was evaluated using J-Image software. The porosity values were found to be approximately 4.5 %, 6.6 %, and 3 % for LaFeO , 3 LaCrO 3 and LaMnO , 3 respectively. Similar observations, characterized by pore-free surfaces, were reported by Ranieri et al [42] for LaFeO 3 thin films and by Pan et al [43] for LaCrO 3 thin films, both prepared using the solgel method. Additionally, Okuyucu et al [44] observed a compact and less porous surface in LaMnO 3 thin film, with a grain size of about 50 nm, aligning with the findings of our study. ) thin films recorded within the UV-vis region ranging from 300 to 800 nm. As shown, all the fabricated films exhibit high absorption in the UV region, which tends to gradually decrease. The absorbance curve of the LaFeO 3 film shows an absorption edge near 585 nm, with a broad high-intensity absorption band observed within -300 500 nm. In addition, a weak absorption band at 672 nm was observed, ascribed to electron transitions from the valence band to the conduction band ( ( ) ), respectively [47]. In the case of the LaMnO , 3 the absorption edge is observed around 420 nm, with the presence of a weak-intensity absorption band centered at around 356 nm. Compared to the other films, the LaMnO 3 film has a lower light absorption in the UV region, which can be ascribed to its lower thickness. Besides, the transmittance spectra ( figure 5(b)) demonstrated that all synthesized films had low UV transparency and high transmittance in the visible range. The maximum transparency in the visible region is recorded for the LaMnO 3 film with an average transmittance of approximately 84 %, which is higher than those found in earlier reports [16,48,49].

Bandgap calculation
The optical bandgaps, E , g of = LaMO M Fe Cr Mn , , 3 ( ) thin films were evaluated from the absorbance spectra using Tauc's equation, given as follows [50]: a n n Here a is the absorption coefficient, n h is the photon energy, k is a constant, and n determines the type of transition, where = n 2 for direct transitions and = n 1 2 for indirect transitions [51]. The absorption coefficient was determined from the absorbance data according to the following relationship: where A is the absorbance and t is the film thickness.  6(b)), and 2.92 eV (figure 6(c)), respectively. According to table 2, we can notice that the estimated bandgaps are different from those found in the earlier findings.

Refractive index n
( ) In the present study, the refractive index of the electrophoretically deposited films is determined using the following models [52,53]: Table 1. The structural parameters, average crystallite size, dislocation density, and lattice strain of the as-prepared thin films.

Sample
Lattice parameters (Å)   Based on the mentioned models, it can be noticed that the refractive index and bandgap energy have an inverse relationship. Figure 7 presents a plot illustrating the refractive index for the different samples using the five models. As shown, all models exhibit the same behavior between the refractive index and the bandgap  energy. Nevertheless, the Moss, Herve-Vandamme, and Kumar models yield almost the same refractive index values, with the LaFeO 3 film displaying the highest refractive index. From these three models, the Kumar and Signh model was selected to evaluate the refractive indices of LaFeO , 3 LaCrO , 3 and LaMnO 3 thin films from previous studies that were synthesized using various techniques. The values obtained are summarized in table 2, along with the results of our study for comparison. The refractive indices of LaFeO LaCrO , 3 3 and LaMnO 3 films in our study were found to be close to those obtained in the reports [54], [15,51], and [43], respectively. In general, the refractive index can be influenced by the microstructure, such as the packing density, the crystalline quality, and the surface morphology of the prepared film. In our study, the higher value recorded for the LaFeO 3 film may be attributed to an increase in the degree of crystallinity, which was confirmed by XRD analysis.

Conclusion
High purity LaMO 3 = M Fe Cr Mn , , ( ) perovskite thin films were successfully synthesized using the electrophoretic deposition technique. XRD analysis confirmed the presence of a single orthorhombic structure in both LaFeO 3 and LaCrO 3 films, whereas a trigonal structure was observed in the LaMnO 3 film. Furthermore, LaFeO 3 has a larger crystalline size ( nm 34.73 ) and lower values of dislocation density and micro-strain, which confirm its high crystallinity. FE-SEM micrographs revealed that LaMnO 3 possessed a nanostructured film with an average particle diameter in the vicinity of nm 50 .The estimated direct bandgaps of LaFeO , 3 LaCrO , 3 and LaMnO 3 perovskite films were found to be in the vicinity of 2.17, 3.18, and eV 2.92 , respectively. Compared to ) thin films calculated using various models. The present study the other films, LaMnO 3 displayed a high refractive index and high transparency in the visible region, suggesting its suitability for optoelectronic device applications.