Combined Effects of Ultraviolet Irradiation and Magnetic Field on the Properties of Dip-coated ZnO thin films

In this study, four ZnO thin films were deposited on FTO substrates using the sol–gel dip coating method to examine their microstructural, morphological, and optical properties through various techniques. Three of them were subjected to ultraviolet (UV) light, magnetic field (MF), and a combination of UV and MF during deposition, referred as ZnO: UV, ZnO: MF, and ZnO: (UV+MF), respectively. The results obtained showed that the simultaneous UV and MF exposure improved the crystallinity and surface homogeneity of the as-deposited film. Moreover, ZnO: (UV+MF) film exhibited an average transparency of 80% in the visible region and a high optical bandgap (3.67 eV). Room-temperature photoluminescence (PL) spectra revealed a weak UV emission and a strong violet emission peaks for all films. However, the violet emission intensity being lower in ZnO: UV and ZnO: MF films due to a reduction in zinc interstitials (Zni) defects, The simultaneous UV and MF exposure did not reduce Zni defects, and the violet emission intensity was almost identical to that of the untreated film. These findings suggest that the ZnO: (UV+MF) film can be a promising candidate for the development of ultraviolet and violet lasers and light-emitting diodes.

To date, various physical and chemical preparation techniques have been used to synthesize ZnO thin films, including chemical vapor deposition [14], pulsed-laser deposition (PLD) [15], glancing angle deposition (GLAD) technique [16], sputtering [17], spray pyrolysis technique [18], electron beam evaporation [19], molecular beam epitaxy [20] and sol-gel deposition [21]. Among these deposition methods, sol-gel dip coating is a versatile and inexpensive method for fabricating thin films [22]. The fabrication of ZnO thin films using dip coating provides an excellent method for the controlled deposition of the films with desirable properties used for a wide range of applications. It offers many advantages such as high uniformity, high purity, the ability to easily control the film thickness, control of the stoichiometry, and coating of large substrate areas [22]. These advantages make the dip-coating an attractive method to fabricate ZnO thin films for many applications. Therefore, extensive studies have investigated the effect of different parameters on the ZnO film properties, such as deposition conditions, doping, and annealing, thereby providing a versatile approach for producing a wide range of ZnO-based materials [23][24][25][26][27][28][29].
Many researchers have investigated the influence of ultraviolet (UV) irradiation on the properties of ZnO thin films deposited using different methods. Lin, et al [30] and Wagata, et al [31] used the spin-spray method to fabricate ZnO thin films and then exposed the as-deposited films to the UV lamp. They observed an increase in the optical bandgap of UV-treated thin films. Hong et al [32] noticed an enhancement in the crystallization of films after UV irradiation. According to Gruzintsev and Volkov [33], UV treatment of as-deposited ZnO thin films using a magnetron-assisted high-vacuum technique increased their transmittance in the visible region.
On the other hand, other researchers have focused on the study the properties of ZnO films synthesized under a magnetic field (MF). AlArfaj and Subhi [34] prepared ZnO thin films deposited using the sol-gel dipcoating technique under the application of an external DC magnetic field placed perpendicular and parallel to the substrate. They found that perpendicular MF led to the coexistence of hexagonal wurtzite and cubic crystal structures and improved the grain texture. Whereas, the MF parallel to the substrate enhanced the film crystallinity. According to Altokka and Yldrm [35], magnetic fields enhanced the crystallinity and increased the transmittance of electrodeposited ZnO films.
The simultaneous exposure of ZnO thin films to UV and MF during deposition provides an important opportunity to enhance their properties. This is the first report on the combined effect of UV and MF during the sol-gel dip coating process on ZnO thin films. The investigation of this combined effect on ZnO thin films is of great scientific significance and can lead to new insights into the fundamental mechanisms behind the improved properties. This approach can potentially pave the way for the development of ZnO-based devices with enhanced performance in various fields such as optoelectronics. For instance, the improved optical and morphological properties of ZnO thin films can enhance their performance in optoelectronic devices such as solar cells and LEDs, where high transmittance and crystallinity are essential for efficient light harvesting and emission.
In the present study, ZnO multilayer thin films were prepared using the sol-gel dip coating method with assistance of UV, MF parallel to the substrate, and both UV and MF in order to investigate the combined effect of UV and MF exposure on the structural, morphological, and optical properties of as-deposited films. This study was undertaken using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UVvisible spectrophotometry, and photoluminescence (PL).

Experimental
as the starting material, isopropanol as the solvent, and monoethanolamine C H NO 2 6 [ ](MEA) as the sol stabilizer. First, zinc acetate dihydrate was dissolved in a mixture of 20 ml isopropanol and the required amount of MEA. The concentration of zinc acetate was 0.6 M and the molar ratio of MEA to zinc acetate was 1:1. The obtained solution was stirred for 1 h at 60°C at to produce a clear and homogeneous solution, and then aged for 24 h before deposition on the substrate.

Thin film preparation
FTO substrates with the dimensions of 1.5 × 5 cm 2 were first cleaned with soap, isopropanol, acetone, and deionized water in an ultrasonic bath for 10 min each. Four multilayer ZnO films were deposited on cleaned substrates using the sol-gel dip-coating method, as shown in figure 1. The first sample was prepared without the assistance of UV or MF and labeled as untreated ZnO ( figure 1(a)). The second film prepared under exposure to a UV lamp (Cole-Parmer AO-09818-0) with a short wavelength of 254 nm and an intensity of 1300 μW cm −2 . The UV lamp was positioned approximately 7 cm away from the substrate. The prepared film is labeled as ZnO: UV ( figure 1(b)). The third sample was prepared under the application of an external DC magnetic field (B = 7 × 10 −3 T) generated by an electromagnetic coil placed parallel to the substrate, and labeled as ZnO: MF ( figure 1(c)). Finally, the fourth sample was prepared under a combination of a UV lamp and magnetic field and labeled as ZnO: (UV+MF) ( figure 1(d)). The cleaned substrates were dipped vertically into the solution and withdrawn at a constant rate of 9 cm min −1 and a dipping time of 10 s. After each coating, the films were dried at 180°C for 10 min in oven before a new coating layer was applied to evaporate the solvent and remove organic residuals. Finally, all films were annealed at 450°C for 2 h. Figure 2 depicts the images of the deposited films.

Characterization techniques
Film thicknesses were estimated using a contact profilometer (Veeco Dektak-150). Structural characterization of as-deposited films was conducted at ambient temperature using an x-ray diffractometer (D-8 Discover, Bruker, 1.5406 Å, 40 kV, 40 mA CuK l = a ). The samples were scanned at a step size and scan speed of 0.0204°/min and 2°/min, respectively. The surface morphologies of ZnO films were characterized using FE-SEM (JEOL; JSM7600F). Optical transmittance spectra of the samples were measured using a UV-visible spectrophotometer (UV-3600 UV-vis-NIR Spectrophotometer). PL emissions were measured at room temperature using a JASCO FP-8200 Spectrofluorometer equipped with Xenon lamp as an excitation source.

Thickness measurements
The film thickness values are listed in table 1, which clearly shows that exposure to UV and/or MF exposure led to a gradual increase in film thickness. Notably, the simultaneous exposure to UV and MF resulted in a significant increase in film thickness (1.76 μm). This can be assigned to the synergistic effect of UV photons and   Table 1. Structural parameters, average crystallite size, micro-strain, dislocation density, and thickness values of as-deposited films. the Lorentz force, produced by the magnetic field, on the precursor solution and promote the formation of defects and vacancies that facilitate the mobility of ions and enhance the film growth. Figure 3 shows typical XRD spectra of all synthesized films recorded within a 2θ range of 20°-80°. As illustrated, all samples were polycrystalline and had a wurtzite (hexagonal) structure that matched well with JCPDS Card No. 89-1397. It is noteworthy to mention that the film deposited under the application of both the magnetic field and UV lamp had the highest peak intensities owing to its higher thickness. The lattice parameters a and c of the wurtzite cell were estimated using the following equation [36]: where l is the X-ray wavelength, q is the diffraction angle. The volume of the unit cell V was calculated using the given equation [37]:

Microstructural study
The average crystallite size (D), microstrain (e), and dislocation density ( ) d of the as-deposited films were estimated using the following equations [38]: The calculated values are listed in table 1. From this table, it can be deduced that the lattice parameters and unit cell volume values for all samples are almost identical, which indicates that UV irradiation and MF have no significant effect on the unit cell dimensions.
In addition, UV and/or FM exposure led to an increase in the crystallite size of the film, pointing a decrease in both the macrostrain and dislocation density. Moreover, the film deposited under both UV light and MF exhibits a larger crystalline size which is a sign of high degree of crystallinity. The increase in the crystallite size can be attributed to the rearrangement of the crystal lattice caused by UV irradiation and/or MF exposure. UV irradiation is known to cause photochemical reactions and the generation of free radicals that can lead to the rearrangement of the crystal lattice [39,40], whereas the magnetic field can cause the alignment of magnetic dipoles along the direction of the field, leading to crystal orientation. These effects may have reduced the number of defects in the film and enhanced the quality of the crystalline structure, resulting in larger crystallite sizes.  (table 2). The rise in average particle size of the synthesized nanostructured ZnO films with the application of UV and magnetic field can be attributed to the Ostwald ripening process [7,41]. During the synthesis process, smaller particles tend to dissolve and re-deposit on larger particles, leading to an increase in particle size over time. It is believed that the UV irradiation and magnetic field may enhance the diffusion of precursor species to facilitate particle growth, resulting in the formation of larger nanoparticles. Figure 4(a) reveals the presence of particle agglomeration in certain areas of the untreated ZnO surface film. In addition, it can be seen that the surfaces of both the untreated and deposited under a magnetic field (figure 4(c)) films show highly porous compared to other samples. Nevertheless, porosity increased when a magnetic field was applied which is linked to the magnetic field acting on the particles, causing them to rearrange and align in the direction of the MF, leading to a higher degree of packing and increased porosity. A similar observation was reported by Sáaedi et al [42] for ZnO/PAni nanocomposites synthesized by spin coating under  different magnetic fields. In contrast, the film surfaces of the ZnO: UV film (figure 4(b)) and ZnO: (UV+ MF) (figure 4(d)) became more compact with fewer pores. Tseng et al [43] noticed that the UV irradiation can enhance the surface of deposited ZnO film because it has sufficient energy to destroy the molecules of the solvent, and therefore, reduces the surface tension of the solvent and organic residues. In general, the UV light energy at a wavelength of 254 nm (472 kJ mol −1 ) is greater than the bonding energy of the organic content and hydrogen, such as C-H (413 kJ mol −1 ), C-C (348 kJ mol −1 ), and C-O (352 kJ mol −1 ) [44], thus it can decompose the oxygen in the air into monatomic oxygen or form ozone O . 3 ( ) Both monatomic oxygen and O 3 have high oxidizing energies and they easily penetrate and diffuse into the thin film owing to their small size. Consequently, the oxidation of the organic components was accelerated to form ZnO particles in the thin film, resulting in a smooth and compact surface of the film and enhanced its crystallinity [43]. Figure 5 illustrates the transmittance and reflectance spectra of the as-deposited thin films measured over a wavelength range 300-800 nm. The transmittance spectra ( figure 5(a)) reveal that all films display transparency to visible light but appear opaque to UV light. The untreated ZnO, ZnO: UV and ZnO: (UV+MF) films exhibit almost the same transparency in visible light with the average value in the vicinity of 80% (table 2). A lower transparency in visible light (73.88%) was recorded in the ZnO: MF film, which may be attributed to the surface porosity ( figure 4(c)). In general, porous or rough surfaces scatter incident visible light, resulting in a decrease in transmittance and an increase in the probability of light absorption or reflection [45]. The reflectance spectra of the deposited films are shown in figure 5(b). It is evident that all films demonstrate a significant increase in reflectance in the UV region, reaching a maximum value of approximately 21%. Subsequently, there is a gradual decrease in reflectance in the visible region. The decrease in reflectance in the visible region indicates that the film surfaces reflect less light, allowing a considerable portion of incident light to pass through.

Band gap calculation
The optical direct band gap energy, E , g of synthesized films was estimated using the well-known Tauc's equation [46]: where hn denotes the incident photon energy, k proportionality constant, n denotes the nature of the electronic in semiconductors (n 1 2 = for a direct band gap and 2 for an indirect band gap), and α is the absorption coefficient calculated from the transmittance data as follows [47]: where d is the thickness of the film and T is the transmittance.   [32]. The broadening of the bandgap may be linked to the Moss-Burstein effect, as photogenerated electrons occupy states in the conduction band, causing an increase in the Fermi level within the ZnO film. Nevertheless, Soleimanpour and Jayatissa [48] observed a decrease in the optical bandgap (3.37 eV to 3.28 eV) after UV irradiation and attributed this decrease to the improvement in film crystallinity. In addition, the dip-coated ZnO film prepared under MF exhibits a remarkable reduction in the bandgap from 3.36 to 3.26 eV. Previous studies have also observed a narrow bandgap in ZnO thin films after MF exposure [34,49]. This can be explained by an increase in the film thickness or an increase in the impurity band as a result of the magnetic field, leading to a decrease in the optical bandgap. In addition, the application of a magnetic field can modify the spin orientation and electronic structure of the material, resulting in the band gap changes. For instance, the magnetic field could modify the density of states near the band edges and affect the band gap through the exchange interaction between the magnetic moments of electrons and the magnetic field. Moreover, the ZnO: (UV+FM) film exhibits a higher optical bandgap (3.67 eV), making it a suitable candidate for ultraviolet lasers and light-emitting diodes (LEDs). The broadening of optical bandgap can be related to changes in the surface or interface properties of the ZnO nanostructure film. ZnO is known to have several types of surface defects, such as oxygen vacancies (V O ) and zinc interstitials (Zn i ) defects, which can act as charge traps and affect the electronic properties of the material [50]. Moreover, UV irradiation and magnetic field could induce changes in the concentration and distribution of these defects, which could alter electronic structure of the ZnO nanostructure film.

Refractive index (n) and extinction coefficient (K)
The calculation of the refractive indices of optical materials is important for applications in optical devices, such as switches, optical communication, and optoelectronic devices [51], [52]. The refractive indices (n) and extinction coefficients (K) of the synthesized samples were evaluated using the following relationships [47]: where R is the reflectance of the film. Figure 7(a) displays the plot of n as a function of the wavelength over the range of 300-800 nm. As shown, the curves of all films follow a similar pattern, increasing in the UV region until reaching a greater value at 340 nm, and then decreasing to become almost constant with the wavelength in the 500-800 nm region. This behavior can be explained by the fact that the electronic transitions in ZnO thin films occur in the UV region, leading to an increase in refractive index [51]. Moreover, the highest refractive index in the visible region was recorded for the ZnO: MF thin film. The rise in refractive index is linked to the decrease in the optical bandgap and transmittance. Besides, the extinction coefficient can be used to measure the surface smoothness and uniformity of synthesized ZnO thin films, where lower values of K denote smoother and more uniform surfaces [53]. The extinction coefficients of the prepared multilayer thin films were plotted against wavelengths ranging from 300 to 800 nm, as illustrated in figure 7(b). According to this figure, the simultaneous UV and MF exposure decreases in K values of the Zn: (UV+MF) film, which signify an improvement in its surface quality and uniformity. These results suggest that the ZnO: (UV+MF) film may be a suitable candidate for applications that require high surface quality and uniformity, such as in optoelectronic devices.

Photoluminescence results
Defects and impurities play an important role in the development of optoelectronic devices in the semiconductor industry. Photoluminescence is a sensitive nondestructive technique that can be used to identify defects in materials. Figure 8 shows PL spectra of as-deposited multilayer ZnO thin films recorded over a wavelength range of 375-500 nm. As illustrated, all samples exhibit two emission bands: a weak emission peak in the UV region and a strong and sharp emission peak in the visible region (400-450 nm). The UV emission peak centered at ∼385 nm corresponds to the near-band-edge (NBE) emission of ZnO and is induced by the radiative recombination of free excitons from localized levels near the conduction band to the valence band [54]. The intensity of the NBE emission is directly proportional to the number of excitons in the material, which is determined by the properties of the film, such as its crystalline structure, thickness, and doping level [55]. While the visible emission (VE) peak is attributed to the emission of electrons and holes that recombine at different defect states or impurities within the ZnO matrix [56]. The VE is sensitive to the presence of impurities or defects in the ZnO matrix and can be used to study their properties. The origin of this peak is still a subject of debate in the scientific community, and it is generally believed to arise from different defect states, such as V O and Zn i [57].    The visible emission appears in the violet luminescence at approximately 405 nm, 412 nm, and 422 nm for all thin films which can be ascribed to the transition of an electron from the Zn i defect level, located below the conduction band, to the valence band [58]. Nevertheless, as compared to the untreated ZnO, the violet emission intensity of the ZnO: UV and ZnO: MF films was found to be 2 times and 1.3 times lower, respectively, owing to a reduction in Zn i defects. The considerable drop in the intensity of violet emission seen in the dip-coated film subjected to UV irradiation was most likely caused by monoatomic oxygen and ozone penetration into the film, which occupied interstitial sites, reducing Zn i defects. In addition, the combined UV and MF exposure did not reduce Zn i defects, and the violet emission intensity was almost identical to that of the untreated film. These findings pave the way for the fabrication of violet LEDs and lasers.

Conclusion
High purity untreated ZnO, ZnO: UV, ZnO: MF, and ZnO: (UV+MF) nanostructured thin films were successfully deposited on FTO substrates using dip coating method. XRD patterns revealed that all as-deposited films had a polycrystalline wurtzite structure. Furthermore, the simultaneous UV and MF exposure improved the film crystallinity. FE-SEM images showed that film surfaces were covered with sphere-like nanoparticles  with the average size ranged from 30.69 to 34.91 nm. The ZnO: (UV+MF) film exhibited a high optical bandgap (3.67 eV) and a lower extinction coefficient in the visible range. The lower extinction coefficient is a sigh of an improvement in its surface quality. PL spectra of all films revealed a UV weak emission at around 385 nm and a strong and sharp violet emission peak. Nevertheless, as compared to the untreated ZnO, the violet emission intensity of ZnO: UV and ZnO: MF films was found to be 2 times and 1.3 times lower, respectively, owing to a reduction in zinc interstitials (Zn i ) defects. The simultaneous application of UV and MF treatment did not diminish Zn i defects, and the intensity of violet emission was almost identical to that of the untreated film. Our findings provide valuable insights into the optimization of ZnO thin films for various optoelectronic device applications and contribute to the advancement of this field.