Comparative Study of the Structural, Electrical, and Antibacterial Properties of ZnO, NiO, Fe2O3 Oxides Prepared by Pulsed Laser Deposition Technique

INTRODUCTION
Over the past decades, the pulse laser deposition (PLD) has become particularly relevant as one of the most sophisticated ways of creating high purity nanostructured films and oxides. This method is characterized by the evaporation of a solid target by laser pulses of high-energy in the prepared medium to allow homogeneous sub layers to be deposited with strictly determined crystalline structure and thickness to be used [1,2]. This method has been employed with much success to synthesize transition metal oxides (including zinc oxide, nickel oxide, and iron oxide) because of their characteristic properties, including: wide band gap, optical activity, and chemical stability since the 1990s [3,4]. One of the most valuable semiconductor oxides (3.3 eV band gap) is zinc oxide and it has found applications in sensing, photonics and in solar cell applications [5,6]. Nickel oxide is one of the P-type semiconductor being magnetic in nature with high bactericidal capacity at nanometer scale which is applicable in electrodes and gas sensors. In the meantime, iron oxide (hematite) has outstanding photo catalytic and electrochemical activity, as well as, anti-bacterial growth activity due to the production of reactive oxygen species (ROS). PDL method enables the optimization of structural and electrical characteristics of these oxides through the control of the laser intensity, the oxygen pressure, and the temperature of substrate hence enhancing both the conductive characteristics and antibacterial actions on a functional basis. The recent reports have indicated that zinc oxide, nickel oxide and iron oxide films synthesized by this method exhibited regular crystal structure, increased conductivity as well as high chemical stability that make them potential application prospects in sensors, optical devices, or biomedicine [7,8]. The importance of this study is that, three metal oxide nanostructures, i.e., NiO, ZnO, and Fe2O3, were prepared and characterized via pulsed laser deposition (PLD) technology, which is a sophisticated and efficient method to prepare a quality film of different materials. This technique helps in enhancing the structural and surface properties of the films making them more suitable to be used in industrial and medical applications. Moreover, the correlation of these films in structure and electrical properties to their antimicrobial activity gives a fresh insight on the application of such films in bioelectronics devices and antimicrobial materials, and the creation of medical applications on the basis of nanotechnology [9,10].

MATERIALS AND METHODS
Laser Deposition Method for Thin Films
Thin films of ZnO, NiO, and Fe2O3 were deposited on glass substrates using a pulsed laser deposition (PLD) system with Nd-YAG laser (1064 nm, 6 Hz). High-purity metal targets of (Zinc oxide, Nickel oxide and Iron oxide) were used laser ablation was performed in an Oxygen atmosphere at pressure of 2.5×10-3 mbar. Laser energy ranged from 500 to 700 mJ. The optimum deposition time was determined to achieve a film thickness of about 200 nm. Note the shape and layout of the system with photos of the samples after deposition (Fig. 1). 

RESULTS AND DISCUSSION
X-ray Diffraction (XRD)
X-ray diffraction analysis was performed to study the crystal structure of the samples prepared using pulsed laser deposition (PLD). This analysis aimed to identify the crystal phases and evaluate the crystallinity and purity of the samples. The XRD patterns of NiO samples prepared by PLD at 500 and 700 mJ show that all peaks correspond to the cubic (rocksalt) structure of NiO (see Fig. 2).  The peaks at 700 mJ exhibit higher intensity and narrower width, particularly for the (111) and (200) planes, indicating improved crystallinity and larger crystallite size in these directions.  In contrast, the (220) and (311) peaks show broader widths and smaller calculated crystallite sizes, suggesting anisotropic growth or internal stress. The crystallite size values were calculated using the Scherrer equation (D=Kλ/(β cosθ )), which depends on the full width at half maximum (FWHM) of the peak and the diffraction angle, and provides an estimate of the average crystallite size along the specific plane. Overall, the average crystallite size decreased from approximately 17.9 nm at 500 mJ to approximately 15.1 nm at 700 mJ (Table 1). This change is attributed to the higher laser energy, which increases particle energy and surface mobility, improving crystallinity in some directions, but may also introduce defects or internal stress, leading to smaller crystallite sizes in other directions.  This is consistent with previous literature, which also suggests the possibility of anisotropic crystal growth or preferred crystallographic orientation [11, 19, 20].
The X-ray diffraction (XRD) results for the zinc oxide samples prepared using the PLD technique at two different laser energies (500 and 700 mJ) (see Fig. 3) showed distinct diffraction peaks at the (100), (002), (101), (102), (110), (103), and (112) planes, corresponding to the hexagonal (wurtzite) structure of ZnO. The peaks at 700 mJ were sharper and more intense, indicating improved crystallinity and larger crystallite size, while the peaks at 500 mJ were broader and less intense, reflecting a smaller crystallite size and lower crystallinity. Calculations of the crystallite size revealed values ​​of approximately 17.5 nm at 500 mJ and 23.4 nm at 700 mJ (Table 2) demonstrating that increasing the laser energy leads to larger crystal growth and improved crystallinity, which is consistent with previous findings in the literature [15,17,21].
XRD analysis of iron oxide nanoparticles synthesized by pulsed laser ablation at different temperatures 500 and 700 mJ indicates that the sample synthesized at 500 mJ has broad peaks and low intensity with higher values of FWHM show in Fig. 4. This shows reduced crystallite sizes, 5 to 24 nm in size and a relatively low level of crystallinity. In contrast, the samples prepared at 700 mJ showed sharper and more intense peaks with smaller FWHM values, reflecting significant crystal growth, an increase in crystallite size to approximately 44 nm, and improved crystallinity, as shown in Table 3. The smaller crystallite size at 500 mJ results in a larger surface area and more crystal defects, thus increasing the chemical activity of the material. Conversely, increasing the temperature promotes crystal growth, reduces grain boundaries and defects, and improves crystallinity at the expense of a reduced specific surface area [16,18].

Morphology analysis using FE-SEM
The surface morphology and grain size distribution of the deposited thin films were studied using the field emission scanning electron microscopy (FE-SEM). Fig. 5 represents the scanning electron microscope (FE-SEM) images of (NiO) nanoparticles prepared by the method of PLD at a wavelength of 500 and 700 nm. The shape of the particles is almost spherical with a certain level of agglomeration of the particles attributed to the close contact of the particles. The particles at 500 mJ are smaller and seem to be more homogeneous and were of a size of about 44.15 nm on average. The particles become bigger and less regular in its shape at 700 mJ and their average diameter remains at about 51.89 nm. Such difference is explained by the higher power of laser which enriches the motion of atoms on the substrate surface, providing the better opportunity of coalescence of nuclei and formation of bigger crystals. This also results in a more widely distributed size composition, as can be seen in the accompanying graphs [11, 19, 23]. The FE-SEM images shown in Fig. 6 of ZnO nanoparticles prepared by pulsed laser deposition (PLD) technique at two different energies, 700 and 500 mJ, as well as the particle size distribution plots, show that increasing the laser energy from 500 to 700 mJ resulted in an increase in the average particle size to about 23.24 nm, indicating the formation of larger particles at higher energies. Morphologically, samples prepared at 500 mJ exhibited relatively homogeneous, fine-grained spherical particles with a partially agglomerated appearance, while samples prepared at 700 mJ exhibited larger and more diverse aggregates in shape and size, indicating that increasing laser energy promotes particle growth and increases their heterogeneity. This behavior can be explained by the fact that increasing the pulse energy during the PLD process leads to an increase in the ejection rate and energy of the ejected species towards the surface, which enhances their dispersion and coalescence, resulting in larger and less homogeneous particles [14, 15, 24]. (FE-SEM) images and size distribution plots of iron oxide particles prepared in the same way at two different energies (500 and 700 mJ) (as shown in Fig. 7) show that laser energy clearly affects both size and shape, with the average particle size at energy 500 mJ at about 35.8 nm. The particles appeared partially clumped with irregular edges and semi-irregular shapes that tended to merge with each other, indicating inhomogeneous growth and a wide distribution in particle size. At 700 nm, the average size decreases to approximately 22.7 nm, and the particles appear more regular and closely shaped, with pronounced spherical tendencies, a more homogeneous distribution, and better surface coverage. This difference is attributed to the fact that increasing the laser power in PLD technology increases the ejection rate and the number of nucleation centers, which leads to the formation of a larger number of small, closely shaped particles, while lower energy allows for the growth of larger, less regular particles, which explains the contrast in images and histograms between the two energies [16,18,25].

DC electrical conductivity
The electrical conductivity curves of (ZnO) ,(NiO) and (Fe2O3) prepared by pulsed laser deposition (PLD) technique at two different energies (500 and 700 mJ) showed a distinct semiconductor behavior for all samples, (as shown in Figs. 8, 9 and 10). The electrical conductivity increased with temperature due to the activation of charge carriers (electrons or holes) and the improvement of their mobility within the crystal lattice. It was observed that the samples prepared at 700 mJ had higher conductivity than their counterparts at 500 mJ due to improved crystallinity, reduced structural defects and increased oxygen vacancies that act as electron doping centers. The 700-ZnO sample showed a clear increase in conductivity compared to the 500-ZnO sample due to fewer defects and grain boundaries [9,27]. While the 700-NiO sample showed the highest thermal response due to increased vacancy density and improved crystal order, which reduced internal resistance and facilitated charge transfer between grains [13,28]. The Fe2O3 sample deposited at 700 mJ showed higher conductivity due to improved crystallinity and increased oxygen vacancies compared to the 500 mJ sample. It can be concluded that increasing the preparation energy from 500 to 700 mJ improved the electrical conductivity of all oxides [4,26]. The three are clearly distinguished, thanks to the improvement of the crystal structure, the reduction of defects, and the increase in the number of effective charge carriers. In comparison between the materials, (NiO) showed the highest conductivity and thermal response, followed by (Fe2O3) and then (ZnO), which reflects the difference in charge transport mechanisms depending on the nature of each oxide and its crystal structure.

Antibacterial activity
Antibacterial activity against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aurous) bacteria was tested using the agar diffusion method. Zones of inhibition were measured after incubating the samples for 24 hours at 37°C. Antibacterial tests of thin films prepared by pulsed laser deposition (PLD) of ZnO, NiO, and Fe₂O₃ oxides at 500 and 700 mJ showed a significant effect against both E. coli (Gram-negative) and Staphylococcus (Gram-positive) bacteria (see Figs. 11, 12, and 13). The inhibition effect increased with increasing sample concentration from 12.5% to 100%. Increasing the laser energy from 500 to 700 mJ improved the biological activity of all samples due to the increased surface area. Zinc oxide (ZnO) showed the highest antibacterial activity compared to other oxides, due to its high production of reactive oxygen species (ROS) [22, 14, 12]. It also exhibited wide inhibition zones and strong activity, especially against E. coli, indicating its high ability to penetrate the cell wall of Gram-negative bacteria. Nickel oxide was the least effective on the list of the other oxides, and had reasonable antibacterial activity. Nevertheless, its action was stronger against Gram-positive bacteria like Staphylococcus aureus as opposed to its action against Gram-negative Escherichia coli. Its limited effectiveness is due to its weak effect at high concentrations, especially when dealing with Gram-positive strains with stronger wall structures. Based on the results, the biological activity of the studied oxides can be ranked according to their efficiency as follows: ZnO, Fe2O3, and NiO. It was also observed that increasing the laser energy used in the preparation from 500 to 700 mJ led to a significant improvement in the ligand capacity of these films, confirming the association between nanoscale properties and molecular structure and its direct role in antibacterial activity [10, 18, 29].Table 4 shows the antibacterial activity of Nickel oxide, Zinc oxide, and Iron oxide nanoparticles prepared using PLD at two different energies, 700 and 500, against Staphylococcus aurous and Escherichia coli, the inhibition zone was measured in millimeters at different concentrations (A constant group, B 2.5%, C 25%, D 50%, E 100%). It is clear that the Zinc oxide nanoparticle sample at 700 PLD showed the largest inhibition zones against bacteria (up to 30-32 mm against S. aurous and 24-25 mm against E. coli), indicating the powerful effect of (ZnO), especially at high energies. On the other hand, (NiO) particles showed relatively less inhibition, with maximum values of 22-24 mm against S. aurous and 18-21 mm against E. coli. Fe₂O₃ showed intermediate activity, achieving concentrations of 27-28 mm against S. aurous and 20-24 mm against E. coli. Overall, the response was stronger against S. aurous (Gram-positive bacteria) than against E. coli (Gram-negative bacteria). (Gram), and increasing the concentration (from A to E) and increasing the preparation energy led to enhanced antibacterial activity, which was associated with an increase in the size of the inhibition zone [30,31].

CONCLUSION
Zinc, nickel, and iron oxide films were prepared by pulsed laser deposition (PLD) at two energy levels (500 and 700 mJ). The results showed that increasing the laser energy improved the structural, morphological, conductive, and biological properties. XRD examinations showed that the samples were crystalline pure, with improved crystallinity as the energy increased, with a clear increase in the crystallite size of ZnO and Fe₂O₃, and a slight decrease in NiO. FE-SEM images showed tightly packed spherical particles, the size of which was affected by increasing energy. The electrical conductivity of all samples increased with temperature and power, and was highest for NiO, followed by Fe₂O₃ and then ZnO. 
ZnO showed the highest antibacterial activity followed by Fe2O3 and NiO, with increased efficacy at higher energy levels. The results confirm that the improvement of the crystal structure resulting from the increase in laser energy enhances the electrical and biological performance of these oxides, making them suitable for bioelectronics and antibacterial applications. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.