INTRODUCTION 
Laser-induced plasma (LIP) is formed through the interaction of intense laser radiation with a material when the laser irradiance exceeds a material-dependent threshold [1]. This phenomenon has attracted considerable interest because of its applications in various scientific and technological fields, including pulsed laser deposition (PLD ([2], laser ablation in liquids for nanostructure synthesis [3], and laser-induced breakdown spectroscopy (LIBS) [4], etc. Pulsed laser deposition (PLD) is a versatile technique widely employed for the in-situ fabrication of thin films and nanoscale materials. In this process, high-energy laser pulses irradiate a solid target inside a vacuum chamber, causing material ablation and the formation of a plasma plume. The ejected species subsequently travel toward a substrate, where they condense to form a thin film that may reach a thickness of several micrometers, depending on the deposition conditions [5, 6]. The window, through which a laser beam enters and hits the target, or the material to deposit, are located in vacuum. This results in energy of the laser-photon beam being absorbed by local solid surface, triggering electronic excitation. The thermal conductivity of the solid target is much too low to permit any significant energy dissipation during the very short duration of the laser pulse, leading to a conversion of photon into thermal and chemical energy. This results in such a high temperature (about 3500 °C) for a small volume of the target, generating instantaneous evaporation, ionization and decomposition of the material [6, 7]. The vapour consists of neutral atoms, positive and negative ions, electrons, molecules and molecular ions, free radicals of the target material in their ground state or excited states. Such correlated whirl forces picturing a mass of plasma particles is known as plume (or wavefront). These are harvested on the substrate to create a thin film [2, 7]. Rapidly-evolving and sensitive plasma processes is transient and observed to be highly dependent on incident laser fluence (laser energy/pulse area), wavelength, ambient gas composition, and pressure [6]. Nanocrystalline metal oxides have received increasing attention because of their potential applications in various fields. Among these materials, nickel oxide (NiO) is a p-type semiconductor characterized by good chemical stability and a wide band gap of approximately 3.6–4.0 eV [8-10]. These properties make NiO a potential candidate for applications in solar cells, catalysis, sensors, and optoelectronic devices [10-12]. NiO thin films have been prepared by various techniques such as sol–gel [13], co-precipitation [14], spray pyrolysis technique (SPT) [15], Dc magnetron sputtering [16], microwave method (MM) [17], electron beam evaporation techniques [18], chemical vapour deposition (CVD) [19], and atomic layer deposition (ALD) [20]. PLD is a versatile process. It can accurately control the composition, structure and growth rate of the film. It is an important technology for preparing high-quality thin films and is well suited to regulate the properties of the film [6, 21]. In this study, the structural, optical and morphological properties of NiO thin films prepared by PLD method were investigated.

MATERIALS AND METHODS
The PLD was performed in a vacuum chamber of 2.5×10-2 mbar pressure. A Q-switched Nd:YAG laser beam was passed through a Pyrex glass window at an incidence angle of 45° to the target surface normal. The substrate was placed opposite the target surface, separated 2 cm from it. It also prevented the substrate holder from interfering with the path of the incoming laser beam. The films were deposited over 200 laser pulses using a Q-switched Nd:YAG laser at λ = 1064 nm (1.16 eV) with pulse energies of either 500,600,700, or 800 mJ. The target was periodically rotated around its axis to reveal a new surface, mitigating the effects of irradiating the same spot multiple times. This focused laser beam ablated a small amount of target material, which has been transported into the substrate and deposited on glass surfaces (Fig. 1).
High-purity nickel oxide (NiO) powder (99.99%) was used to prepare the targets for thin-film deposition. A total of 2 g of powder was compressed into pellets using a hydraulic press. The resulting pellets had a diameter of 10 mm and a thickness of 4 mm. These pressed pellets were used as target materials for the deposition process.
X-ray diffraction (XRD) measurements were performed at room temperature using Cu Kα radiation with a wavelength of 1.5406 Å. The operating voltage and current were set to 40 kV and 30 mA, respectively. The scanning speed was 5°/min. Based on Scherrer equation, average crystallite size (D) for the sample were calculated as follow [22, 23]:

In Scherrer equation, λ, β, and θ are the X-ray wavelength, full width at half maximum (FWHM) of the diffraction peak and Bragg diffraction angle, respectively. The surface morphology and microstructure of the NiO thin films were performed on glass substrates using an Inspect S50 SEM. In addition, atomic force microscopy (AFM) of the NiO thin films was employed to further investigate the surface topography and distribution of features across the samples using a CSPM-5000 scanning probe microscope.

RESULTS AND DISCUSSION
XRD analysis
The XRD patterns for NiO films obtained using varying amounts of laser energy are shown in Fig. 2. It was found that all samples revealed a polycrystalline. The five most intense diffraction peaks centered at 2θ= 37.24o, 43.32o, 62.88o,75.29o and 79.28o which indexed to the planes of (111), (200), (220), (311) and (222), respectively, matched with JCPDS card No. 47-1049 of face center cubic structure of NiO [24]. No additional diffraction peaks arising from secondary crystalline phases observed for NiO films, indicating that all prepared thin films of these samples were pure. With increasing laser pulse energy, slight shifts in peak positions may be related to effects of lattice strain and crystallite size’s changes. The XRD patterns revealed that the peak intensity increases and the peak width decreases with increasing laser pulse energy, suggesting an enhancement in crystallinity and crystallite size (Table 1). Higher particle sizes with laser energy (of more laser energy) are enable to the ablation of larger quantities of the target and also, the higher energy aids to amalgamate small particles between them [25].

Optical properties
The optical absorption spectra of NiO nanostructures prepared at laser pulse energies ranging from 500 to 800 mJ are shown in Fig. 3. The range of the spectra is 300–1100 nm. The absorption peaks at 352, 333, 322 and 311 nm corresponding to the pulse energies of 500,600,700 and 800 mJ, respectively, was associated with the effect of quantum confinement of the nanostructure on carrier generation (electron - hole) [26]. As the laser pulse energy increased, the absorption peak shifted toward shorter wavelengths, indicating a blue shift that may be associated with changes in particle size, as suggested by the SEM and AFM observations [27]. The absorption spectra also showed differences in the visible and near-infrared regions. These variations may reflect changes in the optical properties and morphology of the prepared NiO nanostructures. However, the shift cannot be attributed solely to particle-size reduction. The relationship between wavelength and absorbance depends on the material’s optical properties and the interaction of light with the film [9]. In such a case, absorbance factor increases. The absorption spectra showed an enhancement in absorption intensity and peak broadening as the laser pulse energy increased. These changes may be associated with variations in the size and structural properties of the NiO nanoparticles, as reported in a previous study [28]. In contrast, the transmittance spectra exhibited an opposite trend, as shown in Fig. 4.
The optical band gap energy (Eg) is an important parameter for characterizing semiconductor materials and determining their potential for optical and electronic applications. It is thereby related to material structural and electronic properties, which are determined by the arrangement of the atoms in a crystal lattice. The value of Eg is dictated by the crystal structure, indicating that it responds to local atomic arrangement in semiconductors [29, 30]. The value of band gap energy for NiO films was estimated by using Tauc equation [22]:

In this equation, α represents the absorption coefficient, hν denotes the photon energy, and C is a proportionality constant. The exponent n depends on the type of electronic transition, n =1/2 for a direct allowed transition and n = 2 for an indirect allowed transition [31]. Fig. 5 shows the variation of [(αhν)]2 as a function of incident photon energy (hν). Values of the band gab energy have been extracted from an extrapolation of the linear region for the curve to intersect with the energy axis (hν). The estimated optical band gap values were 2.7, 3.0, 3.1, and 3.4 eV for NiO films prepared at laser pulse energies of 500, 600, 700, and 800 mJ, respectively. The enhancement in band gap may be associated with changes in particle size and the resulting quantum confinement effects, although this interpretation requires supporting evidence of nanoscale dimensions [28]. Furthermore, it is noted that the Eg values obtained for NiO thin films in this work indicated that performance has positively related to film crystallinity, which also verifies with earlier experiments [26, 31].

Morphology
The FESEM images of NiO nanostructures prepared at different laser energies are indicated in Fig. 6. This Figure clarified that at a lower value of laser fluence (500 mJ), the apparent grain size is relatively large due to the coalescence and clustering of smaller particles, a phenomenon known as agglomeration, where the emergence of agglomeration in thin films is associated with PLD [32]. As illustrated in Fig. 6b, and Fig. 6c, the thin films have low agglomeration, as energy increases. Whereas, at the highest value of laser fluence (800 mJ), film surface appears more homogeneous and dense. It geographies a widespread, dense distribution of nano-sized grains. Therefore, as the laser energy increases during PLD process, the produced amount of materials increases and the particle size decreases. Variations in particle size can be explained by the balance between nucleation and growth processes occurring at lower or higher laser energy levels, respectively. When the laser energy is low, fewer nuclei form that allow for the available material to help in forming larger particles. Higher laser energies may favor formation of more nuclei and hence smaller particles. The particle size can be affected by many factors like ablation rate, transport of material and growth conditions [33, 34]. This morphology is anticipated to increase the effective surface area and improve the sensing performance of the NiO film.

EDS analysis
EDS is a general analytical method for the detection and quantitative measurement of any element in trace amounts within a material. The electron beam interacts with the sample and this interaction almost releases the X-ray characteristics. The energies of theses X-rays are used to identify the elements, and their intensities provide information on the elemental composition. Fig. 7(a-d) presents the EDS spectra for the NiO nanoparticles, revealing the presence of peaks corresponding to nickel (Ni) and oxygen (O) at laser pulse energies ranging from 500 to 800 mJ, respectively. EDS analysis provided information on the elemental composition of the synthesized NiO nanoparticles, complementing the structural data obtained via XRD. As shown in Fig. 7, the spectra exhibit characteristic signals for nickel (Ni) and oxygen (O), with their respective weight and atomic percentages provided for each sample. The horizontal axis represents X-ray energy (in keV), while the vertical axis indicates the number of detected X-ray pulses. The measured Ni-to-O ratios deviate from the ideal stoichiometric composition, suggesting a non-stoichiometric structure. These deviations may be linked to defects within the NiO lattice and are consistent with its behavior as a p-type semiconductor. However, EDS analysis is insufficient to definitively identify the specific type of defects or to reveal the origin of the p-type electrical conductivity [35]. 

AFM analysis
AFM was study the characterize NiO’s surface topography obtained by PLD at 500–800 mJ of laser energy. As shown in Fig. 8, at the lower value of laser fluence (500 mJ), the image consists of large agglomerations with smooth upper surfaces. This is agreement with FESEM image. These large clusters reduce the specific surface area and leads to an inhomogeneous grain distribution across the film surface [36]. Contrary, at the highest value of laser fluence (800 mJ), image clearly exhibits highly dense, closely packed nanostructures. This massive number of micro-peaks and valleys indicates a significant enhancement in the number of grains per unit area, accompanied by a reduction to the nanoscale. However, as the energy increases, the grain size tends to decrease, resulting in the formation of smaller crystalline grains [37]. Consequently, this leads to a higher surface area compared to the agglomerated NiO-500 sample, even though the roughness of the current sample is higher than that of the NiO-800 sample. Where, the average roughness may decrease with increasing laser pulse energy due to changes in surface morphology, such as improved surface uniformity or grain refinement [37]. Similarly, the root mean square roughness, which reflects variations in surface height, may also decrease under certain deposition conditions. These trends depend on the material and the laser parameters used [38]. Table 2 includes AFM parameters such as average roughness, mean diameter, and root mean square height deviation. The results showed that the sample prepared at 800 mJ have the highest quality, which is an important factor for gas sensing applications. 

CONCLUSION
NiO thin films were deposited by PLD on glass substrates at different laser energy (500, 600, 700, and 800 mJ), and then examined. Results from XRD revealed that NiO films consists a polycrystalline face center cubic structure and the degree of crystal growth of the films was raised with increasing laser energy. In addition, optical investigation was demonstrated that nanoscale electron confinement (quantum size effect) led to an enhancement in the NiO thin film’s energy gap with decreasing sizes. The FESEM results illustrated more homogeneous and dense films surface of NiO grown at 800 mJ, while the EDS results revealed non-stoichiometric behavior of p-type NiO semiconductor. Similarly, AFM results exhibited minimum grain size and maximum surface area at 800 mJ. Therefore, the optimized NiO thin films with 800 mJ was found to be the best for gas sensors applications.

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