Analysis of Zinc Oxide (ZnO) Thin nano Film Produced Using the Sol-Gel Method

INTRODUCTION
ZnO is a semiconductor that is a member of the II–VI wurtzite family and may be deposited in thin film using a number of methods [1].  Since ZnO is electrically conducting and optically transparent in the visible range, thin nano-films of ZnO have found many uses. The most well-known of these uses is as solar cell windows [2]. ZnO may also be more affordable to produce than other commonly used transparent conducting thin films due to its nontoxicity and comparatively abundant supply. Because of their potential applications in field emitters, photodetectors, biosensors, short wavelength light emitting diodes, field emitters, field emitters, ultraviolet laser emission, and information technology. Considerable research endeavors have been undertaken in recent times to fabricate ZnO thin nano-films that possess a high degree of orientation and transparency [3]. ZnO, a semiconductor material belonging to the II-VI group, possesses a significant band gap of 3.3 electron volts at ambient temperature. Additionally, it exhibits a substantial excitonic binding energy of approximately 60 meV. Various applications extensively use ZnO thin nano-films because of their distinctive optical, electrical, and semiconducting characteristic. Despite the adoption of several approaches for the creation of ZnO thin nano-films, there is still a need to investigate methods for monitoring the shape, size, crystallinity, and various parameters that affect these materials. Consequently, it is imperative to examine the most favorable conditions for the production of ZnO thin coatings that are both transparent and highly oriented [4]. Researchers’ main concern is to improve the quality of material stoichiometry. Different methods, such as magnetron sputtering, MOCVD, spray pyrolysis, and pulsed laser deposition (PLD), are employed to produce ZnO thin nano-films [5]. They have garnered particular attention for their potential as power sources.  Because of its comparatively simple method, which eliminates the need for a costly vacuum system, and its wide-ranging advantages of big area deposition and uniform film thickness, the sol-gel technique has gained widespread adoption [6]. The sol-gel process offers additional advantages for thin film deposition, including superior stoichiometry control and straightforward doping in film composition.  Previous studies have recorded the physical and structural characteristics of ZnO thin nano-films produced via the sol-gel method utilizing various inorganic and/or organic precursors and deposition criteria [7-10]. The optical, vibrational, structural, and photoluminescent characteristics of ZnO thin nano-films synthesized using Sol-gel employing a zinc acetate precursor on a quartz substrate [11]. 

MATERIALS AND METHODS
Analytical grade reagents were used for chemical synthesis in the current investigation. First, 2-methoxyethanol ((CH3)2CHOH) was used as a solvent to dissolve the compound zinc acetate dehydrate (Zn (CH3COO)2·2H2O), and mono-ethanolamine (MEA: H2NCH2CH2OH) was added as a stabilizing agent. The zinc acetate concentration was 0.60 mole per liter, and the stoichiometric ratio between zinc acetate and mono ethanolamine (MEA) was kept at 1.0. After stirring the mixture for an hour at 70 °C the end product was a clear, homogeneous solution that could be coated. The application of the coating was carried out using a solution that had been recently prepared. The films on quartz substrates that had been cleaned using ultrasonic methods were created using a spin-coating equipment. For 40 seconds, the device was rotated at a speed of 3000 revolutions/minute. The nano-films were preheated in a furnace for five minutes at 30 °C. This procedure was used to help the solvent evaporate and get rid of any organic residues that could have been left behind. Ten iterations of the spin coating to the preheating method were performed. After that, the nano-films were allowed to anneal for three hours in the air at 500 °C. The categories of the ZnO thin nano-film were identified using the X-ray diffraction method. This was done using an analytical Diffractometer of type PW3710, using Cu Ka radiation (λ = 0.154 nm). The UV-Visible double beam spectrophotometer model (Jasco-V570) was utilized to measure the optical.
Thickness (d) of the films can be determined by applying the formula [12]:

in where λ is the incident photon ray’s wavelength, ∆x is the shifting between the fringes, and x is the fringe width.  The fundamental absorption area’s absorption coefficient α is found by applying Lambert’s law [13]:

where (A) is the optical absorbance, and (t) is the thickness of thin film. The following equation gives the extinction coefficient (ko) [14]:

The refractive index (n) is defined as the ratio of the innermost part of a substance’s velocity (v) to the vacuum (c) velocity of light.  Refractive index (n) was determined by plugging the values of reflectance and extinction coefficient (ko) into the following calculation [15]:

 
Where R is the reflectance.
Also, the refractive index can be expressed through the following equation [15]:

RESULTS AND DISCUSSION
FESEM results 
For the morphological study of material, FESEM analysis is carried out [16, 17]. The presented FESEM image shows the surface morphology of a zinc oxide (ZnO) thin nano film synthesized via the sol-gel method. The micrograph reveals a highly porous, sponge-like structure characterized by interconnected nanograins and branches (Fig. 1). Such morphology is a typical result of controlled thermal annealing at 500°C, which facilitates the growth of a continuous nanostructured network. This porous texture is indicative of a successful sol-gel process that promotes homogeneous nucleation and results in increased surface area. The nano branches observed are ideal for enhancing surface-dependent properties such as optical absorption and charge transport. This agrees with the optical characterization results reported in the study, where the ZnO films exhibited high UV absorbance (~90%) and a direct optical band gap of 2.84 eV. Furthermore, the grain sizes inferred from the XRD analysis (~20–27 nm) are consistent with the nanoscale features visible in the image. The uniform distribution and compact arrangement of the nanostructures suggest good film crystallinity, which is also confirmed by the XRD peaks aligned with the hexagonal wurtzite phase and preferred orientation along the (002) plane. In conclusion, the FESEM image supports the structural and optical findings of the research, confirming the effective formation of nanostructured ZnO films suitable for applications in solar cells, sensors, and optoelectronic devices [18-22]. 

XRD results 
The crystallinity of material can be determined by XRD study [23-25]. The XRD pattern of ZnO thin film prepared by the sol-gel method on quartz substrates is shown in Fig. 2. All of the diffraction pattern canbe indexed to the hexagonal ZnO phase (Wurtzite Structure) by comparison with thedata from (JCPDS S6-314). However, the XRD patterns of the nanoparticles are considerably broadened due to the very small size of these particles. The strong and narrow diffraction peaks indicated that the product has good crystallinity. The e XRD pattern of ZnO thin nano-films  shows three peaks located at 2θ equal 31.800, 34.590 and 36.22 0 corresponding to (100), (101) and (002) planes with prefferd orientation accured at (002) indicating that the films are oriented along c-axis [6, 26, 27]. The typical hexagonal wurtzite structure of thin nano-films is inferred from the XRD pattern. The crystallites sizes (D) of the films are estimated using the Scherer formula [28]:

where k is the shape factor, which is 0.94.  With a Bragg angle 2θ of approximately 34.44˚, λ is the wavelength of the X-ray utilized, which is 1.54 Å, and β is the full width at half maximum of the (002) peak of the XRD pattern. 
Fig. 2 shows that peak position is affected by uniform strain, whereas peak broadening and intensity are affected by nonuniform strain.  To compute the average strain (𝜺) of nanoparticles, the Stokes-Wilson equation is utilized [28]:

The average grain size is established at 20 nm.  The dislocation density (δ), indicative of the quantity of dislocation lines per unit volume, is calculated using the subsequent equation:

Table 1 displays the structural characteristics of thin nano-films that have been researched. Based on XRD measurements, the lattice features of ZnO thin nano-films such as dislocation and strain show good agreement with the values provided in (JCPDS S6-314) which agreement with references [29, 30]. 

Optical properties 
The absorption spectra of the ZnO film are depicted in Fig. 3. It is cleared that a high level of absorption in the UV area, followed by a quick drop in the visible region [31].

Where c represents the speed of light. The means of the transformation (indirect or direct) is identified by the relationship [32]:

The energy of a photon is denoted as hv, the band gap energy is represented as Eg, and (r and a) are constants. It’s important to mention because of a permissible direct transition, the value of r is 1/2, while for a permissible indirect transition, the value of r is 3/2. In the near infrared region, the absorption experiments displayed that the created films have very low absorption in the visible range, making them highly suited for the production of solar cells [33, 34].  
Fig. 4 displays the absorption coefficient of the ZnO film, as calculated using Eq. 2. The absorption coefficient of this film exhibits a quick increase beyond the absorption edge region, followed by a slow drop due to its inverse relationship with transmittance. It is clear that the ZnO thin film has a greater absorption constant (α≥104cm-1), which promotes the occurrence of direct transitions [35, 36].  
The extinction coefficient (k) has been determined using Eq. 3, and the resulting value is depicted in Fig. 5 for the ZnO film. The extinction coefficient exhibits a quick increase in the UV range, followed by a subsequent decline. Extinction coefficient variation is directly related to light absorption and is expressed as a non-zero value (k) for photon energies below the critical absorption edge.   
The ZnO thin film’s reflectance spectrum is shown in Fig. 6. As the wavelength grows, the reflectance of the film is seen to rise, peaking at energies that correspond to the film’s energy gap. Nevertheless, the reflectance drops with increasing photon energy. The poor absorbance of the film at photon energies and wavelength below the forbidden energy gap is the cause of this phenomenon. An apparent absorbance value is seen when the photon energy is higher than or equal to the energy band gap. This is because the interaction between the material’s electrons and the incident photon has sufficient energy to facilitate electronic transitions, and this result is in agreement with [29, 37]. 
Eq. 7 depicts the refractive index of a ZnO thin film. The connection between the ZnO film’s wavelength and refractive index is shown in Fig. 6. When the wavelength rises in the ultraviolet (UV) and the beginning of the visible range, the refractive index values also increase. The data obtained indicate that the produced film has a maximum refractive index value of 1.701%.   
Fig. 7 shows that the plot curve is linear, which means that there are direct transitions.  It is possible to deduce the direct permissible energy gap of 2.84 eV from the linear connection seen at high photon energy.  The results presented here are in agreement with those in References [9, 32]. 

CONCLUSION
On quartz surfaces, zinc oxide thin nano-films were created using the sol-gel method and 1.0 M zinc acetate solutions. Films have been categorized using structural and optical information. According to the X-ray diffraction analysis, every sample has a hexagonal structure. The sizes of the crystallites are found to be between 20 and 27 nanometers, according to the XRD measurements. The film shows a significant emission peak in the visible range centered at 550 nm, as well as a noticeable emission band at 383 nm. The film exhibits a notable absorption in the ultraviolet spectrum, which rapidly diminishes in the visible and near-infrared areas. The film shows a linear transition with a 2.84 eV permitted energy gap.

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