In this paper, zinc sulfide nanoparticle (ZnS-NP) thin films were deposited onto glass substrates by chemical bath deposition using zinc sulfate as the cation precursor and thiourea as the anionic precursor. Different bath temperatures (65, 70, 75, and 80°C) and different deposition times (20, 30, 40, and 50 min) were selected to study the performance of ZnS thin films. Topographical and optical characterizations of the films were studied using the atomic force microscope (AFM) and UV-Vis spectroscope. The best ZnS thin films were deposited at a bath temperature (70°C) and a deposition time (30 min) with homogeneous distribution, high density, and small average diameter (106 nm). The energy gap () was found to be in the range of 4.05-3.97 eV for the ZnS films. Optical constants (refractive index, , extinction coefficient, , and dielectric constant, ) of the films were obtained in the wavelength range 300-500 nm by using spectrophotometric measurement. The dispersion of the refractive index is analyzed by using a single oscillator model. The oscillator energy and dispersion energy were determined using the Wemple-DiDomenico single oscillator model. Urbach’s energy increases from 0.907 eV to 2.422 eV with increasing of deposition time. The calculated radius of nanoparticles using Brus equation was 1.9, 2.3, 2.45, and 2.51 nm at deposition times 20, 30, 40, and 50 min, respectively.

1. Introduction

Zinc sulfide is one of the most important II-VI semiconductor materials due to its distinct electrical and optical properties. Zinc sulfide nanoparticles (ZnS-NPs) with wide bandgap (3.5-3.8 eV) have recently received intensive attention to be used in many applications [1] such as antimicrobial activity [2, 3], electroluminescence devices, photonic devices [4], field emission devices [5], sensors [6, 7], and applications in infrared windows [8] and lasers [9, 10]. A variety of physical and chemical methods have been used to synthesize ZnS-NPs such as thermal evaporation [11], sputtering [12], spray pyrolysis [13], chemical vapor deposition [14], molecular beam epitaxy [15], pulsed laser deposition [16], microwave [17, 18], wet chemistry processes [19, 20], sonochemical preparation [21], and green synthesis [22]. Among the various methods, chemical bath deposition (CBD) is the most commonly used because of its simplicity, low cost, and coating large area of the semiconductor with good quality and high purity of deposited thin films. The main goal of this present work is to study the influence of bath temperature and time of deposition on surface topography and optical properties of ZnS-NP thin films.

2. Experimental Materials and Methods

2.1. Reaction Mechanism

The fundamental CBD growth mechanism involves mass transport of reactants, adsorption, surface diffusion, reaction, nucleation, and growth. The ZnS-NP thin films can be prepared by decomposition of thiourea [(NH2)2CS] (S2- ion source) in an alkaline solution containing a zinc sulfate (ZnSO4) (Zn2+ ion source) and ammonia as a complexing agent which allow controlling Zn2+ concentration. The deposition process is based on the slow release of Zn2+ and S2- ions into the solution, which then condense on an ion-by-ion basis on the substrate that is properly mounted in a reaction solution. The deposition of ZnS thin film occurs when the ionic product of Zn2+ and S2− ions exceeds the solubility product of ZnS. The reaction mechanism for the deposition of (ZnS) thin film can be given as follows:

Finally, the ZnS films are formed on glass substrates according to the relation:

Heating the chemical bath improves the chemical reaction and accelerates the rate deposition.

2.2. Deposition of ZnS Films

ZnS films were deposited on commercial glass substrates () by chemical bath deposition technique and using a homemade dip-coating apparatus. The substrates were cleaned in ethanol for 10 min, followed by ultrasonically cleaning with double distilled water for another 10 min, finally dried in the air and kept in plastic dry boxes. In this work, we prepare chemical bath using 4.2 ml of ammonia 25% which is added slowly to 15 ml of 0.1 M ZnSO4, after stirring for several minutes the solution becomes colorless and homogeneous; thereafter, 15 ml of 0.179 M thiourea solution was added under stirring. When the water bath temperature () arrive at an appropriate temperature (between 65 and 80°C), the reaction solution was placed in 50 ml beaker into the water bath pot. The glass substrates were then immersed vertically inside the beaker and supported against the wall of the beaker for deposition time (). Each sample are removed from the beaker and cleaned many times with deionized water to remove the white, adherent powder precipitate in the solution during the deposition. Topography of the surface, particle size, and surface roughness of the films are examined using the atomic force microscope (AFM, Nanosurf easyScan 2, Switzerland), tapping mode in air at room temperature. The optical transmission and absorption studies of the deposited ZnS thin films were carried out with a UV-Vis spectrophotometer (model: Varian Carry 5000).

3. Results and Discussion

3.1. Topographical Properties

In this study, we have used AFM to get information on the surface relief and to determine the influence of bath temperature and deposition time on the quality of the as-deposited ZnS films. The AFM micrograph of ZnS films deposited at bath temperature for different times of deposition () is shown in Figure 1.

From AFM images, it is observed that unclear structures formed at lower deposition times (20, 30, and 40 min) so it could not be able to measure their diameters. At , small spherical ZnS-NPs appeared with the mean grain diameter of 120 nm approximately.

In the case of high bath temperature (as shown in Figure 2), the deposited ZnS thin film has a good quality, is uniform, and completely covered the entire substrate surface area.

At lower deposition time , spherical ZnS-NPs with different sizes and unequal distribution formed on the surface of the film, while the particles carpet the substrate at deposition time 30 min with high density and uniform spherical shape. The grains have relatively low and narrow size distributions with the mean diameter 106 nm, mean height 15 nm, and mean roughness 11 nm. The particles covered 65% of the surface. The small particles accumulate continuously at time 40 min and cover the entire surface of the substrate by 85%. At higher deposition time 50 min, the particles start to aggregate and made huge grains (disappeared interval border). The average diameters, heights, and surface roughness of ZnS thin films deposited at bath temperature 70°C for different deposition times (20, 30, 40, and 50 min) were showed in Table 1.

Figure 3 illustrates graphic curves for changes of average diameters (), heights (), and roughness () in term of deposition time for synthesized ZnS thin films at bath temperature 70°C. We notice increasing diameter by 32%, height by 48%, and roughness by 50% with increasing deposition time. This indicates that longer deposition time will mainly affect the growth of particles [23].

For ZnS thin films deposited at bath temperatures more than 70°C, the particle size and growth rate obviously increase as shown in Figures 4 and 5 for bath temperatures 75°C and 80°C, respectively, for different deposition times (20, 30, 40, and 50 min). At , the average grain size was 187 nm and 228 nm for bath temperatures 75°C and 80°C, respectively. The increase of particle size with a temperature of bath is due to increasing ions and particles’ kinetic in the solution, which in turn leads to crystal growth according to Ostwald ripening.

Comparing AFM images at different immersing times, we find that deposition time 30 min is the most suitable time to form ZnS-NPs with a small diameter and uniform spherical shapes.

In Figure 6, we represent the topography of ZnS-NP thin film deposited at 30 min for different bath temperatures (65, 70, 75, and 80°C). The AFM micrographs showed unclear nanostructures formed at a temperature less than 70°C. At high bath temperatures (more than 70°C), nanoparticles began to aggregate each other and form huge grains, while it has homogeneous spherical shape, high density, and regular size distribution at the temperature 70°C.

Table 2 shows a comparison between average grain size (diameters, heights) and mean roughness surface for ZnS thin films deposited at for different bath temperatures.

The reduction of bath temperature from 80°C to 70°C leads to decrease of the average diameter by 53%, the average height by 75%, and the average roughness by 38%. Increasing of the particle size with temperature is related to increase of particle’s kinetic and consequently the rate of nucleation and subsequent growth of ZnS-NPs.

3.2. Optical Properties

UV-Vis transmittance spectra of ZnS-NPs formed at deposition time 30 min for different reaction temperatures (65, 70, 75, and 80°C) are given in Figure 7.

The transmittance of these films decreased with the increasing bath temperature; this result related to increase of the kinetics of particles in the solution leads to new ZnS grains which fill up the voids in the ZnS thin layer that tends to have more thickness and less transmittance. In addition, the decreasing transmittance can be linked with agglomeration and increasing grain size.

Figure 8 shows optical transmittance and reflectance spectra at the UV-Vis region of the as-deposited ZnS films for different deposition times at 70°C.

This figure shows that the films are transparent in the visible region. It can be seen that the reflectance values increase slightly with increasing deposition times as follows: 1%, 1.5%, 3.5%, and 7% for times 20, 30, 40, and 50 min, respectively, whereas the transmittance decreases steadily as follows: 99%, 98.4%, 94%, and 86.6%, respectively. That result is due to the new deposited particles on the surface of the film and growth of grains by increasing the time which leads to increasing film thickness measured by the optical interference fringe method as 70, 89, 118, and 160 nm for 20, 30, 40, and 50 min deposition times, respectively. The sharp absorption edge of the ZnS films which observed at about 290–325 nm demonstrates a narrow grain size distribution as well as a low concentration of defects in the films, and it clearly shifts to higher wavelengths for thicker films. This absorption edge shift is associated with a decrease in the energy bandgap of the larger ZnS nanoparticles. Figure 9 shows the decreasing optical transmittance at the visible region with increasing film thickness (increasing immersion time).

The low reflectance and high transmittance at the visible region emphasize the importance of using ZnS thin films for solar cell applications.

In order to determine optical bandgap, the Tauc relationship is used as follows [24]: where is absorption coefficient , is a constant, is Planck’s constant, is photon frequency, is optical bandgap, and is 1/2 for allowed direct semiconductor bandgap. The bandgap energy of ZnS thin film was estimated by plotting against as in Figure 10; the linear nature of the plot indicates that ZnS is a direct bandgap material. The optical bandgap of the films can be evaluated from extrapolating the linear portion to the axis. was determined to be 4.05, 4.01, 3.99, and 3.97 eV for the ZnS films deposited at different times 20, 30, 40, and 50 min, respectively, which closely agree with the values reported for ZnS thin films obtained by CBD [25, 26].

In Figure11, we have reported the variation of bandgap energy as a function of diameters of deposited ZnS-NPs. It is noticed the increment of the energy gap with decreasing grain size; this is due to the quantum size effect whereas the energy levels are confined to potential wells of small dimension. The distance between energy levels increases as the crystal size becomes smaller [27].

For many amorphous and crystalline semiconductors, an exponential dependence of absorption coefficient may take Urbach’s empirical formula [28]: , where is a constant and (Urbach energy) is an energy characterizing the degree of disorder introduced from defects and grain boundaries; also, it is interpreted as the width of the tail of localized states associated with the amorphous states in forbidden band. Figure 12 represents the logarithm of absorption coefficient as function of the photon energy at different deposition times 20, 30, 40, and 50 min. The value of is calculated from the inverse slope of the linear part of curves and also listed in Table 3.

It has been found that Urbach’s energy increases with time of deposition. This is probably due to increasing structural disorders and defects in prepared films.

The variation of bandgap energy () and Urbach energy () of ZnS thin films with deposition times is shown in Figure 13. The values of ZnS films decrease with increasing deposition time which is due to the agglomeration of the ZnS-NPs. The minimum value of obtained at 20 min indicates a very weak absorption tail due to minimized defects and impurities which improves the transparency and optical conductivity of the film coated at that time.

Applying the confinement effects, particle size could be calculated using the first theoretical calculation for semiconductor nanoparticles given by Brus [29] and Chukwuocha et al. [30] and based on “effective mass approximation” (EMA). In this approximation, an exciton is considered to be confined to a spherical volume of the crystallite and the mass of electron and hole is replaced with effective masses ( and ). The calculations provide an analytical expression for how the electronic bandgap of the semiconductor nanocrystal () is modified relative to that of the bulk semiconductor (). In other words, the energy of quantum confinement () is directly related to the nanocrystal radius () with Brus equation: where is the radius of the nanoparticle, is calculated from the excitonic absorption peak of the nanoparticle, is bulk semiconductor bandgap (3.65 eV), is the effective mass of the electron (), is the effective mass of the hole (), is the charge of the electron, and is the dielectric constant of the material, for and vacuum permittivity constant. The calculated radius for ZnS nanoparticles using Brus equation was 1.9, 2.3, 2.45, and 2.51 nm at different deposition times 20, 30, 40, and 50 min, respectively (less than Bohr radius 2.67 nm). Quantum confinement effect in the nanosemiconductor of ZnS has been studied using the Brus equation. The first term in the left-hand side of the equation represents the bandgap energy of bulk materials, which is characteristic of the material. The second additive term of the equation represents the additional energy due to quantum confinement having dependence on the bandgap energy. It can indeed be thought of as the infinite square-well contribution to the bandgap. The third subtractive term stands for the columbic interaction energy exciton having dependence (often neglected due to the high dielectric constant of semiconductor material).

The graphs of ground state confinement energy against size (radius) for zinc sulfide nanoparticles in Figure 14 show the dependence of confinement on the size of quantum dots. The result shows that ground state confinement energy is inversely proportional to the size (radius). Thus, as one increases the radius (size), the confinement energy decreases, but never reaches zero. That is, the lowest possible energy for the quantum dot sample is not zero. Confinement begins when the radius of the quantum dot sample is comparable or of the order of the exciton Bohr radius.

The extinction coefficient could be calculated using the relation: . Figure 15 shows the variation of extinction coefficient as a function of wavelength; it shows a sharp increase at the ultraviolet region that is due to high absorbance of incident photons near the bandgap.

The refractive index () is the ratio of the velocity of light in a vacuum to its velocity in a specified medium. The value of the refractive index was calculated from the equation: where () is the reflectivity and is extinction coefficient. The variation of refractive index vs. wavelength is shown in Figure 16, which shows that the maximum value of () is ~2.6 for all films at the same wavelength. Also, it shows that the films become more transparent in the visible region. The refractive index dispersion data were evaluated according to the single effective oscillator model proposed by Wemple and DiDomenico [31] and Chiad et al. [32]. It is well known from dispersion theory that, in the region of low absorption, the index of refraction is given in a single oscillator model by the expression: where and are single oscillator constants, is the energy of the effective dispersion oscillator, and is the so-called dispersion energy, which measures the intensity of the interband optical transitions. The oscillator energy is an average of the optical bandgap and can be related to the optical bandgap in close approximation . Figure 17 shows the plot of vs. for the films prepared at different deposited times 20, 30, 40, and 50 min. In all cases, linear dependence was observed. The values of and can then be calculated from the slope of the straight line and the intercept on the vertical axis (). It was found that varies between 1.23 and 8.76 eV, while varies from 4.06 to 5.7 eV for the different deposition times. Furthermore, the values of the static refractive index () can be calculated by extrapolating the Wemple-DiDomenico dispersion equation to . The calculated values of are 1.14, 1.18, 1.34, and 1.59 for the films deposited at different times 20, 30, 40, and 50 min, respectively. The obtained values are given in Table 4.

The real and imaginary parts of dielectric constant were determined using relation: where is the real part and is the normal dielectric constant and is the imaginary part and confirms the free carrier contribution to the absorption. The variation of and versus incident photon energy () is shown in Figures 18 and 19. The variation of and with the increase of the wavelength of the incident radiation is due to the change of reflectance and absorbance.

The behavior of is similar to that of the refractive index because of the smaller value of compared with , while mainly depends on the value, which is related to the variation of absorption coefficient. represents the absorption of radiation by free carriers. The figures revealed that the real part is higher than the imaginary part for all samples.

The optical conductivity was calculated using the following relation: , where is the speed of light. Figure 20 shows optical conductivity for different times of deposition. It was observed that the optical conductivity decreases as the time increases to 50 min. The increased optical conductivity at a low wavelength (ultraviolet region) is due to the high absorbance of ZnS thin films in that region.

4. Conclusion

The nanoparticle ZnS thin films have been successfully synthesized with different deposition times and different bath temperatures. The topographical and optical studies were carried out using the atomic force microscope and UV-visible spectroscope. The particle size and surface roughness, as well as thickness values, were increased with increasing deposition time and bath temperature. However, ZnS thin films prepared at and showed homogeneous nanoparticles with high density, less agglomeration, high transparent, and low reflectance in the visible region which could be recommended for antireflection coating and solar cells. The optical bandgap energy was decreased from 4.05 to 3.97 eV upon increasing the deposition time, while Urbach’ energy was increased from 0.907 eV to 2.422 eV. The quantum confinement effect in nanoparticles ZnS has been studied using the Brus equation. It is noticed that the ground state confinement energy is inversely proportional to the size (radius) of ZnS-NPs.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of the present work.