Synthesis of ZnS Nano-Powders and Fabrication of ZnS Thin Films via Electron-Beam Evaporation: Structural and Optical Characterization

Abstract

Nanoscale zinc sulfide (ZnS) powders have attracted considerable interest due to their unique properties and diverse applications in various fields, including wastewater treatment, optics, electronics, photocatalysis, and solar systems. In this study, nano-powder ZnS was chemically synthetized starting from Zn powder, diluted HCl, and laboratory-prepared Na2S. The obtained ZnS was studied using an SEM coupled with EDS, XRD analysis, UV–Visible spectroscopy, and FTIR techniques. The XRD results showed that the synthesized nanoscale ZnS powder was approximately 2.26 nm. Meanwhile, the EDS and XRD patterns confirmed the high purity of the obtained ZnS powder. In addition, the ZnS powder was compacted and sintered in an argon atmosphere at 400 °C for 8 h to prepare the required pellets for thin-film deposition via E-beam evaporation. The microscopic structure of the sintered pellets was investigated using the SEM/EDS. Furthermore, the optical properties of the deposited thin films were studied using UV–Visible spectroscopy in the wavelength range of 190–1100 nm and the FTIR technique. The bandgap energies of the deposited thin films with thicknesses of 111 nm and 40 nm were determined to be around 4.72 eV and 5.82 eV, respectively. This article offers a facile production route of high-purity ZnS powder, which can be compacted and sintered as a suitable source for thin-film deposition.

1. Introduction

Zinc sulfide (ZnS) with nanoscale crystals has received significant attention because of its properties and its wide range of applications in numerous fields, such as electronics, optics, photocatalysis, infrared devices, solar cells, gas sensors, radiation detectors, transparent conductive films, protective coatings, antireflection coatings, high-reflectivity mirrors, screens, light-emitting diodes, and the purification of wastewater [1,2,3,4,5,6,7]. ZnS inorganic nanoparticles can be produced using different methods, including chemical bath deposition (CBD), sputtering, chemical vapor deposition (CVD), the sol–gel technique, and physical vapor deposition (PVD) [8,9], and with various microstructures, which leads to different properties of this compound.

Zinc compounds, including ZnS and ZnO, are widely used and studied zinc materials. Both ZnS and ZnO are direct wide bandgap materials with comparable bandgap values, and they can be used in sensors, solar cells, and protective coatings and integrated into many semiconductor devices, such as LEDs [10].

A review of the production of nanoscale ZnS via green routes was reported in 2023 [8]. The proposed methods are considered environmentally friendly, meaning that the required inorganic nanoparticle compounds have a minimal environmental footprint for many applications in various fields, such as medicine, optical devices, and engineering applications. In 2024, a comprehensive review of ZnS synthesis using various methods for different nanostructures and their applications was reported by Chakrabarti et al. [9]. They confirmed that the nanoparticle shapes and properties of ZnS determine its uses in various fields, such as catalysts, solar cells, optoelectronics, biomedical, sensors, and batteries. ZnS is highly transparent to infrared radiation (IR) in the wavelength range of 8–12 µm due to its large energy gap (E_g ≥ 3.6 eV). Therefore, this material is widely used in IR sensors such as those found in defensive missile applications [11]. Zinc blende, which is symbolized as ZnS sphalerite, is a stable cubic structure from room temperature (RT) to approximately 1000 °C, where this phase starts to change into the hexagonal close pack (hcp) structure at higher temperatures to form ZnS wurtzite [12]. ZnS thin films, both amorphous and crystalline, are used as antireflection coatings, optical filters, active waveguide materials, a buffer layer in solar cells, an active channel layer in thin-film transistors, and in gas and optical sensors. These applications are attributed to the relatively high bandgap, moderate processing temperatures, possibility of large area deposition, and low production costs of ZnS thin films [13]. Furthermore, doping of ZnS changes its energy gap and leads to improved sensitivity to the visible light spectrum. In addition, the degradation of dye efficiency can be controlled via metal doping of the plain ZnS nanoparticle’s structure. This subject has been fully reviewed by Khan et al. [14].

The chemical precipitation method of ZnS is one of the most widely used due to the ease of controlling of its preparation factors, such as the purity of the starting materials, pH, temperature, and mechanical agitation during precipitation [15,16,17,18,19,20,21,22]. ZnS thin-film technology is also used in various research lines and practical applications since it consumes little material while covering large areas, which makes this technique very suitable for the manufacturing stage [23,24,25,26]. Another route for producing ZnS is powder metallurgy, which starts with the mechanical stoichiometric mixing of bare Zn and S powders to form the compound. The process ends with compaction and sintering to obtain solid and sintered parts of the targeted compound. Two or more types of powders can be milled together in a particular atmosphere for a long time to induce mutual diffusion between the mixed powder particles and yield the required alloy. This method is called mechanical alloying (MA) [4,27,28] and can be generalized to produce several compounds starting from powder components. Most researchers in this field use these chemical compounds to produce ZnS using different production techniques.

It is rare to start with Zinc (Zn) metal, either powder or bulk pieces, when synthesizing ZnS by preparing the required solution, such as zinc chloride (ZnCl₂), and subsequently reducing it to form ZnS nanoparticles. This potentially easier, faster, and more cost-effective means of producing ZnS nano-powder warrants further investigation. The sintering of ZnS powders to form pellets, as a starting material for thin-film deposition, also requires special attention.

The aim of this research project was to produce ZnS nano-powder starting from Zn powder by creating ZnCl₂ from Zn powder and HCl solution and then utilizing it for thin-film deposition. Here, Na₂S was used as a reducing agent. ZnS thin films were produced using the electron-beam (E-beam) evaporation technique and characterized via SEM, XRD, and UV–Visible and FTIR spectroscopy.

2. Materials and Methods

2.1. Instruments and Devices

• A hot plate with magnetic stirring (HS-3000, Camlab Ltd., Cambridge, UK) for heating and mixing as required.
• A four-digit analytical microbalance (Model SEJ-205, Taipei, Taiwan) for high-accuracy weighing.
• A pH meter (EUTEGH, Thermos Scientific, Singapore) for solution pH measurement.
• A scanning electron microscope (SEM) (Inspect F50-FEI company, Eindhoven, The Netherlands) for high-magnification and high-resolution imaging.
• An Agar sputter coater instrument (AGB7340 Agar Scientific, Essex, UK) for specimen coating.
• A vacuum oven (JEIO TECH, MODEL OV-11, Seoul, Republic of Korea) for drying under vacuum.
• A centrifuge system (HERMILE Labortechnik GmbH, Type Z 326 K, Wehingen, Germany) for powder separation.
• A tube furnace (Protherm alumina tube furnace, Model PTF 12/50/450, Protherm Inc., Ankara, Turkey) with a combined home-designed argon gas line and vacuum fitting for powder sintering.
• A digital caliper (Total, TMT 322001, Guangzhou, China) for high-accuracy dimension measurements.
• A milling machine with variable speed (Changsha Tiachuang, Powder Technology Co., Changsha, China) for the milling process.
• A compaction system consisting of a homemade stainless-steel die integrated with a CARVER press (model 4350. L, CARVER, Inc., Wabash, IN, USA) for powder compaction.
• An electron-beam evaporation system (SCT-1800, SCT, System Control Technologies, sctec.com, Battle Ground, WA 98604, USA) for thin-film deposition.
• A UV–Visible spectrophotometer (UV-1601 (PC) S, Shimadzu Corporation, Tokyo, Japan) for the investigation of optical properties.
• An optical reflectance spectrometer (FilmTek 3000, Scientific Computing International, Carlsbad, CA, USA) for thin-film thickness measurements.
• A Fourier Transform Infrared (FTIR, NEXUS, EPS-87, Thermo Fisher Scientific, Waltham, MA, USA) spectrometer in the wavenumber range of 400–4000 cm⁻¹ for powder and thin-film FTIR tests.
• An X-ray diffraction (XRD) instrument (Malvern Panalytical, Aeris, monochromatic Cu kα1, 1.5406 Å, 0.02 step angle, with 2θ ranging from 10° to 60°, Almelo, The Netherlands) for phase and unit cell investigation.

All instruments and devices above are referred to throughout the text.

2.2. Materials

Zinc (Zn) powder of 99.5% purity, hydrochloric acid (HCl) of 37% concentration, sulfur powder of 99.9% purity, and sodium hydroxide (NaOH) of 99.5% purity were used as received. Distilled water and absolute ethanol (99.9%) were used according to the required preparation procedure. All chemicals were purchased from Sandra Chemicals, Amman, Jordan. Glassware, filter papers of 110 mm diameter (Schleicher & Schüll GmbH, 595, Ref: No. 311610, Dassei, Germany), and ceramic funnels were used as required. Glass slides, quartz slides, and silicon wafer substrates were used for the E-beam evaporation of the thin films. Argon gas with a purity of 99.99% was used in these experiments. The research plan implementation is illustrated in Figure 1.

2.3. Production of Na₂S Compound

Na₂S was produced at our laboratory according to the following stoichiometric equation:

$$2{\mathrm{N}\mathrm{a}}^{+}\left(\mathrm{a}\mathrm{q}\right)+\mathrm{S}\left(\mathrm{s}\right)\stackrel{105 °\mathrm{C},\mathrm{x}{\mathrm{H}}_2\mathrm{O}}{\to }{\mathrm{N}\mathrm{a}}_2 \mathrm{S}.\mathrm{x}{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{H}.\mathrm{T}.200 °\mathrm{C},\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{u}\mathrm{u}\mathrm{m}}{\to }{\mathrm{N}\mathrm{a}}_2\mathrm{S}(\mathrm{s})$$

(1)

Exactly 20.0 g of NaOH (molar mass = 39.997 g/mol) was dissolved in 250 mL of distilled water to form an alkaline solution, which was then heated to ~105 °C. With continuous magnetic stirring, 8.0 g of S powder (molar mass = 32.0650 g/mol) was gradually added to the solution to produce a reddish-purple solution. A yellow-orange solid of Na₂S was produced with continued heating at ~105 °C. The produced Na₂S was heated to ~200 °C at a rate of ~2 °C/min in a 10⁻³ Torr vacuum for 2 h. More details about the production of Na₂S have been reported recently [21].

2.4. Synthesis of ZnS

Zn powder was first dissolved in diluted HCl to produce ZnCl₂ solution according to Equation (2).

$$\mathrm{Z}\mathrm{n}\left(\mathrm{s}\right)+2\mathrm{H}\mathrm{C}\mathrm{l}\left(\mathrm{a}\mathrm{q}\right)\to \mathrm{Z}\mathrm{n}\mathrm{C}{\mathrm{l}}_2\left(\mathrm{a}\mathrm{q}\right)+{\mathrm{H}}_2 \left(\mathrm{g}\right)$$

(2)

Zn powder was washed with a solution produced by mixing 1 mL of 37% HCl solution with 100 mL of water in order to remove the oxide layer, if any, during the manufacturing process so that the surface of the particles would be clean and ready for the reaction. Bubbles of H₂ gas were observed during the reaction, and a clear ZnCl₂ solution was obtained at the end of the process. The pH of the solution, measured using a pH meter, was approximately 1.4.

The first run of ZnS precipitation was started by dissolving 10 g of Zn powder in 250 mL of 10% solution of the diluted 37% HCl to form soluble ZnCl₂ (225 mL of distilled water + 25 mL of 37% HCl). After the reaction finished and no more H₂ bubbles were seen, the residual unreacted Zn was dried and weighed, and the measured weight was subtracted from 10 g to find the exact amount of dissolved Zn. The amount of undissolved Zn powder was 0.9382 g, so the actual dissolved mass of Zn was 9.0618 g (~9 g) in 25 mL of 37% HCl.

The obtained ZnCl₂ solution was filtered twice using filter paper into a ceramic funnel to remove any traces of undissolved solids. The pH of the obtained solution was measured to be 1.4. The produced Na₂S was used as a reducing agent to synthesize ZnS from the prepared ZnCl₂ solution according to the following equation:

$$\mathrm{Z}\mathrm{n}\mathrm{C}{\mathrm{l}}_2+\mathrm{N}{\mathrm{a}}_2\mathrm{S}\to \mathrm{Z}\mathrm{n}\mathrm{S}+2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}$$

(3)

The required mass of S to convert 10 g of Zn into ZnS is 4.93 g. The S was dissolved in 200 mL of distilled water, filtered, and then added dropwise (50 drops/min.) to the prepared ZnCl₂ solutions to form the white ZnS powder. The powder was separated from its solution after the supernatant was discharged using the HERMILE centrifuge instrument at a rotating speed of 10,000 rpm for 10 min., and it was washed five times with distilled water and twice with ethanol. The wet powder was then dried under vacuum at ~105 °C for 1 h using the vacuum oven. The dried ZnS was aggregated to form a flake-like material after drying, which was milled to produce fine ZnS powder. Figure 2 contains photographs of the produced Na₂S, synthesized ZnS white powder in the intermediate solution, and the dried milled ZnS powder. The procedure was repeated more than once to ensure its reproducibility.

2.5. Sintering of ZnS Powder

The dried flake-like aggregated ZnS powder was milled before compaction. The fine ZnS powder was compacted at a pressure of 400 MPa with a dwell time of 2 min utilizing a homemade stainless-steel die of 1.2 cm diameter coupled with the CARVER press. The compacted pellets were then sintered at 400 °C for 8 h with a heating rate of approximately 5 °C/min. until reaching the sintering temperature using the Protherm tube furnace. The sintered pellets were then left to cool down overnight at a cooling rate of about 2.5 °C/min. The process was performed in an argon atmosphere with a flow rate of 1 L/min to prevent oxidation. The tube furnace was evacuated first to approximately 10⁻³ Torr before the introduction of the argon gas to reduce the oxygen as much as possible. Figure 3 is a photograph of three compacted pellets at a pressure of 400 MPa.

The green and sintered densities (ρ_g and ρ_s) of the pellets were determined using a digital caliper and an analytical balance. The sintering step was crucial for studying the microstructure of sintered ZnS powder and using the sintered solid pellets for the subsequent E-beam evaporation for thin-film deposition.

2.6. Thin-Film Deposition and UV–Visible Spectrophotometry

Thin films of ZnS were deposited on pre-cleaned silicon, glass, and quartz substrates using the E-beam evaporation technique mentioned in Section 2.1. Sintered pellets of ZnS were loaded into a Boron Nitride crucible, and the deposition chamber was evacuated to a base pressure of the order of 7.0 × 10⁻⁶ Torr prior to the E-beam evaporation process. The deposited thin films’ optical properties were studied using UV–Visible spectrophotometry in the wavelength range of 190–1100 nm. Thin films’ thicknesses were determined using the quartz monitor in the evaporation instrument and confirmed using the FilmTek 3000 spectrometer, Scientific Computing International, Carlsbad, CA, USA.

2.7. Characterizations of the Starting Powders, Synthesized ZnS, and ZnS Thin Films

The XRD patterns of the synthesized ZnS powder and ZnS thin films were measured with the Malvern Panalytical diffractometer (monochromatic Cukα1, 1.5406 Å, 0.02° step, with 2θ ranging from 10° to 60°) using a scanning speed 0.02°/s with a beam current and an acceleration voltage of 7.5 mA and 40 kV, respectively. XRD measurements were carried out for ZnS nano-powder, which was annealed at 200 °C for 2 h in a vacuum of approximately 10⁻³ Torr. Pieces of about 1.5 cm² of the 111 nm thickness thin film were heat-treated at 250 °C and 400 °C for 6 h in a vacuum of 10⁻³ Torr for contrast viewing with the as-evaporated thin films. The results were then analyzed using the software accompanying the XRD system (HighScore Plus version 5.2).

Fourier-transform infrared (FTIR) spectra were measured for both ZnS powder and ZnS thin films using a ThermoFisher Scientific FTIR spectrometer in the wavenumber range of 400–4000 cm⁻¹.

The SEM was used to investigate the microstructures of the Zn and S powders, synthesized ZnS powder, sintered specimens, and deposited thin films. An Agar sputter coater was used for coating with a thin film of platinum (Pt) to enhance the image quality and ensure proper grounding of the specimens to the SEM stage. Specimens were mounted on aluminum stubs of 1.2 cm diameter using double-sided adhesive carbon tapes. Zn powder was examined directly with no coating needed, while the S powder, which is almost an insulator, was coated. The synthesized ZnS powder was Pt sputter-coated and examined for microstructure and chemical analysis. Square pieces of approximately 1 cm² from the thin film of ZnS were also Pt-coated and examined for microstructure and nanoscale particle imaging. The measured thicknesses determined using FilmTek 3000 were confirmed by measuring the thickness of two cross-sectional specimens of deposited thin films using the SEM. The SEM was used to image the fractured surface of the sintered pellet. The selected fractured surface was made as planar as possible for easy examination in the SEM and to produce a high-quality image.

3. Results and Discussion

3.1. SEM Results of Zn Powder, S Powder, and Synthetized ZnS Investigations

The SEM was vital for studying the microstructure and chemical analysis of the starting materials. Figure 4a–d show the shape, size, and EDS spectra of compacted Zn powder at a pressure of 300 MPa and annealed and compacted S powder at 110 °C for an hour under a vacuum of about 10⁻³ Torr. A temperature slightly below the melting point of S element (~115 °C) was chosen to ensure a solid sintered pellet of S powder. The S powder was compacted and heat-treated to prepare a flat specimen for easier examination in the SEM. Likewise, Zn powder was also compacted at a pressure of 300 MPa for the same purpose as above. The results show that both starting powders were almost pure, with no impurities, as confirmed using the EDS spectra. This result encourages the production of high-purity compounds, specifically Na₂S and ZnS.

The obtained ZnS powder was investigated using the SEM for its microstructure and particle size distribution. Figure 5a–d illustrate the images of the produced ZnS powder, its EDS profile with an inserted table of chemical data, and the XRD pattern of the ZnS powder. The figure shows that the particles are of spherical shape, and the particle size is around 50 nm or smaller. In addition, the EDS spectrum and the chemical analysis table confirm the formation of ZnS with atomic conc. % of 47.87 and 52.13 for Zn and S, respectively. Since EDS chemical analysis is semi-quantitative, these values are in very good agreement with the nearly 50 atomic conc. % theoretical values for each of Zn and S in ZnS. The same validation applies to the measured weight conc.% in Figure 5c. This result confirms that our ZnS production method used here is accurate, relatively easy, and effective.

In addition, the XRD pattern peaks for the annealed ZnS powder in the 2θ range of 10°–60° fit the sphalerite cubic structure with the reflection plane indexing of (111), (220), and (311) at 2θ of 28.67°, 48.19°, and 56.17°, respectively. The results are generally similar to published values [5,19,29,30,31,32]. In addition, the crystallite mean diameter of the obtained ZnS powder was determined using Scherrer’s Equation (4) for the three peaks shown in Figure 5d, and the mean value was approximately 2.26 nm [6,33].

$$\mathrm{D}={\displaystyle \frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta } \mathrm{c}\mathrm{o}\mathrm{s} \mathsf{\theta }}}$$

(4)

where D is the mean diameter in nm, K is a constant (0.94 for a spherical particle with cubic symmetry), λ is the X-ray wavelength in nm, β is the full width at half maximum (FWHM) of the XRD peak in radians, and θ is the Bragg angle in (°). The three peaks shown in Figure 5d were used to estimate the crystallite sizes, and the mean value was calculated to be approximately 2.26 nm.

Table 1. XRD data used to determine the crystallite mean diameter.

Parameters		2θ Peaks (in °)	β (FWHM) (in °)	D (in nm)	Mean D (in nm)
K	λ (Å)	28.688	3.139	2.73	2.26
0.94	1.54178	48.187	4.612	1.97
		56.174	4.513	2.09

includes the details used to determine the mean value of D in nm from XRD data.

Moreover, no other peaks appeared in the XRD pattern of the produced ZnS, which verifies that the method successfully produced a high-purity product. This finding supports the EDS analysis, which confirmed that the synthesized ZnS was almost a pure compound. It is clear from the XRD pattern (Figure 5d) that the (002) reflection is hindered by the broad peak of the (111) reflections. This is due to the fact that ZnS is a powder and the (002) reflection is very weak. There seems to be a shoulder at the 2θ value around 33.

The unit cell of the synthesized ZnS was found to be 5.36271 Å as compared to the standard corresponding value of 5.4750 Å (COD 96-153-9415), space group F-43m. Crystallinity can be improved via heat treatment in a high vacuum to avoid any transformation to ZnO [31,32]. The software analysis revealed a cubic structure on average.

3.2. SEM Images of Sintered ZnS Powder

The SEM investigation of the sintered ZnS powder showed particle growth, as can be easily seen in Figure 6a–d. The figure includes the image of the starting nano-powder and the sintered fracture surface specimen. The starting nanoparticles of ZnS powder (Figure 6a) were mutually diffused via solid-state sintering, forming larger particles in the range of 140 nm (Figure 6b–d), which is the usual case during the sintering of powdered material. In addition, many pores appear in the sintered specimen images, indicating that the sintered density is lower than the theoretical value (

Table 2. Green (ρ_g) and sintered (ρ_s) densities of produced ZnS powder with compaction and sintering conditions.

Sample No.	ρg (g/cm3)	ρs (g/cm3)	Compaction and Sintering Conditions
1	2.64	3.19	Compaction pressure 400 MPa and sintering temp. 400 °C for 8 h in an argon atmosphere with a flow rate of approximately 1 L/min, heating rates of 5 °C/min, and furnace cooling at a rate of 2.5 °C/min.
2	2.61	3.15
3	2.69	3.32
4	2.61	3.18
Mean value	2.64	3.21

). Vital information about sintering in various environments has been reported [33].

Sintering in a high vacuum or a high-purity argon atmosphere is vital to preventing the transformation of ZnS powder into ZnO, as reported by Chankhanittha et al. [30]. They showed that ZnS prepared with the hydrothermal method was entirely converted into ZnO via annealing at a temperature of approximately 550 °C and above. Therefore, sintering in a high vacuum of approximately 10⁻⁶ Torr or less is required to maintain the ZnS as the sole synthesized compound. The sintered samples were subsequently used for the E-beam thin-film evaporation process. Table 2 includes the green and sintered densities of ZnS pellets at a sintering temperature of 400 °C for 8 h. The sintered density mean value is lower than the theoretical density of the bulk ZnS (4.09 g/cm³) due to the low sintering temperature. In general, the sintering temperature of any bare powder is at least 70% of its melting point and should be around ~1300 °C for ZnS powder as its melting point is ~1830 °C [33]. The sintered pellets were effectively used in the E-beam thin-film deposition.

3.3. Thin-Film Microstructure at High Magnification

The SEM was used to examine the microstructure of the ZnS thin films produced via E-beam evaporation, as can be seen in Figure 7a–d. Image (a) shows a mat-like structure with a rough surface at a magnification of 40,000×, while image (b) displays the microstructure at a higher magnification of 80,000×, from which it can be seen that the particle size is in the nanoscale with an almost spherical shape. The high roughness may be due to the evaporation discontinuity, as the densities of the sintered ZnS pellets were lower than the theoretical values. The images (c and d) reveal labeled cross-sectional views of 40 nm and 111 nm thin films, and the measured thicknesses were approximately 41 nm and 115 nm, respectively. These values are very close to those obtained using the FilmTek 3000 spectrometer, Scientific Computing International, Carlsbad, CA, USA, which were 40 nm and 111 nm, respectively.

Figure 8a displays the XRD patterns of 111 nm ZnS thin films of the three cases (as-evaporated, annealed at 250 °C for 6 h, and annealed at 400 °C for 6 h). The examined thin films showed an amorphous structure, which could be attributed to the low substrate deposition temperature. There are no peaks indicative of crystallization. The formation of amorphous ZnS thin films is due to two main factors: the low deposition temperature and the E-beam evaporation method. The films were deposited at RT, which is obviously insufficient for forming crystalline ZnS thin films. The E-beam evaporation process vaporizes and may slightly fragment the source material, which usually leads to amorphous thin-film condensation on the substrate unless a higher deposition temperature is used or high-temperature post-annealing is applied after thin-film deposition [34]. The heating of the substrate during deposition is probably a crucial factor for the crystallization of these thin films. The EDS analysis confirmed the formation of the ZnS compound during the deposition process, as shown in Figure 8b. The spectrum reveals the existence of Zn and S peaks and the peak of Si substrate. The used E-beam method confirms the chemical analysis for the deposited thin film of ZnS.

3.4. UV–Visible and FTIR Spectroscopy of the Produced ZnS

3.4.1. UV–Visible Spectroscopy of the Thin Film Samples

The Percent Transmittance (T%) of the UV–Visible spectra for the two evaporated samples is shown in Figure 9. This figure shows clearly that both samples, the 40 nm and the 111 nm samples, are transparent in the near-IR, Visible, and most of the UV regions. This renders ZnS thin films good window materials and protective coatings in the described region. The thickness dependence of T% is also obvious in this figure.

Tauc’s equation was used as the basis to calculate the bandgap energy (E_g) from the T% data [35,36,37,38,39,40].

$${\left(\mathsf{\alpha }\mathrm{h}\mathsf{\nu }\right)}^r=\mathrm{B}(\mathrm{h}\mathsf{\nu }-{\mathrm{E}}_{\mathrm{g}})$$

(5)

where α is the absorption coefficient (in cm⁻¹), h is the Planck’s constant, ν is the light frequency, B is a constant, E_g is the bandgap energy of the material, and r is a constant that depends on the nature of the E_g, whether direct or indirect and whether allowed or forbidden. For ZnS, r = 2 was used for direct allowed transitions.

Figure 10 and Figure 11 present the Tauc’s plots and the linear region fit for the 40 nm and the 111 nm samples, respectively. The E_g calculations show a direct dependence on the thickness of the deposited film. It was found that the E_g increases as the thickness decreases. The reported bulk E_g for ZnS is around 3.6 eV, while the values for the studied thin films were around 4.82 eV and 5.82 eV for the 111 nm and 40 nm thin films, respectively. The E_g values of the thin films are 31%–62% higher than those of thick films and the bulk ZnS material. This thickness dependence of the E_g of ZnS thin films has been reported by many researchers [33,34,35,36,39]. Generally, the E_g of ZnS thin films increases as the film thickness decreases. This behavior is attributed to quantum confinement effects, stresses and strains in the films, defects in the structure of the films, and grain boundaries in the case of polycrystalline thin films. Due to the small thickness, quantum confinement becomes more pronounced, and the electrons and holes become confined, which increases their energy levels and hence increases the E_g. Stresses and strains in the films also alter the band structure and directly affect the E_g [41,42,43]. This attractive feature makes ZnS films a good choice for applications where E_g engineering is needed. Hence, the E_g of ZnS thin films can be simply controlled by varying film thickness.

3.4.2. FTIR of ZnS Powder and Its Thin Film

The FTIR spectra for the powder and thin film samples of ZnS are shown in Figure 12. The correspondence between the powder samples and the thin film samples is clearly seen in this figure. The main peaks associated with ZnS powder are slightly shifted in the thin film samples, which can be attributed to changes in the environment around the ZnS molecules. The peaks observed at 478 cm⁻¹ in the powder sample and the peaks at 533 cm⁻¹ and 646 cm⁻¹ are assigned to ZnS stretching vibrations, and the peak at around 1005 cm⁻¹ is also attributed to ZnS, while the peak at around 905 cm⁻¹ is attributed to metal oxide vibrations. The peaks at around 2370 cm⁻¹ are mainly assigned to C = O of the atmospheric CO₂, while the bands at around 3480 cm⁻¹ are usually assigned to O-H stretching. The powder sample also shows the C-Hx stretching shoulder in the region at around 2900 cm⁻¹. The FTIR spectra confirm the presence of ZnS in the studied samples [44,45,46,47,48,49].

4. Conclusions

The current research project presents a relatively simple but efficient method to chemically synthesize nanoscale ZnS powder. The produced ZnS was characterized using an SEM with EDS, XRD, UV-Vis, and FTIR. The results confirmed the formation of the compound. EDS showed that the atomic percentages (at. %) of Zn and S elements were 47.87% and 52.13%, respectively. The nanoparticle powder was compacted and sintered under vacuum for microstructural investigation, and the sintered pieces were subsequently used to produce thin films using E-beam deposition. The microstructural investigation showed that sintering led to the growth of the initial nanoparticles and that they remained within the nanoscale limit. ZnS thin films of ~40 nm and ~111 nm thickness were E-beam-deposited and optically studied using UV–Visible and FTIR spectroscopy. They showed two different values of E_g: 4.72 eV for the thicker sample (~111 nm) and 5.82 eV for the thinner one (~40 nm). This clearly shows the quantum confinement effects on optical properties and makes ZnS thin films suitable candidates for applications requiring E_g control. In addition, SEM imaging demonstrated that the particle size of the film layer was also in the nano-range with an amorphous state after E-beam deposition at RT, as elucidated by the XRD test results. The bandgap, optical properties, electrical properties, and structure of ZnS thin films can be modified by controlling the dopants, film thickness, and deposition temperature. The effects of film thickness, different dopants, and deposition temperature on the properties of ZnS thin films can be evaluated in future research projects.