Influence of the DEA Concentration on Structural and Optical Properties of Nanodot PbS Thin Films Growth by Chemical Solution Deposition: Unveiling Dual Optical Absorption Edges

Alghoraibi, Ibrahim

doi:https://doi.org/10.1155/2024/9504522

Journal of Nanomaterials

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Abstract Introduction Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2024 | Article ID 9504522 | https://doi.org/10.1155/2024/9504522

Influence of the DEA Concentration on Structural and Optical Properties of Nanodot PbS Thin Films Growth by Chemical Solution Deposition: Unveiling Dual Optical Absorption Edges

Ibrahim Alghoraibi¹

Academic Editor: Valeri P. Tolstoy

Received08 Sept 2023

Revised12 Mar 2024

Accepted02 Apr 2024

Published30 Apr 2024

Abstract

The present paper presents a straightforward method for producing thin film layers of sulfide quantum dots (PbS-QDs) on a glass substrate using chemical solution deposition (CSD) assisted by dipcoating technique. The deposited PbS-QDs films were subjected to a comprehensive analysis using atomic force microscopy (AFM), energy dispersive X-ray (EDX) scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–vis–IR absorption, and photoluminescence spectroscopic (PL) techniques to investigate the effects of varying concentrations of diethanol amine (DEA) on their morphology, crystal structure, elemental composition, light absorption, and emission characteristics. The spherical shape of the PbS-QDs was confirmed by AFM and SEM images with average sizes around 100 and 50 nm, respectively. The energy dispersive X-ray (EDX) analysis provides evidence the existence of Pb and S elements within the PbS matrix. X-ray diffraction (XRD) results validate that the deposited films exhibit high crystallinity, with a preferential orientation along the (111) plane and a face-centered cubic lattice structure of PbS. The crystallite size of PbS is measured to be 46.6 nm. Based on the optical absorption measurements, we have determined the size range of PbS nanocrystals to be between 4.3 and 11.5 nm. The optical studies reveal the presence of two optical absorption edges within the visible and infrared spectrum, two direct band gap energy, two cut-off wavelengths, two confinement energy, two Urbach energy tail, and dual emission peaks of PbS-QDs at room temperature. The analysis reveals the presence of two distinct band gap energies, one in the visible range (1.3–2.28 eV) and the other in the infrared range (0.65–0.88 eV), which can be attributed to the formation of two distinct sizes of quantum dots situated in two different layers. The first layer, deposited directly on the glass substrate, comprises quantum dots with an average size of approximately 5.2 nm, while the second layer contains quantum dots with an average size of about 9.5 nm. This ability to tune the band gap of PbS in the visible range up to the IR band (0.65–2.28 eV) is a critical feature that holds the potential for the development of innovative optoelectronic devices.

1. Introduction

Lead sulfide (PbS) belongs to the binary IVB-VIA group of semiconductors, characterized by a relatively large excitation Bohr radius ranging from 18 to 20 nm [1]. It has a direct, small bulk optical energy band gap of 0.41 eV (excitation edge of 3,200 nm at 300 K), and a high absorption coefficient of 105 cm⁻¹ in visible light [2]. Owing to these properties, PbS thin films hold great potential for a wide range of technological applications, including light-emitting diodes [3], solar cells [4], IR detectors [5–7], photovoltaic [8], photodetectors [9, 10], laser, diode, electronic, optoelectronic devices [11], gas sensing [12], photocatalyst [13], and photosensing. PbS nanostructures can be synthesized in a range of shapes and sizes such as nanowires [12], nanocubes [14], flower-like nanocrystallines [15], star-like [16, 17], nanorods [18], nanoparticles [19], nanofibers [20], nanoclusters [21], and hollow PbS nanospheres [22]. These entire forms of PbS can be prepared by different deposition methods, such as spray pyrolysis [3], vacuum deposition [23], electrochemical deposition [11], chemical bath (solution) deposition (CBD or CSD) [24–26], pulsed laser deposition [27], pulsed femtosecond laser irradiation [28], sonochemical [12, 23], microwave irradiation [29], spray pyrolysis [30], flash thermal evaporation [31], hydrothermal synthesis [16, 17, 32], sol–gel technique [33], solid–vapor deposition [34], successive ionic layer adsorption and reaction [35], green synthesis [36], microwave-assisted chemical bath deposition [37], nebulizer spray technique [38–40], spin coating [41], and radio frequency sputtering [42]. Chemical solution deposition (CSD) is a widely used and popular technique in the field of thin film technology, owing to its numerous advantages. These include its simplicity, low cost, ability to produce thin films with excellent homogeneity, low processing temperature, freedom from the need for complex vacuum equipment, and its suitability for the deposition of materials on a range of substrates. Additionally, it offers the capacity for the deposition of thin films over large areas, control over thin film properties, and the ability to manipulate the grain size of the nanocrystallites by adjusting the deposition parameters [12]: concentration of the reactants [43], reaction time deposition [15, 37, 44], temperature [19, 42, 45, 46], annealing temperature [27], pH of solution [43, 47], number of layer deposition [48, 49], complexing agent [50], coradiation [51], surfactant [52], doping such as Zn [53, 54], Cd [23, 55], Tb³⁺ions [12], Mn [23, 48], Sm [56], Ni [23, 57] Pt [58], Cu [59], Gd [60], Ag [23, 40], Mg [61], La [62], and Cr [23, 63], effect of substrate [64], source precursor [19], and capping ligand [65]. During the chemical bath deposition (CBD) process, controlled chemical reaction kinetics are employed to regulate particle size by manipulating the availability of metal ions (M⁺²) in the solution. It is of utmost importance to meticulously manage the ion-by-ion reaction to avoid any unintended precipitation and ensure the production of premium-quality thin films. This can be achieved by employing a stable metal ion complex, which restricts the number of free ions available in the solution. Particle size of PbS is generally affected by the reaction kinetics in the CSD process. Use of a complexing agent during CSD process alters reaction kinetics and thus particle size by controlling the availability of Pb²⁺ ions in the solution. The objective of this study was to employ the CSD method for synthesizing homogeneous PbS-QDs nanolayers on glass substrates, using three sets of films with varying concentrations of diethanol amine. The synthesized PbS-QDs thin films were characterized using multiple analytical techniques, including AFM, X-ray, EDX, SEM, UV–vis–NIR spectrophotometry, and PL. Additionally, the study presents a detailed discussion of the growth mechanism for the formation of PbS-QDs films. The best of our knowledge, there is no similar results have been reported earlier in the literature and there are not many references that discuss the possibility of controlling the absorption edge values for lead sulfide. The only reference that confirms this idea was provided by Popa et al. [66]. The study aims to prepare PbS-QDs thin films of lead sulfide with tunable absorption edges, extending from the visible to the near-infrared range, to be suitable for photovoltaic cell applications.

2. Experimental Section

2.1. Materials and Chemicals

In the experimental section, PbS-QDs thin films were deposited using a solution that contained HN (CH₂CH₂OH)₂, Pb (CH₃COO)₂, CS (NH₂)₂, and NaOH under various experimental conditions. The diethanol amine (DEA) used in the process was obtained from Aldrich and is a colorless, polyfunctional liquid that acts as a weak base due to being a secondary amine and a diol. DEA is soluble in water and hygroscopic due to the hydrophilic nature of its alcohol groups. Lead acetate (LA) (99.99 + %), thiourea (TU) (≥99.0%), and NaOH (Gadot grade) were also obtained from Aldrich and were used without additional purification. Deionized water was used in all experiments.

2.2. Reaction Mechanism and Deposition of PbS Films

2.2.1. Preparation of Glass Substrate

In order to fabricate PbS films, microglass plates (Ref 7101) with dimensions of 75 mm × 25 mm × 1 mm were utilized as the substrate. The quality and adhesion of the film on the substrate heavily relies on the cleanliness of the substrate surface, as it should exhibit a high level of transparency to visible light. The cleaning process began with carefully washing the glass substrates with acetone and deionized water, followed by ultrasonic cleaning using an ultrasonic bath operating at a frequency of 24 kHz and a power of 150 W at a temperature of 50−60°C for 30 min. Subsequently, the glass slides were dried with clean dry air to enhance the surface nucleation for growth of the films. After this process, the substrate was prepared for film deposition. The glass substrates coated with PbS thin films on both sides were removed and washed thoroughly with distilled water before being left to dry in an open atmosphere. The uniform surface of the glass substrate that faced the wall of the beaker during deposition was preserved for further analysis, while the other substrate surface facing the interior of the bath was treated with a nitric acid solution and removed using a cotton swab.

2.2.2. PbS Precursor Solution

Pb(CH₃COO)₂ and CS(NH₂)₂ were employed as the sources of cations (Pb⁺²) and anions (S⁻²), respectively, for the purpose of depositing PbS thin films onto a glass substrate. Initially, a 0.5 M solution of lead acetate was mixed with 6 ml of deionized water and magnetically stirred for 30 min until a homogeneous mixture was obtained. Following this, 1 M of sodium hydroxide (NaOH) was added to the mixture under stirring conditions for 3 min, leading to the whitening of the solution. The pH of the precursors was subsequently meticulously regulated to 12.4 ± 0.1 using a pH meter. Finally, 7 ml of a 2 M solution of [CS (NH²)₂] was added to the mixture. To avoid precipitation in the bath, control the rate reaction and to enhance the incorporation of ions on the substrate, a small quantity DEA was added as a complexing agent into the precursor solution. Three different molar concentrations (0.4, 1.2, and 2 M) of DEA were prepared separately in the same procedure as given in Table 1. The resultant solution was stirred gently for deposition of good-quality films.

2.2.3. Deposition of Nanofilm

The pretreated microscope glass plates were vertically dipped by using a homemade dipcoating apparatus into a reaction nanosol to have thin film deposition for 30 min at room temperature. Upon completion of the deposition process, the samples were extracted from the solution at a constant speed of 200 µm/s and subsequently underwent ultrasonic rinsing in acetone and distilled water for 2 and 10 min, respectively, in order to eliminate the nonadherent PbS layer. Finally, the samples were air-dried at room temperature. The photographs of PbS thin films are shown in Figure 1. The photograph clearly shows the uniform, grayish-colored PbS thin films, and well adhered to the glass substrate. During the dipping process, thin films are stuck onto the two faces of the substrate. Note that the layers deposited on the side of the substrate that faced the wall of the beaker were used for all measurements. The thickness of deposited film was estimated from the mass change of the glass substrate before and after coating procedure using a high precision weight balance with 0.00001 gr standard deviation and from the known bulk density of PbS (7.569 gr/cm³).

2.3. Mechanism of Thin Film Formation

The chemical reaction mechanism for the formation of lead sulfide is influenced by the experimental conditions. The formation of PbS occurs when the ionic product of Pb²⁺ and S²⁻ ions surpasses the solubility product. The rate of growth for PbS thin films is predominantly influenced by the rate of release of Pb²⁺ ions from the complex and the decomposition of thiourea in an alkaline aqueous medium. The process of (PbS) thin film formation and its growth can be described via the mechanism of the following reaction: In alkaline solution, the hydrolysis of lead acetate leads to the formation of Pb(OH)₂:

Thiourea decomposes in alkaline solution and produces HS⁻ ions:

The solution color was changed to brown and then dark gray after about 2 min, which indicates the PbS formation. Pb²⁺ ion complexes with DEA initially, forming [Pb²⁺DEA⁻]. The complex later decomposes, releasing Pb²⁺ ions that bond with S²⁻ ions to form PbS. The addition of a complexing agent to a Pb salt solution influences the release rate of Pb²⁺ ions. The complexing agent forms lead complex species that, upon dissociation, release variable concentrations of Pb²⁺ ions. These ions, along with liberated sulfide ions, migrate to the substrate surface, where they react to form a PbS thin film. This process modifies the rate of Pb²⁺ ion release, facilitating controlled film formation. The mechanism of this film formation can be elucidated by the ensuing chemical reaction:

3. Results and Discussion

This research paper aimed to optimize the chemical process of depositing semiconducting thin films of PbS-QDs on glass plates for optical applications. The study focused on investigating the impact of varying concentrations of complexing agents on the morphology and optical properties of the PbS-QDs films. Structural characterization, morphological analysis, and elemental composition were performed using AFM, SEM, EDX, XRD, and DLS. Optical properties were evaluated through absorption and transmission measurements, as well as analysis of parameters such as absorption coefficient, extinction coefficient, confinement energy band gap, Urbach energy, and PL.

3.1. Atomic Force Microscopy Analysis

The surface topology, quantum dot size, surface roughness, and film thickness of the PbS were evaluated as a function of concentrations of DEA using atomic force microscopy (AFM, Nanosurf easyScan2, Switzerland) to optimize the chemical process for growing PbS-QDs semiconducting thin films on glass plates. AFM measurements were conducted in tapping mode (Tap190 Al-G, NanoSensors™, Neuchatel, Switzerland) under ambient air conditions. The tip deviation sensitivity and scanner resolution were 0.2 nm, and the scan resolution was set to 256 × 256 points for all observations. AFM images and particle size distributions of the PbS-QDs thin films deposited using different concentrations of DEA (0.4 M, S1; 1.2 M, S2; 2.0 M, S3) are illustrated in Figure 2. The other parameters were kept constant (time deposition 30 min, 0.5 M lead acetate, 2M thiourea, 1 M sodium hydroxide, a pH of 12.4 ± 0.1 for sodium hydroxide and a temperature of 25°C).

These images demonstrate the effect of varying the molar concentration of the complexing agent on the size and quality of the PbS-QDs films. It is evident that the films are of high quality and possess uniform morphology, covering the entire substrate surface. The PbS-QDs adopt a uniformly spherical shape and exhibit a range of dimensions that are dependent on the concentration of the complexing agent. The films prepared using 0.4 M of DEA (Figure 2(S1)) exhibit a uniform distribution of isotropic dot sizes with high density (1,4.10¹⁰ particle/cm²) and an average diameter (d = 89.4 ± 5 nm) and height (h = 6.8 ± 1 nm). Meanwhile, the PbS-QDs grown with intermediate concentration of DEA (1.2 M) (Figure 2(S2)) have an average diameter of 94.6 nm, a mean height of 13.6 nm, and owing to coalesce of smaller dots into larger dots or agglomeration of the dots by the Ostwald ripening process. At higher concentrations of DEA (2.0 M), larger dots tend to coalesce together to form interconnected nanodots without formation of clusters (Figure 2(S3)). When increasing the concentration of DEA, the PbS-QDs mean diameter and height are increased to 117.5 and 18.2 nm, respectively.

Table 1 presents the line (profile) roughness parameters (Ra, Rq), area (surface) roughness parameters (Sa, Sq = RMS), thickness, and growth rate of the PbS thin films grown with varying DEA concentrations. The results show that increasing the molarities of DEA results in an increase in the mean height (h) and roughness (Ra, Rq, Sa, and Sq) of the films due to the formation of larger domed dots and a more complete layer with a decreased fraction of voids. It is noteworthy that the reported values are mean values, subject to statistical fluctuations that are contingent on the position of the measurement on the samples. To minimize these errors, multiple readings of each parameter were performed in various locations on the sample surface. The experimental parameters and AFM data are summarized in Table 1.

As we can see from AFM images, any increase in DEA concentration causes a speed-up formation of PbS dots in the development of average dot sizes and also their agglomeration. This is probably due to the rate of ion–ion exchange being strongly influenced by the amount of DEA, which in turn affects the growth rate of the film. Therefore, at a higher deposition concentration, the deposition rate, and film thickness increase to 0.12 nm/s and 126.8 nm, respectively (Table 1), while the rate of precipitation forming is slow; consequently, the PbS dots have enough time to grow. The presence of DEA with different concentrations during the formation of the PbS probably causes a change in the mechanism and kinetics of the reaction. The thickness of deposited film was estimated from the mass change of the glass substrate before and after the coating procedure using a high precision weight balance with 0.00001 gr standard deviation and from the known bulk density of PbS (7.569 gr/cm³). Thin film growth is controlled by the ion-by-ion deposition kinetics of chalcogenide on nucleating sites that exist on the immersed surfaces. At the outset, the film’s growth rate is low because it takes a certain period of incubation to produce crucial nuclei on a clean surface from a homogeneous system. Once nucleation occurs, the film’s growth rate rapidly increases until the rate of deposition equals the rate of dissolution. The film eventually achieves its maximum thickness, which is referred to as the terminal thickness. An increase in the concentration of complexing (DEA⁻_(complex)) ions leads to a reduction in the concentration of the metal ion (Pb²⁺) and consequent decline in the reaction rate and precipitation. This results in the formation of larger quantum dots and a correspondingly increased terminal thickness of the film. Additionally, the quantum dots tend to exhibit uniform coverage of the substrate.

3.2. Chemical Composition and Scanning Electron Microscopy Analysis

The as-deposited PbS-QDs thin films were subjected to analysis of their elemental composition and morphology using SEM and EDX techniques. The EDX spectra presented in Figure 3 confirm the presence of two elements, Pb and S, in all the thin films deposited on glass substrate at different concentrations of DEA. The Pb/S atomic percentages ratio was found to be 1.2. The EDX analysis results for the as-deposited films, including the weight and atomic percentage concentrations, are presented in the table included in the insets of Figure 4.

The analysis confirms that the PbS thin films exhibit high purity and do not include typical impurities, consistent with the XRD findings. Any extra peaks noticed in the EDX spectra are attributed to X-ray emission from the glass substrate. The morphology and size of the PbS-QDs thin films deposited at different DEA concentrations (0.4, 1.2, and 2.0 M) were examined by SEM imaging, as shown in Figure 4(S1–S3). The SEM images reveal a smooth and uniform surface morphology of the thin films that completely cover the substrate, indicating their high quality. The dots formed were spherical in shape, and their average size was found to be 34.1, 41.8, and 72.2 nm at low, medium, and high concentrations of DEA, respectively. The size of the dots increased with an increase in DEA concentration, and they tend to agglomerate.

3.3. X-Ray Crystallography Analysis

A single concentration (0.4 M) of DEA was analyzed by X-ray diffraction (XRD). The XRD spectra of the deposited PbS thin films are presented in Figure 3, recorded in the range of 20°–80°. The spectrum shows nine characteristic peaks at 2θ values of approximately 26°, 30°, 43°, 51°, 53°, 62°, 68°, 71°, and 78°, corresponding to the indexing Bragg’s reflections planes (111), (200), (220), (311), (222), (400), (331), (420), and (422) orientations. These peaks are consistent with standard JCPDS data card no. 05-0592, confirming the cubic structure of PbS. The sharpness of XRD peaks suggests that the thin films were well crystallized. To determine the crystallite size and broadening (W) for the highest peak (111), modeling was performed for the corresponding preferred peak (111) using Gauss and Lorentz functions (shown in the inset of Figure 3).

The (111) peak is the highest and preferred orientation of all the films, and the average nanocrystallite size of PbS was evaluated using the Scherrer formula from (111) XRD peak (Equation (1)) [67]. The microstrain (Equation (2)) [68] and dislocation density (Equation (3)) [69] were calculated to quantify the defects of the PbS film. The preferential crystallite orientation was determined by calculating TC₍₁₁₁₎ = (I₍₁₁₁₎/(I₀₍₁₁₁₎))/(∑N⁻¹I₍₁₁₁₎/(I₀₍₁₁₁₎)) × 100% (Equation (4)) [70]. The values of β, D, ɛ, δ, and TC were tabulated in Table 2.

The XRD spectra show that the PbS thin films deposited using 0.4 M of DEA had well-crystallized cubic structure with an average nanocrystallite size of 46.6 nm. The abundance of crystals oriented in the (111) direction was higher, as indicated by the higher value of TC (111).

3.4. Optical Studies

The optical characteristics of the thin films were assessed via Varian Cary 5,000 UV–vis–NIR spectrophotometry under normal incidence of light in the wavelength range of 260–3,300 nm. The absorption and transmittance spectra of the as-deposited samples with various concentrations of DEA, 0.4, 1.2, and 2 M, are shown in Figure 5. The spectra exhibit broad and pronounced absorption edges at approximately 750, 900, and 1,130 nm for the films grown with DEA concentrations of 0.4, 1.2, and 2.0 M, respectively. This behavior is attributed to the change in the films’ thickness and the size of PbS-QDs, which leads to a shift toward longer wavelengths due to the quantum size effect [71]. In addition to the absorption maximum, absorption bands were observed in the infrared range of 2,600–3,300 nm for all spectra. The insets of Figure 5(a) illustrate other absorption thresholds, corresponding to electron excitation from the valence band to the conduction band. The transmittance spectra in Figure 5(b) show a change with the increase in DEA molarity. The low transmittance was observed in UV–vis region and this is attributed to high absorption of dots at these wavelengths, while high transmittance was observed in the near-infrared region. There are three peaks at 882, 1,500, and 1,760 nm for 0.4, 1.2, and 2.0 M, respectively.

(a)

(b)

In Figure 6, the optical absorption coefficient of the PbS thin films is presented as a function of photon energy in the range of 280–3,100 nm. The results show that the absorption coefficient values for all samples are within the range of 10⁵−10⁶ cm⁻¹, indicating the presence of a direct optical band gap energy in the prepared PbS-QD thin films. These findings satisfy the criterion for using Tauc-plot analysis, which is suitable for absorption coefficient values greater than 10⁴ cm⁻¹.

The extinction coefficient can be calculated using the relation: k = αλ/4π. Figure 7 illustrates the dependency of the extinction coefficient on wavelength. It shows a sharp increase in the visible region which can be attributed to the strong absorption of incident photons in the near of the band gap.

In order to determine the direct optical band gap of PbS-QD semiconducting thin films synthesized at various concentrations of DEA, the Tauc’s relationship was used to calculate the values of the band gap using an equation () (Equation (5)) [72], where is absorption coefficient () (Equation (6)), A is a constant related to the details of conduction and valence bands, is Planck’s constant, is photon frequency, is optical band gap and parameter depends on the transmission type and is equal to 1/2 for allowed direct transmission.

Using the last data, the direct band gap energy of PbS thin films was estimated by plotting a graph between versus , as shown in Figure 8. The optical band gap of the films can be evaluated by extrapolating the straight-line portion to the axis at α = 0. In fact, the close examination of the precedent curve shows that there are two absorption thresholds for each sample, the first one is in the visible and near-infrared (at about 0.544–0.954 μm) and the other is in the near-infrared and short-infrared (at about 1.409–1.908 μm). This can be clearly seen with the linear adjustment for each region in Figure 8. The optical band gap of the PbS film coated at 0.4 M concentration is the highest (2.28 eV) and it decreases with increases in DEA concentration and attains a minimum value of (1.30 eV) for the film coated at 2.0 M concentration. An increase in the crystallite size of the film coated at 2.0 M concentration (as shown in Table 1) is the reason for the decreased band gap obtained at that concentration. The bigger particle size, the lower quantum size confinement and thus the smaller the band gap, leading to higher wavelength for the absorption edge (Figure 8). The calculated band gap values for each sample (E_g1 and E_g2) are listed in Table 3, the two different absorption thresholds are correlated with the size of nanocrystallites in the thin film, the first one is in the visible region (2.28–1.30 eV, λ_cut-off ≅ 700 nm) corresponds to nanocrystallites of small size (4.3–6.1 nm) and the second one is in the short-infrared (0.88–0.65 eV, λ_cut-off ≅ 1,600 nm) is due to nanocrystallites of larger size (11.5–8.4 nm). This confirms that, for all these cases, the band gap increases with decreasing nanocrystallites size. Obtained band gap values for PbS thin films deposited are higher than those reported in the literature [71].

The Brus’s [73] model, utilizing the effective mass approximation (EMA), can be employed to determine the crystallite size based on the relationship between the band gap of the nanostructure and the crystallite radius. Using the first approximation parabolic bands model, the EMA substitutes the mass of the electron and hole with effective masses ( and ) and considers an exciton confined to a spherical volume of the crystallite. By applying the Brus equation, the energy of quantum confinement () is directly correlated to the nanocrystal radius (R):

The Brus equation was employed to investigate the confinement effect in PbS-QDs, where R represents the radius of the crystallite, and E_g(nano) is the excitonic absorption peak of the nanoparticle. Additionally, (_PbS = 0.085 m_o) and (_PbS = 0.085 m_o) denote the effective masses of the electron and hole, respectively, while e represents the electric charge, and ε = 17.3 is the dielectric constant of PbS, and ε_o is the vacuum permittivity constant. The Brus equation yielded calculated diameters for PbS nanocrystallite at the first absorption thresholds of 5.1, 4.3, and 6.1 nm (≅Bohr radius 18 nm) and at the second absorption thresholds of 11.5, 8.5, and 8.4 nm for different concentrations of DEA (0.4, 1.2, and 2 M, respectively). Investigation revealed two different sizes of quantum dots in two different layers, with the first layer having an average nanocrystal size of about 5.2 nm and the second layer having an average nanocrystal size of about 9.5 nm (as shown in the inset of photographs in Figure 5(b)). The crystallite size PbS-QDs was shown to be smaller than the exciton Bohr radius in PbS (18 nm), thus indicating the strong quantum confinement effect. The graphs in Figure 9 of ground state confinement energy versus the size of PbS-QDs for the first and second absorption thresholds illustrate that the ground state confinement energy is inversely proportional to the size. Consequently, as the diameter increases, the confinement energy decreases but never reaches zero, meaning that the lowest possible energy for the quantum dot sample is not zero. Furthermore, confinement commences when the diameter of PbS-QDs is comparable or of the order of the exciton Bohr radius. As a result, the crystallite size determined by the Brus model differs from those determined using XRD.

(a)

(b)

The relationship between the energy gaps (E_g1 and E_g2) and the diameters of deposited PbS-QDs is shown in Figure 10, which was obtained from the data presented in Figure 8. The results indicate that the energy gap increases as the crystallite size decreases. This observation can be attributed to the quantum size effects [74], which arise due to the confinement of energy levels within small potential wells. As the crystal size becomes smaller, the distance between energy levels increases. The energy gaps in the nanocrystalline films are found to be relatively high compared to that of bulk crystalline PbS (0.41 eV). Notably, a significant change in the energy gap is observed in the first stage of growth.

Urbach’s empirical formula may be used to describe the exponential dependence of the absorption coefficient [75]: (Equation (8)) [76], where is a constant and E_U (Urbach energy) is an energy which is often interpreted as the width of the tail of the localized states associated with the amorphous states in the forbidden band. The relationship between the logarithm of the absorption coefficient (ln(α)) and photon energy for the films deposited at different DEA concentrations (0.4, 1.2, and 2 M) is shown in Figure 11, with E_U1 and E_U2 for the two absorption edges calculated from the reciprocal of the straight-line slopes and presented in Table 3. Table 3 illustrates an inverse relationship between the band gap energy (E_g1 and E_g2), confinement energy (E1_confi and E2_confi), and Urbach energy of PbS thin films. A minimum value of E_U suggests a very weak absorption tail due to minimized defects and impurities, improving the optical properties of the prepared films. The variation in the optical band gap with the complexing agent concentration may be attributed to the change in quantum dot size and structural disorder in the films, as observed from Urbach energy analysis.

3.5. Photoluminescence Studies

Figure 12 displays the photoluminescence emission spectra of the nanocrystalline PbS thin film at room temperature. The emission spectrum exhibits two peaks, one at 540, 542, and 545 nm, which corresponds to the recombination of the electron and hole pair [38], and another as a shoulder at 659, 662, and 691 nm for concentrations of 0.4, 1.2, and 2.0 M, respectively. The shoulder peak is attributed to the various nanocrystallite sizes of PbS. It is noteworthy that these observed peaks are in good agreement with the absorption edges observed in the absorption spectra. The remaining two peaks observed at longer wavelengths, 546 nm and approximately 672 nm, are due to transitions in the nanocrystalline PbS-QDs films [25].

3.6. Characterization PbS Powder

After terminating the reaction and completing the deposition of the films by CSD process, the collected PbS nanopowder was subjected to multiple washes with DI water and ethanol in a sequential manner, followed by drying at 50°C. The morphological and quantitative analysis of as-prepared PbS nanopowder is examined by dynamic light scattering (DLS), zeta value, SEM, and EDX. Particle average size distribution of the resulting powder was analyzed by using DLS. Figure 13 shows the PbS-NPs are mainly uniform and monodisperse with the average range of particle size distribution is 56 nm. The zeta potential of the PbS nanopowder dispersed in water was measured, revealing a sharp peak at −9 mV, as depicted in Figure 13. This negative value suggests that the surface of PbS-NPs is negatively charged, resulting in weak repulsive forces between particles and leading to particle aggregation in the medium. The spherical and uniform morphology of all quantum dots, with an average size of approximately 34 nm, was confirmed by SEM images. Notably, the nanoparticle size measured by DLS was larger than that observed by SEM, reflecting the fact that DLS measures the hydrodynamic diameter of the particles while SEM determines the diameter of dry particles. Additionally, EDX analysis was conducted to determine the elemental composition of the resulting powder, and the results are presented in Figure 13.

In EDX profile, numerous well-defined peaks were evident related to Pb and S, which clearly support the fact that the as-prepared powder is the content of Pb and S. Other than this, while weaker signals from Na and O atoms were detected as well, which may originate from alkaline medium and organic compounds (DEA) or to the exposure of the powder to air, and adsorption of H₂O and CO₂. The inset table displayed the weight and atomic concentrations of Pb, S, Na, and O.

4. Conclusion

We have successfully deposited mirror-like, nanostructured PbS quantum dot films with tunable band-gap energies directly onto glass plates using different molar concentrations of diethanolamine (DEA) as a complexing agent via the CSD dipcoating method. The presence of the complexing agent in the bath has been observed to affect the rate of deposition and the crystallite size of the deposited lead sulfide film. The EDX analysis validates the high quality of the thin films, without any commonly encountered impurities. These findings align with the observations from the XRD analysis. Characterization of the as-deposited thin films has revealed their well-crystalline nature and good optical properties. Our photoluminescence study has indicated the occurrence of dual emission peaks in the visible region, specifically around 546 and 672 nm. The optical measurements indicate that the prepared films are composed of two distinct layers with varying particle dimensions. Also, we have determined that two distinct absorption edges. The first edge, ranging from 0.65 to 0.88 eV, is linked to PbS nanocrystals with sizes ranging from 8.4 to 11.5 nm in the initial layer (glass/PbS). The second absorption edge, observed at 1.3–2.28 eV, indicates the formation of nanocrystals below 6.1 nm in the upper second layer (PbS/PbS). This flexibility in varying the band gap of PbS-QDs in the range of 0.49–2.28 eV makes them a better candidate for solar energy conversion in the near-infrared region and opens the door for the creation of a new types of optoelectronic elements. This study presents a straightforward method for the preparation of lead sulfide quantum dots with potential applications in IR detectors and solar cells.

Data Availability

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

Conflicts of Interest

The author declares that there are no conflicts of interest regarding the publication of this article.

Acknowledgments

The author acknowledges the Higher Commission for Scientific Research (HCSR) in Syria for facilities and funding of this project.

References

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