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Abstract
In this study, CdS, CuS, and Ag2S nanoparticles (NPs) were synthesized by a chemical bath procedure in the presence of polyvinyl alcohol (PVA) as a polymer stabilizer. Morphological studies followed by X-ray diffraction (XRD) and atomic force microscopy (AFM) revealed monolithic and small size NPs. The average crystalline size of CdS, Ag2S, and CuS nanocomposites was 18.1, 26.7, and 21.7 nm, respectively. UV-Vis absorption and photoluminescence (PL) spectra of samples showed a near-infrared region (NIR) emission peak for CuS. The bandgap of samples measured using absorption data was 3.48, 2.75, and 2.30 eV for CdS, Ag2S, and CuS NPs, respectively. Nonlinear optical properties, including nonlinear refractive index and nonlinear absorption of the NPs, were measured by the Z-scan technique under a 632.8 nm wavelength He-Ne CW laser. PVA/Ag2S nanocomposite displayed reverse saturable absorbance (RSA) and self-focusing, while PVA/CdS and PVA/CuS displayed saturable absorbance and self-defocusing behavior under the chemical bath procedure synthesis.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Semiconductor nanoparticles (NPs) are receiving extensive attention owing to their tunable chemical, optical, physical, and electrical properties [1]. The emission wavelength of NPs can vary between ultraviolet (UV), visible, and near-infrared (NIR) regions depending on the selected appropriate core material, which is based on the tuning sizes of NPs and desirable band gap [2]. Semiconductor NPs with tunable bandgap, structure, and linear and nonlinear optical characteristics are excellent candidates for use in diverse applications ranging from energy harvesting, communication and information technology, biology, medicine, sensors, displays, illumination, and cameras [3,4].
Top-down and bottom-up nanoparticle fabrication methods are distinguished [5,6]. Compared to the top-down fabrication methods, wet-chemical approaches, also known as solution processing, are very convenient and inexpensive. The hydrothermal/solvothermal method, hot-injection method, thermolysis, microwave irradiation, electrodeposition, and chemical bath deposition are the main nanomaterial synthesis procedures depending on each specific application [7,8]. By varying precursors in the reaction environment, and employing suitable synthesis procedures, different atomic compounds and structural and morphological nanomaterials like NPs, quantum dots (QDs), nanowires, and nanotubes are obtained [9]. Chemical bath reaction-based methods, which are conducted at atmospheric pressure and low temperatures, are the simplest and environmentally friendly in comparison with all conventional synthesis modalities and are used for the fabrication of powder, colloidal, and thin film platforms [10]. Although chemical bath-based techniques are interesting, they also have several disadvantages, such as wastage of solution and the requirement for a clean substrate. Therefore, developing a new strategy for addressing the current drawbacks of chemical-based methods is an urgent need.
Polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP) were used for growing NPs by producing stable colloidal systems owing to their biodegradability, oxygen resistivity, transparency over a wide range of the visible light spectrum, and biocompatibility [11,12]. The chain of the polymer by applying control on the quantum confinement effect prevents the aggregation of growing nanocrystals with respect to the PVA structure [13]. Hence, PVA controls the particle sizes, resulting in tuned absorption amount and PL emission intensity, and can be introduced to cover some drawbacks of chemical-based synthesis methods [5,14–16].
Ag2S is a semiconductor of the I-VI group with a tunable, narrow, and direct bandgap (∼1.1eV). Owing to the tunable emission, nontoxicity, and high absorption of UV light, promising applications of Ag2S NPs have been reported in near-infrared (NIR), in vivo imaging, and solar cells as an absorber, respectively. In addition, Ag2S-based heterostructure photocatalysts are a great candidate for energy conversion and environmental pollution protection due to their excellent photocatalytic performance. Ag2S colloidal NPs are synthesized using organic and water solvents [17]. For instance, Ag2S was synthesized from a single source (C2H5)2NCS2Ag and oleic acid as a capping agent with the organic solvent, where the 10 nm sized Ag2S particles had a sharp peak PL emission at 1058 nm [18]. Besides, Pang et al. obtained tunable Ag2S NPs using a two-step synthesis procedure, which injected (TMS)2S into silver acetate with toluene solvent under Ar gas flow, and the emission was between 690-1227 nm [19]. Apart from this, in most similar cases, AgNO3 and Na2S are used as precursors and water as a solvent. For example, in 2MPA ligand synthesis, temperature and emission are 90 °C and 780-950 nm [20], whereas BSA and Ribonuclease-A capping agents at room temperature result in 1050-1294 nm and 980 nm PL emissions [21,22]. It is noted that materials systems can be used for widespread applications depending on the band gap tunable. In this regard, Ag2S was synthesized in the presence of PVA at an 80 °C bath temperature. When PVA properties were tuned by varying Ag2S concentrations, the band gap of nanocomposites was between 1.06-5.39 eV [23]. Hence, simple and low-cost methods that can compete with complex methods in terms of efficiency are needed.
Thiourea and cadmium acetate were used as sulfide and cadmium sources to prepare CdS, where the chemical bath deposition (CBD) method is employed [24]. In detail, the pH of the solution was controlled by ammonium hydroxide, and the final product was deposited on the substrate by changing the depositing duration. Regarding the PVA/CdS NPs, the cadmium acetate and PVA were mixed at 70 °C. The band gap and PL emission wavelength are recorded at 2.6 eV and 415 nm, respectively. In comparison to the previous studies, although the used procedure was straightforward, the temperature was high, and synthesis should be conducted in multiple steps [25].
CuS is another I-VI group and p-type semiconductor and has widespread applications in solar cells, sensors, optoelectronic devices, and biomedicine [9] due to its tunable morphological, optical, and chemical properties. Copper sources are low-cost and plentiful in nature, which is another advantage of copper-based nanomaterials. The sodium sulfide and sodium citrate aqueous solution and CuCl2 liquor were mixed at room temperature, then heated up to 90 °C, when sodium citrate acted as a stabilizer of CuS NPs, the size was 10 nm, and the emission peak centered at 930 nm in near NIR region [26]. Moreover, the deposition of CuS NPs by CBD on a glass substrate as a thin film without using any polymer during the reaction is accompanied by a band gap amount of around 2.3-2.4 eV and PL emission at 425 and 530 nm by 370 nm excitation [27]. The research on this material system suffers from a lack of studies on CuS/polymer synthesis and characterization, especially CuS/PVA NPs synthesized by simple and low-cost procedures, which lead to medical applications, e.g., bio-and in-vivo imaging and therapeutic agents.
In the present study, Ag2S, CdS, and CuS as the most important and applicable material systems were synthesized and characterized in PVA aqueous solutions using a single-step and green chemical bath procedure. The structural and optical properties of Ag2S, CuS, and CdS were studied, and the outcomes were discussed for potential applications. Our results demonstrate that all synthesized NPs have similar particle sizes. Moreover, Ag2S NPs showed high third-order nonlinearity in comparison to CuS and CdS NPs. Also, CuS photoluminescence attributes may potentially lead to possible applications in in-vivo imaging that should be confirmed in future research. In essence, Ag2S, CuS, and CdS NPs were successfully synthesized by a single step and low-cost method as an alternative to frequently utilized complex routes, and their structural and optical properties are relevant to diverse applications in electronic, optical, imaging, and other devices.
2. Materials and methods
2.1. Materials
Polyvinyl alcohol (PVA), sodium sulfide (Na2S), copper (II) sulfate (CuSO4), cadmium chloride (CdCl2), and silver nitrate (AgNO3) were purchased from Merck. All chemicals were of analytical grade and were used as received without further purification. The purity was 99% in all used precursors.
2.2. Synthesis of CdS, CuS, and Ag2S NP-polymer nanocomposites
Nanocomposites were prepared by a chemical bath procedure method. Firstly, 2g of PVA was mixed in 100 mL of distilled water and stirred for 30 min at a 50 °C bath temperature. 10 mL precursors of AgNO3, CdCl2, and CuSO4 each in 0.05 M concentration, separately added to 20 mL of PVA on the heater-stirrer at 1700 rpm, and then 10 ml of Na2S, 0.05 M concentration, was added to the solution drop by drop. For more monolithic and homogeneity nanocomposites leave 2 more hours to mix on the stirrer. Then, the prepared samples were centrifuged at 6000 rpm for 30 min. For XRD and AFM studies, each sample was dried on a glass substrate. For absorption spectroscopy and nonlinear measurements, the aqueous form was used. Figure 1 shows the prepared samples of colloidal NP solutions with different colors. The yield of products for Ag2S, CdS, and CuS were 72%, 68%, and 75%, respectively.
2.3. Characterizations
The structure of the samples was analyzed by X-ray diffraction using D8-Advance Bruker, Cu-Kα radiation (λ = 1.5406 °A). An FP-6200 spectrofluorometer (JASCO Corporation, Tokyo; Japan) and a UV-Vis double beam spectrophotometer (T80 UV-Vis, PG Instruments) were used to record fluorescence and absorption spectra, respectively. Atomic force microscopy (AFM) was conducted using the FLEX AFM device by Nanosurf Company, Switzerland. The nonlinear optical properties of nanocomposites were measured by using the single-beam Z-scan technique. For this, a CW He-Ne Gaussian beam with a 632.8 nm wavelength and a 0.07 mm spot size, 2.54 ×106 W/m intensity with a 2.18 cm Rayleigh range, and a sample length of 10−3m (cell thickness) was used for the incitement of samples. The nonlinear refractive index and nonlinear absorption coefficients were measured with a closed and open aperture, respectively. San wa Laser Power Meter Lp1 Mobiken Series with a resolution of +/-5%@calibrated wavelength 633 nm /1mW was used as a detector as well.
3. Results and discussion
3.1. XRD results
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal by studying the strength of diffraction of X-rays from the surface of the crystal at different angles. Each diffracted peak along a different angle corresponds to a plane in a crystal unit cell, which we classify these planes with Miller indexes hkl. The XRD patterns of three nanocomposites are shown in Fig. 2. Compared with the theoretical XRD patterns of powder materials, which were obtained from the Crystallography Open Database (COD), there is a compatible match between prepared samples and their theoretically simulated peaks. As we see, the presence of PVA makes changes in this pattern. Research on the morphology and crystal shape of prepared NPs shows that Ag2S has a bulk shaped crystal structure [28], CdS is spherical [29], and CuS is hexagonal [30]. The average crystallite size of nanocomposites is measured by the Debye-Scherrer equation [31] concerning Miller (hkl) indices:
where K (=0.94) is the geometrical factor in the Scherrer equation, and it depends on the apparent radius of rotation of the crystal taking into account the Bragg angle [32]; β2θ is the full width at half maximum FWHM, and θ is the angle of diffraction. Scherrer relation was obtained under the assumption that each atom scattered the incident x-ray independently from others and that the scattered wave does not interact with atoms again. The relationship is unaffected by the type of atom or structure [33]. Even though there are new ways for measuring NPs sizes such as transition electron microscopy (TEM), dynamic light scattering (DLS), and whole powder pattern modeling, this way is the most commonly used tool for measuring crystallite size from the Scherrer equation, which can be a good approximation of particle sizes too [34].For these three nanomaterials, the average size of the crystals was measured along with the (101) Miller index for Ag2S, CdS, and CuS nanoparticles. The FWHM values were 0.320, 0.385, and 0.313, respectively, and the corresponding crystal sizes were 26.7, 18.1, and 21.7 nm. The lattice constant a can be calculated via the following formula:
where dhkl and β2θ (FWHM) are obtained from the X’pert software for XRD analysis. The dhkl values for Ag2S, CdS, and CuS are 3.283, 3.292, and 3.290 (A°), respectively, resulting in lattice constants of 4.642, 4.655, and 4.652 (A°). Numerical values of the lattice constant and average size of NPs are listed in Table 1. The XRD recorded peaks are in agreement with previous reports. In line with the previous study [35], (101), (111), and (121) peaks were recorded for Ag2S /PVA. Further, CuS/PVA pattern peaks were detected on (102), (103), and (105), which are consistent with [27]. Next, CdS NPs peaks were at (002), (110), and (201) [36].
Table 1. Average crystalline size of nanocomposites. Data are presented as means ± S.D
Dynamic light scattering (DLS) is an efficient tool for measuring particle size distribution. DLS studies applied to NPs to three samples with an average size of 125, 150, and 20 nm were recorded for Ag2S, CdS, and CuS NPs, respectively, which is obvious from Fig. 3.
In the X-ray case, the X waves diffract from the surface of different polycrystals and calculating the size from the Debye-Scherrer relation gives the average crystallite size. However, the scattering of light in the case of DLS from a bunch of (clusters) crystallites gives the average particle size. It is obvious that the average particle size is bigger than the average crystallite size.
3.2. Morphological study
At the micro and nanoscales, atomic force microscopy (AFM) is used to obtain information about functional properties and surface topography. Hence, AFM images of prepared nanocomposites were recorded. From AFM images, the homogeneous distribution of NPs in polymer beds is obvious (Fig. 4). The size of the particles is in agreement with the results obtained from the XRD section 3. The larger particle clusters have appeared in images at 75-120 nm too. The morphology and XRD patterns show a convenient size and morphology that each nanocomposite could be a good candidate for optoelectrical, sensing devices, in vivo imaging, and other applications [5,35,37]. SEM images show the synthesis and distribution of Ag2S, CdS, and CuS NPs in the PVA matrix (Fig. 1–3 Support Information).
3.3. Absorption and bandgap structure
The UV-Vis spectroscopy of samples was recorded in the range of 300-900 nm wavelength of the electromagnetic spectrum to explore the absorption behavior of the nanocomposites. By increasing wavelength from ultraviolet to near-infrared, CdS and Ag2S absorption decrease, and CdS has a minimum of 550 nm in the visible light region (Fig. 5). Our results are in agreement with previous works, where absorption spectra for Ag2S, CuS, and CdS NPs show a high absorption in the NIR region and absorption drastically decreases in the visible light spectrum [23,25,35,38]. Shifting the absorption spectrum to the red or blue side of the spectrum, considering the practical aspect of prepared NPs, is the core of previous research. Regarding the absorption spectrum of PVA/CuS nanocomposites, it should be noted that the strong absorption in the near-IR region corresponds to CuS covellite, which is characterized by the adsorption of surface plasmons.
By using the Tauc relation, the bandgap value is measured [39].
where K and Eg are constant and bandgap energy, respectively. h is the Plank constant, $\vartheta $ is the frequency, and n is equal to 2 for direct band gap and is ½ for indirect band gap semiconductors. α is the optical absorption coefficient, which is calculated from α = 2.303A/d, where A is the absorption amount, and d is the cell thickness. Using the absorption data and Eq. (3) and plotting the ${({\alpha h\vartheta } )^2}$ versus $\textrm{ }h\vartheta $ and extrapolating the linear part of the data gives us the bandgap value [39,40]. The band gap values were between 1.85-2.26 eV in the case of fullerene (C60) containing Ag2S/PVA [41]. In addition, the band gap value of Ag2S/PVA, which was measured for the thin film, was between 1.06-5.39 eV [41]. Figure 6(a) shows the PVA bandgap value of 4.75 eV. NPs and polymer composite together act as a controlling agent on the bandgap. The bandgaps of Ag2S, CdS and CuS NPs were reduced after they were successfully synthesized in the polymer bed, allowing these material systems to be prospective candidates for photovoltaic and optical switching applications. Ag2S and CdS NPs had a direct bandgap of 2.75 ± 0.11 eV, which is consistent with the previous studies. According to previous reports, CuS semiconductors have an indirect band gap. CuS/PVA thin-film polymer-less nanocomposite had a band gap of around 2.3-2.4 eV [27], and the CdS/PVA band gap value was recorded at around 2.54-2.80 eV [25]. As shown in Fig. 6, our results revealed that the bandgap value of CdS/PVA and CuS/PVA is 3.48 ± 0.23 eV and 2.30 ± 0.26 eV, respectively. Regardless of different synthesis procedures, interestingly the obtained bandgap values are the same, which indicates successful synthesis and the high potential of this method for the synthesis of a wide range of material systems.
Fig. 6. Bandgap structure and value of synthesized nanocomposites. (a) PVA, (b) PVA/Ag2S; (c) PVA/CdS; (d) PVA/CuS.
3.4. Photoluminescence spectroscopy (PL)
Some transitions (relaxation) are not radiative because semiconductors absorb light for the energy level structure and incident light spectrum, and only transitions from the conduction band edge to the valence band are radiative and observable (Fig. 7). In this work, Ag2S has an emission peak centered at 407 nm and around 425-430 nm. The PL intensity peak under different excitation modes (370, 380, and 385 nm) increased according to Fig. 8(a). In this regard, the PL emission for Ag2S NPs was controlled by changing the growth time and AgNO3 concentration, so that the PL has an emission in the boundary of NIR and visible region the (400 nm-430 nm) [35], which led to the emerging NIR and LED application.
Fig. 8. Photoluminescence spectrum of a) PVA/Ag2S and b) PVA/CdS in three different excitation modes.
Our findings on CuS/PVA studies show that the PL is not observable for these NPs because of the plasmonic nature of CuS NPs. Even though there are some reports on the PL emission of CuS NPs [27], the majority of these works are focused on reporting the plasmonic properties of CuS NPs. Such PL behavior is not surprising; the absorption spectrum can reveal extreme absorption in all recorded wavelengths [42,43]. Importantly, CuS NPs are suitable for green and medical applications owing to their non-toxicity [44]. Hence, the prepared CuS/PVA NPs can be used as photothermal platforms in the years ahead. The CdS PL spectrum has intense peaks at 500 nm, which are recorded with 325, 330, and 375 nm excitation modes, and the highest one is produced under a 375 nm excitation wavelength [45]. One possibility for differences between the reported amounts of PL is relevant to the presence of PVA.
3.5. Nonlinear optical properties
The nonlinear refractive index, n2, [46] and nonlinear absorption, β, [47] are two important nonlinear optical coefficients due to the intensity-dependent behavior of materials. Calculating and controlling them can be useful from a practical point of view. There are several techniques for measuring n2 and β such as the Morie deflectometer and Z-scan. In this research, we used the Z-scan technique, and Fig. 9 shows the setup for optical measurements. This work follows two layouts: open (without aperture) for nonlinear absorption β measurement shown in Fig. 9 (a) and close (with aperture) for nonlinear refractive index n2 measurement are shown in Fig. 9 (b). Nonlinear refractive index (Fig. 10) and nonlinear absorption (Fig. 11) data were fitted by the following functions (4) and (5), respectively [48,49]:
Fig. 9. Z-scan setup for measuring (a) nonlinear absorption coefficients (open aperture) (b) nonlinear refractive index (close aperture).
Fig. 10. (a) PVA/Ag2S, (b) PVA/CdS, and (c) PVA/CuS close aperture experimental data fitted by the theoretical function.
Fig. 11. (a) PVA/Ag2S, (b) PVA/CdS, and (c) PVA/CuS open aperture experimental data fitted by the theoretical function.
To calculate the values of nonlinear coefficients manually, Eqs. (6) and (7) have been used:
There are different mechanisms for explaining nonlinearity, like thermal, molecular orientation, etc. [50]. In line with previous research [51], we expect a thermal mechanism for this research because of the varying intensity in the profile of the Gaussian beam and different refractive indexes obtained for different points of space by the optical Kerr relation n = n0 + n2I [50]. The nonlinearity can result in a self-focusing/self-defocusing effect. From Fig. 10(a), we show that Ag2S NPs focused on the laser beam have a positive refractive index of 0.275 ×10−12 (m2/W). The observed transition from the transmission peak to the transmission valley marks the beginning of the Ag2S/ PVA's optical limiting behavior. The optical absorption of electrons can quickly be saturated, resulting in valence band depletion and conduction band filling. The thermal nonlinearity, which increases with the input fluence, is most likely the cause of the observed optical limitation. Additionally, CdS and CuS NPs with the refractive index of -0.318×10−12 (m2/W) and -0.195×10−12 (m2/W), exhibit self-defocusing. Ag2S NPs showed the reverse saturable absorbance 2.98×10−4 (m/W) with CW laser in comparison with two other nanocomposites that were saturable absorbers (Table 2). In similar research pulse lasers are used for achieving the nonlinear coefficients of Ag2S [52,53], CdS [54,55], and CuS [56] materials. The RSA attribute can occur in the case of multiphoton absorption and absorption from excited states too related to the mechanisms including the material and light interaction [57]. In general, by changing the intensity and type of laser, nonlinearity will be associated with one or more mechanisms simultaneously [48].
Table 2. Nonlinear refractive index and nonlinear absorption numerical values. Data are presented as means ± S.D.
Numerical values of nonlinear refractive index and nonlinear absorption coefficients were calculated using Eqs. (6) and (7) (Table 2). This different nonlinear optical behavior under the same conditions makes Ag2S NPs a good candidate for use in optoelectronic devices. When comparing our nonlinear results with other studies, it must be pointed out that the coefficient’s order of magnitude is different between this work and previous studies. Differences in calculated coefficients were related to the discrepancy in incident laser intensity, wavelength, type of laser (CW or pulse laser), and the involved mechanism under the input beam. The order of magnitude for the nonlinear refractive index in most comparable works was 10−10 and 10−4 for the nonlinear absorption [51,52]. The PVA is used as a colloid stabilizer and we do not expect sensible contribution in nonlinear response, due to the non-centrosymmetric structure of PVA making the possibility of obtaining third-order nonlinear optical response zero.
4. Conclusions
Semiconductor nanoparticles are receiving extensive attention because of their tunable properties. Here, Ag2S (I-VI), CdS (II-VI), and CuS (I-VI) semiconductor nanoparticles were synthesized by the chemical bath method, which is a single step, low-cost, and environment-friendly synthesis procedure, in the presence of polyvinyl alcohol (PVA) as a stabilizer of NPs. Structural studies demonstrated that all synthesized NPs have an almost identical average crystalline size of between 18-27 nm. Moreover, Ag2S NPs showed high third-order nonlinearity in comparison to CuS and CdS. Also, CuS photoluminescence behavior showed potential for applications in in-vivo imaging. Although the CBD synthetic method is a single step, low-cost, and environmentally-friendly synthesis procedure, similar structural and optical properties were obtained compared with the conventional complex routes and expensive-cost synthetic methods. Collectively, we expect our findings contribute to the rapidly growing body of research aiming to find the best synthetic method for a wide range of advanced functional nanostructured materials.
Acknowledgments
Author contributions: Conceptualization, A.F., M.R. and K.O.; methodology, A.F. and M.R.; software, A.F. and M.R.; validation, A.F., M.R. and D.D.; formal analysis, A.F.; investigation, A.F., M.R. and D.D.; resources, M.G.; data curation, A.F. and M.R.; writing—original draft preparation, A.F. and M.R.; writing—review and editing, M.G., D.D. and K.O.; supervision, M.G., D.D. and K.O.; project administration, M.R.; funding acquisition, M.R. and M.G. All authors have read and agreed to the published version of the manuscript.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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