Facile synthesis of ZnS–Ag₂S core–shell nanospheres with enhanced nonlinear refraction

Figure 3 shows the XRD patterns of Pristine–ZnS NPs and ZnS–Ag₂S CSNHs with a \(2\theta\) scanning angle ranging from 20° to 80°. Five characteristic diffraction peaks were observed in the Pristine–ZnS samples at 29.4°, 48.5°, 57.3°, 70.8°, and 77.2°, which can be indexed to the (111), (220), (311), (400), and (331) planes of cubic ZnS (JCPDS No. 79-0043). No other impurities could be detected in Pristine–ZnS diffractograms, confirming the high purity of the synthesized ZnS NPs. XRD pattern of ZnS–Ag₂S CSNHs can be referred to the structure of monoclinic Ag₂S according to the standard card (JCPDS No. 14-0072). For ZnS–Ag₂S CSNH sample, diffraction peaks appeared at \(2\theta\) values of \(22.5^{ \circ }\), \(25.0^{ \circ }\),\(26.0^{ \circ }\), \(26.3^{ \circ }\), \(29.0^{ \circ }\), \(31.6^{ \circ }\), \(33.7^{ \circ }\), \(34.4^{ \circ }\), \(34.7^{ \circ }\), \(36.9^{ \circ }\), \(37.8^{ \circ }\), \(40.9^{ \circ }\), \(43.4^{ \circ }\), \(44.2^{ \circ }\), \(45.5^{ \circ }\), \(46.2^{ \circ }\), \(47.8^{ \circ }\), \(48.8^{ \circ }\), \(53.3^{ \circ }\), \(56.5^{ \circ }\), \(58.4^{ \circ }\), \(61.2^{ \circ }\) , and \(63.8^{ \circ }\) that can be ascribed to the (\(10\bar{1}\)), (110), (\(11\bar{1}\)), (\(01\bar{2}\)), (111), (\(11\bar{2}\)), (120), (\(12\bar{1}\)), (022), (121), (\(10\bar{3}\)), (031), (200), (103), (\(13\bar{1}\)), (\(12\bar{3}\)), (\(21\bar{2}\)), (211), (\(22\bar{2}\)), (114), (\(04\bar{2}\)), (\(14\bar{2}\)), and (\(13\bar{4}\)) planes. No obvious characteristic peaks related to cubic ZnS phases can be found in ZnS–Ag₂S CSNHs. The absence of diffraction peaks relevant to cubic ZnS NPs in the XRD pattern of ZnS–Ag₂S CSNHs might be attributed to arise due to overlapping peaks of ZnS (111), (220), and Ag₂S (111), (211) [25].

FT-IR spectrum of Pristine–ZnS NPs shows absorption bands situated at 495.16 and 626.20 cm⁻¹ that is assigned to Zn–S vibration [26]. The sharp adsorption band centered at 998.40 cm⁻¹ can be attributed to the resonance interaction between vibrational modes of sulfide ions in ZnS crystal [27]. The bands appearing at 1416.77 and 1585.71 cm⁻¹ correspond to the stretching modes of carboxyl (C=O) groups arising from atmospheric CO₂ adsorbed on the ZnS surface [28]. The bands located at 2342.93, 2359.92, and 2923.82 cm⁻¹ might originate from microstructure formation of Pristine–ZnS NPs [26]. The broad and strong band observed around 3427.25 in the FT-IR spectrum of Pristine–ZnS NPs is from O–H stretching modes, arising from the water adsorbed on the ZnS surface via –COOH group [28]. As shown in Fig. 4, the FT-IR spectrum of ZnS–Ag₂S CSNHs show a typical absorption peak at 620.47 cm⁻¹ corresponding to Ag–N vibrations [29]. In addition, absorptions at 1111.10 and 1383.85 cm⁻¹ can be ascribed to C–O and C=O stretching vibrations, respectively [25]. The absorption peak located at 1622.89 cm⁻¹ and the weak band at 2361.57 cm⁻¹ can belong to aromatic C=C band [25] and microstructure formation of the sample [26], respectively. The broad adsorption peak centered around 3444.15 cm⁻¹ corresponds to O–H group from water molecules adsorbed on the surface of CSNHs [26].

Figure 5 shows the FESEM images captured in two different magnifications for Pristine–ZnS NPs and ZnS–Ag₂S CSNHs. Pristine–ZnS sample exhibits a regular shape with aggregations of many nearly spherical particles, which possess dimensions ranging from 16 to 30 nm (Fig. 5a, b). The enlarged view of Pristine–ZnS NPs (Fig. 5b) further reveals an almost rough surface morphology due to assimilation of nanoparticles. FESEM images of samples after the growth of Ag₂S nanostructures are shown in Fig. 5c, d. It is obvious from Fig. 5c, d that ZnS–Ag₂S CSNHs consists of spherical-shaped smooth morphology in the size ranges of 20–80 nm. It can be also concluded from Fig. 5d that the rod-like shapes formed in FESEM images relevant to ZnS–Ag₂S CSNHs are due to the agglomerated spheres.

EDS spectra obtained for Pristine–ZnS NPs and ZnS–Ag₂S CSNHs are shown as insets in Fig. 5b and d, respectively. The results confirm the presence of zinc (Zn) and sulfur (S) elements in EDS spectrum of Pristine–ZnS NPs and of Zinc (Zn), sulfur (S), and silver (Ag) in that of ZnS–Ag₂S CSNHs without any detectable traces of other impurities. The numerical values related to atom percentages of the constituent elements are summarized in Table 1. The states of surface element dispersion for ZnS–Ag₂S CSNHs are further analyzed by the elemental mapping technique and results are presented in Fig. 6. The elemental maps (Fig. 6b–d) for the surface region (Fig. 6a) suggest homogeneous distributions of Zn, S, and Ag elements in ZnS–Ag₂S CSNHs.

In order to study the core–shell structures of the as-synthesized ZnS–Ag₂S samples, high-resolution transmission electron microscopy (HRTEM) was used. A typical HRTEM image of an individual ZnS–Ag₂S nanostructure is shown in Fig. 7, clearly revealing a core–shell structure of ZnS–Ag₂S sample, displaying a darker core (ZnS) surrounded by a lighter thin layer as a shell (Ag₂S). Careful analysis of the image shows Ag₂S shell thickness of ~ 5.8 nm around a spherical core of ZnS with a diameter ~ 80 nm.

Light absorbance of Pristine–ZnS NPs and ZnS-Ag₂S CSNHs measured in a wavelength range of 200–800 nm is shown in Fig. 8. Pristine–ZnS NPs shows a relatively strong absorption in the wavelength range of 250–350 nm. It is obvious from Fig. 8 that ZnS NPs after being encapsulated in Ag₂S nanocrystals demonstrate enhanced absorption in the visible region as compared to Pristine–ZnS NPs. However, the light absorption in ZnS–Ag₂S CSNHs is weakened in the UV region due to the presence of Ag₂S layers around ZnS nanocrystals. Shells of Ag₂S nanostructures with near-infrared optical gap and intrinsically large absorption coefficient, forming on the surface of ZnS NPs, prevent direct penetration of UV radiation into ZnS structures resulting in a comparatively lower absorption in the UV region [12]. The results indicate that an increase in visible light absorbance for ZnS NPs can successfully occur by encapsulating such a nanostructure coated with layers of Ag₂S nanocrystals.

The room-temperature PL spectra of the as-synthesized samples were recorded in a wavelength range of 300–800 nm under an excitation wavelength of 250 nm (4.96 eV) as shown in Fig. 9a. It is obvious from Fig. 9a that Pristine–ZnS NPs show two similarly broad emission bands located around ~ 400 nm (3.1 eV) and ~ 680 nm (1.82 eV). The first band ranges from 300 to 550 nm with full width at half maximum (FWHM) of 128 nm, while the second band spreads from 650 to 800 nm with a FWHM of 130 nm. Photophysical processes describing the possible emission mechanisms of Pristine–ZnS NPs (Fig. 9b) suggest several possible recombination pathways of the photo-excited excitons. The proposed electronic transitions are the results of defects, which are formed during the growth process of Pristine–ZnS NPs. The structural defects in Pristine–ZnS NPs comprise point, line, and plane defects. The line and plane defects cannot participate in emission processes due to the lack of the quantum confinement effects [30]. Therefore, the presence of emission centers activated in Pristine–ZnS NPs probably originates only from point defects such as Scottky defects (vacancy states) and Frenkel defects (interstitial states) [12, 30]. Zinc interstitial atoms and sulfur vacancies lie in the ZnS forbidden gap and close to the conduction band to act as donor energy states. However, sulfur interstitial atoms and zinc vacancies lead to acceptor levels located above the valence band of Pristine–ZnS NPs [12, 25].

Based on the schematic illustration as in Fig. 9b, in addition to relaxation paths relevant to photo-excited electrons from the conduction band to the valence band and accepter levels formed in the forbidden region, there is a possibility for the recombination of electrons to be trapped in donor levels with photo-excited holes in the valence band and also holes trapped in acceptor states. Therefore, the extended emission pattern of Pristine–ZnS NPs confirms the presence of multiple point defect states in the ZnS nanostructures. A careful survey with a multi-peak Gaussian fit illustrated as an inset in Fig. 9a suggests that the first emission band from Pristine–ZnS NPs could be ascribed to arise from band-to-band emission and defect states including sulfur interstitials, sulfur vacancies, and zinc interstitials, which can be resolved into four Gaussian curves centered at 347, 408, 449, and 524 nm, respectively [25, 31]. However, deep level zinc vacancies are commonly considered to be responsible for a Gaussian emission spectrum centered at 750 nm [31] that appears at the second emission band related to Pristine–ZnS sample. It is noteworthy that the overall theoretical fit estimated in inset of Fig. 9a (dotted curve) comprising five overlapped peaks is in good agreement with experimental observations for Pristine–ZnS NPs.

For comparison, Fig. 9a displays emission patterns relevant to both Pristine–ZnS and ZnS–Ag₂S samples. It is also clear from Fig. 9a that ZnS–Ag₂S CSNHs show a significant decrease in the excitonic PL intensity after the growth of Ag₂S shells on the surface of ZnS nanostructures. This could arise from enhanced charge separation due to the coupling of ZnS and Ag₂S nanostructures as well as due to passivation of surface defects. A schematic diagram of electron–hole separations at the interface of ZnS–Ag₂S CHNHs is proposed in Fig. 9c. The core–shell heterojunction (Fig. 9c) follows an inverse type-I scheme [32] where the conduction band of ZnS is located at a more negative potential compared to that of Ag₂S, while its valence band is more positive than that of Ag₂S [33]. Because electrons and holes tend to migrate, due to the less negative conduction band and the less positive valence band at a semiconductor/semiconductor interface, respectively [34], both electrons and holes are transferred to Ag₂S shell region. In the inverse type-I ZnS–Ag₂S CSNHs, the charge carriers are partially spread within the Ag₂S shells, lowering the probability of charge recombination in ZnS cores. Subsequently, a weaker PL signal is expected for ZnS–Ag₂S CSNHs compared to Pristine–ZnS NPs. On the other hand, the presence of Ag₂S layer on the surface of ZnS core passivates the emission from surface defects of ZnS, which in turn leads to dominating emission characteristics relevant to Ag₂S shell in the emission pattern of ZnS–Ag₂S CSNHs. Weak emission intensities for Ag₂S nanostructures compared to those of ZnS nanostructures have been reported by many researchers, which may be ascribed to the low light emission time span due to the smaller band gap [35]. The weak PL emission may also originate from the thin Ag₂S coating on the ZnS surface lowering optical absorption of excitation photons. A similar trend has also been observed for hollow ZnS/Ag₂S core/shell nanospheres as compared to hollow ZnS nanoparticles alone [35]. Reddy et al. have also reported lower PL intensity for ZnS–Ag₂S nanocomposites compared to ZnS nanostructures [25]. In addition, the lower PL intensity may be explained by multiple reflections, due to the Ag₂S shell overlayers. Since the Ag₂S shell has a smaller refractive index (2.2) than that of the ZnS core (2.35) [36], Ag₂S shell can act as an antireflection layer and confine light rays into the ZnS core [37]. An illustration of possible reflection mechanisms in the interfaces of ZnS–Ag₂S CSNHs is suggested in Fig. 9d. It is concluded from Fig. 9d that because of sequential total internal reflections at the interface of the Ag₂S shell and the ZnS core, most of the light is absorbed by ZnS nanostructures, which is evident because of the thickness of the shell.

In Fig. 10, we have plotted the normalized results obtained from the division of the closed-aperture Z-scan data by the open-aperture Z-scan data for Pristine–ZnS NPs and ZnS–Ag₂S CSNHs indicating that both divided Z-scan traces exhibit a pre-focal peak followed by a post-focal valley. The resultant peak-valley shapes confirm the presence of the third-order nonlinear refractive indices for both Pristine–ZnS and ZnS–Ag₂S samples. The peak-valley configuration shaped in the vicinity of the focus (z = 30 cm) reveals the self-defocusing effects relevant to negative lens effects for such intensity-dependent refractive indices.

Due to the lack of linear absorption (\(\alpha \approx 0\)) in Pristine–ZnS NPs and ZnS–Ag2S CSNHs for the applied wavelength (1064 nm), differences in the magnitude of the nonlinear refractive indices can be directly calculated from the transmittance differences created between the peak and valley (Eq. 1). Therefore, it can be found from Fig. 10 that the nonlinearity of ZnS NPs indicates a remarkable enhancement in the samples encapsulated by a Ag₂S shell. The third-order nonlinear refractive indices of samples tested are calculated from Eq. (1) and the results are summarized in Table 2. An increase in the nonlinear refractive index of ZnS NPs covered by Ag₂S shells increased from \(6.4 \times 10^{ - 12} {\text{m}}^{2} /{\text{W}}\) to \(61.0 \times 10^{ - 12} {\text{m}}^{2} /{\text{W}}\), which is more than nine times higher than that of Pristine–ZnS NPs. This improvement in the resultant third-order refractive index can be understood by considering the effect of the Ag₂S shell growth on ZnS cores. It is noteworthy to point out that nonlinear refractive indices of Ag₂S cores covered by ZnS shells have been previously reported to be in the range of \(0.93 \times 10^{ - 9} {\text{cm}}^{2} /{\text{W}}\) to \(9.16 \times 10^{ - 9} {\text{cm}}^{2} /{\text{W}}\) under excitation of 532 nm, which are relatively small compared what is obtained in this work [38]. However, the third-order refractive indices estimated in this study are comparable to that of Ag/TiO₂ core/shell nanostructures (~ \(40.21 \times 10^{ - 12} {\text{m}}^{2} /{\text{W}}\)) measured at 532 nm [39]. In addition, similar values with a magnitude of \(1.1 \times 10^{ - 12} {\text{m}}^{2} /{\text{W}}\) have been obtained for the nonlinear optical response of reduced graphene oxide decorated with silver nanoparticles at 800 nm [2].