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The Third-Order Nonlinear Optical Properties of Sb2S3/RGO Nanocomposites

The Third-Order Nonlinear Optical Properties of Sb2S3/RGO Nanocomposites hv photonics Article The Third-Order Nonlinear Optical Properties of Sb S /RGO Nanocomposites 2 3 Liushuang Li, Ye Yuan, Jiawen Wu, Baohua Zhu and Yuzong Gu * Physics Research Center for Two-Dimensional Optoelectronic Materials and Devices, School of Physics and Electronics, Henan University, Kaifeng 475004, China; lshuangl08@163.com (L.L.); yuanye_asuka@163.com (Y.Y.); 104753190687@henu.edu.cn (J.W.); bhzhu@henu.edu.cn (B.Z.) * Correspondence: yzgu@vip.henu.edu.cn Abstract: Antimony sulfide/reduced graphene oxide (Sb S /RGO) nanocomposites were synthe- 2 3 sized via a facile, one-step solvothermal method. XRD, SEM, FTIR, and Raman spectroscopy were used to characterize the uniform distribution of Sb S nanoparticles on the surface of graphene 2 3 through partial chemical bonds. The third-order nonlinear optical (NLO) properties of Sb S , RGO, 2 3 and Sb S /RGO samples were investigated by using the Z-scan technique under Nd:YAG picosecond 2 3 pulsed laser at 532 nm. The results showed that pure Sb S particles exhibited two-photon absorption 2 3 (TPA), while the Sb S /RGO composites switched to variable saturated absorption (SA) properties 2 3 due to the addition of different concentrations of graphene. Moreover, the third-order nonlinear susceptibilities of the composites were also tunable with the concentration of the graphene. The third-order nonlinear susceptibility of the Sb S /RGO sample can achieve 8.63  10 esu. The 2 3 mechanism for these properties can be attributed to the change of the band gap and the formation of chemical bonds supplying channels for photo-induced charge transfer between Sb S nanoparticles 2 3 and the graphene. These tunable NLO properties of Sb S /RGO composites can be applicable to 2 3 photonic devices such as Q-switches, mode-locking devices, and optical switches. Keywords: Sb S /RGO composite; graphene; third-order nonlinear optical property; saturable 2 3 absorption; susceptibility Citation: Li, L.; Yuan, Y.; Wu, J.; Zhu, B.; Gu, Y. The Third-Order Nonlinear Optical Properties of Sb S /RGO 2 3 Nanocomposites. Photonics 2022, 9, 1. Introduction 213. https://doi.org/10.3390/ Two-dimensional materials such as graphene, transition metal sulfides, and phos- photonics9040213 phorus have been widely studied in nonlinear optics. In order to explore novel NLO Received: 12 February 2022 properties for suitable applications, these two-dimensional materials are often used to Accepted: 21 March 2022 form composites with other materials. By changing the structure and composition, the Published: 23 March 2022 NLO properties of the composite is expected to be altered or improved. Because graphene has many interesting electrical and optical properties due to its special bandgap struc- Publisher’s Note: MDPI stays neutral ture [1,2], semiconductor–graphene composites have been extensively studied to obtain with regard to jurisdictional claims in excellent electronic, magnetic, optical, catalytic, and mechanical properties [3–5]. In the published maps and institutional affil- semiconductor–graphene composites, graphene has been regarded as an excellent electron iations. 4 2 1 1 acceptor with the fastest known electron mobility of 1.5  10 m V s [6]. In addition, monolayer graphene is easily dispersed in aqueous solution and polar solvent, which is conducive to the construction of graphene-based composites [7–9]. Some studies have Copyright: © 2022 by the authors. shown that combining organic matter and transition metal sulfide with graphene can Licensee MDPI, Basel, Switzerland. adjust its absorption properties to achieve improved optical limiting performances [10–12]. This article is an open access article Some studies have combined transition metal sulfides, such as molybdenum disulfide and distributed under the terms and cadmium sulfide, with graphene to achieve the enhanced NLO properties [13,14]. conditions of the Creative Commons Sulfide semiconductor nanomaterials have special third-order NLO properties [15–17]. Attribution (CC BY) license (https:// Among them, Sb S is one of the promising V-VI semiconductor materials that can be po- 2 3 creativecommons.org/licenses/by/ tentially applied in photoelectric sensors, near-infrared optical devices [18], optoelectronic 4.0/). Photonics 2022, 9, 213. https://doi.org/10.3390/photonics9040213 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 213 2 of 14 devices, and lithium-ion batteries [19]. The third-order NLO properties of Bi S and Sb S 2 3 2 3 monomers and Bi S /RGO composites have been studied [20,21]. The application prospect 2 3 of Sb S /RGO in photo degradation activity and a high-performance sodium ion battery 2 3 has been explored. However, the third-order NLO properties of Sb S /RGO composites 2 3 have been rarely discussed. The main motivation of this work was to explore the third-order nonlinear optical properties of Sb S /RGO by successfully combining Sb S nanoparticles with graphene 2 3 2 3 and to enhance the third-order nonlinear properties of the composite by changing the concentration of GO. In this work, Sb S /RGO composites were synthesized via a facile, 2 3 one-step solvothermal method. The third-order NLO properties were tested by using Z-scan technique with a 30-ps laser at 532 nm. The mechanism for the NLO properties was analyzed. The tunable saturable absorption and positive nonlinear refraction properties of Sb S /RGO composites were obtained, which can be applicable to the Q-switch, mode- 2 3 locking, and all-optical switch. 2. Materials and Methods 2.1. Sample Preparation 2.1.1. Synthesis of GO GO was prepared by a modified Hummers method [22,23]. First, a small amount of concentrated sulfuric acid (H SO ) and concentrated phosphoric acid (H PO ) were 2 4 3 4 mixed into a three-necked flask. The ground mixture of powdered graphite and potassium permanganate was then slowly added to the three-necked flask. Then, the three-necked flask was heated to 50 C and stirred for 24 h. After the reaction was completed, an appropriate amount of diluted hydrogen peroxide solution was added to the reactants. Then, the reaction products were treated with ultrasound and washed with hydrochloric acid and deionized water once and four times, respectively. Finally, the supernatant was removed by centrifugal process and the product was freeze-dried in a vacuum. 2.1.2. Synthesis of Sb S /RGO Composites 2 3 A certain mass of GO was dispersed into the corresponding ethylene glycol (EG) and then treated with ultrasound for 2 h to obtain a clear solution of GO-EG at the concentration of 0.1 mg/mL. Then, 0.114 g of antimony chloride (SbCl ), 0.15 g of polyvinylpyrrolidone (PVP), and 0.114 g of thiourea were added to the GO-EG solution to form a reaction mixture. After the mixed solution was stirred for about 30 min, the reaction mixture was transferred to a stainless steel autoclave with a PTFE lining, heated at 100 C for 12 h, and cooled to ambient temperature naturally. The resulting precipitate was collected by centrifugation, washed with ethanol and water several times, and vacuum-dried overnight for characterization. The final product was a Sb S /RGO composite prepared at 0.1 mg/mL 2 3 GO concentration, expressed as G2. In the control experiment, a series of Sb S /RGO 2 3 3+ composites were synthesized by fixing the amount of Sb and thiourea and changing the GO concentration to 0.05 mg/mL, 0.5 mg/mL, and 1 mg/mL. The products were expressed as G1, G3, and G4, respectively. For comparison, bare Sb S was synthesized 2 3 through the same steps without GO, and graphene was synthesized without using SbCl and thiourea. Figure 1 shows schematically the possible formation mechanism of Sb S 2 3 and the Sb S /RGO materials. 2 3 Photonics 2022, 9, x FOR PEER REVIEW 3 of 14 Photonics 2022, 9, 213 3 of 14 Figure 1. Schematic illustration of the formation of Sb S NPs (a) and Sb S /RGO (b). 2 3 2 3 Figure 1. Schematic illustration of the formation of Sb2S3 NPs (a) and Sb2S3/RGO (b). 2.2. Sample Characterization 2.2. Sample Characterization The morphology and structure of the samples were characterized through field emis- sion scanning electron microscope (SEM, Carl Zeiss Inc., Oberkochen, Baden-Württemberg, The morphology and structure of the samples were characterized through field Germany), transmission electron microscope (TEM, JEOL JEM-2100 working at 200 kV, emission scanning electron microscope (SEM, Carl Zeiss Inc., Oberkochen, Ba- JEOL Ltd. Inc., Akishima, Tokyo, Japan), and X-ray diffraction (XRD, Bruker D8 Advance, den-Württemberg, Germany), transmission electron microscope (TEM, JEOL JEM-2100 Bruker Inc., Karlsruhe, Badensko-Wuertembersko, Germany). The functional groups of working at 200 kV, JEOL Ltd. Inc., Akishima, Tokyo, Japan), and X-ray diffraction (XRD, samples were identified by Fourier transform infrared spectroscopy (FTIR, VERTEX 70v Bruker D8 Advance, Bruker Inc., Karlsruhe, Badensko-Wuertembersko, Germany). The Bruck Optics, Germany). Raman spectra were obtained by a Raman spectrometer. The UV- functional groups of samples were identified by Fourier transform infrared spectroscopy Vis spectra were measured on a PerkinElmer Lambda 35 ultraviolet-visible spectrometer (FTIR, VERTEX 70v Bruck Optics, Germany). Raman spectra were obtained by a Raman (Agilent Inc., Sacramento, CA, USA). The laser source used for the NLO measurement was spectrometer. The UV-Vis spectra were measured on a PerkinElmer Lambda 35 ultravi- an Nd:YAG laser system (EKSPLA, PL2251) with a wavelength of 532 nm, a pulse width of olet-visible spectrometer (Agilent Inc., Sacramento, CA, USA). The laser source used for 30 ps, and a pulse repetition of 10 Hz. the NLO measurement was an Nd:YAG laser system (EKSPLA, PL2251) with a wave- 3. Results length of 532 nm, a pulse width of 30 ps, and a pulse repetition of 10 Hz. 3.1. Structural and Morphology Characterization The XRD patterns of Sb S and Sb S /RGO powders are shown in Figure 2. It can 3. Results 2 3 2 3 be seen that the Sb S /RGO composites showed weak diffraction peaks, in the range of 2 3 3.1. Structural and Morphology Characterization 15  60 [24], and no clear diffraction of other impurities was detected, indicating that The XRD patterns of Sb2S3 and Sb2S3/RGO powders are shown in Figure 2. It can be the Sb S /RGO powders had good composite properties. The morphology and phase of 2 3 Sb seen th S wer ate th afe fected Sb2Sby 3/RGO c solvothermal omposite temperatur s showed weak e. When the diffr solvothermal action peakrs, in eaction the was range of 2 3 carried out at 100 C, the phase of the product was amorphous (Figure 2), indicating that 15°~60° [24], and no clear diffraction of other impurities was detected, indicating that the temperature was too low to crystallize Sb S [25]. The lack of diffraction peaks in the the Sb2S3/RGO powders had good composite proper 2 3 ties. The morphology and phase of sample revealed the amorphous nature. In addition, there was no grapheme diffraction Sb2S3 were affected by solvothermal temperature. When the solvothermal reaction was peak, possibly because the modification of Sb S on the graphene sheet destroyed the 2 3 carried out at 100 °C, the phase of the product was amorphous (Figure 2), indicating that orderly stacking of the graphene sheet and the uneven spacing of the graphene layers led the temperature was too low to crystallize Sb2S3 [25]. The lack of diffraction peaks in the to the disappearance of diffraction belonging to graphene. sample revealed the amorphous nature. In addition, there was no grapheme diffraction peak, possibly because the modification of Sb2S3 on the graphene sheet destroyed the orderly stacking of the graphene sheet and the uneven spacing of the graphene layers led to the disappearance of diffraction belonging to graphene. Photonics 2022, 9, x FOR PEER REVIEW 4 of 14 Photonics 2022, 9, 213 4 of 14 Figure 2. XRD patterns of the Sb S and Sb S /RGO (G1–G3). 2 3 2 3 Figure 2. XRD patterns of the Sb2S3 and Sb2S3/RGO (G1–G3). The morphology and structural characteristics of the Sb S /RGO composites syn- 2 3 thesized The m with orphol differ ogy ent and s concentrations tructural cha of r GO acte wer riste iccharacterized s of the Sb2S3/RGO co by scanning mposites synthe electron - microscopy. Figure 3a shows that the Sb S monomers were spherical structures with a sized with different concentrations of GO we 2 3 re characterized by scanning electron mi- diameter of about 1 m. However, when they were growing on graphene, as shown in croscopy. Figure 3a shows that the Sb2S3 monomers were spherical structures with a Figure 3b–d, the size and distribution of the Sb S particles were affected by the layered 2 3 diameter of about 1 μm. However, when they were growing on graphene, as shown in graphene. Due to the anisotropy of the Sb S nanoparticles, some of them were uniformly 2 3 Figure 3b–d, the size and distribution of the Sb2S3 particles were affected by the layered distributed on the graphene while others were clustered together with a rough surface graphene. Due to the anisotropy of the Sb2S3 nanoparticles, some of them were uniformly and a size of about 120 nm (Figure 3b). A further increase in the GO concentration re- distributed on the graphene while others were clustered together with a rough surface sulted in insufficient coverage of the Sb S nanoparticles on the graphene sheet, and the 2 3 and a size of about 120 nm (Figure 3b). A further increase in the GO concentration re- Sb S nanoparticles sparsely covered the graphene sheet with a slight decrease in size 2 3 sul (Figur ted in ins e 3c,du ).ff Based icienton cover the above age of r the esults, Sb the 2S3 n reaction anopartic of antimony les on the graphene sheet, chloride with thiourea and the may have produced unstable antimony compounds, while the Sb S NPs produced amor- Sb2S3 nanoparticles sparsely covered the graphene sheet wi 2th 3 a slight decrease in size phous particles on the surface of the graphene through the heterogeneous nucleation (Figure 3c,d). Based on the above results, the reaction of antimony chloride with thiourea process [26]. The antimony compounds were dispersed into the Sb S nanoparticles by the 2 3 may have produced unstable antimony compounds, while the Sb2S3 NPs produced solvothermal method. Coordination of COOH and OH groups with Sb (III) promoted good amorphous particles on the surface of the graphene through the heterogeneous nuclea- distribution of the Sb S nanoparticles on graphene sheets [27]. Therefore, the combination 2 3 tion process [26]. The antimony compounds were dispersed into the Sb2S3 nanoparticles of RGO and Sb S not only made Sb S adhere to the graphene layer but also improved the 2 3 2 3 by the solvothermal method. Coordination of COOH and OH groups with Sb (III) pro- conductivity of Sb S . 2 3 moted good distribution of the Sb2S3 nanoparticles on graphene sheets [27]. Therefore, In order to further investigate the structural characteristics of Sb S /RGO, the prepared 2 3 the combination materials were observed of RGO an by transmission d Sb2S3 not o electr nly m on ade micr Sb oscopy 2S3 adhere (TEM) to and the graphene layer but their structures were consistent with the results of the SEM analysis. Figure 3e,f shows the TEM images of also improved the conductivity of Sb2S3. the Sb S /RGO composites; it can be seen that the Sb S monomer synthesized by PVP was 2 3 2 3 spherical and dispersed nanoparticles were attached to the corrugated layered structure of the graphene, while some of them were clustered on the edge of graphene [28].The Sb S NPs were deposited on the surface with oxygen-containing groups in GO reactants 2 3 as anchor sites, indicating that the Sb S NPs were successfully supported on the surface 2 3 of RGO, which prevented aggregation of the Sb S NPs [27]. The sufficient connection 2 3 between the Sb S nanoparticles and graphene maximized the efficiency of electron transfer 2 3 due to the sufficient density and good dispersion of the relatively large Sb S NPs on the 2 3 graphene sheet. Photonics 2022, 9, 213 5 of 14 Photonics 2022, 9, x FOR PEER REVIEW 5 of 14 Figure 3. SEM images of Sb2S3 (a) and the Sb2S3/RGO composites G2 (b), G3 (c), G4 (d); TEM images Figure 3. SEM images of Sb S (a) and the Sb S /RGO composites G2 (b), G3 (c), G4 (d); TEM images 2 3 2 3 of composites G2 (e) and G3 (f). of composites G2 (e) and G3 (f). In order to further investigate the structural characteristics of Sb2S3/RGO, the pre- To investigate the characteristic vibration modes of the prepared samples, Raman spec- pared materials were observed by transmission electron microscopy (TEM) and their troscopy was carried out with the excitation of 532 nm. Figure 4 shows the Raman spectra structures were consistent with the results of the SEM analysis. Figure 3e,f shows the of pure GO, Sb S and Sb S /RGO with different concentrations of GO. Raman spectra of 2 3, 2 3 TEM images of the Sb2S3/RGO composites; it can be seen that the Sb2S3 monomer synthe- pure Sb S and Sb S /RGO showed that the diffraction peaks were about 300 cm . Both 2 3 2 3 1 1 sized by PVP was spherical and dispersed nanoparticles were attached to the corrugated GO and Sb S /RGO showed two main peaks at 1349 cm and 1599 cm , which were 2 3 layered structure of the graphene, while some of them were clustered on the edge of correlated with the disorder (D) band and graphite (G) band of carbon-based materials, re- graphene [28].The Sb2S3 NPs were deposited on the surface with oxygen-containing spectively, and the D band was related to the defect state of graphene. I /I represents the D G 2 3 groups in GO reactants as anchor sites, indicating that the Sb2S3 NPs were successfully carbon atom ratio of SP /SP , indicating that GO had a large number of oxygen-containing supported on the surface of RGO, which prevented aggregation of the Sb2S3 NPs [27]. functional groups [29]. Compared with GO (I /I = 0.95), Sb S /RGO (I /I = 1.01) D G 2 3 D G The sufficient connection between the Sb2S3 nanoparticles and graphene maximized the showed an increased D-G strength ratio, indicating there were more disordered carbons efficiency of electron transfer due to the sufficient density and good dispersion of the and plenty of surface defects [30]. These defects affected the charge distribution and proper- relatively large Sb2S3 NPs on the graphene sheet. ties of the composites. This was caused by the decrease in the average size of SP domains Photonics 2022, 9, x FOR PEER REVIEW 6 of 14 To investigate the characteristic vibration modes of the prepared samples, Raman and the increase in the number of domains. The change in I /I intensity usually indicates D G spectroscopy was carried out with the excitation of 532 nm. Figure 4 shows the Raman the reduction of graphene oxide to graphene [31] and can also be widely used to evaluate spectra of pure GO, Sb2S3, and Sb2S3/RGO with different concentrations of GO. Raman the structural defects of graphene materials [32]. spectra of pure Sb2S3 and Sb2S3/RGO showed that the diffraction peaks were about 300 −1 −1 −1 cm . Both GO and Sb2S3/RGO showed two main peaks at 1349 cm and 1599 cm , which were correlated with the disorder (D) band and graphite (G) band of carbon-based ma- terials, respectively, and the D band was related to the defect state of graphene. ID/IG 2 3 represents the carbon atom ratio of SP /SP , indicating that GO had a large number of oxygen-containing functional groups [29]. Compared with GO (ID/IG = 0.95), Sb2S3/RGO (ID/IG = 1.01) showed an increased D-G strength ratio, indicating there were more disor- dered carbons and plenty of surface defects [30]. These defects affected the charge dis- tribution and properties of the composites. This was caused by the decrease in the aver- age size of SP domains and the increase in the number of domains. The change in ID/IG intensity usually indicates the reduction of graphene oxide to graphene [31] and can also be widely used to evaluate the structural defects of graphene materials [32]. Figure 4. Raman patterns of GO, Sb S and G1 (a); Raman patterns of GO and composites G1–G4 (b). Figure 4. Raman patterns of GO, Sb2S3 and G1 (a); Raman patterns of GO and composites G1–G4 2 3 (b). Figure 5a shows the infrared spectra of GO, Sb2S3, and Sb2S3/RGO composites pre- −1 pared with different GO concentrations. For GO, the broad and strong peak at 3379 cm −1 was attributed to the stretching vibration of the -OH group, the peak at 1701 cm repre- sents the C=O vibration of -COOH located at the edge of the GO sheet, and the peak at −1 1638 cm was attributed to O-H bending vibration, epoxy group, and aromatic C=C −1 −1 skeleton tensile vibration [33]. The peaks at 1207 cm and 1068 cm were attributed to the C-O-C or C-O-H stretching and C-O stretching of the carboxyl group in GO. The FT-IR pattern of G4 (see Figure 5b) confirmed the formation of Sb-S bonds due to the −1 −1 −1 appearance of absorption peaks at 538 cm , 634 cm , and 721 cm . Additionally, a new −1 peak appeared at 994 cm , which may be attributed to C–S stretching vibrations and also demonstrated the strong bonding of GO and Sb2S3. Figure 5. FT−IR spectra of GO, Sb2S3, and the Sb2S3/RGO composites G2–G4 (a); FT-IR spectra of GO and G4 (b). The representative peaks of the carboxyl and hydroxyl groups of GO appeared in the composite materials, indicating that the Sb2S3/RGO composites were successfully synthesized by a solvothermal reaction. The strong covalent bonds and weak van der Waals forces between Sb and S atoms justified the formation of Sb2S3 nanoparticles, which, in turn, succeeded in firmly bonding to graphene due to the strong chemical in- teraction between carbon and sulfur. However, the peaks of oxygen-containing groups in the composites were significantly weakened, which indicated that the solvothermal reaction in EG removed most of the carboxyl and hydroxyl groups [34]. According to the Photonics 2022, 9, x FOR PEER REVIEW 6 of 14 Figure 4. Raman patterns of GO, Sb2S3 and G1 (a); Raman patterns of GO and composites G1–G4 (b). Photonics 2022, 9, 213 6 of 14 Figure 5a shows the infrared spectra of GO, Sb2S3, and Sb2S3/RGO composites pre- −1 pared with different GO concentrations. For GO, the broad and strong peak at 3379 cm −1 Figure 5a shows the infrared spectra of GO, Sb S and Sb S /RGO composites pre- was attributed to the stretching vibration of the -OH 2 3, group2 , the 3 peak at 1701 cm repre- pared with different GO concentrations. For GO, the broad and strong peak at 3379 cm sents the C=O vibration of -COOH located at the edge of the GO sheet, and the peak at was attributed to the stretching vibration of the -OH group, the peak at 1701 cm rep- −1 1638 cm was attributed to O-H bending vibration, epoxy group, and aromatic C=C resents the C=O vibration of -COOH located at the edge of the GO sheet, and the peak −1 −1 skeleton tensile vibration [33]. The peaks at 1207 cm and 1068 cm were attributed to at 1638 cm was attributed to O-H bending vibration, epoxy group, and aromatic C=C the C-O-C or C-O-H stretching and C-O stretching of the carboxyl group in GO. The 1 1 skeleton tensile vibration [33]. The peaks at 1207 cm and 1068 cm were attributed FT-IR pattern of G4 (see Figure 5b) confirmed the formation of Sb-S bonds due to the to the C-O-C or C-O-H stretching and C-O stretching of the carboxyl group in GO. The −1 −1 −1 appearance of absorption peaks at 538 cm , 634 cm , and 721 cm . Additionally, a new FT-IR pattern of G4 (see Figure 5b) confirmed the formation of Sb-S bonds due to the 1 1 1 −1 peak appeared at 994 cm , which may be attributed to C–S stretching vibrations and also appearance of absorption peaks at 538 cm , 634 cm , and 721 cm . Additionally, a new peak appeared at 994 cm , which may be attributed to C–S stretching vibrations and also demonstrated the strong bonding of GO and Sb2S3. demonstrated the strong bonding of GO and Sb S . 2 3 Figure 5. FTIR spectra of GO, Sb S , and the Sb S /RGO composites G2–G4 (a); FT-IR spectra of Figure 5. FT−IR spectra of GO, Sb22S33 , and the Sb2 2S3 3/RGO composites G2–G4 (a); FT-IR spectra of GO and G4 (b). GO and G4 (b). The representative peaks of the carboxyl and hydroxyl groups of GO appeared in The representative peaks of the carboxyl and hydroxyl groups of GO appeared in the composite materials, indicating that the Sb S /RGO composites were successfully 2 3 the composite materials, indicating that the Sb2S3/RGO composites were successfully synthesized by a solvothermal reaction. The strong covalent bonds and weak van der Waals synthesized by a solvothermal reaction. The strong covalent bonds and weak van der forces between Sb and S atoms justified the formation of Sb S nanoparticles, which, in turn, 2 3 Waals forces between Sb and S atoms justified the formation of Sb2S3 nanoparticles, succeeded in firmly bonding to graphene due to the strong chemical interaction between which, in tur carbon and sulfur n, succeede . However d in , the firmly peaksbon of oxygen-containing ding to graphene due groups in to the the strong composites che wer mical e in- significantly weakened, which indicated that the solvothermal reaction in EG removed teraction between carbon and sulfur. However, the peaks of oxygen-containing groups most of the carboxyl and hydroxyl groups [34]. According to the infrared spectrum of G2 in the composites were significantly weakened, which indicated that the solvothermal and G3 in the figure, it can be seen that the C=C framework vibration of the graphene sheet reaction in EG removed most of the carboxyl and hydroxyl groups [34]. According to the (1638 cm ) was prominent in the composite material, which also confirmed the recovery of the aromatic SP hybridized carbon skeleton of graphene [35]. These results indicated that the solvothermal reaction can effectively reduce graphene oxide to graphene. 3.2. Linear Optical Properties To preliminarily understand the nonlinear optical absorption (NOA) mechanism, the linear optical properties of the prepared samples Sb S and Sb S /RGO composites 2 3 2 3 were characterized by UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). As shown in Figure 6,the Sb S NPs had some absorption at the excitation wavelength of 610 nm. 2 3 However, they had more absorption at 240 nm, indicating the probability of excited states’ absorption in two steps or genuine two-photon absorption [36]. The Sb S /RGO had a 2 3 wide shoulder peak after 270 nm, and the absorption peak had a red shift. It was reported that Sb S is a direct band gap semiconductor material and has a high absorption rate [37]. 2 3 The relation between the light absorption coefficient and the energy of incident light 1/2 photon h near the absorption edge was as follows [38]: h = (h E ) (E is the g g band gap width). Therefore, the relation between ( h) and (h) near the absorption edge Photonics 2022, 9, 213 7 of 14 was linear and the band gap E could be calculated. According to Beer ’s law, absorbance A = abc, b is the sample thickness in cm, c is the sample concentration in g/L, and the proportional coefficient alpha is called the absorption coefficient. A can also be defined as A = lgT, where T is the transmission; so, the definition of alpha can also be expressed 1 1 as T = exp(alpha *c*b) in Lg cm . In addition, the value of linear absorption co- 1 1 1 1 efficient can also be obtained: (Sb S ) = 0.77 Lg cm , (G1) = 0.24 Lg cm , 2 3 1 1 1 1 1 1 (G2) = 0.21 Lg cm , (G3) = 0.70 Lg cm , and (G4) = 0.54 Lg cm . The absorption spectra of the samples indicated the presence of different local levels in the forbidden gap, which may have originated from amorphous defects in the antimony sulfide nanostructure, such as vacancies and surface defects. The presence of defects may some- what affect the band gap structure [39]. The Sb S /RGO composites had a reduced band 2 3 gap compared to Sb S , which provided the evidence of a connection between antimony 2 3 sulfide and graphene. The difference in the absorption curves of the composites and the Photonics 2022, 9, x FOR PEER REVIEW 8 of 14 Sb S NPs can be explained by the presence of RGO and the energy interaction between 2 3 the two materials [40]. Figure 6. Figure 6. UV– UV V –V is absorption is absorption spectra of spectra of Sb Sb 2S3S and the and the SbSb 2S3/R S G /RGO O comp composites. osites. 2 3 2 3 3.3. Nonlinear Optical Properties 3.3. Nonlinear Optical Properties The Z-scan curves in Figure 7 show the NLO characteristics of the sample. The NLO The Z-scan curves in Figure 7 show the NLO characteristics of the sample. The NLO properties of the Sb S and Sb S /RGO composites were studied by using Z-scan technique 2 3 2 3 properties of the Sb2S3 and Sb2S3/RGO composites were studied by using Z-scan tech- with a laser at wavelength of 532 nm, pulse width of 30 ps, and repetition rate of 10 Hz. nique with a laser at wavelength of 532 nm, pulse width of 30 ps, and repetition rate of 10 The interval between two pulses was about 0.1 s, which was much longer than the pulse Hz. The interval between two pulses was about 0.1 s, which was much longer than the width. Therefore, a thermal effect can be ignored. Actually, the laser used had the same pulse width. Therefore, a thermal effect can be ignored. Actually, the laser used had the effect as a single-shot mode laser. The samples were dissolved in ethanol solvent to prepare same effect as a single-shot mode laser. The samples were dissolved in ethanol solvent to solutions with a concentration of 0.1 mg/mL. The optical path length (cuvette) was 1 mm. prepare solutions with a concentration of 0.1 mg/mL. The optical path length (cuvette) The actual thickness of the sample was about 0.4 mm, including the total thickness of was 1 mm. The actual thickness of the sample was about 0.4 mm, including the total the cuvette minus its double walls. Therefore, the sample thickness was considered to be thickness of the cuvette minus its double walls. Therefore, the sample thickness was much less than the Rayleigh range; thus, the thin sample approximation was satisfied. It considered to be much less than the Rayleigh range; thus, the thin sample approximation was found that the NLO signal of ethanol solvent at 532 nm was very small and could was satisfied. It was found that the NLO signal of ethanol solvent at 532 nm was very be neglected [41]. Therefore, the nonlinearity of the solvent could be ruled out and the small and could be neglected [41]. Therefore, the nonlinearity of the solvent could be obtained Z-scan curves directly showed the nonlinear response of graphene [42], Sb S , 2 3 ruled out and the obtained Z-scan curves directly showed the nonlinear response of and Sb S /RGO. 2 3 graphene [42], Sb2S3, and Sb2S3/RGO. Photonics 2022, 9, x FOR PEER REVIEW 9 of 14 Photonics 2022, 9, 213 8 of 14 Figure 7. (a) Open-aperture Z-scan curves of Sb S , RGO, and the Sb S /RGO composite G3; Figure 7. (a) Open-aperture Z-scan curves of Sb2S3, RGO, and the Sb2S3/RGO composite G3; (b) 2 3 2 3 (b) open-aperture Z-scan curves of composites G1–G4. (c) Closed-aperture/open-aperture Z-scan open-aperture Z-scan curves of composites G1–G4. (c) Closed-aperture/open-aperture Z-scan curves of Sb S , RGO, and the Sb S /RGO composite G3; (d) closed-aperture/open-aperture Z-scan curves of Sb2S2 3, RGO 3 , and the Sb2 2S3 3/RGO composite G3; (d) closed-aperture/open-aperture Z-scan curves of composites G1–G4. (e,f) Normalized transmission versus input intensity of RGO and G3. curves of composites G1–G4. (e,f) Normalized transmission versus input intensity of RGO and G3. The curves in Figure 7a are the Z-scan curves of Sb S , G3 (Sb S /RGO-0.5), and RGO, 2 3 2 3 The curves in Figure 7a are the Z-scan curves of Sb2S3, G3 (Sb2S3/RGO-0.5), and reflecting the variation of nonlinear absorption characteristics. The curves of RGO and RGO, reflecting the variation of nonlinear absorption characteristics. The curves of RGO Sb S /RGO appear as symmetrical peaks at the focus, indicating the dominant position of 2 3 and Sb2S3/RGO appear as symmetrical peaks at the focus, indicating the dominant posi- saturation absorption (SA) in the NLO absorption mechanism; the saturated absorption tion of saturation absorption (SA) in the NLO absorption mechanism; the saturated ab- sorption of G3 shows an increasing trend compared with that of G1–G2. Generally, SA happens in the resonant region where the photon energy is larger than or equal to the band gap of the materials due to the electrons’ excitation from valence band to conduc- tion band and then the Pauli-blocking induced bleaching effect [43]. In the state of suffi- Photonics 2022, 9, 213 9 of 14 of G3 shows an increasing trend compared with that of G1–G2. Generally, SA happens in the resonant region where the photon energy is larger than or equal to the band gap of the materials due to the electrons’ excitation from valence band to conduction band and then the Pauli-blocking induced bleaching effect [43]. In the state of sufficiently large light intensity, electrons in the valence band can jump to the conduction band through single-photon absorption. The photon bands of the valence band and conduction band were completely occupied by electrons and holes; therefore, further light absorption was blocked and SA occurred [44]. The Sb S NPs exhibited two-photon absorption (TPA), 2 3 which can be seen in Figure 7a, due to the fact that the band gap of Sb S NPs was greater 2 3 than the excitation photon energy. Figure 7b shows the OA Z-scan curves (G1–G4) of Sb S /RGO corresponding to different GO concentrations. From G1–G4, as the graphene 2 3 increased, the peaks first increased and then decreased. This was due to the fact that when the graphene content was increased to a certain amount (G4), the lower Sb S content in 2 3 these composite materials could only generate a low level of photo-excited electron-hole pairs under irradiation. It can be seen from Figure 7c,d that all CA/OA Z-scan curves produced typical valley–peak trajectories, which indicated that all the samples had positive nonlinear refraction and self-focusing characteristics. The theoretical fitting curves, shown as a solid line in Figure 7, and the third-order nonlinear parameters were obtained using the Z-scan theory. The normalized Z-scan curves of OA were well fitted by the following equation [45], which is used to describe a third-order NLO absorptive process: q (z) T(z) = (1) 3/2 (1 + m) m=0 2 2 where q z = I L / 1 + z /z and is the nonlinear absorption coefficient. The ( ) ( ) 0 eff 0 0 z = ! / is the length of Rayleigh diffraction, which can be calculated to be 0.66 mm. I 0 0 is the laser intensity at the focus (1.69 GW/cm ), L = [1 exp( L)]/ is the effective eff length of the sample, L is the actual thickness of the sample, and is the linear absorption coefficient of the sample, which can be obtained from the UV-Vis absorption spectra. When q < 1, the normalized transmittance formula is approximated to the following formula: I L 0 eff T(z)  1 (2) 2 2 1 + z /z 3/2 2 2 According to the above equation, = 2 1 T 1 + z /z /I L can be ob- ( ) z=0 0 eff tained, and the nonlinear absorption coefficient of the sample can be calculated by this formula. The imaginary part of the third-order nonlinear susceptibility is gained by (3) 2 3 Im = cn  /480 and the normalized CA/OA Z-scan transmittance is calculated as [46]: 4xF T(z) = 1 (3) 2 2 (x + 9)(x + 1) where x = z/z ; so, the third-order nonlinear refraction index is n (esu) = (cn /40) m /W . 0 2 0 The is the nonlinear refractive index coefficient in m /W and DT is the peak-to-valley pv difference. The relation between and DT can be expressed by =  DT /[0.812I pv pv 0 0.25 L 1 S) 1 e . S is the linear transmittance of the aperture. The DF is the onaxis phase shift at the center focus, which can be expressed by the formula DF = kDn L = k I L containing . The real part of the third-order nonlinear 0 0 eff 0 eff (3) susceptibility is Re = n n /3 and the third-order nonlinear susceptibility is given by 0 2 1/2 2 2 (3) (3) (3) = Re + Im . The data listed in Table 1 are the third-order nonlinear parameters of the samples (RGO, Sb S , Sb S /RGO) calculated according to the above formula. From Sb S to Sb S /RGO, 2 3 2 3 2 3 2 3 Photonics 2022, 9, 213 10 of 14 (3) the imaginary part of third-order nonlinear optical susceptibility Im changes from (3) negative to positive and the Im values of G1–G3 increase, indicating that the nonlinear absorption can be regulated. It also shows the dominant role of saturated absorption in (3) the recombination process [47]. In addition, the real Re associated with the Kerr effect 1 2 was also regulated and enhanced, and its maximum value was 8.03  10 esu, which (3) (3) appeared in G3. The third-order nonlinear susceptibility  , determined by Im and (3) Re , is thus adjusted and enhanced to a maximum of 8.63  10 esu. The Sb S NPs 2 3 were evenly distributed on the graphene as the graphene increased. Even though the distribution of NPs was random, there existed a Fabry–Perot-like localized resonance due to small-scale ordering within the architecture, which resulted in the local enhancement of the field [48]. This local field enhancement led to an increase in NLO interactions and, hence, higher values for n and . Table 1. The NLO parameters of the samples. (3) (3) (3) Im Re Sample 11 18 12 12 12 (10 m/w) (10 m /w) (10 esu) (10 esu) (10 esu) RGO 2.17 0.03 1.36 0.04 1.36 Sb S 2.60 2.42 1.96 4.31 4.73 2 3 G1 1.32 0.39 0.99 0.69 1.21 G2 2.11 0.68 1.59 1.21 2.00 G3 4.22 4.51 3.18 8.03 8.63 G4 2.42 2.07 1.82 3.69 4.11 Figure 7e,f shows the relationship between normalized transmittance and saturation intensity of samples, which further shows that the composite and RGO exhibited the same absorption characteristics, corresponding to Figure 7a. We can fit the relevant data by the following formula: T = 1 (4) ns 1 + sat where is the non-saturated component, is the modulation depth, and I is the ns s sat saturated intensity. We can obtain the of 6.2%, 11.2%, 21.0%, and 10.7% for the G1, G2, 2 2 2, G3, and G4 corresponding to the I of 0.56 GW/cm , 0.47 GW/cm , 0.36 GW/cm and sat 0.41 GW/cm . According to the relevant nonlinear parameters in Tables 1 and 2, it can be concluded that G3 had the highest nonlinear absorption coefficient = 4.22  10 m/w and the lowest saturation strength I = 0.36 GW/cm . The saturated intensity of the sat sample (G1–G3) decreased, which corresponded to the change in the nonlinear absorption characteristics shown in Figure 7b. Comparing the nonlinear parameters of different materials, we found Sb S /RGO had strong third-order nonlinear optical properties. This 2 3 shows that Sb S /RGO composites have potential applications in mode locking and pulse 2 3 compression. Table 2. The relevant nonlinear parameters of different materials. Sample Reference 11 18 2 2 (nm) (10 m/w) (10 m /w) (GW/cm ) Sb S 532 2.60 2.42 This work 2 3 Sb S /RGO 2 3 532 4.22 4.51 0.36 This work (G3) MoSe 1064 2.05 0.71 [49] G-CuO 1030 1.37 0.48 [50] Photonics 2022, 9, 213 11 of 14 It can be seen from the data that the third-order nonlinearity of Sb S /RGO was 2 3 regulated. It is well known that the NLO process is controlled by the nonlinear susceptibility (3) (3) of NLO materials. The higher the  value is, the better the NLO performance. To Photonics 2022, 9, x FOR PEER REVIEW 12 of 14 further understand the variation of nonlinear optical absorption in the composite structure, we analyzed its possible photo-induced charge carrier transfer behavior [51], as shown in Figure 8. Due to the existence of energy band differences between Sb S and RGO, a 2 3 donor–acceptor electronic structure was formed in the complex. Sb S can be regarded as an garded as an electron donor, and RGO as an electron acc2ep 3tor, so that electrons cannot electron donor, and RGO as an electron acceptor, so that electrons cannot only transit within only transit within each of them, but also transfer between them. Additionally, the pho- each of them, but also transfer between them. Additionally, the photo-generated electrons to-generated electrons in Sb2S3 NPs would immigrate to RGO and further be trapped by in Sb S NPs would immigrate to RGO and further be trapped by the surface defects 2 3 the surface defects in RGO [52], which may minimize the possibility of recombination of in RGO [52], which may minimize the possibility of recombination of photo-generated photo-generated carriers. The excited electrons are transferred from the conduction band carriers. The excited electrons are transferred from the conduction band of Sb S to the 2 3 of Sb2S3 to the graphene and then to the valence band of Sb2S3. The progress may inter- graphene and then to the valence band of Sb S . The progress may interrupt the carrier 2 3 rupt the carrier relaxation in RGO and favor SA. relaxation in RGO and favor SA. Figure 8. Charge-transfer mechanism of the Sb S /RGO composites. Figure 8. Charge-transfer mechanism of the Sb2S3/RGO composites. 2 3 4. Conclusions 4. Conclusions In conclusion, the Sb S /RGO composites were successfully synthesized by the facile, 2 3 In conclusion, the Sb2S3/RGO composites were successfully synthesized by the facile, one-step solvothermal method and the NLO properties of all samples were studied at one-step solvothermal method and the NLO properties of all samples were studied at 532 532 nm by the Z-scan technique. It was found that the addition of GO transformed the two-photon nm by theabsorption Z-scan te of chni Sb q Sue. into It w adjustable as found th saturation at the absorption addition o of Sbf G S O /RGO trancom- sformed the 2 3 2 3 posites, which was attributed to the change of band gap. The tunable positive nonlinear two-photon absorption of Sb2S3 into adjustable saturation absorption of Sb2S3/RGO (3) refraction properties and the enhanced nonlinear susceptibility  of Sb S /RGO compos- 2 3 composites, which was attributed to the change of band gap. The tunable positive non- ites were obtained, which were larger than Sb S and more than six times that of RGO. The () 2 3 linear refraction properties and the enhanced nonlinear susceptibility χ of Sb2S3/RGO mechanism of its nonlinear optical properties was believed to be that the effective charge composites were obtained, which were larger than Sb2S3 and more than six times that of and energy transfer between Sb S NPs and RGO enhance the free carrier absorption and 2 3 RGO. The mechanism of its nonlinear optical properties was believed to be that the nonlinear refraction process. It was directly revealed that the Sb S /RGO composites have 2 3 effective charge and energy (3tr ) ansfer between Sb2S3 NPs and RGO enhance the free tunable nonlinear susceptibility  with different GO concentrations. The results of these carrier absorption and nonlinear refraction process. It was directly revealed that the tunable third-order NLO properties of the Sb S /RGO composites would provide the basis 2 3 () Sb2S3/RGO composites have tunable nonlinear susceptibility χ with different GO for the application in photonic devices. concentrations. The results of these tunable third-order NLO properties of the Sb2S3/RGO composites would provide the basis for the application in photonic devices. Author Contributions: Conceptualization, L.L. and Y.Y.; methodology, L.L.; validation, J.W. and Y.Y.; formal analysis, L.L.; investigation, L.L.; resources, Y.G.; writing—original draft preparation, L.L.; writing—review and editing, Y.G.; supervision, B.Z.; project administration, Y.G.; funding acquisition, Y.G. and B.Z. All authors have read and agreed to the published version of the manu- script. Funding: This research was funded by the National Natural Science Foundation of China (61875053, 61404045, U1404624) and Excellent Youth Project of Henan Province of China (202300410047). Institutional Review Board Statement: Not applicable. Photonics 2022, 9, 213 12 of 14 Author Contributions: Conceptualization, L.L. and Y.Y.; methodology, L.L.; validation, J.W. and Y.Y.; formal analysis, L.L.; investigation, L.L.; resources, Y.G.; writing—original draft preparation, L.L.; writing—review and editing, Y.G.; supervision, B.Z.; project administration, Y.G.; funding acquisition, Y.G. and B.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (61875053, 61404045, U1404624) and Excellent Youth Project of Henan Province of China (202300410047). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. 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[CrossRef] 2 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Photonics Multidisciplinary Digital Publishing Institute

The Third-Order Nonlinear Optical Properties of Sb2S3/RGO Nanocomposites

Photonics , Volume 9 (4) – Mar 23, 2022

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Abstract

hv photonics Article The Third-Order Nonlinear Optical Properties of Sb S /RGO Nanocomposites 2 3 Liushuang Li, Ye Yuan, Jiawen Wu, Baohua Zhu and Yuzong Gu * Physics Research Center for Two-Dimensional Optoelectronic Materials and Devices, School of Physics and Electronics, Henan University, Kaifeng 475004, China; lshuangl08@163.com (L.L.); yuanye_asuka@163.com (Y.Y.); 104753190687@henu.edu.cn (J.W.); bhzhu@henu.edu.cn (B.Z.) * Correspondence: yzgu@vip.henu.edu.cn Abstract: Antimony sulfide/reduced graphene oxide (Sb S /RGO) nanocomposites were synthe- 2 3 sized via a facile, one-step solvothermal method. XRD, SEM, FTIR, and Raman spectroscopy were used to characterize the uniform distribution of Sb S nanoparticles on the surface of graphene 2 3 through partial chemical bonds. The third-order nonlinear optical (NLO) properties of Sb S , RGO, 2 3 and Sb S /RGO samples were investigated by using the Z-scan technique under Nd:YAG picosecond 2 3 pulsed laser at 532 nm. The results showed that pure Sb S particles exhibited two-photon absorption 2 3 (TPA), while the Sb S /RGO composites switched to variable saturated absorption (SA) properties 2 3 due to the addition of different concentrations of graphene. Moreover, the third-order nonlinear susceptibilities of the composites were also tunable with the concentration of the graphene. The third-order nonlinear susceptibility of the Sb S /RGO sample can achieve 8.63  10 esu. The 2 3 mechanism for these properties can be attributed to the change of the band gap and the formation of chemical bonds supplying channels for photo-induced charge transfer between Sb S nanoparticles 2 3 and the graphene. These tunable NLO properties of Sb S /RGO composites can be applicable to 2 3 photonic devices such as Q-switches, mode-locking devices, and optical switches. Keywords: Sb S /RGO composite; graphene; third-order nonlinear optical property; saturable 2 3 absorption; susceptibility Citation: Li, L.; Yuan, Y.; Wu, J.; Zhu, B.; Gu, Y. The Third-Order Nonlinear Optical Properties of Sb S /RGO 2 3 Nanocomposites. Photonics 2022, 9, 1. Introduction 213. https://doi.org/10.3390/ Two-dimensional materials such as graphene, transition metal sulfides, and phos- photonics9040213 phorus have been widely studied in nonlinear optics. In order to explore novel NLO Received: 12 February 2022 properties for suitable applications, these two-dimensional materials are often used to Accepted: 21 March 2022 form composites with other materials. By changing the structure and composition, the Published: 23 March 2022 NLO properties of the composite is expected to be altered or improved. Because graphene has many interesting electrical and optical properties due to its special bandgap struc- Publisher’s Note: MDPI stays neutral ture [1,2], semiconductor–graphene composites have been extensively studied to obtain with regard to jurisdictional claims in excellent electronic, magnetic, optical, catalytic, and mechanical properties [3–5]. In the published maps and institutional affil- semiconductor–graphene composites, graphene has been regarded as an excellent electron iations. 4 2 1 1 acceptor with the fastest known electron mobility of 1.5  10 m V s [6]. In addition, monolayer graphene is easily dispersed in aqueous solution and polar solvent, which is conducive to the construction of graphene-based composites [7–9]. Some studies have Copyright: © 2022 by the authors. shown that combining organic matter and transition metal sulfide with graphene can Licensee MDPI, Basel, Switzerland. adjust its absorption properties to achieve improved optical limiting performances [10–12]. This article is an open access article Some studies have combined transition metal sulfides, such as molybdenum disulfide and distributed under the terms and cadmium sulfide, with graphene to achieve the enhanced NLO properties [13,14]. conditions of the Creative Commons Sulfide semiconductor nanomaterials have special third-order NLO properties [15–17]. Attribution (CC BY) license (https:// Among them, Sb S is one of the promising V-VI semiconductor materials that can be po- 2 3 creativecommons.org/licenses/by/ tentially applied in photoelectric sensors, near-infrared optical devices [18], optoelectronic 4.0/). Photonics 2022, 9, 213. https://doi.org/10.3390/photonics9040213 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 213 2 of 14 devices, and lithium-ion batteries [19]. The third-order NLO properties of Bi S and Sb S 2 3 2 3 monomers and Bi S /RGO composites have been studied [20,21]. The application prospect 2 3 of Sb S /RGO in photo degradation activity and a high-performance sodium ion battery 2 3 has been explored. However, the third-order NLO properties of Sb S /RGO composites 2 3 have been rarely discussed. The main motivation of this work was to explore the third-order nonlinear optical properties of Sb S /RGO by successfully combining Sb S nanoparticles with graphene 2 3 2 3 and to enhance the third-order nonlinear properties of the composite by changing the concentration of GO. In this work, Sb S /RGO composites were synthesized via a facile, 2 3 one-step solvothermal method. The third-order NLO properties were tested by using Z-scan technique with a 30-ps laser at 532 nm. The mechanism for the NLO properties was analyzed. The tunable saturable absorption and positive nonlinear refraction properties of Sb S /RGO composites were obtained, which can be applicable to the Q-switch, mode- 2 3 locking, and all-optical switch. 2. Materials and Methods 2.1. Sample Preparation 2.1.1. Synthesis of GO GO was prepared by a modified Hummers method [22,23]. First, a small amount of concentrated sulfuric acid (H SO ) and concentrated phosphoric acid (H PO ) were 2 4 3 4 mixed into a three-necked flask. The ground mixture of powdered graphite and potassium permanganate was then slowly added to the three-necked flask. Then, the three-necked flask was heated to 50 C and stirred for 24 h. After the reaction was completed, an appropriate amount of diluted hydrogen peroxide solution was added to the reactants. Then, the reaction products were treated with ultrasound and washed with hydrochloric acid and deionized water once and four times, respectively. Finally, the supernatant was removed by centrifugal process and the product was freeze-dried in a vacuum. 2.1.2. Synthesis of Sb S /RGO Composites 2 3 A certain mass of GO was dispersed into the corresponding ethylene glycol (EG) and then treated with ultrasound for 2 h to obtain a clear solution of GO-EG at the concentration of 0.1 mg/mL. Then, 0.114 g of antimony chloride (SbCl ), 0.15 g of polyvinylpyrrolidone (PVP), and 0.114 g of thiourea were added to the GO-EG solution to form a reaction mixture. After the mixed solution was stirred for about 30 min, the reaction mixture was transferred to a stainless steel autoclave with a PTFE lining, heated at 100 C for 12 h, and cooled to ambient temperature naturally. The resulting precipitate was collected by centrifugation, washed with ethanol and water several times, and vacuum-dried overnight for characterization. The final product was a Sb S /RGO composite prepared at 0.1 mg/mL 2 3 GO concentration, expressed as G2. In the control experiment, a series of Sb S /RGO 2 3 3+ composites were synthesized by fixing the amount of Sb and thiourea and changing the GO concentration to 0.05 mg/mL, 0.5 mg/mL, and 1 mg/mL. The products were expressed as G1, G3, and G4, respectively. For comparison, bare Sb S was synthesized 2 3 through the same steps without GO, and graphene was synthesized without using SbCl and thiourea. Figure 1 shows schematically the possible formation mechanism of Sb S 2 3 and the Sb S /RGO materials. 2 3 Photonics 2022, 9, x FOR PEER REVIEW 3 of 14 Photonics 2022, 9, 213 3 of 14 Figure 1. Schematic illustration of the formation of Sb S NPs (a) and Sb S /RGO (b). 2 3 2 3 Figure 1. Schematic illustration of the formation of Sb2S3 NPs (a) and Sb2S3/RGO (b). 2.2. Sample Characterization 2.2. Sample Characterization The morphology and structure of the samples were characterized through field emis- sion scanning electron microscope (SEM, Carl Zeiss Inc., Oberkochen, Baden-Württemberg, The morphology and structure of the samples were characterized through field Germany), transmission electron microscope (TEM, JEOL JEM-2100 working at 200 kV, emission scanning electron microscope (SEM, Carl Zeiss Inc., Oberkochen, Ba- JEOL Ltd. Inc., Akishima, Tokyo, Japan), and X-ray diffraction (XRD, Bruker D8 Advance, den-Württemberg, Germany), transmission electron microscope (TEM, JEOL JEM-2100 Bruker Inc., Karlsruhe, Badensko-Wuertembersko, Germany). The functional groups of working at 200 kV, JEOL Ltd. Inc., Akishima, Tokyo, Japan), and X-ray diffraction (XRD, samples were identified by Fourier transform infrared spectroscopy (FTIR, VERTEX 70v Bruker D8 Advance, Bruker Inc., Karlsruhe, Badensko-Wuertembersko, Germany). The Bruck Optics, Germany). Raman spectra were obtained by a Raman spectrometer. The UV- functional groups of samples were identified by Fourier transform infrared spectroscopy Vis spectra were measured on a PerkinElmer Lambda 35 ultraviolet-visible spectrometer (FTIR, VERTEX 70v Bruck Optics, Germany). Raman spectra were obtained by a Raman (Agilent Inc., Sacramento, CA, USA). The laser source used for the NLO measurement was spectrometer. The UV-Vis spectra were measured on a PerkinElmer Lambda 35 ultravi- an Nd:YAG laser system (EKSPLA, PL2251) with a wavelength of 532 nm, a pulse width of olet-visible spectrometer (Agilent Inc., Sacramento, CA, USA). The laser source used for 30 ps, and a pulse repetition of 10 Hz. the NLO measurement was an Nd:YAG laser system (EKSPLA, PL2251) with a wave- 3. Results length of 532 nm, a pulse width of 30 ps, and a pulse repetition of 10 Hz. 3.1. Structural and Morphology Characterization The XRD patterns of Sb S and Sb S /RGO powders are shown in Figure 2. It can 3. Results 2 3 2 3 be seen that the Sb S /RGO composites showed weak diffraction peaks, in the range of 2 3 3.1. Structural and Morphology Characterization 15  60 [24], and no clear diffraction of other impurities was detected, indicating that The XRD patterns of Sb2S3 and Sb2S3/RGO powders are shown in Figure 2. It can be the Sb S /RGO powders had good composite properties. The morphology and phase of 2 3 Sb seen th S wer ate th afe fected Sb2Sby 3/RGO c solvothermal omposite temperatur s showed weak e. When the diffr solvothermal action peakrs, in eaction the was range of 2 3 carried out at 100 C, the phase of the product was amorphous (Figure 2), indicating that 15°~60° [24], and no clear diffraction of other impurities was detected, indicating that the temperature was too low to crystallize Sb S [25]. The lack of diffraction peaks in the the Sb2S3/RGO powders had good composite proper 2 3 ties. The morphology and phase of sample revealed the amorphous nature. In addition, there was no grapheme diffraction Sb2S3 were affected by solvothermal temperature. When the solvothermal reaction was peak, possibly because the modification of Sb S on the graphene sheet destroyed the 2 3 carried out at 100 °C, the phase of the product was amorphous (Figure 2), indicating that orderly stacking of the graphene sheet and the uneven spacing of the graphene layers led the temperature was too low to crystallize Sb2S3 [25]. The lack of diffraction peaks in the to the disappearance of diffraction belonging to graphene. sample revealed the amorphous nature. In addition, there was no grapheme diffraction peak, possibly because the modification of Sb2S3 on the graphene sheet destroyed the orderly stacking of the graphene sheet and the uneven spacing of the graphene layers led to the disappearance of diffraction belonging to graphene. Photonics 2022, 9, x FOR PEER REVIEW 4 of 14 Photonics 2022, 9, 213 4 of 14 Figure 2. XRD patterns of the Sb S and Sb S /RGO (G1–G3). 2 3 2 3 Figure 2. XRD patterns of the Sb2S3 and Sb2S3/RGO (G1–G3). The morphology and structural characteristics of the Sb S /RGO composites syn- 2 3 thesized The m with orphol differ ogy ent and s concentrations tructural cha of r GO acte wer riste iccharacterized s of the Sb2S3/RGO co by scanning mposites synthe electron - microscopy. Figure 3a shows that the Sb S monomers were spherical structures with a sized with different concentrations of GO we 2 3 re characterized by scanning electron mi- diameter of about 1 m. However, when they were growing on graphene, as shown in croscopy. Figure 3a shows that the Sb2S3 monomers were spherical structures with a Figure 3b–d, the size and distribution of the Sb S particles were affected by the layered 2 3 diameter of about 1 μm. However, when they were growing on graphene, as shown in graphene. Due to the anisotropy of the Sb S nanoparticles, some of them were uniformly 2 3 Figure 3b–d, the size and distribution of the Sb2S3 particles were affected by the layered distributed on the graphene while others were clustered together with a rough surface graphene. Due to the anisotropy of the Sb2S3 nanoparticles, some of them were uniformly and a size of about 120 nm (Figure 3b). A further increase in the GO concentration re- distributed on the graphene while others were clustered together with a rough surface sulted in insufficient coverage of the Sb S nanoparticles on the graphene sheet, and the 2 3 and a size of about 120 nm (Figure 3b). A further increase in the GO concentration re- Sb S nanoparticles sparsely covered the graphene sheet with a slight decrease in size 2 3 sul (Figur ted in ins e 3c,du ).ff Based icienton cover the above age of r the esults, Sb the 2S3 n reaction anopartic of antimony les on the graphene sheet, chloride with thiourea and the may have produced unstable antimony compounds, while the Sb S NPs produced amor- Sb2S3 nanoparticles sparsely covered the graphene sheet wi 2th 3 a slight decrease in size phous particles on the surface of the graphene through the heterogeneous nucleation (Figure 3c,d). Based on the above results, the reaction of antimony chloride with thiourea process [26]. The antimony compounds were dispersed into the Sb S nanoparticles by the 2 3 may have produced unstable antimony compounds, while the Sb2S3 NPs produced solvothermal method. Coordination of COOH and OH groups with Sb (III) promoted good amorphous particles on the surface of the graphene through the heterogeneous nuclea- distribution of the Sb S nanoparticles on graphene sheets [27]. Therefore, the combination 2 3 tion process [26]. The antimony compounds were dispersed into the Sb2S3 nanoparticles of RGO and Sb S not only made Sb S adhere to the graphene layer but also improved the 2 3 2 3 by the solvothermal method. Coordination of COOH and OH groups with Sb (III) pro- conductivity of Sb S . 2 3 moted good distribution of the Sb2S3 nanoparticles on graphene sheets [27]. Therefore, In order to further investigate the structural characteristics of Sb S /RGO, the prepared 2 3 the combination materials were observed of RGO an by transmission d Sb2S3 not o electr nly m on ade micr Sb oscopy 2S3 adhere (TEM) to and the graphene layer but their structures were consistent with the results of the SEM analysis. Figure 3e,f shows the TEM images of also improved the conductivity of Sb2S3. the Sb S /RGO composites; it can be seen that the Sb S monomer synthesized by PVP was 2 3 2 3 spherical and dispersed nanoparticles were attached to the corrugated layered structure of the graphene, while some of them were clustered on the edge of graphene [28].The Sb S NPs were deposited on the surface with oxygen-containing groups in GO reactants 2 3 as anchor sites, indicating that the Sb S NPs were successfully supported on the surface 2 3 of RGO, which prevented aggregation of the Sb S NPs [27]. The sufficient connection 2 3 between the Sb S nanoparticles and graphene maximized the efficiency of electron transfer 2 3 due to the sufficient density and good dispersion of the relatively large Sb S NPs on the 2 3 graphene sheet. Photonics 2022, 9, 213 5 of 14 Photonics 2022, 9, x FOR PEER REVIEW 5 of 14 Figure 3. SEM images of Sb2S3 (a) and the Sb2S3/RGO composites G2 (b), G3 (c), G4 (d); TEM images Figure 3. SEM images of Sb S (a) and the Sb S /RGO composites G2 (b), G3 (c), G4 (d); TEM images 2 3 2 3 of composites G2 (e) and G3 (f). of composites G2 (e) and G3 (f). In order to further investigate the structural characteristics of Sb2S3/RGO, the pre- To investigate the characteristic vibration modes of the prepared samples, Raman spec- pared materials were observed by transmission electron microscopy (TEM) and their troscopy was carried out with the excitation of 532 nm. Figure 4 shows the Raman spectra structures were consistent with the results of the SEM analysis. Figure 3e,f shows the of pure GO, Sb S and Sb S /RGO with different concentrations of GO. Raman spectra of 2 3, 2 3 TEM images of the Sb2S3/RGO composites; it can be seen that the Sb2S3 monomer synthe- pure Sb S and Sb S /RGO showed that the diffraction peaks were about 300 cm . Both 2 3 2 3 1 1 sized by PVP was spherical and dispersed nanoparticles were attached to the corrugated GO and Sb S /RGO showed two main peaks at 1349 cm and 1599 cm , which were 2 3 layered structure of the graphene, while some of them were clustered on the edge of correlated with the disorder (D) band and graphite (G) band of carbon-based materials, re- graphene [28].The Sb2S3 NPs were deposited on the surface with oxygen-containing spectively, and the D band was related to the defect state of graphene. I /I represents the D G 2 3 groups in GO reactants as anchor sites, indicating that the Sb2S3 NPs were successfully carbon atom ratio of SP /SP , indicating that GO had a large number of oxygen-containing supported on the surface of RGO, which prevented aggregation of the Sb2S3 NPs [27]. functional groups [29]. Compared with GO (I /I = 0.95), Sb S /RGO (I /I = 1.01) D G 2 3 D G The sufficient connection between the Sb2S3 nanoparticles and graphene maximized the showed an increased D-G strength ratio, indicating there were more disordered carbons efficiency of electron transfer due to the sufficient density and good dispersion of the and plenty of surface defects [30]. These defects affected the charge distribution and proper- relatively large Sb2S3 NPs on the graphene sheet. ties of the composites. This was caused by the decrease in the average size of SP domains Photonics 2022, 9, x FOR PEER REVIEW 6 of 14 To investigate the characteristic vibration modes of the prepared samples, Raman and the increase in the number of domains. The change in I /I intensity usually indicates D G spectroscopy was carried out with the excitation of 532 nm. Figure 4 shows the Raman the reduction of graphene oxide to graphene [31] and can also be widely used to evaluate spectra of pure GO, Sb2S3, and Sb2S3/RGO with different concentrations of GO. Raman the structural defects of graphene materials [32]. spectra of pure Sb2S3 and Sb2S3/RGO showed that the diffraction peaks were about 300 −1 −1 −1 cm . Both GO and Sb2S3/RGO showed two main peaks at 1349 cm and 1599 cm , which were correlated with the disorder (D) band and graphite (G) band of carbon-based ma- terials, respectively, and the D band was related to the defect state of graphene. ID/IG 2 3 represents the carbon atom ratio of SP /SP , indicating that GO had a large number of oxygen-containing functional groups [29]. Compared with GO (ID/IG = 0.95), Sb2S3/RGO (ID/IG = 1.01) showed an increased D-G strength ratio, indicating there were more disor- dered carbons and plenty of surface defects [30]. These defects affected the charge dis- tribution and properties of the composites. This was caused by the decrease in the aver- age size of SP domains and the increase in the number of domains. The change in ID/IG intensity usually indicates the reduction of graphene oxide to graphene [31] and can also be widely used to evaluate the structural defects of graphene materials [32]. Figure 4. Raman patterns of GO, Sb S and G1 (a); Raman patterns of GO and composites G1–G4 (b). Figure 4. Raman patterns of GO, Sb2S3 and G1 (a); Raman patterns of GO and composites G1–G4 2 3 (b). Figure 5a shows the infrared spectra of GO, Sb2S3, and Sb2S3/RGO composites pre- −1 pared with different GO concentrations. For GO, the broad and strong peak at 3379 cm −1 was attributed to the stretching vibration of the -OH group, the peak at 1701 cm repre- sents the C=O vibration of -COOH located at the edge of the GO sheet, and the peak at −1 1638 cm was attributed to O-H bending vibration, epoxy group, and aromatic C=C −1 −1 skeleton tensile vibration [33]. The peaks at 1207 cm and 1068 cm were attributed to the C-O-C or C-O-H stretching and C-O stretching of the carboxyl group in GO. The FT-IR pattern of G4 (see Figure 5b) confirmed the formation of Sb-S bonds due to the −1 −1 −1 appearance of absorption peaks at 538 cm , 634 cm , and 721 cm . Additionally, a new −1 peak appeared at 994 cm , which may be attributed to C–S stretching vibrations and also demonstrated the strong bonding of GO and Sb2S3. Figure 5. FT−IR spectra of GO, Sb2S3, and the Sb2S3/RGO composites G2–G4 (a); FT-IR spectra of GO and G4 (b). The representative peaks of the carboxyl and hydroxyl groups of GO appeared in the composite materials, indicating that the Sb2S3/RGO composites were successfully synthesized by a solvothermal reaction. The strong covalent bonds and weak van der Waals forces between Sb and S atoms justified the formation of Sb2S3 nanoparticles, which, in turn, succeeded in firmly bonding to graphene due to the strong chemical in- teraction between carbon and sulfur. However, the peaks of oxygen-containing groups in the composites were significantly weakened, which indicated that the solvothermal reaction in EG removed most of the carboxyl and hydroxyl groups [34]. According to the Photonics 2022, 9, x FOR PEER REVIEW 6 of 14 Figure 4. Raman patterns of GO, Sb2S3 and G1 (a); Raman patterns of GO and composites G1–G4 (b). Photonics 2022, 9, 213 6 of 14 Figure 5a shows the infrared spectra of GO, Sb2S3, and Sb2S3/RGO composites pre- −1 pared with different GO concentrations. For GO, the broad and strong peak at 3379 cm −1 Figure 5a shows the infrared spectra of GO, Sb S and Sb S /RGO composites pre- was attributed to the stretching vibration of the -OH 2 3, group2 , the 3 peak at 1701 cm repre- pared with different GO concentrations. For GO, the broad and strong peak at 3379 cm sents the C=O vibration of -COOH located at the edge of the GO sheet, and the peak at was attributed to the stretching vibration of the -OH group, the peak at 1701 cm rep- −1 1638 cm was attributed to O-H bending vibration, epoxy group, and aromatic C=C resents the C=O vibration of -COOH located at the edge of the GO sheet, and the peak −1 −1 skeleton tensile vibration [33]. The peaks at 1207 cm and 1068 cm were attributed to at 1638 cm was attributed to O-H bending vibration, epoxy group, and aromatic C=C the C-O-C or C-O-H stretching and C-O stretching of the carboxyl group in GO. The 1 1 skeleton tensile vibration [33]. The peaks at 1207 cm and 1068 cm were attributed FT-IR pattern of G4 (see Figure 5b) confirmed the formation of Sb-S bonds due to the to the C-O-C or C-O-H stretching and C-O stretching of the carboxyl group in GO. The −1 −1 −1 appearance of absorption peaks at 538 cm , 634 cm , and 721 cm . Additionally, a new FT-IR pattern of G4 (see Figure 5b) confirmed the formation of Sb-S bonds due to the 1 1 1 −1 peak appeared at 994 cm , which may be attributed to C–S stretching vibrations and also appearance of absorption peaks at 538 cm , 634 cm , and 721 cm . Additionally, a new peak appeared at 994 cm , which may be attributed to C–S stretching vibrations and also demonstrated the strong bonding of GO and Sb2S3. demonstrated the strong bonding of GO and Sb S . 2 3 Figure 5. FTIR spectra of GO, Sb S , and the Sb S /RGO composites G2–G4 (a); FT-IR spectra of Figure 5. FT−IR spectra of GO, Sb22S33 , and the Sb2 2S3 3/RGO composites G2–G4 (a); FT-IR spectra of GO and G4 (b). GO and G4 (b). The representative peaks of the carboxyl and hydroxyl groups of GO appeared in The representative peaks of the carboxyl and hydroxyl groups of GO appeared in the composite materials, indicating that the Sb S /RGO composites were successfully 2 3 the composite materials, indicating that the Sb2S3/RGO composites were successfully synthesized by a solvothermal reaction. The strong covalent bonds and weak van der Waals synthesized by a solvothermal reaction. The strong covalent bonds and weak van der forces between Sb and S atoms justified the formation of Sb S nanoparticles, which, in turn, 2 3 Waals forces between Sb and S atoms justified the formation of Sb2S3 nanoparticles, succeeded in firmly bonding to graphene due to the strong chemical interaction between which, in tur carbon and sulfur n, succeede . However d in , the firmly peaksbon of oxygen-containing ding to graphene due groups in to the the strong composites che wer mical e in- significantly weakened, which indicated that the solvothermal reaction in EG removed teraction between carbon and sulfur. However, the peaks of oxygen-containing groups most of the carboxyl and hydroxyl groups [34]. According to the infrared spectrum of G2 in the composites were significantly weakened, which indicated that the solvothermal and G3 in the figure, it can be seen that the C=C framework vibration of the graphene sheet reaction in EG removed most of the carboxyl and hydroxyl groups [34]. According to the (1638 cm ) was prominent in the composite material, which also confirmed the recovery of the aromatic SP hybridized carbon skeleton of graphene [35]. These results indicated that the solvothermal reaction can effectively reduce graphene oxide to graphene. 3.2. Linear Optical Properties To preliminarily understand the nonlinear optical absorption (NOA) mechanism, the linear optical properties of the prepared samples Sb S and Sb S /RGO composites 2 3 2 3 were characterized by UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). As shown in Figure 6,the Sb S NPs had some absorption at the excitation wavelength of 610 nm. 2 3 However, they had more absorption at 240 nm, indicating the probability of excited states’ absorption in two steps or genuine two-photon absorption [36]. The Sb S /RGO had a 2 3 wide shoulder peak after 270 nm, and the absorption peak had a red shift. It was reported that Sb S is a direct band gap semiconductor material and has a high absorption rate [37]. 2 3 The relation between the light absorption coefficient and the energy of incident light 1/2 photon h near the absorption edge was as follows [38]: h = (h E ) (E is the g g band gap width). Therefore, the relation between ( h) and (h) near the absorption edge Photonics 2022, 9, 213 7 of 14 was linear and the band gap E could be calculated. According to Beer ’s law, absorbance A = abc, b is the sample thickness in cm, c is the sample concentration in g/L, and the proportional coefficient alpha is called the absorption coefficient. A can also be defined as A = lgT, where T is the transmission; so, the definition of alpha can also be expressed 1 1 as T = exp(alpha *c*b) in Lg cm . In addition, the value of linear absorption co- 1 1 1 1 efficient can also be obtained: (Sb S ) = 0.77 Lg cm , (G1) = 0.24 Lg cm , 2 3 1 1 1 1 1 1 (G2) = 0.21 Lg cm , (G3) = 0.70 Lg cm , and (G4) = 0.54 Lg cm . The absorption spectra of the samples indicated the presence of different local levels in the forbidden gap, which may have originated from amorphous defects in the antimony sulfide nanostructure, such as vacancies and surface defects. The presence of defects may some- what affect the band gap structure [39]. The Sb S /RGO composites had a reduced band 2 3 gap compared to Sb S , which provided the evidence of a connection between antimony 2 3 sulfide and graphene. The difference in the absorption curves of the composites and the Photonics 2022, 9, x FOR PEER REVIEW 8 of 14 Sb S NPs can be explained by the presence of RGO and the energy interaction between 2 3 the two materials [40]. Figure 6. Figure 6. UV– UV V –V is absorption is absorption spectra of spectra of Sb Sb 2S3S and the and the SbSb 2S3/R S G /RGO O comp composites. osites. 2 3 2 3 3.3. Nonlinear Optical Properties 3.3. Nonlinear Optical Properties The Z-scan curves in Figure 7 show the NLO characteristics of the sample. The NLO The Z-scan curves in Figure 7 show the NLO characteristics of the sample. The NLO properties of the Sb S and Sb S /RGO composites were studied by using Z-scan technique 2 3 2 3 properties of the Sb2S3 and Sb2S3/RGO composites were studied by using Z-scan tech- with a laser at wavelength of 532 nm, pulse width of 30 ps, and repetition rate of 10 Hz. nique with a laser at wavelength of 532 nm, pulse width of 30 ps, and repetition rate of 10 The interval between two pulses was about 0.1 s, which was much longer than the pulse Hz. The interval between two pulses was about 0.1 s, which was much longer than the width. Therefore, a thermal effect can be ignored. Actually, the laser used had the same pulse width. Therefore, a thermal effect can be ignored. Actually, the laser used had the effect as a single-shot mode laser. The samples were dissolved in ethanol solvent to prepare same effect as a single-shot mode laser. The samples were dissolved in ethanol solvent to solutions with a concentration of 0.1 mg/mL. The optical path length (cuvette) was 1 mm. prepare solutions with a concentration of 0.1 mg/mL. The optical path length (cuvette) The actual thickness of the sample was about 0.4 mm, including the total thickness of was 1 mm. The actual thickness of the sample was about 0.4 mm, including the total the cuvette minus its double walls. Therefore, the sample thickness was considered to be thickness of the cuvette minus its double walls. Therefore, the sample thickness was much less than the Rayleigh range; thus, the thin sample approximation was satisfied. It considered to be much less than the Rayleigh range; thus, the thin sample approximation was found that the NLO signal of ethanol solvent at 532 nm was very small and could was satisfied. It was found that the NLO signal of ethanol solvent at 532 nm was very be neglected [41]. Therefore, the nonlinearity of the solvent could be ruled out and the small and could be neglected [41]. Therefore, the nonlinearity of the solvent could be obtained Z-scan curves directly showed the nonlinear response of graphene [42], Sb S , 2 3 ruled out and the obtained Z-scan curves directly showed the nonlinear response of and Sb S /RGO. 2 3 graphene [42], Sb2S3, and Sb2S3/RGO. Photonics 2022, 9, x FOR PEER REVIEW 9 of 14 Photonics 2022, 9, 213 8 of 14 Figure 7. (a) Open-aperture Z-scan curves of Sb S , RGO, and the Sb S /RGO composite G3; Figure 7. (a) Open-aperture Z-scan curves of Sb2S3, RGO, and the Sb2S3/RGO composite G3; (b) 2 3 2 3 (b) open-aperture Z-scan curves of composites G1–G4. (c) Closed-aperture/open-aperture Z-scan open-aperture Z-scan curves of composites G1–G4. (c) Closed-aperture/open-aperture Z-scan curves of Sb S , RGO, and the Sb S /RGO composite G3; (d) closed-aperture/open-aperture Z-scan curves of Sb2S2 3, RGO 3 , and the Sb2 2S3 3/RGO composite G3; (d) closed-aperture/open-aperture Z-scan curves of composites G1–G4. (e,f) Normalized transmission versus input intensity of RGO and G3. curves of composites G1–G4. (e,f) Normalized transmission versus input intensity of RGO and G3. The curves in Figure 7a are the Z-scan curves of Sb S , G3 (Sb S /RGO-0.5), and RGO, 2 3 2 3 The curves in Figure 7a are the Z-scan curves of Sb2S3, G3 (Sb2S3/RGO-0.5), and reflecting the variation of nonlinear absorption characteristics. The curves of RGO and RGO, reflecting the variation of nonlinear absorption characteristics. The curves of RGO Sb S /RGO appear as symmetrical peaks at the focus, indicating the dominant position of 2 3 and Sb2S3/RGO appear as symmetrical peaks at the focus, indicating the dominant posi- saturation absorption (SA) in the NLO absorption mechanism; the saturated absorption tion of saturation absorption (SA) in the NLO absorption mechanism; the saturated ab- sorption of G3 shows an increasing trend compared with that of G1–G2. Generally, SA happens in the resonant region where the photon energy is larger than or equal to the band gap of the materials due to the electrons’ excitation from valence band to conduc- tion band and then the Pauli-blocking induced bleaching effect [43]. In the state of suffi- Photonics 2022, 9, 213 9 of 14 of G3 shows an increasing trend compared with that of G1–G2. Generally, SA happens in the resonant region where the photon energy is larger than or equal to the band gap of the materials due to the electrons’ excitation from valence band to conduction band and then the Pauli-blocking induced bleaching effect [43]. In the state of sufficiently large light intensity, electrons in the valence band can jump to the conduction band through single-photon absorption. The photon bands of the valence band and conduction band were completely occupied by electrons and holes; therefore, further light absorption was blocked and SA occurred [44]. The Sb S NPs exhibited two-photon absorption (TPA), 2 3 which can be seen in Figure 7a, due to the fact that the band gap of Sb S NPs was greater 2 3 than the excitation photon energy. Figure 7b shows the OA Z-scan curves (G1–G4) of Sb S /RGO corresponding to different GO concentrations. From G1–G4, as the graphene 2 3 increased, the peaks first increased and then decreased. This was due to the fact that when the graphene content was increased to a certain amount (G4), the lower Sb S content in 2 3 these composite materials could only generate a low level of photo-excited electron-hole pairs under irradiation. It can be seen from Figure 7c,d that all CA/OA Z-scan curves produced typical valley–peak trajectories, which indicated that all the samples had positive nonlinear refraction and self-focusing characteristics. The theoretical fitting curves, shown as a solid line in Figure 7, and the third-order nonlinear parameters were obtained using the Z-scan theory. The normalized Z-scan curves of OA were well fitted by the following equation [45], which is used to describe a third-order NLO absorptive process: q (z) T(z) = (1) 3/2 (1 + m) m=0 2 2 where q z = I L / 1 + z /z and is the nonlinear absorption coefficient. The ( ) ( ) 0 eff 0 0 z = ! / is the length of Rayleigh diffraction, which can be calculated to be 0.66 mm. I 0 0 is the laser intensity at the focus (1.69 GW/cm ), L = [1 exp( L)]/ is the effective eff length of the sample, L is the actual thickness of the sample, and is the linear absorption coefficient of the sample, which can be obtained from the UV-Vis absorption spectra. When q < 1, the normalized transmittance formula is approximated to the following formula: I L 0 eff T(z)  1 (2) 2 2 1 + z /z 3/2 2 2 According to the above equation, = 2 1 T 1 + z /z /I L can be ob- ( ) z=0 0 eff tained, and the nonlinear absorption coefficient of the sample can be calculated by this formula. The imaginary part of the third-order nonlinear susceptibility is gained by (3) 2 3 Im = cn  /480 and the normalized CA/OA Z-scan transmittance is calculated as [46]: 4xF T(z) = 1 (3) 2 2 (x + 9)(x + 1) where x = z/z ; so, the third-order nonlinear refraction index is n (esu) = (cn /40) m /W . 0 2 0 The is the nonlinear refractive index coefficient in m /W and DT is the peak-to-valley pv difference. The relation between and DT can be expressed by =  DT /[0.812I pv pv 0 0.25 L 1 S) 1 e . S is the linear transmittance of the aperture. The DF is the onaxis phase shift at the center focus, which can be expressed by the formula DF = kDn L = k I L containing . The real part of the third-order nonlinear 0 0 eff 0 eff (3) susceptibility is Re = n n /3 and the third-order nonlinear susceptibility is given by 0 2 1/2 2 2 (3) (3) (3) = Re + Im . The data listed in Table 1 are the third-order nonlinear parameters of the samples (RGO, Sb S , Sb S /RGO) calculated according to the above formula. From Sb S to Sb S /RGO, 2 3 2 3 2 3 2 3 Photonics 2022, 9, 213 10 of 14 (3) the imaginary part of third-order nonlinear optical susceptibility Im changes from (3) negative to positive and the Im values of G1–G3 increase, indicating that the nonlinear absorption can be regulated. It also shows the dominant role of saturated absorption in (3) the recombination process [47]. In addition, the real Re associated with the Kerr effect 1 2 was also regulated and enhanced, and its maximum value was 8.03  10 esu, which (3) (3) appeared in G3. The third-order nonlinear susceptibility  , determined by Im and (3) Re , is thus adjusted and enhanced to a maximum of 8.63  10 esu. The Sb S NPs 2 3 were evenly distributed on the graphene as the graphene increased. Even though the distribution of NPs was random, there existed a Fabry–Perot-like localized resonance due to small-scale ordering within the architecture, which resulted in the local enhancement of the field [48]. This local field enhancement led to an increase in NLO interactions and, hence, higher values for n and . Table 1. The NLO parameters of the samples. (3) (3) (3) Im Re Sample 11 18 12 12 12 (10 m/w) (10 m /w) (10 esu) (10 esu) (10 esu) RGO 2.17 0.03 1.36 0.04 1.36 Sb S 2.60 2.42 1.96 4.31 4.73 2 3 G1 1.32 0.39 0.99 0.69 1.21 G2 2.11 0.68 1.59 1.21 2.00 G3 4.22 4.51 3.18 8.03 8.63 G4 2.42 2.07 1.82 3.69 4.11 Figure 7e,f shows the relationship between normalized transmittance and saturation intensity of samples, which further shows that the composite and RGO exhibited the same absorption characteristics, corresponding to Figure 7a. We can fit the relevant data by the following formula: T = 1 (4) ns 1 + sat where is the non-saturated component, is the modulation depth, and I is the ns s sat saturated intensity. We can obtain the of 6.2%, 11.2%, 21.0%, and 10.7% for the G1, G2, 2 2 2, G3, and G4 corresponding to the I of 0.56 GW/cm , 0.47 GW/cm , 0.36 GW/cm and sat 0.41 GW/cm . According to the relevant nonlinear parameters in Tables 1 and 2, it can be concluded that G3 had the highest nonlinear absorption coefficient = 4.22  10 m/w and the lowest saturation strength I = 0.36 GW/cm . The saturated intensity of the sat sample (G1–G3) decreased, which corresponded to the change in the nonlinear absorption characteristics shown in Figure 7b. Comparing the nonlinear parameters of different materials, we found Sb S /RGO had strong third-order nonlinear optical properties. This 2 3 shows that Sb S /RGO composites have potential applications in mode locking and pulse 2 3 compression. Table 2. The relevant nonlinear parameters of different materials. Sample Reference 11 18 2 2 (nm) (10 m/w) (10 m /w) (GW/cm ) Sb S 532 2.60 2.42 This work 2 3 Sb S /RGO 2 3 532 4.22 4.51 0.36 This work (G3) MoSe 1064 2.05 0.71 [49] G-CuO 1030 1.37 0.48 [50] Photonics 2022, 9, 213 11 of 14 It can be seen from the data that the third-order nonlinearity of Sb S /RGO was 2 3 regulated. It is well known that the NLO process is controlled by the nonlinear susceptibility (3) (3) of NLO materials. The higher the  value is, the better the NLO performance. To Photonics 2022, 9, x FOR PEER REVIEW 12 of 14 further understand the variation of nonlinear optical absorption in the composite structure, we analyzed its possible photo-induced charge carrier transfer behavior [51], as shown in Figure 8. Due to the existence of energy band differences between Sb S and RGO, a 2 3 donor–acceptor electronic structure was formed in the complex. Sb S can be regarded as an garded as an electron donor, and RGO as an electron acc2ep 3tor, so that electrons cannot electron donor, and RGO as an electron acceptor, so that electrons cannot only transit within only transit within each of them, but also transfer between them. Additionally, the pho- each of them, but also transfer between them. Additionally, the photo-generated electrons to-generated electrons in Sb2S3 NPs would immigrate to RGO and further be trapped by in Sb S NPs would immigrate to RGO and further be trapped by the surface defects 2 3 the surface defects in RGO [52], which may minimize the possibility of recombination of in RGO [52], which may minimize the possibility of recombination of photo-generated photo-generated carriers. The excited electrons are transferred from the conduction band carriers. The excited electrons are transferred from the conduction band of Sb S to the 2 3 of Sb2S3 to the graphene and then to the valence band of Sb2S3. The progress may inter- graphene and then to the valence band of Sb S . The progress may interrupt the carrier 2 3 rupt the carrier relaxation in RGO and favor SA. relaxation in RGO and favor SA. Figure 8. Charge-transfer mechanism of the Sb S /RGO composites. Figure 8. Charge-transfer mechanism of the Sb2S3/RGO composites. 2 3 4. Conclusions 4. Conclusions In conclusion, the Sb S /RGO composites were successfully synthesized by the facile, 2 3 In conclusion, the Sb2S3/RGO composites were successfully synthesized by the facile, one-step solvothermal method and the NLO properties of all samples were studied at one-step solvothermal method and the NLO properties of all samples were studied at 532 532 nm by the Z-scan technique. It was found that the addition of GO transformed the two-photon nm by theabsorption Z-scan te of chni Sb q Sue. into It w adjustable as found th saturation at the absorption addition o of Sbf G S O /RGO trancom- sformed the 2 3 2 3 posites, which was attributed to the change of band gap. The tunable positive nonlinear two-photon absorption of Sb2S3 into adjustable saturation absorption of Sb2S3/RGO (3) refraction properties and the enhanced nonlinear susceptibility  of Sb S /RGO compos- 2 3 composites, which was attributed to the change of band gap. The tunable positive non- ites were obtained, which were larger than Sb S and more than six times that of RGO. The () 2 3 linear refraction properties and the enhanced nonlinear susceptibility χ of Sb2S3/RGO mechanism of its nonlinear optical properties was believed to be that the effective charge composites were obtained, which were larger than Sb2S3 and more than six times that of and energy transfer between Sb S NPs and RGO enhance the free carrier absorption and 2 3 RGO. The mechanism of its nonlinear optical properties was believed to be that the nonlinear refraction process. It was directly revealed that the Sb S /RGO composites have 2 3 effective charge and energy (3tr ) ansfer between Sb2S3 NPs and RGO enhance the free tunable nonlinear susceptibility  with different GO concentrations. The results of these carrier absorption and nonlinear refraction process. It was directly revealed that the tunable third-order NLO properties of the Sb S /RGO composites would provide the basis 2 3 () Sb2S3/RGO composites have tunable nonlinear susceptibility χ with different GO for the application in photonic devices. concentrations. The results of these tunable third-order NLO properties of the Sb2S3/RGO composites would provide the basis for the application in photonic devices. Author Contributions: Conceptualization, L.L. and Y.Y.; methodology, L.L.; validation, J.W. and Y.Y.; formal analysis, L.L.; investigation, L.L.; resources, Y.G.; writing—original draft preparation, L.L.; writing—review and editing, Y.G.; supervision, B.Z.; project administration, Y.G.; funding acquisition, Y.G. and B.Z. All authors have read and agreed to the published version of the manu- script. Funding: This research was funded by the National Natural Science Foundation of China (61875053, 61404045, U1404624) and Excellent Youth Project of Henan Province of China (202300410047). Institutional Review Board Statement: Not applicable. Photonics 2022, 9, 213 12 of 14 Author Contributions: Conceptualization, L.L. and Y.Y.; methodology, L.L.; validation, J.W. and Y.Y.; formal analysis, L.L.; investigation, L.L.; resources, Y.G.; writing—original draft preparation, L.L.; writing—review and editing, Y.G.; supervision, B.Z.; project administration, Y.G.; funding acquisition, Y.G. and B.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (61875053, 61404045, U1404624) and Excellent Youth Project of Henan Province of China (202300410047). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. 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Journal

PhotonicsMultidisciplinary Digital Publishing Institute

Published: Mar 23, 2022

Keywords: Sb2S3/RGO composite; graphene; third-order nonlinear optical property; saturable absorption; susceptibility

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