Mixed Salt-Assisted Growth of Large-Size Ultrathin SnS₂ Nanosheets and Their Anisotropy Study

1 Introduction

In recent years, two-dimensional (2D) materials with layered structures have attracted widespread attention in flexible devices, valleytronics, and optoelectronics due to their unique physicochemical properties [1,2]. The family of 2D materials is exceptionally diverse, spanning a continuous spectrum of electronic band structures. This includes zero-bandgap semimetals (e.g., graphene), narrow-bandgap semiconductors (e.g., transition metal dichalcogenides such as MoS2 and WS2), moderate-bandgap semiconductors (e.g., black phosphorus), and wide-bandgap insulators (e.g., hexagonal boron nitride, h-BN) [3,4]. Tin disulfide (SnS2), an important member of the IV-VIA group, features stacked planar triple layers with strong in-plane covalent bonding and weak out-of-plane van der Waals (vdW) interactions, making it a research hotspot [5,6]. SnS2 is an indirect bandgap semiconductor (2.08–2.44 eV) with a light absorption efficiency exceeding 104 cm−1, offering broad prospects in optoelectronics, gas sensing, and photocatalysis [7,8]. Its low cost, earth abundance, non-toxicity, and environmental friendliness also align well with industrial demands for next-generation electronics and optoelectronics [9]. Thus, achieving controllable growth of 2D SnS2 nanosheets is a primary task for advancing their applications. Currently, SnS2 nanosheets are mainly obtained via solution-phase synthesis or mechanical exfoliation [10,11,12]. However, these methods typically yield small lateral sizes (several micrometers) and suffer from poor morphological uniformity, severe agglomeration, low crystallinity, or low yield, severely restricting further applications [13,14].

Chemical vapor deposition (CVD) offers precise control over morphology, defects, and structure, making it attractive for growing large-size 2D materials [15,16,17]. For instance, Zhou et al. [18] provided an improved CVD route to synthesize SnS2 nanosheets with edge lengths up to 150 μm. Phototransistors based on these nanosheets showed high sensitivity (261 A/W) and fast response (20 ms rise time), but the thickness remained above 10 nm, limiting device performance. Ye et al. [19] reported the CVD growth of atomic-layer SnS2 with a large crystal size and uniformity, and external quantum efficiency of the resultant SnS2 crystals is as high as 150%. In addition, assisted CVD for the growth of 2D SnS2 nanomaterials has received extensive attention recently. Wang et al. [20] synthesized in-plane SnS2 nanosheets with sizes up to 280 μm on SiO2/Si substrates via Te-assisted CVD, however, the shape is not regular enough, and the thickness is relatively large. Liu et al. [21] synthesized monolayer SnS2 crystal on SiO2/Si substrates via NaCl-assisted CVD and the edge can be as long as 80 μm, however, the shape is irregular. Shao et al. [22] introduced potassium halide into CVD and grew large-size SnS2 nanosheets with diverse morphologies, finding that more regular morphologies tend to have greater thickness. Fu et al. [23] reported a KI-assisted confined-space CVD method to synthesize multilayer MoS2/SnS2 vertical heterostructure nanosheets composed of monolayer MoS2 and multilayer SnS2, but the thickness of SnS2 is relatively large. During CVD growth, large-size monolayer SnS2 growth is constrained by the high melting point of precursors and low SnS2 adhesion on substrates [24,25]. Consequently, achieving a synergistic balance among morphological regularity, thickness uniformity, and size controllability for large-size SnS2 growth remains a significant challenge.

In this study, a mixed salt-assisted growth strategy was employed to successfully synthesize ultrathin, large-size SnS2 nanosheets. ARPRS and polarized optical microscopy revealed that lattice mismatch between SnS2 and the mica substrate generates interfacial stress, breaking the intrinsic symmetry and inducing significant anisotropy. These results demonstrate that the mixed salt-assisted strategy provides a new approach for the controllable growth of 2D materials and offers a novel method for inducing anisotropy in intrinsically isotropic 2D materials via lattice mismatch.

2 Materials and Methods

2.1 Synthesis Process

A mixed salt-assisted ambient-pressure rapid chemical vapor deposition (CVD) approach was developed for synthesis of large-size, ultrathin SnS2 nanosheets based on rapid CVD methods developed in our reports [26,27]. The typical experimental procedure to grow SnS2 nanostructures with single salt is as follows: 0.020 g SnO2 (Shanghai Aladdin Biochemical Technology Co., Ltd.) powder was uniformly mixed with 0.002 g KI (Shanghai Macklin Biochemical Technology Co., Ltd., 99.9% purity) to serve as the tin source. A freshly cleaved mica sheet (approximately 1 × 2 cm2) was placed above the tin source. Sulfur powder (Shanghai Aladdin Biochemical Technology Co., Ltd.) was placed in a separate quartz boat upstream of the tin source at a certain distance. The quartz tube was evacuated and purged with argon to remove air, then backfilled to atmospheric pressure. A constant argon flow of 50 sccm was maintained, and the outlet was sealed with a water seal. When the furnace reached 540°C, the quartz tube was rapidly moved to position the tin source at the heating center and bring the sulfur powder near the heating zone. After the reaction for 10 min, the furnace was turned off and allowed to cool to room temperature before the sample was removed. The growth process of SnS2 nanostructures using different salts remains the same, with a constant mass ratio of SnO2 to salt fixed at 10:1. All salts are of analytical grade and used as received without further purification (purchased from Shanghai Macklin Biochemical Technology Co., Ltd. or Shanghai Aladdin Biochemical Technology Co., Ltd.). The procedure for growing SnS2 nanosheets with mixed salts follows the same protocol, except that a mixed salt (KI and CsCl) is used instead of a single salt. The mass ratio of SnO2 to the mixed salt is kept constant at 10:1, while the ratio of KI to CsCl can be adjusted according to the experimental design.

2.2 Transfer Method

SnS2 nanosheets were transferred from mica to 300 nm SiO2/Si substrates or Cu grids by a water-assisted ultrasonic transfer as described in our previous reported [27]. The as-grown SnS2 nanosheets were transferred onto mica to characterize polarized optical image using a mechanical transfer method. Briefly, the mica sheet carrying the as-grown SnS2 nanosheets was brought into face-to-face contact with a freshly cleaved mica. The two substrates were gently pressed together and then slowly separated. Consequently, some SnS2 nanosheets were transferred onto the target substrate.

2.3 Characterization

The morphology and spatial distribution of SnS2 nanosheets were examined by reflection-mode optical microscopy (Olympus BX51M) and atomic force microscopy (AFM, Bruker Dimension Icon). Crystal structure, lattice resolution, and compositional uniformity were assessed by X-ray diffraction (XRD, Rigaku Ultima IV, Cu Kα, λ = 0.15418 nm), transmission electron microscopy (TEM, JEOL JEM-F200, 200 kV) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB Xi, Al Kα, hν = 1486.6 eV). The polarized optical imaging was characterized using an OM equipmented with crossed polarizer and analyzer plates. Raman spectra were acquired at room temperature with a HORIBA Jobin-Yvon Evolution spectrometer using a 532 nm excitation laser kept below 5 mW to avoid sample heating.

3 Results and Discussion

3.1 Synthesis Strategy

An ambient-pressure rapid CVD method combined with a salt-assisted growth strategy was used to investigate the effects of various salt additives on the size, morphology, and thickness of SnS2 nanosheets. Fig. 1a shows the schematic of the mixed salt-assisted rapid CVD process [26,27]. It offers several advantages, including rapid heating-up, negligible precursor loss during the heating process, and precise control over growth temperature and duration. In the absence of salt additives, only a few small and thick SnS2 crystal are observed by optical microscopy (OM) as shown in Fig. 1b. In recent years, salts, particularly alkali metal halide salts, have been widely employed as promoters in the CVD growth of 2D materials [28,29]. While certain salts, such as NaCl and KI, have also been utilized in the CVD synthesis of SnS2, however, the existing studies on salt-assisted SnS2 growth remain sporadic and lack a systematic investigation. In this work, we comprehensively investigate the influence of 36 distinct salts, including alkali metal halide salts and other common salt types, on the growth of SnS2 nanosheets (Fig. 1c). Based on careful screening and comparative analysis of these salts, an efficient mixed salt system was successfully identified and established. This optimized system enables the controllable growth of ultrathin, large-area SnS2 nanosheets with excellent crystalline quality, providing a reliable approach for the synthesis of high-quality 2D materials.

Figure 1: Schematic diagram of experiment setup and the salts used. (a) Schematic illustration of the mixed salt-assisted chemical vapor deposition (CVD) process, (b) OM images of SnS₂ nanosheets grown without salt, (c) Periodic table highlighting the metal salts employed in this work. Color code: light blue indicates suppressed growth; blue indicates weak influence; dark blue indicates enhanced growth.

When 12 salts such as AlCl3, Cr(NO3)3·H2O, Mn(CH3COO)2·4H2O, Fe(NO3)3, Co(NO3)2·6H2O, Ni(NO3)2, CuSO4, Zn(CH3COO)2·2H2O, Rb2CO3, SrCl2·4H2O, WCl6 and AgNO3 were introduced into the growth system, the nucleation and subsequent growth of SnS2 were significantly suppressed. As shown in Fig. 2, no observable SnS2 crystal were detected on the sample surface under these conditions, indicating a complete inhibition of crystal formation. This inhibitory effect may be attributed to the strong interaction between the metal cations (such as Al3+, Fe3+, Rb+ and Ag+) or their corresponding anions with the precursors or the growth substrate, which likely hinders the initial nucleation process. These results demonstrate that certain salt additives can act as growth inhibitors rather than promoters, highlighting the dual role of salts in the CVD growth of 2D materials.

Figure 2: OM images of SnS₂ growth by salt-assisted CVD. Salts with suppression: (a) AlCl₃, (b) Cr(NO₃)₃·H₂O, (c) Mn(CH₃COO)₂·4H₂O, (d) Fe(NO₃)₃, (e) Co(NO₃)₂·6H₂O, (f) Ni(NO₃)₂, (g) CuSO₄, (h) Zn(CH₃COO)₂·2H₂O, (i) Rb₂CO₃, (j) SrCl₂·4H₂O, (k) WCl₆ and (l) AgNO₃.

The addition of 12 different salts, including LiCl, MgSO4, CaSO4, MoCl5, Ba(CH3COO)2, SbCl3, BiCl3, InCl3·4H2O, SnCl2·2H2O, HfCl4, CdCl2·2.5H2O, and PbI2, led to distinct morphological outcomes, as shown in Fig. 3. When salts such as LiCl, MgSO4, CaSO4, MoCl5, Ba(CH3COO)2, SbCl3, BiCl3 and InCl3·4H2O were introduced into the precursor, only a few small SnS2 crystals with varying brightness were observed in the OM images (Fig. 3a–h). In contrast, the addition of SnCl2·2H2O and HfCl4 produced significantly larger SnS2 crystals (Fig. 3i,j), suggesting that these salts may promote precursor conversion or facilitate crystal growth under identical conditions. Furthermore, the addition of CdCl2·2.5H2O led to the formation of multilayer SnS2 nanosheets (Fig. 3k), indicating that CdCl2·2.5H2O plays a regulatory role in modulating interlayer van der Waals interactions. These weak interactions govern the vertical assembly of 2D layers, and CdCl2·2.5H2O appears to enable controlled layer-by-layer stacking, offering a promising route for multilayer SnS2 synthesis. Notably, the introduction of PbI2 induced the formation of one-dimensional (1D) nanowires (Fig. 3l). Further analysis overturns the conventional view of PbI2 as merely an auxiliary agent. As demonstrated by the structural and compositional analysis in Fig. S1, PbI2 instead acts as a critical reactant, deeply participating in the lattice formation process. Consequently, the synthesized product is identified as the ternary lead-tin-sulfur compound PbSnS3, not the binary tin disulfide SnS2. These contrasting observations highlight the specificity of salt-assisted growth and underscore the importance of carefully selecting salt additives to achieve desired structural outcomes.

Figure 3: OM images of SnS₂ growth by salt-assisted CVD. Salts with influence: (a) LiCl, (b) MgSO₄, (c) CaSO₄, (d) MoCl₅, (e) Ba(CH₃COO)₂, (f) SbCl₃, (g) BiCl₃, (h) InCl₃·4H₂O, (i) SnCl₂·2H₂O, (j) HfCl₄, (k) CdCl₂·2.5H₂O, and (l) PbI₂, respectively.

The alkali metal halide salts constitute a distinct category of additives. Within this group (NaF, NaCl, NaBr, NaI, KF, KCl, KBr, KI, CsF, CsCl, CsBr, and CsI), only CsCl, CsBr, and KI demonstrated significant regulatory capabilities in the SnS2 growth process (Fig. 4). Specifically, both cesium salts effectively promoted lateral expansion while simultaneously suppressing vertical growth, leading to larger but thinner crystals. CsCl was more potent in this regard than CsBr; however, both yielded products with irregular morphologies. By contrast, KI primarily enhanced the crystalline quality and shape regularity of the nanosheets, although the products remained relatively thick with notable thickness variation. The remaining alkali halides showed little to no effect, with NaF, KF, CsF acting as a strong inhibitor of normal crystal development.

Figure 4: OM images of SnS₂ growth by salt-assisted CVD. alkali metal halide salts: (a) NaF, (b) NaCl, (c) NaBr, (d) NaI, (e) KF, (f) KCl, (g) KBr, (h) KI, (i) CsF, (j) CsCl, (k) CsBr and (l) CsI.

To better understand the effects of salts on the growth of SnS2 nanostructures, a periodic table of the metal salts with different color scheme were draw as shown in Fig. 1c. These salts can be divided into three categories: 15 salts marked in light blue (such as AlCl3, AgNO3, NaF, et al.) represent suppressed growth, 16 salts marked in blue (such as LiCl, CaSO4, BiCl3, NaCl, et al.) represent no significant effect on growth, and 5 salts marked in dark blue (CdCl2·2.5H2O, PbI2, KI, CsCl and CsBr) represent enhanced growth. Most of salts had a weak influence or even suppression on growth, and only a few salts enhanced growth. Among them, CsCl significantly enhance lateral size and reduce thickness of SnS2, while KI optimizes its crystallinity and morphology. This classification strategy enables intuitive screening of auxiliary growth agents with potential application value, providing new insights for the controllable preparation of 2D materials.

To elucidate the synergistic mechanism of CsCl and KI and the effects of their mixing ratio on the growth morphology, size, thickness, and crystalline quality of SnS2, a series of comparative experiments was designed based on single salt-assisted growth shown above. The CsCl/KI ratio was systematically varied while keeping the total amount of SnO2 and mixed salts constant. Five experimental groups were set up, namely CsCl:KI = 0:2, 1:2, 2:2, 2:1, and 2:0, while the total amount of SnO2 and the mixed salts was strictly maintained at a constant. The growth morphology of each group was characterized by OM firstly, and the results are shown in Fig. 5. The results show that the growth characteristics of SnS2, such a morphology, lateral size, and thickness, are strongly depend on the CsCl/KI ratio. Increasing the CsCl proportion promotes lateral growth, leading to larger lateral sizes and reduced thickness, consistent with the effect of CsCl alone. In contrast, a higher KI content (CsCl:KI = 0:2 or 1:2) results in smaller lateral sizes but smoother edges, clearer contours, and a pronounced triangular morphology, aligning with the high crystallinity and regularity achieved with KI alone. To address this, the atomic force microscopy (AFM) of SnS2 growth with assistant of the KI, CsCl and KI/CsCl (KI:CsCl = 1:1) were performed and the results are shown in Fig. S2. For SnS2 nanosheets grown with KI assistance, the average edge length is 34.4 μm, the average thickness is 26.0 nm, and the edges are smooth. In contrast, SnS2 nanosheets grown with CsCl assistance exhibit a significantly larger average edge length of 155.1 μm, a considerably smaller average thickness of 3.7 nm, but with rough edges. When a mixed salt (KI/CsCl) is used, the resulting SnS2 nanosheets show an average edge length of 66.3 μm, an average thickness of 10.6 nm, and smooth edges. Thus, CsCl promotes lateral expansion and thinning, while KI enhances morphological regularity and crystalline quality. In summary, the CsCl/KI ratio synergistically regulates the lateral and vertical growth rates of SnS2, jointly determining its overall morphology and crystalline properties.

To gain mechanistic insight into how CsCl promotes lateral growth and KI improves morphology, theoretical calculations were performed as shown in Fig. S3. First-principles calculations reveal that the formation energies of SnS2 on the (100), (010), and (001) planes are 0.0339 eV, 0.0339 eV and 0.0457 eV, respectively. The equal formation energies of the (100) and (010) planes are consistent with the sixfold symmetric structure of SnS2. Notably, the formation energies of the in-plane (100) and (010) planes are lower than that of the out-of-plane (001) plane, indicating that 2D growth is energetically favorable. When Cs and K ions are introduced onto the (100), (010), and (001) surfaces of SnS2, the binding energies on the (001) surface are the lowest for both ions, suggesting a preferential adsorption on this basal plane. This promotes lateral growth, which is in good agreement with experimental observations. Furthermore, the binding energy of Cs+ on the (001) surface is lower than that of K+, indicating that Cs+ is more effective than K+ in promoting lateral growth and thus facilitating the formation of thinner SnS2 nanosheets. Additionally, the binding energies of Cs+ on the (100) and (010) surfaces are lower than those of K+, implying that K+ exhibits weaker binding on these edge planes. This weaker interaction favors the formation of well-defined, sharp edges, consistent with the experimental observation that KI-assisted growth yields more regular morphologies. These theoretical results are in strong agreement with the experimental findings shown above. These findings provide important guidance for optimizing SnS2 growth processes and preparing high quality samples.

Figure 5: OM images of SnS₂ growth by mixed salt-assisted CVD. OM images of SnS₂ nanosheets grown with CsCl:KI ratios of 0:2 (a), 1:2 (b), 2:2 (c), 2:1 (d), and 2:0 (e), respectively.

3.2 Morphological and Chemical Composition Characterization

To systematically characterize the morphology, structure, and chemical composition of the ultrathin large-size SnS2 nanosheets, atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed. As shown in Fig. 6a, the lateral size of SnS2 nanosheets can reach 248 μm when the salt mixing ratio (CsCl:KI = 2:2) is fixed and growth conditions are further optimized. AFM imaging (Fig. 6b) and cross-sectional analysis reveal a thickness of approximately 1.87 nm, confirming a bilayer structure [30]. XRD (Fig. 6c) exhibits distinct peaks at 14.9°, 30.2°, 46.0°, and 62.9°, corresponding to the (001), (002), (003), and (004) planes of SnS2 (PDF#23-0677), respectively. No secondary phases are detected, confirming the single-phase nature of the as-prepared samples. The survey spectrum on as-grown mica substrate (Fig. 6d) reveals the expected Sn and S as, in addition to the weak C 1s peak always present at 284.8 eV for samples handled in air. Notably, Cs, Cl, K and I are clearly detected, and no other impurities are observed. While only Cs remains detectable, while K, Cl, and I are no longer observed after transfer to SiO2/Si (Fig. S4). High-resolution XPS analysis (Fig. 6e–j) provides detailed chemical state information. The Sn 3d core level splits into two peaks at 495.1 eV and 486.6 eV due to spin-orbit coupling, corresponding to Sn 3d3/2 and Sn 3d5/2, respectively, indicating Sn in the Sn4+ state. The S 2p spectrum shows two peaks at 161.6 eV and 162.8 eV, assigned to S 2p3/2 and S 2p1/2, respectively, confirming sulfur in the S2− state, these values are consistent with the previous reports [31]. The Cs 3d core level splits into two peaks at 738.6 eV and 724.6 eV, corresponding to Cs 3d3/2 and Cs 3d5/2, respectively, indicating Cs in the Cs+ state; the Cl 2p spectrum shows two peaks at 199.0 eV and 197.6 eV, assigned to Cl 2p1/2 and Cl 2p3/2, confirming Cl in the Cl− state [32]. The K 2p spectrum exhibits two peaks at 296.0 eV and 293.2 eV, corresponding to K 2p1/2 and K 2p3/2, confirming K in the K+ state; the I 3d spectrum shows two peaks at 630.5 eV and 618.9 eV, assigned to I 3d3/2 and I 3d5/2, confirming I in the I− state [33]. The clearly detection of Cs, Cl, K, and I further demonstrates that the mixed salt (CsCl/KI) plays an important role in regulating both the lateral and vertical growth rates of SnS2. Collectively, these results confirm that the as-grown SnS2 nanosheets are of high quality.

Figure 6: Morphological and compositional characterization of SnS₂ nanosheets. (a–c) OM image, AFM image, and XRD pattern of SnS₂ nanosheets, respectively; (d) XPS survey spectrum on as-grown mica substrate, confirming all expected elements; (e) Sn 3d region, fitted with a single spin-split doublet; (f) S 2p region, deconvoluted into two peaks; (g–j) Cs 3d, Cl 2p, K 2p, and I 3d regions, fitted with a single spin-split doublet.

3.3 Microstructure Characterization

To investigate the atomic-scale microstructure, growth orientation, and chemical composition distribution of SnS2 nanosheets, transmission electron microscopy (TEM) was employed. Fig. 7a shows a TEM image of a transferred SnS2 nanosheet, revealing its overall morphology. The high-resolution TEM (HR-TEM) image (Fig. 7b) reveals a hexagonal lattice structure free of atomic vacancies or lattice distortions. The measured lattice spacing is approximately 0.32 nm, matching the (100) interplanar spacing of standard SnS2 (0.317 nm) and confirming the high crystal orientation. The selected area electron diffraction (SAED) pattern (Fig. 7c) exhibits regular hexagonal spots, confirming the hexagonal phase and excellent single-crystal quality of the as-prepared SnS2 nanosheets. Moreover, energy-dispersive X-ray spectroscopy (EDS) elemental maps (Fig. 7d–f) show uniform distribution of Sn and S across the nanosheet, with no evidence of elemental segregation or enrichment.

Figure 7: Atomic structure and chemical composition characterization of SnS₂ nanosheets. (a–c) TEM image, HR-TEM image, and SAED pattern of SnS₂ nanosheets, respectively; EDS elemental mapping of Sn (d), S (e), and SnS₂ (f).

3.4 Anisotropic Propertity

3.4.1 Angle-Resolved Polarized Raman Spectroscopy (ARPRS)

Raman spectroscopy utilizes scattered light to gain knowledge about molecular vibrations, which can provide information regarding the structure, symmetry, electronic environment, and chemical bonding of a material. This technique is highly sensitive to symmetry, crystallinity, and stress, making it a powerful tool for characterizing 2D materials. To further confirm that the interfacial stress originates from lattice mismatch between SnS2 and the mica substrate, a series of control experiments were conducted. First, SnS2 nanosheets were transferred onto different substrates, including Si, SiO2/Si (300 nm SiO2) and quartz glass. Raman spectra of the SnS2 nanosheets on both the as-grown (mica) and transferred substrates were then acquired, and the results are shown in Fig. S5. Notably, the characteristic Raman A1g peak of SnS2 on quartz glass, SiO2/Si, and Si appears at 314.6 ± 0.4 cm−1, 314.2 ± 0.2 cm−1, and 314.1 ± 0.1 cm−1, respectively, while on the as-grown mica substrate, it is located at 313.8 ± 0.03 cm−1. Comparing the transferred substrates with the original growth substrate, a clear red-shift of the A1g mode is observed. According to previous reports, the A1g mode in SnS2 is the primary out-of-plane vibrational mode [34,35]. Often described as a “breathing” vibration or an S-Sn-S stretching mode, it is known to be sensitive to applied stress or strain. Therefore, the observed red-shift of the A1g mode on the as-grown substrate is indicative of the presence of stress [36,37]. To further investigate the effect of stress on the properties of SnS2, the angle-resolved polarized Raman spectroscopy (ARPRS) of SnS2 on the growth substrate and the transferred substrates were carried out and the results are shown in Fig. 8. Schematic diagrams of typical configurations for ARPRS are shown in Fig. 8a. For SnS2 on SiO2 (Fig. 8b–d), the Raman peak intensity remains nearly constant and exhibits no significant periodic variation with the polarization angle under parallel configuration (Fig. 8c). In contrast, under perpendicular configuration, the intensity is very low, nearly negligible (Fig. 8d). These observations indicate the intrinsic isotropy of SnS2 [38]. These results are consistent with the calculations based on the semiclassical Placzek model and the corresponding Raman tensor [38,39]. Spectifically, under parallel polarization, the Raman peak intensity remains constant and independent of the polarization angle, while it drops to zero under perpendicular polarization (see Supporting Information for details). This further confirms the intrinsic in-plane isotropy of SnS2. In striking contrast, SnS2 grown on mica exhibits a pronounced anisotropic response: both the mica peak at 263.8 cm−1 and the SnS2 A1g peak at 313.6 cm−1 display regular periodic intensity fluctuations with polarization angle (Fig. 8e), directly confirming strong anisotropy. Raman intensity color maps (Fig. 8f,g) clearly reveal the distribution of intensity. Further analysis shows that both the mica 263.8 cm−1 peak and the SnS2 A1g peak (313.6 cm−1) exhibit a 90° periodic oscillation under parallel and perpendicular configurations. Polar plots (Fig. 8h) confirm that SnS2 on SiO2 shows no intensity variation with polarization angle, further supporting its isotropy. In contrast, SnS2 growth on mica exhibits significant in-plane anisotropy (Fig. 8i–l). For the mica peak (263.8 cm−1), maximum intensity occurs at 61°/151°/237°/325° (parallel) and 20°/108°/196°/288° (perpendicular). For the SnS2 A1g peak (313.6 cm−1), maxima appear at 63°/150°/236°/325° (parallel) and 10°/101°/193°/289° (perpendicular). This periodic phenomenon is attributed to interfacial stress from lattice mismatch between SnS2 and mica, which breaks the intrinsic hexagonal symmetry of SnS2, resulting in pronounced in-plane anisotropy.

Figure 8: The ARPRS of the SnS₂ nanosheets. (a) Schematic diagrams of typical configurations for ARPRS. the ARPRS of the SnS₂ nanosheets transferred on SiO₂ (b) and grown on mica substrates (e) under parallel polarization configuration; False-color intensity maps for SnS₂ nanosheets transferred on SiO₂ under parallel (c) or perpendicular (d) polarization configuration, respectively; False-color intensity maps for SnS₂ nanosheets grown on mica substrates under parallel (f) or perpendicular (g) polarization configuration, respectively; Polar plots of the 314.6 cm−1 peak for SnS₂ nanosheets transferred on SiO₂ under parallel (h) polarization configuration. Polar plots of the 263.8 cm−1 (mica) and 313.6 cm−1 (SnS₂) peaks for SnS₂ nanosheets grown on mica substrates under parallel (i,k) or perpendicular (j,l) polarization configuration.

3.4.2 Polarized Optical Microscopy Characterization

To further verify the transition of SnS2 nanosheets from isotropy to anisotropy, polarized optical imaging was employed [40]. As shown in Fig. 9a, for SnS2 transferred onto SiO2/Si (300 nm SiO2), the SnS2 brightness remains angle-independent and in an extinction state, indicating isotropy. In striking contrast, for SnS2 grown directly on mica (Fig. 9b), its brightness displays a 90° periodic variation, with even greater amplitude than that of mica, confirming significant in-plane anisotropy. To enable quantitative analysis of the intensity-versus-angle relationship, polar plots of polarized reflected light intensity were constructed for SnS2 both on the growth substrate and after transferred to SiO2/Si. The polarized reflected light intensity from SnS2 transferred to SiO2/Si was very low and remained nearly constant as a function of the sample rotation angle θ (Fig. 9c). In contrast, SnS2 on the original growth substrate exhibited a distinct four-folded periodic patterns, confirming the presence of strong in-plane optical anisotropy (Fig. 9d). In summary, combining ARPRS and polarized optical imaging, this study deepens the understanding of interfacial stress effects in 2D materials and demonstrates a strategy for converting intrinsically isotropic 2D materials into anisotropic ones. This work provides important theoretical and experimental support for designing polarization-sensitive optoelectronic devices based on anisotropic SnS2.

Figure 9: Polarized optical imaging of SnS₂ nanosheets. Polarized optical imaging of SnS₂ nanosheets after transferred to SiO₂/Si (a) (the ration angle of 0° is the optical microscopy image) and on the growth substrate (b); polar plots of polarized reflected light intensity of SnS₂ and after transferred to SiO₂/Si (c) and on the growth substrate (d).

4 Conclusion

This study focuses on the controllable synthesis and in-plane anisotropy modulation of 2D SnS2 nanosheets. A CsCl/KI mixed salt-assisted growth strategy was used to synergistically control the morphology, lateral size, and thickness of the nanosheets. Experimental results show that CsCl promotes lateral growth, while KI optimizes crystal morphology. Using this mixed salt system, triangular, ultrathin, large-size SnS2 nanosheets with a lateral size exceeding 200 μm and a thickness of only 1.8 nm were successfully fabricated. ARPRS reveals that SnS2 nanosheets transferred onto SiO2 substrates exhibit intrinsic in-plane isotropy. In contrast, for SnS2 grown directly on mica, lattice mismatch induces interfacial stress that breaks the intrinsic symmetry, causing the A1g Raman peak intensity to exhibit a 90° periodic variation with polarization angle, which demonstrates pronounced in-plane anisotropy and further confirmed by polarized optical microscopy. This work provides a new method for the controllable growth of SnS2 nanosheets and offers experimental evidence for understanding stress-induced anisotropy in 2D materials.