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DOI: 10.1039/D5MH01023C (Communication) Mater. Horiz., 2025, Advance Article

Regulation of metal valence states for enhancing second harmonic generation performance of chiral tin halides

Yue Wang, Puxin Cheng, Wenqing Han, Junjie Guan, Quanwen Li, Zhihua Wang, Peihan Wang, Yongshen Zheng* and Jialiang Xu*
School of Materials Science and Engineering, Tianjin Key Laboratory of Metal and Molecular Materials Chemistry, Frontiers Science Center for New Organic Matter, Academy for Advanced Interdisciplinary Studies, Nankai University, Tongyan Road 38, Tianjin 300350, P. R. China. E-mail: yszheng@nankai.edu.cn; jialiang.xu@nankai.edu.cn

Received 30th May 2025 , Accepted 1st August 2025

First published on 5th August 2025

Abstract

Chiral metal halide perovskites (MHPs) possess an inherent noncentrosymmetric structure, offering a promising platform for the development of second-order nonlinear optical (NLO) materials. Recent research has emphasized the influence of metal valence states on the physical and chemical properties of perovskites. In this work, a series of novel chiral tin-based MHPs were constructed by adjusting the metal valence states, thereby tuning the second-order NLO performance. Through the oxidation strategy, (R-/S-3-methylpiperidine)2SnBr3Br (R-/S-2Sn) can be easily transformed into centimeter-scale (R-/S-3-methylpiperidine)2SnBr6 (R-/S-4Sn) crystals. This transformation significantly enhances the second harmonic generation (SHG) response owing to the reduction in crystal spatial symmetry following the change in the valence states of tin. This work highlighted the potential of low-dimensional chiral tin-based MHPs in high-performance NLO applications, offering a foundation for future exploration in this field.


New concepts

Metal halide perovskites (MHPs) have demonstrated tremendous potential in optoelectronic applications owing to their outstanding photoelectric properties, including long charge carrier diffusion lengths, tunable bandgaps, strong optical absorption coefficients, and structural versatility. Chiral metal halides combine the structural advantages of MHPs with the unique characteristics of chiral materials, enabling effective chirality transfer from organic ligands to inorganic frameworks. This integration endows the materials with novel functionalities such as chiroptical activity and chiral-induced spin selectivity. Studies have shown that incorporating high valence metal ions can significantly enhance material performance, offering new opportunities for developing high efficiency optoelectronic devices. In this work, we enhanced the nonlinear optical (NLO) response of chiral tin halides by modulating their structural symmetry through controlled valence state adjustment. Our findings not only provide a new strategy for designing high-performance and stable NLO materials but also demonstrate the critical role of metal valence state control in the design of functional materials.

Introduction

Metal halide perovskites (MHPs), with the advantages of their diverse crystal structures,1 tunable band gaps,2,3 high dielectric constant4 and improved charge-carrier transport,5 have gained widespread applications in photoelectric detectors,6,7 solar photovoltaic cells,8,9 light-emitting diodes,10,11 high-energy radiation detection12,13 and other fields. For low-dimensional MHPs, the composition regulation of A-, B- and X- sites plays a key role in enhancing their structural diversity.14,15 This approach not only addresses the common challenges faced by traditional 3D MHPs, such as limited battery life, poor air stability, and low efficiency,16–19 but also imparts a range of unique properties to the low-dimensional materials. For example, the large exciton binding energy, large carrier mobility, long photoinduced carrier lifetime and high photoluminescence intensity of low-dimensional MHPs further broaden their potential applications in optoelectronics.20–24 Chiral MHPs are obtained by introducing chiral organic cations into typical perovskite lattices. Since the chiral organic molecules were first incorporated into perovskite structures and their chiral optical activity was further studied,25–27 chiral MHPs have gradually emerged and aroused extensive research interest. In 2018, our group reported the second harmonic generation (SHG) properties of chiral MHP single crystals, and studied their SHG circular dichroism (SHG-CD) response for the first time.28 The chirality was transferred from the chiral organic cations to the hybrid inorganic frameworks through interactions with the inorganic components,29 which endowed the MHPs with an intrinsic noncentrosymmetric structure and numerous unique physical and chemical properties, such as circular dichroism (CD),30,31 circularly polarized luminescence (CPL),32,33 ferroelectricity,34,35 spintronics36,37 and nonlinear optics (NLO).38–40

Studies have shown that the valence states of metals in perovskite materials directly affect their physical and chemical properties.41 Perovskites with metals in higher valence states tend to exhibit superior performance to those with metals in lower valence states.42,43 For instance, iridium-based perovskite electrocatalysts containing high valence Ir5+ show enhanced catalytic activity and electrochemical stability.44 Similarly, CaCoO3 containing Co4+ exhibits better oxygen evolution reaction activity than Co3+-based oxides like LaCoO3 and Co3O4.45 Additionally, Li et al. observed improved luminescence in Sb5+-based single crystal materials with high valence states,46 while Wan et al. showed the surface treatment of high valence metal ions can improve the performance of inorganic perovskite solar cells.47 Dang et al. observed that MHPs with higher valence metal ions exhibited enhanced NLO performance.48 In general, owing to the variation in the valence states of B-site metal cations, perovskite materials have already demonstrated very promising application prospects in catalysis, optoelectronics and other fields.

Tin MHPs, widely regarded as the most promising alternative to lead MHPs, have attracted significant attention in solar cell applications owing to their similar ionic radius and valence electron configuration to lead, along with their excellent photoelectric and environmental properties.49,50 However, due to the easy oxidation of tin(II) by the loss of its 5s2 orbital electrons, the performance of tin(II)-based light-emitting diodes is significantly inferior to that of lead-based perovskites.51 Over the years, researchers have focused on addressing the oxidation issue of tin(II),52–55 while comparatively less attention has been given to tin(IV)-based perovskites. Moreover, tin(IV)-based perovskite materials have been demonstrated to possess good stability thanks to their outer-shell electron configuration of 4d105s0, which enables them to exhibit good performance in fields such as photodetection,56,57 catalysis,58,59 and photoluminescence.60,61 Herein, a series of novel chiral tin-based MHPs, (R-/S-3-methylpiperidine)2SnBr3Br (R-/S-2Sn) and (R-/S-3-methylpiperidine)2SnBr6 (R-/S-4Sn), were synthesized using R-/S-3-methylpiperidine (R-/S-3-MP) as the chiral ligand. Notably, the R-/S-2Sn underwent a dissolution-recrystallization process within the mother liquor, converting into R-/S-4Sn, which resulted in noticeable alterations to its crystal structure, band gap and space group. Surprisingly, the second-order NLO performance of S-4Sn exhibited a significant enhancement. The SHG response of S-4Sn was 65 times that of S-2Sn and 6 times that of Y-cut quartz, accompanied by a higher laser damage threshold of 3.78 mJ cm−2. The centimeter-scale crystals of S-4Sn can be easily obtained, making it a promising candidate for NLO applications. This work provides valuable insights into the design of high-performance NLO materials and paves the way for further exploration of the role of high valence state metals in chiral MHPs.

Results and discussion

Two chiral tin MHP materials, R-/S-2Sn and R-/S-4Sn, were synthesized by reacting R-/S-3-MP with stannous oxide (SnO) in hydrobromic acid (HBr). Orangish and rod-like crystals of R-/S-2Sn were first grown through a temperature-lowering crystallization process. Subsequently, the conversion process from R-/S-2Sn to yellowish and block-shaped crystals R-/S-4Sn was observed in the mother liquor, and this process was irreversible (for more details see the Experimental Section, Fig. S1 and S2, SI). The centimeter-scale S-4Sn crystals could be easily obtained using the room-temperature evaporation method (Fig. S3, SI). Single-crystal X-ray diffraction (SCXRD) analyses revealed that R-/S-2Sn crystallized in the orthorhombic space group P212121, while R-/S-4Sn crystallized in the monoclinic space group C2 (Tables S1 and S2, SI). The overall packing views of S-2Sn and S-4Sn crystals along the crystallographic a- and b-axis were shown in Fig. 1a and b. The asymmetric unit of S-2Sn consisted of a [SnBr3]− pseudo-trigonal pyramidal anion unit, two chiral organic amine cations and one free Br− whose bond length with Sn2+ exceeded the bonding range (Fig. 1a). In contrast, the asymmetric unit in (rac-3-MP)SnBr3 did not contain free Br− and crystallized in P21/n (Fig. S4, SI). This suggests that the introduction of homochiral molecules disrupted the symmetry of the racemic structure, leading to the presence of additional free Br− ions in the crystal to balance the charges of the organic cations. The asymmetric unit of S-4Sn contained one [SnBr6]2− inorganic octahedron and one chiral organic cation (Fig. 1b). The inorganic parts of S-2Sn and S-4Sn were surrounded by chiral (S-3-MP)+ helices via N–H⋯Br hydrogen bonding and electrostatic interactions, and the entire structures belonged to 0D. Specifically, (S-3-MP)+ formed N–H⋯Br hydrogen bonds with Br− in the [SnBr3]− of S-2Sn at distances ranging from 2.71 Å to 2.92 Å, and with the free Br− at distances of 2.39 Å and 2.46 Å (Fig. S5a, SI). The hydrogen bond distances between (S-3-MP)+ and Br− in the [SnBr6]2− octahedron of S-4Sn ranged from 2.58 Å to 2.98 Å (Fig. S5b, SI). Analyses of the Hirshfeld surface and 2D fingerprint plots62,63 revealed that the presence of free Br− in S-2Sn increases the distance between [SnBr3]− and the organic amine, resulting in weaker hydrogen bond interactions compared to those in S-4Sn (Fig. S6 and S7, SI). The coordination mode of the inorganic part transformed from a pseudo-trigonal pyramidal to a distorted octahedral configuration. These differences are mainly influenced by the combined effects of the electron configuration of the central ion, the lone pair effect, and the steric hindrance of the ligand.64,65 In S-2Sn, the lone pair electrons in the 5s orbital of Sn2+ exhibit significant stereochemical activity, making it easier to form low coordination structures; while Sn4+ in S-4Sn has no interference from lone pair electrons and thus has a higher coordination number.66 The change in valence state of Sn atoms is also accompanied by a space group transformation from P212121 to C2, resulting in a significant decrease in crystal structure symmetry. The Sn–Br bond length of S-2Sn ranged from 2.71 Å to 2.74 Å, with an average value of 2.72 Å, while the Br–Sn–Br bond angle between 89.13° and 94.56° (Fig. 1c and Table S3, SI). The average Sn–Br bond length of S-4Sn was 2.60 Å, and the Br–Sn–Br bond angle ranged from 87.70° to 178.68° (Fig. 1c and Table S4, SI). The two tin MHPs exhibit similar characteristic absorption peaks in their infrared spectra owing to their identical organic components (Fig. S8a, SI). In contrast, their Raman spectra display distinct spectral features (Fig. S8b, SI), attributable to the significant differences in the valence state of Sn and its coordination environment. Specifically, S-4Sn exhibits a sharp characteristic peak at 189 cm−1, along with a notable blue shift in the low-frequency region compared to S-2Sn, resulting from the contraction of Sn-Br bonds.67,68 The experimental powder X-ray diffraction (PXRD) patterns of the two synthesized perovskites were in excellent agreement with their theoretical simulation data (Fig. 1d), confirming that the high crystalline quality and phase purity of the obtained materials. PXRD patterns of S-2Sn and S-4Sn after six-month ambient exposure show no significant changes, confirming the superior environmental stability of these tin MHPs (Fig. S9, SI).
image file: d5mh01023c-f1.tif
Fig. 1 Single crystal structures of S-2Sn and S-4Sn. (a) Structural models of S-2Sn in a unit cell viewed along the crystallographic a-axis (H atoms are omitted for clarity) and the asymmetric unit. (b) Structural models of S-4Sn in a unit cell viewed along the crystallographic b-axis (H atoms are omitted for clarity) and the asymmetric unit. (c) [SnBr3]− pseudo-trigonal pyramidal of S-2Sn and [SnBr6]2− octahedron of S-4Sn. (d) Experimental and simulated PXRD patterns of S-2Sn and S-4Sn.

The linear optical properties of the chiral tin MHPs were characterized by ultraviolet-visible (UV-vis) absorption spectrum and circular dichroism (CD) spectrum. Both enantiomers exhibited mirror-imaged CD signals with equivalent intensity, while notable differences were observed in the positions and shapes of the peaks. Specifically, S-2Sn presented a positive Cotton effect at 248 nm and a negative Cotton effect at 376 nm, whereas R-2Sn displayed the reversed signal pattern (Fig. 2a), corresponding to distinct exciton transition states. For R-/S-4Sn enantiomers, three pairs of Cotton effects with opposite signals were identified at 220, 270 and 324 nm (Fig. 2b). Significantly, S-4Sn displayed a positive Cotton effect at 270 nm, contrasting with the negative Cotton effect observed in R-4Sn. The crossover points of these Cotton effects corresponded to the exciton transition wavelengths of R-/S-4Sn. The distinct CD spectral features (peak positions and intensities) of R-/S-2Sn and R-/S-4Sn indicate that tin valence states modulate chiral optical properties through crystal structure modifications. The anisotropic factors (gCD) of R-/S-2Sn and R-/S-4Sn were on the order of 10−4 (Fig. S10 and Table S5, SI). This undistinguished gCD factor could be attributed to the macroscopic disorder of powder samples and the low-degree structural distortions inherent to the materials themselves.69,70 The UV-vis absorption spectra revealed different features for the two compounds. R-/S-2Sn exhibited absorption peaks at 248 and 376 nm with an absorption edge extending to 600 nm, while R-/S-4Sn showed absorption peaks at 270 and 351 nm, reaching a shorter absorption edge at 500 nm. In comparison, R-/S-4Sn demonstrated both lower absorption cutoff edges and a narrower bandwidth compared to R-/S-2Sn. The UV-vis-NIR transmittance spectra demonstrate that both chiral tin MHPs exhibit transmittance exceeding 80% in the 440–2500 nm range (Fig. S11, SI), showcasing a broad transparency window covering the visible to near-infrared region. This feature ensures compatibility with common laser pump sources while effectively reducing the self-absorption between the pump and signal wavelengths.71


image file: d5mh01023c-f2.tif
Fig. 2 Linear optical properties of R-/S-2Sn and R-/S-4Sn. Normalized UV-vis absorption spectra and CD spectra of (a) R-/S-2Sn and (b) R-/S-4Sn. Calculated electronic band structures of (c) S-2Sn and (d) S-4Sn. Calculated DOS of (e) S-2Sn and (f) S-4Sn.

The electronic band structures and density of states (DOS) of S-2Sn and S-4Sn were calculated by DS-PAW software72 based on first-principles density functional theory (DFT). The band dispersion relations along the high-symmetry directions in the Brillouin zone were systematically investigated for both compounds. A significant difference in the band structures of S-2Sn and S-4Sn results in distinct electronic properties. For S-2Sn, both the valence band maximum (VBM) and the conduction band minimum (CBM) are located at the X point in the Brillouin zone, revealing a direct bandgap nature (Fig. 2c). The calculated bandgap was 2.34 eV, which was in good agreement with the experimental value obtained from the UV-vis absorption spectrum (Fig. S12, SI). In contrast, S-4Sn displays an indirect bandgap feature, with the VBM positioned at the E point and the CBM at the Z point in reciprocal space (Fig. 2d). The calculated bandgap of S-4Sn was 2.95 eV, slightly larger than the experimental value (Fig. S12, SI). This discrepancy could potentially be attributed to the specific calculation method employed. Comparative band structure analyses revealed discrete energy bands in S-2Sn and continuous band dispersion in S-4Sn. This difference was attributed to the distinct inorganic components of S-2Sn and S-4Sn. Specifically, the average Sn–Br bond lengths were significantly different, being 2.72 Å for S-2Sn and 2.60 Å for S-4Sn as shown in Fig. 1c. The reduced Sn–Br bond distance in S-4Sn enhanced orbital overlap between the metal Sn and Br atoms, leading to stronger interactions.73–75

The distribution of electronic states within the tin MHPs as a function of energy was displayed by the DOS diagrams. Projected density of states (PDOS) analyses revealed that the hybrid orbitals of Sn and Br atoms predominantly contribute to the VBM and CBM in S-2Sn and S-4Sn. Specifically, the VBM of S-2Sn was primarily formed by Br-p orbitals with minor contributions from Sn-s orbitals, while the CBM was primarily composed of Br-p and Sn-p orbitals (Fig. 2e). Charge density distribution analyses corresponding to the VBM and CBM of S-2Sn show hybridization among Sn-s, Sn-p orbitals and Br-p orbitals, which indicates the presence of interactions between Sn and Br atoms (Fig. S13, SI).76 This observation was further corroborated by Hirshfeld analyses, which confirmed an interaction between Sn and Br atoms (Fig. S6, SI). In the case of S-4Sn, the PDOS analyses showed comparable contributions from Br-p and Sn-s orbitals to the VBM (Fig. 2f). The CBM of S-4Sn was mainly composed of Sn-p and Br-p orbitals. Notably, the pronounced hybridization between Br-p orbitals in the adjacent inorganic octahedra of S-4Sn at the VBM was observed, indicating strong inter-octahedral interactions (Fig. S14, SI).77,78 Furthermore, organic cations contributed slightly to both VBM and CBM in S-4Sn, while this phenomenon was absent in the case of S-2Sn. This suggested that S-4Sn exhibited a more pronounced hydrogen bond interaction compared to S-2Sn. In conclusion, while the optical properties of both compounds were mainly determined by the inorganic components, the organic amine cations played an indirect role, particularly in S-4Sn.

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to systematically evaluate the thermal stability of this series of tin MHPs (Fig. 3 and Fig. S15, S16, SI). TGA curves of S-2Sn and S-4Sn powder samples demonstrated their excellent thermal stability, with decomposition temperatures being 287 and 252 °C, respectively (Fig. 3a and b). The DTA curves of both S-2Sn and S-4Sn exhibited endothermic peaks prior to their decomposition temperatures. To verify whether a thermally induced phase transition occurred, DSC measurements were performed on S-2Sn and S-4Sn (Fig. S16, SI). Both compounds showed an endothermic peak upon heating, followed by an exothermic peak during cooling. The enthalpy integrals for the two compounds were approximately the same, suggesting that the observed process was melting and recrystallization. Additionally, changes during heating and cooling were further investigated using variable temperature PXRD (Fig. 3c and d). When heated to 180 °C, the diffraction peaks of S-2Sn and S-4Sn disappeared, which indicated that they underwent melting. During cooling, S-2Sn exhibited reversible recrystallization, while only a single diffraction peak was observed at around 14° for S-4Sn. These observations indicated that S-4Sn had slower recrystallization kinetics and a longer retention of the amorphous state compared to S-2Sn. Furthermore, in situ heating of the S-2Sn and S-4Sn crystals on a hot plate was consistent with the TGA, DSC and PXRD results, confirming the melting process (Fig. S17, SI).


image file: d5mh01023c-f3.tif
Fig. 3 Thermal stability of S-2Sn and S-4Sn. TGA curves of (a) S-2Sn and (b) S-4Sn. Variable temperature PXRD patterns of (c) S-2Sn and (d) S-4Sn.

The realization of second-order NLO responses, such as SHG, requires that the material possess a strict noncentrosymmetric structure. In this work, two chiral tin MHPs, R-/S-2Sn and R-/S-4Sn, exhibit inherent noncentrosymmetric symmetry due to the introduction of chiral molecules. Since the absolute configuration does not influence the SHG characteristics of these chiral MHPs, the S-2Sn and S-4Sn crystals were chosen as the primary focus of the investigation. The SHG properties of these compounds were investigated by utilizing a homemade femtosecond laser device28,79 (Mai Tai HP, ∼100 fs, 80 MHz, 800–1040 nm). SHG scans were conducted on S-2Sn and S-4Sn under 1040 nm laser excitation, and the resulting mapping images provided a clear demonstration of their SHG activity (Fig. S18, SI). Wavelength-dependent SHG measurements were performed on S-2Sn and S-4Sn single crystals under constant incident power (Fig. 4a and b). The results revealed pronounced SHG signals at half of the pump wavelength within the range of 800–1040 nm, confirming the nonlinear optical response. Furthermore, the SHG spectra of S-2Sn and S-4Sn exhibited distinct trends with varying wavelength. The observed SHG signal attenuation at shorter excitation wavelengths was attributed to the enhanced self-absorption effect within the materials, consistent with their electronic absorption characteristics.


image file: d5mh01023c-f4.tif
Fig. 4 Nonlinear optical properties of S-2Sn and S-4Sn. Wavelength-dependent SHG spectra of (a) S-2Sn and (b) S-4Sn pumped from 800 to 1040 nm laser. (c) Logarithmic plot of SHG intensity as function of the incident power. The red line is the linear fitting of data points with a slope of 2.0. (d) Comparison of SHG intensity between S-2Sn, S-4Sn and Y-cut quartz at 1040 nm. The incident power for S-4Sn and Y-cut quartz is 15 mW and for S-2Sn is 30 mW; the exposure time for all of them is 0.1 s. Polarization dependence of the SHG intensity from the (e) S-2Sn and (f) S-4Sn as a function of the linear polarization angle. The red line is the nonlinear fitting of data points.

To verify the two-photon process for both chiral tin MHPs, the dependence of the SHG signal strength on the input laser power was subsequently tested. The intensity of the SHG signals increased as the power of the incident laser increased (Fig. S19, SI). The slopes of the power-intensity functions of S-2Sn and S-4Sn were 2.0 on a logarithmic scale, confirming the two-photon process (Fig. 4c). Laser damage occurred in S-2Sn when the incident power reached at 732 mW, leading to a decrease in SHG signal intensity. In contrast, S-4Sn exhibited the better laser stability, with damage occurring at 949 mW incident power. The power levels that caused damage to S-2Sn and S-4Sn were defined as their laser damage thresholds, corresponding to pump energy densities of 2.92 and 3.78 mJ cm−2 (pulse width ∼100 fs, laser spot diameter ∼20 μm), respectively. The comparative analyses of the SHG responses demonstrated that S-4Sn exhibited significantly enhanced nonlinear optical activity compared with S-2Sn throughout the entire spectral range (Fig. S20, SI). Specifically, the SHG intensity of S-4Sn was approximately 65 times that of S-2Sn at 1040 nm (Fig. 4d). To evaluate the second-order NLO performance more intuitively, we compared the SHG intensities of S-4Sn with those of commercial Y-cut quartz and KH2PO4 (KDP). Under the same testing conditions, the SHG intensity exhibited by S-4Sn crystal was comparable to that of Y-cut quartz within the broad wavelength range of 800–1040 nm (Fig. S21, SI). Notably, under 940 nm laser excitation, S-4Sn exhibits a 28-fold increase in SHG intensity compared to Y-cut quartz. Meanwhile, at specific excitation wavelengths such as 820 and 840 nm, its SHG response is comparable to that of KDP (Fig. S22, SI). Crystal symmetry and interatomic interaction were identified as crucial factors that affected the SHG response. The change in tin valence states induced modifications to the crystal structure, thereby altering the electronic structure and optical properties of the tin MHPs. The reduced spatial group symmetry of S-4Sn (from P212121 to C2) led to enhanced structural asymmetry, promoting asymmetric electron cloud polarization under optical excitation and consequently enhancing SHG. Furthermore, strong interatomic interactions and organic-inorganic interactions in S-4Sn facilitated asymmetric distortions, which synergistically enhanced the nonlinear optical response.80–82

The anisotropic NLO responses of S-2Sn and S-4Sn were further characterized by measuring the SHG intensity as a function of the linear polarization angle of the incident laser, which was altered by rotating a λ/2 plate. The polarization-dependent SHG measurements showed that both S-2Sn and S-4Sn exhibited a characteristic two-pole dumbbell shape, which could be well fitted by the cos4θ function. The SHG intensity of both compounds respectively presented two maxima and two minima values within a 360° polarization angle change (Fig. 4e and f). The SHG polar plots of S-2Sn and S-4Sn at various wavelengths exhibited identical polarization directions (Fig. S23, SI). For S-2Sn, the maxima SHG intensity occurred at polarization angles of approximately 20° and 200°, with minima at around 110° and 290°. For S-4Sn, the maxima SHG intensity were observed at approximately 10° and 190°, while the minima occurred at around 100° and 280°. The polarization ratios, calculated using the formula ρ = (Imax−Imin)/(Imax + Imin), were 61% for S-2Sn and 98% for S-4Sn. The higher polarization ratio of S-4Sn indicated greater anisotropy compared to S-2Sn, further suggesting that the oxidized tin MHP crystals possess superior crystallinity.

Conclusions

In summary, we have reported a series of 0D chiral tin MHPs. By altering the valence states of tin, the electronic structure and optoelectronic properties of these compounds were effectively tuned. The oxidized S-4Sn crystals exhibited superior second-order NLO performance, including high SHG intensity, a wide bandgap, a high polarization ratio, a large laser damage threshold, excellent crystallinity, and robust air stability. Notably, the SHG response of the S-4Sn at 1040 nm was 65 times that of S-2Sn and 6 times that of Y-cut quartz. These outstanding nonlinear optical properties were attributed to the low spatial symmetry and strong interatomic interactions in the S-4Sn. This work serves as a valuable reference for further exploration of high-performance second-order NLO materials in the field of low-dimensional chiral tin MHPs.

Author contributions

The manuscript was written through the contributions of all authors.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

Experimental section; characterization details; and supporting figures, tables, and references. See DOI: https://doi.org/10.1039/d5mh01023c

CCDC 2380530–2380535 contain the supplementary crystallographic data for this paper.83–88

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22401157, 22475106, 52172045 and 22035003), the National Key R&D Program of China (2022YFA1204500 and 2022YFA1204504), the Natural Science Foundation of Tianjin City (23JCJQJC00110), TCL Science and Technology Innovation Fund, and the Fundamental Research Funds for the Central Universities. We gratefully acknowledge HZWTECH for providing computation facilities.

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