Anisotropy in materials introduces a rich and versatile dimension to advancements in materials science, particularly in emerging semiconductor device technologies (Li L. et al., 2019; Xia et al., 2019; Sarkar and Stratakis, 2020; Chang et al., 2023; Lin et al., 2018). Among these, two-dimensional (2D) anisotropic materials have gained prominence in quantum materials research due to their extreme quantum confinement at the atomic scale (Dwyer et al., 2019; Sarkar et al., 2021). The inherent anisotropy of certain 2D materials gives rise to extraordinary quantum phenomena, unlocking new possibilities for advanced applications. Recent developments in metal monochalcogenides (Sarkar and Stratakis, 2020; Sarkar et al., 2020; 2023), chemical formula (MX, M = metal, X = Chalcogens) have positioned them at the forefront in anisotropic materials research. In particular, 2D tin sulfide (SnS) has attracted significant attention due to its unique direction-dependent properties. This material has been studied from various perspectives, including its electronic, optical, and excitonic behaviors, which are crucial for next-generation energy, optoelectronic, spintronics and quantum devices.

In 2015, the first isolation of few-layer SnS revealed its exotic optical and electronic properties (Brent et al., 2015). The SnS exhibits either a direct or indirect band gap and its polarization dependent anisotropic optical and electronic behaviors have been actively explored (Hanakata et al., 2016; Tian et al., 2017; Chen et al., 2018; Bao et al., 2019; Li F. et al., 2019; Gomes and Carvalho, 2020; Higashitarumizu et al., 2020; Kwon et al., 2020; Gao et al., 2022; Zi et al., 2022; Chang et al., 2023). The anisotropic nature of electronic transport along the armchair (AC) and zigzag (ZZ) directions gives rise to distinct features. Direction dependent nonlinear optical and valley-related properties, including multiferroicity, have also been investigated (Hanakata et al., 2016; Rodin et al., 2016; Wang and Qian, 2017; Chen et al., 2018; Maragkakis et al., 2022; Verma et al., 2023; Sutter et al., 2024a). Monolayer SnS adopts an orthorhombic crystal structure with a Pnma space group (Figures 1a–c). The structure exhibits low crystal symmetry (C _2ν) and broken inversion symmetry. These unique characteristics enable the material to support a range of novel order parameters, including optical nonlinearity, spin-orbit coupling, ferroelectricity, polarization, and synaptic properties.

A variety of exotic phenomena have been predicted or demonstrated in monolayer SnS, such as the bulk photovoltaic effect (Chang et al., 2023; Kumar et al., 2025), synaptic behavior for neuromorphic computing (Kwon et al., 2020; Khot et al., 2024), second- and third- order optical nonlinearity (Maragkakis et al., 2022; Maragkakis et al., 2024), spin-valley polarization (Hanakata et al., 2016; Rodin et al., 2016; Chen et al., 2018; Lin et al., 2018; Tołłoczko et al., 2025), ferroelectricity (Bao et al., 2019; Higashitarumizu et al., 2020; Kwon et al., 2020; Y.-R; Chang et al., 2023), giant piezoelectricity (Khan et al., 2020; Cao et al., 2021; Yoo et al., 2023), multiferroicity (Wang and Qian, 2017), chiral phonons (Skelton et al., 2017), thermoelectric effects (Diyar et al., 2024; Xue et al., 2025), spintronics (Yang et al., 2024), superconductivity (Chen Jianyong et al., 2025), and quantum confinement effects (Dwyer et al., 2019). Additionally, the strong second- and third-harmonic generation observed in mono- and few-layer SnS holds great promise for nonlinear optoelectronics and the generation of entangled photon pairs (EPPs) via spontaneous parametric down-conversion (SPDC) for quantum technological applications (Sarkar, 2025).

This perspective covers the isolation of anisotropic 2D SnS monolayers, their physical properties, advanced characterization techniques, and emerging electronic device applications, including those in quantum technology. More specifically, the isolation of single-layer SnS using conventional methods is explored, along with microscopy and spectroscopic characterizations. Furthermore, applications in energy, optoelectronic, neuromorphic computing, spintronic, and quantum devices are discussed. The perspective also highlights current limitations and explores emerging opportunities for future device applications.

Anisotropy refers to the property of a material exhibiting different physical characteristics depending on the direction of measurement. In particular, structural anisotropy at the crystal level plays a pivotal role in this behavior. The concept is closely related to asymmetry in materials or crystallographic structures, and anisotropy typically arises from differences along specific crystallographic axes. In this context, it discusses the polarization mechanisms in the anisotropic material SnS. For instance, the polarization-dependent optical properties arising from its intrinsic anisotropy. The polarization-dependent optical properties originate from their fundamental physical nature (Sharma et al., 2018). These can generally be classified into three categories: (1) anisotropy due to structural asymmetry, (2) the presence of inequivalent electronic valleys, and (3) interactions among quasiparticles, such as excitons and polarized plasmons or photons.

To understand and describe polarization mechanisms in anisotropic materials, the semiclassical Drude model is employed. In a simplified model for 2D layered materials, polarization contribution along one direction is often considered homogeneous, while the polarization responses in the other two directions can be characterized using the relative permittivity ( ϵ j j ) and optical conductivity ( σ j j ), as described by ϵ j j = ϵ r + i σ j j ϵ 0 ω t (1) σ j j = σ x x σ y x σ x y σ y y (2) j = x, y (considering x-y plane of the material). Here, ϵ 0 and ϵ r represent the vacuum permittivity and the relative permittivity, respectively. ω and t denote the optical frequency and the thickness of the layered material. The polarization- dependent optical and electronic properties of anisotropic layered materials are governed by these parameters. In anisotropic 2D SnS, the atomic arrangement within the crystal leads to highly direction-dependent (anisotropic) carrier effective masses in real space (Figures 1a–c). Specifically, anisotropic 2D materials with linear electronic dispersion exhibit an optical conductivity tensor where σ_xx ≠ σ_yy and σ_{xy =} σ_yx = 0, while isotropic materials follow σ_xx = σ_yy and σ_xy = σ_yx = 0 (Equations 1, 2).

Anisotropy in the physical properties of SnS has been extensively investigated through both theoretical and experimental approaches. For instance, anisotropic absorption and Photoluminescence (PL), including nonlinear optical (NLO) properties of SnS, have been explored (Batool et al., 2022). These studies reveal that the optical properties of SnS are directionally dependent. Notably, second- and third- harmonic generation (SHG and THG) along the AC and ZZ directions exhibit significant differences. The average THG anisotropy in SnS is estimated to be 0.75 ± 0.22 (Maragkakis et al., 2024). In contrast, the anisotropic SHG response of a SnS layer has also been reported. The anisotropic nature of these NLO properties originates from the inherent crystal symmetry of SnS.

In addition to optical anisotropy, anisotropy in the electronic properties of SnS plays a critical role in device applications. Specifically, the anisotropic ratio between the AC and ZZ directions presents opportunities for novel functionalities. The effective mass (m) strongly influences carrier mobility (μ), as μ∝1/m. Importantly, the effective mass along the armchair direction (m _AC ) differs from that along the zigzag direction (m _ZZ ), which contributes significantly to the electronic anisotropy of SnS. For example, the experimentally observed anisotropy ratio of SnS is approximately 1.7, closely matching the theoretically predicted values (Vidal et al., 2012; Guo et al., 2015). Moreover, the theoretical aspects of anisotropy have also been investigated. For instance, Medrano Sandonas et al. (2016) theoretically estimated the anisotropic thermoelectric response in SnS. A significantly high figure of merit (ZT) was obtained at room temperature, reaching 1.6 and 0.95 along the AC and ZZ directions, respectively. These predicted ZT values are in close agreement with the experimental ZT value of approximately 0.7 (Hu et al., 2024).

The synthesis of electronic grade materials is a critical first step toward unlocking the rich physics and exceptional properties required for high performance device applications. Since the first isolation of SnS via liquid phase exfoliation (LPE) (Brent et al., 2015; Sarkar et al., 2020; Sarkar et al., 2023; Sarkar and Stratakis, 2021), various synthesis methodologies have been explored to produce few- and mono-layer SnS (FL/1L-SnS) structures. Conventional LPE methods have been effective in isolating FL-SnS sheets, while non-conventional approaches such as thermally assisted LPE (T-LPE) have been developed to overcome the strong interlayer binding energy, (Sarkar et al., 2023), enabling the 1L-SnS (Figure 1d). However, the lateral dimensions of these monolayers are typically limited to the micrometer scale due to the buckled crystal structure of SnS and the non-uniform shear forces inherent in the exfoliation process. In contrast, vapor-phase techniques such as chemical vapor deposition (CVD) have demonstrated significant potential in producing ultrathin SnS layers with improved crystallinity and uniformity. Monolayer and few-layer SnS have been successfully synthesized using these method (Figure 1e) (Koyama et al., 2025), although challenges remain in achieving consistent layer thickness and large area coverage. The anisotropic crystal structure of SnS, coupled with the differing electron affinities of its constituent atoms, further complicates the growth of high-quality monolayers, often resulting in the presence of crystal defects and residual impurities.

Among all techniques, mechanical exfoliation (ME) of bulk crystals continues to produce the highest purity, electronic grade monolayers. Scotch tape is primarily used to break the weak van der Waals (vdW) bonds between layers for mechanical exfoliation. However, its low exfoliation yield and poor reproducibility render it a non-conventional method. In the case of SnS, the stronger interlayer binding energy compared to conventional van der Waals materials such as TMDs makes mechanical exfoliation particularly challenging. Recent advances in metal assisted mechanical exfoliation (M-ME) have shown notable improvements in yield and layer uniformity. Notably, Rao co-workers demonstrated a M-ME approach that successfully produced monolayer SnS with high structural integrity (Nicolas et al., 2025). Other emerging methods such as chemical synthesis (Li F. et al., 2019), liquid metal (Krishnamurthi et al., 2020), Plasma-induced (Kim et al., 2018), and laser-induced synthesis (Averchenko et al., 2024) are also being actively explored to produce high-quality SnS thin layers. These approaches continue to evolve, aiming to optimize both scalability and material performance for integration into next-generation quantum and optoelectronic devices.

The synthesized anisotropic SnS structures have been critically investigated using various advanced characterization techniques. Initial exfoliation efforts were primarily limited to obtaining few-layer SnS. However, M-ME successfully overcame the strong interlayer forces, enabling isolation down to the monolayer level (Figure 1g). CVD technique by contrast (Sutter et al., 2020; Zhang et al., 2023; Koyama et al., 2025), have generally been limited to producing multilayer films (Figure 1h), as the inherent anisotropy in atomic arrangement restricts the nucleation of tin and sulfur atoms along the AC/ZZ directions. Koyama et al. (2025) addressed this challenge by varying the sulfur vapor concentration relative to tin, using high purity elemental precursors. As a result, they achieved the growth of monolayer SnS crystals (Figure 1i) with lateral dimensions reaching several tens of micrometers. This large scale monolayer growth was enabled by carefully controlling the sublimation rate of bulk SnS crystals.

The wavy nature of the SnS crystal structure has been revealed using scanning tunneling microscopy (STM). Cross-sectional and top-view imaging distinctly highlight the anisotropic structure of monolayer SnS. Notably, the thermal expansion coefficients of SnS are significantly higher than those of conventional TMDs, which facilitates monolayer isolation via T-LPE. The resulting monolayers (Figure 1j) are highly crystalline and exhibit an orthorhombic crystal structure (Figures 1k–m) (Sarkar et al., 2020). The intrinsic anisotropy along the AC/ZZ directions is further confirmed through AFM images. In addition, a variety of optics-based characterization methods have been employed to investigate the anisotropy in SnS. For example, polarized-absorption spectroscopy (Nicolas et al., 2025), Raman spectroscopy (Tian et al., 2017), second harmonic generation (SHG) spectroscopy (Chang et al., 2023), and polarization-resolved optical microscopy (Zhu et al., 2021; Maragkakis et al., 2022) have been utilized to explore the unique anisotropic optoelectronic properties of SnS.

Anisotropic absorption spectra of SnS were acquired under illumination with linearly polarized light along the x- and y- axes. The spectra reveal distinct absorption edges corresponding to the material’s anisotropic band structure, with bandgap energies determined to be 1.173 eV and 1.147 eV, respectively (Chen et al., 2018). Another key technique used to probe anisotropy in SnS is polarized Raman spectroscopy (PRS) (Cong et al., 2020; Gong et al., 2022; Zhao et al., 2024). In this method, the Raman signal intensity (I) is given by the expression I ∞ | e i R e s T 2 , where e i and e_s are the polarization vectors of the incident and scattered light, respectively. As the SnS crystal adopts an orthorhombic structure with C _2ν point group symmetry. This symmetry gives rise to 24 phonon modes at the Brillouin zone center (Γ point), which are classified as Γ = 4 A g + 2 B 1 g + 4 B 2 g + 2 B 3 g + 2 A u + 4 B 1 u + 2 B 2 u + 4 B 3 u (3) where the A g , B 1 g , B 2 g , and B 3 g are the optically active Raman modes. Among all Raman-active modes in SnS (Equation 3), the A _g and B_3g modes are optically active and prominently observed in polarized Raman measurements. SnS belongs to the orthorhombic crystal system, within this symmetry, the optically active Raman tensors for the A _g and B_3g modes are well-defined and govern the PRS scattering (Figures 1n, o). The intensity of these Raman modes under various polarization configurations provides insight into the crystal orientation and anisotropy of the sample. In particular, the angular dependence of the A _g and B _3g mode intensities can be used to distinguish between the AC/ZZ crystallographic directions of the SnS flake.

SHG is a nonlinear optical process that occurs in a non-centrosymmetric crystal. It arises in materials where the electric polarization (P) responds nonlinearly to an applied electric field (E) and described by Equation 4. P t = ϵ 0 χ 1 E t + ϵ 0 χ 2 E 2 t + … . . (4) where, ϵ 0 is free space permittivity, χ 1 and χ 2 is the linear (first order), and nonlinear (2 ^nd order) susceptibility of the material, respectively. To investigate the anisotropic nature of SnS structure particularly along the AC/ZZ directions P-SHG measurements are employed. These measurements provide insight into the crystallographic orientation and symmetry-dependent nonlinear optical response of SnS.

SHG spectroscopy of SnS layers deposited on highly oriented pyrolytic graphite (HOPG) has been investigated to explore their anisotropic optical properties (Figure 1p) (Moqbel et al., 2024). Due to the inherently anisotropic crystal structure of SnS, SHG exhibits a strong angular dependence when excited with linearly polarized light (Figure 1p). Chang et al. (2023) demonstrated polarization resolved SHG (P-SHG) (Figure 1q), where both the incident and detected SHG signals are linearly polarized. The resulting SHG polar plots from FL-SnS showed excellent agreement with theoretical predictions, confirming the distinct optical response along the AC/ZZ crystallographic directions. Beyond conventional SHG spectroscopy, Maragkakis et al. (2022) performed P-SHG microscopy on randomly oriented SnS flakes dispersed on a transparent substrate. Figure 1r presents representative P-SHG images of multiple SnS flakes within the same field of view. Each flake is labeled numerically to examine the polarization dependent anisotropic response and to identify the AC/ZZ orientations. In this study, P-SHG images were recorded as a function of the linear polarization angle (ϕ) of the excitation laser. Regions of interest (ROIs) within individual SnS flakes reflect the angular dependence of the χ 2 tensor with respect to the crystallographic axes. Consequently, the polar plots of P-SHG intensity are directly correlated with the flake’s crystal orientation, allowing for precise identification of the AC/ZZ directions. Furthermore, SHG measurements from large-area monolayer SnS samples have also been employed to determine crystal orientation, as shown in Figure 1s. These findings reinforce the effectiveness of P-SHG as a non-invasive optical tool to probe anisotropic nonlinear responses and to resolve in-plane crystallographic axes in 2D materials.

The recent development of ultrathin SnS layers has generated significant interest due to their rich physical properties and potential for various emerging applications. In particular, the structural anisotropy arising from the puckered structure with low crystal symmetry of SnS makes it highly promising for advanced electronic and optical technologies, including next-generation quantum applications.

In summary, this perspective focuses on anisotropic 2D tin sulfide. In particular, it discussed the anisotropic nature of SnS, its properties, and their fundamentals. Conventional synthesis routes along with the state-of-the-art anisotropic characterization methodologies are highlighted. Emerging electronic device applications are extensively explored, including those in energy, optoelectronics, spintronics, and next-generation quantum-based technologies.

Recent advancements have significantly improved the quality of SnS synthesized through various methods. Techniques such as M-ME, T-LPE, and CVD have made impactful contributions toward achieving ultrathin SnS layers down to the monolayer level. These thin layers exhibit remarkable anisotropic properties, opening pathways for next-generation fundamental studies and technological applications. However, the synthesis of wafer-scale, large-area, highly anisotropic monolayers remains a challenge. This may be achieved through systematic optimization of anisotropic growth along the AC and ZZ directions particularly by precisely controlling the chemical reactions involved in CVD or PVD processes.

The low crystal symmetry and intrinsic anisotropy of SnS give rise to rich electronic and optical properties, which are reflected in a wide range of emerging device applications. SnS has opened new pathways for energy conversion technologies, including the bulk photovoltaic effect, thermoelectric devices, CO₂ conversion, and spin-charge conversion. Moreover, SnS-based synaptic devices are playing a significant role in neuromorphic computing and artificial intelligence applications. These advancements position SnS as a promising material in the development of next-generation quantum technologies. The exceptional anisotropic fundamental properties make it highly suitable for polarized detectors, neuromorphic and synaptic devices, and spin-valley device applications.

The extraordinary properties and rich underlying physics of anisotropic SnS stem from its exceptional crystal symmetry. These unique characteristics are reflected in a range of emerging device applications, particularly in the realm of quantum technologies. For example, its pronounced nonlinear optical properties such as high SHG efficiency make it highly suitable for SPDC and the generation of EPPs. These EPPs play a pivotal role in various quantum applications, including quantum-communication, cryptography, computing, sensing, and imaging. Overall, the anisotropic nature and exceptional fundamental properties of SnS hold great promise for the development of next-generation anisotropic electronic devices, especially in the rapidly evolving field of quantum technology.