News Resource& Photographer: School of Physics

Editor: News Agency of BIT

Translator: Chen Jianan, News Agency of BIT

Transition metal phosphate-sulfur compounds (TMPCs) represent a vast class of two-dimensional layered materials comprising three or more elements, typically exhibiting a quasi-hexagonal structure akin to CdCl₂ crystals. These compounds have garnered significant research attention due to their distinctive electronic structure and exceptional optical, electronic, photoelectric, magnetic, and catalytic properties. However, the majority of studies on 2d TMPCs have primarily relied on liquid phase stripping or mechanical stripping of large crystals. Thus far, achieving direct synthesis of ultra-thin TMPCs through chemical vapor deposition (CVD) methods has proven to be challenging. Firstly, TMPCs are commonly composed of transition metals (TM), phosphorus (P), and sulfur/selenium (X), with interlayer interactions governed by van der Waals forces. Metal sources P and S/Se are typically employed as precursors; however, P exhibits high chemical reactivity and possesses abundant valence states that can lead to the formation of various compounds such as MP, PaXb in addition to TMPCs. Consequently, controlling the preparation process for TMPCs becomes difficult. Secondly, the uncontrolled co-volatilization of these three precursors further complicates the synthesis procedure for terpolymer-based TMPCs compared to binary materials. Therefore, there is an urgent need for developing a universal synthesis method capable of producing high-quality crystals in order to facilitate comprehensive investigations into their properties and applications.

In light of the aforementioned issues, Professor Zhou Jiadong and doctoral student Yang Yang from the School of Physics at Beijing Institute of Technology proposed a growth mechanism based on element reduction, leading to successful controllable preparation of transition metal phosphorous chalcogenides (TMPCs) using the chemical vapor deposition (CVD) method. Various TMPCs such as FePS₃, FePSe₃, MnPS₃, MnPSe₃, CdPS₃, CdPSe₃, In₂P₃S₉ and SnPS₃ were synthesized and further characterized through Raman spectroscopy, second harmonic generation (SHG), and scanning transmission electron exemplary TMPCs-SnPS₃ exhibited a robust SHG signal at 1064 nm with an effective nonlinear magnetic susceptibilityχ(2) of 8.41´10⁻¹¹ m V^-1. The CdPSe3 photodetector demonstrated high responsiveness with a value of 582 mA/W, an impressive detection rate reaching 3.19´10¹¹ Jones, and a rapid rise time of 611 ms—outperforming most previously reported phospho-sulfur compound photodetectors. These significant findings are presented in "A universal strategy for synthesis of 2D ternary transition metal phosphorous chalcogenides", which was published on September 30th in Advanced Materials—a prestigious international academic journal.

Fig.1: Controllable synthesis of diverse TMPCs achieved through the mechanism of elemental reduction

In order to address the aforementioned challenges, this study directly utilizes P₄Se₃/ P₂S₅ and MClx as precursors to reduce the number of precursor types. By simplifying the synthesis process from ternary reactions to binary reactions, it enables the universal growth of multiple ternary TMPCs. The selection of P₄Se₃ and P₂S₅ as suitable precursors is based on their low melting point, high reactivity, and appropriate element composition. This strategy not only effectively avoids the challenge of co-volatilization of precursors in traditional CVD methods but also reduces competitive reactions among multiple precursors, thereby achieving the growth of ternary TMPCs.

Fig.2: Basic characterization of typical TMPCs-SnPS₃

Taking SnPS₃ as an example, the sample exhibits a trapezoidal shape with a transverse size of approximately 30 μm and a thickness of 10.3 nm. Raman spectra have revealed the presence of nine peaks: P1 (38.9 cm^-1), P2 (57.8 cm^-1), P3 (72.1 cm^-1), P4 (120.5 cm^-1), P5 (160.9 cm^-1), P6 (192.8 cm^-1), P7 (248.7 cm^-1), P8 (381.3 cm^-1), and P9 (560.1 cm^-1). Among these peaks, it is noteworthy that Ag mode includes P1, P2, P5 and P7 while Bg mode comprises of P3, P4 and P9. Investigations have shown that all Raman peaks exhibit angle-dependent characteristics. In terms of configuration orientation, when in parallel alignment, six peaks—P1, P2, P3, P5, P7 and P8—displayed a four-fold intensity pattern whereas three other peaks—P4, P6 and P9—exhibited a two-fold intensity pattern. On the other hand, in vertical alignment, eight out of the nine Raman peaks exhibited a four-fold pattern except for peak number six(P6). This phenomenon can be explained by considering the classical Placzek approximate strength which is proportional to |ei•R•es|2 where ei represents the unit polarization vector of incident laser, represents the Raman tensor, and es represents either unit polarization vector for parallel or vertically scattered Raman signal. Furthermore, the consistency between X-ray photoelectron spectroscopy (XPS) experiments and fitting data further confirms the high crystal quality possessed by SnPS₃.

Fig.3: Atomic structure of a typical TMPCs-SnPS₃

The synthesized SnPS3 was further characterized using energy dispersive X-ray spectroscopy (EDX) and high-angle toroidal dark field scanning transmission electron microscopy (HAADF-STEM). The EDX analysis confirmed a stoichiometric ratio of Sn:P:S = 1.1:1:2.95. Moreover, the HAADF-STEM cross-section revealed a well-defined crystalline structure without any atomic vacancies. Additionally, selective electron diffraction (SAED) analysis determined the crystal plane spacing to be 4.89 Å for (002) and 5.50 Å for (100). The periodic variation in HAADF-STEM atomic strength distribution provided insights into the arrangement of Sn, P, and S atoms, with a measured Sn-Sn atomic bond length of 5.80 Å. These findings highlight the high complexity and low symmetry of SnPS3, making it an ideal candidate material for investigating nonlinear optical (NLO) properties.

Fig.4: Nonlinear optical properties of a typical TMPCs - SnPS₃

SHG is derived from the second-order NLO polarization of the material in the incident light field, and is an effective technique to identify the crystal structure of TMPCs. Specifically, when photons with a frequency of w interact with the NLO material, they generate light radiation at 2w frequency. This study focuses on analyzing SHG measurement under a 1064 nm laser (15 ps, 80 MHz). In power-dependent SHG spectrum analysis, a single peak at 532 nm is clearly observed and its slope value of 1.91 linearly fits to the logarithmic coordinates of SHG intensity closely conforming to theoretical value 2, which characterizes second-order NLO process.

In formula (1), ε0 is the dielectric constant, Pω and P2ω are the excited laser power and SHG power, respectively, d is the sample thickness, c is the speed of light in vacuum, A is the incident laser spot area, and nω and n2ω are the linear refractive index of the sample at the fundamental frequency and the subharmonic frequency, respectively. The effective SHG susceptibility χ(2) of SnPS3, calculated using formula (1), is 8.41×10-11 m V-1. This value is approximately eight times higher than the effective SHG susceptibility χ(2) (1.12×10-11 m V-1) observed in single-layer MoS2 under identical experimental conditions, indicating a significantly greater magnitude compared to numerous reported two-dimensional nonlinear materials.

Fig.5: Photoelectric properties of typical TMPCs-CdPSe₃ and SnPS₃

In order to further investigate its photoelectric properties, photodetectors based on CdPSe₃ and SnPS₃ nanosheets were fabricated. All experiments were conducted using a 520 nm laser. The logarithmic representation of the power-dependent photocurrent of CdPSe₃ exhibits a positive correlation with the power density at various intensities. It can be well described by Iph≈Pθ, where θ represents the index associated with light response intensity. Moreover, linear dynamic range (LDR) serves as a performance characterizing the range of light intensities within which the photodetector maintains a consistent response rate. The formula LDR = 20log(Iph/Idark) yields an impressive photosensitive LDR value of 120 dB, significantly surpassing the values of 66 dB for commercial InGaAs photodetectors and 62 dB for other two-dimensional materials like GaPS₄. The responsivity of the CdPSe₃-based photodetector can reach up to 582 mA/W, corresponding to a specific detectivity rate of 3.19´10¹¹ Jones, demonstrating excellent detection capability in visible light range. Furthermore, it exhibits superior rise time and decay time values of 611 ms and 716 ms respectively compared to most other reported two-dimensional TMPCs photodetPSe₃ and SnPS₃ photodetectors exhibit higher switching ratios than other reported two-dimensional materials due to their ultra-low dark currents.

This work not only paves a new pathway for the synthesis of TMPCs, but also demonstrates a universal growth strategy that is of significant importance for understanding the growth mechanism of these materials. Two-dimensional ternary crystals prepared based on the proposed strategy hold great potential for exploring novel properties, including nonlinear optics, ferromagnetism, optoelectronics, and spintronics.

The research presented in this work has received support from the National Key Research and Development Program, the National Natural Science Foundation of China (NSFC), the Innovation Fund of Beijing Institute of Technology, the project funded by Beijing University of Posts and Telecommunications, as well as the project funded by China Postdoctoral Science Foundation.

DOI：10.1021/adma202307237