NiPS3 nanoflakes: a nonlinear optical material for ultrafast photonics

All-optical diode based on asymmetric nonlinear optical absorption in ternary transition metal chalcogenides

As the typical representative of ternary transition metal chalcogenides, CrPS4 and MnPS3 exhibit unique light-matter interactions, demonstrating great potential in photonic devices. In this work, we systematically studied the nonlinear optical (NLO) responses of CrPS4 and MnPS3 flakes by the I-scan technique. CrPS4 and MnPS3 flakes show saturable absorption and reverse saturation absorption excited by fs laser at wavelengths of 600 nm, respectively. Furthermore, utilizing the non-degenerate transient absorption technology, we observed that the fast and slow carrier relaxation times of CrPS4 at different probe wavelengths were components with constants of about 1.4-3 ps and 490-1420 ps, respectively. Their excellent ultrafast NLO properties imply that they can be applied in photoelectronic fields for advanced and functional devices. Here, we designed and demonstrated an all-optical diode based on a CrPS4/MnPS3 tandem structure by breaking the time-reversal symmetry, and it achieved nonreciprocal transmission of light similar to that of a p-n electron diode.

Laser-induced hole coherence in 2D antiferromagnet MPS3 through spatial self-phase modulation

Transition metal phosphorus sulfides (MPS3), a family of two-dimensional magnetic materials with a van der Waals structure, exhibit promising applications in nonlinear optical devices. The emergence of carrier coherence in MPS3 is a fascinating topic in coherently controlling the nonlinear effect (or other novel phenomena). Herein, we systematically investigated the third-order nonlinear optical responses of MPS3 (M = Ni, Fe, Mn) flake suspensions based on spatial self-phase modulation (SSPM) effect. The effective monolayer third-order nonlinear susceptibilities (χmonolayer(3)) of NiPS3 and MnPS3 are obtained for the first time at multiple wavelengths. Our results show that NiPS3 has a higher χmonolayer(3) value (3.59 × 10-9 e.s.u. or 5.01 × 10-17 m2V-2 at 405 nm excitation) than those of FePS3 and MnPS3. Furthermore, we laser-induced non-local hole coherence in MPS3 based on SSPM, where the origin of excited-state holes is analyzed from electronic structures. The relationships between hole mobility μ hole, effective mass m h∗, and χmonolayer(3) for the three materials fulfill the previous investigation results. Because laser-induced hole coherence has rarely been reported, our investigation enriches the coherent regulation of two-dimensional magnetic MPS3 materials, enabling potential applications in all-optical devices.

Q-switched Nd:YVO4 laser operating at 1064 nm with NiPS3 nanoflakes onto a silica metasurface as saturable absorbers

In this work, we show that a metasurface can be used to improve the performance of the two-dimensional (2D) material saturable absorber in a Nd:YVO4 solid-state laser. To our knowledge, the hybrid saturable absorber was fabricated by spraying the NiPS3 nanoflakes onto a silica metasurface for the first time. It is shown that the optical absorption, modulation depth, saturation intensity, and ultrafast recovery time of the metasurface-NiPS3 saturable absorber exhibit better performance than the 2D material control device. In a proof-of-concept experiment, the Q-switched pulses with a pulse duration of 20.5 ns, repetition rate of 4.35 MHz, output power of 2.3 W, peak power of 30.61 W, and pulse energy of 0.63 μJ were experimentally demonstrated. These findings suggest that a hybrid saturable absorber is a promising candidate for developing pulsed laser and optical modulators.

A Universal Strategy for Synthesis of 2D Ternary Transition Metal Phosphorous Chalcogenides

The 2D ternary transition metal phosphorous chalcogenides (TMPCs) have attracted extensive research interest due to their widely tunable band gap, rich electronic properties, inherent magnetic and ferroelectric properties. However, the synthesis of TMPCs via chemical vapor deposition (CVD) is still challenging since it is difficult to control reactions among multi-precursors. Here, a subtractive element growth mechanism is proposed to controllably synthesize the TMPCs. Based on the growth mechanism, the TMPCs including FePS3 , FePSe3 , MnPS3 , MnPSe3 , CdPS3 , CdPSe3 , In2 P3 S9 , and SnPS3 are achieved successfully and further confirmed by Raman, second-harmonic generation (SHG), and scanning transmission electron microscopy (STEM). The typical TMPCs-SnPS3 shows a strong SHG signal at 1064 nm, with an effective nonlinear susceptibility χ(2) of 8.41 × 10-11  m V-1 , which is about 8 times of that in MoS2 . And the photodetector based on CdPSe3 exhibits superior detection performances with responsivity of 582 mA W-1 , high detectivity of 3.19 × 1011  Jones, and fast rise time of 611 µs, which is better than most previously reported TMPCs-based photodetectors. These results demonstrate the high quality of TMPCs and promote the exploration of the optical properties of 2D TMPCs for their applications in optoelectronics.

Nonlinear optical properties and photoexcited carrier dynamics of MnPS3 nanosheets

Here, we systematically report on the preparation of high-quality few-layered MnPS₃ nanosheets (NSs) by chemical vapor transport (CVT) and mechanical stripping method, and its carrier dynamics and third-order nonlinear optical properties were studied. Using the classical technique of open aperture Z-scan, a typical phenomenon of saturable absorption (SA) was observed at 475 nm, which indicates that the material is expected to be used as a saturable absorber in ultrafast lasers. The typical phenomenon of reverse saturation absorption (RSA) is observed at 800 and 1550 nm, which shows its potential in the field of broadband optical limiting. Compared with graphene, BP, MXene, MoS₂ and other typical two-dimensional materials, MnPS₃ NSs has a higher modulation depth. Using the non-degenerate transient absorption spectroscopy technology at room temperature, a slower cooling process of thermal carrier of MnPS₃ was observed. Moreover, the carrier lifetime can be tuned according to the wavelength. This work is of great significance to the improvement of MnPS₃ based devices, and lays a foundation for the application of MnPS₃ in short-wavelength photovoltaic cell, photoelectric detection and other fields.

Controllable Synthesis of Narrow-Gap van der Waals Semiconductor Nb2GeTe4 with Asymmetric Architecture for Ultrafast Photonics

Ultrafast photonics has become an interdisciplinary topic of great consequence due to the spectacular progress of compact and efficient ultrafast pulse generation. Wide spectrum bandwidth is the key element for ultrafast pulse generation due to the Fourier transform limitation. Herein, monoclinic Nb₂GeTe₄, an emerging class of ternary narrow-gap semiconductors, was used as a real saturable absorber (SA), which manifests superior wide-range optical absorption. The crystallization form and growth mechanism of Nb₂GeTe₄ were revealed by a thermodynamic phase diagram. Furthermore, the Nb₂GeTe₄-SA showed reliable saturation intensity and larger modulation depth, ascribed to a built-in electric field driven by the asymmetric crystal architecture confirmed via X-ray diffraction, polarized Raman spectra, and scanning transmission electron microscopy. Based on the Nb₂GeTe₄-SA, femtosecond mode-locked operation with good overall performance was achieved by a properly designed ring cavity. These results suggest that Nb₂GeTe₄ shows great promise for ultrafast photonic applications and arouse interests in exploring the intriguing properties of the ternary van der Waals material family.

High-Stability Hybrid Organic-Inorganic Perovskite (CH3NH3PbBr3) in SiO2 Mesopores: Nonlinear Optics and Applications for Q-Switching Laser Operation

Hybrid organic-inorganic perovskite shows a great potential in the field of photoelectrics. Embedding methyl ammonium lead bromide (MAPbBr₃) in a mesoporous silica (mSiO₂) layer is an effective method for maintaining optical performance of MAPbBr₃ at room temperature. In this work, we synthesized MAPbBr₃ quantum dots, embedding them in the mSiO₂ layer. The nonlinear optical responses of this composite thin film have been investigated by using the Z-scan technique at a wavelength of 800 nm. The results show plural nonlinear responses in different intensities, corresponding to one- and two-photon processing. Our results support that composites possess saturation intensity of ~27.29 GW/cm² and varying nonlinear coefficients. The composite thin films show high stability under ultrafast laser irradiating. By employing the composite as a saturable absorber, a passively Q-switching laser has been achieved on a Nd:YVO₄ all-solid-state laser platform to generate a laser at ~1 μm.

Dual Vacancies Confined in Nickel Phosphosulfide Nanosheets Enabling Robust Overall Water Splitting

Exploring highly efficient electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is of great significance for addressing energy and environmental crises. Vacancy engineering has been regarded as a promising way to optimize the catalytic activity of electrocatalysts. Herein, we put forward a conceptually new dual Ni,S vacancy engineering on 2D NiPS₃ nanosheet (denoted as V-NiPS₃ ) by a simple ball-milling treatment with ultrasonication. This material presents an ideal model for exploring the role of dual vacancies in improving the catalytic activity for overall water splitting. Structural analyses make clear that the formation of dual Ni,S vacancies regulates the electronic structure and catalytic active sites of NiPS₃ nanosheet, leading to the superior HER/OER performance. Smaller overpotentials of 124 mV and 290 mV can be achieved at a current density of 10 mA cm^-2 for HER and OER, respectively. The OER performance of V-NiPS₃ is the best value among all state-of-the-art NiPS₃ catalysts. In addition, the assembled two-electrode cell incorporating V-NiPS₃ exhibits enhanced catalytic performance with a low cell voltage of 1.60 V at 10 mA cm^-2 . This work offers a promising avenue to improve the electrocatalytic performance of the catalysts by engineering dual vacancies for large-scale water splitting.

Phase Transitions and Water Splitting Applications of 2D Transition Metal Dichalcogenides and Metal Phosphorous Trichalcogenides

2D layered materials turn out to be the most attractive hotspot in materials for their unique physical and chemical properties. A special class of 2D layered material refers to materials exhibiting phase transition based on environment variables. Among these materials, transition metal dichalcogenides (TMDs) act as a promising alternative for their unique combination of atomic-scale thickness, direct bandgap, significant spin-orbit coupling and prominent electronic and mechanical properties, enabling them to be applied for fundamental studies as catalyst materials. Metal phosphorous trichalcogenides (MPTs), as another potential catalytic 2D phase transition material, have been employed for their unusual intercalation behavior and electrochemical properties, which act as a secondary electrode in lithium batteries. The preparation of 2D TMD and MPT materials has been extensively conducted by engineering their intrinsic structures at the atomic scale. In this study, advanced synthesis methods of preparing 2D TMD and MPT materials are tested, and their properties are investigated, with stress placed on their phase transition. The surge of this type of report is associated with water-splitting catalysis and other catalytic purposes. This study aims to be a guideline to explore the mentioned 2D TMD and MPT materials for their catalytic applications.

Recent Advances in 2D Layered Phosphorous Compounds

2D layered phosphorous compounds (2D LPCs) have led to explosion of research interest in recent years. With the diversity of valence states of phosphorus, 2D LPCs exist in various material types and possess many novel physical and chemical properties. These properties, including widely adjustable range of bandgap, diverse electronic properties covering metal, semimetal, semiconductor and insulator, together with inherent magnetism and ferroelectricity at atomic level, render 2D LPCs greatly promising in the applications of electronics, spintronics, broad-spectrum optoelectronics, high-performance catalysts, and energy storage, etc. In this review, the recently research progress of 2D LPCs are presented in detail. First, the 2D LPCs are classified according to their elemental composition and the corresponding crystal structures are introduced, followed by their preparation methods. Then, the novel properties are summarized and the potential applications are discussed in detail. Finally, the conclusion and perspective of the promising 2D LPCs are discussed on the foundation of the latest research progress.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

Electric field controlled near-infrared high-speed electro-optic switching modulator integrated with 2D MgO

The electro-optic effect in two-dimensional (2D) MgO nanoflakes synthesized by a microwave-assisted process is demonstrated using a designed optical fiber modulator. The guiding properties of intense core modes excited by the material cavity are modulated by the external electric field. The feasibility of 2D MgO nanoflakes as an effective electro-optic modulator and switching are experimentally verified for the first time, to the best of our knowledge. The proposed optical-fiber-based electro-optic modulator achieves a linear wavelength shift with a high sensitivity of 12.87 pm/V(77.22 nm/kV/mm, in the electric field). The results show that MgO, as a metal oxide 2D material, is a very promising material for electro-optic modulators and switching.