Ultrastrong Optical Harmonic Generations in Layered Platinum Disulfide in the Mid-Infrared

Nonlinear optical activities (e.g., harmonic generations) in two-dimensional (2D) layered materials have attracted much attention due to the great promise in diverse optoelectronic applications such as nonlinear optical modulators, nonreciprocal optical device, and nonlinear optical imaging. Exploration of nonlinear optical response (e.g., frequency conversion) in the infrared, especially the mid-infrared (MIR) region, is highly desirable for ultrafast MIR laser applications ranging from tunable MIR coherent sources, MIR supercontinuum generation, and MIR frequency-comb-based spectroscopy to high harmonic generation. However, nonlinear optical effects in 2D layered materials under MIR pump are rarely reported, mainly due to the lack of suitable 2D layered materials. Van der Waals layered platinum disulfide (PtS₂) with a sizable bandgap from the visible to the infrared region is a promising candidate for realizing MIR nonlinear optical devices. In this work, we investigate the nonlinear optical properties including third-and fifth-harmonic generation (THG and FHG) in thin layered PtS₂ under infrared pump (1550-2510 nm). Strikingly, the ultrastrong third-order nonlinear susceptibility χ⁽³⁾(-3ω;ω,ω,ω) of thin layered PtS₂ in the MIR region was estimated to be over 10^-18 m²/V², which is about one order of that in traditional transition metal chalcogenides. Such excellent performance makes air-stable PtS₂ a potential candidate for developing next-generation MIR nonlinear photonic devices.

Strategy for Fabricating Wafer-Scale Platinum Disulfide

PtS₂ is a newly developed group 10 2D layered material with high carrier mobility, wide band gap tunability, strongly bound excitons, symmetrical metallic and magnetic edge states, and ambient stability, making it attractive in nanoelectronic, optoelectronic, and spintronic fields. To the aim of application, a large-scale synthesis is necessary. For transition-metal dichalcogenide (TMD) compounds, a thermally assisted conversion method has been widely used to fabricate wafer-scale thin films. However, PtS₂ cannot be easily synthesized using the method, as the tetragonal PtS phase is more stable. Here, we use a specified quartz part to locally increase the vapor pressure of sulfur in a chemical vapor deposition furnace and successfully extend this method for the synthesis of PtS₂ thin films in a scalable and controllable manner. Moreover, the PtS and PtS₂ phases can be interchangeably converted through a proposed strategy. Field-effect transistor characterization and photocurrent measurements suggest that PtS₂ is an ambipolar semiconductor with a narrow band gap. Moreover, PtS₂ also shows excellent gas-sensing performance with a detection limit of ∼0.4 ppb for NO₂. Our work presents a relatively simple way of synthesizing PtS₂ thin films and demonstrates their promise for high-performance ultrasensitive gas sensing, broadband optoelectronics, and nanoelectronics in a scalable manner. Furthermore, the proposed strategy is applicable for making other PtX₂ compounds and TMDs which are compatible with modern silicon technologies.

Extraordinarily Strong Interlayer Interaction in 2D Layered PtS2

Platinum disulfide (PtS2 ), a new member of the group-10 transition-metal dichalcogenides, is studied experimentally and theoretically. The indirect bandgap of PtS2 can be drastically tuned from 1.6 eV (monolayer) to 0.25 eV (bulk counterpart), and the interlayer mechanical coupling is almost isotropic. It can be explained by strongly interlayer interaction from the pz orbital hybridization of S atoms.