Scanning near-field optical microscopy study of PtSe₂ flakes

Abstract

Scientists are exploring the mid-infrared (MIR) optoelectronic properties of two-dimensional (2D) materials. Noble metal (Group 10) dichalcogenides MX₂ (M = Pt, Pd, X = S, Se, etc.) are the potential candidates for MIR applications due to their matching bandgaps and high stability. PtSe₂ is an emerging 2D material for MIR optoelectronics and photonics due to its air stability, high electron mobility and layer-dependent band gap. Inspired by the conventional-FTIR spectra of PtSe₂ flakes, which reveals the waveguide supporting properties of PtSe₂, scanning near-field optical microscopy (s-SNOM) and nano-Fourier Transform Infrared Spectroscopy (nano-FTIR) were employed to study the mechanically exfoliated PtSe₂ flakes. Like pervious study on waveguide supporting TMDCs such as WSe₂, MoSe₂ and MoS₂, interference fringe patterns were observed in real space by the use of s-SNOM. The fringes were induced by the MIR waveguide photon modes of PtSe₂, in contrast to previous studies, in where visible light was used. The full-range edge orientation dependence of the fringe patterns was studied for the first time. By tuning the MIR frequency from 1380 to 2519 cm-1, the dispersion relation of the waveguide modes with different thicknesses can be extracted. Theoretical calculations, based on the dielectric constant of PtSe₂ obtained by conventional-FTIR, are in good agreement with the experimental results. Interestingly, in addition to the commonly observed (transverse magnetic) TM modes, the (transverse electric) TE modes were also visualised. This work reveals the applicability of s-SNOM in high-loss MIR waveguide mode imaging, in contrast to the previous studies, in which low-loss visible waveguide modes were observed. In addition, the dielectric constant of the PtSe₂ flakes in MIR range is validated.