Fig. 1: Crystallographic structure and optical characteristics of a few-layer PdSe₂.

a The lattice structure of PdSe₂ flakes with a puckered pentagonal. The unit cell vectors are denoted by the x, y, and z. Blue and yellow globes denote the Pd and Se atoms, respectively. b Top view of the atomic crystal structure of monolayer PdSe₂. The black dotted frame represents a unit cell of PdSe₂. The a and b denote the lattice constants along the x and y directions, respectively. c The optical microscope image of a few-layer PdSe₂ on the silicon substrate with a 285 nm SiO₂ layer. The numbers of sample layers containing 1–6 L were marked with white letters, and the scale bar is 20  μm. d Raman spectra of the 1–6 L and bulk PdSe₂ flakes with the same intensity axis. The low-frequency Raman peaks (0−100 cm⁻¹) were robust and even exceeded that of the high-frequency modes. The Raman vibration modes of bulk PdSe₂ were marked as A_g¹, A_g², A_g³, B_1g¹, B_1g², and B_1g³ mode.