Layer dependent interlayer coherent phonon dynamics in PdSe₂ films

The layered transition metal dichalcogenide has attracted tremendous attention for its unique structure and electrical and optoelectronic properties. As an emerging two-dimensional material, PdSe₂ plays a key role in optoelectronic applications due to its distinct optical and tunable electrical properties. The carrier dynamic and low-frequency phonon modes and how they evolve with the number of layers are important for future device fabrication in photonics, optoelectronics, and nanomechanics. Here, by employing ultrafast optical pump-probe spectroscopy, we have investigated systematically the photocarrier dynamics as well as the thickness dependent interlayer coherent phonon modes in PdSe₂ films. Two low-frequency phonon modes in PdSe₂ films are identified after photoexcitation at 780 nm. The higher-frequency mode is ascribed to the interlayer breathing mode, and the lower one is assigned to the standing wave mode, and both of the mode frequencies decrease with increasing the number of layers of films. Analysis based on simple one-dimensional chain model produces interlayer force constant K = 5.74 × 10¹⁹ N/m³ for the interlayer breathing mode, and sound velocity of v = 8.27 × 10⁴ cm/s for the standing wave mode in PdSe₂ film. Our experimental finding paves the way for designing and developing PdSe₂-based optoelectronic and nanomechanic devices.