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 780nm. 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=5.74x10(19) N/m(3) for the interlayer breathing mode, and sound velocity of v=8.27x10(4)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.

Automated summary

In this study, the photocarrier dynamics and thickness dependent interlayer coherent phonon modes in PdSe2 films were systematically investigated using ultrafast optical pump-probe spectroscopy. The analysis based on a simple one-dimensional chain model revealed important parameters for the interlayer force constant and sound velocity in PdSe2 films. These experimental findings pave the way for designing and developing optoelectronic and nanomechanic devices based on PdSe2.