Layer dependent interlayer coherent phonon dynamics in PdSe2 films

Abstract

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.

Ultrafast Dynamics of Defect-Assisted Auger Process in PdSe2 Films: Synergistic Interaction between Defect Trapping and Auger Effect

“…As reported by Jia et al , the large negative Kerr nonlinearity of 1.33 × 10 –15 m 2 /W for PdSe 2 films, which is two orders of magnitude greater than that of bulk Si, indicating that PdSe 2 films have a unique advantage in high-performance nonlinear photonic devices . Moreover, through the use of ultrafast optical pump–probe spectroscopy, the layer-dependent interlayer breathing mode, and the longitudinal coherent acoustic phonon mode in PdSe 2 films have been studied, and the interlayer force constant and sound velocity were obtained in our previous study . In general, the carrier dynamics in PdSe 2 and other TMD films is conventionally described as defect capture. − The detailed mechanisms, such as the roles of phonon, defect, and Auger process on the photocarriers, remain under debate and to be elucidated.…”

“…The samples used in our experiment were PdSe 2 films with 5, 10, 15, and 30 layers (denoted as 5L, 10L, 15L, and 30L, respectively), grown by chemical vapor deposition (CVD), and were transferred to a sapphire substrate (provided by sixcarbon Tech, Shenzhen, China). The fabrication details and sample characterization are provided in ref . Figure a shows the crystal structure of PdSe 2 ; apparently, PdSe 2 has a puckered pentagonal atomic structure, in which each layer of PdSe 2 consists of two planes of hexagonally arranged Se atoms linked to a hexagonal plane of Pd atoms via covalent bonds and individual PdSe 2 layers are held together by a weak van der Waals (vdW) force.…”

“…Figure a shows the crystal structure of PdSe 2 ; apparently, PdSe 2 has a puckered pentagonal atomic structure, in which each layer of PdSe 2 consists of two planes of hexagonally arranged Se atoms linked to a hexagonal plane of Pd atoms via covalent bonds and individual PdSe 2 layers are held together by a weak van der Waals (vdW) force. It is noted that the interlayer binding energies in PdSe 2 is as high as ∼190 meV/atom, resulting in the interlayer force constant being up to ∼9.57 N/m, so that the film shows highly robust stability in air; all of these qualities indicate the great potential application of PdSe 2 in optoelectronic devices.…”

“…From the UV–visible–IR absorption spectra, 1 μJ/cm 2 pump fluence at 780 nm is estimated to generate an electron (and hole) density of ∼3.08–6.94 × 10 11 cm –2 for 5–30L PdSe 2 . The measured THz transmission, Δ T / T 0 , can be expressed as where σ r (σ i ) is the real (imaginary) part of the conductivity of the sample in THz frequency; Δσ r (Δσ i ) is the change of the real (imaginary) conductivity caused by photoexcitation, i.e.…”

“…19 Moreover, through the use of ultrafast optical pump−probe spectroscopy, the layer-dependent interlayer breathing mode, and the longitudinal coherent acoustic phonon mode in PdSe 2 films have been studied, and the interlayer force constant and sound velocity were obtained in our previous study. 20 In general, the carrier dynamics in PdSe 2 and other TMD films is conventionally described as defect capture. 21−23 The detailed mechanisms, such as the roles of phonon, defect, and Auger process on the photocarriers, remain under debate and to be elucidated.…”