Ultrafast Photocarrier And Coherent Phonon Dynamics Of PdSe₂ Thin Films

The layered transition metal dichalcogenide（TMDs）have always been the focus of two-dimensional（2D）materials due to their unique electrical and optoelectronic properties.Among them,PdSe₂ standing out from other TMDs has been paid specific attention due to their novel electron band structure,high carrier mobility and good thermal conductivity.The pump-probe technology can directly obtain the dynamical transport process along time sequence following optical excitation in the materials,and expound the underlying mechanism of the dynamical phonon oscillation and nonequilibrium evolution of many-body interaction in the time domain.Moreover,the low energy of terahertz（THz）photon makes it very suitable for studying the relaxation dynamics of materials with optical excitation.In this thesis,the coherent phonon dynamics as well as transient carrier dynamics of PdSe₂ films have been studied by using degenerate time-resolved optical spectroscopy and time-resolved THz spectroscopy.The main research contents of this thesis include:（i）The photoexcited coherent phonon dynamics of PdSe₂ films have been systematically investigated at room temperature by using ultrafast time-resolved degenerate optical pump and optical probe spectroscopy.It was found that the relaxation process of PdSe₂ film is superimposed with phonon oscillations.The pure oscillation signal is then extracted from biexponential fitting,and the phonon modes in frequency domain are obtained by applying fast Fourier transformation in PdSe₂ film.We have identified two phonon modes in PdSe₂ films with Mode 1 of～0.316 THz and Mode 2 of～0.064 THz.The frequency of Mode 1 was found to be consistent with that of the lowest breathing mode by comparing it with the low-frequency Raman spectroscopy.Furthermore,the one-dimensional atomic chain model was used to fit the layer dependent Mode 1 frequency,and the fitting results further suggest that the Mode1 arises from the lowest breathing mode in PdSe₂ film,and the interlayer force constant was calculated to be K=5.74×10¹⁹ N/m³.The lifetime of Mode 2 is linearly proportional to the number of layers,which conforms to the longitudinal coherent phonon Mode.As a result,the sound velocity was then calculated to be v=8.27×10⁴cm/s.For 2D TMDs like PdSe₂ films,the low frequency phonon mode and its evolution with number of layers are of great significance for the fabrication of photonics,photoelectronics and nanomechanics devices in the future.（ii）The photoexcited carrier dynamics of PdSe₂ films with different layers were systematically studied by using ultrafast optical pump-THz probe spectroscopy.After the above band gap photoexcitation of 780 nm,the subsequent relaxation is composted of two processes:a 2.5 ps fast process and a 7.3 ps slow process.It is interesting that both processes show neither pump fluence nor temperature dependence,which strongly suggests that any single physical mechanism in PdSe₂ film does not play the role for the relaxation of photocarriers.Due to the strong Coulomb interactions and quantum confinement in 2D materials,the many-body interaction become efficient in 2D materials.In addition,due to the inevitable defects in the process of material growth,the role of defect capture in 2D materials cannot be ignored.Therefore,we proposed that the relaxation process of PdSe₂ film is dominated by the synergistic interaction between defect trapping and Auger effect.Layer dependent PdSe₂ film THz spectroscopy shows that the fast lifetime does not change with the number of layers,while the slow lifetime becomes faster with the increase of the layer number,a rate equation involving both defect trapping and Auger effect is used to fit the relaxation of PdSe₂ film,and the fitting parameters shows that the change of transient lifetime is dominated by slow defect density in PdSe₂ films with different layers,but not affected by the carrier occupancy in the defect state.The results reveal the synergistic interaction between defect trapping and Auger effect on ultrafast photoexcited carrier dynamics of PdSe₂ films,which opens up a new avenue for further optical performance development of PdSe₂-based devices.