Unusual photocarrier and coherent phonon dynamics behaviors of layered PdSe2 unveiled by ultrafast spectroscopy of the edge surface

Layered materials exhibit different electronic and phonon properties along in-plane and out-of-plane directions; existing studies focus on their in-plane behaviors, and the influence of such anisotropies on the dynamics of photocarriers and phonons is unknown. Here, we fabricate layered PdSe₂ crystals with flat edge surfaces and compare the time-resolved ultrafast spectroscopies on their basal and edge surfaces. Pronounced differences in the transient reflection spectroscopies reveal the inconsistent photocarrier and phonon dynamics behaviors on the two surfaces: the slow hot carrier relaxation process is accelerated and the thermoelasticity-induced longitudinal coherent acoustic phonon oscillation completely vanishes on the edge surface, as compared with the basal surface. Theoretical analysis reveals that the inconsistent hot carrier dynamics originate from the anisotropic properties of low-energy phonons in PdSe₂, and the absence of phonon oscillation on the edge surface results from the wavevector-limited sensitivity of acoustic B_1u mode. Moreover, polarization-dependent spectroscopies indicate the diverse optical anisotropies beyond the in-plane of PdSe₂. This work provides a new method to explore unique physical properties and modulate the optical anisotropy of layered materials.