Coherent Electron–Phonon Coupling in Two-Dimensional Bi₂Se₃ Nanoplatelets Studied with Ultrafast Spectroscopy

Electron–phonon coupling in two-dimensional Bi₂Se₃ nanoplatelets was investigated using broadband ultrafast pump–probe spectroscopy (probe 1.6 to 2.8 eV) at 293 and 78 K. The high time resolution resolves coherent phonon oscillations on picosecond time scales, specifically the out-of-plane A_1g⁽¹⁾ (2.12 THz) and A_1g⁽²⁾ (4.95 THz) optical modes, and a low-frequency interlayer breathing mode (∼0.4 THz), arising from standing waves defined by the nanoplatelet thickness. By mapping the probe energy dependence of the oscillation amplitude, we find that the A_1g⁽¹⁾ mode couples most strongly to an electronic transition near 1.97 eV, which we assign using the computed band structure to transitions along the high-symmetry Γ–K line. The absence of Raman-active E_g modes is explained by a symmetry analysis based on the displacive excitation of coherent phonons. These results characterize carrier–lattice interactions in 2D Bi₂Se₃, which are relevant for optoelectronic device applications.