Coherent Electron–Phonon Coupling in Two-Dimensional Bi ₂ Se ₃ 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 A1g(1) (2.12 THz) and A1g(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 A1g(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 Eg 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.