Anisotropic Lattice Expansion of Monolayer WSe2 Revealed by Ultrafast Electron Diffraction

Bulk layered MX2 transition metal chalcogenides (M = Mo, W and X = S, Se) are known to exhibit an indirect to direct band gap transition as the number of layers is reduced. Previous time-resolved work has principally focused on the investigation of the transient evolution of the band structure after photo-excitation, but additional information on the dynamics of the concomitant lattice rearrangement is needed to fully understand this phenomenon. Here, ultrafast electron diffraction is used to probe the atomic motion and bond dilation in monolayer WSe2 with femtosecond temporal resolution. The change in the intensity of the Bragg diffraction spots is characterized by single-exponential dynamics, consistent with a collective response of the lattice during electron-phonon and phonon-phonon equilibration that is repeatable over many hours of illumination with ultrafast pulses. Moreover, a transient and highly anisotropic lattice expansion is observed. A possible explanation for this behavior is axial strain induced by the laser excitation that breaks the degeneracy of the in-plane phonon modes, and that strongly influences the electronic band structure. Such degeneracy-breaking induced by distortions in the lattice are well characterized in static strain measurements, and the results presented here provide valuable insights into the nature and time scales of the structural rearrangement that occurs following optical photo-excitation in monolayer tungsten diselenide.
Comment: Submitted to J. Phys. Chem. C, 21 pages, 7 figures