Vibrational Dynamics in Black Phosphorus Membranes Visualized by Ultrafast Electron Microscopy

In ultrafast electron microscopy (UEM), light-induced temporal changes in electron micrographs provide key insights into physical and chemical processes. However, the microscopic origins of related contrast dynamics require further understanding. Here, we applied stroboscopic UEM and pixel-by-pixel image analysis to study oscillatory bending dynamics in suspended black phosphorus (BP) membranes following femtosecond laser excitation. Experiments at various temperatures showed contrast oscillations exclusively at membrane wrinkle sites, with two frequencies occurring globally across the membrane and increasing upon cooling. Statistical analysis revealed oscillation phases clustered around two values separated by π. Our findings indicate a global membrane vibration rather than coupled localized modes. Periodic lattice distortions at wrinkles modulate the local crystal orientation relative to the electron beam, inducing time-dependent contrast variations. This work provides a clear framework for analyzing contrast dynamics in complex systems, benefiting the development of temperature-sensitive BP-based nanoelectromechanical system (NEMS) applications.