With the rapid development of two-dimensional (2D) matter, orthorhombic black phosphorus (BP), composed of puckered phosphorus layers via van der Waals interactions, recently has been widely studied due to its striking anisotropic properties . Despite huge interests in BP from both fundamental and technical aspects , investigation into its structural dynamics induced by transient strain fields, prevalent for 2D materials and tuning the material physical properties, has been overlooked. Here, we track the course of the morphological deformation and relaxation of suspended BP membranes on timescales spanning nanoseconds to microseconds observed by time-resolved diffractograms and dark-field images in ultrafast electron microscopy (UEM) using pulsed photoexcitation as the source of brief thermal stress [3,4] (Figure 1). The number of BP layers, as well as the excitation polarization, fluence, and probe axis, were all controlled to analyze the anisotropic behavior. Aided by 4D structural reconstruction, we visualize the nonequilibrium bulging of thin BP membranes and reveal the buckling transition driven by impulsive thermal stress upon photoexcitation in real time. Finite element modeling was performed to simulate the stress and strain energy accompanied by the structural transition. The transient bulging, buckling, and flattening of the suspended BP membrane showed anisotropic spatiotemporal behavior. Because the strain field in the form of ripples and buckles on the surface of 2D matter modulates many physical properties, especially the optical and electrical properties, tailoring the transient anisotropic morphology of BP may allow fast spatiotemporal responses to light and unique multi-order functionality.
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