Microscopic Detection and Control of In-plane Surface Mechanical Properties on Anthracene Crystal by Using a Femtosecond Laser and an Atomic Force Microscope

An infrared femtosecond laser pulse was focused on an anthracene crystal surface to generate a stress wave, and its propagation on the crystal was detected by an atomic force microscope (AFM).Additionally, femtosecond laser pulses were employed to modify the crystal surface.The wave propagation velocity was estimated from the delay time between the pulse irradiation time and the first response of cantilever, which depended on the distance between the laser focal point and the contact point of cantilever.Utilizing partial least squares discriminant analysis (PLS-da) of the waveform of the cantilever response, we found that propagation behavior of the wave was modified between unmodified and laser-modified surfaces.These results suggested that the detected wave propagation was dominated by the surface acoustic wave (Rayleigh wave).The propagation velocity of the surface waves provides the Young's modulus of the crystal, which was estimated to be 655 MPa.This is much smaller than that (5 GPa) estimated by the conventional AFM indentation test.These respective values are considered to predominantly reflect the in-plane and out-of-plane mechanical properties of the anthracene crystal surface.We show a novel method for characterizing in-plane mechanical properties of organic crystal surfaces.