UFM observation of lattice defects in highly oriented pyrolytic graphite

Ultrasonic force microscopy (UFM) can be used to image the distribution of elastic modulus up to several tens of GPa, which is not possible by the force modulation mode using a soft cantilever of spring constant less than 1 N m^-1. It was shown that by careful design of a piezoelectric transducer, together with a sample with small friction force, deflection vibration of the cantilever without torsion vibration is achieved, even during scanning perpendicular to the cantilever axis. Using this UFM, we propose that we have observed lattice defects reproducibly under atomically flat terraces of highly oriented pyrolytic graphite. Some defect was bent at surface steps, suggesting an interaction between the defect and the steps. The depth of a defect was found to be more than 3 nm on an assumption that it is continuous across surface steps. Contrast at the edge of terraces was explained by an geometrical effect associated with excitation of torsion vibration.