Choi Jin Sik, Kim Jin-Soo, Byun Ik-Su, Lee Duk Hyun, Hwang In Rok, Park Bae Ho, Choi Taekjib, Park Jeong Young, Salmeron Miquel
Division of Quantum Phases and Devices, Department of Physics, Konkuk University, Seoul 143-701, South Korea.
Rev Sci Instrum. 2012 Jul;83(7):073905. doi: 10.1063/1.4737428.
Ripples in graphene monolayers deposited on SiO(2)/Si wafer substrates were recently shown to give rise to friction anisotropy. High friction appears when the AFM tip slides in a direction perpendicular to the ripple crests and low friction when parallel. The direction of the ripple crest is, however, hard to determine as it is not visible in topographic images and requires elaborate measurements of friction as a function of angle. Here we report a simple method to characterize ripple crests by measuring the cantilever torsion signal while scanning in the non-conventional longitudinal direction (i.e., along the cantilever axis, as opposed to the usual friction measurement). The longitudinal torsion signal provides a much clearer ripple domain contrast than the conventional friction signal, while both signals show respective rotation angle dependences that can be explained using the torsion component of the normal reaction force exerted by the graphene ripples. We can also determine the ripple direction by comparing the contrast in torsion images obtained in longitudinal and lateral scans without sample rotation or complicated normalization.
最近研究表明，沉积在SiO(2)/Si晶圆衬底上的单层石墨烯中的波纹会产生摩擦各向异性。当原子力显微镜（AFM）探针沿垂直于波纹波峰的方向滑动时，摩擦力较大；而沿平行于波纹波峰的方向滑动时，摩擦力较小。然而，波纹波峰的方向很难确定，因为在形貌图像中不可见，需要精确测量摩擦力随角度的变化。在此，我们报告一种简单的方法，通过在非常规纵向方向（即沿着悬臂轴，与通常的摩擦测量方向相反）扫描时测量悬臂扭转信号来表征波纹波峰。纵向扭转信号比传统摩擦信号提供了更清晰的波纹区域对比度，而这两种信号都显示出各自的旋转角度依赖性，这可以通过石墨烯波纹施加的法向反作用力的扭转分量来解释。我们还可以通过比较在纵向和横向扫描中获得的扭转图像的对比度来确定波纹方向，而无需样品旋转或复杂的归一化处理。
