Frictional energy dissipation has been known for thousands of years from the beginning of human beings. For example, a caveman generated fire by using a stick with dry bushes to cause a consequence of frictional energy dissipation. In science, G. Amontons phenomenologically phrased three laws of frictions between hard materials in 1699. Despite its long history and macroscopic understandings, however, frictional energy dissipation has not been thoroughly understood in the microscopic scale, even after ~100 year from the birth of modern physics. After the invention of the scanning probe microscope in 1980’s, frictional phenomena have been systematically investigated at the microscopic scale and even at atomic scale by using the so-called friction force microscopy, which is a modification of the atomic force microscopy. Recently, we have characterized nanoscale frictional behaviors on two-dimensional materials such as graphene, ice, and MoS2 by using frictional force microscopy and density-functional theory analysis. We have found that the frictional energy dissipation on two-dimensional materials is quite unusual, compared to those observed in conventional three-dimensional systems. For example, after fluorination of graphene surface, we found that nanoscale friction is significantly increased [1]. This is against our daily-life experience because fluorinated surface generally becomes inert with causing less friction. The increased friction is due to the change in out-of-plane stiffness after fluorination. Also, water or ice intercalated under the graphene behaves like anti-lubricant [2]. Based on density-functional theory analysis, we have proposed that the increased frictional energy dissipation is mainly attributed to the additional phonon overlaps between graphene and substrate materials due to the intercalated ice layer. Very recently, we have identified that when a two-dimensional material is intervened between three-dimensional materials, the frictional characteristics is dominated by the interfacial out-of-plane stiffness of the two-dimensional material, which is proportional to the contact area [3]. Generally, three-dimensional shear stiffness is proportional to the radius of contact area. This again represents rich science in frictional energy dissipation of two-dimensional materials. In this talk, I will introduce our lastest microscopic understanding of nanoscale friction in two-dimensional materials.
[1] S. Kwon, J.-H. Ko, K.-J. Jeon, Y.-H. Kim, and J. Y. Park, Nano Lett. 12, 6043 (2012).
[2] H. Lee, J.-H. Ko, J. S. Choi, J. H. Hwang, Y.-H. Kim, M. Salmeron, and J. Y. Park, J. Phys. Chem. Lett. 8, 3482 (2017)
[3] J.-H. Ko, S. Kwon, J. Woo, G. E. Yang, W.-D. Kim, J. Y. Park, and Y.-H. Kim, submitted

Prof. Yong-Hyun Kim is currently a tenured professor at Graduate School of Nanoscience and Technology & Department of Physics, KAIST. He received his B.S. (1997), M.S. (1999), and Ph. D. (2003) from Department of Physics, KAIST, and worked for National Renewable Energy Laboratory, Golden, Colorado, USA as a post-doc and regular staff scientist from 2003 to 2009. He specializes on first principles density-functional calculations for nanostructures of real materials including carbon nanotube, graphene, and semiconductor quantum dots. Prof. Kim's current research projects associate with energy storage and utilization including hydrogen storage, battery, water splitting, thermoelectricity, tribology, and quantum dots based solar cells. He published more than 96 SCI peer-reviewed papers including Nature Materials, Phys. Rev. Lett., and J. Am. Chem. Soc. with total citations over 5000 (from Google Scholar). More details can be found at http://qnmsg.kaist.ac.kr.