Image of the Month

March 2013

Enhanced Nanoscale Friction on Fluorinated Graphene

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Figure 1. (b) dI/dV measured at an applied load of 72 nN on fluorinated graphene and pristine graphene. A band gap of 2.9 eV was measured after fluorination.
Figure 2. 500 × 500 nm2 images of (a) topography and (b) friction measured on the fluorinated graphene using contact mode AFM (applied load = 71 nN). (c) Plot of friction force versus applied load measured on pristine and on fluorinated graphene.

Credits:
Sangku Kwon,†,⊥ Jae-Hyeon Ko,‡,⊥ Ki-Joon Jeon,§ Yong-Hyun Kim,*,‡,∥ and Jeong Young Park*,†,∥
Graduate School of EEWS (WCU), KAIST, Daejeon 305-701, Republic of Korea
Graduate School of Nanoscience and Technology (WCU), KAIST, Daejeon 305-701, Korea §School of Electrical Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea
KAIST Institute for the NanoCentury, KAIST, Daejeon 305-701, Republic of Korea

Microscope:
RHK Technology UHV 7500

Control System:
RHK Technology SPM 1000 Control System

Reference:
Nano Lett. 2012, 12, 6043−6048

Abstract:
Atomically thin graphene is an ideal model system for studying nanoscale friction due to its intrinsic two-dimensional (2D) anisotropy. Furthermore, modulating its tribological properties could be an important milestone for graphene-based micro- and nanomechanical devices. Here, we report unexpectedly enhanced nanoscale friction on chemically modified graphene and a relevant theoretical analysis associated with flexural phonons. Ultrahigh vacuum friction force microscopy measurements show that nanoscale friction on the graphene surface increases by a factor of 6 after fluorination of the surface, while the adhesion force is slightly reduced. Density functional theory calculations show that the out-of-plane bending stiffness of graphene increases up to 4-fold after fluorination. Thus, the less compliant F-graphene exhibits more friction. This indicates that the mechanics of tip-to- graphene nanoscale friction would be characteristically different from that of conventional solid-on-solid contact and would be dominated by the out-of-plane bending stiffness of the chemically modified graphene. We propose that damping via flexural phonons could be a main source for frictional energy dissipation in 2D systems such as graphene.