Journal of the Korean Physical Society, Vol. 73, No. 3, August 2018, pp. 388∼391 Brief Reports Gain and Scan Rate Dependence of Friction at the Nanoscale Measured by Lateral Force Microscopy Sunghyun Kim Department of Applied Physics, Hanyang University, Ansan 15588, Korea Suenne Kim * Department of Applied Physics, Hanyang University, Ansan 15588, Korea and Department of Photonics and Nanoelectronics, Hanyang University, Ansan 15588, Korea (Received 4 June 2018, in final form 11 June 2018) Lateral force microscopy (LFM) is used to examine friction at the nanoscale. Although the friction at the tip-sample contact measured by LFM can provide extensive information beyond the friction force itself, such as the crystallographic orientation, the presence of defects, etc., there have been many contradictory reports regarding friction coefficients, nanoscale friction laws, etc. obtained by this technique. Here we investigate the effect of scan rate and gain on the LFM frictional force measurements. We show that the ratio of friction measured on SiO2 to friction measured on graphene can vary from about 1 to approximately 9.2 depending on the combination of these parameters. We discuss how to optimize the associated scan parameters to obtain reliable friction data at the nanoscale. PACS numbers: 06.20.Dk, 68.37.Ps, 68.35.Af Keywords: Lateral Force Microscopy, LFM, Friction, Gain, Scan speed DOI: 10.3938/jkps.73.388 I. INTRODUCTION The invention of the atomic force microscope (AFM) has facilitated many breakthroughs in nanoscale re- search. The AFM can capture various kinds of images simultaneously through different channels. Lateral force microscopy (LFM) is derived from the contact mode AFM and is used for characterizing frictional properties at the nanoscale. Its highly sensitive nature enables LFM to detect subtle frictional differences caused by the chem- ical compositional changes in polymers and oxides [1,2], local surface contaminations [3], directional anisotropies of carbon nanotubes [4] and graphene [5], and the num- ber of layers of two-dimensional materials [6]. However, even though LFM can provide such a wide range of infor- mation, there have been inconsistent reports regarding the normal load dependence [7], the friction coefficient for the same substance, etc. For instance, the friction coefficient of SiO 2 obtained using LFM may differ by a factor of 3 (about 0.25 [8] versus 0.08 [9]). In addition, the friction ratio between silicon oxide and monolayer graphene is found in the range of 1.4 to around 8 in the literature [5,8,10]. The contradictory reports concerning the measure- * E-mail: skim446@hanyang.ac.kr ment of nanoscale friction require an improved under- standing of the LFM technique. Here we investigate the frictional force difference between graphene and silicon oxide using LFM. For comparison studies, LFM friction data on both samples were obtained by the same probe tip. We show that the scan parameters, particularly gain and scan rate, can drastically affect LFM friction mea- surements. We first discuss the reason behind the ob- servations and present a simple parameter-optimization procedure. Eventually, this work will help us peruse quantitative LFM friction studies at the nanoscale. II. EXPERIMENTS AND DISCUSSION Experiments were performed using a commercial AFM (XE-100, Park Systems) in ambient conditions. A soft cantilever (NSC36C, Mikromasch) with a nominal spring constant of 0.6 N/m was used to measure friction. A sin- gle CVD graphene layer on a SiO 2 /Si substrate (p-doped substrate, 300 nm SiO 2 film on Si wafer, Graphene Lab- oratories Inc.) and a height calibration standard (HS- 100MG, BudgetSensors) with rectangular structure ar- rays were examined using LFM. We first present AFM height and LFM friction im- ages of a calibration standard with native silicon oxide pISSN:0374-4884/eISSN:1976-8524 -388- c 2018 The Korean Physical Society