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Tribology and Lubrication Technology November 2011 : Page 12

Student POSTER Abstract Editor’s Note: For a closer look at Xin’s poster abstract, be sure to check out his short video presentation in the November digital version of TLT (available at www.stle.org ) Nanoscale Friction and Adhesion Behavior for Few-Layer Graphene Nanoscale Friction and Adhesion Behavior Xin Z. Liu, 1 Qunyang Li, 1 Changgu Lee, 2 Baolei Zhang, 1 James Hone 3 and Robert W. Carpick, 1 (Advisor) 1 1 Engineering 1 2 1 3 Philadelphia, Pa. Department of Mechanical Applied Lee, Mechanics, University Pennsylvania, Xin Z. Liu, Qunyang Li, and Changgu Baolei Zhang, of James Hone and 1 2 Department of Mechanical Engineering, Sungkyunkwan University, Korea Robert W. Carpick, (Advisor) 1 3 Department of Mechanical Engineering and Applied Mechanics, University Pennsylvania, Philadelphia, Pa. Department of Mechanical Engineering, Columbia University, of New York, N.Y. 2 3 for Few-Layer Graphene Department of Mechanical Engineering, Sungkyunkwan University, Korea Department of Mechanical Engineering, Columbia University, New York, N.Y. 1 of Xin Z. Liu received electronics, his master’s ment. Friction experiments were per-or quantum dot-sized 2 science degree from transistors, the Institute of formed in contact-mode by atomic as well as METHODOLOGY for micro-and For friction experiments, samples were deposited under Physics at Leiden University, the force microscopy (AFM), as depicted nano-electromechanical systems ambient conditions onto SiO 2 /Si substrates, method XE-that Netherlands, under (MEMS/NEMS), the supervision ultrahigh in Figure 1 using a Park a Systems frequency 9 and they were used for all experiments is described in, of professor Joost Frenken. 3,4 100 AFM in ambient conditions (25%-resonators, and high accuracy gas without further treatment. Friction experiments were Currently Xin is and a mass doctoral 50% relative humidity, room tempera-detectors. 5,6 To date, the tri-performed in contact-mode by atomic force microscopy candidate in the group of professor ture), and a RHK UHV350 AFM, bomechanical properties of 2-D mate-(AFM), as depicted in Figure 1 using a Park Systems XE-Robert Carpick at the University of where the sample chamber was purged rials have received less attention com-100 AFM in ambient conditions (25%-50% relative Pennsylvania. His research by dry nitrogen gas (1%-2% relative pared to their electronic and thermal humidity, room temperature), and a RHK UHV350 AFM, interests includes studying the humidity, room temperature). Con-properties, and tribomechanical prop-where the sample chamber was purged by dry nitrogen mechanical properties of various two-dimensional Xin Z. Liu received his master’s of tact-mode silicon AFM probes were remain poorly understood de-relative humidity, room temperature). gas (1%-2% materials, including graphene and other erties carbon-based science degree from the Institute of used for all the experiments. num-spite their importance Contact-mode in determining silicon AFM probes were used The for all the materials, as well as working with surface analytical Physics at Leiden University, the Neth-ber of layers for all sample regions the applicability of 2-D materials to experiments. The number of layers for all sample regions techniques such as AFM, SEM and Raman spectroscopy. erlands, under the supervision of pro-probed was on determined based on AFM the various of was the tri-probed determined based the topographic You can reach him at xinzliu@seas.upenn.edu . devices. The study fessor Joost Frenken. Currently Xin is a topographic AFM images. In several bomechanical properties of 2-D In mate-images. several cases, Raman spectroscopy was also doctoral candidate in the group of pro-cases, Raman the spectroscopy was also rials is scientifically relevant since, as used independently for verifying thicknesses. fessor Robert Carpick at the University used independently for verifying the other studies have shown, materials INTRODUCTION of Pennsylvania. His research interests Photo detector Two-dimensional (2-D) materials, including graphene, thicknesses. will behave very differently from their include studying the mechanical prop-have drawn much attention because of 3-D their notable counterparts due to di-erties of various two-dimensional electronic, thermal, chemical, ma-and mechanical properties, effects. 7,8 mensionality Laser terials, including graphene and other making them outstanding candidates for future electronic Therefore, before 2-D ma-carbon-based as the well as devices. For materials, example, electronic terials properties of can be considered in Tip Cantilever working with surface analytical tech-graphene make it suitable for use in ultrahigh frequency real next-generation appli-1 Sample niques such for as fast AFM, nanoscale SEM and Raman transistors electronics, or quantum dot-mechanical cations, the X 2 sized transistors, well as at for micro-and nano-spectroscopy. You can as reach him and nanotribological prop-Y Substrate electromechanical systems (MEMS/NEMS), xinzliu@seas.upenn.edu. erties ultrahigh of these materials 3,4 gas and mass frequency resonators, and high accuracy must be better understood. detectors. 5,6 To date, the tribomechanical properties of 2-In this study, our goal Figure is to 1. Schematic setup of an AFM that was used for both D materials have received less attention compared to their friction and adhesion measurements in this study. characterize the nanoscale electronic and thermal properties, and tribomechanical Introduction friction and adhesion prop-properties remain poorly understood despite their For adhesion experiments, samples of graphene were Two-dimensional (2-D) materials, erties of of graphene. importance in determining the in-applicability 2-D prepared in the same way as those for friction cluding graphene, have drawn much materials to various devices. The study of the experiments. The tests were performed in the same RHK attention because of their notable of elec-2-D Methodology tribomechanical properties materials is AFM system (1%-2% relative humidity, room tronic, thermal, chemical, me-studies For friction experiments, scientifically relevant since, and as other have shown, temperature) using the regular force-distance (FD) chanical properties, making them out-samples were deposited un-materials will behave very differently from their 3-D spectroscopy, as illustrated in Figure 2. 7,8 ambient standing candidates for future effects. der Therefore, conditions counterparts due to dimensionality electronic For can example, the onto SiO2/Si substrates, a before 2-D devices. materials be considered in real next-Figure 1 | Schematic setup of an AFM that was used for electronic properties of graphene method that is described both friction and adhesion measurements in this generation applications, the mechanical and 9 study. and they were used for make it suitable properties for use in ultrahigh in, nanotribological of these materials must be better understood. In this study, our goal is to characterize the friction and adhesion properties 12 • nanoscale NO VEMBER 2 011 TRIB OL OG Y & L of UBRIC A TION TE CHNOL OG Y WWW .S TLE. OR G graphene. frequency transistors for fast nanoscale all experiments without further treat-

STUDENT POSTER ABSTRACT

Nanoscale Friction and Adhesion Behavior For Few-Layer Graphene <br /> <br /> INTRODUCTION<br /> Two-dimensional (2-D) materials, including graphene, have drawn much attention because of their notable electronic, thermal, chemical, and mechanical properties, making them outstanding candidates for future electronic devices. For example, the electronic properties of graphene make it suitable for use in ultrahigh frequency transistors for fast nanoscale electronics,1 or quantum dot-sized transistors,2 as well as for micro- and nano-electromechanical systems (MEMS/NEMS), ultrahigh frequency resonators,3,4 and high accuracy gas and mass detectors.5,6 To date, the tribomechanical properties of 2-D materials have received less attention compared to their electronic and thermal properties, and tribomechanical properties remain poorly understood despite their importance in determining the applicability of 2-D materials to various devices. The study of the tribomechanical properties of 2-D materials is scientifically relevant since, as other studies have shown, materials will behave very differently from their 3-D counterparts due to dimensionality effects.7,8 Therefore, before 2-D materials can be considered in real next-generation applications, the mechanical and nanotribological properties of these materials must be better understood. In this study, our goal is to characterize the nanoscale friction and adhesion properties of graphene.<br /> <br /> METHODOLOGY<br /> For friction experiments, samples were deposited under ambient conditions onto SiO2/Si substrates, a method that is described in,9 and they were used for all experiments without further treatment. Friction experiments were performed in contact-mode by atomic force microscopy (AFM), as depicted in Figure 1 using a Park Systems XE- 100 AFM in ambient conditions (25%- 50% relative humidity, room temperature), and a RHK UHV350 AFM, where the sample chamber was purged by dry nitrogen gas (1%-2% relative humidity, room temperature). Contact- mode silicon AFM probes were used for all the experiments. The number of layers for all sample regions probed was determined based on the topographic AFM images. In several cases, Raman spectroscopy was also used independently for verifying the thicknesses.<br /> <br /> For adhesion experiments, samples of graphene were prepared in the same way as those for friction experiments. The tests were performed in the same RHK AFM system (1%-2% relative humidity, room temperature) using the regular force-distance (FD) spectroscopy, as illustrated in Figure 2.<br /> <br /> ILLUSTRATIVE RESULTS<br /> From the friction measurements, we observed a trend that the friction force—starting from one single layer— decreases monotonically with increasing number of layers of graphene. Moreover, the friction force on samples of about four layers is approximately the same as on the bulk materials. Those results are shown in Figure 3. A similar trend has been predicted using finite element modeling (FEM) simulations. A puckering effect of the thin sheet has been proposed to cause this phenomenon. In that effect, thinner sheets are more susceptible to out-of-plane elastic deformation than thicker sheets, provided they are not strongly bound to the substrate.10,11<br /> <br /> In contrast to the friction results, the adhesion between an AFM tip and graphene, measured using the regular FD spectroscopy, does not exhibit a significant dependence on the number of layers. This insensitivity to the number of layers is consistent with the results from FEM simulations. The experimental results are plotted in Figure 4. The variation in adhesion for different numbers is within 10%, whereas the friction measured between the tip and one monolayer of graphene is 50% greater than that for four layers or more.<br /> <br /> We also obtained a second set of adhesion tests using a modified form of FD spectroscopy. In that test, adhesion is measured immediately after sliding the AFM tip over the same area for a sufficient distance without breaking the tip-graphene contact until the pull-off itself. Our preliminary results suggest that the adhesion between graphene and the tip has a sliding-history dependence. The adhesion is enhanced by sliding the AFM tip over the same area. This suggests that the enhanced contact area between the graphene and the tip requires a sufficient amount of sliding. This may be emblematic of the fluctuating nature of the graphene sheet, whose geometric configuration is strongly affected by the sliding action.<br /> <br /> SUMMARY <br /> <br /> In this work, we studied the friction and adhesion characteristics between exfoliated 2-D materials and nanoscale single asperity tips using AFM. We observed layer-dependent friction for graphene. This observation coupled with finite element modeling (FEM) suggests that the trend is caused by the puckering effect of the thin graphene sheets. Regular FD spectroscopy also showed that the pull-off force between graphene and the AFM tip did not have an appreciable dependence on the number of layers, which is consistent with FEM predictions. However, our preliminary results obtained using a modified form of FD spectroscopy suggest that the graphene adhesion has a sliding-history dependence.

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