Nanoscale tribology of graphene grown by chemical vapor deposition and transferred onto silicon oxide substrates
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We present a comprehensive nanoscale tribological characterization of single-layer graphene grown by chemical vapor deposition (CVD) and transferred onto silicon oxide (SiO2) substrates. Specifically, the nanotribological properties of graphene samples are studied via atomic force microscopy (AFM) under ambient conditions using calibrated probes, by measuring the evolution of friction force with increasing normal load. The effect of using different probes and post-transfer cleaning procedures on frictional behavior is evaluated. A new method of quantifying lubrication performance based on measured friction coefficient ratios of graphene and SiO2 is introduced. A comparison of lubrication properties with mechanically-exfoliated graphene is performed. Results indicate that CVD-grown graphene constitutes a very good solid lubricant on SiO2, reducing friction coefficients by ;90% for all investigated samples. Finally, the effect of wrinkles associated with CVD-grown graphene on measured friction values is quantitatively analyzed, with results revealing a substantial increase in friction on these structural defects.
I. INTRODUCTION
To extend the lifespan of mechanical systems, wear and consequently, friction must be minimized. Liquidphase lubricants featuring a variety of additives are typically used toward this end in macroscopic mechanical systems. On the other hand, traditional liquid-based lubrication schemes fail in nano- and micro-scale systems comprising mobile components due to increasing surfaceto-volume ratios and the associated enhancement in physical effects such as surface tension.1,2 As such, research efforts in recent years have been aimed at identifying candidates for solid lubricants to be used in nano- and micro-scale mechanical systems and characterizing related structural and tribological properties. Among various candidates for solid lubricants, the two-dimensional material graphene is of particular importance not only due to its outstanding electrical3 and mechanical4 properties, but also because it constitutes the essential building block of bulk graphite, a widely-used solid lubricant in a variety of macroscopic applications.5 Consequently, based on its promise as a single-atom-thick solid lubricant consisting of a sheet of carbon atoms arranged in a honeycomb pattern, the nanotribological properties of graphene have been investigated via several studies in the literature.6–13
Contributing Editor: Mohd Fadzli Bin Abdollah a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.11
In particular, nanotribological characterization of singleand multi-layer graphene samples obtained via mechanical exfoliation has revealed a layer-dependent frictional behavior, with the friction values observed via atomic force microscopy (AFM) decreasing monotonously with increasing number of layers.8 This finding has been explained by the so-called puckering effect that involves the enhanced build-up of graphene in front of the scanning probe at small number of layers
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