RESUMEN
In this work, we study surface functionalization effects of artificially stacked graphene bilayers (ASGBs) to control its wetting properties via low-damage plasma. The ASGBs were prepared on a SiO2/Si substrate by stacking two monolayer graphene, which was grown by chemical vapor deposition. As a result, the low-damage plasma functionalization of ASGBs could hold both the key characteristics of surface functionalization and electrical transport properties of graphene sheets. To characterize ASGBs, Raman and x-ray photoelectron spectroscopy (XPS) were used to determine the degree of defect formation and functionalization. Meanwhile, the degree of the wettability of the ASGBs surface was determined by optical contact angle (CA) measurements. Based on experimental results, the compositional ratio of C-OH + COOH was found to increase 67% based on the analysis of XPS spectra after low-damage plasma treatment. This treatment effect can also be found with 75.3% decrease in the CA of water droplet on graphene. In addition, we found that the ratio of 2D/(D + G') in Raman spectra shows strong correlation to the measured CA; it can be a reliable indicator of ASGBs surface wettability modification. This work showed that we obtained a higher degree functionalization of ASGBs without degrading the under-layer structure of ASGBs due to the moderate low-damage plasma treatment. The presented process technique of controllable wettability through low-damage plasma treatment can be employed for potential application in graphene-based sensors/devices.
RESUMEN
We have investigated the frictional properties of single-layer graphene (SLG) coated rough silica substrate under the influence of nano-confined hydration layer underneath SLG. Through the friction and surface potential measurements by atomic force microscopy (AFM), we found polygonal features in AFM images of SLG-protected silica surface that exhibit simultaneously larger friction and higher surface potential as compared to their surrounding areas due to water layers confined under SLG. Nano-confined water layers at the SLG-silica interface can induce the hole-doping effect in SLG, resulting in a more positively-charged and hydrophilic surface that favors adsorption of ambient water molecules. Therefore, during friction measurements, nanoscale capillary bridges can form within the interstices of AFM probe-SLG contact, leading to larger adhesion and friction. The friction forces were found to respectively have negative and positive dependence on the sliding velocity inside and outside the polygonal regions due to different surface wettability. Hence, it is possible to manipulate the frictional properties of SLG-coated silica by the amount of hydration layer confined underneath SLG. Our results may find applications in friction control for future nano-devices.
RESUMEN
Micrometer sized oxidation patterns were created in chemical vapor deposition grown graphene through scanning probe lithography (SPL) and then subsequently reduced by irradiation using a focused x-ray beam. Throughout the process, the films were characterized by lateral force microscopy, micro-Raman and micro-x-ray photoelectron spectroscopy. Firstly, the density of grain boundaries was found to be crucial in determining the maximum possible oxygen coverage with SPL. Secondly, the dominant factor in SPL oxidation was found to be the bias voltage. At low voltages, only structural defects are formed on grain boundaries. Above a distinct threshold voltage, oxygen coverage increased rapidly, with the duration of applied voltage affecting the final oxygen coverage. Finally, we found that, independent of initial conditions, types of defects or the amount of SPL oxidation, the same set of coupled rate equations describes the reduction dynamics with the limiting reduction step being C-C â C=C.
