RESUMEN
Control of work function (WF) in graphene is crucial for graphene application in electrode material replacement and electrode surface protection in optoelectronic devices. Although efforts have been made to manipulate the effective WF of graphene to optimize its application, most studies have focused on graphene employed in static electrical contact interfaces. In this work, we investigated WF variations of supported single-layer graphene (SLG) in sliding electrical contact under ambient conditions, which was achieved by sliding an electrically biased conductive atomic force microscopy (cAFM) probe on the SLG surface. The effective WF, structural properties, and chemical compositions of rubbed SLG were subsequently measured by Kelvin probe force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, respectively. We found that the effective WF of the rubbed SLG was governed by both the tunneling triboelectric effect (TTE) and tribochemical-induced surface functionalization. The TTE charges generated by the sliding cAFM probe tunneled through the structural defects of the SLG and were trapped underneath the SLG. The SLG will be either p-doped or n-doped depending on the type of TTE charges and the polarity of electric bias applied to the cAFM probe during the rubbing process. However, the applied electric bias also led to the electrolysis of a water meniscus formed at the cAFM probe-SLG contact, resulting in surface oxidation and the increase of SLG WF. Further absorption of ambient water molecules on the oxygenated functional groups gradually reduced the SLG WF. The influence of TTE and surface functionalization on the SLG WF depends on the magnitude and polarity of applied electric biases, relative humidity, and physical properties of the supporting substrates. Our results demonstrate that the effective WF of SLG in a sliding electrical contact interface will vary with time and might need to be considered for related applications.
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
Oxygen vacancy is the most studied point defect and has been found to significantly influence the physical properties of zinc oxide (ZnO). By using atomic force microscopy (AFM), we show that the frictional properties on the ZnO surface at the nanoscale greatly depend on the amount of oxygen vacancies present in the surface layer and the ambient humidity. The photocatalytic effect (PCE) is used to qualitatively control the amount of oxygen vacancies in the surface layer of ZnO and reversibly switch the surface wettability between hydrophobic and superhydrophilic states. Because oxygen vacancies in the ZnO surface can attract ambient water molecules, during the AFM friction measurement, water meniscus can form between the asperities at the AFM tip-ZnO contact due to the capillary condensation, leading to negative dependence of friction on the logarithm of tip sliding velocity. Such dependence is found to be a strong function of relative humidity and can be reversibly manipulated by the PCE. Our results indicate that it is possible to control the frictional properties of ZnO surface at the nanoscale using optical approaches.
