Influence of Oxygen Vacancies on the Frictional Properties of Nanocrystalline Zinc Oxide Thin Films in Ambient Conditions

Abstract

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.

Variations in the Effective Work Function of Graphene in a Sliding Electrical Contact Interface under Ambient Conditions

“…The variations in the surface morphology, roughness, and surface potential of the rubbed SLG were investigated using several AFM-based techniques. The atomic force microscope (Multimode 8, Bruker, Santa Barbara, CA, USA) was placed inside an environmental hood with inlets for dry and moist nitrogen gas flows to enable accurate control of the RH inside the hood for all AFM measurements . The PeakForce tapping mode was first used to examine the surface topography and measure the root-mean-square surface roughness ( R q ) of the SLG on silica (SLG/silica).…”

“…The atomic force microscope (Multimode 8, Bruker, Santa Barbara, CA, USA) was placed inside an environmental hood with inlets for dry and moist nitrogen gas flows to enable accurate control of the RH inside the hood for all AFM measurements. 45 The PeakForce tapping mode was first used to examine the surface topography and measure the root-mean-square surface roughness (R q ) of the SLG on silica (SLG/silica Nanosensors, Neuchatel, Switzerland) coated with platinum (Pt) and iridium (Ir) was used to rub the SLG surface with an applied voltage V Probe of between −10 and +10 V, while the SLG surface was grounded with silver paste. After the rubbing process, frequencymodulated PeakForce KPFM (PF-KPFM) was employed to observe the variations in the contact potential difference V CPD between the cAFM probe and SLG/silica surface.…”

Correlation between Nanoscale Elasticity, Semiconductivity, and Structural Order in Functionalized Polyaniline Thin Films

“…During the PF-QNM experiment, the AFM cantilever approaches the sample from approximately 150 nm above the sample surface. The cantilever is then contacted with the film, indented into the film, and retracted to its initial position at a specific frequency while scanning across the sample surface. ,, The modulation frequency of PF-QNM is known to affect the results of topography imaging and stiffness measurements . Therefore, the tapping frequency of the cantilever was set to be 2 kHz for the following experiments.…”

Self-Propelled Micro-/Nanomotors of ZnO Nanoshuttles Induced by Surface Defects

“…15 As one of the new generation of multifunctional wide-band-gap (3.37 eV) semiconductors, zinc oxide is non-toxic, low cost, easy to prepare and has unique photovoltaic properties. It has aroused great interest in the fields of photocatalysis, 16 sensors, 17 friction power generation, 18 optoelectronics, 19 energy storage, 20,21 and so on. At present, reports on ZnO-based MNMs mainly focus on nanomedicine and photocatalysis.…”