Scan Speed Induced Phenomenon in Contact Resonance Atomic Force Microscopy

Abstract

Atomic Force Microscopy (AFM) has been widely used to measure nanoscale properties in fieldssuch as material science, biology, and medicine. Unexplained phenomena in AFM generate unknown forces that must be understood to make accurate nanoscale measurements. Previous research observed a scan speed dependent phenomenon in Contact Resonance AFM (CR-AFM) on a hydrophilic mica sample and hypothesized it is due to hydrodynamic effects on the cantilever tip and sample surface. At certain relative humidities, a water film can form on a hydrophilic surface. At these relative humidities and at a critical scan speed, the tip can achieve hydrodynamic lift from the sample surface. The scan speed phenomenon decreases the measured frequencies in CR-AFM as the scan speed increases. CR-AFM experiments performed in the literature were replicated and enhanced by expanding the scan speed velocity range by two orders of magnitude, exploring 15% higher relative humidities, and experimenting at an additional scan angle on a hydrophilic mica sample to observe the scan speed phenomenon. A humidity control system was built and the experiments were performed at five different relative humidities ranging from 5% to 85%. At each relative humidity, the scan angles 90° and 0° were tested for six different scan speeds ranging from 0.01 μm/s to 1000 μm/s. The experimental data provided in this work supports the presence of the scan speed phenomenon in CR-AFM where a decreased shift in the measured resonance frequencies is a function of increased scan speed. The scan speed phenomenon appeared at various relative humidities and scan angles, suggesting that other variables may impact the measured resonance frequencies at high scan speeds in CR-AFM.