A Novel Harmonic Atomic Force Microscopy With Tip-Sample Couplings

This article presents a novel harmonic atomic force microscopy with tip-sample couplings (TS-HAFM) working in force modulation mode for imaging samples of different mechanical properties. Compared with traditional harmonic atomic force microscopy (T-HAFM), TS-HAFM has better harmonic characteristics and imaging resolution. T-HAFM does not consider the effect of tip-sample couplings on probe vibration; it can only generate harmonics in the free vibration state. However, after the tip touches the sample, the tip-sample coupling shifts the probe frequency and attenuates the probe's harmonic characteristics. TS-HAFM considers the effect of tip-sample couplings on probe vibration and can prevent problems of image resolution reduction and vibration stability degradation caused by harmonic attenuation. We first tested the Young's modulus of the sample and calculated the tip-sample contact's equivalent contact stiffness based on the Johnson-Kendall-Roberts model. The harmonic probe with tip-sample couplings and the traditional harmonic probe were subsequently designed. Then, the two probes were applied to the topography detection of polypropylene spheres covered with PDMS film and cells on PDMS substrates. Experimental results showed that TS-HAFM was up to 2.01 times more sensitive than T-HAFM in detecting samples of different mechanical properties. When detecting soft samples on PDMS substrates, TS-HAFM still has higher imaging clarity. TS-HAFM has emerged as a powerful platform for high resolution imaging of samples with different mechanical properties, while improving resistance to system interference.