Research on Direct Cantilever Excitations for Dynamic Atomic Force Microscopy in Liquid

Atomic force microscopy (AFM) has become a powerful technique for probing the nano-scale of a sample in various environments such as a vacuum, atmosphere, and liquid. In biochemistry research, molecular structures and the properties of living cells are measured in aqueous solutions for approximating their physiological state. In order to protect the surface structure from damage and to improve the imaging resolution, dynamic AFM is developed and utilized to investigate the surface characteristics. The dynamic mode includes flexural, torsional, lateral resonance modes, etc., which depend on the resonant modal of the AFM cantilever. Flexural resonance (FR) mode, the most fundamental mode of the cantilever, is mostly operated on dynamic AFM. This mode, however, is applied to probing out-of-plane material properties only. For acquiring surface in-plane properties, torsional resonance (TR) or lateral resonance (LR) mode should be used. Moreover, TR and LR modes have much higher resonant frequencies than does the FR mode; hence they are eligible to be developed on high-speed AFM. In TR mode, the sensitivity is also higher, and the contact point can be precisely defined without the influence of long-range force. LR mode is seldom used, since the cantilever’s motion is hardly measured by the AFM detection method. The piezoelectric element is commonly adopted to stimulate the AFM cantilever. In liquid, it is prone to cause the fluid-borne excitation to interfere with cantilever dynamics. Consequently, spurious peaks readily emerge in the frequency spectrum, and the real interaction between the tip and sample surface is difficultly indicated as well. In order to solve the problem, direct cantilever excitations are proposed and developed to vibrate the cantilever in a stable manner with small amplitudes of less than 1 nm. In this way, the signal-to-noise ratio (SNR) of the measured signal is enhanced, thus improving the detection sensitivity. For FR and TR modes, the developed direct cantilever excitations are successfully applied on MultiMode AFM, which include Lorentz force, laser thermal, and Joule driving excitations. Electromagnetic, photothermal, and electrothermal effects are elaborated in their working principles; their excitation performances are also verified experimentally. Lorentz force excitation expresses the best excitation performance for TR mode. Laser thermal excitation has excellent excitation efficiencies in both FR and TR modes. Joule driving excitation demonstrates the simplest method to stimulate the cantilever successfully. In liquid, the resonant peak in the frequency spectrum becomes pure and simple through using direct cantilever excitations. Therefore, MultiMode AFM within the homemade acoustic enclosure, which is built for isolating external influences, can obtain high-resolution images such as periodic trimer structures on purple membrane, atomic arrangement on mica, and ordered layers of gas molecules on highly oriented pyrolytic graphite. In these measurements, the results in TR mode agree with those of previous studies using FR mode.