Chemical bond imaging using torsional and flexural higher eigenmodes of qPlus sensors

Imaging the adsorption sites of organic molecules on metallic surfaces by an adaptive tunnelling current feedback

Atomic force microscopy (AFM) allows submolecular resolution imaging of organic molecules deposited on a surface by using CO-functionalized qPlus sensors under ultrahigh vacuum and low temperature conditions. However, the experimental determination of the adsorption sites of these organic molecules requires the precise identification of the atomic structure of the surface on which they are adsorbed. Here, we develop an automation method for AFM imaging that provides in a single image both, submolecular resolution on organic molecules and atomic resolution on the surrounding metallic surface. The method is based on an adaptive tunnelling current feedback system that is regulated according to the response of the AFM observables, which guarantees that both the molecules and the surface atoms are imaged under optimum conditions. Therewith, the approach is suitable for imaging adsorption sites of several adjacent and highly mobile molecules such as 2-iodotriphenylene on Ag(111) in a single scan. The proposed method with the adaptive feedback system facilitates statistical analysis of molecular adsorption geometries and could in the future contribute to autonomous AFM imaging as it adapts the feedback parameters depending on the sample properties.

Fundamental and higher eigenmodes of qPlus sensors with a long probe for vertical-lateral bimodal atomic force microscopy

The detection of vertical and lateral forces at the nanoscale by atomic force microscopy (AFM) reveals various mechanical properties on surfaces. The qPlus sensor is a widely used force sensor, which is built from a quartz tuning fork (QTF) and a sharpened metal probe, capable of high-resolution imaging in viscous liquids such as lubricant oils. Although a simultaneous detection technique of vertical and lateral forces by using a qPlus sensor is required in the field of nanotribology, it has still been difficult because the torsional oscillations of QTFs cannot be detected. In this paper, we propose a method to simultaneously detect vertical and lateral force components by using a qPlus sensor with a long probe. The first three eigenmodes of the qPlus sensor with a long probe are theoretically studied by solving a set of equations of motion for the QTF prong and probe. The calculation results were in good agreement with the experimental results. It was found that the tip oscillates laterally in the second and third modes. Finally, we performed friction anisotropy measurements on a polymer film by using a bimodal AFM utilizing the qPlus sensor with a long probe to confirm the lateral force detection.

Torsional and lateral eigenmode oscillations for atomic resolution imaging of HOPG in air under ambient conditions

Combined in-plane and out-of-plane multifrequency atomic force microscopy techniques have been demonstrated to be important tools to decipher spatial differences of sample surfaces at the atomic scale. The analysis of physical properties perpendicular to the sample surface is routinely achieved from flexural cantilever oscillations, whereas the interpretation of in-plane sample properties via force microscopy is still challenging. Besides the torsional oscillation, there is the additional option to exploit the lateral oscillation of the cantilever for in-plane surface analysis. In this study, we used different multifrequency force microscopy approaches to attain better understanding of the interactions between a super-sharp tip and an HOPG surface focusing on the discrimination between friction and shear forces. We found that the lateral eigenmode is suitable for the determination of the shear modulus whereas the torsional eigenmode provides information on local friction forces between tip and sample. Based on the results, we propose that the full set of elastic constants of graphite can be determined from combined in-plane and out-of-plane multifrequency atomic force microscopy if ultrasmall amplitudes and high force constants are used.