Hydration Dynamics and the Future of Small-Amplitude AFM Imaging in Air.

Sergio Santos,Matteo Chiesa,Tuza A Olukan,Chia-Yun Lai

doi:10.3390/molecules26237083

Abstract

Here, we discuss the effects that the dynamics of the hydration layer and other variables, such as the tip radius, have on the availability of imaging regimes in dynamic AFM—including multifrequency AFM. Since small amplitudes are required for high-resolution imaging, we focus on these cases. It is possible to fully immerse a sharp tip under the hydration layer and image with amplitudes similar to or smaller than the height of the hydration layer, i.e., ~1 nm. When mica or HOPG surfaces are only cleaved, molecules adhere to their surfaces, and reaching a thermodynamically stable state for imaging might take hours. During these first hours, different possibilities for imaging emerge and change, implying that these conditions must be considered and reported when imaging.

Highlights

The atomic force microscope can be operated in air, ultra-high vacuum (UHV), and liquid environments [1]
(1) such small amplitudes are of the same order of magnitude or smaller than the water films that form on surfaces, i.e., ~0.1–1 nm, and (2) it shows that even recent research fails to report the actual regime of operation, i.e., how many regimes were available for imaging in the experiments and which regime was exploited
We have reviewed and discussed (1) force measurements in air as a function of time and (2) small-amplitude imaging in dynamic AFM in the presence and absence of water layers

Summary

Introduction

The atomic force microscope can be operated in air, ultra-high vacuum (UHV), and liquid environments [1]. We have discussed the variations of force measurements as a function of time [27,34] and the effects on AFM measurements due to the presence of water films on surfaces [35] The latter line of research provides information about the evolution of force profiles, but the connection between such evolution and the emergence of force regimes is stated mostly, or only, with respect to extreme conditions, i.e., fresh samples and aged samples. We emphasize that the connection between the understanding of hydration dynamics, or adsorption kinetics, of contaminants such as water gained from force measurements and the possibilities for imaging is still largely lacking Since these two objects of investigation are connected, it should be possible to understand how and when they affect each other. In this brief article, we discuss possible ways to connect the analysis of force reconstruction in time and as surfaces age on the one hand, and the evolution of imaging regimes in air and the possibilities that these offer on the other

The Problem of Small Amplitude Imaging and Surface Aging

Force Reconstruction and Imaging

Other Possible Dynamic Parameters to Consider

Conclusions