Atomic-resolution imaging in liquid by frequency modulation atomic force microscopy using small cantilevers with megahertz-order resonance frequencies

Abstract

Abstract. In this study, we have investigated the performance of liquid-environment FM-AFM with various cantilevers having different dimensions from theoretical and experimental aspects. The results show that the reduction of the cantilever dimensions provides improvement in the minimum detectable force as long as the tip height is sufficiently long compared with the width of the cantilever. However, we also found two important issues to be overcome to achieve this theoretically-expected performance. The stable photothermal excitation of a small cantilever requires much higher pointing stability of an excitation laser beam than that for a long cantilever. We present a way to satisfy this stringent requirement using a temperature controlled laser diode module and a polarization-maintaining optical fiber. Another issue is associated with the tip. While a small carbon tip formed by electron beam deposition (EBD) is desirable for small cantilevers, we found that an EBD tip is not suitable for atomic-scale applications due to the weak tip-sample interaction. Here we present that the tip-sample interaction can be greatly enhanced by coating the tip with Si. With these improvements, we demonstrate atomic-resolution imaging of mica in liquid using a small cantilever with a megahertz-order resonance frequency. In addition, we experimentally demonstrate the improvement in the minimum detectable force obtained by the small cantilever in the measurements of oscillatory hydration forces.

Dissolution Processes at Step Edges of Calcite in Water Investigated by High-Speed Frequency Modulation Atomic Force Microscopy and Simulation

“…Note that this performance is available only when the noise from the instruments is smaller than that from the thermal vibration of the cantilever. To meet this requirement, we developed a wideband, low noise and highly stable cantilever deflection sensor [22][23][24] , photothermal cantilever excitation system 21,24 and digital phase-locked loop (PLL) circuit 25,26 . With these improvements, we achieved thermal-noise-limited performance even with a small cantilever, as shown in Figure 1b.…”

Advanced Instrumentation of Frequency Modulation AFM for Subnanometer-Scale 2D/3D Measurements at Solid-Liquid Interfaces

“…However, we found that stable excitation of a small cantilever requires a higher stability of the excitation laser beam than that of a conventional cantilever [50]. This is because even a small spatial fluctuation of the laser beam position or shape can give a significant influence on the excitation characteristics of a small cantilever.…”

“…2(c) and 2(d)) [50]. Both the curves show an oscillatory profile corresponding to the hydration structure formed at the mica-water interfaces.…”

“…They recently commercialized these cantilevers with an affordable price. In the meanwhile, we have been working with these companies as a test user and have developed an FM-AFM instrument compatible with the small cantilevers [50].…”

“…Firstly, we should have sufficient F min for a given B. We satisfied this requirement by using the small cantilever [50]. As discussed above, the small cantilever gives seven-fold improvement in F min for a given B.…”