Optimization of qPlus sensor geometry and circuit for high-speed atomic force microscopy in liquid environments

TL;DR

This paper enhances qPlus AFM sensors by optimizing geometry and circuitry to significantly reduce noise and improve high-speed atomic-resolution imaging in liquids.

Contribution

It introduces a low-noise qPlus sensor with a third of the noise density of conventional sensors, enabling faster, high-resolution imaging in liquid environments.

Findings

Achieved an n_{ds} of 9.3 fm Hz^{-1/2}, about one-third of conventional sensors.

Reduced the minimum detectable force gradient by half.

Demonstrated high-speed imaging of a molten gallium interface at 6.6 s frame rate.

Abstract

Atomic force microscopy (AFM) using qPlus sensors is a powerful tool for high-resolution analysis in various liquids, including high-viscosity or opaque environments. However, the relatively high displacement sensor noise density (n_{ds}), combined with the high spring constant and the low resonance frequency, limits force sensitivity and has hindered high-speed imaging. In this paper, we clarify the dominant factors governing n_{ds} and the minimum detectable force gradient (F'_{min}) through a comprehensive analysis of sensor geometry and circuit theory. Based on these findings, we developed a low-noise qPlus sensor that achieves an n_{ds} of 9.3 fm Hz^{-1/2}, which is approximately one-third that of conventional sensors, and reduces F'_{min} by half. Using this sensor, we demonstrated high-speed, atomic-resolution imaging of a molten gallium interface at a frame rate of 6.6 s frame^{-1} (39 lines s^{-1}), proving its advantage for analyzing fast interfacial dynamics in liquid environments.