Sub-nanometer Resolution Imaging with Amplitude-modulation Atomic Force Microscopy in Liquid.

Ethan J Miller,Clodomiro Cafolla,William Trewby,Kislon Voïtchovsky,Luca Piantanida,Amir Farokh Payam

doi:10.3791/54924

Abstract

Atomic force microscopy (AFM) has become a well-established technique for nanoscale imaging of samples in air and in liquid. Recent studies have shown that when operated in amplitude-modulation (tapping) mode, atomic or molecular-level resolution images can be achieved over a wide range of soft and hard samples in liquid. In these situations, small oscillation amplitudes (SAM-AFM) enhance the resolution by exploiting the solvated liquid at the surface of the sample. Although the technique has been successfully applied across fields as diverse as materials science, biology and biophysics and surface chemistry, obtaining high-resolution images in liquid can still remain challenging for novice users. This is partly due to the large number of variables to control and optimize such as the choice of cantilever, the sample preparation, and the correct manipulation of the imaging parameters. Here, we present a protocol for achieving high-resolution images of hard and soft samples in fluid using SAM-AFM on a commercial instrument. Our goal is to provide a step-by-step practical guide to achieving high-resolution images, including the cleaning and preparation of the apparatus and the sample, the choice of cantilever and optimization of the imaging parameters. For each step, we explain the scientific rationale behind our choices to facilitate the adaptation of the methodology to every user's specific system.

Highlights

Since its invention, three decades ago, atomic force microscopy (AFM)[1] has established itself as a technique of choice for investigating samples at the nanoscale, especially where averaging over macroscopic surface areas is not possible and local information is required
Harder crystalline materials such as minerals (Figures 3A, B, D) and single metal ions adsorbed on a surface (Figure 3C) can be imaged using the approach because in every case, it is the interfacial liquid that is effectively imaged with the protocol described
Assuming that the imaging liquid and the cantilever stiffness have been selected appropriately, the most critical steps for achieving successful high-resolution are the adjustment of the imaging amplitude, and the overall cleanliness of the system investigated

Summary

Introduction

Three decades ago, atomic force microscopy (AFM)[1] has established itself as a technique of choice for investigating samples at the nanoscale, especially where averaging over macroscopic surface areas is not possible and local information is required. In a typical AFM measurement, the deflection of a flexible cantilever is used to quantify the interaction force between a small number of molecules and an ultrasharp tip mounted at the end of the cantilever. Part of the success of AFM is its ability to work on a wide range of materials[16] and in multiple environments such as vacuum[17], gas[11,18] or liquid[19,20], because it does not rely on a specific force between probe and sample. An added difficulty comes from the fact that it is usually necessary to operate the AFM in dynamic mode (vibrating tip) in order to preserve both tip and sample by avoiding large friction forces. The presence of liquid around the vibrating cantilever considerably alters its dynamics[45] as well as the interaction between the tip and the sample[29,42]. The liquid can be exploited to enhance the imaging resolution[26,29], with a typical improvement of almost an order of magnitude compared to the best resolution achieved in ambient conditions[46]

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