AFM: A Versatile Tool for High-Resolution Surface Imaging

Advanced Surface Microscopy

Unlike optical microscopy, which requires lenses and beam irradiation, AFM does not have these limitations. This allows the user to obtain a high spatial resolution without needing to stain the sample.

AFM can be operated in three modes: contact mode, tapping mode and non-contact mode. In contact mode, the cantilever is oscillated at its resonant frequency or just above (frequency modulation). A feedback mechanism measures the deflection of the cantilever and adjusts the support-sample separation.

AFM – Atomic Force Microscopy

AFM is a technique that allows the imaging of topography and material properties at micrometer and nanometer scales. It works by rastering a sharp tip on an oscillating cantilever over the sample surface and detecting the force interactions between the tip and the surface. The cantilever deflections are recorded by a photodiode, amplified, and coded into images of the sample.

In non-contact mode, the AFM cantilever is susceptible to lateral forces from ambient air or sticking to the surface contamination layer that is present on most surfaces. These lateral forces distort the image and make it difficult to detect geometrically correct surface structures.

Functional properties are often more important than topography information for many applications, especially in emerging technologies such as solar cells and nonvolatile memory devices. Many AFM modes have been developed to interrogate functional properties by sensing electrostatic, magnetic, and other tip-sample interactions.

AFM Applications

The AFM can image surface topography, measuring the atom-by-atom interaction forces between the sample and the probe tip (as shown in Figure 2). The force data is then converted into an image of the surface. This technique is useful for studying a variety of materials including thin and thick film coatings, polymers, composites, glass, metals, synthetic and biological membranes and other organic structures.

In addition, the AFM can make contactless measurements by modulating the oscillation amplitude or phase to measure changes in the separation distance between the probe tip and the sample. These techniques can be used to perform force spectroscopy and can be used to measure mechanical properties of the sample such as Young’s Modulus, a measurement of stiffness.

The AFM can also measure the geometry of structures such as pillars and valleys in a sample. These structures are often difficult to see with other microscopic techniques. This information can be used to understand and control the material properties of the sample.

AFM Techniques

There are a number of ways to use AFM. It is most common to image surface topography using contact mode, although other techniques are available that can be used in liquids or with a variety of different materials such as magnetic force microscopy or stiffness tomography.

During scanning in contact mode, the interaction between tip and sample (which can be atomic-scale) is converted by the piezoelectric layer to changes of the motion of the cantilever which are detected. These changes can include the value of the deflection, amplitude of an imposed oscillation, or shift in the resonance frequency of the cantilever.

Alternatively, an electric sensor located on the tip can detect tunneling current flowing between the sample and the probe and this is known as scanning tunnelling microscopy (STM). This allows a large amount of geometrical information to be extracted from samples such as mapping the distribution of adhesion forces across the surface. The sensor can also be modified to detect other electrostatic and electrical properties.

AFM Equipment

An AFM is an excellent tool for NP/NM characterization, providing spatial distribution information on composite materials that can be more useful than single point measurements. It also provides quantitative information such as particle size, surface area, and volume distributions.

Despite its advantages, AFM has several limitations. The instrument’s vibration environment limits vertical resolution, and the AFM can be susceptible to contamination or temperature changes that lead to distorted images.

Fortunately, LNF’s AFM can operate at low vibration levels in ambient air and liquids. Moreover, we have developed a processing technique for the AFM tip that maintains the tip’s functionality after the process. Previously, the tip would need to be replaced for each scan to obtain the desired images. With this method, the AFM can operate with the same tip for multiple scans. This is important for acquiring multi-dimensional data sets. It is also possible to gather both topographic and electric/magnetic data in a single scan using the Dual-Pass Mode a.k.a Hover Mode.

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