Exploring Surface Properties and Mapping Topography: The Versatile Technique of Atomic Force Microscopy

How Atomic Force Microscopy Works

Atomic force microscopy is a non-destructive technique that allows the measurement of topographical, electrical, magnetic, chemical, optical and mechanical properties of surfaces with high resolution in air, liquids or ultrahigh vacuum. The technique is also known as scanning probe microscopy (SPM).

AFM utilizes the deflection of an oscillating cantilever. This deflection is measured with an Angstrom-level accuracy using a feedback loop system.

AFM can be used for a variety of applications

AFM is used in a wide range of applications, including solid-state physics, semiconductor science and technology, molecular biology, cell biology, surface chemistry, and biomedical diagnostics. Its unique ability to identify atoms at a surface and change their physicochemical properties through atomic manipulation is particularly useful.

A probe is attached to a cantilever that extends from the surface of the sample. When a force acts on the probe, it causes the cantilever to bend. This bending is detected by a laser diode and split photodetector and can be converted into an electrical signal. A computer uses this signal to create an image of the sample’s surface.

AFM can also measure mechanical properties of samples, such as adhesion and elastic modulus. It can even detect structures of different stiffness buried within the bulk of the material using force spectroscopy. The technique can also be used to map a sample’s surface topography and provide spatial distribution information on composite materials with non-uniform topographies.

It is non-destructive

AFM is unique among classical microscopies in that it senses the force between a sharp probe and the surface it scans. The image is generated by scanning the probe in x and y directions, recording the changes in cantilever deflection, and interpreting the results using force-distance curves. The imaging resolution depends on the imaging force and probe geometry, and can reach sub-nanometer lateral (Z) and sub-Angstrom vertical (X) resolution with high accuracy.

However, due to the minimum force required for imaging, conventional AFM cantilevers can deform and even tear apart living cells. To overcome this limitation, researchers at NIST have developed a new imaging technique that allows them to track the distribution of carbon nanotubes within a polymer composite without damaging it. The technique relies on the difference in the dielectric constant of carbon nanotubes and a polymer. The researchers use this information to identify individual carbon nanotubes, as well as their location below the composite’s surface.

It has high resolution

AFM uses a sharp tip on a flexible cantilever to scan a surface. When the tip contacts a surface, it bends the cantilever and records a response signal that is used to trace the sample’s topography. Unlike optical microscopy, AFM has high resolution in both the x- and y-axes, allowing for accurate measurement of even the most minute features.

AFM also has the ability to measure force at atomic distances using Scanning Tunneling Microscopy (STM) by applying an electrical bias to the probe and measuring the tunneling current between the tip and the sample. Both STM and AFM can achieve a resolution of 0.1 nanometers laterally and 0.01 nm vertically, 1000 times better than the optical diffraction limit.

It is versatile

Atomic force microscopy has demonstrated a resolution of fractions of a nanometer, 1000 times better than the optical diffraction limit. This is possible because it gathers information by “feeling” the surface using a mechanical probe. Piezoelectric elements facilitate very small but precise movements through electronic control for precise scanning.

The cantilever can be scanned by oscillating it at its resonant frequency or just above it (frequency modulation) or it can be pushed against the sample by attractive and repulsive forces. As the tip-surface interactions are recorded, the cantilever deflection and signal amplitude are transmitted to the sensor, which reconstructs the topographic image.

The force-distance curves that are collected can provide valuable information about the surface properties of a sample, such as elasticity, viscoelasticity, and charge densities. These data can be used to optimize biomaterials performance, and it may even lead to new drug delivery methods. It can also help to identify molecular dynamics processes involving porous scaffolds and bioactive molecules in cells, and it may aid in the design of therapeutic strategies.

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