This article was updated on the 3rd December 2019.
Atomic force microscopy (AFM), is a powerful, versatile technique for imaging and studying surfaces, and is used throughout many scientific areas and industries. Not only can it be used in different modes, but the tip can also be changed depending on the material being analyzed. Here in this article, we not only look at what AFM is but how different AFM tips can be used for different functions – namely the standard ‘sharp tip’ and a colloidal probe.
What is AFM?
AFM is a microscopy technique, where a cantilever beam physically touches the sample to image it. The resulting image is a topographic map of the surface. This technique is used across a wide range of applications because the surface chemistry of a material often determines how stable or functional the material is.
So how does it perform the measurements? The tip of a cantilever beam is positioned above the sample and approaches the surface of the material. In doing so, the intermolecular attraction (i.e., the van der Waals forces) between the surface and the tip forces the cantilever to move towards the surface and tap it.
An AFM instrument also possesses a laser beam. When the cantilever moves towards the surface of the material, the laser beam deflects off the back of the cantilever beam. The positional change of the cantilever beam is recorded by the laser hitting a position-sensitive photo-diode (PSPD). AFM instruments then use a feedback loop system to create a high-resolution topographic map, once the deflections of the cantilever have been recorded for both the lateral and vertical displacements.
However, the general mode of operation is not the only beneficial property of AFM. AFM also has non-contact modes, and a variety of tips can be interchanged on the cantilever beam, depending on the material and application.
AFM Sharp Tip
The sharp tip is the standard tip of an AFM instrument. These standard AFM tips are pyramidal (either trigonal or square-based), with the apex of the pyramid facing towards the sample. The sharpness of the point is important, as it relates to the accuracy of the area being analyzed. For this reason, the smaller the tip, the better.
Most tips are less than 10 nm in diameter, but to increase the sharpness of the tip, many AFM tips possess a high aspect ratio with the length of the tip in the micron region. A high aspect ratio is especially useful in the last few hundred nanometers of the tip, as it significantly increases the sharpness. These types of tips come in many forms, including silicon-based tips, carbon-based composite tips and metal-coated tips. The sharp nature of these tips makes them more suited towards ‘harder’ materials and surfaces, as soft surfaces can become damaged by the sharp points of the tip.
If a softer material needs to be imaged, then a sharp tip can be operated in non-contact mode. In this mode, the tip never touches the surface of the material, so there is no risk of surface damage and therefore inaccurate results. In non-contact mode, the tip vibrates above the resonance frequency of the surface, meaning that topological changes can be recorded without needing to engage the sample physically. However, it is not as effective, as it can become privy to interference from long-range interaction forces from the sample.
AFM Colloidal Probe
Colloidal probes do not contain a sharp tip. Instead, they possess a micron-sized sphere glued to the end of the cantilever beam. Colloidal probes are used in a very different way to standard tips and are used to measure a wide range of properties at the surface of a material, including measuring the direct surface force and adhesion forces, as well as studying the colloidal interactions between the particle and the surface.
The internal workings of an AFM instrument don’t change when a colloidal probe is used, and the information is still deduced by deflecting the laser beam to a photodiode. However, unlike a sharp tip, colloidal probes can also be used to interact with particles on a surface, as well as the surface itself. These are known as sphere-sphere and plane-sphere geometries, respectively.
Colloidal probes do have some disadvantages. When measuring the properties of a sphere, the particles in question need to be lined up with the colloidal probe sphere, and this is sometimes approximated. In addition, the gluing of the particle to the probe is not a straightforward procedure, and multiple wetting of the probe can lead to the formation of nanoscale bubbles on the surface of the probe sphere.
Colloidal probes can also be modified with various chemical coatings to introduce specific chemical interactions between the probe and the surface. In addition, colloidal probes can also be adapted to measuring biological phenomena. In all, the colloidal probe is an innovative and non-destructive way to measure the properties of a surface
Thumbnail Image Credit: Georgy Shafeev/Shutterstock