Atomic force microscopes

Technology

“An AFm uses a cantilever with an incredibly fine silicon tip that’s usually micrometres long”

What are atomic force microscopes?

How do these instruments image samples too tiny for normal microscopes?

Atomic force microscopy (AFM) is a branch of scanning probe microscopy that includes several dozen ways of scanning the surface of a tiny specimen to create an image. It can resolve images of objects mere nanometres long, over 1,000 times sharper than the best optical microscopes.

An atomic force microscope uses a cantilever with an incredibly fine silicon tip, or probe, that’s usually micrometres long and whose tip has a radius of under ten nanometres. The tip of the cantilever runs over the surface of the sample, making contact or maintaining a tiny distance depending on the settings dictated.

These microscopes can work in several modes that fall into two categories: static and dynamic. The cantilever is physically dragged over the sample’s surface in static mode and the contours of the specimen are directly measured. In dynamic – or tapping – mode, on the other hand, the cantilever is oscillated and the varying forces that result from its interaction with the sample are recorded. The obvious advantage that dynamic mode has over static is that it can be used on ‘soft’ specimens, where contact might lead to degradation of both the sample and the tip.

AFM: pros and cons

Scanning electron microscopy (SEM) is a different type of microscopy that deals with samples of a similar size to atomic force microscopy. AFM can boast certain advantages over SEM, but it also has some disadvantages.

AFM can create a 3D profile of the sample, unlike SEM’s 2D results and the samples don’t require any special coating either. This means specimens that might be changed or damaged by the SEM process (eg micro-organisms) can be studied. AFM can also be used in conjunction with other optical techniques and can yield higher resolutions even in liquid environments.

On the down side, AFM is limited with regard to the size of the sample scan image it can take – around 1,000 times smaller than the area SEM can capture in a single pass. Its scanning speed is slower too and an atomic force microscope can’t measure samples with particularly steep topography unless it’s specially modified.

Atomic force microscope up close

HIW homes in on the key parts of static and dynamic atomic force microscopes to reveal how they study minute samples

Laser

A beam of laser light shines onto the flat top of the cantilever, reflecting onto the position detector.

Dynamic

Cantilever

A cantilever (a lever fixed at one end) with a very fine tip is scanned across the surface of the sample.

Mode

The AFM can be set to either a static or a dynamic mode; in the latter the tip is vibrated as it passes over the specimen.

A researcher from the University of Leicester, UK, uses an ultra-high vacuum atomic force microscope to study the Casimir effect

Position detector

A light-sensitive photodiode records changes in the position of the reflected laser beam. It is this data which is used to generate an image.

Static

Sample

The microscopic peaks and troughs in the sample deflect the cantilever towards or away from the surface.