Atomic Force Microscopy (AFM)

How does AFM work?

In AFM a sample is scanned by a very sharp micron-sized tip mounted on a cantilever spring. The interaction between the tip and sample can be measured by monitoring the deflection of the cantilever. This is quantified by a laser signal focused on the cantilever tip and reflected onto a position sensitive photodiode. By plotting the deflection of the cantilever against its position on the sample, it is possible to map the sample's topography. Alternatively the height of the translation stage can be mapped while maintaining a constant force against the surface, where a feedback loop is initiated using piezoelectric input. In this way images on the nano-scale can be generated. 

AFM can also be used to measure surface and interfacial forces. If the cantilever and sample are kept in fixed lateral positions, the substrate can be rastered up and down by the piezo translation stage it is carried upon, onto and off the cantilever by applying a voltage. When nearing contact the sample will very often attract the tip prior to a fixed level contact, causing a ‘jump in’ deflection. The piezo input then continues the sample movement until a pre-set value for the stage height (z-coordinate) is reached.

The stage then reverses, and the sample and tip will separate. Separation will occur only when the restoring force of the cantilever (spring constant) surpasses the interaction force of the tip and sample, and as such the negative deflection of the cantilever below the non-contact output can be used to quantify the interaction force by applying Hooke’s law.

By attaching functional materials of interest (pharmaceutical particles, proteins, aptamers etc.) to an AFM cantilever and using this ‘functionalised’ probe to make force measurements it is then possible to measure the interactive forces between that material and a given substrate. In so doing a multitude of systems can be analysed at a particulate to particulate and even molecule to molecule level. Other variations on AFM tip design such as the use of a resistive heater, allow for other modified probing experiments like micro-thermal analysis.