Atomic force microscopy is arguably the most versatile and powerful microscopy technology for studying samples at nanoscale. It is versatile because an atomic force microscope can not only image in three-dimensional topography, but it also provides various types of surface measurements to the needs of scientists and engineers. It is powerful because an AFM can generate images at atomic resolution with angstrom scale resolution height information, with minimum sample preparation.
So, how does an AFM work? In this page, we introduce you to the principles of an AFM with an easy to understand video animations. Feel free to share this page with others, and to email us if you have any questions.
AFM Principle
Nano World
Nano, from the Greek word for 'dwarf', corresponds to a prefix
denoting a factor of 10-9. Thus, a nanometer is one billionth of a meter, which is the length
scale at which intermolecular force and quantum effect take hold. To put the nanoscale in a more
understandable perspective, consider that the size of an atom relative to an apple is similar to the size of
an apple relative to the planet Earth! Atomic Force Microscopes (AFMs) give us a window into this nanoscale
world.
AFM Principle
- Surface Sensing
An AFM uses a cantilever with a very sharp tip to
scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the
surface and the tip cause the cantilever to deflect towards the surface. However, as the cantilever is
brought even closer to the surface, such that the tip makes contact with it, increasingly repulsive force
takes over and causes the cantilever to deflect away from the surface.
- Detection Method
A laser beam is used to detect cantilever deflections towards or away from the surface.
By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause
slight changes in the direction of the reflected beam. A position-sensitive photo diode (PSPD) can be used
to track these changes. Thus, if an AFM tip passes over a raised surface feature, the resulting cantilever
deflection (and the subsequent change in direction of reflected beam) is recorded by the PSPD.
- Imaging
An AFM images the topography of a sample surface by scanning the cantilever over a region of
interest. The raised and lowered features on the sample surface influence the deflection of the cantilever,
which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the
surface—thus maintaining constant laser position—the AFM can generate an accurate topographic map of the
surface features.