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.