Beyond optical microscopy

Atomic force microscope

Another New Microscope

The requirement to have a conducting sample limited the usefulness of the STM. Gerd Binnig, Christoph Gerber, and Calvin Quate solved this problem with the invention of the Atomic Force Microscope (AFM) in 1986. [3] As suggested by its name, the AFM uses atomic forces—not the flow of electrons—to scan a sample, so it can be inductive as well as conductive. Still, the set up of the two microscopes is similar (see Figure 6). The AFM has a sharp tip a few micrometers long and usually a diameter less than 100 Å. It is attached to the end of a flexible tube 100-200 µm in length called a cantilever. The tip is brought close enough to the sample to feel forces that contribute to atomic bonds, called van der Waals forces. These are due to the attraction and repulsion of positively-charged protons and negatively-charged electrons. As electrons zip around an atom, they create temporary regions of positive and negative charges, which attract oppositely-charged regions on other atoms. If the atoms get too close, though, the repulsive force of the electrons overshadows this weaker attraction. In terms of the AFM, the temporary positive and negative charges attract the atoms in the tip and sample when they are far apart (several angstroms), but if they come too close (1-2 Å, less than the length of an atomic bond), the electrons on the tip and sample repel each other. This feature led to the development of two types of AFM: contact and non-contact.

The Contact AFM

A contact AFM is so called because the tip and the sample are closer to each other than atoms of the same molecule are. (It is difficult to define “contact” at the molecular level; bonds form when electrons from different atoms overlap. There is no rubbing together of atoms as we think of it at the macrolevel.) Since the cantilever is flexible, it is sensitive to the mutually repulsive force exerted between the tip and sample. This force varies with the topography of the latter–bumps bring the sample closer to the tip, increasing the force between them, while dips decrease the force. The variance in force is measured in two ways. In “constant-height” mode, the cantilever moves across the sample at a constant height, subjecting the tip to stronger and weaker forces, which cause the cantilever end to bend. This movement is measured by a laser beam that bounces off the reflective cantilever and onto a detector. In “constant-force” mode, the height of the cantilever is adjusted to keep the force between the tip and sample constant. Thus, the bend in the tip stays the same and the height adjustment is measured instead.

The Non-Contact AFM

As suggested by its name, the tip and sample are farther apart in a non-contact AFM. The cantilever vibrates so that the tip is tens to hundreds of angstroms from the sample, greater than the distance of a typical atomic bond, meaning that the force between them is attractive (compare to the 1-2 Å distance of the contact AFM). As the tip vibrates, it is pulled by this force, affecting its vibration frequency. A bump in the sample will cause a greater attractive force than a dip, so the topography is analyzed by recording the vibration frequency.

Comparing the Two

Contact and non-contact AFMs generate similar pictures of a sample, which can be roughly interpreted as a topographical map (though other factors affect the force readings, such as local deviations in the electron density of the sample). However, each has its advantages and disadvantages that better suit it for certain sample types. In non-contact, the sample and tip remain far enough apart that the force between them is low and does not significantly affect the sample itself. This makes changes in topography more difficult to detect, but it also preserves the sample, which is especially important if it is soft and elastic, as well as the tip. In addition, the cantilever must be stiffer than for a contact AFM, otherwise it may bend too much, causing the tip to “contact” the sample. A contact AFM is more useful for sample surfaces that may be covered with a thin layer of water. Even in a high vacuum, this can occur when gaseous water condenses upon it. A non-contact AFM will not penetrate the water layer and will record its topography instead of the sample, but a contact AFM gets close enough to break through this problem.

Questions for review

• What was significant about the invention of the AFM (what could be done that was not possible before)?
• Why is are the names “contact” and “non-contact” associated with these types of AFM?
• AFM tips are commonly composed of silicon or silicon nitride. Given that the latter is a tougher, more durable material, which would be more appropriate for a contact AFM?