AFM Wiki

Atomic Force Microscopy

Atomic force microscopy (AFM) is a scanning probe technique that dispenses surface profiles with nanometer-scale resolution by measuring intermolecular forces between a probe (tip, <10 nm radius) and surfaces at proximal distances (0.2-10 nm). AFM is compatible with both conductive and non-conductive materials, covering a broad range of applications, such as electronics, semiconductors, and biomaterials. Information given by AFM tremendously advances our understanding of the surface chemistry and physics of materials.

The working principle of AFM relies on the detection of the differential van der Waals interactions between the tip and the surface, which manifests itself as attractive and repulsive forces. The force depends on the spring constant of the cantilever and the distance between the probe and the sample surface. The force can be approximated to be linearly proportional to the cantilever displacement, as described by Hooke's law;

F = -kx,

F is force, k is the spring constant, and x is the cantilever deflection. The spring constant of the cantilever typically ranges from 0.1 to 200 N/m, resulting in forces from 10-6 to 10-13 N.

In a general configuration, an atomic force microscope consists of five components, including a piezoelectronic actuator (PZT), a laser source, a position-sensitive photodiode detector, a feedback controller, and a micro-machined sharp tip (Figure 1). AFM is operated by moving the tip laterally over (rasterizing) the surface with feedback mechanisms that enable the PZT scanner to maintain the tip-sample system at constant force or constant separation. As a result of feedback compensation, the PZT scanner moves vertically causing the deviation of laser intensity on the photodiode detector, which is used to construct the differential surface profiles.

Typical AFM modes consist of non-contact mode, contact mode, and tapping mode (Bruker trademark). In non-contact mode (Figure 1-A), the tip is maintained at about 0.1 to 10 nm away from the surface and is oscillated at near its natural resonance frequency. By maintaining its resonance frequency due to varied sample-tip VDW interactions, surface information can be extracted. However, the oscillating tip in native or simulated, biological environment, such as aqueous solutions, causes significant signal interference, making the surface information difficult to interpret. Alternatively, in contact mode (<0.5 nm) (Figure 1-B), the forces between the tip and the surface remain constant by maintaining a constant cantilever deflection. Contact mode measurement offers the advantages of fast scanning and high resolution, provides surface friction analysis, and is suitable for rough samples. Yet, the strong repulsive force exerted by the tip can damage or deform soft samples. Another popular mode of AFM operation is an intermittently contact mode (0.5-2.0 nm), tapping mode, which is the combination of those two techniques (Figure 1-C). While scanning, the oscillating tip periodically touches, "taps", on the sample surface at constant tip-sample interactions by maintaining its oscillation amplitudes. This mode allows for high resolution measurements to be made on sample surfaces, in particular soft biological specimens. In this work, extensive AFM measurements are used to acquire surface topography, analyze protein binding, and examine chemical lift-off lithography.