Atomic Force Microscopy

Atomic Force Microscopy (AFM) is a member of the family of Scanning Probe Microscopy, together with the Scanning Tunneling Microscopy (STM) and the Scanning Near-field Optical Microscopy (SNOM). Unlike them, however, it is also a descendent of a popular instrument from the pre-superresolution era, the Stylus Profilometer (SP). The probe tip in the AFM is analogous to the stylus in the SP, with a few but significant differences. First, the forces probed by the AFM are in the range where the concept of contact becomes hazy: the contact regime can be defined as the situation where the probe is so close to the sample that is repelled by it. At larger distances, the probe and the sample are attracted to each other, for example, by Van der Waals forces. In this case, the operation is said to be in the non-contact regime. Second, the short distance at which the interaction happens in AFM does not allow for the probe to be driven vertically like in the SP. Instead, it is mounted on a cantilever beam. The forces on the surface of the sample are detected through the deflection of the cantilever beam by a suitable method, the most popular consisting of focusing a laser on the back of the cantilever and detecting variations in the direction of the reflection using a four-quadrant photodetector. These differences make the AFM much more versatile than the SP. For example, in the so-called constant-force mode, the force of repulsion is set to a fixed value and an error signal is generated whenever the force deviates from such value. The error signal that is used to correct the distance between probe and sample is used to form an image of the sample. By scanning fast enough, the AFM can work in the so-called constant-distance mode, in which the signal obtained from the deflection of the cantilever is a map of the distribution of the forces throughout the sample. In the dynamic modes of operation, the cantilever is made to oscillate at or close its frequency of resonance. This can involve operation both on the contact and non-contact regimes, that is, the intermittent contact regime. If the cantilever is driven to the sample, the force of interaction changes the amplitude, frequency or phase of the oscillation and the information contained into one or several of these parameters is used to form the image. The sample does not need to be a passive element, as happens in the Piezoresponse Force Microscope (PFM), in which the cantilever is used to provide electrical excitation to a piezoelectric sample. This variation of AFM can collect information about the ferroelectric domains and the topography of a sample, at the same time. In this work, we intend to give a general description of Atomic Force Microscopy, discussing the singularities of the different regimes and modes of operation, as well as of some features and variations of this versatile technique.