Atomic force microscopy (AFM) is a type of scanning probe microscopy (SPM) that has an extremely high resolution, detecting molecules within a fraction of a nanometer. A mechanical probe is used to gather information by touch with the aid of piezoelectric elements that enable very small but precise movements through electronic control for precise scanning.
Before AFM, the scanning tunneling microscope (STM) was developed in the 1980s by IBM, which later resulted in the Nobel Prize in Physics. This led to the development of the AFM microscope, which was made commercially available in the late 1980s for precision imaging at the nanoscale.
What is AFM?
A high-resolution AFM image is generated by the sharp tip at the end of a cantilever scanning over the surface of a sample. When the tip makes contact with the sample, it bends the cantilever and changes the amount of laser light reflected into the detector. The topography of the sample can then be traced by the height of the cantilever being adjusted according to the response signal.
How does it work?
The cantilever in AFM is typically silicon with a nanometer scale tip. When the tip is brought into contact with the surface of a sample, the small spring-like cantilever is deflected by forces between the tip and the surface. This is in accordance with Hooke’s law that states a force needed to extend or contract a spring by a given distance is scaled linearly with respect to that distance.
Furthermore, a piezoelectric element oscillates the cantilever. The detector then records the deflection of the cantilever relative to its equilibrium position and converts it into an electrical signal. The strength of this signal will be relative to the deflection of the cantilever.
Several forces are measured in AFM, some of which include van der Waals forces, mechanical forces, chemical bonding, electrostatic forces, magnetic forces, capillary forces, etc. There are two main imaging modes for AFM. These include static (contact) modes and dynamic (non-contact) modes.
Contact mode involves the tip making continuous contact with the surface of the sample as it moves across the sample. The contours of the sample are measured either by the deflection of the cantilever or by the feedback signal maintaining a constant position of the cantilever. A more flexible cantilever is generally used in contact mode due to it being prone to noise and therefore to keep the interaction force low.
Alternatively, tapping mode can be used to address the problems of the probe sticking in contact mode. This is achieved by oscillating the cantilever up and down as it scans the surface of the sample. This reduces any damage caused to the sample and tip compared to contact mode.
Additionally, there are three types of applications with AFM:
1) force measurement
2) topographic imaging
3) manipulation
Force measurements allow the measurement of forces between the tip of the cantilever and the sample as a function of their mutual separation. This can be used to measure mechanical properties of the sample such as sample rigidity.
High-resolution topographic imaging can be generated by measuring the forces that the sample has on the tip. This is achieved by measuring the height of the probe concerning the position on the sample surface. It is also possible to manipulate and change the properties of the sample using AFM by measuring the forces between the tip and the sample. This has been shown using scanning probe lithography.
Applications
There have been several applications using AFM in a broad range of scientific disciplines. Some of these include the physical sciences, life sciences, solid-state physics, biomedical diagnostics, molecular biology, cell biology, physics, surface chemistry, and medicine.
AFM is commonly used in physics and chemistry to identify and characterize the formation of atoms at a surface. This has big implications in cell biology, molecular