AFM

Atomic force microscopy (AFM) is a type of scanning probe microscopy that images surfaces at the nanometer scale by measuring forces between a sharp tip attached to a flexible cantilever and the sample surface. The tip–sample interaction deflects the cantilever as it scans, and this deflection is converted into a topographic map with sub-nanometer vertical resolution and nanometer-scale lateral resolution determined by the tip diameter and scan path.

The deflection is measured by reflecting a laser off the back of the cantilever onto a position-sensitive photodetector. A feedback loop adjusts the vertical position of the sample or the tip to maintain a constant interaction, producing an image of surface topography. AFM can operate in several modes: in contact mode the tip remains in continuous contact with the surface; in tapping or dynamic mode the cantilever is driven near its resonance and changes in oscillation amplitude, frequency, or phase track the surface; in non-contact mode the tip oscillates at a short distance to detect long-range forces. AFM can function in air, in liquids, or in controlled environments.

Applications include imaging of inorganic and organic materials, polymers, and biological specimens; measurement of surface roughness, adhesion, stiffness, and mechanical properties through force-distance curves and nanoindentation; and functional imaging techniques such as Kelvin probe force microscopy (KPFM) for surface potential, magnetic force microscopy (MFM) for magnetic domains, and electrostatic force microscopy (EFM).

Advantages include high spatial resolution without requiring conductive samples and compatibility with a variety of environments. Limitations include relatively slow imaging speeds, potential tip-induced sample damage, tip wear, and the need for careful calibration and interpretation of force data. The technique requires flat, accessible surfaces and careful sample preparation.

AFM was developed in the 1980s, with early demonstrations by G. Binnig, C. F. Quate, and colleagues; since then it has become a standard tool in materials science, biology, and nanotechnology, with numerous commercial instruments and specialized variants.