Atomic force microscope

An atomic force microscope works by scanning a sharp tip attached to a flexible cantilever across a sample surface. The tip-sample interaction forces cause the cantilever to deflect, and this deflection is measured using a laser beam reflected off the cantilever onto a position-sensitive photodetector. The measured deflection is used to generate a topographic image of the surface.

The AFM can operate in several modes: contact mode, where the tip is in constant contact with the surface; tapping mode (also called intermittent contact), where the cantilever oscillates at its resonance frequency and lightly taps the surface; and non-contact mode, where the tip hovers above the surface and senses attractive forces. Each mode has distinct advantages for different sample types.¹

The atomic force microscope was invented in 1986 by Gerd Binnig, Calvin Quate, and Christoph Gerber at IBM Zurich and Stanford University. It was developed as an extension of the scanning tunneling microscope (STM), which required conductive samples. The AFM overcame this limitation by measuring forces rather than tunneling current, allowing imaging of insulators and biological molecules.²

Binnig and Heinrich Rohrer had already won the Nobel Prize in Physics in 1986 for the STM, and the AFM quickly became a cornerstone of nanotechnology. Early AFMs used simple cantilevers and optical levers, but modern instruments incorporate advanced feedback electronics, piezoelectric scanners, and environmental controls.

Atomic force microscopy is used across many scientific disciplines. In materials science, it characterizes surface roughness, grain structure, and thin-film morphology. In biology, it images DNA, proteins, and living cells in physiological conditions, and can measure mechanical properties of cells and tissues. In semiconductor manufacturing, it is used for critical dimension metrology and defect analysis.

AFM is also employed in polymer science to study surface properties and in electrochemistry to probe electrode surfaces. It has been used to manipulate individual atoms and molecules, and to perform nanolithography.

Beyond conventional imaging, AFM has niche applications. It can be used for force spectroscopy, measuring single-molecule interactions such as ligand-receptor binding or protein unfolding. In the study of two-dimensional materials like graphene, AFM reveals atomic-scale defects and stacking order.

One surprising application is in the analysis of ancient artifacts: AFM has been used to examine the surface of archaeological pottery and paintings to understand ancient manufacturing techniques. Another is in the study of meteorites, where AFM maps mineral surfaces to infer conditions in the early solar system.

AFM has also been combined with other techniques, such as infrared spectroscopy (AFM-IR) and Raman spectroscopy, to provide chemical information at the nanoscale. In the pharmaceutical industry, AFM is used to characterize drug particles and formulations.

Despite its power, AFM has limitations. The scan speed is relatively slow compared to electron microscopy, and the maximum scan area is typically limited to about 100 micrometers. The tip can wear or contaminate, affecting image quality. Also, the technique is sensitive to vibrations and thermal drift, requiring careful isolation.

Interpreting AFM images can be challenging because the image is a convolution of the tip shape and the sample surface. Artifacts such as tip broadening are common. Nevertheless, advances in tip fabrication and image processing continue to mitigate these issues.

Cantilever

A small beam that holds the AFM tip; its deflection is measured to detect forces.

Piezoelectric scanner

A device that moves the tip or sample with sub-nanometer precision using piezoelectric materials.

Tapping mode

An AFM mode where the cantilever oscillates and intermittently contacts the surface, reducing lateral forces.

Force spectroscopy

A technique using AFM to measure force-distance curves to probe molecular interactions.