What is atomic force microscopy?
AFM belongs to the scanning probe family. Gerd Binnig, Calvin Quate and Christoph Gerber presented the first instrument in 1986, in a paper in Physical Review Letters. It grew out of the scanning tunnelling microscope, which earned Binnig and Heinrich Rohrer a share of the 1986 Nobel Prize in Physics. The Nobel went to the tunnelling microscope, not to AFM.
The tunnelling microscope needs a conductive sample, because it measures an electrical current. AFM measures force instead, so it can also image insulators, polymers and biological material. The first AFM images showed features about 3 nm apart on sapphire. Later refinements pushed the technique to atomic resolution under suitable conditions.
How AFM works: the principle step by step
An AFM has four core parts: a probe, a detection system, a piezoelectric scanner and a feedback loop.
- The tip meets the surface. A sharp tip, usually with a radius of a few nanometres to a few tens of nanometres, sits at the free end of a micromachined cantilever.
- The cantilever bends. Forces between tip and sample deflect the cantilever, which acts as a very soft spring.
- A laser reads the bending. A laser beam reflects from the back of the cantilever onto a split photodiode, so that a tiny deflection becomes a measurable shift of the spot.
- The feedback loop reacts. Electronics compare the signal with a setpoint and adjust the height of the tip or sample through the piezo.
- The scanner builds the image. The piezo moves the probe line by line, and the height correction at each point becomes the topography map.
The cantilever obeys Hooke's law, F = k × x. With a spring constant of 0.1 N/m, a deflection of 10 nm corresponds to a force of 1 nN. That simple relationship is what makes AFM a force-measuring tool as well as an imaging one.
AFM imaging modes
The mode describes how the tip interacts with the surface. Choosing it is mostly a question of how delicate the sample is and what you want to measure.
| Mode | How it works | Feedback keeps constant | Best for | Main drawback |
|---|---|---|---|---|
| Contact | Tip stays in contact and drags across the surface | Cantilever deflection | Hard, flat samples, friction studies | Lateral forces can damage or move soft material |
| Tapping (intermittent contact) | Cantilever oscillates near resonance and touches the surface briefly | Oscillation amplitude | Polymers, biomolecules, general imaging | More parameters to tune |
| Non-contact | Cantilever oscillates just above the surface | Frequency or amplitude | Atomic-scale work, often in vacuum | Demanding setup, sensitive to contamination layers |
Tapping mode is the common default for many laboratories because it largely removes the dragging forces of contact mode. Its phase signal adds useful contrast between regions of different stiffness or adhesion, for instance in polymer blends.
Force spectroscopy: measuring instead of imaging
In force spectroscopy the tip stops scanning and moves only up and down at a chosen point. The instrument records a force-distance curve as the tip approaches, touches and pulls away. Forces of a few piconewtons can be detected.
Each part of the curve carries information:
- Approach: long-range attraction or repulsion before contact.
- Contact: the slope reveals sample stiffness, and contact models turn it into an elastic modulus.
- Retraction: the pull-off force gives adhesion, and sawtooth patterns can show single molecules being stretched or unfolded.
Collecting a curve at each point of a grid produces maps of stiffness or adhesion that line up with the topography image.
AFM resolution and what AFM images show
Vertical resolution is the strong point of the technique, with height differences well below a nanometre measurable on flat samples. Lateral resolution depends on tip sharpness and sample type. A typical range quoted for routine work is about 0.1 to 10 nm, and atomic resolution calls for favourable samples and conditions.
A frequency-modulation AFM produced an atomic-scale image of the silicon (111)-(7×7) surface in 1995, and work in liquid has since reached molecular resolution on biological membranes.
Atomic force microscopy images are usually displayed as colour-coded height maps or rendered 3D views. The colours are a scale, not a true appearance, so always read the height bar. From the same data you can extract profiles, step heights and surface roughness values such as RMS.
Read an AFM image as a measurement. A convincing 3D rendering may have a greatly exaggerated vertical scale, and the shape of each feature is partly the shape of the tip.
AFM uses in materials science
- Thin films and coatings: roughness, grain structure and film continuity.
- Semiconductors: surface metrology and step heights.
- Polymers: phase separation and mechanical contrast between domains.
- 2D materials: flake thickness and surface cleanliness.
- Energy materials: smoothness of layers in solar cells and batteries, which affects interface contact.
Specialised modes add functional information. Kelvin probe force microscopy maps surface potential, magnetic force microscopy maps magnetic domains, and conductive AFM maps local current.
AFM uses in biology
AFM's main appeal for biologists is that samples can stay in buffer, close to physiological conditions, with no staining, coating or vacuum. Typical subjects include DNA, membrane proteins, viruses, bacteria and living cells.
Force measurements add mechanical information, such as the stiffness of cells and tissues or the unbinding of individual molecules. High-speed AFM goes further still. In 2010, Noriyuki Kodera, Toshio Ando and colleagues published video of single myosin V molecules walking along actin filaments, a direct view of a motor protein at work.
AFM vs electron microscopy
AFM competes with and complements the scanning electron microscope and the TEM. The right choice depends on whether you need height data, a wide field of view or the interior of the sample.
| Criterion | AFM | SEM | TEM |
|---|---|---|---|
| Data | True 3D height, mechanical properties | Surface image, composition with detectors | Internal structure, diffraction |
| Environment | Air, liquid or vacuum | Vacuum in most instruments | High vacuum |
| Sample preparation | Minimal, sample must be fixed in place | Often conductive coating | Very thin specimen required |
| Field of view | Up to about 150 × 150 µm | Up to millimetres | Small |
| Speed | Slow, often minutes per image | Fast | Fast imaging, slow preparation |
| Live biological samples | Possible in liquid | No | No |
Limitations and common artefacts
AFM is simple to describe and easy to misread. These are the problems that come up most often:
- Tip convolution: a blunt tip makes small features look wider and smooths out fine detail.
- Steep walls and overhangs: the tip cannot follow them accurately.
- Thermal drift: slow scans can shift during acquisition and distort distances.
- Scanner effects: hysteresis and creep in the piezo can distort images unless corrected or calibrated.
- Limited range: height range is usually only 10 to 20 µm, so rough samples may be out of reach.
A practical check is to rescan the same area at a different angle or speed. A real feature stays in place, while an artefact often changes or follows the scan direction.
Atomic force microscopy price and access
Prices depend on the scanner, controller and accessories. A simple AFM can cost as little as a few thousand US dollars, and top-of-range systems can reach half a million dollars or more. Probes wear out and are an ongoing cost. Many users rent time at shared facilities instead, and published hourly rates vary widely between academic and industrial users.
Key takeaways
Atomic force microscopy is the method to reach for when you need real surface height, mechanical properties at the nanoscale or imaging in liquid. Its limits are a small field of view, slow scanning and a dependence on tip quality. Used with electron or light microscopy, it fills a gap that neither can cover alone.