Atomic force microscopy (AFM) is a powerful characterization tool that provides nanoscale resolution of surface properties through a physical probe scanning across a sample. The technique operates by measuring interactions between a sharp tip and the sample surface, enabling multiple imaging modes that reveal different material characteristics. The primary imaging modes include topography, phase, amplitude, and friction, each offering distinct insights into surface properties.

Topography imaging is the most common AFM mode, mapping the height variations of a sample surface with sub-nanometer vertical resolution. A cantilever with a sharp tip scans the surface while maintaining constant tip-sample interaction force, typically through feedback mechanisms adjusting the probe height. This mode is widely used in materials science for measuring surface roughness, step heights, and nanostructure dimensions. For example, it can resolve individual nanoparticles, thin film thickness, or defects in semiconductor devices. The vertical resolution can reach 0.1 nm under optimal conditions, while lateral resolution depends on tip sharpness, usually ranging between 1 nm and 20 nm.

Phase imaging measures the phase lag between the oscillating cantilever and the driving signal when operating in dynamic modes like tapping mode. This lag arises from energy dissipation due to viscoelastic properties, adhesion, or material stiffness variations. Phase contrast highlights differences in material composition, such as polymer blends or contamination on surfaces. It is particularly useful for distinguishing hard and soft domains in composite materials or biological samples where mechanical properties vary locally. The contrast in phase images does not directly correlate with height but provides complementary information about surface heterogeneity.

Amplitude imaging records the oscillation amplitude of the cantilever in dynamic modes. Changes in amplitude reflect variations in surface energy dissipation or stiffness. This mode is sensitive to edges and steep features, often enhancing the visibility of small protrusions or recesses that might be less pronounced in topography images. Amplitude data can also indicate areas of increased adhesion or damping, useful for identifying subsurface features or defects that influence surface mechanics.

Friction imaging, also called lateral force microscopy (LFM), measures torsional bending of the cantilever caused by lateral forces during scanning. Friction contrast arises from variations in surface chemistry, adhesion, or roughness, making it valuable for studying lubricated surfaces, self-assembled monolayers, or tribological properties. For instance, it can differentiate between hydrophobic and hydrophilic regions on a surface or map the distribution of molecular layers with different frictional characteristics.

Several factors influence AFM resolution, with tip-sample interactions being the most critical. The tip geometry directly affects lateral resolution; sharper tips provide finer details but are more prone to wear. Tip contamination or blunting degrades resolution over time. The force applied during scanning must balance between minimizing sample deformation and maintaining stable feedback. Excessive force can distort soft samples or damage delicate structures, while insufficient force may cause tip instability.

Environmental conditions also play a significant role. Ambient humidity can induce capillary forces between the tip and sample, altering measured forces and reducing accuracy. Temperature fluctuations cause thermal drift, leading to image distortion. Vibration isolation is essential to prevent noise in measurements, particularly for high-resolution imaging. In liquid environments, viscous drag on the cantilever affects oscillation dynamics in dynamic modes, requiring adjustments to scanning parameters.

Compared to other microscopy techniques, AFM offers unique advantages and limitations in resolution. Its vertical resolution surpasses that of optical profilometry or scanning electron microscopy (SEM), capable of detecting atomic-scale height differences. However, lateral resolution is generally lower than transmission electron microscopy (TEM), which can achieve sub-angstrom resolution. Unlike electron microscopy, AFM does not require vacuum conditions, enabling imaging in air or liquid, which is critical for biological or soft materials. AFM also provides three-dimensional topography data, whereas SEM offers two-dimensional projections unless combined with tilt-stage tomography.

The choice of imaging mode depends on the specific material properties of interest. Topography is indispensable for quantitative height measurements, while phase and amplitude imaging reveal mechanical or compositional contrasts. Friction imaging adds another dimension for analyzing surface chemistry and tribology. Understanding the factors affecting resolution ensures optimal imaging conditions, balancing tip sharpness, environmental control, and scanning parameters to extract the most accurate nanoscale information. AFM’s versatility makes it indispensable in nanotechnology, materials science, and biological research, where detailed surface characterization is crucial.