How does the AFM work?
The Atomic Force Microscope (AFM) images surfaces by tracing a sharp tip mounted on a flexible cantilever as it scans across a sample. Forces between the tip and the surface deflect the cantilever, and a laser beam reflected from the cantilever into a photodetector converts that deflection into a high-resolution map of the surface. Modern AFMs can also measure mechanical, electrical, and magnetic properties by analyzing how those forces change with distance and time. In recent years, advances such as high-speed AFM and multifrequency tapping have broadened capabilities, particularly in biology and soft materials.
Principle and core components
At the heart of an AFM is a tiny cantilever with a nanometer-scale tip. As the tip encounters the surface, intermolecular forces—van der Waals, electrostatic, capillary, and sometimes chemical interactions—cause the cantilever to bend. A laser beam mounted on the back of the cantilever reflects off its top surface and is detected by a position-sensitive photodetector. The detected deflection is fed to a feedback loop that adjusts the vertical position of the sample or the tip to keep the interaction at a chosen level, producing an image or a force measurement.
The following components are essential to AFM operation:
- Flexible cantilever with a sharp tip (often silicon or silicon nitride)
- Laser diode and an optical detector (usually a four-quadrant photodiode) to monitor cantilever deflection
- Piezoelectric scanner to move the tip in x, y, and z directions with nanometer precision
- Feedback electronics and control software to maintain a constant interaction (setpoint) during scanning
- Calibration tools and probes to determine cantilever stiffness (spring constant) and deflection sensitivity
- Imaging and analysis software for topography, force curves, and material properties
These pieces work in concert to convert tiny forces into a measurable signal and then into a high-resolution surface map. In practice, the same hardware can also be used to probe other properties by analyzing how the tip-surface interaction changes with distance, time, or applied bias. Since the method relies on mechanical interactions, the choice of environment (air, liquid, or vacuum) and the tip geometry strongly influence resolution and interpretation.
Imaging modes and what they measure
AFMs operate in several modes, each optimized for different sample types and information content. The core idea across modes is to detect tip-surface forces while controlling how the tip interacts with the surface, thereby extracting topography or other properties with minimal damage to delicate samples.
Contact mode
In contact mode, the cantilever is kept in constant deflection by adjusting the vertical position as the tip scans laterally. The z-motion tracks surface height, producing a topographic image. This mode provides high resolution but can damage soft or delicate samples due to sustained lateral forces and wear on the tip. It is often used on hard materials and for friction measurements, where lateral forces yield additional information about surface properties.
Tapping (dynamic) mode
In tapping or intermittent-contact mode, the cantilever is driven near its resonance so the tip gently taps the surface with a small oscillation envelope. The feedback loop maintains a constant oscillation amplitude rather than a constant deflection, producing a topographic map with reduced lateral forces and less sample damage. This mode is widely used for a broad range of materials, including biological samples, polymers, and soft tissues. The phase and amplitude signals also provide insight into material properties such as stiffness and adhesion.
Non-contact mode
In non-contact mode, the tip oscillates near a small distance above the surface without making physical contact. Long-range forces (van der Waals and electrostatic interactions) influence the cantilever, and the feedback stabilizes the oscillation amplitude or frequency shift. This mode can image very delicate surfaces but is sensitive to contamination, humidity, and surface cleanliness, and it generally requires stable environmental conditions to avoid capillary effects.
Other advanced modes
Kelvin probe force microscopy (KPFM) maps surface potential by combining AFM with a bias-induced electrostatic force measurement. Force spectroscopy records force versus distance curves at a fixed location to quantify mechanical properties like stiffness, adhesion, and elasticity. Lateral force microscopy (LFM) analyzes frictional forces to map anisotropic or directional properties. Multifrequency and high-speed AFM variants enable simultaneous or faster acquisition of multiple signals, expanding capability in time-resolved studies.
Calibration, performance, and practical considerations
Accurate AFM measurements depend on careful calibration and understanding of limitations. The main calibration tasks include translating deflection signals into physical units and knowing the actual interaction forces involved. Calibration is especially important for quantitative force measurements and for cross-comparing data between instruments or labs.
The following list outlines key calibration and performance aspects:
- Deflection sensitivity calibration to convert photodetector voltage to nanometers of cantilever deflection
- Cantilever spring constant (stiffness) determination, using methods such as thermal tuning or the Sader method
- Tip characterization and wear monitoring, since tip radius and shape affect resolution and force interpretation
- Piezo scanner calibration to correct for nonlinearity, hysteresis, and drift across x, y, and z axes
- selection of imaging mode parameters (setpoint, drive amplitude, scan rate) to balance resolution, speed, and sample integrity
- Environmental considerations, as humidity, temperature, and liquid media alter force ranges and stability
In practice, researchers tailor these parameters to the material under study. For example, soft biological samples benefit from tapping mode with conservative setpoints, while hard materials may tolerate or even require contact mode for the sharpest topographic contrast. High-speed AFM, which uses faster scanners and optimized feedback, has enabled real-time imaging of dynamic processes in biology and materials science.
Applications and limitations
AFM is a versatile tool used across physics, chemistry, materials science, and biology. It can image surface topography with sub-nanometer vertical resolution and often with lateral resolution limited mainly by the tip radius and sample mechanics. It can also measure mechanical properties, map electrical or magnetic fields (with suitable variants), and perform force spectroscopy to reveal interactions at the nanoscale.
The following list highlights common applications and practical constraints:
- Topographic mapping of surfaces such as minerals, nanostructured films, and semiconductor textures
- Mechanical property mapping (stiffness, elasticity, adhesion) via force curves and contact or dynamic modes
- Electrical and potential mapping with modes like KPFM, which reveals surface work function variations
- Friction and wear studies through lateral force measurements and phase imaging
- Characterization of soft biological specimens (proteins, membranes) in liquid environments
- Limitations include tip wear, slow imaging speeds for large areas, and artifacts from tip convolution or adhesion forces; care is needed in interpretation, especially for lateral or phase data
Despite these limitations, AFM remains a staple for nanoscale characterization because it can operate in ambient conditions and liquids, requires minimal sample preparation, and provides rich data beyond simple height maps. The technique continues to evolve with faster scanners, better tips, and integrated multi-parameter measurements.
Summary
AFM works by detecting the tiny forces between a sharp tip and a surface as the tip scans in three dimensions, converted into an image by a laser and photodetector coupled to a feedback loop. Its versatility comes from multiple imaging modes—contact, tapping, non-contact, and specialized variants—that trade off resolution, speed, and sample gentleness. Proper calibration and environmental control are essential for quantitative results. With ongoing advances in high-speed and multifrequency AFM, researchers can visualize and quantify surface structures and properties with increasing precision and in more complex, real-world conditions.
