Atomic force microscopy (AFM) generates three-dimensional maps of a surface with a resolution high enough to discern individual atoms. This technique is a form of scanning probe microscopy that functions by “feeling” a surface with a sharp point, similar to how a person might read braille. This method allows scientists to create a topographical image of a sample without needing to see it.
The Mechanics of Feeling Atoms
The core of an atomic force microscope is a mechanical system. It relies on a microfabricated silicon or silicon nitride cantilever, which is a tiny, flexible lever with a sharp probe, or tip, at its end. This tip, with a radius of just a few nanometers, is scanned across a sample’s surface. As the tip moves, forces between the atoms on the tip and the sample cause the cantilever to bend.
To measure these movements, the system employs a laser beam deflection setup. A laser is aimed at the reflective back of the cantilever, and the beam bounces off onto a position-sensitive photodetector. This detector, often a split photodiode, tracks changes in the laser’s position. Any vertical deflection of the cantilever, caused by the tip encountering a feature, changes the angle of the reflected laser beam.
This optical lever arrangement magnifies the cantilever’s movement, making it possible to detect vertical shifts as small as a fraction of a nanometer. The process is controlled by a piezoelectric scanner that moves the sample or tip with precision, and a feedback loop that maintains a constant interaction force. The system records the cantilever’s deflection at each point, similar to how a record player’s needle traces the grooves in a vinyl record.
Modes of Operation
To accommodate different types of samples, from hard materials to delicate biological specimens, atomic force microscopes operate in several modes. The choice of mode depends on the sample’s characteristics and the information desired. Each mode manages the interaction between the tip and the surface to optimize image quality while preventing damage.
The most direct method is contact mode, where the AFM tip is dragged across the sample’s surface. A feedback loop works to keep the cantilever’s deflection constant, which provides a direct measure of the surface height. While effective for hard, stable samples like ceramics or metals, this contact can exert lateral forces that may damage softer samples.
For soft or delicate samples, non-contact mode offers a gentler alternative. In this mode, the cantilever oscillates near its resonant frequency just above the sample surface without touching it. The tip senses weak, long-range attractive forces, which cause a change in the cantilever’s oscillation frequency or amplitude. This allows for imaging without physical contact, preserving the integrity of biological samples or liquids.
A widely used hybrid is tapping mode, also known as intermittent-contact mode. Here, the cantilever oscillates and the tip lightly “taps” the surface as it scans. This method reduces the lateral, shear forces present in contact mode, making it suitable for a range of samples, including polymers and living cells. Tapping mode provides high-resolution images with minimal sample damage, representing a balance between the other two modes.
Visualizing the Nanoscale World
Atomic force microscopy provides a window into structures far too small to be seen with conventional light microscopes. This capability enables researchers to observe and characterize materials across a wide range of scientific disciplines with a high level of detail.
In biology, AFM is used to visualize life’s building blocks. Scientists can image the double helix of DNA strands, observe proteins as they fold into their functional shapes, and watch living cells in their natural liquid environment. This allows for the study of biological processes in real-time, providing insights into cellular function and disease.
Materials science and nanotechnology also rely on AFM. Engineers use it to inspect the surface of polymers, identify defects in semiconductor wafers, and monitor the growth of crystals with atomic precision. The technique is used to confirm the structure and dimensions of new materials, such as carbon nanotubes or graphene sheets.
Beyond Imaging: Measuring and Manipulating
While AFM is known for its imaging, it is also a tool for measuring physical properties and manipulating matter on an atomic scale. This extends its function beyond that of a simple microscope, turning it into a nanotool for experimentation and fabrication.
One application is force spectroscopy. By pushing the AFM tip into a sample and then retracting it, scientists can measure the forces of interaction at each point. This generates a force-distance curve that reveals material properties like stiffness, elasticity, and adhesion. Researchers can use this technique to measure the difference in stiffness between a healthy cell and a cancerous one, providing potential diagnostic markers.
The AFM tip can also be used with precision to move or arrange individual atoms and molecules on a surface. This “nanomanipulation” is a technique in the field of nanotechnology. By controlling the force and position of the tip, scientists can build molecular structures atom-by-atom, opening pathways for creating new electronic components and nanoscale devices.