Atomic Force Microscopy: AFM Principles, Modes & Uses
- NanoMagnetics Instruments
- Mar 29, 2022
- 8 min read
Updated: Jul 23
Atomic force microscopy (AFM) maps surface topography and measures nanoscale properties by scanning a sharp tip across a sample. This guide explains the AFM operating principle, common imaging modes, applications, advantages, limitations, and how AFM compares with STM and SEM.
History and Background of Atomic Force Microscopy
Scanning probe microscopy began in the early 1980s when Gerd Binnig and Heinrich Rohrer developed the scanning tunneling microscope (STM). Their work received the 1986 Nobel Prize in Physics. That same year, Binnig, Calvin Quate, and Christoph Gerber introduced the atomic force microscope, extending nanoscale surface measurement beyond electrically conductive samples.
AFM is one of the most widely used scanning probe microscopy techniques. Its probe consists of a sharp tip mounted on a flexible cantilever. As the tip moves across the sample, tip-sample forces bend or oscillate the cantilever; the instrument detects that response to reconstruct the surface and measure local properties.
AFM made nanoscale characterization practical across materials science, physics, chemistry, engineering, and biology. Unlike an optical microscope, it does not form an image from light. Instead, it records interactions between the probe and sample to produce quantitative three-dimensional surface data.
AFM is now used to characterize a wide range of material characteristics using a number of approaches in which the probe interacts with the sample in various ways. Mechanical qualities (such as adhesion, stiffness, friction, and dispersion), electrical properties (such as capacitance, electrostatic forces, work function, and electric current), magnetic properties, and optical spectroscopic properties may all be measured with AFM. In lithography and molecular drawing investigations, the AFM probe may be used to manipulate, write, and even draw substrates in addition to imaging.
AFM, like optical and electron microscopy, has become a standard method for materials characterization due to its versatility, allowing for resolutions down to the nanoscale scale and beyond. AFM can function in a variety of conditions, from ultra-high vacuum to liquids, and so is applicable to a wide range of fields, including physics, chemistry, biology, and materials research.
What is atomic force microscopy (AFM)?
Atomic force microscopy is a surface-characterization method that can resolve features from the micrometer scale down to the nanometer scale and, under suitable conditions, the atomic scale. Depending on the probe and operating mode, AFM can measure topography as well as mechanical, electrical, magnetic, and other functional properties.
How does AFM work?
AFM scans surfaces in a small area by raster scanning a sharp microprobe tip controlled by piezoelectric components and a feedback loop via a computer. To capture profile and interaction data, the probe tracks and touches the surface.
A sharp silicon nitrate or silicon tip on a free-moving console installed on a transport chip is the most popular AFM probe. Consoles are thin silicon nitrate or silicon arms with well-defined mechanical qualities and a rectangular or triangular form. The tip of the cantilever has a radius in the nanoscale range and has an extremely sharp protrusion at the end. According to Hooke's law, when the cantilever-mounted tip is brought near a sample surface by the AFM, forces between the tip and the sample alter the deflection of the cantilever.
Scanning the surface with the AFM tip provides a spatial profile or map of the sample region, which is used to gather surface/tip interaction data. On the rear of the console, a laser beam is focused and reflected onto a photodiode. This is shifted, and the photodiode surface records the motion.
A record marker follows the grooves on a vinyl record in the same way the tip interacts with samples. The correct cantilever characteristics for the AFM procedure are critical for effectively profiling the surface at the greatest resolution feasible.
What are the modes of AFM?
The AFM may be operated in a variety of modes depending on the application. Contact or static modes and varied tactile or dynamic modes, in which the probe console vibrates at a certain frequency, are the two types of imaging modalities.
When the tip is dragged across the surface in continuous contact, it is said to be in contact mode. In these instances, the feedback loop keeps the cantilever deflection (force) constant. While scanning over the top surface, the relative Z position of the probe and sample are adjusted. This movement is recorded and depicts the surface's constant force topography. The force generated by the tip's contact with the sample has the potential to harm the sample and wear the tip.
In dynamic Mode, the console oscillates at its resonance frequency. The amplitude of the oscillation is monitored and utilized as an input to the imaging feedback loop. In contact mode, the loop changes the relative Z position of the sample and probe. In this scenario, the goal is to maintain a constant amplitude, which results in a topographical image. When compared to the contact mode, tip-to-specimen forces are considerably reduced, and specimen damage and tip wear are greatly reduced.
In addition, the dynamic mode interaction, including its phase and extra high resonance modes, may be studied in greater depth. These signals provide a lot of information regarding end-sample interactions.
This extra data may be used to 'infer' and quantify a range of material characteristics, including depth of deformation, bond strength, and modulus. This implies that AFM can detect and analyze a variety of components in samples, ranging from biological materials to micro-electric devices and polymers.
Fast Force Matching Mode is another imaging approach that evaluates force-distance curves at high speed (1000 Hz or more) while collecting every curve in the picture scan. The underlying topography, as well as adhesion, modulus, and other characteristics, may then be calculated using both offline and real-time analytical methods.
What does AFM measure?
AFM is a strong imaging technology that can image practically any form of surface, including polymers, ceramics, composites, glass, and biological materials. Many various forces, including as adhesion forces, magnetic forces, and mechanical characteristics, are measured and localized using AFM.
The AFM is made up of a cantilever and a sharp tip with a diameter of 10 to 20 nm. Micro-manufactured AFM tips and brackets are made of Si or Si 3N4. The mobility of the tip is monitored by focussing a laser beam using a photodiode in response to tip-surface interactions.
There are two fundamental modes of operation for an AFM: contact and touch. The AFM tip is always in touch with the surface in contact mode. In dynamic mode, on the other hand, the AFM console vibrates above the sample surface, only coming into touch with the end surface on rare occasions. This procedure aids in the reduction of cutting forces caused by tip movement.
For AFM imaging, dynamic mode is the most widely used and recommended method. Only a few applications, such as force curve measurements, employ contact mode. AFM is a technique for seeing and manipulating atoms and structures on a variety of surfaces. Individual atoms on the surface below are "sensed" by the atom at the tip's top, which creates incipient chemical bonds with each atom. Chemical interactions may be identified and mapped because they modify the tip's vibration frequency exactly.
What can an atomic force microscope observe?
Magnifying observation equipment called an atomic force microscope (AFM) can measure the three-dimensional textures of a very small region. It can gather height data in numerical values, unlike scanning electron microscopes, allowing for the measurement of samples and data post-processing.
How does AFM generate an image?
A tiny cantilever is used in an AFM to scan a sample's surface and produce pictures. The cantilever is bent when its sharp tip makes contact with a surface, altering the quantity of laser light that is reflected into the photodiode.
What is atomic force microscopy used for?
The topography of soft biological materials in their natural habitats may be imaged using the AFM. Additionally, it may be used to examine the mechanical characteristics of cells and extracellular matrix, such as their inherent elastic modulus and interactions between receptors and ligands.
Advantages of atomic force microscopy
The principal functions of the atomic force microscope (AFM), a form of scanning probe microscope, include measuring elements like magnetism, height, and friction. In comparison to the optical diffraction limit, the resolution is measured in nanometers, which is far more precise and efficient.
What is the fundamental principle of AFM?
Surface detection is used in AFM microscopes, which use an extremely sharp tip on a micromachined silicon probe. This tip is used to photograph a sample using a line-by-line scan across the surface, albeit the procedure varies greatly depending on the operating mode. Contact mode and dynamic or tactile mode are the two primary groupings of operating modes that are regularly used.
The nanoscale tip is coupled to a tiny cantilever that generates an arc, which is the underlying principle of AFM. The console bends when the tip makes contact with the surface, and the bend is detected using a laser diode and a split photodetector. The contact force between the tip and the specimen is shown by this bending. The tip is pushed on the surface in contact mode, and an electrical feedback loop monitors the tip-to-sample interaction force to maintain a constant deflection during the scan.
To protect both the sample surface and the tip from damage, the dynamic mode restricts the amount of contact between both. The console vibrates close to its resonance frequency in this mode. The tip then glides up and down in a motion known as sinusoidal. As it approaches the sample, its motion is reduced by attractive or repulsive interactions. A feedback loop is employed in the touch mode, however, instead of quasi-static deflection, the amplitude of the touch gesture is kept constant. As a result, the sample's topography is drawn line by line.
AFM vs. STM
STM stands for Scanning Tunneling Microscope while AFM stands for Atomic Force Microscope. In the atomic and molecular disciplines, the invention of these two microscopes is considered a revolution.
AFM, or atomic force microscopy, takes exact pictures by sliding a nanometer-sized tip across the image's surface. Quantum tunneling is used by STM to capture pictures.
The Scanning Tunneling Microscope was the first of the two microscopes to be created.
In contrast to STM, AFM uses a probe that makes direct contact with the surface and calculates incipient chemical connections. By computing the quantum degree tunnel between the probe and the sample, STM pictures are indirectly shown.
Another distinction is that the tip of the AFM probe softly contacts the surface, whereas the tip of the STM probe is retained at a short distance from the surface.
Unlike STM, AFM simply measures the tiny force between the surface and the tip, not the tunneling current.
AFM and STM have different strengths rather than a universal resolution advantage. STM can achieve atomic resolution on conductive surfaces by measuring tunneling current. AFM works with conductive and nonconductive samples and can provide direct height and force-related measurements, although its lateral resolution is influenced by tip shape and tip-sample interaction.
AFM can examine conductors, semiconductors, and insulators. STM requires an electrically conductive path between the tip and sample. Both techniques can operate in specialized environments, while AFM is particularly versatile in ambient air, controlled gases, vacuum, and liquids.
Choose AFM when you need quantitative three-dimensional topography, force measurements, or characterization of nonconductive materials. Choose STM when atomic-scale electronic structure and conductive surfaces are the primary focus.
AFM vs. SEM
SEM forms high-resolution images using an electron beam and is well suited to rapid imaging over comparatively large areas. AFM scans a physical probe over a smaller area and returns quantitative three-dimensional height data. AFM usually requires less demanding sample preparation, while SEM generally offers faster imaging and a larger field of view.
Limitations of atomic force microscopy
AFM measurements cover a relatively small scan area and can be slower than optical or electron microscopy. Image quality depends on tip condition, scanner calibration, vibration isolation, feedback settings, and sample preparation. Tip convolution can broaden narrow features, and soft or weakly attached samples may deform or move during scanning. Selecting the correct probe and imaging mode is therefore essential.
Choosing and buying an AFM system
NanoMagnetics Instruments offers AFM platforms for education, routine nanoscale characterization, and advanced research. Compare the ezAFM/hpAFM systems to identify the scan range, modes, environmental control, and performance level that match your application.
Speak with an AFM engineer to discuss your samples, required measurements, configuration, and quotation.
Comments