Atomic Force Microscope (AFM) – Definition, Principle, Application

Atomic force microscopy (AFM) is a form of scanning probe microscopy (SPM) with a resolution so high that it can detect molecules within a fraction of a nanometer. A mechanical probe is used to collect information by touch with the assistance of piezoelectric devices that enable very small yet precise motions via electrical control for precision scanning.

Prior to AFM, IBM invented the scanning tunnelling microscope (STM) in the 1980s, resulting in a Nobel Prize for Physics. This resulted in the invention of the AFM microscope, which was commercialised in the late 1980s for nanoscale imaging precision.

What is an Atomic Force Microscope (AFM)?

An atomic force microscope (AFM) is a type of scanning probe microscope that is used to image and manipulate surfaces at the atomic scale. It works by using a sharp probe tip to scan the surface of a sample, and measuring the forces between the tip and the surface. The AFM can produce high-resolution images of a sample’s surface, as well as measure surface roughness and other physical properties. It is a powerful tool for studying the properties of materials, including metals, semiconductors, polymers, and biological samples.

The idea of using a sharp probe to image and manipulate surfaces at the atomic scale dates back to the late 1950s and early 1960s, with the pioneering work of G. Binnig and H. Rohrer, who later received the Nobel Prize in Physics for their inventions. However, the first practical AFM was not developed until the late 1980s, when it was invented independently by three research groups: one led by Gerd Binnig and Calvin Quate at IBM, another led by Christopher Gerber and Art Heinrich at Stanford University, and a third led by Alain Marti and Michel Orrit at Leiden University.

Since then, the AFM has become an important tool in a wide range of fields, including materials science, nanotechnology, and biology. It has also inspired the development of other types of scanning probe microscopes, such as the scanning tunneling microscope (STM) and the scanning near-field optical microscope (SNOM).

Principle of Atomic Force Microscope

Using a micromachined silicon probe with a very fine tip, AFM microscopes do surface sensing. Raster scanning the surface of a sample line by line is how this tip is used to create an image, albeit the specifics of how this is done change drastically across different modes of operation. Contact mode and dynamic mode, sometimes known as tapping mode, are the two main categories of operational modes.

AFM operates on the premise that this nanoscale tip is connected to a tiny cantilever, which acts as a spring. There is a laser diode and a split photodetector in place to detect the cantilever’s bending as the tip makes contact with the surface. The force exerted by the tip on the sample can be seen in this bending. Contact mode involves pressing the tip into the surface while an electrical feedback loop measures the force of the tip-sample interaction to maintain a constant deflection throughout raster scanning.

Tapping mode reduces the amount of time the tip is in contact with the sample surface to ensure the integrity of both the surface and the tip. When operating in this mode, the cantilever is excited to vibrate very close to its natural frequency of resonance. After then, the tip goes through a sinusoidal motion of rising and falling. As this motion approaches the sample, it is slowed by attractive or repulsive interactions. When in contact mode, a feedback loop maintains a constant quasistatic deflection; here, a feedback loop maintains a constant amplitude for the tapping motion. So doing is like drawing a line map of the sample’s geography.

How does an atomic force microscope work?

Similar to an STM, a sharp tip is raster-scanned over a surface as a feedback loop fine-tunes the imaging parameters. Atomic force microscopes, in contrast to scanning tunnelling microscopes, do not require a conducting material. Atomic forces are used to create a map of the tip-sample interaction rather than the quantum mechanical impact of tunnelling.

Atomic force microscopy (AFM), also called scanning probe microscopy (SPM), can be used to measure practically any measurable force interaction, including van der Waals, electrical, magnetic, and thermal forces. Adjustments to the software and tweaks to the advice are necessary for some of the more specific methods.

Atomic force microscopy typically consists of four main parts: deflection, force measurement, angstrom-level positioning, and feedback loop control.

AFM Probe Deflection

Commonly found in AFMs is a laser beam deflection mechanism that works by having the laser beam bounce off the AFM’s reflecting lever and into a position-sensitive detector. Both the tip and the cantilever of an AFM are commonly microfabricated from Si or Si3N4. The typical tip radius ranges from a few nm to the tens of nm.

Measuring Forces

When imaging with an AFM, forces between the tip and sample can’t be ignored because they’re the basis of the technique. Instead of measuring the force directly, we can infer it from the lever’s deflection by knowing the cantilever’s stiffness.

By applying Hooke’s law, we obtain:

F = -kz

where F is the force, k is the lever’s stiffness, and z is the lever’s arc of bending.

Feedback Loop for Atomic Force Microscopy

A feedback loop based on laser deflection regulates the atomic force microscope’s force and tip position. The AFM tip is attached to a cantilever, and a laser is reflected off the cantilever’s back. The laser’s position on the photodetector is fed back into the loop to monitor the surface and take readings as the tip moves across it.

Parts of Atomic Force Microscope

1. Scanning probe: This is a very fine, sharp tip that is mounted on the end of a cantilever and is used to scan the surface of a sample. The probe is typically made of a hard, durable material such as diamond or silicon, and is typically only a few nanometers in size.
2. Cantilever: This is a small, flexible beam that supports the probe and allows it to move freely. The cantilever is typically made of a lightweight material such as silicon or silicon nitride, and is only a few micrometers long and a few hundred nanometers thick.
3. Scanning stage: This is a platform that holds the sample and allows it to be moved relative to the probe. The stage is typically made of a lightweight, rigid material such as aluminum, and is equipped with precision motors or piezoelectric actuators that can move the sample in very small increments.
4. Detection system: This is a system that measures the forces between the probe and the sample, as well as the position of the probe and the sample. There are several different ways to detect these quantities, including optical, capacitive, and piezoresistive methods.
5. Control and data acquisition system: This is a computer system that controls the movement of the probe and the sample, as well as processes the data collected by the detection system. The control and data acquisition system typically includes a computer, software, and various input/output devices such as a keyboard, mouse, and display screen.
6. To detect the surface of a sample, modified tips are bent and flexed.
7. There were tweaks made in the imaging software before we utilised it on the samples.
8. Feedback loop control – They use a laser deflector to regulate the force interactions and the tip positions, creating a feedback loop for precise manipulation. While the cantilever’s tip interacts with the sample, the laser’s position on the photodetector is employed in the feedback loop to track the sample’s surface and make measurements.
9. Deflection – The atomic force microscope incorporates a laser beam deflection device into its design. Back of the AFM lever is reflective surface, which reflects laser to sensitive detector. Their 10nm-sized tips are crafted from silicon compounds.
10. Force measurement – The AFM’s operation and picture quality are both reliant on the force interactions between probe and sample. With knowledge of the cantilever’s rigidity, the deflection lever can be calculated, allowing one to measure the forces. Hooke’s law provides the formula for this computation as follows: F= -kz, where F is the force, k is the stiffness of the lever, and z is the distance the lever is bent.

Operating Procedure of Atomic Force Microscope

An atomic force microscope (AFM) is a complex scientific instrument that requires careful handling and maintenance to ensure reliable and accurate operation. The specific steps involved in using an AFM will depend on the particular model and manufacturer, so it is important to refer to the manual provided by the manufacturer for detailed instructions. In general, however, the following steps are typically involved in using an AFM:

1. Setting up the AFM: This involves unpacking and installing the AFM, setting up the computer and software, and ensuring that all components are properly connected and calibrated.
2. Preparing the sample: This involves cleaning and mounting the sample on the scanning stage, making sure that it is stable and flat, and adjusting the position and focus of the probe.
3. Scanning the sample: This involves using the computer and software to control the movement of the probe and the sample, and collecting data on the forces between the probe and the sample as the probe is scanned across the surface.
4. Analyzing the data: This involves using the software to process the data collected during the scan, and generating images and other data that can be used to study the properties of the sample.
5. Maintaining the AFM: This involves regularly cleaning and checking the probe and other components, and making any necessary repairs or adjustments to ensure optimal performance.

Types of Atomic Force Microscope

There are several different types of atomic force microscopes (AFMs), which are classified based on the type of detection system used:

1. Contact mode AFM: This is the most basic type of AFM, in which the probe tip is held in contact with the sample surface and the forces between the tip and the surface are measured as the tip is moved across the surface. This allows the AFM to produce images of the surface topography, as well as measure surface roughness and other physical properties.
2. Non-contact mode AFM: This type of AFM uses a probe tip that is held just above the surface of the sample, and the forces between the tip and the surface are measured using the changes in the oscillation frequency of the cantilever. Non-contact mode AFM is less damaging to the sample than contact mode AFM, but it is also less sensitive and has a lower resolution.
3. Tapping mode AFM: This type of AFM is similar to non-contact mode AFM, but the probe tip is made to oscillate at or near its resonance frequency while it is scanned across the surface of the sample. Tapping mode AFM is less damaging to the sample than contact mode AFM, and it has a higher resolution than non-contact mode AFM.
4. Dynamic force microscopy (DFM): This type of AFM measures the forces between the probe and the sample as a function of the separation distance between them. DFM can be used to study the forces between atoms and molecules, and it can be used to image surfaces at the atomic scale.
5. Magnetic force microscopy (MFM): This type of AFM is used to study magnetic materials and measure the magnetic forces between the probe and the sample. MFM can be used to image the distribution of magnetic fields on the surface of a sample, as well as measure the magnetic properties of individual atoms and molecules.

Atomic Force Microscope Uses

Atomic force microscopes (AFMs) are used in a wide range of fields, including materials science, nanotechnology, and biology. Some of the key applications of AFMs include:

1. Materials science: AFMs are used to study the properties of materials, including metals, semiconductors, polymers, and ceramics. They can be used to measure surface roughness, surface energy, surface tension, and other physical properties of materials at the nanoscale. Sample identification based on atomic number. Used to Comparing atomic force interactions. Researching the atomic structure and its dynamic physical qualities.
2. Nanotechnology: AFMs are used to fabricate and characterize nanostructures, including nanowires, nanotubes, and nanoparticles. They can also be used to study the properties of individual atoms and molecules, and to manipulate them at the atomic scale.
3. Biology: AFMs are used to study biological samples, such as cells, tissues, and proteins. They can be used to image the surface of biological samples at high resolution, and to measure the forces between biological molecules. Examining the physical and chemical characteristics of protein assemblies and complexes like microtubules. use to tell cancer cells apart from healthy ones. Comparing and contrasting the form and rigidity of the cell walls of nearby cells.
4. Surface science: AFMs are used to study the properties of surfaces, including surface chemistry, surface topography, and surface roughness. They are often used to study the surface properties of materials, and to understand how these properties affect the performance of devices and systems.
5. Industrial inspection: AFMs are used to inspect and test the quality of various industrial products, such as microelectronic devices, coatings, and MEMS devices. They can be used to detect and characterize defects in these products at the nanoscale.

Advantages of the Atomic Force Microscope

Atomic force microscopes (AFMs) have several advantages over other types of microscopes:

1. High resolution: AFMs are capable of producing images with a resolution down to the atomic scale, which is much finer than the resolution of other types of microscopes such as optical microscopes.
2. Non-destructive imaging: AFMs can be used to image and study samples without causing any damage, unlike other techniques such as electron microscopy, which can damage or destroy the sample.
3. Versatility: AFMs can be used to study a wide range of samples, including metals, semiconductors, polymers, ceramics, and biological samples. They can also be used to measure a variety of physical properties, such as surface roughness, surface energy, and surface tension. You may put it to work in a variety of environments, including air, liquid, and vacuum. Useful for both studying living and nonliving things.
4. Three-dimensional imaging: AFMs can produce three-dimensional images of the surface of a sample, which can provide valuable information about the sample’s surface structure and morphology.
5. Nanoscale manipulation: AFMs can be used to manipulate and rearrange individual atoms and molecules, which has a wide range of applications in fields such as nanotechnology and materials science.
6. Sample Preparation: Preparing samples for analysis is a breeze.
7. Reliable: Sample size calculations are reliable.
8. 3D Image: It’s capable of three-dimensional imaging.
9. Surface Study: It’s useful for measuring the roughness of surfaces.

Disadvantages of the Atomic Force Microscope

Atomic force microscopes (AFMs) also have some limitations and disadvantages compared to other types of microscopes:

1. Complexity: AFMs are complex instruments that require specialized training and expertise to operate. They also require careful handling and maintenance to ensure reliable and accurate operation.
2. Cost: AFMs are relatively expensive compared to other types of microscopes, which can make them cost-prohibitive for some users.
3. Sample preparation: AFMs require samples to be prepared and mounted on a scanning stage, which can be time-consuming and may require specialized equipment and techniques. In addition, the sample must be flat and stable, which can be a challenge for some types of samples.
4. Limited imaging depth: AFMs can only image the surface of a sample, and are not able to produce images of the internal structure of the sample. However, it can only scan a single nanosized image at a time, measuring around 150 nm on a side. It’s possible that thermal drift on the sample could occur due to their short scanning time. Magnification and vertical range are both severely restricted.
5. Limited imaging speed: AFMs are relatively slow compared to other types of microscopes, and may take several minutes or longer to produce an image of the sample. This can be a drawback for applications that require rapid imaging or high-throughput imaging.
6. During detection, both the probe and the sample may be harmed.

Atomic force microscope cost

Atomic force microscopes (AFMs) are relatively expensive scientific instruments, with prices ranging from tens of thousands to hundreds of thousands of dollars depending on the model and manufacturer. The cost of an AFM will depend on a variety of factors, including the type of AFM (e.g., contact mode, non-contact mode, tapping mode), the level of automation and features, the size and complexity of the system, and the manufacturer. AFMs can be purchased from scientific instrument companies, or they can be rented or leased from service companies. It is important to carefully consider your research needs and budget when selecting an AFM, and to compare prices from multiple sources before making a purchase.

How Atomic force microscope Works – Animation

Atomic force microscope pictures/atomic force microscope images

References

• Binnig, G.; Quate, C. F.; Gerber, Ch. (1986). “Atomic Force Microscope”. Physical Review Letters. 56 (9): 930–933.
• Cappella, B; Dietler, G (1999). “Force-distance curves by atomic force microscopy”. Surface Science Reports. 34 (1–3): 1–104.
• Zhong, Q; Inniss, D; Kjoller, K; Elings, V (1993). “Fractured polymer/silica fiber surface studied by tapping mode atomic force microscopy”. Surface Science Letters. 290 (1): L688.
• Radmacher, M. (1997). “Measuring the elastic properties of biological samples with the AFM”. IEEE Eng Med Biol Mag. 16 (2): 47–57.
• Galvanetto, Nicola (2018). “Single-cell unroofing: probing topology and nanomechanics of native membranes”. Biochimica et Biophysica Acta (BBA) – Biomembranes. 1860 (12): 2532–2538.
• https://www.first-sensor.com/en/products/optical-sensors/detectors/position-sensitive-diodes-psd/
• https://www.researchgate.net/publication/256195163_The_Atomic_Force_Microscope
•  https://www.sciencedirect.com/topics/nursing-and-health-professions/scanning-probe-microscope
• https://amedleyofpotpourri.blogspot.com/2018/09/atomic-force-microscopy.html
• http://nanoscience.gatech.edu/zlwang/research/afm.html