Atomic Force Microscope (AFM)- Definition, Principle, Parts, Uses

An atomic force microscope (AFM) is a powerful tool that can reveal the nanoscale structure and properties of various materials. It can measure forces as small as a fraction of a piconewton (10^-12 N) and image surfaces with atomic resolution. It can also manipulate individual atoms and molecules, creating new structures and patterns.

An AFM belongs to the family of scanning probe microscopes (SPMs), which use a sharp tip to scan over a sample and interact with it. Unlike conventional optical or electron microscopes, an AFM does not rely on lenses or beams to form an image. Instead, it records the feedback from the tip-sample interaction, such as the deflection of the tip or the change in its electrical properties. By moving the tip across the sample in a raster pattern, an AFM can generate a three-dimensional map of the surface topography or other physical or chemical characteristics.

An AFM can operate in various modes and environments, depending on the type of information desired and the nature of the sample. For example, an AFM can work in contact mode, where the tip is in constant contact with the sample, or in non-contact mode, where the tip oscillates near the sample without touching it. An AFM can also work in air, liquid, or vacuum, allowing for the study of different samples under different conditions.

An AFM has many applications in various fields of science and engineering, such as nanotechnology, biophysics, materials science, chemistry, and medicine. It can reveal the structure and function of biomolecules, such as DNA, proteins, and membranes. It can also characterize the mechanical, electrical, magnetic, thermal, and optical properties of nanomaterials, such as graphene, carbon nanotubes, quantum dots, and nanoparticles. It can also create novel nanostructures by manipulating atoms and molecules on surfaces.

In this article, we will explore the definition and roles of AFM, its history and principle of operation, its main components and modes of operation, its advantages and disadvantages, and its applications in various fields. We will also provide some examples and images of AFM results to illustrate its capabilities and limitations.

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 measure and image various properties of the surface, such as height, friction, magnetism, adhesion, mechanical, electrical, chemical, and optical, with high resolution at the sub-nanometer or nanometer level . It can operate in different environments, such as air, liquids, or ultrahigh vacuum .

The AFM was pioneered in 1986 by Binnig, Quate, and Gerber , following the invention of the scanning tunneling microscope (STM) in 1980 by Binnig and Rohler. The AFM was first used experimentally in 1986 and was commercially available in 1989.

The AFM has various applications in different fields of natural science, such as physics, chemistry, biology, materials science, nanotechnology, and medicine . Some of these applications include:

• Identifying atoms from samples
• Evaluating force interactions between atoms
• Studying the physical changing properties of atoms
• Studying the structural and mechanical properties of protein complexes and assembly
• Differentiating cancer cells and normal cells
• Evaluating and differentiating neighboring cells and their shape and cell wall rigidity

The atomic force microscope (AFM) was invented in 1982 by Gerd Binnig, a physicist who was working at IBM Zurich. He had previously co-invented the scanning tunneling microscope (STM) in 1980 with Heinrich Rohrer, another physicist at IBM Zurich. They received the Nobel Prize in Physics in 1986 for their groundbreaking work on STM, which enabled imaging and manipulating matter at the atomic scale .

Binnig realized that STM had a limitation: it could only image conductive or semiconductive surfaces. He wanted to create a new type of microscope that could image any type of surface, including insulators, polymers, ceramics, and biological samples. He came up with the idea of using a sharp tip attached to a flexible cantilever that would scan over the sample surface and measure the forces between the tip and the surface atoms .

Binnig collaborated with Calvin Quate, a professor of electrical engineering at Stanford University, and Christoph Gerber, a colleague at IBM Zurich, to develop the first experimental implementation of AFM. They published their results in 1986, demonstrating that AFM could image both conductive and non-conductive surfaces with nanometer resolution . The first commercially available AFM was introduced in 1989 by Digital Instruments .

Since then, AFM has become one of the most widely used tools for imaging, measuring, and manipulating matter at the nanoscale. It has also evolved into a versatile platform for various modes of operation, such as contact mode, tapping mode, non-contact mode, force spectroscopy, and nanolithography. AFM has enabled many discoveries and applications in various fields of science and engineering, such as solid-state physics, semiconductor studies, molecular engineering, polymer chemistry, surface chemistry, molecular biology, cell biology, medicine, and nanotechnology .

The Atomic Force Microscope works on the principle of measuring intermolecular forces and seeing atoms by using probed surfaces of the specimen in nanoscale. Its functioning is enabled by three of its major working principles that include Surface sensing, Detection, and Imaging.

Surface sensing

The Atomic Force Microscope performs surface sensing by using a cantilever (an element that is made of a rigid block like a beam or plate, that attaches to the end of support, from which it protrudes making a perpendicularly flat connection that is vertical like a wall). The cantilever has a sharp tip that scans over the sample surface, by forming an attractive force between the surface and the tip when it draws closer to the sample surface. When it draws very close making contact with the surface of the sample, a repulsive force gradually takes control making the cantilever avert from the surface.

Detection

During the deflection of the cantilever away from the sample surface, there is a change in direction of reflection of the beam, and a laser beam detects the aversion, by reflecting off a beam from the flat surface of the cantilever. Using a positive-sensitive photo-diode (PSPD- a component that is based on silicon PIN diode technology and is used to measure the position of the integral focus of an incoming light signal), it tracks these changes of deflection and change in direction of the reflected beam and records them.

Imaging

The Atomic Force Microscope takes the image of the surface topography of the sample by force by scanning the cantilever over a section of interest. Depending on how raised or how low the surface of the sample is, it determines the deflection of the beam, which is monitored by the Positive-sensitive photo-diode (PSDP). The microscope has a feedback loop that controls the length of the cantilever tip just above the sample surface, therefore, it will maintain the laser position thus generating an accurate imaging map of the surface of the image.

One of the key features of AFM is its ability to sense, detect and image the surface topography of a sample at the nanometre scale. This is achieved by using a sharp tip attached to a flexible cantilever that scans over the sample surface and interacts with the atoms or molecules on it . The interaction between the tip and the sample can be attractive or repulsive, depending on the distance and the type of forces involved. These forces cause the cantilever to bend or deflect, which is detected by a laser beam that reflects off the back of the cantilever onto a position-sensitive photodetector (PSPD) . The PSPD measures the changes in the laser position and converts them into electrical signals that are proportional to the cantilever deflection.

The AFM can operate in different modes depending on how the tip-sample interaction is controlled and measured. In contact mode, the tip is in constant contact with the sample surface and the cantilever deflection is kept constant by adjusting the height of the tip using a feedback loop . The height variation of the tip as it scans over the sample reflects the surface topography of the sample . In non-contact mode, the tip is oscillated above the sample surface at a frequency close to its resonance frequency and the changes in the oscillation amplitude or frequency due to the attractive forces between the tip and the sample are measured . In tapping mode, which is a variation of non-contact mode, the tip is oscillated at its resonance frequency and intermittently taps on the sample surface, resulting in changes in both amplitude and phase of the oscillation . The feedback loop adjusts the height of the tip to keep either the amplitude or phase constant, and thus obtains information about the surface topography and properties of the sample .

The AFM can also perform force spectroscopy, which is a technique that measures the force-distance curve between the tip and a specific point on the sample surface . This technique can provide information about the strength and dynamics of molecular interactions, such as adhesion, friction, elasticity, viscosity and binding forces . Force spectroscopy can be performed by either ramping or modulating the tip-sample distance while recording the cantilever deflection . Ramping force spectroscopy involves moving the tip towards and away from the sample surface at a constant speed and measuring the force as a function of distance . Modulating force spectroscopy involves oscillating the tip at a small amplitude around a fixed distance from the sample surface and measuring the changes in amplitude or phase due to nonlinear forces .

The AFM can also perform photoinduced force microscopy (PiFM), which is a technique that measures the optical gradient force between the tip and the sample when they are illuminated by a modulated light source. This technique can provide information about the optical response and properties of nanomaterials, such as quantum dots, plasmonic nanoparticles and molecular complexes. PiFM can be performed by detecting the changes in cantilever oscillation due to the optical gradient force using heterodyne frequency modulation technique. This technique can achieve high spatial resolution (~0.7 nm) and 3D mapping of photoinduced electric field distributions around nanoscale objects.

An atomic force microscope (AFM) consists of four main components: a cantilever, a probe tip, a laser, and a photodetector. These components work together to scan the surface of a sample and measure the atomic forces between the tip and the sample.

• The cantilever is a thin beam of material that is attached to a support at one end and has a sharp tip at the other end. The cantilever acts as a spring that bends when it interacts with the surface of the sample. The amount of bending depends on the distance between the tip and the sample, and the strength of the atomic forces between them. The cantilever can be made of different materials, such as silicon, silicon nitride, or metal, depending on the application and the type of force to be measured.

• The probe tip is the part of the cantilever that makes contact with the sample surface. The tip is usually made of silicon or metal, and has a radius of curvature of about 10 nanometers. The tip shape and size affect the resolution and sensitivity of the AFM. The tip can also be modified or coated with different materials to enhance its performance or functionality. For example, magnetic tips can be used to measure magnetic forces, or biological tips can be used to bind to specific molecules on the sample surface.

• The laser is a source of light that is used to monitor the deflection of the cantilever. The laser beam is reflected from the back side of the cantilever onto a photodetector. As the cantilever bends, the position of the laser spot on the photodetector changes. By measuring this change, the deflection of the cantilever can be calculated.

• The photodetector is a device that converts light into an electrical signal. The photodetector consists of four segments that are arranged in a quadrant. Each segment produces a voltage proportional to the intensity of light it receives. By comparing the voltages from different segments, the position and direction of the laser spot can be determined. The photodetector is connected to a feedback loop that controls the height of the cantilever above the sample surface.

Atomic force microscopy (AFM) is a versatile technique that can image and measure various properties of surfaces and materials at the nanoscale. AFM has been used in many disciplines in natural science and industry, such as physics, chemistry, biology, medicine, engineering, and coatings . AFM can operate in different modes and environments, such as contact, tapping, sub-resonance, air, and liquid . AFM can provide new insights into the structure and function of complex systems, such as bone and bone-related tissues and cells.

Some of the applications of AFM include:

• Identifying atoms from samples. AFM can achieve atomic resolution by using a sharp tip to scan over the sample surface and measure the interatomic forces . AFM can also distinguish different elements by using modified tips that are sensitive to specific chemical interactions.
• Evaluating force interactions between atoms. AFM can measure various types of forces, such as van der Waals, thermal, electrical and magnetic forces, that affect the tip-sample interactions . AFM can also quantify the mechanical properties of materials, such as stiffness, elasticity, adhesion and friction .
• Studying the physical changing properties of atoms. AFM can monitor the dynamic processes that occur on the sample surface, such as phase transitions, molecular rearrangements, chemical reactions and diffusion . AFM can also manipulate individual atoms or molecules by applying controlled forces or electric fields.
• Studying the structural and mechanical properties of protein complexes and assembly, such as microtubules. AFM can image the three-dimensional morphology of biological macromolecules in their native environment and measure their interactions with other molecules or substrates . AFM can also probe the conformational changes and folding dynamics of proteins under different conditions .
• Used to differentiate cancer cells and normal cells. AFM can detect the differences in the morphology, stiffness and adhesion of cancer cells and normal cells at the nanoscale. AFM can also measure the molecular interactions between cancer cells and drugs or antibodies.
• Evaluating and differentiating neighboring cells and their shape and cell wall rigidity. AFM can image the topography of live cells and tissues in their physiological environment and measure their mechanical properties, such as elasticity, viscosity and Young`s modulus . AFM can also study the effects of external stimuli or diseases on the cellular structure and function .
• Application in organic and perovskite solar cells. AFM can characterize the morphology, crystallinity, phase separation and defects of organic and perovskite solar cells at the nanoscale. AFM can also measure the electrical properties, such as current-voltage characteristics, photocurrent generation and charge transport, of these solar cells under illumination.

These are some of the applications of atomic force microscopy in various fields of science and technology. AFM is a powerful tool that can provide valuable information about the nanoscale structure and properties of materials and biological systems.

Atomic force microscopy (AFM) is a powerful and versatile technique that can be used to study various aspects of materials and biological systems at the nanoscale. Some of the advantages of AFM are:

• Easy to prepare samples for observation: Unlike other types of microscopy that require special treatments such as staining, coating, or embedding, AFM can be used to image samples in their natural state, without any modification or damage. This preserves the original structure and properties of the samples and allows for direct correlation with other techniques.
• It can be used in vacuums, air, and liquids: AFM can operate in different environments, depending on the research question and the sample type. For example, AFM can be used to study the surface morphology and chemistry of metals and semiconductors in vacuum, the adhesion and friction of polymers and biomolecules in air, or the hydration and electrostatic forces of cells and membranes in liquid. This flexibility enables AFM to probe various phenomena that are relevant for different applications.
• Measurement of sample sizes is accurate: AFM can measure the height, width, and depth of nanoscale features with high precision and accuracy, thanks to its high resolution and sensitivity. AFM can also measure other properties such as elasticity, stiffness, viscosity, and electrical conductivity by applying different modes of operation and force spectroscopy techniques. These measurements can provide valuable information about the mechanical, thermal, optical, and electrical properties of materials and biological systems at the nanoscale.
• It has a 3D imaging: AFM can generate three-dimensional images of the sample surface by scanning the tip over a grid of points and recording the vertical displacement of the cantilever at each point. This allows for a detailed visualization of the topography and texture of the sample surface, as well as the identification of defects, cracks, pores, or particles. AFM can also produce 3D images of the internal structure of samples by using techniques such as scanning near-field optical microscopy (SNOM) or scanning thermal microscopy (SThM).
• It can be used to study living and nonliving elements: AFM can be used to image both hard and soft materials, ranging from metals and ceramics to polymers and biomolecules. AFM can also be used to image living cells and tissues without killing or damaging them, by using gentle forces and appropriate liquid environments. AFM can reveal the morphology, dynamics, interactions, and functions of living systems at the molecular and cellular level.
• It can be used to quantify the roughness of surfaces: AFM can provide quantitative information about the roughness of surfaces by calculating parameters such as root mean square (RMS) roughness, average roughness (Ra), peak-to-valley height (Rt), or fractal dimension (Df). These parameters can characterize the smoothness or irregularity of surfaces and their influence on properties such as wettability, adhesion, wear, or corrosion.
• It is used in dynamic environments: AFM can be used to study the changes in the sample surface or properties over time or under external stimuli. For example, AFM can monitor the growth or dissolution of crystals or films, the phase transitions or reactions of materials, the folding or unfolding of proteins, or the binding or release of molecules. AFM can also apply forces or voltages to manipulate or modify the sample surface or properties in a controlled manner.

Despite its many advantages, the atomic force microscope also has some limitations and drawbacks that affect its performance and applications. Some of the disadvantages of AFM are:

• It can only scan a small area of the sample surface at a time, typically about 150 x 150 nm. This limits its ability to capture the overall morphology and features of larger samples or specimens.
• It has a low scanning speed, which might cause thermal drift on the sample or the tip during detection. Thermal drift is the change in position or shape of the sample or the tip due to temperature fluctuations, which can affect the accuracy and stability of the measurements and images.
• The tip and the sample can be damaged during detection, especially when using contact mode or high-force modes. The tip can wear out, break, or contaminate the sample surface, while the sample can be scratched, deformed, or altered by the tip. This can reduce the quality and reliability of the data and images obtained by AFM.
• It has a limited magnification and vertical range, which restricts its ability to resolve very small or very high features on the sample surface. The magnification of AFM depends on the ratio of the size of the tip to the size of the feature, which means that smaller features require sharper tips. However, sharper tips are more prone to damage and wear. The vertical range of AFM is determined by the length of the cantilever and the sensitivity of the detector, which means that higher features require longer cantilevers and more sensitive detectors. However, longer cantilevers are more susceptible to noise and vibration, while more sensitive detectors are more expensive and complex.

These disadvantages of AFM can limit its applicability and efficiency in some fields and situations, such as studying large-scale samples, fast-changing processes, or delicate specimens. Therefore, it is important to consider these factors when choosing AFM as a technique for measuring and imaging nanoscale phenomena.

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