Atomic Force Microscopy Explained: Principles, Construction, Working, and Applications
Ever since its invention in the 1980s, Atomic Force Microscopy has been an enabling technology for imaging and conducting a deep analysis of samples. In this article, we explain the most important things you need to know about it.
Atomic Force Microscopy (AFM) is an advanced high-resolution scanning technique used to analyse samples in great detail. The technology can magnify images over a hundred million times. The resolution of scans can go up to the Angstrom scale (10-10 meters).
But the one thing that really sets AFM apart is that the scanned objects are available as 3D structures which can be rotated and observed in real-time. This helps researchers uncover the hidden information which may have been lost otherwise in regular 2D scans.
In this article, we will take a closer look at AFM. We are going to discuss its origin, construction, working principle, modes of operation, applications, and a lot more.
Historical development of Atomic Force Microscopy
By definition, microscopy is the study of objects by magnifying them with the help of a microscope. The technology helps uncover details about the sample that are not visible to the naked human eye.
The earliest microscopy technique (optical microscopy) made use of the magnification property of lenses and has been around for hundreds of years now. Optical microscopes are just like one or more strong magnifying glasses with short focal lengths. These microscopes can work well for magnifying a sample up to around a thousand or a few thousand times at best. But for anything more, we have to turn to other microscopy techniques.
In the ‘30s, Ernst Ruska, a German electrical engineer, realised that the limits of the wavelength of light made it impossible for it to resolve objects beyond a resolution of 10 Angstrom. Ruska then used the electrons, having a much smaller wavelength than light, to build a microscope that later came to be known as Electron Microscope (EM). It was also concluded that electric and magnetic fields affect the electrons just the way lenses affect light. [1] [2]
The ’80s was a crucial decade for nanotechnology and microscopy. In 1981, two researchers from the IBM Zurich research laboratory, Gerd Binnig and Heinrich Rohrer, came up with the concept of Scanning Probe Microscopy (SPM) with the invention of their Scanning Tunneling Microscope (STM). It was the first time quantum tunnelling was demonstrated practically. Five years after creating STM, Gerg Binning and Calvin Quate developed Atomic Force Microscopy (AFM). [3]
When the first Atomic Force Microscope was invented, the inventors managed to create a measurable displacement of 10-4 Armstrong by applying a force of just 10-18 N. This extreme sensitivity of the AFM made it an enabling technology to investigate a wide range of surfaces on an atomic scale. Though the technology was derived from STM, it differed vastly from STM and SEM both. [4]
SEM had to be conducted only in a vacuum, but it was possible to use liquid, gas or vacuum to perform AFM. The primary advantage of AFM, apart from its higher magnification capability, was realised consequently. AFM worked on both conductive and non-conductive surfaces; on the other hand, STM failed on any surface that was not conductive. Apart from all that, AFM had the edge over the other microscopy techniques when it came to measuring the surface’s hardness and friction.
How does AFM work?
The working of AFM is quite straightforward. The sample that needs to be observed is first placed on a stage. Then a cantilever with a sharp tip is made to pass over the surface line by line so as to raster scan the sample in a way. During the scanning process, a laser beam is made to fall on the back of the tip, which is coated with a reflective material. The laser beam gets reflected from the shiny surface and falls on a photodetector screen.
When the tip encounters bumps or depressions on the surface of the sample, it gets deflected from its original position, causing the laser beam to move too. This movement is detected by a photodetector and sent to a high gain amplifier circuit.
After adding some gain and converting the signal into a processable entity, the amplifier passes it on to the computer, collating signals received during the entire scan and ultimately providing a 3D profile of the surface.
Forces affecting the working of AFM
Let us now review the two most important forces acting on the cantilever and tip during the operation.
Van der Waals forces/London Dispersion Forces: The weak intermolecular electric forces that attract two or more electrically neutral bodies towards each other are called Van der Waals forces or London Dispersion Forces. [5]
The forces are inversely proportional to the separation between the bodies. In this case, the two bodies under observation are the sample and the cantilever.
Electrostatic forces: These forces are exerted by two or more charged bodies on one another. The force is repulsive on similarly charged bodies, and on oppositely charged bodies, it is attractive. [6]
When the tip of the cantilever is away from the object’s surface, Van der Waals attractive forces act on it to pull it closer. When this happens, the cantilever is bent towards the sample’s surface.
The tip is made to approach the surface vertically. Once it is close enough, it is practically observed that electrostatic repulsive forces tend to get bigger in magnitude and dominate. The cantilever, which was getting bent towards the surface earlier, is now pushed away from it.
This repulsion is also due to the fact that the tip can not penetrate the sample. This change in deflection can be used as a basis to discover some physical properties like the rigidity of the sample under observation.
So apart from scanning, AFM can also be used to measure the force acting on the cantilever tip due to the sample.
Construction of the Cantilever Tip/Probe
The tip of the device is an extraordinarily delicate and sharp head attached to a probe like structure, also known as a cantilever. The other end of the cantilever is connected to a piezoelectric crystal that acts as a transducer converting the cantilever’s motion into electrical signals and also the other way around.
Silicon (Si) cantilever is used for hard samples, and for soft samples, Silicon Nitride (Si3N4) is used. The factor governing the choice of material for the cantilever is the spring constant of the sample under observation.
The dimensions of the cantilever are set after carefully considering the spring constant requirement in the vertical and lateral directions. Here are the formulas used during the design of the cantilever:
When higher accuracy is required, the probe tip is sharpened with the help of electrochemical etching, ion milling or even with the help of a carbon nanotube. This increases the overall cost of the microscope by quite a bit. But if the choice of cantilever and tip material is not made correctly, the result of the scan will be of poor quality. [7]
Probes sharpened by different methods; Left: Electrochemical etch, Center: Ion milling, Right: Carbon Nanotube Source: [7]Deflection Sensitivity Calibration/Spring Constant Calibration
The calibration of the spring constant is based on Hooke's law. The law establishes a relation between applied force, spring constant and cantilever deflection. The negative sign in the equation is just a sign convention. Here is the equation of Hooke’s law:
The cantilever’s spring constant is adjusted to make it most sensitive to the range of force that it is going to withstand.
Feedback mechanism
To ensure that the measurement accuracy is maintained throughout the process, the relative distance between the tip and the sample must be in a particular range during the whole sweep.
The reason for this being that if the separation between the tip and the sample is high, the strength of the force becomes so weak that noise dominates over the signal. On the other hand, if the distance between the tip and the sample is too small, a large force is exerted by the tip on the surface, which may cause damage to the instrument or the sample itself.
To solve this issue, a feedback loop control is introduced in the AFM device. The Proportional Integral Derivative (PID) control scheme is used to track and maintain the separation between the tip and the sample. The transfer equation of a typical PID controller is given by:
A reference value of separation/oscillating frequency is stored in the PID controller during the manufacturing and calibration of the device. When in use, the current value of separation/oscillation is measured and fed back to the PID controller, which records and limits the difference between the setpoint and the current point.
P and D term facilitates the movement over large surfaces, and I term manages the smaller areas. When the values of P, I and D terms are properly set, the error is minimum. The entire feedback system can be implemented with the help of OpAmps or digital circuits.
But the question remains, how is the relative position of the tip from the sample changed during the operation? The answer is, with the help of a Piezoelectric material. Piezoelectric materials have the ability to expand or contract based on the applied potential difference. Materials like amorphous lead barium titanate (PdBaTiO3) are subjected to changing voltages to manipulate their expansion and contraction in the desired manner to move the cantilever or the sample.
AFM Modes
The modes of operation of the AFM can be broadly classified into two categories.
Static Modes
During contact mode of operation, the tip of the cantilever is literally dragged along the surface of the sample under observation. Contact mode is used when hard surfaces are required to be studied with a resolution of over 50 nanometers (50 X 10-9 meters). The observations made during the process are the basis for readings in the contact mode of operation of AFM.
Static Force Mode: In the static force mode of operation, the strain of the cantilever is measured to sense the structure of the surface. The method is suitable for observing hard surfaces using AFM, but fails in the case of soft and sticky samples like those of biomolecules.
Lateral Force Mode: During the lateral force/material sensing mode, the focus is on studying the mechanical properties of the surface instead of imaging. Frictional and adhesive properties are the two mechanical forces that can be measured with this mode of operation.
When the tip comes in contact with any surface and slides over it, a certain amount of frictional force is applied against the direction of motion.
This force is responsible for keeping the tip a little inclined when sliding. When the frictional and adhesive properties of the sample change, there is a change in the coefficient of friction. This causes an unbalance of forces and changes the tilt angle of the tip.
As the tip passes the sample under observation, the coefficient of friction of the surface it is in contact with changes back to normal, and the tip regains its original orientation.
Dynamic Modes
In dynamic modes of operation, the tip is made to oscillate during the measurement. It either doesn’t touch the sample or touches it intermittently. Either way, it is ensured that the interaction is completely non-destructive.
Dynamic Force Mode (Tapping Mode): Dynamic Force Mode is among the most popular and commonly used modes of operation these days. The cantilever is made to resonate at a high frequency and brought close to the surface under observation.
Phase Imaging Mode: Phase Imaging Mode is a type of dynamic mode in which the tip oscillates at a fixed/resonant frequency above/over the sample’s surface. Due to some properties of the sample (like adhesive force), the tip is not able to instantaneously move up or down based on the input signal.
A certain amount of lag is introduced in the process, which delays the movement of the tip. This lag/delay causes a phase shift, measurement of which can give information about the property that we are testing the sample for.
Field mode: Another interesting application of AFM can be the measurement of electromagnetic fields over a sample’s surface.
The Tip of the AFM device is coated with a conductive or magnetic material for the measurement of the electric and magnetic properties of the sample, respectively. These coatings experience a force when placed in regions with an active electric/magnetic field which ultimately is exerted on the tip and the cantilever.
Due to the extra coating that goes over the tip, the resolution of the instrument takes a hit, the amount of which depends on the quality and thickness of the coat.
If the frequency of vibration is fixed, the amplitude of vibration changes as the coated tip passes over a charged/magnetic region. There is absolutely no contact between the tip and the sample surface during the measurement.
Scanning methods for advanced imaging modes
Depending on the property being measured and the nature of the sample, the method employed for scanning can be different.
Single-pass method
In the single-pass method of AFM, the distance of the tip from the sample is maintained constant. The setup is also called constant height setup and is used for obtaining quick readings. During a scan, the probe is passed just one time over the surface, and that is the reason why the method is called the single-pass method. It can be used to measure either surface properties or field properties of the sample.
Dual-pass method
If non-contact forces are to be measured along with the surface properties, a dual-pass mode of operation is used when scanning the sample by AFM. The only difference between the dual-pass method and the single method is that in dual-pass mode, the probe makes its way over the surface twice during one scan, the first time for finding out the surface structure and the second time for sensing non-contact forces.
When the probe is in contact with the surface, the topography is measured; when it is at a fixed distance from the surface, it measures the field.
Atomic Force Microscopy Applications
The applications of AFM are not limited to a particular field of study. The technology is used for investigating a variety of different samples in the study of physical sciences, life sciences, electronics and engineering in general. [8]
AFM is used to study surface textures, defects, coatings and tons of other physical features. The technology works great for observing cells and biomolecules in their natural environment. [9] AFM can also be used to analyse microelectronics circuits and components. Energy storage materials like batteries and energy generation materials like photovoltaic cells are studied using AFM. Tribology, surface chemistry, genetic engineering, medicine are other prominent fields where the microscopy technique is used as an important tool for observation and research. [10]
Conclusion
The only major drawbacks of the AFM are its low scanning speed and inability to study the chemical properties of the surface under observation. Apart from that, the device is an excellent tool for conducting a study of the samples in detail.
Like any other technology, AFM, which was once used only for research on cutting-edge technologies by PhD holders, has become an irreplaceable tool in industries for product development and quality control. Education institutes, too, are not far behind when it comes to inculcating the latest tech in their curriculum.
Microscopy is no longer limited to gathering raw 2D images of samples. Devices like the AFM will form a basis for research in various disciplines of science and technology in the years to come. With advancements in computing, microscopes are becoming complete systems in themselves to visualise objects and processes. However, the one thing that will not change in microscopy is the contribution of optics. It will continue to remain a key element in microscopy, just like it has always been.
References
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[2] Abdullah, Avin & Mohammed, Azad. (2019). Scanning Electron Microscopy (SEM): A Review.
[3] Binnig, G, Quate, C, Gerber, C. "Atomic Force Microscope". Phys. Rev. Lett. 1986; 56:930–933
[4] Gerber, C., Lang, H. How the doors to the nanoworld were opened. Nature Nanotech 1, 3–5 (2006). https://doi.org/10.1038/nnano.2006.70
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[10] Robert W. Carpick and Miquel Salmeron, Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy, Chemical Reviews 1997 97 (4), 1163-1194, DOI: 10.1021/cr960068q
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[13] Aliofkhazraei, M, Ali, N. "AFM Applications in Micro/Nanostructured Coatings". Comprehensive Materials Processing 2014; 7:191-241.
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