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Atomic Force Microscopy.pptx

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Atomic Force Microscopy
Introduction • The resolution of scans can go up to the Angstrom scale (10^-10 meters). • Scanned objects are available as 3D structures which can be rotated and observed in real-time. • SEM had to be conducted only in a vacuum, but it was possible to use liquid, gas or vacuum to perform AFM. • AFM worked on both conductive and non-conductive surfaces; on the other hand, STM failed on any surface that was not conductive. • AFM can also measure the surface’s hardness and friction. • Atomic forces are used to map the tip-sample interaction. • https://www.youtube.com/watch?v=8gCf1sEn0UU&t=1s • https://www.youtube.com/watch?v=s6KqJS1GZNE
Working • 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.
Working • 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 • Van der Waals forces/London Dispersion Forces: The weak intermolecular electric forces that attract two or more electrically neutral bodies. • Electrostatic forces: Repulsive or attractive. • 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. • Typical tip radius is from a few to 10s of nm.
Deflection Sensitivity Calibration/Spring Constant Calibration • The calibration of the spring constant is based on Hooke's law. F = -ks • F = force, k = Cantilever spring constant, s = Cantilever displacement • 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. • Because 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. • 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.
• 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. • The relative position of the tip from the sample is changed during the operation 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 • 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 1. Static Force Mode: 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. 2. Lateral Force Mode: The focus is on studying the mechanical properties of the surface instead of imaging like Frictional and adhesive properties. • 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. 1. Dynamic Force Mode (Tapping Mode): This is the most popular one. The cantilever is made to resonate at a high frequency and brought close to the surface under observation. 2. 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: Used to measure the electromagnetic fields over a sample’s surface. • The Tip of the AFM device is coated with a conductive or magnetic material. 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.
Scanning methods for advanced imaging modes 1. Single-pass method • 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. It can be used to measure either surface properties or field properties of the sample. 2. Dual-pass method • When non-contact forces are to be measured along with the surface properties. 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 • AFM is used to study surface textures, defects, coatings and other physical features. • The technology works great for observing cells and biomolecules in their natural environment. • 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. • Many material properties can be determined like friction, electrical forces, capacitance, magnetic forces, conductivity, viscoelasticity, surface potential, and resistance.
Applications 1. Identifying atoms from samples 2. Evaluating force interactions between atoms 3. Studying the physical changing properties of atoms 4. Studying the structural and mechanical properties of protein complexes and assembly, such as microtubules. 5. used to differentiate cancer cells and normal cells. 6. Evaluating and differentiating neighboring cells and their shape and cell wall rigidity.