ATOMIC FORCE MICROSCOPE (AFM)

 ATOMIC FORCE MICROSCOPE (AFM)

·       Atomic Force Microscopy (AFM) is a powerful microscopy technique. It is used to acquire high-resolution images at the nanoscale to better understand the properties of matter.

·       The Atomic Force Microscope (AFM) is a type of scanning probe microscope whose primary roles include measuring properties such as magnetism, height and friction.

·       AFM is versatile because it cannot only image in three-dimensional topography, but it also provides various types of surface measurements to the needs of scientists and engineers.

·       Resolution of Atomic Force Microscope (AFM) is 1 nm.

·       AFM can generate images at atomic resolution with Angstrom scale resolution height information, with minimum sample preparation.

·       AFM images can resolve an image a million times smaller than a human hair.

History of Atomic Force Microscopy (AFM)

·       AFM was first introduced in a breakthrough experiment in 1986. Gerd Binnig and Heinrich Rohrer from IMB-Research had earlier garnered a Nobel prize for their invention of Scanning Tunnelling Microscope (STM). However, the STM could only image metal or semiconductor materials. Therefore, Gerd Binnig, along with Christoph Gerber and Calvin Quate developed the AFM in 1986.

·       Atomic Force Microscope was first used experimentally in 1986. It was put on the market for commercial sale in 1989.

Mode of Action of Atomic Force Microscopy (AFM)

i) Static mode or Contact mode

·       The AFM tip is in contact with the sample surface when it is scanned.

·       The tip is “dragged” across the sample like a needle. The tip acts like an elastic, compressing and stretching when it encounters a force induced by the sample surface. The changes in the cantilever deflect the laser beam which is imaged on the photo-detector.

·       Soft materials are deflected further as force is applied, so AFM tips are generally made from soft materials. But since the tip is in contact with the sample it is scanned across, the sample can be damaged easily. The static mode is mostly used to image hard samples to obtain topographical information.

ii) Dynamic mode

·       In Dynamic mode, the cantilever is driven to oscillate at a certain frequency instead of being “static”.

·       Within the Dynamic mode of operation, a) Tapping mode and b) Non-contact mode are widely employed.

a) Tapping mode

·       In Tapping mode, the tip oscillates up and down above the surface of the sample. The tip comes in contact with the sample at its lowest point of oscillation by “tapping” the surface.

·       Repulsive forces are detected when the tip “taps” the surface, while attractive forces are detected at the peak of oscillation.

·       Tapping mode generally lessens the damage done to a surface and the tip compared to Contact mode. This technique is used to image the formation of molecules in chemistry among many other applications.

b) Non-contact mode

·       In Non-contact mode, the tip of the Cantilever does not contact the sample surface. The Cantilever is driven to oscillate above the surface of the sample. Therefore, the tip does not cause any damage to the sample.

·       In Non-contact mode, the driving motion of the cantilever is either amplitude-modulated or frequency-modulated.

 Working Principle of Atomic Force Microscopy (AFM)

·       This image illustrates a typical AFM setup.

·       The Cantilever is used as a force sensor.

·       When the tip of the Cantilever is scanned across a sample, the tip acts like an elastic.

·       Depending on the amount of force between the tip and the sample, the Cantilever compresses and stretches.

·       The Atomic Force Microscope is constructed with a laser beam deflection system. The laser is reflected from the back of the AFM lever to the sensitive detector. They are made from silicon compounds with a tip radius of about 10 nm.

·       The Photo-detector records the changes to the reflected Laser beam position proportional to the movement of the Cantilever.

·       A detailed Topographical image of the sample can be captured by scanning across the surface of the material.

Applications of Atomic Force Microscopy (AFM)

·       Atomic Force Microscopy (AFM) has been used in various disciplines in Natural science such as Solid-state physics, Semiconductor studies, Molecular engineering, Polymer chemistry, Surface chemistry, Molecular biology, Cell biology, Medicine, and Physics.

·       Some of these applications include:

ü  Identifying atoms from samples.

ü  Evaluating force interactions between atoms.

ü  Studying the physical changing properties of atoms.

ü  Studying the structural and mechanical properties of protein complexes and assembly, such as microtubules.

ü  Used to differentiate cancer cells and normal cells.

ü  Evaluating and differentiating neighboring cells and their shape and cell wall rigidity.

Advantages of Atomic Force Microscopy (AFM)

·       Easy to prepare samples for observation.

·       AFM can be used in vacuums, air, and liquids.

·       Measurement of sample sizes is accurate.

·       AFM has a 3D imaging

·       AFM can be used to study living and non-living elements

·       AFM can be used to quantify the roughness of surfaces

·       AFM is used in dynamic environments.

 Disadvantages of Atomic Force Microscopy (AFM)

·       AFM can only scan a single nanosized image at a time of about 150 × 150 nm.

·       AFM have a low scanning time which might cause thermal drift on the sample.

·       The tip and the sample can be damaged during detection.

AFM has a limited magnification and vertical range.