Since its invention, Atomic Force Microscope has significantly evolved into a useful technique for wide spectrum of practical applications.

In 1980, scanning probe microscopy (SPM) was invented. In this SPM class, sharp probe is scanned over surface to sense nanoscale surface features. In 1981, one milestone was achieved with an advancement of Scanning tunneling microscopy (STM)  STM was rst technique to provide 3D topographical images with atomic resolution, also continuously used for individual atoms manipulation to fabricate unique structures. But big limitation of STM is, it requires conductive sample for analysis. Afterwards, in 1986 AFM came into scienti c eld and rev-olutionized the scanning microscopy eld. Atomic Force Microscope uses a sharp tip (radius less than 10 nm) for scanning therefore it is more advanced, developed and compre-hensive version of STM that can image approximately many kind of surface at atomic scale resolution.

Since its invention, Atomic Force Microscope has significantly evolved into a useful technique for wide spectrum of practical applications Like: direct analysis of micro-structural surfaces, study of nanoscale intermolecular forces with atomic scale resolution, helpful in mechanical nanoindentation, electrical, piezoelectrical and elec-tromechanical studies as well as high resolution surface topography, rough-ness and thickness determination of CVD grown 2D materials and there are many others.

Atomic Force Microscope AFM schematic setup

Operation of AFM is based on detection of attractive/repulsive forces between Atomic Force Microscope tip and sample surface. Atomic Force Microscope tip is made of exible cantilever which is re-sponsible for signal transduction. Interaction of tip and surface makes cantilever bending or twisting in a way proportion to interaction force. Small laser diode spot focuses over cantilever, senses any twist or bending of attached cantilever. Any de ection of laser beam is read on segmented position-sensitive photodetec-tor(PSD). During scanning de ection of cantilever because of surface features is monitored and then translated into a 3D image of surface.

Atomic Force Microscope Operational Modes

Atomic Force Microscope has multitude of various operating modes for di erent type of applications since its invention. Details of modes is given below.

Static Mode

Static mode also known as contact mode or constant force mode was rst op-erational mode invented in 1986. Here, AFM tip is in continuous contact with sample. AFM cantilever tends to show de ection when tip comes in contact with the sample. And de ection of cantilever is measured with high precision via photodetector. Feedback loop makes cantilever’s de ection constant through vertical piezo actuator adjustment.

AFM Static mode working principle b tip follows contour path

Then sample is scanned in lateral direction, usually in raster pattern. When de ection is constant, tip follows contour of surface of sample. Finally, by map-ping lateral position against vertical piezo position, topographic view of sample is made. In static mode, de ection of cantilever is proportional to interaction force between tip and sample using Hooke’s law.

Advantages to use this mode are, high scan speed, high atomic resolution is achievable. A challenge of using this mode comes because of high shear and lateral forces (also frictional force) arising during scanning, and can damage both tip or sample (fragile like: biological). Also sharp tip may cause delamination, destruction and removal of material from surface. Also, soft cantilevers snaps with the surface of sample because of tip and sample attractive force, which makes feedback loop instable. These challenges are major motivation to develop AFM dynamic mode.

Dynamic Mode

In 1990, dynamic mode also known as AC mode, tapping mode, intermittent contact mode (tip taps on sample surface) or non contact mode (tip never touch sample surface) invented to address demerits of static mode. In this mode, cantilever oscillates at its mechanical resonance frequency using dither piezo spot-ted at base part of cantilever. Amplitude, frequency and phase of de ection automatically adjusted as cantilever comes closer to sample. So, de ection is de-modulated, and kept constant in feedback loop, and constant tip-sample distance is kept at set point. Then just like static mode, tip scans sample in raster pattern to show its topography.

AFM Dynamic mode working principle

During scanning, this mode minimizes friction and low force is applied to sam-ple surface that results in no damage of tip and sample especially soft samples. Relatively lower lateral resolution as compare to contact mode because of tip-sample distance.

 

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