Week 16 Nanostructures Characterization Techniques (Atomic Force Microscopy)

 Atomic force microscopy (AFM) 

Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very high resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer (atomic size resolution), more than 1000 times better than the optical diffraction limit. Using an atomic force microscope (AFM), it is possible to measure a roughness of a sample surface at a high resolution, to distinguish a sample based on its mechanical properties (for example, hardness and roughness) and, in addition, to perform a microfabrication of a sample (for example, an atomic manipulation).

In a field of semiconductor physics, for example, (a) an identification of atoms at a surface, (b) evaluation of an interaction between a specific atom and its neighboring atoms and (c) a change in physical properties arisen from a change in an atomic arrangement thorough the atomic manipulation have been studied.                             

In a field of a cellular biology, for example, (a) an attempt to distinguish cancer cells and normal cells based on a hardness of cells and (b) an attempt to evaluate of an interaction between a specific cell and its neighboring cells in a competitive culture system have been made. In some variations, electric potentials can also be scanned using conducting cantilevers. In more advanced versions, currents can be passed through the tip to probe the electrical conductivity or transport of the underlying surface.
Basic principles: The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature from few nanometers to few tens of nanometers. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning. When the tip is brought into proximity of a sample surface (Normally few nanometer above the sample surface where interatomic Vander Waals force between tip & sample surface atoms  starts to act or in some cases tip drag on the sample surce where coulomb repulsive force between tip & sample surface atoms stars to act). These attractive and repulsive forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Measuring deflection of cantilever, force as a function of tip position on sample can be measured which indirectly gives surface topography or roughness.

 Depending on the situation, forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces (see magnetic force microscope, MFM), Casimir forces, solvation forces, etc. Along with force, additional quantities may simultaneously bemeasured through the use of specialized types of probes (see scanning thermal microscopy, scanning joule expansion microscopy, photothermal microspectroscopy, etc.). Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes.

In most cases a feedback mechanism is employed to adjust the tip to sample Distance to maintain a constant force between the tip and the sample. Traditionally the tip or sample is mounted on a 'tripod' of  three piezo crystals, with each responsible for scanning in the x, y and z directions.

The resolution of AFM mainly depends on radius of curvature of AFM tip. To get atomic resolution ultra sharp tips withradius of curvature of few nanometer are required.  

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