With neat diagram, explain the principle, construction and working of Atomic Force Microscope

9.A] With neat diagram, explain the principle, construction and working of Atomic Force Microscope.

Answer:-

• Principle of AFM:-
  • The AFM measures the forces acting between a fine tip and a sample.
  • The tip is attached to the free end of a spring cantilever and is brought very close to a surface.
    The inter-atomic potential developed between the tip and the sample at inter-atomic separation results in attractive or repulsive forces.
  • As the tip scans the surface of the sample, the force between the tip and the sample varies which is sensed by the tip.
  • The amount of force between the probe and the sample is dependent on the spring constant of the cantilever and the distance between the probe and the sample surface. This force can be characterized with Hooke’s Law.
    F=Kx (11.4)
    here F = Force, k = spring constant, x = cantilever deflection
  • If the spring constant of cantilever (typically, 0.1 – 1 N/m) is less than surface, cantilever bends and the deflection is monitored.

• Working of AFM :-
  • As the tip travels across the sample surface the interaction with the asperities (Surface Profile) causes up and down motion.
  • These fluctuations are sourced by the interaction (electrostatic, magnetic, capillary, Van der Waals forces) between the tip and the sample.
  • The displacement of the Laser beam reflected from the backside of the tip traces the surface profile.
  • The laser and the photo-detector arrangement detects the position of the cantilever
  • The vertical deflection measures the interaction forces while the horizontal deflection measures the lateral forces.
  • Thus a three-dimensional image of the surface topography of the sample under a constant applied force (as low as nano-Newton range) is obtained.