Explain the working of atomic force microscope in detail.

Answer: an atomic force microscope (AFM) scans a very sharp tip, mounted on a flexible cantilever, over the sample surface; the tiny forces between the tip atoms and surface atoms bend the cantilever, and a laser detects this bending to build a 3-D image of the surface at nanometre (even atomic) resolution.

Main parts

• Cantilever with sharp tip: a tiny silicon or silicon nitride beam (about 100–200 µm long) with a tip only a few nanometres wide at its free end.
• Piezoelectric scanner: moves the sample (or tip) very precisely in x, y and z.
• Laser and position-sensitive photodetector: a laser beam reflects off the back of the cantilever onto a split photodiode.
• Feedback electronics and computer: control the scanner and form the image.

Working

1. The tip is brought very close to the surface. Interatomic forces act between the tip and the surface (van der Waals attraction at slightly larger distances, repulsion at very short distances).
2. As the sample is scanned line by line beneath the tip, the surface hills and valleys change this force and the cantilever bends up or down (Hooke's law, \(F = kx\)).
3. Even a deflection of a fraction of a nanometre shifts the reflected laser spot on the photodiode, and the change in signal measures the deflection.
4. A feedback loop moves the scanner in the z-direction to keep the deflection (force) constant. The z-movement at each (x, y) point gives the height of the surface, and the computer builds a 3-D topographic image.

Modes: contact mode (tip touches the surface, repulsive force), non-contact mode (tip vibrates just above the surface, attractive force), and tapping mode (oscillating tip touches the surface intermittently, reducing damage to soft samples).

Advantages: works on conductors and insulators alike, needs no vacuum (works in air or liquid, so biological samples can be studied), gives true 3-D surface data with very high resolution, and needs little sample preparation.

Limitations: small scan area, slow scanning, and the image can be distorted by tip wear or tip shape.