Question

tungsten diselenide, show your physics reasoning on why there is a shift in the phonon mode as the thickness of the materials reduces?

can someone please explain the physics reasoning of this statement

Answer 1

The phenomenon of shift in the phonon mode is due to change in size is called phonon confinement.

The phonon confinement relates the observed changes in the Raman spectra to the crystal size.

The confinement of optical phonons in nano materials results in asymmetrically broadened Raman lines, which are shifted toward lower wave numbers.

This is very similar to the quantum confinement of electrons in a nano material.

As the size of the particles, reduces, the energy levels becomes quantised and so, the separation between the energy levels changes. As a result, there is a shift in the phonon energies as well.