Question

(For the case of the hexagonal boron nitride and tungsten diselenide, show your physics reasoning on why there is a shift in the phonon mode as the thickness of the materials reduces?)

If thickness of material lattice is reduced , the raman shift increases. So, there is shift in phonon mode as the thickness of materials reduces.

can someone please explain the physics reasoning of this statement

Answer 1

Structure of h-BN:

As an analogy of graphene, B atoms and N atoms of a 2D h-BN are alternately arranged to form a honeycomb structure, following the law of a hexagonal lattice formation (Fig. 12).118,119 The B–N bond length is 1.45 Å, which forms through sp2 hybridization. Three sp2 orbits of each B atom combine with the sp2 orbit of adjacent N atoms to form a strong σ bond, likewise, three sp2 orbits of each N atom combine with the sp2 orbit of adjacent B atoms to form a strong σ bond. Adjacent layers of h-BN are combined with weak van der Waals forces, and in each layer B atoms and N atoms are joined by covalent bonds. The interlayer spacing of graphene is 0.335 nm, and the interlayer spacing of h-BN is 0.333 nm, which is slightly less than that of the graphite. In the c-axis direction of h-BN, the bonding force is small and the interlayer spacing is large, making the interlayer slide easily.

The Raman characteristic peaks of h-BN crystals is at about 1366 cm−1, and a single-layer peak will blue-shift 4 cm−1 due to the BN bond of a single-layer h-BN having slightly shorter phonon modes, causing E2g to harden (Fig. 18).152 The red-shift depends on random strain introduced by the stripping process, which dominates in the double h-BN. Therefore, the h-BN Raman characteristic peak is red-shift 1–2 cm−1.