Substrate induced tuning of compressive strain and phonon modes in large area MoS₂ and WS₂ van der Waals epitaxial thin films

Large area epitaxial thin films of van der Waals compounds MoS₂ and WS₂ have grown on 'c' plane sapphire by pulsed laser deposition (PLD) with control over the number of layers. Substrates induced compressive strain is responsible for the stiffening of both the Raman modes (A_1g and E¹_2g) with implication in increase in the direct band gap of these materials. The results demonstrate a practical way to engineer the optoelectronic property of such materials for large area device fabrication and application. (author)