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

Large area MoS₂ and WS₂ van der Waals epitaxial thin films with control over number of layers including monolayer is grown by pulsed laser deposition utilizing slower growth kinetics. The films grown on c-plane sapphire show stiffening of A_1g and E¹_2g phonon modes with decreasing number of layers for both MoS₂ and WS₂. The observed stiffening translate into the compressive strain of 0.52% & 0.53% with accompanying increase in fundamental direct band gap to 1.74 and 1.68 eV for monolayer MoS₂ and WS₂, respectively. The strain decays with the number of layers. HRTEM imaging directly reveals the nature of atomic registry of van der Waals layers with the substrate and the associated compressive strain. The results demonstrate a practical route to stabilize and engineer strain for this class of material over large area device fabrication.