  Large area MoS2 and WS2 van der Waals epitaxial thin films with complete
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 A1g and E12g 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.
