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

TL;DR

This study demonstrates how substrate-induced effects can control strain and phonon properties in large-area MoS2 and WS2 thin films, enabling strain engineering for device applications.

Contribution

It introduces a pulsed laser deposition method to grow large-area monolayer and multilayer MoS2 and WS2 films with controlled strain via substrate interactions.

Findings

Phonon mode stiffening correlates with layer reduction.

Monolayer films exhibit ~0.5% compressive strain.

Strain influences the band gap, increasing it in monolayers.

Abstract

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.