Substrate Dependent Synergistic Many-Body Effects in Atomically Thin Two Dimensional WS$_2$

Abstract: Mott transition has been realized in atomically thin monolayer (ML) of two dimensional semiconductors (WS$_2$) via optically excited carriers above a critical carrier density through many body interactions. The above nonlinear optical transition occurs when excited electron hole pairs in ML WS2 continuum heavily interact with each other followed by transformation into a collective electron hole plasma phase (EHP), by losing their identity as individual quasiparticles. This is manifested by the alluring red-shift-blue-shift crossover (RBC) phenomena of the excitonic peaks in the emission spectra, resulting from the synergistic attraction-repulsion processes at the Mott-transition point. A systematic investigation of many-body effects is reported on ML WS$_2$, while considering the modulated dielectric screening of three different substrates, viz., silicon dioxide, sapphire, and gold. Substrate doping effects on ML WS$_2$ are discussed using the Raman fingerprints and PL spectral weight, which are further corroborated using theoretical DFT calculations. Further the substrate dependent excitonic Bohr radius of ML WS$_2$ is extracted via modelling the emission energy shift with Lennard-Jones potential. The variation of Mott point as well as excitonic Bohr radius is explained via substrate induced dielectric screening effect for both the dielectric substrates, which is however absent in ML WS$_2$ on Au. Our study therefore reveals diverse many-body ramifications in 2D semiconductors and offers decisive outlooks on selecting the impeccable substrate materials for innovative device engineering.