Universal photoluminescence enhancement/suppression at the vertical van der Waals metal-semiconductor interfaces

Monolayers of transition metal dichalcogenides are considered the prospects for optoelectronic devices and photoluminescence (PL) is one of the key parameters to observe the performance and efficiency of such devices. The PL characteristics of monolayers of semiconducting transition metal dichalcogenides (TMDCs) have been consistently reported to be suppressed in the presence of metal, a phenomenon observed through direct metal evaporation or annealing of heterostructures in prior studies. These methods often resulted in a significant negative charge transfer which creates metal-induced gap states (MIGS) and Fermi level pinning (FLP). These MIGS and FLP provide nonradiative pathways to the excited electrons causing a huge suppression in PL intensity. To address this challenge, we explore heterostructures with a van der Waals gap between the metal and semiconductor surfaces. This design reduces the nonradiative relaxation pathways, allowing for more controlled charge transfer due to the van der Waals gap and the modulation of Schottky barrier height (SBH). The SBH for electrons increases with increasing metal work function and hence provides direct control of charge injection type and magnitude to monolayers of TMDCs. Our research presents a universal methodology for controlling the PL intensity of TMDCs by strategically utilizing the van der Waals gap and tailoring the work function of the interfacing metal. This investigation not only unveils a novel approach to prevent PL quenching but also opens avenues for optimizing opto-electronic devices. By carefully selecting metallic and semiconducting materials, this work offers a pathway to enhance device performance and precisely regulate output characteristics in optoelectronic applications.