Aluminum-rich reconstructed sapphire as a high-quality substrate for tungsten disulfide synthesis

2D transition-metal dichalcogenides have attracted significant attention due to their unique properties, which make them highly promising for a variety of applications ranging from quantum technologies, to electronic and photonic applications. In this context, developing scalable methods for high-quality synthesis is a major research priority, with chemical vapor deposition (CVD) emerging as the most promising approach. In this work, we reveal the critical role of substrate preparation in the CVD growth of tungsten disulfide (WS₂) on sapphire by comparing the aluminum-rich reconstructed surface with a conventionally prepared one. We find that on average the reconstructed sapphire surface provides a significant 50% increase in crystal size and 125% increase in nucleation density. It also has a strong impact on the photoluminescence (PL) of WS₂ crystals, leading to quenching at the center while the edges exhibit intense red-shifted emission. Correlative Raman spectroscopy, atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM) analyses reveal that PL quenching in the center is due to a strong coupling between the Al-rich reconstructed sapphire and the WS₂ crystals, while the edges appear to be decoupled from the substrate, likely due to substrate degradation during the growth process. These findings highlight the importance of substrate reconstruction for tuning the morphology and optoelectronic properties of 2D materials.