  The rise of atomically thin materials has the potential to enable a paradigm
shift in modern technologies by introducing multi-functional materials in the
semiconductor industry. To date the growth of high quality atomically thin
semiconductors (e.g. WS2) is one of the most pressing challenges to unleash the
potential of these materials and the growth of mono- or bi-layers with high
crystal quality is yet to see its full realization. Here, we show that the
novel use of molecular precursors in the controlled synthesis of mono- and
bi-layer WS2 leads to superior material quality compared to the widely used
topotactic transformation of WO3-based precursors. Record high room temperature
charge carrier mobility up to 52 cm2/Vs and ultra-sharp photoluminescence
linewidth of just 36 meV over submillimeter areas demonstrate that the quality
of this material supersedes also that of naturally occurring materials. By
exploiting surface diffusion kinetics of W and S species adsorbed onto a
substrate, a deterministic layer thickness control has also been achieved
promoting the design of scalable synthesis routes.
