Correlating growth time with stoichiometry and electronic structure in WS2 nanoflakes grown by quasi-van der Waals molecular beam epitaxy on quartz

Controlling the electronic properties of two-dimensional tungsten disulfide (WS 2 ) requires precise understanding of how growth conditions govern atomic ordering and stoichiometry. However, direct correlations between growth parameters and the resulting electronic structure of WS 2 grown by molecular beam epitaxy are not yet fully understood, particularly on insulating bulk crystalline substrates. In this study, we investigate the effect of growth time on the structural, chemical, and electronic properties of WS 2 nanoflakes directly synthesized on crystalline α-ST-cut quartz by quasi van der Waals molecular beam epitaxy (q-vdW MBE). Under sulfur-rich conditions, structurally disordered nanoflakes evolve into more ordered WS 2 layers with improved chemical homogeneity and reduced electronic disorder as the growth and annealing time increase. The evolution of bonding and electronic structure was examined using a combination of ex-situ and in-situ characterization techniques, revealing a gradual narrowing and clearer definition of excitonic and plasmonic features. Increasing growth time leads to improved W-S bonding, a decrease in the S:W atomic ratio from 2.58 to 2.34, a shift of the valence band maximum, and an increase in work function from 4.81 to 4.99 eV, indicating a clear reorganization of the surface electronic states. These results establish a direct correlation between growth time, stoichiometry, and electronic structure in q-vdW-MBE-grown WS 2 nanoflakes on a bulk insulating substrate.