Fig. 5: Quality of epitaxially-grown WS₂ monolayer.

a Photograph of a 2-inch sapphire wafer with WS₂ monolayer film; b Line scan of Raman spectra at the sub-mm scale showing the uniformity of the monolayer film; c Line scan of photoluminescence (PL) spectra at the sub-mm scale. d Circular polarized PL spectra of WS₂ grown on c-plane sapphire with \( < 10\bar{1}0 > \) atomic steps. The excitation light is right-handed circularly polarized at 2 eV. Left-handed and right-handed circularly polarized PL spectra are shown in orange and green, respectively. e Circular helicity calculated form the PL spectra in d. The high value over 60% indicates the high quality of our WS₂ monolayer. f Experimental valence band dispersion of WS₂ monolayers at 295 K along the Γ-K point and Γ-M high symmetry directions of the monolayer Brillouin zone obtained from a second derivative filter of the ARPES intensity. g Optical micrograph and schematic illustration of a wafer-scale array of back-gated WS₂ field-effect transistors (FETs). Scale bar, 400 μm. the inset shows a zoom-in of an individual FET device and its corresponding fluorescence image (FL), confirming continuous monolayer channels. Scale bar, 5 μm. h Representative transfer characteristics (I_d-V_gs) of 100 WS₂ FETs at 300 K, showing consistent n-type switching behavior with up to 6 orders of magnitudes in current change. Device yield exceeded 95%. i, Statistical distribution of field-effect mobility (μ) and on/off ratio extracted from the transport data in (h). A strong positive correlation can be observed between mobility and on/off ratio, with an average mobility of 6.3 cm²V⁻¹s⁻¹ and an average on/off ratio of 1.6 ×10⁶.