Extended Data Fig. 8: Effects of TMD quality on the output characteristics.

a, b, Sample-quality-dependent contact performance for the case of monolayer MoS₂. The room-temperature output characteristics of the Bi–MoS₂ transistors fabricated with a CVD-grown defective MoS₂ monolayer (a) and MOCVD-grown MoS₂ monolayer (b). Inset to a, optical image of a typical low-quality MoS₂ crystal with a non-clean surface and curved edges; scale bar, 5 μm. Inset to b, optical image of a typical high-quality MoS₂ crystal with a clean surface; scale bar, 10 μm. c, d, Output characteristics of Bi-contact transistors fabricated with fresh CVD-grown monolayer WS₂ (c) and monolayer WSe₂ (d) FETs, showing that the proposed gap-state-saturation-induced ohmic contact can also be formed on high-quality WS₂ and WSe₂ CVD samples. e, f, Room-temperature output characteristics (e) and transfer curves (f) of the Bi–WSe₂ transistors fabricated with an aged CVD-grown WSe₂ monolayer (low quality). Scale bar, 10 μm. g, h, Room-temperature output characteristics (g) and transfer curves (h) of the Bi–WSe₂ transistors fabricated with a fresh CVD-grown WSe₂ monolayer (medium quality). Scale bar, 10 μm. i, j, Room-temperature output characteristics (i) and transfer curves (j) of the Bi–WSe₂ transistors fabricated with a mechanically exfoliated WSe₂ monolayer (high quality). Scale bar, 5 μm. The results show a clear evolution from p-type conduction to enhanced n-type conduction with the sample quality improvement. These variations could be attributed to the gap-state pinning effect induced by the chalcogen vacancies (Extended Data Fig. 4b). Insets to f, h and j are the optical images of a typical low-quality CVD WSe₂ crystal with an obviously defective surface (f), a medium-quality CVD WSe₂ with an irregular crystal shape (h), and a high-quality, freshly exfoliated WSe₂ with a clean surface (j).