Ultrahigh conductivity and non-trivial band structure in van der Waals Nb dichalcogenides with Ge intercalation

Chemical intercalation has great potential to realize exotic electrical properties in van der Waals materials. Here we report the Ge intercalation in layered Nb dichalcogenides NbX2 (X = Se and S), which can induce ultrahigh conductivity and novel transport properties. Using the chemical vapor transport method, single crystals of two intercalated materials, namely, Ge0.33NbS2 and Ge0.26NbSe2, were grown. Despite the similar structures with an edge-sharing NbX6 triangular prism, the intercalated Ge shows different configurations in these two compounds, which is ordered in sulfide but disordered in selenide. The measurement of transport properties demonstrated that both Ge0.33NbS2 and Ge0.26NbSe2 are metallic and interestingly the conductivity of Ge0.33NbS2 at 300 K reaches 6.83 x 10(4) S cm(-1), which is higher than those of other metallic transition metal dichalcogenides and even comparable to that of the electrode material Pt. Ge0.33NbS2 also shows high carrier mobility (83.40 cm(2) V-1 s(-1) at 300 K) and unsaturated linear magnetoresistance, while negative magnetoresistance was observed in Ge0.26NbSe2 below 10 K. The first-principles calculations indicated that the ultrahigh conductivity in Ge0.33NbS2 is ascribed to the non-trivial topological bands near the Fermi surface. Our work highlights the effective regulation of transport properties of van der Waals materials via modulating the band structures by chemical intercalation.