Two‐Dimensional Haeckelite NbS2: A Diamagnetic High‐Mobility Semiconductor with Nb4+ Ions

In all known Group 5 transition‐metal dichalcogenide monolayers (MLs), the metal centers carry a spin, and their ground‐state phases are either metallic or semiconducting with indirect band gaps. Here, on grounds of first‐principles calculations, we report that the Haeckelite polytypes 1 S ‐NbX₂ (X=S, Se, Te) are diamagnetic direct‐band‐gap semiconductors even though the Nb atoms are in the 4+ oxidation state. In contrast, 1 S ‐VX₂ MLs are antiferromagnetically coupled indirect‐band‐gap semiconductors. The 1 S phases are thermodynamically and dynamically stable but of slightly higher energy than their 1 H and 1 T ML counterparts. 1 S ‐NbX₂ MLs are excellent candidates for optoelectronic applications owing to their small band gaps (between 0.5 and 1 eV). Moreover, 1 S ‐NbS₂ shows a particularly high hole mobility of 2.68×10³ cm² V⁻¹ s⁻¹, which is significantly higher than that of MoS₂ and comparable to that of WSe₂.