A revised mechanism of band gap evolution of TMDC nanotubes and its application to Janus TMDC nanotubes: negative electron and hole compressibility

It is widely accepted that quantum confinement and strain effect display opposite impacts on the band gap size of spherical or tubular transition metal dichalcogenide (TMDC) nanostructures. However, our extensive ab initio calculations and correlation of the band edge evolution of single-wall (SW) TMDC nanotubes (NTs) to their in-plane or out-of-plane orbital characters reveal that the previous interpretation of the band gap evolution behavior (which focused on strain energy) can be revised to the cooperation of deformation potential and flexoelectricity. Specifically, we scrutinize the band profile of multiwall (MW) TMDC NTs and assign the experimentally observed red/blue shift in excitonic transition energy to the decoupling effect arising from flexoelectric field rather than from the commonly expected quantum confinement effect. More importantly, we further apply these novel insights to nested Janus TMDC NTs, which offer an unprecedented platform to realize both negative electron and negative hole compressibilities without the electron correlation effect. Such compressibility gives rise to negative quantum capacitance. This in turn endows these 1D van der Waals heterostructures with emerging applications in hysteresis-free steep-slope transistors and multivalued logic devices.