Pressure-Driven Metallicity in Ångström-Thickness 2D Bismuth and Layer-Selective Ohmic Contact to MoS₂

Recent fabrication of two-dimensional (2D) metallic bismuth (Bi) via van der Waals (vdW) squeezing offers a route to ultrascaling metal into ångström thickness. However, free-standing 2D Bi is typically semiconducting, which contradicts the experimentally observed metallicity in vdW-squeezed 2D Bi. Here we show that this discrepancy originates from the pressure-induced buckled-to-flat structural transition in 2D Bi, changing the electronic structures from semiconducting to semimetallic. Based on the experimentally fabricated MoS2-Bi-MoS2 trilayer heterostructure, we demonstrate the concept of layer-selective Ohmic contact in which one MoS2 layer forms an Ohmic contact to the 2D Bi while the opposite MoS2 exhibits a Schottky barrier. The Ohmic contact can be switched between the two sandwiching MoS2 monolayers by reversing an external gate field, thus enabling charge to be spatially injected into different MoS2 layers. The layer-selective Ohmic contact proposed here represents a layertronic generalization of semimetal/semiconductor contact, paving the way toward layertronic device application.