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.