Gate-induced Metallic Transition in Colloidal Quantum Dots Superlattice

The quantum confinement properties of colloidal quantum dots (QDs) make them emerge as a new class exhibiting intriguing properties, creating artificial giant atoms that can be assembled into coherent superstructures. Controlling individual colloidal QD into superlattice assembly can give rise to novel emerging properties stemming from the collective phenomena, different from the properties of their bulk materials and the isolated QDs. Therefore, systematically understanding the charge transport mechanism in a very clean system of QD assemblies is crucial. Here we demonstrate a fascinating transition from insulating to metallic behavior on lead sulfide (PbS) colloidal QDs. This observation can be attained due to the perfection of the methods to form giant superlattices of epitaxially-connected PbS QDs, in which highly-coherent atomic lattice and minimum energetic disorder are achieved. The induction of extremely high carrier density in these QD-SLs utilizing the electric-double-layer transistor (EDLT) can significantly shift the Fermi level energy, and we can surpass the predicted critical carrier density to achieve metallicity that allows high conductance. This finding will promote QD-SLs as the next-generation platforms for quantum materials and advance the present functionalizations of these low-dimensional materials in optoelectronic devices.