Emergent optical properties in crystalline, semiconducting 2D covalent organic framework / TMD heterostructures

Two dimensional covalent organic frameworks are a new class of Van der Waals materials formed of periodic, covalently-bound lattices of planar organic molecules. The symmetry, lattice constant, optical, and electronic properties of 2D COFs can be controlled via choice of molecular constituents, which give rise to lattice properties not present in the component parts. To date, 2D COFs have been limited by large bandgaps, low coupling, and poor control of material morphology. Here, we present results on a new semiconducting 2D covalent organic framework and the unusual optical properties that emerge in COF / TMD heterostructures and when exfoliated to few-layer sheets. We characterize the structure by X-ray scattering, TEM, AFM, transport, and optical spectroscopy, and demonstrate facile manipulation onto arbitrary experimental platforms. Our data demonstrates that the formation of a highly crystalline and semiconducting 2D COF permits thickness-dependent optical properties not previously observed in 2D covalent organic frameworks. We further demonstrate thickness-dependent energy transfer dynamics in semiconducting COF / TMD heterostructures and outline directions of future research.