ABSTRACT
Fully leveraging the remarkable properties of low-dimensional semiconductors requires developing a deep understanding of how their structure and disorder affect the flow of electronic energy. Here, we study exciton transport in single crystals of the two-dimensional superatomic semiconductor CsRe6Se8I3, which straddles a photophysically rich yet elusive intermediate electronic-coupling regime. Using femtosecond scattering microscopy to directly image exciton transport in CsRe6Se8I3, we reveal the rare coexistence of coherent and incoherent exciton transport, leading to either persistent or transient electronic delocalization depending on temperature. Notably, coherent excitons exhibit ballistic transport at speeds approaching an extraordinary 1600 km/s over 300 fs. Such fast transport is mediated by J-aggregate-like superradiance, owing to the anisotropic structure and long-range order of CsRe6Se8I3. Our results establish superatomic crystals as ideal platforms for studying the intermediate electronic-coupling regime in highly ordered environments, in this case displaying long-range electronic delocalization, ultrafast energy flow, and a tunable dual transport regime.
ABSTRACT
The area of two-dimensional (2D) materials research would benefit greatly from the development of synthetically tunable van der Waals (vdW) materials. While the bottom-up synthesis of 2D frameworks from nanoscale building blocks holds great promise in this quest, there are many remaining hurdles, including the design of building blocks that reliably produce 2D lattices and the growth of macroscopic crystals that can be exfoliated to produce 2D materials. Here we report the regioselective synthesis of the cluster [trans-Co6Se8(CN)4(CO)2]3-/4-, a "superatomic" building block designed to polymerize and assemble into a 2D cyanometalate lattice whose surfaces are chemically addressable. The resulting vdW material, [Co(py)4]2[trans-Co6Se8(CN)4(CO)2], grows as bulk single crystals that can be mechanically exfoliated to produce flakes as thin as bilayers, with photolabile CO ligands on the exfoliated surface. As a proof of concept, we show that these surface CO ligands can be replaced by 4-isocyanoazobenzene under blue light irradiation. This work demonstrates that the bottom-up assembly of layered vdW materials from superatoms is a promising and versatile approach to create 2D materials with tunable physical and chemical properties.
