Collective States in 2D Molecular Monolayers

Molecules (0D) have long been known to interact through the dipole moments of their excited states, which may be exploited to create complex opticaa excitations. In recent years, great interest arose in combining them with 2D materials to design hybrid heterostructures with greatly tunable optical properties. Of particular interest are lattices of molecular monolayers that mimic 2D systems through ordered arrangment of 0D monomers. They can be thought of as a new material platform to replace and expand existing optically active 2D monolayers such as transition metal dichalcogenides (TMDs). Most organic monolayer are grown out of planer dye molecules that tend to form J-aggregates. In J-aggregates the transition dipole moments of the molecules are aligned resulting in a strong coupling of the molecules, forming a collective state. The photonic excitation of the collective states to higher electronic levels will result in a very narrow and strong emission peak.<br/><br/>Theoretically and experimentally, we show how robust the collective state is against spatial disorder excited state quenching, and inhomogeneous broadening of the molecular transitions. Therefore, we grew monolayers of N,N′-Dimethyl-3,4,9,10-Perylentetracarboxylic diimide (MePTCDI), a perylene derivative, on two different van der Waals materials, few-layer hexagonal boron nitride and graphene. Both materials have a flat surface promoting the growth of dye monolayers providing a perfect platform to study the fundamental mechanism of collective states. High resolution AFM was used to determine the lattice structure that the molecules form in a monolayer. Showing a dense packed and highly ordered molecular structure. Optical spectroscopy and light scattering were used to characterize the collective state of the grown 2D material. We were able to measure the characteristic fluorescence for closely packed molecules, further we observed the vanishing Stokes shift between absorption and emission spectrum which is another characteristic for the presence of a collective dipole state. These findings make the molecular monolayers exciting platforms to implement designs of quantum optics in scalable devices.