In-Plane and Out-of-Plane Excitonic Coupling in 2D Molecular Crystals

TL;DR

This study investigates the spatial evolution of excitons in 2D molecular crystals, revealing in-plane and out-of-plane excitonic behaviors, and how these properties change with layer thickness, providing insights for advanced optoelectronic applications.

Contribution

The paper provides the first detailed analysis of excitonic coupling and transition dipole reorientation in 2D molecular crystals at molecular length scales.

Findings

In-plane and out-of-plane excitonic evolution observed

Energy inversion of Frenkel emissions at single-layer limit

Transition dipole moments reorient with increasing thickness

Abstract

Understanding the nature of molecular excitons in low-dimensional molecular solids is of paramount importance in fundamental photophysics and various applications such as energy harvesting, switching electronics and display devices. Despite this, the spatial evolution of molecular excitons and their transition dipoles have not been captured in the precision of molecular length scales. Here we show in-plane and out-of-plane excitonic evolution in quasilayered two-dimensional (2D) perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) crystals assembly-grown on hexagonal boron nitride (BN) crystals. Complete lattice constants with orientations of two herringbone-configured basis molecules are determined with polarization-resolved spectroscopy and electron diffraction methods. In the truly 2D limit of single layers, two Frenkel emissions Davydov-split by Kasha-type intralayer coupling exhibit energy inversion with decreasing temperature, which enhances excitonic coherence. As the thickness increases, the transition dipole moments of newly emerging charge transfer excitons are reoriented because of mixing with the Frenkel states. The current spatial anatomy of 2D molecular excitons will inspire a deeper understanding and groundbreaking applications of low-dimensional molecular systems.