Supertransport of excitons in atomically thin organic semiconductors at the 2D quantum limit

Daniel Macdonald,Miheng Dong,Robert Halbich,Linglong Zhang,Tobias Vogl,Fan Wang,Ping Koy Lam,Kun Liang,Hieu T. Nguyen,Stuart K. Earl,Ankur Sharma,Jonathan O. Tollerud,Yuerui Lu,Jeffrey A. Davis,Yi Zhu,Shilpa Sanwlani

doi:10.1038/s41377-020-00347-y

Abstract

Long-range and fast transport of coherent excitons is important for the development of high-speed excitonic circuits and quantum computing applications. However, most of these coherent excitons have only been observed in some low-dimensional semiconductors when coupled with cavities, as there are large inhomogeneous broadening and dephasing effects on the transport of excitons in their native states in materials. Here, by confining coherent excitons at the 2D quantum limit, we first observed molecular aggregation-enabled ‘supertransport’ of excitons in atomically thin two-dimensional (2D) organic semiconductors between coherent states, with a measured high effective exciton diffusion coefficient of ~346.9 cm2/s at room temperature. This value is one to several orders of magnitude higher than the values reported for other organic molecular aggregates and low-dimensional inorganic materials. Without coupling to any optical cavities, the monolayer pentacene sample, a very clean 2D quantum system (~1.2 nm thick) with high crystallinity (J-type aggregation) and minimal interfacial states, showed superradiant emission from Frenkel excitons, which was experimentally confirmed by the temperature-dependent photoluminescence (PL) emission, highly enhanced radiative decay rate, significantly narrowed PL peak width and strongly directional in-plane emission. The coherence in monolayer pentacene samples was observed to be delocalised over ~135 molecules, which is significantly larger than the values (a few molecules) observed for other organic thin films. In addition, the supertransport of excitons in monolayer pentacene samples showed highly anisotropic behaviour. Our results pave the way for the development of future high-speed excitonic circuits, fast OLEDs, and other optoelectronic devices.

Highlights

There has been increasing interest in harnessing the long-range and fast transport of coherent excitons in solid-state inorganic semiconductors and molecular systems with confined geometries to enhance light-matter interactions[1,2,3]
The first pentacene layer on hexagonal boron nitride (h-BN) was named the wetting layer (WL; ~0.6 nm thick), and the layer of pentacene grown on WL was designated the monolayer (1L; ~1.2 nm thick)[34] (Fig. 1a–c)
The layer-dependent arrangement of pentacene molecules over h-BN and their electrical transport properties were described in a recent report[34]

Summary

Introduction

There has been increasing interest in harnessing the long-range and fast transport of coherent excitons (electron-hole pair quasi-particles) in solid-state inorganic semiconductors and molecular systems with confined geometries to enhance light-matter interactions[1,2,3]. Coherence is critical to engineering the quantum electrodynamics (QED)[4] in low-dimensional systems, such as quantum wells[5], two-dimensional (2D) materials[6], Spontaneous coherent emission from a system of several non-interacting dipole active atoms (excitons) was defined as superradiance (SR) by Dicke[17]. In this phenomenon, interactions between transition dipoles of individual molecules allow coherent delocalisation across multiple sites. The excitation (exciton) can be transferred over much longer distances in a delocalised molecular assembly before annihilation, resulting in large effective exciton diffusion coefficients

Methods

Results

Conclusion