  Long-range and fast transport of coherent excitons is important for
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 exciton transport
in their native states of the materials. Here, by confining coherent excitons
at the 2D quantum limit, we firstly observed molecular aggregation enabled
super-transport of excitons in atomically thin two-dimensional (2D) organic
semiconductors between coherent states, with a measured a high effective
exciton diffusion coefficient of 346.9 cm2/sec at room temperature. This value
is one to several orders of magnitude higher than the reported values from
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 emissions from
the 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 delocalized over 135 molecules, which is significantly larger
than the values (a few molecules) observed from other organic thin films. In
addition, the super-transport 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 opto-electronic
devices.
