1 research outputs found
Collective States in Molecular Monolayers on 2D Materials
Collective excited states form in organic two-dimensional layers through the
Coulomb coupling of the molecular transition dipole moments. They manifest as
characteristic strong and narrow peaks in the excitation and emission spectra
that are shifted to lower energies compared to the monomer transition. We study
experimentally and theoretically how robust the collective states are against
homogeneous and inhomogeneous broadening as well as spatial disorder that occur
in real molecular monolayers. Using a microscopic model for a two-dimensional
dipole lattice in real space we calculate the properties of collective states
and their extinction spectra. We find that the collective states persist even
for 1-10% random variation in the molecular position and in the transition
frequency, with similar peak position and integrated intensity as for the
perfectly ordered system. We measure the optical response of a monolayer of the
perylene-derivative MePTCDI on two-dimensional materials. On the wide band-gap
insulator hexagonal boron nitride it shows strong emission from the collective
state with a line width that is dominated by the inhomogeneous broadening of
the molecular state. When using the semimetal graphene as a substrate, however,
the luminescence is completely quenched. By combining optical absorption,
luminescence, and multi-wavelength Raman scattering we verify that the MePTCDI
molecules form very similar collective monolayer states on hexagonal boron
nitride and graphene substrates, but on graphene the line width is dominated by
non-radiative excitation transfer from the molecules to the substrate. Our
study highlights the transition from the localized molecular state of the
monomer to a delocalized collective state in the two-dimensional molecular
lattice that is entirely based on Coulomb coupling between optically active
excitations of the electrons or molecular vibrations