Correlating Structural and Optical Properties in Aromatic Semiconductor Crystals and Heterostructures
Perylene microcrystals have been grown by continuous resublimation of a perylene layer originally grown by organic molecular beam deposition. Under the correct growth condition, virtually defect free single-crystalline platelets of both the Alpha-and Beta-phase with molecular smooth surfaces were...
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|Summary:||Perylene microcrystals have been grown by continuous resublimation of a perylene
layer originally grown by organic molecular beam deposition. Under the correct growth
condition, virtually defect free single-crystalline platelets of both the Alpha-and Beta-phase
with molecular smooth surfaces were achievable. Both polymorphs are easily distinguishable
by their characteristic rhombic and rectangular shape and their distinct
emission spectra, appearing orange and green to the eye for the Alpha-and Beta-phase, respectively.
Their diameter of up to 100 µm allows for high-resolution polarizationresolved
optical spectroscopy, directly linking the crystalline axis to the anisotropic
optical response of each crystalline phase. To this end, we addressed the in plane
crystalline b and c-axis of both species in absorption spectroscopy at cryogenic temperature.
We obtained information on the excitonic system with unprecedented accuracy.
This enables a comprehensive comparison of the experimental spectra and state of
the art ab initio calculations. Indications for a polaritonic stopband where found by
analyzing the differences between both spectra. The calculated electronic bandstructure
and excitonic wavefunction could be correlated to the measured emission lifetimes of
both perylene polymorphs: Strong dispersion and spatial delocalization translate to
shorter PL lifetimes. The more localized wavefunction of the a-phase could be linked
to the strong intermolecular interaction of the perylene dimers that make up the crystal.
PEN-PFP heterostructures with different molecular alignment at the heterointerface
where grown exploiting templating effects mediated by the substrat and the previously
deposited layer: one intermixed 1:1 molecular blend and two layered heterostructures
with edge-on and face-on molecular alignment at the interface. Comparing the optical
properties of those samples with the corresponding unitary films revealed the interface
specific response of the system. We could show that the interface does not influence
the emission spectra and dynamics of the constituent layers not directly at the interface.
However, completely new interface related emission signal where observed at lower energies,
displaying long lifetimes when compared to the free excitonic emission observed
from the unitary materials. We assign those emission lines to CT-excitons. They form
with great efficiency in the intermixed heterostructure, completely replacing any signal
of the unitary molecules at low temperatures. In the heterostacks, a strong increase of
CT-emission was observable for face-on stacking on the interface, which is linked to an
increase in intermolecular interaction across the interafce due to p-p stacking between
PEN and PFP molecules. Previous studies, especially on the frontier orbitals of the
constituting molecules at the interface, reveals significant deviation from the commonly
discussed discription of CT-excitons.
To gain additional insight into the formation pathways of those CT-states, PLE spectra
of the heterostructures where compared with their respective absorption spectra. The
differences observed in both spectra reveal absorption channels which do not relax into
the CT-subsystem. While all excitons excited in the PEN layer and directly into the CTstate
contribute to CT-emission, any excitation into the PFP layer does not. This further
raises questions about the exact nature of the CT state, as a simple relaxation scheme
based on the frontier orbitals of all involved states does not hold up to the experiments.|
|Physical Description:||116 Pages|