# Highly Ordered T6 Organic Semiconductor Networks on MoS2 Nanosheets for Optoelectronic Applications

## TL;DR

Researchers grew highly ordered organic semiconductor crystals on MoS2 nanosheets, which could be useful for optoelectronic devices.

## Contribution

The study shows that MoS2 enables long-range order in T6 crystals due to alignment with its atomic structure.

## Key findings

- T6 needles on MoS2 show ±7° alignment with zigzag directions.
- T6 on multilayer graphene lacks long-range order.
- Ordered T6 crystals may improve optoelectronic performance.

## Abstract

Van der Waals 2D
crystals, with their dangling bond-free surfaces
and extremely low roughness levels, are highly appealing substrates
for the epitaxial growth of organic semiconductors. The growth has
important consequences in the fabrication of organic electronic components
such as organic light diodes. Here, using a MoS2 nanosheet,
an n-type semiconducting 2D material, as a substrate, we grow highly
ordered crystalline needles made of sexithiophene (T6), a p-type organic
semiconductor. Using atomic force microscopy topographic analysis,
X-ray diffraction, and micro-Raman spectroscopy, we show that the
T6 needles show both short- and long-range order thanks to an alignment
between the T6 long axis and high symmetry directions in MoS2. By statistical analysis we demonstrate that the T6 needles show
a small mismatch of ±7° between their long
axis and the zigzag directions of MoS2. Interestingly,
T6 grown on multilayer graphene does not show such an order, resulting
in a randomly oriented needle network, while T6 grown on atomically
thin MoS2 still show long-range order. Density functional
theory predicts an alignment of the T6 long axis along the zigzag
direction of MoS2 to minimize the total energy of the system
by maximizing the number of thiophene rings positioned on top of sulfur
atoms from MoS2. The ideal interface between T6 molecules
and MoS2 also has implications in the charge transfer of
photoexcited carriers as demonstrated by microphotoluminescence spectroscopy.
These results demonstrate that van der Waals materials are ideal substrates
for the growth of organic molecules and that subtle variations in
the van der Waals long-range potential can influence the long-range
order of the molecular crystals. The ordered one-dimensional T6 crystallites
are interesting for polarized or anisotropic optoelectronic applications.