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

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.