Tuning ordering of organic nanostructures on 2D materials

Two-dimensional (2D) materials only one atom layer thin films have been recently proposed as functional substrates for organic semiconductors. Interfaces between these two types of materials exhibit interesting properties (as low injection barriers and smooth growth morphologies), and could be very important building blocks of novel organic electronics, optoelectronics, and photovoltaic devices. The here proposed project entitled Tuning ordering of organic nanostructures on 2D materials will tackle self-assembly of organic semiconductor nanostructures at the interface with 2D materials, in order to achieve enhanced growth morphologies and novel heterostructures. The project will explore whether an external control over the degree of molecular ordering is possible. In order to achieve this goal, three different approaches will be employed, all using different mechanisms to in situ assist self- assembly, by applying: electric fields, axial strain, and polarized light. The project will employ 2D materials (graphene, phosphorene, molybdenum disulfide, and hexagonal boron nitride) as substrates. Small molecule organic semiconductors will be grown on these substrates using a hot wall epitaxy system, modified to allow for external application of electric fields, strain, or polarized light, during the growth of the molecules. Characterization of the obtained structures will be based on electrical measurements, scanning probe microscopy techniques, confocal and tip-enhanced Raman spectroscopy. Thus far, no one has addressed the issue of external alignment of the molecules supported by 2D or van der Waals materials. The project will explore novel and innovative techniques to grow highly ordered structures of organic semiconductors and will establish cutting-edge fabrication techniques that will allow for the external control over the self-assembly process. Achieving external control over the structure of molecular crystals at the interface with 2D and vdW materials could have strong impact on both the fundamental understanding of these interfaces and on the development, implementation and commercialization of organic semiconductors and 2D materials based technologies. The innovative growth techniques that will be established within the project could be readily integrated into the fabrication processes for organic electronics. Thus the project impact could go beyond the scientific community, reaching consumer electronics and consequently our daily life. Furthermore, the project will strongly impact applicants scientific career allowing him to work at the frontier of his research field and will keep high level of research at the host institution by opening new directions and establishing new methods.

The project under Lise Meitner training and career advancement programme, entitled: "Tuning ordering of organic nanostructures on 2D materials" targeted understanding of the possible means to control molecular self-assembly on two-dimensional (2D) materials. The project helped to increase the knowledge on these interfaces and to bring them closer to future electronics applications. 2D materials are isolated individual sheets of layered crystals. These sheets expose their atomic structure, usually in a honeycomb-like arrangement of atoms, to the surface adsorbents. Small molecular organic semiconductors are in a way similar to 2D materials, since molecular backbones resemble the patterns of the 2D substrates. If these molecules adsorb in high-vacuum and at elevated temperatures, they have "the freedom" to move on the surface and to find the best suited spots for adsorption. These spots minimize interaction energy and can be imagined as the fitting of puzzle pieces. Once the first molecules adsorb, the following molecules will "follow" the seed and form an organic crystal. This is a well-known phenomenon, referred to as epitaxy or registry growth. Considering the particular systems which are targeted by the project (2D materials and small rod-like molecules) there are usually six equivalent registry states. This means that the growing organic crystals will not have random orientation on the substrate, but will grow only in six predefined directions. If we consider applications in integrated electronics, six directions are not too far from completely random, and we cannot fully benefit from these systems. However, if there is inherent ordering present, there might also be a way to control it by external factors. Precisely that was the goal of this project. Within the project, three external factors were controllably and independently introduced during the growth: electric fields, strain of the substrate, and linearly polarized laser light. The first two methods have shown unclear results over the control of ordering of organic crystals. The last method demonstrated a promising pathway for growing self-aligned organic crystals that do not have intrinsic six equal directions, but only one or two. Our results could lead to future development of nanotechnologies and integrated electrical components through light-guided self-assembly. The technologies which are likely to benefit from these advances are flexible and wearable electronics, sensors, and multi-sensing chips.