Growth of polar organic molecules on graphene and sapphire

The development of nanoscale organic devices is in the focus of intense research activities due to their potential to revolutionize the fabrication of inexpensive electronic devices. Electronic newspapers, smart windows, flexible film solar cell sheets, luminescent wallpaper, etc. can be cited as examples of future electronic devices based on organic electronics. In this context two major challenges have been identified: the stability of the organic semiconductors under ambient conditions and the structure of the thin films. The further is very critical in applications since it limits the lifetime of the devices. The latter concerns the charge carrier mobility in the thin films, which is intimately related to the micro- /nano-structure of the films. In this joint project, we plan to address these two challenges by using a multiscale approach, which goes from the organic synthesis of aza-modified molecules, via the characterization of individual molecules and their early stages of growth up to the investigation of the crystalline structure at the micrometer scale. Hereby, we combine the complementary know-how of the project partners: the experience of the French group with scanning tunneling microscopy based imaging and spectroscopy on the atomic and molecular scale with expertise of the Austrian group in scanning probe microscopy based analysis of molecular growth mechanisms and electrical characterization of the films on the nanometer scale. For the project we have chosen two substrates, which mimic application relevant surfaces: ultrathin graphene films as a model for conducting layers potentially relevant for optoelectronics and thin alumina films and sapphire as a model for gate oxides used in microelectronic devices. Concerning the organic semiconductors, we focus our interest to molecules, which are derived from two very popular organic semiconductors: pentacene and parahexaphenyl. These molecules have intensively investigated in the past, and it has been shown that they suffer from low ambient stability and/or low quality of the thin films. Our approach consists of introducing heteroatoms, namely nitrogen, in order to increase the ambient stability of the molecules and to create dipoles, which will increase the intermolecular interactions in thin films. We expect that these dipole-dipole interactions will significantly increase the structural quality of the thin films. The investigation of the relation between molecular structure and the growth of thin films at the nanometer and micrometer scale will lead to a deeper understanding of the mechanisms, which govern thin film growth of polar molecules. Ultimately, this should lead to the development of growth strategies for organic semiconductor films with improved physical properties.

The bilateral project aimed at controlling the growth of novel polar organic semiconductor molecules - provided by the French partner group - on bulk sapphire substrates as well as on two-dimensional (2D) substrates as graphene (Gr) and hexagonal boron nitride (hBN). With respect to the application especially in future flexible and wearable organic electronics devices like electronic paper and electronic skin, it was of particular importance how the dipolar interaction of the molecules influences the molecular arrangement and charge carrier mobility in the resulting organic nanostructures. The project combined the know-how of the French group from Aix-Marseille University in synthesizing polar organic semiconductor molecules and resolving their molecular arrangement with the expertise of the Austrian group on growing nonpolar organic semiconductor molecules on various Gr substrates and in nanostructure characterization by advanced atomic force microscopy (AFM) techniques. In the case of sapphire substrates, growth of island-like crystallites was observed, later forming poly-crystalline layers over the entire substrate's surface. On 2D material substrates, the molecules form extremely elongated needle-like crystallites. These crystallites are self-aligned with the respect to the high-symmetry directions of the 2D material substrates, like Gr or hBN. Initially, the growth of non-polar parahexaphenyl (6P) was investigated on hBN. Analysis of the growth morphologies - together with first-principle calculations - revealed the mechanism behind substrate-mediated self-alignment of the crystalline nanostructures on hBN. These experiments together with experiments of charge transfer at the 6P/Gr interface and nanomanipulation of the 6P needles on Gr and hBN were prerequisites for the growth experiments of the dipolar molecules. The main project results revealed the mechanisms behind self-assembly and self-alignment of dihydrotetraazaheptacene on Gr and hBN. Furthermore, opto-electrical properties of these crystalline nanoneedles on insulating hBN were investigated demonstrating for the first time a "light-gate" to modulate their conductivity. It was found that incident light enhances the conductivity of the nanostructures by almost three orders of magnitude. Even selective propagation of the charges along only one direction within self-assembled crystallite networks could be realized which was controlled by the light's polarization direction.