Synthesis and Characterisation of 2D Organic Materials

“Synthesis and Characterisation of 2D Organic Materials” is a dissertation submitted for the award of a Doctor in Natural Sciences at the Karlsruhe Institute of Technology (KIT), Faculty of Chemistry and Biosciences.
The discovery of graphene triggered an intense general research inter- est in various 2D materials due to their extraordinary properties. For more than a decade, the number of publications dealing with 2D materials has been steadily increasing, along with the development or discovery of other novel 2D structures. Despite all this, graphene is the most studied 2D material in the world due to its remarkable electronic and mechanical properties and is used in various practical applications. The wide range of applications for graphene requires, among other things, the development of new manufacturing methods. In particular, the Chemical Vapour Deposition (CVD) method has established itself for the growth of graphene layers, since this process makes it possible to produce very large graphene areas. In addition, the electronic properties of single-layer graphene, which cannot be tuned for certain applications, have led scientists to search for alternative graphene-like materials. ... mehrOne possible alternative is nanocrystalline graphene (NCG), the properties of which can be controlled during the growth process to allow fine tuning of the electronic band-gap and thus its integration into transistors or other electronic devices. In addition, recent research has also focused on new 2D carbon allotropes that could be used in technological applications. The necessary control in the manufacturing process is the motivation for the development of new methods for the synthesis of graphene and NCG, as well as a novel carbon-based 2D material, known as Graphdiyne (GDY).
In this PhD thesis, the first research project involves the synthesis of graphene using carbon dioxide (CO2), an environmentally problematic gas source. Different metallic substrates were tested to investigate the possibil- ity of CO2 activation and reduction to graphene. The selected metals are copper (Cu), for its properties in graphene synthesis, and palladium (Pd), for its known catalytic activity. The synthesis was performed using an atmospheric pressure CVD reactor (APCVD) and hydrogen as reductant. Raman spectroscopy, Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) confirm the formation of multilayer graphene. Further- more, our study shows that CO2 can be effectively converted to graphene when the concentration of the metal mixture contains more than 82at.% Cu.
In the second research project we try to synthesise nanocrystalline graphene (NCG) from an organic molecule. Common methods of NCG syn- thesis use either photoresists or polymers that require high temperatures, vacuum and long exposure times for successful graphitisation of the precursor. The domain sizes achievable in these processes are typically 8-12 nm. With our process, using the organic molecule hexaethynyl benzene, we are able to produce large areas of NCG with larger crystal domains. These show the expected semiconducting properties in our conductivity studies.
The third research project deals with the synthesis of graphdiyne (GDY), a new carbon-based material. GDY was proposed as the most stable non- natural carbon allotrope with a band gap of 0.46 eV and high porosity. The band gap characteristic offers the possibility to integrate GDY in transistors and transistor-like devices and to use it for gas separation, catalysis, etc. To date, only two GDY precursors have been used for the syn- thesis of GDY, namely hexaethynyl benzene and 1,3,5-trythynyl benzene. Due to the very interesting properties predicted for GDY, we are endeavouring to synthesise new GDY materials from various organic precursors such as hexaethynylbenzene, hexakis[4-(ethynyl)phenyl]benzene, 1,3,5- tris[4-(ethynyl)phenyl]benzene and 1,3,6,8- tetrakis(ethynyl)pyrene using an APCVD reactor and Cu foils as substrate. The structural and physical prop- erties of the produced GDY layers were investigated using a variety of spec- troscopic and surface characterisation techniques. TEM analyses show that the GDY layers obtained are mostly amorphous, while optical bandgap studies show an insulator character. Our results contrast strongly with theoretical predictions and other published reports.
In summary, the points listed below have been successfully implemented: (i) synthesis of graphene from an abundant and problematic carbon source (CO2); (ii) growth of NCG with formation of large crystal domains from an organic molecule; and (iii) fabrication of GDY layers from three different precursors. The syntheses described here offer alternative methods for obtaining high quality 2D materials.