Falke, Yannic
(2022).
Highly Ordered Organic Layers and Wires.
PhD thesis, Universität zu Köln.
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Abstract
This thesis deals with the synthesis of highly ordered organic thin films and the characterization of the molecule-substrate interaction through spectroscopy and diffraction. Organic devices, such as organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs) have become ubiquitous in modern times. The transistor and high frequency performance of such organic devices crucially depends on charge carrier mobility. In inorganic semiconductors, which are bonded covalently, the band masses are typically lower and their crystallinity higher in comparison to their organic counterparts. In a simple Drude model, the charge carrier mobility is inversely proportional to the effective charge carrier mass. The low effective mass and high crystallinity of inorganic semiconductors result in large carrier mobilities of up to 1E7 cm^2/Vs for GaAs at low temperatures. The large effective mass in van der Waals bonded organic materials and their poorer molecular order decrease the carrier mobility. This thesis addresses the limitations of the inherently low mobility and disorder in organic thin films in a twofold way. The first is the introduction of a novel synthesis method for graphene nanoribbons, which are covalently bonded long stripes of graphene. This new method, developed in this thesis, is based on laser induced photothermal heating. It allows for the synthesis of atomically precise graphene nanoribbons with a higher degree of control over the reaction than conventional methods and is shown to work in a multitude of different nanoribbon species. The growth takes place in an area that is solely governed by the spotsize of the incoming laser light (4 µm). This method has an advantage over present methods through the exact control of the growth kinetics with regards to chemical uniformity and local area distribution. Additionally, the physical properties and growth kinetics of photothermally grown graphene nanoribbons are investigated by means of Raman spectroscopy. In a second way, the growth of organic moire structures on a topological insulator is studied. We show the growth of C60 thin films on the topological insulator Bi4Te3 through electron diffraction and observe a moire pattern. This indicates very long range order in the form of a (4x4) on (9x9) superstructure that is observable on the entire 1x1 cm^2 sample surface. The growth of the structure is performed using molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) in ultra-high vacuum (UHV) conditions and the properties of the interface are studied using low energy electron diffraction (LEED), angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). We find that a C60 induced surface reconstruction and the softness of the underlying, layered topological insulator are responsible for the high order. The theoretical calculations find that the structure bonds mostly through physisorption and both the theory and band structure measurements show no perturbation of the electronic states of the topological insulator by the overlayer. Finally, we extend the concept of well ordered growth of organic thin films on topological insulators to superconducting alkali metal doped C60. These organic films are metallic at room temperature but turn into s-wave superconductors at a critical temperature of 28 K. The combination of this relatively high transition temperature in combination with the well defined growth opens up a new playground for both experimental and theoretical studies. The van der Waals bond nature of the interface protects the interface from alloying, which can be a problem for inorganic topological insulator--superconductor interfaces. We show a novel synthesis route for the growth of well ordered superconducting alkali metal doped fullerenes on the topological insulator Bi4Te3. The growth process is studied using LEED and ultra violet photoemission spectroscopy (UPS) and makes the phase pure synthesis of thin film Rb3C60 possible, which is crucial to avoid contamination through an insulating Rb6C60 phase. ARPES spectra confirm the intactness of the interface by measurements of both the Fermi surface of the topological insulator as well as the newly formed Rb3C60 metallic film.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-633160 | ||||||||
| Date: | 7 September 2022 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II | ||||||||
| Subjects: | Natural sciences and mathematics Physics Chemistry and allied sciences |
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| Date of oral exam: | 9 August 2022 | ||||||||
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| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/63316 |
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