De Boni, Francesco (2018) Sintesi per via termica e caratterizzazione di nanoribbons di grafene da precursori molecolari funzionalizzati supportati su superfici di oro e argento. [Magistrali biennali]
Full text disponibile come:
In the field of organic semiconductors, nanostructures based on graphene are very attractive, due to the possibility of finely tuning their band gap in a wide range, with applications in nano- and opto-electronics. Among them, graphene nanoribbons (GNRs) are the most promising. The attention is focused on the bottom-up synthesis of GNRs from molecular precursors supported on metal surfaces (mainly gold, silver and copper): in this way GNRs have atomically-precise widths and edges. In this thesis different types of GNRs depending on the metallic surface used for deposition are synthesized. 4,4”-dibromo-p-terphenyl (DBTP) is chosen as molecular precursor, while Au(110), Au(100) and Ag(110) from single crystal are used as metallic surfaces: they are very less studied than the most important in literature for the growth of nanoribbons, that is Au(111) and Ag(111). The whole syntheses and characterizations of samples are performed in ultra-high-vacuum conditions (1-2 10-10 mbar). The organic precursors are sublimated and deposited on the metallic surface in situ. Each sample undergoes several steps of annealing for a variable time, and after each step the interface is characterized through Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED), in order to gain information on the overlayer topography and local density of states, and overlayer-substrate registry. The synthesis pathway is performed first by the room temperature deposition of the brominated precursor. Then, the following steps are thermoactivated: an annealing at lower temperature promotes Ullmann coupling between the precursor units to form long polymeric wires (poly-p-phenylene, PPP), while another annealing at higher temperature activates cyclodehydrogenation reactions that lead PPP wires to laterally fuse and form GNRs. The metallic surface has a key role because it not only supports and catalyzes the synthesis, but also steers the GNRs final structure (alignment along preferential directions, width distribution, long-range order). The main goal in this field is to study synthesis routes based on different surfaces to produce long-range order and highly unidirectional GNRs with well-defined transport properties for applications in electronics. Au(110) and Au(100) behave quite similarly: Ullmann coupling and cyclodehydrogenation reactions are observed to perform at higher temperature than the same reactions on Au(111) reported in literature. On Au(110) the GNRs are shorter but the width distribution is narrow and there is a quite good alignment of the GNRs along some directions. On Au(100), instead, the GNRs are longer, the width distribution is equally narrow, and the long-range order is very high, so on Au(100) the properties of GNRs are overall better. Ag(110) shows intermediate properties of the GNRs: the main difference is the clear formation of an organometallic intermediate between Ag adatoms present on the surface and debrominated precursors before the formation of PPP wires. Finally, the synthesized GNRs are characterized ex situ by the acquisition of Raman spectra to prove their stability in air: in particular, the presence of the main vibrational modes of GNRs and the absence of fingerprints related to the degradation of GNRs in ambient conditions are verified.
Solo per lo Staff dell Archivio: Modifica questo record