"PhD Defense: "Ullmann coupling reaction in unconventional surfaces""

Who: Mikel Abadia Gutierrez

Place: CFM Auditorium

Date: Friday, 14 July 2017, 11:00

Title: PhD Defense: Ullmann coupling reaction in unconventional surfaces
Speaker: Mikel Abadia Gutierrez
Place: CFM Auditorium
Date: July 14, 2017, 11:00

Abstract: The extremely high electron mobility and its ultimate thinness of just one atomic layer make the graphene an ideal candidate for an all organic field effect transistor (FET). However, prior to the graphene?s implementation in a FET a band gap must be opened to allow high on-off ratios. Towards this end, one of the most promising strategies is quantum confinement, i.e. by reducing the lateral dimensions of the graphene close to the de Broglie wavelength of the charge carries. A promising bottom-up strategy for such confinement is the on-surface synthesis of graphene nanoribons (GNR), where the quasi-one dimensional band structure can be tailored with atomic precision.

The surface assisted Ullmann coupling allows the synthesis of said GNRs. However, reasonable reaction yields and sufficiently extended GNRs can so far only be realized on coinage metals where the GNRs properties are inherently coupled to the surface and therefore inaccessible for device applications such as the FET. Consequently, the next step forward in the field either requires the larger scale synthesis of GNRs for ex situ transfer protocols onto more suitable substrates or the in situ synthesize of GNRs directly on technologically relevant surfaces.

Here, we synthesize poly-p-phenylene (PPP) wires, the smallest possible GNR, via the Ullmann coupling reaction on three unconventional surfaces. First, we probed the formation of PPP wires on a bimetallic GdAu2 surface alloy and demonstrate that the intermixing of elements is a viable strategy to improve the reaction conditions by synergistic effects while maintaining the extraordinary alignment and extensions of individual PPP generally only achievable on Au(111) surfaces. Another strategy to optimize the reaction conditions and alignment of GNRs is the use of surface steps.

We employ a c-Au (111) crystal, where the surface step density is continuously varied across the same sample, thus allowing us to isolate the influence of the steps on the PPP synthesis. The central finding is a reduction of the reaction temperature by 20 K when using the right kind of surface step orientation and density. In the last chapter, we demonstrate the formation of PPP wires on the dielectric TiO2(110) surface, a model surface for the realization of FET. Optimized reaction temperatures and yields are achieved when an external catalyst is employed while simultaneously suppressing unwanted side reactions.

The on-surface synthesized PPPs offer the possibility of characterization by well-established surface science techniques. Specifically, we employ scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) to elucidate geometric structures of the PPPs, angle resolved photoemission spectroscopy (ARPES) to probe the valence band of the PPPs and x-ray photoelectron spectroscopy (XPS), the core technique of this work, to study reaction yields and mechanisms.

The combination of our design strategies and multi-technique approach has unraveled novel substrates for the realization of next generation GNR-based devices such as the FET.

Supervisors: Celia Rogero & Jens Brede.

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