Control de las propiedades electrónicas de nanotiras quirales de grafeno mediante la superficie de soporte

Berdonces Layunta, Alejandro
Serrate Donoso, David (dir.) ; Lobo Checa, Jorge (dir.)

Universidad de Zaragoza, CIEN, 2018
Departamento de Física de la Materia Condensada, Área de Física de la Materia Condensada

Máster en Materiales Nanoestructurados para Aplicaciones Nanotecnológicas (Nanostructured Materials for Nanotechnology Applications)

Resumen: Graphene is a material of great versatility. For instance, the sp2 covalent bonds linking the C atoms in extensive 2D sheets confers great mechanical strength3. Moreover, it exhibits huge electronic mobility and sensitivity of conductivity to external parameters, which makes it interesting towards microelectronics, as well as in the energy sector and data storage. All these properties add to the prospect of numerous applications in the field of materials science. Notably, the electronic properties of graphene are tunable8. When graphene shrinks in one of its dimensions to form ribbons, its conductivity decreases and transforms from metallic to semiconductor below a critical width9, while still preserving the appealing properties of its extended form. Thus, it is of interest to the field of molecular electronics in order to build nanotransistors. The conductivity dependence of graphene nanoribbons on the width has a characteristic length scale in the range of Angstroms8. Therefore, an extremely precise manufacturing method is required. On Surface synthesis is a bottom-up approach that allows graphene manufacturing with atomic precision featuring exact width and edge geometry10. This is nowadays far better than the top-down methods, whose size limit is currently on the scale of tens of nanometers11 and are prone to produce spurious effects due to sample damage. Nanotechnology with such precision requires cutting-edge experimental infrastructures. Based on state of the art Scanning Tunnelling Microscope (STM) of the Laboratorio de Microscopías Avanzadas (LMA), the electronic structure of the different parts of the nanoribbons has been locally studied, facilitating a deeper comprehension of the quantum nature responsible for their superior properties. The aim of this work seeks the experimental accomplishment of these points: - Synthesize chiral (3,1)-GNR from the selected precursor on Au(111). - Study their morphology and electronic structure, focusing on the identification of expected edge states. - To develop new methods to decouple the GNRs from the substrate by addition of NaCl and determine its effectiveness. Decoupling the molecular structures would be used both to study the ribbons with higher resolution, and to release them from the substrate. A new type of chiral nanoribbon, the (3,1)-cGNR, has been successfully synthesised from dBQA precursors. This molecular precursors, the subsequent polimerization steps and the nanoribbons were structurally confirmed by topographic STM images. Specifically, the period and internal structures have been confirmed using topography imaging with normal metallic tips and functionalized ones, which verified to match the theoretical calculations. Furthermore, its electronic properties have been successfully studied, and the higher part of its valence ban, as well as the presence of edge states in the vicinity of the Fermi level have been confirmed. In addition, the NaCl growth and response to temperature after deposition onto Au(111) showed different density agglomeration into islands. This effect, in presence of cGNR caused a generalized displacement of the molecular structures, affecting its orientation with respect to the substrate and causing some of them to stack on top of other cGNRs. This resulted in an effective ribbon decoupling from the metallic substrate, which increased the energy and spatial resolution when studying the ribbons, both on the STM images and the STS spectra. This allowed us to investigate its internal electronic structure without the need for functionalising the tip. Indeed, the performance of a regular metal tip on stacked ribbons parallels the one of functionalized tips on a metal surface, making the burdensome process of tip functionalization unnecessary. In the STS spectra, the decoupling caused a sharpening of the bands that allows its proper identification. As outlook, we propose to use this decoupling method as a simple technique to study the electronic structure of the edge states in cGNR, or as a tool to spontaneously detach the ribbons from the substrate for a possible transferring method to Si-based substrates GNRs on a larger scale, bridging the gap between on-surface synthesis and electronic applications

Tipo de Trabajo Académico: Trabajo Fin de Master