Graphene nanoislands on Ni(111)

Graphene has shown extraordinary electronic and mechanical properties that can be applied in a wide variety of devices and technological fields. Exploring and controlling the different methods to obtain graphene is a crucial step for the successful implementation of this material. Epitaxial growth of graphene on metal surfaces by CVD processes has proven to be a reliable method to obtain graphene layers with good quality and, most importantly, scalable for industrial processes. In this thesis we study using Scanning Tunnel Microscopy at RT and at high temperature the Chemical Vapor Deposition (CVD) reaction that takes place in the Ni(111). We develop a method to grow graphene nanoislands on Ni(111) by tuning the parameters involved in the CVD reaction such as the crystal temperature, propene dose and reaction time. The method consist in dosing the propene at RT and heat the sample once the dosing process is complete during a controlled time. The temperature turns out to be the most determinant parameter. Heating the sample below 400 ºC results in the formation of Ni2C, a surface carbide that inhibits the formation of graphene at the surface. Heating the sample above 400 ºC results in the formation of graphene nanoislands with irregular shape. Above 500 ºC the number of islands diminishes strongly due to gas desorption. To obtain graphene nanoislands in a reproducible manner a minimum dose of 1 L is required, and the coverage of graphene increase with the total dose offered to the surface until it reaches the saturation value at 5 L. The heating time also has an effect in the formation of graphene nanoislands and is found to be optimum for 5 min. A post-reaction thermal treatment can lead to a shape transformaiton of the islands. Annealing at 500 ºC during 20 min forms triangular graphene nanoislands, while annealing the sample at 650 ºC for 10 min leads to hexagonal islands. The structure of the islands is systematically studied paying special attention to the stacking configuration and island edges. Most islands exhibit a 1x1 stacking, although some islands with rotational Moiré patterns were observed. Islands obtained after the thermal treatment have straight edges, which have a crystallographic orientation characteristic of zigzag edges. On the Ni(111) the zigzag hollow (zzh) edges are stable at RT but zigzag top (zzt) edges present a pentagon-heptagon reconstruction named zzt(57). Triangular nanoislands exhibit zzh edges with a predominant top-fcc stacking. The interaction between the substrate and the zzh edges is strong enough to produce stacking changes in some small triangular nanoislands and islands with top-hcp are occasionally observed. Hexagonal nanoislands have zzh and zzt(57) edges alternated