Tailoring the structural and electronic properties of graphene by bottom-up methods
- Autores: Michele Gastaldo
- Directores de la Tesis: Aitor Mugarza Ezpeleta (dir. tes.), Gustavo Adolfo Ceballos Mago (dir. tes.), Jordi Pascual Gainza (tut. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: David Serrate Donoso (presid.), José Miguel Alonso Pruneda (secret.), Cristina Africh (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Resumen
Bottom-up methods such as nanostructuring and interfacing with other materials can be an effective way of tailoring the structural, electronic and magnetic properties of graphene. At the nanoscale, these can dramatically depend on atomic scale variations in size and boundary structure.
In this work we study different bottom-up strategies to tailor the properties of graphene: (i) nanostructuring by the synthesis of graphene nanoislands with controlled shape, internal domain distribution and edge structure; (ii) proximity-induced tailoring of structural and electronic properties by metal intercalation; (iii) synthesis of lateral heterostructures. This is done by using chemical vapour deposition (CVD) to synthesize the nanostructures and metal beam epitaxy (MBE) to intercalate metallic films, scanning tunnelling microscopy (STM) and spectroscopy (STS) to study their atomic and electronic structures, and combining our experimental studies with ab-initio calculations.
The study follows previous work of the group on the synthesis of graphene nanoislands on Ni(111) by CVD. Here we gain a deeper insight in the structure and the growth mechanism of nanoislands on this surface. By high-resolution STM imaging, we access to a complete atomic scale characterization of the stacking symmetry and the edge structure. We also identify polycrystalline nanoislands and characterize both the stacking and orientation of graphene domains and the related boundary atomic structure. We report evidence of different continuous strained and topological defect boundaries and find that the selection of rotational domains is determined by boundary rather than stacking energetics. However, the boundary structure, critical in defining transport properties across, seems to be defined by the substrate interaction. Finally, we find a range of temperature where single-crystal shape-selected graphene nanostructures can be obtained.
Following the synthesis of graphene nanoislands, we intercalate Au and investigate the structural and electronic properties of the Au film and of the graphene nanoislands on top by combined STM and STS. By probing the thickness evolution of field emission resonances and surface states on Au, we find a complex structural evolution of the Au film, which involves alloying at the interface, the formation of a dislocation network, and the gradual strain-relief and formation of the characteristic herringbone reconstruction as we increase the thickness. The interaction of graphene with the substrate is monitored by tracking both the Shockley surface state of Au(111) and the Dirac states of graphene in spectroscopic measurements. By comparing the behaviour of the surface state under graphene at the Ni and Au/Ni surfaces, we evidence how such interaction is reduced after intercalation. An analysis of interference patterns in graphene leads to the conclusion that the Dirac band is recovered after Au intercalation. More interestingly, the decoupled nature of the graphene electronic properties is confirmed by the detection of energy split localized peaks, attributed to the predicted one dimensional spin-split edge states. The energy splitting we obtain is significantly larger than that measured in any other zigzag edge interacting with a metallic surface.
We also explore the synthesis of lateral graphene-hexagonal boron nitride (hBN) heterostructures on Ni(111). We find that heterostructures cannot be grown sequentially when starting with graphene nanoislands, due to the high temperatures required for the CVD growth of hBN. On the other hand, starting from hBN nanoislands leads to heterostructures with well-defined zigzag interfaces that could carry one dimensional electronic states.
The results of this thesis provide a deeper insight on the growth of two-dimensional nanostructures of graphene and hybrid layers and on the tuning of their structural and electronic properties by controlling the interfacial interaction with the underlying metal. These are valuable notions for the realisation of shape-selected graphene quantum dots with tailored properties, which could find applications in optoelectronics and spintronics.
