Functionalizing the electronic properties of graphene through confinement and intercalation

The discovery of graphene, which is essentially the monolayer counterpart of graphite, is regarded as one of the major breakthroughs of the past couple of decades that has revolutionized the eld of condensed matter physics. Once theoretically thought to be unstable in ambient conditions, graphene was experimentally isolated in 2004 for the fist time, and ironically turns out to be one of the strongest materials. This prototype two-dimensional (2D) crystal displays a manifold of unique attributes. Beyond its distinctive structural form, graphene reveals a highly intriguing electronic character. For instance, the low energy excitations in graphene exhibit a strong linear dispersion relation, as opposed to ordinary materials, where a parabolic band structure is expected in the vicinity of the Fermi level. As such, the electrons in graphene behave as massless Dirac fermions, sharing commonalities with quantum electrodynamic (QED) systems, thereby allowing the investigation of QED phenomena in a solid-state environment. Graphene has undoubtedly been a powerful and versatile platform for conducting fundamental research. However, the lack of a sizeable bandgap in its electronic structure is a major obstacle for its incorporation in state-of-the-art complementary metal-oxide-semiconductor (CMOS) technologies. A promising solution to this end is the production of dimensionally reduced narrow stripes of graphene known as armchair graphene nanoribbons (AGNRs). The quantum mechanical effects introduced by the process of one-dimensional (1D) confinement, can transform the 2D metallic graphene system into a semiconducting ribbon displaying a width-dependent gap size. The experimental synthesis of high-quality and gapped AGNRs has been achieved before, though mostly on the surfaces of metallic Au crystals. The latter is thus an impractical avenue when it comes to integrating the ribbons in actual nanoelectronic device concepts. In contrast, the 1D confined AGNRs studied in the scope of the present thesis, are grown on technologically viable semiconducting SiC substrates, making the prospect of developing next-generation graphene-based field-effect transistors a realistic possibility. The theoretical framework used for describing the structural and electronic properties of graphene as well as those of AGNRs is presented in chapter 1. The basic characteristics of the SiC substrate, which play a crucial role in determining the electronic behavior of the overlying epitaxial graphene layer, are also briefly introduced. Being a single-atom thin material, graphene stands as an ideal surface structure with no bulk contribution. Accordingly, a series of surface science techniques are required to investigate its physical properties, the working principles of which are explained in chapter 2. Chapter 3 outlines the central theme and the core subject matter of the present thesis. There, high-quality AGNRs are grown on the sidewalls of 6H-SiC mesa structures. Angle-resolved photoelectron spectroscopy (ARPES) andscanning tunneling spectroscopy measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. Furthermore, an electronic band structure consisting of well-resolved subbands is observed. The subbands disperse only along the ribbons but not across them, clearly demonstrating an ideal 1D electronic character – a real-world “particle in a box”- type behavior – that is for the first time realized in a graphene-based environment. The experimental findings coupled with theoretical tight-binding calculations, set the grounds for a deeper exploration of quantum confinement phenomena and may open intriguing avenues for new low-power electronics. In chapter 4, a scalable method for fabricating periodic nanoarrays of graphene pn-junctions is introduced. Graphene pn-junctions offer a rich portfolio of intriguing physical phenomena. They stand as the potential building blocks for a broad spectrum of future technologies, ranging from electronic lenses analogous to metamaterials in optics, to high-performance photodetectors important for a variety of optoelectronic applications. Via H-intercalation, 1D confined AGNRs are transformed into a single 2D graphene sheet rolling over 6H-SiC mesa structures. Due to the different surface terminations of the basal and vicinal SiC planes constituting the mesa structures, different types of charge carriers are locally induced into the graphene layer. Using ARPES, the electronic band structure of the two graphene regions are selectively measured, finding two symmetrically doped phases with p-type being located on the basal planes and n-type on the facets. The results demonstrate that through a careful structuring of the substrate, combined with H-intercalation, integrated networks of graphene pn-junctions could be engineered at the nanoscale, paving the way for the realization of novel optoelectronic device architectures. The intercalation of foreign atomic elements at the graphene/SiC interface is not only a powerful method for functionalizing the electronic properties of epigraphene, but also an excellent approach for producing atomically thin 2D crystals enabled by con nement heteroepitaxy. In chapter 5, Ge intercalation experiments are performed on epitaxial graphene samples grown on structured and flat SiC surfaces. Even though exactly similar intercalation procedures are used in both sample types, a triangular 2D Ge lattice exhibiting its own electronic band structure is formed on a wafer-scale in the structured sample, but not in the flat one. Low-energy electron microscopy measurements performed in situ during sample preparation demonstrate substantially different intercalation mechanisms in the two systems. In addition, using X-ray photoelectron spectroscopy, a higher concentration of intercalated Ge atoms is detected in the structured as opposed to the at sample. The reported findings suggest that the e ciency of the intercalation process in epigraphene samples could be improved by deliberately structuring the SiC substrate. The latter thus presents a remarkable platform enabling the synthesis of ultrathin 2D crystals that are considerably distinct innature as compared to their bulk counterparts. Finally, as an outlook segment for the present thesis, the epitaxial growth of graphene is attempted on vicinal 6H-SiC(1,1,0,12) surfaces in chapter 6. The subtrates are intentionally miscut at ∼ 26◦ relative to the basal plane, which is the same angle the mesa sidewalls naturally tend to get inclined at after AGNR growth. The electronic structure of the system as probed by ARPES reveals dis- tinct sets of subbands comparable to those of the ribbons observed in chapter 3, suggesting the formation of quasi-1D con ned AGNRs. Moreover, following a successful H-intercalation process, all signatures of quasi 1D-confinement disappear, and a slightly n-doped graphene sheet develops, supporting the findings presented in chapter 4. The epitaxial growth of graphene on this particular off-axis 6H-SiC substrate is a project that is still in its infancy. However, the preliminary data collected thus far, already show promise for both fundamental research as well as future potential technological advancements.