The isolation of single layers of van der Waals materials has shown that their properties can be significantly different compared to their bulk counterparts. These observations, illustrates the importance of interface interactions for determining the materials properties even in weakly interacting materials and raise the question if materials properties of single layer van der Waals materials can be controlled by appropriate hetero-interfaces. To study interface effects in monolayer systems, surface science techniques, such as photoemission spectroscopy and scanning probe microscopy/spectroscopy, are ideally suited. However, before these characterization methods can be employed, approaches for the synthesis of hetero-van der Waals systems must be developed, preferably in-situ with the characterization methods, i.e. in ultra-high vacuum. Therefore, in this thesis, we explored novel approaches for creating van der Waals heterostructures and characterized fundamental structural and electronic properties of such systems. Specifically, we developed an approach to decouple graphene from a Ir(111) growth substrate by intercalation growth of a 2D-FeO layer, and we investigate van der Waals epitaxy of MoSe2 on graphite and other transition metal dichalcogenide substrates.
For the Ir(111)/2D-FeO/graphene heterostructure system, we first demonstrated the growth of 2D-FeO on Ir(111). The FeO monolayer on Ir(111) exhibits a long range moiré structure indicating the locally varying change of the coordination of the Fe atoms with respect to the substrate Ir atoms. This variation also gives rise to modulations in the Fe2+-O2- separation, and thus in the monolayer dipole. We demonstrated that this structure can be intercalated underneath of graphene grown on Ir(111) by chemical vapor deposition. The modulation of the dipole in the 2D-FeO moiré structure consequently gives rise to a modulated charge doping in the graphene. This effect has been studied by C-1s core level broadening. In general, this study demonstrates that modulated substrates can be used to periodically modify 2D materials.
Growth of transition metal dichalcogenides (TMDCs) by molecular beam epitaxy (MBE) is a very versatile approach for growing TMDC heterostructures. However, there may be unforeseen challenges in the synthesis of some of these materials. Here we show that in MBE growth of MoSe2, the formation of twin grain boundaries is very abundant. While this is detrimental in our efforts for characterizing interface properties of TMDC heterostructures, however the twin grain boundaries have exciting properties. Since the twin grain boundaries are aligned in an epitaxial film we were able to characterize their properties by angle resolved photoemission spectroscopy (ARPES), which may be the first time a material’s line defects could be studied by this method. We demonstrate that the line defects are metallic and exhibit a parabolic dispersing band. Because of the 1D nature of the metallic lines, embedded in a semiconducting matrix, the electronic structure follows a Tomonaga Luttinger formalism and our studies showed strong evidence of the predicted so-called spin charge separation in such 1D electron systems. Moreover, a metal-to-insulator Peierls transition has been observed in this system by scanning tunneling microscopy as well as in transport measurements. Finally, we have shown that the defect network that forms at the surface also lends itself for decoration with metal clusters. Although unexpected, the formation of grain boundary networks in MoSe2 marks the discovery of a new material with exciting quantum properties.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-8254 |
| Date | 26 September 2017 |
| Creators | Ma, Yujing |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
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