Investigation of the electronic band structure of 2D transition metal dichalcogenides via angle-resolved photoemission spectroscopy

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are a class of layered van der Waals (vdW) semiconductors that exhibit remarkable properties, including an optical bandgap, high carrier mobility, and tunable electronic properties. These properties, particularly in TMDC heterostructures, make them attractive for future optoelectronic technologies such as tunneling diodes, tunneling transistors, light-emitting diodes, and photovoltaic cells. TMDC devices based on homo- or heterostructures may consist of multi-layered stacks, each layer with a different elemental composition, resulting in devices with diverse properties. With increasing interest in this field and the potential to fabricate TMDC-based devices with multiple configurations, there is enormous potential for exploration, which could uncover interesting phenomena. In this direction, this thesis aims to investigate and provide a thorough understanding of the electronic band structure of WSe${}_2$, an important TMDC semiconductor, using a combination of angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) techniques. In this work, we employed ARPES with a micrometer focused beam spot size, commonly known as micro-ARPES (μ-ARPES), to investigate microstructures containing areas of single-layer (SL) and bilayer (BL) WSe${}_2$ on graphite substrates at different twist angles between the SL WSe${}_2$ and graphite and within the BL WSe${}_2$. In the twisted SL WSe${}_2$/graphite structures the electrons emitted from the graphite π bands scatter in the SL WSe${}_2$ leading to the appearance of multiple copies, which reflect the trigonal symmetry of a SL WSe${}_2$. We investigated two twisted BL WSe${}_2$ at twist angles of ∼ 28° and ∼ 10° and found no evidence of hybridization gaps at the interlayer band crossing points, that could be precursors of the flat bands at smaller twist angles. Similarly, no such gaps were found for SL WSe${}_2$/graphite. For both cases, the global valence band maximum (VBM) is at the K${}_{WS{e}_2}$ point of the Brillouin zone (BZ). We also present the ARPES studies for the parallel momentum k${}_{\parallel }$ beyond the first BZ. Our findings demonstrate the visualization of characteristics features of the individual WSe${}_2$ layers separately in the homo-bilayer structures, which we further use to estimate the twist angle between these SLs. Theoretical density functional theory calculations support the experimental findings by indicating that the formation of hybridization gaps in WSe${}_2$/graphene, a system that closely resembles the experimental WSe${}_2$/graphite system, is sensitively dependent on the WSe${}_2$ band character at the graphene Dirac band crossing point. Additionally, we present the circular dichroism in an angle-resolved photoemission spectroscopy (CD-ARPES) study of WSe${}_2$ bulk, twisted SL WSe${}_2$/graphite and BL WSe${}_2$/graphite structures with different twist angles. Our results show significant CD signals at the K valley in both bulk WSe${}_2$ and twisted SL WSe${}_2$/graphite, while in twisted BL WSe${}_2$, the K valley integrated CD-ARPES is still visible in addition to the dichroic signal asymmetry at Γ. The integrated CD-ARPES at K and Γ valleys in all three sample structures can be attributed to orbital angular momentum (OAM) and geometry-induced effects, respectively. The SL dichroic signal is well described by theoretical one-step model simulations. Initial tests to simulate dichroic signals from twisted bilayers by an incommensurate sum of separate SLs are promising. By delving deeper into TMDC electronic band structure via μ-ARPES, DFT and CD-ARPES, our study unravels the intricate characteristics of WSe${}_2$, advancing our understanding of TMDC semiconductors and fostering innovative applications in tunneling devices, transistors, LEDs and photovoltaics.