Photoemission Studies of Barium Stannate and Two Dimensional Materials

Abstract

Developing optoelectronic devices with increased efficiency and novel functionality requires an understanding of the electronic structure of exotic materials and their interfaces. Perovskite oxides and two-dimensional (2D) materials have emerged as promising classes of materials that exhibit intriguing properties such as high temperature superconductivity and spin-valley locking. Combining these materials into heterostructures offers even more functionality and is facilitated by a shared crystal structure (in the case of perovskites) or van der Waals bonds that do not require epitaxial relationships for clean interfaces (in the case of 2D materials). In this thesis, we present original studies of four materials within these broader classes. Thin films are fabricated by molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) and studied primarily through angle-resolved photoemission (ARPES) measurements conducted at Cornell and the SOLEIL synchrotron in Gif-sur-Yvette, France. LaxBa1-xSnO3 (LBSO) is a high mobility perovskite oxide with a large band gap, enabling promising applications as a transparent conductor for use in solar energy harvesting or fully transparent electronics and as a channel material in all-oxide transistors. LBSO films are grown by MBE and studied by in situ ARPES. While the valence band structure is found to agree well with bulk density functional theory (DFT) calculations, a La-dependent upward band banding is observed at the surface. Additional exposure to ultraviolet (UV) light induces a reduction in the original band bending, offering a route for controlling carrier concentration and band offsets at LBSO interfaces. Graphene is a 2D semimetal with a host of exotic properties, including an extremely high mobility and tunable carrier concentration. Here, monolayer graphene is grown by CVD and studied by ex situ ARPES. Although the films originate from multiple nucleations, all individual graphene grains share the same crystallographic orientation, providing an ideal building block for van der Waals heterostructures with angle-tunable properties. Twisted graphene bilayers are fabricated from these growths and spatially resolved nano-ARPES reveals angle-dependent gaps in the graphene electronic structure, providing a route for creating devices with tunable optical absorption and exotic electronic states. Cu2Si is a 2D Dirac line node semimetal, a newly appreciated form of topological matter. Here we form van der Waals heterostructures of graphene and Cu2Si on Cu substrates by CVD, representing a unique interface between two atomically thin topological materials. SnSe2 is a layered main-group metal dichalcogenide that has exhibited gate-tunable superconductivity and has promising applications as a component in high efficiency two-dimensional heterojunction interlayer tunneling field effect transistors. However, despite decades of study, basic questions about its electronic structure remain unanswered. Here we synthesize thin films of SnSe2 by MBE and study them with ex situ ARPES. A comparison between ARPES and DFT reveals the importance of spin-orbit coupling and out-of-plane dispersion in the SnSe2 valence band structure, critical information for developing new electronic devices based on SnSe2.