Quasi-van der waals epitaxy of low dimensional oxides and halide perovskites

Abstract

With the influx of flexible electronics as well as the emergence or prediction of unique phenomena in two dimensional forms of materials, van der Waals and quasi-van der Waals epitaxy is gaining momentum as a feasible technique to develop nanostructured and two-dimensional materials. The weak substrate-film chemical interaction expected in this method of epitaxy has been believed to be crucial in enabling not only the formation of sharp, strain-free heterostructures but also mechanical exfoliation of the epilayer. In this dissertation, we aim to study van der Waals epitaxy in novel materials systems and unravel an understanding of the growth kinetics. Various layered substrates, epilayers and growth methods are explored, and the resulting growth is studied by optical and electron microscopy. Epitaxial relations are determined and the effect of weak van der Waals interaction at the film-substrate interface on material properties is probed via phase transitions, temperature-dependent and time-resolved photoluminescence studies as well as optoelectronic characterization. Through an attempt to grow of vanadium dioxide (VO2) on a layered Dion-Jacobson perovskite by vapor phase deposition, it is shown that VO2 scavenges ions from the substrate and forms an epitaxial vanadate compound. The crystal anisotropy of the substrate significantly modifies the energy landscape for diffusion of ions and leads to the creation of a unit-cell-thick epitaxial Aurivillius phase at the interface, predicted to exhibit the Rashba-Dresselhaus effect. The scavenging effect, interfaced with an anisotropic low-dimensional substrate, opens a new window to develop two-dimensional flexible components for future electronics. By growing VO2 wires on hexagonal boron nitride (h-BN) substrates via vacuum deposition, it is shown that metal-insulator transition in van der Waals epitaxy grown VO2 proceeds via a single domain within a narrow temperature range of ~2 K. This is contrasted with VO2 wires grown on a rigid Al2O3 substrate. Strong chemical interactions at the VO2/Al2O3 interface can induce significant strain during metal-insulator transition causing the formation of alternating insulating and metallic domains and broadening the temperature range of transition. Hence, van der Waals epitaxy is demonstrated as an effective tool for tuning phase transition kinetics in VO2 or similar systems. Lastly, through an attempt at solution-based growth of two-dimensional hybrid organic-inorganic perovskite (2D-HOIP) flakes on a muscovite mica van der Waals substrate, it is shown that millimeter scale 2D-HOIP flakes can be grown not only on top but also within the interlayer spacings of muscovite mica. It is further demonstrated that such 2D-HOIP flakes buried in mica demonstrate enhanced photostability in comparison to conventional 2D-HOIP flakes. Hence, such liquid phase growth in the interlayer spacings of van der Waals substrates allows controllable growth of single-crystalline 2D-HOIP thin films and opens a new avenue for developing novel materials structure for designing optoelectronic devices.