Adsorption-controlled growth of La-doped BaSnO3 by molecular-beam epitaxy

Epitaxial La doped BaSnO3 films were grown in an adsorption controlled regime by molecular beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A film grown on a (001) DyScO3 substrate exhibited a mobility of 183 cm^2 V^-1 s^-1 at room temperature and 400 cm^2 V^-1 s^-1 at 10 K, despite the high concentration (1.2x10^11 cm^-2) of threading dislocations present. In comparison to other reports, we observe a much lower concentration of (BaO)2 Ruddlesden Popper crystallographic shear faults. This suggests that in addition to threading dislocations that other defects possibly (BaO)2 crystallographic shear defects or point defects significantly reduce the electron mobility

Adsorption-controlled growth of La-doped BaSnO3 by molecular-beam epitaxy

Epitaxial La-doped BaSnO3 films were grown in an adsorption-controlled regime by molecular-beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A film grown on a (001) DyScO3 substrate exhibited a mobility of 183 cm2 V-1 s-1 at room temperature and 400 cm2 V-1 s-1 at 10 K despite the high concentration (1.2 × 1011 cm-2) of threading dislocations present. In comparison to other reports, we observe a much lower concentration of (BaO)2 Ruddlesden-Popper crystallographic shear faults. This suggests that in addition to threading dislocations, other defects - possibly (BaO)2 crystallographic shear defects or point defects - significantly reduce the electron mobility

Photoemission Studies of Barium Stannate and Two Dimensional Materials

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

Soybean-BioCro:a semi-mechanistic model of soybean growth

Abstract Soybean is a major global source of protein and oil. Understanding how soybean crops will respond to the changing climate and identifying the responsible molecular machinery are important for facilitating bioengineering and breeding to meet the growing global food demand. The BioCro family of crop models are semi-mechanistic models scaling from biochemistry to whole crop growth and yield. BioCro was previously parameterized and proved effective for the biomass crops Miscanthus, coppice willow and Brazilian sugarcane. Here, we present Soybean-BioCro, the first food crop to be parameterized for BioCro. Two new module sets were incorporated into the BioCro framework describing the rate of soybean development and carbon partitioning and senescence. The model was parameterized using field measurements collected over the 2002 and 2005 growing seasons at the open air [CO2] enrichment (SoyFACE) facility under ambient atmospheric [CO2]. We demonstrate that Soybean-BioCro successfully predicted how elevated [CO2] impacted field-grown soybean growth without a need for re-parameterization, by predicting soybean growth under elevated atmospheric [CO2] during the 2002 and 2005 growing seasons, and under both ambient and elevated [CO2] for the 2004 and 2006 growing seasons. Soybean-BioCro provides a useful foundational framework for incorporating additional primary and secondary metabolic processes or gene regulatory mechanisms that can further aid our understanding of how future soybean growth will be impacted by climate change

Greater mesophyll conductance and leaf photosynthesis in the field through modified cell wall porosity and thickness via AtCGR3 expression in tobacco

Summary: Mesophyll conductance (gm) describes the ease with which CO2 passes from the sub‐stomatal cavities of the leaf to the primary carboxylase of photosynthesis, Rubisco. Increasing gm is suggested as a means to engineer increases in photosynthesis by increasing [CO2] at Rubisco, inhibiting oxygenation and accelerating carboxylation. Here, tobacco was transgenically up‐regulated with Arabidopsis Cotton Golgi‐related 3 (CGR3), a gene controlling methylesterification of pectin, as a strategy to increase CO2 diffusion across the cell wall and thereby increase gm. Across three independent events in tobacco strongly expressing AtCGR3, mesophyll cell wall thickness was decreased by 7%–13%, wall porosity increased by 75% and gm measured by carbon isotope discrimination increased by 28%. Importantly, field‐grown plants showed an average 8% increase in leaf photosynthetic CO2 uptake. Up‐regulating CGR3 provides a new strategy for increasing gm in dicotyledonous crops, leading to higher CO2 assimilation and a potential means to sustainable crop yield improvement

BioCro II:a Software Package for Modular Crop Growth Simulations

The central motivation for mechanistic crop growth simulation has remained the same for decades: To reliably predict changes in crop yields and water usage in response to previously unexperienced increases in air temperature and CO2 concentration across different environments, species and genotypes. Over the years, individual process-based model components have become more complex and specialized, increasing their fidelity but posing a challenge for integrating them into powerful multiscale models. Combining models is further complicated by the common strategy of hard-coding intertwined parameter values, equations, solution algorithms and user interfaces, rather than treating these each as separate components. It is clear that a more flexible approach is now required. Here we describe a modular crop growth simulator, BioCro II. At its core, BioCro II is a cross-platform representation of models as sets of equations. This facilitates modularity in model building and allows it to harness modern techniques for numerical integration and data visualization. Several crop models have been implemented using the BioCro II framework, but it is a general purpose tool and can be used to model a wide variety of processes

Polycrystalline Graphene with Single Crystalline Electronic Structure

We report the scalable growth of aligned graphene and hexagonal boron nitride on commercial copper foils, where each film originates from multiple nucleations yet exhibits a single orientation. Thorough characterization of our graphene reveals uniform crystallographic and electronic structures on length scales ranging from nanometers to tens of centimeters. As we demonstrate with artificial twisted graphene bilayers, these inexpensive and versatile films are ideal building blocks for large-scale layered heterostructures with angle-tunable optoelectronic properties.11Nsciescopu

Imaging chiral symmetry breaking from Kekulé bond order in graphene

Chirality-or 'handedness'-is a symmetry property crucial to fields as diverse as biology, chemistry and high-energy physics. In graphene, chiral symmetry emerges naturally as a consequence of the carbon honeycomb lattice. This symmetry can be broken by interactions that couple electrons with opposite momenta in graphene. Here we directly visualize the formation of Kekule bond order, one such phase of broken chiral symmetry, in an ultraflat graphene sheet grown epitaxially on a copper substrate. We show that its origin lies in the interactions between individual vacancies in the copper substrate that are mediated electronically by the graphene. We show that this interaction causes the bonds in graphene to distort, creating a phase with broken chiral symmetry. The Kekule ordering is robust at ambient temperature and atmospheric conditions, indicating that intercalated atoms may be harnessed to drive graphene and other two-dimensional materials towards electronically desirable and exotic collective phases.11Nsciescopu