Bulletin of the American Physical Society
83rd Annual Meeting of the APS Southeastern Section
Volume 61, Number 19
Thursday–Saturday, November 10–12, 2016; Charlottesville, Virginia
Session H2: Surfaces and Interfaces |
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Chair: Petra Reinke, University of Virginia Room: Salon C |
Friday, November 11, 2016 1:30PM - 2:00PM |
H2.00001: Interfacial Engineering and Characterization in Polar/Non-Polar Oxide Heterostructures Invited Speaker: Ryan Comes Polar/non-polar interfaces in epitaxial oxide films have been a rich area of research for many years for emergent behavior. Recent work has branched out to explore ways to use these interfaces to engineer optical responses in materials as well. The interfacial dipole that results from the polar discontinuity at such interfaces can generate band bending and a built-in electric field near the interface, which may be used to separate optically-excited electron-hole pairs for enhanced photovoltaic and photocatalytic response. However, detailed characterization of the band structure is needed to understand both the origin of these phenomena and engineer their behavior. In this talk I will discuss our work using interfacial termination in polar/non-polar heterojunctions and superlattices to engineer electric fields in these materials. Recent work has shown short-circuit photocurrents and visible light photocatalysis in LaFeO$_{\mathrm{3}}$/$n$-SrTiO$_{\mathrm{3}}$ heterostructures where the interface is varied between a positively charged TiO$_{\mathrm{2}}$-LaO and a negatively charged SrO-FeO$_{\mathrm{2.}}$ Using \textit{in situ} x-ray photoelectron spectroscopy (XPS) characterization of these heterojunctions, we extract the valence and conduction band alignment between the materials. We show that previous reports of a bulk polarization induced in LaFeO$_{\mathrm{3}}$ due to the SrTiO$_{\mathrm{3}}$ termination are not present, raising questions as to the origin of the previously reported behavior. In related work exploring LaCrO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ superlattices, we employ synchrotron standing wave XPS to examine the electronic dispersion of buried layers and show that by engineering alternating terminations in confined layers between positively charged TiO$_{\mathrm{2}}$-LaO and negative CrO$_{\mathrm{2}}$-SrO interfaces a polarization is induced in each material. This result could open new pathways to engineer electron-hole separation using interfaces. [Preview Abstract] |
Friday, November 11, 2016 2:00PM - 2:30PM |
H2.00002: Local Probe Investigation of Interfaces in 2D Materials Invited Speaker: Chenggang Tao Emerging two-dimensional (2D) materials, such as atomically thin transition metal dichalcogenides and graphene, have been the subject of intense research efforts for their fascinating properties and potential applications in future electronic and optical devices. The interfaces in these 2D materials, including domain boundaries, edges and heterojunctions, strongly govern the electronic and magnetic behavior and may also potentially host new quantum states. On the other hand, the interfaces are more susceptible to thermal fluctuation and external stimuli. In this talk we will present our scanning tunneling microscopy (STM) and spectroscopy (STS) explorations of edges of few layered molybdenum disulfide (MoS$_{\mathrm{2}})$ nanostructures and will show how step edges on titanium diselenide (TiSe$_{\mathrm{2}})$ surfaces change dynamically due to electrical fields. We will also discuss temperature evolution of quasi-1D fullerene nanostructures on graphene. Through careful control of the subtle balance between the C$_{\mathrm{60\thinspace }}$surface mobility and the periodic potential of rippled graphene, C$_{\mathrm{60\thinspace }}$molecules can be arranged into a novel chain structure, and this chain structure can further transition to a compact hexagonal close packed stripe structure by tuning the annealing temperature. [Preview Abstract] |
Friday, November 11, 2016 2:30PM - 3:00PM |
H2.00003: Designing interfaces for Spin Injection into Organic Molecular Solids: A Surface Science Approach Invited Speaker: Daniel Dougherty Organic Spintronics seeks designer materials that exhibit new spin dependent transport effects [1]. However, before the search for new spintronic phenomena can take off, the challenge of spin injection has to be addressed [2]. Our group uses spin polarized scanning tunneling microscopy and spectroscopy to identify the principles governing the formation of magnetic interface states at organic semiconductor-metal interfaces that promote spin injection [3, 4]. In this talk I will discussion recent work on the ``fruitfly'' materials class in organic spintronics, the metal hydoxyquinolates. Within this single class, we identify metal-organic interfaces that exhibit spin polarized interface states sensitive to subtle changes in orbitals. We consider two different metal-\textit{tris}-(8-hydroxyquinolate) compounds, Crq$_{\mathrm{3}}$ and Alq$_{\mathrm{3}}$, which have similar geometries but molecular orbitals with or without minor $d$-orbital mixing from the central metal cation. Surprisingly, this results in large differences in the mechanism of interfacial coupling for the two molecules. The differences give rise to interface states that are either resistive for Crq3 or metallic for Alq3, and are modeled using Density Functional Theory calculations\textbf{. }\textit{This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, under Award No. DE-SC0010324} [1] Sanvito, Chem. Soc. Rev. \textbf{40}, 3336 (2011). [2] Schmidt and Molenkamp, Semicond. Sci. Technol. \textbf{17}, 310 (2002). [3] Barraud et al., Nat. Phys. \textbf{6}, 615 (2010). [4] Raman, Appl. Phys. Rev.~1, 031101~(2014). [Preview Abstract] |
Friday, November 11, 2016 3:00PM - 3:30PM |
H2.00004: Exploring Pd adsorption, diffusion, and nucleation on bilayer SiO$_{\mathrm{2}}$/Ru as a function of hydroxylation and precursor environment: From UHV to catalyst preparation Invited Speaker: William Kaden The hydroxylation-dependent permeability of bilayer SiO$_{\mathrm{2}}$~supported on Ru(0001) was investigated by XPS and TDS studies in a temperature range of 100~K to 600~K. For this, the thermal behavior of Pd evaporated at 100~K, which results in surface and sub-surface (Ru-supported) binding arrangements, was examined relative to the extent of pre-hydroxylation. Samples containing only defect-mediated hydroxyls showed no effect on Pd diffusion through the film at low temperature. If, instead, the concentration of strongly bound hydroxyl groups and associated weakly bound water molecules was enriched by an electron-assisted hydroxylation procedure, the probability for Pd diffusion through the film is decreased via a pore-blocking mechanism. Above room temperature, all samples showed similar behavior, reflective of particle nucleation above the film and eventual agglomeration with any metal atoms initially binding beneath the film. When depositing Pd onto the same SiO$_{\mathrm{2}}$/Ru model support via adsorption of [Pd(NH$_{\mathrm{3}})_{\mathrm{4}}$]Cl$_{\mathrm{2}}$~from alkaline (pH~12) precursor solution, we observe notably different adsorption and nucleation mechanisms. The resultant Pd adsorption complexes follow established decomposition pathways to produce model catalyst systems compatible with those created exclusively within UHV despite lacking the ability to penetrate the film due to the increased size of the initial Pd precursor groups. [Preview Abstract] |
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