Session Y30: Ionically Controlled Transport and Electrooptical Functionalities at Oxide Interfaces

Single atom electrochemical and atomic analytics

In the past decade, advances in electron and scanning-probe based microscopies have led to a wealth of imaging and spectroscopic data with atomic resolution, yielding substantial insight into local physics and chemistry in a diverse range of systems such as oxide catalysts, multiferroics, manganites, and 2D materials. However, typical analysis of atomically resolved images is limited, despite the fact that image intensities and distortions of the atoms from their idealized positions contain unique information on the physical and chemical properties inherent to the system. Here, we present approaches to data mine atomically resolved images in oxides, specifically in the hole-doped manganite La5/8Ca3/8MnO3, on epitaxial films studied by in-situ scanning tunnelling microscopy (STM). Through application of bias to the STM tip, atomic-scale electrochemistry is demonstrated on the manganite surface. STM images are then further analyzed through a suite of algorithms including 2D autocorrelations, sliding window Fourier transforms, and others, and can be combined with basic thermodynamic modelling to reveal relevant physical and chemical descriptors including segregation energies, existence and strength of atomic-scale diffusion barriers, surface energies and sub-surface chemical species identification. These approaches promise to provide tremendous insights from atomically resolved functional imaging, can provide relevant thermodynamic parameters, and auger well for use with first-principles calculations to yield quantitative atomic-level chemical identification and structure-property relations.   

Transport Properties of Exfoliated BSCCO on LAO/STO Heterostructures

We investigate the interaction between high-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (BSCCO) flakes deposited on the oxide heterostructure LaAlO$_3$/SrTiO$_3$ (LAO/STO). Conductive-atomic force microscope (c-AFM) lithography will be used to create nanowires at the LAO/STO interface that couple to the BSCCO. Through coupling of these materials, we will be able to study phenomena such as the proximity effect and coulomb drag.   

Systematic investigation of chemical substitution in BaSnO$_{\mathrm{3}}$ using the combinatorial approach

BaSnO$_{\mathrm{3}}$ has been regarded as a possible material for photo-catalysis, dielectric capacitors, and transparent conductors. We are systematically investigating the effect of chemical substitution for A and B sites in BaSnO$_{\mathrm{3\thinspace }}$using a high-throughput methodology. We have thus far investigated the effect of substituting La and Sr for the Ba-site and Pb and Bi for the Sn-site. The composition spread films were prepared on MgO, SrTiO$_{\mathrm{3}}$ and LaAlO$_{\mathrm{3}}$ using combinatorial pulsed laser deposition. The lattice parameters and band-gap energies were found to continually change as a function of the concentration of each substitutional dopant. We find that the band gap can be tuned from 2.8 eV for BaSn$_{\mathrm{0.05}}$Pb$_{\mathrm{0.95}}$O$_{\mathrm{3}}$ to 4.5 eV for Ba$_{\mathrm{0.05}}$La$_{\mathrm{0.95}}$SnO$_{\mathrm{3}}$. Especially for Ba$_{\mathrm{1-x}}$La$_{\mathrm{x}}$SnO$_{\mathrm{3\thinspace }}$with x in the range of 0.05 \textless x\textless 0.5, we consistently observe resistivity as low as 0.23 m$\Omega $cm at room temperature while maintaining optical transparency with a typical bandgap of \textasciitilde 4 eV. The effect of crystalline defects on electrical properties will also be discussed.   

Correlated Heterostructures for Efficient Solar Cells

Polar$|$non-polar oxide heterostructures such as LaAlO$_3|$SrTiO$_3$ have become well-known for the many intriguing phenomena occurring at the interface, especially the internal potential gradient and the resulting 2d electron gas. We propose to make use of these unique systems as absorbing materials for high-efficiency solar cells [1]. In particular, LaVO$_3|$SrTiO$_3$ ($i$) has a direct band gap $\sim$1.1 eV, nearly optimal for a solar cell; ($ii$) the internal potential gradient serves to efficiently separate the photo-generated electron-hole pairs and reduce recombination losses; ($iii$) the conducting interface offers a natural contact for charge-carrier extraction. Furthermore, ($iv$) oxide heterostructures afford the flexibility to combine layers with different gaps, e.g.~LaVO$_3$ with LaFeO$_3$, in order to achieve even higher efficiencies with band-gap graded solar cells. We use density-functional theory and dynamical mean-field theory to study this strongly correlated heterostructure. \\[2mm] [1] Assmann et al., PRL 110, 078701 (2013)\\ Experimental corroboration: Liang et al., Sci. Rep. 3, 1975 (2013); Wang et al., PR Applied 3, 064015 (2015)   

\textbf{Ferroelectricity and hysterically controlled photocurrent in NaMnF}$_{\mathrm{\mathbf{3}}}$\textbf{ Thin Film}

Abstract: In recent year many fascinating electron correlation phenomena have been discussed, such as two dimensional electron gas, metal-insulator transitions and multiferroic interactions. While most of the researches concentrate on complex oxides, there are strong indications that complex fluorides may have analogous, or even enhanced properties. NaMnF$_{\mathrm{3}}$ is one such example. Theoretical work predicted that NaMnF$_{\mathrm{3}}$ has multiferroic characters and strong magneto-electric coupling. Thin films of NaMnF$_{\mathrm{3}}$ with 50 nm thickness were grown on SrTiO$_{\mathrm{3}}$ substrates via molecular beam epitaxy. By performing piezoelectric force microscopy, rewritable polarizations were manipulated and stable ferroelectric switching was obtained in NaMnF$_{\mathrm{3}}$ at room temperature. At low temperatures, persistent photocurrent was observed under the illumination of 400nm laser. Amplitude and direction of such photocurrent can be hysterically controlled by external biases. This phenomenon is due to the fact that photocarriers generated in SrTiO$_{\mathrm{3}}$ are driven by the controlled built-in electric field in NaMnF$_{\mathrm{3}}$ thin film. These findings indicate great potential of complex fluorides in applications such as ferroelectric switches, photovoltaic devices and memory storages. This work is supported by DMREF-NSF 1434897.   

Linear electro-optic effect in strained BaTiO$_{\mathrm{3}}$

The nominal perovskite ABO$_{\mathrm{3}}$ structure is cubic and centro-symmetric. Therefore, by symmetry there should be no linear electro-opitc (EO) effect. However, perovskites are known to experience various lattice distortions that result in reduced symmetry. The ferroelectric transition in BaTiO$_{\mathrm{3\thinspace }}$(BTO) is one obvious example. Below 393 K the cell becomes tetragonal and Ti shifts away from the center of inversion and produces a dipole moment and a robust linear EO or Pockels effect. In this talk I will discuss how the EO response in BTO can be enhanced by strain and show our recent results obtained with density functional theory. This work was supported by the Air Force Office of Scientific Research (Grant FA9550--12--10494)   

Build-in Electric Field Induced Mechanical Property Change

Mechanical properties describe how materials respond to external stress. Microscopically, many intrinsic and extrinsic factors, such as bond length and strength (intrinsic) and grain boundaries (extrinsic), may affect the mechanical property of the materials. In this study, we observed a change of fracturing behavior of Nb-doped SrTiO$_{3}$ in a Schottky barrier near the interfaces with metallic LaNiO$_{3}$ films. Through cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S) experiments and theoretical analysis, the observed fractured topography could be explained by the change of the bond length caused alternation of mechanical property inside the Schottky barrier. Same model could also explain the widely observed dielectric dead layer for SrTiO$_{3}$ in contact with metal electrodes.   

Ionic and electrochemical phenomena induced by structural and chemical defects in oxide thin films

Interactions at the surfaces/interfaces between complex oxides and gaseous environment are fundamental for the efficiency of many environmental friendly systems and applications. Such interactions can be modified by the intricate interrelationship between microstructure and chemical substitution defects, being their role on functional properties, such as ionic conductivity and surface reaction rates, as particularly relevant as difficult to discriminate. New possibilities in thin film fabrication allow growth of oxide thin films with a more precise control of the structure and chemical stoichiometry, thus unveiling new perspectives in the study of electrochemical effects for physical functionalities, through nanoscale characterizations by complementary state-of-art techniques. As an example of interfacial structural defect effects, we will discuss the case of yttrium doped barium zirconate thin films, where the cation substitutions represent a viable mechanism, alternative to the formation of dislocations near the interface, to relieve the strain building up in the film growing on a highly mismatched substrate, thus providing fast transport pathways together with enhanced interface electrochemical reactivity. The effect of the chemical defects will be further presented in the case of samarium-doped ceria films with different doping concentration. We will explain the role of the trivalent doping on the conduction mechanism, i.e. proton or oxygen ion, which in turns may greatly influence the surface reactivity.   

First principles study of oxygen vacancies and iron impurities on electrical and optical properties of NiO

We are studying the properties of iron doped NiO by Density Functional Theory. NiO is being considered for use in RRAM, based on the reversible switching of a thin transition metal oxide (TMO) layer between a low and high resistance state using the mechanism of soft breakdown. RRAM's high integration density, its high endurance and good retention, its low energy use, and its high speed make it a potential candidate for replacing Flash memory. Switching between the high and low resistance state is inhomogeneous, and low resistance nano-filaments are formed. Fe impurities are introduced to optimize the switching properties. The effects of oxygen vacancies and iron on the electronic structure and optical properties of NiO are calculated and compared with experiment. Antiferromagnetic rhombohedral 108 atom cells with 1.85{\%} Fe concentration are considered. Due to the highly-correlated nature of d orbitals in TMOs, a Hubbard U correction is applied to calculations in this work via the GGA $+$ U method of DFT using VASP. Hybrid HSE06 calculations will also be considered. Localized energy levels from iron and from oxygen vacancies are identified, and their effects on dielectric permittivity are presented.   

\textbf{Investigation on the Mechanism and Application of Nanoscale NiO Memristors}

In contrast to the oxygen-vacancy-based model for the memristive $n$-type metal oxides, the coexistence of cation and anion vacancies had been suggested theoretically to be crucial to the bipolar memristive behavior of NiO. We have revealed the deterministic role of concentration surplus of cation vacancy over anion vacancy in bipolar memristive NiO, with C-AFM measuring the electrical properties, and STEM combined with EELS characterizing the ionic vacancies, giving an experimental support for the first time to the dual-defects-based model, which is of fundamental importance for the comprehensive understanding of memristor mechanisms. Furthermore, we have fabricated NiO nanodots with AAO templates, in which the intrinsically rectifying-resistive switching (IR-RS) has been observed. This is the first work studying the IR-RS in the scale of real devices, where the feasibility for selection device-free memory application has been demonstrated. The IR-RS in NiO nanodots has been ascribed exclusively to the built-in $p-n$ homojunction, differing from previous cases dominated by the interfacial Schottky barriers.   

Nanoscale BaTiO$_{3}$ MOSCAP formation for ferroelectric field effect transistor application

Titanates are an important class of materials with many interesting functional properties and applications for non-volatile memory, i.e. BaTiO$_{3}$, which is a promising candidate for the realization of a ferroelectric field-effect transistor. However, the difficulty of chemically etching titanates has hindered their commercial use in device manufacturing so far. Here, we report a technique to circumvent this problem. Using molecular beam epitaxy, we grew compressively strained ferroelectric BaTiO$_{3}$, within photolithographically defined openings of a sacrificial SiO$_{2}$ layer on germanium (001) with Pt as a top electrode. Etching away the sacrificial SiO$_{2}$ can reveal isolated nanoscale gate stacks circumventing the need to etch the titanate thin film. Using X-ray diffraction we find that the BaTiO$_{3}$ film is tetragonal with the longer $c$-axis being out of plane, which is a requirement for the ferroelectric field effect transistor. The crystal quality of the BaTiO$_{3}$ films grown in the openings is confirmed using RHEED and cross-sectional transmission electron microscopy. Focused ion beam etching of the Pt layer is then used to electrically isolate a Pt/BaTiO$_{3}$/SrTiO$_{3}$/Ge stack to perform electrical measurements.