Epitaxial BiFeO3 multiferroic thin film heterostructures

Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO3, is reported. Structure analysis indicates that the crystal structure of film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The observed enhancement is corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters.     

Van der Waals Epitaxial Growth of agr-Alumina Nanocrystals on Mica

Lattice mismatch stresses, which severely restrict heteroepitaxial growth, are greatly minimized when thin alumina films are grown by means of van der Waals forces on inert mica substrates. A 10-nanometer-thick epitaxial film exhibits crystallographic sixfold symmetry, a lattice constant close to that of the basal plane [0001] of alpha-alumina (sapphire), and an aluminum: oxygen atomic ratio of 1:1.51 +/- 0.02 (measured by x-ray photoelectron spectroscopy), again the same as for bulk sapphire. The film is free of steps and grain boundaries over large areas and appears to be an ideal model system for studying adhesion, tribology, and other surface phenomena at atomic scales.     

Epitaxial and Smooth Films of a-Axis YBa2Cu3O7

YBa(2)Cu(3)O(7) films have been grown epitaxially on SrTiO(3) (100) and LaAlO(3) (100) substrates with nearly pure a-axis orientation and with transition temperature T(c) (R = 0) of 85 K. A unique feature of these films is their smooth surface. These smooth surfaces enable the growth of short-period superlattices with well-defined modulations. The films are untwinned and the grains grow with their c-axis along one of two perpendicular directions on the substrate ([100] or [010]). The fabrication of sandwich-type Josephson junctions with good characteristics may now be possible because unlike c-axis-oriented films, the superconducting coherence length of these smooth films is appreciably large perpendicular to their surfaces.     

Microstructural optimization of a zeolite membrane for organic vapor separation

A seeded growth method for the fabrication of high-permeance, high-separation-factor zeolite (siliceous ZSM-5, [Si96O192]-MFI) membranes is reported. The method consists of growing the crystals of an oriented seed layer to a well-intergrown film by avoiding events that lead to a loss of preferred orientation, such as twin overgrowths and random nucleation. Organic polycations are used as zeolite crystal shape modifiers to enhance relative growth rates along the desirable out-of-plane direction. The polycrystalline films are thin (approximately 1 micrometer) with single grains extending along the film thickness and with large in-plane grain size (approximately 1 micrometer). The preferred orientation is such that straight channels with an open diameter of approximately 5.5 angstroms run down the membrane thickness. Comparison with previously reported membranes shows that these microstructurally optimized films have superior performance for the separation of organic mixtures with components that have small differences in size and shape, such as xylene isomers.     

Atomic-Scale Images of the Growth Surface of Ca1-xSrxCuO2 Thin Films

The surface microstructure of c-axis (Ca,Sr)CuO(2) thin films, grown by laser molecular beam epitaxy on SrTiO(3)(001) substrates, was studied by ultrahigh-vacuum scanning tunneling microscopy (STM). Images were obtained for codeposited Ca1-xSrxCuO(2) thin films, which show a layered-type growth mode. The surfaces consist of atomically flat terraces separated by steps that are one unit cell high. A pronounced dependence of the growth mechanism on the Sr/Ca ratio of the films was observed. Atomic resolution STM images of the CuO(2) sheets in the ab plane show a square lattice with an in-plane spacing of 4 angstroms; the lattice contains different concentrations of point defects, depending on the polarity of the sample-tip bias.     

Single-Crystal Epitaxial Thin Films of the Isotropic Metallic Oxides Sr1-xCaxRuO3 (0 le x le 1)

Single-crystal epitaxial thin films of the isotropic metallic oxides Sr1-xCaxRuO(3) (0 相似文献   

Epitaxial cubic gadolinium oxide as a dielectric for gallium arsenide passivation

Epitaxial growth of single-crystal gadolinium oxide dielectric thin films on gallium arsenide is reported. The gadolinium oxide film has a cubic structure isomorphic to manganese oxide and is (110)-oriented in single domain on the (100) gallium arsenide surface. The gadolinium oxide film has a dielectric constant of approximately 10, with low leakage current densities of about 10(-9) to 10(-10) amperes per square centimeter at zero bias. Typical breakdown field is 4 megavolts per centimeter for an oxide film 185 angstroms thick and 10 megavolts per centimeter for an oxide 45 angstroms thick. Both accumulation and inversion layers were observed in the gadolinium oxide-gallium arsenide metal oxide semiconductor diodes, using capacitance-voltage measurements. The ability to grow thin single-crystal oxide films on gallium arsenide with a low interfacial density of states has great potential impact on the electronic industry of compound semiconductors.     

Growth and erosion of thin solid films

Thin films that are grown by the process of sputtering are, by and large, quite unlike the smooth, featureless structures that one might expect. In general, these films have a complicated surface morphology and an extended network of grooves and voids in their interiors. Such features can have a profound effect on the physical properties of a thin film. The surface irregularities and the bulk defects are the result of a growth instability due to competitive shadowing, an effect that also plays a role in geological processes such as erosion. For amorphous thin films, the shadow instability can be described by a remarkably simple model, which can be shown to reproduce many important observed characteristics of thin film morphology.     

Atomic Control of the SrTiO3 Crystal Surface

The atomically smooth SrTiO(3) (100) with steps one unit cell in height was obtained by treating the crystal surface with a pH-controlled NH(4)F-HF solution. The homoepitaxy of SrTiO(3) film on the crystal surface proceeds in a perfect layer-by-layer mode as verified by reflection high-energy electron diffraction and atomic force microscopy. Ion scattering spectroscopy revealed that the TiO(2) atomic plane terminated the as-treated clean surface and that the terminating atomic layer could be tuned to the SrO atomic plane by homooepitaxial growth. This technology provides a well-defined substrate surface for atomically regulated epitaxial growth of such perovskite oxide films as YBa(2)Cu(3)O(7-delta).     

Epitaxial cuprate superconductor/ferroelectric heterostructures

Thin-film heterostructures of Bi(4)Ti(3)O(12)Bi(2)Sr(2)CuO(6+x), have been grown on single crystals of SrTiO(3), LaAlO(3), and MgAl(2)O(4) by pulsed laser deposition. X-ray diffraction studies show the presence of c-axis orientation only; Rutherford backscattering experiments show the composition to be close to the nominal stoichiometry. The films are ferroelectric and exhibit a symmetric hysteresis loop. The remanent polarization was 1.0 microcoulomb per square centimeter, and the coercive field was 2.0 x 10(5) volts per centimeter. Similar results were obtained with YBa(2)Cu(3)O(7-x) and Bi(2)Sr(2)CaCu(2)O(8+x), and single-crystal Bi(2)Sr(2)CuO(6+x)as the bottom electrodes. These films look promising for use as novel, lattice-matched, epitaxial ferroelectric film/electrode heterostructures in nonvolatile memory applications.     

Polymerization-induced epitaxy: scanning tunneling microscopy of a hydrogen-bonded sheet of polyamide on graphite

A molecularly thin film of a two-dimensional polymer network formed by hydrogen bonding was synthesized and investigated with scanning tunneling microscopy. Poly(in-caprolactam) (nylon 6) was epitaxially grown on the basal plane of graphite and an ultrathin film of the polymer was obtained after the bulk materials had been washed away with solvents. The polymer chain has a planar, all-trans conformation and adjacent chains run in the antiparallel direction. This produces complete pairing of hydrogen bonding groups, with each amide group lying on a straight line perpendicular to the polymer backbone. This hydrogen-bonded sheet is oriented so that each polymer backbone lies in the (1010) direction on the graphite hexagonal lattice, as opposed to the (1120) direction taken by other paraffinic molecules studied so far. This experiment shows that hydrogen bonding can be used to control the orientation of macromolecules in two dimensions.     

Polymer gate dielectric surface viscoelasticity modulates pentacene transistor performance

Nanoscopically confined polymer films are known to exhibit substantially depressed glass transition temperatures (Lg's) as compared to the corresponding bulk materials. We report here that pentacene thin films grown on polymer gate dielectrics at temperatures well below their bulk Tg's exhibit distinctive and abrupt morphological and microstructural transitions and thin-film transistor (TFT) performance discontinuities at well-defined growth temperatures. The changes reflect the higher chain mobility of the dielectric in its rubbery state and are independent of dielectric film thickness. Optimization of organic TFT performance must recognize this fundamental buried interface viscoelasticity effect, which is detectable in the current-voltage response.     

Dislocations and Flux Pinning in YBa2Cu3O7-delta

Bulk YBa(2)Cu(3)O(7-delta) superconductors, under certain processing conditions such as melt texturing, exhibit a very high dislocation density of 10(9) to 10(10) per square centimeter. In addition, the density of low-angle grain boundaries in such samples can be significantly increased (to less than 700-nanometer spacing) through a dispersion of submicrometer-sized Y(2)BaCuO(5) inclusions. These defect densities are comparable to those in high critical current thin films as revealed through scanning tunneling microscopy, and yet the critical current densities in the bulk materials (at 77 kelvin and a field of 1 tesla for example) remain at a 10(4) amperes per square centimeter level, about two orders of magnitude lower than in thin films. The results imply that these defect density levels are not significant enough to explain the difference in flux pinning strength between the thin film and bulk materials. The observation of spiral-like growth of the superconductor phase in bulk Y-Ba-Cu-O is also reported.     

Chemical Solution Routes to Single-Crystal Thin Films

Epitaxial thin films of inorganic single crystals can be grown on single-crystal substrates with a variety of different solution chemistries. This review emphasizes chemical solution deposition, in which a solution is used to deposit a layer of precursor molecules that decompose to low-density, polycrystalline films during heating. Ways to control film cracking during deposition and heat treatment and why many precursors synthesize metastable crystalline structures are discussed, and the different mechanisms that convert the polycrystalline film into a single crystal are reviewed. Hydrothermal epitaxy, in which single crystal thin films are directly synthesized on templating substrates in an aqueous solution at temperatures <150°C, is also discussed.     

Growth Mechanism of Sputtered Films of YBa2Cu3O7 Studied by Scanning Tunneling Microscopy

The surface microstructures of c-axis-oriented films of YBa(2)Cu(3)O(7), deposited by off-axis magnetron sputtering on MgO and SrTiO(3) single crystal (100) substrates, have been investigated with scanning tunneling microscopy and atomic force microscopy. There is strong evidence that the films nucleate as islands and grow by adding material to the edge of a spirally rising step. This results in columnar grains, each of which contains a screw dislocation at its center. This microstructure may be of significance in determining superconducting properties such as critical current, and represents a significant difference between thin films (especially those grown in situ) and bulk materials.     

Quantum electronic stability of atomically uniform films

We have studied the structural stability of thin silver films with thicknesses of N = 1 to 15 monolayers, deposited on an Fe(100) substrate. Photoemission spectroscopy results show that films of N = 1, 2, and 5 monolayer thicknesses are structurally stable for temperatures above 800 kelvin, whereas films of other thicknesses are unstable and bifurcate into a film with N +/- 1 monolayer thicknesses at temperatures around 400 kelvin. The results are in agreement with theoretical predictions that consider the electronic energy of the quantum well associated with a particular film thickness as a significant contribution to the film stability.     

Atomistic Processes in the Early Stages of Thin-Film Growth

Growth of thin films from atoms deposited from the gas phase is intrinsically a nonequilibrium phenomenon governed by a competition between kinetics and thermodynamics. Precise control of the growth and thus of the properties of deposited films becomes possible only after an understanding of this competition is achieved. Here, the atomic nature of the most important kinetic mechanisms of film growth is explored. These mechanisms include adatom diffusion on terraces, along steps, and around island corners; nucleation and dynamics of the stable nucleus; atom attachment to and detachment from terraces and islands; and interlayer mass transport. Ways to manipulate the growth kinetics in order to select a desired growth mode are briefly addressed.     

Transferable GaN layers grown on ZnO-coated graphene layers for optoelectronic devices

We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) using graphene-layered sheets. Heteroepitaxial nitride thin films were grown on graphene layers by using high-density, vertically aligned zinc oxide nanowalls as an intermediate layer. The nitride thin films on graphene layers show excellent optical characteristics at room temperature, such as stimulated emission. As one of the examples for device applications, LEDs that emit strong electroluminescence emission under room illumination were fabricated. Furthermore, the layered structure of a graphene substrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic.     

MgB2 superconducting thin films with a transition temperature of 39 kelvin

We fabricated high-quality c axis-oriented epitaxial MgB2 thin films using a pulsed laser deposition technique. The thin films grown on (1 i 0 2) Al2O3 substrates have a transition temperature of 39 kelvin. The critical current density in zero field is approximately 6 x 10(6) amperes per cubic centimeter at 5 kelvin and approximately 3 x 10(5) amperes per cubic centimeter at 35 kelvin, which suggests that this compound has potential for electronic device applications, such as microwave devices and superconducting quantum interference devices. For the films deposited on Al2O3, x-ray diffraction patterns indicate a highly c axis-oriented crystal structure perpendicular to the substrate surface.     

可见光响应氮杂化TiO2-N薄膜对水源水体的杀菌作用

以尿素为氮源,采用Sol-gel法及浸渍-提拉法制备氮杂化TiO2-N薄膜,并运用正交试验法优化TiO2-N薄膜的最佳制备条件;利用AFM分析技术对TiO2-N薄膜表面形貌进行表征;在太阳光光催化反应器中,探讨TiO2-N薄膜对饮用水供水水源水体中大肠菌群的杀菌作用。结果表明,热处理条件是影响TiO2-N薄膜制备的关键因素,各个影响因素的主次顺序为热处理温度〉停留温度〉热处理时间,在最佳条件下制备的TiO2-N薄膜,在太阳光作用下光解30min时,对饮用水供水水源水体中大肠菌群的杀菌率达到57.7%以上;Ag的协同掺杂（Ag-TiO2-N）或在TiO2-N薄膜表面的沉积（Ag/TiO2-N）,显著提高了TiO2-N薄膜的杀菌作用,对大肠菌群的杀菌率可达90%以上。