Complex Oxide Nanostructures Research Articles

Patterning and epitaxy of large-area arrays of nanoscale complex oxide epitaxial heterostructures

A combination of block copolymer (BCP) lithography and solid-phase epitaxy can be employed to form large areas, on the order of square centimeters, of a high density of epitaxial crystalline complex oxide nanostructures. We have used BCP lithography with a poly(styrene-block-methyl methacrylate) (PS-b-PMMA) copolymer to template a nanohole array either directly on an (001)-oriented SrTiO3 (STO) single crystal substrate or on a 20 nm-thick Si3N4 layer deposited on the STO substrate. BCPs with the selected compositions assembled in a cylindrical phase with 16 nm diameter PMMA cylinders and a cylinder-to-cylinder spacing of 32 nm. The substrate was modified with an energetically non-preferential polymer layer to allow for the vertical alignment of the cylinders. The PMMA cylinders were removed using a subtractive process, leaving an array of cylindrical holes. For BCPs assembled on Si3N4/STO, the pattern was transferred to the Si3N4 layer using reactive ion etching, exposing the underlying STO substrate in the nanoholes. An amorphous LaAlO3 (LAO) layer was deposited on the patterned Si3N4/STO at room temperature. The amorphous LAO epitaxially crystallized within the nanoscale-patterned holes with fully relaxed lattice parameters through solid phase epitaxy, resulting in the formation of nanoscale LAO/STO epitaxial heterostructures.

Unraveling the Complexity of Nano-Dispersoids in the Oxide Dispersion Strengthened Alloy 617.

Nanocrystalline oxides are mainly responsible for Ni-base oxide dispersion strengthened (ODS) superalloys excellent thermo-mechanical properties. To establish the microstructural correlations between the metallic matrix and various oxide dispersoids, we report here the atomic-scale structure and chemistry of the complex nano-oxide dispersoids. Ultrahigh-resolution Cs-aberration-corrected scanning transmission electron microscopy (STEM) based techniques have been used to resolve nano-dispersoids in the Alloy 617 ODS. These nano-oxides, interestingly, possess a variety of high-angle annular dark-field (HAADF) contrasts, that is, bright, dark, and bi-phases. Both the light and heavy atoms have been found to be present in Y–Al–O complex-oxide nanostructures in varying quantities and forming a characteristic interface with the metallic matrix. In overcoming the limitation of conventional STEM-HAADF imaging, the integrated differential phase-contrast imaging technique was employed to investigate the oxygen atoms along with other elements in the dispersoids and its interface with the matrix. The most intriguing aspect of the study is the discovery of a few atoms thick Al2O3 interlayer (shell) around a monoclinic Y–Al–O core in the Ni-matrix. On the other hand, when the dispersoid is a hexagonal type Y–Al–O complex, the interface energy is already low, maintaining a semi-coherent interface and it was devoid of a shell.

PLD growth and characteristics of lead-free NKLNST ferroelectric nanotubes

Lead-free (Na0.5358K0.4042Li0.06)(Nb0.893Sb0.057Ta0.05)O3 (NKLNST) nanotubes were successfully grown on Pt(111)/Ti/SiO2/Si(001) substrates by using pulsed laser deposition (PLD) with home-made ceramic target under relatively high O2 partial pressures. The as-grown NKLNST nanotubes exhibited a perovskite phase and grew along (001) direction. The nanotubes’ outer diameters are ranging between 100 and 200 nm, their typical length is around 1 µm with a wall thickness of 10−20 nm. The films composed of NKLNST nanotubes show well-defined P–E hysteresis loops, with Pr of 3.77 μC/cm2 and Ec of 37.6 kV/cm. While under low electric field, the charge transport is dominated by ohmic conduction, at high electric fields, space charge-limited current turns out to be the governing leakage mechanism in these films. The work shows the potential of PLD in the synthesis of nanostructures of complex oxides under proper conditions.

Shape-imposed anisotropy in antiferromagnetic complex oxide nanostructures

In this study, we report on a shape-imposed magnetic anisotropy in micro- and nanostructures defined in antiferromagnetic (AF) LaFeO3 (LFO) thin films. Two distinct types of structures are investigated: embedded magnets created via ion implantation and free-standing magnets created via ion milling. Using a combination of x-ray photoemission electron microscopy and x-ray absorption spectroscopy, we examine the impact of the structure type, AF layer thickness, and crystal geometry on the Néel vector orientation in these structures. We demonstrate a distinct shape-imposed anisotropy in embedded and free-standing structures alike and show that both parallel and perpendicular alignments of the AF spin axis with respect to structure edges can be achieved by variation of the AF layer thickness and the orientation of the structure edges with respect to the LFO crystalline axes. This work demonstrates how the fabrication procedure affects the magnetic order in thin film AF nanostructures and shows how nanoscale patterning can be used to control the orientation of the Néel vector in epitaxial oxide thin films.

Nanosecond discharge in atmospheric pressure air with ectonic introduction of iron and copper vapor in plasma and its application in nanotechnology

Introduction: Nanosecond discharges in high-pressure gases between metal electrodes are promising for applications in selective UV lamps. The most investigated working media of such lamps are copper and iron vapors in the air, which are excited by a high-voltage discharge of subnanosecond duration. The use of various metal electrodes in one discharge gap and discharge excitation pulses with an amplitude of 20–40 kV and a duration of 50–100 ns is little studied, which is important for controlling the spectral characteristics of the lamp and producing complex oxide nanostructures based on two different metals. Also important is the use of such discharges for the synthesis of nanostructures of transition metal oxides in assisting the deposition process with hard UV radiation, which allows to improve some of their characteristics. Purpose: The aim of the work is to study the electrical and emission characteristics of a bipolar overstressed nanosecond discharge between stainless steel and copper electrodes in atmospheric pressure and to synthesize and study the optical characteristics of thin films deposited on a glass substrate from the sputtered products of metal electrodes in an air plasma. Methods : We used an optical spectroscopic method for studying the discharge plasma. Results : The results of a study of the electrical characteristics of overstressed nanosecond discharge in air at atmospheric pressure between stainless steel electrodes and copper and stainless electrodes found that it is a source of UV radiation from atoms and iron and copper ions in the spectral range 200-300 nm. The peak value of the electric power of the discharge is ~ 4 MW, and the energy contribution to the plasma per pulse was ~ 0.1 J. The developed point source of UV radiation based on the discharge between the copper and stainless electrode is promising for applications in medicine and sanitation. It has been established that under the assistantance by UV-radiation the developed “point” source in the nanostructures of iron oxides, enlightenment bands appear in the visible region of the spectrum. Conclusion: Experiments revealed the suitability of this discharge for use in a selective UV lamp on iron and copper vapor and deposition of nanostructured films based on iron oxides while assisting the process of deposition by UV radiation.

Influence of Fe substitution on structural and magnetic features of BiMn2O5 nanostructures

Nanostructures of complex oxides [BiFexMn2−xO5 (x = 0, 1, 2)] have been designed to study their structural, optical and magnetic behaviour. X-ray diffraction data (XRD) revealed orthorhombic phase with Pbam space group. Noticeable expansion in unit cell parameters has been found from BiMn2O5 (x = 0) to BiFe2O4.5 (x = 2). The observed structural changes via tuning of B-site (x = 0–2) played an important role in overall magnetic properties. Transmission electron microscopic images confirm that the average particle size of all the materials are in nano domain range with different morphologies. From optical studies, it has been found that the observed energy band gap values are strongly related to 3d electron numbers. These values appear to be larger than that reported for bulk. Isothermal magnetization plots (at 5 K) show increase in coercivity (Hc) from x = 0 to x = 2. Temperature dependent magnetization studies implied anti-ferromagnetic interactions for BiMn2O5, frustrated magnet for BiFeMnO5 and ferromagnetic behaviour for BiFe2O4.5. Ferromagnetic state of nanostructured BiFe2O4.5 is in contrast with its bulk counterparts.

Direct observation of oxygen vacancy-driven structural and resistive phase transitions in La2/3Sr1/3MnO3

Resistive switching in transition metal oxides involves intricate physical and chemical behaviours with potential for non-volatile memory and memristive devices. Although oxygen vacancy migration is known to play a crucial role in resistive switching of oxides, an in-depth understanding of oxygen vacancy-driven effects requires direct imaging of atomic-scale dynamic processes and their real-time impact on resistance changes. Here we use in situ transmission electron microscopy to demonstrate reversible switching between three resistance states in epitaxial La2/3Sr1/3MnO3 films. Simultaneous high-resolution imaging and resistance probing indicate that the switching events are caused by the formation of uniform structural phases. Reversible horizontal migration of oxygen vacancies within the manganite film, driven by combined effects of Joule heating and bias voltage, predominantly triggers the structural and resistive transitions. Our findings open prospects for ionotronic devices based on dynamic control of physical properties in complex oxide nanostructures.

Nanostructured CaCu3Ti4O12 for environmental remediation through visible light active catalysis

The unresolved issue of generating well defined nanostructure of calcium copper titanate, a well-known dielectric perovskite, has been overcome through simple molten salt approach. This technique provides a one-step, convenient, low-cost, nontoxic, and mass-production route for the synthesis of nanostructures of complex oxides with important scientific and technological applications. The as-synthesized powder, characterized by powder X-ray diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy was confirmed to be a phase pure calcium copper titanate nanocubes with edge length around 180 nm having sharp edges. The photocatalytic activity of the as-prepared samples was evaluated by the degradation of methyl orange aqueous solution under visible light irradiation and exhibited higher dye degradation ability compared to its bulk counterpart. The enhanced photocatalytic activity could be attributed to increase in both the surface area as well as photo-generated carrier life time.

Mechanisms Of Composite-hydroxide-mediated Approach For The Synthesis Of Functional ZnO Nanostructures And Morphological Dependent Optical Emissions

We report synthesis of the functional ZnO nanostructures (nanowires, nanorods) by a cost effective and efficient method; called composite­hydroxide­mediated (CHM) approach. Effect of the processing temperature on the particle size, morphology, and subsequently morphological dependent optical emissions is investigated. Needle shaped nanowires are obtained at 200 and 220C, of about (500­1500) nm in length, while at 250C; nanorods are formed with length in the range of (200­460) nm and width (10­30) nm. Optical study reveals that ZnO nanorods show only ultra­violet (UV) emission while bent nanowires demonstrate both UV and green emissions simultaneously. The week green emission at 2.4 eV indicates no efficient trapping of the photo­ generated hole in the nanostructures. Phase purity, crystalline structure, size and chemical nature of the product are probed by XRD, EDX, Raman spectroscopy and FT­IR. The particle size estimated from the spatial correlation phonon confinement model for the E2 (high) phonon mode. The applied approach is believed to be an efficient, and a direct route for the synthesis of a wide range simple and complex oxide nanostructures for novel electro­optical nanodevices.

Integration of functional complex oxide nanomaterials on silicon

The combination of standard wafer-scale semiconductor processing with the properties of functional oxides opens up to innovative and more efficient devices with high value applications that can be produced at large scale. This review uncovers the main strategies that are successfully used to monolithically integrate functional complex oxide thin films and nanostructures on silicon: the chemical solution deposition approach (CSD) and the advanced physical vapor deposition techniques such as oxide molecular beam epitaxy (MBE). Special emphasis will be placed on complex oxide nanostructures epitaxially grown on silicon using the combination of CSD and MBE. Several examples will be exposed, with a particular stress on the control of interfaces and crystallization mechanisms on epitaxial perovskite oxide thin films, nanostructured quartz thin films, and octahedral molecular sieve nanowires. This review enlightens on the potential of complex oxide nanostructures and the combination of both chemical and physical elaboration techniques for novel oxide-based integrated devices.

Shape-controlled synthesis of Mn[sbnd]Co complex oxide nanostructures via a polyol-based precursor route and their catalytic properties

A polyol-based precursor route was developed to synthesize MnCo complex oxide with well-defined morphologies, in which ethylene glycol (EG) was treated with metal acetates in the presence of poly(vinyl-pyrrolidone) (PVP). By varying the reaction temperature, the as-obtained precursor was readily regulated its morphologies, which could vary from nanospheres to hierarchically stacked nanoplates. The initial molar ratio of Mn-acetate to Co-acetate in EG solution played a crucial role in determining the chemical composition of the nano/microstructured precursor. On the basis of the experimental results, a possible growth mechanism for the nanostructured precursor was proposed. MnCo2O4 and CoMn2O4 could be obtained from their precursors without changing the morphologies by a simple calcination procedure. The synthetic methodology appears to be general and promises to provide an entryway into other complex oxide materials with various nano/microstructures. As an example of potential applications, the as-obtained MnCo mixed oxide nano/microstructures were used as catalyst in CO oxidation, and the effect of the morphology and composition on catalytic activities was investigated.

Solution based approaches for the morphology control of BaTiO3 particulates

Within the action COST 539 - ELENA our contribution was aimed at studying solution based approaches for the morphology control of BaTiO3 particulates. Initially, our kinetic analysis and systematic structural and morphological studies, demonstrated that during hydrothermal synthesis from layered titanate nanotubes (TiNTS), BaTiO3 forms via two mechanisms depending on the temperature and time. At low temperatures (90?C), 'wild' type BaTiO3 dendritic particles with cubic structure were formed through a phase boundary topotactic reaction. At higher temperatures and/or for longer times time, the reaction is controlled by a dissolution precipitation mechanism and 'seaweed' type BaTiO3 dendrites are formed. Our results unambiguously elucidated why TiNTs do not routinely act as templates for the formation of 1D BaTiO3. In our subsequent investigations, the effect of additives on the aqueous and hydrothermal synthesis of BaTiO3 was assessed. We reported that although the tested additives influenced the growth of BaTiO3, their behaviour varied; poly(acrylic acid) (PAA) adsorbed on specific crystallographic faces changing the growth kinetics and inducing the oriented attachment of the particles; poly(vinyl pyrrolidone) (PVP), sodium dodecylsulfate (SDS) and hydroxypropylmethylcellulose (HPMC) act as growth inhibitors rather than crystal habit modifiers; and DFructose appeared to increase the activation energy for nucleation, resulting in small crystals (26 nm). Our work clearly indicates that the synthesis of 1D nanostructures of complex oxides by chemical methods is non trivial.

Tuning the Magnetic Domain Structure of Spin-polarized Complex Oxide Nanostructures

AbstractTo successfully incorporate the highly spin-polarized material La0.7Sr0.3MnO3 (LSMO) into spin-based electronic devices it is essential to be able to control and tune the magnetic domain structure. In this work, we geometrically confine epitaxial thin films of LSMO into hexagons to examine the effect of magnetostatic and magnetic anisotropy energies on the domain formation. We find through careful choice of hexagon aspect ratio, crystalline direction, and substrate orientation, we can tune the magnetic domain formation to be single, two, six (flux closure), or other domain configurations.

Patterning-controlled morphology of spatially and dimensionally constrained oxide nanostructures

This letter reports a facile approach for morphologic control of complex oxide nanostructures patterned by “soft” electron beam lithography (soft-eBL). The authors demonstrate fabrication of epitaxial nanofrustum and nanopyramidal morphologies of ferroelectric BaTiO3 and magnetic CoFe2O4 lines, with controlled zig-zag or smooth edges. The dimensional and shape control is achieved by simply tuning the patterning parameters such as resist thickness and patterning directions with respect to underlying substrate orientation. The crystal orientation, element distribution, and piezoelectric behavior of BaTiO3 nanofrustums are evaluated with analytical transmission electron microscopy and piezoresponse force microscopy. It is argued that soft-eBL allows for exquisite control over morphology, shape evolution, and orientation of zero- and one-dimensional nanostructures akin to what has been possible in the past with semiconductor heterostructures by thin film approaches.

Composite-Hydroxide-Mediated Approach for the Synthesis of Nanostructures of Complex Functional-Oxides

We demonstrate a generic approach for the synthesis of single-crystal complex oxide nanostructures of various structure types, such as perovskites, spinels, monoclinic, corundum, CaF(2) structured, tetragonal, and even metal hydroxides. The method is based on a reaction between a metallic salt and a metallic oxide in a solution of composite-hydroxide eutectic at approximately 200 degrees C and normal atmosphere without using an organic dispersant or capping agent. The synthesis technique is cost-effective, one-step, easy to control, and is performed at low temperature and normal atomospheric pressure. The technique can be expanded to many material systems, and it provides a general, simple, convenient, and innovative strategy for the synthesis of nanostructures of complex oxides with important scientific and technological applications in ferroelectricity, ferromagnetism, colossal magnetoresistance, fuel cell, optics, and more.

Nanostructure patterns of piezoelectric and ferroelectric complex oxides with variousshapes, obtained by natural lithography and pulsed laser deposition

A variety of patterns of well-ordered perovskite-type complex oxide nanostructures wereobtained by pulsed laser deposition (PLD) on conductive single crystalline substratesthrough latex sphere monolayer and double-layer masks. Patterns obtained via depositionthrough hexagonally close packed (hcp) latex sphere monolayers include the usual arrays ofhexagonally ordered nanoislands, hexagonal arrays of nanorings of much lower heightcoexisting with hexagonal nanoislands, hcp shell-like structures, and hexagonallyinterconnected nanocrown arrays. The formation of the nanopatterns depends on the ambientgas pressure during PLD, with well-separated nanoislands obtained at low pressures, andinterconnected nanocrowns formed at high pressures. After annealing, the obtainedBaTiO3 and SrBi2Ta2O9 nanopatterns were examined by piezoresponse force microscopy using conductivescanning probes. Under an external biasing electric field, polarization domainreversal was observed in the nanopatterns with deep submicron lateral dimensions.BaTiO3 nanocrowns∼9 nm high areferroelectric, while SrBi2Ta2O9 nanorings of ∼5 nm thickness show at least piezoelectric activity. In addition to the patterns ofhexagonally ordered nanostructure arrays, a pattern of rhombohedrally orderednanostructure arrays was also fabricated by deposition through latex sphere double layers.

Complex oxide nanostructures by pulsed laser deposition through nanostencils

We achieved parallel nanoscale patterning of ferroelectric complex oxides by pulsed laser deposition through a nanostencil (i.e., through a pattern of apertures in a thin free-standing membrane). Ordered arrays of nanostructured barium titanate (BaTiO3) were obtained onto different substrates in a single deposition step, at room temperature, replicating accurately the aperture patterns in the stencil membrane. After a postdeposition annealing treatment, x-ray diffraction pattern showed a nanocrystalline BaTiO3 structure close to the perovskite cubic phase with grains 30–35nm in size. Their local ferroelectric properties were detected using piezoresponse force microscopy.

Three-dimensional heteroepitaxy in self-assembled BaTiO3–CoFe2O4 nanostructures

Self-assembled BaTiO3–CoFe2O4 complex oxide nanostructures have been synthesized by pulsed laser deposition. A single ceramic target with a molar ratio of 0.62BaTiO3–0.38CoFe2O4 was used. Spinel CoFe2O4 and perovskite BaTiO3 phases spontaneously separated during heteroepitaxial growth on a single-crystal SrTiO3(001) substrate. The nanostructures are epitaxial in-plane as well as out-of-plane, with CoFe2O4 nanopillar arrays embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and spacing and nearly circular cross section. As the substrate temperature increases from 750 to 950°C, the average diameter of the pillars increases from ∼9 to ∼70nm.