Andriy Lotnyk

Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance

An ultra lightweight carbon microtube material called Aerographite is synthesized by a novel single-step chemical vapor deposition synthesis based on ZnO networks, which is presently the lightest known material with a density smaller than μg/cm(3). Despite its low density, the hierarchical design leads to remarkable mechanical, electrical, and optical properties. The first experiments with Aerographite electrodes confirm its applicability.

Defect chemistry of oxide nanomaterials with high surface area: ordered mesoporous thin films of the oxygen storage catalyst CeO2-ZrO2.

Herein we report the electrical transport properties of a series of ordered mesoporous ceria-zirconia (CexZr1-xO2, referred to as mp-CZO) thin films with both a cubic structure of (17±2) nm diameter pores and nanocrystalline walls. Samples over the whole range of composition, including bare CeO2 and ZrO2, were fabricated by templating strategies using the large diblock copolymer KLE as the structure-directing agent. Both the nanoscale structure and the chemical composition of the mesoporous materials were analyzed by a combination of scanning and transmission electron microscopy, grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The total conductivity as a function of the film composition, temperature, and oxygen partial pressure was measured using impedance spectroscopy. The mesoporous solid solutions of CeO2-ZrO2 prepared in this work showed a higher stability against thermal ripening than both binary oxides, making them ideal model systems to study both the charge transport properties and the oxygen storage at elevated temperatures. We find that the redox properties of nanocrystalline mp-CZO thin films differ significantly from those of bulk CZO materials reported in the literature and, therefore, propose a defect chemical model of surface regions.

Superposition twinning supported by texture in ZnO nanospikes

The morphology and real structure of wurtzite-type ZnO nanospikes grown by the recently introduced flame transport synthesis have been examined by means of advanced transmission electron microscopy (TEM). The rapid synthesis produces nanospikes showing a well defined texture which restricts TEM experiments to a preferred viewing direction of [2 {\overline 1}{\overline 1}3]. Forced by the specific morphology, all of the observed nanospikes show a complicated superposition of twinned domains as an intrinsic real structural feature. The high-resolution contrasts are characterized by lamellar fringes parallel to the (1 {\overline 1} 0 {\overline 1}) planes, and the quasi-kinematic diffraction patterns contain satellite peaks based on multiple scattering. All these phenomena can be interpreted by comparison of experimental and simulated data relying on a supercell approach.

Layered Hydrazinium Titanate: Advanced Reductive Adsorbent and Chemical Toolkit for Design of Titanium Dioxide Nanomaterials

LHT-9, a layered hydrazinium titanate with an interlayer spacing of ~9 Å, is a new nanohybrid compound combining the redox functionality of hydrazine, the ion-exchange properties of layered titanate, the large surface area of quasi-two-dimensional crystallites, surface Brønsted acidity, and the occurrence of surface titanyl bonds. LHT-9, ideally formulated as (N(2)H(5))(1/2)Ti(1.87)O(4), relates to a family of lepidocrocite-type titanates. It possesses a high uptake capacity of ~50 elements of the periodic table. Irreversibility of reductive adsorption allows LHT-9 to be used for cumulative extraction of reducible moieties (noble metals, chromate, mercury, etc.) from industrial solutions and wastewaters. Unlike sodium titanates that do not tolerate an acidic environment, LHT-9 is capable of uptake of transition metals and lanthanides at pH > 3. Adsorption products loaded with the desired elements retain their layered structures and can be used as precursors for tailored titanium dioxide nanomaterials. In this respect, the uptake of metal ions by LHT-9 can be considered as a method complementary to electrostatic self-assembly deposition (ESD) and layer-by-layer self-assembly (LBL) techniques. LHT-9 is readily synthesized in one step by a mild fluoride route involving hydrazine-induced hydrolysis of hexafluorotitanic acid under near-ambient conditions.

Focused high- and low-energy ion milling for TEM specimen preparation

Abstract For atomic-resolution aberration-corrected (Cs-corrected) scanning transmission electron microscopy (STEM) the quality of prepared TEM specimens is of crucial importance. High-energy focused gallium ion beam milling (FIB) is widely used for the production of TEM lamella. However, the specimens after conventional FIB preparation are often still too thick. In addition, damage and amorphization of the TEM specimen surface during the milling process occur. In order to overcome these disadvantages, low-energy Ar ion milling of FIB lamellae can be applied. In this work, we focus on TEM specimen preparation of different thin films (GaN, Ge 2 Sb 2 Te 5 , TiO 2 ) and interface structures (GaN/6H-SiC, SrTiO 3 /TiO 2 , Ge 2 Sb 2 Te 5 /Si) using a combination of FIB with a focused low-energy Ar ion polishing. The results show that this combination enables the routine preparation of high quality TEM lamellae with a smooth surface and uniform thickness, even at the interface region between two different materials and over a lateral range of several micrometres. The prepared lamellae exhibit less surface damage and are well suited for atomic-resolution Cs-corrected STEM/TEM imaging at medium and low accelerating voltages. These results are in a good agreement with Monte Carlo simulations performed by the Stopping and Range of Ions in Matter (SRIM) software.

Crystallization of Ge2Sb2Te5 thin films by nano- and femtosecond single laser pulse irradiation

The amorphous to crystalline phase transformation of Ge2Sb2Te5 (GST) films by UV nanosecond (ns) and femtosecond (fs) single laser pulse irradiation at the same wavelength is compared. Detailed structural information about the phase transformation is collected by x-ray diffraction and high resolution transmission electron microscopy (TEM). The threshold fluences to induce crystallization are determined for both pulse lengths. A large difference between ns and fs pulse irradiation was found regarding the grain size distribution and morphology of the crystallized films. For fs single pulse irradiated GST thin films, columnar grains with a diameter of 20 to 60 nm were obtained as evidenced by cross-sectional TEM analysis. The local atomic arrangement was investigated by high-resolution Cs-corrected scanning TEM. Neither tetrahedral nor off-octahedral positions of Ge-atoms could be observed in the largely defect-free grains. A high optical reflectivity contrast (~25%) between amorphous and completely crystallized GST films was achieved by fs laser irradiation induced at fluences between 13 and 16 mJ/cm2 and by ns laser irradiation induced at fluences between 67 and 130 mJ/cm2. Finally, the fluence dependent increase of the reflectivity is discussed in terms of each photon involved into the crystallization process for ns and fs pulses, respectively.

Artificial Single Variant Martensite in Freestanding Fe70Pd30 Films Obtained by Coherent Epitaxial Growth

2010 WILEY-VCH Verlag Gmb Microactuators and sensors based on magnetic shape-memory (MSM) alloys will benefit from the large strain close to 10% obtained in these materials. These strains exceed the values obtainable by magnetostriction or piezoelectricity by more than one order of magnitude. Thus, they can be used directly for most applications, avoiding additional complications of mechanical amplification. As the highest strains to-date are obtained in bulk single crystals, the use of epitaxial films is most promising for microsystems, owing to their single-crystal-like microstructure. With reduced actuator size, however, the influence of interfaces, and in particular of oxidation, becomes more important. Though the prototype Ni2MnGa system is relatively inert, an oxide surface layer may hinder the martensitic transformation in thin films. For the Fe70Pd30 system [8] oxidation is expected not to be critical due to the high content of a noble element. In Fe70Pd30 the martensitic transformation occurs around room temperature (RT). First reports indicate that epitaxial growth of thin Fe70Pd30 films can be obtained already at RT. For epitaxial growth of the NiMnGa system, a minimum temperature of 350 8C is required. Recently it was shown that epitaxial growth of Fe70Pd30 is possible on various metallic buffers. Due to coherent growth, huge tetragonal distortions were stabilized in 50 nm thick films, covering most of the Bain transformation path from face-centered cubic (fcc) to bodycentered cubic (bcc) structure. Here, we show how this approach can be extended to obtain freestanding films of micrometer thickness, thus fulfilling both key requirements for the integration into microsystems as well as prerequisites for MSM films, that is, martensitic, ferromagnetic at RT, freestanding, and single-crystalline-like. In addition, our experiments reveal that the transformation behavior in these films differs from in the bulk. While this topic has been an extensive playground for theory, experiments are rare, since a detailed analysis often requires epitaxial films. Recently, experiments on epitaxial films, for example, have revealed a variant selection by the rigid interface to the substrate or by reduction of the magnetic stray field energy in freestanding films. The present experiments are more fundamental since they show that circumventing the forward martensitic transformation by forming the martensitic structure directly at RT hinders the nucleation of the reverse transformation to austenite. This remarkable suppression of the transformation not only gives a better understanding of the martensitic transformation but also opens innovative routes for microsensors. Films of Fe70Pd30, 1.2mm thick, were deposited with a low deposition rate of 0.024 nms 1 at 30W sputtering power in a magnetron sputtering system on Au-buffered MgO(001) oriented, epi-polished substrates. The crystal structure of the Fe70Pd30 films was analyzed by four-circle and temperature-dependent two-circle X-ray diffraction (XRD), where x and w denote the tilt and rotational angles, respectively. The u–2u scans (see Fig. 1a) show the 200 Au reflection of the buffer layer (2u1⁄4 44.448) as well as the Fe70Pd30 002 reflection (57.388). Assuming a constant volume of the Fe70Pd30 unit cell compared to the cubic austenite, [15] the lattice parameters a and c were calculated to be 0.287 nm and 0.321 nm ( 0.001 nm), respectively, which constitutes a c/a ratio of 1.12. Temperature-dependent XRD measurements showed no change in the crystal structure in the accessible temperature range between 150 and 375K. The pole figure of the 101 reflection reveals a four-fold symmetry (Fig. 1b). The maximum intensity is obtained at an average of 47.278 at w1⁄4 458. The peak in the w direction is rather sharp with a small full width at half maximum (FWHM), indicating well-oriented growth of the body-centered tetragonal (bct) unit cell rotated by 458compared to the edges of the MgO cell. The larger FWHM in the x direction indicates relaxation of the lattice. There are no indications that twinning has occurred in the film. The sample for transmission electron microscopy (TEM) investigations was prepared by focused ion beam (FIB) lift-out

Van der Waals interfacial bonding and intermixing in GeTe-Sb2Te3-based superlattices

Interfacial phase change memory (iPCM) based on GeTe and Sb2Te3 superlattices (SLs) is an emerging contender for non-volatile data storage applications. A detailed knowledge of the atomic structure of these materials is crucial for further development of SLs and for a better understanding of the resistivity switching characteristics of iPCM devices. In this work, crystalline GeTe-Sb2Te3-based SLs, produced by pulsed laser deposition onto a Si(111) substrate at temperatures lower than in previous studies, are analyzed by advanced scanning transmission electron microscopy. The results reveal the formation of Ge-rich Ge(x+y)Sb(2–y)Tez building blocks with specific numbers of ordered Ge cation layers (between 1 and 5) and disordered cation layers (4) for z = 6–10, as well as intermixed cation layers for z = 5, within the SLs. The G Ge(x+y)Sb(2–y)Tez units are separated from the Sb2Te3 building blocks by van der Waals gaps. In particular, the interlayer bonding is promoted by the formation of outermost cation layers consisting of intermixed GeSb within the building blocks adjacent to the van der Waals gaps. The Ge(x+y)Sb(2–y)Tez units with z > 5 retain metastable crystal structures with two-dimensional bonding within the SLs. The present study shed new light on the possible configurations of the building units that can be formed during the synthesis of GeTe-Sb2Te3-based iPCM materials. In addition, a possible switching mechanism active in iPCM materials is discussed.

An aberration-corrected STEM study of structural defects in epitaxial GaN thin films grown by ion beam assisted MBE.

Ion-beam assisted molecular-beam epitaxy was used for direct growth of epitaxial GaN thin films on super-polished 6H-SiC(0001) substrates. The GaN films with different film thicknesses were studied using reflection high energy electron diffraction, X-ray diffraction, cathodoluminescence and primarily aberration-corrected scanning transmission electron microscopy techniques. Special attention was devoted to the microstructural characterization of GaN thin films and the GaN-SiC interface on the atomic scale. The results show a variety of defect types in the GaN thin films and at the GaN-SiC interface. A high crystalline quality of the produced hexagonal GaN thin films was demonstrated. The gained results are discussed.

Fe–Pd thin films as a model system for self-organized exchange coupled nanomagnets

In equilibrium the Fe–Pd system on the iron rich side of the phase diagram demixes into Fe and L10-ordered FePd. Here, we examine the suitability of the demixing process for self-organized formation of exchange coupled thin film magnets. In this way the benefit of the high magnetization of Fe is combined with the high magnetocrystalline anisotropy of FePd. By using combinatorial methods the influence of composition and thickness on structure, microstructure, and magnetic properties is analyzed. Experiments show the different thermodynamic and kinetic conditions required for demixing and ordering. In particular, for nanostructures the interface energy during demixing must be considered.