Electron Diffraction Studies Research Articles

Structural analysis of the intermetallic surface compoundCePt5/Pt(111)

We report on a detailed low-energy electron diffraction (LEED) and low-temperature scanning tunneling microscopy (STM) study of the intermetallic surface compound ${\mathrm{CePt}}_{5}$ on Pt(111). Depending on the thickness we observe various diffraction patterns and superstructures. In the low-thickness regime a slightly compressed $(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure is aligned along the $\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ direction of the Pt(111) substrate. STM reveals another, much larger superstructure with a periodicity of ($9.02\ifmmode\pm\else\textpm\fi{}0.45$) nm presumably responsible for the strongly broadened LEED spots. At about 3 unit cells (u.c.) the surface is dominated by a $(3\sqrt{3}\ifmmode\times\else\texttimes\fi{}3\sqrt{3})R{30}^{\ensuremath{\circ}}$ pattern as revealed by LEED satellites and Fourier-transformed high-resolution STM images. It is interpreted as a moir\'e pattern between the film and the substrate. We precisely determine the superstructure of the intermetallic film to $(\frac{10}{9}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\frac{10}{9}\sqrt{3})R{30}^{\ensuremath{\circ}}$ with respect to the Pt(111) substrate. Above 3 u.c. the satellites progressively disappear. A model is developed that consistently describes this thickness-dependent transition. For ${\mathrm{CePt}}_{5}$ films with a thickness between 6 and 11 u.c. the lattice of the compressed $(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure rotates back into the substrate's $\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ directions.

Electron diffraction study of the phase formation of Tl-Fe-Se and kinetics of phase transformations of films TlFeSe2

The conditions of phase formation in the Tl-Fe-Se system and the crystallization kinetics of amorphous TlFeSe2 films are investigated by electron-diffraction structural analysis and kinematic electronography. It is shown that the crystallization of amorphous TlFeSe2 films is described by the analytical expression of kinetic phase-transformation curves Vt = V0[1 − exp(ktm)]. The growth dimensionality during the crystallization of amorphous TlFeSe2 equal to three and the activation energies of nucleation and crystal growth are determined.

Charge-order melting in charge-disproportionated perovskite CeCu3Fe4O12.

A novel quadruple perovskite oxide CeCu3Fe4O12 has been synthesized under high-pressure and high-temperature conditions of 15 GPa and 1473 K. (57)Fe Mössbauer spectroscopy displays a charge disproportionation transition of 4Fe(3.5+) → 3Fe(3+) + Fe(5+) below ∼270 K, whereas hard X-ray photoemission and soft X-ray absorption spectroscopy measurements confirm that the Ce and Cu valences are retained at approximately +4 and +2, respectively, over the entire temperature range measured. Electron and X-ray diffraction studies reveal that the body-centered cubic symmetry (space group Im3̅, No. 204) is retained at temperatures as low as 100 K, indicating the absence of any types of charge-ordering in the charge-disproportionated CeCu3Fe4O12 phase. The magnetic susceptibility and neutron powder diffraction data illustrate that the antiferromagnetic ordering of Fe ions is predominant in the charge-disproportionated CeCu3Fe4O12 phase. These findings suggest that CeCu3Fe4O12 undergoes a new type of electronic phase in the ACu3Fe4O12 series and that the melting of the charge-ordering in CeCu3Fe4O12 is caused by the substantial decrease in the Fe valence and the resulting large deviation from the ideal abundance ratio of Fe(3+):Fe(5+) = 1:1 for rock-salt-type charge-ordering.

Manganese oxalate nanorods as ballistic modifier for composite solid propellants

Rod-shaped nanostructures of manganese oxalate (MnC2O4) were synthesized via mild thermal precipitation and aging process. Chemical composition of the MnC2O4 nanorods was confirmed using Fourier transform infra-red (FTIR) spectroscopy and energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) and selected area electron diffraction (SAED) studies revealed the crystal structure. Field emission scanning electron microscopy (FE-SEM) imaging and high resolution transmission electron microscopy (HR-TEM) were employed to study the structural features of the nanorods. The MnC2O4 nanorods were found to be efficient ballistic modifier for the burning rate enhancement of composite solid propellants (CSPs). Thermal analysis using TGA-DSC showed that MnC2O4 nanorods sensitized the thermal decomposition of ammonium perchlorate (AP) and the CSPs. Controlled thermal decomposition of the MnC2O4 nanorods resulted in the formation of managanese oxide nanoparticles with mesoporosity. A plausible mechanism for the burning rate enhancement using MnC2O4 nanorods was proposed.

Study of magnetic, dielectric and magnetodielectric properties of BaTiO3/Fe3O4 core/shell nanocomposite

Multiferroic systems fashioned by involvement of a ferroelectric phase and a ferromagnetic phase have received significant attention because of their wide possibilities for tailoring properties. However, many interrogations persist about how the structure and chemistry of interface mediates magnetodielectric activities. In this study, we have explored the magnetic, dielectric and magnetodielectric behavior of BaTiO3/Fe3O4 core/shell nanocomposite synthesized by combined co-precipitation cum sonochemical method. The phase formation of the core/shell naonocomposite was confirmed independently from X-ray diffraction and selected area electron diffraction studies. The microstructure of the sample was studied by transmission electron microscopy and High resolution TEM revealing a good mixing of two phases in the form of core (BaTiO3)/shell (Fe3O4). Magnetic property of the sample was investigated using a vibrating sample magnetometer (VSM) at room temperature. The magnetic study reveals that the synthesized core/shell nanocompositeis ferromagnetic with saturation magnetization of ~58 emu/g. The dielectric constant of the core/shell nanocomposite, measured as a function of temperature, at frequencies 1, 10, 100, 500 and 1 MHz combines almost the characteristics of BaTiO3 and Fe3O4 in accordance with the Maxwell–Garnett equation. However, near 125 K a clear hump is observed probably due to characteristic Verwey transition (TV) in Fe3O4. The dielectric constant also shows dispersion behavior with increasing frequency due to Maxwell–Wagner interfacial polarization. Furthermore, the shrinkagein dielectric constant (at 20 K) on the application of 1 T external magnetic field depicts the possible coupling between the dielectric (BaTiO3) and the magnetic (Fe3O4) phases of BaTiO3/Fe3O4 core/shell nanocomposite.

Augmented photocatalytic activity and luminescence response of Tb3+ doped nanoscale titania systems

The present work reports on the effect of Tb3+ doping on the luminescence and photocatalytic performance of nano-structured titania derived through a sol-gel route. X-ray diffraction patterns have revealed the existence of anatase phase with and without Tb3+ doping and with an improved orientation factor along (004) and (200) planes. Transmission electron microscopy and selective area electron diffraction studies, while exhibiting ample poly-crystallinity feature, have predicted an average particle size of ∼9 nm and ∼6 nm for the un-doped and 5% Tb3+ doped nano-titania samples; respectively. Apart from emissions accompanied by different types of defects, Tb3+ related transitions, such as, 5D3 → 7F5, 5D3 → 7F4, and 5D4 → 7F6 were identified in the photoluminescence spectra. Brunauer-Emmett-Teller surface area analysis, as carried out on a Tb3+ doped nano-titania system, has demonstrated a more-open hysteretic loop owing to significant difference of N2 adsorption/desorption rates. The photocatalytic activity of nano-titania, as evaluated from the nature of degradation of methyl orange under UV illumination, exhibited the highest efficiency for a Tb3+ doping level of 2.5%. The augmented photocatalytic degradation has also been discussed in the light of a model based on pseudo first-order kinetics.

In-situ observation and atomic resolution imaging of the ion irradiation induced amorphisation of graphene.

Ion irradiation has been observed to induce a macroscopic flattening and in-plane shrinkage of graphene sheets without a complete loss of crystallinity. Electron diffraction studies performed during simultaneous in-situ ion irradiation have allowed identification of the fluence at which the graphene sheet loses long-range order. This approach has facilitated complementary ex-situ investigations, allowing the first atomic resolution scanning transmission electron microscopy images of ion-irradiation induced graphene defect structures together with quantitative analysis of defect densities using Raman spectroscopy.

Interconversion of α-Fe2O3 and Fe3O4 Thin Films: Mechanisms, Morphology, and Evidence for Unexpected Substrate Participation

The reversible transformations of thin magnetite (Fe3O4) and hematite (α-Fe2O3) films grown on Pt(111) and Ag(111) single crystals as support have been investigated by a combined low energy electron microscopy (LEEM) and low-energy electron diffraction (LEED) study. The conversions were driven by oxidation, annealing in ultrahigh vacuum (UHV), or Fe deposition with subsequent annealing. As expected, the oxidation of a Fe3O4 film yielded an α-Fe2O3 structure. Unexpectedly, the annealing in UHV also led to a transformation from Fe3O4 into α-Fe2O3, but only if Pt(111) was used as substrate. In contrast, on a Ag(111) substrate the inverse reaction, a slow transformation from α-Fe2O3 into Fe3O4, was observed, as expected for oxygen desorption. Fe deposition on α-Fe2O3 and subsequent annealing in UHV transformed the film into Fe3O4. As the most probable explanation we propose that the UHV conversion on Pt(111) supports proceeds by Fe cation diffusion through the film and Fe atom dissolution in the substrate, decreasing the Fe concentration within the iron oxide film. This process is not possible for a Ag(111) substrate. The interconversions, which were best observable in mixed films containing domains of both oxides, occurred by growth of one domain type with well-defined boundaries and growth rates.

Relaxor behaviour in BaBi4Ti4O15 ceramics fabricated using the powders obtained by mechanochemically assisted synthesis route

Mechanochemically activated reactants were found to facilitate the synthesis of fine powders comprising 200–400 nm range crystallites of BaBi4Ti4O15 at a significantly lower temperature (700 °C) than that of solid-state reaction route. Reactants (CaCO3, Bi2O3 and TiO2) in stoichiometric ratio were ball milled for 48 h to obtain homogeneous mixture. The evolution of the BaBi4Ti4O15 phase was systematically followed using X-ray powder diffraction (XRD) technique. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to probe its structural and microstructural details. The electron diffraction studies established the presence of correlated octahedral rotations and associated long-range polar ordering. High-resolution TEM imaging nevertheless revealed structural inhomogeneities leading to intergrowth defects. Dense BaBi4Ti4O15 ceramics with an average grain size of 0.9 μm were fabricated using mechanochemically assisted synthesized powders at relatively low temperature (1000 °C). The effect of grain size on the dielectric and relaxor behaviour of BaBi4Ti4O15 ceramics was investigated. Fine-grained ceramics (average grain size ~0.9 μm) showed higher diffusion in phase transition, lower temperature of phase transition, lower Vogel-Fulcher freezing temperature and higher activation energy for the polarization reversal than those for coarse-grained ceramics (average grain size ~7 μm) fabricated via the conventional solid-state reaction route.

Synthesis and crystal structures of gold nanowires with Gemini surfactants as directing agents.

The preparation of crystalline gold nanowires (NWs) by using gemini surfactants as directing agents through a three-step seed-mediated method is reported. Unlike the nanorods with relatively low aspect ratios (typically below 20) obtained by using cetyltrimethylammonium bromide as a directing agent, the NWs obtained in this investigation can reach up to 4.4 μm, and the largest aspect ratio is calculated to be 210. For this, each of seven different gemini surfactants are utilized as directing agents, and the length and/or aspect ratio of the NWs can be tuned by varying the hydrocarbon chain lengths of the gemini surfactants. Both single and twinned crystalline structures are elucidated by selected-area electron diffraction and high-resolution transmission electron microscopy studies. The use of gemini surfactants not only advances the synthesis of gold nanostructures, but improves the understanding of the growth mechanism for seed-mediated growth.

Induced work function changes at Mg-doped MgO/Ag(001) interfaces: Combined Auger electron diffraction and density functional study

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL_23 L_23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atoms incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2- ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.

Structural transition at 360 K in the CaFe₅O₇ ferrite: toward a new charge ordering distribution.

An efficient synthesis route is proposed to obtain single phase powder ceramic of CaFe5O7. This complex structure can be described as an intergrowth between one CaFe2O4 unit and n = 3 slices of FeO Wüstite-type structure. A detailed structural study has been carried out at room temperature combining transmission electron microscopy (TEM) observations (ED, HREM), scanning transmission electron microscopy (STEM-HAADF), and X-ray diffraction data. The analysis of these data has revealed an unexpected supercell with a monoclinic symmetry. From the hkl conditions deduced from the electron diffraction study and the analysis of X-ray diffraction data by simulated annealing, a structural model considering the centrosymmetric P2₁/m setting can be proposed. In addition the first magnetic and electrical transport measurements are reported showing a sharp peak in magnetic susceptibility and a strong localization around 360 K, associated to a structural change from monoclinic setting to orthorhombic one.

Electron diffraction study of ordering in the Er0.715Ca0.285F2.715 tysonite phase

The structure of Er0.715Ca0.285F2.715 crystals as grown from the melt (without heat treatment) has been studied by electron microscopy. It is found that the structure is inhomogeneous, consisting of an ordered tysonite matrix with the hexagonal crystal structure (supercell reflections exhibit the vector 1/8〈203〉 in the units of small tysonite cell), in which crystallites of the other laminar phase with layers ∼10 A thick are incorporated.

Ultraviolet GaN photodetectors on Si via oxide buffer heterostructures with integrated short period oxide-based distributed Bragg reflectors and leakage suppressing metal-oxide-semiconductor contacts

Based on a novel double step oxide buffer heterostructure approach for GaN integration on Si, we present an optimized Metal-Semiconductor-Metal (MSM)-based Ultraviolet (UV) GaN photodetector system with integrated short-period (oxide/Si) Distributed Bragg Reflector (DBR) and leakage suppressing Metal-Oxide-Semiconductor (MOS) electrode contacts. In terms of structural properties, it is demonstrated by in-situ reflection high energy electron diffraction and transmission electron microscopy-energy dispersive x-ray studies that the DBR heterostructure layers grow with high thickness homogeneity and sharp interface structures sufficient for UV applications; only minor Si diffusion into the Y2O3 films is detected under the applied thermal growth budget. As revealed by comparative high resolution x-ray diffraction studies on GaN/oxide buffer/Si systems with and without DBR systems, the final GaN layer structure quality is not significantly influenced by the growth of the integrated DBR heterostructure. In terms of optoelectronic properties, it is demonstrated that—with respect to the basic GaN/oxide/Si system without DBR—the insertion of (a) the DBR heterostructures and (b) dark current suppressing MOS contacts enhances the photoresponsivity below the GaN band-gap related UV cut-off energy by almost up to two orders of magnitude. Given the in-situ oxide passivation capability of grown GaN surfaces and the one order of magnitude lower number of superlattice layers in case of higher refractive index contrast (oxide/Si) systems with respect to classical III-N DBR superlattices, virtual GaN substrates on Si via functional oxide buffer systems are thus a promising robust approach for future GaN-based UV detector technologies.

In-situ high-resolution low energy electron diffraction study of strain relaxation in heteroepitaxy of Bi(111) on Si(001): Interplay of strain state, misfit dislocation array and lattice parameter

The relief of lattice mismatch-induced strain in Bi(111) on Si(001) heteroepitaxial system was investigated in real time as the Bi film relaxes, by means of high resolution low-energy electron diffraction (LEED). The inherent lattice mismatch of 2.5% at room temperature is accommodated through the formation of an ordered misfit dislocation array confined to the interface. The strain fields of the dislocations cause a periodic height undulation of the surface in the sub-Ångström regime, which is observed through spot splitting in LEED. From a simultaneous measurement of the position of the first-order LEED spots, which corresponds to the lattice parameter of the film, and of the separation of satellite spots, which corresponds to the ordering of the dislocation array, the evolution of the strain state during annealing of a 6nm Bi film was determined. The strain is solely relaxed by full edge-type dislocations arranged in the ordered array at the interface. From the remaining strain of ε=0.6% the critical thickness for generation of misfit dislocations under equilibrium conditions can be derived.

A new Li-Mn-Ge-O phase solved by combining 3D electron diffraction methods

Pyroxene compounds are a common form of natural minerals and have been studied as such for a long time. More recently the research on quasi-one-dimensional magnetic and multiferroïc materials has renewed the interest in pyroxenes of the stoichiometry AMX2O6 (A = alkali metal, M = transition metal, X = Si or Ge), since the magnetic M3+ ions form chains. Chemical substitution on the A and M sites can change the magnetic coupling along these chains making this system a rich field for the exploration of new phases of interesting magnetic properties [1]. In this work we report the discovery of a new phase in the Li-Mn-Ge-O system. A HP-HT solid state reaction was performed on a mixture of nominal stoichiometry LiMnGe2O6 during 1 h at a temperature of 8500C and a pressure of 3 GPa in a belt press. Powder X-ray diffraction yielded a diffractogram that could not be indexed by known phases of this system. An electron diffraction study in a transmission electron microscope was conducted in order to identify any unknown phases. In the case of structures that promise interesting properties a more targeted synthesis can then be undertaken. For the purpose of this work, we studied one of several unknown phases in the powder in more detail. From standard selected area electron diffraction the unit cell was determined to be triclinic with cell parameters a = 2.51 nm, b = 1.30 nm, c = 1.30 nm, α = 96.00, β = 98.80and γ = 80.80. No comparable unit cell could be found in the databases neither in this system nor with different A, M or X ions. Intensities were recorded by in-zone axis precession electron diffraction and by electron diffraction tomography. Combining the data from both methods yielded the first model of the structure which we will present here.

Electron diffraction study of the sillenites Bi12SiO20, Bi25FeO39 and Bi25InO39: Evidence of short-range ordering of oxygen-vacancies in the trivalent sillenites

We present an electron diffraction study of three sillenites, Bi12SiO20, Bi25FeO39, and Bi25InO39 synthesized using the solid-state method. We explore a hypothesis, inspired by optical studies in the literature, that suggests that trivalent sillenites have additional disorder not present in the tetravalent compounds. Electron diffraction patterns of Bi25FeO39 and Bi25InO39 show streaks that confirm deviations from the ideal sillenite structure. Multi-slice simulations of electron-diffraction patterns are presented for different perturbations to the sillenite structure - partial substitution of the M site by Bi3+, random and ordered oxygen-vacancies, and a frozen-phonon model. Although comparison of experimental data to simulations cannot be conclusive, we consider the streaks as evidence of short-range ordered oxygen-vacancies.

Dominantly epitaxial growth of graphene on Ni (1 1 1) substrate

Abstract Graphene was grown on a Ni (1 1 1) thin layer, used as a substrate. The Ni layer itself was grown on single crystal sapphire (0 0 0 1). Carbon was deposited by chemical vapor deposition using a mixture of methane, argon and hydrogen at atmospheric pressure implementing a constant gas flow (4.8–5 l/min) varying both the gas composition and the deposition temperature (900–980 °C) and cooling rate (8–16 °C/min) in the different experiments. Formation of uninterruptedly grown epitaxial single layer graphene was observed over the Ni (1 1 1) thin film substrate. Epitaxial growth was proven through STM measurements. Electron diffraction studies, also confirmed by STM, demonstrated that only one dominant orientation exists in the graphene, both results providing evidence of the epitaxial growth. On top of the, continuous, large area graphene flakes were also observed with sizes varying between 10 nm and 10 μm. Most of the top flakes are turbostratically related to the continuous underlying epitaxial graphene layer. The formation of the graphene layer with constant dominant orientation was observed over millimeter wide areas. Large areas (≈20–40 μm in diameter) of continuous, epitaxial graphene, free of additional deposits and flakes were obtained for the best set of growth parameters.

Inhibition of restriction endonucleases by biofunctionalized silver nanoparticles: An in vitro study

Ecofriendly synthesis of metal nanoparticles and their unique properties hold promise for innovative therapeutic applications. In the present study, rapid, novel, low cost method for synthesis of silver nanoparticles (AgNPs) using aqueous leaves extract of medicinal plant Euphorbia heterophylla (E. heterophylla) is reported. AgNPs synthesized by plant extract exhibited absorption peak at 422nm in UV–vis spectroscopy. Transmission electron microscopy micrographs revealed presence of AgNPs with average size of 13nm with grain and triangular shapes. Fourier Transform Infrared Spectroscopy analysis revealed role of leaves metabolites as reducing and capping agent in synthesis of AgNPs. X ray diffraction pattern and selected area electron diffraction study confirmed crystalline nature of AgNPs. Inhibition of DNA cutting activity of restriction endonucleases (EcoRI, HindIII and BamHI) by biofunctionalized AgNPs in agarose gel electrophoresis is first time reported in the present study. This potential of AgNPs can be useful for enhancing effectiveness of phage therapy by acting as synergistic cocktail with bacteriophages to kill bacteria via inhibition of their restriction endonucleases.

Enhancement of thermal properties of polyacrylonitrile by reinforcement of Mg-Al layered double hydroxide

The in situ polymerization technique was used for synthesis of polyacrylonitrile nanocomposites (PAN/LDH) by reinforcement of different concentrations of Mg-Al layered double hydroxide (LDH). Molecular interaction between Mg/Al LDH and PAN matrix was studied by Fourier transform infrared spectroscopy. The morphology of PAN/LDH nanocomposite was investigated by field emission scanning electron microscopy. Energy dispersive spectroscopy of nanocomposites were noticed to establish the elemental composition of the composite. High resolution transmission electron microscopy was used to study the nanostructural distribution of LDH with PAN matrix, whereas crystalline patterns were explained by selected area electron diffraction studies. The thermal properties of nanocomposites were studied by thermo-gravimetric analysis. It was found that, the thermal property of PAN was substantially enhanced due to strong interfacial adhesion of Mg-Al-LDH with PAN matrix. Further, it was noticed that the thermal stability of PAN/LDH nanocomposites were increased with increasing concentration of LDH. POLYM. COMPOS., 2014. © 2014 Society of Plastics Engineers