M. Gajdardziska-Josifovska

Accurate measurements of mean inner potential of crystal wedges using digital electron holograms

Abstract The mean inner potential of a solid is a fundamental property of the material and depends on both composition and structure. By using cleaved crystal wedges of known angle, combined with dogital recording of off-axis electron holograms and with theoretical calculations of dynamical effects, the mean inner potential of Si (9.26±0.08 V), MgO (13.01±0.08 V), GaAs (14.53±0.17 V) and PbS (17.19±0.12 V) is measured with high accuracy of about 1%. Dynamical contributions to the phase of the transmitted beam are found by Bloch wave calculations to be less than 5% when the crystal wedges are titled away from zone-axis orientations and from major Kikuchi bands. The accuracy of the present method is a factor of 3 better than previously achieved by reflection high-energy electron diffraction and electron interferometry. The major causes of uncertainty were specimen imperfections and errors in phase measurement and magnification calibration.

Absolute measurement of normalized thickness, t/λi, from off-axis electron holography

Abstract A simple method is described for determination of the quantity t/λi from off-axis electron holograms, where t is the local thickness and λi is the mean-free-path for inelastic scattering of high energy electrons. The method uses the energy-filtered amplitude reconstructed from a hologram and, when applied to samples with well characterized geometry, allows measurement of the inelastic mean-free-path. Measured values of 71 ± 5 nm for MgO and 92 ± 7 nm for Si for 100 keV beam energy compare favorably with calculated and experimental values from electron-energy-loss spectroscopy. Differences between the two techniques for determining t/λi and the utility of the holographic method are briefly discussed.

Indium-doped SnO2 nanoparticle–graphene nanohybrids: simple one-pot synthesis and their selective detection of NO2

We demonstrate novel nanohybrids of indium- and ruthenium-doped SnO2 nanoparticles (NPs) on a reduced graphene oxide (RGO) surface prepared using a simple one-pot method at a relatively low temperature. The size of the doped SnO2 NPs on the RGO is as small as 2–3 nm with uniform distribution. We find that the introduction of dopants facilitates the NP nucleation on graphene oxide. The gas sensing responses of the resulting nanohybrids demonstrate that the addition of indium in SnO2 significantly enhances the sensitivity to NO2 compared with RGO–SnO2. The sensor also shows excellent selectivity to NO2 when other common gases such as NH3, H2, CO and H2S, are present. The sensing mechanism responsible for the superior sensitivity and selectivity of the nanohybrids is also discussed.

THERMODYNAMICS OF TETRAGONAL ZIRCONIA FORMATION IN A NANOLAMINATE FILM

Zirconia–alumina transformation‐toughening nanolaminates were fabricated by reactive sputter deposition. The average crystallite size and volume fraction of each zirconia polymorph were determined by x‐ray diffraction. The volume fraction of tetragonal zirconia, the phase necessary for transformation toughening, was found to strongly depend upon the zirconia layer thickness. An end‐point thermodynamics model involving hemispherical cap zirconia crystallites was developed to explain this phenomenon. In excellent agreement with experimental results, the model predicts that unity volume fraction of tetragonal zirconia is produced in the nanolaminate when the zirconia layer thickness is less than the radius at which a growing zirconia crystallite spontaneously transforms to the monoclinic phase.

Evidence of nanocrystalline semiconducting graphene monoxide during thermal reduction of graphene oxide in vacuum.

As silicon-based electronics are reaching the nanosize limits of the semiconductor roadmap, carbon-based nanoelectronics has become a rapidly growing field, with great interest in tuning the properties of carbon-based materials. Chemical functionalization is a proposed route, but syntheses of graphene oxide (G-O) produce disordered, nonstoichiometric materials with poor electronic properties. We report synthesis of an ordered, stoichiometric, solid-state carbon oxide that has never been observed in nature and coexists with graphene. Formation of this material, graphene monoxide (GMO), is achieved by annealing multilayered G-O. Our results indicate that the resulting thermally reduced G-O (TRG-O) consists of a two-dimensional nanocrystalline phase segregation: unoxidized graphitic regions are separated from highly oxidized regions of GMO. GMO has a quasi-hexagonal unit cell, an unusually high 1:1 O:C ratio, and a calculated direct band gap of ∼0.9 eV.

Tetragonal zirconia growth by nanolaminate formation

Multilayer films of polycrystalline zirconia and amorphous alumina were grown by reactive sputter deposition and characterized using x‐ray diffraction and high resolution electron microscopy. We demonstrate that the layer spacing can be scaled to insure nanosize crystallites in the zirconia layer. The result is that nanolaminates with a high volume fraction of retained tetragonal zirconia are produced, independent of deposition parameters and without the addition of a stabilizing dopant.

Botanical iron minerals correlation between nanocrystal structure and modes of biological self-assembly

Plants, like animals, use and store iron in their cells. Yet, the composition and structure of the plant-iron biominerals, constituting the inorganic cores of phytoferritin, have remained unknown. Transmission electron microscopy (TEM) and diffraction studies of subcellular phytoferritin, extracted from disrupted plant cells, indicate that phytoferritin occurs as crystalline magnetite (Fe 3 O 4 ) ϵ-Fe 2 O 3 , and hematite (α-Fe 2 O 3 ), with typical sizes of single crystallites in the 1 — 50 nm range and agglomerate grain sizes up to 4 μm. The three-dimensional agglomerates are built with different biomineral nanocrystals in three distinct modes of biological self-assembly: 1) ordered magnetite; 2) semi-ordered mixture of magnetite and ϵFe 2 O 3 ; and 3) random hematite. These self-assemblies correspond to prior TEM reports of crystalline, paracrystalline and amorphous phytoferritin arrangements in sectioned cell samples. A fourth plant-iron biomineral, tentatively assigned as calcium ferrate hexahydrate, has a morphology and diffraction patterns distinct from the phytoferritin aggregates. We do not attribute the plant iron observed in this study to be the results of atmospheric pollution.

Morphology of MgO(111) surfaces: artifacts associated with the faceting of polar oxide surfaces into neutral surfaces

Abstract It is shown by optical, atomic force, scanning and transmission electron microscopies that the MgO(111) surface does not facet into neutral (100) type planes upon annealing. The triangular pyramidal pits that Henrich [V.E. Henrich Surf. Sci., 57 (1976) 385] associates with the polar (111) surface faceting into (100) planes turn out to be artifacts of the acid etch used in the sample preparation process. It is shown that the pyramidal pits have facets sloped at 10.8±2.8° with respect to the (111) surface, corresponding to sets of vicinal surfaces. The pit edges are confirmed by transmission electron microscopy to be along the three equivalent 〈110〉-type directions.

Transmission electron microscopy study of zirconia–alumina nanolaminates grown by reactive sputter deposition. Part I: zirconia nanocrystallite growth morphology

Abstract Pure zirconia films and zirconia–alumina nanolaminate films grown by reactive sputter deposition are studied by high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). The phase composition and morphology associated with zirconia crystallite growth are investigated by examining films containing zirconia layers of varying thickness. These studies, performed at room temperature, suggest that the zirconia crystallites initially grow in the tetragonal phase to a critical size of 6.0±0.2 nm, in agreement with a value of 6.2 nm predicted by end-point thermodynamics. Past the critical size, incorporation of additional zirconia molecules into the zirconia layers is accomplished predominantly by transformation of the growing crystallites to the monoclinic phase, and less frequently by deposition of amorphous zirconia. Transformation to the monoclinic phase is accompanied by a highly faulted intermediary phase. The subsequent growth behavior of monoclinic crystallites is consistent with a three-dimensional interface-controlled, diffusion-limited growth process with a growth exponent between 3 and 4. Nanoindentation measurements of nanolaminates with 5-nm thick zirconia layers give a hardness of ~8 GPa for the upper strata where the morphology of the tetragonal zirconia layers contains an intrinsic roughness. The hardness increases to ~10 GPa closer to the substrate where the laminar morphology is more pronounced. Youngs modulus is between 156 and 195 GPa for these same nanolaminates.

Applications of electron holography to the study of interfaces

Abstract Electron holography has been applied to a variety of layered structures to assess its usefulness for supplying information about composition profiles across heterogeneous interfaces. The phase of the exit-surface electron wave, which to a first approximation is dependent upon the mean inner potential and the specimen thickness, was extracted from electron holograms acquired from suitable cross-sectional multilayer specimens. Line profiles from the reconstructed phase images were analyzed to obtain information about interface diffuseness and layer width with a spatial resolution of about 5 A. Using spatial averaging parallel to the interface, increased measurement precision was obtainable in some special cases. Differences in interdiffusion widths between Mo-Si and Si-Mo interfaces in an Mo/Si multilayer structure were confirmed, and the width of the amorphous layer at Si 3 N 4 grain boundaries was measured to be about 12 A. It was concluded that off-axis electron holography represented a useful complementary technique for characterizing interfaces.