Iryna Andrusenko

Structure analysis of titanate nanorods by automated electron diffraction tomography.

A hitherto unknown phase of sodium titanate, NaTi(3)O(6)(OH)·2H(2)O, was identified as the intermediate species in the synthesis of TiO(2) nanorods. This new phase, prepared as nanorods, was investigated by electron diffraction, X-ray powder diffraction, thermogravimetric analysis and high-resolution transmission electron microscopy. The structure was determined ab initio using electron diffraction data collected by the recently developed automated diffraction tomography technique. NaTi(3)O(6)(OH)·2H(2)O crystallizes in the monoclinic space group C2/m. Corrugated layers of corner- and edge-sharing distorted TiO(6) octahedra are intercalated with Na(+) and water of crystallization. The nanorods are typically affected by pervasive defects, such as mutual layer shifts, that produce diffraction streaks along c*. In addition, edge dislocations were observed in HRTEM images.

Artificial granularity in two-dimensional arrays of nanodots fabricated by focused-electron-beam-induced deposition

We have prepared 2D arrays of nanodots embedded in an insulating matrix by means of focused-electron-beam-induced deposition using the W(CO)(6) precursor. By varying the deposition parameters, i.e. the electron beam current and energy and the raster constant, we obtain an artificial granular material with tunable electrical properties. The analysis of the temperature dependence of the conductivity and of the current-voltage characteristic suggests that the transport mechanism is governed by electron tunneling between artificial grains. In order to understand the nature of the granularity and thus the microstructural origin of the electronic transport behavior, we perform TEM and micro-Raman investigations. Independent of the deposition parameters, TEM measurements show that the dots are constituted of amorphous tungsten carbide clusters embedded in an amorphous carbonaceous matrix. Micro-Raman spectra show two peaks, around 690 and 860 cm(-1) associated with the W-C stretching modes. Higher frequency peaks give information on the composition of the matrix. In particular, we measure a peak at about 1290 cm(-1), which is associated with sp(3) carbon bonds. Furthermore we detect the so-called D and G peaks, at about 1350 and 1560 cm(-1), associated with the vibration modes of the sp(2) carbon bonds. The analysis of the position of the peaks and of their relative intensity suggests that the composition of the matrix is between nanocrystalline graphite and amorphous carbon.

Structural insights into M2O-Al2O3-WO3 (M = Na, K) system by electron diffraction tomography.

The M2O-Al2O3-WO3 (M = alkaline metals) system has attracted the attention of the scientific community because some of its members showed potential applications as single crystalline media for tunable solid-state lasers. These materials behave as promising laser host materials due to their high and continuous transparency in the wide range of the near-IR region. A systematic investigation of these phases is nonetheless hampered because it is impossible to produce large crystals and only in a few cases a pure synthetic product can be achieved. Despite substantial advances in X-ray powder diffraction methods, structure investigation on nanoscale is still challenging, especially when the sample is polycrystalline and the structures are affected by pseudo-symmetry. Electron diffraction has the advantage of collecting data from single nanoscopic crystals, but it is frequently limited by incompleteness and dynamical effects. Automated diffraction tomography (ADT) recently emerged as an alternative approach able to collect more complete three-dimensional electron diffraction data and at the same time to significantly reduce dynamical scattering. ADT data have been shown to be suitable for ab initio structure solution of phases with large cell parameters, and for detecting pseudo-symmetry that was undetected in X-ray powder data. In this work we present the structure investigation of two hitherto undetermined compounds, K5Al(W3O11)2 and NaAl(WO4)2, by a combination of electron diffraction tomography and precession electron diffraction. We also stress how electron diffraction tomography can be used to obtain direct information about symmetry and pseudo-symmetry for nanocrystalline phases, even when available only in polyphasic mixtures.

Snapshots of calcium carbonate formation – a step by step analysis

Abstract Recent advances in our understanding of CaCO3 nucleation from solution have provoked new and challenging questions. We have studied CaCO3 formation using precipitation by carbonate ester hydrolysis which ensures precipitation from a strictly homogeneous solution state and allows “titrating” carbonate to a solution with a given Ca2+ concentration on a timescale suited for kinetic studies. Nucleation and crystallization were traced by combining dynamic light scattering (DLS) and transmission electron microscopy (TEM). DLS served as in situ technique to identify the nucleation time, to monitor particle size evolution, to discriminate different precipitation mechanisms and to validate reproducibility. TEM snapshots taken during different stages of the precipitation process identified different phases and morphologies. At a high level of supersaturation homogeneous nucleation in solution led to the formation of amorphous CaCO3 particles (Ø≈30 nm), which transformed via vaterite to calcite. Nucleation occurred uniformly in solution which appears to be unique for the CaCO3 system. In the presence of Na-polymethacrylate (Na-PMA), heterogeneous nucleation was suppressed and Ca-polymer aggregates were formed in the prenucleation stage. Beyond a critical threshold supersaturation CaCO3 particles formed in solution outside of these aggregates. The nucleation process resembled that without additive, indicating that Na-PMA exerts only a minor effect on the CaCO3 nucleation. In the postnucleation stage, the polymer led to the formation of extended liquid-like networks, which served as a precursor phase for solid ACC particles that formed alongside the network.

Automated electron diffraction tomography (ADT) and X-ray powder diffraction for structure characterization of layered materials

Layered materials attract increasing attention not only from the point of fundamental crystallography due to their structural diversity, but also due to their numerous industrial applications, for instance as gas storage systems or battery elements. Structural characterization is an essential key for understanding and controlling the desired properties. Layered materials have intrinsic features which turn their structure analysis into a challenging task. Large crystals suitable for singly-crystal X-ray analysis with exactly the same internal structure as fine powders can rarely be grown. Due to the internal construction of layered systems, the crystals predominantly have platelet on needle-like morphology causing problems with preferred orientation during the powder X-ray data collection. The systems are typically polyphasic, thus troubling the powder diffraction data analysis, and finally, they often show disorder in the stacking sequence. All above mentioned points make structural analysis of layered materials a complex mission demanding for a combination of advanced methods. One of the most beneficial approaches to the structure analysis of layered materials is the combination of electron diffraction tomography (Automated Diffraction Tomography – ADT [1-3]) and X-ray powder diffraction method [4-5]. The interaction of these complementary methods allowed gaining structural knowledge on a number of layered systems: sodium-titanates [6], carbonsilicates [7], CSH phases and hydrotalcites. Here, the successes and pitfalls of the methods will be demonstrated.

Structure characterization of bio-mineralogical materials by automated electron diffraction tomography: vaterite and hydroxyapatite

In many geological environments an important fraction of rocks consists of nanocrystalline minerals. While some phases form nanocrystals only in special conditions connected with fast cooling or shock events, others are always confined to the nanosize due to kinetic reasons or because related with weathering and biomineralization processes. Similarly, novel synthetic advanced materials and polluting agents in industrial wastes and atmospheric particulate often appear only in form of nanocrystals. Physical and chemical properties of such phases are strictly related with their crystal structure, which is then fundamental for understanding their potential use and toxicity. Electron diffraction is able to pickup information from single nanoscopic grains, allowing the investigation of minority phases in polyphasic samples and of extremely limited amounts of material. In the last years automated diffraction tomography (ADT)[1-3] emerged as an efficient method for routine ab-initio structure determination of nanocrystalline phases.[4-5] ADT characteristics make it particularly suitable for the investigation of biominerals, normally available only in the form of nanoscopic grains. Biominerals can be directly studied inside biological tissues, and their crystal structure and orientation connected with the contiguous organic environment.[6] In this contribution we will show two case studies where ADT was used for the structure characterization of bio-mineralogical materials: vaterite and hydroxyapatite.

Vaterite structure from electron diffraction data – a definitive answer for an old question?

– a definitive answer for an old question? Iryna Andrusenko, Enrico Mugnaioli, Robert E. Dinnebier, Martin Panthöfer, Wolfgang Tremel, Ute Kolb, Institut für Physikalische Chemie, Johannes Gutenberg Universität, Germany, Max-Planck Institut für Festkörperforschung, Germany, Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg Universität ,Germany E-mail: andrusen@uni-mainz.de