Electronic and chemical surface properties of Bi2Se3 derived compoundsZipporah Rini Benher
, doctoral dissertation
Keywords: Bismuth selenide derived compounds, surface chemical and electronic properties, topological insulators, single crystals, phase separation, inorganic bulk heterostructures, ix
Bridgman method, X-ray powder diffraction, scanning electron microscopy, photoelectron spectroscopy
Published in RUNG: 16.12.2022; Views: 768; Downloads: 12
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Structural, morphological and chemical properties of metal/topological insulator interfaces : dissertationKatja Ferfolja
, 2021, doctoral dissertation
Abstract: Topological insulators (TIs) represent a new state of matter that possess a different band structure than regular insulators or conductors. They are characterized with a band gap in the bulk and conductive topological states on the surface, which are spin polarized and robust toward contamination or deformation of the surface. Since the intriguing properties of the TIs are localized at the surface, it is important to obtain knowledge of the possible phenomena happening at the interface between TIs and other materials. This is especially true in the case of metals, due to the fact that such interfaces will be present in the majority of foreseen TI applications.
The presented study combines microscopy and spectroscopy techniques for characterization of morphology, stability and chemical interaction at the interface between TI and metals deposited by means of physical vapor deposition. Our research is based on the interface of Bi2Se3 topological insulator with Ag, Ti and Pt – metals that can be encountered in devices or applications predicted to utilize the special properties of topological insulators.
STM and SEM imaging of Ag/Bi2Se3 interface showed that Ag atoms arrange on the surface in the form of islands, whereas significantly bigger agglomerates are found at the surface steps. The interface was found to be unstable in time and resulted in the absorption of the metal into the crystal at room temperature. Evidences of a chemical reaction at the Ag/Bi2Se3 interface are presented, showing that new phases (Ag2Se, AgBiSe2 and metallic Bi) are formed.
Deposition of Ti on Bi2Se3 resulted in different morphologies depending on the film thickness. At a very low coverage (<1 Å) islands are formed. However, the islands growth is hindered before the completion of a full layer due to the occurrence of a chemical reaction. No surface features could be detected by SEM for Ti coverage up to 20 nm. In contrary, when Ti thickness reached 40 nm, compressive stress triggered buckling of the deposited film. XPS analysis revealed that a redox solid-state reaction occurs at the Ti/Bi2Se3 interface at room temperature forming titanium selenides and metallic Bi. The reaction has significant kinetics even at cryogenic temperature of 130 K.
Pt forms a homogenous film over the whole substrate surface, which is stable in time at room temperature. Although the interface of Pt with Bi2Se3 was found to be
less reactive compared to Ag and Ti, an interfacial phase formed upon annealing to ∼90 °C was detected by TEM cross section experiment.
A model for prediction of interfacial reactions between a metal and Bi2Se3 based on the standard reduction potential of the metals and Gibbs free energy for a model reaction is presented. Based on these two values the reaction can be expected to result in the formation of binary and/or ternary selenides and Bi.
Presented work shows on the importance of metal/topological insulator interfaces characterization taking into account the possibility of a chemical reaction with all of its consequences. Results should be considered for future theoretical and applicative studies involving such interfaces as well as for the possible engineering of 2D TI heterostructures.
Keywords: topological insulators, topological surface states, Bi2Se3, thin films, Ag, Ti, Pt, morphology, interfaces, solid-state reaction, metal selenides, reactivity, stability, electron microscopy, dissertations
Published in RUNG: 09.06.2021; Views: 2742; Downloads: 167
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Molecular characterization of free tropospheric aerosol collected at the Pico Mountain Observatory : a case study with a long-range transported biomass burning plumeKatja Džepina
, Claudio Mazzoleni
, Paulo Fialho
, Swarup China
, Bo Zhang
, R. Chris Owen
, D. Helmig
, J. Hueber
, Sumit Kumar
, J. A. Perlinger
, 2015, original scientific article
Abstract: Free tropospheric aerosol was sampled at the
Pico Mountain Observatory located at 2225 m above mean
sea level on Pico Island of the Azores archipelago in the
North Atlantic. The observatory is located ∼ 3900 km east
and downwind of North America, which enables studies
of free tropospheric air transported over long distances.
Aerosol samples collected on filters from June to October
2012 were analyzed to characterize organic carbon, elemental carbon, and inorganic ions. The average ambient concentration of aerosol was 0.9 ± 0.7 µg m−3
. On average, organic aerosol components represent the largest mass fraction of the total measured aerosol (60 ± 51 %), followed by
sulfate (23 ± 28 %), nitrate (13 ± 10 %), chloride (2 ± 3 %),
and elemental carbon (2 ± 2 %). Water-soluble organic matter (WSOM) extracted from two aerosol samples (9/24 and
9/25) collected consecutively during a pollution event were
analyzed using ultrahigh-resolution electrospray ionization
Fourier transform ion cyclotron resonance mass spectrometry. Approximately 4000 molecular formulas were assigned
to each of the mass spectra in the range of m/z 100–1000.
The majority of the assigned molecular formulas had unsaturated structures with CHO and CHNO elemental compositions. FLEXPART retroplume analyses showed the sampled
air masses were very aged (average plume age > 12 days).
These aged aerosol WSOM compounds had an average O /C
ratio of ∼ 0.45, which is relatively low compared to O /C
ratios of other aged aerosol. The increase in aerosol loading during the measurement period of 9/24 was linked to
biomass burning emissions from North America by FLEXPART retroplume analysis and Moderate Resolution Imaging Spectroradiometer (MODIS) fire counts. This was confirmed with biomass burning markers detected in the WSOM
and with the morphology and mixing state of particles as
determined by scanning electron microscopy. The presence
of markers characteristic of aqueous-phase reactions of phenolic species suggests that the aerosol collected at the Pico Mountain Observatory had undergone cloud processing before reaching the site. Finally, the air masses of 9/25 were
more aged and influenced by marine emissions, as indicated
by the presence of organosulfates and other species characteristic of marine aerosol. The change in the air masses for
the two samples was corroborated by the changes in ethane,
propane, and ozone, morphology of particles, as well as by
the FLEXPART retroplume simulations. This paper presents
the first detailed molecular characterization of free tropospheric aged aerosol intercepted at a lower free troposphere
remote location and provides evidence of low oxygenation
after long-range transport. We hypothesize this is a result of
the selective removal of highly aged and polar species during long-range transport, because the aerosol underwent a
combination of atmospheric processes during transport facilitating aqueous-phase removal (e.g., clouds processing) and
fragmentation (e.g., photolysis) of components.
Keywords: organic aerosol, ultrahigh-resolution FT-ICR MS, electron microscopy, remote marine atmosphere, Pico Mountain Observatory
Published in RUNG: 11.04.2021; Views: 1670; Downloads: 0
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Ice cloud formation potential by free tropospheric particles from long-range transport over the Northern Atlantic OceanSwarup China
, Peter A. Alpert
, Bo Zhang
, Simeon K. Schum
, Katja Džepina
, Kendra Wright
, R. Chris Owen
, Paulo Fialho
, Lynn R. Mazzoleni
, Claudio Mazzoleni
, 2017, original scientific article
Abstract: Long-range transported free tropospheric particles can play a significant role on heterogeneous ice nucleation. Using optical and electron microscopy we examine the physicochemical characteristics of ice nucleating particles (INPs). Particles were collected on substrates from the free troposphere at the remote Pico Mountain Observatory in the Azores Islands, after long-range transport and aging over the Atlantic Ocean. We investigate four specific events to study the ice formation potential by the collected particles with different ages and transport patterns. We use single-particle analysis, as well as bulk analysis to characterize particle populations. Both analyses show substantial differences in particle composition between samples from the four events; in addition, single-particle microscopy analysis indicates that most particles are coated
by organic material. The identified INPs contained mixtures of dust, aged sea salt and soot, and organic material acquired either at the source or during transport. The temperature and relative humidity (RH) at which ice formed, varied only by 5% between samples, despite differences in particle composition, sources, and transport patterns. We hypothesize that this small variation in the onset RH may be due to the coating material on the particles. This study underscores and motivates the need to further investigate how long-range transported and atmospherically aged free tropospheric particles impact ice cloud formation.
Keywords: atmospheric aerosols, ice nucleating particles, long-range transport, optical microscopy, electron microscopy, Pico Mountain Observatory
Published in RUNG: 11.04.2021; Views: 1752; Downloads: 131
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Elucidation of Donor:Acceptor Phase Separation in Nonfullerene Organic Solar Cells and Its Implications on Device Performance and Charge Carrier MobilitySebastian F. Hoefler
, Georg Haberfehlner
, Thomas Rath
, Andreas Keilbach
, Mathias Hobisch
, Alexander Dixon
, Egon Pavlica
, Gvido Bratina
, Gerald Kothleitner
, Ferdinand Hofer
, Gregor Trimmel
, 2019, original scientific article
Abstract: In bulk-heterojunction solar cells, the device performance strongly depends on the donor and acceptor properties, the phase separation in the absorber layer, and the formation of a bicontinuous network. While this phase separation is well explored for polymer:fullerene solar cells, only little is known for polymer:nonfullerene acceptor solar cells. The main hurdle in this regard is often the chemical similarity of the conjugated polymer donor and the organic nonfullerene acceptor (NFA), which makes the analysis of the phase separation via atomic force microscopic (AFM) phase images or conventional transmission electron microscopy difficult. In this work, we use the donor polymer PTB7-Th and the small molecule acceptor O-IDTBR as the model system and visualized the phase separation in PTB7-Th:O-IDTBR bulk-heterojunctions with different donor:acceptor ratios via scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) based elemental mapping, which resulted in a good contrast between the donor and the acceptor despite very low differences in the chemical composition. AFM as well as grazing-incidence wide-angle X-ray scattering (GIWAXS) investigations support the electron microscopic data. Furthermore, we elucidate the implications of the phase separation on the device performance as well as charge carrier mobilities in the bulk-heterojunction layers, and a high performance of the solar cells was found over a relatively broad range of polymer domain sizes. This can be related to the larger domain sizes of the acceptor phase with higher amounts of O-IDTBR in the blend, while the polymer donor phase still forms continuous pathways to the electrode, which keeps the hole mobility at a relatively constant level.
Keywords: nanomorphology bulk-heterojunction scanning transmission electron microscopy organic photovoltaics charge carrier mobility
Published in RUNG: 15.01.2020; Views: 2533; Downloads: 83
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Amorphous nanocomposite of polycarbosilanes and aluminum oxideAndraž Mavrič
, 2018, doctoral dissertation
Abstract: This work presents a paradigm for high temperature stabilization of bulk amorphous aluminium oxide. The thermodynamic stabilization is achieved by preparing a nanocomposite, where polymethylsilane dendritic molecules are dispersed in an aluminium hydroxide gel. Upon heat-treatment the gel transforms to the amorphous aluminium oxide that is stable up to 900°C. The dispersion of the macromolecules and their covalent bonding to the alumina matrix induce homogeneously distributed strain fields that keep the alumina amorphous.
The first part of the thesis focuses on the synthesis, characterization and solubility properties of the dendritic polymethylsilane. The polymethylsilane is synthetized by electrochemical polymerization from trichloromethylsilane monomer. The polymerization mechanism, involving a single polymerization pathway, is identified. The polymer growth proceeds through reduction of the monomers to the silyl anions and their addition to the growing polymer.
The solubility of three chemically related but topologically different polysilanes (linear, dendritic and network) were studied by dynamic light scattering. At room temperature the agglomerates in a range from 500 to 1300 nm are present. They undergo de-agglomeration at slightly elevated temperatures of around 40°C. The de-agglomeration results in formation of stable solutions, where a hydrodynamic diameter of the individual polymer molecules was measured to be in a range from 20 to 40 nm.
The obtained diameters of two dendritic polymethylsilane macromolecules, synthesized under different electrolysis conditions, are much larger than the theoretical size estimated for an ideal dendrimer. We determined by 29Si NMR that the reason for this is in a large number of branching irregularities (defects) contained in the molecular structure. Combining the experimental values obtained by DLS and density measurements with a structural model that considers the branching irregularities, it is shown that the inclusion of the defects allows the dendritic polymer to exceed the sterical limitations and form the hyperbranched dendritic structure. The final size depends on a relative amount of the branching defects.
In the second part, the synthetized polymethylsilane molecules were successfully used for the nanocomposite formation. The aluminium hydroxide gel with the dispersed polymethylsilane molecules was prepared as a precursor. Upon heat-treatment it gives the amorphous aluminium oxide stable up to 900°C. The dispersed macromolecules induce homogeneously distributed strain fields that keep the aluminium oxide amorphous during the thermal treatment the dispersed macromolecules covalently bind to the matrix, inducing the interface strain. The amorphous state was confirmed by the presence of penta-coordinated aluminium detected by 27Al NMR and a low bandgap measured by UV-vis absorption spectroscopy.
Keywords: amorphous aluminium oxide, polymethylsilane, nanocomposite, electropolymerization, solubility, agglomeration, de-agglomeration, dendrimer, hyperbranched dendritic structure, dynamic light scattering, thermal analysis, transmission electron microscopy, scanning electron microscopy, X-ray diffraction, infrared spectroscopy, UV-Vis spectroscopy
Published in RUNG: 19.07.2018; Views: 5307; Downloads: 206
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