121. CVD Growth of Molybdenum Diselenide Surface Structures with Tailored MorphologyM. Naeem Sial, Usman Muhammad, Binjie Zheng, Yanan Yu, Andraž Mavrič, Fangzhu Qing, Matjaž Valant, Zhiming M. Wang, 2018, original scientific article Abstract: Controllable atmospheric pressure CVD has been optimized to grow transition metal dichalcogenide MoSe2 with tunable morphology at 750 °C on a silicon substrate with a native oxide layer of 250 nm. Utilizing tetrapotassium perylene-3,4,9,10-tetracarboxylate (PTAS) as a seed promoter and varying the vertical distance between the substrate and the precursor MoO3, different morphologies of MoSe2 were achieved, including 2D triangles, hexagons, 3D pyramids and vertically aligned MoSe2 sheets. We find that the shape of MoSe2 is highly dependent upon the distance h between the substrate and the precursor. The change in the morphology is attributed to the confinement of vapor (MoO3 and Se) precursors and their concentrations due to the change in h. These results are helpful in improving our understanding about the factors which influence the morphology (shape evolution) and also the continuous growth of MoSe2 films.
Keywords: Transition metal dichalcogenides, 2D materials, Seed promotor, Chemical vapor deposition, Molybdenum diselenide
Published in RUNG: 20.08.2018; Views: 3730; Downloads: 14
 Full text (2,68 MB) |
122. High-temperature stabilization of bulk amorphous Al2O3Andraž Mavrič, Mattia Fanetti, Gregor Mali, Matjaž Valant, 2018, original scientific article Abstract: We present a method for high temperature stabilization of bulk amorphous aluminium oxide. The stabilization is achieved by dispersing polysilane dendritic molecules in aluminium hydroxide gel, which upon thermal treatment gives amorphous aluminium oxide stable up to 900 °C. The dispersed macromolecules covalently bind to the alumina matrix and induce homogeneously distributed strain fields that keep the alumina amorphous. The thermal conversion of the precursor system was followed by thermogravimetry with an evolved gas analysis, infrared spectroscopy and 29Si NMR. The amorphous structure of aluminium oxide was confirmed with an X-ray and electron diffraction. Additionally, the amorphous state was supported by presence of penta-coordinated aluminium detected by 27Al NMR and a low bandgap measured by a UV–visible absorption spectroscopy.
Keywords: Amorphous aluminium oxide, Polysilane, Nanocomposite, Stabilization
Published in RUNG: 30.07.2018; Views: 4283; Downloads: 16
Full text (2,67 MB) |
123. 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: 6058; Downloads: 210
Full text (5,07 MB) |
124. Optoelektronske in fotokatalitske lastnosti z vanadijem dopiranega Fe2TeO6Mojca Vrčon Mihelj, 2018, master's thesis Abstract: V tem magistrskem delu sem raziskovala fotokatalitske lastnosti Fe2TeO6 in Fe2-ZTeVZO6 (0z0.7). Za ta sistem sem se odločila, ker ima rutilu sorodno kristalno strukturo, ki predstavlja trikratno osnovno celico rutila, doslej najučinkovitejšega fotokatalizatorja. Fe2TeO6 sem pripravila s sintezo v trdnem, strukturo sem karakterizirala s XRD in SEM. Fotokatalitsko aktivnost sem spremljala z razgradnjo barvila metiloranž v odvisnosti od časa pod UV in vidno svetlobo. Da bi dosegla boljšo fotokatalitsko aktivnost sem se odločila, da v sistemu sistematično zamenjam Fe3+ ione z V3+. S sintezo v trdnem sem pripravila sisteme Fe2-ZTeVZO6 (0z0.7) in jih karakterizirala s XRD, SEM in TEM. Pripravljeni sistemi so bili dvofazni, s kristalno in steklasto amorfno fazo. S homogenizacijo prahov in daljšanjem reakcijskega časa ni bilo možno doseči enofaznega sistema. Kristalna faza je vsebovala vse tri kovinske ione, medtem ko je steklasta amorfna faza vsebovala le kovinski ion vanadija. Kristalna faza je imela strukturo trirutila, intenziteta vrhov v praškovnem difraktogramu je pokazala, da je prišlo do vključitve vanadijevih ionov, vendar ne po izovalentnem mehanizmu. Vključitev vanadijevih ionov v strukturo ni povečalo fotokatalitske aktivnosti pod UV svetlobo.
Keywords: Fe2TeO6, trirutil, vanadij, fotoaktivnost, metiloranž
Published in RUNG: 22.05.2018; Views: 7053; Downloads: 166
Full text (2,43 MB) |
125. Polymer Nanoparticle Sizes from Dynamic Light Scattering and Size Exclusion Chromatography: The Case Study of Polysilanes.Artem Badasyan, Andraž Mavrič, Irena Kralj Cigić, Tim Bencik, Matjaž Valant, 2018, original scientific article Abstract: Dynamic Light Scattering (DLS) and Size Exclusion Chromatography (SEC) are among the most popular methods for determining polymer sizes in solution. Taking dendritic and network polysilanes as a group of least soluble polymer substances, we critically compare and discuss the difference between nanoparticle sizes, obtained by DLS and SEC. Polymer nanoparticles are typically in poor solution conditions below the theta point and are in globular conformation therefore. Determination of particle sizes in presence of attractive interactions is not a trivial task. The only possibility to measure aggregation-free, a true molecular size of polymer nanoparticles in such regime of solution, is to operate with the dilute solution of globules (below theta point and above the miscibility line). Basing on results of our polysilane measurements, we come to a conclusion that DLS provides more reliable results than SEC for the dilute solution of globules. General implications for the size measurements of polymer nanoparticles in solutions are discussed.
Keywords: Polymer Nanoparticle, Dynamic Light Scattering, Size Exclusion Chromatography, Polysilanes
Published in RUNG: 16.05.2018; Views: 4231; Downloads: 16
Full text (3,17 MB) |
126. |
127. |
128. |
129. |
130. |
