21. Room-temperature solid-state reaction at the Ag/Bi[sub]2Se[sub]3 interfaceKatja Ferfolja, Mattia Fanetti, Iuliia Mikulska, Sandra Gardonio, Matjaž Valant, 2017, published scientific conference contribution abstract Abstract: Topological insulators (TI) are materials that, while having a forbidden bandgap in bulk, are conductors at their surface due to presence of surface-localized electronic states crossing the band gap. [1] TIs are possible because of a time reversal symmetry and spin-orbit coupling, which invert bulk band states in their energy positions and make the bulk band structure non trivial. Consequently, the topological surface states (TSS) emerge on the surface of these materials. Unlike ordinary surface states, TSS cannot be destroyed by contamination or defects on the surface. Additionally, TSS are also spin polarized, which means that when applying current to TI, the current will have a well defined direction of the electron spins.
The topological insulators - a relatively new class of materials - are being widely studied not only from fundamental aspects, but also from their applicative perspectives. It has been predicted that TIs could be used in fields of spintronics, electronics and catalysis. [2,3] Interestingly, only a few studies about metal/TI interfaces have been reported. This is surprising since integration of TI in the applications will often necessitate an interface with the metal, therefore, detailed knowledge on chemistry and electrical conditions at the interface will be required.
In this contribution results on research on the chemistry of the Ag/Bi2Se interface will be presented, in particularonthesolid-statereactionbetweennanoparticles.IthasbeenobservedthatwhenBi2Se3 andAgare put in contact a chemical reaction occurs at the interface, producing AgBiSe2 and Ag2Se. Interestingly, the reaction already occurs at room temperature, which is not usual for solid-state reactions. In literature this reaction has not been properly described. The authors rather described it as intercalation of the silver atoms, which we have disproved and showed that recrystallization of the new phases occurs. [3,4,5] The results will alsobediscussedincomparisonwithothertwoAg/Bi2Se3 systemsunderourinvestigation:i)Agdepositedby achemicalrouteonBi2Se3 nanoflakesandii)AgdepositedfromavapourphaseinvacuumonaBi2Se3.single crystal
Keywords: topological insulators, topological surface states, solid-state reaction, TI/metal interface
Published in RUNG: 20.08.2021; Views: 2240; Downloads: 0
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22. Improved photocatalytic activity of SnO[sub]2-TiO[sub]2 nanocomposite thin films prepared by low-temperature sol-gel methodKsenija Maver, Iztok Arčon, Mattia Fanetti, Samar Al Jitan, Giovanni Palmisano, Matjaž Valant, Urška Lavrenčič Štangar, 2021, original scientific article Abstract: The objective of this research was to investigate how the photocatalytic activity of pure TiO2 can be improved by
SnO2 modification. Different molar ratios of tin to titanium were prepared. The correlation between tin concentration
and structural properties was investigated to explain the mechanism of photocatalytic efficiency and
to optimize the synthesis conditions to obtain enhanced activity of the SnO2-modified TiO2 photocatalysts under
UV-irradiation. The SnO2-modified TiO2 photocatalysts were prepared by a low-temperature sol-gel method
based on organic tin and titanium precursors. The precursors underwent sol-gel reactions separately to form
SnO2-TiO2 sol. The sol-gels were deposited on a glass substrate by a dip-coating technique and dried at 150 ◦C to
obtain the photocatalysts in the form of a thin film. To test the thermal stability of the material, an additional set
of photocatalysts was prepared by calcining the dried samples in air at 500 ◦C. The photocatalytic activity of the
samples was determined by measuring the degradation rate of an azo dye. An increase of up to 30% in the
photocatalytic activity of the air-dried samples was obtained when the TiO2 was modified with the SnO2 in a
concentration range of 0.1–1 mol.%. At higher SnO2 loadings, the photocatalytic activity of the photocatalyst
was reduced compared to the unmodified TiO2. The calcined samples showed an overall reduced photocatalytic
activity compared to the air-dried samples. Various characterization techniques (UV-Vis, XRD, N2-physisorption,
TEM, EDX, SEM, XAS and photoelectrochemical characterization) were used to explain the mechanism for the
enhanced and hindered photocatalytic performances of the SnO2-modified TiO2 photocatalysts. The results
showed that the nanocrystalline cassiterite SnO2 is attached to the TiO2 nanocrystallites through the Sn-O-Ti
bonds. In this way, the coupling of two semiconductors, SnO2 and TiO2, was demonstrated. Compared to
single-phase photocatalysts, the coupling of semiconductors has a beneficial effect on the separation of charge
carriers, which prolongs their lifetime for accessibility to participate in the redox reactions. The maximum increase
in activity of the thin films was achieved in the low concentration range (0.1–1 mol.%), which means that
an optimal ratio and contact of the two phases is achieved for the given physical parameters such as particle size,
shape and specific surface area of the catalyst.
Keywords: photocatalytic activity, Sn-modified TiO2, low-temperature, thin films, XAS analysis
Published in RUNG: 05.07.2021; Views: 2696; Downloads: 0
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23. 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
i
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: 4060; Downloads: 176
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25. Electronic properties of phases in the quasi-binary Bi[sub]2Se[sub]3-Bi[sub]2S[sub]3 systemZipporah Rini Benher, Sandra Gardonio, Mattia Fanetti, Paolo Moras, Asish K. Kundu, Chiara Bigi, Matjaž Valant, 2021, original scientific article Abstract: We explored the properties of the quasi-binary Bi2Se3–Bi2S3 system over a wide compositional range. X-ray diffraction analysis demonstrates that rhombohedral crystals can be synthesized within the solid solution interval 0–22 mol% Bi2S3, while at 33 mol% Bi2S3 only orthorhombic crystals are obtained. Core level photoemission spectroscopy reveals the presence of Bi3+, Se2− and S2− species and the absence of metallic species, thus indicating that S incorporation into Bi2Se3 proceeds prevalently through the substitution of Se with S. Spin- and angle-resolved photoemission spectroscopy shows that topological surface states develop on the surfaces of the Bi2Se3−ySy (y ≤0.66) rhombohedral crystals, in close analogy with the prototypical case of Bi2Se3, while the orthorhombic crystals with higher S content turn out to be trivial semiconductors. Our results connect unambiguously the phase diagram and electronic properties of the Bi2Se3–Bi2S3 system.
Keywords: topological insulator, quasi-binary Bi2Se3-Bi2S3 system, electronic properties
Published in RUNG: 29.03.2021; Views: 2389; Downloads: 0
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26. Stoichiometry and disorder influence over electronic structure in nanostructured VO[sub]x filmsAlessandro D'Elia, S. J. Rezvani, Nicola Zema, F. Zuccaro, Mattia Fanetti, Blaž Belec, B. W. Li, C. W. Zou, Carlo Spezzani, Maurizio Sacchi, Augusto Marcelli, Marcello Coreno, 2021, original scientific article Keywords: nanoparticles, VOx films, electronic structure, nanostructured films
Published in RUNG: 31.01.2021; Views: 2528; Downloads: 0
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27. Ga2Se3 Nanowires via Au-Assisted Heterovalent Exchange Reaction on GaAsFederico Berto, Niloofar Haghighian, Katja Ferfolja, Sandra Gardonio, Mattia Fanetti, Faustino Martelli, Valentina Mussi, Vladimir G. Dubrovskii, Igor V. Shtrom, Alfonso Franciosi, Silvia Rubini, 2020, original scientific article Keywords: nanowires, Gallium arsenide, Gallium Selenide, gold, molecular beam epitaxy
Published in RUNG: 15.12.2020; Views: 2608; Downloads: 0
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28. Household and road dust as indicators of airborne particulate matter elemental composition : dissertationKlemen Teran, 2020, doctoral dissertation Abstract: Household dust (HD) and road dust (RD) are widespread and easily accessible urban sediments, which are influenced by deposition of airborne particulate matter (PM). Since airborne PM is considered to be one of the most important pollutants in urban areas, with significant adverse effects on human health, a better understanding of its elemental composition and dispersion mechanisms is needed. The present study examines whether the HD and RD elemental composition can be used as a quick alternative method for the determination of corresponding PM elemental composition over a selected area.
In summer 2016, HD, RD, and topsoil samples were collected from 249 sampling locations distributed across rural, urban, and industrial areas in Slovenia. The collected samples were sieved for particle diameters below 63 μm and analysed for 53 elements with ICP-MS after aqua regia digestion. SEM/EDS analyses were applied for the determination of characteristic particles at the microlevel. Finally, the elemental composition of HD and RD was compared with the PM10 elemental composition obtained from National Network for Ambient Air Quality Monitoring governed by Slovenian Environmental Agency (ARSO) to determine any potential connection between them.
The results show that HD and RD are considerably enriched with a large set of elements compared to the topsoil. Correlations and factor analyses show that spatial distribution of factor scores in RD revealed strong regional trends, connected with soil resuspension (Al, Ce, Ga, K, La, Li, P, Rb, Sc and Y) or with anthropogenic sources such as traffic (Ag, Bi, Cu, Sb and Sn), steel mill emissions (Cr, Mo, Mn, Ni and W) and construction material decomposition (Ca and Sr). In contrast, HD elemental composition was highly variable between sampling locations. Variability was probably caused by indoor sources, such as smoking (Ce and La), biomass combustion (K, P and Rb), construction material decomposition (Ca and Sr) and residents’ professional occupation: dental care employees (Ag, Au, Pd) and employees in the metal-processing industry (Cr, Mo, Ni).
Among deposited particles in RD, urbanization processes, such as fossil fuel combustion and traffic emissions, including brake pad abrasion and tyre wear, contributed the largest share of particles with anomalous elemental composition. Brake pad abrasion, for instance, contributed Ba-, Cu-, Sn-, and Zn-enriched irregular, angular and tabular particles, while tyre wear produced elongated rubber particles with traces of Ba, Cu, and Zn. RD from urban areas showed significantly higher elemental levels of Ag, Bi, Ca, Cd, Cr, Cu, Hg, Fe, Mo, Nb, Pb, Pt, Sb, Sn, Sr, Ti, Zn, and W in comparison to the rural environment, indicating the strong impact of urbanization on RD elemental composition.
Another important anthropogenic source of deposited particles in RD were steel mills. Strong anomalies of Cr, Mo and Ni were detected in their vicinity. Their elemental levels decreased with distancing from the plan location, reaching urban background levels between 15 and 20 km from the mills. SEM/EDS analyses identified enrichments of Cr, Mn, Mo, Ni, V, and W in spherical particles and particles with partially melted surfaces, which were found only in the proximity of steel mills, indicating their influence of the PM deposition.
Comparison of RD and the corresponding PM10 elemental composition showed that the RD fraction with particle diameters below 63 μm reflects PM10 elemental composition for the last 30 to 90 days for Cr, Cu, Mo and Zn and can be used as a predictor for PM10 elemental levels. This is not true for HD, as indoor particle sources prevail over the deposition of ambient PM10.
Keywords: household dust, road dust, particulate matter, PM10, pollution, Slovenia, steel mills, topsoil, traffic, urbanization
Published in RUNG: 02.12.2020; Views: 3908; Downloads: 187
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