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Crystal habit modification of Cu(II) isonicotinate-N-oxide complexes using gel phase crystallisation
Jonathan Steed, Žygimantas Drabavičius, Katja Ferfolja, Dipankar Ghosh, Krishna Kumar Damodaran, 20, izvirni znanstveni članek

Opis: We report the crystallisation of three forms of copper(II) isonicotinate-N-oxide complex and their phase interconversion via solvent-mediated crystal to crystal transformation. The different forms of copper complex have been isolated and characterised by single crystal X-ray diffraction. Gel phase crystallisation performed in hydrogels, low molecular weight gels and gels of tailored gelator showed crystal habit modification. Crystallisation in aqueous ethanol resulted in concomitant formation of blue (form-I) and green (form-II/IV) crystals while use of low molecular weight gel results in selective crystallization of the blue form-I under identical conditions. Comparison of gel phase and the solution state crystallisation in various solvent compositions reveals that the blue form-I is the thermodynamically stable form under ambient conditions.
Najdeno v: osebi
Ključne besede: copper(II) complex, gel phase crystallisation, crystal habit modification, isonicotinate-N-oxide, X-ray diffraction
Objavljeno: 08.11.2018; Ogledov: 1731; Prenosov: 0
.pdf Polno besedilo (968,24 KB)

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Interfacial reaction, morphology and growth mode of metals on topological insulator surfaces
Sandra Gardonio, Mattia Fanetti, Katja Ferfolja, Matjaž Valant, 2019, objavljeni povzetek znanstvenega prispevka na konferenci

Najdeno v: osebi
Ključne besede: topological insulators, surfaces, metals
Objavljeno: 19.12.2019; Ogledov: 1608; Prenosov: 0
.pdf Polno besedilo (18,68 MB)

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Structural, morphological and chemical properties of metal/topological insulator interfaces
Katja Ferfolja, 2021, doktorska disertacija

Opis: 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.
Najdeno v: osebi
Ključne besede: topological insulators, topological surface states, Bi2Se3, thin films, Ag, Ti, Pt, morphology, interfaces, solid-state reaction, metal selenides, reactivity, stability, electron microscopy, dissertations
Objavljeno: 09.06.2021; Ogledov: 716; Prenosov: 48
URL Polno besedilo (0,00 KB)
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Room-temperature solid-state reaction at the Ag/Bi[sub]2Se[sub]3 interface
Matjaž Valant, Iuliia Mikulska, Sandra Gardonio, Katja Ferfolja, Mattia Fanetti, 2017, objavljeni povzetek znanstvenega prispevka na konferenci

Opis: 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
Najdeno v: osebi
Ključne besede: topological insulators, topological surface states, solid-state reaction, TI/metal interface
Objavljeno: 20.08.2021; Ogledov: 231; Prenosov: 0
.pdf Polno besedilo (1,11 MB)

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Chemical (in)stability of interfaces between different metals and Bi[sub]2Se[sub]3 topological insulator
Sandra Gardonio, Matjaž Valant, Katja Ferfolja, Mattia Fanetti, 2018, objavljeni povzetek znanstvenega prispevka na konferenci

Opis: In recent years a classification of materials based on their topological order gained popularity due to the discovery of materials with special topological character – topological insulators (TI). TI have different band structure than regular insulators or conductors. They are characterized by a band gap in the bulk of the material, but at the surface they possess conductive topological surface states (TSS) that cross the Fermi level. TSS are a consequence of the non-trivial bulk band structure and have properties that differ from ordinary surface states. They are robust toward contamination and deformation of the surface. Additionally, they are also spin polarized, which means that an electron spin is locked to a crystal momentum and, therefore, backscattering during transport is suppressed [1]. Due to their specific properties the TI could be used in fields of spintronics, quantum computing and catalysis [2]. The investigation of the interfaces between metals and the TI has not been given much attention even though its characterization is interesting from fundamental physics and applicative point of view. (In)stability of the contacts with metal electrodes, in a form of a chemical reaction or diffusion, has to be taken into account since it can affect the transport properties of the material or increase the contact resistance. Our research is dedicated to the study of the metal/TI interfaces, in particular to Bi2Se3 with deposited metals that are relevant for electrical contacts (Au, Ag, Pt, Cr, Ti). The thermal and chemical stability of the interfaces are of fundamental importance for understanding the contact behavior, therefore, we focused our work to the characterization of these properties. The metal/TI interfaces are investigated mainly with an electron microscopy (SEM, TEM, STM), EDX microanalysis and XRD. Our previous studies showed that the interface between Bi2Se3, and Ag deposited either chemically or from a vapor phase, results in the formation of new phases already at room temperature [3]. On the contrary, Au deposited on the Bi2Se3 surface shows very limited reactivity and is stable at RT, but diffusion and coalescence of the metal are observed starting from 100 °C [4]. In this contribution, we will present further characterization on the evolution of the Ag/Bi2Se3 and Au/Bi2Se3 interfaces, show preliminary results about recently investigated systems (Pt/Bi2Se3, Ti/Bi2Se3) and compare the thermal and chemical stability of the systems under investigation.
Najdeno v: osebi
Ključne besede: thermal lens spectrometry, photothermal beam deflection spectroscopy, dye remediation, photothermal technique, photocatalytic degradation, reactive blue 19, TiO2 modification
Objavljeno: 20.08.2021; Ogledov: 323; Prenosov: 0
.pdf Polno besedilo (100,42 KB)

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Chemical (in)stability of an interface between metals and Bi[sub]2Se[sub]3 topological insulator
Sandra Gardonio, Matjaž Valant, Katja Ferfolja, Mattia Fanetti, 2019, objavljeni povzetek znanstvenega prispevka na konferenci

Opis: Our research is dedicated to a study of an interface between a Bi2Se3 topological insulator (TI) and various metals due to the essential need for providing a metal contact for devices. The main objective is to characterize structural and chemical properties at the interface, where the electronic properties of the TI can be affected. The structure of the interface and processes happening at it are investigated by microscopy (SEM, TEM, STM) and spectroscopy techniques (EDX, XPS). The research started with the noble metals: Ag, Au and Pt. A good stability was observed for Au and Pt, whereas Ag reacted with Bi2Se3 already at room temperature, producing Ag2Se and AgBiSe2 phase. Interface stability was also checked at high temperature and results showed that the Au coating undergoes a coalescence process starting from 100 °C whereas the interface with Pt does not show any change at least up to 350 °C. At present we are focused on the interface with Ti, a metal which is regularly used as an adhesive layer in electrical contacts. At low coverage (<30 nm) Ti forms an extremely flat film, smoother than Au, Ag or Pt. At higher coverage the film undergoes buckle delamination, likely induced by stress release. The observed morphology indicates that a chemical interaction leads to the growth of the initial smooth Ti epitaxial film. Se interdiffusion and formation of interfacial TixSey phase is envisaged, as suggested from preliminary TEM observations of the interface structure. The presented results show the importance of the processes happening at the interface, especially solid-state chemical reactions, which are often neglected in the study of systems with metal/TI interfaces. Such instability has to be taken into account since the produced phases can affect transport properties of the material, increase a contact resistance or affect functionality of devices.
Najdeno v: osebi
Ključne besede: Bi2Se3, interface, topological insulator, chemical instability
Objavljeno: 20.08.2021; Ogledov: 245; Prenosov: 1
.pdf Polno besedilo (133,34 KB)
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10.
Au and Ag on the Bi2Se3(0001) Surface: Experimental Electronic and Physical Properties
Sandra Gardonio, Mattia Fanetti, Katja Ferfolja, Matjaž Valant, objavljeni povzetek znanstvenega prispevka na konferenci

Opis: Binary bismuth chalcogenides, Bi2Se3 and Bi2Te3, have been extensively studied as reference topological insulators (TIs). These materials are bulk insulators with topological surface states (TSS) crossing the Fermi level. In contrast to conventional surface states of metals, the TSS are extremely robust against local modifications at the surface, such as adsorbed adatoms, localized defects or changes in the surface termination. This aspect makes the TIs attractive for applications in spintronics, plasmonics, quantum computing and catalysis. A theoretical model of charge transport by the TI surface states predicts that the TSS survive, provided that bonding at the metal/TI interface is weak. Ab-initio calculations have been done to understand the electronic properties of Au, Ni, Pt, Pd and graphene layers in a contact with Bi2Se3. These calculations showed that for Au and graphene the spin-momentum locking of TSS is maintained at the interface. In another theoretical study, Ag and Au thin layers on Bi2Se3 have been predicted to show a large Rashba splitting and a high spin polarization of the Ag quantum wells, providing a great potential for development of the spintronic devices. Finally, the calculations have foreseen that the presence of the robust TSS affects the adsorption properties of metals (Au bi-layer and clusters of Au, Ag, Cu, Pt, and Pd) supported on TI, in some cases resulting in the enhancement of the catalytic processes. Despite the fundamental importance of the metal/TI interfaces and a number of theoretical studies predicting exotic interfacial phenomena, the experimental knowledge about the metals on the TI surfaces is surprisingly limited, especially concerning combined study of morphology, growth mode, electronic and chemical properties. In order to exploit the predicted physical properties of such systems, it is especially important to extend the study above the diluted coverage regime and to understand what is the growth morphology of the metal on the TI surface, to what extent the metal overlayer interacts with the TI substrate, how the TSS change with the presence of the metal overlayer and what is the reactivity of the system at the different stages of the overlayer growth. Within this frame, we present a comprehensive surface sensitive study, of Au and Ag on Bi2Se3 by means of ARPES, XPS, SEM, LEED and XRD. The obtained results allow us to discuss the relation between electronic and physical properties at two of the most important model metal/TI interfaces
Najdeno v: osebi
Ključne besede: topological insulator, electronic properties, synchrotron radiation
Objavljeno: 27.06.2019; Ogledov: 1629; Prenosov: 0
.pdf Polno besedilo (5,72 MB)

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