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2. Electronic and chemical surface properties of Bi2Se3 derived compoundsZipporah Rini Benher, doktorska disertacija Ključne besede: 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
Objavljeno v RUNG: 16.12.2022; Ogledov: 459; Prenosov: 6
 Celotno besedilo (9,56 MB) |
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5. Hot-carrier and optical-phonon ultrafast dynamics in the topological insulator Bi2Te3 upon iron deposition on its surfaceM Weis, K Balin, T Sobol, A Ciavardini, G Vaudel, V Juvè, B Arnaud, B Ressel, M Stupar, K.C. Prince, G De Ninno, P Ruello, J Szade, 2021, izvirni znanstveni članek Opis: This paper presents a complete study of electronic structures and photoexcited carrier dynamics in topological insulators capped with iron and iron oxide. We combine static and time-resolved angle-resolved photoemission spectroscopies (ARPES, TR-ARPES) with time-resolved optical methods (transient optical reflectivity and transmission). Both single crystal and thin films of Bi2Te3 are studied. We show that monolayers of iron and iron oxide significantly affect the electronic band structure at the interface by shifting the Fermi level into the conduction band, which we explain by a band bending effect, and is confirmed by in situ XPS measurements
Ključne besede: time resolved spectroscopies, topological insulators, interfaces
Objavljeno v RUNG: 13.12.2021; Ogledov: 1123; Prenosov: 20
Celotno besedilo (2,90 MB) |
6. Room-temperature solid-state reaction at the Ag/Bi[sub]2Se[sub]3 interfaceKatja Ferfolja, Mattia Fanetti, Iuliia Mikulska, Sandra Gardonio, Matjaž Valant, 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
Ključne besede: topological insulators, topological surface states, solid-state reaction, TI/metal interface
Objavljeno v RUNG: 20.08.2021; Ogledov: 1236; Prenosov: 0
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7. Structural, morphological and chemical properties of metal/topological insulator interfaces : dissertationKatja 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
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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.
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 v RUNG: 09.06.2021; Ogledov: 2337; Prenosov: 160
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8. Chemical Instability of an Interface between Silver and Bi2Se3 Topological Insulator at Room TemperatureKatja Ferfolja, Matjaž Valant, Iuliia Mikulska, Sandra Gardonio, Mattia Fanetti, 2018, izvirni znanstveni članek Opis: Understanding an interaction at an interface between a topological insulator and a metal is of critical importance when designing electronic and spintronic devices or when such systems are used in catalysis. In this paper, we report on a chemical instability of the interface between Bi2Se3 and Ag studied by X-ray powder diffraction and electron microscopy. We present strong experimental evidence of a redox solid-state reaction occurring at the interface with kinetics that is significant already at room temperature. The reaction yields Ag2Se, AgBiSe2, and Bi. The unexpected room-temperature chemical instability of the interface should be considered for all future theoretical and applicative studies involving the interface between Bi2Se3 and Ag.
Ključne besede: topological insulators, Ag, thin metal films, interfaces, redox reaction
Objavljeno v RUNG: 17.06.2020; Ogledov: 2182; Prenosov: 0
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10. Looking for a topological insulator in the tetradymite familyZipporah Rini Benher, Sandra Gardonio, Mattia Fanetti, P. M. Sheverdyaeva, Paolo Moras, Matjaž Valant, 2019, objavljeni povzetek znanstvenega prispevka na konferenci Opis: Materials that are topological insulators (TI) manifest a novel state for their electrons. They possess topological surface states that are not destroyed by the presence of non-magnetic impurities on their surfaces. This unique property lies in the bulk band structure and it is typically found in narrow gap semiconductor with strong spin-orbit coupling.
Bi2Se3 and Bi2Te3 belong to the class of compounds called tetradymites and are considered as the 3D-prototypical TI materials. However, these compounds are not usually insulators but have metallic bulk conductivity as a consequence of intrinsic defect doping: vacancies and anti-site defects. For these reasons, it is difficult to electrically gate these materials for the manipulation and control of charge carriers for realizing devices. This led to the search for other topological materials, which might have better insulating behavior in their bulk.
Theoretical studies have pointed out that ternary variants of the Bi2Se3 and Bi2Te3, such as Bi2Te2Se, Bi2Te2S, Bi2Se2S Sb2Te2Se and Sb2Te2S, should be stable TIs and potentially offer a chemical way to control TI behavior, in particular by lowering native doping. Among the cited ternary compounds, Bi2Se2S should manifest a genuine topological spin-transport regime hosting an isolated Dirac cone with the Dirac point in the gap as well. However, it has been poorly studied from the TI experimental perspective.
Therefore, to uncover the full potential of the predicted topological electronic properties of the Bi-Se-S system, in this presentation we will revisit the crystallographic and electronic structure of Bi2Se3-Bi2S3 solid solutions. The combined use of bulk and surface sensitive techniques such as X-ray diffraction (XRD), low energy electron diffraction (LEED), scanning electron microscopy (SEM) with Energy Dispersive X-ray spectroscopy (EDX) and X-ray photoemission spectroscopy (XPS) was applied to analyze single crystal samples grown by us. The quality of the single crystals was suitable for rigorous measurement of the electronic properties by means of Angle Resolved Photoemission Spectroscopy. We unambiguously showed that within a certain solid solution range, the single crystals of Bi-Se-S have a rombohedral structure with the topological surface states as theoretically predicted.
Ključne besede: topological insulators, ternary tetradymite, electronic properties.
Objavljeno v RUNG: 19.12.2019; Ogledov: 2996; Prenosov: 0
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