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Title:Structural, morphological and chemical properties of metal/topological insulator interfaces : dissertation
Authors:ID Ferfolja, Katja (Author)
ID Fanetti, Mattia (Mentor) More about this mentor... New window
Files:.pdf Katja_Ferfolja.pdf (26,44 MB)
MD5: 6340D7DF21A05693357CBDE80441BBD3
 
URL http://repozitorij.ung.si/IzpisGradiva.php?id=6515
 
Language:English
Work type:Doctoral dissertation
Typology:2.08 - Doctoral Dissertation
Organization:FPŠ - Graduate School
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
Place of publishing:Nova Gorica
Place of performance:Nova Gorica
Publisher:K. Ferfolja
Year of publishing:2021
Year of performance:2021
Number of pages:VIII, 153 str.
PID:20.500.12556/RUNG-6515 New window
COBISS.SI-ID:65896707 New window
UDC:620.1/.2
NUK URN:URN:SI:UNG:REP:UXYK7PTV
Publication date in RUNG:09.06.2021
Views:6252
Downloads:193
Metadata:XML DC-XML DC-RDF
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FERFOLJA, Katja, 2021, Structural, morphological and chemical properties of metal/topological insulator interfaces : dissertation [online]. Doctoral dissertation. Nova Gorica : K. Ferfolja. [Accessed 23 April 2025]. Retrieved from: https://repozitorij.ung.si/IzpisGradiva.php?lang=eng&id=6515
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Secondary language

Language:Slovenian
Title:Strukturne, morfološke in kemijske lastnosti stičnih ploskev med kovino in topološkim izolatorjem
Abstract:V doktorski disertaciji je predstavljena študija stičnih ploskev med kovino in topološkim izolatorjem (TI). Topološki izolatorji predstavljajo novo vrsto materialov, ki se po svoji elektronsko pasovno strukturi razlikujejo od običajnih izolatorjev ali prevodnikov. Za notranjost materiala je značilen prepovedan pas, medtem ko so na površini prisotna prevodna topološka stanja. Stanja na površini so topološko zaščitena in posledično veliko bolj obstojna v primeru kontaminacije ali deformacije v primerjavi z običajnimi elektronskimi stanji na površini. Poleg tega so topološka stanja na površini spinsko polarizirana, kar preprečuje povratno sipanje elektronov pri transportu. Ker so posebne lastnosti TI lokalizirane na površini, so študije stičnih ploskev med TI in drugimi materiali izrednega pomena. To še posebej velja za primer stične ploskve s kovino, saj je takšen kontakt predviden za večino aplikacij, kjer bi se TI lahko uporabljali. Raziskava zdužuje mikroskopske in spektroskopske metode za karakterizacijo morfologije, stabilnosti in kemijskih interakcij na stični površini med TI in kovinami, ki so nanešene s pomočjo metod fizikalnega nanosa iz parne faze. Raziskava opisuje stično ploskev med topološkim izolatorjem Bi2Se3 ter Ag, Ti in Pt – kovinami, ki jih srečamo v napravah ali drugih aplikacijah, predvidenih za izkoriščanje posebnih lastnosti topoloških izolatorjev. STM in SEM meritve stične ploskve Ag/Bi2Se3 so pokazale, da atomi Ag na ravni površini Bi2Se3 tvorijo skupke, medtem ko se na prehodih med posameznimi plastmi tvorijo veliko večji aglomerati. Stik med Ag in Bi2Se3 se je izkazal kot nestabilen, saj sčasoma pride do absorpcije kovine v substrat. Posledično se morfologija površine spremeni. Zaznane spremembe so posledica kemijske reakcije na stični ploskvi. Uporaba različnih spektroskopskih metod je omogočila določitev produktov, ki nastanejo v reakciji, in sicer Ag2Se, AgBiSe2 in Bi. Morfološka struktura Ti na Bi2Se3 je odvisna od debeline nanešenega filma. Pri nanosu zelo majhne količine Ti (<1 Å) na površino Bi2Se3 nastanejo skupki. V začetku se ti skupki z večanjem debeline nanosa Ti povečujejo, vendar se rast nato ustavi zaradi pojava kemijske reakcije, ki onemogoča nastanek homogenega sloja. SEM analiza nanometrskih filmov Ti (>5 nm) je pokazala enako morfologijo površine kot v primeru čistega Bi2Se3. Pri večjih debelinah nanešenega Ti (okrog 40 nm) pa se je v filmu ustvarila tlačna napetost, ki je sprožila nagubanje in mehanski razpad filma. XPS iii analiza je pokazala, da pride na stiku Ti/Bi2Se3 do kemijske redoks reakcije v trdnem, pri kateri se tvorijo titanovi selenidi in kovinski Bi. Reakcija poteče hitro tudi pri nizki temperaturi (130 K). Pt tvori homogen film čez celotno površino Bi2Se3. Film ne kaže vidnih morfoloških sprememb v času po hranjenju pri sobnih pogojih. Čeprav se je stična ploskev med Pt in Bi2Se3 izkazala za veliko manj reaktivno v primerjavi z Ag in Ti, je segrevanje vzorca na temperaturo ∼90 °C privedlo do nastanka nove faze, ki je bila detektirana s pomočjo TEM. Na osnovi standardnega redukcijskega potenciala in Gibbsove proste energije za teoretično reakcijo smo izdelali model za napovedovanje reakcije na stični ploskvi med kovino in Bi2Se3. Glede na omenjeni vrednosti lahko predvidimo ali bo kemijska reakcija privedla do nastanka binarnih in/ali ternarnih selenidov in Bi. Predstavljeno delo kaže na pomen karakterizacije stičnih ploskev kovina/topološki izolator z ozirom na nastanek kemijske reakcije in posledic, ki jih le- ta povzroči. Dobljene rezultate je smiselno upoštevati za nadaljne teoretične in eksperimentalne študije, ki vključujejo podobne stične površine, kot tudi za možnost načrtovanja in sintezo 2D heterostruktur s topološkimi izolatorji.
Keywords:topološki izolatorji, topološka površinska stanja, Bi2Se3, tanki filmi, Ag, Ti, Pt, morfologija, stične ploskve, reakcija v trdnem, kovinski selenidi, reaktivnost, stabilnost, elektronska mikroskopija, disertacije


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