Scanning Tunneling Spectroscopy of correlated electron materials and molecular structures

Midden, Marion van (author), Škarabot, Miha (mentor), Zupanič, Erik (co-mentor)

In this thesis, Scanning tunneling microscopy (STM) and spectroscopy were used to study the topography, electronic properties, and band structure of the surfaces of complex correlated electron materials and self-assembled molecular structures. First, the setpoint artifacts that can occur when acquiring band structure data using quasi-particle interference (QPI) are studied, applied to the surface states of (111) metal surfaces. Next, QPI is applied to study the topological semi-metal Sb(111), enabling the reconstruction of its band structure. The second part of the thesis concerned materials exhibiting charge density waves (CDW). Combining STM imaging with Fourier transform analysis along the columns of the quasi-one-dimensional material NbSe$_3$, the real-space distribution of the two modulation wave-vectors is studied. Our measurements support a new model of CDW sliding, in which the two modulations alternate along two of the three types of columns comprising the unit cell. Measurements of the topography and electronic properties of the layered material 1T-TaSeS, exhibiting both CDWs and superconductivity, are presented. In contrast to expectations, high energy resolution measurements below its superconducting transition temperature reveal no spatial correlation between the observed superconducting gap and the domain wall structure. In the end, the self-assembly of 2-mercaptobenzimidazole, an organic corrosion inhibitor, on the surface of copper Cu(111) is investigated. Evaporation in UHV enabled controlled preparation of high-quality samples with (sub-)monolayer coverages. By tuning the substrate temperature and surface coverage, different self-assembled structures could be obtained and characterized using STM and DFT, elucidating the bonding of 2-mercaptobenzimidazole on Cu(111). Atomic real-space and high energy resolution in combination with a detailed understanding of the measuring modes and possible artifacts, make STM a powerful experimental technique, providing several possibilities for investigating the surfaces of complex materials.

• Language: English
• Keywords: Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), Quasi-particle interference, charge density waves, molecular self-assembly, Density Functional Theory (DFT)
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2021
• PID: 20.500.12556/RUL-127848
• COBISS.SI-ID: 72761603
• Publication date in RUL: 25.06.2021

Secondary language

• Language: Slovenian
• Title: Vrstična tunelska spektroskopija koreliranih elektronskih sistemov in molekulskih struktur

Abstract

V disertaciji so predstavljeni rezultati uporabe vrstične tunelske mikroskopije (VTM) in spektroskopije za preučevanje topografije ter elektronskih lastnosti koreliranih elektronskih materialov in samourejenih molekulskih struktur na površinah. Najprej smo se osredotočili na karakterizacijo merilnih napak, ki se lahko pojavijo pri merjenju pasovne strukture površin s pomočjo interference kvazi delcev na primeru površinskega stanja enostavnih kovin. Nato smo s pomočjo te metode rekonstruirali pasovno strukturo topološke polkovine Sb(111). V drugem delu doktorata smo se osredotočili na preučevanje materialov, ki kažejo valove gostote naboja (VGN). Z uporabo VTM in analize s pomočjo Fourierjeve transformacije vzdolž kolon kvazi-enodimenzionalnih kristalov NbSe$_3$ smo preučevali urejanje VGN v tem kristalu, ki kaže zanimive transportne lastnosti imenovane drsenje VGN. Naši rezultati so v skladu z novim modelom transporta, ki predvideva izmenjavo obeh modulacij vzdolž dveh od treh kolon. Na primeru plastnih kristalov 1T-TaSeS smo preučevali soobstoj superprevodnosti in VGN. Prek visoko ločljivih meritev lokalne gostote stanj pod temperaturo prehoda v superprevodno stanje, smo pokazali, da v nasprotju s pričakovanji superprevodne lastnosti in domenska struktura tega materiala nista povezani. V zadnjem delu doktorata so predstavljeni rezultati meritev samourejanja organskega inhibitorja, 2-merkaptobenzimidazola, na površini bakra. S pomočjo naparevanja v ultra visokem vakuumu smo nadzorovano pripravili visoko kvalitetne vzorce s (pod-)enoplastno pokritostjo. Z nadzorom nad temperaturo substrata in površinsko pokritostjo smo pridobili različne samourejene strukture in jih analizirali z VTM ter simulacijami s pomočjo teorije gostotnih funkcionalov in tako izboljšali razumevanje mehanizmov vezave 2-merkaptobenzimidazola na površino bakra. Zaradi atomske in visoke energijske ločljivosti je VTM skupaj s podrobnim poznavanjem merilnih načinov in možnih napak močna eksperimentalna tehnika, ki omogoča raznolike raziskave površin kompleksnih materialov.

• Keywords: Vrstična tunelska mikroskopija in spektroskopija, interferenca kvazi delcev, valovi gostote naboja, samourejanje molekul na površinah, teorija gostotnih funkcionalov