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Geometrical Structure, Phase Diagrams, and Core-Level Binding Energiesof Metal Surfaces: Calculations, Simulations, and Experiments

Borg, Mikael LU (2003)
Abstract (Swedish)
Popular Abstract in Swedish

Många fysiska egenskaper för ett material beror på den atomära strukturen, det vill säga vilka atomer som sitter var.



För att bestämma den geometriska strukturen måste ofta en kombination av flera metoder användas. I detta arbete har vi använt högupplöst fotoelektronspektroskopi (HRCLS), diffraktion av lågenergetiska elektroner (LEED) samt beräkningar baserade på täthetsfunktionalteori (DFT).



Med HRCLS låter man ljus träffa provet, varefter man mäter de så kallade fotoelektroner som exciteras från bundna tillstånd i provet till fria tillstånd. Eftersom man vet energin på ljuset som man använder samt mäter energin hos de elektroner som exciteras, kan man räkna... (More)
Popular Abstract in Swedish

Många fysiska egenskaper för ett material beror på den atomära strukturen, det vill säga vilka atomer som sitter var.



För att bestämma den geometriska strukturen måste ofta en kombination av flera metoder användas. I detta arbete har vi använt högupplöst fotoelektronspektroskopi (HRCLS), diffraktion av lågenergetiska elektroner (LEED) samt beräkningar baserade på täthetsfunktionalteori (DFT).



Med HRCLS låter man ljus träffa provet, varefter man mäter de så kallade fotoelektroner som exciteras från bundna tillstånd i provet till fria tillstånd. Eftersom man vet energin på ljuset som man använder samt mäter energin hos de elektroner som exciteras, kan man räkna ut hur hårt bundna elektronerna varit i materialet. Denna bindningsenergi beror dels på vilken sorts atom som elektronerna kommer ifrån, men även på den atomära geometrin, hur många och vilka atomer som finns runt atomen som elektronen exciterats ifrån. Man kan t ex se skillnad på elektronens bindningsenergi om atomen sitter i ytlagret eller längre in i provet. Ibland är det inte möjligt att förstå den atomära geometrin endast utifrån fördelningen av dessa bindningsenergier. Då kan man ställa upp olika modeller och räkna ut de olika bindningsenergier som de olika modellerna ger och utifrån detta hitta den som stämmer överens med det experimentella resultatet. Till detta har vi använt täthetsfunktionalteori, som är en metod för att räkna ut energin och elektronernas fördelning i ettmaterial. Metoden, som belönades med Nobel-priset i kemi 1998, använder inga empiriska parametrar utan bygger på grundläggande kvantmekaniska principer.



Med LEED låter man elektroner studsa mot provets yta och sedan träffa en fluoroscerande skärm. Det mönster som då uppstår på skärmen innehåller information om symmetrin på provets yta och kan även användas för att finna den geometriska strukturen.



I denna avhandling har vi studerat metallytor av aluminium och rhodium samt ytlegeringar av aluminium och alkalimetaller. Metallytor är av stor betydelse för många katalysatorer, men kan även fungera som modell-system för att utvärdera nya metoder.



Vi har även studerat strukturen som funktion av sammansättning och temperatur för aluminium-natrium ytlegeringar, det så kallade fasdiagrammet. Eftersom natrium inte är lösbart i aluminium stannar natriumatomerna på ytan, där de kan bilda en legering som därmed är ett atomlager tjockt. Detta tvådimensionella system har för en viss sammansättning en struktur som visar sig smälta vid ca -35°C. Vi har karakteriserat detta system experimentellt och dessutom gjort omfattande beräkningar och simuleringar som visar att vi kan återskapa hela fasdiagrammet med hjälp av DFT. (Less)
Abstract
This thesis concerns clean and adsorbate covered metal surfaces and surface alloys. These systems have been studied using theoretical and experimental methods. In the first part of the thesis, the different techniques that have been utilized in this work are presented, and the included publications are summarized. The second part of the thesis consists of seven original publications.



In the first two papers, the structure of two aluminum-alkali surface alloys are determined using low energy electron diffraction, high resolution core level photoemission spectroscopy and density functional theory calculations. The first is a three-layer Al<sub>3</sub>Li surface alloy which is found to have a... (More)
This thesis concerns clean and adsorbate covered metal surfaces and surface alloys. These systems have been studied using theoretical and experimental methods. In the first part of the thesis, the different techniques that have been utilized in this work are presented, and the included publications are summarized. The second part of the thesis consists of seven original publications.



In the first two papers, the structure of two aluminum-alkali surface alloys are determined using low energy electron diffraction, high resolution core level photoemission spectroscopy and density functional theory calculations. The first is a three-layer Al<sub>3</sub>Li surface alloy which is found to have a Al<sub>3</sub>Ti-type structure. In the second paper, the structure and formation of an Al(100)-(√5x√5)R27°-Na surface alloy is investigated.



In the third paper, the phase diagram of Al(100)-Na surface alloys is studied as a function of stoichiometry and temperature. An order-disorder phase transition is characterized using LEED and HRCLS and this, as well as the phase diagram, is reproduced in first principles Monte Carlo simulations with input parameters from DFT calculations.



Paper IV is an experimental and theoretical study of the surface core level shifts (SCLS) of two low-index aluminum surfaces, Al(100) and Al(111). The experimental and theoretical results are in excellent agreement. These measurements provide the first experimental determination of the SCLS od Al(111), For Al(100), the core level shift of the second layer is also resolved. Furthermore, the Al 2p photemission peak is found to consist of a no-phonon line and satellite structures at higher binding energies corresponding to phonon losses.



In Paper V, the SCLS of two stepped rhodium surfaces are studied using HRCLS. The components from terraces and step edges of a vicinal surface are resolved for the first time. We also demonstrate how this may be used to show that initial oxygen adsorption occurs on the steps and not on the terraces of the vicinal surfaces.



The last two papers are experimental and theoretical studies of CO adsorption on Rh(111). In the experimental study, the fine structure components due to vibrational excitation of the C-O stretch mode are clearly resolved in the C1s emission peaks and are found to be site specific. Furthermore, a new C1s HRCLS component is resolved at intermediate coverages. It is suggested that the new component is due to CO adsorbed in bridge sites on the surface. This is supported by the calculated core level shifts in paper VII. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Prof Hammer, Bjørk, Aarhus University, Denmark
organization
publishing date
type
Thesis
publication status
published
subject
keywords
adsorption, chemical shifts, low energy electron diffraction, Monte Carlo simulations, Condensed matter:electronic structure, electrical, magnetic and optical properties, Kondenserade materiens egenskaper:elektronstruktur, spectroscopy, relaxation, magnetic resonance, supraconductors, spektroskopi, magnetisk resonans, supraledare, egenskaper (elektriska, magnetiska och optiska), molecular vibrations, density functional theory, single crystal surfaces, core-level photoemission, metal surfaces, Fysicumarkivet A:2003:Borg
pages
210 pages
publisher
Div. of Synchrotron Radiation Research, Lund University, Box 118, SE-221 00 Lund, Sweden,
defense location
Lecture Hall B, Department of Physics
defense date
2003-12-17 10:15
ISBN
91-628-5884-X
language
English
LU publication?
yes
id
b4746155-f5fb-454d-bc03-89b5a72a2ff2 (old id 466423)
date added to LUP
2007-09-28 09:38:08
date last changed
2016-09-19 08:45:09
@phdthesis{b4746155-f5fb-454d-bc03-89b5a72a2ff2,
  abstract     = {This thesis concerns clean and adsorbate covered metal surfaces and surface alloys. These systems have been studied using theoretical and experimental methods. In the first part of the thesis, the different techniques that have been utilized in this work are presented, and the included publications are summarized. The second part of the thesis consists of seven original publications.<br/><br>
<br/><br>
In the first two papers, the structure of two aluminum-alkali surface alloys are determined using low energy electron diffraction, high resolution core level photoemission spectroscopy and density functional theory calculations. The first is a three-layer Al&lt;sub&gt;3&lt;/sub&gt;Li surface alloy which is found to have a Al&lt;sub&gt;3&lt;/sub&gt;Ti-type structure. In the second paper, the structure and formation of an Al(100)-(√5x√5)R27°-Na surface alloy is investigated.<br/><br>
<br/><br>
In the third paper, the phase diagram of Al(100)-Na surface alloys is studied as a function of stoichiometry and temperature. An order-disorder phase transition is characterized using LEED and HRCLS and this, as well as the phase diagram, is reproduced in first principles Monte Carlo simulations with input parameters from DFT calculations.<br/><br>
<br/><br>
Paper IV is an experimental and theoretical study of the surface core level shifts (SCLS) of two low-index aluminum surfaces, Al(100) and Al(111). The experimental and theoretical results are in excellent agreement. These measurements provide the first experimental determination of the SCLS od Al(111), For Al(100), the core level shift of the second layer is also resolved. Furthermore, the Al 2p photemission peak is found to consist of a no-phonon line and satellite structures at higher binding energies corresponding to phonon losses.<br/><br>
<br/><br>
In Paper V, the SCLS of two stepped rhodium surfaces are studied using HRCLS. The components from terraces and step edges of a vicinal surface are resolved for the first time. We also demonstrate how this may be used to show that initial oxygen adsorption occurs on the steps and not on the terraces of the vicinal surfaces.<br/><br>
<br/><br>
The last two papers are experimental and theoretical studies of CO adsorption on Rh(111). In the experimental study, the fine structure components due to vibrational excitation of the C-O stretch mode are clearly resolved in the C1s emission peaks and are found to be site specific. Furthermore, a new C1s HRCLS component is resolved at intermediate coverages. It is suggested that the new component is due to CO adsorbed in bridge sites on the surface. This is supported by the calculated core level shifts in paper VII.},
  author       = {Borg, Mikael},
  isbn         = {91-628-5884-X},
  keyword      = {adsorption,chemical shifts,low energy electron diffraction,Monte Carlo simulations,Condensed matter:electronic structure,electrical,magnetic and optical properties,Kondenserade materiens egenskaper:elektronstruktur,spectroscopy,relaxation,magnetic resonance,supraconductors,spektroskopi,magnetisk resonans,supraledare,egenskaper (elektriska,magnetiska och optiska),molecular vibrations,density functional theory,single crystal surfaces,core-level photoemission,metal surfaces,Fysicumarkivet A:2003:Borg},
  language     = {eng},
  pages        = {210},
  publisher    = {Div. of Synchrotron Radiation Research, Lund University, Box 118, SE-221 00 Lund, Sweden,},
  school       = {Lund University},
  title        = {Geometrical Structure, Phase Diagrams, and Core-Level Binding Energiesof Metal Surfaces: Calculations, Simulations, and Experiments},
  year         = {2003},
}