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Dissertation
zur Erlangung des Doktorgrades Dr. rer. nat.
der Fakultät für Naturwissenschaften der
Universität Ulm
Atomic Structure, Stability
and Formation of
Surface Confined Alloys
vorgelegt von
Andreas Bergbreiter
geboren in Günzburg
2011Amtierender Dekan: Herr Prof. Dr. Axel Groß
1. Gutachter: Herr Prof. Dr. Rolf Jürgen Behm
2. Herr Prof. Dr. Axel Groß
Dissertation eingereicht am: 02.02.2011
Mündliche Prüfung am: 25.05.2011Contents
1 Introduction 1
1.1 Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Historical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Actual requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 State of research for bimetallic surfaces at the beginning of the present thesis 5
1.2.1 Chemical properties – relevance for catalysis . . . . . . . . . . . . . 5
1.2.2 Bimetallic model surfaces. . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2.3 Surface confined alloy formation . . . . . . . . . . . . . . . . . . . . 11
1.2.4 Structure characterization of surface alloys . . . . . . . . . . . . . . 13
1.3 Open questions tackled in the current thesis . . . . . . . . . . . . . . . . . . 19
1.3.1 Surface alloy structure – experiment, theory and simulation . . . . 19
1.3.2 Stability of surface alloys . . . . . . . . . . . . . . . . . . . . . . . . 20
1.3.3 Formation of surface alloys . . . . . . . . . . . . . . . . . . . . . . . 20
2 Quantitative characterization of surface alloys 21
2.1 Short-range order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2 Energy model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.1 Effective cluster interactions . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.2 2D lattice-gas Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . 26
2.3 Monte-Carlo simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.1 Metropolis algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.2 Simplex downhill algorithm . . . . . . . . . . . . . . . . . . . . . . . 29
3 Experimental 33
3.1 setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2 Experimental problems and solutions . . . . . . . . . . . . . . . . . . . . . 37
3.3 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4 Results 43
iContents
4.1 The equilibrium structure of surface alloys – experiment and simulation . 45
4.1.1 Energetics driving the short-range order in Cu Pd /Ru(0001) . 46x 1 x
4.1.2 Atomic distribution in Pt Ru /Ru(0001) surface alloys . . . . . 58x 1 x
4.1.3 in Pd Ru /Ru(0001) surface alloys . . . . . 75x 1 x
4.1.4 Atomic distribution in Au Pt /Pt(111) surface alloys . . . . . . 91x 1 x
4.1.5 Simulation of the atom distribution in Au Pt /Pt(111) based onx 1 x
DFT total energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.1.6 2D phase diagrams of binary surface confined alloys . . . . . . . . 119
4.2 The surface alloy formation process . . . . . . . . . . . . . . . . . . . . . . 133
4.2.1 On the origin of Ru bilayer island growth on Pt(111) . . . . . . . . 136
4.2.2 From bilayer to monolayer growth: temperature effects in the
growth of Ru on Pt(111) . . . . . . . . . . . . . . . . . . . . . . . . . 146
4.2.3 Initial alloy formation in Pt Ru /Ru(0001) . . . . . . . . . . . . . 163x 1 x
4.2.4 Segregation and Stability in Surface Alloys: Pd Ru /Ru(0001)x 1 x
and Pt Ru /Ru(0001) . . . . . . . . . . . . . . . . . . . . . . . . . 175x 1 x
5 Summary 189
5.1 Surface alloy structure – experiment, theory and simulation . . . . . . . . 189
5.2 Stability of surface alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
5.3 Formation of surface alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
6 Zusammenfassung 195
7 Appendix 201
8 Bibliography 209
Publications 227
Conference Contributions 229
Erklärung 235
Danksagung 237
iiList of Figures
1.1 Artifacts of the Bronze Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Illustration of ligand and ensemble effect: CO adsorption on AuPd/Pd(111) 6
1.3 of the ensemble requirement for H recombination / desorption 72
1.4 Schematic description of different single crystal model systems . . . . . . 9
1.5 The three classical metal on metal growth modes . . . . . . . . . . . . . . . 10
1.6 Schematic description for the preparation of (multi-/sub-)surface alloys . 12
1.7 Atom distribution in different disordered surface alloys . . . . . . . . . . . 14
1.8 Composition dependent atom distribution in surface alloys . . . . . . . . . 15
1.9 The four generic classes of surface energy diagrams . . . . . . . . . . . . . 17
2.1 Algorithm scheme to deduce the EPIs from experimental distribution . . . 30
3.1 Ultra-high vacuum chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2 Manipulator head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3 Home-built pocket size STM . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.4 Sample and sample holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.1 Cu Pd /Ru(0001): effective pair interactions as function of thex 1 x
nearest-neighbor distance/Cu content . . . . . . . . . . . . . . . . . . . . . 50
4.2 Cu Pd /Ru(0001): experimental and simulated short-range orderx 1 x
parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3 Cu Pd /Ru(0001): ordered ground state structures search; mixingx 1 x
enthalpies deduced from experiment and DFT . . . . . . . . . . . . . . . . 52
4.4 Cu Pd /Ru(0001): simulated annealing . . . . . . . . . . . . . . . . . . 55x 1 x
4.5 Pt Ru /Ru(0001): STM images after the consecutive preparation steps . 61x 1 x
4.6 Pt content counted in the surface alloys as a function of initial Pt coverage 62
4.7 Pt Ru /Ru(0001): atomically resolved STM images with chemicalx 1 x
contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.8 Pt Ru : short-range order coefficients . . . . . . . . . . . . . . . . . . . . 66x 1 x
iiiListofFigures
4.9 Pt Ru /Ru(0001): abundance of compact 2-, 3-, 4-, and 5-foldx 1 x
ensembles as a function of Pt-coverage . . . . . . . . . . . . . . . . . . . . . 68
4.10 Pt Ru /Ru(0001): ligand statistics . . . . . . . . . . . . . . . . . . . . . . 69x 1 x
4.11 Pd Ru /Ru(0001): STM images after the consecutive preparation steps 77x 1 x
4.12 Pd Ru /Ru(0001): atomically resolved STM images with chemicalx 1 x
contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.13 Pd Ru /Ru(0001): Pd content in the surface alloys as function of initialx 1 x
Pd coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.14 Pd Ru /Ru(0001): Pt Ru : short-range order coefficients . . . . . . . 83x 1 x x 1 x
4.15 Pd Ru /Ru(0001): simulated atom distribution . . . . . . . . . . . . . . 85x 1 x
4.16 Pd Ru /Ru(0001): abundance of compact 2-, 3-, 4-, and 5-foldx 1 x
ensembles as a function of Pd-coverage . . . . . . . . . . . . . . . . . . . . 86
4.17 Pd Ru /Ru(0001): ligand statistics . . . . . . . . . . . . . . . . . . . . . 87x 1 x
4.18 Au Pt /Pt(111): STM images after the consecutive preparation steps . . 94x 1 x
4.19 Au Pt /Pt(111): Au content in the surface alloys as function of initialx 1 x
Au coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.20 Au Pt /Pt(111): atomically resolved STM images with chemical contrast 96x 1 x
4.21 Au Pt /Pt(111): short-range order coefficients . . . . . . . . . . . . . . . 98x 1 x
4.22 Au Pt /Pt(111): effective pair interactions as function of thex 1 x
nearest-neighbor distance / Au content . . . . . . . . . . . . . . . . . . . . 99
4.23 Au Pt /Pt(111): comparison of experimental and simulated atomx 1 x
distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.24 Au Pt /Pt(111): abundance of compact 2-, 3-, 4-, and 5-fold ensemblesx 1 x
as a function of Au-coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.25 Au Pt /Pt(111): ligand statistics derived experiment and EPIs basedx 1 x
on atomic distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
4.26 Correlation between DFT-energies and interaction parameters deduced
from experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.27 Au Pt /Pt(111): effective pair interactions derived from experimentx 1 x
and DFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4.28 Au Pt /Pt(111): simulated atom distribution using EPIs deduced fromx 1 x
DFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
4.29 Au Pt /Pt(111): SRO values derived from experiment and DFT totalx 1 x
energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
ivListofFigures
4.30 Au Pt /Pt(111): abundance of compact 2-, 3-, 4-, and 5-fold ensemblesx 1 x
as a function of Au-coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
4.31 Au Pt /Pt(111): ligand statistics derived from experiment and DFTx 1 x
based EPIs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
4.32 DH for a phase separating binary system as function of the composition 123mix
4.33 Common tangent construction to deduce a 2D phase diagram . . . . . . . 125
4.34 2D phase diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
4.35 Surface, segregation and 2D mixing energies . . . . . . . . . . . . . . . . . 130
4.36 Comparison of the alloying behavior of Ru/Pt(111) and Pt/Ru(0001) . . . 134
4.37 Ru on Pt(111): preference for Ru bilayer island growth . . . . . . . . . . . 138
4.38 Ru on Pt/Ru(0001): preference of Ru bilayer growth on monolayer Pt
islands covering Ru(0001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.39 Ru on Pt/Ru(0001): plot of Ru distribution on Ru(0001) vs Pt/Ru(0001) . . 142
st nd rd4.40 Ru on Ru amount in 1 , 2 and 3 layer as function of Ru
coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
4.41 Ru on Pt(111): Morphology after deposition of variable Ru amounts at
different temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
4.42 Ru on Pt(111): temperature dependent Ru distribution in different layers 153
4.43 Ru on Pt(111):e Ru island density . . . . . . . . . . 155
4.44 Ru on Pt(111): STM details of temperature dependent growth . . . . . . . 157
4.45 PtRu/Pt(111) initial alloy formation process: Pt islands deposited at
600 K on Ru(0001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
4.46 PtRu/Pt(111) initial alloy formation process: morphology of Pt on
Ru(0001) after subsequent annealings . . . . . . . . . . . . . . . . . . . . . 167
4.47 PtRu/Pt(111) initial alloy formation process: morphology and atomic
resolution after initial alloy . . . . . . . . . . . . . . . . . . . . . 169
4.48 PtRu/Pt(111) initial alloy formation process: atomic resolution of
vacancy islands at different temperatures . . . . . . . . . . . . . . . . . . . 171
4.49 PtRu/Pt(111) initial alloy formation process: Pt! Ru exchange scheme . 172
4.50 PtRu/Ru(0001): floating effect; detailed STM imaging of the floating
sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
4.51 PdRu/Ru(0001): floating effect; detailed STM imaging of the floating
sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
4.52 PtRu and PdRu on Ru(0001): floating effect; Auger spectra of the floating
sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
vListofFigures
4.53 Ru on PtRu/Ru(0001) and PdRu/Ru(0001): Ru islands; STM image and
height profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
4.54 PtRu/Ru(0001) and PdRu/Ru(0001): scheme of the predicted floating
process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
viList of Tables
4.1 Ru on Pt(111): characteristic features of the islands formed by deposition
of 0.40 ( 0.04) ML Ru on Pt(111) at different deposition temperatures . . 149
7.1 Cu Pd /Ru(0001): unit cells of the calculated DFT total energies /x 1 x
mixing energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
7.2 Au Pt /Pt(111): unit cells of the calculated DFT total energies / mixingx 1 x
energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
vii

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