Quantum size effects in the electronic structure of novel self organized systems with reduced dimensionality [Elektronische Ressource] / von Andrei Varykhalov

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• Physik

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• Publié par: universitat_potsdam
• Publié le: 01 janvier 2005
• Nombre de lectures: 17
• Langue: English
• Poids de l'ouvrage: 5 Mo

Extrait

BESSY G.m.b.H. und Institut fu¨r Physik, Universit¨at Potsdam
Arbeitsgruppe Prof. Dr. W. Gudat
Quantum-size effects in the electronic structure
of novel self-organized systems with reduced
dimensionality
Dissertation
zur Erlangung des akademischen Grades
”doctor rerum naturalium” (Dr. rer. nat.)
in der Wissenschaftsdisziplin Experimentalphysik
eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakult¨at
der Universit¨at Potsdam
von
Andrei Varykhalov
Universit¨at Potsdam, 16 Juni, 2005Gutachter
Prof. Dr. Wolfgang Gudat, BESSY (Berlin) und Universit¨at Potsdam (Potsdam)
Prof. Dr. Ullrich Pietsch, Universit¨at Potsdam (Potsdam)
Prof. Dr. Karsten Horn, FHI der MPG (Berlin)
Datum der Disputation: 16 Juni 2005Abbreviations
ARPES angle-resolved photoelectron spectroscopy
BZ Brillouin zone
b.c.c. bulk centered cubic (crystal structure)
DOS density of states
f.c.c. face centered cubic (crystal structure)
HOMO highest occupied molecular orbital
KKR Korringa-Kohn-Rostoker approach
LCGO linear combination of Gaussian orbitals
LDOS local density of states
LDA local density approximation
LUMO lowest unoccupied molecular orbital
LEED low-energy electron diffraction
ML monolayer
PES photoelectron spectroscopy
QWS quantum-well state
RAPW relativistic augmented plane-wave
STM scanning tunneling microscopy
UHV ultra-high vacuum
XPS x-ray photoelectron spectroscopySymbols, units, definitions
−10˚ ˚A Angstr¨om = 10 m, length unit
−9nm nanometer = 10 m, length unit
−9nA nanoampere = 10 A, current unit
−19eV electron-volt ∼ 1.6· 10 J, energy unit
˚a (or a ) [A] lattice constant of a cubic crystal0
k wave vector of the electron in the crystal
k , k components of k relative to the crystal surfacek ⊥
−1˚k, k , k [A ] scalar (absolute) values of wave-vectorsk ⊥
−31m ∼ 9.1·10 kg, mass of the free electrone
∗m effective mass of the electron is the lattice
∗m in-plane effective mass of the electron
k
E(k) energy-momentum dispersion, band structure
E [eV] kinetic energy of the electronkin
E [eV] electron energy in the initial state of photoemissioni
E [eV] electron energy in the final state of photoemissionf
E Fermi level, Fermi energyF
V [V] bias voltage in STMt
I [nA] tunneling current in STMt
λ [nm] wave-length
hν (~ω) [eV] energy of photon
Φ [eV] work function of a solid surface
ρ(E) density of states (DOS)
ψ wave function of an electron
M matrix element of electron transitionν,μ
T(E) transition tunneling coefficient
1f(E)= Fermi-Dirac distributionE−EF
kTe +1Contents
1 Introduction 7
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 Methodical background . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.2.1 Technique of Photoemission . . . . . . . . . . . . . . . . . . 13
1.2.2 Technique of STM . . . . . . . . . . . . . . . . . . . . . . . 19
1.2.3 LEED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.3 Experimental systems . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.3.1 Photoemission set-up . . . . . . . . . . . . . . . . . . . . . . 24
1.3.2 STM set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2 Quantum-Well States in Ultra-Thin Metallic Films 31
2.1 Ground state of a correlated electron system probed by quantum
well states: Ag/Ni(111) . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.1.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . 33
2.1.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.1.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.2 Summary of conclusions . . . . . . . . . . . . . . . . . . . . . . . . 44
3 Towards Lower Dimensionality: Stepped Surfaces 45
3.1 Electronic structure of vicinal W(nn1) . . . . . . . . . . . . . . . . 46
3.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.1.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . 47
3.1.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.1.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2 Lateral electron confinement in Au
step-decorations on W(nn1) . . . . . . . . . . . . . . . . . . . . . . 55
3.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.2.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . 56
3.2.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.2.5 Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . 72
3.3 Summary of conclusions . . . . . . . . . . . . . . . . . . . . . . . . 73
54 Self-Organization on Large-Scale Surface Reconstructions 75
4.1 Structure and electronic properties of a surface carbide: W(110)/C-
R(15×3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.1.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . 77
4.1.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.1.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.2 Self-assembled Au nanowires and their electronic structure . . . . . 91
4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.2.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . 91
4.2.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.3 Self-organized nanoclusters of fullerene
molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.3.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . 104
4.3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.4 Summary of conclusions . . . . . . . . . . . . . . . . . . . . . . . . 121
Summary 122
Bibliography 123
Publication List 138
Refereed Papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Conferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Brief Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Curriculum Vitae 141
Zusammenfassung 143
Acknowledgements 145
6Chapter 1
Introduction
1.1 Motivation
When single atoms of one or more chemical elements assemble to form a solid,
the electrons of outer shells become delocalized and may propagate over the whole
lattice of the crystal. A very important characteristics is their energy-momentum
dispersion E(k)[1,2], which definesfundamental electrical, mechanical andoptical
properties of a solid [3]. The typical situation in solids is that the valence band
electrons form Bloch waves of sp- and d-type. In metals they are weakly bound
to the lattice and can be well described in terms of the nearly free electron [1]
approximation, where the function E(k) possesses the form of a simple parabola
at high-symmetry points of the Brillouin zone (Fig. 1.2(b)). The coefficient of
parabolic dispersion is inversely proportional to the effective mass of the electron
∗m , which reflects a moderate perturbation introduced into the Hamiltonian of
the free electron by the periodic potential of the lattice. The crystal potential
is also responsible for diffraction of propagating electron waves, which leads to
the appearance of forbidden energy regions which separate the electron bands [4].
What makes the band structure of real solids appear considerably more compli-
cated than Fig. 1.2(b) is the three-dimensional crystal lattice (in reciprocal space)
and the non-degenerate orbitals of p- and d-electrons which contribute individual
bands. This hold also for each constituent of a compound. The description of lo-
calized states requires computational approaches like the tight-binding formalism
or density functional methods. Additional atomic and molecular phenomena, like
spin-orbit splitting [5] as well as effects of hybridization [6] can lead to the lifting
of degeneracies and in this way affect the electronic structure of the real solid.
So far the electronic properties of infinitely extended crystals are well un-
derstood experimentally and also theoretically [7], and another effect related to
ultra-smallcrystals attractedtheattentionofthescientific community aboutthree
decades ago [9]. Most generally this phenomenon can be described as quantiza-
tion of the spectrum of trapped electrons into discrete energy levels, when the
dimensions of the potential well are comparable with the wave-length of the elec-
tron. Concerning the novelty of the effects, one has to admit that the theoretical
7background has already been developed at the early ages of quantum physics. For
instance, the classical solution of the particle-in-a-box problem can be found in
any textbook on wave mechanics. A sketch is shown in Fig. 1.1. If an electron is
placed into a one-dimensional potential box with infinitely high w