Epitaxial graphene and cluster lattices on iridium(111) [Elektronische Ressource] = Epitaktisches Graphen und Clustergitter auf Iridium(111) / vorgelegt von Alpha T. N'Diaye

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2010
• Nombre de lectures: 63
• Langue: Deutsch
• Poids de l'ouvrage: 11 Mo

Extrait

Epitaxial Graphene and
Cluster Lattices on
Iridium(111)
Epitaktisches Graphen und
Clustergitter auf Iridium(111)
Von der Fakult¨at fu¨r Mathematik, Informatik und
Naturwissenschaften der RWTH Aachen University zur
Erlangung des akademischen Grades eines Doktors der
Naturwissenschaften genehmigte Dissertation
vorgelegt von
Dipl.–Phys. Alpha T. N’Diaye
aus Duisburg
Berichter: Univ. Prof. Dr. Thomas Michely
Univ. Prof. Dr. Markus Morgenstern
Tag der mu¨ndlichen Pru¨fung: 1. Februar 2010
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfu¨gbar.Contents
1 Introduction 7
2 Background 11
2.1 Peculiarities of graphene . . . . . . . . . . . . . . . . . . . . . 12
2.1.1 Band Structure . . . . . . . . . . . . . . . . . . . . . . 12
2.1.2 Half Integer Quantum Hall Effect and Dirac Fermions . 15
2.1.3 Edge state . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2 Preparation of Graphene . . . . . . . . . . . . . . . . . . . . . 23
2.2.1 Exfoliation . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.2 Epitaxial graphene on SiC . . . . . . . . . . . . . . . . 27
2.2.3 Epitaxial graphene on metal substrates . . . . . . . . . 37
2.3 Envisioned Applications . . . . . . . . . . . . . . . . . . . . . 52
2.3.1 Transistor . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.3.2 Spintronics . . . . . . . . . . . . . . . . . . . . . . . . 57
2.3.3 Chemical sensing . . . . . . . . . . . . . . . . . . . . . 60
2.4 Cluster materials . . . . . . . . . . . . . . . . . . . . . . . . . 63
2.4.1 Weakly interacting substrates . . . . . . . . . . . . . . 64
2.4.2 Template substrates . . . . . . . . . . . . . . . . . . . 64
3 Experimental 69
3.1 STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.1.1 TuMA III . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.1.2 Reservoir cooling . . . . . . . . . . . . . . . . . . . . . 71
3.1.3 Gas dosing . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.1.4 Metal deposition . . . . . . . . . . . . . . . . . . . . . 72
3.2 LEEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724 Contents
3.3 Experimental procedures . . . . . . . . . . . . . . . . . . . . . 73
4 Growth of graphene on Ir(111) 75
4.1 Temperature programmed growth . . . . . . . . . . . . . . . . 77
4.1.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.1.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.2 Chemical vapor deposition . . . . . . . . . . . . . . . . . . . . 86
4.2.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.2.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 96
4.3 Selectingasingleorientationformillimetersizedgraphenesheets101
4.3.1 Full layer chemical vapor deposition . . . . . . . . . . 101
4.3.2 Cyclic etching . . . . . . . . . . . . . . . . . . . . . . . 104
4.3.3 TPG + CVD combination . . . . . . . . . . . . . . . . 106
4.4 Industry compatible CVD growth . . . . . . . . . . . . . . . . 109
4.4.1 Preparation . . . . . . . . . . . . . . . . . . . . . . . . 109
4.4.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.5 In-situ observation of stress relaxation and wrinkle formation . 111
4.5.1 Wrinkle formation . . . . . . . . . . . . . . . . . . . . 111
4.5.2 Lattice expansion . . . . . . . . . . . . . . . . . . . . . 116
4.5.3 Local stress evolution . . . . . . . . . . . . . . . . . . . 118
4.5.4 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
5 Structure of Graphene on Ir(111) 127
5.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.1.1 Graphene preparation . . . . . . . . . . . . . . . . . . 128
5.1.2 LEED and STM results . . . . . . . . . . . . . . . . . 130
5.1.3 The moir´e of graphene on Ir(111) . . . . . . . . . . . . 131
5.1.4 Moir´e unit cell . . . . . . . . . . . . . . . . . . . . . . 135
5.1.5 Chemical inhomogenity . . . . . . . . . . . . . . . . . . 140
5.1.6 Contrast inversion . . . . . . . . . . . . . . . . . . . . 141
5.2 Structural Coherency . . . . . . . . . . . . . . . . . . . . . . . 146
5.2.1 Coherency across small angle domain boundaries . . . 146
5.2.2 Coherency across step edges . . . . . . . . . . . . . . . 150Contents 5
5.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
6 Clusters 157
6.1 Preliminary considerations . . . . . . . . . . . . . . . . . . . . 158
6.2 Cluster structure and superlattice formation at 300K . . . . . 161
6.2.1 Low temperature cluster superlattice growth and an-
nealing . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
6.2.2 Cluster seeding . . . . . . . . . . . . . . . . . . . . . . 167
6.2.3 Temperature stability . . . . . . . . . . . . . . . . . . . 169
6.2.4 Binding sites of clusters in the superlattice . . . . . . . 175
6.2.5 Towards cluster superlattice materials. . . . . . . . . . 177
6.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
7 Summary and Outlook 179
7.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
7.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
A Frequently used abbreviations 209
B The band structure of graphene 211
C Deutsche Zusammenfassung / German short summary 217
D Curriculum Vitae 219
E Publications 221
E.1 Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
E.2 Awards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2226 Contents1 Introduction
Graphene is a two dimensional sheet of carbon, just as it occours naturally
in graphite. Intense research on the electronic properties of graphene was
initiated in 2004 using both exfoliated [1] and epitaxial [2] graphene. From
then on, theoretical and experimental efforts enabled one to unravel some of
the unique physical properties of graphene [3,4]. The chemical deposition of
carbon on metal substrates has been extensively studied from the 1970s to
1990s. A major motivation for studying these graphite films was the passi-
vation of catalysts by carbon films known as poisoning, but the properties of
graphene were not investigated in detail. The studies of the electronic elec-
tronic properties which brought to light the field effect, theexistence of Dirac
fermions, and the half integer quantum hall effect fueled an enormous growth
of the interest in graphene. While in May 2006 when Berger et al. published
their results on the Hall effect in graphene in SiC, there were eight papers
with the word graphene in the title, three years later in May 2009, there were
about one hundred such papers.
At the heart of the scientific interest are the consequences of the unique
band structure of graphene which arises from its lattice symmetry and its
monoatomicthickness[5]. Thehighmobilityofelectronsingrapheneandthe
strongelectricfieldeffectencourageworktorealizegraphenebasedelectronics
[4]. Moreover,theuseofgraphenefortransparentconductingelectrodes[6,7],
to realize photosensitive transistors [8], ultracapacitors [9], or novel chemical
sensors [10] is envisioned. Due to the increasing importance of graphene, it
is desirable to obtain a thorough understanding of the structure of graphene
on metals and of its interplay with the underlying substrate.
Grapheneismanufacturedmainlyinthreeways: byexfoliationfromhighly
oriented pyrolytic graphite (HOPG) [1,11], by the epitaxial growth in sili-8 Introduction
con carbide [2,12,13] and by the epitaxial growth on metals. Exfoliation of
graphene was the basis for the exploration of the exciting electronic proper-
ties of graphene. They were primarily explored with transport measurements
of devices built on flakes of exfoliated graphene on SiO . Nevertheless there2
appears to be consensus, that for future scientific exploration and technolog-
ical applications, epitaxial growth of high quality graphene over large areas
is a prerequisite [12,14–19].
The graphene-metal interface is a model system where the interaction be-
tween the graphene π-bands and the metal bands can be investigated. This
has relevance to contacting of graphene with metal electrodes. The carbon
hybridization and also the epitaxial relationship with the metal substrate
were proposed to influence the contact transmittance [20,21] and cause local
doping of graphene [22,23]. A variety of situations regarding the interplay
of graphene and its metal support is realized depending on the support ma-
terial. This ranges from almost no interaction in the case of Ir(111) [24] to
deepamodificationoftheelectronicstructureinthecaseofagraphenemono-
layer on Ni(111) [25,26] or Ru(0001) [15,27]. Other fundamental questions
come along with the graphene/metal interface, in particular considering su-
percurrent with Dirac-like electrons flowing between two metallic electrodes
coupled through a graphene layer to a superconductor [28]. Regarding spin-
tronics, recent work focussing on spin filtering highlighted the relevance of
epitaxial graphene on a ferromagnetic metal, like Ni or Co [29–31]. Graphene
potentially constitutes a new material for electronic circuitry with vastly im-
proved transport properties compared to traditional silicon [1]. A large scale
application of graphene crucial