(hetero)aromatics on transition metal surfaces

182 Pages·2013·12.77 MB·English

Tailoring on-surface chemistry
of (hetero)aromatics on

transition metal surfaces

Memoria presentada para optar al grado de Doctor en Ciencias Físicas por
Anna Lisa Pinardi

Dirigida por Prof. José Ángel Martín Gago
Tutor Dr. Alejandro Gutiérrez
Madrid, Junio 2013

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Abstract ................................................................................................................................................................. i
Resumen .............................................................................................................................................................. iii
1 Introduction and motivation ....................................................................................................................... 1
2 Experimental methods ................................................................................................................................ 9
2.1 Experimental systems used in this thesis .......................................................................................... 12
2.2 Substrate preparation ....................................................................................................................... 13
2.3 Deposition of organic molecules ....................................................................................................... 14
2.4 Scanning Tunneling Microscopy (STM) ............................................................................................. 16
2.5 Low-Energy Electron Diffraction (LEED) ............................................................................................ 19
2.6 X-ray Photoelectron Spectroscopy (XPS) ........................................................................................... 21
2.7 Near Edge X-ray Absorption Fine Structure (NEXAFS)....................................................................... 23
2.8 Temperature Programmed Desorption (TPD) ................................................................................... 25
2.9 Density Functional Theory (DFT) ....................................................................................................... 26
3 Tailored formation of N-doped nanoarchitectures by diffusion-controlled on-surface
(cyclo)dehydrogenation of heteroaromatics .................................................................................................... 27
3.1 Introduction ....................................................................................................................................... 30
3.2 Design of the model precursors 1 and 4 ........................................................................................... 31
3.3 Cyclodehydrogenation of 1 and 4 on Pt(111): a highly reactive surface .......................................... 32
3.4 Dehydrogenation of 1 and 4 on Au(111): a weakly interacting surface ............................................ 33
3.5 Conclusion ......................................................................................................................................... 37
4 Spectroscopic and morphological study of DiPy[5]DBH on different transition metals ........................... 39
4.1 DiPy[5]DBH on highly interacting metals: Pt(111) ............................................................................ 42
4.1.1 Deposition on the surface at room temperature ...................................................................... 42
4.1.2 DiPy[5]DBH/Pt(111): deposition on the hot surface ................................................................. 52
4.2 DiPy[5]DBH on coinage metals .......................................................................................................... 54
4.2.1 DiPy[5]DBH/Cu(110) .................................................................................................................. 55
4.2.2 DiPy[5]DBH/Cu(111) .................................................................................................................. 68
4.2.3 DiPy[5]DBH/Au(111) .................................................................................................................. 80
4.2.4 Discussion: DiPy[5]DBH on coinage metals ............................................................................... 87
4.3 DiPy[5]DBH on different transition metals: conclusions ................................................................... 93

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5 C H N : on-surface cyclodehydrogenation steps towards the formation of triaza-fullerenes ............... 95
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5.1 Introduction ....................................................................................................................................... 98
5.2 Scanning Tunneling Microscopy of C H N /Pt(111): the formation of triaza-fullerenes in steps 100
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5.2.1 Planar molecules: crushed fullerenes ...................................................................................... 100
5.2.2 Open-cage triaza-fullerenes .................................................................................................... 104
5.2.3 The formation of triaza-fullerenes .......................................................................................... 106
5.2.4 N-doped graphene ................................................................................................................... 107
5.3 High resolution and temperature programmed XPS ....................................................................... 108
5.3.1 Temperature Programmed XPS of C1s .................................................................................... 109
5.3.2 High-resolution XPS ................................................................................................................. 110
5.4 Conclusions ...................................................................................................................................... 119
6 The mechanism of the atomic vacancy formation on the C /Pt(111) system ....................................... 121
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6.1 Introduction ..................................................................................................................................... 124
6.2 Results ............................................................................................................................................. 128
6.2.1 STM .......................................................................................................................................... 128
6.2.2 LEED ......................................................................................................................................... 138
6.2.3 XPS ........................................................................................................................................... 140
6.2.4 TPD ........................................................................................................................................... 145
6.3 DISCUSSION ..................................................................................................................................... 147
6.4 Conclusions ...................................................................................................................................... 149
Conclusions ...................................................................................................................................................... 151
Conclusiones .................................................................................................................................................... 155
7 Bibliography ............................................................................................................................................. 161

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Abstract
Organic molecules such as Polycyclic Aromatic Hydrocarbons (PAH) have the potential of substituting the
silicon-based technology in electronic circuits, and are also widely spread in Organic Light Emitting Diodes
(OLED) and in new designs of solar cells. The bottom-up construction of such devices with organic molecules
as the building blocks is one of the bases of nanotechnology. The ability of manipulating the electronic
properties and the relative position of the molecules with respect to each other is a great challenge for
controlling the outcome at the atomic scale to obtain new nanoarchitectures.
On-surface chemical modification of molecules is an excellent approach to obtain a controlled manipulation
of the organic functional molecules. In particular, transition metal surfaces are uniquely prone to engineer
desired outcomes from the pristine molecules, as some of the resultant structures cannot be achieved
otherwise. To this extent, many scientists focused in manipulating precursors in perfect systems such as on
single crystal metal surfaces in Ultra High Vacuum (UHV). The employment of such tools has the object to
control separately each parameter to achieve a desired outcome.
Depending on the electronic structure of a particular transition metal, the effect that the surface may have
on a PAH varies completely. In this thesis we focus on three organic molecules, namely C H N (a crushed
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triaza-fullerene), C H N (an open N-doped nanographene flake) and C (fullerene) on different metal
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surfaces. The latter molecule is in general very stable due to its symmetric shape; however, the first two
precursors differ from the fullerene, as when deposited on a surface they undergo chemical modification
upon thermal activation. The metal surfaces catalyse the cleavage of the C-H bonds, thereby leaving
unstable C dangling bonds which combine with either dangling bonds of the same or of another molecule,
creating new C-C bonds, or with the surface, forming new bonds between the carbon and the metal. This
process is named cyclodehydrogenation if new aromatic rings are generated, and dehydrogenation if no new
rings arise from the new bond.
When depositing submonolayer coverage of a PAH on a highly reactive surface such as Pt(111), the
molecules stick where they land, and therefore, as the C-H bonds cleave upon annealing, only intramolecular
modification is allowed because the molecules are fixed. Due to a bonding with the surface, there is no
interaction with other precursors and the molecule cyclodehydrogenates to modify its structure and its
electronic properties. C H N transforms into a triaza-fullerene, while C H N into a N-doped
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nanographene.
On the other hand, if the PAHs are deposited on a weakly interacting surface such as Au(111) or on other
coinage metals, the molecules are free to move at room temperature as they do not bind with the metal.
Upon annealing, they (cyclo)dehydrogenate and link together to form covalent chains: this is possible
because the activated molecules can meet each other because surface diffusion allows intermolecular
interaction.

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Abstract

The intermediate structures formed by C H N and C H N on different metals upon annealing have been
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investigated in details with different surface science experimental and theoretical techniques such as
Scanning Tunneling Microscopy (STM), high resolution X-ray Photoelectron Spectroscopy (XPS), Near-Edge X-
ray Absorption Fine Structure (NEXAFS) and Density Functional Theory (DFT) calculations. The shape of the
precursors has been selected on purpose to obtain triaza-fullerenes and N-doped nanographene, and the
employment of hetero-aromatic precursors is an elegant way of introducing dopants on the resultant
outcome. Before obtaining triaza-fullerenes, the partial cyclodehydrogenation of C H N allows to observe
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open-cage fullerenes. On the other hand, C H N undergoes a two-steps hierarchical transformation: due
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to steric reasons, the first C-H bonds to cleave upon annealing are the internal ones, hence forming
nanographene; however, further thermal energy injection also causes the edge C-H atoms to cleave, forming
N-doped nanodomes. Both precursors transform into N-doped graphene when enough thermal energy is
provided.
The third precursor, namely the fullerene, differs from the other two as it is much more stable, since it is
highly symmetric and it lacks of C-H bonds. In this thesis, we analyse the effect of the presence of this
molecule on the metal, rather than the opposite. C is well known to induce a single- or a multi-atom
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vacancy on transition metals, even though the mechanism of the vacancy formation is still under discussion.
According to the theoretical calculations, the energy needed for the surface to reconstruct is noticeably
lower in the presence of the fullerene. Our experimental observations reveal that the as-deposited
fullerenes at room temperature are weakly interacting with the surface, however annealing at 450 K causes
a charge transfer from the surface to the molecule. This is a metastable state in which an adatom-vacancy
pair is formed, as predicted by DFT calculations. Further annealing at 700 K forces the formation of a single
atom vacancy beneath each fullerene.

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