Electronic, geometric and functional group effects in the adsorption of organic molecules: STM and STS of ultra-thin layers of phthalocyanines and naphthalocyanines on graphite (0001) [Elektronische Ressource] / vorgelegt von Thiruvancheril G. Gopakumar
114 pages
Deutsch

Electronic, geometric and functional group effects in the adsorption of organic molecules: STM and STS of ultra-thin layers of phthalocyanines and naphthalocyanines on graphite (0001) [Elektronische Ressource] / vorgelegt von Thiruvancheril G. Gopakumar

-

Le téléchargement nécessite un accès à la bibliothèque YouScribe
Tout savoir sur nos offres

Description

Electronic, Geometric and Functional-Group Effects in the Adsorption of Organic Molecules: STM and STS of Ultra-Thin Layers of Phthalocyanines and Naphthalocyanines on Graphite (0001) von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz genehmigte Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) vorgelegt von M. Sc (Master of Science) Thiruvancheril G. Gopakumar geboren am 10. May 1978 in Kurattikadu (Kerala, Indien) eingereicht am 30 Januar 2006 Gutachter: Prof. Dr. Michael Hietschold Prof. Dr. Dr. h.c. Dietrich R.T. Zahn Prof. Dr. Wolfgang M.

Sujets

Informations

Publié par
Publié le 01 janvier 2006
Nombre de lectures 36
Langue Deutsch
Poids de l'ouvrage 8 Mo

Exrait


Electronic, Geometric and Functional-Group
Effects in the Adsorption of Organic Molecules:
STM and STS of Ultra-Thin Layers of
Phthalocyanines and Naphthalocyanines on
Graphite (0001)



von der Fakultät für Naturwissenschaften der Technischen Universität
Chemnitz genehmigte Dissertation zur Erlangung des akademischen Grades

doctor rerum naturalium
(Dr. rer. nat.)


vorgelegt von
M. Sc (Master of Science) Thiruvancheril G. Gopakumar
geboren am 10. May 1978 in Kurattikadu (Kerala, Indien)

eingereicht am 30 Januar 2006


Gutachter: Prof. Dr. Michael Hietschold
Prof. Dr. Dr. h.c. Dietrich R.T. Zahn
Prof. Dr. Wolfgang M. Heckl


Tag der Verteidigung: 28 Juni 2006









































“There is nothing new to be discovered in physics now, All that remains is more and more precise
measurement”
Lord Kelvin (1824 -1907)

“The most fundamental and lasting objective of synthesis is not production of new compounds, but
production of new properties”
George Hammond (1921-2005)

Bibliographische Beschreibung

Thiruvancheril Gopalakrishnan Gopakumar
Electronic, Geometric and Functional-Group Effects in the Adsorption of Organic Molecules:
STM and STS of Ultra-Thin Layers of Phthalocyanines and Naphthalocyanines on Graphite
(0001)
Dissertation (in englischer Sprache), Technische Universität Chemnitz,
Fakultät für Naturwissenschaften, Chemnitz, Januar 2006
114 Seiten, 62 Abbildungen, 06 Tabellen, 01 Schema

Referat

Aus der riesigen Vielfalt organischer Materialien sind gerade die Phthalocyanine dafür bekannt
geworden, auf verschiedenen kristallinen Substraten geordnete Strukturen auszubilden. Außerdem
dienen diese Moleküle als Modellsysteme für grundlegende Untersuchungen zur Einstellung
elektronischer und struktureller Eigenschaften durch gezielten Einbau eines Metallatoms in den
zentralen Hohlraum. Die Strukturen der Adsorptionsschichten von verschiedenen
Metallphthalocyaninen, funktionalisierten Phthalocyaninen und Naphthalocyaninen auf der
Basaldebene des Graphits werden verglichen, um die Adsorptionsstruktur der einzelnen Moleküle
innerhalb der Adsorptionsschicht zu verstehen. Das erlaubt uns die Untersuchung der Molekül-
Molekül- und Molekül-Substrat-Wechselwirkungen in Abhängigkeit von Molekularadsorption des
zentralen Metallatoms, der Geometrie, einzelner funktionellen Gruppen oder ähnlichem am Molekül.
8Der Vergleich der Adsorptionsstrukturen von Phthalocyaninen wie PdPc und PtPc, welche d -Metalle
enthalten, dient dem Verständnis für den Effekt des Metallatoms, speziell bei großen
Ordnungszahlen. Während beide Moleküle ähnliche Arten von Adsorptionsstrukturen ausbilden
weist die PtPc-Adsorptionsschicht eine außergewöhnlich hohe thermische Stabilität auf. Das wurde
auf die stark Molekül-Molekül-Wechselwirkung zurückgeführt, die durch die Metallatome in der
Adsorptionsschicht vermittelt wird. Der Effekt langer molekularer ‚Flügel’ wird durch den Vergleich
der Adsorptionsstrukturen ebener Naphthalocyanine mit denen von ebenen Metallphthalocyaninen
demonstriert. Naphthalocyanine bilden viel lockerer gepackte Adsorptionsschichten als
Phthalocyanine, was von der stärkeren sterischen Abstoßung zwischen den Wasserstoffatomen in
benachbarten molekularen ‚Flügeln’ herrührt. Cyano-funktionalisierte metallfreie Phthalocyanine
zeigen als Adsorptionsschicht eine poröse Netzwerkstruktur. Es konnte gezeigt werden, dass durch
die Sondenspitze hervorgerufene Störungen dieser Struktur durch die elektrostatische
Wechselwirkung zwischen den Molekülen in der Adsorptionsschicht bald weider ausheilen.
Schließlich ist durch den Vergleich von Adsorptionsstrukturen ebener Naphthalocyanine und
nichtebener Zinn-Naphthalocyanine auch der Geometrieeffekt untersucht worden. Abweichend von
allen anderen untersuchten ebenen Molekülen haben Zinn-Naphthalocyanine eine
Adsorptionstruktur, die der des Graphits ähnlich ist (hexagonal). Deshalb überwiegt in diesem Fall
die Wechselwirkung zwischen Molekül und Substrat, und die Adsorptionsstruktur folgt der
Geometrie des Graphitsubstates.
Darüberhinaus sind mit der Tunnelspektroskopie in Abhängigkeit vom Abstand zwischen der Spitze
und der Probe die elektronischen Eigenschaften der Molekül-Substrat-Grenzfläche für
Naphthalocyanin und Zinn-Naphthalocyanin untersucht worden.

Schlagwörter

Adsorption, Graphit, LEED, Naphthalocyanin, Phthalocyanin, Rastertunnelmikroskopie, STM, STS,
Selbstassemblierung, Tunnelspektroskopie, Orbitale, HOMO, LUMO.
___________________________________________________________________Contents

Contents

List of Abbreviations………………………………………………………………….…. 7

1.0. Introduction 8
1.1. Outline of the Thesis…...………………………………………………….…. 9

2.0. Experimental
2.1. Experimental Methods…………………………………………………….… 11
2.1.1. Scanning Tunneling Microscope…………….………………….. 11
2.1.1.1. General Principle……………………..………………… 12
2.1.1.2. Tunneling Effect………………………..………………. 14
2.1.1.3. What Does STM Measure?…….………..…….………. 15
2.1.1.4. Orbital-Mediated Tunneling and Molecular Imaging…. 17
2.1.2. Low Energy Electron Diffraction………………………….……. 18
2.1.2.1. Working Principle…………………………………..….. 19
2.1.2.2. Theory of LEED………………..…. 20
2.2. Sample Preparation………………………………………………………..…. 21
2.2.1. Purification of Organic Molecules………………….… 21
2.2.2. Preparation of Substrate…………………………… 21
2.2.3. Preparation of Tunneling Tips…………………………………... 22
2.2.4. Epitaxy of Organic Molecules…………………………………... 23
2.2.5. General Principles of Molecular Epitaxy……………………….. 24
2.3. Experimental Set-up……………………………………………………….… 28
2.4. Theoretical Methods……………………………………………………….… 29
2.4.1. Hartree-Fock Theory…………………………………………..… 29
2.4.2. Density functional theory……………………….…….. 30
2.4.3. Gaussian Program……………………… 31

83.0. Effect of Metal Atoms in the Adlayer Structure and Electronic Properties of d

MePcs on Graphite
3.1. Introduction…………………………………………………………….……. 32
3.2. Palladium Phthalocyanine……………………………………………….…... 33
3.2.1. Adlayer Structure…………………………………………….….. 33
3.2.2. Tip-Induced Molecular Diffusion………………………….……. 37
3.2.3. LEED Experiments of PdPc at Graphite Single Crystal…...……. 39
___________________________________________________________________Contents
3.3. Platinum Phthalocyanine……………………………………………….……. 41
3.3.1. Adlayer Structure…………………….…….. 41
3.3.2. Single-Molecular Adsorption Structure…………..………….….. 43
3.4. Comparison of Molecular Orbital Structure of PdPc and PtPc……….……... 45
3.5. Conclusion…………………………………………………………….……... 49

4.0. Porous Network Structure of Octacyano-Metal-Free Phthalocyanine on the
Basal Plane of HOPG: Effect of Functional Group in the Adlayer Structure of
Phthalocyanine
4.1. Introduction………………………………………………………….………. 50
4.2. Adlayer Structure of H Pc(CN) ……………………………………..………. 2 8 51
4.2.1. Different Models……………………………………………….... 53
4.2.2. Different Phases and Defects………………………………….… 56
4.3. Tip-Induced Molecular Rearrangement…...……………………………….... 58
4.4. Conclusion………………………………………………………………….... 60

5.0. Effect of Geometry in the Adlayer Structure of Naphthalocyanine at HOPG
5.1. Introduction……………………………………………………………….…. 61
5.2. Adsorption Geometry of Planar Metal-Free Naphthalocyanine……...…..….. 62
5.2.1. Monolayer………………………………………………….……. 62
5.2.2. LEED Experiments on the Ultra-Thin Film…………..…….…... 65
5.2.3. Multilayer……………………………………………………..…. 66
5.3. Adsorption Geometry of Non-Planar Tin-Naphthalocyanine……...…….….. 68
5.3.1. Monolay 68
5.3.1.1. Pure Phases………………………………………..……. 71
5.3.1.2. One-Dimensional Selectivity………...…………..……... 74
5.3.2. Multilayer……………………………………………….………. 79
5.4. Voltage-Induced Flipping of Non-Planar Tin-Naphthalocyanine……....…… 81
5.5. Conclusion…………………………………………………….……………... 84

6.0. Tip-Sample Distance-Dependent STS on Planar and Non-Planar

Naphthalocyanines at HOPG
6.1. Introduction……………………………………………………….…………. 85
6.2. Tunneling Spectroscopy of Naphthalocyanine at HOPG…………....………. 86
6.2.1. Current-Voltage Characteristics………………………….……… 86
6.2.2. Normalized Differential Characteristics………………….……... 87
6.2.3. Comparison with Theoretical Calculations………….…….......... 89
___________________________________________________________________Contents
6.2.4. Model for HOMO-LUMO Gap Shrinking…………………..…... 90
6.3. Tunneling Spectroscopy of Tin-Naphthalocyanine at HOPG……………….. 93
6.3.1. Current-Voltage and Normalized Differential Characteristics….. 93
6.3.2. Comparison with Theoretical Calculations……………………… 94
6.4. Conclusion………………………………………………………………….... 96

7.0. Summary and Outlook…………………………………………………………….... 97

Bibliography……………………………………………………………………………… 101

Erklärung…………………………………………………………………………………. 112

Curriculum Vitae……………………………………………………………………….... 113

Acknowledgement……………………………………………………………...………..... 114

____________________________________________________________List of abbreviations

List of Abbreviations

CoPc Cobalt Phthalocyanine
CuPc Copper Phthalocyani
DFT Density Functional Theory
FePc Iron Phthalocyanine
FFT Fourier Transform
H Hydrogen
H Pc(CN) Octacyano-Hydrogen Phthalocyanine 2 8
HF Hartree-Fock
HOMO Highest Occupied Molecular Orbital
HOPG Highly-Oriented Pyrolytic Graphite
HV High Vacuum
LEED Low-Energy Electron Diffraction
LUMO Lowest Unoccupied Molecular Orbital
MePcs Metal Phthalocyanines
MO’s Molecular Orbitals
Nc Naphthalocyanine
NiPc Nickel Phthalocyanine
OMBD Organic Molecular Beam Deposition
OMBE Organic Molecular Beam Epitaxy
OMCs Organic Molecular Crystals
Pc Phthalocyanine
Pcs yanines
PdPc Palladium Phthalocyanine
PtPc PlatinumPhthalocyani
SnNc Tin Naphthalocyanine
SnPc Phthalocyanine
STM Scanning Tunneling Microscope
STS Scanning Tunneling Spectroscopy
TEM Transmission Electron Microscope
UHV Ultra High Vacuum
vdW van der Waals
VOPc Vanadyl Phthalocyanine

7__________________________________________________________________Introduction

Chapter 1

1.0. Introduction

Today’s technological approach (top-down), which is based on building the fundamental
units of electronic devices by cutting the materials down to micrometer or nanometer dimensions, is
limited when the size of the building blocks approaches the quantum size, effects like discretization
[Yoff02] [Anto97, Chem87, Wada94, of energy, confinement of electrons in limited dimensions, non-linear effects,
Zhel04] [Hiro00, Rao03] [Min98, Wong05] gate leakage through tunneling, contact resistance, poly depletion layers,
[Min98]diffusion of materials (dopants) at interfaces, temperature stability etc. come in to play. This
creates a technological road block, which does not permit to use the current approach for further
developments into the future. Therefore the future should be based on approaches and materials
different from those used now. A new approach called the “bottom-up” approach can be employed to
resolve this problem, where one brings different nano-sized objects like nanoparticles, organic
molecules (zero-dimension), nanowires (one-dimension), thin-films of inorganic/organic
semiconductors (two-dimension) etc. together and forms the fundamental units of electronic devices.
The selection of these materials is purely based on their electronic functionalities like switching
effects, rectifying, capacitance, storage, conductivity (wires and insulators) and so on.
Organic molecules are suitable candidates as building blocks for the future electronics due to
[Welt03]their size, easy availability in high purity, tunable optical, electrical and structural properties
[Böhr99, Dmit03, Grie04a, Hipp02, and moreover due to their ability to self-organize into different nanostructures.
Iked04, Lack02b, Lein00, Safa04, Spil03, Scud03, Step04, Wei04, Yabl02, Yang05, Yoko01b] The wide range of organic molecular
[Avir74, Lara03] [Coll00, Korn01, Stew04]species offers different electronic components like diodes, switches,
[Chen99, Sess93] [Davi98, Pato98, Reic02]storage, wires and insulators, which fueled the hope that electronic
devices can be shrunk from the current micrometer-length scale all the way down to the molecular
[Joac00]scale. Applications based on inorganic thin films like low-threshold current laser diodes, low-
[Shia94, noise avalanche photo-detectors, high band-width optical modulators are already demonstrated
Wada94]. Similarly organic multiple quantum wells (OMQW) based on self-organised thin films of
[Lam91]organic molecules have shown their applications in non-linear optics , organic light emitting
[Bald98, Frie99, Leun00, Shah99, Stru96, Tang87] [Alt92, Li98]devices, field effect transistors and molecular
[Adle94, Coll99, Heat98]memories . The potential of an entirely new class of materials therefore offers
unprecedented opportunities for extending our understanding of the fundamental properties of a large
class of material (nanostructured organic molecular crystals). These structures, in turn, promise to
have many new and useful properties which will undoubtedly be heavily exploited in the next
decades.
8__________________________________________________________________Introduction
The design strategies of the above mentioned molecular-nanostructures purely depend on
their self-assembling ability. Therefore, to consider a real architecture of the molecular species down
to nanostructure, it is essential to have the knowledge about their self-assembling properties and the
type of interactions in the interlayer and intralayer. Knowledge about the self-assembly is however
limited due to the incomplete understanding of the non-covalent interactions like hydrogen bonds,
van der Waals interactions, π- π interactions, electrostatic interactions, chelating effect, halogen
bonding, self-organised criticality and so on, between the molecules. Moreover the wide range of
molecular species involved increases the complexity in understanding different effects like
geometric, electronic, functionality etc during the process of self-assembly. Therefore, it is essential
to converge the study of self-assembly into certain model systems which would offer an opportunity
to change its chemical and physical nature in a controlled way.
Phthalocyanines are well known for the formation of ordered structures on different
[Gimz87, Suto03, Yash03]crystalline substrates. They belong to an interesting class of organic dye molecules,
which has a stable extended π-electron system and a structural analogy with biomolecules such as
chlorophyll, vitamine B , hemoglobin, and so on. The tunability of their electronic and structural 12
properties by the selective addition of a metal atom to the central cavity makes these molecules ideal
[Fan78a, Fan78b, for fundamental scientific and technological applications such as optoelectronic devices,
Hone02, Mina91, Wang03, Xiao03, Yong02, Yama98] [Bao96, Zhan04] [Coll88] organic field effect transistors, sensors, solar
[Amao03, Ghos74]cells, etc. In the current thesis the adlayer structures of different phthalocyanines and
naphthalocyanines are compared to understand the adsorption structure of individual molecules
within the adlayer. Different metal atoms inside the central cavity allowed studying the effect of
geometry (in the case of SnNc) and effect of metal electronic structure in the adlayer formation.
Naphthalocyanine has been studied to find the influence of molecular lobes in the adlayer and
thereby the understanding of steric-construction in weakly interacting systems. The cyano-
functionalized metal phthalocyanine has been studied to demonstrate the effect of functional groups
and the stability resulting from the electrostatic interaction between the molecules in the adlayer.

1.1. Outline of the Thesis
The current thesis is sub divided into seven main chapters out of which the first chapter is an
introduction to the molecular systems and their applicability to different fields. Moreover this chapter
shows the limitations of current technological approach and the requirement of new materials and
approaches. Chapter 2 discusses the experimental methods like LEED, STM, OMBE etc. employed
in the study, a theoretical background for the molecular epitaxy, and some basics to the theoretical
approaches used to calculate the single-molecular electronic structure like HF method, DFT etc.
8Chapter 3 deals with the effect of metal atoms in the adlayer structure where d transition metal
complexes, PdPc and PtPc are studied. Chapter 4 demonstrates the effect of a functional group in the
adlayer formation of phthalocyanine where a cyano-functionalised metal-free phthalocyanine is
9
B__________________________________________________________________Introduction
studied. Chapter 5 shows the influence of geometry in the adlayer structure where planar
naphthalocyanines (phthalocyanines with one additional benzene ring on each molecular lobe) and
non-planar SnNc are studied. Voltage-induced flipping of the non-planar SnNc from one adsorption
geometry to another is also demonstrated in this chapter. In chapter 6, the molecule-substrate
interface properties of the above mentioned molecules (Nc and SnNc) are studied using tip-sample
distance dependent tunneling spectroscopy. The main body of the thesis ends with the summary and
outlook as chapter 7, which is then followed by the bibliography, Erklärung (declaration), bio data
and acknowledgement.
10