Surface-confined molecular self-assembly [Elektronische Ressource] / Rico Gutzler

Ricogutzler

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2010
• Nombre de lectures: 31
• Langue: English
• Poids de l'ouvrage: 29 Mo

Extrait

Surface-Confined Molecular
Self-Assembly
Dissertation der Fakultät für Geowissenschaften der
Ludwig-Maximilians-Universität München
Rico Gutzler
München, 14. September 2010Supervisor: Prof. Dr. Markus Lackinger
Second Referee: Prof. Dr. Wolfgang M. Heckl
Disputation: 19. November 2010Abstract
The design and fabrication of nanometer-sized entities of defined structure on the
nanometer scale is a challenge hardly achievable by large-scale top-down procedures.
In particular, alternative techniques are needed to structure surfaces at very small
length scales. In nanotechnology the process of self-assembly has become a potent
method which allows to vanquish these difficulties. Self-assembly is a mechanism by
which randomly distributed small units arrange into large, ordered structures under
equilibrium conditions. Information about the final morphology of the self-assembled
structure is encoded at least partially in the basic building units. The environment
constitutesanadditionalparameterwhichinfluencestheself-assemblyprocess, impair-
ing both its thermodynamics and kinetics. Thus, any assembly has to be interpreted
in view of the boundary conditions which the environment inflicts.
Inthefollowing, self-assemblyofmoleculesiselucidatedbasedonexperimentalresults.
The formation of molecular monolayers at surfaces, under ambient conditions as well
as in vacuum, is discussed in the light of the parameters that govern self-assembly.
Thermodynamic and kinetic considerations are employed to gain deeper insight into
the growth mechanisms of the monolayers. The primary instrument applied is the
scanning tunneling microscope, a device which facilitates the real-space observation
of surface-bound molecular structures with submolecular resolution. The assembly
of various organic molecules is studied at the liquid/solid interface under ambient
conditions. Especially the effects of molecular structure and molecular interactions,
type of solvent, concentration, and temperature on the self-assembly process are in-
vestigated. Additionally, monolayer formation under ultra-high vacuum conditions
is explored. Particularly the influence of temperature and of the type and crystal-
lographic orientation of the crystal surface that serves as substrate for monolayer
growth is studied. Both cases, liquid/solid and ultra-high vacuum, are fundamentally
different with respect to the environment in which molecular self-assembly takes place.
Hence they allow for comparative and complementary conclusions when adequately
analyzed. This knowledge provides a basis for the deliberate tuning of the morphol-
ogy of self-assembled monolayers, and eventually facilitates the defined manipulation
of surfaces at the molecular level. Although much research has been done on self-
assembled molecular monolayers, the delicate interplay between building blocks and
environment in molecular self-assembly is still far from being fully understood. Only
if fundamental principles and interactions in self-assembly are well comprehended, a
controlled approach to deliberate surface patterning and functionalization becomes
feasible.Contents
1 Introduction 1
1.1 Self-Assembly in Nanotechnology . . . . . . . . . . . . . . . . . . . . . 1
1.2bly, a Brief Definition . . . . . . . . . . . . . . . . . . . . . . 3
2 Scanning Tunneling Microscopy 4
2.1 Contrast Formation in Molecular Adsorbates . . . . . . . . . . . . . . . 4
2.2 The Low-Temperature Ultra-High Vacuum Scanning Tunneling Micro-
scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3 Interfacial Self-Assembly, Thermodynamics, and Kinetics 14
3.1 Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4 Molecular Self-Assembly 25
4.1 Molecular Building Blocks . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.2 Intermolecular Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.3 Reversibility of Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.4 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.5 Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.6 Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.7 Vacuum/Solid Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.8 Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5 Conclusion 39
List of Figures 41
Bibliography 42
List of Publications 51
6.1 Aromatic Interaction vs. Hydrogen Bonding in Self-Assembly at the
Liquid-Solid Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.2 Surface Mediated Synthesis of 2D Covalent Organic Frameworks: 1,3,5-
Tris(4-Bromophenyl)Benzene on Graphite(001), Cu(111), and Ag(110) . 736.3 Combination of a Knudsen Effusion Cell With a Quartz Crystal Mi-
crobalance: InSitu MeasurementofMolecularEvaporationRatesWith
a Fully Functional Deposition Source . . . . . . . . . . . . . . . . . . . 86
6.4 Influence of Solvophobic Effects on Self-Assembly of Trimesic Acid at
the Liquid-Solid Interface . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.5 Reversible Phase Transitions in Self-Assembled Monolayers at the
Liquid-Solid Interface: Temperature-Controlled Opening and Closing
of Nanopores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
6.6 Material and Orientation Dependent Activity for Heterogeneously Cat-
alyzed Carbon-Bromine Bond Homolysis . . . . . . . . . . . . . . . . . 117
Acknowledgements 123
CV 124Chapter 1
Introduction
1.1 Self-Assembly in Nanotechnology
A great part of current advances in nanoscience and nanotechnology is based on a
process termed self-assembly, a mechanism by which disordered small building units
spontaneously arrange into large, ordered phases. For example, self-assembly steers
[1] [2]the growth of molecular crystals and molecular monolayers, is responsible for the
[3,4] [5]folding of proteins, and affects the morphology of lipid bilayers and colloidal par-
[6]ticles. The importance of self-assembly thus manifests itself not only in the advances
in material science, but also through its role in biology, life science, and the physics
and chemistry of soft matter. Further examples for the successful use of self-assembly
as a bottom-up approach for engineering at very small dimensions include the fabri-
[7] [8]cation of nanoscopic structures like quantum dots, photonic-bandgap materials,
[9]and the design of hybrid organic-inorganic materials. Supramolecular chemistry at
surfaces, i.e., the association of molecules through rather weak non-covalent bonds, is
[10–14]yet another case where molecular self-assembly is decisive. In this work the for-
mation of molecular monolayers will be discussed in some detail, and the importance
of self-assembly for this process will be elucidated.
Understanding how molecular crystals emerge in self-assembly from a disordered state
requires more than just knowledge about the forces between two molecules. Rather
rarely it is appreciated that kinetic and entropic considerations are equally important.
Owing to the fact that material synthesis through self-assembly is still mostly a trial-
and-error process, a profound understanding of its underlying concepts is desirable,
and will eventually lead to more educated approaches to various problems in nan-
otechnology. Particularly supramolecular crystal engineering in two-dimensions (2D)
istheprincipalobjectiveforstructuringandfunctionalizingsurfaces. Atthemolecular
level, self-assembly is responsible for the formation of ordered molecular monolayers
at the liquid/solid interface as well as at the vacuum/solid interface. A prominent
1Chapter 1 Introduction
example is the formation of self-assembled monolayers (SAM) on gold surfaces from
[2]thiol-functionalized molecules. These layers permit to alter the surface termination,
and thus to change the surface’s physical and chemical properties. Studies on molecu-
lar thin films are naturally conducted with surface sensitive techniques like low-energy
electron diffraction (LEED), atomic force microscopy (AFM), and scanning tunneling
microscopy (STM). They cover a whole variety of research interests like the identi-
[15,16]fication of exact epitaxial relations of adsorbate to substrate lattice, dewetting
[17] [18]of organic molecular layers on insulators, molecular transport on surfaces, or
[19–21]probing properties like molecular orbital distributions. Especially the STM has
proven to be a beneficial tool for probing molecules with high spatial resolution. Its
use is not limited to imaging single molecules, islands of molecular aggregates, and
monolayers in real space, but furthermore allows to acquire information about the lo-
[22,23]cal electronic structure of a molecule by scanning tunneling spectroscopy (STS).
This work continues with a brief convenient definition of self-assembly and its detailed
description for molecular systems at surfaces. The next section gives a short descrip-
tion of scanning tunneling microscopy of molecular monolayers, since it was the pri-
mary technique used in this work. This includes a description of the low-temperature
ultra-high vacuum STM which was set up as a complementary instrument