Samorí , Paolo : Self-assembly of conjugated (macro)molecules: nanostructures for molecular electronics


Kapitel 1. Introduction

In the last two decades there has been a growing interest towards the nanoworld. The scientific community was prying into casting new light on the structure of organic, inorganic and biological materials, probing their chemical and physical properties on a molecular scale and comparing the properties of a single molecule with those of an ensemble or Avogadro number of molecule. Manipulating single molecules at room temperature [Jun96], visualizing [Sta95a] and stimulating chemical reactions [Hei94] at surfaces are just few examples of how the scientific community has been able to approach to the nanoworld. Particularly interesting in this new scientific approach is what one may call nanochemistry and nanophysics at interfaces.

Until few years ago information on the sub-micrometer scale length was accessible only using indirect techniques such as electron or X-ray diffraction or with electron microscopies that required vacuum environment and conductive materials.

Figure 1.1: Spatial resolution of some microscopical techniques

In 1982 a breakthrough occurred: the Scanning Tunneling Microscope (STM) was invented by Binnig, Rohrer, Gerber and Weibel [Bin82a]. Their project aimed at developing a microscopical technique able to generate real-space images of surfaces with a resolution on the nanometer scale. Four years later they were awarded the Nobel prize in physics for their invention. This discovery represented also a big improvement for the development of miniaturized electronic devices. Even greater importance had the invention of the Atomic Force Microscope (AFM) [Bin86], known also as Scanning Force Microscope (SFM), that made it possible also to investigate insulating materials such as polymers and biomolecules.


On the other hand in the last two decades conjugated molecular species have received a great deal of attention in the chemical and physics community because they are building blocks of materials with high electronic conductivity and even superconductivity [Chi77].

In view of these two developments, the aim of this thesis is to grow highly ordered molecular nanostructures of conjugated (macro)molecules with well defined chemical functionalities and physical properties that arise from the molecules their selves. These architectures could be useful for building molecular based electronic devices, in particular a molecular nanowire. Scanning Probe Microscopies played a pivotal role in this project because they allowed to investigate self-assembled architectures on flat solid substrates and to monitor dynamic processes at surfaces.

This thesis is divided into seven chapters: after this brief introduction, the second chapter will introduce the different Scanning Probe Microscopies, focusing on the Scanning Tunneling Microscopy and Scanning Force Microscopy. The third chapter introduces to the conjugated molecular systems, their synthesis and their application in the development of molecular based electronic devices. In the fourth chapter the self-assembly of molecular species on flat solid substrates and the development of nanoelectrodes are addressed. The fifth chapter is dedicated to the experimental procedures. The sixth chapter will discuss the results of this research, first chemisorbed monolayers on metallic substrates and then physisorbed layers and multilayers both at the solid-liquid interface and in dry films produced from solution or by sublimation in vacuum. The seventh chapter will summarize and give outlooks of the project. Last but not least, the acknowledgments underline the importance of the collaborative efforts for an interdisciplinary work such as that presented here.

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