Molecular electronic building blocks based on self-assembled monolayers

Lüssem, Björn

Dissertation / PhD Thesis/Book

Molecular electronic building blocks based on self-assembled monolayers

Lüssem, B. (Corresponding author)FZJ*

2006
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 3-89336-454-4

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich. Reihe Informationstechnik / Information Technology 12, 138 S. (2006) = RWTH Aachen, Diss., 2006

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Please use a persistent id in citations: http://hdl.handle.net/2128/2542

Abstract: The evolution of microelectronics often described by Moore’s law is driven by a continuous downscaling of all electronic devices. However, it is expected that the miniaturisation of the CMOS technology as the currently used technique for the fabrication of transistors and integrated circuits will face fundamental physical limitations within the next 15 to 20 years. Molecular electronics is one alternative to CMOS. The aim of molecular electronics is to contact single molecules, to utilise the current transport properties of these molecules for information technology and to build complex logical circuits. One method of contacting single molecules is described in this thesis: the method of contacting molecules embedded in a molecular monolayer by scanning tunnelling microscopy (STM). A self-assembled monolayer of insulating alkanethiols is deposited onto an atomically flat gold film. The molecules of interest, in this case biphenylthiols, are embedded into this monolayer in a second deposition step. The gold film represents the bottom contact to the molecule, and the top contact is formed by the STM tip resulting in a current flow from gold across the molecule to the tip. The bottom and top contact are slightly different: Whereas the molecule is covalently bound to the gold, the top contact results from a weak coupling of the tip with the molecule across a vacuum gap. All necessary steps towards the completion of the aforementioned device setup are carried out in this thesis. In particular, the gold film is optimised, different methods for depositing monolayers of alkanethiols are evaluated, monolayers of biphenylthiols are studied by STM, and finally biphenylthiols are embedded into a self-assembled monolayer of alkanethiols. One focus of the thesis is the characterisation of the current transport properties of biphenylthiols. Two different methods are used. By current vs. distance spectroscopy it is shown that the current through the monolayer of biphenylthiols is best described by a tunnelling process. The decay length of the biphenyl group can be obtained from these measurements ($\beta$ = 4.7±0.8nm$^{-1}$ ). Furthermore, the height differences between alkanethiols and biphenylthiols as seen in the STM images of mixed monolayers are interpreted in terms of a tunnelling model. As for the preceding method, the decay constant of the biphenyl group is determined ($\beta$ ~ 5nm$^{-1}$). The significance of this thesis is not only based on the characterisation of the current transport through biphenylthiols, but also on the development of a setup by which the conductance of a whole class of molecules can be measured. Thereby, a method is developed that helps to gain a broader understanding of current transport through single molecules.

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