Regarding the phase of molecules, there are two distinct possibilities of investiga-tion. They may either be characterized in a dry phase, or in a liquid environment.
Dry molecules have few degrees of freedom and therefore rapidly assume a fixed configuration. In order to explore the full range of possible chemical reactions, molecules must be dissolved in a liquid solvent. Only within a solvent molecules have sufficient time and spatial freedom to form a monolayer, for instance, or to undergo more complex chemical changes. It must therefore be a primary aim of research in molecular electronics to design experimental configurations which facilitate measurements in liquid environment. Such a setup allows systematic investigation of the effect of different, pure solvents as well as measurements on various kinds of molecules.
2.4 Measurements in Liquid Environment
Figure 2.15: I vs. gap size curves in decanethiol. The solid lines indicate two regions with different tunneling de-cay constants [34].
Figure 2.16: Conductance vs. gap size curves in thiolated C60 [34].
Measurements in Liquid with an MCB
L. Gr¨uter has constructed the first system that integrates a liquid cell into an MCB setup [34]. During her PhD thesis, measurements on various solvents were performed. By variation of the electrode distance within the solvent, it was found that the solvents differ mainly in their tunneling decay constant.
Monolayers of alkanethiols were assembled on both electrodes of the MCB. The dependency of the current on the gap size was investigated. The measurement revealed two distinct ranges of gap size with different tunneling decay constants (cf. figure 2.15). This behavior is attributed to deformation of the monolayers during reduction of the gap size. When approaching each other, the monolayers may interlock and act as a mechanical resistance opposing further compression [34].
Furthermore, C60 molecules functionalized with one thiol group were investi-gated. The measurements of solvents mentioned above served as control experi-ment for the characterization of C60 molecules. It was found that the shape of the conductance curves depends strongly on the solvent used.
The C60molecules are bound by chemisorption of the thiol group to one electrode only. This results in different coupling strengths at each end of the molecule. Plots of the conductance versus the gap size exhibit a local maximum, as presented in figure 2.16. By application of a simple Breit-Wigner tunneling model, it was possible to extract the electronic tunneling rates.
This model for resonant tunneling in a double-barrier junction assumes one specific tunneling constant for each end of the molecule and one discrete molecular energy level between the ends. The thiolated C60 molecule allows for tuning the
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coupling to the electrode, because one tunneling rate is fixed while the other varies exponentially with the gap size. The resonant tunneling model results in a peak in the conductance once the molecular energy level aligns with the Fermi level of the leads.
Measurements in Liquid with an STM
Only a few measurements have been performed with an STM in liquid solution.
B. Xu et al. investigated bipyridine and alkanedithiols of different lengths [35]. The tunneling decay constants of alkanedithiols obtained from these measurements are consistent with values obtained from first-principles calculations. Furthermore, the characterization yields a single-molecule conductance of 2×10−5G0 for de-canedithiol and 1.2×10−3G0 for hexanedithiol.
L. Venkataraman et al. characterized amine-terminated molecules in solution be-tween gold contacts [36]. Conductance histograms obtained from this experiment exhibit much sharper peaks than conductance diagrams obtained from dithiol-gold junctions. Therefore, the amine linkage provides well-defined conductance mea-surements of a single molecule. The tunneling decay constant obtained from these measurements is in good agreement with calculation based on density-functional theory.
3
Experimental Section
There is still no routine procedure for the investigation of electrical properties of single molecules. Electrodes require small and sharp tips so that a single mole-cule can be trapped between them. J. v. Ruitenbeek and others have developed one method which suits this task, called mechanically controllable breakjunction technique.
A metal wire only 100 nm in width is produced on a substrate. The gold wire is then carefully stretched by applying mechanical pressure on the substrate, as seen in figure 3.1. Thus, the wire successively narrows until it finally tears apart, thereby providing two atomically sharp electrodes facing each other at the two ends of the metal wire. A detailed sketch of the wire structure may be found on page 24.
In order to trap a single molecule, in the next step, both electrodes are covered with identical monolayers. If the gap measurement between the electrodes equals the total length of two molecules in the monolayer, then one molecule on each electrode may chemically bind to the opposing one and form a dimer1. The final molecule that links the electrodes is therefore chemically assembled in situ from two identical monolayer molecules. For details, cf. page 32.
Another type of molecule investigated consists of two identical monolayers plus a central linking group which is inserted between the monolayers in situ as well.
The chemical reaction is displayed on page 31.
Provided that the electrode tips are sufficiently sharp, one single molecule will span the gap. For less defined tips, one may expect a few parallel molecules bridging the gap.
1Dimer generally denotes a two-compound structure in chemistry, while trimer accordingly denotes a three-compound structure.
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substrate gold
2!m
L u
t
"u
"x
Figure 3.1: Left: Principle of the mechanically controllable breakjunction tech-nique, with substrate thickness t, distance between counter supports L, free sus-pended length u, elongation of the free sussus-pended length δu, and movement of the pushing rod δx. Center: Colored scanning electron microscope image of the bridge. Right: Illustration of the central point contact.
This chapter contains a brief description of the breaking mechanism and the setup utilized, followed by an overview of the manufacturing process of the samples.
At the end of the chapter, a description of the molecules investigated is given as well as information about molecule deposition and about the chemical processes involved in the investigation.