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Small organic molecules as potential building-blocks for future nanoelectronic devices [1][2][3][4][5] will require new types of sensors able to identify/quantify molecules in appropriate solutions on a single-molecule level. Recently, an elegant new method has been proposed [6] that allows detection of small aromatic units like 4-aminothiophenol (4-ATP) molecules by measuring the tunneling resistance between two metal electrodes separated by a short distance (a few nanometers). While, from a simple point of view, an increase in conductivity should be expected because of the bridging of the tunnelling gap by one or more molecules (thus offering molecular orbitals as additional transport channels), a decreased conductivity was observed experimentally with a reduction factor that depended on the type of molecule present in the solution. Here, we report on combined experimental and theoretical efforts aimed at unravelling this phenomenon by studying the electronic properties of one of the metal electrodes in such a molecular junction.For this purpose, a 4-ATP self-assembled monolayer (SAM) has been prepared on top of a Au(111) crystal, which, in a second step, has been metallized by a nearly closed Pd overlayer of monoatomic height by means of a recently developed electrochemical approach. [7][8][9][10] Photoelectron spectroscopy together with density functional theory (DFT) taking into account all contributing parts of the molecular junction finally allowed analysis of its structural setup and its electronic properties. Angle-resolved X-ray photoelectron spectroscopy (XPS) reveals that the 4-ATP SAM actually consists of a minimum of two molecular layers. Most importantly, using ultraviolet photoelectron spectroscopy (UPS) and DFT simulations, strong chemical interactions between the metal overlayer and the amino groups are found to play a decisive role in determining the overall electronic properties, and thus the transport properties of the SAM/metal contact, as will be demonstrated in the following.It is well-known that 4-ATP has a strong tendency to form multilayers on Au(111), which lead to scanning tunnelling microscopy (STM) images with considerable height variations and of blurred contrast when it comes to molecular-scale resolution.[11] Even for highly diluted solutions (sub-millimolar concentrations of the 4-ATP), more than just one layer is generally formed. On the other hand, reductive desorption of thiols from gold surfaces is known to occur at electrode potentials negative of À0.1 V vs. standard calomel electrode (SCE), which may be considered an appropriate means for desolving any thiol in excess of the first layer.[12] In Figure 1 cyclic voltammograms are shown for Au(111) in 0.1 M H 2 SO 4 , after the electrode had been immersed for 15 min in a 0.1 Â 10 À3 M 4-ATP/0.1 M H 2 SO 4 modification solution. While the first cycle (dash-dotted line, start at þ0.2 V vs. SCE in the negative direction) was restricted to the stability range of the 4-ATP adlayer, i.e., 0 and þ0.4 V, the second cycle (solid line)...

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Since the first proposal [1] by Aviram and Ratner to use organic molecules as new building blocks for information technology, tremendous efforts have been spent to implement basic electronic units such as rectifiers or transistors.[2] To fabricate appropriate metal-molecule-metal hybrid structures, two strategies have been followed in recent years, resulting in either planar or sandwich arrangements of the involved materials.[3] In the planar design, nanolithography at its current technological limits is required to manufacture metal electrodes with gaps precisely corresponding to the length of the molecules involved (1-3 nm). In the sandwich design, counter electrodes need to be deposited on top of molecular layers without destroying the assembly by interdiffusion. [3,4] The successful metallization of self-assembled organic monolayers (SAMs) can reliably be achieved with only a few methods.[3] Among those, electrochemical techniques [5,6] proved to be quite powerful, as they allow variation of both the type of molecules and the metals involved. [5][6][7][8][9] To increase the functionality of molecule-based nanoelectronic devices in the future, a significant increase in the complexity of their architecture might be required. As a vision, combinations of different molecular layers which can be electrically contacted by individual metal electrodes could serve as a new platform for this ambitious aim. Thus, it appears rather appealing to extend the sandwich design (one organic layer involved) to a molecular double decker (two separate organic layers) and, finally, to a molecular multilayer. Herein, we present theoretical and experimental evidence that a stable molecular double decker can be prepared, representing a proof-of-principle for the first step towards a 3D metal-molecule hybrid structure. The sample consists of a Au(111) single crystal as base electrode, a layer of 4-mercaptopyridine (Mpy) molecules forming the first SAM, a nearly closed Pd monolayer as a spacer, and a second Mpy SAM, which is metallized by a (sub-)monolayer of Pt atoms as the terminal electrode. We demonstrate that the Pd interlayer still reveals metallic properties despite the presence of two SAMs attached to it by chemical bonds. Such metallic interlayers might be useful as "intermediate electrodes" in future experiments, thus offering a new possibility to influence and control charge transport through metal-molecule hybrid structures.

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A new method is described for immobilisation of enzymes on polymer-coated Pt islands. These islands are deposited on top of a SAM-covered Au(111) electrode by a combination of electroless and electrochemical deposition, which allows for a variation of island size and distance between the islands. Here we describe the immobilisation of pyranose-2-oxidase (P2Ox) and the catalytic response to D-glucose on such a nanopatterned surface, which provides optimum access to the active centres of the enzyme.

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