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Fig. 3: Schematic of the photovoltaic-relevant valence states in the rigid band model a) as prepared nanocrystalline TiO2 anatase film, b) after dye adsorption from ethanol solution with HOMO position, c) after coadsorption of the solvent acetonitrile with the HOMO shifted by 150meV to higher EB. Using the optical absorption maximum the LUMO is found 0.17 eV above the Fermi level. A schematic representation of the photovoltaic-relevant valence states as deduced in the simple rigid band model is displayed in Fig.3. The measured HOMO corresponds to the position of the lowest energy holestate created by the photoemission of an electron i.e. the HOMO of the molecular cation. Except for a Franck Condon shift of approximately 0.1 eV away from the Fermi level due to vibrational excitation in the photoemission process, this is the relevant energy position for rereduction by the redox system. For the electron injection process the alignment of the LUMO to the conduction band edge is crucial. Using the energy of the optical absorption maximum (535nm = 2.32eV), the LUMO is found 0.17 eV above the Fermi level. While the shape of the S2p line of the dye NCS group does not change when benzene is coadsorbed (Fig 4a right), the S2p emission counterintuitively sharpens upon CH3CN coadsorption (Fig 4a left). In a simple model (Fig. 4b left) the polar solvent acetonitrile penetrates the dye layer and the interaction of the dye NCS groups with the TiO2 surface and with neighbouring dye molecules is suppressed due to a solvation shell. The S2p intensity development in the course of CH3CN adsorption suggests reorientation of the dye molecules pointing the NCS groups away from the surface towards the electrolyte. On the other hand nonpolar benzene just covers the dye layer (Fig. 4b right). The solvation and orientation of the dye by acetonitrile may be important for obtaining vectorial photovoltaic charge transfer of the electron from the dye LUMO to the TiO2 conduction band and the hole from the dye HOMO to the electrolyte. Fig. 4a: Ru(N3)-dye S2p level in the course of acetonitrile (left) and benzene (right) coadsorption and desorption. - 74 - Fig. 4b: Cartoon of the Ru(N3)-dye/solvent interaction in the course of acetonitrile (left) and benzene (right) coadsorption.
Alkyl- and aryl-modified Si(111) for silicon/organic hybrid devices Ralf Hunger, Wolfram Jaegermann Silicon/Organic junctions attract interest in several application perspectives: for silicon/organic hybrid devices such as biosensors, storage devices, or solar cells, as well as in the perspective of molecular electronics. It combines the advantages of modularity and tailoring capabilities of organic chemistry with the established, industrial-scale proven silicon technology. An example silicon/organic hybrid device is shown in Figure 1. It is a “wet” field-effect transistor (w-FET) acting as a biochemical sensor. The central idea of this device is that biochemical sensor molecules with highly specific binding properties are attached on the gate electrode of a w-FET. This is the active region of the electrochemical sensor, which is contacted by an electrolyte. Realizing this device in silicon technology requires two technological preconditions: a) The gate electrode surface needs to be stable against the electrolyte (corrosion resistance). b) sensor molecules need to be fixed to the gate electrode in a defined way, and (lateral) spatial pattern, thus “chemical lithography” is required. Fig. 1: Schematics of an electrochemical, “wet” field effect transistor, w-FET, for biosensing applications. Alkyl-modified silicon surfaces are one interesting class of materials considered for surface passivation and structuring. Various surface terminations such as methyl-, ethyl, or butylfunctionalized Si(111) were realized by our collaborator groups at CalTech, Pasadena (N.S. Lewis), the Waseda University Tokyo (T. Osaka) or at the HMI Berlin (J. Rappich), employing wet chemical as well as electrochemical processing. A schematic representation of these surfaces is shown in Figure 3. The methyl-terminated surface is particular in comparison to the other, longer-chain alkyl-terminations in two aspects: (1) The methyl (-CH3) termination is the only alkyl species the van-der-Waals radius of which is small enough such that the passivation of every silicon surface atom by a methyl group is possible. Our measurements showed that already for the next largest alkyl, i.e. ethyl (C2H5), only a fractional coverage of about 2/3 of a monolayer are achieved. (2) Methylmodified n-Si(111) surfaces in the presence of water affect a p-conductive surface channel, which makes them particularly promising for the implementation as passivation layers in w-FET devices. The mechanism leading to p-type surface channels is not yet understood and is a focal point of our studies. The chemical, electronic, and structural properties of alkylated Si(111) surfaces are analysed in our “Solid/Liquid Analysis System” (SoLiAS) at the 3 rd generation synchrotron facility BESSY II in Berlin. SoLiAS is equipped with the analytical methods of highresolution photoelectron spectroscopy and low energy electron diffraction (LEED). Figure 2 shows the LEED pattern of ethylated Si(111) which proofs an exceptionally high degree of lateral ordering of the wet chemically processed surfaces. These surfaces are subjected to further processing steps within the SoLiAS station such as vacuum thin film deposition, electrodeposition or gas phase adsorption, etc. which allow for an in-situ - 75 -
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ANNUAL REPORT 2 0 0 6 FACULTY OF MA
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Annual Report 2006 Faculty 11 Mater
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Preface The year 2006 of the Depart
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difference of numerical and analyti
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Institute of Materials Science Phys
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Research Projects Bulk Amorphous Al
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el Dsoki, C.; Landersheim, V. ; Kau
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Das, J.; Kim, K. B.; Xu, W.; Wei, B
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Staff Members Head Prof. Dr. Jürge
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Zhou L.; Rixecker G.; Aldinger F.;
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concerned with the synthesis and ch
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Publications Fleissner, A.; Schmid,
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tower stripping, and therefore much
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Microstructures in Omphacite and Ot
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As an example, Fig. 2 shows the che
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MATERIALS SCIENCE Physical Metallur
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