Structural and Electronic Properties of Self-Assembled Supramolecular Grid Structures: Doping of Supramolecular Thin Fil
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Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
trochemical underpotential deposition. The electronic valence band structure was determined by photoelectron spectroscopy with vacuum-UV-radiation (UPS) and molecular orbital calculations. The grids reveal semiconducting behavior with an insulating gap of approximately 2.5 eV. Nevertheless, the conductivity of the films was too low for reliable electronic functionalizations. Therefore, the conducting properties had to be "tuned" by the incorporation of additional metal ions in the films. The size of the insulating gap could be reduced by doping 24 of the thin films by addition of Cd ions, which results in a strongly increased conductivity. EXPERIMENT The molecules The structure of the 4,6-bis(6-(2,2'-bipyridyl)-pyrimidine) ligands and the crystal structure [7] of the supramolecular Co-[2x2] grid are depicted in Fig. 1. The ligands were synthesized as published [8). The grids form by self-assembly in methanol and were investigated in acetonitrile solution. They were characterized by X-ray crystallography, UV spectroscopy, mass spectrometry, elemental analysis and electrochemical methods [9]. Characterization on solid substrates was performed with XPS and mass spectrometry. The length of the ligand is approximately 16 A, the distance between the metal centers is 6.4 A. The positive charges are countered by PF 6 ions.
Figure 1: Crystal structure of the supramolecular Co-[2x2] grid (bottom), consisting of four Co2" ions and four 4,6-bis(6-(2,2'-bipyridyl)-pyrimidine) ligands (top). The grids form by selfassembly in methanol. The positive charges are countered by PF 6 ions Rj= -H, R2= -CH 3. Preparation and characterization of the thin films; molecular orbital calculations Electrochemical preparation was performed using a EG&G model 362 potentiostat. Multilayer systems like 50 monolayers for UPS were also produced by depositing a defined vol448
ume on the substrate with a microliter syringe and slow evaporation of the solvent in a saturated solvent atmosphere. The same method was used for the doped films. The dopant was dissolved with the grids in acetonitrile in a defined stoichiometry. Electrochemical in-situ STM measurements were performed using a commercial Nanoscope ECM with a Pt wire as counter-, and a Ag wire as reference electrode. UPS measurements were carried out at the HASYLAB,DESY/Hamburg using vacuum UV synchrotron radiation in an energy range between 40 eV and 110 eV. For detection of the photoelectrons, a cylindrical mirror analyzer was used. Base pressure was 510". mbar. Semiempirical molecular orbital calculations with a VAMP 6.5 program package [10] were performed using the PM3 method [11]. The positions of the dopants were optimized using the constant crystal structure of the grids. Conductivity measurements were carried out using a Keithley 238 Source measure unit in a four point geometry. RESULTS Electrochemical in-situ STM One important condition for physical functionalizations of the supramolecular architectures on solid sub
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