Force microscopy on insulators: Imaging of organic molecules

O. Pfeiffer, E. Gnecco, L. Zimmerli, S. Maier, E. Meyer, L. Nony, R. Bennewitz, F. Diederich, H. Fang, D. Bonifazi,J. Phys.: Conf. Ser. 2005, 19, 166-174DOI: 10.1088/1742-6596/19/1/027 Abstract So far, most of the high resolution scanning probe microscopy studies of organic molecules were restricted to metallic substrates. Insulating substrates are mandatory when the molecules need to be electrically decoupled in a electronic circuit. In such a case, atomic force microscopy is required. In this paper we will discuss our recent studies on different organic molecules deposited on KBr surfaces in ultra-high vacuum, and then imaged by AFM at room temperature. The distance between tip and surface was controlled either by the frequency-shift of the cantilever resonance or by the excitation signal required to keep the oscillation amplitude constant. Advantages and drawbacks of both techniques are discussed. The high mobility of the molecules, due to their weak interaction with the substrate, hinders the formation of regular self assembled structures. To overcome this problem we created artificial structures on the surface by annealing and by electron irradiation, which made possible the growth of the molecules onto step edges and their confinement into rectangular pits.

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Self-assembly on surfaces: Supramolecular patterned surfaces driven by cooperative assembly of C60 and porphyrins on metal substrates

D. Bonifazi, H. Spillmann, A. Kiebele, M. De Wild, P. Seiler, F. Cheng, J. Guntherodt, T. Jung, F. Diederich,Angew. Chem., Int. Ed. 2004, 43, 4759-4763DOI: 10.1002/anie.200460562 Abstract Schicke Muster: Neuartige ein- und zweidimensionale Fullerenmuster wurden durch Aufdampfen von reinem C60 auf vororganisierte Porphyrin-Monoschichten auf Silberoberflächen aufgebaut. Die Anordnung der Fulleren-Moleküle auf der gemusterten Schicht kann durch die Porphyrin-Struktur gesteuert werden (siehe rastertunnelmikroskopische Aufnahmen)

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Chromophoric interactions in [60]fullerene-porphyrin dyads investigated by solid-state UV-Vis and IR spectroscopies

A. Graja, I. Olejniczak, A. Bogucki, D. Bonifazi, F. Diederich,Chem. Phys. 2004, 300, 227-232DOI: 10.1016/j.chemphys.2004.02.005 .Abstract Solid-state electronic (UV–Vis) and vibrational (IR) spectra of several fullerene–porphyrin conjugates were recorded in KBr in the spectral range between 400 and 50,000 cm−1. It was observed that significant redistribution of the charges occurs in both fullerene and porphyrin moieties upon covalent linkage. This effect is mainly observed as shifts of both electronic and vibrational bands in comparison with the respective free porphyrin and fullerene features. Variable temperature IR measurements were also performed.

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Exceptional redox and photophysical properties of a triply fused diporphyrin-C60 conjugate: Novel scaffolds for multicharge storage in molecular scale electronics

D. Bonifazi, M. Scholl, F. Song, L. Echegoyen, G. Accorsi, N. Armaroli, F. Diederich,Angew. Chem., Int. Ed.  2003, 42, 4966-4970DOI: 10.1002/anie.200352265 . Abstract Up to fifteen electrons are reversibly accommodated in a triply fused porphyrin dimer conjugated to two [60]fullerene moieties. Its photophysical properties differ completely from those of the many known porphyrin–fullerene dyads: Photoexcitation of the C60 moieties results in quantitative sensitization of the low-lying (about 1 eV) and very short lived lowest singlet level of the porphyrin sheet (see scheme).

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Strong intramolecular chromophore interactions in novel bis([60]fullerene)-oligoporphyrin nanoarrays

D. Bonifazi, F. Diederich,Chem. Commun. (Cambridge, U. K.) 2002, 2178-2179DOI: 10.1039/B206624F Abstract Novel nanodimensional dyads were prepared by Bingel reaction of meso, meso-linked oligoporphyrin bis-malonates with C60. Distinct conformational effects were observed as a result of strong fullerene–porphyrin interactions, which also lead to efficient quenching of the porphyrin luminescence.

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Tuning electronic properties of semiconductors by adsorption of [60] fullerene carboxylic acid derivatives

D. Bonifazi, A. Salomon, O. Enger, F. Diederich, D. Cahen, Adv. Mater. (Weinheim, Ger.) 2002, 14, 802-805 DOI: 10.1002/1521-4095(20020605)14:11<802::AID-ADMA802>3.0.CO;2-# Abstract Changing the electronic surface properties of GaAs and ZnO can be achieved by chemisorption of partial monolayers of C60 derivatives. This effect appears to depend on the way the C60 moiety is coupled to the semiconductor surface (see Figure), as shown by preliminary I–V investigations of (n-GaAs C60 derivative)/Au solid-state diodes.

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Lateral Electron Transport inside a Monolayer of Derivatized Fullerenes Anchored on Nanocrystalline Metal Oxide Films

N. Papageorgiou, M. Graetzel, O. Enger, D. Bonifazi, F. Diederich,J. Phys. Chem. B 2002, 106, 3813-3822DOI: 10.1021/jp012657s Abstract Carboxylated fullerene derivatives adsorbed on nanocrystalline ZrO2 films deposited on conducting glass, display reversible electrochemical behavior with currents being 200 times higher than measured on a monolayer of the redox active fullerene molecule adsorbed on the conducting support. Despite the insulating oxide layer on which the fullerene is adsorbed, cross-surface charge transfer results in the participation of the internal surface of the oxide in the redox process. A mechanism of charge transport involving electron injection from the conducting support, followed by lateral electron hopping within the fullerene monolayer on the oxide, is proposed. Apparent diffusion coefficients as high as 1.5 × 10-8 cm2 s-1 were measured for the electron hopping process. A percolation threshold for electronic conductivity was found at a surface coverage between 40 and 60% of a full monolayer. Co-adsorbed spacer molecules, however, were seen to modify the percolation limit, as no threshold was observed up to 60% fullerene coverage. Significant implications are envisaged with regard to prospective applications in e.g. nano-optoelectronics, electrochemical devices, sensors, solar cells, and redox targeting of adsorbed biomolecules.

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Synthesis and Coordination Chemistry of Lower Rim Cavitand Ligands

L. Pirondini, D. Bonifazi, E. Menozzi, E. Wegelius, K. Rissanen, C. Massera, E. Dalcanale,Eur. J. Org. Chem. 2001, 2311-2320DOI: 10.1002/1099-0690(200106)2001:12<2311::AID-EJOC2311>3.0.CO;2- Abstract Four new cavitand ligands, modified at the lower rim with cyano and pyridyl functional groups, have been synthesized and their coordinative behaviour toward transition metal ions has been explored. The outcome of the self-assembly of these compounds in the presence of different metal precursors is biased toward the formation of intramolecular complexes, independent of the coordination geometry imposed by the metal centre. Dimeric species have been obtained by decreasing the number of ligands at the lower rim from four to two and by using a metal precursor capable of exchanging four ligands simultaneously.

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