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Tuning Electronic Properties of Semiconductors by Adsorption of [60]Fullerene Carboxylic Acid Derivatives** By Davide Bonifazi, Adi Salomon, Olivier Enger, François Diederich,* and David Cahen* Semiconductor device performance depends on the physics and chemistry of the device interfaces because electronic transport must cross these interfaces. Specifically, this trans- port is controlled by the electron energy band (and level) ar- rangements at the interface. [1,2] These depend on the interface dipoles and the density and energy distribution of the surface states. [3] The adsorption of organic molecules holds great promise for fine-tuning of the relevant electronic properties. It has been shown that chemisorption of simple mono- and di- carboxylic acid or cyclic disulfide derivatives on the surfaces of semiconductors can change the band bending and espe- cially the electron affinity in a controllable manner. [4±7] It was demonstrated that the electrical behavior of the simplest semiconductor devices, the metal/semiconductor diodes, can be controlled by the same molecules, adsorbed as a partial or complete monolayer on either component of the device. [8±10] The interesting excited state properties and strong electron- accepting capacity of [60]fullerene and its covalent deriva- tives [11±15] suggest such compounds as promising candidates for semiconductor surface modifications. Reports on supra- molecular assemblies, such as self-assembled monolayers (SAMs), of fullerene derivatives on Au [16±18] and Si [19,20] pro- vide an additional basis for such work. We report here on how the adsorption of [60]fullerene car- boxylic acid conjugates on p- and n-GaAs and ZnO semicon- ductor surfaces, can affect the electronic properties of these novel hybrid materials. The fullerene components have differ- ently sized and shaped linkers between the C 60 moiety and the surface-binding carboxylate groups, to investigate effects resulting from changes in distance and orientation between the carbon sphere and the surface. Compounds 1±5 were assembled onto p- and n-GaAs and ZnO crystals by adsorption from THF solutions. The resulting surfaces were characterized by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), ellipsometry, and contact angle of wetting. Contact potential difference (CPD) measurements by the Kelvin probe method served to examine how chemi- sorption of the molecules affected the surface electron ener- getics. [21] Taken together, the data from these measurements confirm formation of roughly a monolayer of 1±5 on GaAs surfaces. Water contact angles and film thickness on modified n-GaAs, obtained from ellipsometry data, for adsorbed 1±5 are given in Table 1. These contact angles correspond closely to the values reported in the literature for C 60 -SAMs. [20] The modified surfaces were further evaluated by AFM. Stable and reproducible images were obtained in air. Not surprisingly, no well-defined organization could be detected on a molecular scale. The fullerene spheres appear to control the molecular arrangement in the monolayer. 802 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1106-0802 $ 17.50+.50/0 Adv.Mater. 2002, 14, No. 11, June 5 COMMUNICATIONS ± [*] Prof. F. Diederich, D. Bonifazi, O. Enger Laboratorium für Organische Chemie, ETH-Hönggerberg Wolfgang-Pauli-Strasse 10, CH-8093 Zürich (Switzerland) E-mail: diederich@org.chem.ethz.ch Prof. D. Cahen, A. Salomon Department of Materials and Interfaces Weizmann Institute of Science Rehovot 76100 (Israel) E-mail: david.cahen@weizmann.ac.il [**] We thank Drs. Luana Scheffer and Sidney Cohen (WIS) for the AFM analyses, Jamal Ghabboun (WIS) for helping us with dipole moment cal- culations and electrical transport measurements, and Dr. Yoram Selzer (WIS) for helpful discussions. DC thanks the Israel Science Foundation for partial support, and FD acknowledges funding by the Swiss National Science Foundation. Table 1. Characterization of monolayers on n-GaAs by ellipsometry and ad- vancing contact angle measurements. Compound Water contact angles [] Film thickness [nm] [a] 1 96 ± 2 1.2 ± 0.4 2 81 ± 3 2.3 ± 0.4 3 99 ± 4 2.2 ± 0.4 4 84 ± 1 2.3 ± 0.4 5 83 ± 2 1.9 ± 0.4 etched n-GaAs 45 ± 4 ± malonic acid 22 ± 3 ± [a] Calculated from ellipsometric data.