Electrochemical Grafting of Boron-Doped Single-Crystalline Chemical Vapor Deposition Diamond with Nitrophenyl Molecules Hiroshi Uetsuka, Dongchan Shin, Norio Tokuda, Kazuhiko Saeki, and Christoph E. Nebel* Diamond Research Center, AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan ReceiVed NoVember 6, 2006. In Final Form: December 12, 2006 The growth of covalently bonded nitrophenyl layers on atomically smooth boron-doped single-crystalline diamond surfaces is characterized using cyclic voltammetric attachment and constant-potential grafting by electrochemical reduction of aryl diazonium salts. We apply atomic force microscopy (AFM) in contact mode to remove phenyl layers and measure phenyl layer thicknesses by oscillatory AFM. Angle-resolved X-ray photoelectron spectroscopy is applied to reveal the bonding arrangement of phenyl molecules, and transient current measurements during the grafting are used to investigate the dynamics of chemical bonding. Nitrophenyl groups at an initial stage of attachment grow three-dimensional (3D), forming layers of varying heights and densities. Layer thicknesses of up to 80 Å are detected for cyclic voltammetry attachment after five cycles, whereas the layer becomes denser and only about 25 Å thick in the case of constant-potential attachment. No monomolecular closed layer can be detected. The data are discussed taking into account established growth models. Redox systems such as Fe(CN) 6 3-/4- and Ru(NH 3 ) 6 2+/3+ are used to probe the electrochemical barrier properties of nitrophenyl groups grafted onto diamond. I. Introduction Biosensor devices based on diamond attract increasing attention as diamond is known to be biocompatible and chemically inert and shows excellent electrochemical properties and long-term chemical stability of biomolecules bonded to it. 1 Controlled chemical modification of diamond surfaces, however, has proven difficult. The pristine diamond surface is generally inert to most chemical reagents. Exceptions to the low reactivity include reaction with atomic species, including hydrogen, 2 fluorine, 3 and chlorine. 4 For the realization of biosensors from diamond, covalently bonded linker molecular layers are required. When Takahashi et al. in 2000 first introduced a photochemical chlorination/amination/carboxylation process of the initially H-terminated diamond surface, a giant step toward biofunc- tionalization of diamond was taken, as the obstacle of “chemical inertness” had finally been removed. 5,6 This triggered more activity, so that 2 years later, Yang and co-workers in 2002 introduced a new photochemical method to modify nanocrys- talline diamond surfaces using alkenes, 1 followed by electro- chemical reduction of diazonium salts, which has been suc- cessfully applied to functionalize boron-doped ultrananocrystalline diamond, 7 and in 2006 a direct amination was introduced by Zhang and co-workers. 8 Grafting of a variety of substrate materials by aryl groups using the reduction of diazonium salts is a rather popular electrochemical technique. 9 Modification of metal surfaces, 10-12 of silicon, 13-16 and of many carbon materials 9,17-28 has meanwhile been performed. A few studies discuss attachment to polycrys- talline boron-doped diamond, 29,30 to ultrananocrystalline diamond (UNCD), 7,31 and to nanocrystalline diamond. 32 These surfaces are all relatively rough and do not allow characterization in nanoscale dimensions. This is however required as the growth of phenyl layers is fast and complex and can result in multilayer formation as reported on hydrogen-terminated silicon. 14 In this study, we apply cyclic voltammetric and constant- potential attachment experiments to electrochemically grow nitrophenyl films on atomically smooth metallically doped single- crystalline diamond grown by plasma-enhanced chemical vapor deposition (PE-CVD). The growth is characterized using cyclic voltammograms, transient attachment currents, atomic force (1) Yang, W.; Auciello, O.; Butler, J. E.; Cai, W.; Carlisle, J. A.; Gerbi, J. E.; Gruen, D. M.; Knickerbocker, T.; Lasseter, T. L.; Russell, J. N., Jr.; Smith, L. M.; Hamers, R. J. Nat. Mater. 2002, 1, 253. 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