Electrochemical Charge Transfer to Diamond and Other Materials
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1203-J07-01
Electrochemical Charge Transfer to Diamond and Other Materials Vidhya Chakrapani1, John C. Angus1, a, Kathleen Kash2, Alfred B. Anderson3 1 Chemical Engineering Department, 2Department of Physics, 3Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, U.S.A. Sharvil Desai4 and Gamini U. Sumanasekera4 4 Department of Physics, University of Louisville, Louisville, KY, 40292, U.S.A. a
corresponding author
ABSTRACT The oxygen redox couple in adsorbed water films acts as an “electrochemical ground” that tends to pin the Fermi level in solids at the electrochemical potential of the redox couple. We discuss this effect on the conductivity of diamond; the conductivity type of sp2-based carbons including single-walled, semiconducting carbon nanotubes and graphene; the photoluminescence of GaN and ZnO; and the contact charging of metals. INTRODUCTION Early studies
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In 1989 Maurice Landstrass and K.V. Ravi reported that hydrogen-terminated diamond showed a p-type surface conductivity when exposed to air [1]. This observation was extremely unusual because no similar observation had been reported in more than a century of studying the properties of diamond. Also unusual was the observation that the surface conductivity is not seen in diamond synthesized at high pressure and high temperature. This set of highly unusual characteristics has made the surface conductivity a subject of much discussion and its mechanism is still somewhat controversial. The first studies that led to eventual understanding of the effect were performed by Gi and co-workers [2-4]. They showed that the sheet hole concentration could be as great as 1013 cm−2, and that the conductivity increased on exposure to acidic gases and decreased with exposure to basic gases. They proposed that the effect arose from surface oxidation of the diamond by the hydronium ion, H3O+. Subsequently, Maier et al. [5] described this oxidation process in terms of the hydrogen redox couple, 2H+ + 2e" = H 2 , operating in an adsorbed water film. Later studies by Foord [6] et al. and Chakrapani [7] et al. indicated that the oxygen redox couple was responsible for the effect. Chakrapani confirmed this conclusion by demonstrating that the changes in pH and dissolved oxygen concentration upon adding diamond powder to ! consistent with the four-electron oxygen redox couple, macroscopic size aqueous solutions were O 2 + 4H+ + 4e" = 2H 2O [8]. It is of interest that electron transfer to aqueous redox couples is not the first report of surface transfer doping in diamond. Shapoval et al. of the Institute of General and Inorganic Chemistry in Kiev in 1995 invoked electron transfer to an electrochemical couple to explain the surface conductivity of diamond immersed in molten salts [9].
Electrochemically mediated surface transfer doping of diamond to an aqueous redox couple is a particular example of a more general process [10]. Other adsorbed acceptors have been used. For example, Strobel et al. [11] used an adsorbed layer of C60 molecules and Qi and
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