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0956-J15-01

Electrochemical Charge Transfer to Diamond Vidhya Chakrapani1, John C. Angus1, Alfred B. Anderson2, and Gamini U. Sumanasekera3 1 Chemical Engineering, Case Western Reserve University, Cleveland, OH, 44106 2 Chemistry, Case Western Reserve University, Cleveland, OH, 44106 3 Physics, University of Louisville, Louisville, KY, 40291

ABSTRACT Electrochemically mediated charge transfer has been primarily studied by its effect on the surface conductivity of diamond. In this paper we show that the effect is not restricted to diamond, but may occur in other material systems as well, for example, semiconducting singlewalled carbon nanotubes and gallium nitride.

INTRODUCTION Landstrass and Ravi [1] first reported that a conducting, p-type surface layer appears on hydrogen-terminated diamond that depends on the ambient atmosphere. There have been numerous proposals explaining this effect including acceptor levels arising from hydrogen and direct oxidation of the diamond by highly oxidizing gas phase species such as ozone, atomic fluorine or NO2. An electrochemical explanation was proposed by Gi et al. [2, 3, 4], who suggested that hydronium ions oxidize the diamond surface leaving holes in the valence band. Maier et al. [5] proposed that the electron transfer from the diamond is to the H2/H3O+ electrochemical redox couple in an adsorbed water film. It was later proposed that the oxygen may be an active species and the oxygen redox couple is responsible for the effect [6, 7]. No mechanism has been widely accepted and the source of the p-type conductivity remains controversial.

THEORY Direct gas phase oxidation of diamond Figure 1 shows the electron affinities of several strong gas phase oxidizing agents: F, NO2, O2 and O3 [8]. Also shown in Figure 1 is the position of the band edges of diamond. The value of the electron affinity of hydrogen-terminated diamond is taken to be −1.3eV, which is the value obtained in vacuum [9]. It is clear from Figure 2 that these oxidizing agents will not remove an electron from the valence band of diamond. It appears that direct oxidation by gas phase species is not the source of the p-type surface conductivity of diamond.

ε [eV]

Hydrogen-terminated diamond EC = +1.3 eV

+1

χdia = -1.3 eV

0 -1

χ = 3.45 eV

-2 -3

F

-4 -5

χ = 0.45 eV O2

5.5 eV

χ = 2.3 eV NO2

χ = 2.1 eV O3

EV = -4.2 eV

Figure 1. The band edge energies of diamond and the electron affinities of several strong gas phase oxidizing agents. The electron affinity of hydrogen-terminated diamond is the value obtained in high vacuum [9].

Electrochemically mediated charge transfer Aqueous electrochemical redox couples are able to accept electrons from the valence band of diamond. We discuss this in the context of redox reactions (1) and (2) below. O 2 + 4 H 3 O + + 4e − = 6H 2 O

(1)

At high pH the potential is determined by the reaction O 2 + 2H 2 O + 4e − = 4OH −

(2)

Reactions (1) and (2) are not independent; they are related by the water equilibrium. H 3 O + + OH − = 2H 2 O

(3)

For the oxygen redox couple