Semiconductors exposed to humid air can reversibly exchange electrons with the oxygen redox couple in a thin adsorbed water layer on the surface. This effect was first discovered in diamond,[1,2,3] but recent work shows that the process is general and can affect the properties of many materials [4,5]. In diamond, p-type conductivity arises when the Fermi level of the diamond is higher than the electrochemical potential of the oxygen redox couple. For semiconducting, single walled carbon nanotubes, the conductivity is reversibly changed between p-type to n-type by changing the electrochemical potential of the water film by exposure to HCl and NH3 vapors [5]. For GaN and ZnO, charge transfer between mid gap states and the redox couple affects both defect-related and band-edge luminescence in a systematic way with change in electrochemical potential [6]. In all cases the process leads to a charged interface with compensating ions present in the aqueous film. Electrochemically mediated charge transfer may play a previously unrecognized role in other processes where solids are exposed to humid air, e.g., particulate processing, contact electrification and mechanical friction.

[1]. M. I. Landstrass, and K. V. Ravi, Appl. Phys. Lett., 55 (1989) 975.

[2]. F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, Phys. Rev. Lett., 85 (2000) 3472.

[3]. V. Chakrapani, J. C. Angus, A. B. Anderson, S. D. Wolter, B. R. Stoner and G. U. Sumanasekera, Science 318, (2007) 1424.

[4]. V. Chakrapani, J. C. Angus, A. B. Anderson and G. Sumanasekera, Mat. Res. Soc. Symp. Proc. 956, paper 0956-J15-01 (2007).

[5]. V. Chakrapani, PhD thesis, Case Western Reserve University, Cleveland, OH (2007).

[6]. V. Chakrapani, C. Pendyala, K. Kash, A. B. Anderson, M. K. Sunkara and J. C. Angus, Submitted.