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Angus and Chakrapani performed a series of controlled experiments in which they showed that electron exchange occurs between diamond and the aqueous redox couple in an adjacent water phase. They further showed that the electron exchange not only causes conductivity, but influences other properties -- both the contact angle and the charge on the diamond are changed by changing the composition of the surrounding atmosphere. Furthermore, the work of adhesion of water to diamond is enhanced by electrostatic attraction after the charge transfer, which enhances the ability of water films to adsorb on otherwise hydrophobic surfaces. These observations have important ramifications because the key components are all derived from humid air: the water film that provides the medium for the electrochemical reaction, the oxygen molecules and the protons (that arise from the acidity generated by the carbon dioxide in the air). This means that the effect can occur whenever semiconductors or other solids are exposed to humid air. It is highly likely that the effects of this process have remained unrecognized in many common situations. In this light, it will be of great interest to re-examine the literature on mechanical sliding friction and contact electrification, both of which depend in complex ways on relative humidity and impurities in the ambient. Even more speculative is the possibility that certain animals and insects have evolved the capability of modulating the electrochemical potential in their feet to change the adhesive force to solid surfaces. Finally, it should be noted that charge transfer to nano-structures not only can affect their properties, but will vary with the size of the structure because of quantum confinement effects. The paper can be accessed from the publisher's web site here (subscription required). Vidhya Chakrapani, John C. Angus, Alfred B. Anderson, Scott D. Wolter, Brian R. Stoner, Gamini Sumanasekera, Science 318, 1424-1430 (2007) |
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In 1989 a very curious phenomenon was observed--undoped diamond, known to be an exceedingly good insulator, showed substantial conductivity when exposed to air. The observation has remained a subject of controversy since that time. Recent studies by Professor John Angus, graduate student Vidhya Chakrapani, and colleagues confirmed that the effect is electrochemical in origin, and is not confined to conductivity changes in diamond, but is a more general phenomenon that can influence a wide range of materials and processes. 
The figure shows how the electron energies of various solids are related to the electron energies in a water film. The oxygen redox couple in the water film fixes the electron energy of diamond at the top of the valence band, which generates positive charge carriers (holes). The energy range of the couple spans the band gap of semiconducting single-walled carbon nanotubes (s-SWNT). For GaN the electrochemical energies are at the energy of the mid-gap states responsible for the "yellow band" luminescence. Indeed, Angus and Chakrapani have found that the conductivity of carbon nanotubes can be changed from n-type to p-type and that the intensity of luminescence from GaN can be modulated by changing the electrochemical potential of the surrounding air.