E-Letter responses to:

r-articles: Vidhya Chakrapani, John C. Angus, Alfred B. Anderson, Scott D. Wolter, Brian R. Stoner, and Gamini U. Sumanasekera Charge Transfer Equilibria Between Diamond and an Aqueous Oxygen Electrochemical Redox Couple Science 2007; 318: 1424-1430 [Abstract] [Full text] [PDF]

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Response to A. P. Sommer et al.'s E-Letter

John C. Angus, Vidhya Chakrapani, Alfred B. Anderson, Scott D. Wolter, Brian R. Stoner, Gamini U. Sumanasekera   (28 February 2008)

Breathing Conductivity into Diamonds

Andrei P. Sommer, Dan Zhu and Horst-Dieter Försterling   (28 February 2008)

Response to A. P. Sommer et al.'s E-Letter 28 February 2008
John C. Angus Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106-7217, USA, Vidhya Chakrapani, Alfred B. Anderson, Scott D. Wolter, Brian R. Stoner, Gamini U. Sumanasekera Respond to this E-Letter: Re: Response to A. P. Sommer et al.'s E-Letter	A. P. Sommer et al. find that the conductance of a nanocrystalline diamond film decreases when they increase the humidity by breathing on it (1), which they suggest is counter to the predictions of the surface transfer doping model espoused by us and others (2). We first point out that our results (2) were obtained on diamond in contact with a macroscopic water phase and therefore at fixed, saturated water activity, so changes in humidity played no role. Second, in an earlier work by us, controlled measurements showed that the conductance of single crystal, natural diamond increased with increasing humidity as predicted by the surface transfer doping mechanism (3). We believe that the striking difference in the results of Sommer et al. from our results is caused by large differences in the electronic properties of the synthetic, nanocrystalline diamond used by them and the natural, single crystal diamond used by us. We note that the nanocrystalline diamond used by Sommer et al. contains up to 10% sp2 carbon (4), primarily in the grain boundaries. The fact that they report conductivity through their nanocrystalline diamond film to the silicon substrate is highly suggestive that grain boundary conductivity was indeed present in their sample. Breathing on the sample will likely increase the thickness of the adsorbed water film and also decrease its pH. Both of these effects should increase electron transfer from the diamond. In light of this, we suggest that the decrease in conductance upon breathing on the sample is caused by increased transfer of electrons from the grain boundaries to the oxygen redox couple in the enhanced, adsorbed water film. This explanation is in agreement with the observations of Sommer et al. and the predictions of the surface transfer doping mechanism. Confirmation or denial of this explanation will require controlled experiments on the effect of humidity, CO2 level and pH on the conductance of nanocrystalline films with well-characterized chemical composition (including nitrogen content) and electrical properties. However, we do note that compensation of electronic conductivity by the surface transfer doping mechanism has been observed in single-walled carbon nanotubes (5), a process that is closely analogous to our suggestion above. John C. Angus, Vidhya Chakrapani, Alfred B. Anderson Case Western Reserve University, Cleveland, OH 44106, USA. Scott D. Wolter Duke University, Raleigh-Durham, NC 27708, USA. Brian R. Stoner Research Triangle Institute, Research Triangle Park, NC 27709, USA. Gamini U. Sumanasekera University of Louisville, Louisville, KY 40291, USA. References 1. A. P. Sommer, D. Zhu, K. Bruhne, Cryst. Growth and Des. 7, 2298 (2007). 2. V. Chakrapani et al., Science 318, 1424 (2007). 3. V. Chakrapani, S. C. Eaton, A. B. Anderson, M. Tabib-Azar, J. C. Angus, Electrochem. and Solid State Lett. 8, E4-E8 (2005). 4. S. O. Kucheyev et al., Appl. Phys. Lett. 86, 221914 (2005). 5. V. Chakrapani, J. C. Angus, A. B. Anderson, G. Sumanasekera, Mat. Res. Soc. Symp. Proc. 956, paper J15-01 (2007).
Breathing Conductivity into Diamonds 28 February 2008
Andrei P. Sommer, senior scientist University of Ulm, Dan Zhu and Horst-Dieter Försterling Respond to this E-Letter: Re: Breathing Conductivity into Diamonds	We were surprised to read in the synopsis of your November 30 issue (This Week in Science, 30 November 2007, p. 1345) the title “Breathing conductivity into diamonds.” Indeed, an increase in the electrical conductivity upon breathing onto hydrogen-terminated diamond is exactly what one would expect to observe, if the model proposed by Chakrapani et al. (Research Articles, “Charge transfer equilibria between diamond and an aqueous oxygen electrochemical redox couple,” 30 November 2007, p. 1424) for the explanation of the conductivity effect would be the final solution to the almost 20-years-old puzzle (1). Instead, we observed that breathing upon hydrogen-terminated diamond resulted instantaneously in a significant decrease in conductivity (2), thereby strongly challenging the validity of concept of the surface transfer doping mechanism (3). According to it, electrons transfer from the diamond to the water layer adsorbed from the air (acceptor layer), resulting in the formation of a positive space charge layer (holes) in the diamond. Clearly, an increase in humidity on the hydrogen-terminated diamond site (i.e., by breathing) is equivalent with a thicker acceptor layer, which should facilitate the liberation of more holes in the diamond, and thereby increase the conductivity. Albeit, we observed a manifest decrease in holes both upon breathing and upon exposing the samples to higher humidity levels via tissues wetted with ultra-pure water. In view of the promising technical applications of hydrogen-terminated diamond, including but not limited to biosensors and chemical sensors (4), as well as sensitive and robust humidity sensors for the exploration of water reservoirs on Mars (2), it is important to consider in future research the humidity levels at which the conductivity measurements are performed. Andrei P. Sommer, Dan Zhu Institute of Micro and Nanomaterials, University of Ulm, 89081 Ulm, Germany. Horst-Dieter Försterling Department of Physical Chemistry, Philipps University, 35032 Marburg, Germany. References 1. M. I. Landstrass, K. V. Ravi, Appl. Phys. Lett. 55, 975 (1989). 2. A. P. Sommer, D. Zhu, K. Brühne, Cryst. Growth Des. 7, 2298 (2007). 3. F. Maier, M. Riedel, J. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85, 3472 (2000). 4. C. E. Nebel, Science 318, 1391 (2007).