John Angus presumes in his letter that the nanocrystalline diamond used in our work (Cryst. Growth Des. 2007, 7, 2298) contained up to 10% sp² carbon in the grain boundaries and concludes that the source of the conductivity in our diamond samples was due to grain boundary material and not doping by hydrogen (C&EN, March 10, page 9). Actually, we reported that surface conductivity was detectable only on the diamond samples terminated with hydrogen, thus excluding that the effect was due to sp² carbon.

To further explore the conductivity in diamond, we hydrogenated a natural diamond cube and applied platinum electrodes at its two opposite sides. Using an ohmmeter, we obtained in-air resistances on the order of 1 MΩ. After application of an equatorial belt of Baysilone paste, a highly insulating sticky material, we repeated the measurement by applying the electrodes at the same sides of the diamond cube (i.e., the paste-free sides normal to the belt). Although the diamond cube had an edge length of 5 mm, we observed resistances on the same order and even smaller ones. Because of the electrically insulating belt interrupting the water layer on the diamond surface, we can only assume that in the present scenario, the conductivity on it cannot be understood on the basis of the transfer doping model (Science 2007, 318, 1424), which requires a superficial water layer on the hydrogenated site as a prerequisite.

Andrei P. Sommer, Dan Zhu
Ulm, Germany
Horst-Dieter Försterling
Marburg, Germany