Diamond is an insulator

Contrary to the assertion of Andrei P. Sommer and Dan Zhu in their letter, hydrogen-terminated diamond is indeed an insulator (C&EN, Jan. 28, page 11). This fact has been confirmed in numerous studies over many years (see, for example, Phys. Rev. Lett. 2000, 85, 3472). Sommer and Zhu observed bulk conductivity in thin films of so-called nanocrystalline diamond, a form of carbon that contains up to 10% nondiamond carbon, primarily in the grain boundaries (Appl. Phys. Lett. 2005, 86, 221914).

The nondiamond component is mainly trigonally bonded sp² carbon, a form similar to graphite. This grain boundary material is the likely source of the bulk conductivity observed by Sommer and Zhu, not doping by hydrogen. The interested reader can find further discussions of diamond-surface conductivity in a recent Science article (2007, 318, 1424).

John C. Angus
Cleveland