C hyperfine interactions in the ground state of the negatively charged nitrogen vacancy ͑NV − ͒ center have been investigated using electron-paramagnetic-resonance spectroscopy. The previously published parameters for the 14 N hyperfine interaction do not produce a satisfactory fit to the experimental NV − electron-paramagnetic-resonance data. The small anisotropic component of the NV − hyperfine interaction can be explained from dipolar interaction between the nitrogen nucleus and the unpaired-electron probability density localized on the three carbon atoms neighboring the vacancy. Optical spin polarization of the NV − ground state was used to enhance the electron-paramagnetic-resonance sensitivity enabling detailed study of the hyperfine interaction with 13 C neighbors. The data confirmed the identification of three equivalent carbon nearest neighbors but indicated the next largest 13 C interaction is with six, rather than as previously assumed three, equivalent neighboring carbon atoms.

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ExperimentalCharacterization: The UV-visible spectra were recorded on a Shimadzu 3101 spectrophotometer. Dynamic light scattering studies were carried out using a Horiba LB-550 instrument. Transmission electron micrographs of the clusters were recorded by applying a drop of the sample to a carbon-coated copper grid (JEOL JEM-200CX). AFM measurements were obtained using a Digital Nanoscope III in the tapping mode.Photoelectrochemical Measurements: The photoelectrochemical measurements were performed in a one-compartment Pyrex UV cell with a standard three electrode arrangement consisting of a working electrode, a Pt wire counter electrode, and a Ag/AgNO 3 reference electrode (0.5 M LiI and 0.01 M I 2 in acetonitrile as the electrolyte). Photocurrent measurements were made with an ALS 630A electrochemical analyzer. Monochromatic light obtained by passing light from a 500 W xenon lamp (Ushio XB-50101AA-A) through a monochromator (Ritsu MC-10N) was used for excitation.