The characterization of the VN x H y defects in diamond through the infrared vibrational spectrum. A quantum mechanical investigation
Résumé
The six possible VNxHy defects in diamond (a vacancy surrounded by to 3 a vacancy surrounded by x = 1 to 3 nitrogens and y = 1 to 4-x hydrogens) are investigated at the quantum mechanical level by using the periodic supercell approach (64 atoms), an all electron Gaussian-type basis set, and hybrid functionals. It turns out that steric hindrance and short range repulsion is not such to prevent hydrogen atoms (up to 3 in VNH3) from fitting in the V cavity and saturating the dangling bonds of the carbon atoms first neighbors of the vacancy. All the investigated systems present specific IR spectra, with bending and stretching peaks above the nearly continuous band (from 400 to about 1340 cm−1) resulting from the perturbation of the perfect diamond spectrum by the vacancy and the nitrogen substitutions. These peaks, ranging from 1340 to 1700 cm−1 (bending; data refer to the harmonic approximation) and from 3100 to nearly 4000 cm−1 (stretching), might be considered fingerprints of the various defects. The C–H anharmonicity, evaluated numerically, turns out to be very sensitive to the N and H load, ranging from −202 cm−1 (redshift of the triplet state of VNH) to +57 cm−1 (blueshift of the symmetric stretching in VNH3).