The 1.40 eV and 2.56 eV centres in synthetic diamond

Abstract

Cathodoluminescence in the 1.40 eV and 2.56 eV systems is observed from most nitrogen-containing synthetic diamonds. The authors show that there is a tendency for the 2.56 eV luminescence to be strong when the 1.4 eV luminescence is strong, and that both are strongest for diamonds grown in the presence of nickel. Absorption due to the 1.4 eV centre has been observed for the first time, using diamonds grown with a nickel catalyst-solvent. Temperature-dependence measurements confirm that the 1.4 eV zero-phonon doublet is due to a split ground state, and show that the 2.56 eV system has a complex excited state structure with a group of four well defined levels at 2.5634, 2.5618, 2.5602 and 2.5587 eV and a diffuse level consisting of one excited state, or two or more overlapping states at approximately 2.588 eV. No absorption has been detected in the 2.56 eV system, and in most diamonds the 1.4 eV absorption is too weak to detect. This indicates that the centres are present in low concentrations, and the circumstantial evidence suggests that they both involve nickel complexes. The latter conclusion is strongly supported by recently reported studies on diamonds implanted with nickel.