Origin of the asymmetric zero-phonon line shape of the silicon-vacancy center in nanocrystalline diamond films

The spectral line shape of the 1.68 eV (738 nm) emission line – usually associated with the negatively charged silicon-vacancy (SiV) center in diamond and promoted as a strong candidate for many quantum technology and nanobiology related applications – was studied by luminescence spectroscopy in different nanocrystalline diamond films prepared by MW CVD technique. An asymmetric line profile, expressed as a long tail on the low-energy side of the narrow luminescence band located around 1.68 eV, have been observed in all samples. By applying a multi-wavelength laser excitation, it was found that the asymmetric shape of the 1.68 eV emission line varies by changing the excitation energy and becomes more pronounced under red (1.95 eV) excitation, especially for nanodiamond film characterized by larger average grain size. Based on the observed excitation dependent line shape behavior and on the analysis of the fine-structured emission lines, recorded by photoluminescence mapping experiments, the asymmetry of the ZPL was assigned to another optically active defect, simultaneously presented in diamond nanocrystals, namely to the GR1 center. The common presence of the GR1 and SiV centers within the CVD diamond structures even in the case of high-quality microcrystals was supported by photoluminescence excitation measurements also. •The asymmetric line shape of the SiV ZPL was studied in CVD nanodiamond films.•The sensitivity of ZPL asymmetry on the applied excitation energy was revealed.•The larger the grain size, the asymmetry of the 1.68 eV PL line is more pronounced.•Simultaneous presence of SiV and GR1 centers proved by PL and PLE measurements.•Contribution of GR1 centers to the PL asymmetry suppressed by the subsequent annealing.