Effect of substrates on the temperature dependence of fluorescence spectra of nitrogen vacancy centers in diamond nanocrystals

The temperature dependence of fluorescence spectra of nitrogen vacancy (NV) centers in diamond nanocrystals is investigated in terms of the ability of the substrate to suppress the fluorescence phonon sideband of negatively charged NV (NV ^-) centers. Three substrates were used: a glass coverslip, undoped silicon (Si), and thermally oxidized silicon (SiO2/Si). Nanocrystals were distributed on these substrates and the temperature dependence of fluorescence spectra of NV ^- centers of individual nanocrystals were measured in the temperature range 3.5-230 K. We found that the zero-phonon line (ZPL) of NV ^- centers in nanocrystals on the SiO ²/Si substrate remained intense as the temperature was increased from 3.5 to 160 K, whereas it monotonically decreased for the Si substrate. Diamond nanocrystals dispersed on a glass coverslip did not exhibit prominent ZPLs of NV ^- centers, even at 3.5 K. The temperature dependencies of the peak intensities of NV ^- centers were quantitatively analyzed by determining the Debye-Waller factors of the three substrates. As the temperature was increased from 3.5 to 230 K, the Debye-Waller factor decreased almost linearly from 10 to 1.7% for the SiO ₂/Si substrate, decreased from 6.1 to 1.6% for the Si substrate, and decreased from 0.9 to 0.62% for the glass coverslip. These results will be useful for nanophotonic device applications of nanodiamond NV centers in quantum information science and sensing.