Optical Properties of Silicon Vacancy (SiV) Centers in Nanodiamonds Fabricated by Detonation Process

Silicon vacancy (SiV) center-containing nanodiamonds (SiV-NDs) have attracted much attention as promising fluorescent probes due to their high biocompatibility and fluorescence properties. SiV centers in bulk diamond exhibit a sharp zero-phonon line (ZPL) with a linewidth of 5 nm in the biomaterial window (650-950nm) [1] at room temperature [2][3]. In addition, the SiV center emission has a large Debye-Waller factor (DWF), concentrating about 70% of the total emission in the ZPL [4]. This sharp photoluminescence is excellent for multicolor bioimaging and for distinguishing of autofluorescence. Recently, Daicel Corporation has demonstrated the synthesis of SiV-NDs by detonation process (SiV-DNDs) [5]. This method has the potential to pave the way for bioimaging applications because it can efficiently produce NDs with a single-digit nm size, which is extremely small compared to NDs produced by other methods. The small size allows non-invasive imaging. However, there is a concern that SiV centers in DNDs may show optical properties different from those of SiV centers in bulk diamond due to their extremely small size. For example, a nitrogen-vacancy (NV) center, another well-studied color center, in DNDs has been reported to show different optical properties from NVs in bulk diamond, such as the extinction of fluorescence caused by defects near the surface and inhomogeneous distribution of the ZPL due to energy level shifts depending on the position of the NV centers [6].<br/>In the present study, we measured photoluminescence and photoluminescence excitation spectra (PLE) and conducted time-resolved photoluminescence measurements to investigate the optical properties of SiV centers in DNDs.<br/>The optical properties were compared with those of typical SiV centers in bulk diamond; in the case of SiV centers in DNDs, the linewidth of ZPL is 14.4 nm at room temperature, which is wider than the line width of SiV centers in bulk diamond. Furthermore, the DWF of the SiV center in DNDs is 0.47, which is reduced to about 70% of that of the SiV center in bulk diamond. The luminescence lifetime of the SiV center in DNDs is 0.56 ns, which is about half of that of the SiV center in bulk diamond. PLE measurements confirmed SiV-DND emission under excitation wavelengths from 390 to 690 nm. PLE spectra is broad and more efficiently excited at longer wavelengths compared to SiV centers in bulk diamond.<br/>We attribute these differences in optical properties to surface effects due to the very small particle size (~10 nm). The enhanced non-radiative transition probability and electron-phonon coupling resulting from the surface effects lead to homogeneous broadening of the ZPL and shorter fluorescence lifetime of the SiV centers in the DNDs. The rise in the low-energy side of the PLE spectrum is also considered to be the tail of the ZPL of the absorption spectrum. Moreover, surface effects cause fluctuations in the energy levels of the SiV centers in each DND. This fluctuation gives rise to the inhomogeneous broadening of the ZPL. However, we demonstrated that the photostability does not change, irrespective of the particle size. In summary, SiV-DNDs show a sharp photostable emission, a wide excitation wavelength range and a high excitation efficiency in the biomaterial window region, suggesting that they are promising fluorescent probes in the field of bioimaging.<br/> <br/>[1] R. Weissleder, et al. Nat. Biotechnol. 19, 316 (2001).<br/>[2] G. Thiering, A. Gali, Phys. Rev. X, 8, 021063 (2018).<br/>[3] S. V. Bolshedvorski, et al., ACS Appl. Nano Mater. 2, 4765 (2019).<br/>[4] S. Häußler, et al., New J. Phys. 19, 063036 (2017).<br/>[5] Y. Makino, et al., Diam. Relat. Mater. 112, 108248 (2021).<br/>[6] V.Y. Osipov, et al., Nanoscale Res. Lett. 14, 1 (2019).