  We report fluorescence investigations and Raman spectroscopy on colloidal
nanodiamonds (NDs) obtained via bead assisted sonic disintegration (BASD) of a
polycrystalline chemical vapor deposition film. The BASD NDs contain in situ
created silicon vacancy (SiV) centers. Whereas many NDs exhibit emission from
SiV ensembles, we also identify NDs featuring predominant emission from a
single bright SiV center. We demonstrate oxidation of the NDs in air as a tool
to optimize the crystalline quality of the NDs via removing damaged regions
resulting in a reduced ensemble linewidth as well as single photon emission
with increased purity. We furthermore investigate the temperature dependent
zero-phonon-line fine-structure of a bright single SiV center as well as the
polarization properties of its emission and absorption.
