Micro-photoluminescence spectroscopy of detonation nanodiamonds containing germanium-vacancy centres.

A germanium-vacancy (GeV) centre in nanodiamond is a promising candidate for bright single-photon emitters, fluorescence labelling and quantum sensing due to its narrow zero-phonon line (ZPL) and large Debye-Waller factor. Recently, the GeV centre-containing nanodiamond (GeV-ND) was fabricated by a detonation process, which provides a scalable method capable of producing large quantities of ND particles. However, the optical properties of the GeV-ND at the single-particle level have not been well studied. In this study, we conducted spatially resolved photoluminescence (PL) mapping on sparsely distributed GeV-ND particles to examine luminescence characteristics across individual particles. The GeV-ND exhibited a narrow ZPL accompanied with a broad emission band. By comparing with well-characterized PL spectra of nitrogen-vacancy (NV) centres in detonation NDs reported in previous single-particle-level studies for magnetometry sensing applications, we attribute the broad background to luminescence from the NV centres coexisting with the GeV centres in NDs. By subtracting this background, we identified the pure GeV emission. Some GeV-NDs showed ZPL linewidths as narrow as 28 meV at room temperature, suggesting the presence of optically promising GeV centres. These findings clarify the origins of PL observed in GeV-NDs and demonstrate their potential as scalable quantum emitters.