Fluorescent HPHT Nanodiamonds Have Disk- and Rod-Like Shapes

Fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) centers can be used as nanoscale sensors for temperature and electromagnetic fields and find increasing application in many areas of science and technology from biology to quantum metrology. Decreasing the separation between the NV centers and their sensing target often enhances the measurement sensitivity. FND shape strongly affects this distance from NV centers to the particle surface and therefore properties such as brightness and fluorescence spectrum, and can limit sensor applications. Here, we demonstrate that FNDs made from high-pressure high-temperature (HPHT) diamond have predominantly disk-like shapes. Using single-particle atomic force microscopy in combination with ensemble X-ray and light scattering techniques, we show that a typical FND in the 50-150 nm size range has an aspect ratio of three i.e. is three times thinner (e.g. in z) than it is wide (e.g. in the x-y plane). This high aspect ratio of FNDs is important for many quantum sensing measurements as it will enable enhanced sensitivities compared to spherical or other isotropic particle geometries. We investigate FND shape, fluorescence properties, T1 spin relaxation time and T1 fluorescence contrast as functions of particle size and discuss the implications of FND particle shape on quantum sensing applications.