Nanodiamonds (NDs) hold promise for a vast range of various applications related to their surface properties (drug delivery, selective biomolecule binding), volumetric properties (cellular labeling and imaging, quantum sensing), and optoelectronic properties (energy harvesting and storage, photocatalysis). Although researchers employ nanodiamonds in all these applications, due to the great variety of this nanomaterial, the actual nanoparticles used can differ substantially. Sometimes to the extent that they could be defined as a different material.
In this contribution, two most common ND types, detonation NDs (DNDs) and high-pressure high-temperature (HPHT) NDs will be introduced and their specifics highlighted. We will review recent experimental advances in the preparation of sub-5 nm NDs of both detonation and HPHT origin, including the HPHT NDs synthesized from molecular precursors¹. The presentation aims to critically compare these two ND types from the Raman spectroscopy point of view as this technique is broadly used to assess not only the ND phase composition (C-sp3 and C-sp2 content) but also the ND size. We show that the Raman spectra of DND and HPHT NDs on the 3-5 nm scale are size-insensitive yet significantly different from each other which limits the application of the current phonon confinement models for the ND size distribution analysis². Finally, we show how to control the size of the ND and answer the question: How small can nanodiamonds be³?

1. E. Ekimov et al., Nanomaterials 2022, 12, 351.
2. S. Stehlik et al., J. Phys. Chem. C 2021, 125, 5647-5669.
3. S. Stehlik et al., J. Phys. Chem. C 2015, 119, 27708-27720
   
   *corresponding author: e-mail: stehlik@fzu.cz