Real Effect of Size on The Raman Spectrum of sub-5 nm Nanodiamonds

Nanodiamonds (NDs) hold promise for a vast range of various distinct applications including drug delivery, cellular labeling and imaging, quantum sensing (such as thermometry and magnetic field/spin sensing), selective biomolecule binding or energy harvesting, and (photo)catalysis - to name a few. 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.<br/>In this contribution, specifics of the two most common ND types, detonation NDs (DNDs) and high-pressure high-temperature (HPHT) NDs will be 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 precursors (E. Ekimov et al., <i>Nanomaterials</i> 2022, 12, 351). 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 analysis (S. Stehlik et al., <i>J. Phys. Chem. C</i> 2021, 125, 5647−5669). The diamond structural quality, and temperature (in)stability caused by laser irradiation during the measurement are the key features reflected in the NDs’ Raman spectrum. The first time observed low-frequency (20−200 cm^-1) Raman scattering signals may correspond to “breathing-like” modes of NDs as these signals exhibit clear size dependence. The low-frequency Raman scattering may thus provide another way for size distribution analysis of nanodiamonds (A. Vlk et al., <i>J. Phys. Chem. C</i> 2022, 126, 6318−6324).