Locating inherent unpaired orbital spins in detonation nanodiamonds through the targeted surface decoration by paramagnetic probes

Detonation nanodiamond (DND) produced by explosive method has been successfully modified by divalent copper ions via their exchange with protons of neighboring carboxyl groups in water suspension. These ions interact magnetically with all diamond defect sites (both surface and bulk) causing changes in parameters of electron paramagnetic resonance (EPR) signal originating from spins (S = 1/2) of dangling C–C bonds — i.e. unpaired lone orbital paramagnetic centers (PC). EPR on the series of well purified aggregated and disintegrated DND samples in powders and suspensions showed that EPR parameters of PC in DND are unique features characterizing the nanodiamond particle as an isolated object. Surface Cu2+ ions located on the DND surface have been used for probing the location of PC. Double component analysis of PC's EPR spectra showed quasilinear dependence of line broadening for both EPR spectra components on probe concentration. The concentration changes for the broader component were found to be more prominent than that for the narrower one. It allows attributing PC characterized by the broader and narrower components to different types of defects located closer to the DND surface and deeper towards the diamond core. The estimated depths of occurrence for two types of intrinsic PC are ~ 0.8 nm and ~ 1.5 nm from the DND surface for the shallow and deeper PC, respectively.

Research Highlights
► Strong EPR signal of sp3 defects bears unique signature of isolated DND particle.
► Targeted decoration of DND surface by paramagnetic probes to study defects’ location.
► Cu2+ ions as paramagnetic probes modify DND surface and broad defects’ EPR signal.
► Broadening allows estimatiing depths of occurrence for two types of intrinsic defects.
► ~ 0.8 nm and ~ 1.1 nm from DND surface for shallow and deeper defects, respectively.