Size and Purity Control of
HPHT Nanodiamonds down to 1 nm
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Abstract
High-pressure
high-temperature (HPHT) nanodiamonds originate from grinding of diamond
microcrystals obtained by HPHT synthesis. Here we report on a simple
two-step approach to obtain as small as 1.1 nm HPHT nanodiamonds of
excellent purity and crystallinity, which are among the smallest artificially
prepared nanodiamonds ever shown and characterized. Moreover we provide
experimental evidence of diamond stability down to 1 nm. Controlled
annealing at 450 °C in air leads to efficient purification from
the nondiamond carbon (shells and dots), as evidenced by X-ray photoelectron
spectroscopy, Raman spectroscopy, photoluminescence spectroscopy,
and scanning transmission electron microscopy. Annealing at 500 °C
promotes, besides of purification, also size reduction of nanodiamonds
down to ∼1 nm. Comparably short (1 h) centrifugation of the
nanodiamonds aqueous colloidal solution ensures separation of the
sub-10 nm fraction. Calculations show that an asymmetry of Raman diamond
peak of sub-10 nm HPHT nanodiamonds can be well explained by modified
phonon confinement model when the actual particle size distribution
is taken into account. In contrast, larger Raman peak asymmetry commonly
observed in Raman spectra of detonation nanodiamonds is mainly attributed
to defects rather than to the phonon confinement. Thus, the obtained
characteristics reflect high material quality including nanoscale
effects in sub-10 nm HPHT nanodiamonds prepared by the presented method