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.
Detonation nanodiamonds (DNDs) with a typical size of 5 nm have attracted broad interest in science and technology. Further size reduction of DNDs would bring these nanoparticles to the molecular-size level and open new prospects for research and applications in various fields, ranging from quantum physics to biomedicine. Here we show a controllable size reduction of the DND mean size down to 1.4 nm without significant particle loss and with additional disintegration of DND core agglutinates by air annealing, leading to a significantly narrowed size distribution (±0.7 nm). This process is scalable to large quantities. Such molecular-sized DNDs keep their diamond structure and characteristic DND features as shown by Raman spectroscopy, infrared spectroscopy, STEM and EELS. The size of 1 nm is identified as a limit, below which the DNDs become amorphous.
Color centers in diamonds have shown excellent potential for applications in quantum information processing, photonics, and biology. Here we report chemical vapor deposition (CVD) growth of nanocrystalline diamond (NCD) films as thin as 5-6 nm with photoluminescence (PL) from silicon-vacancy (SiV) centers at 739 nm. Instead of conventional 4-6 nm detonation nanodiamonds (DNDs), we prepared and employed hydrogenated 2 nm DNDs (zeta potential = +36 mV) to form extremely dense (∼1.3 × 10 cm), thin (2 ± 1 nm), and smooth (RMS roughness < 0.8 nm) nucleation layers on an Si/SiO substrate, which enabled the CVD growth of such ultrathin NCD films in two different and complementary microwave (MW) CVD systems: (i) focused MW plasma with an ellipsoidal cavity resonator and (ii) pulsed MW plasma with a linear antenna arrangement. Analytical ultracentrifuge, infrared and Raman spectroscopies, atomic force microscopy, and scanning electron microscopy are used for detailed characterization of the 2 nm H-DNDs and the nucleation layer as well as the ultrathin NCD films. We also demonstrate on/off switching of the SiV center PL in the NCD films thinner than 10 nm, which is achieved by changing their surface chemistry.
Aberration-corrected
transmission electron microscopy of the atomic
structure of diamond–graphite interface after Ni-induced catalytic
transformation reveals graphitic planes bound covalently to the diamond
in the upright orientation. The covalent attachment, together with
a significant volume expansion of graphite transformed from diamond,
gives rise to uniaxial stress that is released through plastic deformation.
We propose a comprehensive model explaining the Ni-mediated transformation
of diamond to graphite and covalent bonding at the interface as well
as the mechanism of relaxation of uniaxial stress. We also explain
the mechanism of electrical transport through the graphitized surface
of diamond. The result may thus provide a foundation for the catalytically
driven formation of graphene–diamond nanodevices.
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