posted on 2021-03-03, 23:13authored byStepan Stehlik, Michel Mermoux, Bernhard Schummer, Ondrej Vanek, Katerina Kolarova, Pavla Stenclova, Ales Vlk, Martin Ledinsky, Rene Pfeifer, Oleksandr Romanyuk, Ivan Gordeev, Francine Roussel-Dherbey, Zuzana Nemeckova, Jiri Henych, Petr Bezdicka, Alexander Kromka, Bohuslav Rezek
Understanding
materials with dimensions down to a few nanometers
is of major importance for fundamental science as well as prospective
applications. Structural transformation and phonon-confinement effects
in the nanodiamonds (NDs) have been theoretically predicted below
3 nm in size. Here, we investigate the effect of size on the surface
chemistry, microscopic structure, and Raman scattering of high-pressure
high-temperature (HPHT) and detonation nanodiamonds (DNDs) down to
2–3 nm. The surface and size of NDs are controlled by annealing
in air and ultracentrifugation resulting in three ND fractions. Particle
size distribution (PSD) of the fractions is analyzed by combining
dynamic light scattering, analytical ultracentrifugation, small-angle
X-ray scattering, X-ray diffraction, and transmission electron microscopy
as complementary techniques. Based on the obtained PSD, we identify
size-dependent and synthesis-dependent differences of ND properties.
In particular, interpretation of Raman scattering on NDs is revisited.
Comprehensive comparison of detonation and pure monocrystalline HPHT
NDs reveals effects of diamond core size and defects, chemical and
temperature (in)stability, and limitations of current phonon confinement
models. In addition, low-frequency Raman scattering in the 20–200
cm–1 range is experimentally observed. The size
dependence of this signal for both HPHT NDs and DNDs suggests that
it may correspond to confined acoustic vibrational, “breathing-like”
modes of NDs.