Surface Tamm States of 2-5 nm Nanodiamond via Raman Spectroscopy.

We observed resonance effects in the Raman scattering of nanodiamonds with an average size of 2-5 nm excited at a wavelength of 1064 nm (1.16 eV). The resonant Raman spectrum of the 2-5 nm nanodiamonds consists of bands at wavelengths of 1325 and 1600 cm-1, a band at 1100-1250 cm-1, and a plateau in the range from 1420 to 1630 cm-1. When excited away from the resonance (at a wavelength of 405 nm, 3.1 eV), the Raman spectrum consists of only three bands at 1325, 1500, and 1600 cm-1. It is important to note that the additional lines (1500 and 1600 cm-1) belong to the sp3-hybridized carbon bonds. The phonon density of states for the nanodiamonds (~1 nm) was calculated using moment tensor potentials (MTP), a class of machine-learning interatomic potentials. The presence of these modes in agreement with the lattice dynamics indicates the existence of bonds with force constants higher than in single-crystal diamonds. The observed resonant phenomena of the Raman scattering and the increase in the bulk modulus are explained by the presence of Tamm states with an energy of electronic transitions of approximately 1 eV, previously observed on the surface of single-crystal diamonds.

The Effect of Shear Deformation on C-N Structure under Pressure up to 80 GPa.

We studythe effect of shear deformation on graphitic g-C3N4 under pressures of up to 80 GPa at room temperature. g-C3N4 samples are transformed from initial amorphous flakes into onion-like structures, in which the nitrogen content in the quenched samples decreases with increasing pressure (from 42% in the initial conditions to 1% at 80 GPa). The concentration of the sp2 bonds also decreases from 1 (the initial sample) to 0.62 with increasing pressure to 80 GPa. This transformation of the sample is due to the fact that in the pressure range of 55-115 GPa, the equilibrium phase is not a diamond, but instead, carbon onions cross-linked by sp3 bonds, which are denser than diamonds. The results of our study show that the presence of nitrogen in sp3-bonded structures at pressures of higher than 55 GPa reduces the density and, accordingly, carbon structures without nitrogen become thermodynamically favorable.

Transformation of diamond to fullerene-type onions at pressure 70 GPa and temperature 2400 K.

We report the observation of a phase transition of diamond to denser than diamond carbon phase composed from 2 to 3 fullerene-type shells of onions. Raman spectra indicate the fullerene-type of the onions shells. The onions phase is a stable phase in a diamond instability zone of a phase diagram of carbon at pressure 70 GPa and temperature 2400 K. A mixture of diamond and Ni powders was heated by a laser beam under pressure in a diamond anvil cell. Both direct and catalytic diamond to onions transitions were observed during heating. The catalytic transformation includes the following steps. Melting of Ni during the laser heating at pressure 70 GPa, a 'diamond solution' (a transfer of carbon atoms from diamond) in liquid Ni and the formation of an equilibrium carbon phase from the supersaturated solution upon cooling. The catalytic process is a reverse one relative to the catalytic synthesis of diamond in a diamond stability zone at pressure around 6 GPa. The main result of our study is the presence of fullerene-type structures in the phase diagram of carbon in the region of diamond instability under high sub-Mbar pressure and wide range of temperatures.

Phase diagram of carbon and the factors limiting the quantity and size of natural diamonds.

Phase diagrams of carbon, and those focusing on the graphite-to-diamond transitional conditions in particular, are of great interest for fundamental and applied research. The present study introduces a number of experiments carried out to convert graphite under high-pressure conditions, showing a formation of stable phase of fullerene-type onions cross-linked by sp3-bonds in the 55-115 GPa pressure range instead of diamonds formation (even at temperature 2000-3000 K) and the already formed diamonds turn into carbon onions. Our results refute the widespread idea that diamonds can form at any pressure from 2.2 to 1000 GPa. The phase diagram built within this study allows us not only to explain the existing numerous experimental data on the formation of diamond from graphite, but also to make assumptions about the conditions of its growth in Earth's crust.

Raman Spectra and Bulk Modulus of Nanodiamond in a Size Interval of 2-5 nm.

Nanodiamond in a 2-5-nm size interval (which is typical for an appearance of quantum confinement effect) show Raman spectra composed of 3 bands at 1325, 1600, and 1500 cm-1 (at the 458-nm laser excitation) which shifts to 1630 cm-1 at the 257-nm laser excitation. Contrary to sp2-bonded carbon, relative intensities of the bands do not depend on the 458- and 257-nm excitation wavelengths, and a halfwidth and the intensity of the 1600 cm-1 band does not change visibly under pressure at least up to 50 GPa. Bulk modulus of the 2-5-nm nanodiamond determined from the high-pressure study is around 560 GPa. Studied 2-5-nm nanodiamond was purified from contamination layers and dispersed in Si or NaCl.