Transformation of C60 polymers to a superelastic hard carbon (nanoclustered graphene phase (NGP)) occurring in metal matrix at 5 GPa in a temperature interval of 1000–1100 K was studied by optical, scanning electron microscopy (SEM), and Raman spectroscopy. Raman spectral scan across the sample surface allowed us to identify different stages of the structural transformation. The SEM and Raman spectroscopy data testify for the NGP appearance at the defects concentration sites in the parent fullerite structure. We propose that the buckyballs collapse/formation of the NGP is governed by nucleation and growth (diffusive) mechanism unlike earlier discussed in the literature possibility of the martensitic-type (displacive) character of this transformation.

In this paper, a study of the C₆₀ – nanoclustered graphene phase (NGP) transformation via characterization of non-completely transformed carbon particles is presented. High-resolution (∽ 1 µm) Raman spectroscopy and nanoindentation measurements were performed on the same pre-selected sample area. The results evidence different steps of the transformation that allows establishing correspondence between the NGP/C₆₀ ratio and the nanohardness: an abrupt increase in nanohardness from 2 GPa to 20 GPa for a stepwise NGP/C₆₀ ratio change in the transformation zone. These results demonstrate that (I) at a micro-level (1 µm), the transformation of C₆₀ into NGP does not occur simultaneously in the entire volume and (II) the residual C₆₀ polymer is not desirable in superhard amorphous carbon materials. This work demonstrates importance of advanced experimental methodologies to characterization of disordered carbon phases.

The paper compares the Raman spectroscopy, HRSTEM, and indentation hardness results of disordered systems synthesized by squeezing (8 GPa, 850 °C) C₆₀ and ball-milled C₆₀. Mechanical activation introduces substantial damage to the C₆₀ crystal leading to the rupture of van der Waals interaction between the C₆₀ units and a chemical reaction between the balls creating C₆₀ dimers. The multi-wavelength Raman spectra of both, the compressed mechanical activated phase (MA-Phase) and without the mechanical activation phase (wMA phase) reveal that fused aromatic rings with a low fraction of nanographene clusters dominate the MA phase, whereas wMA phase is composed of nanographene clusters. Moreover, a Raman model is presented which introduces fullerene-like structures because of fivefold (F-band) and sevenfold carbon rings-like defects for the wMA phase and part of fused aromatic rings for the MA phase. HRSTEM-EELS data confirm that: nanographene clusters present in wMA (I) are smaller and not abundant in the MA phase (II). (III) EELS data reveal a higher fraction of sp3 bonds in the MA phase compared to that in wMA. The hardness of the MA Phase (37 GPa) is twice its value (18 GPa) in the wMA (IV). The extensive analysis of the Raman data yielded empirical dependences of Hardness vs ID/IG/Hardness vs ID/IF that can be useful for prediction of the hardness of sp2-dominant disordered carbon systems based on their spectroscopic data.         

New carbon forms that exhibit extraordinary physicochemical properties can be generated from nanostructured precursors under extreme pressure. Nevertheless, synthesis of such fascinating materials is often not well understood. That is the case of the C₆₀ precursor, with irreproducible results that impede further progress in the materials design. Here, the semiconducting amorphous carbon, having band gaps of 0.1–0.3 eV and the advantages of isotropic superhardness and superior toughness over single-crystal diamond and inorganic glasses, is produced from fullerene at high pressure and moderate temperatures. A systematic investigation of the structure and bonding evolution is carried out with complementary characterization methods, which helps to build a model of the transformation that can be used in further high-pressure/high-temperature (high p,T) synthesis of novel nano-carbon systems for advanced applications. The amorphous carbon materials produced have the potential of accomplishing the demanding optoelectronic applications that diamond and graphene cannot achieve.

Carbon is one of the most fascinating elements due to its structurally diverse allotropic forms stemming from its bonding varieties (sp, sp2, and sp3). Exploring new forms of carbon has always been the eternal theme of scientific research. Herein, we report the amorphous (AM) carbon materials with high fraction of sp3 bonding recovered from compression of fullerene C60 under high pressure and high temperature previously unexplored. Analysis of photoluminescence and absorption spectra demonstrates that they are semiconducting with a bandgap range of 1.5–2.2 eV, comparable to that of widely used amorphous silicon. Comprehensive mechanical tests demonstrate that the synthesized AM-III carbon is the hardest and strongest amorphous material known so far, which can scratch diamond crystal and approach its strength. The produced AM carbon materials combine outstanding mechanical and electronic properties, and may potentially be used in photovoltaic applications that require ultrahigh strength and wear resistance.

A key factor that determines the mechanical and electrical performance of graphene-based materials and devices is how graphene behaves under extreme conditions, yet the response of few-layer graphene to high shear stress has not been investigated experimentally. Here we applied high pressure and shear to graphene powder using a rotational diamond anvil cell and studied the recovered sample with multiple means of characterization. Sustaining high pressure and shear, graphene breaks into nanometer-long clusters with generation of large number of defects. At a certain stress level, it transforms to amorphous state and carbon onions. The reduction of infrared reflectivity in the severely sheared phase indicates the decrease in conductivity. Our results unveil the shear sensitive nature of graphene, point out the effects of shear on its physical properties, and provide a potential method to manipulate this promising material.