We show that non-hydrostatic compression of glassy carbon to 145 GPa at room temperature produces cubic, hexagonal and an amorphous form of diamond in the same recovered sample. The amorphous diamond phase exhibits a microstructure comparable to tetrahedral amorphous carbon thin films and to that formed from fullerene samples subjected to high pressure treatment. The crystallinity and preferred orientation of the phases indicate that shear stress is critical in driving the phase transformations from the randomly oriented graphitic glassy carbon precursor. A Gibbs free energy landscape, mapped as a function of density and crystallinity, illustrates proposed transformation pathways from glassy carbon to a common high-density parent phase that forms under compression that then converts to the phases observed, depending on the local stress environment. Modelling demonstrates that the common high-density parent phase has a preferred orientation in its atomic structure which explains the microstructure in many of the recovered daughter phases. Our results demonstrate that non-hydrostatic stress can be used to synthesise desirable diamond-like carbon phases without heating, including amorphous diamond and ultra-fine nanodiamonds with sizes down to 2 nm in diameter.

Nanoporous carbons and carbon nanostructures can store hydrogen at cryogenic temperatures but lack the volumetric and gravimetric capacity to be industrially significant. Recent inelastic neutron scattering experiments suggest a highly dense phase of hydrogen at temperatures well above the melting point of solid hydrogen. However, it remains unclear how pore geometry and intermolecular interactions enable these dense phases to exist, with dispersion (van der Waals) or electrostatic/induction suggested to be the key effects in slit and curved pores but their relative contributions have yet to be quantified. In this paper, we perform benchmark electronic structure calculations allowing the interactions between planar and curved aromatic molecules with hydrogen to be accurately determined. Dispersion was found to dominate over electrostatic and inductive effects with some many-body charge transfer (Dobson type-A) effects needed to capture the most highly curved structures. Density functional methods that include type-A many-body effects were found to accurately describe the intermolecular interactions at a fraction of the cost of coupled-cluster simulations and these approaches were used to calculate the energies inside large carbon bowl and slit pores. The interaction energies inside the bowl pores were found to depend on the orientation of the hydrogen molecule. This rotational barrier, modeled as a quantum hindered rotor, could reproduce the peak splitting observed in inelastic neutron scattering experiments, with weak splitting arising from bowl-like fullerene pores and strong splitting from highly confining nanotubelike pores. Increasing the fraction of such curved pores in nanoporous carbons may therefore offer a pathway to enhance their hydrogen-storage capacity. Moreover, the preferential adsorption of ortho hydrogen on nanotubelike pores could enable the storage of high-density hydrogen without the need to remove heat produced during the ortho-para hydrogen conversion.

This work examines how screw dislocations disrupt ideal stacking in graphenic materials, using a machine learning interatomic potential to model screw dislocation dipoles within different periodic cells. A novel tool is developed to assess local interlayer registration to quantify and visualise regions exhibiting stacking order. Using molecular dynamics simulations, we demonstrate that single screws exhibit greater stability in rhombohedral stacking compared to AA stacking, whereas double screws exhibit greater stability in Bernal AB stacking. The investigation reveals several mechanisms through which the ideal stacking configuration is achieved despite the presence of screw dislocations, including shearing, bond length distortion, and buckling. Furthermore, an upper threshold for the density of screw dislocations is calculated, beyond which ideal stacking cannot be realised, potentially offering an explanation for certain forms of turbostratic carbon. The findings also indicate that single screw dislocations hinder Bernal AB stacking in graphite, whereas double screw dislocations support Bernal stacking, which is significant for understanding the formation processes of graphite.

Carbon nanomembranes (CNMs) are nanometer-thin disordered carbon materials that are suitable for a range of applications, from energy generation and storage through to water filtration. The structure-property relationships of these nanomembranes are challenging to study using traditional experimental characterization techniques, primarily due to the radiation sensitivity of the free-standing membrane. Highly charged ion spectroscopy is a novel characterization method that is able to infer structural details of the carbon nanomembrane without concern about induced damage affecting the measurements. Here we employ molecular dynamics simulations to produce candidate structural models of terphenylthiol-based CNMs with varying degrees of nanoscale porosity and compare predicted ion charge exchange data and tensile moduli to experiment. The results suggest that the in-vacuum CNM composition likely comprises a significant fraction of under-coordinated carbon, with an open subnanometer porous structure. Such a carbon network would be reactive in the atmosphere and would be presumably stabilized by hydrogen and oxygen groups under atmospheric conditions.

Understanding the adsorption behavior of hydrogen and deuterium in nanoporous carbons is critical for advancing gas storage and separation technologies. In this study, neutron scattering, gas adsorption, and molecular simulations were combined to unravel the complex interplay between pore structure, spatial confinement, and adsorption mechanisms. By simulating the adsorption in realistic 3D molecular structures of nanoporous carbons, preferred adsorption sites were identified, revealing that highly confining geometries-rich in defects-enhance adsorption. Denser carbons exhibit stronger confinement but lower overall uptake due to limited pore space. Despite accounting for isotope-specific effects, significant deviations between simulated and experimental scattering data suggest distinct molecular arrangements, particularly for H2. The findings of this study underscore the need for refined atomistic models incorporating surface chemistry and spin-isomer effects to bridge the gap between experiment and simulation, guiding the design of optimized nanoporous materials for hydrogen storage.

The transformation of carbon into sp3-rich phases is of broad interest for developing superhard materials, yet the role of non-hydrostatic pressures during such transformations remains poorly understood. While buckminsterfullerene (C60) is known to collapse under high pressures and temperatures, the structural evolution pathways under non-hydrostatic conditions have not been fully established. Here, we report the formation of transparent, amorphous diamond-like carbon (a-D) from C60 compressed to 49 GPa at room temperature under non-hydrostatic stress. Transmission electron microscopy of the recovered material reveals a predominantly sp3-bonded network with (85 +/- 5)% sp3 bonds and a density of (3.05 +/- 0.10) g cm-3, interlaced with residual C60 molecules and narrow bands of nanocrystalline diamond. To understand the transformation mechanism, we performed molecular dynamics simulations using an atomic cluster expansion potential. The simulations show that under uniaxial stress, fullerene cages collapse at lower pressures (29-32 GPa) than under hydrostatic conditions (42-45 GPa), forming a structure consistent with amorphous diamond-like carbon. Annealing simulations on the amorphous structure at 50 GPa result in nanocrystalline diamond formation, suggesting that localised heat spikes generated by an adiabatic shear band process drive the formation of observed nanodiamond regions.

Molecular dynamics simulations are used to study the structure and removal of prismatic dislocation loops during graphitization. The carbon models contain a mixture of screw and edge dislocations, and are created by self-assembly at high temperature. Four mesh-based analysis tools are used to track the time-evolution of the dislocation loops, providing insight into loop structure and allowing quantification of kinetics. We find that the loop structure is complex, being dispersed in three dimensions with alternating screw and edge components in multiple slip planes. Loop removal involves edge glide through kink formation and propagation, followed by a slower screw glide mechanism. All analysis tools yield similar activation energies, in the range of 2.9 ± 0.3 eV, consistent with recent experimental work by ourselves. Literature values for the kinetics of graphitization fall into two brackets, a lower range of 2.8–3.9 eV, and a higher range of 7.8–11.6 eV. This work supports the lower range and suggests that prismatic dislocation loops are the key defect removed during graphitization.

Understanding new mechanisms for phase transformation in carbon is of considerable interest. This study investigates on the compression conditions required to create recoverable diamond during room-temperature high-pressure compression of glassy carbon. Under non-hydrostatic compression conditions when shear is present, glassy carbon transforms into an oriented graphitic structure at ∼45 GPa, and then forms mixed diamond and lonsdaleite nanocrystals when the pressure is higher than ∼80 GPa. In contrast, during hydrostatic compression no significant changes in the microstructure was observed, highlighting glassy carbon’s resilience under compression. Molecular dynamics modelling supports the proposed model that shear drives the phase transition mechanism and causes a temperature spike that drives crystallisation. Our work demonstrates that shear is key to high-pressure diamond formation in the absence of heating.

We explore the onset of phase transformation, at the nanoscale, in single-crystal diamond-cubic silicon (dc-Si) subjected to pressures of 13 GPa using a diamond anvil cell with a methanol-ethanol pressure medium. Transmission electron microscopy reveals two distinct structural features along {111} planes: (1) thin bands of defective dc-Si and (2) thicker bands of body-centered cubic silicon (bc8), surrounded by defective dc-Si. We propose that these features are consistent with shear bands that have been formed by slip along the low energy {111} planes and have a range of thicknesses depending on how much plastic deformation has occurred. The presence of bc8-Si within the thicker bands can be explained by localized regions of high pressure or energy at their center facilitating phase transformation to the metastable metallic beta-Sn phase, which in turn, transforms to bc8 on pressure release. Our observations reveal that phase formation in silicon can be shear-activated, the transformation is not nucleation-limited, and its sluggish nature may be due to the slow growth of the metallic phase. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/).

We explore the onset of phase transformation, at the nanoscale, in single-crystal diamond-cubic silicon (dc-Si) subjected to pressures of 13 GPa using a diamond anvil cell with a methanol-ethanol pressure medium. Transmission electron microscopy reveals two distinct structural features along {111} planes: (1) thin bands of defective dc-Si and (2) thicker bands of body-centered cubic silicon (bc8), surrounded by defective dc-Si. We propose that these features are consistent with shear bands that have been formed by slip along the low energy {111} planes and have a range of thicknesses depending on how much plastic deformation has occurred. The presence of bc8-Si within the thicker bands can be explained by localized regions of high pressure or energy at their center facilitating phase transformation to the metastable metallic β-Sn phase, which in turn, transforms to bc8 on pressure release. Our observations reveal that phase formation in silicon can be shear-activated, the transformation is not nucleation-limited, and its sluggish nature may be due to the slow growth of the metallic phase.

We explore the onset of phase transformation, at the nanoscale, in single-crystal diamond-cubic silicon (dc-Si) subjected to pressures of 13 GPa using a diamond anvil cell with a methanol-ethanol pressure medium. Transmission electron microscopy reveals two distinct structural features along {111} planes: (1) thin bands of defective dc-Si and (2) thicker bands of body-centered cubic silicon (bc8), surrounded by defective dc-Si. We propose that these features are consistent with shear bands that have been formed by slip along the low energy {111} planes and have a range of thicknesses depending on how much plastic deformation has occurred. The presence of bc8-Si within the thicker bands can be explained by localized regions of high pressure or energy at their center facilitating phase transformation to the metastable metallic β-Sn phase, which in turn, transforms to bc8 on pressure release. Our observations reveal that phase formation in silicon can be shear-activated, the transformation is not nucleation-limited, and its sluggish nature may be due to the slow growth of the metallic phase.

We develop a picture of graphenic crystallites within disordered carbons that goes beyond the traditional model of graphitic platelets at random orientations. Using large atomistic structures containing one million atoms, we redefine the meaning of the quantity La extracted from X-ray diffraction (XRD) patterns. Two complimentary approaches are used to measure the size of graphenic crystallites, which are defined as regions of regularly arranged hexagons. Firstly, we calculate the X-ray diffraction pattern directly from the atomistic coordinates of the structures and analyse them following a typical experimental process. Second, the graphenic crystallites are identified from a direct geometrical approach. By mapping the structure directly, we replace the idealised picture of the crystallite with a more realistic representation of the material and provide a well-defined interpretation for La measurements of disordered carbon. A key insight is that the size distribution is skewed heavily towards small fragments, with more than 75% of crystallites smaller than half of La.

Structural characterization of porous carbon materials is critical for the evaluation of their synthesis procedures and performance. Throughout the last decades, many methods have been employed to determine porosity properties from gas adsorption such as surface area, pore size distribution (PSD) and real density. However, gas adsorption models use 1D structures of carbon nanopores, although adsorption and separation properties of nanoporous carbons are governed by 3D pore parameters. Estimating the 3D nanostructure of nanoporous carbons using gas adsorption would accelerate progress in research and implementation of nanoporous carbons.We report here a promising 3D pore nanostructural characterization from gas adsorption. Using atomistic simulations, we have generated a database of realistic 3D porous carbon structures spanning a wide range of pore sizes and geometries. After calculating their gas adsorption isotherms, we employed a numerical procedure to find the relative contribution for each of the structures to the adsorption isotherm of a nanoporous carbon sample. These contributions allowed us to estimate the surface area and pore size distribution of carbon materials; moreover (and perhaps more importantly!), we will show that the plausible 3D pore structures correlate very well with the local carbon structure as experimentally determined by high-resolution TEM observations and can successfully predict adsorption of different molecules. This is a powerful procedure that can be extended to other materials, and with enough computer power, to larger pore sizes.

The wastewater release from the Fukushima Daiichi nuclear plant is expected to have negligible effects on people and the ocean.

Graphite is the thermodynamically stable form of carbon and yet is remarkably difficult to synthesize. We show the annihilation of screw dislocations is critical for graphitization. These dislocations wind through the layers like a spiral staircase, inhibiting lateral growth of the graphenic crystallites (La) and preventing AB stacking of Bernal graphite. High-resolution transmission electron microscopy identifies screws as interdigitated fringes with narrow focal depth in graphitizing polyvinyl chloride. Molecular dynamics simulations of parallel graphenic fragments confirm that screws spontaneously form during heating, with higher annealing temperature driving screw annihilation and crystallite growth. The time evolution and kinetics of graphitization is tracked via X-ray diffraction, showing the growth of La and reduction of the interlayer spacing, consistent with screw annihilation. We find that graphite forms orders of magnitude faster than previously assumed, taking less than ten seconds at 3000 degrees C and just minutes at 2500 degrees C. This rapid transformation suggests major cost savings in synthetic graphite production, important for lithium-ion batteries and smelting electrodes. By reducing the time spent at ultra-high temperatures, energy costs and component degradation can be significantly lower.

Aligned graphenic fragments of varying size, mimicking those found in carbons carbonized through a mesophase, are found to self-assemble towards structures of varying anisotropy when annealed using molecular dynamics simulations. These models enable us to probe the range of topological features present in carbons of varying anisotropy where we find significant differences in the defects present. We conclude that the screw dislocation is the dominant annealable defect in graphitizable carbons, while a persistence of saddle-shaped defects may make some carbons less graphitizable. These findings contribute to the ongoing questions surrounding the factors which determine precursor graphitizability.

High pressure experiments and ab initio calculations are used to investigate unexpected crystallographic preferred orientation in the bc8 phase of silicon formed under non-hydrostatic conditions. Microstructural characterization in two orthogonal directions reveals that the preferred orientation is only visible when the sample is viewed perpendicular to the compression axis. Curiously, the elastic constants of bc8-Si are almost perfectly isotropic, making it counter-intuitive that preferred crystallographic orientation is observed. This conundrum is resolved by tracking the phase transformation pathway and computing the three-dimensional Young's modulus. We find the preferred orientation most likely originates from the highly anisotropic simple-hexagonal phase and is passed on to subsequent daughter phases via displacive phase transformations. Our investigation of preferred orientation in bc8-Si complements other high pressure studies where preferred orientation in silicon phases is often observed but not explained.