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Glassy carbon is a technologically important material with isotropic properties that is nongraphitizing up to ∼3000 °C and displays complete or “superelastic” recovery from large compression. The pressure limit of these properties is not yet known. Here we use experiments and modeling to show permanent densification, and preferred orientation occurs in glassy carbon loaded to 45 GPa and above, where 45 GPa represents the limit to the superelastic and nongraphitizing properties of the material. The changes are explained by a transformation from its sp2 rich starting structure to a sp3 rich phase that reverts to fully sp2 bonded oriented graphite during pressure release.

Diamond is an attractive material due to its extreme hardness, high thermal conductivity, quantum optical, and biomedical applications. There is still much that is not understood about how diamonds form, particularly at room temperature and without catalysts. In this work, a new route for the formation of nanocrystalline diamond and the diamond-like phase lonsdaleite is presented. Both diamond phases are found to form together within bands with a core-shell structure following the high pressure treatment of a glassy carbon precursor at room temperature. The crystallographic arrangements of the diamond phases revealed that shear is the driving force for their formation and growth. This study gives new understanding of how shear can lead to crystallization in materials and helps elucidate how diamonds can form on Earth, in meteorite impacts and on other planets. Finally, the new shear induced formation mechanism works at room temperature, a key finding that may enable diamond and other technically important nanomaterials to be synthesized more readily.

Glassy carbon (GC) is a class of disordered carbon materials that is known to be superelastic and non-graphitizing up to 3000 °C. The maximum heat treatment temperature is often used as a proxy to denote structure and physical properties. GC synthesised at low temperatures (~1000 °C) is often classified as Type I GC which has advantages of higher elastic modulus, resistance to oxidation, and lower permeability to gases. Type II GC is synthesised at higher temperatures (>2000 °C), has fewer impurities, is more electrically conductive, and is rated to a higher service temperature. Here Type I and II GC samples sourced from two suppliers are investigated using Rutherford backscattering spectrometry and elastic recoil detection analysis for composition, Raman spectroscopy, transmission electron microscope imaging, X-ray and neutron diffraction for structure determination, nanoindentation for mechanical properties, and Van der Pauw measurements for resistivity. The results show that the broad classifications of Type I or Type II do not correlate with the physical properties of the samples. We conclude that the quoted maximum heat treatment temperature alone is not sufficient to specify the properties of GC and that a careful microstructural examination of the material should be used to inform materials selection.

The precise measurement of mechanical stress at the nanoscale is of fundamental and technological importance. In principle, all six independent variables of the stress tensor, which describe the direction and magnitude of compression/tension and shear stress in a solid, can be exploited to tune or enhance the properties of materials and devices. However, existing techniques to probe the local stress are generally incapable of measuring the entire stress tensor. Here, we make use of an ensemble of atomic-sized in situ strain sensors in diamond (nitrogen-vacancy defects) to achieve spatial mapping of the full stress tensor, with a submicrometer spatial resolution and a sensitivity of the order of 1 MPa (10 MPa) for the shear (axial) stress components. To illustrate the effectiveness and versatility of the technique, we apply it to a broad range of experimental situations, including mapping the stress induced by localized implantation damage, nanoindents, and scratches. In addition, we observe surprisingly large stress contributions from functional electronic devices fabricated on the diamond and also demonstrate sensitivity to deformations of materials in contact with the diamond. Our technique could enable in situ measurements of the mechanical response of diamond nanostructures under various stimuli, with potential applications in strain engineering for diamond-based quantum technologies and in nanomechanical sensing for on-chip mass spectroscopy.

Room temperature compression of graphitic materials leads to interesting superhard sp3 rich phases which are sometimes transparent. In the case of graphite itself, the sp3 rich phase is proposed to be monoclinic M-carbon; however, for disordered materials such as glassy carbon the nature of the transformation is unknown. We compress glassy carbon at room temperature in a diamond anvil cell, examine the structure in situ using x-ray diffraction, and interpret the findings with molecular dynamics modeling. Experiment and modeling both predict a two-stage transformation. First, the isotropic glassy carbon undergoes a reversible transformation to an oriented compressed graphitic structure. This is followed by a phase transformation at ∼35 GPa to an unstable, disordered sp3 rich structure that reverts on decompression to an oriented graphitic structure. Analysis of the simulated sp3 rich material formed at high pressure reveals a noncrystalline structure with two different sp3 bond lengths.

The rhombohedral phase of Si (r8-Si), a promising semiconducting material, is formed by indentation together with the body-centered cubic phase (bc8-Si). Using a novel sample preparation method, x-ray diffraction is used to determine the relative volume of these phases in indented Si and allow observation of a distorted unit cell along the direction of indentation loading. Theoretical calculations together with these observations suggest the indent contains an intrinsic compression of ∼4 GPa that stabilizes the r8 phase.

Hexagonal diamond, a potentially superhard material, forms from a glassy carbon precursor at pressuresof ~100 GPa at the relatively low temperature of 400 C. The formation mechanism of the hexagonaldiamond phase was investigated by performing microstructural analysis on cross-sections of therecovered samples. Three distinct structures have been observed, a graphitic region near the centre of thesample with low density, a hexagonal diamond region at the edge of the sample with high density, and amixed region containing significant proportions of both the graphitic structure and hexagonal diamond.The hexagonal diamond was more likely to occur at greater radial distance from the centre of the samplewith some evidence for greater amounts also near the diamond anvil faces. The observed distribution ofthe hexagonal phase correlates well to regions of greatest shear strain expected from modelling studiesof strainfields in diamond anvil cells. Thefindings support the proposition that shear strain plays animportant role in the formation of hexagonal diamond, and that it may be a driving force for the naturaloccurrence of hexagonal diamond in the shear zone of meteorite impact craters.

Si nanowires of 80–150 nm and 200–250 nm diameter are pressurized up to 22 GPa using a diamond anvil cell. Raman and x-ray diffraction data were collected during both compression and decompression. Electron microscopy images reveal that the nanowires retain a nanowire-like morphology (after high pressure treatment). On compression, dc-Si was observed to persist at pressures up to 19 GPa compared to 11 GPa for bulk-Si. On decompression, the metallic b-Sn phase was found to be more stable for Si nanowires compared with bulk-Si when lowering the pressure and was observed as low as 6 GPa. For the smallest nanowires studied (80–150 nm), predominately a-Si was obtained on decompression, whereas for larger nanowires (200–250 nm), clear evidence for the r8/bc8-Si phase was obtained. We suggest that the small volume of the individual Si nanowires compared with bulk-Si inhibits the nucleation of the r8-Si phase on decompression. This study shows that there is a size dependence in the high pressure behavior of Si nanowires during both compression and decompression

Exotic phases of germanium, that form under high pressure but persist under ambient conditions, are of technological interest due to their unique optical and electrical properties. The thermal evolution and stability of two of these exotic Ge phases, the simple tetragonal (st12) and hexagonal diamond (hd) phases, are investigated in detail. These metastable phases, formed by high pressure decompression in either a diamond anvil cell or by nanoindentation, are annealed at temperatures ranging from 280 to 320 C for st12-Ge and 200 to 550 C for hd-Ge. In both cases, the exotic phases originated from entirely pure Ge precursor materials. Raman microspectroscopy is used to monitor the phase changes ex situ following annealing. Our results show that hd-Ge synthesized via a pure form of a-Ge first undergoes a subtle change in structure and then an irreversible phase transformation to dc-Ge with an activation energy of (4.3 6 0.2) eV at higher temperatures. St12-Ge was found to transform to dc-Ge with an activation energy of (1.44 6 0.08) eV. Taken together with results from previous studies, this study allows for intriguing comparisons with silicon and suggests promising technological applications.

Diamond cubic Ge is subjected to high pressures via nanoindentation at temperatures between 45 C and 20 C. The residual impressions are studied using ex-situ Raman microspectroscopy and cross-sectional transmission electron microscopy. The deformation mechanism at 20 C is predominately via the generation of crystalline defects. However, when the temperature is lowered, the analysis of residual indentation impressions provides evidence for deformation by phase transformation and formation of additional phases such as r8-Ge, hd-Ge, and amorphous Ge. Furthermore, these results show that at 0 C and below, dc-Ge will reliably phase transform via nanoindentation. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4993163]

Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by “locking in” favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.

The Group 14 element silicon possesses a complex free-energy landscape with many (local) minima, allowing for the formation of a variety of unusual structures, some of which may be stabilized at ambient conditions. Such exotic silicon allotropes represent a significant opportunity to address the ever-increasing demand for novel materials with tailored functionality since these exotic forms are expected to exhibit superlative properties including optimized band gaps for solar power conversion. The application of pressure is a well-recognized and uniquely powerful method to access exotic states of silicon since it promotes large changes to atomic bonding. Conventional high-pressure syntheses, however, lack the capability to access many of these local minima and only four forms of exotic silicon allotropes have been recovered over the last 50 years. However, more recently, significant advances in high pressure methodologies and the use of novel precursor materials have yielded at least three more recoverable exotic Si structures. This review aims to give an overview of these innovative methods of high-pressure application and precursor selection and the recent discoveries of new Si allotropes. The background context of the conventional pressure methods and multitude of predicted new phases are also provided. This review also offers a perspective for possible access to many further exotic functional allotropes not only of silicon but also of other materials, in a technologically feasible manner.

Anthropogenic inputs of CO2 are changing ocean chemistry and will likely affect calcifying marine organisms, particularly aragonite producers such as pteropods. This work seeks to set a benchmark analysis of pteropod shell properties and variability using nanoindentation and electron microscopy to measure the structural and mechanical properties of Subantarctic pteropod shells (Limacina helicina antarctica) collected in 1998 and 2007. The 1998 shells were collected by a sediment trap deployed at 2000 m, 47S, 142E, and the 2007 shells were collected using nets from mixed-layer waters in the region (44– 54S, 140–155E). Transmission electron microscopy revealed that the shells are composed of a polycrystalline structure, and no obvious porosity was visible. The hardness and modulus of the shells were measured using shell cross-section nanoindentation, across various regions of the shell from the inner to outer whorl. No change in mechanical properties was found with respect to the region of the shell cross-section probed. There was no statistically significant difference in the mean modulus or hardness of the shells between the 1998 and 2007 data sets. No major changes in the mechanical properties of these pteropod shells were detected between the 1998 and 2007 data sets, and we discuss the possible biases in the sampling techniques in complicating our analysis. However, quantifying the mechanical properties and microstructure of calcified may still provide insights into the responses of calcification to environmental changes, such as ocean acidification.