Search

Glassy carbon (GC) is usually considered the prototypical super-elastic material, which can almost fully recover its shape after compression of several gigapascals (GPa). In this work, nanoindentation is used to study the mechanical response of GC, which was subjected to a range of high pressures using a diamond anvil cell (DAC). We show that GC starts to lose its elasticity after compression to 6 GPa and becomes clearly mechanically anisotropic after being compressed beyond ∼30 GPa. Molecular dynamics (MD) simulations are used to calculate Young's modulus before and after compression. Through our experimental results and MD simulations, we show that the elasticity of GC is at a minimum around 30 GPa but recovers after compression to higher pressures along the DAC compression axis. [ABSTRACT FROM AUTHOR]

In the framework of the kinetic-collective model of phonon heat transport, we analyze how each range of the phonon frequency spectrum contributes to the total thermal conductivity both in the macro and the nanoscale. For this purpose, we use two case study samples: naturally occurring bulk silicon and a 115 nm of diameter silicon nanowire. We show that the contribution of high-energy phonons (optic branches) is non-negligible only when N-collisions are strongly present. This contribution increases when the effective size of the sample decreases, and it is found to be up to a 10% at room temperature for the 115 nm nanowire, corroborating preliminar ab-initio predictions. [ABSTRACT FROM AUTHOR]

We present a model which deepens into the role that normal scattering has on the thermal conductivity in semiconductor bulk, micro, and nanoscale samples. Thermal conductivity as a function of the temperature undergoes a smooth transition from a kinetic to a collective regime that depends on the importance of normal scattering events. We demonstrate that in this transition, the key point to fit experimental data is changing the way to perform the average on the scattering rates. We apply the model to bulk Si with different isotopic compositions obtaining an accurate fit. Then we calculate the thermal conductivity of Si thin films and nanowires by only introducing the effective size as additional parameter. The model provides a better prediction of the thermal conductivity behavior valid for all temperatures and sizes above 30 nm with a single expression. Avoiding the introduction of confinement or quantum effects, the model permits to establish the limit of classical theories in the study of the thermal conductivity in nanoscopic systems. [ABSTRACT FROM AUTHOR]

Isotopes of heavier gases including carbon ( 13 C/ 14 C), nitrogen ( 13 N), and oxygen ( 18 O) are highly important because they can be substituted for naturally occurring atoms without significantly perturbing the biochemical properties of the radiolabelled parent molecules. These labelled molecules are employed in clinical radiopharmaceuticals, in studies of brain disease and as imaging probes for advanced medical imaging techniques such as positron-emission tomography (PET). Established distillation-based isotope gas separation methods have a separation factor (S) below 1.05 and incur very high operating costs due to high energy consumption and long processing times, highlighting the need for new separation technologies. Here, we show a rapid and highly selective adsorption-based separation of 18 O 2 from 16 O 2 with S above 60 using nanoporous adsorbents operating near the boiling point of methane (112 K), which is accessible through cryogenic liquefied-natural-gas technology. A collective-nuclear-quantum effect difference between the ordered 18 O 2 and 16 O 2 molecular assemblies confined in subnanometer pores can explain the observed equilibrium separation and is applicable to other isotopic gases.

Disordered carbons comprise graphene fragments assembled into three-dimensional networks. It has long been debated whether these networks contain positive curvature, as seen in fullerenes, negative curvature, as proposed for the schwarzite structures, or zero curvature, as in ribbons. We present a mesh-based approach to analyze the topology of a set of nanoporous and glassy carbon models that accurately reproduce experimental properties. Although all three topological elements are present, negatively curved structures dominate. At the atomic level, analysis of local environments shows that sp- and sp^{3}-bonded atoms are associated with line defects and screw dislocations that resolve topological complexities such as termination of free edges and stacking of low curvature regions into ribbons. These results provide insight into the synthesis of porous carbon materials, glassy carbon and the graphitizability of carbon materials.

Two-dimensional graphene has remarkable properties that are revolutionary in many applications. Scrolling monolayer graphene with precise tunability would create further potential for niche applications but this has proved challenging. We have now established the ability to fabricate monolayer graphene scrolls in high yield directly from graphite flakes under non-equilibrium conditions at room temperature in dynamic thin films of liquid. Using conductive atomic force microscopy we demonstrate that the graphene scrolls form highly conducting electrical contacts to highly oriented pyrolytic graphite (HOPG). These highly conducting graphite-graphene contacts are attractive for the fabrication of interconnects in microcircuits and align with the increasing interest in building all sp 2 -carbon circuits. Above a temperature of 450 °C the scrolls unravel into buckled graphene sheets, and this process is understood on a theoretical basis. These findings augur well for new applications, in particular for incorporating the scrolls into miniaturized electronic devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)