3D Strain in 2D Materials: To What Extent is Monolayer Graphene Graphite?

This work addresses a fundamental question: To what extent is graphene graphite? In particular does 2D graphene have many of the same 3D mechanical properties as graphite, such as the bulk modulus and elastic constant c_{33}? We have obtained, for the first time, unambiguous Raman spectra from unsupported monolayer graphene under pressure. We have used these data to quantify the out-of-plane stiffness of monolayer graphene, which is hard to define due to its 2D nature. Our data indicate a first physically meaningful out-of-plane stiffness of monolayer graphene, and find it to be consistent with that of graphite. We also report a shift rate of the in-plane phonon frequency of unsupported monolayer graphene to be 5.4 cm^{-1} GPa^{-1}, very close to that of graphite (4.7 cm^{-1} GPa^{-1}), contrary to the previous value for supported graphene. Our results imply that monolayer graphene has similar in-plane and out-of-plane stiffnesses, and anharmonicities to graphite.

Temporal mapping of photochemical reactions and molecular excited states with carbon specificity.

Photochemical reactions are essential to a large number of important industrial and biological processes. A method for monitoring photochemical reaction kinetics and the dynamics of molecular excitations with spatial resolution within the active molecule would allow a rigorous exploration of the pathway and mechanism of photophysical and photochemical processes. Here we demonstrate that laser-excited muon pump-probe spin spectroscopy (photo-µSR) can temporally and spatially map these processes with a spatial resolution at the single-carbon level in a molecule with a pentacene backbone. The observed time-dependent light-induced changes of an avoided level crossing resonance demonstrate that the photochemical reactivity of a specific carbon atom is modified as a result of the presence of the excited state wavefunction. This demonstrates the sensitivity and potential of this technique in probing molecular excitations and photochemistry.

Anomalous plasticity in the cyclic torsion of micron scale metallic wires.

The plasticity of micron scale Cu and Au wires under cyclic torsion is investigated for the first time by using a torsion balance technique. In addition to a size effect, a distinct Bauschinger effect and an anomalous plastic recovery, wherein reverse plasticity even occurs upon unloading, are unambiguously revealed. The Bauschinger effect and plastic recovery have been observed in molecular dynamics and discrete dislocation dynamics simulations of ideal single-crystal wires; the results here are an excellent confirmation that these effects also occur in experiment in nonideal polycrystalline wires. A physical model consistent with the simulations is described in which the geometrically necessary dislocations induced by the nonuniform deformation in torsion play the key role in these anomalous plastic behaviors.

Easy computation of the Bayes factor to fully quantify Occam's razor in least-squares fitting and to guide actions.

The Bayes factor is the gold-standard figure of merit for comparing fits of models to data, for hypothesis selection and parameter estimation. However, it is little-used because it has been considered to be subjective, and to be computationally very intensive. A simple computational method has been known for at least 30 years, but has been dismissed as an approximation. We show here that all three criticisms are misplaced. The method should be used to complement and augment all least-squares fitting, because it can give very different, and better outcomes than classical methods. It can discriminate between models with equal numbers of parameters and equally good fits to data. It quantifies the Occam's Razor injunction against over-fitting, and it demands that physically-meaningful parameters rejected by classical significance testing be included in the fitting, to avoid spurious precision and incorrect values for the other parameters. It strongly discourages the use of physically-meaningless parameters, thereby satisfying the Occam's Razor injunction to use existing entities for explanation rather than multiplying new ones. More generally, as a relative probability, the Bayes factor combines naturally with other quantitative information to guide action in the absence of certain knowledge.

Nanostrain sensitivity in a wire torsion experiment.

The feasibility of a thin-wire torsion stress-strain experiment with nanostrain sensitivity is demonstrated. A gauge length of 50 m was made possible by using The Monument, London, thereby restoring it to its original purpose as a scientific instrument. A wire of 150 µm diameter was studied using the load-unload method, and data were recorded in the elastic regime and through the elastic-plastic transition. Analysis of this preliminary experiment suggested some necessary improvements to the equipment and methods. Progress towards definitive experiments is described together with difficulties still to be overcome.

Elastic limit and strain hardening of thin wires in torsion.

A theory for the size effect in the strength of wires under torsion is reported and compared with data from thin copper wires. Critical thickness theory is solved rigorously and used to validate a useful approximation which is combined with slip-distance theory modified for a finite structure size. Experimental data with high accuracy around and above the elastic limit show excellent agreement with the theory. The results strongly imply that the physical principle is the constraint that size, whether grain size or structure size, puts on allowed dislocation curvature.

Spider dragline silk as torsional actuator driven by humidity.

Self-powered actuation driven by ambient humidity is of practical interest for applications such as hygroscopic artificial muscles. We demonstrate that spider dragline silk exhibits a humidity-induced torsional deformation of more than 300°/mm. When the relative humidity reaches a threshold of about 70%, the dragline silk starts to generate a large twist deformation independent of spider species. The torsional actuation can be precisely controlled by regulating the relative humidity. The behavior of humidity-induced twist is related to the supercontraction behavior of spider dragline silk. Specifically, molecular simulations of MaSp1 and MaSp2 proteins in dragline silk reveal that the unique torsional property originates from the presence of proline in MaSp2. The large proline rings also contribute to steric exclusion and disruption of hydrogen bonding in the molecule. This property of dragline silk and its structural origin can inspire novel design of torsional actuators or artificial muscles and enable the development of designer biomaterials.

The Hall-Petch effect as a manifestation of the general size effect.

The experimental evidence for the Hall-Petch dependence of strength on the inverse square-root of grain size is reviewed critically. Both the classic data and more recent results are considered. While the data are traditionally fitted to the inverse square-root dependence, they also fit well to many other functions, both power law and non-power law. There have been difficulties, recognized for half-a-century, in the inverse square-root expression. It is now explained as an artefact of faulty data analysis. A Bayesian meta-analysis shows that the data strongly support the simple inverse or lnd/d expressions. Since these expressions derive from underlying theory, they are also more readily explicable. It is concluded that the Hall-Petch effect is not to be explained by the variety of theories found in the literature, but is a manifestation of, or to be underlain by the general size effect observed throughout micromechanics, owing to the inverse relationship between the stress required and the space available for dislocation sources to operate.

The new high field photoexcitation muon spectrometer at the ISIS pulsed neutron and muon source.

A high power pulsed laser system has been installed on the high magnetic field muon spectrometer (HiFi) at the International Science Information Service pulsed neutron and muon source, situated at the STFC Rutherford Appleton Laboratory in the UK. The upgrade enables one to perform light-pump muon-probe experiments under a high magnetic field, which opens new applications of muon spin spectroscopy. In this report we give an overview of the principle of the HiFi laser system and describe the newly developed techniques and devices that enable precisely controlled photoexcitation of samples in the muon instrument. A demonstration experiment illustrates the potential of this unique combination of the photoexcited system and avoided level crossing technique.

Micromechanical testing with microstrain resolution.

Simple test equipment has been developed for studying the elastic limit and plastic deformation of thin metal wires and thin foils (down to 10 µm) under torsion, tension, and bending. Using load-unload methods and gauge lengths up to 1 m, plastic strain as low as 10(-6) can be measured accurately.

Derivation of special relativity from Maxwell and Newton.

Special relativity derives directly from the principle of relativity and from Newton's laws of motion with a single undetermined parameter, which is found from Faraday's and Ampère's experimental work and from Maxwell's own introduction of the displacement current to be the -c(-2) term in the Lorentz transformations. The axiom of the constancy of the speed of light is quite unnecessary. The behaviour and the mechanism of the propagation of light are not at the foundations of special relativity.

Solvation pressure in chloroform.

Molecular dynamics (MD) simulations of chloroform vapor and liquid at normal temperature and pressure and liquid under hydrostatic pressure are presented, giving bond lengths and vibrational frequencies as functions of pressure. The change in bond lengths between vapor and liquid at normal temperature and pressure is consistent with a pressure equivalent to the cohesive energy density (CED) of the liquid, supporting the solvation pressure model which predicts that solvated molecules or nanoparticles experience a pressure equal to the CED of the liquid. Experimental data for certain Raman frequencies of chloroform in the vapor phase, in the liquid, and in the liquid under pressure are presented and compared to MD. Results for C-Cl vibrational modes are in general agreement with the solvation pressure model whereas frequencies associated with the C-H bond are not. The results demonstrate that masking interactions exist in the real liquid that can be reduced or eliminated in simplified simulations.

Discontinuous tangential stress in double wall carbon nanotubes.

We have examined the stability of double wall carbon nanotubes under hydrostatic pressures up to 10 GPa. The tangential optical phonon mode observed by inelastic light scattering is sensitive to the in-plane stress and splits into a contribution associated with the external and internal tube. While the pressure coefficient from the external tube is the same as in single wall carbon nanotubes, the pressure coefficient from the internal tube is found to be 45% smaller. The phonon band from the external tube broadens considerably with applied pressure in contrast with the phonon band of the internal tube which stays constant. These pressure dependent phonon shifts of the external and internal tubes and the contrasting phonon line broadening are explained by the elastic continuum shell model which takes into account both the continuous radial and discontinuous tangential stress components.