Mechanical properties of carbynes investigated by ab initio total-energy calculations

As sp carbon chains (carbynes) are relatively rigid molecular objects, can we exploit them as construction elements in nanomechanics? To answer this question, we investigate their remarkable mechanical properties by ab-initio total-energy simulations. In particular, we evaluate their linear response to small longitudinal and bending deformations and their failure limits for longitudinal compression and elongation.Comment: 6 pages, 4 figures, 1 tabl

Tribology of the lubricant quantized-sliding state

In the framework of Langevin dynamics, we demonstrate clear evidence of the peculiar quantized sliding state, previously found in a simple 1D boundary lubricated model [Phys. Rev. Lett. 97, 056101 (2006)], for a substantially less idealized 2D description of a confined multi-layer solid lubricant under shear. This dynamical state, marked by a nontrivial ``quantized'' ratio of the averaged lubricant center-of-mass velocity to the externally imposed sliding speed, is recovered, and shown to be robust against the effects of thermal fluctuations, quenched disorder in the confining substrates, and over a wide range of loading forces. The lubricant softness, setting the width of the propagating solitonic structures, is found to play a major role in promoting in-registry commensurate regions beneficial to this quantized sliding. By evaluating the force instantaneously exerted on the top plate, we find that this quantized sliding represents a dynamical ``pinned'' state, characterized by significantly low values of the kinetic friction. While the quantized sliding occurs due to solitons being driven gently, the transition to ordinary unpinned sliding regimes can involve lubricant melting due to large shear-induced Joule heating, for example at large speed.Comment: 11 pages, 11 figure

On the thermoelectric properties of Nb-doped SrTiO3epitaxial thin films

The exploration for thermoelectric thin films of complex oxides such as SrTiO3-based oxides is driven by the need for miniaturized harvesting devices for powering the Internet of Things (IoT). However, there is still not a clear consensus in the literature for the underlying influence of film thickness on thermoelectric properties. Here, we report the fabrication of epitaxial thin films of 6% Nb-doped SrTiO3 on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) single crystal using pulsed laser deposition (PLD) where the film thickness was varied from 2 nm to 68 nm. The thickness dependence shows a subtle increase of tetragonality of the thin film lattice and a gradual drop of the electrical conductivity, the density of charge carriers, and the thermoelectric Seebeck coefficient as the film thickness decreases. DFT-based calculations show that ∼2.8% increase in tetragonality results in an increased splitting between t2g and eg orbitals to ∼42.3 meV. However, experimentally observed tetragonality for films between 68 to 13 nm is only 0.06%. Hence, the effect of thickness on tetragonality is neglected. We have discussed the decrease of conductivity and the Seebeck coefficient based on the decrease of carriers and change in the scattering mechanism, respectively.The research leading to these results has received funding from the European Union's H2020 Programme under Grant Agreement no 824072 – HARVESTORE

Carbon sp chains in graphene nanoholes

Nowadays sp carbon chains terminated by graphene or graphitic-like carbon are synthesized routinely in several nanotech labs. We propose an ab-initio study of such carbon-only materials, by computing their structure and stability, as well as their electronic, vibrational and magnetic properties. We adopt a fair compromise of microscopic realism with a certain level of idealization in the model configurations, and predict a number of properties susceptible to comparison with experiment.Comment: 34 pages, 27 figure

Fe-Doping in Double Perovskite PrBaCo2(1-x)Fe2xO6-δ: Insights into Structural and Electronic Effects to Enhance Oxygen Evolution Catalyst Stability

Perovskite oxides have been gaining attention for its capability to be designed as an ideal electrocatalyst for oxygen evolution reaction (OER). Among promising candidates, the layered double perovskite—PrBaCo2O6-δ (PBC)—has been identified as the most active perovskite electrocatalyst for OER in alkaline media. For a single transition metal oxide catalyst, the addition of Fe enhances its electrocatalytic performance towards OER. To understand the role of Fe, herein, Fe is incorporated in PBC in different ratios, which yielded PrBaCo2(1-x)Fe2xCo6-δ (x = 0, 0.2 and 0.5). Fe-doped PBCF’s demonstrate enhanced OER activities and stabilities. Operando X-ray absorption spectroscopy (XAS) revealed that Co is more stable in a lower oxidation state upon Fe incorporation by establishing charge stability. Hence, the degradation of Co is inhibited such that the perovskite structure is prolonged under the OER conditions, which allows it to serve as a platform for the oxy(hydroxide) layer formation. Overall, our findings underline synergetic effects of incorporating Fe into Co-based layered double perovskite in achieving a higher activity and stability during oxygen evolution reaction

The role of an interface in stabilizing reaction intermediates for hydrogen evolution in aprotic electrolytes

By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electro-reduction reactions of HF, water and methanesulfonic acid (MSA) on the single crystal (111) facets of Au, Pt, Ir and Cu in organic aprotic electrolytes, 1 M LiPF(6) in EC/EMC 3:7W (LP57), the aprotic electrolyte commonly used in Li-ion batteries, 1 M LiClO(4) in EC/EMC 3:7W and 0.2 M TBAPF(6) in 3 : 7 EC/EMC. In our previous work, we have established that LiF formation, accompanied by H(2) evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li(+) adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having adsorbed Li(+). Li(+) is needed to facilitate the dissociation of HF which is the source of protons. Further experiments on other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F(−) or HO(−), a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy

Anisotropic Proton and Oxygen Ion Conductivity in Epitaxial Ba₂In₂O₅ Thin Films

Solid oxide oxygen ion and proton conductors are a highly important class of materials for renewable energy conversion devices like solid oxide fuel cells. Ba₂In₂O₅ (BIO) exhibits both oxygen ion and proton conduction, in a dry and humid environment, respectively. In a dry environment, the brownmillerite crystal structure of BIO exhibits an ordered oxygen ion sublattice, which has been speculated to result in anisotropic oxygen ion conduction. The hydrated structure of BIO, however, resembles a perovskite and the protons in it were predicted to be ordered in layers. To complement the significant theoretical and experimental efforts recently reported on the potentially anisotropic conductive properties in BIO, we measure here both the proton and oxygen ion conductivity along different crystallographic directions. Using epitaxial thin films with different crystallographic orientations, the charge transport for both charge carriers is shown to be anisotropic. The anisotropy of the oxygen ion conduction can indeed be explained by the layered structure of the oxygen sublattice of BIO. The anisotropic proton conduction, however, further supports the suggested ordering of the protonic defects in the material. The differences in proton conduction along different crystallographic directions attributed to proton ordering in BIO are of a similar extent as those observed along different crystallographic directions in materials where proton ordering is not present but where protons find preferential conduction pathways through chainlike or layered structures