Thermal runaway of metal nano-tips during intense electron emission

When an electron emitting tip is subjected to very high electric fields, plasma forms even under ultra high vacuum conditions. This phenomenon, known as vacuum arc, causes catastrophic surface modifications and constitutes a major limiting factor not only for modern electron sources, but also for many large-scale applications such as particle accelerators, fusion reactors etc. Although vacuum arcs have been studied thoroughly, the physical mechanisms that lead from intense electron emission to plasma ignition are still unclear. In this article, we give insights to the atomic scale processes taking place in metal nanotips under intense field emission conditions. We use multi-scale atomistic simulations that concurrently include field-induced forces, electron emission with finite-size and space-charge effects, Nottingham and Joule heating. We find that when a sufficiently high electric field is applied to the tip, the emission-generated heat partially melts it and the field-induced force elongates and sharpens it. This initiates a positive feedback thermal runaway process, which eventually causes evaporation of large fractions of the tip. The reported mechanism can explain the origin of neutral atoms necessary to initiate plasma, a missing key process required to explain the ignition of a vacuum arc. Our simulations provide a quantitative description of in the conditions leading to runaway, which shall be valuable for both field emission applications and vacuum arc studies.Peer reviewe

Large-Cavity Coronoids with Different Inner and Outer Edge Structures

Coronoids, polycyclic aromatic hydrocarbons with geometrically defined cavities, are promising model structures of porous graphene. Here, we report the on-surface synthesis of C168 and C140 coronoids, referred to as [6]- and [5]coronoid, respectively, using 5,9-dibromo-14-phenylbenzo[m]tetraphene as the precursor. These coronoids entail large cavities (>1 nm) with inner zigzag edges, distinct from their outer armchair edges. While [6]coronoid is planar, [5]coronoid is not. Low-temperature scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy unveil structural and electronic properties in accordance with those obtained from density functional theory calculations. Detailed analysis of ring current effects identifies the rings with the highest aromaticity of these coronoids, whose pattern matches their Clar structure. The pores of the obtained coronoids offer intriguing possibilities of further functionalization toward advanced host-guest applications

Ab initio calculation of field emission from metal surfaces with atomic-scale defects

In this work we combine density functional theory and quantum transport calculations to study the influence of atomic-scale defects on the work function and field emission characteristics of metal surfaces. We develop a general methodology for the calculation of the field emitted current density from nanofeatured surfaces, which is then used to study specific defects on a Cu(111) surface. Our results show that the inclusion of a defect can significantly locally enhance the field emitted current density. However, this increase is attributed solely to the decrease of the work function due to the defect, with the effective field enhancement being minute. Finally, the Fowler-Nordheim equation is found to be valid when the modified value for the work function is used, with only an approximately constant factor separating the computed currents from those predicted by the Fowler-Nordheim equation.Peer reviewe

How to verify the precision of density-functional-theory implementations via reproducible and universal workflows

In the past decades many density-functional theory methods and codes adopting periodic boundary conditions have been developed and are now extensively used in condensed matter physics and materials science research. Only in 2016, however, their precision (i.e., to which extent properties computed with different codes agree among each other) was systematically assessed on elemental crystals: a first crucial step to evaluate the reliability of such computations. We discuss here general recommendations for verification studies aiming at further testing precision and transferability of density-functional-theory computational approaches and codes. We illustrate such recommendations using a greatly expanded protocol covering the whole periodic table from Z=1 to 96 and characterizing 10 prototypical cubic compounds for each element: 4 unaries and 6 oxides, spanning a wide range of coordination numbers and oxidation states. The primary outcome is a reference dataset of 960 equations of state cross-checked between two all-electron codes, then used to verify and improve nine pseudopotential-based approaches. Such effort is facilitated by deploying AiiDA common workflows that perform automatic input parameter selection, provide identical input/output interfaces across codes, and ensure full reproducibility. Finally, we discuss the extent to which the current results for total energies can be reused for different goals (e.g., obtaining formation energies).Comment: Main text: 23 pages, 4 figures. Supplementary: 68 page

Changes of mitochondrial function and energy transfer enzymes in muscles of mice with deleted wolframin (wfs1) gene

Aim: To assess changes of mitochondrial function and activities of enzymes involved in the transport of energy in different muscles of wfs1 deficient mice. Methods: Mitochondrial function was assayed by high resolution oxygraphy of permeabilized muscle fibers. Respiration related to oxidative phosphorylation was calculated by subtracting initial basal respiration in the presence of pyruvate and malate from respiration with ADP. Proton leak related respiration was found by subtracting residual oxygen consumption with rotenone from basal respiration. The difference between the respiration rates in the presence of ADP+pyruvate+malate and rotenone was considered to represent the activity of Compex I in the electron transfer chain, and the difference between ADP+succinate and rotenone represented the activity of Complex II. Activities of enzymes were measured by spectroscopy of muscle homogenates. Results: Compared to controls, there was no change of proton leak and hexokinase activity in the wfs1 deficient heart and m. soleus, but in m. rectus femoris 16.1-fold (p<0.002) and 1.7-fold (p<0.01) increases were found respectively. However, oxidative phosphorylation was not changed in any muscle group. The Complex I / Complex II ratio was decreased in heart by 15% (p<0.03) and in musculus rectus femoris by 21% (p<0.01). Activities of creatine and adenylate kinase were decreased in m. rectus femoris by 34% (p<0.01) and 48% (p<0.02) respectively. Conclusions: In heart and m. rectus femoris of wfs1 deficient mice the ratio of Complex I and Complex II activities was decreased. In m. rectus femoris of wfs1 deficient mice proton leak and hexokinase activity were increased, but activity of other energy tranfer enzymes was decreased

Energy production and transfer in oxidative muscles of mice with deleted wolframin (wfs1) gene

Poster sessio

On-surface synthesis of polyazulene with 2,6-connectivity

Azulene, the smallest neutral nonalternant aromatic hydrocarbon, serves as not only a prototype for fundamental studies but also a versatile building block for functional materials because of its unique opto(electronic) properties. Here, we report the on-surface synthesis and characterization of the homopolymer of azulene connected exclusively at the 2,6-positions using 2,6-diiodoazulene as the monomer precursor. As an intermediate to the formation of polyazulene, a gold-(2,6-azulenylene) chain is observed