Suppression of material transfer at contacting surfaces: The effect of adsorbates on Al/TiN and Cu/diamond interfaces from first-principles calculations

The effect of monolayers of oxygen (O) and hydrogen (H) on the possibility of material transfer at aluminium/titanium nitride (Al/TiN) and copper/diamond (Cu/C$_{\text{dia}}$) interfaces, respectively, were investigated within the framework of density functional theory (DFT). To this end the approach, contact, and subsequent separation of two atomically flat surfaces consisting of the aforementioned pairs of materials were simulated. These calculations were performed for the clean as well as oxygenated and hydrogenated Al and C$_{\text{dia}}$ surfaces, respectively. Various contact configurations were considered by studying several lateral arrangements of the involved surfaces at the interface. Material transfer is typically possible at interfaces between the investigated clean surfaces; however, the addition of O to the Al and H to the C$_{\text{dia}}$ surfaces was found to hinder material transfer. This passivation occurs because of a significant reduction of the adhesion energy at the examined interfaces, which can be explained by the distinct bonding situations.Comment: 27 pages, 8 figure

High-throughput generation of potential energy surfaces for solid interfaces

A robust, modular, and ab initio high-throughput workflow is presented to automatically match and characterize solid–solid interfaces using density functional theory calculations with automatic error corrections. The potential energy surface of the interface is computed in a highly efficient manner, exploiting the high- symmetry points of the two mated surfaces. A database is automatically populated with results to ensure that already available data are not unnecessarily recomputed. Computational parameters and slab thicknesses are converged automatically to minimize computational cost while ensuring accurate results. The surfaces are matched according to user-specified maximal cross-section area and mismatches. Example results are presented as a proof of concept and to show the capabilities of our approach that will serve as the basis for many more interface studies

Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$-point depend critically on the Fe magnetic moment and atomic volume. Analyzing these non vibrational modes leads to the discovery of a stable monoclinic ground state structure which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice degrees of freedom contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as the electronic and magnetic subsystems and therefore needs to be included in thermodynamic modeling.Comment: 15 pages, 12 figure

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

Gleitreibung und Kontaktfläche in nano tribologischen Systemen

Abweichender Titel laut Übersetzung der Verfasserin/des VerfassersZsfassung in dt. SpracheObwohl sich Wissenschafterinnen und Wissenschafter seit Jahrhunderten mit Reibung, Schmierung und Verschleiß beschäftigen, sind viele wichtige Probleme in diesem, heutzutage als Tribologie bekannten, Feld noch immer ungelöst. Die Definition der Kontaktfläche und die Abhängigkeit der Reibkraft von der Reibrichtung, jeweils auf der atomarer Ebene, sind zwei dieser offenen Fragen und werden in dieser Dissertation behandelt. Die Erfindung des Rasterkraftmikroskops (AFM), 1986 von Binnig, Quate und Gerber, ermöglichte eine rasante Entwicklung auf dem jungen Feld der Nanotribologie, da nun Reibkräfte auf atomarer Ebene untersucht werden konnten. Auf theoretischer Seite wurden diese Experimente bald durch Molekulardynamik Simulationen beschrieben, die in diesem Feld immer noch von überragender Bedeutung sind. Ab-inito Methoden werden allerdings erst seit kurzer Zeit vermehrt verwendet um tribologische Phänomene im Nanobereich zu modellieren. Die vorliegende Arbeit beschäftigt sich im Rahmen der Dichtefunktionaltheorie mit der parameterfreien Beschreibung von tribologischen Problemen. Seit der Entdeckung, dass die reale Kontaktfläche zwischen zwei Körpern um Größenordnungen kleiner sein kann als die scheinbare, wurden viele Versuche unternommen, um die reale Kontaktfläche zu beschreiben. Klassische Kontaktmechanik ist geeignet um das Verhalten makroskopische Objekte, eventuell mit mikroskopischer Rauigkeit, zu erklären, aber da die Annahme kontinuierlicher, elastischer Körper auf atomarer Ebene nicht mehr gegeben ist, ist es unwahrscheinlich dass solche Theorien auch im Nanobereich gültig bleiben. In dieser Arbeit wird eine Methode vorgestellt um die reale Kontaktfläche auf atomarer Ebene zu definieren und zu berechnen. Dazu werden plötzliche Relaxationen, wie der "jump to contact", der häufig in AFM Experimenten beobachtet wird, verwendet um den Punkt des ersten Kontakts zu definieren. Baders Quantum Theory of Atoms in Molecules wird dann verwendet, um die Größe und Form der Kontaktfläche parameterfrei zu berechnen. Baders Methode teilt die Elektronendichte - eindeutig den involvierten Atomen zu, die theoretisch den ganzen Raum ausfüllen. Da dies zu Kontakt bei jedem beliebigen Abstand führen würde, muss die Elektronendichte ab einem bestimmten, kleinen Wert --cut auf null gesetzt werden, um Oberflächenatome mit begrenzter Ausdehnung zu erhalten. Dieser Parameter kann über die erwähnte Diskontinuität, z.B. den "jump to contact", bestimmt werden, indem man -cut so einstellt, dass Kontakt nur bei Abständen auftritt, bei denen sich die Geometrie bereits geändert hat. Das Konzept wurde an einer Pyramide aus 10 Wolfram Atomen, die eine AFM Spitze simulieren, erprobt, die mit zwei unterschiedliche Oberflächen in Kontakt gebracht wurde. Die zwei getesteten Systeme, Graphene auf fcc Iridium (111) und fcc Kupfer (111), besitzen unterschiedliche Eigenschaften, der berechnete Wert für -cut ist, mit ~ 5 × 10-2 Elektronen pro Å3, allerdings dennoch in beiden Fällen sehr ähnlich. Für beide Systeme konnte ein linearer Zuwachs der realen Kontaktfläche mit veringertem Abstand zwischen der Spitzenhalterung festgestellt werden. Betrachtet man jedoch den realen Abstand zwischen den relaxierten Atomen der Spitze und der Oberfläche, ist der Zusammenhang in beiden Fällen exponentiell. Die Beschreibung von Reibkräften mit parameterfreien Methoden ist, obwohl einige Modelle existieren, immer noch eine nur unzufriedenstellend gelöste Aufgabe. In der vorliegenden Arbeit entwickeln wir eine Raster-basierte, quasi-statische Methode, um die Richtungsabhängigkeit von Reibkräften zwischen glatten Flächen in trockenem Kontakt zu berechnen. Das Verfahren beruht auf der Annahme, dass der Energieverlust durch Reibung durch die Relaxation der Atome in der Kontaktzone abgeschätzt werden kann. Zwei unabhängige Wege wurden entwickelt, um die Reibkräfte bei konstanter Last aus den mit DFT gewonnenen Daten zu bestimmen. Beide führen zu einem exponentiellen Reibgesetz für alle Gleitrichtungen. Da unsere Methode keine Limits bezüglich der Länge der Reibpfade setzt, konnten auch aperiodische Pfade untersucht werden, bei denen die Reibkraft erst nach sehr langen Distanzen konvergiert. Für alle untersuchten Metall auf Metall Grenzflächen, konnten wir zwei periodische Gleitrichtungen finden, die obere und untere Grenzen für die Reibkraft bilden. Die aperiodischen Pfade konvergieren alle zum gleichen Wert zwischen diesen Limits. Für geringe Last erhalten wir die von Derjaguin generalisierte Form des Reibgesetzes von Amontons-Coulomb, welches auch auf atomarer Ebene gültig zu sein scheint. Wir beobachten auch einen nicht verschwindenden Derjaguin Offset für atomare glatte Metalle in schmiermittelfreiem Kontakt. Berücksichtigen wir unsere Methode zur exakten Berechnung der realen Kontaktfläche bei der Berechnung der Last, erhalten wir bei jedem Druck eine vergleichsweise höhere Last. Dies führt zu einer Streckung der Kurven im exponentiellen Reibgesetz und verringert in Folge die Reibzahlen um 25%-35%.Although friction, wear, and lubrication have been investigated for hundreds of years, many important questions in the field, now dubbed tribology, remain open. Two of these are the determination of the real area of contact and the direction dependence of friction forces on the nanoscale, which are investigated in this thesis. The invention of the atomic force microscope (AFM) in 1986 by Binnig , Quate, and Gerber was invaluable for the investigation of frictional forces on the atomic scale, giving a boost to the emerging field of nanotribology. While experimental AFM set-ups where quickly modeled with classical molecular dynamic (MD) simulations, the use of ab-initio methods on tribological problems remained scarce until recently. In this thesis density functional theory (DFT) is used to develop parameter free methods in the field of nanotribology. Since the discovery that the apparent area of contact can be orders of magnitude larger than the true area of contact, the definition and determination of the latter has been an important topic of research. While classical contact mechanics provides satisfactory theories and results for various macroscopic systems, the application of these methods to atomistic systems is dubious, as the contacting bodies are not continuous at this length scale. We developed a parameter free approach to define and calculate the real area of contact between two bodies depending on distance. Strong relaxations at distinct distances, like the "jump to contact", which is often observed in AFM experiments, are used to define the onset of contact and Bader-s Quantum Theory of Atoms in Molecules is used to calculate the real area of contact at a given distance. Bader's method partitions the charge density - unambiguously into atoms, which theoretically fill all space and thus give non zero contact areas for all distances. It is therefore necessary to use a density cutoff -cut which assigns all regions in space where - < -cut to the vacuum, resulting in surface atoms of finite size. In our proposed method the parameter -cut is calculated by allowing contact only after the "jump to contact", or a similar discontinuity, has taken place, which can be clearly observed in our DFT simulations. We demonstrate the method by lowering a ten atom tungsten pyramid, which serves as a model of an AFM tip, onto a smooth surface. Two systems are examined, the first being moiré graphene on iridium (111), the second a clean copper (111) surface. Although the surfaces are very different, a similiar cutoff parameter -cut of about 5 × 10-2 electrons per Å3 is computed in both cases. The calculated area of contact is found to increase linearly with lowering of the tip support while increasing exponentially with the true relaxed distance between the tip apex atom and the surface atom below it. Although there exist a number of models that tackle the problem of calculating friction forces on the atomic level, providing a completely parameter-free approach remains a challenge. To examine the direction dependence of dry sliding friction we developed a quasi-static grid method with a mechanism to allow dissipative sliding, which relies on atomic relaxations. We define two different ways of calculating the mean nanofriction force, both leading to an exponential friction-versus-load behavior for all sliding directions. Since our approach does not impose any limits on lengths and directions of the sliding paths, we investigate arbitrary sliding directions for several metal interfaces and detect two periodic paths which form the upper and lower bound of nanofriction in all cases. For long aperiodic paths the friction force convergences to a value in between these limits. For low loads we retrieve the Derjaguin generalization of the Amontons-Coulomb kinetic friction law which appears to be valid all the way down to the nanoscale. We observe a non-vanishing Derjaguin-offset even for atomically flat surfaces in dry contact. Incorporating our approach to determine the true area of contact into the evaluation of the loading force leads to higher loads for each selected pressure. This influences the friction versus load curves by stretching them laterally and thus reducing the coefficients of friction by about 25%-35%.11