334 research outputs found
The nature of the observed free-electron-like state in a PTCDA monolayer on Ag(111)
A free-electron like band has recently been observed in a monolayer of PTCDA
(3,4,9,10-perylene tetracarboxylic dianhydride) molecules on Ag(111) by
two-photon photoemission [Schwalb et al., Phys. Rev. Lett. 101, 146801 (2008)]
and scanning tunneling spectroscopy [Temirov et al., Nature 444, 350 (2006)].
Using density functional theory calculations, we find that the observed
free-electron like band originates from the Shockley surface state band being
dramatically shifted up in energy by the interaction with the adsorbed
molecules while it acquires also a substantial admixture with a molecular band
The roles of Eu during the growth of eutectic Si in Al-Si alloys
Controlling the growth of eutectic Si and thereby modifying the eutectic Si from flake-like to fibrous is a key factor in improving the properties of Al-Si alloys. To date, it is generally accepted that the impurity-induced twinning (IIT) mechanism and the twin plane re-entrant edge (TPRE) mechanism as well as poisoning of the TPRE mechanism are valid under certain conditions. However, IIT, TPRE or poisoning of the TPRE mechanism cannot be used to interpret all observations. Here, we report an atomic-scale experimental and theoretical investigation on the roles of Eu during the growth of eutectic Si in Al-Si alloys. Both experimental and theoretical investigations reveal three different roles: (i) the adsorption at the intersection of Si facets, inducing IIT mechanism, (ii) the adsorption at the twin plane re-entrant edge, inducing TPRE mechanism or poisoning of the TPRE mechanism, and (iii) the segregation ahead of the growing Si twins, inducing a solute entrainment within eutectic Si. This investigation not only demonstrates a direct experimental support to the well-accepted poisoning of the TPRE and IIT mechanisms, but also provides a full picture about the roles of Eu atoms during the growth of eutectic Si, including the solute entrainment within eutectic Si
Energies and structures of Cu/Nb and Cu/W interfaces from density functional theory and semi-empirical calculations
Cu/Me multilayer systems, with Me referring to a body-centered cubic () metal, such as Nb and W, are widely used for nuclear, electrical, and electronic applications. Despite making up only a small percentage of the volume, interfaces in such systems play a major role in determining their electrical, mechanical, thermal and diffusion properties. Face-centered cubic () Cu often forms Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) type interfaces with metals or variations thereof. For the Cu/Nb system, these interface relationships have been extensively studied with semi-empirical methods. Surprisingly, the energetics and interface properties of Cu/W have not yet been studied in detail, in spite of extensive applications. In this study, we employ both periodic Embedded Atom Method (EAM) and Density Functional Theory (DFT) simulations to explore the geometric and energetic properties of the KS and NW interfaces of Cu/Nb and Cu/W. To assess the reliability of our approach, the dependence of the results on the size of periodic cells is examined for coherent and incoherent interfaces. We provide the interface energies and the work of separation for the Cu/W and Cu/Nb interfaces at DFT accuracy. The results of calculations with two EAM potentials are in qualitative agreement with those obtained using DFT and allow investigating the convergence of interfacial properties. These key energetic quantities can be used for future thermodynamic and mechanical modeling of Cu/Me interfaces
Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
We performed a systematic density functional study of the adsorption of
copper, silver, and gold adatoms on graphene, especially accounting for van der
Waals interactions by the vdW-DF and the PBE+D2 methods. In particular, we
analyze the preferred adsorption site (among top, bridge, and hollow positions)
together with the corresponding distortion of the graphene sheet and identify
diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to
the graphene sheet and that in some cases the buckling of the graphene can be
significant. The results for silver are at variance with those obtained with
GGA, which gives no binding in this case. However, we observe some quantitative
differences between the vdW-DF and the PBE+D2 methods. For instance the
adsorption energies calculated with the PBE+D2 method are systematically higher
than the ones obtained with vdW-DF. Moreover, the equilibrium distances
computed with PBE+D2 are shorter than those calculated with the vdW-DF method
Aluminum depletion induced by complex co-segregation of carbon and boron in a {\Sigma} 5 [3 1 0] bcc-iron grain boundary
The local variation of grain boundary atomic structure and chemistry caused
by segregation of impurities influences the macroscopic properties of
poylcrystalline materials. Here, the effect of co-segregation of carbon and
boron on the depletion of aluminum at a tilt
grain boundary in a Fe-Al bicrystal was studied by combining
atomic resolution scanning transmission electron microscopy, atom probe
tomography and density functional theory calculations. The atomic grain
boundary structural units mostly resemble kite-type motifs and the structure
appears disrupted by atomic scale defects. Atom probe tomography reveals that
carbon and boron impurities are co-segregating to the grain boundary reaching
levels of >1.5 at.\%, whereas aluminum is locally depleted by approx. 2~at.\%.
First-principles calculations indicate that carbon and boron exhibit the
strongest segregation tendency and their repulsive interaction with aluminum
promotes its depletion from the grain boundary. It is also predicted that
substitutional segregation of boron atoms may contribute to local distortions
of the kite-type structural units. These results suggest that the
co-segregation and interaction of interstitial impurities with substitutional
solutes strongly influences grain boundary composition and with this the
properties of the interface.Comment: 26 pages, 10 Figures, 1 Tabl
Structure and Migration Mechanisms of Small Vacancy Clusters in Cu: A Combined EAM and DFT Study
Voids in face-centered cubic (fcc) metals are commonly assumed to form via the aggregation of vacancies; however, the mechanisms of vacancy clustering and diffusion are not fully understood. In this study, we use computational modeling to provide a detailed insight into the structures and formation energies of primary vacancy clusters, mechanisms and barriers for their migration in bulk copper, and how these properties are affected at simple grain boundaries. The calculations were carried out using embedded atom method (EAM) potentials and density functional theory (DFT) and employed the site-occupation disorder code (SOD), the activation relaxation technique nouveau (ARTn) and the knowledge led master code (KLMC). We investigate stable structures and migration paths and barriers for clusters of up to six vacancies. The migration of vacancy clusters occurs via hops of individual constituent vacancies with di-vacancies having a significantly smaller migration barrier than mono-vacancies and other clusters. This barrier is further reduced when di-vacancies interact with grain boundaries. This interaction leads to the formation of self-interstitial atoms and introduces significant changes into the boundary structure. Tetra-, penta-, and hexa-vacancy clusters exhibit increasingly complex migration paths and higher barriers than smaller clusters. Finally, a direct comparison with the DFT results shows that EAM can accurately describe the vacancy-induced relaxation effects in the Cu bulk and in grain boundaries. Significant discrepancies between the two methods were found in structures with a higher number of low-coordinated atoms, such as penta-vacancies and di-vacancy absortion by grain boundary. These results will be useful for modeling the mechanisms of diffusion of complex defect structures and provide further insights into the structural evolution of metal films under thermal and mechanical stress
Interstitial segregation has the potential to mitigate liquid metal embrittlement in iron
The embrittlement of metallic alloys by liquid metals leads to catastrophic
material failure and severely impacts their structural integrity. The weakening
of grain boundaries by the ingress of liquid metal and preceding segregation in
the solid are thought to promote early fracture. However, the potential of
balancing between the segregation of cohesion-enhancing interstitial solutes
and embrittling elements inducing grain boundary decohesion is not understood.
Here, we unveil the mechanisms of how boron segregation mitigates the
detrimental effects of the prime embrittler, zinc, in a
tilt grain boundary in Fe ( Al). Zinc forms nanoscale
segregation patterns inducing structurally and compositionally complex grain
boundary states. Ab-initio simulations reveal that boron hinders zinc
segregation and compensates for the zinc induced loss in grain boundary
cohesion. Our work sheds new light on how interstitial solutes intimately
modify grain boundaries, thereby opening pathways to use them as dopants for
preventing disastrous material failure.Comment: 29 pages, 6 figures in the main text and 10 figures in the
supplementar
Unraveling the temperature dependence of the yield strength in single-crystal tungsten using atomistically-informed crystal plasticity calculations
We use a physically-based crystal plasticity model to predict the yield
strength of body-centered cubic (bcc) tungsten single crystals subjected to
uniaxial loading. Our model captures the thermally-activated character of screw
dislocation motion and full non-Schmid effects, both of which are known to play
a critical role in bcc plasticity. The model uses atomistic calculations as the
sole source of constitutive information, with no parameter fitting of any kind
to experimental data. Our results are in excellent agreement with experimental
measurements of the yield stress as a function of temperature for a number of
loading orientations. The validated methodology is then employed to calculate
the temperature and strain-rate dependence of the yield strength for 231
crystallographic orientations within the standard stereographic triangle. We
extract the strain-rate sensitivity of W crystals at different temperatures,
and finish with the calculation of yield surfaces under biaxial loading
conditions that can be used to define effective yield criteria for engineering
design models
Analysis of Bonding between Conjugated Organic Molecules and Noble Metal Surfaces Using Orbital Overlap Populations
The electronic structure of metal−organic interfaces is of paramount importance for the properties of organic electronic and single-molecule devices. Here, we use so-called orbital overlap populations derived from slab-type band-structure calculations to analyze the covalent contribution to the bonding between an adsorbate layer and a metal. Using two prototypical molecules, the strong acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) on Ag(111) and the strong donor 1H,1′H-[4,4′]bipyridinylidene (HV0) on Au(111), we present overlap populations as particularly versatile tools for describing the metal−organic interaction. Going beyond traditional approaches, in which overlap populations are represented in an atomic orbital basis, we also explore the use of a molecular orbital basis to gain significant additional insight. On the basis of the derived quantities, it is possible to identify the parts of the molecules responsible for the bonding and to analyze which of the molecular orbitals and metal bands most strongly contribute to the interaction and where on the energy scale they interact in bonding or antibonding fashion
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