Ideal Work Of Separation Research Articles

Adhesion and bonding at the Ag(110)/Au(110) interface, a DFT study

This work reports our results on the theoretical investigation of the Ag(110)/Au(110) interface system using periodic density functional theory within the generalized gradient approximation. We considered pristine, non-reconstructed surfaces in our supercell models for the (110) surfaces. The surface energy and structural relaxation were calculated for the clean surfaces while the ideal work of separation (∼2.0 J/m2) and interfacial energy (∼–0.19 J/m2) were determined for the Ag/Au interface. Computational tensile tests in the rigid grain shift (RGS) framework were also performed and a generalized universal binding energy relation (UBER) was used to describe the energy-displacement data. Our energy calculations indicated a stable interface in the Ag(110)/Au(110) through attractive interactions between the metals characterized by the expected d-d orbital interactions.

Bond strengthening in lateral heterostructures of transition metal dichalcogenides

The adhesion strength of lateral heterostructures composed of transition metal dichalcogenides ($\mathrm{Mo}{\mathrm{S}}_{2}, \mathrm{Mo}{\mathrm{Se}}_{2}, {\mathrm{WS}}_{2}$, and $\mathrm{W}{\mathrm{Se}}_{2}$) monolayers is investigated with first-principles electronic structure calculations. Our density functional theory calculations demonstrate that the adhesion strength, which is gauged by the ideal work of separation (${W}_{\mathrm{sep}}$), strongly depends on the local atomic configuration, and that ${W}_{\mathrm{sep}}$ becomes enhanced (diminished) at the interface where a chalcogen atom forms bonds with one (two) W and two (one) Mo atoms compared to those of the homogeneous cases. It is further shown that the increase (decrease) in ${W}_{\mathrm{sep}}$ from the homogeneous value is caused by the charge redistribution among the interfacial bonds, which is determined by the differences in the electronegativity of the transition metal species at the interface. Such geometrically controlled interfacial strength presents a route to control the materials' mechanical characteristics through structural engineering.

Doping and interfacial modification of the P3HT/ZnO photovoltaic interface: Insights from density functional theory

In this work, we examine how the properties of the poly-3-hexylthiophene(P3HT)/ZnO solar cell are affected by interfacial modification with phenyl-C61-butyric acid (PCBA) and doping with Sr. Using density functional theory calculations on ideal models of the P3HT/ZnO(101¯0) interface, we characterize the equilibrium structures of the pure and modified interfaces, evaluate their ideal work of separation, and examine the energy level alignments and electronic structure. We find that doping with Sr and introducing PCBA have little impact on interfacial distances and do not strengthen interfacial adhesion. Rather, the individual and cumulative impacts of Sr-doping and PCBA are observed in changes to the energy level alignments, including the open circuit voltage and the energy offset that drives exciton separation. Sr dopants and PCBA cause these energetic shifts through differing effects on the electrostatic potential offset and interfacial dipole moments.

Structural details of Al/Al2O3 junctions and their role in the formation of electron tunnel barriers

We present a computational study of the adhesive and structural properties of the Al/Al2O3 interfaces as building blocks of the Metal-Insulator-Metal (MIM) tunnel devices, where electron transport is accomplished via tunnelling mechanism through the sandwiched insulating barrier. The main goal of this paper is to understand, on the atomic scale, the role of the geometrical details in the formation of the tunnel barrier profiles. To provide reliable results, we carefully assess the accuracy of the traditional methods used to examine Al/Al2O3 interfaces. These are the most widely employed exchange-correlation functionals, LDA, PBE and PW91, the Universal Binding Energy Relation (UBER) for predicting equilibrium interfacial distances and adhesion energies, and the ideal work of separation as a measure of junction stability. Finally, we perform a detailed analysis of the atomic and interplanar relaxations in each junction. Our results imply that the structural irregularities on the surface of the Al film have a significant contribution to lowering the tunnel barrier height, while interplanar relaxations in the Al film, away from the immediate interface do not have a notable impact on the tunnelling properties. On the other hand, up to 5-7 layers of Al2O3 may be involved in shaping the tunnel barriers. Interplanar relaxations of these layers depend on the geometry of the interface and may result in the net contraction by 13% relative to the corresponding thickness in the bulk oxide. This is a significant amount as the tunnelling probability depends exponentially on the barrier width.

First-principles-aided design of a new Ni-base superalloy: Influence of transition metal alloying elements on grain boundary and bulk cohesion

A new approach to the design of Ni-base polycrystalline superalloys is proposed. In this approach, we assume that the creep–rupture characteristics of a superalloy are mostly determined by the strength of interatomic bonding at grain boundaries (GBs) and in the bulk of γ matrix. The ideal work of separation, Wsep, of a GB is used as a fundamental thermodynamic quantity that controls the mechanical strength of an interface, whereas the partial cohesive energy, χ, of an alloy component serves to characterize its contribution into the strength of the bulk. Using the Σ5 (210)[100] symmetric tilt GB as a representative high-angle GB in Ni, we calculate Wsep,χ, and GB segregation energies, Eseg, for the complete set of 4d and 5d transition metal impurities, to which we add B (a typical microalloying addition), S and Bi (notoriously known as harmful impurities in Ni-base superalloys). The purpose of the analysis is to identify the elements that demonstrate a high tendency to segregate to GBs, have positive (preferably high) partial cohesive energies in the bulk, and have positive impact on Wsep of GBs. We refer to these elements as low-alloying additions. Our study reveals Zr, Hf, Nb, Ta and B as the most promising low-alloying additions. Our next step is to introduce the elements found in the first step into a new powder metallurgy (P/M) Ni-base superalloy. The results of the subsequent testing confirm that the newly created P/M superalloy indeed demonstrates superior mechanical properties at high temperatures compared to the existing Russian P/M alloy EP741NP.

First-principles study of thermodynamical and mechanical stabilities of thin copper film on tantalum

The adhesion, stability, and wetting behavior at interfaces between thin Cu films and clean Ta (110) substrates are investigated by first-principles calculations using density functional theory (DFT) in the local-density approximation. Interfaces between pseudomorphic body-centered-tetragonal thin films of Cu, strained face-centered-cubic thin films of Cu, and a single pseudomorphic monolayer of Cu on body-centered-cubic Ta (110) surfaces are studied. Various high-symmetry interface configurations are considered for each case. The mechanical stability of the interfaces is studied by the ideal work of separation, while the thermodynamic stability is investigated by Gibbs' excess interface energy. All three interfaces are found to be thermodynamically unstable. An energy-weighting scheme extends the use of the DFT calculations to the case of an incoherent misfitting interface. The incoherent monolayer of Cu on Ta is thereby found to be thermodynamically stable. For coverages by more than a monolayer, the Cu atoms are expected to form three-dimensional islands on top of the Cu monolayer. With respect to interface separation, the monolayer is found to be bound more strongly to the Ta substrate than the thin film. Hence, failure is expected to occur not at the $\mathrm{Cu}∕\mathrm{Ta}$ interface but inside the Cu.

Microscopic characteristics of theAg(111)∕ZnO(0001)interface present in optical coatings

A first-principles computational method is used to investigate the microscopic properties of the $\mathrm{Ag}(111)∕\mathrm{Zn}\mathrm{O}(0001)$ interface that is often present in optical coatings designed for solar-control windows. The mechanical stability of the interface is important and therefore the ideal work of separation has been calculated for several structural variants of the interface which have different lattice mismatches and in-plane orientations. The process by which silver atoms are deposited, cluster, and form layers on the ZnO(0001) surface has also been studied. It is found that interfaces with the O-terminated ZnO surface are stronger than those with the Zn-terminated surface. In addition, incoherent interfaces with small lattice mismatch and minimal strain are preferred. In particular, the large period $(9\ifmmode\times\else\texttimes\fi{}8)$ $\mathrm{Ag}∕\mathrm{Zn}\mathrm{O}$ coincidence superstructure (0.1% mismatch) is found to have a significantly higher work of separation than the coherent $(1\ifmmode\times\else\texttimes\fi{}1)$ interface (11% mismatch). A rotated variant of the interface $(2\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)$ $R30$ (2.6% mismatch) has a work of separation that is comparable with the coincidence superstructure. Both the $(9\ifmmode\times\else\texttimes\fi{}8)$ and $(2\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)$ $R30$ $\mathrm{Ag}∕\mathrm{Zn}\mathrm{O}$ interfaces have been observed in deposition experiments and which one is seen depends on the ambient conditions and strain state of the interface. The calculated works of separation are consistent with measured works of adhesion obtained from cantilever beam experiments.