Atomistic impurities

The microstmcture and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce stmctures and properties similar to or totally different from bulk processed materials. In the latter case, the deposited materials may have high intrinsic stress, high point-defect concentration, extremely fine grain size, oriented microstmcture, metastable phases, incorporated impurities, and macro-and microporosity. AH of these may affect the physical, chemical, and mechanical properties of the coating. 

In this situation computer simulation is useful, since the conditions of the simulation can be chosen such that full equihbrium is established, and one can test the theoretical concepts more stringently than by experiment. Also, it is possible to deal with ideal and perfectly flat surfaces, very suitable for testing the general mechanisms alluded to above, and to disregard in a first step all the complications that real substrate surfaces have (corrugation on the atomistic scale, roughness on the mesoscopic scale, surface steps, adsorbed impurities, etc.). Of course, it may be desirable to add such complications at a later stage, but this will not be considered here. In fact, computer simulations, i.e., molecular dynamics (MD) and Monte Carlo (MC) calculations, have been extensively used to study both static and dynamic properties [11] in particular, structural properties at interfaces have been considered in detail [12]. 

For the deformation of NiAl in a soft orientation our calculations give by far the lowest Peierls barriers for the (100) 011 glide system. This glide system is also found in many experimental observations and generally accepted as the primary slip system in NiAl [18], Compared to previous atomistic modelling [6], we obtain Peierls stresses which are markedly lower. The calculated Peierls stresses (see table 1) are in the range of 40-150 MPa which is clearly at the lower end of the experimental low temperature deformation data [18]. This may either be attributed to an insufficiency of the interaction model used here or one may speculate that the low temperature deformation of NiAl is not limited by the Peierls stresses but by the interaction of the dislocations with other obstacles (possibly point defects and impurities). 

In this context, Sayle et al. have used atomistic simulation methods to investigate the interaction of ceria with impurities, particularly rhodium, palladium and platinum. The energetics of the most common valence states for the metal atoms were investigated, as well as the variation of the energy of the impurities with depth below the surface and the tendency of defects to segregate to the surfaces. Fig. 8.9. shows the substitutional defect formation energies as a function of the distance from the (111) and (110) surfaces of Ce02 for C e " ", PcP+ and Sayle etal. found that... 

Light-induced processes are described quite differently in molecular photochemistry and solid-state photophysics. In photochemistry one is used to an atomistic picture in which the arrangement of the atoms in the structure of a single molecule determines the electronic levels and thus the photochemical behavior. In contrast, the electronic levels of a solid are determined by the infinite periodicity of the atomic sequence in the crystal lattice. This leads to a basic concept according to which the solid can be treated as a dielectric continuum. Atomistic irregularities in the crystalline structure, such as lattice defects or impurities, are treated as perturbations of the spatially independent states in the energy bands. 

Silicon is, of course, the most important materials for electronics. Despite the tremendous amount of work done both from experiment and theory, many issues still remain unresolved concerning the atomistic details of the structure and dynamics of defects and impurities in silicon. Several SIESTA studies have focused on resolving some of these open problems. 

In every approach one finds a wide range of sophistication. In the continuum approach, the simplest (and most common) models are based on linear elastic fracture mechanics (LEFM), a well developed discipline that requires a linear elastic behaviour and brittle fracture, not always exhibited by fibres. Ductility and the presence of interfaces, not to mention hierarchical structures, make modelling much more involved. The same is true of the atomistic approach fracture models based on bond breaking of perfect crystals, using well established techniques of solid state physics, allow relatively simple predictions of theoretical tensile stresses, but as soon as real crystals, with defects and impurities, are considered, the problem becomes awkward. Nevertheless solutions provided by these simple models — LEFM or ideal crystals — are valuable upper or lower bounds to fibre tensile strength. 

The probe atoms can be one of the constituents of the material, or they can be impurities introduced into the material from the outside by melting, by diffusion, or by implantation. The former processes make use of the thermal motion of the atoms, while the implantation process injects energetic probe atoms using an accelerator. In this tutorial we will further discuss implantation Mossbauer Spectroscopy , i.e., the probe atoms will be first implanted into a material, and subsequently Mossbauer spectra will be measured by detecting emitted y-rays and electrons. The spectra will provide us with atomistic information on the probe atoms through the hyperfine interactions. This situation may be well compared with an analogy of a spy which is sent to a place to gather information, and he/she will... 

We hope to have demonstrated the enormous resolving power on atomistic scale of emission Mossbauer spectroscopy for such smdies. For Fe impurities in Si, with their extremely complex behavior, these techniques have clearly shown their merits and have substantially contributed to our understanding of the behavior of Fe impurities in Si. 

An atomistic deposition process is one in which the overlay material is deposited atom-byatom. The resulting film can range from single crystal to amorphous, fully dense to less than fully dense, pure to impure, and thin to thick. Generally the term thin film is applied to layers which have thicknesses on the order of a micron or less (1 micron = 10 meters) and may be as thin as a few atomic layers. Thicker deposits are called coatings. The term thick film is usually not used for thick atomistically deposited vacuum deposits as that term is used for paint-on, fire-on types of deposition. 

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