Ch.E. Lekka

Tensile deformation accommodation in microscopic metallic glasses via subnanocluster reconstructions

We present results on the structure and the atomistic mechanisms for tensile deformation accommodation of the Cu46Zr54 microscopic metallic glass. At equilibrium, 23% of the atoms belong to tiny Cu-centered icosahedral clusters (Cu-ICO) and approximately 41% Zr centered slightly larger ICO-like clusters. Under deformation, the number of Cu-ICOs remains dynamically constant until yielding through a continuous cluster destruction-recreation process. Plastic deformation occurs homogeneously and is locally accommodated through the formation of rhombic dodecahedral clusters with significant (∼2%) atomic density drop. These findings explain very recent experimental results demonstrating the fundamental differences of plastic deformation mechanisms between bulk metallic and microscopic glasses.

Conducting transition metal nitride thin films with tailored cell sizes: The case of δ-TixTa1−xN

We present results on the stability and tailoring of the cell size of conducting δ-TixTa1−xN obtained by film growth and ab initio calculations. Despite the limited solubility of Ta in Ti, we show that TiN and TaN are soluble due to the hybrization of the d and sp electrons of the metal and N, respectively, that stabilizes the ternary system to the rocksalt structure. The stress-free cell sizes follow the Vegard’s rule; nevertheless, process-dependent stresses expand the cell size of the as-grown films. The electronic properties of δ-TixTa1−xN films (ρ=180Ωcm) are similar to those of TiN and TaN.

Structure and electronic properties of conducting, ternary TixTa1−xN films

We report on the electronic structure and optical properties of conducting ternary transition metal nitrides consisting of metals of different groups of the periodic table of elements. For the study of the bonding, electronic structure, and optical properties of conducting TixTa1−xN film growth, optical spectroscopy and ab initio calculations were used. Despite the different valence electron configuration of the constituent elements, Ta(d3s2) and Ti(d2s2), we show that TiN and TaN are completely soluble due to the hybridization of the d and sp electrons of the metals and N, respectively, that stabilizes the ternary TixTa1−xN systems to the rocksalt structure. The optical properties of TixTa1−xN have been studied using spectroscopic methods and detailed electronic structure calculations, revealing that the plasma energy of the fully dense TixTa1−xN is varying between 7.8 and 9.45 eV. Additional optical absorption bands are manifested due to the N p→Ti/Ta d interband transition the t2g→eg transition due to ...

Optical properties, structural parameters, and bonding of highly textured rocksalt tantalum nitride films

Tantalum nitride is an interesting solid with exceptional properties and it might be considered as a representative model system of the d3s2 transition metal nitrides. In this work highly textured, stoichiometric, rocksalt TaN(111) films have been grown on Si(100) by pulsed laser deposition. The films were under a triaxial stress, which has been determined by the sin2 ψ method. The stress-free lattice parameter was found to be 0.433±0.001 nm, a value which has been also determined by ab initio calculations within the local spin density approximation. The optical properties of TaN have been studied using spectroscopic ellipsometry and detailed band structure calculations. The electron conductivity of TaN is due to the Ta 5dt2g band that intercepts the Fermi level and is the source of intraband absorption. The plasma energies of fully dense rocksalt TaN were found to be 9.45 and 9.7 eV based on the experimental results and ab initio calculations, respectively. Additional optical absorption bands were also o...

Self-diffusion processes of copper adatom on Cu(110) surface by molecular dynamics simulations

Abstract The self-diffusion mechanisms of single adatoms on the Cu(110) surface have been studied using molecular dynamics simulation and a many-body potential within the second-moment approximation of the tight-binding model. From a detail trajectory analysis we found a variety of diffusion mechanisms, the hopping being the favoured one and we deduced the migration energies for the most important among them. At high temperatures, saturation in diffusion frequency for both hopping and exchange mechanisms is observed, indicating that the diffusion proceeds via complicated and concerted movements. In addition, we estimated the formation energy for the spontaneous creation of the vacancy-adatom pair, in good agreement with the experiment. Furthermore, from the temperature dependence of the relaxed adatom positions we found that the adatom exhibits strong contraction compared to the bulk interlayer spacing, attaining −20% at high temperatures.

Electronic properties and bonding characteristics of AlN:Ag thin film nanocomposites

We present theoretical and experimental results on the bonding and structural characteristics of AlN:Ag thin film nanocomposites obtained by means of density functional theory (DFT) computations, high resolution transmission electron microscopy (HRTEM) observations, Auger electron spectroscopy (AES), and x-ray diffraction (XRD) measurements. From the theoretical calculations it was determined that the presence of the Ag substitutional of N or Al atoms affects the electronic density of states (EDOS) of the resulting systems. In particular, occupied energy states are introduced (between others) that lie within the energy gap of the AlN matrix due to Ag-d, Al-p (accompanied with a charge transfer from Al to Ag), Ag-p, and N-p hybridizations, respectively. The effect is predicted to be even more pronounced in the case of Ag nanoparticle inclusions affecting the EDOS of the composite system. These predictions were verified by the HRTEM images that gave unequivocal evidence for the presence and stability of Ag ...

Fe–Co magnetic nanoclusters by density functional theory calculations

ABSTRACT We present density functional theory results referring to the structural, electronic and magnetic properties of 13, 55, 147 and 309 Fe–Co (magnetic–magnetic) icosahedral nanoclusters (ICO) comparing with our previous results on Fe–Cu (magnetic–nonmagnetic). It came out that the Fe atoms always favour the edge surface sites exhibiting higher average magnetic moment (MM) for Fe and FeCo ICOs than FeCu while the local Fe MM is greater for FeCu12 and Fe6Cu49 ones. This is due to the strong hybridisation of Fe 3d–Co 3d states, while in Fe–Cu the Fe spin down states are restricted close to fermi without been affected by the corresponding Cu states. These results could be used for the design of environmental sustainable smart magnetic alloys. This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.

Transferability of Slater-Koster parameters

Abstract In this work, we present a method of improving the transferability of the Slater–Koster Tight-Binding (SK-TB) parameters given in [Handbook of the Band Structure of Elemental Solids, Plenum Press, New York, 1986] to describe the electronic properties at other lattice constants and structures. First of all, we express the Hamiltonian and on-site atom parameters as a function of distance. We validate our method by calculating the electronic structure for various lattice constants for Nb and Mo (bcc) and for Cu and Au (fcc) and we find good agreement with the APW results. Furthermore, we transfer successfully the SK-TB parameters of the bcc to the fcc structure of Nb and Mo and vice versa for Cu and Au. Moreover, we apply a uniform shift Vo, which equates the sum of SK-TB eigenvalues to the APW total energy for the available volumes. We find that Vo presents a smooth volume dependence. Finally, using the volume dependence of Vo and the SK-TB theory we calculate the total energy for every lattice constant.

Vibrational and Structural Properties of the Nb (001) Surface With and Without a Nb Adatom by Tight-Binding Molecular Dynamics

We present results of tight-binding molecular dynamics simulations of the Nb(001) surface. We find that the surface energies of the unrelaxed and relaxed system are in good agreement with the available experimental and theoretical data. From the calculated phonon density of states, we found that the bulk phonon mode around 6.4 THz is altered by the presence of the surface, indicating looser coupling between the surface and the bulk atoms. Furthermore, it emerges that the surface atoms are more tightly bound in the direction normal to the surface than in the in- plane directions. We find that a Nb. adatom exhibits a contraction, while from its phonon density of states, it appears to be more tightly bound in the direction normal to the surface, introducing modes at higher frequencies than those of surface atoms. From these results, we conclude that our method reproduces satisfactorily the main features of the Nb (001) surface and that it is a promising approach for describing this class of materials.

Adatoms and Vacancies on A3B(001) Surfaces

We have investigated the vibrational and diffusive properties of adatoms and vacancies on the (001) surfaces of CU3AU and Ni3Al alloys using molecular dynamics. For both materials, we found that the low-energy vibrations of the clean ordered surface and of adatoms are due mainly to the heavier atom and that the surfaces are rumpled. We also found that the behavior of the adatoms is marked by the availability of two equilibrium positions: one over the four-fold site and another one on top of a Cu (or Ni) surface atom. The Cu adatom on Cu3Au(001) is found to diffuse mainly via a new hopping mechanism between neighboring four-fold sites, passing over the on-top adatom position, and a new exchange mechanism involving second-layer atoms. The Au adatom is unstable on Cu3Au(001), segregating into the surface layer via an exchange mechanism similar to that of the Cu adatom, resulting in loss of local surface order. On Ni3Al(001), both adatoms are stable and show considerable diffusion with a variety of hopping mechanisms, but exchange-type mechanisms are absent. For both alloys surfaces, vacancy diffusion is favored in the [110] in-plane direction. On Cu3Au(001), this process results in new stable atomic re-arrangements, with the surface atoms at anti-sites, while for Ni3Al(001), vacancy hops lead the system to unstable atomic conformations. Thus, the presence of adatoms or vacancies on Ni3Al(001) does not affect surface order, at least at relatively low temperatures, while on Cu3Au(001), they stimulate phenomena that are related to the order-disorder transition at higher temperatures.