Computational Materials Science Lab
University of Ioannina
Dept. of Materials Science and Engineering



May 15, 2016:
1 Postdoc position available

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Clustering and mechanical properties in Cu/Zr-based glassy models

CuZr glass

It is now accepted that Cu-Zr Metallic Glasses (MG) are mainly composed by small Icosahedral-like (ICO) clusters that may be interconnected and/or interpenetrating. Using Molecular Dynamics and Reverse Monte Carlo simulations we show that the interpenetrating ICOs carrying the compositional signature of the system form SuperClusters (SC) which obey particular sequences of magic numbers. It comes out that these SCs are reproducing very well the structural features of the experimental Radial Distribution Functions (RDF). Interestingly, we found that the atomic pair distances do not depend on the respective compositions of the systems, suggesting that the experimental shift of the RDFs when changing the stoichiometry is due only to the alterations of the relative heights of the peaks. Upon deformation, the ICO clusters are destructed and recreated while similar behavior follow the SCs explaining the way the Cu-Zr MGs accommodate the stress upon deformation. In addition, we addressed the issue of Microalloying in CuxZr12-xY (Y=Mg,Be,Al,Si,P,Nb,Ag) ICO cases and in selective CuZrAl and CuZrNb SCs by means of Density Functional Theory calculations. We found significant alterations in the electronic structures of the CuZr clusters and SCs manifested by new low-energy states and charge transfer near the Fermi level. In the CuZrAl ICO and SC cases these new states are due to covalent-like bonding between Al core and Cu shell atoms and Cu-Al core-core atoms, respectively. In the case of Nb substitution the effects in the electronic structure are similar, but the interactions between Nb core and Cu/Zr shell atoms are characterized by directional π-like bonds. In addition, we found that a p-electron type dopant, as central atom, results in the creation of a plane with free of core–shell atomic bonds, at certain energies and weak interactions at the Fermi level, which could be viewed as a slip plane. s or d-electron type dopants may behave similarly due to significant charge transfer towards unoccupied p-electrons occurring upon alloying. These results could explain the experimental findings referring to the short range order, the elucidation of the micro-alloying effect and the modifications induced in the mechanical properties of these MGs by small Al or Nb additions.

Collaboration: Ch.E. Lekka and D.G. Papageorgiou

Plasmonic nanocomposites for solar harvesting


Study and utilization of metallic nanoparticles in solar harvesting applications. When metallic nanoparticles are illuminated by electromagnetic waves, their free electrons oscillate in response to the electric field. At the right frequency this oscillation is driven into resonance (Surface Plasmon Resonance), exhibiting huge polarization fields on the nanoparticle surface. These fields induce strong light scattering and promote photo-electrical processes (e.g. fluorescence, Raman scattering, light absorption). In addition, these fields are very sensitive to the metal type as well as the nanoparticle shape, size and dielectric environment, allowing great flexibility in designing applications. Metallic nanoparticles are in the heart on nanotechnology, with applications in medicine, energy and photonics. In this project we use the properties of the metallic nanoparticles to improve and optimize the performance of: 1) photovoltaic cells, through a) enhancing the semiconductor absorption, and b) solar spectrum conversion for better utilization of all available photons, 2) solar collectors, through designing a nanostructured metallodielctric coating exhibiting total absorption to the solar spectrum and minimal losses from thermal re-radiation.
Principal Investigator: E. Lidorikis
Project funded from: IRAKLEITOS II (2011-2014) smartonics
and FP7 CP-IP 310229 "SMARTONICS" (2013-2016)

TiN decoration of Single Wall Carbon Nanotubes and Graphene by Density Functional Theory computations

Ti_n decoration

Ti nanostructures on Single Wall Carbon Nanotubes (SWCNTs) attracted considerable attention due to their potential applications in electronic nanodevices and molecular adsorption. We report on Density Functional Theory results referring to TiN (N=1,2,3,7,13) supported on SWCNTs and graphene. Two new equivalent positions emerged that trisect the line joining the hexagon along the tube’s axis sides (TSH). These sites accommodate dimmers and trimmers in compact linear and 2D triangular forms, respectively, and 3D Ti7 and Ti13 conformations. Ti adsorbates introduce new electronic states close and at the Fermi level. Despite the significant charge transfer from adsorbates to substrates, these otherwise reduced TiN induce substantial charge screening in their surrounding substrate’s ldlstoms and appear eventually as charged locations. These findings enlighten the early stages of Ti deposition, predict possible active sites and may be of use for the design of metal-carbon coatings for applications in catalysis and nano-electronics.

Principal Investigator: Ch.E. Lekka

Optical properties and applications of carbon-based materials

Project graphene

Carbon nanotubes (CNTs), and more recently graphene, have been at the center of nanotechology research, with the search for new technologies based on their mechanical and electrical properties ever increasing. Graphene, a two-dimensional honeycomb lattice of carbon atoms, can be thought of as the "building block" of other carbon allotropes: it can be "wrapped" into fullerenes, "rolled" into CNTs or "stacked up" into graphite, with many of their properties deriving from graphene. In this project we study different aspects of the photonic response and applications of graphene and CNTs. We have developed a theoretical understanding for graphene's enhanced visibility on certain substrates, as well as interference-enhanced (IERS) and surface-enhanced Raman scattering (SERS) phenomena in graphene. We have investigated the photonic properties of two-dimensional CNT arrays for photon energies up to 40eV and unveiled the physics of two distinct applications, namely deep-UV photonic crystals and total visible absorbers. We study graphene's optical absorption, and design graphene and hybrid graphene/plasmonic optical detectors and photovoltaic systems.
Principal Investigator: E. Lidorikis
Project funded from: FP7 "GRAPHENE FLAGSHIP" (2013-2016)

Theory-guided bottom-up design of low-rigidity Ti-based alloys


The project objective is to develop a computational procedure (from ab-initio towards large scale molecular dynamics simulations) for the investigation and design of low rigidity Ti-based alloys. This project aims in studying and understanding the relations between the mechanical properties and the structure and chemistry of existing and/or proposed β-type Ti-based alloys, goal being the design of new β-type Ti-based alloys. Tasks: a) physical and chemical insight of existing and/or proposed Ti-based systems by means of the prediction of the basic thermodynamic, mechanical and chemical properties based on ab-initio calculations for the crystalline alloys, b) Design of new Ti-based crystalline systems from ab-initio, c) Scale-up to nanocrystalline via modeling of suitable semi-empirical approaches: from total energy and ab-initio computations to interatomic model interactions and large scale parallel Computations and Simulations and d) Simulation into real cases (e.g. β-type and nano-structured Ti-based, as well as glassy alloys) and comparison with the other experimental data.

Collaboration: Ch. E. Lekka and D. G. Papageorgiou
Project : FP7-PEOPLE-2010-ITN (# 264635), Academic-Industrial Initial Training Network on Innovative Biocompatible Titanium-based Structures for Orthopaedics (BioTiNet) 2011-2014

Density functional study of small bimetallic Ag–Pd clusters


The geometric and electronic properties of small AgmPdn clusters with m+n = 2–5 are studied within the framework of density functional theory in conjunction with two hybrid and one GGA exchange–correlation functional. For every composition, the global minimum is identified by using geometry optimization for a collection of initial structures. Results indicate that, for bimetallic tetramers and pentamers, the clusters shift from two-dimensional to three-dimensional structures with the addition of a second Pd atom. Ag2Pd2 is identified as the most stable tetramer by the calculation of the excess energy and second energy difference of bimetallic clusters. Concerning the fragmentation channels it is seen that the most favourable route in the majority of cases is via the evaporation of a single atom. Density of states calculations reveal that the increase of Pd content depletes the isolated s states close to the Fermi level, while at the same time shifts the d states to higher energies.

Principal Investigator: D. Papageorgiou