Publications

2009

• Local atomic order and optical properties in amorphous and laser-crystallized GeTe
  
  • Wełnic Wojciech
  • Wuttig Matthias
  • Botti Silvana
  • Reining Lucia
  Comptes Rendus. Physique, Académie des sciences (Paris), 2009, 10, pp.514. In this work we study the role of short-range order changes upon amorphization on the optical properties of GeTe – a prototype phase change material employed for optical data storage. It is found that the profound change in the absorption is due to changes in the matrix elements of the optical transitions. The importance of the local distortions in the crystalline phase for the optical absorption are revealed as well. Modifying the degree of the distortions has a significant impact on the optical properties of the crystalline state and should therefore become a promising instrument to improve material properties for storage applications. Furthermore we study the effect of electron–electron and electron–hole interactions on the optical properties. This is achieved by evaluating many-body effects in the crystalline phase through a GW correction of eigenvalues and the solution of the Bethe–Salpeter equation. (10.1016/j.crhy.2008.09.002)
  DOI : 10.1016/j.crhy.2008.09.002
• Charles Crussard's early contributions: Recrystallization in situ and a Grain Boundary study with J. Friedel and B. Cullity
  
  • Hardouin Duparc Olivier
  International Journal of Materials Research, 2009, 100, pp.1382-1388. Charles Crussard's discovery of what he called Recrystallization in situ in 1944 is compared with what Egon Orowan and Robert Cahn named Polygonization. The context of the previous research is recalled in order to show the novelty of these contributions. Progress in accurate deformation and observation techniques such as tensile machines and X-ray methods were important, as were theoretical models about plausible microscopic mechanisms. With Jacques Friedel and Barney Cullity, Ch. Crussard also wrote a pioneering article on grain boundaries, which contains an experimental study of the relative energies of both symmetrical and asymmetical grain boundaries observed in aluminium together with the first atomistic calculation of energies of symmetrical low and high angle tilt boundaries in several metals. This less well-known contribution is also recalled. (10.3139/146.110193)
  DOI : 10.3139/146.110193
• Ion engineering of embedded nanostructures: From spherical to facetted nanoparticles
  
  • Rizza G.
  • Dawi E. A.
  • Vredenberg A. M.
  • Monnet Isabelle
  Applied Physics Letters, American Institute of Physics, 2009, 95, pp.043105. We show that the high-energy ion irradiation of embedded metallic spherical nanoparticles (NPs) is not limited to their transformation into prolate nanorods or nanowires. Depending on their pristine size, the three following morphologies can be obtained: (i) nanorods, (ii) facettedlike, and (iii) almost spherical nanostructures. Planar silica films containing nearly monodisperse gold NPs (8–100 nm) were irradiated with swift heavy ions (5 GeV Pb) at room temperature for fluences up to 5x1013 cm−2. The experimental results are accounted for by considering a liquid-solid transformation of the premelted NP surface driven by the in-plane stress within the ion-deformed host matrix. This work demonstrates the interest of using ion-engineering techniques to shape embedded nanostructures into nonconventional configurations.
• Alloying effects on the optical properties of Ge1−xSix nanocrystals from time-dependent density functional theory and comparison with effective-medium theory
  
  • Botti Silvana
  • Weissker Hans-Christian
  • A. L. Marques Miguel
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2009, 79, pp.155440. We present the optical spectra of Ge1−xSix alloy nanocrystals of a fixed size calculated with time-dependent density functional theory in the adiabatic local-density approximation (TDLDA). The spectra change smoothly as a function of the composition x. On the Ge side of the composition range, the lowest excitations at the absorption edge are almost pure Kohn-Sham independent-particle highest occupied molecular orbital–lowest occupied molecular orbital transitions, while for higher Si contents strong mixing of transitions is found. Within TDLDA the first peak is slightly higher in energy than in earlier independent-particle calculations. However, the absorption onset and in particular its composition dependence is similar to independent-particle results. Moreover, classical depolarization effects are responsible for a very strong suppression of the absorption intensity. We show that they can be taken into account in a simpler way using Maxwell-Garnett classical effective-medium theory. Emission spectra are investigated by calculating the absorption of excited nanocrystals at their relaxed geometry. The structural contribution to the Stokes shift is about 0.5 eV. The decomposition of the emission spectra in terms of independent-particle transitions is similar to what is found for absorption. For the emission, very weak transitions are found in Ge-rich clusters well below the strong absorption onset. (10.1103/PhysRevB.79.155440)
  DOI : 10.1103/PhysRevB.79.155440