Publications

2009

• 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
• 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
• Evidence of transient species occurring in the reduction process of trivalent lanthanides under 2.5 MeV electron irradiation by in situ cathodoluminescence and time-resolved photoluminescence
  
  • Ollier Nadège
  • Boizot B.
  • L'Henoret P.
  • Guillous Stephane
  • Petite G.
  Journal of Applied Physics, American Institute of Physics, 2009, 105, pp.113515. In situ cathodoluminescence and time-resolved photoluminescence measurements were carried out on Sm-doped glasses during a 2.5 MeV electron irradiation. These experiments allow characterizing more precisely the mechanisms operating under irradiation and more specifically the reduction processes of Sm3+ to Sm2+ ions. Sm2+ emission lines appear in the first steps of the irradiation, however, the 5D0→7F0–2 emission lines of Sm2+ were not observed on the cathodoluminescence spectrum. Moreover, two sites of Sm2+ have been evidenced in the glasses; the formation of each species is clearly different. Relaxation processes of Sm3+ and Sm2+ leads to consider the existence of different transient states of Sm3+ and Sm2+ species which are annealed after irradiation. We propose some synthetic schemes of the reduction mechanisms produced during the irradiation.
• Organisation of carbon nanotubes and semiconductor nanowires using lateral alumina templates
  
  • Pribat Didier
  • Cojocaru Costel Sorin
  • Gowtham M.
  • Marquardt B.
  • Wade T.
  • Wegrowe Jean Eric
  • Kim B.S.
  Comptes Rendus. Physique, Académie des sciences (Paris), 2009, 10 (4), pp.320--329. Carbon nanotubes and semiconductor nanowires have been thoroughly studied for the future replacement of silicon-based com- plementary metal oxide semiconductor (CMOS) devices and circuits. However, the organisation of these nanomaterials in dense transistor arrays, where each device is capable of delivering drive currents comparable with those of their silicon counterparts is still a big challenge. Here, we present a novel approach to the organisation of carbon nanotubes and semiconductor nanowires, based on the use of porous lateral alumina templates obtained by the controlled anodic oxidation of aluminium thin films. We discuss the growth of nanomaterials inside the pores of such templates and show the feasibility of our approach. Our first results point to further work on controlling the synthesis of catalyst nanoparticles at the bottom of the pores, these particles being necessary to nucleate and sustain the growth of carbon nanotubes or semiconductor nanowires. (10.1016/j.crhy.2009.05.007)
  DOI : 10.1016/j.crhy.2009.05.007
• Interpolation between spectra satisfying sum rules
  
  • Weissker Hans-Christian
  • Hambach Ralf
  • Olevano Valerio
  • Reining Lucia
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2009, 79 (9), pp.094102. A method is presented which is able to interpolate between spectra depending parametrically on one variable and obeying a sum rule. This enables the description of experiments with a finite resolution in that parameter because integrals over certain parameter ranges are easily obtained, as for instance in the case of inelastic x-ray scattering with finite resolution in momentum transfer. Beyond the sum rule, the method does not use further assumptions about the physics of the system. It is applicable to a wide range of spectra as for instance the dynamic structure factor or the dielectric function for different moduli or directions of momentum transfer, absorption spectra for different alloy compositions or for a range of nanocrystal sizes, probability distributions, etc. The method is expected to be useful not only in the simulation of experimental spectra but also in calculations where the determination of certain spectral quantities is numerically cumbersome. A code carrying out the interpolation is provided. (10.1103/PhysRevB.79.094102)
  DOI : 10.1103/PhysRevB.79.094102