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Niquet Y.-M.,Laboratoire Of Simulation Atomistique L Sim | Delerue C.,1 boulevard Vauban
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Epitaxially grown semiconductor heterostructures make it possible to tailor the potential landscape for the carriers in a very controlled way. In planar lattice-matched heterostructures, the potential has indeed a very simple and easily predictable behavior: it is constant everywhere except at the interfaces, where there is a step (discontinuity) that only depends on the composition of the semiconductors in contact. In this paper, we show that this universally accepted picture can be invalid in nanoscale heterostructures (e.g., quantum dots, rods, nanowires), which can presently be fabricated in a large variety of forms. Self-consistent tight-binding calculations applied to systems containing up to 75 000 atoms indeed demonstrate that the potential may have a more complex behavior in axial heteronanostructures: The band edges can show significant variations far from the interfaces if the nanostructures are not capped with a homogeneous shell. These results suggest new strategies to engineer the electronic properties of nanoscale objects, e.g., for sensors and photovoltaics. © 2011 American Physical Society. Source

Londos C.A.,National and Kapodistrian University of Athens | Aliprantis D.,National and Kapodistrian University of Athens | Sgourou E.N.,National and Kapodistrian University of Athens | Chroneos A.,Imperial College London | Pochet P.,Laboratoire Of Simulation Atomistique L Sim
Journal of Applied Physics | Year: 2012

Infrared spectroscopy (IR) measurements were used to investigate the effect of lead (Pb), tin (Sn), and (Pb, Sn) codoping on electron radiation-induced defects in silicon (Si). The study was mainly focused on oxygen-vacancy (VO n) clusters and in particular their formation and evolution upon annealing. It was determined that Pb causes a larger reduction in the production of the VO defect than Sn. In (Pb, Sn) co-doped Si isochronal anneals revealed that the evolution of VO increases substantially at ∼170 C. This is attributed to the release of V from the SnV pair. Interestingly, in the corresponding evolution curves of VO in the Sn- and the Pb-doped samples, this inverse annealing stage is also present for the former while it is not present for the latter. This is attributed to the formation of PbV pairs that do not dissociate below 280°C. The partial capture of V by Sn in co-doped samples is rationalized through the higher compressive local strain around Pb atoms that leads to a retardation of vacancy diffusion. The conversion of VO to the VO2 defect is substantially reduced in the Pb-doped sample. The evolution curves of VO and VO2 clusters in the isovalent doped Si samples hint the production of VO2 from other mechanisms (i.e., besides VO+Oi → VO2). For larger VOn clusters (n=3,4), the signals are very weak in the Pb-doped sample, whereas for n ≥ 5, they are not present in the spectra. Conversely, bands related with the VO5 and VOnCs defects are present in the spectra of the Sn-doped and (Pb, Sn) codoped Si. © 2012 American Institute of Physics. Source

Groeneveld E.,University Utrecht | Delerue C.,IEMN | Allan G.,IEMN | Niquet Y.-M.,Laboratoire Of Simulation Atomistique L Sim | De Mello Donega C.,University Utrecht
Journal of Physical Chemistry C | Year: 2012

In this paper, we present a detailed investigation of the size dependence of the optical transitions of colloidal CdTe QDs ranging in diameter from 2.9 to 14.8 nm. The energy integrated absorption cross section per CdTe unit is investigated in detail for the lowest two exciton transitions (1S 3/2(h)-1S (e) and 2S 3/2(h)-1S (e)) and shown to increase with decreasing size, although the size dependence of the 2S 3/2(h)-1S (e) is less pronounced. The experimental absorption spectra are compared to spectra calculated by using a tight-binding approach. The calculations were carried out with electron-hole configuration interaction (CI) and without (single-particle, SP). The optical absorption spectra calculated by using the CI approach are in excellent agreement with the experiment, as well as the evolution of the optical gap and the optical transitions with nanocrystal size. © 2012 American Chemical Society. Source

Pochet P.,Laboratoire Of Simulation Atomistique L Sim | Genovese L.,European Synchrotron Radiation Facility | De S.,University of Basel | Goedecker S.,University of Basel | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We show by means of first-principles calculations that in boron nanostructures a large variety of two-dimensional structures can be obtained, all with similar energetic properties. Some of these new structures are more stable than both the B80 fullerenes initially proposed by Szwacki and boron nanotubes. At variance from other systems like carbon, disordered configurations are energetically comparable with ordered ones. Cage-like structures that are not ordered are thus comparable in energy to the more ordered original B80 fullerene. A comparison with other more disordered structures like bulk-like boron clusters is also presented. We found that in the presence of other seed structures (like Sc3 or Sc 3N), some endohedral cage-like structures are energetically preferred over bulk-like clusters. This result opens a new pathway for the synthesis of the B80 fullerene as an endohedral fullerene as was done in the case of the C80 fullerene. © 2011 American Physical Society. Source

De S.,University of Basel | Willand A.,University of Basel | Amsler M.,University of Basel | Pochet P.,Laboratoire Of Simulation Atomistique L Sim | And 2 more authors.
Physical Review Letters | Year: 2011

Using the minima hopping global geometry optimization method on the density functional potential energy surface we show that the energy landscape of boron clusters is glasslike. Larger boron clusters have many structures which are lower in energy than the cages. This is in contrast to carbon and boron nitride systems which can be clearly identified as structure seekers. The differences in the potential energy landscape explain why carbon and boron nitride systems are found in nature whereas pure boron fullerenes have not been found. We thus present a methodology which can make predictions on the feasibility of the synthesis of new nanostructures. © 2011 American Physical Society. Source

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