S. Serra

Self-trapping vs. non-trapping of electrons and holes in organic insulators: polyethylene

We show, by electronic structure based molecular dynamics simulations, that an extra electron injected in crystalline polyethylene should fall spontaneously into a self-trapped state, a shallow donor with a large novel distortion pattern involving a pair of trans-gauche defects. Parallel calculations show instead that a hole will remain free and delocalized. We trace the difference of behavior to the intrachain nature of the hole, as opposed to the interchain one of the electron, and argue that applicability of this concept could be more general. Thus electrons (but not holes) should tend to self-trap in saturated organic insulators, but not for example in aromatic insulators, where both carriers are intrachain.

Exciton self-trapping in bulk polyethylene

We studied the behaviour of an injected electron–hole pair i nc rystalline polyethylene theoretically. Time-dependent adiabatic evolution by ab initio molecular dynamics simulations show that the pair will become self-trapped in th ep erfect crystal, with a trapping energy of about 0.38 eV, with formation of a pair of trans-gauche conformational defects, three C2H4 units apart on the same chain. The electro ni s confined in the interchain pocket created by a local, 120 ◦ rotation of the chain between the two defects, while the hole resides on the chain and is much less bound. Despite the large energy stored in the trapped excitation, there does not appear to be a direct non-radiative channel for electron–hole recombination. This suggests that intrinsic self-trapping of electron–hole pairs inside the ideal quasi-crystalline fraction of polyethylene might not be directly relevant for electrical damage in high-voltage cables.

Trapping of excitons at chemical defects in polyethylene

In a previous paper we studied an injected electron-hole pair in crystalline polyethylene (PE) and found that the exciton becomes weakly self-trapped in a narrow interchain pocket comprised between two gauche defects. Despite the large energy stored in the trapped excitation, there did not appear to be a direct nonradiative channel for electron-hole recombination. Actual polyethylene systems of practical use are, however, neither crystalline nor pure. To understand the fate of an electron-hole pair in the impure case, we studied by ab initio simulations the evolution of an exciton trapped on three common chemical defects found in polyethylene: a grafted carbonyl (C=O); an intrachain vinyl group (C=C); a grafted carboxyl (COOH). Ab initio simulations lead to predict three different outcomes: trapping, nonradiative recombination, and homolitic bond-breaking, respectively. This suggests that extrinsic self-trapping of electron-hole pairs over chemical defects inside the quasicrystalline fraction of PE could be relevant for electrical damage in high-voltage cables.

Interchain states and the negative electron affinity of polyethylene

The negative electron affinity of polyethylene well established experimentally is investigated theoretically by first-principles electronic structure calculations. We hypothesize that the nature of the lowest conduction band could be interchain, instead of intrachain, explaining the negative electron affinity. We focus in particular on the nature of states on either sides of the band gap, and on their behavior for variable interchain spacing. Preliminary results indicate that while the valence bands are clearly intrachain, the lowest conduction band in fact appears to display an interchain character extending about 5 /spl Aring/ away from the chain. The consequences on the possible formation mechanism of space charge region are discussed.

First principles calculations of charge and spin density waves of 3-adsorbates on semiconductors

Abstract We present ab-initio electronic structure results on the surface of 3 × 3 adsorbates. In particular, we address the issue of metal–insulator instabilities, charge-density waves (CDWs) or spin-density waves (SDWs), driven by partly filled surface states and their 2D Fermi surface, and/or by the onset of magnetic instabilities. The focus is both on the newly discovered commensurate CDW transitions in the Pb/Ge(111) and Sn/Ge(111) structures, and on the puzzling semiconducting behavior of the Pb/Ge(111), K/Si(111):B and SiC(0001) surfaces. In all cases, the main factor driving the instability appears to be an extremely narrow surface state band. So far, we have carried out preliminary calculations for the Si/Si(111) surface, chosen as our model system, within the gradient corrected local density (LDA+GC) and local spin density (LSD+GC) approximations, with the aim of understanding the possible interplay between 2D Fermi surface and electron correlations in the surface+adsorbate system. Our spin-unrestricted results show that the 3 × 3 paramagnetic surface is unstable towards a commensurate density wave with periodicity 3×3 and magnetization 1/3.