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We use the Mott-Littleton approach to evaluate polarisation energies in LaMnO$_3$ lattice associated with holes localized on both Mn$^{3+}$ cation and O$^{2-}$ anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O-site is estimated as 2.4 eV which is appreciably greater than that of 0.8 eV for a hole localized at the Mn-site, indicating on the strong electron-phonon interaction in the former case. Using a Born-Haber cycle we examine thermal and optical energies of the hole formation associated with electron ionization from Mn$^{3+}$, O$^{2-}$ and La$^{3+}$ ions in LaMnO$_3$ lattice. For these calculations we derive a phenomenological value for the second electron affinity of oxygen in LaMnO$_3$ lattice by matching the optical energies of La$^{4+}$ and O$^-$ hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn$^{4+}$ state in LaMnO$_3$ host lattice, but the energy of a hole in the O$^-$ state is only higher by a small amount, 0.75 eV, rather suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn$^{4+}$ and O$^-$ in LaMnO$_3$ lattice. The reasonable agreement with experiment of our predicted energies, linewidths and oscillator strengths leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with O(2p) - Mn(3d) transition of charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn$^{4+}$ and/or O$^-$ self-trapped holes in non-stoichiometric LaMnO$_3$ compound., Comment: 18 pages, 6 figures, it was presented partially at SCES-2001 conference in Ann Arbor, Michigan

We present the results of ab initio Hartree-Fock and configuration interaction calculations performed on selected small silicon clusters with a view to study their luminescent properties. We found that the covalent character of bonding is much reduced at the surface of the silicon clusters. The charge density of the clusters considered tends to be polarized towards their bulk-like core. This reflects the ‘self-passivation’ character of the chemical bonding in clusters. The optical transitions in the Si 18 cluster are found to have essentially many-electron character. The energies of three lowest optical transitions are found to be in the near-IR region and the corresponding oscillator strengths are 0.0001–0.004. Our results suggest that there is a strong dependence of the optical properties of silicon nanostructures not only on their size but also on their shape, which strongly affects the charge density and the character of correlation between the excited electrons.

In order to study the electronic properties of porous silicon (por-Si) we have investigated silicon nanoclusters with atomic configurations satisfying the general features of the radial and angular distribution functions of the por-Si structure modelled using the diffusion-limited aggregation model. The electronic structure calculations were performed within the intermediate neglect of differential overlap approximation of the Hartree-Fock-Roothaan scheme, while the restricted configuration interaction was employed in calculating excited states and transition rates. Both the electronic structure and optical transition strength are found to depend strongly on the cluster size and local environment. Importantly, the inclusion of many-electron effects is crucial in determining the transition rates. The low reactivity of some nanoclusters, together with their intermediate size and well-determined electronic properties, make them prospective initial blocks for the manufacturing of optoelectronic materials with desired characteristics.

We have implemented a modified diffusion-limited aggregation model to simulate the porous silicon structure obtained by electrochemical dissolution. The resulting fractal structures were fully equilibrated using the molecular dynamics method. An analysis of the relaxed structure shows it to be quite stable with the presence of one-, two- and three-coordinated atoms as well as the four-coordinated atoms found in bulk silicon. It is suggested that the different substructures or nanocrystals might be responsible for the observed photoluminescence

Quantum dots show a range of time–dependent behaviours. We show that the polarity of II–VI nanoparticles has important dynamical implications for electronic, vibrational and other phenomena. Polarity–dependent phenomena are found even for nearly spherical stoichiometric clusters of ZnO and ZnS in studies based on interatomic potentials or on a plane–wave density–functional approach. We find a substantial dipole moment for free nanoparticles, whether of the zinc blende or wurtzite structure. This dipole causes a highly non–uniform spin–density distribution on electronic excitation or after a change in the dot9s electronic charge state. The spin density of the triplet exciton shows that the dipole aligns so as to reduce the dipole moment in the electronically excited state. The polarity of II–VI dots also affects their vibrational properties. High– and low–frequency tails of the vibration density of states arise from modes strongly localized at surface atoms, near the poles of the dipole. These features, first noted for free clusters, also hold for particles embedded in a wide–gap dielectric, a –SiO 2 . We present the results of molecular dynamics of the ZnS particle embedded into the silica glass, and consider the role played by the soft modes in energy–dissipation processes such as dephasing during non–radiative recombination of excitons, and energy transfer from the dot to the matrix.

We apply the frozen phonon and molecular dynamics methods within the semiempirical orthogonal tight-binding framework to study the anomalous behaviour of the (0001) optical longitudinal (LO) and transverse (TO) phonons in the low temperature hep phase of Ti, and the ⅔[111]L and ½[110]T1 phonons in the high temperature bec phase. We demonstrate that, in agreement with previous findings in Zr, the anomalous thermal frequency shifts in hep Ti are related to the strong coupling of the electron density of states (DOS) to the particular lattice distortions. The distortions along the bec ⅔[111]L and ½[110]T1 phonons also significantly affect the DOS, resulting in the instability of these modes at low temperatures and triggering the bcc-hep and bcc-ω phase transformations.

The changes in the radial distribution function (RDF) and vibrational density of states (DOS) of porous silicon (p-Si) with change of porosity are studied within a modified diffusion limited aggregation model and molecular dynamics simulations. By decomposing the first peak of the radial distribution function of p-Si on to partial RDFs, for atoms having different coordinations, and partial RDFs, for bonds connecting atoms with different coordinations, we show that appearance of the structure in the first peak of the RDF in p-Si is stipulated by bonds between undercoordinated surface atoms. The vibrational DOS projected on surface atoms are also shown to be different from that corresponding to crystalline phonons. It is characterised by the relative increase of the intensity of vibrations in the acoustic region and by appearance of surface-like vibrations split from the optical band.