Quantum Theory of Advanced Materials

Here{,} by utilizing the special interaction between metal Cu and multi-walled carbon nanotubes (CNTs){,} we have successfully realized the in situ growth of Cu2Se on the surface of CNTs and then fabricated a series of Cu2Se/CNT hybrid materials. Due to the high degree of homogeneously dispersed molecular CNTs inside the Cu2Se matrix{,} a record-high thermoelectric figure of merit zT of 2.4 at 1000 K has been achieved.

We present a first-principles investigation of the structural properties, electronic structure, and the chemical stability of the complex hydrides NaAlH(4) and Na(3)AlH(6). The calculations are performed within the density functional framework employing norm conserving pseudopotentials. The structural properties of both hydrides compare well with experimental data. A detailed study of the electronic structure and the charge-density redistribution reveal the features of an ionic covalent bonding between Al and H in the (AlH(4))(-) and (AlH(6))(-3) anionic complexes embedded in the matrix of Na(+) cations. The orbital hybridization and the characteristics of bonding orbitals within the complexes are identified. The calculated reaction energies of these complex hydrides are in good agreement with the experimentally determined values.

Quasiparticle band structures for the cubic trihydrides YH3 and LaH3 have been calculated by evaluating the self-energy in the GW approximation using ab initio pseudopotentials and plane waves. These are the prototype metal hydrides that exhibit switchable optical properties. For both materials, the local-density approximation (LDA) yields semimetallic energy bands with a direct overlap of about 1 eV. We find the self-energy correction to the LDA energies opens a gap at Gamma of 0.8-0.9 eV for LaH3 and 0.2-0.3 eV for YH3, where the latter is in sharp contrast to a previous study using linear-muffin-tin orbitals. The quasiparticle band gaps are analyzed as a function of an initial shift in the LDA bands used to evaluate the random-phase approximation screening in constructing the self-energy. We also make a comparison of results obtained by using two different pseudopotentials, each designed to better approximate exchange and correlation between the semicore states and valence states of Y and La.