Search

We have performed scanning probe microscopy investigations of ZnO and Co-substituted ZnO under dark/UV conditions as well as in air and an ultra-high vacuum environment to shine a light on the change in electronic structure and surface reactivity as a consequence of Zn substitution with Co. We have achieved two major results: first, Co substituting Zn atoms significantly downward shifts by about 400 meV the Fermi level, which is close to the conduction band in the as-grown n-type ZnO. Second, a thoroughly novel result, Co substitution strongly reduces the absorption of negative oxygen species (NOS) at the ZnO surface. These two experimental findings are fully explained by a phenomenological model assuming the formation of Co-defect (Co-D) complexes that induce the appearance of an unoccupied impurity band in the ZnO energy gap. NOS play a central role in both the operating principles of UV photodetectors and applications in nanomedicine. Thus, the inhibiting effect of Co-D complexes on NOS formation has many applicative implications since it suggests that defect-engineering procedures might be devised for realizing nano-patterned Co-doped ZnO surfaces with regions showing different surface properties.

Experiments suggest that atomic H neutralizes the effects of N (i.e., it recovers the host energy gap) without inducing any band-filling effect (i.e., without behaving as a shallow donor) in the GaAsN alloy, while the vice versa is true in the InAsN one. Moreover, theoretical results on H in GaAsN contradict some experiments. These facts motivated the present study, where the role of N-H complexes has been investigated by performing density functional theory-Heyd-Scuseria-Emzerhof calculations. Present results confirm and explain the H properties; N neutralization is certain only in GaAsN, while H behaves as a shallow donor only in InAsN. They also show that, despite an identical geometry, single-H complexes neutralize the N effects in GaAsN, not in InAsN. This result is accounted for by a simple model showing that: (i) band gap recovery is directly related to a recovery of the atomic charge of the Ga/In cations neighboring the N atom in a N-H complex and (ii) a larger extent of charge recovery is found for the Ga cations with respect to the In ones, consistently with an electronegativity larger for Ga than for In. Remarkably, the same reason is at the ground of the different H behavior, deep versus shallow, in the two GaAsN and InAsN alloys.

By combining ab initio density functional theory (DFT) and time-dependent density functional perturbation theory (TDDFPT) methods, we investigate the structural, electronic and optical properties of a zinc phthalocyanine (ZnPc) molecule interacting with the zinc oxide (ZnO) wurtzite (10-10) surface. Our results reveal the existence of a strong molecule-surface coupling whose major effect is the appearance of a new unoccupied electronic level, deriving from an intimate mixing of ZnPc and ZnO electronic states and strategically located within the ZnO conduction band and below the ZnPc LUMO. This level induces appreciable changes in the ZnPc absorption spectrum and is expected to significantly favor a molecule-to-surface transfer of photo-excited electrons, a key process in the functioning of hybrid photovoltaic devices. The molecule-surface interactions are also characterized by significant van der Waals forces and by the formation of molecule-surface chemical bonds, thus resulting in appreciable molecular adhesion to the surface.

Substitution of constituent atoms and/or changes of crystal structure are routinely used to tailor the fundamental properties of a semiconductor. Here, it is shown that such a tailoring can also be realized thanks to a novel hydrogen effect. Four hydrogen atoms can screen the effect the crystal potential has on a constituent cation, thus generating a solitary cation: an effectively isolated impurity, so chemically different from the unscreened constituent cations that it strongly perturbs the electronic properties of the material by increasing its fundamental band-gap energy. Such a hydrogen-induced screening effect is removed by thermal treatments, thus permitting reversible modifications of both the "crystal chemistry" and material's properties. This phenomenon, observed in InN and other topical nitrides, should permit the development of a new class of materials as well as the fabrication of photonic devices and optical integrated circuits with distinct, tailor-made r egions emitting or absorbing light, all integrated onto a monolithic semiconductor structure. Fundamental properties of nitrides are changed through deep modifications of the chemistry of their constituent atoms induced by atomic hydrogen. In InN, for example, an In cation is isolated from its crystalline environment by the cooperative action of four hydrogen atoms that locally screen the crystal potential, thus generating a "solitary cation" (In) that remarkably increases the InN fundamental band-gap energy.

The structural, electronic, and optical properties of a hybrid interface formed by zinc phthalocyanine (ZnPc) molecules adsorbed on the (101?0) zinc oxide (ZnO) surface have been investigated by using ab initio and model potential theoretical methods. In particular, the attention has been focused on the effects of molecular assembling on the interface properties by considering cofacial and planar molecular aggregates on the surface. Present results show that planar aggregations provide a remarkable molecule-to-surface electronic coupling which can favor electron injection toward the substrate. Furthermore, we predict a blue shift of absorption bands in the case of cofacial aggregation and a red shift in the case of nanostructured planar J-stripes, which are in agreement with previous phenomenological models and give a firm theoretical support to observed relationships between red shift and molecular assembling. All together, present results indicate that structural and electronic properties can be achieved in ZnPc-sensitized ZnO surfaces of high potential interest for improving the efficiency of different kinds of hybrid photovoltaic cells.

The structural and electronic properties of oxygen vacancies (VOx) and titanium interstitials (Ti(i)) in the bulk of the rutile and anatase forms of TiO2 have been investigated with LSD-GGA+U ab initio simulations. In particular, formation energies of the charged and neutral forms of the VOx and Ti(i) defects as well as the corresponding vertical and thermodynamic transition levels have been estimated. The achieved results can reconcile the apparent inconsistency of experimentally observed deep donor levels with the n-type conductivity observed in reduced TiO2. They show indeed that both defects give rise to vertical transition levels about 1 eV below the conduction band (CB), in agreement with experimental measures, and to thermodynamic transition levels close to the CB. That is, these defects behave as deep donors, when looking at vertical transitions, and as shallow donors, when the effects of the structural relaxations are taken into account. A major part of the explanation of this behavior is played by the polaron-like character of the defect states, which was already noted, but not deepened, in literature. Finally, it is shown that the application of the U correction to both Ti and O species gives qualitatively similar results, but with a better agreement to experimental findings, with respect to the application to Ti only. The former approach gives pretty similar results, for both rutile and anatase bulk properties, to those coming from HSE hybrid functional calculations.

The equilibrium sites of atomic hydrogen and muonium in both doped and undoped elemental semiconductors, as investigated by means of several theoretical methods and experimental techniques, are outlined. A particular emphasis is given to the undoped c-Si results. Two controversial points regarding the H and muonium equilibrium sites have been carefully discussed. The bond centered model has been favored with respect to the vacancy associated one. The T site quite reasonably results to be a metastable site and the most favored candidate for the normal muonium center.

The structure and thermodynamics of some gas phase molecular reactants [As 2 , As 4 , Ga(CH 3 ) 3 ] used in MBE (molecular beam epitaxy) and MO-MBE (metal-organic molecular beam epitaxy) has been studied by Hartree-Fock-Roothaan molecular-orbitals linear-combination-of-atomic-orbitals self-consistent-field (HFR-MO-LCAO-SCF) ab initio methods. A first attempt to simulate the sticking of a gallium atom at the (100) surface of GaAs has also been made.

The energetics of bond reconstruction at the core of a 90° partial dislocation in silicon has been studied by means of HFR-MO-LCAO-SCF computations on Si5H10, Si9H18 and SilOH18 model molecular clusters’. These studies show the reconstructed geometry to be the most favourable one.

The results of detailed theoretical investigations of the properties of atomic and diatomic H in GaAs were analyzed with the effort to give a unified picture of the H behavior in this semiconductor. All calculations were performed in the psuedopotential density-functional framework using a supercell approach. The authors studied both shallow impurities (Si and C) and deep points defects (As antisite and Ga vacancy). Generally, a simple scheme may be applied in order to describe the H interaction with shallow impurities, where a key role is played by the amphoteric character of H. More complex mechanisms are involved in the deep impurity case that are related to new, interesting effects of H incorporation in GaAs. 39 refs., 14 figs., 4 tabs.