Spectroscopic identification of the chemical interplay between defects and dopants in Al-doped ZnO.

The conduction and optoelectronic properties of transparent conductive oxides can be largely modified by intentional inclusion of dopants over a very large range of concentrations. However, the simultaneous presence of structural defects results in an unpredictable complexity that prevents a clear identification of chemical and structural properties of the final samples. By exploiting the unique chemical sensitivity of Hard X-ray Photoelectron Spectra and Near Edge X-ray Absorption Fine Structure in combination with Density Functional Theory, we determine the contribution to the spectroscopic response of defects in Al-doped ZnO films. Satellite peaks in O1s and modifications at the O K-edge allow the determination of the presence of H embedded in ZnO and the very low concentration of Zn vacancies and O interstitials in undoped ZnO. Contributions coming from substitutional and (above the solubility limit) interstitial Al atoms have been clearly identified and have been related to changes in the oxide stoichiometry and increased oxygen coordination, together with small lattice distortions. In this way defects and doping in oxide films can be controlled, in order to tune their properties and improve their performances.

Zn vacancy induced green luminescence on non-polar surfaces in ZnO nanostructures.

Although generally ascribed to the presence of defects, an ultimate assignment of the different contributions to the emission spectrum in terms of surface states and deep levels in ZnO nanostructures is still lacking. In this work we unambiguously give first evidence that zinc vacancies at the (1010) nonpolar surfaces are responsible for the green luminescence of ZnO nanostructures. The result is obtained by performing an exhaustive comparison between spatially resolved cathodoluminescence spectroscopy and imaging and ab initio simulations. Our findings are crucial to control undesired recombinations in nanostructured devices.

Piezoelectric properties of zinc oxide nanowires: an ab initio study.

Nanowires made of materials with non-centrosymmetric crystal structures are expected to be ideal building blocks for self-powered nanodevices due to their piezoelectric properties, yet a controversial explanation of the effective operational mechanisms and size effects still delays their real exploitation. To solve this controversy, we propose a methodology based on DFT calculations of the response of nanostructures to external deformations that allows us to distinguish between the different (bulk and surface) contributions: we apply this scheme to evaluate the piezoelectric properties of ZnO [0001] nanowires, with a diameter up to 2.3 nm. Our results reveal that, while surface and confinement effects are negligible, effective strain energies, and thus the nanowire mechanical response, are dependent on size. Our unified approach allows for a proper definition of piezoelectric coefficients for nanostructures, and explains in a rigorous way the reason why nanowires are found to be more sensitive to mechanical deformation than the corresponding bulk material.

Optoelectronic properties of natural cyanin dyes.

An integrated theoretical/experimental study of the natural cyanin dye is presented in terms of its structural and optoelectronic properties for different gas-phase and prototypical device configurations. Our microscopic analysis reveals the impact of hydration and hydroxylation reactions, as well as of the attached sugar, on ground and optically excited states, and it illustrates the visible-light harvesting capability of the dye. Our optical experiments at different and controlled pH concentrations allow for a direct comparison with theoretical results. We analyze the many different contributions to photocurrent of the various portions of a prototypical device and, as a proof of principle, we propose the addition of specific ligands to control the increase of the photocurrent yield in the cyanin-based electrochemical device.

Hydrogen-induced surface metallization of beta-SiC(100)-(3x2) revisited by density functional theory calculations.

Recent experiments on the silicon terminated (3 x 2)-SiC(100) surface indicated an unexpected metallic character upon hydrogen adsorption. This effect was attributed to the bonding of hydrogen to a row of Si atoms and to the stabilization of a neighboring dangling bond row. Here, on the basis of density-functional calculations, we show that multiple-layer adsorption of H at the reconstructed surface is compatible with a different geometry: in addition to saturating the topmost Si dangling bonds, H atoms are adsorbed at rather unusual sites, i.e., stable bridge positions above third-layer Si dimers. The results thus suggest an alternative interpretation for the electronic structure of the metallic surface.

Ab initio simulations of homoepitaxial SiC growth.

We present first-principle calculations on the initial stages of SiC homoepitaxial growth on the beta-SiC(111)-(sqrt[3]xsqrt[3]) surface. We show that the nonstoichiometric reconstruction plays a relevant role in favoring the attainment of high-quality films. The motivation is twofold: On one hand, we find that the reconstruction controls the kinetics of adatom incorporation; on the other hand, we observe that the energy gain upon surface stability can induce the reorganization of the deposited material into a crystalline structure, thus revealing that a surface-driven mechanism is able to stabilize defect-free layer deposition on Si-rich surfaces.