Alessandra Catellani

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.

Anchor group versus conjugation: toward the gap-state engineering of functionalized ZnO(1010) surface for optoelectronic applications.

Molecular sensitization of the single-crystal ZnO (1010) surface through absorption of the catechol chromophore is investigated by means of density functional approaches. The resulting type II staggered interface is recovered in agreement with experiments, and its origin is traced back to the presence of molecular-related states in the gap of metal-oxide electronic structure. A systematic analysis carried out for further catecholate adsorbates allows us to identify the basic mechanisms that dictate the energy position of the gap states. The peculiar level alignment is demonstrated to be originated from the simultaneous interplay among the specific anchoring group, the backbone conjugation, and the lateral functional groups. The picture derived from our results provides efficient strategies for tuning the lineup between molecular and oxide states in hybrid interfaces with potential impact for ZnO-based optoelectronic applications.

Reconstruction and Thermal Stability of the Cubic SiC (001) Surfaces.

The (001) surfaces of cubic SiC were investigated with ab-initio molecular dynamics simulations. We show that C-terminated surfaces can have different c(2x2) and p(2x1) reconstructions, depending on preparation conditions and thermal treatment, and we suggest experimental probes to identify the various reconstructed geometries. Furthermore we show that Si-terminated surfaces exhibit a p(2x1) reconstruction at T=0, whereas above room temperature they oscillate between a dimer row and an ideal geometry below 500 K, and sample several patterns including a c(4x2) above 500 K.

Hydroxyl-Rich β-Sheet Adhesion to the Gold Surface in Water by First-Principle Simulations

Proteins able to recognize inorganic surfaces are of paramount importance for living organisms. Mimicking nature, surface-recognizing proteins and peptides have also been man-made by combinatorial biochemistry. However, to date the recognition mechanisms remain elusive, and the underlying physicochemical principles are still unknown. Selectivity of gold-binding peptides (cysteine-free and rich in hydroxyl amino acids) is particularly puzzling, since the most relevant gold surface, Au(111), is known to be chemically inert and atomically flat. Using atomistic first-principle simulations we show that weak chemical interactions of dative-bond character confer to a prototype secondary structure (an antiparallel beta-sheet made of hydroxyl amino acids) and its hydration layer the capability of discriminating among gold surface sites. Our results highlight the unexpected role of hydration water in this process, suggesting that hydrophilic amino acids and their hydration shell cooperate to contribute to protein-gold surface recognition.

Optoelectronic properties of Al:ZnO: Critical dosage for an optimal transparent conductive oxide

We study the effects of aluminum doping on the electronic and optical properties of ZnO, via density functional simulations. We discuss the bandstructure and absorption properties of Al:ZnO as a function of the dopant concentration, and compare with recent experimental data. Our results support the formation of a transparent conductive oxide compound up to an incorporation of Al of about 3% in substitutional Zn sites. We propose an explanation to the observed degradation of conductivity in terms of interstitial defects expected to occur at high doping concentrations, beyond the Al solubility limit.

Theoretical studies of silicon carbide surfaces

Recent results on ab initio calculations of electronic and structural properties of SiC surfaces are reviewed. Particular attention is given to cubic SiC surfaces, and to the (001) plane of the cubic polytype, which is still open to controversy from both the theoretical and experimental side. Furthermore, new exciting evidence of self-aggregating low-dimensional structures has been reported, which is well characterized by first-principles simulations. Results on both stoichiometric and non-stoichiometric surface reconstructions for the cubic and hexagonal polytypes are presented, as well as some recent calculations regarding adsorption and initial stages of nitride and oxide growth on crystalline SiC.

Towards SiC surface functionalization: an ab initio study

We present a microscopic model of the interaction and adsorption mechanism of simple organic molecules on SiC surfaces as obtained from ab initio molecular-dynamics simulations. Our results open the way to functionalization of silicon carbide, a leading candidate material for biocompatible devices.

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 Study of Misfit Dislocations at the SiC/Si(001) Interface

The high lattice mismatched SiC/Si(001) interface was investigated by means of combined classical and ab initio molecular dynamics. Among the several configurations analyzed, a dislocation network pinned at the interface was found to be the most efficient mechanism for strain relief. A detailed description of the dislocation core is given, and the related electronic properties are discussed for the most stable geometry: we found interface states localized in the gap that may be a source of failure of electronic devices.

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.