A simple hemi-squaraine dye (CT1) has been studied as a TiO2 sensitizer for application in dye sensitized solar cells (DSCs) by means of a combined experimental and theoretical investigation. This molecule is a prototype dye presenting an innovative anchoring group: the squaric acid moiety. Ab initio calculations based on Density Functional Theory (DFT) predict that this acid spontaneously deprotonates at the anatase (101) surface forming chemical bonds that are stronger than the ones formed by other linkers (e.g. cathecol and isonicotinic acid). Moreover an analysis of the electronic structure of the hybrid interface reveals the formation of a type II heterostructure ensuring adiabatic electron transfer from the molecule to the oxide. DSCs containing hemi-squaraine dyes were assembled, characterized and their performances compared to state of the art cells. Experimental results (large incident photon-to-electron conversion efficiency and an efficiency of 3.54%) confirmed the theoretical prediction that even a simple hemi-squaraine is an effective sensitizer for TiO2. Our study paves the way to the design of more efficient sensitizers based on a squaric acid linker and specifically engineered to absorb light in a larger part of the visible range.

A ZnO nanowires memristor switching between multiple resistance states. The conductivity of nanowires is tuned by changes in ZnO surface states that are induced at ZnO/polymer interfaces by redox reactions guided by an external bias.

In this work we present a theoretical investigation of the attachment of catechol and isonicotinic acid to the rutile-TiO(2)(110) surface. These molecules can be considered as prototypical dyes for use in Grätzel type dye sensitised solar cells (DSCs) and are often employed as anchoring groups in both organic and organo-metallic sensitisers of TiO(2). Our study focuses on determining the lowest energy adsorption mode and discussing the electronic properties of the resultant hybrid interface by means of density functional theory (DFT) calculations using the hybrid exchange (B3LYP) functional. We find that both molecules adsorb dissociatively at the TiO(2) surface giving a type II (staggered) heterojunction. Compared to isonicotinic acid, catechol, due to the greater hybridisation of its molecular orbitals with the states of the substrate, is seen to enhance performance when employed as an anchoring group in dye sensitised solar cells.

Graphene oxide (GO) is a versatile 2D material whose properties can be tuned by changing the type and concentration of oxygen-containing functional groups attached to its surface. However, a detailed knowledge of the dependence of the chemo/physical features of this material on its chemical composition is largely unknown. We combine classical molecular dynamics and density functional theory simulations to predict the structural and electronic properties of GO at low degree of oxidation and suggest a revision of the Lerf-Klinowski model. We find that layer deformation is larger for samples containing high concentrations of epoxy groups and that correspondingly the band gap increases. Targeted chemical modification of the GO surface appears to be an effective route to tailor the electronic properties of the monolayer for given applications. Our simulations also show that the chemical shift of the C-1s XPS peak allows one to unambiguously characterize GO composition, resolving the peak attribution  uncertainty often encountered in experiments.

Resistive switching memory operation is generally described in terms of formation and rupture of a conductive filament connecting two metal electrodes. Although this model was reported for several device types, its applicability is not guaranteed to all of them. On the basis of density functional theory calculations, we propose a novel switching mechanism suitable to nanowire-based resistive switching memories. For thick devices in particular, the current is highly unlikely to flow through a metallic filament connecting the electrodes. We demonstrate that in the case of ZnO nanowires metal adatoms, spread on the nanowire surface, locally dope the insulating oxide allowing surface conductance even for small metal concentrations.

The interface between the semiconductor and the dye is one of the fundamental parameters that directly impact the dye sensitized solar cell (DSSC) performance. In this paper the coupling between a prototype organic sensitizer and inorganic oxides is studied by a combined experimental and theoretical approach. In particular, the interface properties of the hemi-squaraine molecule (CT1) anchored onto the TiO 2 and ZnO surfaces are investigated. Experimental results evidence that, beside the comparable surface coverage of the dye on both the oxides and the very fast chemisorption kinetics, TiO 2 photoanodes give much larger solar cell efficiency values. Theoretical calculations based on density functional theory and time dependent density functional theory show that this difference is due to the stronger electronic coupling occurring between the CT1 anchoring group (the squaric acid) and the TiO 2 surface. In this case, chemisorption induces a larger red-shift in the dye optical absorption which extends the range of harvestable frequencies if compared to the isolated dye. Moreover, the CT1/TiO 2 system is characterized by an extended electron delocalization of the lowest unoccupied molecular orbital involving the substrate cations, which gives rise to easier electron injection, as confirmed by the incident photon-to-electron conversion efficiency measurements. This study demonstrates that a given dye anchoring group, although being able to form strong chemical bonds with different oxide surfaces, may be responsible for very different DSSCs performances depending on the electronic rearrangement that it undergoes upon attachment to the substrate.

Comparative analysis of electronic structure and optical properties of crystalline and amorphous silicon nitrides J. Appl. Phys. 106, 053717 (2009); 10.1063/1.3213359Electron cyclotron resonance deposition, structure, and properties of oxygen incorporated hydrogenated diamondlike amorphous carbon films

Copper (Cu)‐based materials for efficient CO2 conversion to adjustable syngas are reported. We employ an energy‐efficient microwave‐assisted route to synthesize copper/copper oxides with different structures and morphologies. The initial oxidation state of the copper on the surface significantly influences the performance of the materials in the CO2 reduction reaction (CO2RR). On the as‐prepared material surface, a high content of Cu2O improves the selectivity toward CO and consequently the catalyzed CO2RR produce a syngas with a low H2/CO ratio, while the CuO performs poorly. In particular, Cu2O particles with a cubic shape show the highest selectivity for the CO formation. The superior catalytic activity of the Cu2O cubes for the CO2RR is attributed to the surface roughening under potential, which offers abundant electrochemically active sites and facilitates mass transport.