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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.

In dilute nitride InyGa1-yAs1-xNx alloys, a spatially controlled tuning of the energy gap can be realized by combining the introduction of N atoms-inducing a significant reduction of this parameter-with that of hydrogen atoms, which neutralize the effect of N. In these alloys, hydrogen forms N-H complexes in both Ga-rich and In-rich N environments. Here, photoluminescence measurements and thermal annealing treatments show that, surprisingly, N neutralization by H is significantly inhibited when the number of In-N bonds increases. Density functional theory calculations account for this result and reveal an original, physical phenomenon: only in the In-rich N environment, the InyGa1-yAs host matrix exerts a selective action on the N-H complexes by hindering the formation of the complexes more effective in the N passivation. This thoroughly overturns the usual perspective of defect-engineering by proposing a novel paradigm where a major role pertains to the defect-surrounding matrix.

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.

Titanium dioxide and TiO2-based materials are widely used in environmental- and energy-related applications like photocatalysis and photovoltaics, where they are usually employed as nanocrystals or nanostructures. The present contribution is aimed at filling the gap between the vast literature devoted to the simulation of electronic and photochemical properties of TiO2 crystals and surfaces, and the few theoretical studies of photoactivated processes involving instead TiO2 nanostructures. More specifically, photocatalytic and photovoltaic processes promoted by model TiO2 nanoparticles (NPs) have been investigated by using ab initio simulations based on the U-corrected density functional theory, and on the time-dependent density functional perturbation theory. We focus on well-investigated processes like the photogeneration of charge carriers in UV-irradiated NPs, the photoreduction of dioxygen and photooxidation of methanol catalyzed by NPs, and the splitting of photogenerated charge carriers occurring at a model NP-dye interface. Our results provide indications on some crucial points of such processes, showing that (i) excited charge carriers photogenerated within bare NPs are preferentially trapped as small polarons at surface undercoordinated Ti3+ and O(-)sites; (ii) dioxygen and methanol are efficient scavengers of such electrons and holes, respectively, and trigger surface redox processes likely involving proton coupled electron transfer (PCET) steps; (iii) dye-sensitized NPs are instead characterized by low-energy excited states in which electrons and holes photogenerated within the dye are efficiently split by the TiO2/dye junction, thus confirming the expected spontaneous formation of charge-separated states in TiO2-based photovoltaic devices; (iv) cost-effective theoretical tools can be fruitfully employed to obtain reliable predictions of the photocatalytic properties of nanostructured metal oxides and of the photovoltaic properties of hybrid organic photovoltaic devices.

A number of studies have investigated the properties of monomeric and double-decker phthalocyanines (Pcs) adsorbed on metal surfaces, in view of applications in spintronics devices. In a combined experimental and theoretical study, we consider here a different member of the Pcs family, the (RuPc)(2) dimer, whose structure is characterized by two paired up magnetic centers embedded in a double-decker architecture. For (RuPc)(2) on Ag(111), we show that this architecture works as a preserving cage by shielding the Ru-Ru pair from a direct interaction with the surface atoms. In fact, while noticeable surface-to-molecule charge transfer occurs with the ensuing quenching of the molecular magnetic moment, such phenomena occur here in the absence of a direct Ru-Ag coupling or structural rearrangement, at variance with other Pcs and thanks to the above shielding effect. These unique properties of the (RuPc)(2) architecture are expected to permit an easy control of the surface-to-molecule charge-transfer process as well as of the molecular magnetic properties, thus making the (RuPc)(2) dimer a significant paradigm for innovative "cage" structures as well as a promising candidate for applications in spintronics nano or single-molecule devices.

In InN, a genuine band gap opening observed after hydrogenation has been explained by means of the "solitary cation" model, a multi-H complex in which the central cation, In*, is fully separated from the structure [Pettinari et al., Adv. Funct. Mater. 25, 5353 (2015)]. Similar effects of H on the host band gap have been observed in In-rich In1-xGaxN alloys. Paying attention to these materials, we have theoretically investigated the In* properties against three kinds of disorder, structural, compositional, and configurational, all of them possibly occurring in In1-xGaxN alloys. As a first major result we have found that a same, general solitary-cation model and mechanism explain the effects of hydrogenation on the electronic properties of both InN and In-rich In1-xGaxN alloys. Even more interestingly, in these alloys, both the energetics of the In* solitary cations and their effects on the band gap result to be thoroughly independent of their atomic neighborhood, in particular, of the number and spatial distribution of their cation neighbors. Significantly, this implies that band-gap opening effects can be safely predicted in whatever hydrogenated In-rich nitride alloy containing different In companions (e.g., B, Al, or Ga) as well as in InN-containing, unconventional compounds (e.g., ZnO-InN), thus offering novel opportunities for material engineering.

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.

The effects of low-energy irradiation by light ions (H and He) on the properties of In-rich InxGa1−xN alloys are investigated by optical and structural techniques. H-irradiation gives rise to a remarkable blue-shift of light emission and absorption edge energies. X-ray absorption measurements and first-principle calculations address the microscopic origin of these effects.

The absorption properties of p-extended macrocycles such as porphyrins and phthalocyanines have been intensively applied to the light -harvesting process related to organic photovoltaics, with particular focus on Dye-Sensitized Solar Cells1. Even if in the last few years new emerging materials have shown very promising results in terms of efficiencies, it is still extremely important to study and test new dyes because of their potential application in building-integrated-photovoltaics. In particular, no blue or green dye have yet shown an outstanding stability once exposed to thermal stress and UV irradiation into the device. Phthalocyanines are of course great candidates because of their great chemical stability and high molar extinction coefficients (e> 105) and these properties justify the intensive study still carried on these molecules. Amongst them an interesting sterically hindered push-pullstructure, having a record efficiency value of 6.4%2 has been prepared and published. It is well known that the metal centers of these macrocyclic compounds can affect their absorption peaks, their HOMO/LUMO energy values as well as their possible charge transfer properties in a DSSC device. In this context we have chosen to investigate the performances of dyes possessing the same molecular structure but different central metal ions, and in particular we focused on copper derivatives, because of their extremely high stability and color intensity. We have synthesized the 9(10),16(17),23(24)-tri-ter-butyl-2-[acetynyl-(4-carboxy)phenyl]phthalocyaninato copper complex and its related free base and then investigated their optical and electrochemical properties. Very surprisingly, even for the copper derivative we have measured a considerable fluorescence decay that should be in principle almost completely depleted due to the spin-orbital interaction arising from its d9 electronic configuration. The photovoltaic performances as well as IPCE measurements will be presented for both the molecules and compared with those of the zinc derivative4 that has recently reached efficiency values around 2%.

At variance with the mechanisms responsible for the passivation of typical shallow dopants in GaAs, where H bonding with the impurity or to its neighbors permits a recovery of the "natural" chemical valence of the impurity itself, in the case of the N isoelectronic impurity in GaAs, H induces no changes in the N chemical valence. It breaks, instead, two Ga-N bonds to form complexes sharing a common, quite stable NH2 core characterized by a local C2v-like symmetry, thus strongly perturbing the interactions of N with its four Ga neighbors. A theoretical picture is presented here, where the N effects on the GaAs electronic properties, as well as their passivation upon hydrogenation, are closely related to the local N-Ga interactions and to the perturbations induced by H bonding on the N environment. The formation of multi-H complexes with N, predicted by theory, is also discussed togegher with the support given to their existence by recent experimental findings.

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 investigation of the electrical properties of Co-doped ZnO thin films provides two unexpected results: a decrease of the electrical conductivity and the contemporary occurrence of a reduction of conductivity and of an enhancement of ferromagnetic order. The former result is surprising since Zn atoms are replaced with iso-valent Co atoms. The latter finding questions previously suggested beneficial effects of n-type doping on the ZnO:Co magnetic behavior. While morphological and structural characterization permits us to exclude an influence of the morphology and of the presence of Co metal or Co-oxide phases on present experimental findings, with the aid of first-principles electronic structure calculations, we propose a qualitative picture which can explain in a coherent way both the changes in conductivity and ferromagnetic behavior ensuing from the Co doping and the above-mentioned, beneficial effects of n-type doping. This journal is © The Royal Society of Chemistry.

A new unsymmetrical zinc phthalocyanine sensitizer has been synthesised. The anchoring of the molecule to nanocrystalline TiO(2) films is realised by a carboxylic group connected to a phenyl ethynyl moiety. Density Functional Theory (DFT) calculations show significant and positive effects of such a functionalization. Electron injection into the semiconductor and photocurrent generation in DSSC are also presented.

Multi-component organic chromophores having intense visible absorption characteristics covering large part of solar radiation are currently an object of much attention in view of their exploitation as artificial light-harvesting antenna. In particular those containing the phthalocyanine (Pc) and porphyrin (P) macrocyclic ligands have been recently studied being interesting for the peculiar photoinduced energy transfer processes[1] and being also attractive for their potential use in the field of molecular photovoltaics, artificial photosynthesis and molecular electronic applications.[2-4] Several strategies can be used to link each other the two different subunits to obtain a hetero phthalocyanine-porphyrin dyads, resulting in different geometries and properties. It is possible for example to link the different moieties through a conjugated or not spacer between the peripheral positions of one macrocycle[5] toward either the periphery or the central metal of the other macrocycle.[1] Moreover single atom (O, N, C) bridged systems as μ-oxo, μ-nitrido and μ-carbido can also allow a combination, via metal centres, leading to homo/heterometallic and homo/heteroleptic systems.[6] To the aim recalled above, it should be important that the orbital interactions could promote an electronic communication along the stack thereby optical absorption will change and, in the case of complementary chromophores, extend. As other analogues mixed-ligand porphyrin/phthalocyanine bridged systems are expected to mutate the electronic configuration and to extend the visible absorption to the near IR wavelength. Among the μ-nitrido species, the mixed-ligand tetraphenylporphyrin-phthalocyanine μ-nitrido complex of iron was synthesised and fully characterised as solid material by Ercolani et al.;[7,8] it showed a good thermal stability (> 300°C) and an interesting electronic and magnetic exchange between the iron centres trough the linear Fe-N-Fe bond system. Unfortunately its optical (UV-vis) spectrum was not reported likely because of its very scarce solubility in non-donor solvent. Being this feature indispensable for the processability of these material in view of technological applications, we attempted to increase the solubility of such species and we present here preliminary investigations on the substituted analogous (μ-nitrido)[((tetraphenylporphyrinato) iron)-(tetra-tert-butylphthalocyaninato)iron], I.§

Nonplanar titanyl phthalocyanine (TiOPc) molecules, characterized by a central, highly dipolar Ti-O group, can offer further degrees of freedom in tailoring the properties of hybrid organic-inorganic structures. Here, we combine scanning tunneling microscopy and low-energy electron diffraction measurements with ab initio density functional theory calculations to investigate the interaction of TiOPc molecules with the Ag(100) surface. Isolated molecules are adsorbed with the macrocycle parallel to the surface in two different configurations: with the O atom pointing outward (UP configuration) or with the O atom pointing toward the surface (DOWN configuration). A different interaction of UP and DOWN molecules with the surface can account for their different orientation on the surface as well as for the observation of marked chiral effects only for UP molecules. Self-assembled domains of randomly mixed UP and DOWN molecules form at the monolayer coverage, driven by a subtle interplay between the molecule-surface interaction, the intermolecular dipolar attraction, and the side interaction between adjacent molecules. Chiral patterns of isolated molecules, observed by STM, are transferred to these domains. Remarkably, theoretical calculations disclose an interfacial nature of such chiral properties. Finally, a metallic behavior observed for the monolayer disappears for TiOPc molecules in a second layer, which are electronically decoupled from the substrate.

The present work[1] combines atomistic simulations and experimental investigations to study heterojunction interfaces of hybrid polymer solar cells, with the aim to better understand and precisely predict their photovoltaic properties. A state-of-the-art theoretical multiscale approach rooted on ab initio calculations[2], has been used to model an original, hybrid ternary system based on a P3HT/Zinc Phthalocyanine (ZnPc)/ZnO interface[3,4], in which a ZnPc interlayer is applied to improve the performance of the hybrid interface. The theoretical findings have been then validated against the properties of concrete P3HT/ZnPc/ZnO planar heterojunction devices. Our theoretical predictions are in close agreement with the photovoltaic properties obtained in such a kind of P3HT/ZnPc/ZnO solar cells, indicating the strength of the present approach for modeling hybrid heterojunction interfaces. In detail, our jointly obtained theoretical and experimental results reveal that: (i) ZnPc molecules in direct contact with a ZnO surface insert new energy levels due to a strong ZnPc/ZnO coupling[3] (ii) electron injection from these new energy levels of ZnPc into ZnO is highly efficient, (iii) the ZnPc/ZnO coupling strongly influences the energy levels of the ZnO and P3HT leading to a reduction of the open circuit voltage, and (iv) charge carrier recombination at the P3HT/ZnO interface is reduced by the ZnPc interlayer. In practice, the intercalation of ZnPc leads to an increase in photocurrent as well as to an overall increase in power conversion of the planar hybrid heterojunction by 50%, thus stressing the importance of interfacial engineering in hybrid solar cells. This work was developed as a collaboration between CNR-ISM Montelibretti (RM), CNR-IOM Cagliari, and CNRS-CINaM (Marseille,France) within the IIT-SEED Project POLYPHEMO funded by Italian Institute of Technology. References [1] G. Mattioli, et al.; Adv. Energy Mater., (2014) 1301694, DOI: 10.1002/aenm.201301694 [2] C. Melis, et al.; ACS NANO, 5, (2011) 9639 [3] G. Mattioli, et al.; J. Phys. Chem. C, 116 (2012) 15439 [4] C. Melis, et al.; J. Phys. Chem. C, 115 (2011) 18208

Several experimental and theoretical studies have related the peculiar mechanical properties of the macro-defect-free polymer−cement composites with the existence of a cross-linking of the polymer chains by Al or Ca ions coming from the hydration reactions of the cement. In the present study, the interaction of Ca ions with the −COO- groups of one or two poly(acrylic acid) (PAA) chains has been investigated with first-principles theoretical methods in order to achieve evidences of PAA cross-linking. The present results show that a stable CaO4 complex can be formed by a Ca ion and the O atoms belonging to the −COO- groups of two polymer chains. This implies the existence of a quite efficient cross-linking because a Ca ion becomes anchored to four points of two PAA chains. The stretching frequencies of several Ca−O and CO bonds have also been estimated. The achieved results suggest that indirect evidence of the PAA cross-linking could be obtained by infrared spectroscopy measurements.

The significant increases in flexural strength achieved in macro-defect-free cements have been attributed to a reduction in the density of defects in the materials as well as to chemical reactions occurring between the organic constituent, i.e., the poly(vinyl alcohol) (PVA) polymer, and inorganic ions coming from the hydration products of the cement. In particular, it has been suggested that PVA chains are partially linked together by Al ions. This phenomenon has been investigated here by studying the structure and the energetics of Al-PVA models with ab initio theoretical methods. The achieved results strongly support the existence of a cross-linking of PVA chains by Al ions, even in the presence of structural deformations of the PVA chains. The vibrational frequency of a stretching mode related to the O atoms bonded with Al has been also evaluated, which may represent a fingerprint for an experimental identification of cross-linking.

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 field of dilute magnetic semiconductors is gaining increasing interest for the potential technological applications in spintronic devices, in which the spin degree of freedom of the charge carriers can be used to enhance the capabilities of conventional electronics, combining semiconducting and magnetic properties. Among dilute magnetic semiconductors, compounds based on ZnO (Zn1-xCoxO and Zn1-xMnxO) are especially appealing since they exhibit ferromagnetism at room temperature and have low toxicity. Despite this great potential, the origin of ferromagnetism in dilute ferromagnetic oxides has been mysterious for years. In this present work, we investigated the local structure of ferromagnetic Zn1-xCoxO epilayers by coupling polarization-dependent x-ray absorption spectroscopy and ab initio density functional theory calculations of selected defect structures. This approach allowed us to perform a full quantitative investigation of the local environment of Co atoms and the three-dimensional structure of the defect complexes. We gave clear evidence of the presence of oxygen vacancies, located close to the Co atoms in a specific complex configuration [1]. We also established the upper concentration limit of metallic parasitic nanophases and their contribution to magnetism. Our results led to the conclusion that cobalt-oxygen vacancy complexes play an important role in originating the high temperature ferromagnetism of Zn1-xCoxO. These results provided significant contribution to the understanding of the origin of ferromagnetism in dilute ferromagnetic oxides.

The stability and magnetic properties of Fe clusters in the (Ga,Fe)N magnetic semiconductor is investigated by using first-principles density functional theory and local spin density+Hubbard U theoretical methods. The present results reveal the existence of ferrimagnetic clusters formed by three or four peripheral Fe atoms neighboring a central Fe atom acting as a robust magnetic anchoring point. These clusters have magnetic moments 2 or 3 times that of a single Fe atom and, when connected by sharing peripheral Fe atoms, can form stable, ordered magnetic regions where all of the central atoms are ferromagnetically coupled. The formation of these ferrimagnetic clusters is proposed here to be at the origin of the ferromagnetic behavior observed in (Ga,Fe)N samples showing chemical phase separation.

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.

X-ray absorption fine-structure (XAFS) measurements supported by {\em ab initio} computations within the density functional theory (DFT) are employed to systematically characterize Fe-doped as well as Fe and Si-co-doped films grown by metalorganic vapour phase epitaxy. The analysis of extended-XAFS data shows that depending on the growth conditions, Fe atoms either occupy Ga substitutional sites in GaN or precipitate in the form of $\epsilon$-Fe$_3$N nanocrystals, which are ferromagnetic and metallic according to the DFT results. Precipitation can be hampered by reducing the Fe content, or by increasing the growth rate or by co-doping with Si. The near-edge region of the XAFS spectra provides information on the Fe charge state and shows its partial reduction from Fe$^{+3}$ to Fe$^{+2}$ upon Si co-doping, in agreement with the Fe electronic configurations expected within various implementations of DFT., Comment: Minor changes: corrected references [3], [29] and [52]

Ab initio density functional calculations in the beyond-LSD (Local Spin Density), LSD + U approximation have been performed to investigate the properties of bridging hydroxyls (OHb) at the rutile (1 1 0) and anatase (1 0 1) surfaces as well as of bulk oxygen vacancies (VOx) and Ti–OH defects in the same two polymorphs. The aim of this study is to deepen our understanding of the properties of these defects both in rutile and anatase as well as to reveal similar behaviours of VOx and OH defects. In the case of rutile OHbs, a good agreement is found between present results and those of a recent STM study. Anatase OHbs show properties similar to those of rutile OHbs. On the contrary, a remarkable difference is found between the structural and electronic properties of bulk VOxs in rutile and anatase, which could explain the higher electron mobility and photocatalytic activity observed for the latter compound. Bulk Ti–OH defects are predicted to easily form through insertion of atomic H in TiO2 lattices. Moreover, in the case of rutile, these defects should present properties quite similar to those of VOxs. Finally, the results achieved here stress the importance of beyond-LSD methods when dealing with defects in TiO2.

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 structural, electronic, and vibrational properties of intermediates of the O(2) photoreduction at the (101) TiO(2) (anatase) surface have been investigated by performing ab initio density functional calculations. In detail, a recently proposed approach has been used where molecules on the surface are treated like surface defects. Thus, by applying theoretical methods generally used in the physics of semiconductors, we successfully estimate the location and donor/acceptor character of the electronic levels induced by an adsorbed molecule in the TiO(2) energy gap, both crucial for the surface-molecule charge-transfer processes, and investigate the formation and the properties of charged intermediates. The present approach permits a view of the O(2) photoreduction process through several facets, which elucidates the molecule-surface charge-transfer conditions and reveals the key role played by charged intermediates. A comparison of present results with those of a highly sensitive IR (infrared) spectroscopy study of intermediates of the O(2) photoreduction leads to a deeper understanding of this process and to revised vibrational-line assignments and reaction paths.

In the macro-defect-free cements (MDFs), a cross-linking of the polymer chains by Al or Ca ions coming from the hydration reactions of the cement paste may have crucial effects on the mechanical properties achieved by the final materials as well as on the material processing. In the present study, the cross-linking of poly(vinyl alcohol) (PVA) chains by Ca ions has been investigated with first-principles theoretical methods and compared with a previously investigated case of cross-linking of poly(acrylic acid) (PAA) chains. The achieved results show the existence of a Ca−PAA cross-linking that is significantly more efficient than that realized in the case of the Ca−PVA system. This result implies that the Ca−O bonds involved in the cross-linking of the PVA and PAA chains are affected by the different functional groups present in the two polymer chains and agrees with the improvement of the mechanical properties observed when the PVA polymer is replaced by the PAA polymer in Portland-cement-based MDFs. The...

Although it is well known that dilute species often significantly modify the properties of solids in which they are hosted, a basic question remains open: how critical are their local properties in determining the macroscopic ones of the host material? Here, we address this issue by taking N in dilute III-V-N alloys as a paradigmatic case and propose an original approach based on synergic use of x-ray spectroscopies (XANES and valence – to – core XES at the N K-edge), density functional theory simulations and hydrogen exposure as a tool to modify N local bonding. Our results disclose the key role of the local N – Ga interactions in determining both N-related band states and the macroscopic effects of dilute N on the GaAs energy gap. The success of our approach indicates a new way to connect local and macroscopic properties.

We present an investigation of the close relationship between chemical structure, physical properties and reactivity of the three nitrotoluene isomers: a joint experimental and theoretical study, based on X-ray photoelectron spectroscopy (XPS) measurements and ab initio calculations, addressing the complex interplay between the competing electron-donor and electron-acceptor effects of the nitro- and methyl-substituents on the chemical properties of the nitrotoluene isomers. As the main results of the investigation we: (i) point out that accurate ab initio calculations play a key role in the complete assignment of photoemission measurements, as well as in the estimate of proton affinities in the case of all the eligible sites; (ii) revisit, at a more quantitative level, textbook models based on inductive and resonant effects of different substituents of the aromatic ring, as well as on the hyper-conjugative connection of the methyl group to the π-conjugated system; (iii) provide an accurate analysis of correlation patterns between calculated proton affinities and core-ionization energies, which represent a powerful tool, capable of predicting site-specific reactivities of polysubstituted molecules in the case of electrophilic aromatic substitution reactions. © 2014 The Royal Society of Chemistry.

Ultraviolet photoelectron spectra of NH2–NH–C(S)SCH3, NH2–NCH3–C(S)SCH3, N(CH3)2–NH–C(S)SCH3, NHC6H5–NH–C(S)SCH3, N(C6H5)2–NH–C(S)SCH3, and [Ni{N(CH3)2–NC(–S)SCH3}2] have been recorded, and the ionisation energies related to the results of quantum-mechanical calculations. Assignments are made on the grounds of this comparison and by correlation with the parent molecules.

The energetics of bond reconstruction at the core of a 90° partial dislocation in diamond has been studied by means of HFR-MO-LCAO-SCF computations on C5H10 and C10H18 model molecular clusters. The results obtained agree with previous VFF investigations and they confirm that the reconstructed geometry is energetically favoured with respect to the unreconstructed one.

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.

ZnO is a well-known semiconducting material showing a wide bandgap and an n-type intrinsic behavior of high interest in applications such as transparent electronics, piezoelectricity, optoelectronics, and photovoltaics. This semiconductor becomes even more attractive when doped with a few atomic percent of a transition metal. Indeed, e.g., the introduction of substitutional Co atoms in ZnO (ZCO) induces the appearance of room temperature ferromagnetism (RT-FM) and magneto-optical effects, making this material one of the most important representatives of so-called dilute magnetic semiconductors (DMSs). In the present review, we discuss the magnetic and magneto-optical properties of Co-doped ZnO thin films by considering also the significant improvements in the properties induced by post-growth irradiation with atomic hydrogen. We also show how all of these properties can be accounted for by a theoretical model based on the formation of Co-VO (oxygen vacancy) complexes and the concurrent presence of shallow donor defects, thus giving a sound support to this model to explain the RT-FM in ZCO DMSs.