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Structure, electronic properties and formation mechanisms of Hydrogen-Nitrogen complexes in GaP(y)N(1-y) alloys
No full textPhotocatalytic and Photovoltaic Properties of TiO2 Nanoparticles Investigated by Ab Initio Simulations
No full textTitanium 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
Protonation States in a Cobalt-Oxide Catalyst for Water Oxidation: Fine Comparison of Ab Initio Molecular Dynamics and X-ray Absorption Spectroscopy Results
No full textAb initio molecular dynamics simulations of a recently proposed
cobalt-based catalyst for water oxidation provide insight into the
properties of protons at the water/oxide interface. Calculations
and X-ray absorption spectroscopy data indicate a cubane-like
structure of the catalyst, support the occurrence of protonated
l2-O atoms, suggest deprotonated l3-O atoms and the presence
of sites promoting low-barrier hydrogen bonds
Hydrogen-nitrogen Complexes in GaAsN Alloys: the Role of Doping in the Formation of Mono- and Di-hydrogen Complexes
No full textTheory of hydrogen complexes in Mn(x)Ga(1-x)As dilute magnetic semiconductors
No full textAtomic hydrogen diffuses in semiconductor lattices and binds to impurities by forming complexes that can lead to a full neutralization of the impurity effects. In the present paper, the structural, vibrational, electronic and magnetic properties of complexes formed by H in the Mn xGa1-xAs (x=0.03) dilute magnetic semiconductor have been investigated by using first-principles DFT-LSD and LDA+U theoretical methods. The results account for recent experimental findings showing a H passivation of the electronic and magnetic properties of Mn in GaAs. Moreover, they show that electron correlation has crucial effects on the properties of H-Mn complexes
Effects of cobalt substitution on ZnO surface reactivity and electronic structure
No full textWe 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
Hybrid Zinc Phthalocyanine/Zinc Oxide System for Photovoltaic devices: a DFT and TD-DFPT Theoretical Investigation
No full textTransport mechanisms in Co-doped ZnO (ZCO) and H-irradiated ZCO polycrystalline thin films
No full textIn the present study, the electrical resistivity (ρ) as a function of the temperature (T) has been measured in polycrystalline ZnO, Co-doped ZnO (ZCO) and H irradiated ZCO (HZCO) samples, in the 300-20 K range. The achieved results show impressive effects of Co doping and H irradiation on the ZnO transport properties. The Co dopant increases the ZnO resistivity at highT(HT), whereas it has an opposite effect at lowT(LT). H balances the Co effects by neutralizing theρincrease at HT and strengthening its decrease at LT. A careful analysis of theρdata permits to identify two different thermally activated processes as those governing the charge transport in the three materials at HT and LT, respectively. The occurrence of such processes has been fully explained in terms of a previously proposed model based on an acceptor impurity band, induced by the formation of Co-oxygen vacancy complexes, as well as known effects produced by H on the ZnO properties. The same analysis shows that both Co and H reduce the effects of grain boundaries on the transport processes. The high conductivity of HZCO in the wholeT-range and its low noise level resulting from electric noise spectroscopy make this material a very interesting one for technological applications
Giant magneto-optical response in H+ irradiated Zn1−xCoxO thin films
Get PDFRemarkable improvements in the magneto-optical properties of Zn1-xCoxO thin films were achieved by post-growth hydrogen irradiation at temperatures of 400 degrees C. Hydrogen incorporation increases the magnetic circular dichroism signal resulting in large values of ellipticity and Faraday rotation at room temperature. The hysteretic behavior of the magneto-optical field dependent loops at different sub-bandgap wavelengths indicates an intrinsic ferromagnetic regime. A giant Faraday rotation of 3000 deg cm(-1) was observed at 400 nm. Optical, structural and microstructural characterizations pointed out the Zn substitution by Co, ruling out the presence of noticeable Co-related secondary phases down to the nanoscale. The increased values of saturation magnetization, ellipticity and Faraday rotation have been explained based on an impurity band close to the conduction band minimum, which is induced by Co-V-O (O vacancies) complexes. A phenomenological model founded on such an impurity band and electronic states induced by other Co-defect complexes in the ZnO energy gap allows a thoroughly novel, fine interpretation of the ellipticity spectra
Nitrogen-hydrogen complex in GaAsN revealed by X ray absorption spectroscopy
No full textGaAsN alloys belong to a class of semiconductors with fascinating physical properties. Indeed, a small amount of nitrogen incorporation in GaAs leads to a counterintuitive and large band-gap reduction, and to an unexpected sudden increase in the effective mass of electrons. Even more surprisingly, both electronic and structural changes can be reversed fully and in a tunable manner by hydrogen incorporation. In this paper, we combine x-ray absorption spectroscopy at the nitrogen edge with ab initio simulations to investigate the atomic geometry of N-H complexes in hydrogenated GaAsN. In this way, we provide experimental evidence that dihydrogen-nitrogen complexes with C2v symmetry are the most abundant species in hydrogenated GaAsN. This finding contradicts previous predictions of “in-line” N-H2* complexes as the predominant species, and accounts for recent infrared absorption experiments
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