Mariana Stefan | National Institute of Materials Physics (original) (raw)
Papers by Mariana Stefan
Radiation Effects and Defects in Solids, 1999
ESR measurements in the 9 and 94 GHz microwave frequency bands were performed on thick free-stand... more ESR measurements in the 9 and 94 GHz microwave frequency bands were performed on thick free-standing polycrystalline diamond films grown by microwave plasma enhanced CVD from CH4/H2 mixtures under variable deposition conditions. An exponential decrease in the concentration of the N0 centres with increase in the H2 flow rate was found. Neither H1 nor H2 centers could be detected.
Optical Materials, 2010
A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce 3þ impurities in PbWO ... more A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce 3þ impurities in PbWO 4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43 GHz) and at the high frequencies/fields (HF) 95, 190 and 285 GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g k and g ? , are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g ?-parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S 4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce 3þ ions with tetragonal symmetry S 4 in the PbWO 4 scintillator.
![Research paper thumbnail of Erratum: “Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd[sup 3+] in PbWO[sub 4] single-crystals” [J. Chem. Phys. 131, 034505 (2009)]](https://attachments.academia-assets.com/98697980/thumbnails/1.jpg)
](The Journal of Chemical Physics, 2009
In this paper, 1 the acknowledgments section was incomplete. The missing information should have ... more In this paper, 1 the acknowledgments section was incomplete. The missing information should have read as follows:
ACS Omega
The morphology, structure, composition, and conduction electron properties of quasi-spherical tin... more The morphology, structure, composition, and conduction electron properties of quasi-spherical tin nanocrystals (NCs) of 2.5 nm average diameter, with unstrained, bulk-like α-Sn diamond cubic structure, observed in dark cubic boron nitride (cBN) crystallites, were determined by correlated analytical high-resolution scanning transmission electron microscopy and multifrequency electron spin resonance (ESR) investigations. The narrow Lorentzian ESR line with g = 2.0028 is attributed to the conduction ESR of the α-Sn NCs, consistent with the temperature-and frequency-independent small g-shift and intensity reduction under high temperature (950°C) vacuum annealing when the α-Sn NCs are thermally dissolved in the host cBN crystallites. The ESR linewidth and line intensity vs temperature dependences recorded in the 20 to 295 K range are quantitatively described considering the presence of discrete, quantum confinement-induced conduction electron energy levels with Δ QC /k B = 125 K separation, close to the theoretical value for conductive α-Sn NCs of 2.5 nm in diameter. The observed properties are tentatively explained with the predicted nanosize induced band-gap opening and change of band ordering from bulk α-Sn to small unstrained α-Sn NCs, resulting in a topological phase transition that also explains the predominantly s-like character of the conduction band electron orbitals.
Nanomaterials
Special attention has recently been paid to surface-defective titanium dioxide and black TiO2 wit... more Special attention has recently been paid to surface-defective titanium dioxide and black TiO2 with advanced optical, electrical, and photocatalytic properties. Synthesis of these materials for photodegradation and mineralization of persistent organic pollutants in water, especially under visible radiation, presents interest from scientific and application points of view. Chemical reduction by heating a TiO2 and NaBH4 mixture at 350 °C successfully introduced Ti3+ defects and oxygen vacancies at the surface of TiO2, with an increase in the photocatalytic degradation of amoxicillin—an antibiotic that is present in wastewater due to its intense use in human and animal medicine. Three TiO2 samples were prepared at different annealing temperatures to control the ratio between anatase and rutile and were subjected to chemical reduction. Electron paramagnetic resonance investigations showed that the formation of surface Ti3+ defects in a high concentration occurred mainly in the anatase sa...
Sensors and Actuators B: Chemical, 2019
Highlights: Pd decorated nanocrystalline Zn doped SnO2 Hydrothermal synthesis route using Bri... more Highlights: Pd decorated nanocrystalline Zn doped SnO2 Hydrothermal synthesis route using Brij 52 and Tripropylamine as co-templates The intrinsic aspects of the materials were correlated with the sensing properties The catalytic role of Pd was highlighted towards NO2 detection with Pd/SnO2_10Zn The selective sensitivity to NO2 envisage the applicative potential of Pd/SnO2_10Zn
RSC Advances, 2016
Evidence of manganese induced redirection of the Zn(OH)2 synthesis towards nanostructured ZnO:Mn,... more Evidence of manganese induced redirection of the Zn(OH)2 synthesis towards nanostructured ZnO:Mn, not observed for other M(ii)(OH)2 compounds.
The Journal of Physical Chemistry C, 2016
The source of collective magnetism in II–VI semiconductor quantum dots (QDs) doped with Mn2+ ions... more The source of collective magnetism in II–VI semiconductor quantum dots (QDs) doped with Mn2+ ions at high nominal impurity levels is still under debate. In the particular case of mesoporous, self-assembled cubic ZnS:Mn QDs, quantitative electron paramagnetic resonance (EPR) studies have shown that the Mn2+ ions incorporated in the core and on the surface of the QDs cannot be responsible for the observed collective magnetism because they remain in a diluted paramagnetic state up to the 50 000 ppm nominal concentration. Here we investigate the composition, localization, structure, and magnetic properties of the aggregates of Mn2+ ions incorporated in the mesoporous cZnS:Mn as a possible source of the observed collective magnetism. Samples of mesoporous cubic ZnS:Mn prepared by coprecipitation at several nominal impurity levels from 200 to 50 000 ppm are investigated by EPR, magnetometry, and analytical high resolution (scanning) transmission electron microscopy. The low temperature magnetic properties of th...
Journal of Alloys and Compounds, 2016
The distribution and interaction of isolated Mn 2þ impurity ions incorporated in 2.9 nm average d... more The distribution and interaction of isolated Mn 2þ impurity ions incorporated in 2.9 nm average diameter cubic ZnS quantum dots (QDs), prepared by surfactanteassisted liquideliquid synthesis with initial impurity concentrations in the 20e50,000 ppm range, has been investigated by electron paramagnetic resonance (EPR) spectroscopy. The well resolved spectra, observed in the whole investigated concentration range, reflect the localization of the Mn 2þ ions in the core and on the surface of the cZnS:Mn QDs at isolated sites. According to the analysis of the dependences of the concentration of incorporated isolated Mn 2þ ions vs. initial impurity concentration and of the core localized Mn 2þ ions spectra linewidth vs. actual concentration, the isolated Mn 2þ ions remain in the whole concentrations range in a diluted paramagnetic state characterized by dipolar magnetic interactions. Pulse EPR measurements of the spinespin dipolar interaction for the incorporated Mn 2þ ions confirm their diluted distribution, which excludes these ions as a possible source of collective magnetism properties.
The Journal of Physical Chemistry C, 2015
Although impurity doping of nanocrystals is essential in controlling their physical properties fo... more Although impurity doping of nanocrystals is essential in controlling their physical properties for various applications, the doping mechanism of ultrasmall, colloidal II–VI semiconductor nanocrystals, corresponding to the initial stages of growth, is not yet understood. In this study the concentrations of Mn2+ ions in the core, on the surface, and as an agglomerated separate phase in 2.9 nm cubic ZnS nanocrystals, prepared by a surfactant-assisted liquid–liquid synthesis within 20 to 20 000 ppm nominal impurity concentration range, have been determined by quantitative multifrequency electron paramagnetic resonance. The unexpected strong decrease in the core doping efficiency with the nominal concentration increase, in contrast to the small variation of the doping efficiency for the surface-bound Mn2+ ions, and the sizable core doping efficiency observed for 1.8 nm nanocrystals were explained with the extended lattice defect assisted mechanism of incorporation. According to this mechanism, which is not siz...
Springer Series in Materials Science, 2014
Electron paramagnetic resonance (EPR) spectroscopy has been extensively employed to investigate t... more Electron paramagnetic resonance (EPR) spectroscopy has been extensively employed to investigate the presence, localization, distribution and interaction with the host crystalline lattice of the paramagnetic point defects (intrinsic defects and transition metal ions) in semiconductors. The retrieval of such information for nanostructured semiconductors is considerably more difficult, due to the high disorder level in such systems, reflected in broad, featureless EPR spectra. We show here how, with proper adjustments of the EPR experiments and accurate numerical analysis of the resulting spectra, it was possible to obtain more accurate information regarding the localization and structure of various Mn2+ centers in ZnS and ZnO semiconductor nanoparticles (NPs). This lead to the observation and investigation of size related effects such as the presence of the extended lattice defect assisted incorporation of impurities in small (~3 nm) cubic ZnS NPs, the dominant size induced lattice disorder observed for ZnO NPs, independent of the synthesis procedures, or the three steps decomposition of the e-Zn(OH)2 disordered shell of ZnS NPs with formation of new oxy-hydrated zinc compounds. These effects can be used to synthesize semiconductor nanoparticles with controlled size distribution, doping level and functionalized surfaces for specific technological applications.
Journal of Magnetic Resonance, 2011
Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagne... more Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagnetic resonance (EPR) spectra of paramagnetic impurity ions in wide band gap semiconductor nanocrystals, is essential for determining their localization and quantum properties. Here we present a procedure, based on publicly available software, for determining with higher accuracy the SH parameters of isolated Mn(2+) impurity ions in small cubic ZnS nanocrystals. The procedure, which can be applied to other cubic II-VI semiconductor nanocrystals as well, is based on the analysis of both low and high frequency EPR spectra with line shape simulation and fitting computing programs, which include the hyperfine forbidden transitions and line broadening effects. The difficulties, limitations and errors which can affect the accuracy in determining some of the SH parameters are also discussed.
![Research paper thumbnail of Fourth-order zero-field splitting parameters of [Mn(cyclam)Br2]Br determined by single-crystal W-band EPR](https://attachments.academia-assets.com/91469910/thumbnails/1.jpg)
](Applied Magnetic Resonance, 2001
Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed ... more Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spin S = 2 ground state and have been attributed to equivalent centers related by fourfold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values Bá = 0.0009(3) cm-', B = 0.0006(2) cm-' and B = 0.0017(3) cm-', to be considered together with the second-order parameters D =-1.1677(7) cm-' and E =-0.0135(6) cm-'.
arXiv: Strongly Correlated Electrons, 2019
Understanding the interplay between the structural, chemical and physical properties of nanomater... more Understanding the interplay between the structural, chemical and physical properties of nanomaterials is crucial for designing new devices with enhanced performance. In this regards, doping of metal oxides is a general strategy to tune size, morphology, charge, lattice, orbital and spin degrees of freedoms and has been shown to affect nanomaterials properties for photoelectrochemical water splitting, batteries, catalysis, magnetic applications and optics. Here we report the role of lattice small polaron in driving the morphological transition from nearly isotropic to nanowire crystals in Si doped hematite ($\alpha-Fe_2O_3$). Lattice small polaron formation is well evidenced by the increase of hexagonal strain and degree of distortion of FeO_6FeO_6FeO_6 showing a hyperbolic trend with increasing Si content. Local analysis via pair distribution function highlights an unreported crossover from small to large polarons, which affects the correlation length of the polaronic distortion from short ...
Scientific Reports
The present study concerns the in vitro oxidative stress responses of non-malignant murine cells ... more The present study concerns the in vitro oxidative stress responses of non-malignant murine cells exposed to surfactant-tailored ZnO nanoparticles (NPs) with distinct morphologies and different levels of manganese doping. Two series of Mn-doped ZnO NPs were obtained by coprecipitation synthesis method, in the presence of either polyvinylpyrrolidone (PVP) or sodium hexametaphosphate (SHMTP). The samples were investigated by powder X-ray Diffraction, Transmission Electron Microscopy, Fourier-Transform Infrared and Electron Paramagnetic Resonance spectroscopic methods, and N2 adsorption–desorption analysis. The observed surfactant-dependent effects concerned: i) particle size and morphology; ii) Mn-doping level; iii) specific surface area and porosity. The relationship between the surfactant dependent characteristics of the Mn-doped ZnO NPs and their in vitro toxicity was assessed by studying the cell viability, intracellular reactive oxygen species (ROS) generation, and DNA fragmentati...
Chemosensors
Nanocomposite thin films, sensitive to methane at the room temperature (25–30 °C), have been prep... more Nanocomposite thin films, sensitive to methane at the room temperature (25–30 °C), have been prepared, starting from SnSe2 powder and Zn(II)-5,10,15,20-tetrakis-(4-aminophenyl)- -porphyrin (ZnTAPP) powder, that were fully characterized by XRD, UV-VIS, FT-IR, Nuclear Magnetic Resonance (1H-NMR and 13C-NMR), Atomic Force Microscopy (AFM), SEM and Electron Paramagnetic Resonance (EPR) techniques. Film deposition was made by drop casting from a suitable solvent for the two starting materials, after mixing them in an ultrasonic bath. The thickness of these films were estimated from SEM images, and found to be around 1.3 μm. These thin films proved to be sensitive to a threshold methane (CH4) concentration as low as 1000 ppm, at a room temperature of about 25 °C, without the need for heating the sensing element. The nanocomposite material has a prompt and reproducible response to methane in the case of air, with 50% relative humidity (RH) as well. A comparison of the methane sensing perfo...
Scientific Reports
the synthesis of semiconductor nanocrystals with controlled doping is highly challenging, as ofte... more the synthesis of semiconductor nanocrystals with controlled doping is highly challenging, as often a significant part of the doping ions are found segregated at nanocrystals surface, even forming secondary phases, rather than incorporated in the core. We have investigated the dopant distribution dynamics under slight changes in the preparation procedure of nanocrystalline Zno doped with manganese in low concentration by electron paramagnetic resonance spectroscopy, paying attention to the formation of transient secondary phases and their transformation into doped ZnO. The acidification of the starting solution in the co-precipitation synthesis from nitrate precursors lead to the decrease of the Mn 2+ ions concentration in the core of the Zno nanocrystals and their accumulation in minority phases, until ~79% of the Mn 2+ ions were localized in a thin disordered shell of zinc hydroxynitrate (ZHN). A lower synthesis temperature resulted in polycrystalline Mn-doped ZHN. Under isochronal annealing up to 250 °C the bulk ZHN and the minority phases from the ZnO samples decomposed into Zno. the Mn 2+ ions distribution in the annealed nanocrystals was significantly altered, varying from a uniform volume distribution to a preferential localization in the outer layers of the nanocrystals. Our results provide a synthesis strategy for tailoring the dopant distribution in ZnO nanocrystals for applications ranging from surface based to ones involving core properties.
Applied Microbiology and Biotechnology
Radiation Effects and Defects in Solids, 1999
ESR measurements in the 9 and 94 GHz microwave frequency bands were performed on thick free-stand... more ESR measurements in the 9 and 94 GHz microwave frequency bands were performed on thick free-standing polycrystalline diamond films grown by microwave plasma enhanced CVD from CH4/H2 mixtures under variable deposition conditions. An exponential decrease in the concentration of the N0 centres with increase in the H2 flow rate was found. Neither H1 nor H2 centers could be detected.
Optical Materials, 2010
A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce 3þ impurities in PbWO ... more A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce 3þ impurities in PbWO 4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43 GHz) and at the high frequencies/fields (HF) 95, 190 and 285 GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g k and g ? , are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g ?-parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S 4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce 3þ ions with tetragonal symmetry S 4 in the PbWO 4 scintillator.
The Journal of Chemical Physics, 2009
In this paper, 1 the acknowledgments section was incomplete. The missing information should have ... more In this paper, 1 the acknowledgments section was incomplete. The missing information should have read as follows:
ACS Omega
The morphology, structure, composition, and conduction electron properties of quasi-spherical tin... more The morphology, structure, composition, and conduction electron properties of quasi-spherical tin nanocrystals (NCs) of 2.5 nm average diameter, with unstrained, bulk-like α-Sn diamond cubic structure, observed in dark cubic boron nitride (cBN) crystallites, were determined by correlated analytical high-resolution scanning transmission electron microscopy and multifrequency electron spin resonance (ESR) investigations. The narrow Lorentzian ESR line with g = 2.0028 is attributed to the conduction ESR of the α-Sn NCs, consistent with the temperature-and frequency-independent small g-shift and intensity reduction under high temperature (950°C) vacuum annealing when the α-Sn NCs are thermally dissolved in the host cBN crystallites. The ESR linewidth and line intensity vs temperature dependences recorded in the 20 to 295 K range are quantitatively described considering the presence of discrete, quantum confinement-induced conduction electron energy levels with Δ QC /k B = 125 K separation, close to the theoretical value for conductive α-Sn NCs of 2.5 nm in diameter. The observed properties are tentatively explained with the predicted nanosize induced band-gap opening and change of band ordering from bulk α-Sn to small unstrained α-Sn NCs, resulting in a topological phase transition that also explains the predominantly s-like character of the conduction band electron orbitals.
Nanomaterials
Special attention has recently been paid to surface-defective titanium dioxide and black TiO2 wit... more Special attention has recently been paid to surface-defective titanium dioxide and black TiO2 with advanced optical, electrical, and photocatalytic properties. Synthesis of these materials for photodegradation and mineralization of persistent organic pollutants in water, especially under visible radiation, presents interest from scientific and application points of view. Chemical reduction by heating a TiO2 and NaBH4 mixture at 350 °C successfully introduced Ti3+ defects and oxygen vacancies at the surface of TiO2, with an increase in the photocatalytic degradation of amoxicillin—an antibiotic that is present in wastewater due to its intense use in human and animal medicine. Three TiO2 samples were prepared at different annealing temperatures to control the ratio between anatase and rutile and were subjected to chemical reduction. Electron paramagnetic resonance investigations showed that the formation of surface Ti3+ defects in a high concentration occurred mainly in the anatase sa...
Sensors and Actuators B: Chemical, 2019
Highlights: Pd decorated nanocrystalline Zn doped SnO2 Hydrothermal synthesis route using Bri... more Highlights: Pd decorated nanocrystalline Zn doped SnO2 Hydrothermal synthesis route using Brij 52 and Tripropylamine as co-templates The intrinsic aspects of the materials were correlated with the sensing properties The catalytic role of Pd was highlighted towards NO2 detection with Pd/SnO2_10Zn The selective sensitivity to NO2 envisage the applicative potential of Pd/SnO2_10Zn
RSC Advances, 2016
Evidence of manganese induced redirection of the Zn(OH)2 synthesis towards nanostructured ZnO:Mn,... more Evidence of manganese induced redirection of the Zn(OH)2 synthesis towards nanostructured ZnO:Mn, not observed for other M(ii)(OH)2 compounds.
The Journal of Physical Chemistry C, 2016
The source of collective magnetism in II–VI semiconductor quantum dots (QDs) doped with Mn2+ ions... more The source of collective magnetism in II–VI semiconductor quantum dots (QDs) doped with Mn2+ ions at high nominal impurity levels is still under debate. In the particular case of mesoporous, self-assembled cubic ZnS:Mn QDs, quantitative electron paramagnetic resonance (EPR) studies have shown that the Mn2+ ions incorporated in the core and on the surface of the QDs cannot be responsible for the observed collective magnetism because they remain in a diluted paramagnetic state up to the 50 000 ppm nominal concentration. Here we investigate the composition, localization, structure, and magnetic properties of the aggregates of Mn2+ ions incorporated in the mesoporous cZnS:Mn as a possible source of the observed collective magnetism. Samples of mesoporous cubic ZnS:Mn prepared by coprecipitation at several nominal impurity levels from 200 to 50 000 ppm are investigated by EPR, magnetometry, and analytical high resolution (scanning) transmission electron microscopy. The low temperature magnetic properties of th...
Journal of Alloys and Compounds, 2016
The distribution and interaction of isolated Mn 2þ impurity ions incorporated in 2.9 nm average d... more The distribution and interaction of isolated Mn 2þ impurity ions incorporated in 2.9 nm average diameter cubic ZnS quantum dots (QDs), prepared by surfactanteassisted liquideliquid synthesis with initial impurity concentrations in the 20e50,000 ppm range, has been investigated by electron paramagnetic resonance (EPR) spectroscopy. The well resolved spectra, observed in the whole investigated concentration range, reflect the localization of the Mn 2þ ions in the core and on the surface of the cZnS:Mn QDs at isolated sites. According to the analysis of the dependences of the concentration of incorporated isolated Mn 2þ ions vs. initial impurity concentration and of the core localized Mn 2þ ions spectra linewidth vs. actual concentration, the isolated Mn 2þ ions remain in the whole concentrations range in a diluted paramagnetic state characterized by dipolar magnetic interactions. Pulse EPR measurements of the spinespin dipolar interaction for the incorporated Mn 2þ ions confirm their diluted distribution, which excludes these ions as a possible source of collective magnetism properties.
The Journal of Physical Chemistry C, 2015
Although impurity doping of nanocrystals is essential in controlling their physical properties fo... more Although impurity doping of nanocrystals is essential in controlling their physical properties for various applications, the doping mechanism of ultrasmall, colloidal II–VI semiconductor nanocrystals, corresponding to the initial stages of growth, is not yet understood. In this study the concentrations of Mn2+ ions in the core, on the surface, and as an agglomerated separate phase in 2.9 nm cubic ZnS nanocrystals, prepared by a surfactant-assisted liquid–liquid synthesis within 20 to 20 000 ppm nominal impurity concentration range, have been determined by quantitative multifrequency electron paramagnetic resonance. The unexpected strong decrease in the core doping efficiency with the nominal concentration increase, in contrast to the small variation of the doping efficiency for the surface-bound Mn2+ ions, and the sizable core doping efficiency observed for 1.8 nm nanocrystals were explained with the extended lattice defect assisted mechanism of incorporation. According to this mechanism, which is not siz...
Springer Series in Materials Science, 2014
Electron paramagnetic resonance (EPR) spectroscopy has been extensively employed to investigate t... more Electron paramagnetic resonance (EPR) spectroscopy has been extensively employed to investigate the presence, localization, distribution and interaction with the host crystalline lattice of the paramagnetic point defects (intrinsic defects and transition metal ions) in semiconductors. The retrieval of such information for nanostructured semiconductors is considerably more difficult, due to the high disorder level in such systems, reflected in broad, featureless EPR spectra. We show here how, with proper adjustments of the EPR experiments and accurate numerical analysis of the resulting spectra, it was possible to obtain more accurate information regarding the localization and structure of various Mn2+ centers in ZnS and ZnO semiconductor nanoparticles (NPs). This lead to the observation and investigation of size related effects such as the presence of the extended lattice defect assisted incorporation of impurities in small (~3 nm) cubic ZnS NPs, the dominant size induced lattice disorder observed for ZnO NPs, independent of the synthesis procedures, or the three steps decomposition of the e-Zn(OH)2 disordered shell of ZnS NPs with formation of new oxy-hydrated zinc compounds. These effects can be used to synthesize semiconductor nanoparticles with controlled size distribution, doping level and functionalized surfaces for specific technological applications.
Journal of Magnetic Resonance, 2011
Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagne... more Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagnetic resonance (EPR) spectra of paramagnetic impurity ions in wide band gap semiconductor nanocrystals, is essential for determining their localization and quantum properties. Here we present a procedure, based on publicly available software, for determining with higher accuracy the SH parameters of isolated Mn(2+) impurity ions in small cubic ZnS nanocrystals. The procedure, which can be applied to other cubic II-VI semiconductor nanocrystals as well, is based on the analysis of both low and high frequency EPR spectra with line shape simulation and fitting computing programs, which include the hyperfine forbidden transitions and line broadening effects. The difficulties, limitations and errors which can affect the accuracy in determining some of the SH parameters are also discussed.
Applied Magnetic Resonance, 2001
Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed ... more Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spin S = 2 ground state and have been attributed to equivalent centers related by fourfold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values Bá = 0.0009(3) cm-', B = 0.0006(2) cm-' and B = 0.0017(3) cm-', to be considered together with the second-order parameters D =-1.1677(7) cm-' and E =-0.0135(6) cm-'.
arXiv: Strongly Correlated Electrons, 2019
Understanding the interplay between the structural, chemical and physical properties of nanomater... more Understanding the interplay between the structural, chemical and physical properties of nanomaterials is crucial for designing new devices with enhanced performance. In this regards, doping of metal oxides is a general strategy to tune size, morphology, charge, lattice, orbital and spin degrees of freedoms and has been shown to affect nanomaterials properties for photoelectrochemical water splitting, batteries, catalysis, magnetic applications and optics. Here we report the role of lattice small polaron in driving the morphological transition from nearly isotropic to nanowire crystals in Si doped hematite ($\alpha-Fe_2O_3$). Lattice small polaron formation is well evidenced by the increase of hexagonal strain and degree of distortion of FeO_6FeO_6FeO_6 showing a hyperbolic trend with increasing Si content. Local analysis via pair distribution function highlights an unreported crossover from small to large polarons, which affects the correlation length of the polaronic distortion from short ...
Scientific Reports
The present study concerns the in vitro oxidative stress responses of non-malignant murine cells ... more The present study concerns the in vitro oxidative stress responses of non-malignant murine cells exposed to surfactant-tailored ZnO nanoparticles (NPs) with distinct morphologies and different levels of manganese doping. Two series of Mn-doped ZnO NPs were obtained by coprecipitation synthesis method, in the presence of either polyvinylpyrrolidone (PVP) or sodium hexametaphosphate (SHMTP). The samples were investigated by powder X-ray Diffraction, Transmission Electron Microscopy, Fourier-Transform Infrared and Electron Paramagnetic Resonance spectroscopic methods, and N2 adsorption–desorption analysis. The observed surfactant-dependent effects concerned: i) particle size and morphology; ii) Mn-doping level; iii) specific surface area and porosity. The relationship between the surfactant dependent characteristics of the Mn-doped ZnO NPs and their in vitro toxicity was assessed by studying the cell viability, intracellular reactive oxygen species (ROS) generation, and DNA fragmentati...
Chemosensors
Nanocomposite thin films, sensitive to methane at the room temperature (25–30 °C), have been prep... more Nanocomposite thin films, sensitive to methane at the room temperature (25–30 °C), have been prepared, starting from SnSe2 powder and Zn(II)-5,10,15,20-tetrakis-(4-aminophenyl)- -porphyrin (ZnTAPP) powder, that were fully characterized by XRD, UV-VIS, FT-IR, Nuclear Magnetic Resonance (1H-NMR and 13C-NMR), Atomic Force Microscopy (AFM), SEM and Electron Paramagnetic Resonance (EPR) techniques. Film deposition was made by drop casting from a suitable solvent for the two starting materials, after mixing them in an ultrasonic bath. The thickness of these films were estimated from SEM images, and found to be around 1.3 μm. These thin films proved to be sensitive to a threshold methane (CH4) concentration as low as 1000 ppm, at a room temperature of about 25 °C, without the need for heating the sensing element. The nanocomposite material has a prompt and reproducible response to methane in the case of air, with 50% relative humidity (RH) as well. A comparison of the methane sensing perfo...
Scientific Reports
the synthesis of semiconductor nanocrystals with controlled doping is highly challenging, as ofte... more the synthesis of semiconductor nanocrystals with controlled doping is highly challenging, as often a significant part of the doping ions are found segregated at nanocrystals surface, even forming secondary phases, rather than incorporated in the core. We have investigated the dopant distribution dynamics under slight changes in the preparation procedure of nanocrystalline Zno doped with manganese in low concentration by electron paramagnetic resonance spectroscopy, paying attention to the formation of transient secondary phases and their transformation into doped ZnO. The acidification of the starting solution in the co-precipitation synthesis from nitrate precursors lead to the decrease of the Mn 2+ ions concentration in the core of the Zno nanocrystals and their accumulation in minority phases, until ~79% of the Mn 2+ ions were localized in a thin disordered shell of zinc hydroxynitrate (ZHN). A lower synthesis temperature resulted in polycrystalline Mn-doped ZHN. Under isochronal annealing up to 250 °C the bulk ZHN and the minority phases from the ZnO samples decomposed into Zno. the Mn 2+ ions distribution in the annealed nanocrystals was significantly altered, varying from a uniform volume distribution to a preferential localization in the outer layers of the nanocrystals. Our results provide a synthesis strategy for tailoring the dopant distribution in ZnO nanocrystals for applications ranging from surface based to ones involving core properties.
Applied Microbiology and Biotechnology