41. In-depth structural characterization and magnetic properties ofquaternary ferrite systems Co0.5Zn0.25M0.25Fe2O4(M¼Ni, Cu, Mn, Mg)Radu-G. Ciocarlan, Iztok Arčon, Aurel Pui, Myrjam Mertens, Nataša Novak Tušar, Elena M. Seftel, Pegie Cool, 2019, original scientific article Abstract: This paper investigates the structural and magnetic properties of a mixed series of ferrites having generalformula Co0.5Zn0.25M0.25Fe2O4(M¼Ni, Cu, Mn, Mg). Insights on surface chemistry, structural andmorphological parameters are presented in order to achieve highly tuned ferrite systems with specificcharacteristics. The site occupancy of the cations in the spinel structure was determined using XAS, XRDand Raman spectroscopy. A 100% occupancy of the tetrahedral sites was observed for the Zn cations forall the samples. Co cations occupied the octahedral sites in proportion of 100% for CoeZneCu, CoeZneMg and around 80% for CoeZneNi, CoeZneMn. Ni cations were found only in the octahedral sites,while Cu 80% in the octahedral sites and Mn 60% in the octahedral sites. The cation distribution resultswere correlated with the magnetic properties data. At the same time, not only the cation distribution, butalso the particles size distribution was observed to have a great influence on magnetic and opticalproperties of the ferrites. Given the importance of the surface chemistry of the nanoparticles, XPS and FT-IR analysis were used to prove the functionalization with groups belonging to the surfactant. Moreover,the peculiar behavior of manganese in spinel structures was clarified and the origin of the multiple states(Mn2þand Mn4þ) was elucidated. Finally, parallels have been drawn between the use of different cationsin mixed ferrites in order to gain specific properties. Keywords: FerritesMagnetic propertiesSpinel nanoparticlesX-ray absorption spectroscopyCo-precipitation Published in RUNG: 23.10.2019; Views: 3313; Downloads: 0 This document has many files! More... |
42. Effects of the molecular potential on coexcitations of valence electrons in the K-shell photoeffect of 3p and 4p elementsRobert Hauko, Jana Padežnik Gomilšek, Alojz Kodre, Iztok Arčon, Giuliana Aquilanti, 2019, original scientific article Abstract: Photoabsorption spectra of gaseous hydrides of 3p (PH3, H2S, HCl) and 4p elements (GeH4, AsH3 , H2Se,
HBr) are measured in the energy region within 50 eV above the K edge, to study coexcitations of valence
electrons by photoeffect in the K shell. The analysis of the valence coexcitations is extended to Ar, Kr, and
SiH4. Relative probabilities and energies of states in the individual coexcitation channels are recovered by
modeling the spectral features with a minimal ansatz based on the features in the contiguous noble gas. The
extracted parameters are compared to the results of theoretical calculations for molecules (ORCA code) and
free atoms (Hartree-Fock code). The experimental results confirm that the valence coexcitations in the 3p and
4p hydride molecules can be satisfactorily described by a two-step process, with the shake of the outer electron
following the excitation of the core electron. The total probability—relative to the K-edge jump—of the shake-up
processes shows a steady decrease from 19% in Si to 14% in Cl, and from 15% in Ge to 12% in Br. The
experimental values for Ar (12%) and Kr (10%) are in accord with the trend. The dominant contribution is
the transition to quasiatomic orbitals, in contrast with the deeper coexcitation channels in hydride molecules
where transition to molecular orbitals prevails. Keywords: X-ray absorption spectra, gaseous hydrides, 3p K-edge spectra, DFT Published in RUNG: 05.09.2019; Views: 2998; Downloads: 0 This document has many files! More... |
43. Multi-wavelength aerosol absorption coefficient measurements: instrument inter-comparison and results of source and source-component modellingVera Bernardoni, Luca Ferrero, Francesca Soldan, Sara Valentin, Mario Massabò, Griša Močnik, Asta Gregorič, Marco Cataldi A., Ezio Bolzacchini, Paolo Prati, Gianluigi Valli, Roberta Vecchi, 2019, published scientific conference contribution abstract Keywords: aerosol absorption, method intercomparison Published in RUNG: 17.07.2019; Views: 3547; Downloads: 0 This document has many files! More... |
44. Trace detection of C2H2 in ambient air using continuous wave cavity ring-down spectroscopy combined with sample pre-concentrationManik Pradhan, Ruth Lindley, Roberto Grilli, Iain R. White, Damien Martin, Orr-Ewing Andrew, 2008, original scientific article Abstract: Continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled with sample pre-concentration has been used to measure acetylene (C2H2) mixing ratios in ambient air. Measurements were made in the near-infrared region (λ∼1535.393 nm), using the P(17) rotational line of the (ν1+ν3) vibrational combination band, a region free from interference by overlapping spectral absorption features of other air constituents. The spectrometer is shown to be capable of fast, quantitative and precise C2H2 mixing ratio determinations without the need for gas chromatographic (GC) separation. The current detection limit of the spectrometer following sample pre-concentration is estimated to be 35 parts per trillion by volume (pptv), which is sufficient for direct atmospheric detection of C2H2 at concentrations typical of both urban and rural environments. The CRDS apparatus performance was compared with an instrument using GC separation and flame ionization detection (GC-FID); both techniques were used to analyze air samples collected within and outside the laboratory. These measurements were shown to be in quantitative agreement. The indoor air sample was found to contain C2H2 at a mixing ratio of 3.87±0.22 ppbv (3.90±0.23 ppbv by GC-FID), and the C2H2 fractions in the outside air samples collected on two separate days from urban locations were 1.83±0.20 and 0.69±0.14 ppbv (1.18±0.09 and 0.60±0.04 ppbv by GC-FID). The discrepancy in the first outdoor air sample is attributed to degradation over a 2-month interval between the cw-CRDS and GC-FID analyses. Keywords: Rotational Line, Cavity Enhance Absorption Spectroscopy, Adsorbent Trap, Trace Atmospheric Constituent, CRDS Instrument Published in RUNG: 15.07.2019; Views: 3326; Downloads: 0 This document has many files! More... |
45. OXYGEN-EXCESS RELATED DEFECTS IN SiO2-BASED MATERIALS: COUPLING THEORY AND EXPERIMENTSBlaž Winkler, 2019, doctoral dissertation Abstract: This work is primarily focused on application of standard first-principle computational approaches to model oxygen excess related point defects in amorphous silica. Atomic models with their respective electronic and optical properties are explored together with some conversion mechanisms between defect models.
The first chapter overviews extensive literature about the already known properties of oxygen related defects. Second chapter briefly introduces main methods that have been used in this research, in particular Density Functional Theory (DFT) as energy and force engine with short description of minimal energy path (MEP) algorithm used for modeling chemical/migration reactions, GW approximation for charged electronic excitations (band structure) and Bethe-Salpeter Equation (BSE) for neutral excitations (optical absorption and excitonic structure including electron hole interaction). The third chapter is devoted to the presentation of results. Thanks to the calculation of optical properties of peroxy bridge (POL), a correlation has been found between structural disorder, specifically dihedral angle dispersion, and low coupling with light, which has been identified as main reason why no clear absorption bands have been assigned to the POL. Structure and stability of some other defects, like interstitial ozone molecule (ozonyl) and dioxasilirane (silicon analogy of dioxirane), have been studied. These defects are usually not considered as most important species, however their calculated formation energies are lower compared to some known defects, which indicates they might be present in silica.
From a detailed study on possible reaction mechanisms, it has been found that ozonyl might be one of the most important intermediate steps for oxygen exchange reactions. Results also show that dioxasilirane can be spontaneously created during the interaction of oxygen with lone pair defects. By exploring different reactions between oxygen and pre-existing oxygen deficiency centers (ODCs), calculations predict two kinds of passivation behaviors: single-barrier reversible mechanisms with the formation of dioxasilirane-like groups, for which the network keeps the memory of the precursory lone pair defects, and single or multiple-barrier mechanisms, for which the network loses its memory, either because of the high reverse barrier or because of a reconstruction.
Final part of this research has been devoted to experimental characterization of the response and tolerance of optical fibers loaded with oxygen under irradiation. These include experiments on commercial fiber along with canonical samples (Optical fibers developed with the intention of studying correlations between different fabrication parameters, dopant/impurity concentration and doping concentrations). Studied fibers also include rare-earth doped fibers. Keywords: Silica, DFT, GW-approximation, Bethe-Salpeter equation, NEB, defect, oxygen, oxygen excess centers, oxygen deficiency centers, optical absorption, optical fibers, radiation induced attenuation. Published in RUNG: 07.05.2019; Views: 4500; Downloads: 201 Full text (13,18 MB) |
46. PHOTO-EXCITATION ENERGY INFLUENCE ON THE PHOTOCONDUCTIVITY OF ORGANIC SEMICONDUCTORSNadiia Pastukhova, 2018, doctoral dissertation Abstract: In this work, we experimentally studied the influence of photoexcitation energy
influence on the charge transport in organic semiconductors. Organic semiconductors
were small molecules like corannulene, perylene and pentacene derivatives, polymers
such as polythiophene and benzothiophene derivatives, and graphene, along with
combinations of these materials in heterojunctions or composites.
The first part of this study is focused on the photoexcitation energy influence on
the transient photoconductivity of non-crystalline curved π-conjugated corannulene
layers. The enhanced photoconductivity, in the energy range where optical absorption
is absent, is deduced from theoretical predictions of corannulene gas-phase excited
state spectra. Theoretical analysis reveals a consistent contribution involving
transitions to Super Atomic Molecular Orbitals (SAMOs), a unique set of diffuse
orbitals typical of curved π-conjugated molecules. More, the photoconductivity of the
curved corannulene was compared to the π-conjugated planar N,N′-1H,1H-
perfluorobutyldicyanoperylene-carboxydi-imide
(PDIF-CN2),
where
the
photoexcitation energy dependence of photocurrent closely follows the optical
absorption spectrum.
We next characterized charge transport in poly(3-hexylthiophene) (P3HT) layers
deposited from solution. Our results indicate that time-of-flight (TOF) mobility
depends on the photoexcitation energy. It is 0.4× 10 −3 cm 2 /Vs at 2.3 eV (530 nm) and
doubles at 4.8 eV (260 nm). TOF mobility was compared to field-effect (FET) mobility
of P3HT field-effect transistors (OFETs). The FET mobility was similar to the 2.3 eV
excitation TOF mobility. In order to improve charge mobility, graphene nanoparticles
were blended within a P3HT solution before the deposition. We found that the mobility
significantly improves upon the addition of graphene nanoparticles of a weight ratio
as low as 0.2 %. FET mobility increases with graphene concentration up to a value of
2.3× 10 −2 cm 2 /Vs at 3.2 %. The results demonstrate that phase segregation starts to
influence charge transport at graphene concentration of 0.8 % and above. Hence, the
graphene cannot form a bridged conduction channel between electrodes, which would
cancel the semiconducting effect of the polymer composite.
An alternative approach to enhance mobility is to optimize the molecular ordering
of organic semiconductors. For that purpose, we studied an innovative nanomesh
device. Free-standing nanomesh devices were used to form nanojunctions of N,N′-
iiDioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowires and crystalline
bis(triisopropylsilylethinyl)pentacene (TIPS-PEN). We characterized the photocurrent
response time of this novel nanomesh scaffold device. The photoresponse time
depends on the photon energy. It is between 4.5 − 5.6 ns at 500 nm excitation
wavelength and between 6.7 − 7.7 ns at 700 nm excitation wavelength. In addition, we
found that thermal annealing reduces charge carrier trapping in crystalline nanowires.
This confirms that the structural defects are crucial to obtaining high photon-to-charge
conversion efficiency and subsequent transport from pn junction in heterostructured
materials.
Structural defects also influence the power conversion efficiency of organic
heterostructured photovoltaics (OPVs). Anticipating that polymers with different
backbone lengths produce different level of structural defects, we examined charge
transport
dependence
on
the
molecular
weight
of
poly[4,8-bis(5-(2-
ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-
ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]
(PTB7-Th)
from 50 kDa to 300 kDa. We found p-type hopping transport in PTB7-Th,
characterized by 0.1 – 3× 10 −2 cm 2 /Vs mobility, which increases with temperature and
electric field. The polymer molecular weight exhibits a non-trivial influence on charge
transport. FET mobility in the saturation regime increases with molecular weight. A
similar trend is observed in TOF mobility and FET mobility in the linear regime,
except for the 100kDa polymer, which manifests in the highest mobility due to reduced
charge trapping. The lowest trapping at the dielectric interface of OFET is observed at
200 kDa. In addition, the 200 kDa polymer exhibits the lowest activation energy of the
charge transport. Although the 100 kDa polymer indicates the highest mobility, OPVs
using the 200 kDa polymer exhibit the best performance in terms of power conversion
efficiency. Keywords: organic
semiconductors, optical
absorption
spectroscopy, time-of-flight
photoconductivity, transient photocurrent spectroscopy, organic thin film transistors, atomic force microscopy, superatomic molecular orbitals, pn heterojunction, organic
nanowires, graphene, composites, charge mobility, charge trapping, temperature
dependence, photodetector, photovoltaic, solar cell, organic electronics Published in RUNG: 08.10.2018; Views: 5578; Downloads: 165 Full text (4,56 MB) |
47. Absorption Spectra of Ethanol and Water Using a Photothermal Lens SpectrophotometerHumberto Cabrera, Jehan Akbar, Dorota Korte, Imrana Ashraf, Evelio E. Ramírez- Miquet, Ernesto Marin, Joseph Niemela, 2018, original scientific article Keywords: Thermal lens spectrometry, TLS, absorption of water, absorption of ethanol, vibration overtones Published in RUNG: 08.06.2018; Views: 4444; Downloads: 0 This document has many files! More... |
48. Giant magneto–electric coupling in 100 nm thick Co capped by ZnO nanorodsGiovanni Vinai, Barbara Ressel, Piero Torelli, Federico Loi, Benoit Gobaut, Regina Ciancio, Barbara Casarin, Antonio Caretta, Luca Capasso, Fulvio Parmigiani, Francesco Cugini, Massimo Solzi, Marco Malvestuto, Roberta Ciprian, 2018, original scientific article Keywords: ZnO nanorods, Cobalt, X-rays absorption near edge spectroscopy Published in RUNG: 07.02.2018; Views: 3987; Downloads: 0 This document has many files! More... |
49. New strategy for magnetic gas sensingRoberta Ciprian, Piero Torelli, Angelo Giglia, B Gobaut, Barbara Ressel, Janez Štupar, Matija Stupar, Antonio Caretta, Giovanni De Ninno, Tommaso Pincelli, Barbara Casarin, Ganesh Adhikari, G Sberviglieri, C Baratto, Marco Malvestuto, 2016, original scientific article Keywords: hybrid nanostructures, sensor, absorption spectroscopy Published in RUNG: 05.02.2018; Views: 4098; Downloads: 0 This document has many files! More... |
50. Design of a highly photocatalytically active ZnO/CuWO4 nanocompositeSaim Emin, Matjaž Valant, 2017, published scientific conference contribution abstract (invited lecture) Abstract: We report the synthesis, photocatalytic activity and mechanistic study of a novel heterostructure (HTS) with an efficient charge separation. A ZnO/CuWO4 HTS material is reported for the first time. The nanocomposite (NC) consists of CuWO4 nanoparticles (ca. 200-400 nm) decorated with ZnO nanorods (ca. 30 nm, 100 nm length) and is shown to be a highly active photocatalyst for decomposition of model contaminants including methyl orange and terephthalic acid. The ZnO/CuWO4 interface is shown to be the key for controlling the enhanced activity of the composite material. Transient absorption spectroscopy studies demonstrated that a photoinduced charge transfer across the ZnO/CuWO4 interface increased electron-hole lifetime by 3 orders of magnitude, from < 20 s in ZnO to 30 ms in the ZnO/CuWO4 NC sample. Our findings show that through interface design efficient HTS materials can be prepared for a wide range of photocatalytic applications. Keywords: CuWO4, nanocomposite, transient absorption spectroscopy Published in RUNG: 28.08.2017; Views: 5174; Downloads: 0 This document has many files! More... |