147 research outputs found
Detection of the Glass Transition of Polymers Used in Art and Art-Conservation Using Raman Spectroscopy
In this work, Raman spectroscopy was employed for the
detection of the glass transition temperatures (Tg) of some
thermoplastic polymers and natural terpenoid resins. In
particular, our attention was focused on evaluating the Tg of
polystyrene and colophony. The measurements returned Tg
values in accordance with those reported in the literature
obtained using the DSC technique, thus confirming the
reliability of the approach proposed herein. Further studies
will be focused on the evaluation of Tg temperature changes
depending on materials treatments and ageing
Ligand exchange on CdSe nanoplatelets for the solar light sensitization of TiO2 and ZnO nanorod arrays
In quantum dot (QD) solar cells, the ex situ sensitization of wide band gap semiconductors (WBSCs) makes it possible to control the shape and the passivation of the nanosized sensitizer. Hence, ex situ techniques can be used to investigate how the band gap of the sensitizers affects the performance of quantum dot solar cells. The latter can be precisely controlled in 1D confined structures such as quasi-2D nanoplatelets (NPLs), the thickness of which is defined with an atomic precision. In this work, we tested and thoroughly characterized the attachment of 7, 9 and 11 monolayers thick CdSe NPLs (as well as QDs for the sake of comparison) to ZnO and to TiO2 nanorods. A crucial point of the ex situ techniques is the choice of bifunctional ligands that link the nanosized sensitizers to the WBSCs. Besides the well-known mercaptopropionic acid, we also studied two ‘atomic linkers’ (OH− and SH−) to minimize the distance between the sensitizer and the oxide. The as-prepared systems have been analyzed by UV/VIS absorption and Raman spectroscopy. Among them, SH− was found to be the most versatile linker that enabled the efficient attachment of all types of CdSe nanocrystals on ZnO and TiO2 nanorods.Fil: Szemjonov, A.. PSL Research University; Francia. Centre National de la Recherche Scientifique; FranciaFil: Tasso, Mariana Patricia. Laboratoire de Physique Et D'etude Des Materiaux; Francia. Centre National de la Recherche Scientifique; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Ithurria, S.. Laboratoire de Physique Et D'etude Des Materiaux; FranciaFil: Ciofini, I.. PSL Research University; Francia. Centre National de la Recherche Scientifique; FranciaFil: Labat, F.. PSL Research University; Francia. Centre National de la Recherche Scientifique; FranciaFil: Pauporté, T.. PSL Research University; Francia. Centre National de la Recherche Scientifique; Franci
Aggregation effects on pigment coatings: Pigment red 179 as a case study
Here, we have studied, with a combined experimental and computational approach, the effect of the crystal environment and aggregation on the electronic properties of Pigment Red 179, which affect both its color and optical energy gap. Spectra acquired in the near-infrared and visible range of energies suggest that this molecule is indeed a "cool" dye, which can be employed as a red pigment that provides effective color coverage to different substrates without contributing to their heating during light irradiation. Spectra acquired on different polymer mixtures at different pigment concentrations (i.e., 2.5-10 wt %) suggest that absorption features depend on chromophoric arrangements promoted by the strong intermolecular I -\u3c0interactions. Calculations, performed at the time-dependent density functional theory level, allowed to both attribute the nature of the electronic transitions causing the observed spectra involved and understand the effect of the environment. Indeed, the visible spectra of the pigment is dominated by two localized transitions, with negligible charge transfer for both a dye monomer and dimer either in vacuum or acetonitrile solution. Instead, models including the crystal environment of the pigment show the presence of a high-wavelength S1 \ue2 S0 charge transfer transition between two adjacent molecules, in quantitative agreement with the experimental absorption energy of the crystal pigment
Performance of local orbital basis sets in the self-consistent Sternheimer method for dielectric matrices of extended systems
We present a systematic study of the performance of numerical pseudo-atomic
orbital basis sets in the calculation of dielectric matrices of extended
systems using the self-consistent Sternheimer approach of [F. Giustino et al.,
Phys. Rev. B 81 (11), 115105 (2010)]. In order to cover a range of systems,
from more insulating to more metallic character, we discuss results for the
three semiconductors diamond, silicon, and germanium. Dielectric matrices
calculated using our method fall within 1-3% of reference planewaves
calculations, demonstrating that this method is promising. We find that
polarization orbitals are critical for achieving good agreement with planewaves
calculations, and that only a few additional \zeta 's are required for
obtaining converged results, provided the split norm is properly optimized. Our
present work establishes the validity of local orbital basis sets and the
self-consistent Sternheimer approach for the calculation of dielectric matrices
in extended systems, and prepares the ground for future studies of electronic
excitations using these methods.Comment: 10 pages, 8 figure
DFT exchange: sharing perspectives on the workhorse of quantum chemistry and materials science
In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022
Complete structural and magnetic characterization of biological radicals in solution by an integrated quantum mechanical approach. Glycyl radical as a case study
Effect of self-interaction error in the evaluation of the bond length alternation in trans -polyacetylene using density-functional theory
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