15 research outputs found
Spettroscopia computazionale di cluster metallici: sviluppo metodi ed applicazioni
No full textI cluster di metalli hanno iniziato a svolgere un ruolo importante nella scienza dei materiali negli ultimi decenni. Ciò è dovuto al comportamento peculiare mostrato dalla materia a dimensioni nanometriche. Tali differenti comportamenti vanno da proprietà tipiche (energie di coesione, potenziali di ionizzazione, numeri medi di coordinazione) fino alla reattività (catalisi) e alla risposta ai campi elettromagnetici esterni (comportamenti ottici). Inoltre la materia a nanodimensione pone domande interessanti ai modelli interpretativi: spesso è difficile descrivere i nanosistemi con i paradigmi tipici delle molecole o dei sistemi bulk. Per tali ragioni in questo campo la teoria e in generale gli studi computazionali possono essere molto importanti per razionalizzare i comportamenti osservati. Nel presente studio ci siamo concentrati sulle proprietà ottiche dei cluster metallici, che includono transizioni molecolari standard e fenomeni collettivi come i plasmoni. La necessità di descrivere i plasmoni e quindi il moto collettivo degli elettroni impone di andare oltre l'una particella (approssimazione di particelle indipendenti), considerando un modello di risposta lineare come TDDFT, che offre all'economia computazionale della DFT insieme la capacità di descrivere fenomeni collettivi. In particolare gran parte degli sforzi è stata dedicata al miglioramento dell'algoritmo TDDFT di polarizzabilità complessa (POLTDDFT), un metodo efficiente recentemente sviluppato nel gruppo di chimica teorica dell'Università di Trieste per risolvere le equazioni TDDFT per sistemi di grandi dimensioni e per un ampio intervallo di energia, in per evitare i limiti di schemi più tradizionali come quello dovuto a Casida. A questo proposito un nuovo schema chiamato Hybrid Diagonal Appromation (HDA) ha permesso di applicare il funzionale ibrido nei calcoli TDDFT di grandi sistemi, ottenendo un'ottima corrispondenza con l'esperimento. Anche in questo lavoro è stato realizzato lo sviluppo di un nuovo set di base per il fit della densità, ottimizzato per POLTDDFT, che fornisce all'utente generico uno strumento valido per applicare il metodo ad una gamma molto ampia di target. Oltre allo sviluppo di metodi, anche applicazioni a campi aperti della scienza dei cluster, come il dicroismo circolare elettronico (ECD) di cluster metallici chirali in collaborazione con Thomas Bürgi (Università di Ginevra) e esperimenti di fotoassorbimento a bassa temperatura molto accurati in collaborazione con Flavio Maran (Università di Padova) ) sono stati presi in considerazione. Nel campo specifico della plasmonica è stato studiato anche l'effetto dell'accoppiamento di due sistemi chirali plasmonici sull'ECD ed è stato identificato il meccanismo di accoppiamento sottostante. L'accuratezza del metodo suggerito insieme alla sua efficienza computazionale rendono POLTDDFT un buon candidato per applicazioni ancora più ampie, ma, cosa più importante, l'accuratezza quantitativa ottenuta rende questo metodo predittivo, e quindi un buon candidato anche per la progettazione in-silico di nuovi materiali con le proprietà ottiche richieste.Metal clusters have started to play a major role in material science in the last decades. This is due to the peculiar behaviour showed by the matter at nanosize. Such different behaviours range from typical properties (cohesive energies, ionization potentials, average coordination numbers) up to reactivity (catalysis) and response to external electromagnetic fields (optical behaviours). Moreover matter at nanosize poses interesting questions to interpretative models: it is often hard to describe nanosystem with the paradigms typical of molecules or bulk systems. For such reasons in this field theory and in general computational studies can be very important to rationalize the observed behaviours. In the present study we have focussed on the optical properties of metal clusters, which include standard molecular transitions as well as collective phenomena like plasmons. The necessity to describe plasmons and therefore electron collective motion imposes to go beyond the one-particle (independent particle approximation), considering a linear response model like TDDFT, which offers the computational economy of DFT together the ability to describe collective phenomena. In particular a large part of efforts have been devoted to improve the complex polarizability TDDFT algorithm (POLTDDFT), an efficient method recently developed in the theoretical chemistry group of Trieste University to solve the TDDFT equations for large systems and for a wide energy interval, in order to avoid the limits of more traditional schemes like that due to Casida. In this respect a new scheme called Hybrid Diagonal Approximation (HDA) has allowed to apply hybrid functional in TDDFT calculations of large systems, obtaining excellent match with the experiment. Also in this work the development of a new density fitting basis set, optimized for POLTDDFT, has been carried out, giving to the generic user a valid tool to apply the method to a very wide range of targets. Besides method development, also applications to open fields of cluster science, like Electronic Circular Dichroism (ECD) of chiral metal clusters in collaboration with Thomas Bürgi (Geneve University) as well as very accurate low temperature photoabsorption experiments in collaboration with Flavio Maran (Padua University) have been considered. In the specific field of plasmonics, also the effect of coupling two plasmonic chiral systems on the ECD has been studied, and the underlying coupling mechanism has been identified. The accuracy of the suggested method together with its computational efficiency make POLTDDFT a good candidate for even wider applications, but, more important, the quantitative accuracy obtained makes this method predictive, and therefore a good candidate also for the in-silico design of novel materials with requested optical properties
Optimization of density fitting auxiliary Slater‐type basis functions for time‐dependent density functional theory
No full textA new set of auxiliary basis function suitable to fit the induced electron density is presented. Such set has been optimized in order to furnish accurate absorption spectra using the complex polarizability algorithm of time‐dependent density functional theory (TDDFT). An automatic procedure has been set up, able, thanks to the definition of suitable descriptors, to evaluate the resemblance of the auxiliary basis‐dependent calculated spectra with respect to a reference. In this way, it has been possible to reduce the size of the basis set maximizing the basis set accuracy. Thanks to the choice to employ a collection of molecules for each element, such basis has proven transferable to molecules outside the collection. The final sets are therefore much more accurate and smaller than the previously optimized ones and have been already included in the database of the last release of the AMS suite of programs. The availability of the present new set will allow to improve drastically the applicability range of the polTDDFT method with higher accuracy and less computational effort
Coupling between Plasmonic and Molecular Excitations: TDDFT Investigation of an Ag-Nanorod/BODIPY-Dye Interaction
No full textA time-dependent density functional theory (TDDFT) computational approach is employed to study the optical coupling between a plasmonic system (a Ag-50 nanorod) and a fluorescent dye (BODIPY). It is found that the BODIPY dye can interact with a plasmonic system in a rather different and selective way according to the mutual orientation of the fragments. Indeed, (i) the plasmon excitation turns out to be sensitive to the presence of the BODIPY transition and (ii) this can lead to amplify or suppress the resonance accordingly to the relative orientation of the corresponding transition dipoles. To understand the coupling mechanism, we analyze the shape of the induced density in real space and the Individual Component Map of the Oscillator Strength (ICM-OS) plots and achieve a simple rationalization and insight on the origin and features of the coupling. The resulting possibility of understanding plasmon/fluorophore interactions by simple qualitative arguments opens the way to a rational design of hybrid (plasmon + dye) systems with the desired optical behavior
A Resolution of Identity Technique to Speed up TDDFT with Hybrid Functionals:Implementation and Application to the Magic Cluster Series Au<sub>8n+4</sub>(SC<sub>6</sub>H<sub>5</sub>)<sub>4n+8</sub> (n = 3-6)
No full textThe Resolution of Identity (RI) technique has been employed to speed up the use of hybrid exchange-correlation (xc) functionals at the TDDFT level using the Hybrid Diagonal Approximation. The RI has been implemented within the polTDDFT algorithm (a complex damped polarization method) in the AMS/ADF suite of programs. A speedup factor of 30 has been obtained with respect to a previous numerical implementation, albeit with the same level of accuracy. Comparison of TDDFT simulations with the experimental photoabsorption spectra of the cluster series Au8n+4(SR)4n+8(n = 3-6; R = C6H5) showed the excellent accuracy and efficiency of the method. Results were compared with those obtained via the more simplified and computationally cheaper TDDFT+TB and sTDDFT methods. The present method represents an accurate as well as computationally affordable approach to predict photoabsorption spectra of complex species, realizing an optimal compromise between accuracy and computational efficiency, and is suitable for applications to large metal clusters with sizes up to several hundreds of atoms.</p
Chiral Functionalization of an Atomically Precise Noble Metal Cluster: Insights into the Origin of Chirality and Photoluminescence
No full textWe probe the origin of photoluminescence of an atomically precise noble metal cluster, Ag24Au1(DMBT)18 (DMBT = 2,4-dimethylbenzenethiolate), and the origin of chirality in its chirally functionalized derivatives, Ag24Au1(R/S-BINAS)
x
(DMBT)18-2x, with x = 1-7 (R/S-BINAS = R/S-1,1'-[binaphthalene]-2,2'-dithiol), using chiroptical spectroscopic measurements and density functional theory (DFT) calculations. Combination of chiroptical and luminescence spectroscopies to understand the nature of electronic transitions has not been applied to such molecule-like metal clusters. In order to impart chirality to the achiral Ag24Au1(DMBT)18 cluster, the chiral ligand, R/S-BINAS, was incorporated into it. A series of clusters, Ag24Au1(R/S-BINAS)
x
(DMBT)18-2x, with x = 1-7, were synthesized. We demonstrate that the low-energy electronic transitions undergo an unexpected achiral to chiral and back to achiral transition from pure Ag24Au1(DMBT)18 to Ag24Au1(R/S-BINAS)
x
(DMBT)18-2x, by increasing the number of BINAS ligands. The UV/vis, luminescence, circular dichroism, and circularly polarized luminescence spectroscopic measurements, in conjunction with DFT calculations, suggest that the photoluminescence in Ag24Au1(DMBT)18 and its chirally functionalized derivatives originates from the transitions involving the whole Ag24Au1S18 framework and not merely from the icosahedral Ag12Au1 core. These results suggest that the chiroptical signatures and photoluminescence in these cluster systems cannot be solely attributed to any one of the structural components, that is, the metal core or the protecting metal-ligand oligomeric units, but rather to their interaction and that the ligand shell plays a crucial role. Our work demonstrates that chiroptical spectroscopic techniques such as circular dichroism and circularly polarized luminescence represent useful tools to understand the nature of electronic transitions in ligand-protected metal clusters and that this approach can be utilized for gaining deeper insights into the structure-property relationships of the electronic transitions of such molecule-like clusters
Predictive optical photoabsorption of Ag24Au(DMBT)18 - via efficient TDDFT simulations
No full textWe report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn-Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn-Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches
Back to meaning - information structuring in the PEER project
No full textTest supprimer ce commentaire Alain M. 04/06 3eme testInternational audienceWe present the infrastructure for mapping publishers' metadata formats into a standardized TEI representation in the context of the EU PEER project. Initiated as an experiment to observe the consequence of large scale author manuscript deposit in publication repositories, the project led to the design and implementation of an information HUB (the PEER Depot) where all publishers' data (the author's manuscript and, when available, publishers' metadata) would be normalized so that they could be further uploaded to a series of trusted publication repositories to be put eventually in open access
Plasmonic Circular Dichroism in Chiral Gold Nanowire Dimers
No full textWe report a computational study at the time-dependent density functional theory (TDDFT) level of the chiro-optical spectra of chiral gold nanowires coupled in dimers. Our goal is to explore whether it is possible to overcome destructive interference in single nanowires that damp chiral response in these systems and to achieve intense plasmonic circular dichroism (CD) through a coupling between the nanostructures. We predict a huge enhancement of circular dichroism at the plasmon resonance when two chiral nanowires are intimately coupled in an achiral relative arrangement. Such an effect is even more pronounced when two chiral nanowires are coupled in a chiral relative arrangement. Individual component maps of rotator strength, partial contributions according to the magnetic dipole component, and induced densities allow us to fully rationalize these findings, thus opening the way to the field of plasmonic CD and its rational design
Florisztikai felmérések hibaforrásai I. „Jószándékú” flórahamisítások: „kakukk” a Medvesen
No full textThe aim of the planned thematic studies is to point out the main biases and/or errors of floristic surveys in Hungary. In this first article of the series we focus on those examples when native species were planted out of their original habitats and/or range by a non-professional naturalist. The sources of information are those letters that were posted by Ferenc Varga to the first author of this article in 2005. The circumstances of the transplantation of 10 phytogeographycally significant species of the investigated area and/or the Pannonicum ( Adonis vernalis , Arum orientale , Asarum europaeum , Genista germanica , Phragmites australis , Pulsatilla grandis , P. nigricans , Salvia aethiopis , Viola alba , Xeranthemum annuum ) are detailed in the text and summarised in a table. The correspondence between the first author and Ferenc Varga provides insight how the ambition of a non-professional naturalist can influence the flora of an area. Most of the species were removed from South and transplanted to North, almost 10 km away from their original stands. With the exception of Adonis vernalis and Genista germanica at least one transplantation of the species was successful. In the case of Salvia aethiopis , the naturalized populations came 1.25 km close to Slovakia, where this species is a very rare and endangered (EN) taxon.A tervezett cikksorozat a florisztikai térképezések, illetve az így keletkezett adatbázisok hibaforrásaira szeretné felhívni a figyelmet. E dolgozatban elsőként a „jószándékú” flórahamisítás esetét mutatjuk be egy kistáj, a Medves vidék példáján. Az adatok forrását azok a 2005-ben postázott levelek képezik, amelyeket Varga Ferenc az elsőszerző kérésére fogalmazott meg. A dolgozatban 10, a tájegység és/vagy a Pannonicum szempontjából növényföldrajzilag is jelentős faj ( Adonis vernalis , Arum orientale , Asarum europaeum , Genista germanica , Phragmites australis , Pulsatilla grandis , P. nigricans , Salvia aethiopis , Viola alba , Xeranthemum annuum ) átültetésének körülményeit listázzuk. A levelezésekből kiderül, hogy amatőr botanikusként milyen módon szólhatunk bele egy terület flórájának alakulásába. A fajok döntő részét Varga Ferenc déli irányból észak felé mozdította el, átlagosan mintegy 10 km-nyire az eredeti termőhelyüktől. Az Adonis vernalis és a Genista germanica kivételével legalább egy áttelepítés sikeresen, meghonosodással végződött. A Salvia aethiopis esetében az új állomány csupán 1,25 km-nyire található Szlovákiától, ahol a faj már kifejezetten ritka, veszélyeztetett (EN) taxon
