arXiv subjects

Fausto Cargnoni

Publications and source records attributed to Fausto Cargnoni.

5 recordsLinked to original sources

arXiv2024

Revisiting Thomson's model with multiply charged superfluid helium nanodroplets

We study superfluid helium droplets multiply charged with ions. When stable, the charges are found to reside in equilibrium close to the droplet surface, thus representing a physical realization of Thomson's model. We find the minimum radius of the helium droplet that can host a given number of ions using a model whose physical ingredients are the solvation energy of the cations, calculated within the He-DFT approach, and their mutual Coulomb repulsion energy. Our model goes beyond the often used liquid drop model, where charges are smeared out either within the droplet or on its surface, and which neglects the solid-like helium shell around the ions. We find that below a threshold droplet radius R_0, the total energy of the system becomes higher than that of the separated system of the pristine helium droplet and the charges embedded in their solvation microcluster ("snowball"). However, the ions are still kept within the droplet by the presence of energy barriers which hinder Coulomb explosion. A further reduction of the droplet radius below a value R_expl eventually results in the disappearance of such barrier, leading to Coulomb explosion. Surprisingly, our results are rather insensitive to the ion atomic species. This makes room to discuss them in the context of intrinsic multicharged helium droplets, where the charges are triatomic He3+ ions. Our calculated values for R_expl display the correct scaling with the number of cations compared to available experimental results, at variance with other estimates for the critical radii.

cond-mat.mes-hall↗

arXiv2020

Ab Initio Many-Body Perturbation Theory Calculations of the Electronic and Optical Properties of Cyclometalated Ir(III) Complexes

Cyclometalled Ir(III) compounds are the preferred choice as organic emitters in Organic Light Emitting Diodes. In practice, the presence of the transition metals surrounded by carefully designed ligands allows the fine tuning of the emission frequency as well as a good efficiency of the device. To support the development of new compounds the experimental measurements are generally compared with ab-initio calculation of the absorption and emission spectra. The standard approach for these calculations is TDDFT with hybrid exchange and correlation functional like the B3LYP. Due to the size of these compounds the application of more complex quantum chemistry approaches can be challenging. In this work we used Many Body Perturbation Theory approaches (in particular the GW approximation with the Bethe-Salpeter equation) implemented in gaussian basis sets, to calculate the quasiparticle properties and the adsorption spectra of six cyclometalled Ir(III) complexes going behind TDDFT. In the presented results we compared standard TDDFT simulation with BSE calculations performed on top on perturbative G 0 W 0 and accounting for eigenvalue self consistency. Moreover, in order to investigate in detail the effect of the DFT starting point, we concentrate on Ir(ppy) 3 performing GW-BSE simulations starting from different DFT exchange and correlation potentials.

cond-mat.mtrl-sci↗

arXiv2019

Unraveling the degradation mechanism of FIrpic based blue OLEDs: I. A theoretical investigation

We report a detailed ab-initio study of the of the microscopic degradation mechanism of FIrpic, a popular blue emitter in OLED devices. We simulate the \emph{operando} conditions of FIrpic by adding an electron-hole pair (exciton) to the system. We perform both static calculations with the TDDFT framework and we also simulate the evolution of the system at finite temperature via Car-Parrinello molecular dynamics. We found triplet excitons are very effective in reducing the Ir-N bond breaking barrier of the picolinate moiety. After the first bond breaking, the two oxygen of picolinate swap their position and FIrpic can either remain stable in an "open" configuration, or loose a picolinate fragment, which at a later stage can evolve a CO$_2$ molecule. Our method can be applied to other light emitting Ir-complexes in order to quickly estimate their stability in OLED devices. In Paper~II we complement our theoretical study with a parallel experimental investigation of the key degradation steps of FIrpic in an aged device.

cond-mat.mtrl-sci↗

arXiv2019

Unraveling the degradation mechanism in FIrpic based Blue OLEDs: II. Trap and detect molecules at the interfaces

The impact of organic light emitting diodes (OLEDs) in modern life is witnessed by their wide employment in full-color, energy-saving, flat panel displays and smart-screens; a bright future is likewise expected in the field of solid state lighting. Cyclometalated iridium complexes are the most used phosphorescent emitters in OLEDs due to their widely tunable photophysical properties and their versatile synthesis. Blue-emitting OLEDs, suffer from intrinsic instability issues hampering their long term stability. Backed by computational studies, in this work we studied the sky-blue emitter FIrpic in both ex-situ and in-situ degradation experiments combining complementary, mutually independent, experiments including chemical metathesis reactions, in liquid phase and solid state, thermal and spectroscopic studies and LC-MS investigations. We developed a straightforward protocol to evaluate the degradation pathways in iridium complexes, finding that FIrpic degrades through the loss of the picolinate ancillary ligand. The resulting iridium fragment was than efficiently trapped "in-situ" as BPhen derivative 1. This process is found to be well mirrored when a suitably engineered, FIrpic-based, OLED is operated and aged. In this paper we (i) describe how it is possible to effectively study OLED materials with a small set of readily accessible experiments and (ii) evidence the central role of host matrix in trapping experiments.

physics.app-ph↗

arXiv2005

Predicting atomic dopant solvation in helium clusters: the MgHe$_n$ case

We present a quantum Monte Carlo study of the solvation and spectroscopic properties of the Mg doped helium clusters MgHe$_n$ with $n=2-50$. Three high level (MP4, CCSD(T) and CCSDT) MgHe interaction potentials have been used to study the sensitivity of the dopant location on the shape of the pair interaction. Despite the similar MgHe well depth, the pair distribution functions obtained in the diffusion Monte Carlo simulations markedly differ for the three pair potentials, therefore indicating different solubility properties for Mg in He$_n$. Moreover, we found interesting size effects for the behavior of the Mg impurity. As a sensitive probe of the solvation properties, the Mg excitation spectra have been simulated for various cluster sizes and compared with the available experimental results. The interaction between the excited $^1$P Mg atom and the He moiety has been approximated using the Diatomics-in-Molecules method and the two excited $^1Π$ and $^1Σ$ MgHe potentials. The shape of the simulated MgHe$_{50}$ spectra show a substantial dependency on the location of the Mg impurity, and hence on the MgHe pair interaction employed. To unravel the dependency of the solvation behavior on the shape of the computed potentials, exact Density Functional Theory has been adapted to the case of doped He$_n$ and various energy distributions have been computed. The results indicate the shape of the repulsive part of the MgHe potential as an important cause of the different behaviours.

physics.atm-clus↗