Combining rare events techniques: phase change in Si nanoparticles

We introduce a combined Restrained MD/Parallel Tempering approach to study the difference in free energy as a function of a set of collective variables between two states in presence of unknown slow degrees of freedom. We applied this method to study the relative stability of the amorphous vs crystalline nanoparticles of size ranging between 0.8 and 1.8 nm as a function of the temperature. We found that, at variance with bulk systems, at low T small nanoparticles are amorphous and undergo an amorphous-to-crystalline phase transition at higher T. On the contrary, large nanoparticles recover the bulk-like behavior: crystalline at low $T$ and amorphous at high T

Order-disorder phase change in embedded Si nano-particles

We investigated the relative stability of the amorphous vs crystalline nanoparticles of size ranging between 0.8 and 1.8 nm. We found that, at variance from bulk systems, at low T small nanoparticles are amorphous and they undergo to an amorphous-to-crystalline phase transition at high T. On the contrary, large nanoparticles recover the bulk-like behavior: crystalline at low T and amorphous at high T. We also investigated the structure of crystalline nanoparticles, providing evidence that they are formed by an ordered core surrounded by a disordered periphery. Furthermore, we also provide evidence that the details of the structure of the crystalline core depend on the size of the nanoparticleComment: 8 pages, 5 figure

Collapse of superhydrophobicity on nanopillared surfaces

The mechanism of the collapse of the superhydrophobic state is elucidated for submerged nanoscale textures forming a three-dimensional interconnected vapor domain. This key issue for the design of nanotextures poses significant simulation challenges as it is characterized by diverse time and length scales. State-of-the-art atomistic rare events simulations are applied for overcoming the long time scales connected with the large free energy barriers. In such interconnected surface cavities wetting starts with the formation of a liquid finger between two pillars. This break of symmetry induces a more gentle bend in the rest of the liquid-vapor interface, which triggers the wetting of the neighboring pillars. This collective mechanism, involving the wetting of several pillars at the same time, could not be captured by previous atomistic simulations using surface models comprising a small number of pillars (often just one). Atomistic results are interpreted in terms of a sharp-interface continuum model which suggests that line tension, condensation, and other nanoscale phenomena play a minor role in the simulated conditions

Unraveling the Salvinia paradox: design principles for submerged superhydrophobicity

The complex structure of the Salvinia molesta is investigated via rare event molecular dynamics simulations. Results show that a hydrophilic/hydrophobic patterning together with a re-entrant geometry control the free energy barriers for bubble nucleation and for the Cassie-Wenzel transition. This natural paradigm is translated into simple macroscopic design criteria for engineering robust superhydrophobicity in submerged applications

The Influence of Silicon Nanoclusters on the Optical Properties of a-SiNx Samples: A Theoretical Study

By means of ab-initio calculations we investigate the optical properties of pure a-SiN$_x$ samples, with $x \in [0.4, 1.8]$, and samples embedding silicon nanoclusters (NCs) of diameter $0.5 \leq d \leq 1.0$ nm. In the pure samples the optical absorption gap and the radiative recombination rate vary according to the concentration of Si-N bonds. In the presence of NCs the radiative rate of the samples is barely affected, indicating that the intense photoluminescence of experimental samples is mostly due to the matrix itself rather than to the NCs. Besides, we evidence an important role of Si-N-Si bonds at the NC/matrix interface in the observed photoluminescence trend

An observable for vacancy characterization and diffusion in crystals

To locate the position and characterize the dynamics of a vacancy in a crystal, we propose to represent it by the ground state density of a quantum probe quasi-particle for the Hamiltonian associated to the potential energy field generated by the atoms in the sample. In this description, the h^2/2mu coefficient of the kinetic energy term is a tunable parameter controlling the density localization in the regions of relevant minima of the potential energy field. Based on this description, we derive a set of collective variables that we use in rare event simulations to identify some of the vacancy diffusion paths in a 2D crystal. Our simulations reveal, in addition to the simple and expected nearest neighbor hopping path, a collective migration mechanism of the vacancy. This mechanism involves several lattice sites and produces a long range migration of the vacancy. Finally, we also observed a vacancy induced crystal reorientation process

Relaxation of a steep density gradient in a simple fluid: comparison between atomistic and continuum modeling

We compare dynamical nonequilibrium molecular dynamics and continuum simulations of the dynamics of relaxation of a fluid system characterized by a non uniform density profile. Results match quite well as long as the lengthscale of density nonuniformities are greater than the molecular scale (10 times the molecular size). In presence of molecular scale features some of the continuum fields (e.g. density and momentum) are in good agreement with atomistic counterparts, but are smoother. On the contrary, other fields, such at the temperature field, present very large difference with respect to reference (atomistic) ones. This is due to the limited accuracy of some of the empirical relations used in continuum models, the equation of state of the fluid in the example considered

La condizione di insularità nell’Unione Europea: accessibilità e incidenza del trasporto marittimo

Questo paper esamina gli aspetti relativi all’accessibilità delle isole, così come definite dall’U.E., in ambito Europeo. L’accessibilità, nell’Unione Europea (in accordo con lo studio ESPON "Atlas" del 2006) è stata legata al concetto di “cuore” del territorio Europeo e di “periferia”; in questo modo l’ubicazione geografica e la distanza fisica sono divenuti i parametri significativi in relazione all’accessibilità in termini di infrastrutture e di sistema di trasporti. L’obiettivo del seguente studio è quello di indagare in che modo l’insularità possa essere analizzata, caratterizzata e misurata in relazione alle peculiarità endogene ed ai requisiti strutturali e funzionali del sistema dei collegamenti ed in che modo questa misura possa garantire un confronto quantitativo, semplice da interpretare, dell’accessibilità con realtà e regioni della terraferma, anche periferiche. In particolare, vengono proposti una serie di indicatori che descrivono l’accessibilità delle isole in riferimento al sistema dei trasporti marittimi, attraverso la specificazione di una serie di attributi dell’accessibilità che fanno riferimento ai parametri di lontananza (distanza reale), isolamento e discontinuità geografica (frequenza e tempi di attesa), parametri che caratterizzano le realtà insulari

Mechanism of the Cassie-Wenzel transition via the atomistic and continuum string methods

The string method is a general and flexible strategy to compute the most probable transition path for an activated process (rare event). We apply here the atomistic string method in the density field to the Cassie-Wenzel transition, a central problem in the field of superhydrophobicity. We discuss in detail the mechanism of wetting of a submerged hydrophobic cavity of nanometer size and its dependence on the geometry of the cavity. Furthermore, we discuss the algorithmic analogies between the string method and CREaM [Giacomello et al., Phys. Rev. Lett. 109, 226102 (2012)], a method inspired by the string that allows for a faster and simpler computation of the mechanism and of the free-energy profiles of the wetting process. This approach is general and can be employed in mesoscale and macroscopic calculations

Liquid intrusion in and extrusion from non-wettable nanopores for technological applications

In this article, we review some recent theoretical results about intrusion and extrusion of non-wetting liquids in and out of cavities of nanotextured surfaces and nanoporous materials. Nanoscale confinement allows these processes to happen at conditions which significantly differ from bulk phase coexistence. In particular, the pressure at which a liquid penetrates in and exits from cavities is of interest for many technological applications such as energy storage, dissipation, and conversion, materials with negative compressibility, ion channels, liquid chromatography, and more. Notwithstanding its technological interest, intrusion/extrusion processes are difficult to understand and control solely via experiments: the missing step is often a simple theory capable of providing a microscopic interpretation of the results, e.g., of liquid porosimetry or other techniques used in the field, especially in the case of complex nanoporous media. In this context, simulations can help shedding light on the relation between the morphology of pores, the chemical composition of the solids and liquids, and the thermodynamics and kinetics of intrusion and extrusion. Indeed, the intrusion/extrusion kinetics is determined by the presence of free energy barriers and special approaches, the so-called rare event techniques, must be used to study these processes. Usually, rare event techniques are employed to investigate processes occurring in relatively simple molecular systems, while intrusion/extrusion concerns the collective dynamics of hundreds to thousands of degrees of freedom, the molecules of a liquid entering in or exiting from a cavity, which, from the methodological point of view, is itself a challenge