1,721,323 research outputs found
Alloy nanoclusters in dielectric matrix
No full textSequential ion implantation in dielectric matrix determines three different cluster morphologies: separated systems, alloy clusters and core-shell clusters. Post-implantation thermal treatments influence the alloy stability as a function of the annealing atmosphere (oxidizing, inert or reducing). Case studies will be given for the three different structures
Calculated high resolution electron microscopy images of nano-sized structures obtained by molecular dynamics
No full textThe limits of high resolution electron microscopy (HREM) applied to quantum-sized structures were investigated by designing a simulation method which combines molecular dynamics and HREM imaging. Standard molecular dynamics is used to simulate either disorder due to ion irradiation or the formation of layered structures by beam-assisted deposition. The HREM images of the structures thus obtained are computed by a multislice dynamic procedure. For both cases the simulation results indicate that HREM images reproduce closely the actual lattice structure
Potentialities of ion implantation for the synthesis and modification of metal nanoclusters
No full textThis work reviews the basic features of ion implantation as a tool for synthesizing and manipulating nanostructured materials with particular emphasis to nanocomposites made by dielectric matrices embedding metallic nanoparticles. After a brief introduction on the physics of ion implantation, two operating modes of ion implantation will be presented: the first is related to the direct synthesis of nanoclusters as resulting from the precipitation of the supersaturated solid solution produced by the implanted ions; the second is related to the modification induced by the ion beam on already formed nanostructures and is therefore referred to as indirect synthesis. Throughout the paper also various possible technological applications of the different kind of nanoclusters (mono-elemental or bimetallic) obtained by both approaches are presented: examples are given in the field of nonlinear optics, nanophotonics, magnetism and gas sensing
Analysis of the capability of HREM applied to quantum heterostructures
No full textThis work analyzes the possibilities offered by HREM for the observation of quantum heterostructures. Multilayers of Cu/Pd, Fe/Ag and Ag/Si are constructed by using a molecular dynamics simulation method and the configuration of the electronic charge at the interfaces is obtained from a Hartree-Fock calculation of the type extended Debye-Hueckel approximation. The HREM image of the multilayers is calculated by using a standard dynamical diffraction method. MD simulations show that in all cases good epitaxis can be achieved. However the interfacial regions have a complex structural and electronic configuration which is significantly different from the bulk reference state. As these regions span few monolayers across their observation is extremely critical. The HREM image of the films is clear. The interfaces and defective epitaxial growth can be observed. An interesting possibility for future developments appears to be offered by the availability of the interfacial potential obtained from the HREM image intensity
Introduzione alle Nanotecnologie e ai Nanosistemi
No full textPresentare un quadro completo e scientificamente avanzato dello stato dell’arte nel settore delle nanotecnologie, che peraltro si sta velocemente e “quotidianamente” modificando, necessiterebbe della partecipazione nella stesura di esperti in differenti settori tecnologici, la produzione di un testo di rilevanti dimensioni, con tempi di consegna della versione finale non consistenti con il piano temporale inizialmente previsto per la pubblicazione.
Questo peraltro non era lo scopo dell’AssoAutomazione nel realizzare la presente documentazione.
Le finalità indicate dall’AssoAutomazione erano quelle di fornire, in particolare alle PMI, un quadro generale sulle prospettive offerte dalle nanotecnologie, evitando di introdurre tematiche scientifiche avanzate, che potranno essere approfondite rivolgendosi a Centri di Ricerca specialistici, citati nel testo, o consultando articoli scientifici e testi specifici nel settore di interesse, riportati nei riferimenti bibliografici, anche se non in forma esaustiva.
Questo lavoro non è quindi configurato come un testo scientifico indirizzato al settore della ricerca avanzata, ma come una selezione introduttiva alle proprietà chimico-fisiche di determinati materiali nanostrutturati o di nanostrutture, che attualmente presentano specifiche applicazioni e che, in prospettiva, possono trovare nuove ricadute tecnologiche. Il livello di presentazione è stato quindi adeguato ad un lettore non esperto nel settore ma, piuttosto, interessato ai possibili campi di applicazione dei nanomateriali.
Si è evitato di approfondire inoltre alcune tematiche, attualmente affrontate da grandi industrie italiane o straniere. Per specifici interessi in tali settori si suggerisce di fare riferimento alle Industrie operanti nelle corrispondenti aree tecnologiche.
Abbiamo riportato, nella parte finale, l’indicazione di Centri, Laboratori ed Istituti Universitari operanti nei vari settori dei materiali nanostrutturati
Multiple cascades regime in fcc metals. Molecular dynamics simulations and calculated HREM imaging
No full textThis study presents a simulation method which consists of three parts. The first one is classic molecular dynamics and is used to evaluate disordered atomic configurations. The second one is an evaluation of the electronic charge configuration of the type extended Debye-Hueckel. The third one calculates the HREM image of the lattice structure obtained from molecular dynamics. The simulation method is applied to collective motions of PKAs displaced in a fee lattice with energies near the displacement threshold. These conditions, which represent in a simplified manner e(-) irradiation, are aimed at investigating whether the defect growth kinetics shows ordering phenomena similar to the ones studied in phase transition. Embryonic forms of ordered interstitial agglomeration and unusual oscillatory behaviours of recrystallization are observed. The HREM calculations show that disorder is visible during its entire evolution
Some aspects of ion implantation technique in nanostructured materials
No full textThe paper will discuss the synthesis of metal nanoclusters (NCs) composites, in silica matrix, in which the cluster concentration is below the percolation limit (dispersed clusters). Some case studies will be presented in which ion-beam-based techniques are exploited either for the synthesis, but also for the structural or compositional modification of composites. As regards the technological properties of the studied systems, some results will be presented about the nonlinear properties and energy transfer mechanisms in silica containing Erbium ions and molecular-like metal structures
Analysis of irradiated Ag and Si by molecular dynamics and calculated HREM imaging
No full textIn order to clarify the limits of HREM observations of defects due to radiation damage, a simulation method has been designed which combines molecular dynamics and HREM simulation. Disorder due to ion irradiation in Si and Ag is simulated by using a molecular dynamics technique and HREM images are computed by a multislice dynamic procedure. The conditions to achieve a goad HREM visibility of such defective structures have been investigated by analyzing the effects of the physical inputs of molecular dynamics as well as the ones of the instrumental parameters of TEM observations. This analysis indicates that the structure of the damaged region is not critical. On the contrary, the fractional disorder is of paramount importance and determines the conditions leading to an appreciable contrast of the disordered region against the crystalline background
Shape and stability of heteroepitaxial metallic islands: effects of the electronic configuration
No full textThe stability of metallic islands, epitaxially grown on a metallic substrate, is Studied by the evaluation of the island total energy. Hartree-Fock calculations at semiempirical level have been used to describe the island electronic configuration with quantum mechanical detail. The functional dependence of the island energy on the elements forming the island and the substrate (i.e. Ag, Cu and Fe). on the island geometry and on its state of stress, is analyzed and discussed. A similar analysis is also carried out for the barrier experienced by adatoms at descending steps
Point defects observations by molecular dynamics and calculated TEM imaging
No full textA simulation method which combines molecular dynamics and calculated TEM imaging is presented. The simulation is applied to irradiated crystalline silicon and silver to clarify the minimum amount of disorder which could be experimentally observed. It is found that in Si isolated cascades formed at energies near the displacement threshold are clearly visible, while in Ag observable disordered regions are generated only by the overlapping of many cascades launched with energy well above the energy displacement threshold
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