Mixed ab initio quantum mechanical and Monte Carlo calculations of secondary emission from SiO2 nanoclusters

A mixed quantum mechanical and Monte Carlo method for calculating Auger spectra from nanoclusters is presented. The approach, based on a cluster method, consists of two steps. Ab initio quantum mechanical calculations are first performed to obtain accurate energy and probability distributions of the generated Auger electrons. In a second step, using the calculated line shape as electron source, the Monte Carlo method is used to simulate the effect of inelastic losses on the original Auger line shape. The resulting spectrum can be directly compared to 'as-acquired' experimental spectra, thus avoiding background subtraction or deconvolution procedures. As a case study, the O K-LL spectrum from solid SiO2 is considered. Spectra computed before or after the electron has traveled through the solid, i.e., unaffected or affected by extrinsic energy losses, are compared to the pertinent experimental spectra measured within our group. Both transition energies and relative intensities are well reproduced.Comment: 9 pageg, 5 figure

The role of low-energy electrons in the charging process of LISA test masses

The estimate of the total electron yield is fundamental for our understanding of the test-mass charging associated with cosmic rays in the Laser Interferometer Space Antenna (LISA) Pathfinder mission and in the forthcoming gravitational wave observatory LISA. To unveil the role of low energy electrons in this process owing to galactic and solar energetic particle events, in this work we study the interaction of keV and sub-keV electrons with a gold slab using a mixed Monte Carlo (MC) and ab-initio framework. We determine the energy spectrum of the electrons emerging from such a gold slab hit by a primary electron beam by considering the relevant energy loss mechanisms as well as the elastic scattering events. We also show that our results are consistent with experimental data and MC simulations carried out with the GEANT4-DNA toolkit

Monte Carlo simulations of measured electron energy-loss spectra of diamond and graphite: Role of dielectric-response models

N.M.P. is supported by the European Research Council PoC 2015 ”Silkene” No. 693670, by the European Commission H2020 under the Graphene Flagship Core 1 No. 696656 (WP14 ”Polymer Nanocomposites”) and under the FET Proactive ”Neurofibres” No. 732344. M.D., G.G., and S.T acknowledge funding from the Graphene Flagship (WP14 “Polymer composites”, no. 696656). This work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk). Access to computing and storage facilities owned by parties and projects contributing to the National Grid Infrastructure MetaCentrum provided under the programme ”Projects of Large Research, Development, and Innovations Infrastructures” (CESNET LM2015042), is greatly appreciated (https://www.metacentrum.cz/en/). Furthermore, we acknowledge FBK for providing unlimited access to the KORE computing facility

Formation of Si Nanocrystals for Single Electron Transistors by Ion Beam Mixing and Self-Organization – Modeling and Simulation

The replacement of the conventional field effect transistor (FET) by single electron transistors (SET) would lead to high energy savings and to devices with significantly longer battery life. There are many production approaches, but mostly for specimens in the laboratory. Most of them suffer from the fact that they either only work at cryogenic temperatures, have a low production yield or are not reproducible and each unit works in a unique way. A room temperature (RT) operating SET can be configured by inserting a small (few nm diameters) Si-Nanocrystal (NC) into a thin (<10 nm) SiO2 interlayer in Si. Industrial production has so far been excluded due to a lack of manufacturing processes. Classical technologies such as lithography fail to produce structures in this small scale. Even electron beam lithography or extreme ultraviolet lithography are far from being able to realize these structures in mass production. However, self-organization processes enable structures to be produced in any order of magnitude down to atomic sizes. Earlier studies realized similar systems using a layer of Si-NCs to fabricate a non-volatile memory by using the charge of the NCs for data storage. Based on this, it is very promising to use it for the realization of the SET. The self-organization depends only on the start configuration of the system and the boundary conditions during the process. These macroscopic conditions control the self-formed structures. In this work, ion beam irradiation is used to form the initial configuration, and thermal annealing is used to drive self-organization. A Si/SiO2/Si stack is irradiated and transforms the stack into Si/SiOx/Si by ion beam mixing (IBM) of the two Si/SiO2 interfaces. The oxide becomes metastable and the subsequent thermal treatment induces selforganization, which might leave a single Si-NC in the SiO2 layer for a sufficiently small mixing volume. The transformation of the planar SiOx layer (restriction only in one dimension) into a small SiOx volume (restriction in all three dimensions) is done by etching nanopillars with a diameter of less than 10nm. This forms a small SiOx plate embedded between two Si layers. The challenge is to control the self-organization process. In this work, simulation was used to investigate dependencies and parameter optimization. The ion mixing simulations were performed using binary collision approximation (BCA), followed by kinetic Monte Carlo (KMC) simulations of the decomposition process, which gave good qualitative agreement with the structures observed in related experiments. Quantitatively, however, the BCA simulation seemed to overestimate the mixing effect. This is due to the neglect of the positive entropy of the Si-SiO2 system mixing, i.e. the immiscibility counteracts the collisional mixing. The influence of this mechanism increases with increasing ion fluence. Compared to the combined BCA and KMC simulations, a larger ion mixing fluence has to be applied experimentally to obtain the predicted nanocluster morphology. To model the ion beam mixing of the Si/SiO2 interface, phase field methods have been applied to describe the influence of chemical effects during the irradiation of buried SiO2 layers by 60 keV Si+ ions at RT and thermal annealing at 1050°C. The ballistic collisional mixing was modeled by an approach using Fick’s diffusion equation, and the chemical effects and the annealing were described by the Cahn Hilliard equation. By that, it is now possible to predict composition profiles of Si/SiO2 interfaces during irradiation. The results are in good agreement with the experiment and are used for the predictions of the NCs formation in the nanopillar. For the thermal treatment model extensions were also necessary. The KMC simulations of Si-SiO2 systems in the past were based on normed time and temperature, so that the diffusion velocity of the components was not considered. However, the diffusion of Si in SiO2 and SiO2 in Si differs by several orders of magnitude. This cannot be neglected in the thermal treatment of the Si/SiO2 interface, because the processes that differ in speed in this order of magnitude are only a few nanometers apart. The KMC method was extended to include the different diffusion coefficients of the Si-SiO2 system. This allows to extensively investigate the influence of the diffusion. The phase diagram over temperature and composition was examined regarding decomposition (nucleation as well as spinodal decomposition) and growing of NCs. Using the methods and the knowledge gained about the system, basic simulations for the individual NC formation in the nanopillar were carried out. The influence of temperature, diameter, and radiation fluence was discussed in detail on the basis of simulation results

Effects of nuclear and electronic stopping power on ion irradiation of silicon-based compounds

Ion irradiation is used to analyze and modify the structure of condensed matter. It can for instance be used to form and shape nanocrystals in solids. In research on materials for high radiation environments, ion beams function as a controlled source of irradiation for studying the basic mechanisms of ion-solid interactions and for analyzing the structure of materials by methods like Rutherford backscattering spectrometry. Understanding the fundamental processes that take place in a material under ion irradiation is important for all these applications of ion beams, and of great interest from a basic science point of view. The mechanisms involved during ion irradiation-induced displacement of atoms in uniform bulk solids are fairly well understood and described in the literature, but many unresolved questions remain regarding the structural modification caused by electronic interactions, and the radiation response of materials with phase boundaries. Especially ion irradiation of nanomaterials is a topic that is under active research. The short-lived collision cascades caused by energetic ions in solids cannot be studied in experiments and are therefore often modeled in computer simulations. Such simulations can give a host of valuable information about processes that occur in nature. It is necessary to validate simulation results by either some other computational method, or ideally by experiments. Ions lose energy by elastic collisions with the atomic nuclei as well as to the electronic system through excitation and ionization. Both energy loss mechanisms - nuclear and electronic stopping - can cause modifications to the structure of the material. In this thesis, molecular dynamics simulations are carried out in close collaboration with experimental scientists in order to study the effects of nuclear and electronic stopping during ion irradiation on nanoclusters and bulk materials. The amorphization of germanium and silicon nanocrystals in silica under ion irradiation is studied in simulations. The amorphization dose of nanocrystals is much lower than for bulk materials and it is furthermore found to depend on the size of the nanocrystals. The inelastic thermal spike model is explored as a method of incorporating electronic stopping effects into molecular dynamics. The simulations predict that local heating due to electronic stopping contributes to irradiation damage in both nanocrystals in silica and bulk silica. In silicon carbide, high electronic stopping is found to recrystallize irradiation damaged samples. Molecular dynamics simulations of inelastic thermal spikes support the hypothesis that the observed recrystallization is induced by local heating due to electronic stopping. We need a combination of computer simulations and experimental observations to explain many of the complex processes that take place during ion irradiation. The results in this thesis give insight into some experimentally observed phenomena of the effect that nuclear and electronic energy loss have in materials, but especially the research on combined effects is still in its infancy and further progress can be expected in the near future.Joner är atomer med elektrisk laddning vilka kan uppnå hög hastighet i en partikelaccelerator. När sådana jonstrålar träffar materia kommer de att kollidera med materialets atomer. Jonbestrålning kan därför användas för att förändra, och analysera, strukturen hos fasta material. Inom forskning i material som ska utstå höga strålningsdoser, till exempel för kärnkraftverk eller ute i rymden, fungerar jonstrålar som en kontrollerad källa till strålning, med vilken man kan studera mekanismerna för växelverkan mellan joner och material. Att förstå de fundamentala processer som sker i ett material under jonbestrålning är viktigt för dessa tillämpningar av jonbestrålning, och också av stort intresse inom grundforskning. Vi är idag relativt insatta i de mekanismer som sker vid växelverkan mellan joner och atomer i ett bulkmaterial, men det finns ännu många olösta frågor gällande hur jon-elektronväxelverkningar kan ändra strukturen hos material, och hur material med fasgränser beter sig under jonbestrålning. Speciellt jonbestrålning av nanomaterial är ett ämne som det forskas aktivt i. De kortvariga kollisionskaskaderna som orsakas av energetiska joner i fasta ämnen kan inte observeras i experiment och simuleras därför ofta med hjälp av datormodeller. Sådana simuleringar kan ge en mängd av information om processer som sker i naturen. Det är dock nödvändigt att bekräfta resultat från simuleringar med antingen en annan beräkningsmetod, eller företrädesvis i experiment. Joner förbrukar sin energi i elastiska kollisioner med atomernas kärnor samt till materialets elektroner genom excitation och jonisering. Båda växelverkningsmekanismer nukleär och elektronisk bromskraft kan orsaka förändringar i strukturen hos material. I den här avhandlingen utförs molekyldynamiska simuleringar, i nära anslutning till experiment, för att studera effekterna av nukleär och elektronisk bromskraft under jonbestrålning av nanoklustrar och bulkmaterial. Under jonbestrålning kan kristallina ämnen förlora sin kristallstruktur. Genom molekyldynamiska simuleringar visar vi att halvledarmaterial i nanostorlek förlorar sin kristallstruktur vid betydligt lägre strålningsdoser än det motsvarande bulkmaterialet, och att den så kallade amorfiseringsdosen är lägre ju mindre nanostrukturerna är. Den elektroniska bromskraften visar sig också bidra betydligt till strålskador i nanoklustrar och i kiseldioxidglas. I kiselkarbid däremot, observerar vi att jon-elektronväxelverkningarna har en återställande effekt på kristaller som tidigare blivit strålskadade av joner med mycket låg elektronisk bromskraft. Att simuleringarna överensstämmer mycket väl med resultaten från jonbestrålningsexperiment visar på att de använda datormodellerna är tillförlitliga. En kombination av datorsimuleringar och experimentella observationer är nödvändigt för att förklara många av de komplexa processer som äger rum vid jonbestrålning. Resultaten i den här avhandlingen ger insikt i några experimentellt observerade fenomen av effekten som av nukleär och elektronisk bromskraft har i material. Dock är forskningen i kombinerade effekter ännu i startgroparna och nya framsteg kan väntas inom en snar framtid

Molecular dynamics simulation of elastic and sputtering properties of nanowires

Nanotechnology became an emerging field during the last few decades. The possibility to create elements having sizes in the nanometer range provides new opportunities for medical applications, various sensors and detectors, and composite materials technologies. However, at the nanoscale the basic physical properties may change unexpectedly including chemical, mechanical, optical and electronic properties. There is still no clear understanding of all possible consequences of miniaturization on the behavior of nanostructures. This thesis is focused on the analysis of mechanical and structural (including sputtering under irradiation) properties of nanorods. By nanorods we imply structures like beams or rods, with their cross-sectional diameter measuring in nanometers and having a length several times larger than the diameter. At such sizes it becomes possible to simulate the structures atom by atom using the molecular dynamics (MD) method. In the first part of the thesis, we analyze the elastic properties of Si nanorods: how the variation in size may change the elastic moduli, the effects of oxidation and intrinsic stresses. We also check the validity of the classical continuum mechanics approach by modeling the same nanorods with the finite elements method (FEM). In the second part we investigate sputtering from Au nanorods under ion irradiation. Recent experiments had shown that there is a big enhancement of sputtering yields from Au nanorods in contrast with those from a flat surface. The yields can be as much as 1000 per individual impact. MD gives us an opportunity to analyze the sputtering process with a femtosecond resolution which is impossible by any of the existing experimental methods. We find that an explosive ejection of nanoclusters is the main factor causing such large sputtering yields

Low Energy Ion Beam Synthesis of SiNanocrystals for Nonvolatile Memories – Modeling and Process Simulations

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