Size effect in cluster collision on solid surfaces

Quantum Science and Engineering Center, Kyoto University, Kioto, Kyōto, Japan
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms (Impact Factor: 1.19). 04/2007; 257(1-2). DOI: 10.1016/j.nimb.2007.01.164
Source: OAI

ABSTRACT New surface modification processes have been demonstrated using gas cluster ion irradiations. Multiple collision and high energy density collision of cluster ions are responsible for “non-linear phenomena”, which play an important role in the surface modification process. Because of the unique interaction between cluster ions and surface atoms, atomistic mechanisms of cluster ion bombardment must be understood for the further developments of this technology. Cluster size is a unique parameter for cluster ions. One of the fundamental questions in this surface modification technique is the cluster size effect. It is important to use appropriate cluster size in each process. Size dependence of sputtering yields and secondary ion yields with large Ar cluster (N>300) have been measured.

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    ABSTRACT: This thesis concerns the dynamics of nanoparticle impacts on solid surfaces. These impacts occur, for instance, in space, where micro- and nanometeoroids hit surfaces of planets, moons, and spacecraft. On Earth, materials are bombarded with nanoparticles in cluster ion beam devices, in order to clean or smooth their surfaces, or to analyse their elemental composition. In both cases, the result depends on the combined effects of countless single impacts. However, the dynamics of single impacts must be understood before the overall effects of nanoparticle radiation can be modelled. In addition to applications, nanoparticle impacts are also important to basic research in the nanoscience field, because the impacts provide an excellent case to test the applicability of atomic-level interaction models to very dynamic conditions. In this thesis, the stopping of nanoparticles in matter is explored using classical molecular dynamics computer simulations. The materials investigated are gold, silicon, and silica. Impacts on silicon through a native oxide layer and formation of complex craters are also simulated. Nanoparticles up to a diameter of 20 nm (315000 atoms) were used as projectiles. The molecular dynamics method and interatomic potentials for silicon and gold are examined in this thesis. It is shown that the displacement cascade expansionmechanism and crater crown formation are very sensitive to the choice of atomic interaction model. However, the best of the current interatomic models can be utilized in nanoparticle impact simulation, if caution is exercised. The stopping of monatomic ions in matter is understood very well nowadays. However, interactions become very complex when several atoms impact on a surface simultaneously and within a short distance, as happens in a nanoparticle impact. A high energy density is deposited in a relatively small volume, which induces ejection of material and formation of a crater. Very high yields of excavated material are observed experimentally. In addition, the yields scale nonlinearly with the cluster size and impact energy at small cluster sizes, whereas in macroscopic hypervelocity impacts, the scaling 2 is linear. The aim of this thesis is to explore the atomistic mechanisms behind the nonlinear scaling at small cluster sizes. It is shown here that the nonlinear scaling of ejected material yield disappears at large impactor sizes because the stopping mechanism of nanoparticles gradually changes to the same mechanism as in macroscopic hypervelocity impacts. The high yields at small impactor size are due to the early escape of energetic atoms from the hot region. In addition, the sputtering yield is shown to depend very much on the spatial initial energy and momentum distributions that the nanoparticle induces in the material in the first phase of the impact. At the later phases, the ejection of material occurs by several mechanisms. The most important mechanism at high energies or at large cluster sizes is atomic cluster ejection from the transient liquid crown that surrounds the crater. The cluster impact dynamics detected in the simulations are in agreement with several recent experimental results. In addition, it is shown that relatively weak impacts can induce modifications on the surface of an amorphous target over a larger area than was previously expected. This is a probable explanation for the formation of the complex crater shapes observed on these surfaces with atomic force microscopy. Clusters that consist of hundreds of thousands of atoms induce long-range modifications in crystalline gold. Nanohiukkaset ovat pieniä atomiryppäitä, jotka sisältävät muutamia kymmeniä tai korkeintaan miljoona atomia. Niitä käytetään teollisuudessa pintojen työstämisessä. Nanohiukkasten muodostamilla suihkulla esimerkiksi siloitetaan pintoja tai muodostetaan pinnoille hienojakoisia kuviointeja, joita sitten käytettään pohjana myöhemmissä valmistusvaiheissa. Hiukkassuihkujen avulla istutetaan epäpuhtausatomeja puolijohteisiin. Suihkujen irrottamaa ainetta analysoimalla selvitetään pinnan kemiallista koostumusta. Avaruudessa pienimmät meteoriitit ovat kooltaan nanohiukkasten luokkaa. Niiden törmäyksiä tutkitaan, jotta avaruusalukset voitaisiin paremmin pienten meteoriittien törmäyksiltä. Törmäykset myös muokkaavat aikojen kuluessa ilmakehättömien taivaankappaleiden pintarakennetta. Yksittäisen törmäyksen mekanismit ja vaikutukset on tunnettava, jotta voitaisiin laskea kokonaisen hiukkassuihkun vaikutus pintaan. Tässä työssä on tutkittu nanohiukkasten törmäysten dynamiikkaa tietokonesimulointien avulla. Simulointeja on tehty hyvin eri kokoisilla nanohiukkasilla (2-315000 atomia) ja laajalla törmäysenergiaskaalalla (0.1 1500 keV/atomi). Näin on muodostunut kuva, miten törmäysten dynamiikka muuttuu nanohiukkasen koon ja energian kasvaessa. Jopa alle 20 nm halkaisijaltaan oleva hiukkanen voi saada aikaan törmäyksen, jonka dynamiikka on makroskooppisen törmäyksen kaltainen. Hiukkanen ja sen eteen osunut aine puristuvat tiiviiksi alueeksi, jonka energia purkautuu räjähdyksen tavoin irrottaen ainetta. Pinnalle muodostuu kraateri, joka muistuttaa meteoriittikraatereita. Törmäävän hiukkasen koon pienetessä muuttuu törmäysten dynamiikka toisenlaiseksi. Pakkautumisen sijasta törmäävän nanohiukkasen atomit tunkeutuvat kohtion atomien väliin. Törmäysenergia muodostaa erittäin kuuman alueen, josta atomeja virtaa pois. Myös tässä tapauksessa muodostuu kraateri, mutta irronneen aineen määrä on eri tavalla verrannollinen törmäävän hiukkasen kokoon ja törmäysenergiaan kuin suurempien hiukkasten tapauksessa. Kokeellisesti on osoitettu, että suurten nanohiukkasten ja makroskooppisten kappaleiden törmäyksissä kraaterin koko on suoraan verrannollinen törmäävän kappaleen kokoon, mutta pienten nanohiukkasten tapauksessa verrannollisuus on usein epälineaarinen. Tässä työssä tehtyjen simulointien avulla voidaan selittää, mitkä atomaariset mekanismit ovat tämän eroavaisuuden syynä. Simuloinnin osoittavat lisäksi, että suhteellisen pienetkin törmäysenergiat saavat aikaan laaja-alaisempia muutoksia esimerkiksi puolijohteiden pinnoilla kuin aikaisemmin on oletettu. Törmäyskraatereiden ympärille muodostuu vyöhykkeitä, joissa pintarakenne on muuttunut tavalla joka riippuu materiaalista ja törmäysenergiasta. Tällä on merkitystä laskettaessa hiukkassuihkun kokonaisvaikutuksia. Tulokset osoittavat, että molekyylidynaamisia simulointeja voidaan tulevaisuudessa käyttää entistä laajemmin uusien materiaalien kehittämisessä. Makroskooppisten törmäysten simulointi on nyt mahdollista lähtien atomien välisistä vuorovaikutuksista. Tämä avaa uusia mahdollisuuksia tutkia materiaalien murtumista ja rakenteen muuttumista törmäyksissä.
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