Jantunen, Ville, University of Helsinki, Faculty of Science, Doctoral Programme in Materials Research and Nanoscience, Helsingin yliopisto, matemaattis-luonnontieteellinen tiedekunta, Materiaalitutkimuksen ja nanotieteiden tohtoriohjelma, Helsingfors universitet, matematisk-naturvetenskapliga fakulteten, Doktorandprogrammet i materialforskning och nanovetenskap, Murphy, Samuel, Djurabekova, Flyura, Nordlund, Kai, and Leino, Aleksi
Familiar materials show interesting properties when their size is reduced to the nanoscale. At the same time, controlling and tuning something so small becomes a challenge of its own. Metal nanoparticles, for instance, show interesting optical properties such as narrow peaks in their absorption spectra. The location of this peak depends on not only the material of the nanoparticle, but also on its shape. Controlling the shape of a nanoparticle can be done in different ways. One of them is irradiation using swift heavy ions. The latter are able to penetrate deep into the material, allowing for modification of nanoparticles embedded in insulators to protect them from environment. Swift heavy ion irradiation is known to affect the aspect ratio of embedded metal nanoparticles. Because the process happens in a picosecond timescale, the only plausible way to study the process with sufficient resolution is to use computer simulations. In this work we use both the multiscale atomistic simulations which include atomic and electronic dynamics, as well as a continuum model for larger structures to understand the driving mechanisms for swift heavy ion modification of Au nanoparticles embedded in silica. First we look into the shape modification in the subtrack regime. A cylindrical damage region – an underdense core-overdense shell track – created in the wake of the passing ion was believed to be the prerequisite for nanoparticle shape modification. The explanation is based on material flow of a molten nanoparticle into the underdense ion track. In the subtrack region, however, these tracks are not experimentally measurable, but nanoparticles still elongate. This observation was in apparent contradiction to the previous explanation. We show that the shape change is still possible, because of the transient tracks that form on a picosecond time scale. This confirms that the suggested earlier mechanism holds, while no permanent tracks are required. In our study, we have also included shape modification of nanorods. Such elongated nanoparticles have not been yet as extensively studied as spherical ones. Experimentally, we observed an apparent rotation of the nanorods that were initially positioned perpendicular to the ion beam. After irradiation, however, all nanorods were found to be aligned with the ion beam direction. Using atomistic simulations of the multiple subsequent impacts, we explained the "rotation" by re-orientation of the nanorod in a series of incremental shape modifications that cumulatively made the nanorod "rotate". Furthermore, we improved our atomistic simulation models by introducing a new interatomic potential for the interface between the nanoparticle and the surrounding insulator. Earlier atomistic simulations had the weakness that while they could show shape change after one simulated ion impact, these changes did not accumulate with multiple impacts without additional tweaks between the impacts. The latter was necessary to account for processes not accessible within the existing models in order for the elongation process to continue. With the use of the new potential, and by including side impacts around the nanoparticle as well as longer cooling between the ion impacts, we were able to accumulate the shape changes without the use of any poorly motivated ad hoc models. The multiscale nature of atomistic simulations of swift heavy ion impacts includes the solution of two coupled partial differential equations, a so-called two-temperature model that describes the heat dynamics in both atomic and electronic subsystems. Although these equations are routinely solved for Au-SiO2 systems, the complex nature of possible effects in the interface between the metal and insulator had not been this far taken into account. These models suggested that a nanoparticle melts fully or partially via the deposition of energy to silica, which is a prerequisite for their shape transformation. The excited electrons from the nanoparticle, in these models, reach the surface without a barrier, depositing their energy to the atoms of the insulator material close to the interface and subsequently melting the nanoparticle from the outside. We found that taking into account that an insulator around the gold nanoparticle effectively traps the electrons of gold in a potential well, and reduces the energy that can leak from the nanoparticle when its electrons are excited. We also saw that including interface scattering in the heat simulations, increases the heating in the surface of the nanoparticle. With these two effects also taken into account, we studied the effects of thermal insulation caused by a material interface between gold and silica. The atomic heat conduction between the nanoparticle and its surrounding became negligibly small. The only source of heating was the energy trapped inside the nanoparticle because of the barrier. Our study showed that when this energy was sufficiently high in medium-sized < 80 nm diameter nanoparticles, large regions of the nanoparticle still melted, consistent with experimental results. Overall, in this work, we have improved the realism of both the two-temperature model and the atomistic simulations that are used together to simulate swift heavy ion impacts on materials. With these improvements, we can better understand the shape modification of metal nanoparticles under swift heavy ion irradiation. This new understanding can be used to better control the processthat can be used in the production of future nanoscale optical devices. Monet arkiset ja tutut materiaalit muuttuvat mielenkiintoisiksi kun niiden kokoluokkaa skaalataan nanometrimittaluokkaan. Esimerkiksi kultaiset nanopartikkelit reagoivat valoon eri tavalla kuin paljaalla silmällä erottuvat kultaiset kappaleet. Nämä pienestä koosta johtuvat ominaisuudet voivat osoittautua hyödyllisiksi tulevaisuuden elektronisissa sovelluksissa. Nanomittakaavassa olevien kappaleiden muodon tarkka säätäminen ei ole helppoa, mutta muoto vaikuttaa kuitenkin nanopartikkelien ominaisuuksiin. Eräs tapa muokata eristeisiin upotettuja metallisia nanopartikkeleita on niiden säteilyttäminen ripeillä raskas-ioneilla (Swift heavy ion). Tämä työ käsittelee tällaisen nanopartikkelien säteilytyksen teoreettista mallintamista tietokonesimulaatioiden avulla. Suhteessa aiempiin aiheesta tehtyihin tutkimuksiin, uusina tuloksina tässä työssä on nanosauvojen kääntymisen selittäminen, sekä sellainen säteilytys, joka ei vaikuta jättävän jälkiä nanopartikkeleita ympäröivään eristemateriaaliin, mutta kuitenkin muuttaa sisällä olevien nanopartikkelien muotoa. Lisäksi tässä työssä on tutkittu nanopartikkelien ja niitä ympäröivät eristemateriaalin rajapinnassa tapahtuvia fysikaalisia ilmiöitä. Vaikka tämä työ on keskittynyt aiheen teoreettiseen tutkimukseen, voi sen tuloksista olla hyötyä tulevaisuudessa, erityisesti jos nanopartikkelipohjaiset elektroniset sovellutukset yleistyvät.