Säteilytyksen vaikutukset grafeenissa ja sen sukulaismateriaaleissa

Nanomaterials with a hexagonally ordered atomic structure, e.g., graphene, carbon and boron nitride nanotubes, and white graphene (a monolayer of hexagonal boron nitride) possess many impressive properties. For example, the mechanical stiffness and strength of these materials are unprecedented. Also, the extraordinary electronic properties of graphene and carbon nanotubes suggest that these materials may serve as building blocks of next generation electronics. However, the properties of pristine materials are not always what is needed in applications, but careful manipulation of their atomic structure, e.g., via particle irradiation can be used to tailor the properties. On the other hand, inadvertently introduced defects can deteriorate the useful properties of these materials in radiation hostile environments, such as outer space. In this thesis, defect production via energetic particle bombardment in the aforementioned materials is investigated. The effects of ion irradiation on multi-walled carbon and boron nitride nanotubes are studied experimentally by first conducting controlled irradiation treatments of the samples using an ion accelerator and subsequently characterizing the induced changes by transmission electron microscopy and Raman spectroscopy. The usefulness of the characterization methods is critically evaluated and a damage grading scale is proposed, based on transmission electron microscopy images. Theoretical predictions are made on defect production in graphene and white graphene under particle bombardment. A stochastic model based on first-principles molecular dynamics simulations is used together with electron irradiation experiments for understanding the formation of peculiar triangular defect structures in white graphene. An extensive set of classical molecular dynamics simulations is conducted, in order to study defect production under ion irradiation in graphene and white graphene. In the experimental studies the response of carbon and boron nitride multi-walled nanotubes to irradiation with a wide range of ion types, energies and fluences is explored. The stabilities of these structures under ion irradiation are investigated, as well as the issue of how the mechanism of energy transfer affects the irradiation-induced damage. An irradiation fluence of 5.5x10^15 ions/cm^2 with 40 keV Ar+ ions is established to be sufficient to amorphize a multi-walled nanotube. In the case of 350 keV He+ ion irradiation, where most of the energy transfer happens through inelastic collisions between the ion and the target electrons, an irradiation fluence of 1.4x10^17 ions/cm^2 heavily damages carbon nanotubes, whereas a larger irradiation fluence of 1.2x10^18 ions/cm^2 leaves a boron nitride nanotube in much better condition, indicating that carbon nanotubes might be more susceptible to damage via electronic excitations than their boron nitride counterparts. An elevated temperature was discovered to considerably reduce the accumulated damage created by energetic ions in both carbon and boron nitride nanotubes, attributed to enhanced defect mobility and efficient recombination at high temperatures. Additionally, cobalt nanorods encapsulated inside multi-walled carbon nanotubes were observed to transform into spherical nanoparticles after ion irradiation at an elevated temperature, which can be explained by the inverse Ostwald ripening effect. The simulation studies on ion irradiation of the hexagonal monolayers yielded quantitative estimates on types and abundances of defects produced within a large range of irradiation parameters. He, Ne, Ar, Kr, Xe, and Ga ions were considered in the simulations with kinetic energies ranging from 35 eV to 10 MeV, and the role of the angle of incidence of the ions was studied in detail. A stochastic model was developed for utilizing the large amount of data produced by the molecular dynamics simulations. It was discovered that a high degree of selectivity over the types and abundances of defects can be achieved by carefully selecting the irradiation parameters, which can be of great use when precise pattering of graphene or white graphene using focused ion beams is planned.Hexagonaalisesti järjestäytyneillä nanomateriaaleilla, mukaanlukien grafeenilla, hiili- ja boori-nitridinanoputkilla ja valkoisella grafeenilla (yksittäiskerros hexagonaalista boori-nitridiä), on lukuisia vaikuttavia ominaisuuksia. Nämä materiaalit ovat esimerkiksi ennennäkemättömän jäykkiä ja lujia. Grafeenin ja hiilinanoputkien erikoislaatuiset elektroniset ominaisuudet herättävät toiveita siitä, että näitä materiaaleja voitaisiin hyödyntää tulevaisuuden elektroniikassa. Virheettömässä muodossaan kyseisten materiaalien ominaisuudet eivtä kuitenkaan ole aina sitä, mitä sovellutuksissa tarvitaan, mutta näiden rakenteiden huolellinen muokkaaminen, esimerkiksi hiukkassäteilytyksen avulla, voi olla tapa räätälöidä kyseisten materiaalien ominaisuuksia. Toisaalta materiaalit voivat altistua sovellutuksissa ei-toivotulle säteilylle ja tällöin on tärkeää tuntea materiaalien kestävyys kyseisessä tilanteessa. Tässä väitöskirjassa tarkastellaan vaurionmuodostusta edellämainituissa materiaaleissa niiden altistuessa hiukkassäteilytykselle. Ionisäteilytyksen vaikutuksia moniseinäisissä hiili- ja boori-nitridinanoputkissa tutkitaan altistamalla näytteitä ensin hiukkaspommitukselle käyttäen hiukkaskiihdytintä, minkä jälkeen näytteet karakterisoidaan läpäisyelektronimikroskopialla ja Raman spektroskopialla. Näiden karakterisointimenetelmien käyttökelpoisuutta tarkastellaan kriittisesti ja läpäisyelektronimikroskooppikuviin perustuva asteikko esitetään käytettäväksi analyysin apuvälineenä. Hiukkassäteilytyksen vaikutuksia grafeenissa ja valkoisessa grafeenissa tarkastellaan teoreettisesti. Kvanttimekaniikkaan perustuvien molekyylidynaamisten simulaatioiden pohjalta rakennettua tilastollista mallia sovelletaan yhdessä elektronisäteilytyskokeiden kanssa mallinnettaessa erikoisen muotoisten kolmion muotoisten vauriorakenteiden muodostumista valkoisessa grafeenissa. Suuri määrä klassisia molekyylidynaamisia simulaatoita suoritettaan, tavoitteena mallintaa vauriontuottoa ionisäteilytykselle altistetussa grafeenissa ja valkoisessa grafeenissa. Näiden tulosten pohjalta toteutetaan tilastollinen malli, jota voidaan hyödyntää esimerkisi mikäli grafeenia tai valkoista grafeenia halutaan muokata käyttäen fokusoituja ionisuihkuja

Implantation and atomic scale investigation of self-interstitials in graphene

Crystallographic defects play a key role in determining the properties of crystalline materials. The new class of two-dimensional materials, foremost graphene, have enabled atomically resolved studies of defects, such as vacancies, grain boundaries, dislocations, and foreign atom substitutions. However, atomic resolution imaging of implanted self-interstitials has so far not been reported in any three- but also not in any two-dimensional material. Here, we deposit extra carbon into single-layer graphene at soft landing energies of ~1 eV using a standard carbon coater. We identify all the self-interstitial dimer structures theoretically predicted earlier, employing 80 kV aberration-corrected high-resolution transmission electron microscopy. We demonstrate accumulation of the interstitials into larger aggregates and dislocation dipoles, which we predict to have strong local curvature by atomistic modeling, and to be energetically favourable configurations as compared to isolated interstitial dimers. Our results contribute to the basic knowledge on crystallographic defects, and lay out a pathway into engineering the properties of graphene by pushing the crystal into a state of metastable supersaturation

Numerical correction of anti-symmetric aberrations in single HRTEM images of weakly scattering 2D-objects

Here, we present a numerical post-processing method for removing the effect of anti-symmetric residual aberrations in high-resolution transmission electron microscopy (HRTEM) images of weakly scattering 2D-objects. The method is based on applying the same aberrations with the opposite phase to the Fourier transform of the recorded image intensity and subsequently inverting the Fourier transform. We present the theoretical justification of the method and its verification based on simulated images in the case of low-order anti-symmetric aberrations. Ultimately the method is applied to experimental hardware aberration-corrected HRTEM images of single-layer graphene and MoSe2 resulting in images with strongly reduced residual low-order aberrations, and consequently improved interpretability. Alternatively, this method can be used to estimate by trial and error the residual anti-symmetric aberrations in HRTEM images of weakly scattering objects

Lähi- ja luomuruoan saavutettavuus Pohjois-Pohjanmaalla

Lähi- ja luomuruoan saavutettavuus Pohjois-Pohjanmaalla (RuokaGIS) -hankkeen tavoitteena oli selvittää lähi- ja luomuruokaan liittyviä sijainti- ja saavutettavuustekijöitä Pohjois-Pohjanmaalla lähi- ja luomuruoan saatavuuden kehittämiseksi. Hanke oli Euroopan maaseudun kehittämisen maatalousrahaston ja Pohjois-Pohjamaan ELY-keskuksen rahoittama ja kestoltaan kaksivuotinen (1.10.2012–30.9.2014). Hankkeen tavoitteena oli 1) selvittää lähi- ja luomuruoan saavutettavuuden alueellisia eroja 2) tarkastella mahdollista tarjontaylijäämää tai tyydyttymätöntä kysyntää lähi- ja luomuruoan suhteen ja tuottaa aiheesta helppolukuisia karttoja ja 3) tunnistaa aluetekijöitä lähi- ja luomuruoan tuotannon riittävyyden varmistamiseksi. Suomessa ei ole aikaisemmin toteutettu ruoan maantieteelliseen saavutettavuuteen liittyvää tarkastelua, vaikka korkeatasoiset ja kattavat paikkatietoaineistot alkutuotannosta, liikenneverkosta ja väestöstä mahdollistavat huomattavan tarkkojen saavutettavuusanalyysien tekemisen. Ruoan arvoketjusta paikkatietotarkasteluiden keskiöön valittiin kysely- ja haastatteluaineistojen pohjalta ruoan tuotannon ja kulutuksen alueellisten ulottuvuuksien esiin nostaminen tilastollisen kysynnän ja tarjonnan kannalta sekä saavutettavuustarkastelut pienten ja keskisuurten yritysten logistiikan edistämiseksi. Paikkatietoperusteista ja toimijalähtöistä tarkastelua tehtiin vuorovaikutuksessa ja hankkeen toteutuksessa yhdistettiin verkosto-, tuottaja- ja väestötietojen paikkatietotarkastelu kysely- ja haastattelututkimuksiin. Saavutettavuustarkastelut toteutettiin soveltamalla ja kehittämällä yleisesti käytettävissä olevia saavutettavuusmenetelmiä. Keskeisimmät aineistot olivat Maa- ja metsätalousministeriön Tietopalvelukeskuksen (Tike) toimittamista tiedoista koottu tuottajakohtainen paikkatietokanta, Tilastokeskuksen väestöruututietoaineistot ja Liikenneviraston tieverkon paikkatietomalli (digiroad) sekä tuottajille, ammattikeittiöille, yrityksille ja ruokapiireille tehdyt kyselyt sekä niitä täydentävät haastattelut. Hankkeen keskeisinä tuloksina olivat toimijoiden näkemykset lähi- ja luomuruoan määrittelyistä, kilpailukyvystä, markkinoista, logistiikasta ja näiden kehittämisen tarpeista. Paikkatieto- ja saavutettavuustarkastelut osoittivat Pohjois-Pohjanmaan eteläosien vahvuudet alueen kaupunkikeskuksien kysynnän tyydyttämisessä. Saavutettavuustarkasteluiden perusteella Pohjois-Pohjanmaan eteläosat ja Oulun ympäristö ovat sijanniltaan suotuisia alueellisen keräilyverkoston solmukohdille, joita voidaan hyödyntää pienten toimijoiden logistiikan tehostamiseen. Tulosten perusteella suositellaan, että lähiruoka-alan toimijoiden yhteistyötä ja verkostoitumista kehitetään etenkin logistiikan suhteen kilpailukyvyn lisäämiseksi ja markkinoille pääsyn helpottamiseksi.This study is based on the “Accessibility of local and organic food in Northern Ostrobothnia (RuokaGIS)” project, which aim was to analyse local and organic food in geographic context and to develop their availability and access to markets at Northern Ostrobothnia. The project was funded by European Agricultural Fund for Rural Development and Centre for Economic Development, Transport and the Environment of Northern Ostrobothnia. The project covered the period from 1. Oct. 2012 to 30. Sept. 2014. The project combines quantitative and qualitative approaches by applying geographic information system (GIS) based accessibility analyses and structured questionnaires and semi-structured interviews. The aim of the project was to 1) explore areal patterns of local on organic food accessibility, 2) visualise supply surplus and unsaturated demand and 3) recognise factors to develop local and organic food production in geographic context. Due to a long logistic chain of food markets, the emphasis of the project designed on the basis of survey results. Primary aspects were to analyse areal balance of statistical production and consumption at accurate scale and to develop logistics and networking of small and medium size companies in food sector by analysis of potential hub node locations. GIS based accessibility analyses and qualitative surveys were conducted interactively during the project with aim to generate novel approaches for analysing accessibility in the context of local and organic food markets and logistics. Key GIS data of the study covered farms with production attributes supplied by Information Centre of the Ministry of Agriculture and Forestry, Statistics Finland’s grid cell data of population and Finnish Transport Agency’s digital model of road network. Questionnaires and supporting interviews covered agricultural producers, companies in processing and refining, institutional kitchens, public procurements and food circles. Main findings of the project consider local and organic food concept, competitiveness, markets, logistics and their development aspects. Accessibility analyses indicated that regions in the southern parts of Northern Ostrobothnia have a strong role in saturating areal demand, which emphasises to towns and city regions. Analysis of suitable hub locations for collecting local production refers that most suitable locations are in southern parts of Northern Ostrobothnia and also near Oulu. On the basis of results, cooperation and networking between actors, particularly in logistics, may benefit competitiveness of local food and increase its access to markets

Non-invasive transmission electron microscopy of vacancy defects in graphene produced by ion irradiation

Irradiation with high-energy ions has been widely suggested as a tool to engineer properties of graphene. Experiments show that it indeed has a strong effect on its transport, magnetic and mechanical characteristics. However, to use ion irradiation as an engineering tool requires understanding of the type and detailed characteristics of the produced defects which is still lacking, as the use of high-resolution transmission electron microscopy (HRTEM) - the only technique allowing direct imaging of atomic-scale defects - often modifies or even creates defects during imaging, thus making it impossible to determine the intrinsic atomic structure. Here we show that encapsulating the studied graphene sample between two other (protective) graphene sheets allows non-invasive HRTEM imaging and reliable identification of atomic-scale defects. Using this simple technique, we demonstrate that proton irradiation of graphene produces reconstructed monovacancies, which explains the profound effect that such defects have on magnetic and transport properties. This finding resolves the existing uncertainty with regard to the effect of ion irradiation on the electronic structure of graphene.Comment: 18 pages, 6 figures and Supplementary Information (4 supplementary figures

Ion irradiation of multi-walled boron nitride nanotubes

Non Peer reviewe

Electron beam controlled covalent attachment of small organic molecules to graphene

Markevich A, Kurasch S, Lehtinen O, et al. Electron beam controlled covalent attachment of small organic molecules to graphene. NANOSCALE. 2016;8(5):2711-2719.The electron beam induced functionalization of graphene through the formation of covalent bonds between free radicals of polyaromatic molecules and C=C bonds of pristine graphene surface has been explored using first principles calculations and high-resolution transmission electron microscopy. We show that the energetically strongest attachment of the radicals occurs along the armchair direction in graphene to carbon atoms residing in different graphene sub-lattices. The radicals tend to assume vertical position on graphene substrate irrespective of direction of the bonding and the initial configuration. The "standing up" molecules, covalently anchored to graphene, exhibit two types of oscillatory motion bending and twisting - caused by the presence of acoustic phonons in graphene and dispersion attraction to the substrate. The theoretically derived mechanisms are confirmed by near atomic resolution imaging of individual perchlorocoronene (C24Cl12) molecules on graphene. Our results facilitate the understanding of controlled functionalization of graphene employing electron irradiation as well as mechanisms of attachment of impurities via the processing of graphene nanoelectronic devices by electron beam lithography

Characterization of ion-irradiation-induced defects in multi-walled carbon nanotubes

Peer reviewe

Työn fyysisen kuormittavuuden yhteys fyysiseen toimintakykyyn alle 40-vuotiailla kunta-alan työntekijöillä

Vertaisarvioitu. English summary.Peer reviewe