Mechanical properties of polycrystalline graphene based on a realistic atomistic model

Graphene can at present be grown at large quantities only by the chemical vapor deposition method, which produces polycrystalline samples. Here, we describe a method for constructing realistic polycrystalline graphene samples for atomistic simulations, and apply it for studying their mechanical properties. We show that cracks initiate at points where grain boundaries meet and then propagate through grains predominantly in zigzag or armchair directions, in agreement with recent experimental work. Contrary to earlier theoretical predictions, we observe normally distributed intrinsic strength (~ 50% of that of the mono-crystalline graphene) and failure strain which do not depend on the misorientation angles between the grains. Extrapolating for grain sizes above 15 nm results in a failure strain of ~ 0.09 and a Young's modulus of ~ 600 GPa. The decreased strength can be adequately explained with a conventional continuum model when the grain boundary meeting points are identified as Griffith cracks.Comment: Accepted for Physical Review B; 5 pages, 4 figure

Irradiation-mediated tailoring of carbon nanotubes

The ever-increasing demand for faster computers in various areas, ranging from entertaining electronics to computational science, is pushing the semiconductor industry towards its limits on decreasing the sizes of electronic devices based on conventional materials. According to the famous law by Gordon E. Moore, a co-founder of the world s largest semiconductor company Intel, the transistor sizes should decrease to the atomic level during the next few decades to maintain the present rate of increase in the computational power. As leakage currents become a problem for traditional silicon-based devices already at sizes in the nanometer scale, an approach other than further miniaturization is needed to accomplish the needs of the future electronics. A relatively recently proposed possibility for further progress in electronics is to replace silicon with carbon, another element from the same group in the periodic table. Carbon is an especially interesting material for nanometer-sized devices because it forms naturally different nanostructures. Furthermore, some of these structures have unique properties. The most widely suggested allotrope of carbon to be used for electronics is a tubular molecule having an atomic structure resembling that of graphite. These carbon nanotubes are popular both among scientists and in industry because of a wide list of exciting properties. For example, carbon nanotubes are electronically unique and have uncommonly high strength versus mass ratio, which have resulted in a multitude of proposed applications in several fields. In fact, due to some remaining difficulties regarding large-scale production of nanotube-based electronic devices, fields other than electronics have been faster to develop profitable nanotube applications. In this thesis, the possibility of using low-energy ion irradiation to ease the route towards nanotube applications is studied through atomistic simulations on different levels of theory. Specifically, molecular dynamic simulations with analytical interaction models are used to follow the irradiation process of nanotubes to introduce different impurity atoms into these structures, in order to gain control on their electronic character. Ion irradiation is shown to be a very efficient method to replace carbon atoms with boron or nitrogen impurities in single-walled nanotubes. Furthermore, potassium irradiation of multi-walled and fullerene-filled nanotubes is demonstrated to result in small potassium clusters in the hollow parts of these structures. Molecular dynamic simulations are further used to give an example on using irradiation to improve contacts between a nanotube and a silicon substrate. Methods based on the density-functional theory are used to gain insight on the defect structures inevitably created during the irradiation. Finally, a new simulation code utilizing the kinetic Monte Carlo method is introduced to follow the time evolution of irradiation-induced defects on carbon nanotubes on macroscopic time scales. Overall, the molecular dynamic simulations presented in this thesis show that ion irradiation is a promisingmethod for tailoring the nanotube properties in a controlled manner. The calculations made with density-functional-theory based methods indicate that it is energetically favorable for even relatively large defects to transform to keep the atomic configuration as close to the pristine nanotube as possible. The kinetic Monte Carlo studies reveal that elevated temperatures during the processing enhance the self-healing of nanotubes significantly, ensuring low defect concentrations after the treatment with energetic ions. Thereby, nanotubes can retain their desired properties also after the irradiation. Throughout the thesis, atomistic simulations combining different levels of theory are demonstrated to be an important tool for determining the optimal conditions for irradiation experiments, because the atomic-scale processes at short time scales are extremely difficult to study by any other means.Yhä realistisemmat tietokonepelit sekä tarkemmat tieteelliset mallinnusmenetelmät vaativat jatkuvasti lisää laskentatehoa. Tämän tehon kasvu ajan funktiona on hämmästyttävän hyvin seurannut niin sanottua Mooren lakia jo useiden vuosikymmenten ajan. Lain mukaan komponenttien koko (joka on kääntäen verrannollinen tehoon) tulee kuitenkin lähivuosikymmeninä niin pieneksi, että käytössä olevien piirakenteiden edelleenpienentäminen ei enää onnistu. Yhtenä ratkaisuna tähän ongelmaan on esitetty komponenttien uudelleen suunnittelemista käyttäen materiaalina piin sijaan hiiltä. Tämän vaihtoehdon tekee erityisen kiinnostavaksi se tosiasia, että hiili muodostaa luonnostaan joitakin erittäin pieniä (kokoluokkaa yksi miljardisosa metristä) rakenteita, joita voitaisiin suoraan käyttää komponenttien rakentamiseen. Eniten huomiota on saanut lieriömäisten rakenteiden, hiilinanoputkien, käyttäminen tähän tarkoitukseen. Nämä putket ovat atomitason rakenteestaan riippuen sähköisiltä ominaisuuksiltaan joko puolijohteita tai metallin kaltaisia. Niillä on myös kiinnostavia muita ominaisuuksia ja siten sovelluksia useilla eri aloilla myös elektroniikkateollisuuden ulkopuolella. Koska näiden putkien valmistaminen täsmälleen halutuilla ominaisuuksilla on äärimmäisen vaikeaa, ovat nanoputkiin perustuvat kaupallisesti kannattavat elektroniikkasovellutukset vielä olleet saavuttamattomissa. Tässä väitöskirjassa esitellään tutkimusta, jonka lähtökohtana on käyttää varatuilla atomeilla (ioneilla) säteilyttämistä keinona muokata nanoputkia siten, että niille saadaan halutut ominaisuudet esimerkiksi tuomalla rakenteisiin epäpuhtausatomeja. Koska tällaisten prosessien tutkiminen on vaikeaa suorilla kokeellisilla mittauksilla, on tässä työssä käytetty erilaisia laskennallisia menetelmiä kvanttimekaanisista malleista karkeampiin tilastollisiin menetelmiin. Väitöskirjassa esitetyt tulokset osoittavat, että ionisäteilytys on erinomainen menetelmä nanoputkien seostamiseen typpi- ja boori-epäpuhtauksilla siten, että ne asettuvat rakenteessa hiiliatomin paikalle. Lisäksi säteilytyksellä voidaan tuottaa pieniä kaliumryppäitä nanoputkirakenteiden onkaloihin. Työssä esitetään myös miten säteilytystä voidaan käyttää hitsaamaan nanoputkia pintoihin ja siten parantaa tällaisen rajapinnan sähköisiä ja lämmönjohtumiseen liittyviä ominaisuuksia. Lopuksi osoitetaan, että vaikka säteilytyksellä aina myös tuotetaan rakenteelle vahinkoa, on nanoputkien itsekorjautumismekanismi niin tehokas, että vahinko pysyy vähäisenä kunhan säteilytys suoritetaan tarpeeksi korkeassa lämpötilassa

Substitutional Si impurities in monolayer hexagonal boron nitride

We report the first observation of substitutional silicon atoms in single-layer hexagonal boron nitride (h-BN) using aberration corrected scanning transmission electron microscopy (STEM). The medium angle annular dark field (MAADF) images reveal silicon atoms exclusively filling boron vacancies. This structure is stable enough under electron beam for repeated imaging. Density functional theory (DFT) is used to study the energetics, structure and properties of the experimentally observed structure. The formation energies of all possible charge states of the different silicon substitutions (Si$_\mathrm{B}$, Si$_\mathrm{N}$ and Si$_\mathrm{{BN}}$) are calculated. The results reveal Si$_\mathrm{B}^{+1}$ as the most stable substitutional configuration. In this case, silicon atom elevates by 0.66{\AA} out of the lattice with unoccupied defect levels in the electronic band gap above the Fermi level. The formation energy shows a slightly exothermic process. Our results unequivocally show that heteroatoms can be incorporated into the h-BN lattice opening way for applications ranging from single-atom catalysis to atomically precise magnetic structures

Implanting germanium into graphene

Incorporating heteroatoms into the graphene lattice may be used to tailor its electronic, mechanical and chemical properties. Direct substitutions have thus far been limited to incidental Si impurities and P, N and B dopants introduced using low-energy ion implantation. We present here the heaviest impurity to date, namely $^{74}$Ge$^+$ ions implanted into monolayer graphene. Although sample contamination remains an issue, atomic resolution scanning transmission electron microscopy imaging and quantitative image simulations show that Ge can either directly substitute single atoms, bonding to three carbon neighbors in a buckled out-of-plane configuration, or occupy an in-plane position in a divacancy. First principles molecular dynamics provides further atomistic insight into the implantation process, revealing a strong chemical effect that enables implantation below the graphene displacement threshold energy. Our results show that heavy atoms can be implanted into the graphene lattice, pointing a way towards advanced applications such as single-atom catalysis with graphene as the template.Comment: 20 pages, 5 figure