Browsing by Author "Åhlgren, Elina Harriet"

Graphene is the ultimately thin membrane composed of carbon atoms, for which future possibilities vary from desalinating sea water to fast electronics. When studying the properties of this material, molecular dynamics has proven to be a reliable way to simulate the effects of ion irradiation of graphene. As ion beam irradiation can be used to introduce defects into a membrane, it can also be used to add substitutional impurities and adatoms into the structure. In the first study introduced in this thesis, I presented results of doping graphene with boron and nitrogen. The most important message of this study was that doping of graphene with ion beam is possible and can be applied not only to bulk targets but also to a only one atomic layer thick sheet of carbon atoms. Another important result was that different defect types have characteristic energy ranges that differ from each other. Because of this, it is possible to control the defect types created during the irradiation by varying the ion energy. The optimum energy for creating a substitution for N ion is at about 50 eV (55%) and for B ion it is ca. 40% at about the same energy. Single vacancies are most probably created at an energy of about 125 eV for N (55%) and for B at ca. 180 eV (35%). For double vacancies, the maximum probabilities are roughly at 110 eV for N (16%) and at 70 eV for B (6%). The probabilities for adatoms are the highest at very small energies. A one atom thick graphene membrane is reportedly impermeable to standard gases. Hence, graphene's selectivity for gas molecules trying to pass through the membrane is determined only by the size of the defects and vacancies in the membrane. Gas separation using graphene membranes requires knowledge of the properties of defected graphene structures. In this thesis, I presented results of the accumulation of damage on graphene by ion irradiation using MD simulations. According to our results, graphene can withstand up to 35% vacancy concentrations without breakage of the material. Also, a simple model was introduced to predict the influence of the irradiation during the experiments. In addition to the specific results regarding ion irradiation manipulation of graphene, this work shows that MD is a valuable tool for material research, providing information on atomic scale rarely accessible for experimental research, e.g., during irradiation. Using realistic interatomic potentials MD provides a computational microscope helping to understand how materials behave at the atomic level.