ТЕОРЕТИЧНА МОДЕЛЬ ФОРМУВАННЯ ДВОВИМІРНИХ НАНОСТРУКТУР ВЕРТИКАЛЬНОГО ГРАФЕНУ ПІД ДІЄЮ ПЛАЗМИ

Abstract

Vertically oriented graphene nanostructures have been grown for more than decade, but the mechanisms of their formation are still unclear. A multifactor model is proposed, which is verified by comparison with experimental data and describes the processes of growth of the structure of vertical graphene in plasma. The role of chemical and physical processes that cannot be directly characterized by available experimental methods, such as surface diffusion of adatoms and radicals under the action of ions, has been studied. Ion bombardment is a key factor that significantly accelerates the growth rate through the formation of surface defects and, consequently, increases the energy of surface adsorption. Hydrocarbon radicals formed on the substrate under the bombardment diffuse to the graphene nanosheets and serve as the main source of the construction material. Thus, the leading role in the formation of vertical graphene belongs to surface diffusion, rather than direct deposition from the gas phase. The temperature of the sample is also an important parameter, which affects the growth process according to the following mechanism: at low temperatures the adsorption from the gas phase is more intense, but the diffusion processes are slowed down; elevated temperatures have the opposite effect. The surface density of graphene nanosheets, which can be controlled at the stage of nucleation, strongly affects the height of the structure due to the redistribution of ion fluxes during the growth: as the nanosheets grow, the ion current density decreases to the side edge of the sheet and increases to the upper edge. This process leads to a decrease in the ion current density at the side edge of the nanosheet, and, as a consequence, to a change in the dependence of the graphene sheet length on time: from a saturated curve or a quasilinear time dependence to a parabolic dependence. The assumption of surface diffusion of hydrocarbon radicals as the dominant growth mechanism is consistent with existing experimental data; these results confirm the physical model, and also bring a deeper understanding of the physics of growth of vertical graphene.