Theoretical study on surface properties of colloidal quantum dots and graphene

Abstract

In this thesis, we study surface chemistry of colloidal quantum dots and nanoscale friction of graphene by investigating their surface properties using first-principles density-functional theory calculations. For colloidal quantum dots, our calculation results show that cation-rich surfaces are necessary for proper ligand-surface coordination to be stabilized. In addition to ligand-surface coordination, the cation-rich surface stabilization is affected by steric hindrance between coordinating ligands. For IV-VI colloidal quantum dots, the steric hindrance between passivating ligands has influence on shape transition from oleate-capped octahedron to cuboctahedron truncated with (100) surface that prone to surface oxidation. The shape transition leads to size-dependent air-stability of IV-VI colloidal quantum dots. For graphene, we examine nanoscale friction properties of chemically modified graphene, epitaxial graphene on SiC(0001), and water-intercalated graphene on mica. By calculating elastic properties of graphene and investigating 3D/2D contact through first-principles calculations and contact mechanics theory, we find that nanoscale frictional energy of graphene is mainly dissipated by out-of-plane bending deformation of graphene and shear deformations between graphene and substrate. In addition, for water-intercalated graphene, we propose that the increased friction by water layer is attributed to phonon excitation enhancement and improved energy transfer based on calculated phonon properties.