Two-dimensional (2D) materials present a new class of materials whose structures and properties can differ from their bulk counterparts. We perform a genetic algorithm structure search using density-functional theory to identify low-energy structures of 2D group-IV dioxides AO2 (A=Si, Ge, Sn, Pb). We find that 2D SiO2 is most stable in the experimentally determined bi-tetrahedral structure, while 2D SnO2 and PbO2 are most stable in the 1T structure. For 2D GeO2, the genetic algorithm finds a new low-energy 2D structure with monoclinic symmetry. Each system exhibits 2D structures with formation energies ranging from 26 to 151 meV/atom, below those of certain already synthesized 2D materials. The phonon spectra confirm their dynamic stability. Using the HSE06 hybrid functional, we determine that the 2D dioxides are insulators or semiconductors, with a direct band gap of 7.2 eV at Γ for 2D SiO2, and indirect band gaps of 4.8 -2.7 eV for the other dioxides. To guide future applications of these 2D materials in nano-electronic devices, we determine their band-edge alignment with graphene, phosphorene, and single-layer BN and MoS2. An assessment of the dielectric properties and electrochemical stability of the 2D group-IV dioxides shows that 2D GeO2 and SnO2 are particularly promising candidates for gate oxides and SnO2 also as a protective layer in heterostructure nanoelectronic devices.

The classic models of metal oxidation developed by Wagner and Cabrera and Mott presuppose the existence of a planar oxide film and develop expressions for the rate at which the film thickens. Missing from those models is a description of how that initially planar film forms. Using scanning tunneling microscopy, we study the growth of NiO islands on the (100) surface of a Ni-5Cr alloy during the oxidation regime where the initial planar film is formed as oxide islands. The island height and area distributions as a function of the oxygen exposure in Langmuir (1 L = 10 Torr s) are measured. Lateral island growth and thickening occur as seemingly separate processes, and after a critical thickness of ≈0.4 nm is achieved, growth is purely in the lateral direction. We develop a surface diffusion model for the evolution of the island size distribution that accounts for the lateral growth and coalescence of the NiO islands. Our results indicate that the oxygen surface diffusion screening length [Formula: see text] controls the island evolution. The screening length is found to be 0.3-0.4 nm, which suggests that the processes leading to island growth are highly localized to the island edge.

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