Bilayers on Ni-Pd Substrates*

Silicates in zeolite form have many applications in catalysis. They have porous structures with large internal surface area where chemical reactions can occur. However, it is hard to characterize such internal surfaces with atomic resolution in real-space experimentally. Therefore, a two-dimensional (2D) form of silica has been created experimentally to serve as a model system that can be probed using surface science techniques such as scanning tunneling microscopy (STM). Silica can either form monolayer or bilayer forms depending on the substrate on which it is grown. When 2D silica forms a bilayer, it interacts weakly with substrates due to the fully saturated Si-O chemical bond. Therefore, bilayer SiO₂ can be used to simulate the internal surface of zeolites. Experimentally, one can create Ni-Pd random alloys which have tunable composition-dependent lattice constants and can serve as a set of substrates for 2D silica. In this work, we investigate the stable morphologies of 2D SiO₂ on Ni-Pd using density functional theory (DFT). We also describe the role of the weak interaction between bilayer silica and Ni-Pd in terms of how much epitaxial strain it can enforce before the silica overlayer becomes incommensurate to the substrate.