Chin-Lung Kuo, Chemical Engineering, The University of Texas at Austin, 1 University Station Stop C0400, Austin, TX 78712 and Gyeong S. Hwang, Chemical Engineering, Dept. of Chem. Eng.,The University of Texas at Austin, 1 University Station Stop C0400, Austin, TX 78712.
Atomic-level control and determination of the surface reactivity of amorphous silica materials has long been an issue of great importance due to their many applications in electronics, photonics, heterogeneous catalysis and sensors. While a defect-free silica surface is relatively inert towards chemical reactivity, previous experiments (1,2) have evidenced that the surface reactivity can be enhanced in the presence of intrinsic stress. For instance, the number density of Si nanocrystals on a thin thermally oxidized silica film has been found to increase substantially with lowering the oxidation temperature or decreasing the silica film thickness. The observed reactivity change apparently originates from the intrinsic compressive stress in the silica film due to the molar volume increase during the conversion of silicon to silica. In addition, it has been demonstrated that an ordered array of silicon nanoparticles can be synthesized by imposing a corresponding anisotropic strain on the silica surface, for instance through a buried array of dislocations near the silicon/silica interface. While it is evident that a compressive stress leads to creation of certain favorable nucleation sites for silicon nanoparticle growth on the silica surface, the underlying reason still remains a subject of speculation. In this presentation, we will focus on addressing changes in the surface structure and reactivity of thin amorphous silica films under biaxial compressive strain, based on combined extensive Monte Carlo and density functional theory calculations. We have found that the externally imposed strain can be lowered by creating edge-sharing tetrahedra and/or silanone groups at the surface of amorphous silica, while the former is prevailing than the latter. The surface defect formation is considered as two-dimensional densification because it leads to the reduction of Si-O-Si linkages on the surface. This also turns out to assist in relieving the impose strain with irreversible structural changes in the silica slab. Our results also suggest that the surface reactivity enhancement of amorphous silica towards silicon nanoparticle growth under compressive strain conditions could be attributed to the formation of surface defects, particularly edge-sharing tetrahedra.