Amorphous Networks at the Atomic-Scale: Comparing Two-Dimensional Silica and Germania

Recently, new insight into the atomic-scale structure of amorphous networks was established through the development of 2D thin-films of known glass formers using scanning tunneling microscopy (STM) in combination with density functional theory (DFT) [1-3]. Here we present a study of two prominent glass-formers, silica and germania, using high-resolution ultra-high vacuum STM to characterize structure. Both SiO_x and GeO_x films were grown on Ru(0001) by physical vapor deposition and subsequent annealing in oxygen, yielding either crystalline or amorphous structures. STM images reveal a hexagonal network with domain boundary structures in atomically flat monolayer films for both glass formers, with germania showing a greater variety of structures. Bilayer films provide for the development of amorphous structure and similar ring structures are seen for both materials. A few exceptions to the structural analogy are seen experimentally and described by DFT, which predicts a stronger interaction with the metal support for germania. This work reveals commonalities and species dependency for glassy structures.

[1] L. Lichtenstein et al. Angew. Chem. Int. Ed. 51, 404 (2012)
[2] A. Lewandowski et al. Phys. Rev. B. 97, 115406 (2018)
[3] A. Lewandowski et al. J. Phys. Chem. C, accepted (2018)