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Abstract

Dynamics related to thin silicon- and germanium dioxide films that are grown on Ru(0001) crystals are investigated. Between the film and the metal support oxygen species are present that play a crucial role for these film systems. First, these oxygen adlayers on Ru(0001) are analyzed by high-speed scanning tunneling microscopy (STM) with the focus on dynamic processes. In a next step, the monolayer of germanium dioxide, termed also germania, supported on Ru(0001) is studied at elevated frame rates. For the structural analysis of the two-dimensional oxide network, a semi-automated network detection tool is developed. Finally, the bilayer of silicon dioxide, termed also silica, on Ru(0001) is studied by conventional and by high-speed STM both at room temperature and at elevated temperatures of 600 K.
To realize fast STM measurements at elevated temperatures, a high-speed STM is designed and built that can operate at variable temperatures. Mechanical and electronic components are custom designed. For high-speed STM measurements, an unconventional spiral scan pattern is implemented for the first time. Frame rates of 120 Hz are achieved without reaching the limits of the mechanical setup. The high-speed images are free of drift and distortion artifacts.
With the spiral scan approach, the dynamics in oxygen adlayers are investigated for the first time at elevated frame rates. Experimental results are supported by density functional theory (DFT) calculations performed externally. Dynamic events are observed in the oxygen ad-layers that are stable at room temperature, namely O(2×2)/Ru(0001), O(2×1)/Ru(0001), and 3O(2×2)/Ru(0001). The occupation of an intermediate state along the oxygen diffusion pathway and fast "flipping" events of atomic one-dimensional stripe patterns are observed.
On the germania monolayer on Ru(0001), complex domain bound-ary structures are resolved with high-speed STM. In high-speed measurements on the silica bilayer on Ru(0001), dynamic changes of the imaging contrast are observed that may relate to the mobile species in the oxygen interfacial layer. Measurements at elevated temperature reveal dynamic contrast changes of mesoscopic features. These measurements constitute the first high-speed STM scans on the silica film at elevated temperatures and form the basis for future studies with the focus on dynamic processes in thin oxide film systems. One of the most encouraging long-term objectives is the observation of dynamic processes at the atomic scale in real space and in real time resulting in the structural transformation between crystalline and vitreous networks.