Preparation of silica films on Ru(0001): A LEEM/PEEM study
Graphical abstract
Introduction
The most abundant minerals on the earth's crust are silicates. The mother compound, of course, is quartz and it comes as crystalline and vitreous or glassy phases [1]. The discovery of the details of the vitreous-crystal transition is still to come, and we and collaborators have recently made a step towards unraveling the structure of both, the crystalline as well as the vitreous phase in real space by scanning tunneling (STM) and atomic force microscopy (AFM) of a bilayer silica film grown on Ru (0001) as shown in Fig. 1 [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. One can clearly identify the silica ring structure composed of SiO4-tetrahedra connected into a hexagonal network of rings in the crystalline phase, while in the vitreous phase the connectivity changes and allows for larger and smaller ring systems, as predicted 80 years ago by Zachariasen [14]. After our discovery similar films have been prepared and characterized on graphene [15], [16], Pd(100) [17], and Pt(111) [18] using STM and transmission electron microscopy. However, those characterization techniques usually do not allow a sufficiently wide scanning range so that conclusions can only be drawn on small areas of silica covered Ru surfaces. On the other hand, if one is interested in properties of such systems, which have to be investigated using ensemble-averaging techniques, this requires information on at least a mesoscopic length scale in order to draw representative conclusions. For example, we are interested in the investigation of diffusion of molecules of different sizes through the silica film in order to study chemistry in constrained space, a topic that came up first with zeolites [19], but has now been also applied to diffusion and reaction under metal supported graphene sheets [20], [21], [22], [23], [24]. Graphene on the other hand is considerably more strongly coupled to the metal than the van-der-Waals bound silica bi-layers. Therefore, for the latter one expects a less pronounced influence of the covering layer beyond representing a membrane for diffusion of species and a means to constrain the space. We have undertaken first attempts to study diffusion underneath the silica film using infra-red spectroscopy [25]. For such studies it is essential to know the structure and morphology of the bilayer on a considerably larger, mesoscopic length scale, because holes in the layer would influence diffusion underneath the layer considerably, and would lead to false conclusions if interpreted at a microscopic level. In the present study, we follow the preparation of a bilayer silica film at the mesoscopic level using low energy electron microscopy in conjunction with photoelectron emission spectroscopy (LEEM/PEEM), using synchrotron radiation [26], [27], [28], [29]. Ultimately, this would allow to study the distribution of crystalline and vitreous phases across the surface and to investigate changes as a function of temperature. In conjunction with atomically resolved scanning probe techniques, this would bring us closer to a detailed understanding of the vitreous–crystal phase transition. In this paper, we present the first steps towards this goal by investigating the preparation conditions for crystalline and vitreous films at the mesoscopic scale.
Section snippets
Experimental
The experiments were carried out in the SMART microscope operating at the UE49-PGM beam line of the synchrotron light source BESSY II of the Helmholtz Center Berlin (HZB). This aberration corrected and energy filtered LEEM/PEEM instrument combines microscopy, diffraction, and spectroscopy techniques for comprehensive characterization of surfaces. The base pressure of the system is 10− 10 mbar; however, operation at oxygen pressure of up to 10− 5 mbar and at temperature above 1300 K is possible.
The
Results and discussion
Two different preparation recipes were applied to study the influence of the various parameters and the properties of mesoscopic defects. Whereas in the so-called “standard recipe” the Si deposition and the oxidation are done in separate steps, the silicon is oxidized during deposition in the second recipe, i.e. reactive deposition.
Discussion
The experiments concerning Si deposition in an oxygen atmosphere, presented in this work, (Section 3.2) yield three main results: (1) temperature dependent nucleation density, (2) dendrite shape of growing silica islands, and (3) reduced reactive sticking coefficient at higher temperature.
From the temperature dependence of the island density an apparent activation energy of Ea = 1.65 eV may be derived, which is clearly higher than observed for the nucleation process in metal epitaxy[34]. The
Conclusion
This study has shown that it is possible to grow large scale silica films on Ru(0001) and analyze structure and morphology on a mesoscopic scale. This provides valuable input for the use of ensemble averaging techniques to study properties of such films, including the diffusion of molecules through the film and at the interface between the silica film and the Ru metal surface.
Acknowledgments
We thank the BESSY II crew for their technical support and the Helmholtz-Center Berlin for Material and Energy for the allocation of synchrotron radiation beamtime. We gratefully acknowledge the financial support by the Federal German Ministry of Education and Science (BMBF) under Contract no. 05KS4WWB/4 and the Deutsche Forschungsgemeinschaft through CRC 1109, as well as by the Fonds der Chemischen Industrie.
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