Self-organization of Ge tetramers on Ag(0 0 1) surface: A 2D realization of unusual substrate mediated interactions
Introduction
The temperature dependence of the morphology and kinetics of ultra thin films deposited on metallic substrates is still the subject of many investigations in relation to the formation of surface alloys which generally present specific and original chemical and/or physical properties [1]. Generally, the main parameters which drive the thermal behaviour of the deposited element are the chemical interactions between the adsorbate and the substrate elements and their respective tendency to segregate or not at the surface of the corresponding alloy [2]. Thus, although a complete dissolution of the deposited element into the substrate is expected at sufficiently high temperature, one can observe at intermediate temperatures some blocking of the dissolution process on metastable confined phases, the so-called surface alloys [3]. In the case of the latter, it has been shown that the dissolution kinetics is governed by a local equilibrium between the surface and its selvedge [3], [4] and that it is related to both its chemical composition and the atomic structure [5], with a slowing down being the signature of the formation of a surface alloy. Recently such a behaviour has been observed during the dissolution of one monolayer (ML) of Ge deposited on Ag(0 0 1) [6]. Using Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) for 1 ML of Ge in the temperature range 250–320 °C, Oughaddou et al. [6] find that kinetics always show a first part corresponding to a rapid dissolution in the bulk followed by a strong slowing down to a plateau with a concentration that depends on the temperature: at 250 °C the plateau corresponds to about 0.5 ML with a surface superstructure (which has been characterized elsewhere [7]) while at 264 °C the kinetics stops on a second plateau (which corresponds to about 0.3 ML) but surprisingly without any surface superstructure present in LEED. Since a slowing down of the kinetics is generally the signature of the formation of a specific structure, an investigation of this dissolution process using scanning tunneling microscopy (STM) is necessary to shed light on the nanoscopic mechanisms behind surface alloys formation and stability. This STM study is the subject of the present paper.
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Experimental procedure
The sample was prepared in an ultrahigh-vacuum system ( Torr) with a set of surface preparation and characterization facilities: LEED, AES and STM. The surface of the Ag single-crystal used was mechanically and chemically polished and presented a [0 0 1] orientation (to within 0.1° as checked by X-rays diffraction). The surface preparation consisted of a clean-up of the surface by repeated cycles of sputtering with ions (around 500 eV), followed by extended annealing at 500 °C for a few
Lattice gas analysis
We will not discuss here the apparent enhanced stability of Ge tetramers with respect to the other clusters, but we will try to identify the nature of tetramer–tetramer interaction which could explain the ordered tetramer structures observed in the previous STM images. Indeed, it is known that dissolution blocking on such surface alloys is only possible if they are linked to at least metastable ordered compounds [2], [3]. Therefore, the pavements displayed in Fig. 1, Fig. 2, Fig. 3 can be
Conclusion
To summarize, we have presented here an STM study of dissolution of Ge/Ag(0 0 1) which shows that it proceeds by successive disappearance of tetramer entities. Moreover, it has allowed us to assign the successive slowing down of the kinetics reported during this dissolution to the formation of ordered 2D arrangements (pavements) of tetramer vacancies. A lattice gas analysis of the latter reveals a very unusual sequence of pair interactions between these vacancies, probably mediated by the
Acknowledgements
The work of A.K. and T.S.R. was partially supported by the US Department of Energy under Contract No. DE-FG02-03ER46354.
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