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It is known that the electronic and optical properties of nanocrystals depend on their geometrical shape. However, the stability of the crystallite shape is not always preserved. During the growth or synthesis process, nanostructures can evolve from a well-defined shape to another one as a result of external perturbations such as misfit strain, surface or interface energy. The precise nature of the shape transitions is, therefore, very important both for scientific interest and for technologic controlling of the island-size distribution, especially for the shape control. In this work, we describe two kinds of shape transitions recognized during the growth of Er silicide nanostructures on Si(001) surface. The experiments were carried out in an Omicron ultrahigh-vacuum scanning tunneling microscopy (STM) system. Er was evaporated onto the substrate at room temperature. After Er deposition, the samples were post-annealed at 580 ~ 750 oC for between 10 and 180 min, and then cooled down to room temperature for STM imaging.
By carefully choosing the appropriate post-annealing temperature or duration time, long nanowires in AlB2-type structure and compact square nanodots in ThSi2-type structure of Er silicide can be formed on the surface, indicating a shape transition. This shape transition actually is caused by a structural phase transition for Er silicide from AlB2-type to ThSi2-type crystalline structure, reflecting the role of surface and interface energy. The transition is an outcome of a competition between the interaction of silicide atoms and the interaction of silicide / substrate, and stress is believed having a small effect. When ThSi2-type square nanodots increase in size with the increasing of post-annealing time, they will undergo another shape transition to rectangular nanodots at the critical size of 32 nm, which is governed by the tradeoff between surface energy and strain in the nanodots. A quantitative analysis will show that the ThSi2-type nanodots have widely varying length, but similar width. The two shape transitions described here shed light on the ways to fabricate Er silicide nanostructures with different dimension extents, shapes and crystalline structures.
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