Wednesday, September 28, 2011

Abrupt Escape From Flatness: Nano-Lead Shows Rapid Transition From 'Pancake' To Hemisphere

A lead coating on a nickel surface has unusual electronic properties which cause it to form flat “pancakes”, consisting of billions of atoms arranged in a crystalline structure. These “pancakes” of solid lead are quantum mechanically stabilized and just a couple of dozen atoms thick. When exposed to gradual heating, nothing much changes at first. 

When subjected to very gradual heating, the “slivers” of lead suddenly switch to a hemispherical shape. 
Credit: University of Twente

At about 520 Kelvin (247 degrees Celsius), however, the lead coating suddenly seems to disappear completely. Within the space of a few milliseconds, the lead “slivers” transform into hemispheres with a radius (or “height”) of a few micrometers. Interestingly, this all takes place at a temperature below the melting point of lead. The hemispheres, too, consist of solid lead. So no mass has been lost, the material has simply taken on a different spatial configuration.

Low energy electron microscope

The technique used by the researchers to observe this process is known as Low Energy Electron Microscopy (LEEM). There are only a few such microscopes in existence, but two have recently been installed in the Netherlands. They are designed to bombard surfaces with low energy electrons. This makes them especially well suited to making accurate observations of surface phenomena and events in thin films.
From 2D to 3D within milliseconds from University of Twente on Vimeo.

Beyond the scope of our current knowledge

The abrupt transformation from flat to spherical can be explained in terms of the most energetically favourable shape. From this viewpoint, hemispheres make much more effective use of surfaces, whereas pancakes are not very stable. There has recently been a massive expansion in our understanding of atomic processes right down to the level of single atoms, facilitated by experimental techniques such as Scanning Tunnelling Microscopy (STM), together with newly developed theories. Even so, we cannot account for the sheer speed at which this transition takes place.

Group process

However, this recently discovered super-fast transition from two to three dimensions is based on a delicate interplay between several atoms, a kind of group process. In their published article, these researchers from Twente express the view that a more detailed explanation of the very rapid transition from flat to spherical will only be possible when we have a better fundamental theoretical understanding of meso-level phenomena. LEEM can be used to make direct observations of new phenomena at the meso-scale, thereby generating data crucial to our knowledge of this field. The importance of these results is that they will give us a more profound understanding of the stability of nanostructures.

This study was carried out by Prof. Harold Zandvliet’s Physics of Interfaces and Nanomaterials group. Funding was provided by the FOM Institute. The group is part of the University of Twente’s MESA+ Institute for Nanotechnology. The LEEM equipment used in this study was purchased with funds provided by the Dutch Technology Foundation (STW).

Contacts and sources:
University of Twente 

Citation: The article entitled “Anomalous decay of electronically stabilized lead mesas on Ni(111)” by Tjeerd Bollmann, Raoul van Gastel, Harold Zandvliet and Bene Poelsema has been published in Physical Review Letters (online 23 Sept).

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