Crystal structure of bismuth varies
Thin bismuth films modify their structure under the influence of conducting electrons
The poorly conducting semi-metal bismuth assumes completely different crystal structures in thin films than in bulk form. The structure of the layers, distance between layers and number of layers appear to be formed such that exactly one standing wave of conducting electrons can develop. FOM PhD researcher Tjeerd Bollmann and his colleagues at the University of Twente discovered this when they grew ultra-thin bismuth films on a nickel substrate. Thin bismuth films are therefore probably far better conductors than the bulk crystal. The researchers published their unexpected result online today in Physical Review Letters.
The researchers from the MESA+ Institute for Nanotechnology investigated thin films of lead on nickel and bismuth on nickel. The morphology of a thin film depends upon various factors, including the growth temperature and lattice constants of the film material and the substrate. For some metals the wavelength of the conducting electrons (Fermi electrons) also exerts an effect. The most important example of this is lead. A thin lead film does not grow layer by layer, but with an unequal number of layers at once. Film thicknesses of three, five or seven layers are energetically advantageous as these precisely accommodate a standing wave of the Fermi electrons.
Now the researchers have observed this phenomenon in thin bismuth films as well. They were surprised to discover that bismuth grows with an uneven number of layers at once: the Fermi wavelength of bismuth in bulk is about one hundred times greater than that of lead and so layers of bismuth just a few atoms thick were not expected to influence the Fermi wavelength at all. However, they did. Using a low-energy electron microscope, the researchers could see that the layers in a thin bismuth film had a distinctly different crystal structure than the bulk crystal. This crystal structure is always in balance with the distance between layers in the film and the Fermi wavelength: the bismuth modifies its structure and with that the distance between layers such that the total film thickness (of three or five layers) equals a multiple of the Fermi wavelength associated with the structure concerned.
Bismuth is an ambiguous material at several levels. A piece of bismuth ’hovers’ between being a metal or a semiconductor and is therefore termed a ‘semi-metal’. It has now been demonstrated that in thin films, bismuth modifies its structure, density and distance between layers, to maximise the energetic advantage that can be obtained from standing Fermi waves with a wavelength far smaller than in the bulk material. The researchers expect that the structures found in the thin bismuth films are far better conductors than ordinary bismuth. Other physical and chemical properties of the newly discovered crystal structures probably differ from those in the bulk bismuth as well. The researchers aim to study these properties by means of controlled growth.
Contact
Bene Poelsema, +31 (0)53 489 30 60
Reference
'Quantum size effect driven structure modifications of Bi films on Ni(III)'
Tjeerd R.J. Bollmann, Raoul van Gastel, Harold J.W. Zandvliet and Bene Poelsema
Physical Review Letters, 14 October 2011 (online 13 October 2011)