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Effect of Electronic Structure on the Lengths of Atomic Chains

Self assembled nanostructures hold great potential for a diverse set of applications, ranging from atomic-scale devices to material additives. However, the lack of control of the sizes and structures during the assembly process remains a critical obstacle. As nanostructures become smaller and smaller, approaching atomic dimensions, their electronic structure provides a possible path for obtaining the required control. When electrons are confined to reduced dimensions, the geometry of the confinement leads to the formation of quantized electronic states. In turn, these quantized states determine the energetic stability of a particular geometry. For systems that are self assembled, where thermodynamics and the cohesive energy can play a key role in the formation process, this interplay between the geometry of the confinement and the electronic states leads to the formation of nanostructures with "magical sizes".

In the present work we show how these electronic effects are manifest for chains of atoms self-assembled at surfaces. For Si(553)-Au chains, where quantized electronic states have been observed in finite segments the electronic structure leads to particular chain lengths that are favored. To demonstrate this, we measured the distribution of lengths via scanning tunneling microscopy, compiling the lengths of over 20,000 chains from high-resolution images (see Figure 1).

Figure 1. Click for larger version.
Figure 1. Example STM topography image (50 nm × 150 nm) of self assembled atom chains on the Si(553) surface used to compile the distribution of chain lengths.

When normalized to a random distribution function, the distribution of chain lengths exhibits long-range oscillations as a function of length that extend to at least 15 lattice constants (see Figure 2). Even chain lengths are favored over odd lengths out to lengths of at least fifteen atoms and a long-range beating is observed at a period of roughly eight lattice constants.

Figure 2. Click for larger version.
Figure 2. (top) The normalized distribution function shows oscillations as a function of length indicating particular chain lengths that are favored. The inset demonstrates the two major components of the oscillations, at periods of 2.0 and 2.63 lattice constants.

To understand how the electronic structure determines preferred lengths, consider electronic states that scatter at the ends of finite chain segments. A particular chain length exhibits increased stability when scattered electron waves fit neatly inside the chain. To connect the observed oscillations in the length distribution to the electronic structure of the chains, we extracted the periods of the oscillations and compared them to the wave lengths of the scattered electronic states at the Fermi level. The two periods observed in the length distribution were linked to two scattering vectors from the electronic states at the Fermi surface.


Related publications listing
Electronic Effects in the Length Distribution of Atom Chains

Staff listings
Daniel T. Pierce
Joseph A. Stroscio
Mark D. Stiles

Former Staff listing
Jason N. Crain

Supported in part by the Office of Naval Research

Online: April 2007
Last Updated: February 2008

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