Researchers control nature's self-organization (11/16/2007)

Scanning Tunneling Microscopy image showing a self-organized array of atomic vacancy lines that meander as dark trenches through the atom wires on the Ga/Si(112)nx1 surface. Area about 20x20 nm.

Atom vacancies in a monatomic layer of gallium atoms on a silicon surface are shown to "selforganize" into a nanoscale pattern of almost perfectly straight lines. The vacancy lines are about 0.38 nm wide and are almost perfectly straight for up to 50 nm. The average spacing between the lines can be controlled (within certain limits) with a precision better than 0.05 nm. Kavli researchers publish their results in Physical Review Letters this week.

Self-organization is considered a promising route for the "bottom-up" fabrication of functional nano-devices. For instance, arrays of self-organized vacancy lines in hetero-epitaxial growth (i.e., growth of material A on B) may be used as a template for fabricating atomic wires. However, the laws of thermodynamics dictate that the structural uniformity of such nanostructures will be compromised by entropy, refering to nature's tendency to maximize disorder. Entropic fluctuations are especially profound in low-dimensional systems.

Here, we carefully controlled the line spacing by adjusting the chemical potential of the gallium atoms. We used atomic-resolution imaging to study the resulting line patterns. While current mean field models describing vacancy line arrays breakd down in the limit of ultrasmall (~2 nm) line spacings, we were able to quantitatively analyze the energetic driving force of the selforganization process, as well as the entropic factors that are responsible for the observed deviations from the perfect line structure. Our theoretical analysis involves a novel hybrid approach of first-principles density functional theory calculations and statistical mechanical modeling, and may be applicable to similar problems in thin film growth and nanoscience.

Note: This story has been adapted from a news release issued by the Kavli Institute of Nanoscience

Post Comments: