Magnetotactic bacteria utilize strings of magnetite nanocrystals as an aid to navigation. These 'nanodevices' have been fabricated on the earth for roughly two billion years, and we may have even seen similar nanodevices fabricated on Mars. The very existence of these 'natural' nanostructures raises questions about how they were made and why they appear to be so stable against Ostwald ripening. If thermodynamically stable nanodevices can be fabricated by bacteria, then we should be able to learn from them how to fabricate 'quantum dot' and wire structures for use in man-made devices.
I explore the possibilities and limitations of utilizing the energetics of surfaces and interfaces, as well as the symmetry relation between overlayer and substrate, to devise self-assembled epitaxial nanostructures. First, I examine the well-studied system of coherently strained Ge islands grown on Si (001), and develop a thermodynamic model for the growth and evolution of the nanocrystals. This model generalizes the zero-temperature energetic picture of Shchukin et al.  by including the principle of detailed balance for different sized nanocrystals into the expression for the free energy of the island ensemble . Next, I demonstrate that the classes of Ge nanocrystals that form can be modified dramatically via the presence of a surfactant. Finally, I show how to grow regular arrays of uniformly sized epitaxial nanowires on a surface by combining this thermodynamic understanding with the basic crystal symmetries of the ovelayer and substrate as a constraint on the system.
V. A. Shchukin, N. N. Ledentsov, P. S. Kop'ev and D. Bimberg, Phys. Rev. Lett.75 (1995) 2968.
R. S. Williams, G. Medeiros-Ribeiro, T. I. Kamins, and D. A. A. Ohlberg, Acc. Chem. Res.32 (1999) 425.
R. Stanley Williams
1501 Page Mill Road, MS 1L-14, Palo Alto, CA 94304 USA