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Surface Structures Far-from-Equilibrium


The emergence of novel properties in nanostructures can be related to various factors, important ones being electron confinement and lower atom coordination. The goals of this FWP are two-fold, first to grow epitaxially controllable nanostructures and second to use their novel, selectable properties on several technologically important problems. Achieving these goals it is also essential to find ways to tune atomistic processes (diffusion, adsorption) and use them to grow  perfect nanoscale patterns easily and in short times. Such studies have been carried out in several specific systems. Understanding metal growth on graphene, graphite and other carbon coated substrates is one of the areas of interest because graphene based devices require stable metal contacts of low electrical resistance. Novel graphene properties can emerge after metal intercalation. Robust ways were found to speed up adatom diffusion, from the electric field generated in regions of different workfunction on metal islands or when 2-dimensional metallic overlayers become extremely mobile after compressed to densities higher than their crystalline densities. Understanding the growth mechanism for defect-free nanostructures with high and tunable aspect ratios on carbon coated surfaces is relevant for magnetic and optoelectronic applications.

See Tringides Group and Thiel group web pages for more information.

This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

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  • Schematic representation of how the adatoms (small spheres) are guided by the electric field. The non-intercalated domain shown in the middle of the figure has higher electrostatic potential. An electric field is generated at the boundary and, in this case, the middle region attracts the adatoms.

    Researchers have shown how the motion of individual atoms on surfaces of graphene—a one atom thick layer of carbon—can be controlled. The adatom diffusion rate and hopping direction can be tuned by lowering the diffusion barrier using an effective electric field. This was shown using in situ scanning tunneling microscopy at low temperatures and the mechanism was elucidated using first-principles calculations. The electric field is locally tuned by inserting metal atoms below graphene.

  • In a matter of seconds, fully-formed metal islands have been observed exploding out of a silicon surface wetted with lead atoms. This island formation vastly differs from the classical model of nucleation, where islands form from the gradual aggregation of randomly moving surface atoms and can grow slowly over hours. At temperatures well below freezing, lead atoms are systematically deposited onto a thick piece of silicon wafer, forming an amorphous layer known as a wetting layer.

  • Scientists have discovered that the rare earth element dysprosium grown on graphene — a one atom thick layer of carbon — forms triangular-shaped islands, whereas other magnetic metals form hexagonal-shaped islands. Based on the hexagonal closed packed (hcp) bulk crystal structure of dysprosium, hexagonal islands would also have been expected. Researchers used scanning tunneling microscopy to identify the crystal structure of dysprosium on graphene. The results indicate that dysprosium grows as face centered cubic (fcc) crystals on graphene rather than hcp.

  • The self-organization of lead on silicon stands out for its remarkable efficiency and surprising new results suggest why. Most atoms sitting on surfaces like to go about their business by themselves. Alone they walk in random directions. Rarely do they move together, so when a billion atoms collectively decide to move 0.05 mm within 1 second below room temperature, it is exceptional. Researchers have found evidence of this 'superdiffusion' for lead on silicon using a technique known as low energy electron microscopy.

  • Scientists have discovered that the growth of iron on graphene — a one atom thick layer of carbon — occurs in an unusual way. For other metals the first atoms to arrive form small clusters on the graphene surface, and then the clusters migrate across the surface, seemingly at random. Whenever two clusters encounter each other, they merge to form a larger cluster, which moves a little slower. Growing these larger clusters is important for making electronic connections to graphene for microelectronic applications.

  • A layer of lead on clean silicon moves in a surprising way — in waves like a caterpillar. This explains the unexpected ultrafast mass transport observed even at low temperatures for this system. Although solid these single layers of atoms move as fast as molten lead. Computer simulations show that the lead layer forms waves that require almost no energy to keep moving thus explaining the quickness of mass transport. Other metals on surfaces typically move much slower by one atom at a time hopping along the surface.

  • Graphene is supposed to have the potential to replace silicon in electronic devices, making them thinner and faster, but making such devices depends on making electrical contacts. Researchers have deposited two metals onto graphene — a one atom thick layer of carbon — to see what kinds of elements might work best. Metals like lead were predicted to attach weakly, while rare earth metals were predicted to stick strongly giving better results. Scanning tunneling microscopy experiments confirmed the predictions.