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We are very interested in new materials that have interesting surface properties and that have phases that form only on surfaces.
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| In this part of our research program, we have focused on determining the surface atomic and electronic structure of low-index faces of FeAl, in particular the (110), (210), and (310). It is well known that surfaces of solids can differ significantly from the bulk in regard to the atomic and electronic structure. In addition to the geometrical reordering in the surface, metal alloy surfaces can exhibit a deviation in chemical composition. Surface atoms may have chemically different atomic neighbors compared to those in the bulk and this leads to a chemical reordering in the surface. When the chemical reordering is not stoichiometric (not preserving the quantity of constituents present in the bulk) due to segregation of one of the constituents to the surface, new ordered/disordered phases emerge in the surface/surface selvage. This leads to a "surface" phase diagram which can be radically different from its bulk phase diagram counterpart. STM and angle-resolved photoemission (ARPES) techniques have proven to be indispensable for revealing the atomic and electronic structure of these novel surface alloy phases. |
 | To clean the surface, we sputter with Ne. Although this cleans the surface of contaminants, Al is also preferentially removed. Subsequent annealing then restores the near surface concentration through Al segregation. For example when the sputtered FeAl(110) surface is annealed to 600-800C, there is an increase in the Al concentration at the surface. Our STM results indicate that the ensuing increased compressive stress along the [110] direction is relieved through the formation of strain-relieving missing row reconstruction. |
| However, this is surface structural phase is metastable. Upon annealing above 850C, the surface undergoes another type of reconstruction. In this case the surface stoichiometry is FeAl2. As seen in the STM image, the lowest energy atomic structure of this novel surface phase is nearly incommensurate with the underlying lattice. A quasi-hexagonal arrangement of atoms is atomically resolved in the image. The structural model of the ordered quasi-hexagonal overlayer which has a unit mesh containing two Al and one Fe atoms is also displayed |  |
| Previous preliminary studies by our research group showed that up to a Ag coverage of 1 ML, silver forms a nearly incommensurate overlayer
structure nearly equivalent to a bulk Ag(111) structure. Second and subsequent layers cluster in a Ag(110) pseudoepitaxial arrangement to the underlying Cu(110) structure. The driving force to this layer-cluster growth is dictated by the electronic properties (2-D to quasi 1-D).
The growth of the Ag(110) is quite novel. It is believed that due to anisotropic strain effects, the Ag(110) clusters, if grown under the proper growth conditions, are pyramidal-shaped with nanoscale widths (7-10 nm) along the [001] crystallographic direction and they can grow very long along the perpendicular [110] direction (1000's nm). The height of these Ag-nanowires is but quite uniform at about 2 nm. |
 Ag nanowire grown on Cu. 2 ML of Ag has been deposited on the Cu(110) surface at ~200 K and subsequently annealed to 400 K. On the flat terraces, individual atomic-high steps are seen on the Cu substrate. |  As seen from the 3D projection the Ag clusters are anisotropic (in other words, they are long and thin like wires), but all of the nanowire clusters are approximately 7 nm wide. |
 If higher coverages are deposited ( at 240 K), the clusters coalesce forming micron long (1000 nm) and 70 nanometer wide Ag structures. The flux rate and temperature play a key role in determining the resulting morphology. |  If Ag is deposited at 100 K, thermal diffusion is minimized. This results in a morphology characterized by small critical-size clusters which are only 1-3 nm in diameter and 0.5 nm in height. |
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