September 1, 2001
Rational design of interfacial structure
The interaction of rare gases with single crystal surfaces has been a topic of much interest over the past three decades. These systems have served as a testbed for refining our ideas on adsorption dynamics, structure formation, and low-dimensional phase behavior. Adsorption characteristics of xenon in particular have been widely examined, including an abundance of studies on smooth and complex metallic, semiconducting, and insulating substrates. Particular emphasis has been placed on elucidating the phenomena which lead to ordering on (111) metallic surfaces; the geometric simplicity and theoretical tractability of these surfaces facilitates this effort. These studies have shown that xenon virtually always forms a close-packed (3x 3)R30º superlattice, governed by a delicate balance of interadsorbate and adsorbate-substrate interactions. We show that it is possible, using an intentionally atomically-patterned surface, to guide the formation of a novel non-close-packed xenon structure in which the rare gas overlayer is templated by the symmetry of the underlying interface. We believe this procedure to be generally applicable to the rational design of custom interfaces and materials.
We have studied both xenon on Ni(977), a stepped surface with (111) terraces, and xenon on Ni(977) with hydrogen pre-adsorbed in a (2x2)-2H overlayer (Figure 1). Our low energy electron diffraction (LEED) results show that this surface supports a commensurate p(2x2)-Xe structure that is highly reinforced by a templating pre-adsorbed hydrogen overlayer.
The poorly ordered xenon domains which form on the clean metal are quite small, less than 10 Å, or approximately two xenon unit cells. Therefore, introducing regular steps to this system shifts the energetics allowing a p(2x2)-Xe rather than the typical (3x3)R30º-Xe to form, but the ordering is very weak (Figure 2a).
We have recently studied the H/Ni(977) phase diagram and have a reliable algorithm for forming a well-ordered (2x2)-2H overlayer. This particular system was deliberately selected because its honeycomb structure provides an accessible array of nickel on-top sites for the xenon atoms having the targeted symmetry and dimensions. After pre-adsorbing this overlayer, a subsequent application of xenon also results in a p(2x2)-Xe superstructure. A LEED pattern of this structure, obtained using the same parameters as with the clean metal, is presented in Figure 2b. This pattern differs both qualitatively and quantitatively from that for adsorption on the clean metal. A visual inspection of the LEED photographs clearly reveals the diffuse nature of the xenon spots on clean Ni(977) relative to the same spots from the surface containing pre-adsorbed hydrogen. Note that the diffuse background, a measure of disorder, is significantly lower with hydrogen present. Also, the ordered xenon domains are significantly larger, approximately by a factor of four, when hydrogen is present. Domain sizes have increased from a modest 9.4 Å to 39.3 Å, or from two to eight xenon unit cells.
Our results have shown that one can engineer the symmetry and dimensions of a desired atomic scale structure by using a pre-adsorbed guiding overlayer. Tuning the corrugation and structure of substrates can be used to induce the formation of novel interfaces. The templating phenomenon described here suggests a universal route to building new rationally-constructed and self-organizing two- and three-dimensional structures.