Low‐energy electron diffraction (LEED) is a common and powerful method for determining the geometric structure of solid surfaces. It has the advantage of being fast and inexpensive relative to many other surface techniques. LEED can provide quick information on the surface unit cell size and geometry of single crystal surfaces, and with more effort can be used to determine the complete surface geometry, i.e. composition, bond lengths and angles. Although LEED has been used primarily as a structural technique, it can be used to determine other surface properties such as atomic and molecular vibration and libration amplitudes and energies. LEED has dominated the study of surface geometries for relatively simple structures, and is expected to become increasingly important in the study of nanostructures, molecular adsorbates, and insulating surfaces.

We have studied the incommensurate moiré structure of epitaxial graphene grown on iridium(111) by dynamic low-energy electron diffraction [LEED I (V )] and noncontact atomic force microscopy (AFM) with a COterminated tip. Our LEED I (V ) results yield the average positions of all the atoms in the surface unit cell and are in qualitative agreement with the structure obtained from density functional theory. The AFM experiments reveal local variations of the moiré structure: The corrugation varies smoothly over several moiré unit cells between 42 and 56 pm. We attribute these variations to the varying registry between the moiré symmetry sites and the underlying substrate. We also observe isolated outliers, where the moiré top sites can be offset by an additional 10 pm. This study demonstrates that AFM imaging can be used to directly yield the local surface topography with pm accuracy even on incommensurate two-dimensional structures with varying chemical reactivity.

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.

No abstract

No abstract