S S Dhesi

STM observations of Cu(100)−c(2×2)N surfaces: evidence for attractive interactions and an incommensurate c(2 × 2) structure

Abstract STM studies of N on Cu (100), adsorbed via activated ion bombardment, reveal behaviour consistent with attractive interactions and an incommensurate c(2×2)N structure. For extremely low N doses, images of the resultant surface show roughly square shaped islands with island edges running along the 〈001〉 directions. The islands have an internal c(2 × 2) periodicity and while tending to group together do not coalesce. For higher N doses, island coalescence occurs simultaneously with the appearance of trench-like defect structures running in the 〈011〉) directions and small one-atomic layer high islands. Lack of island coalescence at low coverages and the appearance of trench-like defects at higher coverages are interpreted as strain-relief mechanisms for an incommensurate c(2×2)N structure. To derive the N adsorption site, S has also been coadsorbed along with the c(2×2)N islands to form coexisting p(2×2)S domains. The spatial relationships between features in both domains show that the protrusions observed in the c(2 × 2)N areas are located above four-fold hollow sites. Finally, in order to compare our work with previous studies, we have obtained detailed LEED I–V data from the various Cu(100)−c(2×2)N surfaces studied with STM.

The Structure of Rare-Earth Metal Surfaces

Introduction to the Rare Earths The Basics of Surface Structure Surface Structure Techniques Crystal Growth and Surface Preparation Rare-Earth Surface Science Quantitative Low-Energy Electron Diffraction Quantitative LEED Results Summary - Past, Present and Future.

Electron-stimulated disordering in c(8 × 2) benzoate/Cu(110): a combined STM, LEED and HREELS study

Abstract We demonstrate a novel, negative-ion-mediated, electron-stimulated disordering process in LEED and, at nanoscopic scales, in the STM. Electron-stimulated disordering of the c(8 × 2) benzoate/Cu(110) structure in LEED is reversible, and HREELS shows that it is non-destructive to the adsorbed species. The disordering kinetics in LEED are pseudo first-order above a threshold between 5 and 7 eV with a large cross-section (∼ 10 −16 cm 2 ). The threshold correlates with strong negative ion resonance scattering in HREELS centred at 7–8 eV. Defect-controlled kinetics in LEED are observed at 5 eV and correlate with the spatial dependence in the STM disordering, which initiates preferentially at domain boundaries and is inhibited at step edges. Both the STM- and electron-beam-induced disordering are consistent with a negative ion resonance mechanism for efficient energy transfer from the tunnelling/incident electron to adsorbate internal vibrational modes which then relax by excitation of the frustrated translation. The influence of field effects on the potential energy surface is also considered.

Determination of the surface relaxation of Sc(0001) by video LEED analysis

The authors describe a low-cost system of video data acquisition as an extension to qualitative LEED studies that allows quantitative LEED intensity-voltage (I-V) analysis. The type of equipment used and the procedures developed to align the samples, acquire the data, extract the LEED spot intensities and determine the surface layer spacing from a simple kinematic analysis are outlined. As a test case, the authors determine the surface layer spacing d12 of the rare earth metal Sc(0001) using kinematic analysis of I-V spectra of the (00) LEED spot. Values derived from spectra taken on different days are reproducible to within 0.5% (about 1 pm), and the mean value of d12 is found to be 0.2633+or-0.0005 nm. This compares to the spacing for bulk Sc of 0.2634 nm, indicating that there is no surface relaxation.

Surface structure of rare earth metals

Abstract We have performed LEED, angle-resolved UV photoemission (ARUPS) and inverse photoemission (IPE) measurements on a representative set of hcp rare earth surfaces in order to obtain basic structural information. LEED patterns from the (0001) surfaces show six-fold symmetry due to the two equally probable bulk terminations. ARUPS data show a large non-dispersing peak at ~ 10 eV binding energy, with intensity extremely sensitive to surface order, which is not reproduced in one-electron calculations. Suggestions that this many-body feature is due to an unoccupied surface state are supported by our IPE data, which appear to show such a state just above the Fermi level. LEED patterns from the (1120) surfaces of Y. Ho and Er also show six-fold symmetry, with no visible substrate spots, implying that these relatively open surfaces have collapsed to a (7 × 1) close-packed structure many layers deep. ARUPS data from the reconstructed (1120) surface of Ho are virtually indistinguishable from those obtained from Ho(0001). This suggests that the two structures are identical within the probing depth of this technique.

LEED and STM studies of structures formed by NO dissociation on Cu(100) surfaces

Abstract We have recorded LEED I - V spectra from the clean Cu(100), Cu(100)-(√2 × 2√2)R45°-O, Cu(100)-c(2 × 2)N surfaces and Cu(100) surfaces exposed to NO at varying pressures. The dissociation of NO on the clean Cu(100) surface, at room temperature and at a pressure of 1.2 × 10 −4 mbar, exhibits a weak c(2 × 2) LEED pattern with similar amounts of both O and N present on the surface. By contrast, the dissociation of NO on the clean Cu(100) surface, at room temperature and at a pressure of 6 × 10 −4 mbar, results in a (√2 × 2√2)R45° LEED pattern with mostly oxygen remaining on the surface. Annealing this structure to ∼ 780 K results in the disappearance of the (√2 × 2√2)R45° pattern and the appearance of another LEED pattern indicating a two-domain hexagonal structure on the surface. A similar LEED pattern has been observed by Ertl (following the decomposition of N 2 O on the clean Cu(100) surface at 800 K). Comparisons of the LEED I - V curves for all of these structures allows qualitative predictions to be made regarding their surface structures. Scanning tunneling microscopy has been used to test these predictions.

Angle-resolved UV photoemission from Pr(0001)

We observe a number of well defined peaks in the photoemission spectra from Y(0001) over the photon energy range 25 eV<hv<60 eV. We believe that one of the features with a binding energy of 1.7 eV is due to emission from states near the upper band edge at the Γ point and that a second nearEForiginates from a surface state. We find a large peak at a binding energy of 9.6 eV which is sensitive to the quality of the surface.

Determination of the Tb A4- critical point energy from angle-resolved UV photoemission of Tb(0001)

The authors report an angle-resolved UV photoemission study of the clean (0001) surface of the rare-earth metal terbium. Normal emission spectra were recorded in the photon energy range 12-50 eV using synchrotron radiation. A number of well-defined features were observed which showed negligible dispersion with photon energy. Close similarity is found between these spectra and those from the (0001) surfaces of Y, Gd and Ho. The binding energy of the Gamma 4- critical point is determined to be 1.5(+or-0.1) eV. They suggest that the apparent shift of the Tb Delta 1 band to higher binding energy reported by previous workers was a result of the significant Fe contamination of their sample.

Observation of a (7*1) surface reconstruction on Y(1120)

A study of the surface reconstruction of Y(1120) has shown a sixfold-symmetric low-energy electron diffraction pattern in contrast to the twelvefold-symmetric pattern reported earlier. This indicates that only one orientation of close-packed atoms exists on the surface; the collapse of the surface unit cell is uniaxial along the (0001) direction. This produces the same commensurate surface structure as found previously for the (1120) (7*1) reconstructions of Ho and Er. Why the earlier study of Y showed a two-domain reconstruction has not been determined, but they suggest that this structure was influenced by the presence of surface defects, inducing the collapse of the unit cell along the (1010) direction to form an incommensurate domain with approximately equal probability to that of the commensurate (7*1) domain.

Surface electronic structure of rare earth metals

Abstract Angle-resolved UV photoemission has been used to investigate the electronic structure of the (0001) surfaces of scandium, yttrium, praseodymium and gadolinium. Off-normal emission spectra were recorded with high angular resolution, enabling detailed mapping of the dispersion of valence band features. Yttrium and gadolinium show similar results to published data from Ho(0001), suggesting minimal 4f influence in the lanthanide bandstructures. Differences seen on praseodymium and scandium may be due to 4f derived states and surface states respectively.