LEED crystallographic studies for chemisorption on the Cu(100) surface
Creators: name: Zeng, Hua Chun
Publisher: name: University of British Columbia
Year: 2010

description: The work in this thesis includes LEED crystallographic studies with low-energy electron diffraction (LEED) for the surface structures designated Cu(100)-(2x2)-S, Cu(100)-c(2x2)-N, Cu(100)-(√2x√2)45°-O and Cu(100)-(2x√2x√2)45°-O formed by chemisorption on the (100) surface of copper. In each case the intensity-versus-energy curves (I(E) curves) were measured with a video LEED analyser for a set of independent diffracted beams, and comparisons were made with the results of multiple-scattering calculations for various structural models. Levels of correspondence between experimental and calculated I(E) curves were assessed with visual comparisons as well as with the reliability indices introduced by Zanazzi and Jona and by Pendry. Surface structures were determined by the conditions for the best correspondence between the measured and calculated I(E) curves, and were analysed with bond length-bond order relationships to check chemical reasonableness. The Cu(100)-(2x2)-S surface structure was obtained by the adsorption and presumed dissociation of H₂S on the (100) surface of the copper. An initial LEED intensity analysis which assumed the metal atoms remain in their regular bulk positions was made for 16 diffracted beams measured at normal incidence and at one off-normal incidence direction. This study indicated that S atoms adsorb 1.32 Å above the "expected" four-fold adsorption sites, which corresponds to a nearest-neighour S-Cu bond distance equal to 2.24 Å. The second LEED crystallographic study used data measured for the 5 beams at normal incidence to investigate the lateral and vertical relaxations experienced by the four Cu atoms which bond to S. It was found these atoms relax outwards by about 0.04 Å, a result that contrasts with a report from a recent analysis with the angle-resolved-photoemission extended-fine-structure (ARPEFS) method, which indicated an inward relaxation by 0.05 Å for this surface structure. The interlayer spacing between S and the first copper layer obtained by this work differs substantially from that by ARPEFS (1.29 Å and 1.42 Å respectively). Further LEED observations were performed for the S on Cu(100) system to address some beam splitting phenomena observed at higher sulphur coverage. In addition investigations were made to study the constancy of I(E) curves for corresponding beams with varying coverages of sulphur. This can be used to simplify some calculations, and to check the polar angle for a LEED analysis at off-normal incidence. The Cu(100)-c(2x2)-N structure was prepared by the adsorption of nitrogen activated by an ion gun. The structural conclusions from LEED intensity studies differ markedly from those in two earlier reports in the literature. An initial intensity analysis with ten independent beams (at normal and off-normal incidences) showed that this surface structure involves N atoms incorporated deeply into the expected four-fold sites to become closely coplanar with the topmost copper layer. Each N atom then becomes 5-fold coordinated with bonding to the atom directly below in the second copper layer. A further analysis investigated the adsorbate induced relaxations. This indicated that the N atoms adsorb about 0.06 Å above the topmost copper layer, the Cu-Cu interlayer spacing between the first and the second metal layers increases by 0.04 Å from the bulk value (1.81 Å), and those Cu atoms in the second metal layer directly below N relax downward by about 0.09 Å. A range of geometrical models have been considered for the oxygen on Cu(100) system. A LEED multiple-scattering analysis with ten independent diffracted beams for the Cu(100)-(√2x√2)45°-O surface indicates for chemisorption of O atoms on the fourfold hollow sites that the agreement is slightly better for coplanar adsorption than for adsorption about 0.75 Å above the topmost copper layer as reported by earlier ARPEFS and SEXAFS studies. A new model proposed here for the Cu(100)-(2√2x√2)45°-O surface has missing rows of copper atoms parallel to the [010] direction. Several versions of this model were considered in a LEED analysis which used 6 diffracted beams for normal incidence. This analysis indicated the O atoms adsorb at 0.5 monolayer coverage in sites adjacent to the missing rows (these sites would correspond to four-fold sites in the regular surface). O is held at close to 0.15 Å above the topmost copper layer, while the topmost Cu-Cu interlayer spacing increases to 2.02 Å; each oxygen atom then bonds to four neighbouring copper atoms with an average O-Cu bond length of 1.91 Å.