- Email: [email protected]
Effects of madelung potentials at surfaces of ZnO on photoemission spectra and work functions
A105 Surface Science 115 (1982) 117-140 North-Holland Publishing Company EFFECTS OF MADELUNG ZnO ON PHOTOEMISSION R.E. WATSON,
117 POTENTIALS AT SURFACES OF SPECTRA AND WORK FUNCTIONS
M.L. PERLMAN
*
and J.W. DAVENPORT
Departments of Physics and Chemistry, Brookhaven National Laboratory, Upton, New York 11973, USA Received 4 August 1981; accepted for publication 27 October 1981 Madelung potentials have been calculated for sites in and near polar and nonpolar, primitive and reconstructed surfaces of wurtzite ZnO. Surface versus bulk potential differences result in electron binding energy differences which should be observable in photoemission spectra even on nonpolar surfaces. In polar planes, the average site charges in the outermost ion layer are "naturally" reduced relative to charges in the crystal interior, and observable effects should be larger. Results for polar surfaces of singly bonded atoms are indicative of the Madelung potential effects expected for dense layers of chemisorbed species. These potentials are especially sensitive to details of charge transfer. If a crystal has uniform site charges except for the above-mentioned charge reduction on polar surfaces, there exist surface dipoles which would cause considerably greater variation in work function from one crystal face to another than that observed experimentally; this discrepancy is resolved if, in the outermost cation-anion layer, charges are further reduced to three-quarters to one-half the "natural" values.
Surface Science 115 (1982) 141-160 North-Holland Publishing Company A STRUCTURAL AND KINETIC STUDY OF CHLORINE CHEMISORPTION AND SURFACE CHLORIDE FORMATION
141
ON
Cr(lO0) J.S. F O O R D *
and R.M. LAMBERT
Department o/Physical Chemistry, University of Cambridge, Cambridge, CB2 I EP, UK Received 17 June 1981 ; accepted for publication 26 October 1981 Chlorine adsorption on ( I X I)-Cr(100) has been studied using AES, UPS, XPS, LEED, TDS and A~ measurements. For gas exposures of < 8 . 0 x 10 Is molecules m -2, C12 adsorbs dissociatively with high sticking probability into an atomic overlayer. Continuous compression of an initial c(2 X2) phase occurs as coverage increases, until a p(2X5) structure is formed with a CI-CI nearest neighbour separation of 3.41 ,~. In this coverage regime a single adsorbate induced band appears at - 6 . 0 eV in UPS and the Cr(2p) and C1(2p) binding energies remain constant in XPS. Thermal desorption occurs as CrCI, and the TD spectra exhibit a single peak which corresponds to an activation energy of 300-340 kJ tool - t. After higher gas exposures, chemical shifts in XPS and the appearance of additional bands in UPS reveal that epitaxial growth of chromium dichloride occurs. This halide phase has a twinned layer structure; it desorbs a s C r C i 2 with an activation energy of ~ 240 kJ tool - i. The system now exhibits three-dimensional island growth, and the rate of corrosion of the chromium substrate is shown to be ~0.02 that of overlayer formation.
117 POTENTIALS AT SURFACES OF SPECTRA AND WORK FUNCTIONS
M.L. PERLMAN
*
and J.W. DAVENPORT
Departments of Physics and Chemistry, Brookhaven National Laboratory, Upton, New York 11973, USA Received 4 August 1981; accepted for publication 27 October 1981 Madelung potentials have been calculated for sites in and near polar and nonpolar, primitive and reconstructed surfaces of wurtzite ZnO. Surface versus bulk potential differences result in electron binding energy differences which should be observable in photoemission spectra even on nonpolar surfaces. In polar planes, the average site charges in the outermost ion layer are "naturally" reduced relative to charges in the crystal interior, and observable effects should be larger. Results for polar surfaces of singly bonded atoms are indicative of the Madelung potential effects expected for dense layers of chemisorbed species. These potentials are especially sensitive to details of charge transfer. If a crystal has uniform site charges except for the above-mentioned charge reduction on polar surfaces, there exist surface dipoles which would cause considerably greater variation in work function from one crystal face to another than that observed experimentally; this discrepancy is resolved if, in the outermost cation-anion layer, charges are further reduced to three-quarters to one-half the "natural" values.
Surface Science 115 (1982) 141-160 North-Holland Publishing Company A STRUCTURAL AND KINETIC STUDY OF CHLORINE CHEMISORPTION AND SURFACE CHLORIDE FORMATION
141
ON
Cr(lO0) J.S. F O O R D *
and R.M. LAMBERT
Department o/Physical Chemistry, University of Cambridge, Cambridge, CB2 I EP, UK Received 17 June 1981 ; accepted for publication 26 October 1981 Chlorine adsorption on ( I X I)-Cr(100) has been studied using AES, UPS, XPS, LEED, TDS and A~ measurements. For gas exposures of < 8 . 0 x 10 Is molecules m -2, C12 adsorbs dissociatively with high sticking probability into an atomic overlayer. Continuous compression of an initial c(2 X2) phase occurs as coverage increases, until a p(2X5) structure is formed with a CI-CI nearest neighbour separation of 3.41 ,~. In this coverage regime a single adsorbate induced band appears at - 6 . 0 eV in UPS and the Cr(2p) and C1(2p) binding energies remain constant in XPS. Thermal desorption occurs as CrCI, and the TD spectra exhibit a single peak which corresponds to an activation energy of 300-340 kJ tool - t. After higher gas exposures, chemical shifts in XPS and the appearance of additional bands in UPS reveal that epitaxial growth of chromium dichloride occurs. This halide phase has a twinned layer structure; it desorbs a s C r C i 2 with an activation energy of ~ 240 kJ tool - i. The system now exhibits three-dimensional island growth, and the rate of corrosion of the chromium substrate is shown to be ~0.02 that of overlayer formation.