Douglas G. Frank

Characterization of single-crystal electrode surfaces as a function of potential and pH by Auger spectroscopy and LEED: Pt (111) in aqueous CaCl2 and HCl solutions

Experiments were performed in which a well-characterized Pt (111) surface was immersed into aqueous CaCl2/HCl solutions at controlled potential, after which the surface was removed from solution, evacuated, and characterized by LEED, Auger spectroscopy and related techniques. Potential-dependent and pH-dependent adsorption of halogen was observed, predominantly as Cl atoms. Stable chemisorption of Cl occurred only when the pH was less than about 4 with the electrode potential more positive than about 0.5 V (vs. Ag/AgCl reference). An order adsorbed layer, Pt (111)(3×3)−Cl, containing θCl = 0.25, was formed at potentials at which Cl was the predominant adsorbed species. Chemisorbed oxides or hydroxides were formed at the more positive potentials. Retention of Ca2+ by the surface was potential-dependent, with a minimum at about 0.3 V, and never exceeded θCa = 0.08 (Ca2+ ions per surface atom). Water was retained by the surface following evacuation, to the extent of 10–20 water molecules per Ca2+ ion, depending upon the pH.

Oriented adsorption at well-defined electrode surfaces studied by Auger, leed, and eels spectroscopy

Abstract Quantitation by use of Auger spectroscopy and cyclic voltammetry of molecular layers adsorbed at Pt(111) and Pt(100) surfaces from aqueous electrolytes is examined in this work for the following compounds: hydroquinone (HQ); phenol (PL); catechol (CT); 3,4-dihydroxyphenylacetic acid (DOPAC); L-3,4-dihydroxyphenylalanine (DOPA); l -tyrosine (TYR); l -phenylalanine (PHE); nicotinic acid (NA); 2,5-dihydroxy-4-methyl-benzyl mercaptan (DMBM); thiophenol (TP); benzylmercaptan (BM); 3-thiophene carboxylic acid; and 2,5,2′,5′-tetrahydroxybiphenyl (THBP). Two independent methods of measurement of packing density based upon Auger spectroscopy, and one based upon cyclic voltammetry are employed and the results compared. Voltammetric oxidation/reduction of adsorbed layers formed from these compounds at Pt surfaces in aqueous electrolyte is found to be essentially the same whether carried out before or after lengthy evacuation. Therefore, the results of surface spectroscopy in UHV are directly applicable to the liquid—solid chemistry and electrochemistry of these adsorbed compounds. Packing densities measured by means of two Auger spectroscopic methods were in good agreement with each other and with the voltammetric measurements.

Adsorption of ferricycanide at Pt (111) as a function of electrode potential studied by Auger spectroscopy

Measurements by means of Auger spectroscopy of the composition of Pt (111) surfaces after immersion into aqueous potassium ferricyanide solution (pH = 4) as a function of electrode potential are reported. The ferrocyanide complex was adsorbed with retention of its usual stoichiometry and ionic charge while ferricyanide was partially displaced by adsorption of an iron-free species such as cyanide. Hydrogen predominated at negative extremes of potential, while iron oxides and chlorides were present at positive extremes. Cations such as H+, Cs+ and K+ were retained relatively strongly at Pt (111) in ferricyanide/ferrocyanide solutions. The LEED patterns contained only beams due to the Pt (111) surface; no fractional-index beams were observed. Evidently, the layer of iron complexes did not possess long-range order.

Direct Imaging of Epitaxial Layers by Auger Electrons

It has recently been demonstrated that the surface atomic structure of materials can be imaged by means of Auger electrons. Angular distribution Auger microscopy (ADAM) produces subatomic resolution images of atomic structure by measuring and displaying the complete angular distribution of Auger electrons emitted from atoms near the surface of a solid material or thin film. Auger angular distributions contain the “silhouettes” of surface atoms “backlit” by emission from atoms located deeper in the solid. The locations and shapes of these silhouettes directly reveal the relative positions of atoms near the surface. Consequently, ADAM is an exciting new technique for characterizing materials surfaces and epitaxially grown surfaces. Auger electrons have been employed for surface elemental analysis and depth- profiling for many years. The limited escape-depth of Auger electrons makes Auger spectroscopy inherently sensitive to the surface region. Depth profiling of materials, often performed in combination with ion etching, is widely used. ADAM offers the possibility of nondestructively obtaining profiles and structural information concerning buried interfaces and can also yield images of atomic structure.

Imaging surface atomic structure by means of Auger electrons

It has recently been demonstrated that the surface atomic structure of single crystals, monolayers, and thin films can be imaged by means of Auger electrons. Angular distribution Auger microscopy (ADAM) produces subatomic resolution images of atomic structure by measuring and displaying the complete angular distribution of low‐energy Auger electrons emitted from atoms near the surface of a substrate or thin film. Auger angular distributions contain the silhouettes of surface atoms backlit by emission from atoms located deeper in the solid, revealing the relative positions of atoms near the surface. High‐energy Auger electrons have a larger escape depth and are therefore less surface sensitive but contain crystallographic information, particularly the locations of channels in the crystal. Consequently, ADAM is a powerful new technique for the characterization of semiconductor surfaces and epitaxial thin films, and also for fundamental studies of electron physics provided that certain important experimental...

Direct imaging of thin film atomic structure by angular distribution Auger microscopy (ADAM)

Abstract Angular distributions of Ayger electrons emitted from a thin film of electrodeposited Ag on Pt(111) are measured and displayed in order to investigate the structure and growth of the film. Complete angular distributions of 355 eV Ag Auger electrons are presented for deposition of 1, 2, 3, 4 and 100 monolayer (ML) quantities of Ag. In the early stages of deposition the observed angular distribution charge significantly, revealing the symmetry and structure of the growing film. As deposition proceeds, the observed angular distribution converges to one essentially identical to that measured from Ag(111), except for a 180° rotation with respect to the Pt(111) substrate. Theoretical angular distributions based on atomic point emitters and spherical scatterers of Auger electrons predict many of the features in the observed distributions. These results illustrate the applicability of this methodology to the investigation of thin film atomic structure.

Measurement of complete Auger electron emission angular distributions from β‐SiC films on Si(100)

The structure of epitaxial β‐SiC thin films grown on Si(100) has been investigated by measuring Auger electron emission angular distributions over an essentially complete hemisphere of angles of emission above the film surface. The β‐SiC films were grown by rapid thermal chemical vapor deposition, in which the heated Si(100) surface was carbonized with propane. Auger emission angular distributions were measured for carbon at 268 eV, and for silicon at 86 and 1605 eV, allowing the film structure to be probed from the viewpoint of each element. The Auger measurements probe the film structure to a depth of several atomic layers. Each of the distributions displayed distinct, fourfold symmetric features, demonstrating the crystalline character of the β‐SiC films. Comparison of the measured angular distributions with geometric projections and simulations for the known β‐SiC structure indicates that the films consist of interspersed [100] crystalline domains (each domain having twofold symmetry), with 90° in‐pla...

Imaging monolayer structure by means of Auger electrons

We report the complete angular distribution of 507 and 518 eV Auger electrons emitted from a monolayer of iodine atoms on single‐crystal platinum, Pt(111) ((7)1/2×(7)1/2) R19.1°–I. This monolayer is of particular interest because: (i) it provides an opportunity to measure an angular distribution of relatively high kinetic energy Auger electrons emitted from a single monolayer of known structure; and (ii) the structure of the monolayer has been independently determined from scanning tunneling microscopy and low‐energy electron diffraction, permitting one to unambiguously relate features in the angular distribution to the structure. The experimental angular distribution contains intensity minima along trajectories corresponding to the I–I internuclear directions and intensity maxima along trajectories corresponding to gaps between neighboring iodine atoms. A simulation of the angular distribution in which iodine atoms are treated as point‐emitters and spherical blockers of Auger electrons predicts the featu...

Direct imaging of monolayer and surface atomic structure by angular distribution Auger microscopy

Presented here are complete angular distributions of Auger electrons emitted from Pt(111), Ag(111), Cu(100), and an adsorbed monolayer of iodine on Pt(111). These samples were chosen because there is agreement as to their surface structure, providing a foundation on which to compare the nature of the observed distributions to the structural properties of the surface. In particular, the dependence of the angular distribution on the Auger electron kinetic energy was investigated by obtaining data at more than one energy for each metal. In each case, angular distributions from crystalline samples contain intensity minima (‘‘silhouettes’’) along the internuclear directions for Auger kinetic energies <100 eV. Above 100 eV, the intensity minima are gradually replaced with maxima along directions parallel to the clearest channels through the crystal. Near grazing angles of detection, the angular distributions of 507 and 518 eV iodine Auger electrons emitted from a monolayer of iodine on Pt(111) contains intensit...

Surface Electrochemistry of Amino Acids: Voltammetry Assisted by Eels, Auger Spectroscopy and Leed

Recent studies by means of thin-layer electrochemistry of the chemisorption at polycrystalline Pt of hydroquinone, catechol and more than 50 related compounds have revealed that a layer of oriented molecules is formed in virtually all instances1–4. Variables affecting adsorbate orientation include: adsorbate molecular structure, adsorbate concentration, electrode potential, nature of the electrolyte anion, temperature, solvent and structure of the Pt surface. Adsorbate orientation strongly influences the course of electrocatalytic oxidation and reduction3. References 1 and 2 are recent reviews; Reference 4 includes subsequent work.