Theodore E. Madey

Electron-Stimulated Desorption as a Tool for Studies of Chemisorption: A Review

Bombardment of solids by low-energy electrons (<500 eV) can cause various changes in the surface region, particularly if the surface contains an adsorbed layer. Electron bombardment can promote the desorption of energetic neutral and ionic fragments from the surface, can alter the bonding of surface species, and in some cases can cause decomposition of the surface region. These processes are termed electron-stimulated desorption (ESD) phenomena, and this review is primarily concerned with the use of ESD in studies of species adsorbed on surfaces. Topics covered include experimental methods, a theoretical discussion of physical mechanisms, and a detailed discussion of certain specific and well-studied adsorbate–substrate systems.

Interaction of water, oxygen, and hydrogen with TiO2(110) surfaces having different defect densities

We have studied the stoichiometric (annealed in oxygen), the slightly oxygen‐deficient (annealed in vacuum), and the highly defective (sputtered with Ar+) TiO2(110) surfaces and their reactivities to molecular oxygen, molecular water, and 50‐eV hydrogen ions using x‐ray photoelectron spectroscopy (XPS) and low‐energy ion scattering spectroscopy (LEIS). The use of isotopically labeled 18O enables us to distinguish adsorbed oxygen from lattice oxygen, and the concentration of surface oxygen vacancies is titrated by 18O2 adsorption. LEIS (1‐keV He+) is used to analyze the chemical composition of the outermost surface layer before and after 18O2 and H218O exposure. Water adsorbs on both stoichiometric and slightly O‐deficient surfaces [with oxygen vacancies ∼0 and 0.08 monolayer (ML), respectively] at room temperature. There is little or no dependence of saturation water coverage (lower limit of ∼0.07 ML for both surfaces) on the concentration of surface oxygen vacancies. On the highly defective surfaces, the...

Titanium and reduced titania overlayers on titanium dioxide(110)

Abstract The adsorption of titanium on titanium dioxide TiO 2 (110) has been studied by X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). The XPS data for Ti overlayers are interpreted using peak fitting based on experimental standard spectra. 4 A of Ti deposited at 150 K reacts with the substrate to produce ≈ 12 A of intermediate oxidation state Ti. Adsorption of neutral metal begins on top of this interface oxide film, but 20 A of deposited Ti are needed to cover the oxide completely. LEIS data indicate a tendency for clustering of Ti on top of the interface oxide. Ar + sputtering of stoichiometric TiO 2 leads to preferential loss of O from the near surface region. This reduction of the clean, annealed oxide surface by Ar + ion bombardment starts immediately and does not reach a steady state until 3 × 10 17 ions cm −2 , at which point the reduced overlayer is 17 A thick.

Thermal stability of Pt films on TiO2(110): evidence for encapsulation

We have studied the thermal stability of ultrathin platinum films on a rutile TiO2(110) surface, using low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). At room temperature, Pt grows in three-dimensional islands on the TiO2 surface, with little indication of an interface reaction. Upon annealing to temperatures above 450 K in UHV, encapsulation of the Pt islands by Ti suboxides is observed; the rate of this process increases with annealing temperature and decreases with island thickness. The Ti layer on top of the Pt islands is identified as a reduced Tin+ species (1 ⩽ n ⩽ 3) with the degree of reduction depending on the thickness of the Pt islands. These results are discussed in the framework of the strong metal-support interaction (SMSI) effect.

Desorption of alkali atoms and ions from oxide surfaces: Relevance to origins of Na and K in atmospheres of Mercury and the Moon

This paper begins with a brief survey of the literature dealing with the adsorption and desorption of alkalis on oxide surfaces. Emphasis is on desorption phenomena: thermal desorption, electron- and photon-stimulated desorption, and ion-induced desorption (sputtering). Then the relevance of these data to the desorption of alkalis from mineral surfaces and to the origins of alkali vapors in tenuous planetary atmospheres is discussed. The data presented for Na and K indicate that desorption processes initiated by thermal or electronic excitations do not depend strongly on whether the Na returns to the surface or diffuses up through the regolith, and that neutral yields dominate ion yields in all cases. Although the desorbed neutral energy distributions are not well approximated by Maxwell-Boltzmann distributions, the mean energies of the desorbed neutral Na and K are seen to be consistent with the temperatures extracted for the “hot” component of the lunar atmosphere. This suggests that the “hot” component may be produced by electronically stimulated desorption (e.g., electron-stimulated desorption and/or photon-stimulated desorption). If this is the case, a possible “size effect” may be operative, in which desorbed neutral K atoms are somewhat more energetic than desorbed Na. In such desorption processes a low-energy component may be generated by scattering of desorbing atoms in the porous regolith; thermal desorption can also generate low-energy atoms. The data further indicate that thermal desorption should be rapid in the equatorial regions of Mercury, possibly depleting this region of alkalis, whereas thermal desorption should be less efficient on the Moon. Surface charging may be important at the surface of the Moon, by accelerating the solar electrons to energies above the threshold for initiating alkali desorption. Suggestions are made for future laboratory work.

Ultrathin metal film growth on TiO2(110): an overview

Abstract The interface between an oxide substrate and a metal overlayer may crucially influence the macroscopic behavior of technological devices such as sensors, catalysts, ceramics, and semiconductor chips. Hence investigations of adsorption, nucleation and growth of ultrathin metal films on metal oxide surfaces have attracted increasing interest during recent years. Experimental results on the growth of metal overlayers on a model oxide, TiO 2 (110), are reviewed. The emphasis is on the very initial stages of overlayer growth and on extracting general trends on metal/metal-oxide interaction by comparing results from different metal overlayers. The electronic and geometric structure, growth mode, interface formation, and thermal stability of metal films are discussed. The strength of the oxidation/reduction reaction at the interface, the wetting ability and the tendency to form a disordered layer are all related to the reactivity of the overlayer metal towards oxygen. We propose that ‘reactive adsorption’ accounts for the observed trends.

TiO2 by XPS

The surfaces of titanium oxide belong to the most-studied oxide systems in the surface science literature. This is in part because TiO2 surfaces and interfaces play a major role in several technological applications, e.g., as promoters in catalysis, as photocatalysts, and as gas sensors. TiO2 is a reducible oxide, i.e., several phases with different stoichiometries exist. Because Ti is highly reactive towards oxygen, titanium oxides are formed readily when Ti is exposed to an atmosphere containing water or oxygen. The oxidation behavior of the metal is of interest for the properties of protective coatings. Although accurate knowledge of the XPS binding energies of different oxidation states is necessary for XPS investigations of titanium oxides, a recent review of the 16 literature data of the binding energy of Ti 2p3/2 from Ti4+ showed wide scatter of the reported values with a mean of 458.7 eV and a standard deviation of 1.3 eV [J. Mayer, E. Garfunkel, T. E. Madey, and U. Diebold, J. Electron Spectrosc....

Far-out surface science: radiation-induced surface processes in the solar system

Interplanetary space is a cosmic laboratory for surface scientists. Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids. Many of these bodies exist in ultrahigh vacuum environments, so that direct particle–surface collisions dominate the interactions. In this article, we discuss the origins of the very tenuous planetary atmospheres observed on a number of bodies, space weathering of the surface of asteroids and comets, and magnetospheric processing of the surfaces of Jupiter’s icy satellites. We emphasize non-thermal processes and the important relationships between surface composition and the gas phase species observed. We also discuss what laboratory and computational modeling should be done to support the current and future space missions––e.g. the Genesis mission to recover solar wind particles, the Cassini mission to probe Saturn, the Europa Lander mission to explore the subsurface ocean hypothesis, and the Pluto/Kuiper Express to sample the outer reaches of the solar system. 2001Elsevier Science B.V. All rights reserved.

STRUCTURE OF ULTRATHIN SIO2/SI(111) INTERFACES STUDIED BY PHOTOELECTRON SPECTROSCOPY

Device-grade ultrathin (9–22 A) films of silicon dioxide, prepared from crystalline silicon by remote-plasma oxidation, are studied by soft x-ray photoelectron spectroscopy (SXPS). The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxidation states of silicon between Si0 (the Si substrate) and Si4+ (the thin SiO2 film). The relative binding energy shifts for peaks Si1+ through Si4+ (with respect to Si0) are in agreement with earlier work. The relatively weaker signals found for the three intermediate states (I1, I2, and I3) are attributed to silicon atoms at the abrupt interface between the thin SiO2 film and substrate. Estimates of the interface state density from these interface signals agree with the values reported earlier of ∼2 monolayers (ML). The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and exposure to He/N2 plasma. We find that annealing produces more abrup...

Band offsets for ultrathin SiO2 and Si3N4 films on Si(111) and Si(100) from photoemission spectroscopy

High resolution soft x-ray photoelectron spectroscopy with synchrotron radiation is used to study the interfaces of SiO2/Si(111), SiO2/Si(100), Si(111)/Si3N4, and SiO2/Si3N4 for device-quality ultrathin gate oxides and nitrides. The thin oxides and nitrides were grown by remote plasma deposition at a temperature of 300 °C. Aftergrowth samples were further processed by rapid thermal annealing for 30 s at various temperatures from 700 to 950 °C. The Si(111)/Si3N4 samples were air exposed and formed a thin ∼6 A SiO2 layer with a Si(2p) core-level shift of 3.9 eV, thus allowing us to study both the Si(111)/Si3N4 and SiO2/Si3N4 interfaces with a single type of sample. We obtain band offsets of 4.54±0.06 eV for SiO2/Si(111) and 4.35±0.06 eV for SiO2/Si(100) with film thicknesses in the range 8–12 A. The Si(111)/Si3N4 nitrides show 1.78±0.09 eV valence-band offset for 15–21 A films. This value agrees using the additivity relationship with our independent photoemission measurements of the nitride–oxide valence-ba...