Jason S. Corneille

Synthesis and characterization of ultra-thin MgO films on Mo(100)

Abstract Ultra-thin MgO films have been synthesized under UHV conditions by evaporating Mg onto Mo(100) in various background pressure of oxygen. Low-energy electron diffraction (LEED) studies show that MgO films grow epitaxially in the 200–600 K substrate temperature range with the (100) face of MgO oriented parallel to Mo(100). The one-to-one stoichiometry of the MgO films has been confirmed using Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). A typical loss pattern, characteristic of single-crystal MgO, has been obtained by high-resolution electron energy-loss spectroscopy (HREELS). At low oxygen pressures, the MgO film grows via a mechanism of island nucleation with domains that coexist with metallic Mg particles. The heat of sublimation of three-dimensional MgO islands is dependent on the oxygen pressure during growth and relates to the coordination number of the Mg cations.

XPS characterization of ultra-thin MgO films on a Mo( 100) surface

Abstract The oxidation of ultra-thin Mg films supported on a Mo(100) surface has been studied using X-ray photoelectron spectroscopy (XPS) in the 90–1300 K sample temperature range. Upon adsorption of oxygen onto Mg thin films or deposition of Mg in the presence of oxygen, a Mg(2p) XPS feature at ~ 50.5–50.8 eV is observed. The binding energy of this peak is higher than that of metallic Mg(2p) (at 49.6 eV) and is assigned to oxidized magnesium. The associated O(1s) XPS spectra exhibit two peaks which can be attributed to a dioxygen species concluded to be magnesium peroxide and the lattice oxygen in MgO. Upon annealing the peroxide containing film to ~ 700 K, the magnesium peroxide is reduced to MgO through the loss of oxygen and metallic magnesium existing within the film is subsequently oxidized to MgO. Mg deposition in an oxygen background (~ 10 −6 Torr) onto the Mo(100) surface at 300 K produces essentially stoichiometric MgO films.

Model surface studies of metal oxides : adsorption of water and methanol on ultrathin MgO films on Mo(100)

Model surface studies of magnesium oxide have been carried out using surface sensitive techniques. Ultrathin MgO films have been synthesized under ultrahigh vacuum (UHV) conditions by thermally evaporating Mg onto Mo(100) in the presence of oxygen. Low‐energy electron diffraction (LEED) studies indicate that the MgO films grow epitaxially with the (100) face of MgO oriented parallel to Mo(100). The MgO films, prepared under optimum synthesis conditions, have essentially one‐to‐one stoichiometry, are nearly free from pointlike surface defects, and have properties essentially identical to those of bulk, single‐crystal MgO. Adsorption of water and methanol onto the MgO films has been studied using high‐resolution electron energy‐loss spectroscopy (HREELS) and temperature programmed desorption (TPD). In order to circumvent the difficulty associated with intense multiple surface optical phonon (Fuchs–Kliewer modes) losses, a new approach to acquisition of HREELS data has been demonstrated. This new approach en...

CO adsorption on ultrathin MgO films grown on a Mo(100) surface: an IRAS study

Abstract CO adsorption on MgO ultrathin films grown on a Mo(100) surface is studied using infrared reflection absorption spectroscopy (IRAS). CO adsorbed on 7 ML of MgO shows a stretch frequency at 2178 cm −1 which is blue-shifted relative to that of gas-phase CO (2143 cm −1 ). This blue-shift is believed to arise from electron charge donation from the CO 5σ orbitals to the MgO surface. The heat of adsorption of CO on 7 ML of MgO is estimated to be 9.9 kcal/mol using an isothermal adsorption method. CO adsorbed on the MgO thin films desorbs between 100 and 180 K, as indicated by temperature-programmed desorption.

X-ray photoelectron spectroscopic characterization of ultra-thin silicon oxide films on a Mo(100) surface

Ultra-thin films of silicon oxides supported on a Mo(100) surface have been studied using X-ray photoelectron spectroscopy (XPS). The films were synthesized by evaporating Si onto the Mo surface in oxygen ambient and were subsequently characterized using XPS with respect to the chemical states of silicon and the composition of the film. It has been found that the silicon oxide, prepared at room temperature with a silicon deposition rate of ∼ 1.2 A/min and an oxygen pressure of 2 × 10−5 Torr, consisted of predominantly silicon dioxide with a small fraction of suboxides. Annealing to ∼ 1300 K yielded a stoichiometric film of SiO2. The suboxides are believed to further react with oxygen forming SiO2 at an elevated temperature.

Preparation and characterization of ultra-thin iron oxide films on a Mo(100) surface

Abstract The synthesis and characterization of ultra-thin films of iron and iron oxides on a Mo(100) surface, have been carried out under ultrahigh vacuum conditions in the 100–1500 K substrate temperature range. The oxides were prepared by both post-oxidation of pure Fe ultra-thin films and by evaporating Fe onto the Mo surface in oxygen ambient (in situ oxidation). The characterization of the iron oxide films, with respect to the chemical states and composition, was made using X-ray photoelectron spectroscopy as well as temperature programmed desorption. By varying the molybdenum substrate temperature and the oxygen background pressure during either the post-or the in situ oxidation process, oxide films consisting of virtually pure phases of Fe 2 O 3 and FeO can be successfully obtained as well as intermediate phases including Fe 3 O 4 . In addition, discrete phase changes were found corresponding to the reduction of Fe 2 O 3 to Fe 3 O 4 at ∼ 550 K and Fe 3 O 4 to FeO at ∼ 750 K by means of thermally induced oxygen loss.

Preparation, characterization, and chemical properties of ultrathin MgO films on Mo(100)

Ultrathin MgO films have been synthesized under ultrahigh vacuum conditions by evaporating Mg onto Mo(100) in various background pressures of oxygen. Low‐energy electron diffraction and surface spectroscopic studies show that the MgO films, prepared under optimum oxidation conditions, grow epitaxially in the 200–600 K substrate temperature range and have an essentially one‐to‐one stoichiometry. The nature of the near‐surface defects of the MgO films grown at low oxygen pressures has been explored using electron energy‐loss spectroscopy. Finally, the chemical properties of the stoichiometric MgO films have been investigated using high‐resolution electron energy‐loss spectroscopy.

Molecular beam studies of ethanol oxidation on Pd(110)

The adsorption and decomposition of ethanol on Pd(110) has been studied by use of a molecular beam reactor and temperature programmed desorption. It is found that the major pathway for ethanol decomposition occurs via a surface ethoxy to a methyl group, carbon monoxide and hydrogen adatoms. The methyl groups can either produce methane (which they do with a high selectivity for adsorption below 250 K) or can further decompose (which they do with a high selectivity for adsorption above 350 K) resulting in surface carbon. If adsorption occurs above 250 K a high temperature (450 K) hydrogen peak is observed in TPD, resulting from the decomposition of stable hydrocarbon fragments. A competing pathway also exists which involves C-O bond scission of the ethoxy, probably caused by a critical ensemble of palladium atoms at steps, defects or due to a local surface reconstruction. The presence of oxygen does not significantly alter the decomposition pathway above 250 K except that water and, above 380 K, carbon dioxide are produced by reaction of the oxygen adatoms with hydrogen adatoms and adsorbed carbon monoxide respectively. Below 250 K, some ethanol can form acetate which decomposes around 400 K to produce carbon dioxide and hydrogen.

CO interaction with ultrathin MgO films on a Mo(100) surface studied by infrared reflection–absorption spectroscopy, temperature programmed desorption, and x‐ray photoelectron spectroscopy

The interaction of CO with MgO ultrathin films grown on a Mo(100) surface is studied using infrared reflection–absorption spectroscopy, temperature programmed thermal desorption (TPD), and x‐ray photoelectron spectroscopy. CO adsorbed on 7 ML of MgO shows an infrared band at 2178 cm−1. This blue‐shift of the CO stretching frequency relative to that of CO in the gas phase (2143 cm−1) is attributed to electron charge transfer from the CO 5σ orbital to the MgO surface. It is further shown that CO adsorption induces a 0.4 eV shift in the Mg(2p) and O(1s) core levels of the MgO thin films to lower binding energy, consistent with the charge transfer from CO molecules to the MgO thin films. The TPD spectra indicate that CO molecules on the MgO thin films desorb in the 100–180 K sample temperature range. The CO adsorption heat on 7 ML of MgO is deduced to be 9.9 kcal/mol using an isothermal adsorption method.

The adsorption and oxidation of methanol on thin palladium films

Abstract The adsorption of methanol (MeOH) on clean and oxygen covered Pd films, deposited on a multicrystalline Re substrate, has been studied using TPD, XPS and ISS. Significant differences in the chemistry are seen between thin (1 monolayer) and multilayer (> 10 monolayers) films, the thicker film behaving in a very similar way to the Pd {111} surface and showing a higher reactivity to MeOH decomposition, but also exhibiting higher desorption temperatures for the decomposition products when compared to the thin layer case. This behaviour is described in terms of charge transfer between the Re substrate and the Pd overlayer. The preadsorption of oxygen leads to significant structural differences in the Pd film as determined by XPS and ISS depending on the preparation temperature, and this in turn leads to significant differences in the reactivity of MeOH. When the oxygen-covered film is prepared at 300 K and exposed to MeOH, no formaldehyde is formed on desorption, whereas following a preparation temperature of 1000 K, significant amounts of formaldehyde are evolved.