J.-W. He

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.

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.

Overlayer growth and chemisorptve properties of ultra-thin Fe films on W(110) and W(100)

Abstract The metal overlayer growth of Fe on W(110) and W(100) surfaces and the desorption of CO and H2 from the prepared bimetallic surfaces have been studied by Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and low energy electron diffraction (LEED). TD spectra of Fe/W(110) and Fe/W(100) reveal two desorption peaks (β1 and β2). The multilayer desorption peaks (β1s) follow zero order kinetics, with activation energies of ∼ 86 and ∼ 87 kcal mol−1 for Fe/W(110) and Fe/W(100), respectively, values close to the sublimation energy of bulk Fe. AES results indicate that Fe grows layer-by-layer on both W(110) and W(100). Fe overlayers of 1 ML or less on W(110) and 2 ML or less on W(100) are stabe to a 1000 K anneal. Annealing Fe/W(110) with θFe > 1 ML and Fe/W(100) with θFe > 2 ML to T > 500 K causes nucleation of the deposits into 3D clusters. A 10 × 10 LEED pattern was observed on Fe/W(110) with θFe > 1 ML, after the surface was annealed to 723 K. Annealing the surface further to 950 K produces a pattern which is likely a superposition of the W(110) substrate and epitaxial Fe(110) islands. Fe appears to grow epitaxially on W(110) with the 〈110〉 orientation parallel to the 〈110〉 orientation of the W substrate. A complex, streaked pattern was observed for a 2 ML Fe covered W(100) following an anneal to 200 K. TD spectra of CO from 1 ML Fe covered W(110) and W(100) both show a shift of the molecular state to a lower temperature compared to CO from bulk Fe, implying a lower binding energy of CO on the ultra-thin Fe films. The desorption states from dissociated CO, however, are stabilized on 1 ML Fe/W(110) and Fe/W(100) compared to clean Fe. Hydrogen desorption also shows a low temperature shift for 1 ML Fe/W(110) compared to bulk Fe. Fe is found to block effectively the sites on W(110) for H2 adsorption. A desorption state at ∼ 520 K is observed for ∼ 4–6 ML Fe/W(100) annealed to 1100 K. This state is interpreted to arise from hydrogen adsorbed at W sites or at the Fe-W interface.

Adsorption of CO on Rh(100) studied by infrared reflection–absorption spectroscopy

The interaction of CO with a Rh(100) surface at 90 and 300 K has been investigated with infrared reflection–absorption spectroscopy (IRAS). Absorption bands due to the C–O stretch are found in both the linear and bridging regions at all coverages for both adsorption temperatures. For adsorption at 300 K, an ordered CO layer is formed at ∼0.4 monolayers (ML) as evidenced by a sharp, highly symmetrical, linear‐CO band. At θCO>0.45 ML, the reduction in intermolecular separation and increasing intermolecular repulsive forces in the CO adlayer are evidenced by the increased broadness of both the linear‐ and the bridged‐CO bands. CO adsorption at 90 K yields a nonuniform adlayer dominated by island formation at θCO<0.5 as indicated by peak splitting of the linear‐CO band and the presence of a broad bridged‐CO band. At θCO∼0.5, a highly ordered CO adlayer is suggested by the appearance of a highly symmetrical linear‐CO band as well as the formation of a sharp c(2×2) LEED pattern. The presence of multiple compone...

A surface spectroscopic study of metal-support interactions: model studies of copper on thin SiO2 films

The metal-support interactions for a model silica-supported copper catalyst have been studied using temperature-programmed desorption, X-ray photoelectron and infrared reflection-absorption spectroscopies under ultrahigh vacuum conditions. The model catalyst support was prepared by synthesizing a thin SiO2 film (∼ 100 A) on a Mo(110) surface. Copper was then evaporated onto the SiO2 films. A small amount of copper (< 0.1 monolayer) is partially oxidized at the CuSiO2 interface with the remainder forming 3D-clusters. The desorption energy of copper from the SiO2 is found to depend markedly on copper coverage (cluster size).

Surface chemistry of monolayer metallic films on Re(0001) and Mo(110)

The overlayer growth of Fe, Ni, and Cu on a Mo(110) surface and Fe and Cu on Re(0001) as well as adsorption of D2 and CO on the prepared bimetallic surfaces have been studied in the sample temperature (Ts ) range 115–1500 K, using Auger electron spectroscopy (AES), low‐energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). Fe, Ni, and Cu on Mo(110) and Fe and Cu on Re(0001) were found to grow layer by layer upon deposition at Ts =115 K whereas multilayer deposits, upon annealing to Ts >330 K, nucleate into three‐dimensional (3D) clusters. The 3D clusters of Fe and Cu are shown by LEED to be epitaxial with respect to the Mo(110) and Re(0001) substrates. Fe at submonolayer coverages is believed to form a strained pseudomorphic superlattice on Mo(110) while similar coverages of Ni on Mo(110) and of Cu on Re(0001) form a superstructure. The results of chemisorption of D2 and CO on the prepared bimetallic surfaces Fe/Mo(110), Ni/Mo(110), and Cu/Re(0001) are presented and discussed. It i...

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.