T.J. Chuang

Interpretation of the x-ray photoemission spectra of cobalt oxides and cobalt oxide surfaces

Abstract CoO and Co3O4 have been studied by high-resolution X-ray photoemission. The characteristic binding energies in the Co 2p 3 2 , 2p 1 2 , and 3s regions, their band shapes and widths, the associated shake-up structure, the O(1s) and O(2s) BEs, and the valence band spectra have been examined. The two oxides are readily distinguished from their spectra though it is shown that the O(1s) BEs are identical at 529.50 ± 0.14 eV. Argon and oxygen ion sputtered surfaces were examined to establish the integrity of the oxides. A higher BE O(1s) component (530.7–531.6 eV), the intensity and BE of which vary with the treatments mentioned above, corresponds to non-stoichiometric surface oxygen. The results are discussed with respect to the electronic structures of the oxides and the often conflicting earlier studies of these oxides.

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of FexO (x ~ 0.90–0.95); Fe2O3 (α and γ); Fe3O4; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). FeII and FeIII states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below EF, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for FexO and Fe3O4. Fe surfaces (films, foils, (100) face) yield predominantly FeIII species when exposed to high exposures of oxygen or air, though there is evidence for some FeII also. At low exposures the FeII/FeIII ratio increases.

X- ray photoemission study of the interaction of oxygen and air with clean cobalt surfaces

Abstract The interaction of oxygen with polycrystalline cobalt surfaces has been studied at 300 K (1 × 10−6 to 1 × 10−5 Torr) using high-resolution (monochromatized) X-ray photoemission. At high exposures (> 100 L nominal) CoO is identified as the product from the nature of the Co 2 p 3 2 , 2 p 1 2 , 3 s , and valence band spectra. There is no evidence for measurable amounts of Co3O4 or Co2O3. Two O 1s features are observed at both high and low (10L) exposures. The dominant O 1s feature at 529.5 ± 0.2 eV corresponds to the oxide and a minor feature at 531.3 ± 0.2 eV is attributed to non-stoichiometric surface oxygen. Exposure to air produces quite different results, with a dominant O 1s feature at 531.5 ± 0.2 eV and dominant Co 2 p 3 2 and 2 p 1 2 features centered at 781.3 ± 0.2 eV and 797.1 ± 0.2 eV. These three values are very close to those reported here for bulk Co(OH)2. Ion etching of the air-exposed surface removes this dominant surface product rapidly revealing some oxide and finally metal.

Infrared photodesorption: Vibrational excitation and energy transfer processes on surfaces

Abstract Photon-stimulated desorption due to internal vibrational excitation of adsorbed molecules is a resonant phenomenon and has recently attracted considerable experimental and theoretical attention. n this paper, a brief review of prior studies is given. New results on NH3, ND3 and Xe in neatly adsorbed and co-adsorbed states excited by a tunable infrared laser and photodesorbed from Cu(100), NaCl and Ag films are presented to elucidate desorption mechanisms. Both single-photon and multiphoton excitation processes are examined and the velocity distributions of desorbed particles are analyzed. Various energy transfer and relaxation processes related to desorption are evaluated and a model for infrared photodesorption with particular emphasis on assessing the quantum and thermally-assisted effects is also discussed.

An X-ray photoelectron spectroscopy study of the chemical changes in oxide and hydroxide surfaces induced by Ar+ ion bombardment☆

In the course of X-ray photoemission studies on the oxidation of metals and alloys and on bulk oxides, we found that in addition to physical sputtering Ar+ ion bombardment can in many cases reduce an oxide to a mixture of the original oxide, lower oxides and metal. The effect is even more pronounced in systems containing hydroxyl groups which are readily destroyed by the ion beam. Specific examples for oxidized cobalt, nickel and iron surfaces and their bulk oxides and hydroxides are given. The relative reduction rates of CoII and FeIII in CoFe2O4 are also examined. From these observations, it is clear that any depth compositional profiling using ion sputtering in conjunction with Auger or X-ray photoelectron spectroscopy should be treated with extreme caution. The mechanism for the chemical changes induced by ion bombardment is briefly discussed.

Laser-photoetching characteristics of polymers with dopants

The photoetching behavior of poly(methylmethacrylate), poly(dimethylglutarimide) and chlorinated poly(methylstyrene) doped with pyrene and 4-aminobenzoylhydrazide excited by 308 nm excimer-laser pulses has been studied. Some common laser-etching characteristics including the reduction of the threshold fluence for ablation, the enhancement of etching efficiency and the existence of optimal conditions regarding the laser fluence and dopant concentration for generating clean and smooth etching patterns are identified. The photoetching mechanism and the potential application of the doping technique to material processing are discussed.

The interaction of chlorine with copper: II. Bulk diffusion

Abstract Auger depth profiling combined with X-ray photoemission and Rutherford backscattering has been used to study diffusion processes in Cu films exposed to C1 2 at 25°C under a large range of exposure conditions. The diffusion depths and coefficients as a function of the gas pressure and the exposure time are determined and analyzed. It is found that the pressure and the time dependences of the diffusion properties are rather complex and there is a drastic change in the diffusion behavior in the pressure range between 10 −3 and 10 −2 Torr. Below 10 −3 Torr, the average Cl concentrations in the surface regions are small and the diffusion depths are shallow. Above 10 −2 Torr, the surface Cl concentrations are high and rather thick surface layers containing constant Cl concentrations extending several thousand A or more into the bulk of the metal are found. The diffusion processes clearly affect the surface chlorination reaction of copper. These and other effects including the depth profiling technique are discussed.

Laser-induced desorption and etching processes on chlorinated Cu and solid CuCl surfaces

Time-resolved mass spectrometry is used to study the desorbed species due to laser-induced etching of a solid CuCl and a chlorinated Cu surface. The observed desorption threshold, mass distribution and kinetic energies of the desorbed atoms and molecules at 355 and 532 nm radiation show that the laser-induced etching process is not simply thermal evaporation. It is suggested that competing nonthermal mechanisms due to electronic excitations may be very important in laser-induced desorption and etching. These processes are different for a solid CuCl and a chlorinated Cu surface. For laser-induced etching of Cu surfaces, chlorination of Cu is essential; however, formation of stoichiometric CuCl is not necessary. Excess Cu in the surface layer is responsible for the observed different etching behavior of a chlorinated Cu and a solid CuCl surface. The effect of laser radiation on these surfaces and possible etching mechanisms are discussed based on the experimental observations.

UV-laser-induced desorption of NO from Pt(111)

Abstract Ultraviolet-laser-induced desorption of NO molecules from a Pt(111) surface is investigated at 80 K by state-selective detection. We find two distinctive desorption channels are operative on an as-adsorbed surface and a surface annealed at 220 K. Rotational energy distributions are Boltzmann on as-adsorbed Pt(111) and non-Boltzmann on annealed Pt(111) with inverted population of spin-orbit states. Rotational, vibrational, and translational temperatures of NO from an as-adsorbed surface are increased with the incident photon energy. The effect of the adsorption state on desorption and the desorption mechanism are discussed.

Interactions of deuterium and hydrocarbon species with the diamond C(111) surface

Abstract AES, XPS, TDS and LEED have been used to study the adsorption, the desorption and the reaction behavior of D atoms and CHx (1 ⩽ x ⩽ 3) species on the diamond C(111) surface. It is found that a small amount of chemisorbed D atoms can induce the (2 × 1) to (1 × 1) phase transition and hold the sp3 structure up to nearly 1400 K. The structural transformation can also be monitored by the optical second-harmonic generation technique. Thermal desorption of deuterium occurs mainly around 1300 K. Adsorption of CHx can also induce the phase change but not as readily as the D atoms. Annealing of the adlayer at a high temperature results in the partial conversion to graphitic carbon. D atoms can react with both the adsorbed CHx and the surface graphite. The surface bonding characteristics and the relevance of present results to the diamond CVD process are briefly discussed.