Monolayer Of Oxygen Research Articles

Thermal oxidation of the hydrogenated diamond ( [formula omitted]) surface

Hydrogenated single-crystal diamond (1 0 0) surfaces were exposed to dry O 2 under UHV conditions at temperatures ranging from room temperature to 1000 °C. The surfaces were largely inert to O 2 at 850<T sub <950 ° C , but they reacted rapidly with O 2 at higher temperatures. Thermal oxidation with unactivated molecular O 2 occurred only after the surface hydrogen had desorbed, usually at about 950 °C but depended partially on the condition of the surface. Oxidation at T sub >950 ° C generated an overlayer of graphitic sp 2 carbon on the diamond surface, as confirmed by low energy electron diffraction, Auger electron spectroscopy and C-KLL lineshape analysis, electron loss spectroscopy and high resolution electron energy loss spectroscopy. Relatively little surface oxygen (or hydrogen) remained after oxidation, probably due to thermal desorption. By comparison, the surfaces of unheated diamonds exposed to oxygen activated by a hot metal filament (O 2 ∗ ) were terminated with a full monolayer of oxygen in the form of ether or carbonyl groups, and had only a small amount of sp 2 carbon. Deuterium- and hydrogen-terminated surfaces behaved identically towards oxygen addition.

CO2 Formation over a Catalytic Surface Containing Free Oxygen in the Subsurface Monolayers

In this work, we developed Monte Carlo simulations to understand the transient experiments on CO 2 formation at the oxygen-rich ruthenium surface observed in recent experiments [A. Bottcher et al., J. Phys. Chem. B 193, 6267 (1999)]. We modeled this very complex system by a semi-infinite cubic lattice, the catalyst having been represented by the (001) free surface, and the typical profile shapes obtained by employing the TRIM (Transport and Range of Ions in Matter). When the surface is exposed to a constant flux of CO molecules we observe the formation of CO 2 due to the Langmuir-Hinshelwood process. The number of active sites over the surface is taken proportional to the number of monolayers of oxygen deposited due to the sputtering mechanism. Increasing the temperature, the active sites on the free surface become more mobile, and the subsurface oxygen atoms migrate to surface through diffusion. We show that desorption of CO molecules from the surface is very important at higher temperatures. Even taking high values for the diffusion of active sites and subsurface oxygen, a small desorption rate of CO leads the surface to become quickly poisoned by these molecules, therefore blocking the diffusion of oxygen atoms towards the topmost layer.

Photo-stimulated desorption of rare gas atoms adsorbed on Si(100) surfaces modified with oxygen and deuterium

Photo-stimulated desorption of rare gas atoms (Xe and Kr) adsorbed on Si(100) surfaces has been investigated in the photon energy range of 1.16 eV to 6.43 eV. Rare gas atoms are photodesorbed from a clean surface and their velocity distribution is well represented by a Maxwellian with the average kinetic energy, 〈EK〉=0.07 eV. When the surface is modified by oxygen or deuterium, the desorption yield increases and the velocity distribution changes dramatically. Upon oxidation with one monolayer oxygen the velocity distribution shows two nonthermal components with 〈EK〉=0.85 and 0.25 eV, while only one component with 〈EK〉=0.17 eV for the mono-deuterated surface. The velocity component with 〈EK〉=0.85 eV has a threshold at hν∼3.5 eV. The origin of the component is understood by assuming the excitation from the ground state of a rare gas adsorbate to an excited state where electron transfer occurs from the adsorbate to the substrate. On the other hand, the other nonthermal desorption components do not show such remarkable wavelength dependence; they appear in the entire range of photon energies used in this study and do not show any significant changes in their velocity distributions. The origins of these components are discussed in terms of substrate-mediated excitation.

Initial Oxidation Kinetics of Copper (110) Thin Films As Investigated By IN SITU UHV-TEM

In the study of metal oxidation, there is a wide gap between information provided by surface science methods and that provided by bulk oxidation studies. The former have mostly examined the adsorption of up to ∽1 monolayer (ML) of oxygen on the metal surface, where as both low and high temperature bulk oxidation studies have mainly focused on the growth of an oxide layer at the later stages of oxidation. Hence, we are visualizing the initial oxidation stages of a model metal system by in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM), where the surfaces are atomically clean, in order to gain new understanding of these ambiguous stages of oxidation. We have previously studied the growth of Cu2O islands during initial oxidation of Cu(100) film. We are presently investigating the initial stages of Cu(110) oxidation, from 10−4 Torr O2 to atmospheric pressures and temperature range from room temperature to 700 °C.

Influence of Surface Properties on the Quantum Photoyield of Diamond Photocathodes

The quantum efficiency and chemical stability of CVD diamond photocathodes has been examined. As-grown or microwave plasma hydrogenated boron-doped diamond films display a quantum photoyield of approximately 0.05% at 190 nm, which degrades gradually as the material is left in ambient atmosphere, due to slow oxidation. Rapid degradation in performance occurs when exposed to atomic or electronically excited oxygen. X-ray photoelectron spectroscopy shows that the yield drops close to zero at around monolayer oxygen coverage, and that the main oxygen species on the surface is hydroxyl or isolated ether linkages.

Structure of MBE grown semiconductor–atomic superlattices

A new type of superlattice, semiconductor–atomic superlattice, consisting of multiple periods of semiconductor layers separated by adsorbed atomic/molecular monolayers, requires that the growth remains epitaxial. Specifically, the Si/O superlattices have been fabricated with epitaxially grown thin silicon layers (1–2 nm thick) separated by periodically introduced monolayers of oxygen. High-resolution TEM shows defect density at the Si/O interface is below 10 9/cm 2. Electroluminescence diodes show a peak in the yellow–green of the visible spectrum, and have been life-tested for over a year without signs of degradation. Measured effective barrier height is between 0.5 and 1 eV. Models of the structures, showing how epitaxy can be continued with a monolayer of oxygen, are presented. Preliminary calculation shows that the strain is relatively low. The door is opened for device applications in silicon optoelectronics.

Diffusion-limited growth of single- and double-octahedral voids in silicon and the effect of surface oxygen monolayer

The density of grown-in voids produced in silicon is controlled by a diffusion-limited flux of vacancies to the voids. This flux was computed for both single- and double-octahedral voids. It was found that if a void is initially formed as a double octahedron it would evolve into stable shape which is different from that which is observed experimentally. It is concluded that voids are initially of single-octahedral shape, but can become double-octahedral at a later stage of evolution. This morphological change is most likely caused by an oxygen monolayer at the void facets. Initially this monolayer covers only a fraction of the void surface but gradually spreads over the whole surface. The remaining spot of non-covered surface is the source of growth steps produced by 2D nucleation. When the non-covered spot becomes small, the steps may pile-up at the boundary between the non-covered and covered parts of a facet, thus giving rise to a new void. The criterion for the single-to-double transition includes oxygen concentration, cooling rate and vacancy concentration.

In Situ X-Ray Reflectivity and Voltammetry Study of Ru(0001) Surface Oxidation in Electrolyte Solutions

The electrochemical surface oxidation of Ru(0001) in acid solutions is limited to a one-electron process resulting in one monolayer oxygen uptake at potentials below the onset of bulk oxidation at ...

Transient oxidation of CO over a catalyst in the presence of subsurface oxygen

We consider a lattice model to study the transient oxidation of CO molecules over a catalytic surface. A precovered surface containing a variable number of monolayers of oxygen is taken at the initial time. When a flux of CO molecules impinges the surface, we observe the formation of CO 2, due to the Langmuir–Hinshelwood mechanism. The flux of CO 2 molecules depends on the number of monolayers of oxygen and on temperature. The temperature enters through the desorption of CO molecules, diffusion of active sites over surface and diffusion of subsurface O atoms towards the topmost layer. We employ Monte Carlo simulations in our calculations, and the (0 0 1) surface and subsurface planes of a simple cubic lattice are considered. Our results explain the main features of the reactive scattering of CO molecules at the oxygen-rich Ruthenium surface observed in the experiments performed by Böttcher and coworkers.

Nb interaction with hydrogen plasma

A niobium membrane was immersed in hydrogen plasma and could be electrically biased to vary the energy of bombarding ions in the range of 1–200 eV. The fluxes of plasma driven absorption and permeation were almost entirely governed by incident suprathermal neutrals (mostly, thermal atoms), whose energy does not depend on membrane bias, but the ions of controllable energy do affect the neutral-induced permeation through modifying the membrane surface. At the zero bias a high temperature-independent plasma driven permeation (superpermeation) was observed alongside of an enhanced absorption. Bombardment by ions of an energy higher than 50 eV resulted in a sharp decrease of the plasma driven permeation/retention and in an acceleration of boundary processes of absorption/reemission of thermal molecules. At ion energies below 50 eV, the effect of ion bombardment on the plasma driven permeation and the kinetic coefficients of boundary processes were nonmonotonic in ion energy, having a maximum at ∼10 eV. Both an in situ doping with O of the bulk of Nb and a membrane temperature increase reduced the effects of ion bombardment to their complete disappearance. Responsible for that was the replenishment by means of surface segregation of an oxygen monolayer sputtered by ion bombardment.

Reaction between CO and a pre-adsorbed oxygen layer on supported palladium clusters

The transient kinetics of reaction between carbon monoxide and an oxygen monolayer pre-adsorbed on palladium clusters supported on MgO(100), has been studied for various cluster sizes (4–15 nm), in the temperature range 120–400°C, using molecular beam and mass spectrometry under ultrahigh vacuum. When the CO beam is opened, the CO 2 production rate first increases instantaneously, and then increases slowly to its maximum, before decreasing to zero due to the lack of oxygen. The period of slow increase of the reaction rate, namely the induction period, appears at about 200°C and becomes longer when temperature increases. Although it is not observed on Pd(111), this peculiar reaction kinetics does not depend on cluster size, and is attributed to a precursor state of CO chemisorption. The oxygen coverage at saturation is found equal to 0.4. At this high oxygen coverage, gaseous CO physisorbs above the oxygen adlayer. At high temperature, the precursor is more likely to desorb, which reduces its chemisorption probability, and thus the CO 2 production rate. The temperature-dependant kinetics of CO adsorption and reaction with oxygen has been simulated thanks to a simple kinetic model accounting for the precursor mechanism.

Yield resonances in electron-stimulated desorption of europium from tungsten

The electron-stimulated desorption of europium atoms from layers adsorbed on the surface of tungsten covered with a monolayer of oxygen was observed for the first time. The yield of Eu atoms as function of the energy of bombarding electrons exhibits a pronounced resonance character. The resonances correspond to ionization energies of the core electron levels of Eu and W atoms.

The role of O and Cl adsorbates on the secondary emission properties of tungsten

Absolute probabilities for generating secondary electrons and negative ions have been measured as a function of adsorbate coverage and impact energy for 50–500 eV Na + incident upon a tungsten surface. Adsorbate coverage ranges from none up to a monolayer or more of oxygen or chlorine. Kinetic energy distributions of the emitted anions and secondary electrons have also been measured, and the identity of the anions has been determined. The presence of an adsorbate is observed to significantly enhance secondary emission of electrons and anions for both adsorbates. The dependence of the emission probability on Na + impact energy differs markedly for the two adsorbates. The results are discussed in terms of a model in which a molecular anion residing on the surface is collisionally excited, its subsequent decay giving rise to both negative ion and electron emission into the vacuum.

Periodic density-functional study on oxidation of diamond (100) surfaces

The chemical reactions of the diamond surfaces with oxygen play important roles in the chemical-vapor deposition process, etching, and wear of the surface. In the present study, periodic density-functional calculations have been performed to clarify the oxidation mechanisms of the hydrogenated diamond (100) surfaces. The oxidation processes have been simulated in terms of the reaction heats. The ether, hydroxyl, and ketone structures are found to be stable on the diamond (100) surfaces. At the initial stage of the oxidation, the ether structures are priory formed at monohydride dimer bonds on the diamond (100) surfaces. The insertions of oxygen atoms into the lower layers are difficult to occur. As the coverage of oxygen atoms on the diamond surface is increased, the formation of ketone structures becomes easier. The stable structure of the oxygen monolayer sensitively depends on the lattice parameters. As the cell parameters are decreased, the bridge ether becomes more stable and the on-top ketone becomes more unstable.

Electron-stimulated desorption of lithium atoms from oxidized molybdenum surface

The yield and energy distributions of lithium atoms upon electron-stimulated desorption from lithium layers adsorbed on the molybdenum surface coated with an oxygen monolayer have been measured as functions of the impact electron energy and lithium coverage. The measurements are performed using the time-of-flight technique and a surface ionization detector. The threshold of the electron-stimulated desorption of lithium atoms is equal to 25 eV, which is close to the ionization energy of the O 2s level. Above a threshold of 25 eV, the yield of lithium atoms linearly increases with an increase in the lithium coverage. In the coverage range from 0 to 0.45, an additional threshold is observed at an energy of 55 eV. This threshold can be associated with the ionization energy of the Li 1s level. At the electron energies above a threshold of 55 eV, as the coverage increases, the yield of lithium atoms passes through a maximum at a coverage of about 0.1. Additional thresholds for the electron-stimulated desorption of the lithium atoms are observed at electron energies of 40 and 70 eV for the coverages larger than 0.6 and 0.75, respectively. These thresholds correlate with the ionization energies of the Mo 4s and Mo 4p levels. Relatively broad peaks in the range of these thresholds indicate the resonance excitation of the bond and can be explained by the excitation of electrons toward the band of free states above the Fermi level. The mean kinetic energy of the lithium atoms is equal to several tenths of an electronvolt. At electron energies less than 55 eV, the energy distributions of lithium atoms involve one peak with a maximum at about 0.18 eV. For the lithium coverages less than 0.45 and electron energies higher than 55 eV, the second peak with a maximum at 0.25 eV appears in the energy distributions of the lithium atoms. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model, in which the adsorbed lithium ions are neutralized after filling holes inside inner shells of the substrate and lithium atoms.

A Study of the Kinetics and Mechanism of the Adsorption and Anaerobic Partial Oxidation of n-Butane over a Vanadyl Pyrophosphate Catalyst

The interaction of n-butane with a ((VO)2P2O7) catalyst has been investigated by temperature-programmed desorption and anaerobic temperature-programmed reaction. n-Butane has been shown to adsorb on the (VO)2P2O7 to as a butyl–hydroxyl pair. When adsorption is carried out at 223 K, upon temperature programming some of the butyl–hydroxyl species recombine resulting in butane desorption at 260 K. However, when adsorption is carried out at 423 K, the hydroxyl species of the butyl–hydroxyl pair migrate away from the butyl species during the adsorption, forming water which is detected in the gas phase. Butane therefore is not observed to desorb at 260 K after we lowered the temperature to 223 K under the butane/helium from the adsorption temperature of 423 K prior to temperature programming from that temperature to 1100 K under a helium stream. Anaerobic temperature-programmed oxidation of n-butane produces butene and butadiene at a peak maximum temperature of 1000 K; this is exactly the temperature at which, upon temperature programming, oxygen evolves from the lattice and desorbs as O2. This, and the fact that the amount of oxygen desorbing from the (VO)2P2O7 at ∼1000 K is the same as that required for the oxidation of the n-butane to butene and butadiene, strongly suggests (i) that lattice oxygen as it emerges at the surface is the selective oxidant and (ii) that its appearance at the surface is the rate-determining step in the selective oxidation of n-butane. The surface of the (VO)2P2O7 catalyst on which this selective oxidation takes place has had approximately two monolayers of oxygen removed from it by unselective oxidation of the n-butane to CO, CO2, and H2O between 550 and 950 K and has had approximately one monolayer of carbon deposited on it at ∼1000 K. It is apparent, therefore, that the original crystallography of the (VO)2P2O7 catalyst will not exist during this selective oxidation and that theories that relate selectivity in partial oxidation to the (100) face of the (VO)2P2O7 catalyst cannot apply in this case.

Applications of in-line oxygen monitoring to a rapid thermal processing tool: diagnosing gas flow dynamics and silicidation processes

Two types of application of real-time oxygen monitoring on a production-type rapid thermal processor (RTP) are reported. Diagnosing of gas flow dynamics in an RTP chamber is described by using a tracer technique. Deviation from plug flow, which decreases purge efficiency, was observed as a function of gas flow rate. Purge design is performed on the basis of the gas flow dynamics, and the efficiency of several purge patterns was comparatively studied. Monitoring of a typical two-step RTP process for silicidation of Ti or Co was then demonstrated. Both adsorption and desorption were found to take place during annealing. Oxygen desorbs first at lower temperature (during temperature ramp) and then starts to adsorb at higher temperature. A few monolayers of oxygen is adsorbed to Ti during annealing, while less oxygen is adsorbed during Co RTP.

Oxygen adsorbed on oxidized Ru(0001)

An atomic oxygen species adsorbed on Ru(0001) containing subsurface oxygen has been found and is characterized by the means of thermal description spectroscopy (TDS), ultraviolet photoemission spectroscopy, and reactive CO scattering. The surplus oxygen can be adsorbed on an oxygen-rich ruthenium crystal at room temperature when more than two monolayers of oxygen are deposited in the subsurface region $(1 \mathrm{ML}=1.58\ifmmode\times\else\texttimes\fi{}{10}^{15} {\mathrm{cm}}^{\ensuremath{-}2}).$ The maximum number of oxygen atoms occupying this state depends on the oxygen content in the subsurface region. It saturates for oxygen contents higher than about 10 ML at a level of 0.25 ML. The initial binding energy of an adsorbed oxygen atom, as derived from TDS data, is lower than 0.75 eV per atom. It continuously decreases with increasing lateral adatom density. The work function $\ensuremath{\varphi}$ varies with the increasing population of this oxygen state. For saturating oxygen coverage it reaches a maximum that can be by 1.3 eV higher than the value known for a clean metallic Ru surface. The CO oxidation was performed in a beam scattering experiment and it reveals a high reactivity of this weakly bound state. At room temperature the $\mathrm{CO}/{\mathrm{CO}}_{2}$-conversion probability reaches a value of 0.18, which is nearly by one order of magnitude higher than the one found for subsurface oxygen at much higher temperatures.

Oxygen surface studies in ultra-thin diamond using a resonance reaction and transmission channelled Rutherford forward scattering

Polished low-index diamond planes are often taken to be hydrogen terminated. Previous work has shown a partial monolayer of oxygen on the surface. We have made use of the 16O(α,α)16O resonance at 3.045 MeV to study the oxygen on the diamond surface with increased sensitivity. With a method for producing ultra-thin diamonds, new research in diamond is possible. Ultra-thin diamonds were made by damaging thick diamonds with MeV carbon ions, annealing the damage out of the surface region leaving a graphitised (Bragg peak) layer below, and electrochemical etching with de-ionised water. This produced a few μm thick and a few mm diameter sized samples. Transmission channelling, with a 4 MeV α-microbeam was used to determine the position of the surface oxygen on a (100) oriented diamond.

Mechanism of the Initial Stage of the Oxidation of the Clean and Precovered with Nonmetals Iron Surface

The oxidation of the iron surface has been studied using the Auger electron spectroscopy method, particularly the intermediate state of the process proceedings between a monolayer of oxygen and an oxide layer. The oxidation of the clean iron surface has been compared to that of the iron surface precovered with carbon, sulfur, nitrogen, and phosphorus. The monolayer of carbon or nitrogen does not affect the rate of the oxidation process; however the monolayer of sulfur or phosphorus inhibits the oxidation. The mechanism of the oxidation stage between the monolayer of oxygen and the oxide layer has been elucidated taking into account the incorporation of oxygen atoms into the first iron layer. The concentration of defects in the bidimensional adsorbate layer has been taken into consideration. The number of surface defects in the 2D lattice of sulfur or phosphorus is 20 times lower than in the layer of oxygen.