Initial Oxidation Process Research Articles

First-principles calculations for initial oxidation processes of SiC surfaces: Effect of crystalline surface orientations

The initial oxidation processes on both the C-face and Si-face SiC surfaces are investigated by performing electronic-structure calculations within the density functional theory. We find the characteristic features of stable structures and adsorption–desorption behavior depending on the surface orientation of SiC. The desorption of C atoms as CO molecules on the C-face occasionally occurs in addition to the adsorption of O atoms even for the surfaces with low oxygen coverage, whereas the adsorption of O atoms preferentially occurs on the Si-face. These calculated results indicate that the incorporation of O atoms along with the desorption of C atoms is predominant even during the initial oxidation processes on the C-face of SiC. Moreover, on the C-face, three-coordinated O atoms can be formed at the topmost layer of the SiC surface, resulting in the formation of SiO2-like layers whose characteristics are different from those on the Si-face. These findings suggest that the differences in carbon-desorption behavior and resultant structural difference of SiO2 layers between the C-face and the Si-face are possible origins of the orientation dependence of oxidation rate observed in SiC oxidation.

シリコン高指数面の酸化過程

Our recent studies of oxidation processes on high-index silicon surfaces are reviewed. Interface trap densities, Dit, at the thermally oxidized Si surfaces were systematically investigated for (001), (111), (110), (120), (331) and (113) orientations. Structure model of the SiO2/Si interface on (113) substrate was proposed based on the analysis of the optical measurements. The initial oxidation processes on high-index silicon surfaces with (113), (120) and (331) orientations at high temperatures have been investigated by X-ray photoelectron spectroscopy, and the results were compared with the case of (001) orientation. It has been shown that monolayer oxidation on high-index silicon surfaces can be characterized through chemical shift in the core level of the Si 2p and O 1s states, and in addition through evaluation of oxide thickness, oxide composition and band bending.

X‐ray Absorption Spectroscopy Investigation of Lithium‐Rich, Cobalt‐Poor Layered‐Oxide Cathode Material with High Capacity

AbstractWe present an ex situ X‐ray absorption spectroscopy investigation into lithiation–delithiation of the lithium‐rich, but cobalt‐poor cathode material Li[Li0.2Ni0.16Mn0.56Co0.08]O2. The main focus of this work is to address the role of manganese and oxygen in the electrochemical redox process and, especially, to identify the phenomena occurring during the initial activation, that is, during the first delithiation–lithiation cycle. Although manganese is not taking part in the initial electrochemical oxidation process, the results indicate that complete Ni2+/Ni4+ and partial Co3+/Co4+ redox processes occur. The electrochemical performance of the material, considering the full and partial redox inactivity of Mn and Co, reveals the participation of oxygen in the overall electrochemical redox process. Finally, it is found, surprisingly, that a partial reduction of Co and Ni takes place at the end of the initial activation.

Energetic Studies on the B Effect on the Oxidation of γ-TiAl Alloys

The energetic study of B effects on the oxidation of γ-TiAl alloys are performed by using the first-principles method based on Density Functional Theory (DFT) in this paper. The surface and interface segregatation of B as well as of the surface adsorption of O are discussed. B is found to preferentially segregat to TiAl subsurface with respect to γ-TiAl bulk. The B segregation at surface decreases oxygen coverage in the initial oxidation process of γ –TiAl alloys, thereby it is beneficial to the decrease of the growth of γ–TiAl alloys oxide film. In the initial oxidation process, oxygen prefers to stay in the vicinity of surface Ti atoms, and B addition is beneficial for the decrease of the growth of A12O3and TiO2. After the formation of Al2O3oxide film, B is energetically favoured stayed at interstitial site of α-Al2O3(0001)/γ-TiAl (111) interface, and enhances the adhesion of this interface.

Initial oxidation of TiFe1−xMnx (x = 0–0.3) by low dose exposures to H2O and O2

The very initial room-temperature oxidation processes of the ternary pseudo-binary TiFe1−xMnx (x=0–0.3) intermetallics by trace amounts of H2O vapor and O2 were studied utilizing XPS and AES techniques. Different reactivities of the two gases were obtained, with a lower oxidation ability of H2O, relative to O2, as anticipated from thermodynamic considerations. The exposure to O2 results in a two stage oxidation of the Ti ingredient, which first converts into a divalent TiO (up to exposures of about 2L), then proceeds into a tetravalent TiO2 form. Unlike oxygen, water exposure produces only the divalent oxide through the whole exposure range studied (11L). The Mn component in these compounds is oxidized only by O2 and not by H2O. The Fe ingredient is not oxidized at all and remains in its metallic form up to exposures of 30L.

Atomic scale insights into the initial oxidation of Ru(0001) using atomic oxygen

With scanning tunneling microscopy (STM) we studied the initial oxidation of Ru(0001) using atomic oxygen (O′) as oxidizing agent at various sample temperatures and exposures. In contrast to the oxidation of Ru(0001) by molecular oxygen, exposure of atomic oxygen is remarkably corrosive even at temperatures as low as 300K. The initial oxidation process is accompanied by the formation of small three-dimensional Ru-oxide domains (clusters) primarily formed at single steps and the formation of larger clusters in the step bunching regions. With increasing O′ exposure both the corrosion at the step edges and the size of the produced clusters progress. The corrosion of the steps leads to the formation of a mobile Ru–O precursor species preceding the actual nucleation and growth of Ru–O clusters. From this study we conclude that the activation of molecular oxygen is the true bottleneck in the initial oxidation of Ru(0001) by O2 exposure. Applying a fixed O′ exposure in the oxidation of Ru(0001), the size of clusters at single steps depends critically on the preparation temperature: For lower temperatures the clusters are small, while the cluster size increases with increasing sample temperature up to 700K. At 700K the Ru-oxide cluster growth accelerated drastically. Anneal of Ru-oxide clusters to 700K for 30min in vacuum leads to thermal decomposition of the Ru-oxide clusters so that this process may modify also the oxidation process at 700K. Flat precursor domains of RuO2 are found to grow at 600K and 700K. At higher O′ exposures (80L of O′) small RuO2(110) domains are observed on a densely with clusters covered Ru(0001) surface. At a sample temperature of 800K corrosion of the Ru(0001) surface is substantial but almost no clusters are observed with STM.

The influence of oxidation on the tribological performance of diester lubricant

ABSTRACTThe oxidative degradation test of di‐2‐ethylhexyl sebacate (DEHS) was carried out by a sealed stainless steel reactor. Fourier transform infrared spectroscopy and gas chromatography–mass spectrometry (GC–MS) were utilised to characterise the chemical composition and molecular structure of the oxidation products. The results revealed that 2‐ethyl‐1‐hexanol and mono(2‐ethylhexyl)sebacate were major degradation products in the liquid phase. Tribological behaviour was measured using a four‐ball tester. Scanning electron microscopy was employed to investigate the morphologies of the worn surface. Tribological results showed that anti‐wear and friction‐reducing performances of DEHS were highly dependent on the chemical composition of the oxidation products generated during the test. The hydroperoxides formed during the initial oxidation process could deteriorate lubrication properties of DEHS. However, certain amount of highly polar degradation products (acids and alcohols) generated could significantly improve the friction‐reducing performance of DEHS by the formation of boundary adsorption films on the rubbing surfaces. Copyright © 2013 John Wiley & Sons, Ltd.

Oxidation of TiAl surface with hyperthermal oxygen molecular beams

We report results of our detailed studies on the initial oxidation process of TiAl with a 2eV hyperthermal oxygen molecular beam (HOMB) and thermal O2 in the backfilling. The oxidation processes are monitored by X-ray photoemission spectroscopy (XPS) measurements in conjunction with synchrotron radiation (SR). Oxidation of both Al and Ti occurs during the oxidation. The incident-energy and surface-temperature dependences of oxidation reveal that the precursor-mediated dissociative adsorption is the dominant initial step of oxidation in the thermal O2 backfilling. Thus, the efficiency of oxidation is higher for the thermal O2 backfilling than for the HOMB dose. The result is quite different from that on TiNi where the HOMB dose has the advantages in oxide layer growth at high O coverage. We succeeded in fabricating blue colored TiO2 and Al2O3 containing layers, combining HOMB and surface annealing.

Atomic Scale Insights into the Initial Oxidation of Ru(0001) Using Molecular Oxygen: A Scanning Tunneling Microscopy Study

The initial oxidation of Ru(0001) has been studied by scanning tunneling microscopy (STM) using molecular oxygen as the oxidizing agent. The initial oxidation process reveals a complex behavior in which three-dimensional RuO2 domains (clusters) are exclusively formed at multiple steps, and only few of these clusters are able to initiate the growth of flat RuO2 domains with its (110) orientation along the (0001) direction of the Ru substrate. The oxide formation requires a minimum temperature of 500 K and a minimum pressure of about 1 × 10–5 mbar of oxygen. Below 1 × 10–5 mbar, oxide growth is very slow, although even at pressure of 10–6 mbar oxidation in step bunching regions is occasionally observed. The modified nucleation and growth mode of Ru(0001) oxidation is corroborated by growing the oxide in two separate steps: Starting with oxygen exposure of 1.5 × 10–5 mbar of O2 for 960 s at a sample temperature of 680 K, RuO2 nuclei are preferably formed. Subsequently, the further growth of flat oxide domain...

Oxidation of Silicon Carbide by O2and H2O: A ReaxFF Reactive Molecular Dynamics Study, Part I

Simulations of the initial oxidation process of a SiC surface exposed to O2 and H2O molecules was studied with ReaxFF, an atomically detailed reactive molecular dynamics method that naturally models the breaking and forming of bonds. In this work, the ReaxFF forcefield was first expanded by training it with new quantum mechanics data of the binding energy, equation of state, and heat of formation of the SiC crystal, along with data from earlier studies that describes Si – Si, Si – O, and Si – H interactions. This expanded ReaxFF forcefield is capable of simultaneously describing both Si–C–O and Si–O–H bonding interactions. Using the forcefield, oxidation simulations were performed at various temperatures (in the range of 500 to 5000 K), and the trajectories were analyzed. The analyses showed that SiC gradually transforms into the oxides of silicon with simultaneous formation of a graphite-like layer. In presence of excess O2, the graphite-like layer was further oxidized to CO and CO2. We also analyzed the...

Adsorption of O2 on Ge(100): Atomic Geometry and Site-Specific Electronic Structure

Germanium is considered to be a potential semiconductor to replace silicon in future high-performance microelectronic devices. Yet, when compared to Si, very little is known about the surface chemistry of Ge surfaces. In this article, we report on the oxygen adsorption on Ge(100)-(2 × 1) surfaces and the related changes in the density of surface states. In particular, the adsorption geometry is examined both in reciprocal- and real-space using reflection high-energy electron diffraction and scanning tunneling microscopy, respectively. Our findings reveal that the insertion of oxygen atoms into the Ge backbonds is not favored under the investigated reaction conditions, i.e., at a moderate O2 exposure. The O2-exposed surface exhibits a local (1 × 1) surface reconstruction, which implies that dimer bonds are destroyed upon oxygen adsorption. Surface states related to Ge dangling surface bonds present on the clean Ge(100) surface are found to be passivated. In addition, a high density of expelled substrate atoms is observed on the surface, leading to a drastic increase in surface roughness. These Ge adatoms induce distinctly different electronic properties compared to the well-ordered, oxidized surface regions. Our findings significantly contribute to the understanding of semiconductor-oxide interfaces, in particular with respect to the initial oxidation processes and the atomic and electronic structure of oxide complexes.

水素終端Si(111)及びSi(110)表面の初期酸化過程

水素終端Si(111)及びSi(110)表面の初期酸化過程

Multistate Redox‐Active Metalated Triarylamines

Abstracttrans‐[(η5‐C5H5)Fe(η5‐C5H4‐η1‐C≡C)Ru(C≡C‐4‐C6H4NPh2)(dppe)2] [4; dppe = 1,2‐bis(diphenylphosphanyl)ethane] and trans,trans,trans‐[{(η5‐C5H5)Fe(η5‐C5H4‐η1‐C≡C)Ru(dppe)2(C≡C‐4‐C6H4)}3N] (7) have been synthesized from trans‐[Ru(C≡C‐4‐C6H4NPh2)Cl(dppe)2] (3) and trans,trans,trans‐[{(dppe)2ClRu(C≡C‐4‐C6H4)}3N] (6), respectively, and the identities of trans‐[Ru(C=CH‐4‐C6H4NPh2)Cl(dppe)2][PF6] (2, precursor to 3), 3, and 4 have been confirmed crystallographically. Chemical oxidation of 4 and 7 afforded the isolable mixed‐valence species 4[PF6] and 7[PF6]3. The CV of 4 reveals sequential loss of three electrons in fully reversible oxidation steps, whereas the CV of 7 shows five reversible redox waves; in contrast, oxidation of the precursor amines HC≡C‐4‐C6H4NPh2 and (HC≡C‐4‐C6H4)3N are irreversible processes. All oxidation processes afford reversible changes in the linear optical properties. Complementary time‐dependent density functional theory (TD‐DFT) studies suggest that the initial oxidation process for 4 and 7 is iron‐centered and is followed by one (for 4) or three (for 7) ruthenium‐centered oxidations. The final reversible oxidation is assigned by TD‐DFT as delocalized along the metalla‐ethynylarylamine moiety. The intense optical changes consequent on reversible oxidation, together with their charge‐transfer character, suggest that 4 and 7 have potential as nonlinear as well as linear optical multistate switches.

Anthraquinone‐Sensitized Photooxidation of 5‐Methylcytosine in DNA Leading to Piperidine‐Induced Efficient Strand Cleavage

One-electron photooxidations of 5-methyl-2'-deoxycytidine (d(m)C) and 5-trideuteriomethyl-2'-deoxycytidine ([D(3)]d(m)C) by sensitization with anthraquinone (AQ) derivatives were investigated. Photoirradiation of an aerated aqueous solution containing d(m)C and anthraquinone 2-sulfonate (AQS) afforded 5-formyl-2'-deoxycytidine (d(f)C) and 5-hydroxymethyl-2'-deoxycytidine (d(hm)C) in good yield through an initial one-electron oxidation process. The deuterium isotope effect on the AQS-sensitized photooxidation of d(m)C suggests that the rate-determining step in the photosensitized oxidation of d(m)C involves internal transfer of the C5-hydrogen atom of a d(m)C-tetroxide intermediate to produce d(f)C and d(hm)C. In the case of a 5-methylcytosine ((m)C)-containing duplex DNA with an AQ chromophore that is incorporated into the backbone of the DNA strand so as to be immobilized at a specific position, (m)C underwent efficient direct one-electron oxidation by the photoexcited AQ, which resulted in an exclusive DNA strand cleavage at the target (m)C site upon hot piperidine treatment. In accordance with the suppression of the strand cleavage at 5-trideuterio-methylcytosine observed in a similar AQ photosensitization, it is suggested that deprotonation at the C5-methyl group of an intermediate (m)C radical cation may occur as a key elementary reaction in the photooxidative strand cleavage at the (m)C site. Incorporation of an AQ sensitizer into the interior of a strand of the duplex enhanced the one-electron photooxidation of (m)C, presumably because of an increased intersystem crossing efficiency that may lead to efficient piperidine-induced strand cleavage at an (m)C site in a DNA duplex.

Synchrotron X-ray Photoemission Study of NiTi Surface Oxidation with Hyperthermal Oxygen Molecular Beam

We report results of our detailed studies on the initial oxidation process of NiTi with a 2 eV hyperthermal oxygen molecular beam (HOMB) and thermal O2 in the backfilling. The oxidation processes are monitored by X-ray photoemission spectroscopy measurements in conjunction with synchrotron radiation. In the oxide formation process at higher O coverage, HOMB has the advantage in the dissociation process of O2 molecule and can grow TiO2 layers with the underlying TiOx-rich and/or Ni-rich layers even at room temperature. Surface annealing enhances the diffusion atoms and as a result, the efficiency of Ti oxide formation. We succeeded in fabricating thick Ni-free TiO2 layer, possibly blue colored rutile TiO2, combining HOMB and surface annealing.

Oxidation of TiNi surface with hyperthermal oxygen molecular beams

We report results of our detailed studies on the initial oxidation process of TiNi with a 2eV hyperthermal oxygen molecular beam (HOMB) and thermal O2 in the backfilling. The oxidation processes are monitored by X-ray photoemission spectroscopy (XPS) measurements in conjunction with synchrotron radiation (SR). In the early stage of oxidation, the precursor mediated dissociative adsorption is the dominant reaction mechanism. In the oxide formation process at higher O coverage, HOMB has the advantage in the dissociation process of O2 molecule and can grow TiO2 layers with the underlying TiOx-rich and/or Ni-rich layers. We succeeded in fabricating thick Ni-free TiO2 layer, possibly blue colored rutile TiO2, combining HOMB and surface annealing.

Real-time Analysis of Initial Oxidation Process on Si(001) by Means of Surface Differential Reflectance Spectroscopy and Reflectance Difference Spectroscopy

We introduce quantitative investigation of initial oxidation process on Si(001)-(2×1) by means of linear optical spectroscopic methods, namely, surface differential reflectance spectroscopy and reflectance difference spectroscopy. Our recent results obtained with these real-time measurement techniques revealed that the transition between two different oxide growth modes, Langmuir-type adsorption and two-dimensional island growth, could be identified. The activation energies were estimated from Arrhenius plots of the oxidation periods for the growth of an oxide monolayer. Our results suggest that a finite activation energy exists for monolayer oxide formation on Si(001)-(2×1) at high temperatures.

水素終端シリコン (100) 表面の酸化初期過程

水素終端シリコン (100) 表面の酸化初期過程

Initial oxidation of Si(110) as studied by real-time synchrotron-radiation x-ray photomission spectroscopy

Initial oxidation processes of the Si(110) surface and the chemical bonding states of silicon atoms in the initial oxides have been investigated by using real-time synchrotron-radiation photoemission spectroscopy. Time evolutions of the Sin+ (n=1–4) components in the Si 2p spectrum indicates that the Si3+ component always overwhelms the Si4+ component during the oxidation up to one monolayer. This is in sharp contrast to the Si(001) surface where Si4+>Si3+ always holds. The dominance of the Si3+ component is related to presence of two types of bonds on the Si(110) surface and to their possible different reactivity against insertion of oxygen atoms.

Origin of unusual rapid oxidation process for ultrathin oxidation (<2 nm) of silicon

We propose a kinetic model to reveal the origin of unusual rapid oxidation behavior for the ultrathin (<2 nm) oxidation of silicon demonstrated by [Enta et al., Appl. Phys. Lett. 92, 012110 (2008)] by considering the space-charge drift of O ions and the intrinsic concentration gradient diffusion of residual O ions that dominate the initial rapid oxidation process following the slow oxidation state, respectively. Importantly, the present model well describes the whole experiment data with different temperatures and pressures and diffusion activation energies for two regimes attained by our model, which coincides with the reported experiment data and theory calculation.