Hyperthermal Atomic Oxygen Research Articles

Structural comparisons of SiOx and Si∕SiOx formed by the exposure of silicon (100) to molecular oxygen and to hyperthermal atomic oxygen

The SiOx layer and Si∕SiOx interface formed on Si(100) substrates, held at 493K, by oxidation from a beam of hyperthermal (5eV) atomic oxygen and upon exposure to thermal molecular oxygen were characterized by a variety of microcharacterization and surface science techniques. The amorphous silica formed by atomic oxygen was nearly twice as thick, more ordered (similar to a quartz structure), and more homogeneous in composition, than the oxide formed by molecular oxygen. The Si∕SiO2 interface formed by atomic oxygen was atomically abrupt and no suboxides were detected near the interface or throughout the oxide. In contrast, the Si∕SiOx interface that developed from molecular oxygen exposure was atomically rough, and a large fraction of suboxides was found near the interface. The differences in the oxide films grown by exposure to atomic and molecular oxygen are discussed in the context of a thermionic emission model of silicon oxidation.

Hyperthermal Atomic Oxygen Interaction with MoS2 Lubricants Relevance to Space Environmental Effects in Low Earth Orbit?Atomic Oxygen-Induced Oxidation

Effect of 5 eV atomic oxygen beam exposure on the surface properties of sputter-deposited and single-crystal molybdenum disulfide (MoS2) were evaluated in the light of space environmental effects in low earth orbit. X-ray photoelectron spectra indicated that the loss of sulfur from the atomic oxygen exposed MoS2 surface was significant, especially for the sputter-deposited samples. This is due to the formation and gasification of the volatile product, SO. It was also identified that Mo atoms at the surface were oxidized to form MoO3. The amount of oxygen increased within a depth of 22 nm from the surface, whereas loss of sulfur was only observed within 3 nm. It was thus concluded that the chemical change of MoS2 due to atomic oxygen attack is limited to the surface layer of the MoS2-sputtered lubricant.

Chemical Alteration of Poly(tetrafluoroethylene) (TFE) Teflon Induced by Exposure to Hyperthermal Atomic Oxygen

In this study, the erosion of poly(tetrafluoroethylene) TFE Teflon by hyperthermal atomic oxygen (AO) has been examined using X-ray photoelectron spectroscopy (XPS). The initial F/C atom ratio of 1.66 decreases to 1.15 after a 2 h exposure to a flux of 2 × 1015 atoms/cm2 s AO with an average kinetic energy ∼5 eV. The F/C atom ratio is further reduced to a value of 0.90 after a 25 h exposure. The high-resolution XPS C 1s data indicate that new chemical states of carbon form as the F is removed, and that the relative amounts of these states depend on the F content of the near-surface region. The states are most likely due to C bonded only to one F atom and C bonded only to other C atoms. Exposures of the AO-damaged surface to research-grade O2 results in chemisorption of a very small amount of O (∼0.8 at. %); this indicates that large quantities of reactive sites are not formed during the chemical erosion by AO. Further exposure to AO removes this chemisorbed oxygen. After exposing the AO-exposed surface to air for 90 min, O2 is chemisorbed and the F/C atom ratio is reduced to 0.68. Another 46 h of AO exposure results in removal of this O and a further decrease in the F/C atom ratio to 0.58.

Structural Changes of Ag Single Crystals by Exposure to Hyper-thermal Atomic Oxygen

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Chemical alteration of poly(vinyl fluoride) Tedlar by hyperthermal atomic oxygen

In this study the erosion of poly(vinyl fluoride) Tedlar by hyperthermal atomic oxygen (AO) has been examined using X-ray photoelectron spectroscopy (XPS). Initially the Tedlar film had F/C and O/C atom ratios of 0.45 and 0.11, which decrease to 0.018 and 0.04, respectively, after a 2-h exposure to a flux of 2 × 10 15 atoms/cm 2 s AO with an average kinetic energy of 5 eV. This exposure essentially produced a graphitic or amorphous carbon-like layer with a carbon content greater than 90 at.%. Longer AO exposures do not alter the composition of this layer significantly. Exposure to O 2 or air nearly doubles the oxygen content in the near-surface region. This is due to dissociative oxygen adsorption at reactive sites formed at the polymer surface during AO exposure. Further exposure to AO removes this chemisorbed oxygen.

Erosion Study of Poly(ethylene tetrafluoroethylene) (Tefzel) by Hyperthermal Atomic Oxygen

In this study the erosion of poly(ethylene tetrafluoroethylene) (ETFE) (Tefzel) by hyperthermal atomic oxygen (AO) has been examined using X-ray photoelectron spectroscopy (XPS). Initially, the Tefzel film had F/C and O/C atom ratios of 0.74 and 0.04, which decrease to 0.17 and 0.01, respectively, after a 2 h exposure to a flux of 2 × 1015 atoms/(cm2 s) AO with an average kinetic energy of 5 eV. The F/C atom ratio is further reduced to 0.02 with longer AO exposures, essentially producing a graphitic or amorphous carbon-like layer with a carbon content greater than 90 at. %. Longer AO exposures do not alter the composition of this layer significantly. Exposure of the AO-damaged surface to O2 or air nearly doubles the oxygen content in the near-surface region. This is due to dissociative oxygen adsorption at reactive sites formed at the polymer surface during AO exposure. Further exposure to AO removes this chemisorbed oxygen. C−H bonds are important sites for attack during erosion by hyperthermal AO.

Hyperthermal chemistry in the gas phase and on surfaces: theoretical studies

We review recent theoretical studies aimed at understanding gas/surface and gas-phase collisions at hyperthermal energies. The review is restricted to interactions between neutral species, and particular attention is given to the interactions of hyperthermal ground-state atomic oxygen (O(3P)) with hydrocarbons. Quantum mechanical and molecular dynamics calculations are used to simulate collisions of O(3P) with gas-phase methane, ethane, and propane molecules and with condensed-phase alkanethiolate self-assembled monolayers. The results of such studies are examined in the light of atomic-oxygen degradation of polymeric materials in low Earth orbit (LEO).

Nucleation and Growth of Nanoscale to Microscale Cylindrical Pits in Highly-ordered Pyrolytic Graphite upon Hyperthermal Atomic Oxygen Exposure

The erosion of highly-ordered pyrolytic graphite (HOPG) caused by exposure to hyperthermal atomic 0(3P) atoms was explored. Profilometry has revealed that the overall erosion yield is linear with respect to increasing atomic oxygen exposure at a constant sample temperature of 373 K. Atomic force microscopy (AFM) was employed to image the eroded material that contains numerous nanoscale to microscale cylindrical etch pits. As there is also a linear relationship between the etch pit diameter and atomic oxygen fluence, it is proposed that the largest cylinders are nucleated at or near the topmost sheet of the original graphite material. There is a large distribution of depths for cylinders of a chosen diameter. This suggests that the chemical and physical nature of the nucleation event plays a key role in the final depth of the etch pit.

Vacuum Ultraviolet Radiation in Sources of Hyperthermal Atomic Oxygen. Photodestruction of Polytetrafluoroethylene (PTFE) and Teflon FEP for hdication of this Radiation

Most facilities for testing the stability of spacecraft materials under attack by fast atomic oxygen (AO) are based on the generation of hypersonic beams of AO in nozzle sources with the use of continuous electrical discharges or in laser pulse-induced breakdown of the gaseous oxygen. In both cases, the sources of fast AO are simultaneously quite intense sources of vacuum ultraviolet (VUV) radiation. In the present study the intensity of VUV radiation with wavelengths > 115 nm from fast AO source were directly measured in the Central Aerohydrodynamic Institute facility VAT- 103. The intensity of this VUV radiation at the nozzle exit was shown to be almost equal to the intensity of the commercial lamp XeR-2 emitting at 147.0 nm (at its window). This VUV radiation can take part in destruction and erosion of polymer materials simulating the oxydative effects of fast AO. Its contribution to observed mass losses is greatly dependent on the material under test. For polymers consisting of carbon, hydrogen, oxygen and nitrogen the main role in erosion is played by fast AO. The decisive role of VUV radiation is shown in the erosion of fully fluorinated polymers. The mass losses of Teflon and Teflon FEP films in facilities for generation of fast AO are suggested as a means of measuring the VUV radiation dose in these facilities. The main features of mass losses that depend on duration of VUV irradiation are formulated. They are based on the previously proposed model of mass losses from these polymers during VUV photodestruction. The key feature of this model is the limited rate of photofragment evaporation (sublimation) into vacuum.

In situ atomic oxygen erosion study of fluoropolymer films using X‐ray photoelectron spectroscopy

AbstractThe surfaces of a homologous series of fluoropolymers were characterized in situ using X‐ray photoelectron spectroscopy before and after a 15‐min exposure to the flux produced by a unique hyperthermal atomic oxygen (AO) source, which produces a flux of about of 1015 atoms cm−2 s−1. The linear polymers investigated in this study include high‐density polyethylene (HDPE), poly(vinyl fluoride) (PVF), poly(vinylidene fluoride) (PVdF), and poly(tetrafluoroethylene) (PTFE). They possess a similar base structure with increasing fluorine‐to‐carbon ratios of 0, 1 : 2, 1 : 1, and 2 : 1, respectively. No interaction of the AO with the nonfluorine‐containing linear polymer HDPE was detected over this short exposure. However, a correlation exists between the chemical composition of the fluorinated polymers and the induced chemical and structural alterations occurring in the near‐surface region as a result of exposure to AO. The data indicate that AO initially attacks the fluorine portion of the polymers, resulting in a substantial decrease in the near‐surface fluorine concentration. The near‐surface fluorine‐to‐carbon ratios of PVF, PVdF, and PTFE decreased during the 15‐min AO exposure by 68, 39, and 18.5%, respectively. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1977–1983, 2004

Increased Ordering in the Amorphous SiOx due to Hyperthermal Atomic Oxygen.

ABSTRACTWe had studied the effects of hyperthermal (5.1eV) atomic oxygen (AO) on the structural characteristics of the silica layer and Si/SiOx interface formed by the oxidation of Si-single crystal by a variety of microcharacterization techniques. A laser detonation source was used to produce atomic oxygen with 5.1eV kinetic energy. High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED) demonstrated that the silica layer formed on Si(100) by atomic oxygen is thicker, more homogeneous, and less amorphous, compared to the oxide layer created by molecular oxygen (MO). High spatial resolution Electron Energy Loss Spectroscopy (EELS) study confirmed that the Si/SiOx interface created by atomic oxygen is abrupt containing no suboxides as opposed to the broad interface with transitional states formed by molecular oxygen. SAED technique was used to observe sharper diffraction rings present in the diffraction pattern of Si(100) oxidized by reactive atomic oxygen as opposed to the diffused haloes present in the diffraction pattern of Si(100) oxidized by molecular oxygen. Radial Distribution Function (RDF) analyses were performed on the SAED patterns of Si(100) oxidized in atomic and molecular oxygen, indicating that a more ordered oxide is formed by atomic oxygen. Initial Fluctuation Electron Microscopy (FEM) results confirmed an increased medium range ordering in SiOx formed by atomic oxygen when compared to the non-regular arrangement present in the amorphous oxide formed by the oxidation of Si(100) in molecular oxygen.

Chemical alteration of poly(tetrafluoroethylene) Teflon induced by exposure to vacuum ultraviolet radiation and comparison with exposure to hyperthermal atomic oxygen

The chemical alteration of poly(tetrafluoroethylene) Teflon by vacuum ultraviolet radiation (VUV) (115–400 nm) has been examined with X-ray photoelectron spectroscopy (XPS). The initial F/C atom ratio of 1.98 decreases to 1.65 after a 2-h exposure. The F/C atom ratio is further reduced to a steady-state value of 1.60 after a 74-h exposure. The high-resolution XPS C1s data indicate that new chemical states of carbon form as F is removed and that the relative amounts of these states depend on the F content of the near-surface region. The states are most likely due to C bonded only to one F atom, C bonded only to other C atoms, and C that has lost a pair of electrons through the emission of F−. The exposure of the VUV-damaged surface to research-grade O2 results in the chemisorption of a very small amount of O, and this indicates that large quantities of reactive sites are not formed during the chemical erosion by VUV. Further exposure to VUV removes this chemisorbed oxygen. A comparison of the XPS data indicates that the mechanisms of chemical alteration by VUV radiation and hyperthermal (∼5 eV) atomic oxygen are different, as expected, because the excitation sources are quite different. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 552–561, 2005

Spatially resolved wettability control of polymer surfaces using laser-detonated hyperthermal atomic beams

The fundamental properties of hyperthermal atomic oxygen or atomic fluorine-exposed polyimide and polyethylene surfaces, based on the analytical results by X-ray photoelectron spectroscopy and contact angle measurements, are reported. It was observed that CF, CF2 or CF3 moieties were formed at the atomic-fluorine-exposed polymer surfaces depending on the fluence, whereas carbonyl and carboxyl groups were formed at the atomic oxygen-exposed polymer surfaces. Advancing contact angles of water can be controlled from 60 to 150 degrees by varying the atomic oxygen or atomic fluorine fluence. It was also demonstrated that a selected area on the target surface could be processed with this method using a fine metal mask. The behavior of a water droplet on polymer surfaces could be controlled by the spatially resolved surface modification using hyperthermal atomic beams.

Structural Characterization of Oxide layers on Aluminum Formed by Exposure to Hyperthermal Atomic Oxygen

ABSTRACTSingle crystal Al (100) was exposed to 5 eV atomic oxygen beam. The sample was maintained at a temperature of 220°C and the total atomic oxygen fluence was 8×1019 atom.cm-2. We have characterized the resulting oxide and interface structures by cross-sectional (scanning) transmission electron microscopy ((S)TEM) and scanning electron microscope(SEM). Our TEM results show that an amorphous aluminum oxide layer with ∼6 nm thickness formed on the aluminum crystal, and a rough alumina/Al(100) interface forms. For a systematic study of the evolution of the oxide, a unique Physical Sciences, Inc. Pitt FASTTM AO laser detonation atomic oxygen source in a UHV chamber is employed. The system is equipped with a Maxtek RQCMTM system, a research quartz crystal microbalance (QCM) with a dual-sensor head, to dynamically measure the mass change of an aluminum film coated on the sensor crystal during exposure to atomic oxygen. The Al film initially experiences mass loss, and then parabolic mass gain. To observe the structural evolution of the oxide, a very thin Al (100) single crystal was exposed inside the AO source, characterized by SEM and TEM. The surface morphology changed from flat to rough after 5.5 minutes of exposure. This surface roughening could be related to the initial mass loss measured by QCM.

Complex oxide structures formed by oxidation of Ag(100) and Ag(111) by hyperthermal atomic oxygen

Complex oxide structures formed by oxidation of Ag(100) and Ag(111) by hyperthermal atomic oxygen

Structural comparisons of passivated SI(100) by atomic and molecular oxygen

We compared the structural characteristics of a silica layer formed on Si(100) by oxidation in hyperthermal atomic oxygen and molecular oxygen at 493K. The laser detonation method was used to create primarily neutral atomic oxygen with kinetic energy of 5.1eV. The silicon oxides were characterized by High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). We determined that atomic oxygen forms amorphous silica that is almost twice as thick and nearly double the surface roughness as compared to molecular oxygen – formed silica at the same temperature and time conditions

Structural Characterization of Broad Oxide Layers of Ag Single Crystals Caused by Hyperthermal Atomic Oxygen by SEM and TEM

Structural Characterization of Broad Oxide Layers of Ag Single Crystals Caused by Hyperthermal Atomic Oxygen by SEM and TEM

Formation of Nanosized Metallic Ag Grains by Oxidation of Ag Single Crystals with Hyperthermal Atomic Oxygen

ABSTRACTSilver single crystals (Ag(100), Ag(111)) were exposed to 5eV hyperthermal atomic oxygen, created by a laser detonation of molecular oxygen at a substrate temperature of 220°C for 7 hours. Oxide scales of more than ten microns formed on both Ag (100) and Ag (111) substrates. The microstructural investigation of the oxide layers by high resolution transmission electron microscopy (HRTEM) revealed that the “oxide” scales are predominately composed of nanosized polycrystalline silver grains (5–100nm) as well as a small amount of nanosized silver oxides. The results were remarkably different than the O2 oxidation. The HRTEM investigation suggested that the grains of polycrystalline silver were first carved off from the substrate into “oxide” scale by lattice expansion and decohesion, which are driven by atomic oxygen diffusion in Ag lattice, occupation of oxygen atoms at the interstitial sites of Ag lattice, and partially internal oxidation. The grains in the scale were also subject to continuing oxidations with the atomic oxygen--secondary poly-crystallization, and changed to smaller grains. The preferred oxidation fronts in silver lattice is along the {111} planes.

Structural comparisons of passivated SI(100) by atomic and molecular oxygen

We compared the structural characteristics of a silica layer formed on Si(100) by oxidation in hyperthermal atomic oxygen and molecular oxygen at 493K. The laser detonation method was used to create primarily neutral atomic oxygen with kinetic energy of 5.1eV. The silicon oxides were characterized by High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). We determined that atomic oxygen forms amorphous silica that is almost twice as thick and nearly double the surface roughness as compared to molecular oxygen – formed silica at the same temperature and time conditions

Structural Comparisons of SiOx and Si/SiOx Formed by Passivation of Single-Crystal Silicon by Atomic and Molecular Oxygen

ABSTRACTThe structural characteristics of a silica layer and Si/SiO2 interface formed on Si single-crystal by oxidation in hyperthermal atomic oxygen (AO) and molecular oxygen (MO) at 493K were compared by wide variety of experimental techniques. The hyperthermal AO with kinetic energy of 5.1eV was created by the pulsed laser detonation of oxygen gas. The oxide formed by AO and MO on Si single crystal is amorphous as observed by HRTEM and selected area electron diffraction (SAED). However, the oxide formed by AO has a less random distribution of silicon and oxygen atoms as compared to the oxide formed by MO, as evidenced by the SAED patterns and EELS spectra. In contrast to MO formed silica, initial EELS results across the Si/SiO2 interface revealed no region of suboxides exists near the interface in the AO formed silica. The Si/SiO2 interface formed by AO species was found to be very abrupt and the oxide homogeneous, as opposed to the broad interface and non-homogeneous oxide created by MO, as determined by HRTEM and EELS.