The surface reaction kinetics of Si thermal oxidation was investigated by a real-time monitoring method of Auger electron spectroscopy combined with reflection high-energy electron diffraction. From the time evolution of O KLL Auger electron intensity measured simultaneously with that of RHEED intensity, thermal oxidation on the Si(001) surface under an O2 pressure of 2×10-7 Torr was divided into three temperature regions: (1) Langmuir-type adsorption at T < 630oC, (2) two-dimensional (2D) SiO2 island growth at 630oC < T < 800oC, and (3) etching (active oxidation) at 800oC < T. In the temperature region of 2D SiO2 island growth, an oscillatory behavior of RHEED half-order spot intensity of (1/2, 0) and (0, 1/2) was observed, indicating layer-by-layer etching of the surface between SiO2 islands. The RHEED intensity oscillation was accompanied with an appearance of bulk diffraction spots in RHEED patterns, suggesting a development of protrusions under the SiO2 islands, however no bulk diffraction spots were observed in other two regions. On the basis of the correlation between SiO2 coverage and RHEED intensity of specular, half-order and bulk diffraction spots, the time evolution of surface morphology is discussed for Langmuir-type adsorption and 2D SiO2 island growth.
Etching of graphite and hydrogenated diamond C(100) $2\ifmmode\times\else\texttimes\fi{}1$ surfaces by irradiating atomic hydrogen, which is one of the key reactions to promote epitaxial diamond growth by chemical vapor deposition, has been investigated by ab initio pseudopotential calculations. We demonstrate the reaction pathways and determine the activation energies for breaking C-C bonds on the surfaces by irradiating hydrogen atoms. The activation energy for C-C bond breaking on graphite is found to be only one-half of that on the hydrogenated diamond surface. This indicates that graphite, which is a typical nondiamond phase unnecessarily generated on the diamond surface during epitaxial growth, can be selectively eliminated by atomic hydrogen, resulting in methane desorption. Our result supports the growth rate enhancement in diamond epitaxy observed in a recent experiment by gas-source molecular beam epitaxy under hydrogen beam irradiation.
Graphene gas-barrier performance holds great interest from both scientific and technological perspectives. Using in situ synchrotron X-ray photoelectron spectroscopy, we demonstrate that chemical vapor-deposited monolayer graphene loses its gas-barrier performance almost completely when oxygen molecules are imparted with sub-electronvolt kinetic energy but retains its gas-barrier performance when the molecules are not energized. The permeation process is nondestructive. Molecular dynamics-based simulation suggests kinetic energy-mediated chemical reactions catalyzed by common graphene defects as a responsible mechanism.
Abstract Synchrotron radiation photoelectron spectroscopy during the oxidation of a Si(100)2 × 1 surface at room temperature revealed the existence of molecularly adsorbed oxygen, which was considered to be absent. The O 1s spectrum of such oxidation was found to be similar to that of Si(111)7 × 7 surface oxidation. Also, molecular oxygen appeared after the initial surface oxides were formed, indicating that it was not a precursor for dissociation oxygen adsorption on a clean surface. Considering this finding, we have proposed presumable structural models for atomic configurations, where molecular oxygen resided on the oxidized silicon with two oxygen atoms at the backbonds.
We report valence‐band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. The degree of oxygen functionalization was controlled by annealing temperature, and an electronic structure evolution was monitored using real‐time ultraviolet photoelectron spectroscopy. We observed a drastic increase in the density of states around the Fermi level upon thermal annealing at ∼600 °C. The result indicates that while there is an apparent bandgap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of bandgap closure was correlated with the electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ∼500 °C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to an as‐synthesized counterpart.
The chemical structure of diamond-like carbon (DLC) films was analyzed by Raman spectroscopy. The samples were DLC films synthesized by photoemission-assisted Townsend discharge (PATD). Group theory of the fundamental molecular structures suggested that the Raman spectrum consists of five bands with specific vibration modes, and the lineshape was represented by a modified Voigt-type formula. Analysis of the areas and positions of the bands resulted in the chemical structure of the DLC films with the sp2 cluster model. The model comprises conjugated and conductive clusters of sp2 carbons (sp2 clusters) floating in a non-conjugated and dielectric matrix of sp2 carbon, sp3 carbon, and hydrogen. The sp2 clusters were rather aliphatic for DLC films formed in low concentration of methane. The clusters grew to become aromatic with increasing methane concentration. The number of defects or dangling bonds increased similarly but were terminated with hydrogen for the films formed in a high methane concentration. The essential structure of DLC is the result of the development of random conjugation represented by the sp2 cluster model. We consider that DLC is a carbonaceous material in which conjugation increases slowly with time during the deposition process and which exhibits dielectric characteristics.
