Xidong Chen

Fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films.

In this letter, we report fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films and the effect of annealing on this ordering. Annealed and unannealed diamond-like carbon films have almost identical short-range order. Our fluctuation microscopy results, however, indicate the presence of medium range order or clustering in the films on a lateral length scale that exceeds 1 nm. Within the clustered regions, the dominant local ordering appears to be diamond-like, and graphite-like ordering is not observed. Thermal annealing up to 600 °C leads to an increase in diamond-like clustering with no onset of graphite-like clustering. However, after high temperature annealing up to 1000 °C, graphite-like clustering becomes apparent as a result of the conversion of diamond-like carbon to graphite-like carbon. The results on the as-deposited films and films annealed up to 600 °C suggest that a spontaneous medium range ordering process occurs in diamond-like carbon films during an...

Oxidation-driven surface dynamics on NiAl(100).

Significance Oxidation of metals involves the reaction of oxygen with the metallic substrate, requiring sources or sinks of substrate atoms. This study provides direct evidence that atomic steps serve as the sources of Al surface atoms for aluminum oxide growth in the oxidation of NiAl(100), which results in coordinated migration and bunching of steps that progressively impedes the oxide growth. Comparison with the oxidation of NiAl(110) that exhibits unimpeded oxide film growth over substrate steps suggests that whenever steps are the source of atoms used for oxide growth they limit the oxidation process; when atoms are supplied from the bulk, the oxidation rate is not limited by the motion of surface steps. Atomic steps, a defect common to all crystal surfaces, can play an important role in many physical and chemical processes. However, attempts to predict surface dynamics under nonequilibrium conditions are usually frustrated by poor knowledge of the atomic processes of surface motion arising from mass transport from/to surface steps. Using low-energy electron microscopy that spatially and temporally resolves oxide film growth during the oxidation of NiAl(100) we demonstrate that surface steps are impermeable to oxide film growth. The advancement of the oxide occurs exclusively on the same terrace and requires the coordinated migration of surface steps. The resulting piling up of surface steps ahead of the oxide growth front progressively impedes the oxide growth. This process is reversed during oxide decomposition. The migration of the substrate steps is found to be a surface-step version of the well-known Hele-Shaw problem, governed by detachment (attachment) of Al atoms at step edges induced by the oxide growth (decomposition). By comparing with the oxidation of NiAl(110) that exhibits unimpeded oxide film growth over substrate steps we suggest that whenever steps are the source of atoms used for oxide growth they limit the oxidation process; when atoms are supplied from the bulk, the oxidation rate is not limited by the motion of surface steps.

Percolating Oxide Film Growth During Cu(111) Oxidation

We report in situ transmission electron microscopy dynamic observations of the early stage oxidation of Cu(111) surfaces at ∼450 °C, which show that the Cu2O film morphology evolves with continued oxidation from initially ramified islands to irregularly connected clusters. The geometrical features of these noncompact oxide films are analyzed in terms of the scaling theory of percolation. We show by kinetic Monte Carlo simulations that the percolating oxide film growth is related to a mechanism of neighbor-dependent site percolation.

Homogeneous Silica Formed by the Oxidation of Si(100) in Hyperthermal Atomic Oxygen

We review and summarize all of our microstructural comparisons of the silica and Si/SiO x interface created by the oxidation of Si(100) in atomic oxygen and molecular oxygen, using primarily electron microscopy techniques. A laser detonation source was used to produce atomic oxygen with kinetic energy 5.1 eV, whereas a conventional furnace was used to expose Si single crystal to thermal molecular oxygen. The silica formed on Si(100) by atomic oxygen is thicker, more homogeneous, and less amorphous (similar to alpha-quartz), as compared to the oxide layer created by molecular oxygen. High-angle annular dark field imaging and high-spatial-resolution electron energy loss spectroscopy confirmed that the Si/SiO x interface created by atomic oxygen is abrupt, containing no suboxides, as opposed to the broad interface with transitional states formed by molecular oxygen. Preliminary fluctuation electron microscopy results confirmed increased medium-range ordering in SiO x formed by atomic oxygen compared to the nonregular arrangement present in the amorphous oxide formed by the oxidation of Si(100) in molecular oxygen. Differences in the oxide films grown by exposure to atomic and molecular oxygen are discussed in the context of the thermionic emission model of silicon oxidation.

Making superior corrosion resistant aluminum oxide films using ozone-electrochemical and electron microscopy studies

Aluminum surfaces have been subjected to oxidation at 300 K, using a molecular beam of ozone of 97% purity. An enhancement of the kinetics of oxidation is found compared to oxidation by O2. The oxide film produced exhibits enhanced corrosion resistance as measured by several electrochemical methods compared to comparable thickness oxide films made by oxygen exposure. Transmission electron microscopy measurements show that the ozone-grown films exhibit small pore sizes and also increased average density as measured by the behavior of the radial distribution function derived from electron diffraction measurements on films made from ozone and oxygen. These effects may be due to the lower oxygen vacancy defect density in films made from ozone. The special properties of ozone-grown aluminum oxide films are preserved even after the films are removed from vacuum and exposed to the atmosphere.

Medium-Range Order Structures of Amorphous Diamond-Like Carbon Films

Abstract : In this paper, we have studied medium-range order structures in amorphous diamond-like carbon films with fluctuation microscopy. Medium-range order structures are quantified by a specific form of a general four-body correlation. We name this function as a pair-pair alignment correlation function. By analyzing speckle dark-field images taken over different areas as a function of momentum transfer in reciprocal space, we measured the pair-pair alignment correlation function for both thermally annealed samples and unannealed samples. We have found that thermal annealing introduces medium-range order in amorphous diamond-like carbon films, causing more pairs of atoms to be aligned. These results agree with density-functional simulations. Larger-scale simulations will be needed to filly understand our experimental results.

Bubble formation and growth in glasses

In this study, we simulated gas bubble formation in glasses by in-situ ion implantation. Alkali silicate glass and Na-borosilicate glass were implanted in situ with 50 keV Xe ions at temperatures at 200°C in a Hitachi-9000 electron microscope. Bubble formation was studied by transmission electron microscopy images taken during interruptions of the ion beam after discrete implanted-ion dose steps. We present a possible mechanism of bubble formation and growth based on amorphous network structures.

Stress relaxation and medium-range order in diamond-like amorphous carbon films

In this paper, we have studied the relationship between medium-range order structures in and stress relaxation in amorphous diamond-like carbon films with fluctuation microscopy. Our preliminary results show strong correlation between stress relaxation and medium-range order. Our previous results showed that annealing films that had been through stress relaxation procedures caused great increase of medium-ranger order. In this paper, we have found that the increase of medium-range order in films that have been annealed before going through stress relaxation through removal of substrates is less pronounced than that of films that have been annealed after removal of substrates. We will discuss interpretations and implications of these results.

Fluctuation Microscopy Studies Of Aluminum Oxides Exposed To CL Ions

Fluctuation electron microscopy studies have been performed on several aluminum oxides exposed to different electrochemical conditions. Little is known about amorphous aluminum oxide structures and their relationship with their passivation behaviors. Corrosion studies have shown that exposure of aluminum oxide films to Cl ions in solution reduces the oxides passivity, and this results in the onset of pitting corrosion. The physical changes that occur in the oxide as a result of Cl exposure have not been previously identified due to the difficulty in investigating the structure of this amorphous material. Fluctuation microscopy is a new electron microscopy technique that is able to detect the presence of medium range order structures in amorphous systems. In this paper, we will report fluctuation microscopy results on amorphous aluminum oxides that have been exposed to Cl ions in solution and compare them with oxides that have seen no electrolyte exposure or that have been exposed to electrolytes that do not contain Cl, such as SO4 2containing electrolytes. We will also compare the Cl-exposed oxides with oxides that have been implanted with Cl ions. The differences in pitting behaviors for these oxides are consistent with our previous speculation on the effect of medium range order on the passivation behavior of aluminum oxides grown using ozone.