J. C. Yang

Self-limiting oxidation of copper

The classical theory of Cabrera–Mott describes passivation film formation on metals, where they predicted that this film grows as a uniform layer due to a field-enhanced ionic transport mechanism. Here we present experimental evidence, based on in situ transmission electron microscopy of copper oxidation, that the passivation film nucleates and grows as oxide islands, not as a uniform layer. We propose an alternative phenomenological theory to describe passivation film formation, based on island growth followed by coalescence.

“Contact epitaxy” observed in supported nanoparticles

We have observed the formation of heteroepitaxial interfacial layers between silver nanoparticles and a single crystal copper surface by a phenomenon we term “contact epitaxy.” Upon depositing Ag nanoparticles (5–20 nm diameter) onto clean (001) Cu in an ultrahigh vacuum in situ transmission electron microscope, a thin (111)-oriented layer of Ag was detected at the interface between the substrate and particles. Molecular dynamics simulations reveal that the epitaxial layers form within picoseconds of impact, with rapid alignment arising from mechanical relaxation of the highly stressed interface formed upon initial contact. The simulations also show that multiple grains form in the nanoparticle as a consequence of this relaxation process. The unique structure of the nanoparticles, induced by contact epitaxy, is expected to significantly influence physical properties such as interfacial bonding, diffusion, chemical activity, and electrical transport, as well as forming a nucleus for grain growth and epitax...

Oxygen surface diffusion in three-dimensional Cu2O growth on Cu(001) thin films

By studying the growth of Cu2O islands during the initial oxidation stage of Cu(001) with in situ transmission electron microscopy, it is found that the dominant mechanism for the growth of three-dimensional islands is surface diffusion of oxygen. However, there exists a non-negligible contribution to the metal oxide growth by another mechanism, probably direct impingement of the oxygen atoms on the oxide island. These results demonstrate the importance of surface conditions in oxidation.

STEM-based mass spectroscopy of supported Re clusters

Abstract Annular dark-field imaging in a scanning transmission electron microscope (STEM) is known to enhance contrast of small supported particles of heavy elements. At high scattering angles (∼ 100 mrad), this imaging technique can be used quantitatively to measure absolute particle size by comparing measured and theoretical elastic scattering cross-sections. Here, we report quantitative size measurement of small rhenium particles supported on thin graphite films. Statistical measurement of absolute cross-sections of prepared Re-6 clusters show good agreement with theoretical cross-sections, within a ±2 atom random experimental error. This is the first report of such measurements using angles sufficiently high that coherent effects can be neglected. Our experiments confirm the exceptional stability of the Re-6 organometallic compound relative to other compounds with 7, and 8-Re atoms. Details of the high angle annular dark-field STEM experiments are presented including image processing/analysis, annular dark-field detector calibration, and sources of error in the absolute intensity measurements. We discuss the future possibility of individual atomic sensititity.

In-situ observations of classical grain growth mechanisms during sintering of copper nanoparticles on (001) copper

The sintering of randomly oriented copper nanoparticles in the size range 4–20 nm with a single crystal (001) copper substrate has been studied in real time using a novel in situ ultrahigh vacuum (UHV) transmission electron microscope. The particles were generated in situ using an UHV DC sputtering attachment and deposited directly onto an electron transparent copper foil inside the microscope. We demonstrate that these particles reorient upon heating to assume the same orientation as the substrate by a classical mechanism involving neck growth and grain boundary motion.

Sintering of silver and copper nanoparticles on (001) copper observed by in-situ ultrahigh vacuum transmission electron microscopy

The sintering of copper and silver nanoparticles with single crystal copper substrates has been studied using a novel in-situ ultrahigh vacuum transmission electron microscope (UHV TEM). The system is equipped with a UHV DC sputtering attachment enabling metal nanoparticles to be generated in-situ and transferred directly into the microscope in the gas phase. In both cases, we find the particles to be of initially random orientation on the substrate. Upon annealing, however, the particles reorient and assume the same orientation as the substrate. The process apparently occurs by a mechanism involving sintering and grain growth. In the case of silver on copper, grain growth cannot occur since the metals are immiscible. Our observations show that, upon annealing, the particles wet the substrate surface and form epitaxially oriented islands by surface diffusion and grain boundary migration. The post-anneal islands exhibit the orientation relationship (111)Ag∥001)Cu, [110]Ag∥[110]Cu.

Rapid and semi-automated method for analysis of the number of atoms of ultra-small platinum clusters on carbon.

Very high angle ( approximately 100 mrad) annular dark-field (HAADF) images in a dedicated scanning transmission electron microscope (STEM) can be used to quantitatively measure the number of atoms in a cluster on a support material. We have developed a computer program which will automatically find the location of the particles and then integrate the intensity to find the number of atoms per cluster. We have examined ultra-small Pt clusters on a C substrate by this novel mass-spectroscopic technique. We discovered that the Pt clusters maintain their three-dimensional shape, and are probably spherical.

Novel interactions of supported clusters: contact epitaxy

Abstract The study of clusters of ‘model’ metal systems such as Cu and Ag provide a valuable route to explore critical issues in materials epitaxy. Our investigations have led to observations of novel interactions between supported metal clusters in both homo- and heteroepitaxial configurations. In the experiments, clusters of both Cu and Ag were produced by inert gas condensation and deposited on the clean (001)Cu surface under ultrahigh vacuum. Following deposition, the Cu clusters were observed to be of initially random orientation on the substrate surface, undergoing reorientation upon annealing by a mechanism involving sintering and grain growth. In the case of Ag clusters, the formation of a heteroepitaxial layer between the particle and substrate was observed upon initial contact. The phenomenon, which we call ‘contact epitaxy’, may be understood from molecular dynamics simulations of a ‘soft impact’ between the nanoparticle and substrate which indicate that the ordered layers form within picoseconds of impact. The experiments were performed in an ultrahigh vacuum transmission electron microscope equipped with an in-situ nanoparticle sputtering system.

Surface Reconstruction and Oxide Nucleation Due to Oxygen Interaction with Cu(001) Observed by In Situ Ultra-High Vacuum Transmission Electron Microscopy

: Reconstruction of the Cu(001) surface due to oxygen gas impingement on a clean copper surface was directly observed by in situ UHV-TEM. Strain contrast between differently oriented surface reconstruction domains, assumed to be radical2 x 2 radical2 R45, were clearly visible by this method. The reconstruction precedes the nucleation of Cu2O islands. When Cu2O islands were partially reduced and then re-oxidized, a dwell time before formation of Cu2O was noted, demonstrating that a reconstructed Cu-O surface monolayer is necessary before oxide formation.

Techniques for Studying Nanoparticle Sintering by Plan-View In Situ Transmission Electron Microscopy.

: We discuss various techniques for the characterization of supported nanoparticles by in situ plan-view transmission electron microscopy. In particular, we discuss here mechanisms of image contrast formation by particles undergoing reorientation on the surface of a single crystal substrate. We consider reorientation by a variety of mechanisms including rotation, sintering and grain growth, and surface diffusion. Experimental observations are presented and the data compared with theoretical predictions.