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Stephen J Pennycook

Publications and source records attributed to Stephen J Pennycook.

10 recordsLinked to original sources
PubMed2006

Three-dimensional ADF imaging of individual atoms by through-focal series scanning transmission electron microscopy.

Aberration correction in scanning transmission electron microscopy has more than doubled the lateral resolution, greatly improving the visibility of individual impurity or dopant atoms. Depth resolution is increased five-fold, to the nanometer level. We show how a through-focal series of images enables single Hf atoms to be located inside an advanced gate dielectric device structure to a precision of better than 0.1 x 0.1 x 0.5 nm. This depth sectioning method for three-dimensional characterization has potential applications to many other fields, including polycrystalline materials, catalysts and biological structures.

Journal Article↗
PubMed2006

Depth sectioning with the aberration-corrected scanning transmission electron microscope.

The ability to correct the aberrations of the probe-forming lens in the scanning transmission electron microscope provides not only a significant improvement in transverse resolution but in addition brings depth resolution at the nanometer scale. Aberration correction therefore opens up the possibility of 3D imaging by optical sectioning. Here we develop a definition for the depth resolution for scanning transmission electron microscope depth sectioning and present initial results from this method. Objects such as catalytic metal clusters and single atoms on various support materials are imaged in three dimensions with a resolution of several nanometers. Effective focal depth is determined by statistical analysis and the contributing factors are discussed. Finally, current challenges and future capabilities available through new instruments are discussed.

Journal Article↗
PubMed2006

Structural basis for near unity quantum yield core/shell nanostructures.

Aberration-corrected Z-contrast scanning transmission electron microscopy of core/shell nanocrystals shows clear correlations between structure and quantum efficiency. Uniform shell coverage is obtained only for a graded CdS/ZnS shell material and is found to be critical to achieving near 100% quantum yield. The sublattice sensitivity of the images confirms that preferential growth takes place on the anion-terminated surfaces. This explains the three-dimensional "nanobullet" shape observed in the case of core/shell nanorods.

Cadmium Compounds↗
PubMed2004

Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions.

Silicon nitride (Si3N4) ceramics are used in numerous applications because of their superior mechanical properties. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives. Understanding the origin of dopant effects, central to the design of high-performance Si3N4 ceramics, has been sought for many years. Here we show direct images of dopant atoms (La) within the nanometre-scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La for the crystalline surfaces, which is essential for forming elongated grains and a toughened microstructure. Whereas principles of micrometre-scale structural design are currently used to improve the mechanical properties of ceramics, this work represents a step towards the atomic-level structural engineering required for the next generation of ceramics.

Journal Article↗
PubMed2004

Dopants adsorbed as single atoms prevent degradation of catalysts.

The design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers. For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous gamma-Al(2)O(3), a widely used catalyst and catalytic support, transforms to non-porous alpha-Al(2)O(3) at approximately 1,100 degrees C (refs 7-10). Doping with La raises the transformation temperature to approximately 1,250 degrees C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds. Here, we use direct imaging by aberration-corrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and first-principles calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to alpha-Al(2)O(3). The results provide the first guidelines for the choice of dopants to prevent thermal degradation of catalysts and other porous materials.

Aluminum Oxide↗
PubMed2004

Quasicrystals as cluster aggregates.

Quasicrystals are solids that exhibit symmetries long thought forbidden in nature. Since their discovery in a rapidly solidified Al-Mn alloy in 1984, the central issue in the field has been to understand why they form. Are they energetically stable compounds or stabilized by entropy? In recent years, major strides have been made in determining atomic structure, largely by direct imaging using advanced electron microscopy. One system is now known to be energetically stabilized, and quasicrystals are therefore firmly established as a new physical state of matter. They represent a unique packing of atomic clusters some tens of atoms in size, with substantial localized fluctuations, referred to as phasons. Understanding phasons may in future allow their unique macroscopic properties to be tailored for useful materials applications.

Crystallization↗
PubMed2004

HAADF-STEM imaging with sub-angstrom probes: a full Bloch wave analysis.

A full coherent Bloch wave calculation is presented to investigate high-angle annular dark-field image formation for sub-angstrom probes in scanning transmission electron microscopy (STEM). With increasing illumination angle, the contribution of the 1s bound state increases to a maximum at an optimum probe angle, after which we find increasing contributions from high-angle plane wave states around the periphery of the objective aperture. Examination of image contributions from different depths within a crystal shows an oscillatory behavior due to the beating between 1s and non-1s states. The oscillation period reduces with decreasing probe size, while the relative contribution from a specific depth increases. This signifies a changeover from a projection mode of imaging to a depth-slicing mode of imaging. This new mode appears capable of resolving three-dimensional atomic structures in future generation aberration-corrected STEM.

Algorithms↗