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Michael A O'Keefe

Publications and source records attributed to Michael A O'Keefe.

6 recordsLinked to original sources
PubMed2005

HRTEM imaging of atoms at sub-Angström resolution.

John Cowley and his group at Arizona State University pioneered the use of transmission electron microscopy for high-resolution imaging. Images were achieved three decades ago showing the crystal unit cell content at better than 4 A resolution. This achievement enabled researchers to pinpoint the positions of heavy atom columns within the unit cell. Lighter atoms appear as resolution is improved to sub-Angström levels. Currently, advanced microscopes can image the columns of the light atoms (carbon, oxygen, nitrogen) that are present in many complex structures, and even the lithium atoms present in some battery materials. Sub-Angström imaging, initially achieved by focal-series reconstruction of the specimen exit surface wave, will become commonplace for next-generation electron microscopes with C(S)-corrected lenses and monochromated electron beams. Resolution can be quantified in terms of peak separation and inter-peak minimum, but the limits imposed on the attainable resolution by the properties of the microscope specimen need to be considered. At extreme resolution the 'size' of atoms can mean that they will not be resolved even when spaced farther apart than the resolution of the microscope.

Journal Article↗
PubMed2004

Sub-angstrom atomic-resolution imaging from heavy atoms to light atoms.

John Cowley and his group at Arizona State University pioneered the use of transmission electron microscopy (TEM) for high-resolution imaging. Three decades ago they achieved images showing the crystal unit cell content at better than 4 angstroms resolution. Over the years, this achievement has inspired improvements in resolution that have enabled researchers to pinpoint the positions of heavy atom columns within the cell. More recently, this ability has been extended to light atoms as resolution has improved. Sub-angstrom resolution has enabled researchers to image the columns of light atoms (carbon, oxygen, and nitrogen) that are present in many complex structures. By using sub-angstrom focal-series reconstruction of the specimen exit surface wave to image columns of cobalt, oxygen, and lithium atoms in a transition metal oxide structure commonly used as positive electrodes in lithium rechargeable batteries, we show that the range of detectable light atoms extends to lithium. HRTEM at sub-angstrom resolution will provide the essential role of experimental verification for the emergent nanotech revolution. Our results foreshadow those to be expected from next-generation TEMs with CS-corrected lenses and monochromated electron beams.

Carbon↗
PubMed2003

Atomic resolution of lithium ions in LiCoO2.

LiCoO2 is the most common lithium storage material for lithium rechargeable batteries, used widely to power portable electronic devices such as laptop computers. Operation of lithium rechargeable batteries is dependent on reversible lithium insertion and extraction processes into and from the host materials of lithium storage. Ordering of lithium and vacancies has a profound effect on the physical properties of the host materials and the electrochemical performance of lithium batteries. However, probing lithium ions has been difficult when using traditional X-ray and neutron powder diffraction techniques due to lithium's relatively low scattering power when compared with those of oxygen and transition metals. In the work presented here, we have succeeded in simultaneously resolving columns of cobalt, oxygen and lithium atoms in layered LiCoO2 battery material, using experimental focal series of LiCoO2 images obtained at sub-ångstrom resolution in a mid-voltage transmission electron microscope. Lithium atoms are the smallest and lightest metal atoms, and scatter electrons only very weakly. We believe our observations of lithium to be the first by electron microscopy, and that they show promise for direct visualization of the ordering of lithium and vacancies in transition metal oxides.

Lithium↗
PubMed2002

Atomic resolution three-dimensional electron diffraction microscopy.

We report the development of a novel form of diffraction-based 3D microscopy to overcome resolution barriers inherent in high-resolution electron microscopy and tomography. By combining coherent electron diffraction with the oversampling phasing method, we show that the 3D structure of a nanocrystal can be determined ab initio at a resolution of 1 A from 29 simulated noisy diffraction patterns. This new form of microscopy can be used to image the 3D structures of nanocrystals and noncrystalline samples, with resolution limited only by the quality of sample diffraction.

Journal Article↗
PubMed2002

HRTEM image simulations for the study of ultrathin gate oxides.

We have performed high resolution transmission electron microscope (HRTEM) image simulations to qualitatively assess the visibility of various structural defects in ultrathin gate oxides of MOSFET devices, and to quantitatively examine the accuracy of HRTEM in performing gate oxide metrology. Structural models contained crystalline defects embedded in an amorphous 16-A-thick gate oxide. Simulated images were calculated for structures viewed in cross section. Defect visibility was assessed as a function of specimen thickness and defect morphology, composition, size, and orientation. Defect morphologies included asperities lying on the substrate surface, as well as "bridging" defects connecting the substrate to the gate electrode. Measurements of gate oxide thickness extracted from simulated images were compared to actual dimensions in the model structure to assess TEM accuracy for metrology. The effects of specimen tilt, specimen thickness, objective lens defocus, and coefficient of spherical aberration (Cs) on measurement accuracy were explored for nominal 10-A gate oxide thickness. Results from this work suggest that accurate metrology of ultrathin gate oxides (i.e., limited to several percent error) is feasible on a consistent basis only by using a Cs-corrected microscope. However, fundamental limitations remain for characterizing defects in gate oxides using HRTEM, even with the new generation of Cs-corrected microscopes.

Crystallization↗