Biomedical subjects

M A O'Keefe

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

8 recordsLinked to original sources

PubMed2004

Atomic structure of defects in GaN:Mg grown with Ga polarity.

The atomic structure of characteristic defects (Mg-rich hexagonal pyramids and truncated pyramids) in GaN:Mg thin films grown with Ga polarity was determined at atomic resolution by reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities within the defects have inside walls covered by GaN of reverse polarity. We propose that lateral overgrowth of the cavities restores matrix polarity on the defect base. From matrix to defect, exchange of Ga and N sublattices leads to a 0.6+/-0.2 A displacement of Ga sublattices. We observe a [1100]/3 shift from matrix AB stacking to BC stacking for the entire pyramid. Electron energy loss spectroscopy detected changes in N edge and presence of oxygen on the defect walls. Our results explain commonly observed decrease of acceptor concentration in heavily doped GaN:Mg.

Journal Article↗

PubMed2004

Screw dislocations in GaN grown by different methods.

A study of screw dislocations in hydride-vapor-phase-epitaxy (HVPE) template and molecular-beam-epitaxy (MBE) overlayers was performed using transmission electron microscopy (TEM) in plan view and in cross section. It was observed that screw dislocations in the HVPE layers were decorated by small voids arranged along the screw axis. However, no voids were observed along screw dislocations in MBE overlayers. This was true both for MBE samples grown under Ga-lean and Ga-rich conditions. Dislocation core structures have been studied in these samples in the plan-view configuration. These experiments were supported by image simulation using the most recent models. A direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high-resolution images was applied. It was shown that the core structures of screw dislocations in the studied materials were filled. The filed dislocation cores in an MBE samples were stoichiometric. However, in HVPE materials, single atomic columns show substantial differences in intensities and might indicate the possibility of higher Ga concentration in the core than in the matrix. A much lower intensity of the atomic column at the tip of the void was observed. This might suggest presence of lighter elements, such as oxygen, responsible for their formation.

Gallium↗

PubMed2001

Imaging columns of the light elements carbon, nitrogen and oxygen with sub Angstrom resolution.

It is reported that lattice imaging with a 300 kV field emission microscope in combination with numerical reconstruction procedures can be used to reach an interpretable resolution of about 80 pm for the first time. A retrieval of the electron exit wave from focal series allows for the resolution of single atomic columns of the light elements carbon, nitrogen, and oxygen at a projected nearest neighbor spacing down to 85 pm. Lens aberrations are corrected on-line during the experiment and by hardware such that resulting image distortions are below 80 pm. Consequently, the imaging can be aberration-free to this extent. The resolution enhancement results from increased electrical and mechanical stability of the instrument coupled with a low spherical aberration coefficient of 0.595 + 0.005 mm.

Journal Article↗

PubMed2001

Sub-Angstrom high-resolution transmission electron microscopy at 300 keV.

Sub-Angstrom transmission electron microscopy has been achieved at the National Center for Electron Microscopy (NCEM) by a one-Angstrom microscope (OAM) project using software and enhanced hardware developed within a Brite-Euram project (Ultramicroscopy 64 (1996) 1). The NCEM OAM provides materials scientists with transmission electron microscopy at a resolution better than 1 A by using extensive image reconstruction to exploit the significantly higher information limit of an FEG-TEM over its Scherzer resolution limit. Reconstruction methods chosen used off-axis holograms and focal series of underfocused images. Measured values of coherence parameters predict an information limit of 0.78 A. Images from a [1 1 0] diamond test specimen show that sub-Angstrom resolution of 0.89 A has been achieved with the OAM using focal series reconstruction.

Journal Article↗

PubMed2001

A program description of health care interventions for homeless teenagers.

This prospective review was designed to determine the effectiveness of a broad-spectrum health intervention program for homeless and runaway youth. Diagnosis, treatment, and counseling for drug use, sexually transmitted diseases (STDs), and other health issues were provided all new admissions to a residential care facility during a 2-month enrollment. Education was continued during a 9-month follow-up period based on the program entitled Bright Futures, previously developed and published by the National Center for Education in Maternal and Child Health. Sixty percent of the 106 study residents had STDs on admission and 7% developed new STDs after completing therapy and undergoing counseling. Drug dependence was reduced from 41% to 3%, and 42% achieved full-time or part-time employment. Fifty-nine percent completed hepatitis B immunization with the 3-dose series. This experience suggests that an organized program of interventions in a residential care facility for homeless teenagers can significantly reduce drug dependence and STDs.

Adolescent↗

PubMed1992

3D structure determination from electron-microscope images: electron crystallography of staurolite.

Resolution of better than 2 A has been obtained in many crystals by high-resolution electron microscopy. Although this resolution is sufficient to resolve interatomic spacings, structures are traditionally interpreted by comparing experimental images with contrast calculations. A drawback of this method is that images are 2D projections in which information is invariably obscured by overlap of atoms. 3D electron crystallography, developed by biophysicists to study proteins, has been used to investigate the crystal structure of staurolite. Amplitudes and phases of structure factors are obtained experimentally from high-resolution images (JEOL ARM 1000 at the National Center for Electron Microscopy at LBL), taken in different directions from thin regions where dynamic scattering is minimal. From images in five orientations (containing 59 independent reflections to a resolution of 1.38 A), a 3D electron potential map is constructed which resolves clearly all cations (Al, Si, Fe, including those with partial occupancy) and all O atoms. This method has great potential in crystal structure determinations of small domains in heterogeneous crystals which are inaccessible to X-ray analysis. It is estimated that 3D structure determinations should be possible on regions only about ten unit cells wide and should resolve not only atom positions but also site occupancies. The method is also applicable to space-group determination.

Chemical Phenomena↗

PubMed1991

Use of nonanatomical dolls in the sexual abuse interview.

Controversy exists regarding use of anatomically detailed dolls in child sexual abuse evaluations because of concerns that such dolls may provoke false positive demonstrations of sexually explicit behavior. This study shows that children referred for medical evaluation of sexual abuse will use sexually explicit behavior to demonstrate what has happened to them with nonanatomical dolls as frequently as when they are interviewed with anatomically detailed ones. Over a two-year period, 136 children (aged 24 months to 10 years) were interviewed by the same pediatric interviewer. During the first year sexually anatomically detailed dolls (SAD) were used, and in the subsequent year nonanatomic dolls (NAD) were used. Data was analyzed according to age, sex, and demonstration of sexually explicit behavior. There were 67 children in the NAD group and 69 in the SAD group. Of the NAD group, 72% showed sexually explicit behavior compared to 68% in the SAD group. Comparisons using chi-square analysis revealed no significant differences between NAD and SAD. Results indicated that in the sexual abuse interview setting, use of sexually detailed dolls did not increase children's use of sexually explicit behavior to describe what had happened to them when compared to use of nonanatomical dolls, and that use of either type of doll provides similar information in the interview setting.

Child↗

PubMedDate not supplied

"Helicopter" fracture.

An interesting fracture of the fibula is described which occurs very close to the radiographic isthmus of the bone. This fracture is produced by a rotational maneuver but does not result in a rotational type injury. It is our conclusion that these fractures were produced by a force distributed along the long axis of the fibula and bear a direct relationship to that fibula's anatomical ability or inability to transmit the force required of it at the moment of forcible dorsiflexion of the foot.

Adolescent↗