Biomedical subjects

JM Gibson

Publications and source records attributed to JM Gibson.

18 recordsLinked to original sources

PubMed2000

Strain evolution in coherent Ge/Si islands

Strain evolution of coherent Ge islands on Si(001) is measured using a newly developed transmission electron microscopy technique based on two-beam dark-field strain imaging. The strain measurements show that a metastable Ge island shape is involved in the shape transition between pyramids and domes; this shape is more readily observed for growth at 550 than 600 degrees C because of the slower rate at which islands cross the kinetic barrier between shapes. The strain relaxation changes discontinuously between pyramids and domes, indicating that the underlying shape transition is first order.

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PubMed2000

Measurement of internuclear distances in solid-state NMR by a background-filtered REDOR experiment

A background-filtered version of the rotational-echo double resonance (REDOR) experiment is demonstrated. The experiment combines a traditional REDOR pulse sequence with a double-cross-polarization (DCP) sequence to select only those signals coming from spin pairs of interest. The relatively inefficient DCP sequence, which transfers polarization from (1)H to (15)N and subsequently to (13)C, is improved by the use of adiabatic passages through the (-1) sideband of the Hartmann-Hahn matching condition. The result is an efficient 2D-REDOR pulse sequence that does not require a reference experiment for removal of background signals. The data produced by the experiment are ideally suited to analysis by newly developed dipolar transform methods, such as the REDOR transform. The relevant features of the experiment are demonstrated on simple labeled amino acids. Relative efficiencies of several other potential filtering methods are also compared. Copyright 2000 Academic Press.

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PubMed2000

Atom pair persistence in disordered materials from fluctuation microscopy

We show that the kinematical theory of fluctuation microscopy for a disordered specimen can be reformulated in terms of an atom pair-pair correlation function. Resolution and coherence are two experimental variables which yield a two-dimensional fluctuation map. This map is a transformation of the pair-pair correlation function, but can be directly interpreted in terms of a pair persistence function (PPF). We show that the PPF reveals the correlation length and structure for a simple paracrystalline model.

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PubMed2000

TEM measurement of strain in coherent quantum heterostructures

We report on a transmission electron microscopy (TEM) technique that can be used to measure strain due to individual nanometer-scale coherent heterostructures such as quantum dots or inclusions. The measurement relies on two-beam imaging and on an approximation that employs a universal model for lattice plane bending. We demonstrate that analysis is simple and accurate. Using this method, we measured the average strain in dome-shaped Ge islands grown on Si (0 0 1). We found that the method of specimen preparation can significantly affect the observed strain in these islands.

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PubMed2000

Transmission electron microscopy of semiconductor quantum dots

We report on plan-view transmission electron microscopy techniques, by which the size, the actual shape and the strain of a coherent quantum dot in semiconductor heterostuctures can be measured very accurately. The bright-field suppressed-diffraction imaging condition where no strong diffracted beam is excited in the sample provides reliable size measurement. Using suppressed-diffraction imaging condition, the intensity contour in a coherent island is related to the height, and thus the detailed shape and the aspect ratio can be extracted. The strain contrast of a coherent island imaged using an exact two-beam dynamical diffraction condition is useful for strain measurement and the corresponding features is related to the shape of an island. The physical origins and accuracy of interpretation of the image contrast are discussed, using the simulations and experimental examples.

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PubMed2000

Fluctuation microscopy: a probe of atomic correlations in disordered materials

Fluctuation microscopy is a new technique to study medium-range structure in disordered materials. Low-resolution dark-field transmission electron microscope images are interpreted as spatially resolved diffraction maps. Statistical analysis of intensity variations in these maps provides information about higher-order atom position correlation functions. Here we give a qualitative description of the technique and explore the properties and utility of higher-order correlation functions in describing the structure of disordered materials.

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PubMed1998

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.

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PubMed1998

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.

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PubMed1996