Daniel P. Fogarty

The design and application of an in-laboratory diffraction-enhanced x-ray imaging instrument

We describe the design and application of a new in-laboratory diffraction-enhanced x-ray imaging (DEXI) instrument that uses a nonsynchrotron, conventional x-ray source to image the internal structure of an object. In the work presented here, a human cadaveric thumb is used as a test-sample to demonstrate the imaging capability of our instrument. A 22 keV monochromatic x-ray beam is prepared using a mismatched, two-crystal monochromator; a silicon analyzer crystal is placed in a parallel crystal geometry with the monochromator allowing both diffraction-enhanced imaging and multiple-imaging radiography to be performed. The DEXI instrument was found to have an experimentally determined spatial resolution of 160+/-7 mum in the horizontal direction and 153+/-7 mum in the vertical direction. As applied to biomedical imaging, the DEXI instrument can detect soft tissues, such as tendons and other connective tissues, that are normally difficult or impossible to image via conventional x-ray techniques.

In‐laboratory diffraction‐enhanced X‐ray imaging for articular cartilage

The loss of articular cartilage characteristic of osteoarthritis can only be diagnosed by joint space narrowing when conventional radiography is used. This is due to the lack of X‐ray contrast of soft tissues. Whereas conventional radiography harnesses the X‐ray attenuation properties of tissues, Diffraction Enhanced Imaging (DEI), a novel radiographic technique, allows the visualization of soft tissues simultaneous with calcified tissues by virtue of its ability to not only harness X‐ray attenuation but also the X‐ray refraction from tissue boundaries. Previously, DEI was dependent upon synchrotron X‐rays, but more recently, the development of nonsynchrotron DEI units has been explored. These developments serve to elaborate the full potential of radiography. Here, we tested the potential of an in‐laboratory DEI system, called Diffraction‐Enhanced X‐ray Imaging (DEXI), to render images of articular cartilage displaying varying degrees of degradation, ex vivo. DEXI allowed visualization of even early stages of cartilage degeneration such as surface fibrillation. This may be of eventual clinical significance for the diagnosis of early stages of degeneration, or at the very least, to visualize soft tissue degeneration simultaneous with bone changes. Clin. Anat. 23:530–538, 2010.

Minimizing image-processing artifacts in scanning tunneling microscopy using linear-regression fitting

We present a method for removing noise from scanning tunneling microscopy images based on least-squares fitting of spatial data. Modeling the known structure of the surface, including isolated features and surface steps, allows for effective discrimination of signal from noise and produces minimal processing artifacts, even for very noisy images. This approach is effective for removing external noise due to vibrational or acoustic interference, and can also be applied to correct for tip-related height jumps as well as to flatten images warped by thermal effects or nonlinearity of the microscope scanner.

Design of a scanning tunneling microscope for in situ observation of the interactions of molecular beams with surfaces

We describe an ultrahigh vacuum scanning tunneling microscope (UHV-STM) that is interfaced to a pulsed molecular-beam source. Optimization of the vibration isolation of the STM and molecular beam source allows a sample to be imaged before, during, and after molecular-beam dosing, without ever having to remove the sample from the microscope. A helium-seeded argon beam was used to effect collision-induced mobility of C60 molecules adsorbed on the Au(111) surface. Changes in the sample were monitored using STM. The ability to image a sample during exposure to a molecular beam opens up new avenues for looking at physical and chemical processes on highly heterogeneous surfaces.

Structural changes of an octanethiol monolayer via hyperthermal rare-gas collisions

In situ scanning tunneling microscopy is used to measure the effect of hyperthermal rare-gas bombardment on octanethiol self-assembled monolayers. Close-packed monolayers remain largely unchanged, even after repeated collisions with 0.4 eV argon and 1.3 eV xenon atoms. In contrast, gas-surface collisions do induce structural changes in the octanethiol film near defects, domain boundaries, and disordered regions, with relatively larger changes observed for xenon-atom bombardment.

Collision-induced annealing of octanethiol self-assembled monolayers by high-kinetic-energy xenon atoms

Collisions with high-energy xenon atoms (1.3 eV) induce structural changes in octanethiol self-assembled monolayers on Au(111). These changes are characterized at the molecular scale using an in situ scanning tunneling microscope. Gas-surface collisions induce three types of structural transformations: domain boundary annealing, vacancy island diffusion, and phase changes. Collision-induced changes that occur tend to increase order and create more stable structures on the surface. We propose a mechanism where monolayer transformations are driven by large amounts of vibrational energy localized in the alkanethiol molecules. Because we monitor incremental changes over small regions of the surface, we can obtain structural information about octanethiol monolayers that cannot be observed directly in scanning tunneling microscopy images.

Structure and dynamics of fullerenes adsorbed on the Au(111) surface

Scanning tunneling microscopy was used to characterize the structure of partial monolayers of C60 and C70 on the Au(111) surface. Both 2√3 × 2√3 R30° and 7 × 7 lattice symmetries were observed for C60 monolayers, in accordance with previous results. For C70 monolayers, structures are observed with rotation angles of 0°, 30°, and 14° with respect to the underlying substrate; we propose a previously unreported √13 × √13 R13.9° lattice structure to explain this last observation. Time sequences of STM images show that while fullerene monolayers are largely stable, molecular motion can be observed on the timescale of minutes or hours. For C60, thermal diffusion is the predominant cause of this motion, and STM perturbation of the sample is negligible. In contrast, C70 is observed to diffuse far more slowly; under normal scanning conditions, tip-induced motion is the major effect.